The Late Cenozoic of Patagonia and Tierra del Fuego (Developments in Quaternary Sciences, Volume 11)

DEVELOPMENTS IN QUATERNARY SCIENCES SERIES EDITOR: JAAP J.M. VAN DER MEER

11

THE LATE CENOZOIC OF PATAGONIA AND TIERRA DEL FUEGO

Developments in Quaternary Sciences Series editor: Jaap J.M. van der Meer Volumes in this series 1.

The Quaternary Period in the United States Edited by A.R. Gillespie, S.C. Porter, B.F. Atwater 0-444-51470-8 (hardbound); 0-444-51471-6 (paperback) – 2004

2.

Quaternary Glaciations – Extent and Chronology Edited by J. Ehlers, P.L. Gibbard Part I: Europe ISBN 0-444-51462-7 (hardbound) – 2004 Part II: North America ISBN 0-444-51592-5 (hardbound) – 2004 Part III: South America, Asia, Australasia, Antarctica ISBN 0-444-51593-3 (hardbound) – 2004

3.

Ice Age Southern Andes – A Chronicle of Paleoecological Events By C.J. Heusser 0-444-51478-3 (hardbound) – 2003

4.

Spitsbergen Push Moraines – Including a translation of K. Gripp: Glaciologische und geologische Ergebnisse der Hamburgischen Spitzbergen-Expedition 1927 Edited by J.J.M. van der Meer 0-444-51544-5 (hardbound) – 2004

5.

Iceland – Modern Processes and Past Environments ´ . Knudsen Edited by C. Caseldine, A. Russell, J. Hardardo´ttir, O 0-444-50652-7 (hardbound) – 2005

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Glaciotectonism By J.S. Aber, A. Ber 978-0-444-52943-5 (hardbound) – 2007

7.

The Climate of Past Interglacials Edited by F. Sirocko, M. Claussen, M.F. Sa´nchez Gon˜i, T. Litt 978-0-444-52955-8 (hardbound) – 2007

8.

Juneau Icefield Research Project (1949–1958) – A Retrospective By C.J. Heusser † 978-0-444-52951-0 (hardbound) – 2007

9.

Late Quaternary Climate Change and Human Adaptation in Arid China By David B. Madsen, Chen Fa-Hu, Gao Xing 978-0-444-52962-6 (hardbound) – 2007

10. Tropical and Sub-Tropical West Africa – Marine and Continental Changes During the Late Quaternary By P. Giresse 978-0-444-52984-8 – 2008 11. The Late Cenozoic of Patagonia and Tierra del Fuego Edited by J. Rabassa 978-0-444-52954-1 (hardbound) – 2008 For further information as well as other related products, please visit the Elsevier homepage (http://www.elsevier.com)

Developments in Quaternary Sciences, 11 Series editor: Jaap J.M. van der Meer

THE LATE CENOZOIC OF PATAGONIA AND TIERRA DEL FUEGO Edited by

J. Rabassa Laboratorio de Geologı´a del Cuaternario; CADIC-CONICET, Universidad Nacional de la Patagonia, Ushuaia, Argentina

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To the memory of my beloved father, Professor Roger J.B. Rabassa, still missing him after 25 years since his much mourned passing away. To my mother, Lila Sarraillet, for her permanent support and encouragement during as much as the last 60 years. To Martı´n, Mariano and Marina, in love. To Teo, Vero´nica and Damion, in affection. To Manuel, in hope and dreams. To Mo´nica, in devotion and gratitude. To my teachers, instructors and professors at all education levels, with many special thanks to Francisco Fidalgo, Fe´lix Gonza´lez Bonorino, Edgardo Rolleri, Donald R. Coates and Marie Morisawa.

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Contents

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Introduction Jorge Rabassa

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Calvin John Heusser (1924–2006): A Life Devoted to the Quaternary of Patagonia and Tierra del Fuego Jorge Rabassa

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The Physical Geography of Patagonia and Tierra del Fuego Andrea M.J. Coronato, Fernando Coronato, Elizabeth Mazzoni and Mirian Va´zquez

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Tectonic Evolution of the Patagonian Andes Vı´ctor A. Ramos and Matı´as C. Ghiglione

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Neotectonics, Seismology and Paleoseismology Laura Perucca and Hugo Bastias

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Late Cenozoic Quaternary Volcanism in Patagonia and Tierra del Fuego Hugo Corbella and Luis E. Lara

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Late Cenozoic Paleomagnetic Studies in Patagonia Guillermo H. Re, Mario Mena and Juan Francisco Vilas

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Late Cenozoic Glaciations in Patagonia and Tierra del Fuego Jorge Rabassa

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The Late Cenozoic Fluvial Deposits of Argentine Patagonia Oscar A. Martı´nez and Andrea M.J. Coronato

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Coastal Geology and Morphology of Patagonia and the Fuegian Archipelago Federico I. Isla and Gustavo G. Bujalesky

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Late Pleistocene Environmental Change in Eastern Patagonia and Tierra del Fuego – A Limnogeological Approach Daniel Ariztegui, Flavio S. Anselmetti, Adrian Gilli and Nicola´s Waldmann

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Geocryology of Southern South America Darı´o Trombotto Liaudat

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Neogene Vertebrates from Argentine Patagonia: Their Relationship with the Most Significant Climatic Changes Eduardo P. Tonni and Alfredo A. Carlini

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Late Cenozoic Invertebrate Paleontology of Patagonia and Tierra del Fuego, with Emphasis on Molluscs Marina L. Aguirre, Julio C. Hlebszevitsch Savalscky and Florencia Dellatorre

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Calcareous Microfossils (Foraminifera and Ostracoda) of the Late Cenozoic from Patagonia and Tierra del Fuego: A Review Gabriela C. Cusminsky and Robin C. Whatley

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Late Miocene Continental and Marine Palynological Assemblages from Patagonia Viviana Barreda, Vero´nica Guler and Luis Palazzesi

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Contents Late Quaternary Vegetation and Climate of Patagonia Marı´a Virginia Manzini, Aldo R. Prieto, Marta Mercedes Paez and Frank Scha¨bitz

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Late and Postglacial Paleoenvironments of Tierra del Fuego: Terrestrial and Marine Palynological Evidence Ana M. Borromei and Mirta Quattrocchio

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Diatoms from Patagonia and Tierra del Fuego Marcela A. Espinosa

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Quaternary Fossil Insects from Patagonia Julieta Massaferro, Allan Ashworth and Stephen Brooks

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Understanding Climate from Patagonian Tree Rings Fidel A. Roig and Ricardo Villalba

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Archeological Hunter-Gatherer Landscapes Since the Latest Pleistocene in Fuego-Patagonia Mo´nica C. Salemme and Laura L. Miotti

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Late Cenozoic Mineral Resources of Argentine Patagonia Isidoro B. Schalamuk, Rau´l E. de Barrio and Miguel A. Del Blanco

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Late Cenozoic Geohydrology of Extra-Andean Patagonia, Argentina Mario A. Herna´ndez, Nilda Gonza´lez and Lisandro Herna´ndez

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1 Introduction Jorge Rabassa Laboratorio de Geologı´a del Cuaternario, CADIC-CONICET, Universidad Nacional de la Patagonia, Ushuaia and the ancient forests became limited to both slopes of the Andean mountain ranges. Then, in the Late Miocene, Patagonia’s endless plains became the realm of the roaring westerlies as the land changed into impressive mountains, fertile piedmont slopes and eastern arid plateaus, bearing endemic faunas, living fossil trees and countless deflation hollows and salt lakes. These are the reasons why this book, which is included in a Quaternary series, starts its narration in the Late Miocene, due to the proven geographical, geological and biological continuity of Patagonia since those quite ancient times. Patagonia has magic connotations for the outside world. With a total surface of slightly over 1 million square kilometer, Patagonia has the equivalent area of France and Spain combined, or Texas and New Mexico together. Being a land of adventure, mystery and opportunity, it is one of the least populated regions in the world and the southernmost territories with temperate continental ecosystems. It is today a chosen destination by thousands of tourists that flock from all over the world, searching for the well-promoted Patagonian enchantments. Patagonia is the last corner of the Americas that was colonized by humans. It was one of the last places in the Americas to be occupied by European immigrants, with the exception of the Amazonian forests. As a proof of the recent disentanglement of the Patagonian mysteries, there were ethnic groups in central Tierra del Fuego still living under Stone Age conditions as late as the 1930s. Patagonia was discovered and explored by southward-moving Asian-origin hunter-gatherer communities, more than 12,000 yrs ago. The waterless infinite plains were traversed on foot, generation after generation, until humans arrived at the southernmost end, yet even before modern man settled in Scandinavia as ice receded. For a very long time, perhaps unmatched anywhere else in the world with the exception of Australia, they lived as stable communities in magnificent harmony with a tough, severe, demanding environment, with relatively minor changes in their cultural background. A very long time indeed spanned until the first Hispanic explorers surveyed the Patagonian coasts. Fernando de Magallanes and Sebastia´n El Cano brought notice to the western Europeans about a world of strange, harsh lands, conveying legends of colossal giants and bigfeeted monsters, of opulent splendor and golden cities. They expanded the promise of a hypothetical Terra Incognita that was assumed to be spanning the entire southern pole.

Patagonia is the southernmost portion of the Southern Cone. It is defined as the geographical region east of the Andean Range extending between the Rı´o Colorado (35–36 S), a major river descending from the eastern side of the Andes to the Atlantic Ocean, and the Cape Horn, the southernmost point of South America (56 S). The island of Tierra del Fuego and the Fuegian Archipelago are thus considered as part of the Patagonian region and hereinafter, when using the term ‘‘Patagonia’’ it will be assumed that the Fuegian lands are included in this concept. By extension, this name has been applied also to the adjacent lands in Chile, in comparable latitudes west of the Andes. In fact, a small portion of Chile is actually part of those lands corresponding to the classical definition of Patagonia, east of the Andes, within the Magellan Strait area. Patagonia is an ancient, buoyant fragment of the Gondwana supercontinent, which merged with the South American shield core sometime in the Paleozoic. Present-day Patagonia is mostly located within the South American plate, but the southernmost part of the Patagonian Andes and the Fuegian Archipelago is included in the Scotia plate, south of the major Magallanes–Fagnano fault zone. From many points of view, Patagonia is a rather unique region not only in the Southern Hemisphere but also at the global level. A mosaic of Paleozoic cratons and Mesozoic rift basins related to the opening of the South Atlantic Ocean are backed by a very massive mountain range, the Patagonian Andes, of highly complex lithological, structural and geomorphological distinctiveness and history. The tectonic chronicle of Patagonia is exposed by very active and extensive volcanism, almost recurrently from the Late Permian until today as several active volcanoes appear mostly along the Chilean slope of the Andes. Volcanism is highly relevant to Late Cenozoic studies, due both to the possibility of radiometric dating and determining their paleomagnetic signal, thus allowing not only for local chronologies but also for global correlation of geological and paleontological events. With a long history behind it, Patagonia became what it is today during the Late Miocene, when it reached its present latitudinal position, becoming separated from the Antarctic Peninsula as the Drake Passage was opened by the eastward push of the Scotia plate. Then, the Andes achieved their present morphology and perhaps their current elevation, blocking the wet winds coming from the south Pacific oceanic anticyclone. Subsequently, the greenish, Miocene subtropical savannas were rapidly replaced by the gray, dry steppes of today, the large, shallow marine basins of Mid-Cenozoic times emerged,

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DEVELOPMENTS IN QUATERNARY SCIENCES VOLUME 11 ISSN 1571-0866 1

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In the 1830s, Charles Darwin set the first, modern scientific eye on this region. The Beagle vessel was the messenger of the expanding empire and Robert Fitz Roy was its competent captain. Darwin was the brain of that expedition which splendidly informed the world about the Patagonian landscape, nature and anthropology. Thus, Patagonia became known to the scientists of the world and the nineteenth-century explorers started to cross deserts and plains, climb inaccessible mountains, negotiate deep forests and scrutinize endless coastal fringes. The early explorers were followed by the modern scientists. Francisco P. Moreno, Florentino Ameghino, Otto Nordenskjo¨ld, Guido Rovereto, Carl Skottsberg, Carl Czon Caldenius, Egidio Feruglio, Erik Ljungner, Pablo Groeber, Vaı¨no Auer, among many others, unlocked Patagonia for contemporary science. Patagonia has a crucial geographical location. It is the closest temperate land to Antarctica, and particularly to the Antarctic Peninsula, which is found just 1000 km south of Ushuaia, the southernmost city of the world. It is the closest landmass to the Antarctic Circumpolar Current, which drives the climate of the southern oceans by means of the emerging cold streams such as the Humboldt, Malvinas/Falklands or Benguela currents, and therefore rules the climate of the entire Southern Hemisphere. Patagonia is the continental wedge that protrudes into the southern oceans. Its present climate reflects the enormous oceanic climate influence, and most likely this has been the case since the Late Miocene. Thus, it is assumed that the paleoclimate record of Patagonia is closely depicting the climate of Antarctica and the Southern Hemisphere during coeval periods. Most probably, the Late Cenozoic climate of the world has been determined by the thermohaline marine circulation, closely related to the behavior of the Antarctic oceanic masses and the marine currents originating there. Therefore, paleoclimatic and paleoenvironmental studies in Patagonia shall be decisive to expose the world paleoclimate framework, or at least they should allow global correlation and interpretation of foremost, key events. This is the main hypothesis underlying the aim and contents of this book. It is hoped that the objective of revealing this hypothesis has been achieved. Patagonia is today the scenario of many geological, tectonic and paleontological studies related to Late Cenozoic times, both by Argentine and Chilean investigators as well as by foreign scientists, at a scale never imagined before in Patagonian history. It is nowadays the battleground of fascinating discussions, of endless debate, of careful, comprehensive and meticulous research projects, which bring the scientists into barely populated regions, to almost unknown areas and highly inaccessible sites, of unheard beauty and loveliness. It is sincerely expected that this book will stimulate the strengthening of future regional studies of global significance, bringing together local and foreign scientists, trying to solve the major paleoclimate and paleoenvironmental problems, at times when mankind is fighting to understand the global climate change dilemma and its future consequences.

The idea of this book started many years ago during an informal conversation with Professor Jaap van der Meer. More recently, Jaap requested me to submit a proposal for such a book which might be included in the present series. I am extremely thankful to Jaap for his kind offer, for his confidence in my potential to lead this editorial process to a fruitful end, for his generosity in allowing me to choose the main authors for each of the chapters, for his priceless assistance in the editing and reviewing process and for his endless understanding patience when awaiting for the forever delayed manuscripts. Most improvements on the text are the result of his proficiency, and all mistakes and inaccuracies are exclusively my fault. I do thank Elsevier Publishing Co. for the opportunity offered, and to all officers and personnel involved in the production of this book for their confidence, permanent deference and reliable guidance. As the reader can promptly observe, all chapters have a leading Argentine author. There is a long, more than centenary scientific tradition on Patagonian studies in Argentina, and I understood that the local scientists deserved the opportunity of presenting their work and their points of view. Once the main authors were contacted, they were offered the option of finding their respective coauthors, if any, among other distinguished Argentine or Chilean colleagues as well as important foreign scientists who have been active in the region. Another reason for this choice is that an extensive scientific production is available in review books, local journals and scientific meetings published in Spanish (some of them in French and German, the science languages of old times), which started in the late 1800s. This extensive production is either unavailable to foreign scientists because such bibliography is not found outside Argentina and Chile or because language limitations of non-Latin speakers would preclude them to read it. Thus, thanks to the efforts of the respective authors, this book brings also a major collection of scientific references about the studied regions which has never been compiled before and which is now available to all scientists. Finally, it was intended that the Argentine and Chilean authors would convey the South American vision to the problems studied, a comprehensive, ample view over Patagonia and its close relationship with the nature and processes of the Pampas and the tropical regions. I am greatly indebted to all authors for their great efforts, plentiful generosity and endless patience and collaboration during the editing process. This book is dedicated to Calvin J. Heusser, who devoted most of his life to the study of the Quaternary of Patagonia and Tierra del Fuego, leading and supporting me as well as many other Argentine and Chilean scientists during almost the last four decades. In an unpretentious, modest, certainly incomplete and probably unfair overview of his life (Chapter 2), I have tried to expose our thankfulness and deep feelings for his recent and mourned death. This is certainly inadequate to clearly display the vastness of Cal’s contributions to Patagonian science, but at least we want to recall the reader’s attention to his paramount professional life and our enduring gratitude for all his help, bigheartedness and encouragement.

Introduction The first chapters of the book provide a general framework to understand the geographical, geological, tectonic, and geomorphological background of Patagonia and Tierra del Fuego. In Chapter 3, Andrea Coronato, Fernando Coronato, Elizabeth Mazzoni, and Mirian Va´zquez offer an updated, modern, and innovative overview of the physical geography of Patagonia and Tierra del Fuego, linking it with the regional geology and geomorphology as well as to human activities along the endless landscapes. The tectonic evolution of the Patagonian Andes has been discussed by Vı´ctor Ramos and Matı´as Ghiglione (Chapter 4), who provided information about the general bedrock and structure characteristics of the region, with an modernized vision of the regional structural units, geological provinces and tectonic history. Laura Perucca and Hugo Bastias present an ample summary of the neotectonics of the region, with emphasis on the seismology and paleoseismology (Chapter 5). Being an area deeply influenced by the Pacific tectonic plate activity, Patagonia is highly sensitive to recent movements and the evidence of neotectonic activity is fully considered in the chapter. The Late Cenozoic Quaternary volcanism of the region, by Hugo Corbella and Luis Lara (Chapter 6), is highly significant, as it is part of the Pacific volcanic belt. With many volcanoes active today and many more that were clearly so during the studied period, Patagonia is extremely noteworthy at the continental and hemispheric level to reconstruct the volcanic and tectonic history of the Pacific rim. Mostly due to the availability of volcanic rocks, Late Cenozoic paleomagnetic studies in Patagonia have been long-lived and frequent, being this region one of the first areas in the world were these techniques were fully applied, thanks to the indefatigable work of the late Daniel Valencio, a geophysics professor at Universidad de Buenos Aires. Guillermo Re´, Mario Mena, and Juan Francisco Vilas provide an extensive report of the available information in this field (Chapter 7). As stated before, paleomagnetic studies are extremely significant since they allow us to correlate the Patagonian rocks and sedimentary units with the Pampean loess beds as well as with the oceanic record. I have written myself the chapter on the Late Cenozoic glaciations of Patagonia and Tierra del Fuego (Chapter 8), trying to provide a general overview of the glacial events in the southern Andes, which started in the latest Miocene and extending until recent times and today, with the surviving Patagonian and Fuegian ice caps. This region has been the object of many research projects in this field during the last three decades, both by local and foreign scientists, following the pioneer work of John H. Mercer, who settled the bases of original, resourceful, innovative glacial studies not only in Patagonia but everywhere else in the Southern Hemisphere. The objective of this chapter is to take into account such significant work, trying to update the essential review made by Chalmers Clapperton in his paramount book of 1993. The Late Cenozoic fluvial deposits of Argentine Patagonia have been discussed by Oscar A. Martı´nez and Andrea Coronato (Chapter 9). In fact, very little is

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known about these deposits, usually neglected in Patagonian studies. The foremost issue taken into account is the problem (could it be said, the enigma?) of the Patagonian gravels, a matter of research already for Charles Darwin, Otto Nordenskjo¨ld and Carl Caldenius. One hundred and seventy years have passed since Darwin was puzzled by these sedimentary and geomorphological units, including perhaps some of the largest, single and continuous landforms in the world, extending for several hundred of kilometers. This chapter aims to organize the available information and discuss the origin and evolution of these perplexing units. Federico I. Isla and Gustavo G. Bujalesky have discussed the coastal geology and morphology of Patagonia and the Fueguian Archipelago in Chapter 10. Being one of the most extensive coastal environments in the Southern Hemisphere, with a huge adjacent submarine platform which became largely exposed during glacial events, the features studied include one of the longest records of raised marine terraces in the world. This evidence of ancient sea-level fluctuations starts already in the Pliocene, thanks to crustal stability and epeirogenic elevation of the cratonic areas, comprising many different interglacial episodes throughout the Quaternary times as well. A limnogeological approach to Late Pleistocene environmental change in eastern Patagonia and Tierra del Fuego is presented in Chapter 11 by Daniel Ariztegui, Flavio S. Anselmetti, Adrian Gilli, and Nicolas Waldmann. Patagonia is a land of many large lakes, most of them of glacial origin at the foot of the Andes, but some other sizeable ones and thousands of smaller ponds, hollows, and depressions extend over the plains. The methodology exposed by the authors is modern, fresh, comprehensive, and challenging, unlocking the way for the application of this approach to many different ecosystems and geomorphological environments. The geocryology of southern South America is described by Darı´o Trombotto Liaudat in Chapter 12. Trombotto discussed the problem of the number, extension and chronology of the cold episodes that generated periglacial environments in some areas of Patagonia. At least half a million square kilometer, almost the actual surface of France, were under permafrost conditions in Patagonia during the last glaciation, not counting the subaerially exposed portions of the submarine platform. The extent of the areas under these same permafrost conditions during pre-Late Pleistocene glacial events was perhaps even larger, but this is still a matter of intense debate and passionate argument. Although Trombotto’s stimulating ideas, concepts, and observations may not be fully shared and even be challenged by the reader, key goals of this chapter are to expose the magnitude of the scientific problem and to encourage research on this thought-provoking field in the near future. Eduardo P. Tonni and Alfredo A. Carlini present the study of neogene Patagonian vertebrates and their relationship with the most significant climatic changes in Chapter 13. Vertebrate paleontology has a long and skilled tradition in Argentina, particularly since the original works by Florentino Ameghino at the Museo de La Plata toward the end of the nineteenth century and

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continued later by the far-reaching efforts of many other paleontologists. Having been continuously exposed during the Tertiary, Patagonian continental depocenters have a paramount record of vertebrate remains, which become even enhanced during the Late Cenozoic, due to the widespread occurrence of ashfall beds and eolian units bearing curious and remarkable, sometimes bizarre, terrestrial faunas – a proof of long isolation and unique environments. The Late Cenozoic invertebrate paleontology, with emphasis on mollusks, is discussed by Marina L. Aguirre, Julio C. Hlebszevitsch Savalscky, and Florencia Dellatorre in Chapter 14. The transgressive marine deposits are found almost everywhere along the Patagonian coasts. Their fossiliferous content is extremely important to determine both the age and the environmental conditions when the bearing deposit was formed. Moreover, the species distribution is representative not only of the regional climate, but also of the marine current distribution and intensity. Similar considerations can be applied to Chapter 15, where Gabriela C. Cusminsky and Robin Whatley present a review of the nature, taxonomy, ecology, distribution, and chronology of the Late Cenozoic calcareous microfossils (Foraminifera and Ostracoda) from Patagonia and Tierra del Fuego. Marine environments and current distribution are inferred from the characteristics of the studied microfossils. The Late Miocene palynological assemblages from Patagonia are described in Chapter 16 by Viviana Barreda, Vero´nica Guler, and Luis Palazzesi. This chapter starts a series of palynological chapters, with the intention of describing the paleoenvironments of Late Miocene times, when it is assumed that Patagonia was reaching its modern characteristics and the plant communities were achieving their present conditions. The palynological history of Patagonia continues with Chapter 17, on the Late Quaternary vegetation and climate of Patagonia, by Marı´a Virginia Mancini, Aldo R. Prieto, Marı´a Marta Paez, and Frank Scha¨bitz. The contrast between the dense forests of the Andean slopes and foothills and the bushy steppes sparsely covering the drier plains is investigated by means of the palynological record, which helps in the reconstruction of Late Pleistocene and Holocene climate, vegetation, and ecosystems. Likewise, Chapter 18, by Ana Marı´a Borromei and Mirta Quattrocchio, describes the Late and postglacial paleoenvironments of Tierra del Fuego, both considering the terrestrial and marine palynological evidence. Very few places in the Southern Hemisphere are showing such complete palynological record as Tierra del Fuego, where very wet, cool temperate climate has allowed the preservation of pollen-bearing organic sediments in alluvial sediments, peat layers, and littoral marine deposits. These chapters provide a brief and complete review on the palynology, which are very useful, not only for the specialist but also for the interested Quaternary scientist and the general reader. These chapters are excellent, comprehensible and talented introductions to Cal Heusser’s benchmark book on the Palynology of the Southern Andes (2003), which has been published in this same series.

Inadequately known, only little and surficially investigated, the diatoms from Patagonia and Tierra del Fuego are discussed by Marcela A. Espinosa in Chapter 19. Ubiquitous dwellers of fresh and salt waters, in lakes, alluvial plains, marshes, and marine environments, diatoms have been only exceptionally studied since the pioneer studies of Vaı¨no Auer, from Helsinki University, and Joaquı´n Frenguelli, a distinguished professor at University of La Plata, both active during the first half of last century. A promising future is imagined for diatom research, considered as indispensable tools for paleoenvironment reconstruction. Our present knowledge of the Quaternary fossil insects from Patagonia is described by Julieta Massaferro, Allan Ashworth, and Stephen Brooks in Chapter 20. Mostly concentrated on chironomids and beetles, the study of these frequent inhabitants of lakes and ponds is highly promising, particularly if the investigations are extended in the future toward the ecotone environments, where ecological stress conditions may have affected the more sensitive taxonomic groups. Understanding the climate on the basis of tree rings is the subject of Chapter 21, by Fidel A. Roig and Ricardo Villalba. Long-lived tree genus, an appropriate coexistence of conifer and broad-leaf species and suitable humid environments for wood preservation are the desired conditions for dendrochronological studies, based on both standing trees and sub-fossil wood fragments. The possibilities of developing long-term tree-ring chronologies, perhaps the most complete in the Southern Hemisphere over most of the Holocene, is clearly exposed in this chapter. A summary of the archeology of Patagonia, with emphasis on the early peopling by hunter-gatherer societies, is presented by Mo´nica Salemme and Laura Miotti in Chapter 22. Such a chapter could not be disregarded in a book like this. Its content is mostly oriented toward the chronological and paleoenvironmental aspects of published archeological sites and the nature of human cultural relationships with the hostile, harsh, and unfriendly Patagonian climate and landscape. The problem of the very early peopling of Patagonia (before 12 ka 14C yr BP) and the arising conflicts with the timely later North American ‘‘Clovis-first’’ model are discussed by the authors. Finally, the last two chapters are dedicated to applied aspects of Quaternary sciences. In Chapter 23, the Late Cenozoic mineral resources of Argentine Patagonia are considered by Isidoro B. Schalamuk, Rau´l E. de Barrio, and Miguel A. Del Blanco. Rocks for building and technical purposes, sand and gravel for construction and road activities, saline materials and gold-bearing fluvial and coastal deposits are some of the mineral resources widely distributed in Patagonia. Because the Patagonian tablelands are mostly a semiarid region, fresh water is a limited comodity. In Chapter 24, Mario A. Herna´ndez, Nilda Gonza´lez, and Lisandro Herna´ndez (parents and son, a quite unique ‘‘groundwater family’’) discuss the Late Cenozoic geohydrological resources. Future development of extra-Andean Patagonia is powerfully restricted by the availability of water resources, either for domestic, industrial, mining, or

Introduction agricultural consumption. This chapter argues these possibilities and the accessibility of economic water supplies. In summary, this book is the result of dedicated efforts of more than 50 capable, competent, gifted authors. I am extremely thankful to all of them for their skillful contributions, their responsible labor, their committed perseverance to write in a language which is not their own for most cases, their limitless

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patience, and their boundless endurance. Their papers are actually the backbone of this book. Theirs are the valuable inputs, but flaws and defects should be blamed only upon me. I have tried my best to stand at the worthy level of their laudable work, at the precious rank of this influential series, and at the quality of this publisher production. I do hope to have accomplished it.

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2 Calvin John Heusser (1924–2006): A Life Devoted to the Quaternary of Patagonia and Tierra del Fuego Jorge Rabassa Laboratorio de Geologı´a del Cuaternario, CADIC-CONICET, Universidad Nacional de la Patagonia, Ushuaia

His activity in professional organizations was skillful and competent. He had been in the editorial board of several major journals, such as Ecology (1957–1958) and Quaternary Research (1970–1976), and contributed as a reviewer to many others. Being one of the most prestigious members of the editorial board, Cal constantly supported and helped me as Editor of Quaternary of South America & Antarctic Peninsula (1983–1999). He had been Head Editor of Torreya (1971–1977), President of the Editorial Board of the Bulletin of the Torrey Botanical Club (continued by Journal of the Torrey Botanical Society) (1975–1976) and Associate Editor of Radiocarbon (1987–1999). He had been a Theresa Seessel Fellow at Yale University (1952–1953), a John Simon Guggenheim Memorial Foundation Fellow at Universidad de Chile (1962–1963), a fellow of the Fulbright Commission at Universidad de Chile, Santiago (1962–1963), and a visiting fellow (1985) and a life fellow (since 1986) at Clare Hall, University of Cambridge. He received the David Livingstone Centenary Medal of the American Geographical Society (1987). His publication record is outstanding, with benchmark books such as Late-Pleistocene Environments of North Pacific North America (American Geographical Society, Special Publication, 35, 1960), Pollen & Spores of Chile. Modern Types of the Pteridophyta, Gymnospermae and Angiospermae (University of Arizona Press, Tucson, 1971) and, recently, Ice Age Southern Andes: A Chronicle of Paleoecological Events (Elsevier, 2003). He also published more than 175 papers in refereed journals, some of which are major contributions to the palynology and paleoclimatology of North Pacific North America and most significant for us since it is highly pertinent to the nature of this book, southern South America. The following biographical highlights, mostly based in his personal notes and vitae, portray a long and talented scientific career centered on stratigraphic palynology. I am deeply grateful to Linda Heusser for giving authorization to use these writings, some of which have been published previously. At Rutgers University, being a chemistry major and while completing undergraduate work, Cal came in contact with Murray Buell, who had come from North Carolina State to the Department of Botany at Rutgers. In Cal’s own words, ‘‘Murray was friendly, unassuming, and accessible. A fine teacher, he was a person I greatly respected and was inclined to emulate. His knowledge of botany was in-depth and broad, and his enthusiasm and love for field work most infectious. At the University of

Professor Calvin J. Heusser, Cal for all his friends, was born in North Bergen, New Jersey, USA, on September 10, 1924. He obtained his Bachelor of Science (1947) and Master of Science (1949) degrees in botany at Rutgers University, and later, his PhD degrees both in botany and in geology at Oregon State College (1952). While he was a university student he was drafted and he fought in World War II, in the European theater as a rifleman, 137th Infantry, 35th Division, where he took part in the Battle of the Ardennes, where he was wounded in one of his legs, receiving the Combat Infantry Badge, and the Battle Star and the Purple Heart medals. Cal was very proud of his participation in that battle, though he was not usually willing to talk about his European memories, certainly due to the sadness of the long war. His teaching and research career is outstanding, starting as a teaching fellow at Rutgers University (1947–1949) and continuing as a research fellow at Oregon State College (1949–1952). After completing his education, Cal joined the American Geographical Society as a research associate, from 1952 to 1967, and as such, Cal was on his first scientific expedition to South America and Patagonia in 1959. Then he became an associate professor at New York University (1967–1971), the renowned college of downtown Manhattan, where he stayed until his death. He achieved professorship in 1971, until 1991, when he formally retired and he was honored as Professor Emeritus of this university. In fact, he never retired from science and he continued working, together with Linda, his beloved wife and colleague, in their own laboratory in his gorgeous house in Tuxedo, New York, bounded by his dearly loved forests and the Appalachian, with a large garden where wild deer usually roam to feed on the lawn. He really esteemed those beautiful animals and he was full of pride about his neighbors from the wild and the peaceful tranquility of his home, in spite of being adjacent to the Big Apple. His main fields of research were the Quaternary vegetation and phytogeography of North Pacific North America and, particularly, the southern end of South America in Chile and Argentina. He was very interested in Quaternary climates and glaciation of the middle latitudes on both hemispheres. He developed original techniques on the quantification of climatic parameters used in modeling and different aspects of paleoclimatic theory. Strictly in botanical terms, he studied pollen and spore morphology, particularly in relation to plant systematics and evolution.

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Minnesota, he had been a student of William Skinner Cooper, famed for his work on plant succession following glacier recession in the Canadian Rockies and Glacier Bay, Alaska. During 25 yrs at Rutgers (1946–1971), Murray was exceedingly industrious, guiding 39 students through their doctorate degrees and 23 through MS degrees’’. Cal reserved a great admiration for his advisor through his entire life. Once, at the time of spring break, Cal traveled with Murray Buell to collect sediment cores in Dial Bay in South Carolina. Cal’s job was to cut brush along transects into the interior of the bay, which turned out to be an unforgettable adventure sloshing through knee-deep water infested with snakes. During this trip to the bay, Buell asked Cal if he would like to be a teaching assistant in botany, concurrently working toward an MS in the department. For Cal, this was the opportunity that he had been looking forward to since he returned from the war, and thus he wholeheartedly accepted. Before beginning graduate work in the fall of 1947 at Rutgers, Cal traveled in the summer with the Buell family (three adults and two children in a two-door 1936 Ford) on an astonishing trip from Massachusetts to Minnesota. In summer, Buell taught at the University of Minnesota Biological Station at Lake Itasca, so that this was a chance for Cal to benefit as a student at the station not only from courses in ecological field methods and aquatic plants but also from exposure to the flora and vegetation of Midwestern United States. Buell generated his initial interest in paleoecology. Cal did an MS thesis having to do with the ‘‘History of an estuarine bog at Secaucus, New Jersey’’ (1949). Plant fossil macroremains, the focus of the work, traced sealevel change and progressive demise of a freshwater white cedar bog. Frequented in the early nineteenth century by the botanist John Torrey, the site in addition to white cedar contained black spruce and larch. Although Buell had become active working with fossil pollen, Cal did not involve himself in palynology until later, when he went to Oregon State University in Corvallis, to work for a PhD. Three years at Oregon State (1949–1952), as both a teaching and research assistant, provided him a wider botanical horizon through contact with plants of the Pacific Northwest. Cal was fortunate because Henry Hansen, with whom he had come to study, became Dean of the Graduate School the year Cal arrived. Because he was busy with administrative work, as usually happens, Cal was much on his own. Cal would major in botany, but because his thesis was to be in palynology, he need to minor in geology, which meant picking up necessary credits in earth science courses. Cal chose ‘‘Pollen profiles from Southeastern Alaska’’ for a dissertation. This decision came about upon his joining the American Geographical Society’s Juneau Icefield Research Project in southeastern Alaska as plant ecologist. During 1950 and 1951, the project enabled him to collect cores for the thesis from muskegs in the Juneau region. Although circumstances reduced the amount of time they otherwise might have spent together, Cal thoroughly enjoyed and profited from the contacts had with Hansen

during the years at Oregon State and later in the course of a lifelong friendship. Consistently available for counsel on a variety of matters, Hansen treated him more as a friend than as a graduate student and Cal enjoyed many unforgettable social events at the Hansen family home. Following the doctorate that Cal obtained at Oregon State in 1952, he joined the American Geographical Society in New York as a research associate. In 1953, early in his career at the society, however, Cal was the recipient of the Theresa Seessel Fellowship for postdoctoral study at Yale University and took leave for a year. At Yale, he came in contact with Paul Sears and Ed Deevey, both recognized early American palynologists. Graduate students, Dan Livingstone and Estella Leopold, along with Heikki Ignatius, who was visiting from Finland, added to the nucleus of pollen people. Dan was finishing, having gained considerable notoriety with his development of the piston sampler named after him, while Estella like Cal himself had just arrived. Following a trip of the Yale Group to the University of Massachusetts, where L.R. Wilson was training students in palynology, it was decided that Cal was to be responsible for the organization of the First Palynology Conference (1953). Held in February and attended by palynologists from different parts of the United States, the conference included presentation of 17 papers and a tour of the Geochronometric Laboratory by Ed Deevey, its Director. Working at the American Geographical Society at the start was largely in connection with administration of the Juneau Icefield Research Project. Cal was at the society in the Department of Exploration and Field Research, whose director, W.O. Field, had been much involved with Alaskan glacier fluctuations for many years. He and M.M. Miller were responsible for initially setting the Juneau Icefield Research Project in motion. The Juneau Project concentrated on the setting and fluctuations of the glaciers issuing from the 750-mile-square source of ice in the Coast Mountains of southeastern Alaska. For several periods during eight consecutive summers, Cal worked mostly on the upper Taku Glacier and on Lemon Creek Glacier. Opportunity was afforded to study pollen in the annual snow cover and also to investigate the nunatak flora (1954). In 1953, Cal was not directly involved with work on the ice field because of his study of glacier variations in the Canadian Rockies, while time spent with the Juneau Project in 1956 was short because of additional research commitments. The summer of 1956 was given over to the collection of 114 lake and mire pollen sections on the North Pacific coast ranging from Alaska to northern California. Focus shifted to South America with the society’s Southern Chile Expedition in 1959. Cal organized and led the expedition to Laguna San Rafael in 1959, and since then, he carried out field studies in Chile (1963–1998) and in Argentina (1978–1993). At Laguna San Rafael, Cal initiated a research program involving the study of pollen records that continued in the southern Andes down to the late 1990s. The expedition was an international undertaking with cooperation from the Chilean government. Field personnel included D.B. and E.G. Lawrence of the University of Minnesota (plant ecology), E.H. Muller of Syracuse University

Calvin John Heusser (glacial geology), Carlos Mun˜oz and Federico Schlegel of the Universidad de Chile (plant taxonomists), Augusto Grosse of Puerto Ayse´n (explorer) and Shoji Horie, Otsu Hydrobiological Station, Kyoto, Japan (limnology). In 1962–1963, as a fellow of both the John Simon Guggenheim Foundation and Fulbright Commission in the Escuela de Geologı´a at the Universidad de Chile in Santiago, Cal was able to work in the field and collect additional cores for fossil pollen stratigraphy in the Chilean Lake District in the province of Llanquihue. Perhaps the most important outcome during his tenure at the university, however, was the preparation of a modern pollen and spore reference collection of the Chilean vascular flora. Under the guidance of Professor Carlos Mun˜oz, Director of the Herbarium at the Museo Nacional de Historia Natural, Santiago, Cal prepared pollen/spore-bearing material of 698 species of plants, representing 624 genera and 78 families. Descriptions and illustrations constituting the collection resulted in ‘‘Pollen and spores of Chile’’ (1971). Up to this point, Cal had not offered yet any coursework in palynology. In 1961, he began teaching palynology at the graduate level in the Department of Geology at New York University. Cal taught at first as an adjunct, while at the same time employed as a research associate at the American Geographical Society. The course was given at night, so that there was no conflict of interest. In 1964, also as an adjunct, Cal began offering courses in the Department of Biology. After 15 yrs at the American Geographical Society, Cal resigned in 1967 and joined the faculty of New York University as a full-time member of the Department of Biology. He was Associate Professor, 1967–1974, and subsequently Professor until his retirement as Professor Emeritus in 1991. Courses offered during the period included field biology and ecology, ecological botany, plant systematics and palynology. Thirteen graduate students received the PhD under his direction and there were seven who took the MS. Two graduate students who took palynology at New York University, Doug Nichols and Al Solomon, went on to receive their doctorates, respectively, under Al Traverse at Penn State and Murray Buell at Rutgers. With support from the National Science Foundation, Cal continued his studies of the Quaternary of the North Pacific coast with emphasis on stratigraphic palynology of the Olympic Peninsula of western Washington. The semester break in December–January at the university also allowed him to continue doing fieldwork in Chile and Argentina during the austral summer. Gradually, greater attention was given to the Quaternary of southern South America with vegetation–paleoenvironmental reconstruction ranging from subtropical central Chile to subantarctic Tierra del Fuego. Studies to date are summed up in his 2003 book, Ice Age Southern Andes: A Chronicle of Paleoecological Events. In 1985, Cal was elected a fellow of Clare Hall, University of Cambridge, with research facilities in the Department of Botany and at the Godwin Laboratory of the Department of Earth Sciences. With his wife, Linda, who was elected a fellow of Clare Hall in 1995, he made frequent visits to take up residence there. The Godwin Laboratory, under the direction of the late N.J. Shackleton,

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was an exceedingly viable research center for Quaternary– Tertiary studies, attracting scientists from all over the world. During more than 50 yrs, his professional activity involved national and international meetings, conferences, and presentation of papers throughout the United States, Canada, Japan, Australia, New Zealand, several countries in Europe, South Africa, and South America, in Chile (Santiago, Valdivia), Argentina (Mendoza, Neuque´n, Ushuaia) and Brazil (Sa˜o Paulo). I met Cal and Linda for the first time at the 1976 Friends of the Pleistocene Meeting in Delaware, USA. I was then a young postdoctoral Fulbright Fellow at S.U.N.Y.–Binghamton working under Don Coates and Marie Morisawa at the Department of Geological Sciences. Cal and Linda were sitting in the first seats of the fieldtrip bus and I was overjoyed to see the name of such famed palynologist on his tag, which I knew from his exceptional work in southern Chile. I timidly introduced myself, but they greeted me wholeheartedly, thus beginning a lifelong friendship and a most rewarding scientific bond for me and several of my Argentine colleagues. Cal and Steve Porter (University of Washington) gently indeed invited me to join them during fieldwork in the Chilean Lake District, during the following Southern Hemisphere summer (Fig. 1). I still keep fondest memories of that trip, which instantly opened my mind to the problem of Late Pleistocene Andean glaciations. Cal and Linda came to Argentina for the first time in March 1982, when I organized our first INQUA Till Commission Meeting in Neuque´n, northern Patagonia (see Chapter 8, Fig. 45). This stay was very important because Cal envisaged the wide potential of working on Patagonian palynology, and thus, this was the opening step for a long-term scientific collaboration with Argentine researchers in different institutions. In the middle of the late 1980s, invited by CONICET, the National Research Council of Argentina, Cal visited the CONICET Research Center at Mendoza, where he advised the palynological work of Dr Mo´nica Wingenroth, at IANIGLA, the Argentine Institute for Nivology and Glaciology.

Fig. 1. Two giants of South American Quaternary sciences: Cal Heusser and Steve Porter, discussing the deformed glaciolacustrine deposits at the shore of Lake Llanquihue, Puerto Varas, Chile (Photo by J. Rabassa, February 1977).

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Fig. 2. Cal and Linda Heusser, sampling Mid-Pleistocene glaciolacustrine sediments at the classic site of La Pilila, originally described by Richard Foster Flint and Francisco Fidalgo in 1963, located 50 km southeast of San Carlos de Bariloche, northern Patagonia (Photo by J. Rabassa, February 1983).

the investigation of fine-grained glacigenic sediments of Pleistocene age (Fig. 2) and Late Pleistocene–Holocene peatlands. The history of Araucaria, the monkey-puzzle tree, was interpreted from cores taken in the Lanı´n National Park, province of Neuque´n (Fig. 3). After I moved to CADIC, Ushuaia, in 1986, Cal and Linda visited frequently our laboratory during the late 1980s and the early 1990s. During these activities, Cal founded the bases of Fuegian palynology, through indefatigable fieldwork seasons and major paramount papers (Figs 4–6). His contributions oriented the work of all of us, and particularly that of Dr Ana Borromei (Chapter 18), who has continued with Cal’s research in the Fuegian Archipelago, extending his concepts, methodology and findings to many other Patagonian sites. Cal generously gave me and my group at Ushuaia everything he had at reach, all he could make available in terms of scientific guidance, research advice and human feelings, with wonderful and endless openhandedness. We all owe Cal a great share of our awareness and comprehension of the Fuegian paleoclimates, and many of his creative ideas and innovative projects will keep us busy for a long time. Cal was a prominent scientist and a celebrated professor, but most important, a sincere, truthful and heartfelt friend, a splendid, magnificent human being, an enchanting individual and a deep lover of the arts, particularly classical music. Most, if not all, of his stupendous contributions to South American palynology are listed in different chapters

Important contributions on the palynology of the high central Andes were the precious outcome of those years. The Heussers started fieldwork in Northern Patagonia in 1983, where palynological research was oriented toward

Fig. 3. Cal and Linda Heusser, resting after a hardcoring day, surrounded by flowering Empetrum bushes, during fieldwork at Lanı´n National Park, province of Neuque´n, northern Patagonia. The snowy summit of Volca´n Lanı´n is the backdrop (Photo by J. Rabassa, February 1984).

Fig. 4. Cal Heusser watchfully revises Early Pleistocene glacigenic sediments outcropping at Can˜ado´n Beta, along the cliffs at the Atlantic coast of northern Tierra del Fuego (Photo by J. Rabassa, February 1987).

Calvin John Heusser

Fig. 5. Cal Heusser, sampling from a Holocene core taken from a peatland at Cabo San Pablo, eastern Tierra del Fuego. Note that Cal is using his famous, secret, almost magic sampling tool: a silver teaspoon that traveled with him around the world. This particular stainless steel corer (which he had specially designed and built for him) was later that season regrettably lost due to a bolt failure while drilling into a peat bog near Lago Yehuin, central Argentine Tierra del Fuego, where it waits to be rediscovered by Quaternary scientists, perhaps during the twenty-second century (Photo by J. Rabassa, March 1988).

Fig. 6. Cal Heusser looking for possible pollen-bearing sediments in Late Pleistocene glacigenic sediments, at the Beagle Channel, Tierra del Fuego. Note that Cal is wearing his interminable Alpine sweater, an everlasting component of his fieldwork gear (Photo by J. Rabassa, March 1988).

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Fig. 7. Cal Heusser, one of his very last portraits (Photo by Linda Heusser, September 2005).

of this book. His outstanding papers are cited here, his key findings are herein quoted by several authors and his stimulating ideas are reflected either in the scientific discussion or in the opening of new, future lines of research. But, certainly, we will miss forever his charming smile, his affectionate company, his captivating conversation, his fascinating personality and his enlightening guidance (Fig. 7).

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3 The Physical Geography of Patagonia and Tierra del Fuego Andrea M.J. Coronato1,2, Fernando Coronato3, Elizabeth Mazzoni4 and Mirian Va´zquez4 1

2

Centro Austral de Investigaciones Cientı´ficas (CADIC-CONICET), B. Houssay 200, (9410) Ushuaia, Argentina. Universidad Nacional de la Patagonia San Juan Bosco, Sede Ushuaia, Darwin y Canga, (9410) Ushuaia, Argentina. 3 Centro Nacional Patago´nico (CENPAT-CONICET), Boulevard Brown s/n, (9120) Puerto Madryn, Argentina. 4 Universidad Nacional de la Patagonia, Unidad Acade´mica Rı´o Gallegos, Lisandro de la Torre 1070, (9400) Rı´o Gallegos, Argentina. provinces are affected by this structure, north and south of this latitude. The northern portions of these geological provinces form the central and northern portions of Neuque´n Province and, since they border with other geological provinces north of the Barrancas and Colorado rivers, they are considered as part of Patagonia. According to Feruglio (1949), the northern boundary would be considered at the heads of the Rı´o Alumine´ (39 S), where the Andean Cordillera changes its main characteristics. From a climatic point of view, the domination of the South Pacific anticyclone over the South Atlantic anticyclone depicts a NW–SE boundary, cutting across the Rı´o Negro east of its origins and reaching the Atlantic coast at 41 S. South of this imaginary line, the air masses determine the almost permanent westerlies and the precipitation distribution that characterize the Patagonian climate (Godoy Manrı´quez, 1997). In terms of vegetation distribution, there is a continuity north of the Barrancas and Colorado rivers. Thus, the semiarid steppes and high mountain grasslands occupying the provinces of Neuque´n and Rı´o Negro extend almost continuously northwards up to 34 S, in southern and central Mendoza (Roig, 1998). Patagonia was originally known by the European world from its Atlantic face, solitary, arid and windy. From AD 1520, the term ‘‘Patagonia’’ identified the vast plains of the American southern end, inhabited by nomad natives, where cliffy and waterless coasts were inaccessible. It was only in the nineteenth century that this name was applied to the Andean Cordillera and the Pacific coast. For Argentina and Chile, Patagonia is a sort of peripheral region, lately incorporated to their respective national identities, scarcely populated and for which full integration to their national spaces is still in progress. This chapter discusses a junction of natural elements in the southern end of South America. Bedrock substratum and structure, climate, distribution of the superficial runoff, soils and vegetation, all act together in a systematic, cause–effect relationship and form a group of homogeneous geographical spaces that, although they have obvious differences between them, are joined under first-order geological and atmospheric elements, such as tectonic plates and the general atmospheric circulation. Beyond the scientific criteria that may be used to identify this region, Patagonia is defined as a geographical space by the deep feeling of regional belonging of its inhabitants, linked by a common history and a natural

1. Introduction Patagonia, the southernmost region of the South American continent, extends from 37 S to Cape Horn, at 56 S, the latter located at less than 1000 km from the northernmost tip of the Antarctic Peninsula. The Patagonian region is the only continental landmass emerging along the midlatitudes in the Southern Hemisphere. Its main geographic feature is the Andean Cordillera, which is both the continental watershed and, in many areas, the international boundary between Argentina and Chile. It includes the Pacific and Atlantic lowlands and coasts, the southern archipelagos, and the valleys, tablelands and high plains extending between the Andes and the Atlantic Ocean. The establishment of the boundaries of this portion of South America, especially in its northern portion, varies according to accepted criteria, from natural characteristics to the administrative and political organization. Following its administrative regional ordering, the Argentine Republic considers as part of Patagonia the provinces of Neuque´n, Rı´o Negro, Chubut, Santa Cruz, Tierra del Fuego and the southern portion of Buenos Aires Province. The total superficial extent of Argentine Patagonia is 790,000 km2, not including the adjacent seas. Its natural boundaries are the Barrancas and Colorado rivers, the provinces of Mendoza and La Pampa and the south portion of Buenos Aires Province to the north, the Andean Cordillera and the Republic of Chile to the west, the Atlantic Ocean to the east and the Beagle Channel and the Chilean Navarino Island archipelago to the south. The Republic of Chile considers its Patagonian sector as that territory extending along the western slope of the Andes, from 43 S, in the province of Palena, and the southernmost archipelagos in the Cape Horn area, including the XIth (Ayse´n) and XIIth (Magallanes) administrative regions. Some Chilean geographers also include the continental portion located next to Isla de Chiloe´, from Seno Reloncavı´ southward to the X Region (Los Lagos) (J. Negrete Sepu´lveda, 2005, personal communication; Fig. 1). From a geological point of view, Patagonia extends over a basement of varied lithology, structure and age, with respect to the other structures of the rest of Argentina and Chile. The transversal structure expanding west and north of the Rı´o Limay, at 39 S, is known as the Huincul Dorsal that separates Patagonia from the rest of Argentina, making up the northern boundary of this region (Ramos et al., 2004). However, other geological

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Fig. 1. Patagonia, southernmost end of South America, extending at intermediate latitudes of the Southern Hemisphere, between the Atlantic and Pacific oceans. East of the Andean Cordillera, Patagonia includes five Argentine provinces, whereas in the western sector, it comprises the two southernmost Chilean administrative regions.

landscape, whose basic characteristics will be defined below. The aim of this chapter is, therefore, to provide an ample geographical basis to the reader interested in the Late Cenozoic of Patagonia and Tierra del Fuego. The Patagonian landscapes, particularly their relief, hydrographic network and soils, are the results of different processes that have been acting during the last 10 Myr, crudely the time span covered by this book, and, particularly, during the last 2.5 Myr, the Quaternary epoch.

2. Geological Provinces Patagonia evolved on top of an ancient basement, a Gondwanic remnant, affected by epirogenic movements

that marked the basic geological features of the region. Over this initial relief, endogenic and exogenic processes were active throughout geological history until reaching the present structural configuration. During the Paleozoic, the western margin of the ancient massif underwent holding due to western forces, forming a positive relief complex known as the ‘‘Patago´nides’’. In the downwarped blocks, thick, continental and marine sedimentary packages were deposited during the Mesozoic and Cenozoic, which contain fossil-bearing rocks, oil and gas and mineral reserves. During the Tertiary, the Andean orogeny formed the most important positive relief of the South American continent, the Andean Cordillera. It is known as the ‘‘Principal Cordillera’’ in central Argentina and northern

Physical Geography of Patagonia and Tierra del Fuego Patagonia, and as ‘‘Patagonian Andes’’ in the southern part. The volcanic eruptions that characterized the Andean orogeny extended on the uplifting mountain ranges and formed tablelands on the basement and Mesozoic sedimentary rocks. A repeated Plio–Pleistocene glaciation of the Patagonian Andes and volcanic eruptions generated a landscape of Alpine glaciation in the mountains and glaciofluvial plains and volcanic plateaus in the lowlands. The present fluvial system, developed on the landscape during and after the Last Glacial Maximum (LGM; ca. 25 ka BP), is the result of the continued incision of the tableland system. The coastline kept a similar position to the present throughout the different interglacial periods, modeling cliffs along the sedimentary and volcanic rocks tablelands, as well as bays and drowned fluvial valleys in the lowlands. The geological structure of Patagonia offers two large, well-defined sub-regions: (a) Andean Patagonia, composed of mountain ranges due to intense plutonic and volcanic activity and Tertiary folding and (b) extraAndean Patagonia, characterized by tablelands or ‘‘mesetas’’, with wide depressions and fluvial valleys, resulting from the Mesozoic and Cenozoic sedimentary and volcanic filling of the tectonic blocks of the ancient basement. The southern Chilean Archipelago and the Fuegian Archipelago are included in the concept of Patagonia because their geological composition is deeply linked to that of southern Patagonia, and that their separation of the continent by the Magellan Straits and many other marine channels and fjords is only the consequence of the erosive effect of Pleistocene glaciers during the last million years and, particularly, during the LGM. On this morphostructural configuration, different geological provinces may be recognized, in the sense of Rolleri (1975), defined as natural units characterized by a certain stratigraphic sequence, peculiar structural and geomorphological features and a particular geological history. The regional division adopted in this chapter follows that proposed by Ramos (1999), though has been complemented by the work of other authors (Fig. 2).

2.1. Principal Cordillera (Ramos, 1999) This unit extends from extra-Patagonian latitudes and occupies the northwestern end of the provinces of Neuque´n, until the Rı´o Agrio. The Cordillera del Viento is included in this unit, a raised tectonic block that exposes outcrops of sedimentary, volcanic and plutonic rocks of the Late Paleozoic basement, affected by fractures with eastern and western orientation and also vertical faults. The outcropping substrate includes a complex of rhyolitic tuffs overlying massive mudstones and siltstones interbedded with quartzitic sandstones and underlying Late Paleozoic porphyritic tuffs. This volcano-sedimentary complex was intruded by granitic and granodioritic plutons during the Early to Middle Permian. An important, continental, evaporitic and marine sedimentary complex filled the basins within the basement elevations during the Jurassic, following a N–S orientation. The easternmost basins, between the towns of Chos Malal and

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Zapala, were filled by sedimentary sequences of transitional and continental environments during the Early to Middle Cretaceous. Late Tertiary basaltic rocks (in the Domuyo Massif) and Early to Middle Quaternary andesitic and basaltic rocks are dominant in the region (Tromen basalts; Yrigoyen, 1980). The present volcanic activity is a postvolcanic, fumarole and hydrothermal phase (Volca´n Copahue). During the Quaternary, this area was glaciated several times, generating an erosive glacial landscape on the high summits and moraine ridges, and glaciofluvial and glaciolacustrine terraces along the valleys. Lago Caviahue represents the ice modeling action during the LGM.

2.2 The Mountain Sector of the ‘‘Neuque´n Basin’’ (Digregorio and Uliana, 1980) This unit occupies the center of the province of Neuque´n (38–39 S; 70–72 W) and is the transitional area between the geological provinces Principal Cordillera and Northern Patagonian Andes (Ramos, 1999). It includes a region of lower ranges with N–S orientation (Sierras de Cata´n Lil, 2440 m a.s.l., and Sierras de Chachil, 2444 m a.s.l). Eastward, it merges with the Jurassic–Cretaceous sedimentary complex that forms the representative sequence of the Neuque´n Basin, in extraAndean Patagonia. The older rocks are Precambrian and Paleozoic granodiorites and low-grade metamorphic rocks, with limited igneous contribution. Gray granodiorites and a complex sequence of andesitic lavas, breccias and tuffs occupy the western part between the southern end and Cordillera del Viento, representing Permian and Triassic intrusive and volcanic events. Early Jurassic– Paleocene outcrops are sparsely distributed toward the east of this unit, composed of marine and continental, clastic, carbonatic and evaporitic rocks, corresponding to the first two cycles of sedimentation, including localized volcanic processes. Pyroclastic rocks and continental deposits form a sedimentary complex of mid Tertiary age; mesosilicic and basaltic eruptions came from preAndean emission centers. Quaternary basaltic rocks complete the sequence, forming tableland lava fields. The Pleistocene glaciers deposited stratified and nonstratified drift along the transversal valleys, tributaries to the Rı´o Agrio along its middle and lower reaches.

2.3. The Northern Patagonian Andes (Ramos, 1999) This unit extends from Lago Alumine´ (latitude 39 S) until latitude 45 S, including in its northern section the unit described as ‘‘Cordillera Neuquina’’ by Gonza´lez Dı´az and Nullo (1980). It occupies the southwestern portion of Neuque´n Province and the northwestern of Rı´o Negro, between the Rı´o Alumine´, the middle and lower valley of the Rı´o Collo´n Cura´ and the lower valley of the Rı´o Limay; westward, it extends into Chile. Morphologically, it is composed of high mountain ranges (between 1600 and 1900 m a.s.l.) with N–S orientation, separated by deep parallel and transverse valleys; within the latter, glacial lakes and wide,

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Fig. 2. Patagonian geological provinces (modified from Ramos, 1999).

Quaternary glaciofluvial valleys are located. Large volcanoes, such as Lanı´n (3776 m a.s.l.) and Tronador (3554 m a.s.l.), developed on top of the mountain ranges. Low relief plains, such as Pampa de Lonco Loan or Planicie Alicura´, are exposed N and E of the main mountain range, adding some extra-Andean features to this unit. The oldest rocks correspond to medium to low grade metamorphic units, with a N–S extent, with type outcrops in the Lago Alumine´ zone; these metamorphic rocks were affected by Paleozoic intrusive processes and later covered by Paleogene volcanosedimentary sequences. Granitic bodies, of Late Paleozoic age, are very common and they appear widely distributed along the N–S axis of the region, hosting the deep, glacially eroded transversal valleys of this region, excavated during Quaternary glaciations. In the area of the Espejo and Gallardo lakes, hornblende-biotite

volcanic rocks and conglomeratic ortoquartzites appear, together with interbedded siltstones, waterlain tuffs and pyroclastic beds, of a probable Late Paleozoic age. East of the Rı´o Alumine´ fault, andesitic tuffs and breccias, cineritic tuffs and basal conglomerates with granite clasts are exposed, of an estimated Early Triassic age. Between the Traful and Nahuel Huapi lakes, the grayish granitic rocks (hornblende-bearing granodiorites) extend along the mountain summits, with an intrusive character into the Late Paleozoic metamorphic rocks. Eocene volcanosedimentary rocks are overlying them unconformably. Miocene–Pliocene basalts and andesites with interbedded breccias, volcanic conglomerates and andesitic tuffs are widely distributed along the north and central portions of this region, whereas a chrono-stratigraphic equivalent occupies the tectonic block limited by the alignments of the Rı´o Alumine´ in the east, Rı´o Malleo in

Physical Geography of Patagonia and Tierra del Fuego the north and another that transversally cuts the eastern portion of Lago Huechulafquen. The grayish sandstones with cross-bedding, diatomite layers and carbonatecemented conglomerates, located in the area of the Chimehuı´n, Caleufu and Collo´n Cura´ rivers, and the andesitic rocks of the summits of Lago Paimu´n have been assigned to the Middle Pliocene. Distributed in the entire region, basaltic flows and pyroclastic rocks form structural plains of Pliocene age, such as the Pampa de Lonco Luan and Pampa de Chenqueniyeu. In the central-eastern portion of the region, west of Rı´o Collo´n Cura´ and north of Rı´o Limay, wide plains of boulders, gravels and sands are developed (Pampa de Alicura´, 150–200 m a.s.l.), with materials of varied lithology and covered by volcanic rocks. These basaltic flows had been considered for a long time as Holocene in age; radiometric dating has proven that they erupted in the latest Miocene or earliest Pliocene (Schlieder, 1988; Rabassa et al., 2005). These sedimentary rocks have been interpreted as Quaternary piedmont levels (Gonza´lez Bonorino, 1944; Flint and Fidalgo, 1968), as Quaternary glaciofluvial deposits (Feruglio, 1941, 1949) and also included within the ‘‘Rodados Tehuelches’’ or ‘‘Patagonian gravels’’ of extra-Andean Patagonia (Dessanti, 1972). These units are more extensively treated in another chapter of this book (see Chapter 9). Recently, Rabassa et al. (2005) have suggested that the Alicura´ Formation may be related to valley trains formed during very early mountain glaciation in latest Miocene times. Likewise, the glacial drift is widely distributed in this region and it is considered in another chapter (see Chapter 8). The Pleistocene basalts with pyroclastic fractions have a restricted distribution in the western part of this unit, well represented at Volca´n Lanı´n, where they appear interbedded with glacigenic deposits. Holocene volcanic activity is more widely present in the region, either as small cones on top of the tablelands or as lava flows in the valleys, Volca´n Lanı´n and Lago Epulafquen. The modern deposits include gravels and alluvial silts and sands in tributary valleys to the large regional streams such as Collo´n Cura´ and Limay, as well as mass movement deposits everywhere, with slumped blocks along the structural plain slopes, soil creep, rock falls and rock and soil slides, and piedmont deposits along the more important ranges. Ramos (1999) identified (a) a northern segment in this unit (down to latitude 43 S), structurally characterized by a strong tectonic inversion of the Paleogene basins during the Miocene, and (b) a southern segment (between latitude 43 and 45 S), with a structure of grabens formed during the Jurassic–Early Cretaceous in an oblique position with respect to the Andean chains, partially inverted during the Andean orogeny, but without the low-angle thrusts that occur in the northern segment. The southern sector of this geological province has been ˜ irihuau-N ˜ ordescribed as a separate unit, ‘‘Cuenca de N quinco-Cushamen’’ (Cazau, 1980). Its substratum is characterized by Late Jurassic volcanic rocks interbedded toward the north with Late Jurassic marine sedimentary rocks, underlying Early Cretaceous volcanics. This is the region where the Patagonian Batholith has been emplaced,

17

extending southwards in discontinuous outcrops. The Tertiary rocks correspond to continental clastic sequences with interbedded pyroclastic deposits of the same age.

2.4. The Southern Patagonian Andes (Ramos, 1999) This unit extends from the area of Lago Fontana (44580 S) until Seno Otway (53550 S), in the Magellan Straits, Chile. This unit has been defined with the same name by Riccardi and Rolleri (1980), including also the northern Patagonian Andes unit, previously described here. It is characterized by the presence of the Patagonian Batholith with axial development, with better outcrops in the Chilean sector of the Andean Cordillera. At 46300 S, it is divided into two sectors with different structure, geological composition and topography. The northern sector, although developing a volcanic arc, has a lower relief than the southern one. Scarce metamorphic basement outcrops have been observed south of Lago Fontana, where the older rocks correspond to Late Jurassic andesites and dacites. Prograding deltaic facies toward the west represent the Late Jurassic marine sedimentary environment and form a paleo-gulf in the Rı´o Mayo region, composed of hemigrabens transversally located to the Andean Cordillera. Early Cretaceous granodiorites and granites intrude the continental and marine deposits. Continental sedimentary rocks with tuff levels represent the valley sedimentary fill, which took place during the Miocene due to Plinian-type eruptions from the western volcanoes such as Volca´n Hudson (Ramos, 1999). The structure corresponds to blocks with oblique orientation with respect to the Cordilleran axis, with compressional effect and slight inversion. The southernmost sector of the southern Patagonian Andes extends south of Lago Buenos Aires, northern Santa Cruz Province (46300 S), including the granitic peaks such as San Lorenzo, Fitz Roy, Stockes and Torres del Paine, whose elevations vary between 2000 and 3400 m a.s.l. The regional basement is characterized by Middle to Late Paleozoic flysch sequences, in which non-metamorphic sedimentary facies or low-grade metamorphic rocks are present. The volcanic rocks are forming thick Late Jurassic sequences and correspond to dacitic and rhyolitic rocks, with scarce andesitic bodies, overlying gravel sequences. Late Jurassic to Early Tertiary marine sedimentary rocks, including prograding deltas, are developed between Lago Pueyrredo´n (47200 S) and Lago Argentino (50100 S), showing a gradual recession of the marine environment with basin continentalization during the Paleocene. Around 98 Ma, the Patagonian Batholith was emplaced and the first orogenic compression of the region took place (Ramos et al., 1982). During the Early to Middle Tertiary, Atlantic marine transgressions occurred, forming marine sedimentary rocks in which coal deposits and basalts are interbedded. Miocene stocks, like the one forming famous Cerro Fitz Roy (3405 m a.s.l.), are associated to the uplifting of this portion of the Andes. Structurally, it is composed of a folded and thrusted belt that underwent

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Andrea M.J. Coronato et al.

shortening and uprising of the mountain range, which was in turn originated by the Chilean Dorsal collision (Ramos, 1989) and the emplacement of the Lautaro, Aguilera and Cook volcanoes.

2.5. The ‘‘Patago´nides’’ (Ramos, 1999) This unit extends from north of Rı´o Limay, in centralsouth Neuque´n Province until the Rı´o Senguerr, on the boundary between the Provinces de Chubut and Santa Cruz, including the low-altitude ranges of Piedra del ´ guila, Lipetren, Tecka, Tepuel, Agnia, Languin˜eo and A San Bernardo. It was originally described by Frenguelli (1946). This unit represents the mountain ranges located away from the Andean Cordillera, with elevations of 1200–1700 m a.s.l.; it transitionally bounds with the tableland environment of extra-Andean Patagonia. It is composed of Jurassic and Early Cretaceous marine and continental sediments, with associated volcanic and plutonic rocks. Ramos (1999) identified the subunits as ‘‘Precordillera Patago´nica’’ (Sierras de An˜ueque, Pire Mahuida, Taquetren) and ‘‘Berna´rdides’’ (Sierra Nevada and Sierra de San Bernardo). The first subunit includes pre-Andean extensions with ancient metamorphic basement rocks, intruded by the Early Paleozoic plutonism, with overlying marine, continental and glacial sediments of Late Paleozoic age. Over the aforementioned strata, Early Jurassic marine and continental sediments corresponding to a Pacific marine transgression are developed. These sedimentary beds are interbedded with marginal, andesitic volcanic rocks. During the Late Triassic and Early Jurassic, and due to a W–NW trending fault system, the central Patagonian Batholith would have been emplaced in Rı´o Negro and in northern Chubut provinces. During the Cretaceous and the Early Tertiary, an important continental sedimentary cover developed. Volcanic activity during the Cretaceous and the Early Tertiary is represented by basaltic lavas and tuffs. Structurally, this subunit is characterized by superimposed tectonic inversion on a system of Mesozoic hemi-grabens and compressive and thrusting structures. The ‘‘Berna´rdides’’ exposes its range-like landscape N and E of the Rı´o Senguer, transitionally north-bounding with the northern Patagonian Andes and eastward with the Lago Colhue´-Huapi depression. These ranges are characterized by the development of anticlinal structures separated by N–NW faults and formed by hemi-graben tectonic inversion during the Early Mesozoic. These depressions were in-filled with Early Jurassic deposits, Jurassic volcanics and Early Cretaceous lacustrine sedimentary rocks. Basaltic flows are frequent and together with alkaline, basic intrusions in the shape of volcanic necks and domes, they represent the igneous activity of this region from the Cretaceous to the Paleogene. The marine sedimentary rocks of the Tertiary transgressions are scarcely represented, since this subunit was part of a positive relief during those times. It shows a compressive-type structure formed in several pulses during the Late Cretaceous and the Tertiary (Barcat et al., 1984).

2.6. The Fuegian Andes (Borrello, 1972) This is the only Andean segment that extends in a W–E direction, from the Magellan fault to the Isla de los Estados (Staaten Island), in the South Atlantic Ocean (54–56 S; 63–72 W). This geological province has been regionally described by Caminos (1980) and Olivero and Martinioni (2001), among other authors. The Fuegian Andes forms a mountain system that is bounded by W–E and NW–SE fault, loosing elevation from W to E and from S to N. These are the lowest Andean summits in Patagonia; their highest peaks reach 1476 m a.s.l. in Monte Olivia and 1490 m a.s.l. at Monte Cornu´, in southern Argentine Tierra del Fuego, whereas westward, in the Chilean sector of the Darwin Cordillera, the maximum elevations are present at Monte Darwin (2488 m a.s.l.). The Fuegian Andes is characterized by three areas: (a) the Fuegian Archipelago, in the western and southern Chilean sector, formed by plutonic rocks resulting from several intrusions during the Cretaceous and the Cenozoic, (b) the Fuegian Cordillera and (c) the foothills of the Fuegian Cordillera, north of Lago Fagnano, until 53300 S, approximately. The basement rocks form the core of the Fuegian Cordillera, outcropping at Darwin Cordillera (Chile) and Bahı´a Lapataia (Argentina). They correspond to Paleozoic metamorphic rocks (Borrello, 1969), strongly folded with quartz injections. They are overlain with acid volcanic rocks in pyroclastic and lava facies, dating from the Middle to Late Jurassic, that integrate the Sierras de Alvear. Associated to these rhyolitic-dacitic lavas, metalliferous, polisulphide minerals exist, forming some of the orange and yellowish summits of Sierras de Sorondo and Alvear (Ametrano et al., 1999). Metamorphic rocks of Early Cretaceous marine sedimentary origin, partly associated to igneous rocks and with intense deformation and folding (Quartino et al., 1989), form the more extensive mountain ranges of Tierra del Fuego (Sierras de Martial, Sorondo and Lucio Lo´pez, between 1000 and 1200 m a.s.l.). On the northern coast of the Beagle Channel, the metamorphic rocks present ultramaphic intrusions of the hornblendepyroxene type (Acevedo et al., 1989), whereas granitoid intrusions are found in the Cordilleran inner portion, crossed by basaltic dykes, which were emplaced during the Andean orogeny and corresponding to a marginal position of the Fuegian Batholith (Acevedo et al., 2000). The mountain systems, located north of Lago Fagnano (such as the Sierras de Apen and Beauviour), are composed of Early Cretaceous mudstones, slates and dark limestones and Late Cretaceous mudstones and sandstones. North of Lago Fagnano, NW–SE alignments are emplaced in fan shapes, originating a system with elevations decreasing from S to N from 500 to 200 m a.s.l. They are composed of marine sandstones, limestones, mudstones, claystones and clayey siltstones, Paleocene to Early Oligocene in age. The structure of the Fuegian Andes is characterized by a series of east-trending faults, with northeast and southeast deviations, and by northoriented thrusts, associated to dynamic metamorphism of pre-Late Cretaceous age (Caminos, 1980; Ramos, 1999). The transcurrent faults in the Andean region affect the entire Paleozoic–Quaternary complex.

Physical Geography of Patagonia and Tierra del Fuego 2.7. The ‘‘Neuquen Embayment’’ (Ramos, 1999) or ‘‘Neuquen Basin’’ (Digregorio and Uliana, 1980) This unit includes the central and NE sectors of Neuque´n Province, from the Rı´o Colorado to the north until the lower Rı´o Limay valley in the SE and the middle Rı´o Negro valley to the south. Westward, it separates the Principal Cordillera from the Northern patagonian Andes, around 39 S. In its eastern sector, tableland landscapes are developed, with elevations between 800 and 200 m a.s.l., which are original surfaces that represent the sedimentary processes of the Neuquen Embayment. The subsurface is formed by Jurassic and Early Cretaceous marine sedimentary rocks which wedge eastward, a product of Pacific Ocean transgressions. In this unit, Cretaceous continental sedimentary rocks appear, together with sand and gravel, thin sedimentary cover of Pliocene and Quaternary age. In the tableland landscape, the Sierra de Auca Mahuida (2253 m a.s.l.) stands out, a Quaternary volcanic complex composed of a main strato-volcano and more than 100 minor vents which provided basaltic flows that overlie the sedimentary deposits. The structure is characterized by very gentle folding controlled by the basement and affected by the Andean orogeny (Ramos, 1999).

2.8. Somun Cura Massif or Northern Patagonian Massif (Ramos, 1999) This unit forms the landscape of ranges and mesetas of hard rocks from the south-central portion of Rı´o Negro Province and north and east-central areas of Chubut Province. Some of the oldest rocks of Patagonia, of Middle Proterozoic age, are found here. The metamorphic basement includes gneisses, mica schists and granitoid rocks associated with low-grade metamorphics. Clastic, marine sedimentary rocks of Early to Middle Paleozoic age are located in the eastern zone and they are penetrated by various Paleozoic plutonic rocks. The basement is covered by acid lavas and pyroclastic rocks of Early to Middle Mesozoic age, in some areas interbedded or covered by continental deposits. Toward the east and southeast of the massif, marine sedimentary rocks corresponding to the Tertiary transgressions are located. The intense volcanism during the Middle Tertiary created the present relief by means of the formation of necks, domes and alkaline-type basalt flows. The structure is characterized by the existence of large basement blocks with inclined hemi-grabens, affected by the Andean orogeny.

2.9. The Northern Patagonian Tablelands (Ramos, 1999) The meseta-type landscape is the dominant morphological feature in extra-Andean Patagonia. This unit extends south of the Somun Cura Massif, east of the Patago´nides and north of the Deseado Massif. The tablelands are formed by Paleocene marine and continental sedimentary rocks, covered by Eocene–Oligocene pyroclastic rocks. Overlying these are marine rocks of the Patagonian

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transgression and Middle Miocene fluvial sediments. Of smaller extent, but characteristic of this unit landscape, the basaltic tablelands formed by Eocene to Miocene basaltic flows and necks are exposed here. The mesetas are developed over the Golfo San Jorge basin, which is an important oil- and gas-bearing, Jurassic–Cretaceous sedimentary sequence. In the eastern sector, the plains are covered by gravel deposits of possible glaciofluvial origin, deposited during the Pleistocene (see Chapter 9).

2.10. ‘‘Deseado Massif’’ (Leanza, 1958) This unit includes the territory located between the Deseado and Chico rivers, in Santa Cruz Province. It presents a sub-positive relief, stable since the Paleozoic. The basement is formed by phyllites and schists of Late Proterozoic to Early Paleozoic age, intruded by granitoids and sub-volcanic rocks during the Middle Paleozoic. Over these rocks, continental sedimentary rocks occur, which were deposited during the Late Paleozoic and the Early Mesozoic, outcropping in reduced sectors of the east-central portion of the area, as well as the acid plutonic rocks that intruded them during the Triassic and Jurassic periods, forming the Patagonian Central Batholith. Tertiary marine and continental sedimentary rocks are interbedded with rhyolitic volcanics and pyroclastic flows forming a plateau and which are locally related with alkaline basic volcanic rocks. These volcanic sequences would have been produced during the Jurassic until the Early Cretaceous. Jurassic–Cretaceous continental sedimentary rocks are located along the northern sector of the massif, whereas the younger sedimentary rocks are exposed along all its margins. Volcanic activity continued during the Cenozoic, erupting basaltic flows that form the center of the massif. Over these rocks, alluvial Early to Middle Pleistocene sediments, probably of glaciofluvial origin, have been deposited. The structure is characterized by subhorizontal sequences over an extended time, from the Late Cretaceous to the Cenozoic. The Jurassic–Cretaceous rocks show intense fracturing whereas the basement has been inclined, showing a strong deformation. The massif presents evidence of a very long crustal stability.

2.11. The Southern Patagonian Tablelands (Ramos, 1999) This unit develops south of the Deseado Massif and east of the southern Patagonian Andes, also including the northern part of Isla Grande de Tierra del Fuego. The substratum corresponds to Mesozoic and Tertiary sedimentary rocks that form the so-called ‘‘Cuenca Austral’’, but in this region only Late Cretaceous and Tertiary marine and continental rocks are outcropping. Alkaline basalt flows form the typical feature of the western central sector and correspond to volcanic events that took place during the Miocene. Relatively low level mesetas and interbedded lava flows and till deposits are located toward the south, showing intense volcanic activity during the Pliocene and Early Pleistocene, followed by cold periods that forced the

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expansion of the Andean glaciers (Rabassa et al., 2005). Of the same age, the Pali-Aike volcanic field is located north of the Magellan Straits, where the positive relief of maars and basaltic flows and glacial and glaciofluvial landforms break the monotony of the tableland landscape (Corbella, 2002). The relief of high plains built upon the Tertiary marine rocks continues south of the Magellan Straits, where the volcanic rocks disappear but the Early to Middle Pleistocene landscape is enhanced. There, moraines, beheaded and dry valleys, erratic boulder fields, eroded hills and glaciofluvial terraces are forming the landscape north of the Fuegian Andes. It should be noted here that the extra-Andean portion of Tierra del Fuego is located north of the Andes and not east of the Andes as in the rest of Patagonia. This is due to the change in orientation of the Andean Cordillera, due to the tectonic relationship between the crustal lithospheric plates. During the Late Oligocene and the Early Miocene, silts, clays, sandstones and conglomerates with an abundant mollusk fauna were deposited in a deltaic environment along the northern zone of the island, whereas during the Middle Miocene and Pliocene, silty sandstones corresponding to coastal environments affected by a marine transgression accumulated (Olivero et al., 1999). The substratum of this basin is formed by marine clastic rocks known as the Springhill Formation (Robles, 1982), indicating a marine transgression during the Late Jurassic and the Early Cretaceous. Several Atlantic transgressions also took place during the Tertiary.

2.12. Islas Malvinas/Falkland Islands (Turner, 1980) These islands form a separate geological province (Turner, 1980), surrounded by the oceanic basins known as ‘‘Malvinas’’, ‘‘Eastern Malvinas’’ and ‘‘Northern Malvinas’’, and the so-called ‘‘Malvinas Plateau’’, a continental substratum on which the Argentine continental platform is based (Ramos, 1999). They are located nearby the northern dorsal of the Scotia plate (Biddle et al., 1996; Ramos, 1996), between 51–52 S and 58–62 W. The Precambrian basement (with ages in between 1124 and 1100 Ma, according to Cingolani and Varela, 1976) of this unit is composed of metamorphic and intrusive rocks, which are outcropping in the southern end of the Isla Occidental or Gran Malvina Island. The rest of the rocky outcrops correspond to quartzites, sandstones and mudstones of marine origin, with trilobite fauna, and other continental rocks, with terrestrial fossil plants of Devonian age, associated by faunal affinity with similar rocks in Cape Town (South Africa) and Ponta Grossa (Brazil). Toward the Late Paleozoic, a sandy-silty sedimentary complex is formed, though no fossils have been found yet in it. These rocks, of shallow marine environments, are the base of the Gondwanic sequence. Well lithified, dark diamictites, composed of coarse fractions with striated and flattened clasts in sandy–clayey matrix correspond to Late Paleozoic glacial environments. In Isla Soledad or Isla Oriental, black shales and mudstones, and grayish shales with fossil plants occur. In the Gran Malvina Island or Isla Occidental, the Paleozoic sedimentary rocks are intruded by basic dykes composed of green

porphyritic diabases corresponding to intrusive episodes of Late Triassic to Early Jurassic age (Cingolani and Varela, 1976). Quaternary deposits and other features are unconsolidated sand and mud, of fluvial, lacustrine and marine origin, raised beaches with abundant mollusk fauna, peat deposits indicative of lacustrine environments and stone rivers and ice wedges of periglacial origin. These features are proof of the very cold environments that dominated this region when it was part of the emerged lands of South America, when sea level was lowered down to 150 m below present sea level during different glacial events. The geological structure of the Islas Malvinas is characterized by a series of E–W thrusts, leaning to the S–SE. The Straits of San Carlos is an important transcurrent fault that separates both islands.

3. Climate According to its latitudinal location, Patagonia is placed between the subtropical high pressure belt and the subpolar low pressure zone. Therefore, it is completely included in the circulation zone of the southern westerlies. South of 40 S, these winds meet no other continent in their way, thus they reach a strong intensity, unknown in the Northern Hemisphere. In the words of Prohaska (1976), ‘‘in few parts of the world is the climate of the region and its life so determined by a single meteorological element, as is the climate of Patagonia by the constancy and strength of the wind’’. The Andean Cordillera intersects the westerlies in a perpendicular position, creating a marked climatic contrast between the Pacific (windward side) and the Atlantic slopes – a contrast exposed by one of the sharper vegetation gradients in the world (Endlichter and Santana, 1988; Warren and Sugden, 1993). Following the 46 S parallel, along a 400 km, W–E transect, a wet-temperate forest grades into Alpine forests and grasslands, changing again into moderate continental forests to merge finally into an arid environment, with steppes and deserts in continental climate (Bailey, 1989). South of 52 S, the Andean ranges have a lesser height and loose continuity, shifting to a W–E orientation. Thus, the rain shadow effect diminishes and the Nothofagus pumilio and Nothofagus antarctica deciduous forests and the grasslands reach for the first time the Atlantic coast at the center of Isla Grande de Tierra del Fuego. Along the Beagle Channel, in the Fuegian Archipelago, rainfall decrease eastward, due to the influence of the W–SW winds. South of the Beagle Channel, the lack of mountain obstacles determines that these winds generate an increase in rainfall toward the eastern portion of Tierra del Fuego. Rainfall drastically changes at both sides of the Andes, with a relationship varying from 5:1 to 10:1, but its seasonality and the patterns of cloudiness and temperature do not behave in such a contrasting way. Moreover, due to the intensity and persistence of the weather flow, perpendicular to the mountain ranges, the westerlies shifting mechanic effect is fully developed, as described by Flohn (1969). Such deviation is shown by a much larger frequency of southwest winds on the

Physical Geography of Patagonia and Tierra del Fuego Argentine side compared with the Chilean slope, where the northwest winds are dominant (Carrasco et al., 1998). The absence of another continental mass in these latitudes determines that the general circulation patterns affecting Patagonia are simpler and more persistent than in the Northern Hemisphere at equivalent latitudes, although seasonal changes are observed in the high and low pressure centers. In summer, the subtropical high pressure zone (the eastern South Pacific and South Atlantic anticyclones) is moved a few latitudinal degrees southwards, whereas the subpolar low pressure zone has almost no displacement, due to the stability of the underlying oceanic conditions. As a consequence of this differential displacement of the low and high pressure belts, the barometric gradient between them grows as spring comes nearer (Lamb, 1972). During the last quarter of the year, this gradient is coincident with thermal differences, which are higher between the South American subtropical sector, becoming warmer earlier, and the Antarctic sea ice, which persists for 3 or 4 months more at 60 S (Burgos, 1985). For this reason, although in Patagonia the average wind speed is very high during the entire year, in almost all the region it reaches a maximum in spring; however, in some Pacific highly exposed meteorological stations, the maximum takes place in winter (Zamora and Santana, 1979). Because of the high wind velocities, the wind chill effect is an important bioclimatic factor in the whole region. Due to a mean wind speed that is larger in the summer half of the year, the cooling effect diminishes the sensitive temperature range. From an ecological point of view the Patagonian climate is felt as more oceanic than really is (Coronato, 1993). There is no uniformity of criteria between different authors about the role of the ocean in Patagonian climate. Whereas for some of them, the climate is definitively maritime (Walter and Box, 1983), for others it has evident continental characteristics (Mensching and Akhtar, 1995). Besides, the windy conditions seem to have a double influence, as supporting both positions. On one side, the continental characteristics are very smoothed by the reduced continental width and the wind intensity (Miller, 1946) or, in other terms, South America south of 40 S is too narrow to allow the formation of continental air masses, specially due to the dominance of the characteristic fresh to strong western winds (Taljaard, 1969). Moreover, Prohaska (1976) indicated that the continental influence on temperature is also noted in the tableland zones, since, due to the prevailing western winds, the refreshing influence of the Atlantic Ocean is not felt in the hinterland. Nevertheless, more than contradictory, the statements of these authors are in fact supplementary. After crossing the Andean Cordillera, the westerlies create rain shadow conditions in eastern Patagonia and they limit the Atlantic influence, but expand the Pacific impact across the region at the same time. Mostly, the middle and high cloudiness recorded in eastern Patagonia until the Atlantic coast is only residual cloudiness generated by the orographic precipitations along the Pacific coast. In fact, because of the geographic factors already mentioned, it is difficult to define the Patagonian climate

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in a global climatic classification. In any other site of the Earth, the eastern side of a continent at equivalent latitude presents a cool-temperate climate, with a noted degree of continentality and moderate rainfall (Cfb or Dfb in Ko¨ppen’s (1936) classification). Contrarily, eastern Patagonia has a dry climate with moderate thermal amplitude. The opposite side, western Patagonia, has a markedly oceanic climate, cooler than their counterparts in other continents, particularly due to the summer heat absence. The principal cause for this relatively low, thermal level in southern Patagonia is the strong influence of the Antarctic continent. The air masses originated there are modified during their crossing of the Drake Passage and they reach Patagonia as maritime polar air masses during the whole year (Weischet, 1985).

3.1. Temperature As has already been mentioned, Patagonia extends over more than 20 in latitude, that is, over more than 2200 km in a N–S direction. In Europe, it would be equivalent to the distance between Copenhagen and the island of Malta. This implies significant differences in the incoming solar radiation, which changes from little above 180 W/m2 (annual average) in the northernmost stations, such as Neuquen, to only 100 W/m2 in Tierra del Fuego (Paruelo et al., 1998) or even less in the more exposed islands of the Magellanic archipelago. There, the average amount of sunshine hours is among the lowest in the world (Lamb, 1972), being in the order of only 1 hour daily during June (Tuhkanen, 1992). According to the increase in latitude, the relationship between the incoming summer solar radiation and that of winter increases progressively as well, from 4:1 at Neuquen (39 S) to 13:1 at Ushuaia (55 S). However, due to the continental narrowing, the temperature pattern follows an opposite pattern. The mean annual thermal amplitude varies from 16C in the north to 8C in the south, or even down to 4C in the outermost Magellanic Islands. The first value is almost equal to those of the Argentine continentality nuclei (17C, located 400 km farther north from Neuquen, at 35 S), whereas the SW archipelagos have been considered as ‘‘hyper-oceanic’’ (Tuhkanen, 1992). The extreme temperatures follow the same pattern, with maxima of 38C recorded at 46 S in eastern Patagonia, but in Tierra del Fuego they do not go over 30C, not even reaching 20C in the hyper-oceanic islands. Minimum temperatures of –30C are recorded in the central tablelands at 41 S. Along the Pacific coast, the absolute minimum readings are between –5 and –7C (Zamora and Santana, 1979). The interannual variation of the temperature is not in phase all over the region, but two main areas of isofluctuation, north and south, are detected. These areas are independent of the Andean Cordillera which in this matter behaves as a second-order differentiation factor. The meteorological stations of these principal areas in which the oscillations are better correlated are Trelew and Rı´o Gallegos, both located on the Atlantic coast at 43 S and 51 S, respectively (Coronato and Bisigato, 1998).

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3.2. Precipitation

1

subhumid 6 2 semiarid arid 10 10

3

humid 12

7

4

°C

The marked differences in rainfall condition on both sides of the Andean ranges have already been mentioned. Almost all of extra-Andean Patagonia gets less than 250 mm per year. Most of this region is located within the winter maritime rainfall pattern, which is ruled by the northward displacement of the Southwestern Pacific anticyclone, during that season and the consequent increase of the front activity from the SW. This Mediterranean-type pattern extends up to 46 S. The winter rainfall dominance is more noted along a meridian belt in central Patagonia (5:1–3:1), since toward the east, a modest influence of the Atlantic Ocean is observed (2:1–5:1). In fact, in eastern Chubut there is not a defined rainfall season and toward the northern portions of the region (east of Rı´o Negro), the increase of summer rainfall anticipates the subtropical continental pattern of central and northern Argentina. In the southern end of Patagonia and on the Pacific coast until 47 S, a summer rainfall pattern is also recorded. In this maritime pattern, the slight increase in summer and fall rainfall, conditioned by the sea temperature reflects characteristics like those in subpolar climates.

15

9

8 5

13

11

5

0 0

1000

2000

3000

mm

Fig. 3. Temperature-precipitation data of Patagonian climatic stations quoted in this chapter. Although the climatic range is quite large in both senses, it is clearly noticeable that most of Patagonia lies within the 5–10C mean annual air temperature and 150–1000 mm annual rainfall amount. Code stations are (1) Cipoletti; (2) Trelew; (3) Maquinchao; (4) Gobernador Gregores; (5) Rı´o Gallegos; (6) Patagones; (7) Esquel; (8) Punta Arenas; (9) San Carlos de Bariloche; (10) Lago Puelo; (11) Ushuaia; (12) Puerto Montt; (13) Evangelistas.

3.3. Climatic Classification

4. Hydrographic Basins

The present classification is based on the superposition of three significant climatic elements that describe the thermal level and pattern (mean temperature and thermal amplitude, respectively) and the hydrological pattern (aridity index). The intersection or superposition of the respective isolines defines climatically homogeneous areas, whose denomination of three terms responds to the following code:

The Andean Cordillera acts as an extensive, continental water divide to the Atlantic Ocean in the Argentine side and to the Pacific in the Chilean. However, there are many streams, such as the Rı´o Hua-Hum in Neuquen, the Rı´o Puelo in Chubut or the Rı´o Vizcachas in Santa Cruz, that have their heads in Argentine territory, but discharge into the Pacific Ocean. These anomalies are due to drainage diversion during postglacial times. On the other hand, there are others, such as the Rı´o Chico de Santa Cruz (locally known as Chillik Aike) or the Rı´o Grande of Tierra del Fuego, which have their heads in the morainic systems in Chile but run through Argentine territory toward the Atlantic coast. The intensive modeling of glacier action during the Pleistocene has caused these diversions in drainage direction. The endorheic basins control the drainage in the extra-Andean Patagonian tablelands, but in the more arid zones, the drainage systems do not become truly organized. Fig. 6 presents the distribution of the four major drainage basins of the region: Atlantic, Pacific, Magellan Straits and the endorheic depressions, as well as the location of the main streams, lakes, ponds, salt lakes and closed basins of the region.

First term: Thermal level: Mean Annual Temperature (MAT) MAT > 10C = Temperate (T) MAT < 10C = Cold (C) Second term: Hydric Re´gime: Aridity Index (AI): mean annual rainfall/potential evapotranspiration (AI = p/PET; UNESCO, 1977) AI < 0.2 = Arid (A) 0.2 < AI < 0.5 = Semiarid (sA) 0.5 < AI < 0.75 = Subhumid (sH) AI > 0.75 = Humid (H) Third term: Thermal Regime: Mean Annual Temperature Range (MATR) MATR > 16C = Continental (c) 16C > MATR > 10C= Transitional (t) 10C > MATR > 5C = Oceanic (o) MATR < 5C = Hyper-oceanic (oþ) Not all the 32 resulting climates actually exist in Patagonia. We have considered only 13 climates affecting sizeable areas at the scale of this work (Figs 3–5).

4.1. The Atlantic Ocean Slope This drainage slope is composed of the large stream basins that are generated in the Andean environment, where they receive abundant precipitation, both as rain and snow. In this sector, the drainage network is formed by many permanent streams, with a dendritic pattern. Eastward, the rivers drastically lose the amount of

Physical Geography of Patagonia and Tierra del Fuego

23

Fig. 4. Climates of Patagonia. tributaries and they become of allochtonous nature, seldom fed by very scarce precipitation. In Table 1, the basic data of the more important regional drainage basins is presented. The basins of the Colorado, Negro, Chubut, Deseado, Santa Cruz, Coyle and Gallegos rivers cross the region in NW–SE and W–E directions (Fig. 6), developing ample fluvial valleys in between the tableland margins. In Tierra del Fuego, the Rı´o Cullen drains a wide area of glacial plains, and the Rı´o Carmen Sylva (or Rı´o Chico de Tierra del Fuego) and the Rı´o Grande flow in wide valleys eroded in a low hill system formed by Tertiary marine sediments. These three streams flow in a W–E direction, whereas the Fuego, Ewan, San Pablo and Irigoyen rivers, among others, drain important basins located in the northern slopes of the Fuegian Andes and extend in ample, terraced valleys with a mostly SW–NE drainage orientation. The Patagonian stream pattern is mixed, rain and snowfall in most cases, with some exceptions as the

Colorado and Deseado rivers, whose feeding is mostly by snow, or the Rı´o Coyle, which collects the discharge of springs located at the foot of the basaltic plateaus. The annual floods take place in spring, as a result of snow melting in the higher portion of the basins, although the northern streams, such as the Chubut and Negro rivers have also flooding in the fall, due to seasonal precipitation. The Rı´o Santa Cruz, on the other hand, has summerfall flooding, due to the abundance of seasonal rainfall as well as the intense ablation of the outlet glaciers feeding the Viedma and Argentino lakes (Fig. 7). Flooding is controlled in almost all drainage basins by the existing lakes, some of them inter-connected, a product of glacial erosion during the last Pleistocene glaciation (Caballero, 2000). The Rı´o Neuquen, one of the larger Patagonian rivers that does not have a natural, high basin, regulating lacustrine system, had violent and damaging fast floods in the past, due to fall–winter torrential runoff, or because of accelerated snow melting in warm spring weather. These

24

Andrea M.J. Coronato et al. 1. Cipoletti. T A c.

2. Trelew. T A t.

38°57′S, 67°59′W, 265 masl

43°14′S, 65°19′W, 39 masl

300

300 20

20

250

250 15

200 150

10

100

100 5

5

50

50 0

0

0 J

F

M

A

M

J

J

A

S

O

N

0

J

D

F

3. Maquinchao. C A c

M

A

M

J

J

A

S

O

N

D

4. Gobernador Gregores. C A t.

41°15′S, 68°44′W, 888 masl

48°47′S, 70°10′W, 358 masl 300

300

20

20

250

250 15

200 150

10

100

100

5

5 50

50 0

0

0

J

F

M

A

M

J

J

A

S

O

N

0

J

D

F

M

A

M

J

J

A

S

O

5. Río Gallegos. C sA t.

6. Patagones. T sA t.

51°37′S, 69°17′W,19 masl

40°47′S, 62°59′W, 40 masl

N

D

300

300

20

20

250 15

250 15

200 150

10

100

100

5

5

50 0

50

0 J

F

M

A

M

J

J

A

S

O

N

0

D

0 J

F

7. Esquel. C sH t.

M

A

M

J

J

A

S

O

N

D

8. Punta Arenas. C sH o.

42°54′S, 71°09′W, 785 masl

53°10′S, 70°54′W, 8 masl

300

300

20

20

250 15

250

200

200 150

10

100 5 50 0

0 F

M

A

M

J

J

A

S

O

N

D

Fig. 5. Representative climograms of Patagonian climate.

100 5 50 0

0 J

F

M

A

M

J

J

A

S

O

N

D

mm

°C

15

mm

150

10

J

mm

150

10

°C

200

mm

°C

mm

150

10

°C

200

mm

°C

15

°C

mm

150

10

°C

200

mm

°C

15

Physical Geography of Patagonia and Tierra del Fuego 9. Bariloche. C H t.

10. Lago Puelo. T H t.

41°09′S, 71°10′W, 810 masl

42°06′S, 71°38′W, 260 masl 300

300

20

20

250

250

200

°C

200 150

10

100

100

5

5

50 0

50

0 J

F

M

A

M

J

J

A

S

O

N

0

0

D

J

F

M

11. Ushuaia. C H o.

A

M

J

J

A

S

O

N

D

12. Puerto Montt. T H o.

54°48′S, 67°47′W, 22 masl

41°28′S, 72°57′W, 13 masl 300

300

20

20 250

15

250 15

200

°C

200 150

10

100

100

5

5

50 0

50

0 J

F

M

A

M

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O

N

mm

150

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mm

°C

mm

150

10

15

mm

°C

15

25

0

D

0 J

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A

M

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D

13. Evangelistas. C O+ 52°24′S, 75°06′W, 55 masl 300 20 250 200 150

10

mm

°C

15

100 5 50 0

0 J

F

M

A

M

J

J

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D

Fig. 5. Continued.

conditions were modified with the development in the 1960s of a large system of regulating dams in its lower valleys. Similarly, the catastrophic flooding of the Rı´o Chubut was controlled as well around 1960. Natural lakes in the higher basin and/or artificial dams in the middle basin collect the sedimentary load of these drainage systems, lowering the sedimentary discharge in the lower basins, giving the waters a great transparency. The Fuegian streams and the main Patagonian trunk streams have a characteristic meandering pattern and an underfit stream nature, conditions that have been inherited from the gently sloping, glaciofluvial landscape that they cross. A braided stream pattern is found in the lower valleys of the Neuque´n, Limay, Chubut, and Santa Cruz rivers and all along the Rı´o Negro valley. Fluvial islands, bars and active and abandoned meanders are forming part of the

present fluvial landscape. They are the landforms that allow the development of abundant riparian vegetation, highly contrasting with the shrubby steppe that occupies the alluvial valley bottoms or the surrounding tablelands. The Rı´o Negro, in northern Patagonia, is one of the more important ones in Argentine hydrography, because it has the largest discharge (Table 1) with the exception of those of the Rı´o de la Plata Basin, that is, the Parana´, Uruguay and Paraguay rivers (Grondona, 1975). It is entirely located within Argentine territory. The allochtonous nature of the Patagonian streams generates a significant discharge decrease as they traverse the arid tablelands. That is the case of the Rı´o Deseado, which even disappears in some reaches, forming again downslope thanks to springs located at the stream bed, or being fed by underground outcrops at the foot of the volcanic plateaus.

26

Andrea M.J. Coronato et al.

Fig. 6. Patagonian hydrographic basins.

The rivers of the Malvinas-Falkland Islands are short, in wide channels that discharge smoothly undulated terrain and whose annual precipitation is smaller than 600 mm. Those more important are the Rı´o San Carlos on Isla Soledad or East Falkland and the Rı´o Warrah on Isla Gran Malvina or West Falkland. Some of these streams have a discontinuous flow over the year.

4.2. The Beagle Channel Slope This marine channel, with water circulation from west to east, connects the Pacific and the Atlantic oceans at 55 S. It is the base level for many short mountain basins of the Fuegian Andes. The Pipo, Grande, Olivia, Lasifashaj and Moat rivers are draining the inner valley basins,

which generally are extending in a longitudinal way, following the Andean axis, or are placed on faults or other geological structures. The scarce data available about these basins are shown in Table 2. Isla et al. (1999) defined five different sectors concerning stream discharge in the Beagle Channel: (a) in the westernmost end, there are short rivers and outlet glaciers coming from the Cordillera Darwin mountain ice sheet that reach the sea, sometimes providing small icebergs that melt in the inner fjords; (b) in between Yendegaia and Ushuaia bays, short collectors from inner basins occur, with lacustrine and cirque glacier spillways; (c) in between Ushuaia and Brown bays, streams are draining slopes and high mountain valleys of the

Physical Geography of Patagonia and Tierra del Fuego

27

Table 1. Hydrographical data of the Patagonian stream basins of the Atlantic slope, based upon Grondona (1975) and Medus and Rey (1982). The discharges are measured along the middle portion of the stream channels, with the exception of Rı´o Santa Cruz*, measured 25 km downstream from its heads, because there are no gauge stations in the rest of the basin. Trunk stream

Main tributaries

Colorado Neuquen

Grande (N) and Barrancas (NW) Varvarco (N), Nahueve (NW), Agrio (W), Covunco (SW)

923 510

Spring Fall and spring

Limay

Traful (W), Collon Cura (W), Pichileufu (S), Comallo (S), Picun Leufu (N) Neuquen (NW) and Limay (SW)

430

Chubut

˜ orquinco Leleque and Tecka (N), N and Chico (S)

820

Deseado

Fe´nix Grande (NW), Fe´nix Chico (W) and Pinturas (SW) Belgrano (NW), Lista (W) Shehuen (W) La Leona (N) and Bote (S)

615

Winter and spring Winter and spring Winter and spring Spring

420 383

Pelque (NW) and Brazo Sur (W) Turbio (NW), Penitente (S), Zurdo (SE), Chico (SW)

250 300

Negro

Chico Santa Cruz Coyle Gallegos

Length (km)

637

Sierras de Sorondo, being shorter and with less discharge than the previous ones; (d) in between Bahı´a Brown and Punta Navarro, the inner basin collectors are flowing along glaciofluvial and glaciolacustrine valleys, with frequent peatlands and have a meandering pattern, receiving the discharge of many tributaries from the higher valleys, some of them starting at cirque glaciers; (e) in the eastern portion of the Beagle Channel, up to Bahı´a Sloggett, the streams drain basins occupied by extensive and thick peatlands, with ponds, flow with meandering pattern and collect the runoff of the easternmost Fuegian Andes. The streams of the Beagle Channel slope are fed by rainfall and snow melting, with flooding from October to December, with seasonal snowfall the most important storage factor, which usually persists until February. Lowest runoff is achieved between February and April, when the streams are mostly fed by cirque glacier ablation and underground runoff stored in the debris cover of the mountain slopes and talus (Iturraspe et al., 1998). Unique characteristics are observed in the streams of Tierra del Fuego when compared with the Patagonian streams. This is due to dam construction by Castor canadensis, an exotic, introduced species. The aquatic behavior of this species have generated significant changes along the longitudinal stream profiles, modifying the

Floods

Total basin surface (km2)

Mean annual discharge (m3/sec)

Mouth

34,040 34,100

133.68 302.8

63,700

723.1

125,500

1021.4

Delta Merging with Rı´o Limay Merging with Rı´o Neuquen Submarine delta

31,000

48.6

Estuary

14,450

–

Estuary

Spring

16,880

24

Estuary

Summer and fall Spring Spring

24,510

696*

Estuary

14,600 8400

6 33.6

Estuary Estuary

local base levels, increasing retention of finer sediments, widening channels, intensifying erosion and bank flooding, modifying channel orientation and changing the transport-accumulation rates along the basins (Coronato et al., 2003).

4.3. The Pacific Ocean Slope This slope is also formed by streams and lacustrine systems, originated in the Argentine territory but which discharge in the Pacific Ocean, extending, in a discontinuous manner, between 40 S in the province of Neuque´n until 54 S in Tierra del Fuego. These are the basins of the Hua-Hum, Manso, Puelo, Futaleufu, Carrenleufu, Pico and Simpson rivers, among others. Several lakes, as the La´car, Buenos Aires, Pueyrredo´n-Posadas, San Martı´n and Fagnano are head basins of rivers mostly developed in the western Andean slope. All together, they represent a surface of as much as 37,400 km2 (Daus, 1975). The existence of streams with Pacific discharge on the eastern slope of the Andes was already noted by the first Patagonian explorers (Musters, 1871; Moreno, 1889) and it was an issue of long debate in academic and diplomatic international meetings. The Pacific slope of some Patagonian rivers with origin in the eastern piedmont areas dismantled the ‘‘divortium aquarum’’ theory, when the international boundary

28

Andrea M.J. Coronato et al.

Fig. 7. Hydrograms of Atlantic and Pacific slope streams, according to their mean monthly discharge. Flooding in the Negro, Futaleufu and Manso rivers shows the fall and winter precipitation and snow melting influence. The early summer Rı´o Colorado flooding is due to the snowy re´gime in its upper basin, whereas the Rı´o Santa Cruz raises its discharge in the fall, when glacier ablation, feeding the mountain lakes and creeks, takes place.

Table 2. Hydrographical data of the stream basins of Argentine Tierra del Fuego. The Rı´o Grande belongs to the Atlantic slope whereas the Lasifashaj and Olivia rivers discharge into the Beagle Channel. Trunk stream of the basin

Main tributaries

Rı´o Grande

Bella Vista (E), Herminita (), de la Turba (SW) de los Onas (S), Moneta (NW), Candelaria (S) Las Cotorras (N) Tristen (N), de la Quebrada (S) Beban (N) and Esmeralda (E)

Lasifashaj Olivia

Length (km)

Floods

Total area (km2)

Mean annual discharge (m3/sec)

Mouth

230

Spring

7021

60

Estuary

44

Spring

592

4.6*

Delta

30

Spring

624

6**

Delta

* corresponds to the upper basin, no data on the middle and lower basins; ** near the mouth.

Physical Geography of Patagonia and Tierra del Fuego between Chile and Argentina was settled in the late nineteenth Century. This theory assumed the hydrological basins as orographic boxes, perfectly bounded by high mountain rocky watersheds. The geological origin of this problem is related to the differential uplift of some Andean blocks and the generation, as early as the Miocene, of antecedent valleys (Groeber, 1927), the differential uplifting of the extra-Andean terrains with respect to the mountain valleys and the Quaternary basaltic flows that blocked the Atlantic drainage and forced relief inversion. Intense glacial erosion and the construction of terminal moraine systems since the earliest Pliocene should also be considered as one of the main causes for these drainage anomalies. Once the postglacial drainage network was established, some streams developed intense capture and headwater erosion, thanks to the abundant precipitation of the region (Grondona, 1975). The Pacific slope streams are typical of mountain environments, with narrow channels, steep gradient, riffles and waterfalls, and high erosive power. The base level is locally controlled by the existence of glacial lakes along their basins (Table 3). Frequent short and steep streams are draining the islands of the Austral and Fuegian–Magellanic archipelagos. Their pattern is based on rain and snowfall, though those streams located south of 45 S, have a larger pluvial contribution due to the high precipitation values of this Patagonian region. Discharge varies between 50 and 296 m3/sec (Table 4). Flooding is produced by March and July precipitation, and snow melting mostly during November (Fig. 6). The Rı´o Futaleufu (or ‘‘Big Stream’’ in the native Mapuche language) is the most important of the Pacific slope streams due to the wide extension of its watershed, composed of eight major lakes.

4.4. The Magellan Straits Slope The marine channel known as the Magellan Straits extends in a NW–SE direction from the Pacific Ocean

29

Table 4. Measured and estimated discharge of some Patagonian rivers of Pacific slope, in their upper valleys (Medus and Rey, 1982). Stream Hua-Hum Manso Epuye´n Puelo Futaleufu´ Carrenleufu´

Measured discharge (m3/sec)

Estimated discharge (m3/sec)

– 67 15 – 296 31

50 – – 100 – –

to Isla Dawson (53550 S) and switches then to the N–NE until reaching the Atlantic Ocean at Punta Du´ngenes (52250 S; 68300 W). It has the name of its discoverer Hernando de Magallanes, a Portuguese sailor working for Spain, who in 1520 found the long searched physical communication between both oceans and identified the existence of a vast archipelago separated from the southernmost end of the South American continent. Water circulates through the Straits from W to E, having low salinity values due to meltwater supply from the glaciers of the southern part of the region. When reaching the Atlantic Ocean, currents take a northern direction, merging with the Patagonian coastal current (Piola and Rivas, 1997). The Rı´o Dinamarquero and other smaller streams discharge on the northern coast of the Straits, coming from the semiarid lands of southern Patagonia. Their basins are limited by the San Gregorio and Monte Alto ranges. On the west coast, the morainic hills of Penı´nsula Brunswick lead the flow of the Oro, San Juan and Amarillo rivers. From the south, the Almirantazgo (Admiralty) Sound collects the meltwaters of the outlet glaciers of the Cordillera Darwin while the Rı´o Azopardo, a spillway of the extensive Lago Fagnano Basin, joins the Magellan Straits through the Whiteside Channel, where the short

Table 3. Fluvio-lacustrine systems discharging toward the Pacific Ocean, based on Grondona (1975) and Medus and Rey (1982). Lakes

Streams discharging towards the West

Height of the trans-Andean pass (m a.s.l.)

La´car Mascardi, Guillermo, Fonck Puelo Futalaufquen-Cholila General Vintter –

Hua-Hum- Valdivia (Chile) Manso

645 400

– 53

Epuyen- Manso (Chile) Futaleufu-Yelcho (Chile) Carrenleufu-Palena (Chile) Pico- Figueroa and Palena (Chile) Baker (Chile) Baker (Chile)

210 518 940 no data

15–55 70 20 70

217 200

60 60

Rı´o Vizcachas Azopardo (Chile)

no data 26

35 –

Lago Buenos Aires Pueyrredo´n-PosadasSan Martı´n – Fagnano

Distance of the heads in the extra-Andean environment (km)

30

Andrea M.J. Coronato et al.

but torrential streams of Isla Dawson discharge as well. From Isla Grande de Tierra del Fuego, the Paralelo, Co´ndor and Nuevo rivers reach the marine channel from the east, because of terminal moraines that block Atlantic drainage. Other streams with a SW–NE direction, longer than those coming from the south and with very well developed valleys are draining the northern slope of Cordo´n Baquedano and its terminal morainic systems, forming the basins of the Verde, de Oro, Oscar and O’Higgins rivers, which reach the southern coast of the straits.

4.5. The Endorheic Basins In extra-Andean Patagonia, tectonic or erosive basins acting as local base level to the ephemeral streams activated during winter precipitation or spring meltwater are very common. Many of these depressions are occupied by temporary ponds and small lakes that form noted places for biological concentration, in marked contrast with the surrounding aridity (Laguna Blanca and Laguna Tromen, in Neuquen Province, Laguna Cari Laufquen in Rı´o Negro, Laguna Aleusco in Chubut). Other depressions are occupied by salt lakes, some of them of large dimensions, as the Salina del Gualicho (Rı´o Negro), the An˜elo Basin (Neuque´n) or the Grande and Chica salt lakes in the Penı´nsula Valde´s, Chubut. Some of these depressions have been used for the construction of artificial lakes that retain the excessive discharge of the fluvial network, as the Cuenca Vidal, Rı´o Negro, that controls the violent flooding of the Rı´o Neuquen or those of Los Barreales and Mari Menuco, in Neuquen, which are also used for hydroelectricity generation (Calcagno et al., 1995). Some central depressions control very extensive endorheic basins, such as the Bajo de la Tierra Colorada, with an area of 21,000 km2, in Chubut. Absolute depressions such as the Salina del Gualicho, in Rı´o Negro [5 m b.s.l. (below sea level)], Salina Grande, in Penı´nsula Valde´s (51 m b.s.l.), the Gran Bajo de San Julia´n, in Santa Cruz (105 m b.s.l.) are important landscape features. Among these large depressions, the Nuevo and San Jose´ gulfs at the Atlantic coast should be included, since they were originally subaerial endorheic basins later invaded by the sea, sometime in the Holocene (Mouzo et al., 1978). The tectonic depressions that are occupied by the large Musters and Colhue-Huapi lakes receive the runoff of the Rı´o Senguerr which, after a 340 km long channel, drains a 17,500 km2 basin (Grondona, 1975). In extraordinary flooding conditions, the Rı´o Chico del Sur, a spillway of Lago Colhue-Huapi, temporarily discharges into the Atlantic Ocean through the Rı´o Chubut. Another depression occupied by a permanent fresh water lake in the tableland environment is the Lago Cardiel, which receives the runoff of the river of the same name and other minor tributaries. In extra-Andean Patagonia, including northern Tierra del Fuego, there are many depressions of varied genesis, size and age, which are given the general name of ‘‘bajos sin salida’’ (endorheic hollows). Their origin has been considered by several authors (Feruglio, 1949;

Frenguelli, 1957; Methol, 1967; Fidalgo, 1972), among others, who considered multiple possible origins, mostly deflation and hydro-eolian activity, particularly in the sedimentary rock tablelands. The spatial distribution and the morphometric parameters of 220 depressions with a surface over 5 ha located in southern Santa Cruz were studied by Mazzoni (2001). The depression dimensions are highly variable, from very small ones to 100 km2, but the modal interval is found between 10 and 25 ha. Their depth ranges between less than 3 m in those depressions smaller than 50 ha and a maximum of 60 m in the larger basins, with a mean value of 13 m. The spatial distribution of the endorheic basins varies with each landscape unit considered; thus, for example, in glacial environments, there is a high density of smallsized depressions, whereas in the sedimentary tablelands, basin density is smaller, though they have a larger size. These differences are related both to genetic and morphoevolutionary processes and to lithological differences, plant cover and intensity of erosive processes that are active in these areas. Concerning the geomorphological features of these landforms, the coastlines have a particularly environmental interest (Mazzoni, 2001) as well as the ‘‘eolian plumes’’ (Mazzoni et al., 2002). As for the coastlines, up to six different levels have been identified in the larger basins, which show the important recession of water bodies since their maximum extent. Similar conditions have been described also in other large Patagonian closed basins, such as Laguna Cari Lafquen, Rı´o Negro (Gonza´lez Bonorino and Rabassa, 1973) and Lago Cardiel, Santa Cruz (Galloway et al., 1988), which would confirm the restrictions in moisture conditions for the entire region. Stine and Stine (1990), based on radiocarbon dating of the ancient coastlines of the latter lake, estimated that a depth reduction of 55 m has taken place in the last 10,000 yrs. The ‘‘eolian plumes’’ are a clear indicator of the intense erosive processes that affect the extra-Andean region, associated to desertification processes. These are landforms generated by wind action that removes clastic particles from the depressions, when these are dry or with a very shallow water level. The largest ones reach up to 4–5 km in length along the eastern side of the basins, following the dominant wind direction. In southern Santa Cruz, it has been observed that 75% of the depressions have a plume and their number increases east of 70 W, where those of larger dimensions show a growth of up to 800 m per year (Mazzoni et al., 2002). Likewise, in southwestern Chubut, there are erosion tongues of a perfect straight shape, with a W–E orientation of up to 60 km in length, like those of Laguna del Coyte (Movia, 1972, 1980).

5. Morphoclimatic Units According to Sayago (1982), there are natural, climaterelated processes that modify the land surface and contribute to the genesis and evolution of the landscape, providing distinctive characteristics. To characterize from a geomorphological point of view, a geographical

Physical Geography of Patagonia and Tierra del Fuego space means to consider the geological structures under the influence of modeling agents imposed by physical– chemical processes developed in the low atmosphere and their interaction with the ground surface. The geological structures modify their original superficial features with time under the action of weathering, erosion, transport and accumulation processes, triggered by the transformation of solar energy into thermal, mechanical and kinetic energy. From a genetic classification, the morphoclimatic agents responsible of the landscape modeling are running water, ice, wind and wave action. The geomorphological processes acting in Patagonia and their resulting landforms which are noted at the scale of this work are presented in Table 5, whereas in Table 6, these are ordered according to the geological units and climatic types shown in Figs 2 and 4, respectively. The unequal resistance to erosion and debris production of the different lithologies present in the region, the topographic gradients and the varying moisture, temperature and wind conditions generate different morphoclimatic units. In northwestern Patagonia, the Principal Cordillera (see Section 2.1) in Neuquen Province, under a transitional subarid climate, presents mountains formed by intrusive and sedimentary rocks. The latter are affected by erosion due to superficial runoff during fall precipitation and spring melting and to eolian erosion during the dry periods at the end of the spring and summer. In the higher zones, cryoclastic processes and seasonal frost

31

provides the debris for talus, cones and fan formation. Eastward, the thermal contrast and the wind frequency generate soil drying and deflation. The fluvial systems of the Neuquen and Agrio rivers have developed wide valleys with landforms of varied type. The present glacial modeling is limited to the highest summits, including the volcanoes, where nivation hollows and a few cirque glaciers are present; however, the relict glacial modeling has generated moraine systems, glaciofluvial plains and lakes. This imprint is even more noted southwards (39 S), in the mountain zone of the Neuque´n Basin (see Section 2.2), where the subarid climate participates in basalt thermal contraction, as well as in the deflation of sedimentary and volcanic rock fragments, and the summer flash floods generate rills in the morainic hills. In the easternmost mountain ranges, cryoclastic and seasonal soil-frosting processes, small cirque glaciers and snowfields produce debris and contribute to the ice modeling. The intrusive and metamorphic rocks that form the northern Patagonian Andes (see Section 2.3) are exposed to glacial erosion, cryoclastics and seasonal frost, due to the combination of temperature and moisture at higher elevations. The wet transitional climate provides enough water to generate slope erosion and channel erosion in the inner valleys. The large, overdeepened glacial basins, excavated in the transversal valleys, are sites for many glacial lakes that behave as both water reservoirs and runoff regulators in the Rı´o Limay Basin. Along their

Table 5. Dominant processes in Patagonian landscape modeling and resulting landforms. Types 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Dominant process

Geomorphological product

Physical weathering Thermal Contraction – Expansion Soil frost Cryoplanation Cryoclastics Earth slide, snow avalanche, slumping Torrential floods Pedimentation Eolian erosion Eolian deposition Dessication Concentrated discharge Fluvial deposition Glacial erosion

Regolith Talus cones, cracking, talus fans

Relict glacial erosion 15 16 17 18 19

Glacial deposition Relict glacial deposition Coastal erosion Coastal deposition

Creep, patterned ground, gelifluction lobes, rock glaciers Cryoplanation terraces, cryopediments Stratified regolith, tors Head scars, channels, foot hill deposits Alluvial cones and fans, uadis Bajadas, playas Eroded soils, deflation hollows, desert pavements dunes, eolian plumes Salt concentration depressions Rill erosion, deepening and widening of channels Bars, islands, valley trains, alluvial plains, terraces, alluvial fans Cirques, areˆtes, horns, needles, truncated spurs, roches moutonne´es, glacial troughs Troughs, hanging valleys, isolated and beaded lakes or patter-noster lakes, truncated spurs, cirques, areˆtes, horns, roches moutonne´es, drumlins, smallscale erosion features Push, frontal and lateral moraines, proglacial lakes, glaciofluvial plains Moraine systems of various types, kames, kame-deltas, glaciofluvial terraces, drumlins, glaciolacustrine plains Cliffs Sand and gravel beaches, spits, tombolos, littoral bars

Note: drumlins are indicated both as erosional and depositional glacial features.

32

P r ocesses + L a n d for m s G eologica l p r ovin ces C l im a t i c t yp es 1. Principal Cordillera

C sA t

2. Neuquen Embayment mountains 3. Northern Patagonian Andes

C sH t

4. Southern Patagonian Andes 5. Fuegian Andes 6. Patagónides 7. Neuquen Embayment 8. Somun Cura Massif 9. Northern Patagonian Tablelands 10. Deseado Massif 11. Southern Patagonian Tablelands 12. Malvinas Islands

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

CHt 1. C H o 2. C H t 3. C sH t 1. C H o 2. C o + 1. C sH t 2. C sA t 3. C A t 1. C sA t 2. T A c 1. C A c 2. T A c 3. T A t 1. C A t 2. T A t CAt 1. C A t 2. C sA t 3. C sH t 4. C sH o CHo

?

?

18

19

Andrea M.J. Coronato et al.

Table 6. Patagonian morphoclimatic systems, based upon the active processes and resulting landforms (Table 5). The squares in grey indicate presence of those active processes and resulting landforms in the corresponding geological province. In some morphoclimatic units, the modeling processes do not generate all those landforms included in the cited Table.

Physical Geography of Patagonia and Tierra del Fuego intricate coastlines, erosion is produced on the rocky cliffs and the accumulation of gravels and sands obtained from the morainic slopes generates paraglacial beaches. The processes of relict glacial erosion and deposition are dominant in landscape modeling. On the trough slopes, diverse mass-movement processes take place, among which snow avalanches, landslides, rock fall and soil creep may be considered. The eastward precipitation gradient determines a gradual increase in eolian erosive processes, as the active agent on drier soils or polishing volcanic rock. The southern Patagonian Andes (see Section 2.4) have similar morphoclimatic characteristics, though a larger oceanic influence provides several differences. The dominance of oceanic and hyper-oceanic climatic types over this higher elevation section of the Andes generates abundant snowfall that contributes to two mountain ice sheets, known as the ‘‘Hielo Patago´nico Norte’’ and ‘‘Hielo Patago´nico Sur’’ (Fig. 1). These ice fields, with a total area of 17,200 Km2 (Skvarca, 2002), together with the Cordillera Darwin Ice Field, are the only ones in South America of such large size and the biggest of the Southern Hemisphere outside Antarctica. They are a very important water reserve for southern Patagonia. The glaciers of the western slope end in fjords, whereas those of the eastern side reach glacial lakes, located in areas with subhumid to semiarid climates. These present glacial processes occur as well in the Fuegian Andes, though on a smaller scale; they are typical geomorphological features of southern Patagonia and Tierra del Fuego. The relict glacial landscape is responsible for the existence of an intricate marine, fjord-like, channel network and the rocky archipelago with an abrupt relief, both features a result of glacial modeling during Pleistocene glaciations. On the other hand, east from the present ice margin, troughs occupied by glacial lakes are developed, surrounded by volcanic tablelands and morainic systems, the latter formed during the LGM. Fluvial processes are mostly of erosive character, due to the kind of mountain streams, but toward the eastern part of the region, the rivers flow along graded terrains in glaciofluvial settings, loss of energy and the deposition of alluvium in the valley bottoms. Mass-movement processes take place in the mountain area, under oceanic to subhumid climate, whereas cryoclastic processes and rock falls occur in the transitional subhumid environments during spring, when exposure conditions favor melting along the joints. In these sectors, littoral processes are responsible for the modeling of the glacial lake coasts and eolian erosion shapes and polishes the volcanic and metamorphic rocks. The metamorphic, granitic and volcanic mountains of the Fuegian Andes (see Section 2.5) are located in areas with cold and humid climates that vary from hyperoceanic to oceanic. However, the landscape modeling processes are similar in both zones. The higher precipitation values generate snow and ice accumulation in the summit areas and channeled fluvial erosion in slopes of steep gradient along the westernmost sector. These fluvial processes also generate depositional landforms at the bottom of the valleys in the inner part of the larger islands, such as Isla Grande de Tierra del Fuego, Dawson, Navarino, and Hoste. The cryoclastic and seasonal soil-

33

freezing processes generate high altitude landforms, above upper tree limit, and in the vegetated slopes, landslides, snow avalanches and soil creep are the most common processes. The present glacial landscape changes from west to east on Isla Grande de Tierra del Fuego. Toward the west, the Darwin Cordillera mountain ice sheet is located, which has outlet glaciers that flow toward the Magellan Straits and the northwest branch of the Beagle Channel. In the eastern portion, instead, the glaciers are much reduced in size, occupying cirques in the highest mountains. The relict glacial landscape is dominant in the regional physiognomy, where fjords are found in the northwestern part and extensive troughs of varied size on the larger islands. Littoral erosion predominates over accumulation along the coast, due to the constant occurrence of waves and the rocky nature of the high shores. Beaches are small, generated between the rocky outcrops and they are formed by coarse debris and gravels, taken from the rocky slopes and relict glacial deposits, respectively. The cold climate affects also the region of hills and tablelands of extra-Andean Patagonia. The hilly ranges that form the so-called ‘‘Patago´nides’’ (see Section 2.4) are under the influence of the cold climate that ranges from subhumid to transitional arid, although in the northern extreme, a small portion of Rı´o Negro Province is under continental cold arid climate. The W–E precipitation gradient affects those areas in which the hilly ranges are oriented N–S, whereas an arid climate is dominant in the ‘‘Berna´rdides’’ ranges trending NW–SE. This climatic transition does not determine important differences in the existing modeling processes; however, it intensifies their relative action, according to the increase of continentality and aridity. Soil freezing generates clast fracturing and slow slope movements, as creep during spring. Landsliding is of small magnitude due to the low water availability, but they should not be ruled out as modeling agents on the rocky slopes. The seasonal superficial runoff created by precipitation or snowmelt generates channeled runoff, thus forming alluvial cones and fans at the outlet of the sierras valleys. The cryopedimentation processes are favored by soil freezing during several months a year and they generate low slope surfaces that are interfingering with the tableland landscapes of the neighbouring geological provinces. Erosive and accumulative fluvial action is important in the heads of the Rı´o Chubut and its tributaries. During the summer, eolian erosion generates abrasion and soil deflation. The Quaternary glaciations imposed cryoclastic weathering in the peripheral tableland environments, whereas in the valleys glaciofluvial erosion and deposition took place. The tablelands and depressions of the Neuque´n Basin (see Section 2.6) are exposed to a transitional, cold, semiarid climate that changes into a continental temperate arid climate toward the east, which increases the effect of low temperatures in soil and rocks during the winter and the thermal contrasts between winter and summer. The modeling processes of the volcanic and sedimentary rocks, including the uppermost conglomerates forming the tablelands, are the same under both climatic types, though cryoclastics and soil drying are more effective under a continental climate. At the tablelands covered by Tertiary

34

Andrea M.J. Coronato et al.

and Quaternary basalts, slope processes are slumping and soil creep during spring. The pedimentation processes generate wide erosion and accumulation surfaces and playas between the tablelands and the valley bottoms. Rill formation by headwater erosion, debris cones, alluvial fans and bajadas cutting the scarps of the sedimentary rock tablelands, form hills locally known as ‘‘bardas’’. The Colorado, Neuquen and Limay rivers have developed wide valleys with terrace systems, floodplain, bars and islands; in them, the erosive effect is locally concentrated at the foot of the erosive scarps in the sedimentary rocks. The torrential summer runoff develops extensive ‘‘uadis’’, some of which have suffered base-level changes due to the damming of the main rivers at the bottom of the valleys. In these lakes, the littoral erosion processes on the sedimentary rocks are very active and the formation of sand beaches has been a significant process during the last three decades and still continues. The relict glacial deposition is responsible for the gravel cover, sometimes cemented by calcium carbonate that mantles the sedimentary tablelands. Eolian erosion contributes with erosive pavement formation with genesis of ventifacts, soil drying, deflation in depressions and salt lakes, and clay dunes or ‘‘lunettes’’ of small dimensions. The Somun Cura Massif (see Section 2.7) is under the influence of continental and transitional arid climates, which grade eastward into temperate. The predominant landscape is composed of tablelands, although low ranges of volcanic origin break the landscape monotony. In the western part, under a continental, cold arid climate, the minimum winter temperatures generate freezing and soil organization, cryoclastic debris and cryoplanation in low slope surfaces; slumps interrupt the regularity of the lava capped tablelands. Snow precipitation provides the necessary moisture for the occurrence of these processes and the existence of seasonal channeled runoff that generates channel deepening. The maximum summer temperatures induce the contraction and drying of soils that trigger deflation and abrasion, with structural depressions being influenced by these processes. Streams are seasonal and they form endorheic and arheic basins, and their contribution to landscape modeling is through ‘‘uadi’’ and alluvial fan formation. Fluvial depositional processes are restricted to a few examples, for instance the Arroyo Verde system, which drains the southern portion of Sierra Grande and the lower valley of Rı´o Chubut, under a transitional climate. In the latter case the runoff from the basin of this allochtonous stream originates in a more humid, morphoclimatic system. The dominant littoral process is cliff erosion on igneous and sedimentary rocks, in this case with a significant recession. The abrasion platform in front of some medium to coarse sand beaches is significant. The most conspicuous geomorphological feature is Penı´nsula Valde´s, with a surface of 3600 km2, a bedrock remnant linked by an isthmus to the continent having sand and gravel spits. In the southern sector of this peninsula, there is a dune field of over 520 km2, in which active and stabilized parabolic and bargan dunes of different ages are found (Lapido and Pereyra, 1999). The northern Patagonian tablelands (see Section 2.8) largely extend under the influence of a transitional, cold

arid climate, though the eastern sector is under the influence of temperate climatic conditions that lower the daily and seasonal thermal contrast. The larger moisture supply appears in a small area toward the west, in contact with the ‘‘Berna´rdides’’ ranges, which are blocking the path of the humid air masses toward the eastern sector of the region. The dominant landscape forming processes in the inner part of the tablelands are physical weathering and soil freezing, which generates clast fracturing and sorting, due to nivation, cryoplanation and cryopedimentation. During the summer, expansion and contraction also lead to clast fracturing, cracking, and soil drying; however, the scarce precipitation, which may be occasionally very intense, start torrential flow with rill activity, and ‘‘uadi’’ and alluvial fan formation. The pedimentation and bajada formation is produced from the base of the tableland scarps toward the central depressions or ‘‘playas’’, and in some cases, all the way to the terraces of the most important fluvial systems, such as the Rı´o Chubut. The smaller streams control the bajada development by means of sediment transportation over the pediment planes. Fluvial erosion is seasonally important in the short streams that drain the tablelands, and it acts in a pulsatory way, with long inactivity intervals (Rostagno et al., 1999). The bar, island, floodplain and terrace system deposition is restricted to the more important streams such as the Rı´o Chubut and some areas of the Rı´o Senguerr. Eolian erosion is the dominant process during spring and summer, including abrasion and deflation from depressions and salt lakes. The moderating sea effect diminishes the impact of low temperatures as a weathering agent; notwithstanding, regolith is produced along the tableland slopes and debris cones at the mouth of the ‘‘uadis’’. Fluvial and eolian processes are also active in this sector, to which it should be added sand accumulation in the shape of mantles and dunes. Other landforms are those related to littoral wave action. This process generates receding cliffs on the sedimentary rocks with extensive abrasion platforms and sand and gravel beaches among the rocky outcrops. Relict glacial deposition is evidenced by sheets of glaciofluvial gravels that mantle the inner and littoral tablelands, generally Ca carbonate cemented. The igneous and metamorphic rocks, of which the Deseado Massif landscape is basically composed of (see Section 2.9), are under the influence of the transitional arid climate, in the same way as the sedimentary and pyroclastic rocks of the northern Patagonian tablelands. The landscape modeling processes in both units are essentially the same, with the difference that there is more erosion resistant, hard rock in the Deseado Massif and a larger tectonic stability over time, which has generated a landscape of a less dynamic aspect. Perhaps, the most distinctive landscape feature is the Bajo de San Julia´n, although its origin corresponds to tectonic processes. Eolian erosion contributes to its modeling as much as in the rest of the region. Another interesting feature of this unit is the temporary disappearance of the Rı´o Deseado Channel, due to the intense evaporation imposed by the transitional arid climate. This implies dominance of eolian accumulation landforms within the channel environments.

Physical Geography of Patagonia and Tierra del Fuego The similar morphoclimatic conditions between the northern Patagonian tablelands and the Deseado Massif are responsible for the Patagonian landscape homogeneity, with the difference that the harder rocks of the Deseado Massif form an irregular coast of capes, and abrasion platforms which, in spite of being defined as a low coast, reach elevations of up to 15 m. From the Somun Cura Massif to the Deseado Massif and along a coastal fringe of less than 10 km wide, different relict, littoral aggradation and erosion levels are found, corresponding to marine terraces formed during previous interglacial periods. Feruglio (1949) described at least five different levels between þ185 and þ6 m above present sea level; below this elevation, the youngest, Holocene relict marine levels are developed. The tabular landscape continues in the southern Patagonian tablelands (see Section 2.10), where the sedimentary rocks are mantled by volcanic rocks, developing different steps under the influence of a climatic diversity that goes from the transitional cold arid climate in the northern part of the unit to the oceanic cold subhumid climate of the Tierra del Fuego hills and plains, including the transitional subhumid and semiarid types of central and southern Santa Cruz Province. The arid conditions determine the occurrence of clast fracturing processes under conditions of freezing and seasonal snow accumulation, and of eolian erosion and accumulation during spring and summer. Total drying occurs in the shallow hollows whereas evaporation lowers the level of endorheic lakes such as Lago Cardiel. On the hills of the lavaflow covered, sedimentary rock tablelands, slumping takes place, giving them an irregular shape. Relict glacial processes include erosion producing troughs in which proglacial lakes are found nowadays, like Lago Argentino and Viedma. Furthermore by deposition of lateral and frontal moraines damming these lakes, while till and lava flow sequences form meseta-like landscapes and glaciofluvial deposits can be found on the tablelands or as high terraces in the fluvial valleys, depending upon their chronostratigraphic position. The littoral processes are of lacustrine and marine nature. In the first of them, there is gravel beach formation, with particles obtained from the till on the slopes. In the marine environment, the estuaries of the Chico, Santa Cruz, Coyle and Gallegos rivers are ancient fluvial channels deepened by glaciofluvial erosion, during Pleistocene glaciations in the southern Patagonian Andes. The cliff erosion on sedimentary rocks and the formation of sand and gravel beaches and littoral ridges are active processes everywhere along the coast. In the case of the southernmost cliffs, headward erosion affects the lower section of the stream channels and they then occur as hanging valleys over the beach. Relict soil freezing (not included in Table 5) has generated patterned ground, today demonstrated by ice and sand wedge casts in the higher levels of the eastern tablelands, composed of sandy-silty, gravelly or till materials. Another landscape forming process, not included in Table 5 since it is not of morphoclimatic nature, is the Pliocene–Quaternary volcanism. Volcanic processes have resulted in a positive relief of cone shapes with radial runoff and it has scoria-covered wide portions of the relict glaciofluvial plains, mitigating the effect of

35

eolian erosion while producing desert pavements upon them. The impact of subhumid climate is restricted to the southwestern sector of the unit, in an environment of morainic hills and glaciofluvial and glaciolacustrine plains that form the headwater area of the Rı´o Gallegos tributaries. The processes dominant in this case are those of channeled runoff, eolian and relict glacial erosion; with occasional soil creep and solifluction on the steep slope moraines. The marine littoral processes are predominantly erosive. The most noted littoral accumulation feature is the southeasternmost end of the South American continent, the Punta Dungeness, a cuspate spit of gravel and mud, N–S oriented, which tends to close the eastern mouth of the Magellan Straits. It formed from a glaciofluvial gravel deposit on the Atlantic side, starting between 5 and 7 14C ka BP, and from the Magellan Straits materials between 2.5 and 0.9 14C ka BP (Uribe and Zamora, 1981; Gonza´lez Bonorino, 2002). During the Holocene marine transgression along the Magellan Straits, the relict accumulation has formed a system of at least three raised beaches, composed of shelly gravels. The morphogenetic variety, including a subhumid zone, that characterizes the center and north of Tierra del Fuego where the northern plains are formed by Tertiary sedimentary rocks, is basically modeled by relict glacial erosion and accumulation. The glaciofluvial accumulation has built lateral terrace systems and wide valley bottoms; the hill and depression landscapes generated by ice disintegration are a distinctive feature of central Tierra del Fuego and different from the rest of Patagonia. The precipitation gradient from south to north controls the eolian erosive and accumulation effect, concentrating this in the northern zone, where drying of temporary lakes and ponds occurs. Fluvial erosion and deposition are present in the whole unit. The Atlantic shore presents intense cliff erosion, with the formation of arcs and windows on Tertiary sedimentary rocks, extensive gravel beach accumulation and coastal features of sand and gravel like the Pa´ramo and Popper spits, respectively. The Malvinas/Falkland Islands (see Section 2.11), with their Precambrian basement and the Paleozoic sedimentary rocks outcropping in an oceanic environment, are affected by fluvial and mass-movement processes on the higher slopes. The lower zones are formed by mineral and biogenic, alluvial and lacustrine in-fillings, whereas littoral erosion on the cliffs is dominant with respect to the genesis of gravel beaches, restricted to small embayments between capes and rocky outcrops. Eolian erosion polishes the outcropping slopes. The relict processes associated with soil frost, such as patterned soils and stone streams, indicate a predominantly tundra-type paleoclimate when the peripheral sea floor emerged during the last Pleistocene glaciation. As a synthesis, it should be noted that the constant westerlies are intercepted by the Patagonian Andes barrier, forcing eolian erosion to be the dominant process in the region, providing characteristic Patagonian landforms such as deflation hollows. On the other hand, the same morphoclimatic condition, together with the global temperature lowering, favored glaciation of the extra-Andean lowlands. The glaciofluvial accumulation reinforced the terracing characteristic of Patagonia, originally imposed over

36

Andrea M.J. Coronato et al.

structural tablelands and basaltic plateaus. Marine littoral erosion is dominant along the Atlantic coast, due to a continuous continental uplift, whereas on the western side, tectonic subsidence together with relict glacial action is responsible for the formation of the southern archipelagos. Superficial runoff is basically erosive in western mountain streams while ‘‘uadis’’ and temporary channels characterize the extra-Andean area. The dominant weathering process is cryoclastic activity due to soil freezing. Glacial modeling today is restricted to limited areas of the region, although the mountainous zone owes its present morphology fully to past glacial action.

6. Soils The lithological, morphological and climatic variations along a total distance of 20 in latitude and 12 in longitude have determined the existence of a wide variety of soil types in Patagonia. Nine of the eleven taxonomic orders included in the North American Soil Taxonomy System (Soil Survey Staff, 1997) are present in Patagonia (Scoppa, 1998; del Valle, 1998). The occurrence of largescale geomorphological processes, such as glacial erosion, volcanic ash deposition and eolian deflation, have temporarily interrupted the pedogenetic processes on several occasions during the Quaternary, even in recent times, thus explaining why the soils are poorly developed. Along the Principal Cordillera, the Northern Patagonian Andes and the northernmost extreme of the Southern Patagonian Andes, with different rock types and volcanic deposits under humid climates, andisols have developed, whereas in the center and southern parts of the southernmost Andes, mollisols and inceptisols are found as well. The areas with bare rock and glacial ice do not present soil development. In addition to those aforementioned, spodosols and histosols are present in the Fuegian Andes. In the morphoclimatic units composed of tablelands, pediments and valleys under arid to semiarid climates (AI  0.2), continental or transitional, aridisols, entisols, mollisols, alfisols and inceptisols developed, all of them strongly affected by seasonal eolian deflation. The spatial distribution of the soil types in Argentine Patagonia is shown in Fig. 8, based upon the ‘‘Atlas de Suelos de la Repu´blica Argentina’’ (Aeroterra, 1995), whereas Fig. 9 represents the surface extension of the soil orders (del Valle, 1998). Aridisols developed under semiarid and arid climate conditions, thus occupying more than 50% of the Patagonian surface, east of 71 W to the Atlantic coast, over tableland landscapes of varied lithology. The soil temperature re´gime varies from frigid to isothermal, and the moisture re´gime is aridic and torric, but the ustic or xeric varieties are locally important. The more representative suborders are •

Argids: This is the most extended subgroup and it has as a diagnostic characteristic a natric or argilic horizon, generally superimposed on a calcareous level. They occur on plains, pediments, terraces and valley bottoms of the Somun Cura Massif, the

•

•

•

northern Patagonian tablelands, the Deseado Massif and the northern portion of the southern Patagonian tablelands. Calcids: They have a petrocalcic horizon and are developed on the smooth to steep, hilly slopes, the structural tablelands and the pediments of the ‘‘Patago´nides’’, the ‘‘Neuquen Embayment’’ and the Somun Cura Massif, under transitional arid climate. Cambids: They appear as small patches in the Rı´o Negro tablelands and other morphoclimatic units such as the Somun Cura tablelands and the ‘‘Neuque´n Embayment’’, under rigorous cold-temperate, arid continental climates. They show petrocalcic, calcic or gypsic horizons, and aquic conditions, at least during 1 month per year. Gypsic: They occur over larger parts of the northern end of the ‘‘Neuquen Embayment’’, under aridtemperate, continental climate and have a gypsic or petrogypsic horizon at a depth of 100 cm under the surface. They form pseudo-hexagonal soils if this horizon is located in a superficial position.

Entisols are developed in the central sector of this region, from northern Neuquen to western Santa Cruz, in second place with respect to their surface extent. These soils are better represented in the hilly areas with strong slopes. They extend under a subarid to arid, continental and transitional climate. The soil thermal re´gime is cryic, mesic and thermic; the moisture re´gime is aquic, ustic, xeric and torric. The most representative suborders are •

•

•

•

Acquents: they are located in the margins of water filled depressions, at the foot of volcanic mesetas and floodplains. Fluvents: They occur along streams with annual floods and mineral deposition along their margins, either in floodplains or in alluvial fans, along the edges of the tablelands. Orthents: They represent poorly developed soils on erosive surfaces, either by deflation, mass-movement processes or anthropic action. Psamments: These coarse-grained, sandy soils appear along some fluvial valleys in the ‘‘Neuquen Embayment’’ and the southern Patagonian tablelands, as well as in wetlands such as ‘‘mallines’’ and ‘‘vegas’’ of the western tablelands region.

Mollisols are the darker soils of Patagonia. They developed on low to medium gradient slopes of the western mountain ranges and on plains with Quaternary glaciofluvial deposits, along the northern and southern Patagonian Andes, under subhumid climate and in a SW–NE band between the northern Patagonian tablelands and the Deseado Massif, under transitional semiarid climate. The soil thermal re´gime is cryic, frigid, mesic or thermic and the moisture re´gime is aquic, udic, ustic or xeric. Those suborders with larger surface extension are • •

Aquolls: These are restricted to transverse valleys of northern Santa Cruz. Cryolls: These are restricted to mountain and highland environments of SW Chubut, Santa Cruz and Tierra

Physical Geography of Patagonia and Tierra del Fuego

37

Fig. 8. Distribution of the soil types at the order level [based upon Aeroterra (1995) and del Valle (1998)]. Spodosols and vertisols are not represented due to scale and because their distribution is very limited and localized.

•

•

del Fuego, and developed on glaciogenic, valley bottom and slope deposits of Pleistocene or Holocene age. Ustolls: These are the soils of the tablelands and plains of Santa Cruz and Tierra del Fuego, with calcareous and clayey horizons. Xerolls: These soils are developed on parent materials of glacial, glaciofluvial, volcanic and alluvial origin, on terraces and plains, slopes of low-to-medium gradient, valleys and alluvial fans.

Andisols are located in the Principal Cordillera and the Northern Patagonian Andes, under subarid and subhumid climate conditions. They developed on parent materials of volcanic origin, on any type of landforms, not showing an altitudinal range. The thermal re´gime is cryic and the moisture re´gime may be aquic, dic, ustic or xeric. Inceptisols are found in the regional subhumid to humid climate, along the Patagonian and the Fuegian

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Andrea M.J. Coronato et al. Soils identified in Patagonia (after Del Valle, 1998) 1%

0%

0%

The poor surface extension of the Spodosols, Histosols and Vertisols in the Patagonian region does not allow their cartographic representation at the adopted scale.

0%

1%

8%

7. Vegetation

5% 13%

50%

22% Aridisols

Entisols

Mollisols

Andisols

Inceptisols

Alfisols

Spodosols

Histosols

Vertisols

No soils

Fig. 9. Soil type percentage distribution, as identified in Argentine Patagonia. Aridisols and entisols, soil types affected by eolian erosion, fluvial transport and massmovement processes are those occupying the largest extension in this region. Mollisols, more appropriate for agricultural development, occupy the third place but with a much lower percentage. The bare rock surfaces, lakes or various kinds of glaciers, on which there is no soil, are altogether more extended than the six soil orders identified.

Andes. They show cambic horizons and ocric epipedon. They are developed on smooth to strong gradient slopes, in younger sediments or under cold or unstable conditions. The thermal re´gime is cryic, mesic or isomesic; the moisture re´gime is aquic, ustic or xeric. Alfisols are distributed in an irregular and highly localized way where there is water availability in the soil during the summer months, generally at the foot of lava-capped tablelands. They also occur on unconsolidated deposits, on flat landforms or on moderate to strong gradient slopes. The thermal re´gime varies from cryic to mesic and the moisture re´gime may be aquic, ustic or xeric. Spodosols are those soils with high volcanic ash content, coming from the eruptive centers of the Patagonian Andes. They have been described in Tierra del Fuego and southwestern Santa Cruz. The thermal re´gime is cryic and the moisture re´gime is aquic or udic. Histosols are organic soils that occur in the hyperoceanic to subhumid morphoclimatic units, in western and southern Patagonia and are usually associated to other soil types. These are the soils locally developed in wetlands such as ‘‘mallines’’, near the Andean Cordillera or in peatlands of Tierra del Fuego. They have a moisture re´gime of the peraquic type. Vertisols are specifically located in some areas of the Rı´o Chubut lower valley and in the Sarmiento plains, near the Musters and Colhue-Huapi lakes, with parental materials derived from lacustrine or marine clays in the tablelands.

The Patagonian vegetation is heterogeneous and of a high floristic richness. More than 65% of all families present in the south of South America are found in this region (FAO, 2004). This large variety is a consequence of the heterogeneity in the geomorphological conditions, climate and soils. The largest differences, both in physiognomy and relative dominant species abundance, are explained mainly by the average annual precipitation, analyzed at the scale of provinces and phytogeographic districts. Thus, the more humid western regions are occupied by forest and grasslands, whereas the more arid ones, located to the east of the Andean Cordillera, have a semidesert physiognomy, dominated by small bushes, dwarf shrubs and cushion shrubs. In intermediate environments, it is common to find a grass and shrub co-dominated vegetation. In each of these regions, there are azonal biomes as well, true oasis locally known as ‘‘mallines’’ (Boelcke, 1957; Cabrera, 1976, among others). In a small-scale analysis, four phytogeographic provinces have been distinguished (Cabrera and Willink, 1973): (1) the ‘‘Monte’’ Province, (2) the ‘‘Patagonian’’ Province, (3) the ‘‘Sub-Antarctic’’ Province and (4) the ‘‘High-Andean’’ Province. The first three form the ‘‘Neotropical’’ region, whereas the last one is part of the ‘‘Antarctic’’ region. The spatial distribution of these large units is depicted in Fig. 10. In a large portion of the Patagonian territory, the natural vegetation shows some degree of alteration due to human action. The extra-Andean environment is seriously affected by desertification processes (Soriano and Movia, 1986; del Valle et al., 1998), which lead to the impoverishment of the floral diversity of the region, exposed by the gradual replacement of palatable species by others which are adapted to more arid conditions (Leo´n and Aguiar, 1985). In medium to serious desertification situations, erosive processes are triggered, leading to the loss of soil superficial horizons due to water and wind erosion.

7.1. The ‘‘Monte’’ Province The ‘‘Monte’’ Province extends from central-northern Argentina to northeastern extra-Andean Patagonia, reaching a latitude of 43200 S (the Rı´o Chubut lower valley), although under impoverishment conditions. It is developed under temperate climate conditions, with precipitation concentrated in the summer and which does not exceed 200 mm per year (Marchetti and Prudkin, 1982). The water deficit limits the vegetation development, which reaches a temperate semidesert physiognomy, composed of a shrubby steppe with high percentage of bare soil (>50%) and species adapted to drought conditions. The bushes are lower than 2 m in height and they

Physical Geography of Patagonia and Tierra del Fuego

39

Fig. 10. Vegetation units. Modified from Cabrera (1958) and Roig (1998).

show branching almost from their base or have a very short trunk. Various species of Larrea (‘‘jarillas’’ or ‘‘creosote bush’’) and some species of the genus Prosopis (‘‘mesquite’’), low and extended, dominate. There are very few permanent grasses and no trees. In addition to the poorly developed soil conditions, saline soils or dune fields are also present, in which some adapted species appear. ‘‘Monte’’ is present in Patagonia as the ‘‘Patagonian Monte’’ and the ‘‘Atlantic Shrubby’’ Districts, being the first the most representative and of the larger surface extension. It is composed of ‘‘jarillales’’ that form the semiarid, Larrea shrubby steppe (Roig, 1998), being interrupted only by halophyte vegetation patches in endorheic basins. According to their topographic,

geomorphological and edaphic differences, Larrea divaricata dominates, preferably in low areas and sandy soils, and Larrea cuneifolia in the more xeric and higher tablelands. This genus is accompanied by different species of Chuquiraga, Atriplex lampa (‘‘zampa’’) or Patagonian elements such as Retanilla patagonica or Maihuenia patagonica, among others (Movia et al., 1982). The Atlantic Shrubby District includes the vegetation of Penı´nsula Valde´s (42300 S–64100 W) and Punta Ninfas (42550 S–64200 W) in Chubut Province. These areas penetrate into the Atlantic Ocean thus being exposed to a larger marine influence, with smaller thermal amplitude and more abundant rain (230–250 mm/yr) than in the hinterland. This climatic condition explains the existence of floristic elements as members of both the

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Andrea M.J. Coronato et al.

‘‘Monte’’ and ‘‘Patagonian’’ phytogeographical provinces, combining species characteristics of the semiarid and arid steppes (Soriano, 1950; Roig, 1998). The dominant species in this district is Chuquiraga avellanadea (‘‘quilimbai’’), a cushion shrub that alternates with sandloving grasslands, located on the dune systems, where Panicum urvilleanum and Sporobolus rigens also appear.

7.2. The Patagonian Province The Patagonian phytogeographical province is distributed in most of the extra-Cordilleran environment of Rı´o Negro, Chubut and Santa Cruz. It is also found in a narrow fringe east of the Andean Cordillera of Neuque´n Province. The dominant vegetation is the medium height (20–80 cm), low density (1 plant every 6 m2), shrubbyherbaceous steppe (Leo´n et al., 1998) with a W–E gradient in the floristic composition, following the precipitation index. As the floristic composition increases, the low bushes are replaced by grasses. The shrubby vegetation presents diverse adaptations to moisture deficit and winds in the driest places, such as thorns, hairs and protective waxes. The cushion plants and the dwarf bushes, such as Azorella sp. (‘‘len˜a de piedra’’), Mulinum spinosum (‘‘neneo’’) and Nassauvia glomerulosa (‘‘colapiche’’), are frequent in this unit, which also shows a particular abundance of endemic genera, such as Ameghinoa, Duseniella, Neobaclea Crispifolia, Panthacantha and Lepidophylum (Cabrera, 1947; Soriano, 1956; Leo´n et al., 1998). In the more humid regions, grasses such as Festuca sp. and Stipa sp. (‘‘coirones’’) are dominant. In some particular landforms of this wide territory dominated by tablelands, different vegetation types appear, forming patches in the dominant steppe. Thus, usually covering small surfaces, halophytic and sandloving steppes and wet grasslands (‘‘mallines’’) occur, which are characterized by a different floristic composition and cover. The ‘‘mallines’’ are remarkable because they are the areas with larger biological productivity within the ecosystems of the Patagonian arid and semiarid fringe. They are present in places where there is some permanent water supply, such as valley floors and basaltic tableland slopes (Mazzoni and Va´zquez, 2004). In these units, a community composed of Cyperacea, Juncacea and Gramineae dominates, in which Juncus balticus appears as the most representative hydromorphic species. In these areas, intensively used for cattle and sheep rising, overgrazing has generated strong salt concentration processes and water and eolian erosion, driving them into semideserts and generating ‘‘erosion tongues’’ (Movia, 1972). Many of the vegetation types of this physiographic unit have narrow affinities with the Puna and AltoAndean provinces, present in the high landscapes of northern Chile and Argentina (Cabrera, 1958). Three districts may be differentiated: the western Patagonian District (Section 2.1), the central Patagonian District or the ‘‘Erial Patago´nico’’ (Section 2.1) and the Golfo San Jorge District (Section 2.3).

The first of them is located in western Rı´o Negro and Chubut, extending toward Neuque´n and Santa Cruz in narrower belts. The northernmost portion is identified as the ‘‘Payenia District’’ (Roig, 1998), or, alternatively, it has been considered as a single phytogeographic province due to its high number of endemic forms (Ruiz Leal, 1972). As a unit, the Western Patagonian District is composed of a 60–180 cm tall, grassy–shrubby steppe, with an approximate total cover of 50% (Leo´n et al., 1998). Most of the plant cover corresponds to grasses (‘‘coirones’’), for this reason being also known as the ‘‘coiro´n amargo grassland’’ (Soriano et al., 1976). It shows differences in floristic composition according to the various places where it occurs. The most important community in its austral portion has a mean floristic richness of 26 species (Golluscio et al., 1982), of which the most representative are Stipa speciosa (‘‘coiro´n amargo’’), Stipa humilis (‘‘coiro´n llama’’), Adesmia campestris (‘‘mamuel choique’’), Berberis heterophylla (‘‘calafate’’) and Poa lanuginosa (‘‘pasto hilo’’). In the central part of this district, some floristic elements occur that are not common in the southern parts, such as Nassauvia axillaris (‘‘un˜a de gato’’) and Stillingia patagonica (‘‘mata crespa’’). The first one of these plants is an Andean Puna species which descends to northwest Patagonia coming from the higher Andes of northern Argentina. According to Cabrera (1958), it is in this Patagonian sector where the influence of the Andean flora is more significant. Among other representative species, N. glomerulosa, Tetraglochin ameghinoi, Nardophylum parvifolium, Grindelia chiloensis (‘‘melosa’’), M. spinosum (‘‘neneo’’), Colliguaja integerrima (‘‘duraznillo’’) and Trevoa patagonica (‘‘malaspina’’) appear (Speck, 1982; Lores et al., 1983). The latter ones are very distinctive in the bushland of central western Neuquen (Movia et al., 1982). In the northern sector, a low (40–60 cm tall) shrubby steppe develops, dominated by C. integerrima (‘‘duraznillo’’) and R. patagonica in the first stratum, and by N. axillaris (‘‘un˜a de gato’’) in the second one, named by Roig (1998) ‘‘Payenia shrubby steppe’’. This formation occupies the rugged, very arid, extra-Andean landscapes, including mountain ranges, piedmont areas, volcanic tablelands and inter-mountainous plains. The Central District or ‘‘Erial Patago´nico’’ occupies the central part of Chubut and extends over almost all of Santa Cruz, until the Rı´o Coyle (latitude 51 S). The dominant vegetation type is the very xerophytic, shrubby steppe (Cabrera, 1958), composed of low cushions and small grass patches. The dominant community is the N. glomerulosa erial (Roig, 1998). On the sedimentary tablelands, Chuquiraga avellanedae (‘‘quilimbay’’), Prosopis patagonica, Junellia tridens (‘‘mata negra’’) and some herbaceous plants such as S. humilis and Poa sp. also appear. On the hilly slopes, Ameghinoa patagonica is abundant and endemic of this phytogeographical province. On the volcanic tablelands, somewhat higher, the proportion of grasses increases, with high frequency of Poa ligularis. Likewise, in the salty soils of the Atlantic coastal area, in the southernmost end of the district, Lepidophillum cupressiforme, Spartina patagonica and several species of Atriplex appear as well. South of this

Physical Geography of Patagonia and Tierra del Fuego district, in Santa Cruz Province, Juniella tridens (‘‘mata negra’’) becomes dominant, forming shrubs of 70 cm tall and 60% cover with poor herbaceous strata, extending as a continuous mantle along the high plains located from 300 to 500 m a.s.l., between the Santa Cruz and Coyle rivers, only interrupted by numerous shallow depressions (Movia et al., 1987). In highly degraded environments, as in central Santa Cruz, it appears as a colonizing species. The third district occupies the tablelands of the Golfo San Jorge (Fig. 1). The Atlantic influence allows the development of a high shrubby steppe that has C. integerrima as the dominant plant, reaching up to 3 m tall (Soriano, 1956; Soriano et al., 1983). R. patagonica and Schinus marchandii (Roig, 1998) appear as well. In the higher pampas, located above 700 m a.s.l., the herbaceous stratum reaches higher importance, forming a very homogeneous, shrubby grassy steppe, with 80% cover, dominated by xerophytic grasses forming rigid patches, such as several species of Festuca and Stipa. (Bertiller et al., 1981; Leo´n et al., 1998). Some species of the Monte phytogeographical province reach their southern boundary, such as Stipa tenuis and Prosopis denudans (‘‘Patagonian algarrobo’’). The genus Larrea is also present in this district with a dwarf, bushy species with its branches creeping to the soil: Larrea ameghinoi (Soriano, 1956).

7.3. The Sub-Antarctic Province The Sub-Antarctic Province extends along the Andean Cordillera and the Chilean Archipelago, from approximately 37 S to Cape Horn 56 S, including the minor islands of the Magellanic and Fuegian Archipelagoes. The dominant vegetation types are the evergreen and deciduous forests, grassy steppes and peatlands. This diversity has allowed the identification of four phytogeographical districts in the Patagonian and Fuegian continental territory, including an environmental complex composed of different forest types and three units of grassy steppe (Roig, 1998). The forest biome shows a great variety of elements, with the differentiation of hydrophytic, mesophytic and xerophytic forests, distributed from west to east following the moisture gradient. Among the first ones, the Valdivian and the Magellanic forests are determined by the dispersal of Nothofagus dombeyii (‘‘coihue’’) and Nothofagus betuloides (‘‘guindo’’), respectively. The Valdivian forest is located in the more humid sectors of the Cordillera, mainly in its western side. It extends from 35 to 48 S, in a narrow fringe, 150–250 km wide and 1600 km long. It covers the southern portion of the Chilean territory, including the whole VIIth to Xth Regions, and part of the XIth Region, and, in Argentina, the western portion of the provinces of Neuque´n, Rı´o Negro and Chubut. This unit is developed under temperate climate environmental conditions, with mean annual temperature varying between 7 and 15C and very high precipitation, which may even exceed 4000 mm/yr. It is an evergreen hydrophytic forest that looks like a jungle. It has three forest strata and a very dense understory of ferns, bamboos, epiphytes and lianas. The first stratum is

41

dominated by Nothofagus dombeyi, the second by Saxegothea conspicua (‘‘man˜iu´ hembra’’) and Laureliopsis philippiana and the third by Podocarpus nubigenus (‘‘man˜iu´ macho’’), Weinmannia trichosperma and Dasyphyllum diacanthoides, among others. It has a very high floristic richness, with many red and orange flowering plants. In some unfavorable sites such as very high and rugged rocky slopes, or marshes and wetlands, communities of Fitzroya cupressoides (‘‘alerce’’) appear (Roig, 1998). The Magellanic forest extends along the Fuegian Cordillera and the Magellan–Fuegian Archipelago, from 47 S until Cape Horn. In Argentina, it occupies small portions of westernmost Santa Cruz and southern Tierra del Fuego. It is also an evergreen forest, but with a poorer flora than the Valdivian forest. The most characteristic species is N. betuloides as tall as 20–30 m. At a second level, 8–12 m tall Drymis winteri (‘‘canelo’’) occurs, and at a third level, smaller trees of these species, 4–6 m tall. D. winteri and Embothrium coccineum (‘‘notro’’ or ‘‘ciruelillo’’) are found in the moist environments of the Fuegian Channel coasts (Moore, 1983). The arboreal vegetation is also accompanied by some epiphytes, ferns, mosses and lichens. The mesophytic forests are distributed along the eastern slopes of the southern Patagonian Andes, with precipitation close to 1000 mm/yr. These units are composed primarily by deciduous trees such as N. pumilio (‘‘lenga’’) and N. antarctica (‘‘n˜ire’’), of wide distribution, and Nothofagus obliqua (‘‘roble pellı´n’’) and Nothofagus alpina (‘‘raulı´’’), which are present only in certain areas of Neuquen Province. The ‘‘lenga’’ forest is distributed with a noted homogeneity along the Andean Cordillera, south of 35 S, occupying the higher forest stage which is located between 1700 and 1900 m a.s.l., in the provinces of Neuque´n, Rı´o Negro and Chubut, descending toward Tierra del Fuego down to 500 m a.s.l. and even to sea level (Donoso, 1994). The ‘‘n˜ire’’ forest presents a similar latitudinal distribution to that of ‘‘lenga’’, but it occupies a different ecological niche: Whereas ‘‘lenga’’ requires very well-drained soil, ‘‘n˜ire’’ presents a great adaptability, being present in both hydromorphic soils and steppe marginal areas, where it receives as low as 400 mm/yr precipitation (Ramı´rez et al., 1985). Frequently, it is located in a lower topographic stage, although in Tierra del Fuego it makes up the upper forest limit (ca. 600–700 m a.s.l.), adapting to thermal stress conditions and developing dwarf, chaparral-like shrubs. The xerophytic forests are located along the eastern slopes of the Patagonian Cordillera where precipitation is lower than 1000 mm/yr. All trees show xeromorphic structures such as coriaceous and thick leaves, a compact mesophyll and an epidermis covered by a thick cuticle. In some cases, they appear as shrubs. The most significant units are the Araucaria araucana (‘‘araucaria’’ or ‘‘pehue´n’’) forest, which occurs only between 36470 and 40230 S, in Neuquen Province and the adjacent Chilean regions; the Austrocedrus chilensis (‘‘cipre´s’’) forest, found between 39300 S in Neuquen and 43450 S in Chubut; the Maytenus boaria (‘‘maite´n’’) forest, which is present in reduced populations in some areas of Rı´o

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Andrea M.J. Coronato et al.

Negro and Chubut; and the Lomatia hirsuta (‘‘radal’’) shrubbery, which appears dominant in highly localized places of northern Neuquen (Cordillera del Viento, 37450 S) until the Rı´o Carrenleufu in Chubut (43300 S). The Sub-Antarctic Province includes a district of wet and xeric grassy steppes, in contact with the forest, with exceptions such as the Punta Vı´rgenes and the Malvinas/ Falkland Islands (Roig, 1998). They are mainly located in three regions: (1) south of the Rı´o Coyle, in Santa Cruz Province and northern Tierra del Fuego, (2) in a narrow belt at the eastern foot of the Andes, between 41 and 47 S and (3) in the Malvinas/Falkland Islands. The first two are composed of grassy associations of Festuca gracillima (‘‘coiro´n fueguino’’) and Festuca pallecens (‘‘coiro´n blanco’’), whose proportion may vary with the moisture gradient (Boelcke et al., 1985). The plant cover oscillates between 50 and 70%. It may present a few isolated bushes of Berberis buxifolia (‘‘calafate’’) or J. tridens (‘‘mata negra’’). It also has a cover of low Gramineae, of palatable interest, such as Poa dusenii, Bromus setifolius, Hordeum comosum and graminoids of the genus Carex (C. andina and C. argentina). The transition to wetter climates allows the development of a grassy steppe dominated by Festuca pallescens, in contact with the Sub-Antarctic forests. It is an almost pure herbaceous steppe with accompanying species such as B. setifolius and Carex sp. and some bushes of Berberis sp. The homogeneous vegetation covers the morainic hills, whereas the valley bottoms are occupied by the hygromorphic ‘‘mallines’’ community. The primary productivity of these Festuca grasses (festucaetum) puts them among the richest in extra-Andean Patagonia, thus being under very intense grazing pressure. Besides, in this forest-steppe ecotone fringe at the foot of the Andes, a tendency toward the recession in the forest positions can be observed, probably due to a very recent and constant anthropic pressure (Roig, 1998). Due to the differences with the rest of the western district, some authors consider that ‘‘festucaetum’’ is a district separated from the so-called ‘‘Sub-Andean’’ district (Ares et al., 1990). The grassy steppe present in the Malvinas/Falklands Islands is developed on poorly drained soils. It is formed by Cortaderia pilosa (tussock grass), in 30–40 cm tall patches, and accompanied by other Gramineae such as Deschampsia flexuosa, Festuca magellanica and Trisetum spicatum. In certain places with a better drainage, Empetrum rubrum (‘‘murtilla’’) forms low shrubs which may appear alone or accompanied by other shrubby plants. Due to the geographical distribution of this species, it may be assumed that it played a pioneer role in occupying those areas from which the ice was receding at the end of the Last glaciation (Roig, 1998). The peatlands are located in various sectors of the Patagonian Andes associated to the Sub-Antarctic forest, where there is a positive hydrologic balance generally in the higher mountains, terraced slopes and valley bottoms. In the extra-Andean regions, wetlands or ‘‘mallines’’ – which are not always peat producers – and peaty prairies over the peatlands are dominating (Malva´rez et al., 2004). These units are better and more abundantly exposed in Tierra del Fuego, in southern Santa Cruz and in southwestern Chile, both on the continent and on the islands. The peatlands are ecosystems in which peat is

produced and progressively accumulated as the sedentary accumulation of organic matter under anaerobic, water saturated conditions. The accumulation, when sustained in time, increases the thickness and volume of the organic deposit (Roig and Roig, 2004), reaching depths between 0.50 m in the Northern Patagonian Andes and 13 m in the Fuegian Andes. Peatlands have a different floristic composition according to the precipitation conditions and the landforms on which they are developed. Moss peatlands are dominating in environments with precipitation in between 400–2000 mm/yr and over glacigenic landforms, whereas the Gramineae peatlands occupy steppe environments, preferably the floodplains and footslope, on which runoff water is abundant (Coronato et al., 2006). The species characteristic of ombrotrophic peatlands, that is, those nourished directly by precipitation of Tierra del Fuego is Sphagnum magellanicum, an invader moss that generates spongy and soft mounds (Roig, 1998). When it grows, this moss covers older plants, which are subjected to humidification and decomposition, generating peat at an accumulation rate specific for each region, which has been estimated as 1 mm/yr (Rabassa et al., 1989) for peatlands in Tierra del Fuego and the island of Chiloe´ over the last 15,000 14C yr. The process of peat formation started in Patagonia and Tierra del Fuego in postglacial times and continues today. The oldest peatland in Argentine Tierra del Fuego, and one of the oldest in the region, is that of Estancia Harberton (54520 S, 67130 W), with a basal age of at least 14,600 14C yr BP (Heusser, 1989). The mounds of S. magellanicum provide support for different phanerogam plants which find there a permanently wet environment. This is the case of E. rubrum and Pernettya pumila. The species characteristic of the minerotrophic peatlands (i.e. those nourished by runoff or underground waters) are Carex curta and Carex gayana, accompanied by Marssiporpermun grandiflorum. These peatlands occupy the fluvial environments of the Fuegian forest and steppe. In the eastern end of Tierra del Fuego and at Isla de los Estados (Staaten Island; Fig. 1), blanket peatlands formed by Astelia pumila, a Lileaceae with rigid leaves that generate a flat, dense and hard surface, are common, with abundant ponds and marshes occupied by Tetroncium magellanicum (Roig and Collado, 2004). Some trees like the ‘‘guindo’’ and ‘‘canelo’’ may grow on them, but they develop poorly. This species forms also the larger peatlands of the Malvinas/Falklands Islands, where it is found in depressions as well as on hill slopes (Moore, 1974). In southern Santa Cruz, minerotrophic peatlands of smaller thickness are found, formed by Carex gayana var. densa, on which irregularly distributed 30–50 cm high domes or mounds are observed, formed by Azorella trifurcada and Bryum pseudotrichetrum. In other cases, they may be formed by Cyperaceae and Dicotiledoneae.

7.4. The Alto-Andean Province The Alto-Andean Province occupies the highest mountains, with elevations that get progressively lower from

Physical Geography of Patagonia and Tierra del Fuego north to south, between elevations above 1600 and 600 m a.s.l., respectively. In Santa Cruz it also includes the higher western tablelands, such as the Meseta de Lago Buenos Aires (47000 S; 71250 W) and the Meseta Latorre (51300 S; 72000 W), among others. This phytogeographical unit is located above the upper tree limit, on poorly developed, stony or sandy soils. The dominant physiognomic types are the shrubby steppe along the slopes and tundra at the summits, in which Bryophyta and lichens reach great significance. The vegetation is composed of low plants, cushion-like, adapted to extreme xeric conditions and with very short vegetative periods. In general, it presents an extremely low overall cover. Among the characteristic species, the following may be cited: E. rubrum, Nassauvia pygmaea, Viola columnaris and Azorella ameghinoi. Occasionally, in places where the soil and higher moisture allow it, high altitude grasslands, called ‘‘vegas’’, appear showing good Gramineae covering, with F. gracillima and Poa regidifolia as dominant species in the grasslands of southern Patagonia, and F. pallescens, being abundant in the northern part of the region. Other herbaceous species, characteristic of these wetlands, are found as well. Tundra is developed in those environments in which the mean annual temperature is lower than 10C and where the vegetation is affected by processes of seasonal soil freezing in the active (upper) zone. In Tierra del Fuego, these conditions are reached above 700– 800 m a.s.l., over 6–7 months; in western Santa Cruz between 1000 and 1100 m a.s.l. and in northern Patagonia between 1900 and 2100 m elevations. The floristic composition of the tundra varies according to the precipitation gradient; in very humid and humid zones, peatlands that have as dominant species Donatia fascicularis and S. magellanicum, respectively, are found as well (Roig, 1998). In the Chilean sector of the Andes on summits with maritime influence, high elevation ‘‘pa´ramos’’ are developed (Burgos, 1985), with very scattered vegetation where Phyllachne uliginosa is dominant, whereas in dry environments, steppe formed by different species of Nassauvia are developed, which extend along the higher Andean summits.

7.5. The Azonal Vegetation: The ‘‘Mallines’’ Ecosystems Distinctive from the zonal vegetation described phytogeographically, in those landforms favorable to water concentration, such as valley bottoms, endorheic basins and slopes where underground waters are outcropping, wet prairies composed of Gramineae and Cyperaceae (Pisano, 1977) are developed, in which the high coverage is the more important structural characteristic (Movia et al., 1987). On these landforms, depending on the available water, the following units may develop: (a) ‘‘junquillales’’ of Schoenophectus californicus, in marshes and swamps of shallow depth, margins of lakes and ponds, tranquil water meanders and so on with water on the surface during all year around; (b) very dense, hydrophillous prairies in places that are drowned most of the year, with J. balticus, Pratia repens, Carex sp. and Caltha

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sagittata, among others; (c) prairies with F. pallescens and J. balticus, as dominant species, with a cover close to 70% on soils with good water supply but not drowned; and (d) ‘‘coironales’’ in the areas of the wetland-steppe ecotone. The proportion between the different Gramineae species (Festuca spp., Stipa spp.) varies according to the soil characteristics and land-use conditions. The variable percentage of vegetation cover is around 50%. In the provinces of northern Patagonia, ‘‘cortaderales’’ also occur, with Cortaderia araucana and Cortaderia rudiuscula. In saline wetlands, short and strong grasses appear, dominated by the genus Distichlis. The proportion of wetlands for the different Patagonian natural environments varies according to the physiographic characteristics of each landscape and the local interaction of lithological, topographic and hydrological factors (Mazzoni and Va´zquez, 2004). Values estimated for different sectors of Patagonia provide figures around 1 and 4% (Mazzoni, 1987; Bran, 2004; among others). In the more favorable environments (landscape units formed by glacial plains, floodplains and volcanic tableland slopes), the surface occupied by ‘‘mallines’’ is smaller than 7%, considering vegetation types with good water availability and abundant vegetal cover (Mazzoni and Va´zquez, 2004).

8. Landscape Units The physiographic synthesis of the natural elements – described in the previous sections – and the human intervention on them is spatially exposed in landscape units. Historically, Patagonia had a very low population density, from 0 to 0.5 inhabitants per square kilometer in 1895 to 1.88 inhabitants per square kilometer in 1998 (Godoy Manrı´quez, 1997), mainly concentrated in coastal or piedmont urban centers. Human impact is more notorious in the extra-Andean sectors, paradoxically the least inhabited, where landscape fragility is determined by aridity and the omnipresent winds. The development of economic activities such as sheep farming, intensive agriculture in the irrigated valleys, oil and gas exploitation and hydroelectric energy generation was determinant in the modification of the natural environment. The noted physical contrasts between the Andean and extra-Andean sectors of this region determine the existence of two large groups of landscape units, each of them with multiple subdivisions, according to the scale of analysis. Given the level of detail with which the natural elements have been presented in this chapter, the following units are defined.

8.1. Arid Mountain Landscape in the Shrubby Steppe This unit is located in the northwestern end of the region, in Neuquen Province. It is composed of the mountains and tablelands of the Cordillera Principal and the Cordillera del Viento, and extinct volcanoes or with secondary activity only. It is crossed by powerful streams belonging to the Rı´o Neuquen Basin. The vegetation corresponds to

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Andrea M.J. Coronato et al. Patagonian Andes (Fig. 14) and the Fuegian Cordillera (Fig. 15). This is the emblematic landscape of Andean Patagonia. It is dominated by rugged mountain ranges, with cirque glaciers, longitudinal and transversal valleys occupied by lacustrine basins and stream network composed of the upper reaches of the allochtonous streams that discharge in the Atlantic Ocean and the Pacific slope streams. The forest occupies the slopes and valley bottoms with several varieties of hygrophyllous, mesophyllous and xeric communities, according

Fig. 11. Tromen volcano (4114 m a.s.l.) and Laguna Tromen (province of Neuquen) in the Cordillera Principal, with a community of grassy steppe and patches of bare soil. The lake hosts a bird fauna of high ecological interest. (Photo by E. Mazzoni, 2001).

Fig. 12. Landscape of volcanic relief of the Cordillera Principal, showing mass-movement processes, fluvial erosion on slopes, streams with waterfalls and riffles, colonized by the xeric forest of A. araucana and accompanied by grasslands of the shrubby steppe. On the highest slopes, the cushion grassy vegetation of the Alto-Andean Desert occurs. Caviahue, province of Neuquen (1600 m a.s.l.) (Photo by A.M.J. Coronato, 1998).

Fig. 13. General view of the northern Patagonian Andes, whose summits and valleys have been modeled by glacier action during the Quaternary. Note Lago Nahuel Huapı´, the largest lake in northern Patagonia, and its surrounding moraines. The vegetal cover is composed of dense deciduous forest, mainly consisting of several species of the genus Nothofagus (Photo. by E. Mazzoni, 2001).

the Patagonian Province, though in the summits, it extends to the High-Andean Province (Fig. 11). Southwards, in a mesa-like volcanic environment, and transitionally to the wetter mountains, the xerophytic forest of A. araucana occurs, a biome that makes this landscape unique at the global scale (Fig. 12). Human presence is scarce. Occupation is rural, with nomad extensive sheep and goat grazing. The urban centers have low hierarchy political and administrative functions, with most of them having less than 5000 inhabitants.

8.2. Humid Mountains Landscape in the Sub-Antarctic Mixed Forest This unit is located along both slopes of the northern Patagonian Andes (Fig. 13), portions of the southern

Fig. 14. Ecosystems of the Sub-Antarctic forest and ‘‘mallines’’ (wetlands) in the southern Patagonian Andes (El Chalten, province of Santa Cruz). In the lower flooding areas, wetlands with high grassy cover are used for cattle grazing. The slopes are covered by a dense Nothofagus forest, in which ‘‘lenga’’ is the dominant species (Photo by E. Mazzoni, 1998).

Physical Geography of Patagonia and Tierra del Fuego

Fig. 15. Alto-Andean Desert, Sub-Antarctic forest and raised reddish bogs in the Fuegian Cordillera. The Sierras de Alvear (1200 m a.s.l., Argentine Tierra del Fuego) shows the modeling effect of the Pleistocene glaciers and the present cirque glaciers and snowfields. The streams have waterfalls in their upper reaches, and lakes and ponds in the valley floors; the latter produced by the dam building activity of an invader species, C. canadensis (Canadian beaver). Note the light colored patch of dead forest, caused by flooding. (Photo by A.M.J. Coronato, 1998).

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Fig. 16. Oblique aerial view of the Perito Moreno Glacier in Lago Argentino (Santa Cruz Province), one of the many outlet glaciers that drain the Southern Patagonian Ice Field, worldwide known because of its peculiar, permanently advancing icefront. This photograph was obtained before the last breaking of the ice wall caused by damming one of Lago Argentino fjord-like branches. The breach took place on March 11, 2004. Note that the icefront had not yet gotten to the opposing shore on Penı´nsula Magallanes. The ice-modeled mountain summits and the snow line position can be observed as well (Photo by Juan C. Carrizo, 2003).

to the influence of the W–E precipitation gradient. Human activity is concentrated in small to medium towns, which are mostly devoted to tourism and winter sports. Marginally, there is some timbering and extensive sheep and cattle grazing in the natural pastures of the lowlands. In the Fuegian Andes, this type of landscape also includes the wide glacial valleys occupied by reddish peatlands surrounded by the Sub-Antarctic forest, elements that provide particular characteristics compared to the rest of Patagonia. The high scenic value of this landscape, its unique biodiversity and the abundance of natural resources have been the reasons to develop various protected areas.

8.3. Glacier Covered Mountain Landscape in Cold Desert This landscape is located in the southern Patagonian Andes (Fig. 16) and a reduced portion of the Fuegian Cordillera (Fig. 17). The presence of three mountain ice sheets (‘‘Hielo Patago´nico Norte’’, ‘‘Hielo Patago´nico Sur’’ and Cordillera Darwin), with their large outlet glaciers, and countless, smaller mountain glaciers and snowfields impedes the development of plant communities at high elevations. Only a few lichens and mosses colonize the rocky walls of cirques and nunataks. Along the slopes of the medium and terminal zones of the outlet glaciers, patches of the deciduous Nothofagus spp. forest of the Sub-Antarctic Province develop. The glacier snouts reach much lower elevations and cross other landscape units. In the fjord archipelago on the Pacific slope and the Beagle Channel, they reach sea level while in the Patagonian

Fig. 17. Outlet glacier of the Cordillera Darwin ice sheet, in the western Fuegian Andes. The ice tongues reach sea level and lose mass by calving in the Beagle Channel. The rugged coast forms fjords with noted erosive features. The slopes are occupied by SubAntarctic forest (Photo by C. Roig, 1996). Province they reach the large piedmont lakes. Human presence is transitory, restricted to tourism and high mountain expeditions.

8.4. Arid Ranges Landscape in the Shrubby Steppe This unit occupies the ‘‘Patago´nides’’ ranges and the northern Patagonian Massif hilly systems. The wide lithological variety of the outcrops in these ranges

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Fig. 18. Sierra de San Bernardo, the southernmost of the ‘‘Patago´nides’’, with summits lower than 1200 m a.s.l. In the Patagonian phytogeographical province, these ranges are the borderline between the central and western district. This image, taken in the latter district, shows an extreme case of desertification, where only a desert pavement remains (Photo by F. Coronato, 1996). generates a different morphological response to the channeled, concentrated runoff, mostly forced by torrential summer rainfall together with eolian erosion it makes this landscape highly variable from a morphological point of view. Homogeneity is provided by the Patagonian Province vegetation, with its varieties of shrubby steppe and Patagonian semidesert. Human occupation is mostly rural, extremely dispersed over large areas, dedicated to extensive sheep farming. The only urban concentrations are related to mining activities, which have been recently reactivated (Fig. 18).

Fig. 19. A range system between 400 and 100 m a.s.l. with N. antarctica deciduous forest, separated by wide glaciofluvial valleys with meandering streams of low discharge in the Atlantic Ocean. The grasslands of the grassy steppe are located on the valleyfloor, where water infiltration in the gravel substratum is higher than on the slopes. In the foreground, dead trees with abundant lichens are observed. Lower valley of the Rı´o Ewan (Argentine Tierra del Fuego) in the Sub-Antarctic Province (Photo by A.M.J. Coronato, 2005).

8.5. Landscape of the Wet Ranges with Deciduous Forest This unit is developed along the northern portion of the Fuegian Cordillera, where it bounds with the Southern Patagonian Tablelands. These are systems of low hills formed by sedimentary rocks, with rounded summits and convex slopes, covered by the deciduous mesophyllic forest of the Sub-Antarctic Province. Wide, terraced valleys occupied by peaty prairies and grassy steppe separate the hilly systems. The streams of the region are part of the Atlantic slope basins. The noted contrast between the forested hills and the grassy valleys defines this particular landscape, originated by differences in soil water availability, following bedrock lithology. Human settlement is rural, scarce and temporary, devoted to extensive cattle and sheep farming, without urban concentrations (Fig. 19).

8.6. Landscape of the Arid Tablelands with Grassy and Shrubby Steppe This is the emblematic landscape of the region and it occupies the largest portion of extra-Andean Patagonia. It covers the Neuquen Basin (Fig. 20), the Somun Cura Massif, the northern Patagonian tablelands (Fig. 21), the

Fig. 20. Tableland and piedmont landscape in the Neuquen Basin. The tablelands are formed by continental red clays, gray tuffs and lava flows. Under a continental semiarid climate, the shrubby steppe, with patches of deflated bare rock, is covered by sand and pebbly clasts, most of them ventifacts. Meseta de la Barda Negra, province of Neuquen, that borders the Huincul depression, the northern geological boundary of Patagonia (see the first section in this chapter) (Photo by A.M.J. Coronato, 2002). Deseado Massif and the Southern Patagonian Tablelands (Figs 22 and 23). With variations imposed by lithological and geomorphological characteristics, the stepped tableland landscape, with erosion scarps, rotational slumps, pediments and deflation hollows, presents a noted homogeneity along 15 in latitude and 6 in longitude. The monotonous tableland landscape, with far horizons and distant ranges is due to the uniform development of the

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Fig. 21. Golfo San Jorge shrubby steppe, in the subhumid littoral of the northern Patagonian tablelands. The Atlantic influence allows the development of a steppe of greater extent than is possible in the hinterland. In the central plain, the effect of human activity on the environment can be observed: an extensive eolian plume extending from a deflated dirt road made by the oil industry (Photo by E. Mazzoni, 2004).

Fig. 23. Aerial view of the Southern Patagonian Tablelands, close to the Atlantic coast. A high density of deflation hollows can be observed on the tableland. These hollows are occupied by fall and winter precipitation, and the shallower ones get thoroughly frozen in winter. Note also the ‘‘uadi’’ systems, locally called ‘‘can˜adones’’, with receding erosion toward the depression areas. Littoral erosive activity develops a cliff coast, in which the ‘‘uadis’’ appear as hanging valleys above the present beach (Photo by A.M.J. Coronato, 2002).

Fig. 22. Patagonian Erial, in the Southern Patagonian Tablelands, province of Santa Cruz, seriously affected by desertification processes. On the slopes, the vegetation is highly degraded, with a very sparse cover. The ‘‘mata negra’’ (J. tridens) shrubs seem to be colonizing the areas lacking vegetation. In the lower area, a small ‘‘mallı´n’’ (wetland) is located (Photo by E. Mazzoni, 2004).

to medium in size, dedicated to supplying the ranches and the oil industry field camps. Some towns of the hinterland of the tablelands of Rı´o Negro, Chubut and Santa Cruz are remnants of ancient, regional communication lines which served wool production during the first decades of the twentieth century.

8.7. Landscape of Semiarid Hills with Grassy Steppe Patagonian semidesert, here and there interrupted by ‘‘mallines’’, ephemeral lakes, salt lakes surrounded by herbaceous and/or halophillous vegetation, and a few permanent lakes such as Cardiel, Musters and Colhue´ Huapi. In the latter two, marsh formation due to flooding generates a green oasis which has allowed the development of cattle farming of good quality as well as small urban centers. The larger region is occupied by ranches devoted to extensive sheep farming, but since 1970 most of them converted to rural tourism. Some sectors of the Neuquen Basin and the Northern and Southern Patagonian Tablelands suffer oil and gas exploration and exploitation activities, which generates a vast network of dirt roads and fields without vegetation cover in which eolian erosion is dominant. The urban concentrations are small

This unit occurs at the foot of the southern Patagonian Andes (Fig. 24), along the Magellan Straits and the northern zone of Tierra del Fuego. The landscape consists of various morainic hills, glaciofluvial and glaciolacustrine plains and terraces, sometimes associated with low ranges. It is spatially dominated by grassy steppe, although along the creeks, humid grasslands and peaty prairies are found, used for extensive cattle and sheep farming. Human settlement is concentrated in small urban sites, dedicated to rural supplies and, to a lower extent, to tourism, with the exception of the administrative capital of the Chilean XIIth Region, Punta Arenas. This city leads the economy, administration and frontier control of southern Chile.

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Fig. 24. A system of low ranges, covered by glaciofluvial deposits of Middle Pleistocene age, corresponding to the glaciation of the Magellan Straits, in the northern part of Chilean Tierra del Fuego. The plains and hills are occupied by shrubby steppe vegetation of cold, subarid climate, whereas the floodplains are covered by grasslands and ‘‘mallines’’ that form around the springs and groundwater outcrops at the foot of gravel terraces. The low discharge streams drain into the Magellan Straits. Sheep farming generates ‘‘terracettes’’ on slopes and desertification of the plains (Photo by A.M.J. Coronato, 2002).

Fig. 25. Eastern sector of Lago Viedma, Santa Cruz Province. The extensive lakes of glacial origin of southern Patagonia to the east reach the semiarid steppe environment. Starting in the lacustrine basins, longitudinal dunes extend, and their advance has been controlled by planting sand-loving grasses.

The morphology of Pliocene and Quaternary volcanic cones and scoria flows at Pali-Aike, in southern Santa Cruz, generates a particular hilly landscape, with positive relief units spread like rocky islands over a huge plain.

8.8. Landscape of Glacial Lakes with Arid Coasts It is developed between 44 and 50 S, at the foot of the southern Patagonian Andes. The strong precipitation gradient ascertains that the mesophyllous forest of the SubAntarctic Province reaches the coast at the heads of glacial lakes, but their distal coasts are surrounded by the grassy steppe and the arid shrubby steppe of the Patagonian Province. The lacustrine basins are surrounded by volcanic tablelands and lateral and frontal moraines. Human presence is basically dispersed, in rural settlements traditionally dedicated to extensive sheep farming and some of them, later reconverted to tourism. The urban concentrations are small to medium in size, and originated as supply centers for agricultural services but presently, the most prosperous are devoted to tourism and the service of high mountain expeditions to the Patagonian ice fields (Fig. 25).

8.9. Landscape of Allochtonous Stream Valleys This landscape unit is spatially concentrated. It is the one that presents larger evidence of physical transformation by human action. Along the lower valleys of the Neuquen, Limay (Fig. 26) and Chubut rivers and the entire Rı´o Negro valley, the landscape of the

Fig. 26. Lower valley of the Rı´o Limay during a winter flood, caused by precipitation in its higher and middle basins. Although the flood is regulated by a system of upstream dams, the intense human occupation of the floodplains and islands for intensive agriculture does not allow the free evacuation of the excess discharge, thus causing plantation and infrastructure damage. Note the cultivation areas separated by windbreaks, a typical form of rural space organization in irrigated oasis in Argentina (Photo by A.M.J. Coronato, 2001).

Physical Geography of Patagonia and Tierra del Fuego shrubby steppe in contact with the riparian forest vegetation has been transformed into an almost continuous irrigated oasis, with intensive rural settlement dedicated to fruit and vegetable production. The pediments, fluvial terraces and floodplains are occupied by intensive crops fed by a complex system of irrigation channels, connected to the main streams. Although the rural habitat is very dense at the regional scale, the human settlement is basically agglomerated, with cities devoted to agricultural and oil industry services, and political-administrative centers. It should be noted that urban settlements are separated by rural patches along 150 km in the lower basins of the Limay and Neuquen rivers and the upper basin of the Rı´o Negro. The landscape of the Rı´o Santa Cruz is similar to the upper basins of the Neuquen and Chubut rivers, because of its very little environmental transformation, although the relief on which the latter have carved their valleys is different from the Santa Cruz valley, due to the absence of moraines. The Limay, Neuquen and, to a lesser extent, the Chubut rivers, have suffered the transformation of their channels due to the construction of dams and artificial lakes or the reduction of their discharge. Both modifications are related to the need for hydroelectric power generation in Argentina for regional and national use, and the extraordinary flood control, to mitigate damage to the fruit and vegetable production. The presence of extensive artificial lakes in the arid and semiarid shrubby steppe has produced the break of the desert landscape monotony and has created new habitats for migratory birds.

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Fig. 28. Upper reach of the Ayse´n Fjord, between the city of such name and Puerto Chacabuco (Chile). Although scarce, human occupation is denser than in the distal zone of the fjords that along the Pacific coast, more hostile in climate and topography. In this image, some aquiculture development is observed, an economic activity that profits from calm, clean and nutrient rich waters (Photo by F. Coronato).

8.10. Landscape of Hyper-Humid Archipelago and Fjords, with Evergreen Forest and Tundra This unit corresponds to the intricate complex of rugged, rocky islands separated by deep fjords that compose the Magellanic–Fuegian Archipelago (Fig. 27), the Chilean Fig. 29. The Beagle Channel joins the Pacific and Atlantic oceans and extends between the mountain coast of Isla Grande de Tierra del Fuego (to the N) and Hoste and Navarino islands (to the S) over more than 200 km. The rocky slopes show the glacial erosive action on which poor, shallow soils bearing the Nothofagus spp. forest develop (Photo by A.M.J. Coronato, 2002).

Fig. 27. Isla Carlos III (Chile). Deep fjords with many islands and rugged coasts dominate the insular landscape of western Tierra del Fuego, where the relict subglacial erosion is the main landscape modeling agent. The evergreen hygrophillous forest reaches sea level, and abundant precipitation and poorly drained soils favor the Gramineae peatland and high altitude grassland formation (Photo by C. Roig, 1996).

slope of the southern Patagonian Andes (Fig. 28) and the Fuegian Cordillera (Fig. 29), including the westernmost sector of the Magellan Straits and Isla de los Estados (Staaten Island). The glaciers coming down from the Patagonian and Fuegian ice sheets feed the head of the fjords. The evergreen, hydrophyllous forests, the peatlands and the tundra correspond to the western, hyperhumid, cold climate. Perhaps due to the lack of other present land-use possibilities, several national parks and natural reserves have been established in large portions of these archipelagos. The whole of Isla de los Estados is a single provincial natural reserve of the Argentine province of Tierra del Fuego. The region has a few small

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urban settlements on the continent’s coast, especially in the northern sector, from which the rest of southern Chile may be reached. Southwards, the islands are inhabited only by military and a very small civilian population.

8.11. Atlantic Coastal Landscape Along the entire region and in marked contrast with the western coast, the Atlantic coast is developed as tall cliffs (Fig. 30), formed mostly by stratified sedimentary rocks, with a few exceptions in the coastal portions of the Somun Cura and Deseado Massifs. The erosive action has formed rocky windows, caves and arcs in some areas and it has truncated the outlet of the smaller streams, which appear as hanging valleys over the beach. Dunes are developed along the more extensive coasts, in the larger embayments and gulfs (Fig. 31). The larger rivers, on the contrary, discharge in estuaries which, because they were used as harbors, were the occupation nuclei of the region. Abrasion platforms, mixed or gravel beaches are developed at the base of the cliffs. The littoral vegetation is very poor

Fig. 31. Penı´nsula Valde´s (3600 km2) presents a singular superposition of geomorphological features not frequent in Patagonia: the large spit formed by littoral ridges along the eastern coast, the two large, absolute (their bottom is below sea level) depressions in its central part and the large active dune fields in the southern area (seen in this image). These dunes, which are advancing at a rate of 25 m/yr, are formed with materials coming from the cliffs of the southwestern peninsula coast, which faces the dominant winds. (Photo by P. Blanco, 1990). and consists of shrubs on the supralittoral dune fields or along the bottom of the ‘‘uadis’’. The most important floristic richness is concentrated in the estuaries and abrasion platforms. A landscape variety develops along the coasts of Tierra del Fuego where the deciduous forest of N. antarctica grows on the hills that form the cliffy coasts (Fig. 32). Most of the Patagonian population is concentrated along the Atlantic coast, where most of the urban centers are located, with political-administration functions and oil

Fig. 30. The rectilinear, sedimentary rock cliffs, characteristic of the Patagonian Atlantic coast are interrupted between 44 and 45 S, changing into a hard rock littoral topography, as in Cabo Dos Bahı´as, visible in this picture. The outcrops of Jurassic volcanic rocks determine the existence of a very rugged coast with small rocky islands, a very rare feature along the Atlantic coast. Although the climate is warmer and drier and the rock geology is not the same as in the Malvinas-Falkland islands, both landscapes have general similarities (Photo by F. Coronato, 1998).

Fig. 32. Receding cliffs on the Atlantic coast of Tierra del Fuego, occupied by grassland steppe deteriorated by sheep farming. On the hills, the N. antarctica deciduous forest develops. The beach is formed by medium sand and gravel taken from glaciofluvial valleys like the one observed in the image. Gravel is transported by littoral drift and deposited in cuspate prisms (Photo by A. Schiavini, 1998).

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industry, tourism, fisheries and harbor services. Rural space is occupied everywhere and dedicated to extensive sheep and cattle farming, although the effective number of inhabitants is extremely small.

southernmost Patagonia, together with the glacierized mountains and the frozen desert, are the least known landscape units and a reservoir of pristine environments still surviving in Patagonia.

8.12. Steppe Landscape of the Atlantic Islands

9. Final Comments

The Malvinas/Falklands Islands show a relatively flat landscape, with rounded mountains and slopes and plains covered by grassy steppe and hollows occupied by grassy peatlands and marshes. The constant presence of maritime winds inhibits forest growth. The human occupation is rural, dedicated to extensive sheep farming, with one urban settlement dedicated to goods and services supply and located on the eastern island. In Chubut Province, between 44 S and the northern portion of the Golfo San Jorge, a similar landscape can be found, created by the contact of volcanic rocks of the Late Paleozoic–Mesozoic Porphyritic Complex and the ocean. In this area, the coast is very rugged, with small islands, which is in sharp contrast with the rectilinear cliffs of the Patagonian coast.

From the previous sections it may be concluded that the interaction of the wetter Pacific winds and the Andean Cordillera is the basic natural condition that determines the geographical aspects of the natural, physical environment of the southernmost end of South America. The pluvial and snow re´gime of the streams, the allochtonous character of the tableland streams, the development of contrasting biomes such as jungle-like forests and rainshadow deserts, with a modest thermal re´gime are a consequence of the orographic barrier interaction with the air masses of the Southern Pacific anticyclonic centers and the polar front. The action of the relief modeling agents today and during the Quaternary has been active over a lithological substratum which is the result of preorogenic and postorogenic marine embayments, orogenic uplifting, intense fracturing, subduction, volcanism and continental block downwarping. Among the past modeling agents, the Pliocene– Quaternary glaciations are the most important, together with their periglacial environments, affecting also the neighboring non-glaciated regions. Likewise, systematic sea level rise events during interglacial periods have intervened in the modeling of the coasts and the mouths of the big rivers. Concerning the present modeling agents, wind is dominant in most of the region, creating a singular regional morphology with abundant deflation hollows and eolian plumes, water runoff in the humid areas, and seasonal soil frost of the higher areas and sea waves that generate active cliff recession. Patagonian and Fuegian biomes present peculiar elements, both for their density and for the occurrence of unique communities and species. The occupation of the Patagonian space by Europeans and their American offspring displaced the native, american indian population since the end of the nineteenth century, replacing the nomadic, hunter-gatherer cultures by modern economic activities such as extensive cattle and sheep farming, oil and gas industry, and intensive agriculture. These changes have generated harsh social and cultural conflict together with serious environmental stress, of which desertification is the most evident. Patagonia is not only the remote, located in the southernmost end of the American continent, land of myth and legend, but it is a portion of the Southern Hemisphere with a great scenic richness, a reservoir of important natural resources and fresh water, and lightly inhabited, pristine lands. The physical environment of Patagonia offers Quaternary scientists a unique opportunity to investigate and understand the main questions of the paleoenvironmental evolution of the Southern Hemisphere and the entire planet during the Late Cenozoic, and particularly, during the last 2 Myr.

8.13. Landscape of Low Relief Covered by Blanket Peatlands This landscape unit is located exclusively in the eastern end of Tierra del Fuego. It is a vast extension of ombrotrophic reddish peatlands with abundant ponds, surrounded by low coastal hills and rocky cliffs. The streams are organized in intricate circuits across ponds and lakes, until they reach the Atlantic Ocean. Mineral soils are absent, the hygrophyllous and mesophyllous forest is found in the transitional environments between hills and peatlands. There is no population at all, with the exception of a few military outposts along the Atlantic coast (Fig. 33). This is another singular landscape unit for

Fig. 33. Reddish moss peatlands, with abundant ponds, creeks and lakes in Penı´nsula Mitre, Argentine Tierra del Fuego. The hills form coastal cliffs and are occupied by the Nothofagus sp. hygrophillous forest. This landscape is repeated in all directions along the eastern portion of Isla Grande de Tierra del Fuego.

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Skvarca, P. (2002). Importancia de los glaciares del Hielo Patago´nico Sur para el desarrollo regional. In: Haller, M.J. (ed.), Geologı´a y Recursos Naturales de Santa Cruz. Relatorio del XV Congreso Geolo´gico Argentino 5, 1. El Calafate & Buenos Aires, 785–798. Soil Survey Staff (1997). National Soils Handbook. USDA-Natural Resources Conservations Service. Washington, D.C. Soriano, A. (1950). La vegetacio´n del Chubut. Revista Argentina de Agronomı´a 17, 30–66. Buenos Aires. Soriano, A. (1956). Los distritos florı´sticos de la Provincia Patago´nica. Investigaciones Agropecuarias 10, 323–347. Buenos Aires. Soriano, A. and Movia, C. (1986). Erosio´n y desertizacio´n en la Patagonia. Interciencia 1, 77–83. Soriano, A., Movia, C. and Leo´n, R. (1983). Deserts and semideserts of Patagonia (Vegetation). In: Goodall, D. (ed.), Ecosystems of the world 17. Elsevier, Amsterdam, 440–454. Soriano, A., Trabuco, R. and Deregibus, V. (1976). Ecologı´a de un pastizal de coiro´n amargo en el SW del Chubut. Anales Academia Nacional de Agronomı´a y Veterinaria 30, 5–13. Buenos Aires. Speck, N. (1982). Vegetacio´n y pasturas de la zona Ingeniero Jacobacci-Maquinchao. In: INTA (ed.), Sistemas Fisiogra´ficos de la Zona Ingeniero Jacobacci-Maquinchao (Prov. Rı´o Negro). INTA, Buenos Aires, 157–208. Stine, S. and Stine, M. (1990). A record from Lake Cardiel of climatic change in southern South America. Nature 345, 705–708. Taljaard, J. (1969). Air masses of the Southern Hemisphere. Notos 18, 79–104. Tuhkanen, S. (1992). The climate of Tierra del Fuego from a vegetation geographical point of view and its ecoclimatic counterparts elsewhere. Acta Botanica Fennica 145, 1–64. Turner, J.C.M. (1980). Islas Malvinas. In: Turner, J.C.M. (ed.), Geologı´a Regional Argentina, Academia Nacional de Ciencias de Co´rdoba 2. Co´rdoba, Argentina, 1503–1527. UNESCO (1977). Carte de la re´partition mondiale des re´gions arides. Notes techniques du MAB, 7. Paris. Uribe, P. and Zamora, E. (1981). Origen y geomorfologı´a de la Punta Dungeness, Patagonia. Anales del Instituto de la Patagonia 12, 143–158. Punta Arenas, Chile. Walter, H. and Box, E. (1983). Climate of Patagonia. In: West, N. (ed.), Deserts and semideserts of Patagonia. Ecosystems of the World Elsevier, Amsterdam, 5, 440–454. Warren, C. and Sugden, D. (1993). The Patagonian Icefields: a glaciological review. Arctic & Alpine Research 25, 316–331. Weischet, W. (1985). Climatic constraints for the development of the Far South of Latin America. GeoJournal 11, 1, 79–87. Yrigoyen, M. (1980). Cordillera Principal. In: Turner, J.C.M. (ed.), Geologı´a Regional Argentina. Academia Nacional de Ciencias de Co´rdoba 1. Co´rdoba, Argentina, 651–694. Zamora, E. and Santana, A. (1979). Caracterı´sticas clima´ticas de la costa occidental de la Patagonia entre las latitudes 46400 y 56300 . Anales del Instituto de la Patagonia 10, 109–144. Punta Arenas, Chile.

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4 Tectonic Evolution of the Patagonian Andes Vı´ctor A. Ramos and Matı´as C. Ghiglione Laboratorio de Tecto´nica Andina, Facultad de Ciencias Exactas y Naturales (FCEN), Universidad de Buenos Aires, Pabello´n 2, Ciudad Universitaria, Buenos Aires 1428, Argentina, and CONICET

3. The Patagonian Cordillera has an anomalous isostatic rebound, near to 20 mm/yr along the axis of the cordillera, which has been associated with the last glacial maximum (LGM) deglaciation. This anomalous behavior is related to a hot upper mantle, generated as a consequence of several episodes of ridge subduction, which favors the important isostatic rebound. Because of that, this segment of the Andes records the first glaciations during the Late Cenozoic.

1. Introduction Throughout the Cenozoic, the southern Andes have had a distinctive feature when compared to the central Andes. Since the early definition of Windhausen (1931) and Feruglio (1949–1950), the presence of a continuous batholith belt that starts at nearly 39 S and ends in Cape Horn Islands in the southernmost tip of the Andes at about 52 S was evident (Fig. 1). These granitic rocks of Andean age are not exposed in a continuous belt further north in the Principal Cordillera. Dessanti (1972) used that feature to define the northern limit of the Patagonian Cordillera as a different geological province – the criterion used in later regional studies by Leanza (1958, 1972), Sua´rez (1976), Haller and Lapido (1980), Ramos et al. (1982) and Ramos (1999). However, the presence of a batholith in the backbone of the cordillera was taken as a natural fact, without questioning why it was there, what controls its continuity and why it is not a common feature in other segments of the Andes. The objective of this chapter is to focus on the importance of the different geological and tectonic processes that uplifted the Andes at these latitudes, and how the climate has had an important role not only in carving the landscape, but also in controlling the uplift mechanisms through time. This interaction between climate and tectonics can be addressed in three distinct topics:

Prior to the discussion of these topics a brief summary of the geologic and tectonic framework of the Andes at these latitudes will be presented. This description does not aim to be a comprehensive review of the geology of this region, but it will focus on its main characteristics in order to show the effective control of climate in the tectonic history of this segment of the Andes.

2. Geologic Framework The Patagonian Cordillera can be subdivided into three distinct segments that have different geological histories, which reflect the diverse tectonic evolution of these sectors of the southern Andes. The northern segment extends from 39000 to 43300 S, the central segment from 43300 to 46300 S and the southern segment south of this latitude. All of these segments have in common a continuous batholith belt along the western slope of the Andes, but each one of them possesses a particular geologic setting.

1. The present structural volume of the Patagonian Cordillera is relatively small when compared with the central segments of the Andes. This difference has been attributed to the sediment fill of the trench, which lubricates the friction in the subduction channel and therefore produced less coupling between the continental upper plate and the subducted slab. This sediment supply is related to the dominant wet winds from the southwest, which produce the rain shadow and asymmetric erosion in the Patagonian Cordillera. 2. The continuous batholith belt is also a consequence of uplift and climatic interaction. As soon as the uplift of the Patagonian Cordillera initiated in Miocene times, most of the western slope of this part of the cordillera was subjected to an extreme erosional gradient as a consequence of rain shadow. As a result, the magmatic arc was deeply eroded and the batholith was denudated.

2.1. Northern Patagonian Andes This northern segment is characterized by a mid- to high-grade metamorphic basement associated with Late Paleozoic granitoids. These rocks are overlain by marine sedimentary and volcanic sequences of Jurassic age in isolated outcrops, which in turn are covered by thick sequences of volcanic, volcaniclastic and marine rocks of Paleogene age. All of these rocks are intruded by the Patagonian Batholith that at these latitudes is predominantly Cretaceous in age. The metamorphic basement, characterized by gneisses and amphibolites, has been considered for many years as Precambrian in age, due to the higher grade of metamorphism when compared with the basement of the southern sector of the Patagonian Cordillera.  2008 ELSEVIER B.V. ALL RIGHTS RESERVED

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Fig. 1. The extension of the Patagonian Batholith and the different segments of the Patagonian Cordillera, with indication of the approximate present water divide (see discussion in the text). However, since the early work of Gonza´lez Bonorino (1944), a Paleozoic age was suspected based on the correlation with the basement on the western slope. Recent geochronologic studies have demonstrated a Late Carboniferous–Early Permian age for these rocks, based on U/Pb ages in zircons (Basei et al., 2002). This basement has a fabric with a northwestern trend, which continues in the adjacent Somuncura Massif. There, the igneous and metamorphic basement is composed of gneisses, biotite schists, marbles, amphibolites, foliated granitoids and late tectonic granitoids (Chernicoff and Caminos, 1996). Crystallization ages of these rocks vary between 304 and 281 Ma (Basei et al., 2002), which indicate a maximum age for the deformation of 280 Ma, within the Lower Permian. These rocks were part of an extensive Late Paleozoic batholith that crosses northern Patagonia with a northwestern trend into the Patagonian Andes. The orthogneisses and foliated granitoids are intruded by nondeformed granites with an age of 244 – 9 Ma (Basei et al., 2002). These rocks have similar metamorphic and igneous ages like those in the area surrounding San Carlos de Bariloche, where zircons of Early Permian age have been found in the basement rocks (Basei et al., 1999). The basement has been interpreted as a

deformed magmatic arc, exposed at middle to lower crustal levels through collision processes (Ramos, 1984, 2004a). The Jurassic sequences exposed in the Cerro Piltriquitro´n were described by Petersen and Gonza´lez Bonorino (1947) and their poor marine fauna studied by Mancen˜ido and Damborenea (1984). The black shales and sandstones are interbedded with volcanic rocks of Early Jurassic age, representing remnants of the Mesozoic magmatic arc and associated intraarc basin. Most of the foothills are formed by thick sequences of Paleogene andesitic volcanics and associated volcaniclastic rocks, with some intercalated marine deposits (Gonza´lez Bonorino, 1973), typical of a magmatic arc setting (Dalla Salda et al., 1981; Rapela et al., 1988). The marine deposits of Late Oligocene–Early Miocene age in both slopes of the cordillera indicate that Pacific transgressions were able to go across the Andes, previous to the present uplift (Ramos, 1982). The plutonic rocks that constitute the batholith at these latitudes present three major different pulses: A Paleozoic pulse detected by Lizuaı´n (1981) in Lago Puelo is Late Devonian in age (367–380 Ma, K/Ar),

Tectonic Evolution of the Patagonian Andes recently restricted to the Carboniferous (335–320 Ma, U/Pb in zircons; Pankhurst et al., 2005); a Jurassic– Cretaceous pulse (170–90 Ma, K/Ar ages; Lizuaı´n, 1981; Gonza´lez Dı´az and Lizuaı´n, 1984); and a Tertiary pulse represented by Paleogene (55–37 Ma, K/Ar) and Middle Miocene (13–15 Ma, K/Ar) granitoids reported by Gonza´lez Dı´az and Lizuaı´n (1984). A series of stratovolcanoes of Late Cenozoic age are spread mainly along the western slope of the Andes.

2.2. Central Patagonian Andes The main characteristic of this segment, which runs from 43300 to 46300 S, is the absence of Paleogene volcanic and plutonic rocks on both slopes of the Patagonian Cordillera. The northernmost outcrops are at the latitude of Corcovado (43300 S). The basement is poorly exposed along the Esquel region and is formed by heavily deformed sedimentary rocks of Late Paleozoic age (Lo´pez Gamundi, 1980). Most of the cordillera is characterized by thick sequences of Middle–Late Jurassic to Early Cretaceous volcanic and volcaniclastic rocks of andesitic composition (Ramos, 1979; Haller and Lapido, 1980; Sua´rez and de la Cruz, 1997). These rocks are interfingering marine successions along intraarc and retroarc basins, bearing Tithonian to Neocomian ammonites of the Pacific realm (Ramos and Palma, 1983; Aguirre-Urreta, 2002). These deposits are well preserved along the Rı´o Mayo embayment (Aguirre-Urreta and Ramos, 1981; Mpodozis and Ramos, 1989; Sua´rez et al., 1999). The Tertiary deposits are represented by continental sediments, which indicate that the Andes were, at that time, an effective topographic barrier for the Pacific marine transgressions that were restricted to the western slope of the Andes. This segment of the Patagonian Batholith is characterized by a complex set of pulses that started along the western Chilean side with Early Cretaceous ages (140–124 Ma, Rb/Sr ages), which migrated during the Middle Cretaceous (117–98 Ma) to the eastern side (Pankhurst et al., 1999). They are represented in the Argentine side by a series of minor stocks east of the batholith, ranging in age from 110 to 87 Ma (K/Ar and U-SHRIMP ages), which were interpreted as eastern apophyses of the main batholith (Ramos, 1981; Ramos et al., 1982; Rolando et al., 2002). The magmatic activity is restricted to the central part of the batholith during the Paleogene in the Chonos Archipelago, while during Neogene times it is associated with the Liquin˜e–Ofqui fault zone (Herve´ et al., 1996). These data can be interpreted as a shallowing of the paleo-Benioff zone during Middle Cretaceous times, associated with a main phase of deformation in the Late Cretaceous (Ramos and Alema´n, 2000; Sua´rez et al., 2000). Tertiary synorogenic deposits are restricted to some local depocenters, which filled the low relief during Middle Miocene times. Volcanic activity is developed in the arc along the Chilean side where a series of stratovolcanoes are aligned with the Liquin˜e–Ofqui fault zone.

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2.3. Southern Patagonian Andes The Patagonian Cordillera, south of the triple junction among the Nazca, Antarctic and South American plates (south of 46300 S), comprises a very distinct segment of the Andes (Fig. 1). The Paleozoic basement is again exposed along the foothills of the Cordillera, but it is mainly composed of metasediments of Devonian to Carboniferous age (Herve´ et al., 2004), deposited in an accretionary setting along the continental margin (Mpodozis and Ramos, 1989; Herve´ et al., 2000). The magmatic rocks are preserved along the batholith, and with the exception of the Middle Jurassic El Quemado Complex, volcanic rocks are absent in this segment. These volcanics are coeval with the extraAndean silicic magmatism of the Deseado Massif, formed during a period of generalized extension between 177 and 168 Ma (Uliana et al., 1985; Alric et al., 1996; Feraud et al., 1999). This early extension is related to the opening of the Weddell Sea further south (Ghidella et al., 2002; Ramos, 2004b). The Cretaceous sedimentary deposits are well preserved in the Austral (or Magallanes) basin, and further north in the Rı´o Mayo embayment (Ramos and Aguirre-Urreta, 1994). A thick sequence of marine clastic sediments interbedded with scarce, thin limestones documents the first marine transgression from the Pacific side during Tithonian times. The regression was diachronically produced from north to south, starting in the Lago Belgrano region (47 S) during Barremian times, and prograding to Lago San Martı´n (49 S) in the Aptian, and ending in the Campanian–Maastrichtian in Lago Argentino (51 S). By the end of the Cretaceous, the first Atlantic transgression flooded the Patagonian Cordillera (AguirreUrreta, 2002; Arbe, 2002). The deposits were accumulated in a retroarc setting, and the first evidence of deformation has been dated on detrital zircons as Turonian (Fildani et al., 2003, 2005), when the earliest provenance from the axial area indicates the start of uplift of the cordillera at these latitudes (Macellari, 1988). The granitoids of the batholith, which are mainly exposed along the Chilean slope at these latitudes, have the oldest record (157 Ma) along the Magellan Strait, and comprise bimodal leucogranites and gabbros of earliest Cretaceous age further north (145–137 Ma), late Early Cretaceous granitoids along the western margin (136–111 Ma) and Late Cretaceous plutons at about 52450 S between 99 and 78 Ma. The Cenozoic plutons are confined to the axial region and indicate two pulses: a Paleogene pulse between 65 and 40 Ma and a Neogene pulse between 22 and 16 Ma (all these ages U-SHRIMP in zircons; Herve´ et al., 2004). Further to the east, isolated plutons of reduced size are slightly younger, like the San Valentı´n, San Lorenzo, Fitz Roy and Paine stocks of Middle Miocene age (Ramos, 2002). The structural study of some of these stocks indicates that the emplacement was coeval with deformation and was interpreted as syntectonic (Skarmeta and Castelli, 1997). Synorogenic deposits of Paleogene and Miocene ages are outstanding in this segment, and indicate important generation of relief that will be discussed below.

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South of the triple junction, there is a volcanic gap, but south of Lautaro volcano (49 S), the activity resumes in Pliocene–Quaternary times, and is represented by few and isolated volcanoes of the Austral volcanic zone of characteristic adakitic composition (Stern, 2004), which extends further south to Mount Cook volcano along the western side of the Beagle Channel (Fig. 3).

3. Tectonic Setting The distinct geologic characteristics of the different segments are heavily controlled by tectonic processes. The boundary between the northern and central segments is controlled by the collision and subduction along the trench during the Paleogene of a seismic oceanic ridge, the Aluk–Farallon spreading center (Cande and Leslie, 1986). The Eocene volcanic gap of arc-related rocks that starts south of Corcovado (43300 S, Fig. 2) coincides with the beginning of an extensive development of intraplate alkaline basalts, known as the Posadas Basalts, which reached their maximum extension further south. These retroarc basalts have been interpreted by Ramos and Kay (1992), Kay et al. (2002) and Ramos (2005) as plateau basalts linked to the formation of asthenospheric

Fig. 2. Paleogene paleogeography of the Patagonian Cordillera, with indication of the main magmatic domains. Note that south of 43300 S there is no evidence of an active magmatic arc (modified from Ramos, 2000).

windows associated with the subducted spreading center. North of these latitudes, extensive Eocene to Early Oligocene subduction-related andesitic and dacitic rocks represent orogenic volcanic arc products (Rapela et al., 1988). The boundary between these two different segments indicates the inception of the oceanic ridge–trench interactions in the foreland, as proposed by Ramos and Kay (1992). The boundary between the central and southern segments is also related to the collision of a seismic oceanic ridge against the trench, presently observed as the interaction of the Chile ridge and the Taitao Peninsula (Guivel et al., 1999). The subduction of the Chile ridge occurred in discrete segments, from south to north, since 15 Ma. Bathymetric data from the oceanic crust region adjacent to the Chile ridge, together with the digital topography, indicate a 2000 m uplift south of the Taitao fault zone along the axis of the cordilleran region. The Cerro San Valentı´n (4070 m a.s.l.), the highest peak in the Patagonian Cordillera, is just south of the Chile ridge. This drastic change in elevation north and south of 46300 S latitude coincides with the collision of the Chile ridge. South of Cerro San Valentı´n, there are the Cerro San Lorenzo (3706 m) and the mountain chain encompassed by the Hielo Continental Patago´nico Norte (North Patagonian Ice Field). Several peaks within this chain are over 3000 m a.s.l. (Cerro Fitz Roy: 3375 m; Cerro Bertrand: 3200 m, among others). The present elevation of these granitic mountains indicates a minimum uplift, as these Miocene intrusives have been unroofed by erosion that eliminated at least 4–5 km of country rock, as demonstrated further south in the Torres del Paine by vitrinite studies of the sedimentary cover (Skarmeta and Castelli, 1997). Crustal stacking was the uplift mechanism, as there is a spatial coincidence between this southern segment and the development of the Patagonian fold and thrust belt (Ramos, 1989; Alvarez-Marro´n et al., 1993; Kraemer et al., 2002). South of the triple junction, there is a substantial amount of shortening in the foothills absorbed by the sedimentary cover, which can be correlated with the basement shortening and uplift of the inner part of the Patagonian Andes (Klepeis, 1994; Ghiglione and Cristallini, 2007). This orogenic shortening varies from north to south from 25 to 45 km (Ramos, 1989). This high topography is maintained 14–12 Ma after the ridge collision at these latitudes (Cande and Leslie, 1986), as can be seen in the southern segment of the Patagonian Cordillera in Torres del Paine area. This fact rules out a thermal uplift as the dominant cause. The elevation of these granitic plutons is between 2670 (Cerro Almirante Nieto) and 3050 m a.s.l. (Cerro Payne Grande). However, the main factor controlling the uplift of the southern Patagonian Andes seems to have been a strong increase in the convergence rate together with a decrease in convergence obliquity between the Nazca and South America plates at 26–28 Ma (Pardo-Casas and Molnar, 1987; Somoza, 1998; Silver et al., 1998). As a consequence of this increase, the spreading center was finally subducted in the trench, as proposed by Folguera and

Tectonic Evolution of the Patagonian Andes Ramos (2002) and Blisniuk et al. (2005). Apatite fission track data show that increased denudation started at ca. 30–23 Ma in the region near the Pacific coast and subsequently migrated 200 km eastward to the region of the present-day topographic axis of this cordilleran segment until 12 Ma, most likely as the result of subduction erosion (Thomson et al., 2001) combined with the ridge collision at this time. The evidence for increasing uplift and denudation since the Oligocene is in good agreement with the stratigraphic sequence in the eastern foreland basin, which includes marine molasses deposits of the Centinela Formation (Ramos, 1982), continental synorogenic clastic Miocene deposits of the Santa Cruz Formation and Miocene to Pleistocene plateau basalts that have been related to slab windows associated with the Cenozoic ridge subductions (Ramos and Kay, 1992). North of the triple junction, there is only modest deformation, with minor shortening. Crustal stacking in this region was controlled by partial tectonic inversion, and large areas of the extensional Mesozoic basin are still preserved beneath the surface at these latitudes (Ramos, 1989, 2005). The collision of the Neogene ridge progressed from south to north (Fig. 3), as evidenced by oceanic magnetic anomalies (Cande and Leslie, 1986), the alkaline plateau basalts related to asthenospheric windows (Ramos and Kay, 1992; Gorring et al., 1997) and the emplacement of adakites at 14.67 Ma at 49250 S (Chalten Adakite), 12.97 Ma at 48590 S (Puesto Nuevo Adakite) and 11.79 Ma at 47550 S (Cerro Pampa Adakite) latitudes (40Ar/39Ar ages, Kay et al., 1993; Ramos et al., 2004).

Fig. 3. Present setting of the Patagonian Cordillera in relation to the collision of the Chile ridge.

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As a result of all these collisions of active oceanic ridges during the Cenozoic, and the consequent subduction of these thermal active sea floor spreading centers, the lithospheric mantle became anomalously heated, decreasing its viscosity. Studies performed by Ivins and James (2002) show that mantle viscosities seem to be much lower in this sector of the Patagonian Cordillera (on the order of 5–0.2  1018 Pa s) than in normal continental cratons (approx. 1021 Pa s).

4. Miocene Uplift The spatial and temporal constraints of the Neogene uplift of the Patagonian Cordillera are established by the distribution and age of the molasse synorogenic deposits. It is well established that the thickness of the Neogene synorogenic deposits suddenly increases south of the Ayse´n triple junction at the latitude of Taitao Peninsula (46300 S; Ramos, 1989) (Fig. 4). The lower part of the molasse deposits is represented by the Oligocene to Early Miocene Centinela Formation, nearshore marine conglomerates, sandstones and shales deposited after a transgression from the Atlantic (Ramos, 1982; Malumia´n and Ramos, 1984). These marine beds are conformably followed by Early to Middle Miocene fluvial deposits of the Santa Cruz Formation, which have been related to the main phase of Cenozoic deformation and uplift of the cordillera to the west. Maximum thicknesses of these continental deposits are reached along the northern part of the Patagonian Cordillera foothills with values up to 1500 m, decreasing to 225 m along the Atlantic coast (Tauber, 1997). The sedimentation of the Santa Cruz Formation clearly indicates that the regression of the marine deposits of Centinela Formation was forced by the cordillera uplift. The deposition of the Santa Cruz Formation occurred in a foreland basin where sediment supply exceeded the accommodation space. As a result, a prograding sequence expanded to the Atlantic coast (Nullo and Combina, 2002). The Miocene sequences of the Santa Cruz Formation are important in determining the timing of deformation in the Patagonian Cordillera. The base of the continental synorogenic deposits marks the beginning of the Cenozoic uplift at about 19–18 Ma (Marshall et al., 1977, 1983, 1986; Ramos, 1989). The upper section of these deposits at Lago Pueyrredo´n has been dated at about 15 Ma (Marshall et al., 1986). Based on K/Ar ages of tuffs from several scattered outcrops of the Santa Cruz Formation, the age range of these deposits has been estimated between 19 and 15 Ma (Marshall and Salinas, 1990). Feagle et al. (1995) 40Ar/39Ar-dated the tuffs intercalated in this formation in Cerro Observacio´n, near the Atlantic coast. The new values yielded ages between 19.33 – 0.18 and 16.16 – 0.27 Ma. More recent 40 Ar/39Ar ages obtained by Blisniuk et al. (2005) in the lower part of this unit in the Manantiales section, along the foothills of the Patagonian Cordillera, south of Lago Belgrano (see Ramos, 1983, for location), yielded

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Vı´ctor A. Ramos and Matı´as C. Ghiglione b

a

c

d

13

14.24 ± 0.78 Ma

500

500

δ C (PDB) [‰]

18

O (SMOW) [‰]

500

500

400

400

300

300

200

200

100

100

136.2 ± 94.4 m/Myr 400 m/Myr ± 24.5

300

84.9 ± 43.4 m/Myr

271.2 ± 448.5 m/Myr 200

averag ed 114 .0

sediment thickness [m]

400

15.51 ± 0.41 Ma 300 16.45 ± 0.25 Ma 200 16.71 ± 0.63 Ma

85.0 ± 27.6 m/Myr 100

100

18.15 ± 0.31 Ma 11.6 ± 2.2 m/Myr

22.36 ± 0.73 Ma

0 24 23 22 21 20 19 18 17 16 15

sediment age [Myr]

0

–30 m covered interval –15 average from several nodules single sample 5-pt running average

OLIGOCENE

dated tuff conglomerate sandstone

0 13 14 15 16 17 18 19 20 21 22 23 average from several nodules single sample 5-pt running average

volcanic ash / tuff siltstone

Fig. 4. Isotopic and geochronologic data from the Manantiales section (after Blisniuk et al., 2005). (a) Plot of stratigraphic position versus age of dated tuffs from the Santa Cruz Formation with sediment accumulation rates. (b) Stratigraphic log of the studied sediment section, with positions and ages of dated tuffs. The age for the top of the section is 14.2 – 0.8 Ma (tuff sampled 3m beneath the top of the Santa Cruz Formation where it is overlain by basalts of 12.1 – 0.7 Ma). (c) Carbon isotope data of pedogenic carbonate nodules in paleosoils. Note solid dashed line highlighting a d13C value of –8ø, with higher values strongly indicative of the presence of a significant proportion of C4 vegetation. (d) Oxygen isotope data of pedogenic carbonate nodules in paleosoils.

22.36 – 0.73 Ma for the base of the Santa Cruz Formation, 30 m above the contact with the Centinela Formation (Fig. 4). The younger 40Ar/39Ar date for the upper section yielded an age of 14.24 – 0.78 Ma (Blisniuk et al., 2005). A minimum age of 10–12 Ma is implied by the age of the oldest overlying plateau basalts (Ramos, 1989; Ramos and Kay, 1992; Gorring et al., 1997). These data indicate that the continental foreland basin started at about 23 Ma along the foothills, and that progradation reached the Atlantic coast at about 19 Ma. Maximum rates of deposition associated with a threefold increase in the uplift rate of the Patagonian Cordillera have been obtained at 16.5 Ma (0.27 mm/a, Blisniuk et al., 2005). The increase in uplift rate coincides with a considerable desertification in the leeward eastern foreland at 16.5 Ma, according to recent isotopic studies of calcretes and paleosols within the Santa Cruz Formation, which provided an independent good resolution for the cordilleran uplift (Stern and Blisniuk, 2002). The stable isotope data presented by these authors imply that surface uplift of the southern Patagonian Cordillera led to a climatic deterioration in the foreland, also recorded by changes in the mammal communities during the Miocene, as recognized by Pascual (1984). There is no evidence for an increase in the convergence rate between the Nazca and South American

plates at that time. Therefore, the most likely reason for this surface uplift could have been an increase in compression and tectonic shortening due to (1) the subduction of progressively younger and more buoyant oceanic lithosphere as the Chile ridge approached the trench (Folguera and Ramos, 2002) and (2) an increase in the strength of coupling between the Nazca and South America plates because of the absence of a significant sediment fill in the trench (Bourgois et al., 2000; Behrmann and Kopf, 2001; Blisniuk and Strecker, 2001; Blisniuk et al., 2005). The buoyancydriven surface uplift caused by subduction of a spreading center relative to older oceanic lithosphere can produce a surface uplift of > 1 km within the expected range of tectonic responses to spreading ridge subduction (Cloos, 1993). Sediments postdating the Santa Cruz Formation are relatively limited in extent and volume. Final deformation of these deposits occurred prior to the eruption of the Miocene plateau basalts at 10–12 Ma, as a strong angular unconformity separates the folded and thrust Mesozoic and Tertiary deposits from the undeformed basalts (Ramos, 1989). Subsequent sedimentation was dominated by coarse conglomerates related to Pleistocene and older glaciations (Mercer, 1976; Mercer and Sutter, 1982; Ramos, 1982; Rabassa and Coronato, 2002).

Tectonic Evolution of the Patagonian Andes A much more important change in erosion rates at the eastern foreland occurred at about 14 Ma, when deposition of the Santa Cruz Formation ended. Since then, sedimentation in that region has been almost exclusively limited to short-lived episodes of conglomerate deposition like the Caban˜a conglomerates (10–12 Ma; Ramos, 1982), or during and immediately after glacial periods (Mercer and Sutter, 1982), implying a drastic increase in aridity at that time. As compression and surface uplift in the southern Patagonian Andes presumably increased when the ridge–trench collision started (<15 Ma), it seems possible that at about 14 Ma a threshold elevation was reached at which the orographic rain shadow effect became much stronger (Blisniuk et al., 2005). As the age of ridge collision decreases to the north with the segments north of Esmeralda fault zone (Fig. 4) colliding in the last 6 Ma, the final uplift is closely related to the beginning of the oldest glaciation recorded at these latitudes in South America (Ton-That et al., 1999).

5. Sediment Supply to the Trench The Miocene uplift resulted in an increased sediment flux along the western side of the mountains into the trench. Increasing surface elevation would have blocked more moisture from reaching the leeward eastern side of the

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mountains (Fig. 5), while leading to an increase of precipitation and erosion rates on their windward western side (Thomson, 2002; Blisniuk et al., 2005). The effective rain shadow has been obtained in the last 12 Ma, based on the interruption of erosion and sedimentation in the eastern foreland side of the cordillera. Thicker trench fill promotes weaker coupling along the plate interface (e.g. Lamb and Davis, 2003; Sobolev and Babeyko, 2005) and therefore the increase in sediment supply into the trench may have contributed to the apparent cessation of uplift in the southern Patagonian Cordillera that occurred at 12–10 Ma (Ramos, 1989). Prior to the Chile ridge collision, crustal erosion by subduction and eastward migration were active between 30–23 and 12 Ma, as proposed by Thomson et al. (2001) based on fission track data. The ridge collision has a profound influence in controlling the tectonic regime. Based on the studies of Bourgois et al. (2000), three factors are important: (1) sediment supply into the trench governed by climate variations; (2) plate reorganization after postsubduction ridge jump to the north; and (3) underthrusting of slab positive topography and subsequent tectonic accretion. The detailed survey of the Ayse´n triple junction by these authors demonstrates that an increase in sediment supply caused the margin to switch from subduction erosion to subduction accretion, attributed to a change in the dynamics of the subduction channel.

–6

–10

precipitation non-evaporated spring/stream evaporated spring/stream

–12

18O SMOW

SURFACE WATER SAMPLES

(‰)

–8

–14 –16

West

2000

Elevation [m]

4000

4000

Precipitation [mm/yr]

6000

East

studied sediment section

precipitation

Maximum elevation

Mean elevation

3000 2000

Minimum elevation

76°

75°

74°

73°

72°

71°

70°

69°

Fig. 5. Present-day rain shadow effect across the southern Patagonian Andes (after Blisniuk et al., 2005) Top: Oxygen isotope values of present-day surface waters collected along a transect across the Patagonian Cordillera. Note that surface waters from the leeward eastern side have significantly lower d18O values. Also note that deuterium excess values demonstrate that a relatively high proportion of surface waters east of the mountains have experienced substantial evaporation, which causes a d18O increase (based on Stern and Blisniuk, 2002). Bottom: maximum, mean and minimum elevation along an E–W oriented and 70 km wide swath between 47200 and 48000 S, which contains water sampling locations and the studied sediment section.

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Vı´ctor A. Ramos and Matı´as C. Ghiglione

The mass balance estimation north and south of the Ayse´n triple junction shows important variation in frontal accretion, underplating, and sediment subduction according to Behrmann and Kopf (2001). Almost 80% of the sediment on the downgoing Nazca plate is subducted 200 km north of the triple junction. At the triple junction, the forearc is almost completely destroyed by spreading ridge collision and subduction erosion. On the contrary, about 60% of the sediment on the incoming Antarctic plate has been scraped off and was frontally accreted to the Chile Forearc (Fig. 6).

The change in the tectonic regime is deeply influenced by the sediment supply and the amount of sediment filling the trench. The deeper and starving trench along the Peru margin has a strong coupling and concentrates crustal erosion by subduction (Von Huene et al., 1996). This part of the central Andes, where the dominant wet winds are coming from the east, has increased aridity on the oceanward side of the mountains and sediment starvation in the trench favoring strong coupling, deformation and uplift. On the contrary, on the southern Andes where the dominant trade winds are coming from the

Pre-collision zone (north of the Chile triple junction) EAST

WEST

(a) Line 734

(b) Line 745

Collision zone (c) Line 751

Post-collision zone (south of the Chile triple junction) (d) Line 769

Fig. 6. Depth-converted cross section of seismic lines 734, 745, 751 and 769 showing the change from subduction erosion to subduction accretion north and south of the Chile ridge collision (after Behrmann and Kopf, 2001). See location of seismic lines in Fig. 3.

Tectonic Evolution of the Patagonian Andes southwest, precipitation and erosion rates are higher on the oceanward side of the mountains, and abundant sediment supply is provided by fluvial and glacial denudation of the Patagonian Cordillera. Based on these facts, it can be assumed that the increased elevation trend of the southern Andes, when going from south to north, is mainly controlled by the dominant winds and the capacity to create an effective rain shadow, as in the Patagonian Cordillera. The largest contrast is observed south of the triple junction, as ridge collision produced important uplift in the last 12 Ma, south of 46300 S. This general trend of uplift and shortening is not only controlled by the climatic conditions as proposed by Lamb and Davis (2003), but also by several other complex interactions such as the geometry of Benioff zone, an overridden velocity and the age of oceanic crust being subducted, among others (Ramos et al., 2004; Sobolev and Babeyko, 2005; Spagnuolo et al., 2005). However, in cases where the effective rain shadow reaches the extreme values presently obtained in the southern Patagonia Cordillera, ranging from several meters of precipitation on the western side to a few hundred of millimeters in extra-Andean Patagonia, the climatic conditions may have a first-order control in the tectonic regime. This tectonic regime produces a relatively small volume of the southern Andes when compared with the volume of the central Andes.

6. Denudation Rates An intriguing fact of the Patagonian Cordillera is related to the continuous exposure of the Patagonian Batholith along the main axis of the cordillera. North of 38300 S, the batholith is only represented by isolated stocks until its disappearance further north. At 30–33 S the batholith outcrops are reduced, composed mainly of Paleozoic rocks, and are controlled by Andean thrust showing

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minor denudation. Combined apatite and zircon fission track data obtained along the Patagonian Batholith between 44 and 51 S by Thomson et al. (2001) clearly show low denudation rates in the batholith from Late Cretaceous to Late Oligocene times. Denudation rates assuming a thermal gradient of 30C/km are around 0.03 mm/yr (equivalent to a cooling rate of 1C/Myr), contrasting with values of 0.50 mm/yr in the last 8 Myr, which corresponds to a cooling rate of 15C/Myr. This very rapid increase in denudation rate coincides with the age of uplift and the establishment of an effective rain shadow as discussed in the previous chapter. Granitic stocks of Early Miocene age like the Mount Fitz Roy granite (18 – 3 Ma; Nullo et al., 1978), and several other intrusions of these ages (16–18 Ma) identified by Thomson et al. (2001) in the main batholith have been rapidly exhumed. This exhumation was the result of combined tectonic emplacement and rapid denudation. Deformation of the Patagonian fold and thrust belt uplifted several peaks as the San Valentı´n, San Lorenzo, Fitz Roy and Paine, with present highs exceeding 3 km elevation, east of the main cordillera. North of the triple junction, new fission track data (Thomson, 2002) indicate a very rapid uplift in the last 7 Myr related to transpressional displacement associated with the Liquin˜e–Ofqui fault zone (Fig. 7). This deformation is closely linked with the younger collision and ridge jumps of the northern segments along the triple junction. These data contradict the previous proposal of Cembrano et al. (1999) that a dextral transpression regime driven by plate coupling related to oblique subduction of the Nazca plate has existed in the Patagonian Cordillera for much of the Cenozoic. As pointed out by the foreland deformation, the Patagonian fold and thrust belt, south of the triple junction, is controlled by dextral transpression formed after the ridge collision (Ramos, 1989; fig. 11). The deformation peak was at about 12–10 Ma (Kraemer et al., 2002), with reactivation along the Cosmelli basin,

Present day land surface

Rock mass removed by erosion since ca. 10 Ma

E

W

Prevailing weather

Liquiñe-Ofqui fault Azul-Tigre fault Cisnes pluton Rio Mañihuales fault

ca. 10 km

Note: Horizontal and vertical ~45° S scale only approximate

Canal Costa fault

Positive flower structure Brittle-Ductile transition

Transpressional shear zone ~26° Subducting Nazca plate

Trend of Nazca-South America plate boundary (approx. north-south) Nazca-South America plate convergence vector (approx. N 075° E) Trench orthogonal shortening direction

Fig. 7. Denudation of ca. 10 km of the Patagonian Batholith related to prevailing winds immediately north of the triple junction (after Thomson, 2002).

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Vı´ctor A. Ramos and Matı´as C. Ghiglione

in segments immediately south of the triple junction (Flint et al., 1994). Modern fieldwork in this area identified important recent deformation related to the collision of the shorter and younger segments along the triple junction latitudes (Lagabrielle et al., 2004). Most of the southern larger segment of the Patagonian Cordillera does not record any neotectonic movements. As one moves to the north, there is a parallel evolution of decreasing effective rain shadow, and less denudation. The Andean volume of the mountain chain increases to the north, but the Andean Batholith is not exhumed, which clearly indicates that denudation and not tectonic uplift is the main factor to expose the batholith. Fission track data indicate more than 10 km denudation occurring since Late Miocene times (Thomson, 2002). The denudation rate reaches values up to 1.3 mm/ yr (equivalent to a cooling rate of > 25C/Myr). Recent numerical modeling has confirmed that rain shadows within the present range of precipitation can produce these values of denudation, just by the climatic gradient controlled by the prevailing wet winds between the western and the eastern side of Patagonian Cordillera (Quinteros et al., 2004).

7. Tectonics and Glaciation The collision of the Chile ridge with the trench in Late Cenozoic times, in addition to all the processes described above, also triggered the beginning of glaciation in southern South America. There is a striking coincidence in time and space between tectonic uplift and the beginning of glaciation in the latest Miocene. The oldest till deposits identified by Mercer and Sutter (1982) were located south of Meseta Buenos Aires (approx. 46300 S), within the segment comprised between the Taitao and Esmeralda fault zones (Ramos, 1989; Gorring et al., 1997). The glacial deposits are sandwiched between two lavas. K/Ar dating by these authors indicates ages of 7.03–6.75 Ma for the lower basaltic flow, and 5.05–4.43 Ma for the upper flows, bracketing the first glaciation at about 4.6 Ma. More recent 40Ar/39Ar ages of these flows (7.4 – 0.1 and 5.04 – 0.04–Ma) constrained the age of the first till between 6 and 5 Ma (Ton-That et al., 1999). Although this glaciation had not reached the extension of the Great Patagonian Glaciation according to Ton-That et al. (1999) and Rabassa and Coronato (2002), it is located at the latitude south of the foreland

(a)

(c)

(b)

Fig. 8. Present tectonic setting of the Patagonian Cordillera. (a) North of the triple junction, prior to the seismic ridge subduction. Note the active volcanic arc, and the mild deformation of the retroarc region. (b) South of the triple junction. Note that shortening and uplift are favored by a ductile lower crust and the low viscosity of the mantle indicated with a stipple pattern (modified from Ramos, 1989). (c) Inset indicating the plate kinematics of the triple junction between Nazca, South America and Antarctic plates (after Ramos, 2005).

Tectonic Evolution of the Patagonian Andes projection of present triple junction, where important uplift had occurred at 6 Ma (Ramos, 1989; Gorring et al., 1997). The effective rain shadow of the prevailing winds could be an important factor in controlling the beginning of the glaciation and the further development of the Patagonian Ice Fields, in those areas where relief attained the necessary threshold elevation for glaciation. Wenzens (2002) showed a neat coincidence between the northern and southern ice caps, and the major segmentation produced by collision of the two major segments of the spreading Chile ridge. A similar effect is described for the LGM at about 19 ka by Wenzens (2002), who found a close correlation between the topography originated by ridge collisions and the extension of the continental ice fields. These studies, together with the model of Ivins and James (2002) on the isostatic response to deglaciation, mainly based on the last 5 ka mass fluctuation of the Patagonian Ice Fields, show that mantle viscosities seem to be lower (on the order of 5–0.2  1018 Pa s) than in normal continental shields (approx. 1021 Pa s). The results of the Little Ice Age (LIA, AD 1400–1750) show an abnormally rapid response, related by these authors to an abnormally hot mantle, a consequence of slab window formation due to oceanic spreading center subduction beneath the Patagonian Cordillera during the Cenozoic. Recent GPS measurements of vertical displacement indicate ongoing vertical uplift related to LIA and younger ice retreats up to 20 mm/yr (Bevis et al., 2002), more than two times the uplift rates currently recorded in Fennoscandia and Hudson Bay (5–10 mm/yr). This is a clear indication of abnormal heat flow beneath the Patagonian Cordillera (Lagabrielle et al., 2000), probably controlling a low viscosity in the mantle. The thermal setting near the triple junction has an estimated heat flow higher than 100 mW/m2, which would have enhanced the uplift and deformation during the last Cenozoic seismic ridge subduction. This thermal state is incompatible with flat subduction, which implies a cold regime in the mantle and lower crust (Fig. 8). There is strong evidence that several spreading centers have interacted with the trench at the margin of the South American plate in the last 50 Myr along the subduction zone of the Patagonian Cordillera. This interaction is documented by abrupt changes in uplift, deformation and magmatism. As the collisions of these active seismic ridges were associated with periods of rapid convergence in the Paleogene and the Neogene, their effects were superimposed to an increase of orogenic activity generated by acceleration of the convergence rates. However, the climax of this deformation and the rapid uplift coincided in time and space with the ridge collision, as documented in the most recent interactions between the spreading center and the trench. These last uplifts triggered glaciation in the southernmost Andes, as a response to the topographic barriers to the westerly winds, and even LIA deglaciation had a rapid and abnormal 20 mm/yr uplift, controlled by the low viscosity of the mantle associated with an abnormal thermal regime as a consequence of the oceanic ridge collisions.

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8. Conclusions The Patagonian Cordillera records a complex Pre-Cenozoic history that controls the Andean structures. During the Cenozoic, the interaction of spreading ridges and subduction at the trench controlled the main deformation episodes, the volcanic arc gaps and the eruption of foreland plateau basalts. As a result, most of the Patagonian Cordillera south of the Ayse´n triple junction had an abnormal heat flux through time, which substantially decreased the viscosity of the underlying mantle. This fact enhanced the structural response to different climatic changes that affected the area since the Late Miocene. The relation between tectonics and climate can be summarized in the following processes: 1. Uplift related to ridge subduction created an effective barrier to the prevailing winds, producing one of the most severe rain shadows in the world (Blisniuk et al., 2005). As a result of that strong denudation that affected the axial part of the Patagonian Cordillera, south and north of the Ayse´n triple junction, the Patagonian Batholith has continuous exposure. 2. The severe denudation produced an overfilled trench south of the triple junction, switching the tectonic regime from subduction erosion prior to collision to subduction accretion after collision. The sediment flux to the trench inhibited a strong coupling between the oceanic and continental plates, which resulted in a small Andean volume of the Patagonian Cordillera, and the lack of neotectonics in the foothills south of 47 S. 3. The Late Miocene uplift controlled the inception of the first glaciation in the Andes, and one of the oldest glaciation in the Southern Hemisphere. This uplift also controlled the inception of the northern and southern ice fields. 4. The large vertical uplift recorded by GPS measurements is a consequence of the isostatic response to the low viscosity of the mantle triggered by a series of ridge collisions through the Cenozoic. Acknowledgments The authors are indebted to the Agencia Nacional de Promocio´n Cientı´fica y Tecnolo´gica for continuous funding through those years that permitted fieldwork and subsequent analyses. M.C. Ghiglione was supported by a postdoctoral grant from CONICET. The members of the Laboratorio de Tecto´nica Andina of the Universidad de Buenos Aires and the Andes Project of the Cornell University are also thanked for many years of fruitful discussions and collaborative work. References Aguirre-Urreta, M.B. (2002). Invertebrados del Creta´cico inferior. In: Haller, M.J. (ed.), Geologı´a y Recursos Naturales de Santa Cruz. XV Congreso Geolo´gico Argentino, Relatorio 2, 439–459. El Calafate. Argentina.

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5 Neotectonics, Seismology and Paleoseismology Laura Perucca1,2 and Hugo Bastias2 1

2

CONICET Gabinete de Neotecto´nica – INGEO – FCEFN – UNSJ. Av. Ignacio de La Roza y Meglioli. 5400 San Juan, Argentina Instead, they suggested that there is evidence of sinistral displacement all along the fault areas in this part of the northern Patagonian Massif. At 46 S, in the central-southern part of the Chubut Province, a NW–SE trending megafracture is located, which can be identified by an extensional depression with evidence of recent tectonic activity. In this area, one can distinguish a series of aligned springs and a shear zone that is part of the Genoa megafracture (Fig. 3). Another megafracture, conjugated with the previous one and bearing a NE–SW trending, reveals as well a shearing zone with a strike-slip displacement. In Pampa de Agnia, in mid-western Chubut Province, abundant evidence of modern tectonic activity has been found, with south trending faults that affect not only the piedmont but also the Neogene lava flows. The Rı´o Chubut displays a marked structural control along this section. The Magallanes–Fagnano fault is located in Isla Grande de Tierra del Fuego (Fig. 2), trending WNW– ESE over more than 600 km from the Atlantic to the Pacific. All along this active transcurrent fault, with a sinistral movement, the South American continent has a slow westward displacement in relation to the Andean region in Tierra del Fuego. This displacement is shown by strong earthquakes and low seismicity, mainly on the Chilean side. In Argentina a classic model considering, from west to east, two distinctive seismotectonic environments has prevailed for many years. The Andean western portion is described as a seismic area with a present active tectonic zone, and an eastern section that extends from the Andean front eastward with a non-seismic regime. Studies of neotectonic activity are scarce due to the lack of seismographic equipment and a low population density, and thus the absence of historical records. However, the presence of remarkable morphotectonic features associated with active faulting fronts in several of the defined seismotectonic regions indicates modern tectonic activity for this region of Patagonia. This evidence should be completed and studied in detail with associated disciplines like paleoseismology and the morphological evolution of landscapes with special climatic conditions and high erosion rates. The present work does not analyze individual features of Neogene faulting, but rather tries to define environments or regions where Neogene faulting apparently has similar characteristics. From the point of view of seismic risk studies, it is logical to assume that these regions have

1. Introduction The purpose of the present chapter, dedicated to the neotectonics of Argentine–Chilean Patagonia, is to complete an outlook of active tectonics of the southernmost region of South America, which constitutes a field of relatively recent development on a national standard. The present state of our knowledge can be summarized in Fig. 2, where the main faults active during the Late Neogene and the main Neogene geologic units, sediment cover and Miocene–Holocene volcanic cover have been represented. The main regional alignments related to outstanding morphological features are shown in Fig. 1. Based upon the interpretation of satellite images, at least 70% of the moderately preserved Neogene volcanic structures were identified (Fig. 2). In Figs 3 and 4, earthquakes of magnitude over 4 that have been recorded in the region since the mid-twentieth century have been represented. In these figures, it can be seen that there is a marked lack of uniformity in the distribution of seismic activity, in the localization as well as in the epicenter depth. Nevertheless, some regional patterns governing the major morphostructural domains have been found. Apparently, there is a certain correspondence between these great structures and the location of seismic events, such as it has been observed with earthquakes related to the Liquin˜e–Ofqui fault system (Figs 1 and 2), and to the Magallanes–Fagnano fault system (Fig. 2). It may also be observed that the major structures set the boundaries of the volcanic environments, especially during the Late Neogene; such is the case of the Liquin˜e– Ofqui fault system in western South America and its assumed northern continuation into the province of Neuque´n. This is the main fault system in western Patagonia, in which the Chilean territory is called the ‘‘Liquin˜e– Ofqui fault system’’ (Figs 2 and 3), a N–S trending intraarc fault that extends over hundreds of kilometers and described by Lavenu and Cembrano (1999a) as a dextral strike-slip duplex. In the province of Chubut, southern Argentine Patagonia, a megafracture with a NW–SE trend is located; this is known as the ‘‘Gastre Megafault’’ (Rapela et al., 1991) which has been described by Rapela (1997) as an intra-continental fault with a dextral strike-slip displacement. However, other authors, such as Franzese and Martino (1998) or von Gosen and Loske (2004), claimed that there is insufficient evidence to assert the presence of an intra-continental fault system with a dextral heading.

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DEVELOPMENTS IN QUATERNARY SCIENCES VOLUME 11 ISSN 1571-0866 73

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Fig. 1. Location map. The numbered points refer to sites described in this chapter.

been subjected to various seismic effects, and therefore they can be considered as units of potential risk. The product is a map showing the major regions where seismic activity has specific characteristics, and for which the probable occurrence of earthquakes, their maximum intensity and the seismic hazard are also

different. This primary zonation allows the definition of seismotectonic regions, in which the faulting and the stresses within the present tectonic framework are approximately uniform. From the point of view of seismicity, Patagonia has a pre-Hispanic earthquake record of almost nil and in the

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Fig. 2. Preliminary tectonic map of Patagonia showing the location of main geographic features: 1. Macizo de Somuncura´; 2. Lagos Musters and Colhue´-Huapi; 3. Isla de Chiloe´; 4. Lago Nahuel Huapi; 5. Volca´n Copahue; 6. Lago Viedma; 7. El Chalte´n; 8. Lago General Carrera – Buenos Aires; 9. Ing. Jacobacci; 10. Pali-Aike field; 11. Liquin˜e–Ofqui fault system; 12. Puerto Iba´n˜ez–Chile Chico fault system; 13. Gastre fault system; 14. Desde´mona fault; 15. Isla de Los Estados; 16. Lago San Martı´n; 17. Lago Argentino; 18. Pampa de Gastre; 19. Sierra de Traquetre´n; 20. Macizo del Deseado; 21. Lago Fagnano; 22. Magallanes–Fagnano fault system.

case of Argentina it offers very little information even during the 300 yrs following the Spanish colonization. This lack of information affects the interval of the regional seismological record. In the Chilean sector, the first earthquake was registered on October 28, 1562, whereas on the Argentine territory, the first record appeared only on February 1, 1879. Prior to these dates, only a few events are recorded in old stories of the indigenous population of Isla Grande de Tierra del Fuego. The southern portion of South America has very poor information in comparison to other regions and,

unfortunately, this leads to an underestimation of its seismicity. Moreover, as the distribution of seismological stations is poor, focal mechanisms for low-magnitude earthquakes cannot be determined anywhere. In 1991, the Direccio´n Nacional del Anta´rtico (Argentine National Antarctic Bureau) organized a cooperation project with the Instituti Nazionale di Oceanografia e Geofisica Sperimentale of Trieste, Italy, to carry out seismological studies in the Antarctic Peninsula and Isla Grande de Tierra del Fuego (Russi et al., 1994).

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Fig. 3. Simplified map indicating Neogene volcanism and main fracture zones of southern South America.

The Universidad Nacional de La Plata in the last 5 yrs set up three seismological stations, one in La Plata, another in Trelew (eastern Chubut Province) and the third in Estancia La Despedida, 40 km from the city of Rı´o Grande in Isla Grande de Tierra del Fuego. Following the installation of these stations, seismograms were obtained, thus contributing to the seismic knowledge of these regions (Sabbione, 2004). From a structural point of view, studies began in the middle of past century, when the basic essentials of the regional stratigraphy had been established. The Servicio Geolo´gico Nacional (Argentine National Geological Survey) carried out the first systematic structural mapping of the Patagonian Andes, at a scale of 1:200,000, and produced many geological sheets in this region.

Until now, there are very few studies which have the Quaternary or recent deformations as central topics, or in which in a broad sense these deformations have been treated separately from Neogene or Late Cenozoic events. A seismotectonic analysis of the Argentine territory between 20 and 30 S and its application to seismic hazard evaluation, a pioneer work in Argentina, was performed by Castano and Bastias (1981). Another early report on active faults in the Argentine territory was produced by Amos et al. (1981). However, these authors only mentioned faults in the northern and central Argentine provinces, and made no reference to the presence of recent deformation in Patagonia. Among the first studies in the Argentine Patagonia referring to active tectonics, it is possible to quote Steffen (1944), who discovered the Liquin˜e–Ofqui fault toward

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Fig. 4. Simplified map of the seismicity of Patagonia showing the inhomogeneity of shallow earthquakes (M > 4).

the end of the nineteenth century and beginning of last century. Other important research papers are those of Hauser (1991), Lavenu and Cembrano (1999b), Garcı´a et al. (1988), Cembrano and Moreno (1994) and Melnick (2000). The Magallanes–Fagnano fault, which is situated in the Fuegian Andes and forms the border between the Scotian and the South American plates, was also described by Winslow (1981), Winslow and Prieto (1991) and Cunningham (1993), among others. Bastias (1986) evaluated the seismic activity in the western portion of Argentina, north of 34 S, dividing the area into ‘‘seismotectonic regions’’, based upon the relationship between active faults and seismicity (Fig. 5).

2. Seismotectonic Regions Once the different aspects of tectonics, large morphological features, seismic activity and active volcanism have been analyzed individually, it is important to consider in combination since their occurrence indicates those regions where the earth’s crust is being exposed to greater stresses and the resulting tension release. As the purpose is to properly estimate seismic risk in the Patagonian region, it is necessary to delimit these regions in such a way that they can be identified as independent units called ‘‘seismotectonic regions’’ (Bastias, 1986; Bastias et al., 1990, 1993). They are areas in which the probability of an important seismic event has

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–75

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Santa Rosa

Villa Mercedes

Valcheta

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Chillán

Valdivia

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Peruvian–Chilean Trench

Seismicity distribution between 33.5° and 46° S

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–64 0 50 100 150 200 250 300 350 400 450 500 550 600 650 700

Fig. 5. West–East seismic cross-section between 33.5 and 46 S.

similar statistical values. The boundaries of a seismotectonic region are generally first-order morphostructural features and they are associated with regional seismicity. The analysis of the tectonic features in Fig. 2 indicates that the major alignments such as mountain ranges follow preferential directions. The volcanic areas are also well defined, thus revealing differential stress actions according to the cortical area under consideration. The border lines are not clear and intertwining of shape and phenomena does occur. The established seismotectonic regions are not necessarily coinciding with geological provinces or morphostructural units previously known to exist in the Patagonian region. The correct evaluation of the geological and seismological data must be the object of future research work on potential hazards. Generally, the studied tectonic features show that the territory analyzed may be divided into domains or areas. In each of these, some structural features are particularly noted, making them the main area characteristics. Four large areas with different seismotectonic characteristics have been defined. In each of them the principal fracture systems have been analyzed, as well as their possible relation to present-day seismicity, destructive earthquakes and active volcanoes, leading to their recognition as independent units for a subsequent seismic risk evaluation. The border lines established for the proposed seismotectonic regions in Fig. 6 are not well defined and may be modified in the future when ongoing studies on modern

fault for the different regions are completed and a better knowledge of present tectonic movements is achieved. The identified Patagonian seismotectonic regions are the following: 1. 2. 3. 4.

Liquin˜e–Ofqui–Fagnano Somuncura´ Agnia Deseado.

2.1. Liquin˜e–Ofqui–Fagnano Seismotectonic Region The Liquin˜e–Ofqui–Fagnano region is located in the southwestern portion of South America, between 39 and 54 S. This seismotectonic unit extends over different geological provinces, having been established as regions with their own sedimentological and structural characteristics. The region is characterized by a high surface seismicity, especially in the Chilean sector, with earthquakes of a magnitude over 7.0 on the Richter scale (Servicio Sismolo´gico Universidad de Chile), and associated Neogene volcanism, particularly related to the Liquin˜e–Ofqui fault area. The Payunia–Colorado megafracture is its northern boundary, whereas it is in contact with the Genoa megafracture to the west and with the Gallegos–Calafate fault to the south. The northern Patagonian Andes are located south of the central Andes, stretching over more than 500 km between 39 and 45 S. This segment of the Andes is

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Fig. 6. Simplified map of the main seismotectonic regions in Patagonia with major fracture zones.

characterized by a mild oblique subduction, with a convergence vector of 79 azimuth of the Nazca plate below the South American plate. This oblique component produces transcurrent movements in the volcanic arc area, with a front parallel to the subduction trench. The structural system is characterized by the development of transpressional and transtensive faults in a transcurrent dextral setting, along the Quaternary volcanic arc. The northern Patagonian Andes are a transpressional range with a subordinated transtension (Lavenu and Cembrano, 1999a). The northern Patagonian Andes (37300 –46 S) are a relatively low mountain chain with a normal crustal thickness and a volcanic arc controlled by the active Liquin˜e–Ofqui fault zone, a dextral strike-slip fault system that seems to rule the main eruptions of Quaternary volcanism. This is indicated by fissural stratovolcanoes

and the alignment of volcanic cones, following the westward shift of the arc toward the trench, possibly due to the steepening of the Nazca plate during the Late Pliocene (Stern, 1989). On both sides of the mountain-range axis, the Cenozoic structure of the northern Patagonian Andes has two typical features. To the west, in the Chilean territory, Cretaceous deformations with N–S trending displacements are prevailing (Herve´, 1994; Cembrano et al., 1996), whereas to the east, in the area farther away from the plate border, the northern Patagonian Andes reach their highest elevations and are characterized by a fold-and-thrust belt structure (Giacosa and Heredia, 2000). Where the Liquin˜e–Ofqui fault zone cuts the Bio Bı´o–Alumine´ fault zone, there are transtensional volcanic

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alignments like the pull-apart basin of the El Agrio Caldera. Volca´n Copahue, situated in the Cordillera Principal Neuquina at 38 S, southwest of the Agrio Caldera, is controlled by two sets of faults of N 70 W and N 30 E orientation that create a depression area of approximately 20 km. Within the basin, grabens develop with a N 70 W trending and folding and reverse faults do so with a N 60 E trend. The up to 2 m high fault scarps of a N 60 W to E–W curving affect the lava flows on the northern side of the volcano. Folguera and Ramos (2000) described an important fault scarp with a N 40 W trending, overlain by the Volca´n Copahue postglacial lava flows. Folguera and Ramos (2000) indicated a folded belt of approximately 4 km wide, with a N 60 E bearing, eastern vergency of Pliocene age and stepped geometry, which would be the result of the local stress field. They concluded that a progressive deformation pulse occurred at an Early Quaternary age in the Volca´n Copahue area. During the Late Pliocene–Early Pleistocene, quadrangular transtensional basins were formed. Successive reactivation would have continued to deform later lava flows. The orogenic front is situated about 300 km from the subduction trench and is characterized by the folding of Quaternary alluvial deposits. Tunstall et al. (2005) established that the Liquin˜e– Ofqui fault system, which runs along the Quaternary volcanic front between 37300 and 39 S possesses at least three branches directly controlling calderas, stratovolcano and small volcanic domes emplacement. Between 38 and 42300 S, the continental forearc area embraces the Cordillera de la Costa and the Central Depression, which are parallel to the plate edge. This depression’s northern boundary is in the San Felipe region, near Santiago, and ends south of Aise´n in the Ofqui Isthmus, at the triple point junction latitude. The Central Depression extends for over 1400 km with a width less than 75 km. Fault scarps form the western boundary between the depression and the Cordillera de la Costa. Its eastern border with the Cordillera Principal, between 33 and 36 S, corresponds to a fault relief. From 39 S southwards, the boundary is not so clear, as it is covered by volcanic deposits. This depression was originally described as a graben of Pliocene age (Aubouin et al., 1973). In this region, the present volcanic arc develops over the cortical alignments of the Liquin˜e–Ofqui fault area. It is characterized by a series of NNE–SSW oriented alignments, faults and ductile shear zones, which follow the direction of the present and Mio–Pliocene magmatic arcs. Steffen (1944) discovered and described it toward the end of the nineteenth century and Hauser (1991) did so at the end of the twentieth century. Many detailed studies have confirmed and clearly established its kinematics: a ductile dextral shear zone to reverse-dextral during the Late Miocene and part of the Pliocene (Cembrano, 1992; Cembrano and Herve´, 1993; Lo´pez et al., 1997; Cembrano, 1998; Arancibia et al., 1999) and fragile transpressional shear zone in Pliocene and Pleistocene times (Lavenu and Cembrano, 1994; Cembrano et al., 1996; Lavenu et al., 1996, 1997; Lavenu and Cembrano, 1999b). Iaffa et al. (2002) described reverse faults in the Rı´o Picun-leo valley, affecting postglacial lacustrine sediments,

temporally delimiting movements, and restricting them to the most recent activity of this orogenic segment. The latest activity of the normal fault situated to the east of the Rı´o Laguna, a tributary of the Rı´o Picun-leo, genetically associated to the postglacial slide deposits that created a dam, is necessarily older than the compressive activity and represents one of the last episodes of the Cordilleran collapse. Ramos and Folguera (1998) and Folguera and Ramos (2000) interpreted that the reverse and normal faults described here, and probably many others in the region with similar morphological characteristics, have been active during postglacial times. Between 41 and 42 S, the structure is characterized by Tertiary thrust-and-fold belts which reactivated during the Plio–Pleistocene. Giacosa and Heredia (2004) found no relevant evidence that the dextral strike-slip faults played a major part in the formation of the Andes at these latitudes, as suggested (Diraison et al., 1997) or as it occurs in the Chilean forearc region. In the southern end of the northern Patagonian Andes (44–46 S), the mechanisms for superficial shortening in the external orogenic areas include Cretaceous to Paleogene piling up on Mesozoic extensional detached blocks (Homovc et al., 1996). Bed piling is controlled by transpressional systems (Lavenu and Cembrano, 1999b) during the Neogene and Quaternary (Cembrano et al., 2000) and are the cause of the location of magmatic activity in the upper crust by means of transcurrent faults (Herve´ et al., 1993). The hydrographic basin of the La Plata and Fontana lakes at 46 S and 72 W outlines a series of north-northwest alignments of hundreds of meters to kilometers showing graben morphology. These faults cut volcanic, sedimentary and plutonic sequences of Mesozoic age. The sequences were deposited in a basin developed in the Mesozoic volcanic arc area and accumulated in depocenters of extensional origin (Folguera, 2002, Folguera et al., 2003). The geometry of these extensional depocenters on the eastern front of the Patagonian Andes is similar to the one of the extra-Andean area. The main normal faults oriented west–northwest bound the main expanse of the Early Cretaceous sequences. Folguera et al. (2004) inferred that the Quaternary tectonic activity reflects reactivation of the Mesozoic structures.

The Liquin˜e Ofqui Fault Zone Bastias (1996) recognized three systems of parallel faults along the Pacific border of the South American coast, with evidence of tectonic activity during the Quaternary on a regional scale. Two of them are found at the western margin of the Andean Cordillera, the third on the eastern side. The Liquin˜e–Ofqui fault system is located farther south, between 30 and 47 S, with a length of over 1800 km and is related to the location of the volcanic arc, probably as a result of the Nazca plate subduction underneath the South American plate. The El Tigre fault system is situated between 36 and 26 S, with a length of 1000 km, and some 300 km to the east of the subduction trench, without active volcanism. On the northern

Neotectonics, Seismology and Paleoseismology Chilean coast, the Atacama fault system (Fig. 1) is placed between 20 and 29 S, with a length of 1200 km. Evidence of the displacement of Quaternary units suggests two segments, of which the northern one is the most active one. For all three segments, the Quaternary tectonic activity shows important movements over the last hundreds of years, but regional seismicity records do not indicate any recent activity. Historical evidence for great earthquakes in the last 500 yrs is scarce, mostly due to the low population density in these areas. These discontinuities have played an important role in the development of the Andes, in the origin of compressional and extensional areas and the location of volcanic belts. The Liquin˜e–Ofqui area is an intraarc megafault, situated in southern Chile and characterized by regional alignments of hundreds of kilometers and volcanic centers along these alignments. Several authors have studied this fault (Garcı´a et al., 1988; Hauser, 1991; Cembrano and Moreno, 1994; Lavenu and Cembrano, 1999b; Melnick, 2000) describing it as the result of the oblique subduction of the Nazca plate beneath the South American plate. Lavenu and Cembrano (1999a) described it as a dextral duplex and found a NE–SW compressive event during the Quaternary. The geometry of the Liquin˜e–Ofqui area consists of NNW-trending straight segments, hundreds of kilometers long, joined by northeast-oriented en echelon faults. Herve´ (1976) and Cembrano (1992) considered the oblique subduction as the main cause of lateral shear deformation of the Liquin˜e–Ofqui fault over all its length. Nelson et al. (1994) established that the generation and activation of the fault area is a mechanical and thermal response of the continental border to the collision of the Chilean ridge with the South American plate at 40300 S. Limited seismic data from the area as well as microtectonic and paleomagnetic studies confirm dextral displacement for the Liquin˜e–Ofqui fault. Forsythe and Diemer (2000) described two active faults with 10–30 km long, clearly defined scarps in the southern portion of the Liquin˜e–Ofqui fault (46 S). The geometry of major alignments shows a ‘‘horse tail’’ type structure at 38 S. The NE–SW volcanic fissure is arranged like a tension fracture in a transtensive tectonic regime, which was active during the Plio–Quaternary. Along the fissure postglacial crater cones and the Agrio Crater are lined up. A low-angled reverse fault is found on the southern side of the Puco´n Mahuida valley, and this brings the Pliocene lavas in contact with Miocene sedimentary deposits. The existence of dextral faults, with secondary Riedel structures in postglacial lava flows from Volca´n Copahue, and a marked stream displacement indicate dextral displacement for the Liquin˜e–Ofqui fault at 38 S, during the Late Neogene (Melnick, 2000). The Isla de Chiloe´ (Fig. 2) is structurally divided into three segments that each has distinctive morphological, geological and structural features, and that influence Pleistocene and Holocene glacial, fluvial, estuarine and beach deposits. The central and southern portions are low, whereas the central segment is higher. NW–SE oriented

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transtensive faults have affected old deposits that have been reactivated in the Neogene (Mun˜oz et al., 1999). The seismicity in this region is low though superficial earthquakes of a magnitude of over 4 were registered nearby Lago Argentino (Fig. 2). Significant events are unknown from the past, possibly due to the lack of population in the region. Ramos (2002) indicated that the Patagonian fold-andthrust belt does not show evidence of neotectonic reactivation between the Pueyrredo´n and Argentino lakes, with the exception of the evidence described by Ramos (1982), south of Lago Salitroso (Fig. 1), with a fault whose geomorphological features would indicate modern reactivation. The present-day relative motion vector of the Nazca plate beneath the South American plate, north of the Chile Triple Junction is oriented N 80, and has a displacement magnitude of 84 mm/yr (DeMets et al., 1990). Part of the convergence has been accommodated along the Liquin˜e–Ofqui fault area which is connected to a former position of the Chilean Triple Point, south of the Golfo de Penas (Fig. 1). The principal feature at the Triple Junction latitude is the Lago General Carrera (Fig. 2), perpendicular to the Cordillera de Los Andes whose steep sides may be either the result of glacial erosion or have been originated by normal or transtensional fault oriented N 50–60, N 140–160 and N 90, as in other transversal depressions of Patagonia (Lagabrielle et al., 2004), or both. The regional neotectonic activity has been assessed starting from the study of the fluvio-lacustrine terraces and the drainage network design, which show anomalies in their development. In the proximities of the Magellan Strait, vertical displacements were registered during 1949 and 1950 earthquakes all along the fractures belonging to the system, allowing to consider it as active (Winslow and Prieto, 1991).

Magallanes–Fagnano fault zone The seismic activity in this region is related to a deformation and fracture belt that, at the latitude of Lago Fagnano, extends E–W for over 600 km, from one ocean to the other, and is known as the Magallanes–Fagnano fault system (Fig. 2). All along this active fault, the South American continent moves very slowly to the west, with respect to the Tierra del Fuego Andean region. The border between the South American and Scotia plates extends for more than 3000 km, from the western section of North Scotia ridge, in the Chilean southern trench, at 50 S, through the islands of Tierra del Fuego. The Magallanes–Fagnano fault system, with a sinistral E–W displacement, evolved as a component of relative plate movements between the southern end of South America and the Antarctic Peninsula. This regional alignment can be seen mainly in the eastern branch of the Estrecho de Magallanes, along the northern shore of Lago Fagnano and along the Atlantic coast (Winslow, 1981; Winslow and Prieto, 1991). It has a length of 165 km and trends N 89 W.

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The present deformation, measured at stations located on both sides of the main faults of the South American and Scotia plates, shows a sliding rate of around 0.5 cm/yr (Del Cogliano et al., 2000). The Magallanes–Fagnano fault system emerges on the Chilean side of Isla Grande de Tierra del Fuego, linked to Mount Hope (Fig. 1), where the fault plane can be seen in the Quaternary alluvial cover by the scarp’s alignment, truncated vegetation and by sag ponds. It is formed by different segments of the transform system and that are represented by near vertical faults, with polarities that change along the fault. The sedimentary architecture of the asymmetric basins formed within the main displacement area suggests simultaneous strike-slip and extensive movement, which is a common feature of other continental transtensive systems (Klepeis, 1994). To the east of Lago Fagnano, morphological evidence of Quaternary active fault activity may be found associated with truncated meanders and changes of direction of the streams. Some of the features are very recent, such as the scarp created by the 1949 earthquake associated with a gravel bed that enclosed a pond. Lago Fagnano is probably the superficial expression of a large pull-apart basin, formed by segments of the Magallanes–Fagnano fault system. Its length is comparable to some of the largest strike-slip basins, located along transform borders (Ben-Avraham and Zoback, 1992). It is formed by at least two subparallel, disconnected and in echelon segments. The western branch is outlined by a narrow depression occupied by the Rı´o Turbio valley, and the eastern branch reaches the Atlantic coast. The complex sedimentary architecture of the basin reflects different tectonic mechanisms in which periods of oblique and normal extension are alternated (Ben-Avraham and Zoback, 1992). Comparable features are known in many sites around the world, such as the Dead Sea rift, the San Andreas fault, the Polochic fault (Guatemala), the El Pilar fault (Venezuela) and the El Tigre fault (San Juan–La Rioja, Argentina), among other locations (Bastias, 1986). The fault system evolution has a close relationship with the complex tectonic events responsible for the development of the oceanic bed in the West Scotia Sea during the Late Oligocene. These events caused the definitive separation of the Antarctic Peninsula from the South American continent. The role played by the fault in the adjustment of the movement between the South American and Scotia plates must have been essential after the oceanic expansion stopped in the West Scotia Sea (9 Ma), though some displacement may have taken place a long time before (Cunningham et al., 1995, Ramas et al., 1986). The analysis of numerous faults on the Chilean side of Isla Grande de Tierra del Fuego (Klepeis, 1994) indicates that these zones have adjusted a sinistral strike slip from the Cretaceous (Grunow et al., 1991). On a larger scale, the mechanism of regional deformation has generated associated topographic alignments with offsets in the central region of the Estrecho de Magallanes, with evidence of Holocene activity (Winslow, 1981, 1983). The seismicity along the entire fault system is low (M < 3.5) and mainly superficial. The focal mechanisms

indicate a distensive component and a strike-slip feature (Pelayo and Wiens, 1989). The Isla Grande de Tierra del Fuego was affected in 1949 by an earthquake of M = 7.8 (Richter scale), which caused many strike slips on the shores of Lago Fagnano, and a local tsunami in the western branch of the Estrecho de Magallanes (Jaschek et al., 1982). Schwartz et al. (2002) described a superficial split in two sections, one on the edge of Lago Fagnano and the other in the Rı´o San Pablo. In Lago Fagnano, a quite degraded scarp is mentioned, whose height varies between 0.50 and 1 m. The downfaulted block has dead trees still standing, which are the result of floods caused by the seiche associated with the 1949 earthquake. In the Rı´o San Pablo, some 30 km from the town of Tolhuin, the scarp measures from 5 to 11 m, with an elevated block to the north that exposes Quaternary glaciofluvial deposits and the formation of successive terrace levels. Schwartz et al. (2001) identified en echelon tensional cracks, coaxial grabens and sags. The fault crosses peatlands and to the east of Lago Fagnano a scarp older than the 1949 event is found. The stratigraphic evidence associated to a secondary fault of the main one allows the interpretation of possibly three seismic events during the last 8 ka (Schwartz et al., 2002), with a recurrence interval of 2–2.7 ka. However, it is most likely that the recurrence interval is shorter than that suggested by these authors, thus requiring further detailed studies of trenches in other sections of the fault. Klepeis (1994) determined that from Mount Hope in Chile up to the central section of Lago Fagnano in Argentina, an accumulated sinistral displacement of 20–25 km, combined with a vertical displacement of 3 km has been recorded since 30 Ma. The El Deseado fault area (Fig. 2), located to the north of Lago Fagnano shows its presence by a linear, west–east trending, intermountain valley of about 3.5 km wide. This narrow valley is characterized by the presence of Lago Deseado and an alluvial Quaternary cover, with faults that truncate the vegetation and form sag ponds that are taken as evidence of the recent tectonic activity. The mountain ranges that surround the depression are about 1000 m from the heads of the valley, and are dissected by secondary synthetic faults, in an area of approximately 10 km, mainly to the northeast of Lago Deseado. Klepeis (1994) also described a group of antithetic dextral faults, subordinated to the previous ones, to the north of the Deseado valley. In the eastern portion of Isla Grande de Tierra del Fuego, ENE–WSW trending thrusts, such as the Castor thrust, are observed. It is a synthetic fault of the Fagnano fault area, with a transpressive sinistral component that controls the drainage network.

2.2. Somuncura´ Seismotectonic Region The Somuncura´ region (Fig. 2) is located in the centralnorthern portion of Patagonia, its northern border is the Payunia–Colorado megafracture, and to the southwest is the Gastre fault system. It is a special region because it has a very low seismicity, with a few superficial

Neotectonics, Seismology and Paleoseismology earthquakes, and with only one recorded historic event in the Choele Choel area (Fig. 1), where an earthquake was recorded on June 31, 1960 (Volponi, 1976). Neogene volcanic vents are well preserved and mainly situated in the central area of this region (Figs 2 and 3). In the Sierra de Taquetre´n (Fig. 1; 43 S and 69300 W), Costa et al. (1996) described a fracture area with a general trend of N 320–340, with scarps diminishing toward the further end of the hills and a topographic cross profile of marked asymmetry. From a morphotectonic point of view, the hilly front appears to be a fault scarp with a rejuvenated drainage network with the Rı´o Chubut as its base level. These authors suggested that, given the scarp’s short evolution, the uplift would have had to continue during the Quaternary.

2.3. Agnia Seismotectonic Region The narrow seismotectonic region known as Agnia is located in the western part of Chubut Province and trends NE–SW. It borders to the east with the Gastre megafracture, whereas to the west it does so with the Genoa fault zone. The Pampa de Agnia depression shows evidence of active fault on its borders, which has affected the Holocene piedmont deposits and Neogene lava flows. The recent volcanic bodies are principally related to the shear zones. Seismicity is very low, and only one superficial seismic event of a magnitude of over 4 has been recorded (Figs 2 and 3). A shear zone with a NW–SE orientation has been identified in this region, and it has been named Gobernador Costa (Fig. 1) due to its closeness to this locality. It shows aligned springs and strong evidence of tectonic activity during the Neogene (Fig. 7). Another shear zone, related to the previous one and oriented NE– SW, also shows signs of recent activity, since it is possible to observe aligned springs and vegetation coincident with the fault trace.

2.4. Deseado Seismotectonic Region This region is located in the center of the province of Santa Cruz, between 45 and 50 S. It borders to the north with the megafracture Comodoro–Las Heras, which has a

Fig. 7. Satellite image showing the Gobernador Costa shear zone with aligned springs.

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northeast–southwest orientation, and in the south with the NW–SE Gallegos–Calafate fault area. The Deseado region has been considered as an ancient massif, independent of the northern Patagonian or Somuncura´ Massif (Feruglio, 1946) and it is characterized by a great tectonic stability. Seismicity in this region is practically nonexisting, and historical earthquakes are not known (Figs 4 and 5). To the west of Rı´o Gallegos, between 50 and 25 S, in a NNW oriented zone, there are many aligned volcanic structures along extensive fractures, the Pali-Aike tectonovolcanic belt of Plio–Pleistocene age. Corbella (2002) described a predominant fault system, both at the surface and in the underground, with a NW trend, accompanied by faults with an east and northeast orientation, normal gravitational faults, with near vertical dips (70–80). Some of these faults were active during the Tertiary, ceasing their movement during the Late Tertiary. These structures reactivated toward the end of the Pliocene, enabling their opening by transtensive movements and allowing the escape toward the surface of basaltic magmas of deep origin. Another fracture system located to the east–southwest of the Laguna Azul (Fig. 1) shows a lava flow, presumably of Holocene age, deviated toward a graben formed by two ENE parallel faults, which were activated by the eruption and would indicate recent movements.

3. Seismicity The western South American region possesses a complex morphology, with an active western margin, represented by typical topography and seismicity, due to the convergence of the Nazca, Antarctic and South American plates. This convergence began approximately 200 Myr ago, with the subduction of the oceanic plate beneath the continental plate, dipping toward the east, and a displacement to the west of the ocean–continent contact, at an absolute velocity or 2.2 cm/yr (Uyeda and Kanamori, 1979) The west coast of South America is outlined by the eastern edge of the Nazca tectonic plate and is characterized by high seismicity. A very narrow, 100 to 150 km wide, active seismic strip is found between the Cordillera de los Andes and the Peruvian–Chilean trench. The distribution of hypocenters all along the Chilean coast shows great differences in their seismicity. Seismicity notably diminishes south of the Penı´nsula de Taitao and from west to east (Figs 4 and 5). The northern region, located between 18 and 27 S, is characterized by a high seismicity and an active earthquake history, the same as the central region, situated between 27 and 37 S (Lomnitz, 1970). South of 37 S seismicity decreases, though important earthquakes have been recorded. The superficial earthquakes seem to be related to the Liquin˜e–Ofqui fault system and to minor associated faults. An increase in surface seismic activity can be observed at the latitude of the Taitao Peninsula, coinciding with the Chilean Triple Junction (Fig. 4). The Chilean central–south region, between 37 and 41 S, is characterized by part of the main seismic activity having moved out to sea. Great earthquakes with

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Fig. 8. Simplified map showing historical earthquakes, with magnitude and seismotectonic regions.

destructive tsunamis have been recorded in this region between 1562 (M = 8.0) and 1985 (M = 7.8) on the Richter scale. Historical records (Fig. 8) begin with the earthquake of October 28, 1562, with a magnitude of 8 (Richter scale) and epicenter at 38 S and 73.5 W, south of Concepcio´n (Fig. 1), which then generated a major tsunami. On February 8, 1570, around 9 a.m. (local time) a religious celebration at Concepcio´n was interrupted by a strong earthquake that destroyed most of the city, only 20 yrs after its foundation. The chronicles recorded that the ground opened up in various places, throwing out blackish waters with a sulfurous smell. A few moments later, a seaquake destroyed all remaining buildings. The earthquake magnitude was 8.5 and the epicenter was located at 37 S and 73 W.

On December 16, 1575, a new earthquake of 8.5 magnitude and epicenter in the town of Valdivia, at 39.8 S and 73.2 W, brought to ruins all villages in southern Chile, also triggering a tsunami. A priest named Escobar, based on manuscripts by Marin˜o de Lobera, Valdivia’s Corregidor, made an account of those days explaining some of the effects caused by the quake: ‘‘Furthermore, while the Earth was trembling for a space of a quarter of an hour, a remarkable thing was seen in the big stream, and that was that in a certain part the waters divided, one part running toward the sea, while the other ran upstream, exposing the naked streambed. Afterwards, the sea went over its boundaries running inland at such a speed as the river with the greatest onrush in the world, entering three leagues inland’’ (Encina, 1955). Its characteristics were very similar to

Neotectonics, Seismology and Paleoseismology those of the big earthquake of May 22, 1960, that occurred in the same region. Other historical earthquakes were those of March 15, 1657, of M = 8.0, in Concepcio´n, and in April 1949, in Angol (Fig. 1), of M = 7.3 (Richter scale). The biggest earthquake that ever happened in this region took place on May 22, 1960, at 3:11 p.m. (7:11 p.m. GMT) and it was felt throughout southern South America, with an epicenter at 39.5 S and 74.5 W. Valdivia was strongly affected by this earthquake, where it recorded an intensity range of XI to XII on the Mercalli scale and 9.5 on the Richter Scale. The hipocenter was located at a depth of 60 km. Two thousand people died in Valdivia (4000 to 5000 in the entire region) and over 2 million lost their homes. Rivers changed their courses, new lakes were formed and there were many landslides. A few minutes later, a huge tsunami devastated anything that was still left standing, leaving some boats many kilometers inland. The southern Chilean region, between 41 and 60 S, is divided into different seismic areas. From 41 to 45 S, it is characterized by the occurrence of destructive earthquakes, like the one of May 14, 1633 that generated a small tsunami; another on December 24, 1737, of M = 7.5/8 (Richter scale) and an epicenter at 43 S and 74 W and the earthquake of November 7, 1837, with an epicenter at 42.5 S and 74 W, with a magnitude of 8.5, and which also caused a big tsunami. On November 21, 1927, at 44.6 S and 73 W, an earthquake of a magnitude of M = 7 occurred, also causing a tsunami. Another important earthquake was recorded on October 11, 1940, with M = 7 (USGS/NEIC, 2002). Between 45 and 60 S, south of the Triple Junction and the Chilean ridge, the oceanic portion of the Antarctic plate is transported beneath the South American plate, at an annual velocity of 2 cm per year. Nevertheless, only two destructive earthquakes have been recorded, one in 1879 and another on December 17, 1949. The seismic records are incomplete due to the rather recent settlement of European people; nevertheless, the present seismicity is low. According to the seismic zonation of Argentina by INPRES (1993), Patagonia shows in its continental sector the following: (a) A very low seismic hazard in extra-Andean Patagonia (b) Low seismic hazard west of 68 W (c) Moderate seismic hazard in the Patagonian Andes, between 70 W and the Chilean border (d) High seismic hazard on Isla Grande de Tierra del Fuego. However, since 1969 to the present date, more than 400 superficial earthquakes of magnitude over 4 have been recorded. Due to poor historical seismic data in Santa Cruz Province and to its proximity to regions with earthquakes of a magnitude higher than 7 (Isla Grande de Tierra del Fuego), Gonza´lez Bonorino (2002) assumed that the eastern region of the province is moderately seismic. In short, although the magnitude of earthquakes in Patagonia is generally moderate in strength, most of these

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events were superficial and could be related to active faults, which indicates potential future seismogenic sources. As shown in the previous paragraphs, the large number of records corresponding to the second half of the past century, in comparison with previous years, is an indicator of the scarcity of seismological stations in the region and not the absence of seismic activity in the past. The historical record of Isla Grande de Tierra del Fuego is also very brief due to the relatively recent European settlement and its low population density. The seismic activity record in Tierra del Fuego goes back to 1879 and continues to the present date, recording 1600 events, although most of them are of a very low magnitude (Southern Catalogue, 52–62 S–24–71 W). There are references to an ancient earthquake that took place before the European colonization, according to a Yaghan (Fuegian indigenous) legend mentioned by Lucas Bridges (2000): They used to say that very long ago, the moon dropped into the sea, which rose up in a huge turmoil. The only survivors of the flood were the inhabitants of the island of Gable, which detached from the ocean bed and floated on the sea. The surrounding mountains were submerged and the inhabitants, looking around, could not see but the horizon . . . . (In: Isla and Bujalesky, 2004) Alternatively, it is also possible that this account was related to a meteorite falling in the sea, which would have caused a huge wave, or to the sudden flooding of the Beagle Channel by the Mid-Holocene rising sea level, and not to a seismic event. The first recorded earthquake occurred on February 1, 1879, at 5 a.m. (local time). It was described by Thomas Bridges (1879), an Anglican missionary, the first European settler in Tierra del Fuego, as follows: ‘‘We had a succession of shocks, sufficiently strong to wake almost everybody and to make walking somewhat difficult. It split largely the milk in the pans and was felt all over the country’’ (In: Isla and Bujalesky, 2004). Its intensity was of grade VI in the MM (Modified Mercalli) scale. Its epicenter was located at 54 S and 65 W. On November 11, 1902, an earthquake of grade VI in the MM scale occurred with its epicenter at 53.0 S and 71.0 W. On February 2, 1929, an earthquake was recorded at 10:30 a.m. (local time) with an epicenter at 54 S and 62 W. On July 13, 1930, at 1:12 a.m. (local time), another event of a magnitude of 6.3 and with its epicenter at 56 S and 67 W occurred. On December 17, 1949, a strong earthquake occurred at 6:53 a.m. (local time) with a magnitude of 7.8 on the Richter scale and intensity of grade VII in the MM scale, which caused subsidence of the shore of Lago Fagnano and generated a gravel bank along the eastern margin, which enclosed a pond. The Cabo San Pablo lighthouse (Fig. 1) tilted up to 15 from its original position. The earthquake was noted in Ushuaia (Fig. 1), where there was one fatal victim and minor damage, as well as in Rı´o Grande. It was also felt in Rı´o Gallegos and San Julia´n (Fig. 1), in southern Santa Cruz Province. According to INPRES (2004), ‘‘La Prensa’’ and ‘‘La Nacio´n’’, important newspapers in Buenos Aires,

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mentioned major damage in Punta Arenas (Chile) (Fig. 1), with houses collapsing and cracking, together with a strong seaswell that threw the boats against the shore. In Ushuaia, the epicenter was first located at the Isla Dawson (Fig. 1), at 54060 S, 70300 W; later on, it was relocated at 53240 S and 69130 1200 W. At 3:07 p.m. (local time), there was an aftershock of certain intensity with the epicenter relocated at 53590 2400 S and 68460 1200 W. During several months in 1950, various aftershocks were recorded. On February 15, 1950, another event of M = 5.2 was recorded at 54 S and 68 W. At 2:52 p.m. (local time) on June 15, 1970, an earthquake of magnitude 7 (Richter scale) with epicenter in the northern sector of Isla de Los Estados (Fig. 1) occurred (54180 S and 63360 W), at a depth of 6 km. It was felt by the inhabitants of Ushuaia and Rı´o Grande. Schwartz et al. (2002) described a settler story, where they mention the formation of a scarp of 1 m height, coinciding with a fault, which interrupted road traffic on the eastern coast of Lago Fagnano. On November 27, 1975, an earthquake occurred, with a magnitude of 6.5 and epicenter in the Drake Passage (Pasaje de Drake; Fig. 1) at 56480 S and 68300 W and a depth of 11 km. On October 15, 1977, at 2 p.m. (local time) an earthquake of M = 4.8 was recorded. On November 30, 1977, at approximately 11 p.m. (local time), an earthquake of a magnitude of 3.8 took place. Its epicenter was situated at 54480 5700 S and 68040 2000 W, some 10 km east of Ushuaia. Its intensity was of grade III on the MM scale. On April 26, 2003 (at 1:13 a.m., local time), an earthquake of a magnitude of 2.7 occurred, with its epicenter on the northeastern margin of Lago Fagnano, and was felt by the inhabitants of the small town of Tolhuin. Seven minutes later a strong aftershock was recorded.

4. Paleoseismicity Paleoseismology is the study of prehistoric earthquakes and specially refers to their location, magnitude and age. This discipline studies the sudden deformation of sediments and geoforms during earthquakes, for example those events of M > 6. It also enables to extend the historic and instrumental catalogs, since it includes great prehistoric earthquakes. These studies are of vital importance in Patagonia, where historic and instrumental recordings are very scarce and where these events are known only after the Spanish colonization. Evidence of paleoearthquakes can be classified in two great groups: 1. Primary paleoseismic evidence refers to the tectonic deformation resulting from coseismic sliding along one fault plane. For example, fault scarps, fault beds and so on. 2. Secondary paleoseismic evidence refers to the features resulting from a seismic shock, such as landslides, sand dikes and sediment deformation. These effects will be discussed at the following paragraphs.

5. Secondary Paleoseismic Evidence This term is used in neotectonic superficial processes directly related to seismic movement. The secondary effects are generally the more outstanding expressions of an earthquake and may cause the largest damage. The preservation of these phenomena in the geological record is the paleoseismic evidence of historical earthquakes. Landslides are one of the most common geomorphic expressions of earthquakes, as well as rock falls, block slides and rock avalanches. Lateral spreading normally produced by liquefaction is generated on slopes with a gradient lesser than 0.1. Other expressions are sand dykes, sills and clastic dykes that result from water and sediment ejections during the seismic event.

5.1. Landslides Landsliding is one of the most common geological processes in the Patagonian Andes environment, triggered by both seismic movements and rainfall. The occurrence of these geological events is related to regional lithological features, presence of active faults, lacking of stability on steep slopes, geomorphologic processes and torrential drainage systems. Vegetation and soil development, climate and so on are also important. These natural phenomena are generally unpredictable and usually causing disasters, with loss of human life, destruction of urban centers, infrastructure, agricultural lands, or affecting mining production, with negative social and economic consequences for the population. Earthquakes are the principal cause of sliding. Keefer (1984) classified these features, with the weakest earthquake capable of causing a minor slide of M = 4. These earthquakes can cause rock falls, rockslides and movement of other loose materials. In order to cause falls and landslides, the minimum intensity must be M = 4.5. To cause rock falls, block slides, slow-moving flows, lateral spreading, soil liquefaction, the minimum earthquake intensity is of M = 5.0. For rock avalanches, M = 6.0 and for soil avalanches M = 6.5. There are several examples of landslides in Argentina and Chile triggered by strong earthquakes. For example, Perucca (1995) described numerous Holocene and recent landslides in the Sierra de la Punilla (Fig. 1) in the San Juan and La Rioja Provinces, related to the El Tigre fault system. Some were generated during the so-called ‘‘Terremoto Argentino’’, October 27, 1894 (M = 7.8 Richter scale), which caused enormous upheaval in areas as far as 200 km from the epicenter. Perucca and Moreiras (2003) recognized two rock avalanches in the Rı´o Acequio´n area, south of San Juan Province, related to at least two seismic events that occurred in the region during the Late Pleistocene–Holocene. In Neuque´n Province, the main cause of large landslides are seismic movements (Gonza´lez Dı´az, 2001), though it is possible that rainfall and earthquakes resulting from volcanic eruptions may have triggered sliding as well. Snowfall generally only has a very local effect, and may cause small alluvial

Neotectonics, Seismology and Paleoseismology accumulations behind temporal dams. Gonza´lez Dı´az et al. (2005a) considered Neuque´n as the region of Argentina with the largest number of Holocene landslides, assigning all of them to a seismic origin. Gonza´lez Dı´az et al. (2005b) recognized the occurrence of 47 Pleistocene–Holocene rock avalanches in the Cordillera Neuquina, between 36 and 38 S. These rock avalanches show a spatial relationship between landslides and neotectonic structures like the Antin˜ir–Copahue fault system (Folguera et al., 2004). At the boundary between the Mendoza and Neuque´n provinces, during the Holocene, the mountain slope became unstable, which caused a great landslide that blocked the Rı´o Barrancas valley (Fig. 1) resulting in a lake. Gonza´lez Dı´az et al. (2001) inferred that the cause of this huge rock avalanche was a seismic shock. In 1914, collapse of this natural dam caused a flash flood that devastated the lower valley of the Rı´o Barrancas and continued into the Rı´o Colorado valley. The Moncol rock avalanche took place at 37220 S and 71 W, in northwestern Neuque´n Province, covering an area of approximately 8 km2, with a maximum length of 6.75 km and a maximum width of 2.5 km (Ecosteguy et al., 1999). The regional lithological, geomorphological and structural setting may have favored the generation of this Holocene avalanche. The most frequent landslide processes in Santa Cruz Province are rock falls, landslides (complex) and flows. Near the town of Chalte´n, Inbar (2002) described megablocks of basaltic composition ( >100 m3) close to Cerro Chalte´n (Fig. 1), which are accompanied by smaller blocks, stemming from a cliff. He assumed that these falls were the result of recent seismic events. Landslides are frequent on almost all the structural lava plains, which are a very common component of the Patagonian tablelands. For example, on Meseta del Lago Buenos Aires they affect both the lava flows as well as the Mesozoic sedimentary rocks and the Late Cenozoic glacial deposits. Pereyra et al. (2002) described large rotating landslides in Santa Cruz Province that affect the lava surfaces in the Cardiel and Strobel lake region. Even though these authors did not assign an origin to these landslides, it is probable that, in some cases, they were caused by seismic events in the adjacent Chilean region. Hauser (2000) mentioned small rotational landslides in the south of Chile due to the May 1960 earthquake in Valdivia (Fig 1). He also described important multirotational landslides comprising a total volume of approximately 38,000 m3. In this region, several features assigned to ancient landslides are common, possibly generated by the seismic event of great magnitude that occurred on December 16, 1575. Fuenzalida and Skarmeta (1976) mentioned a great landslide that occurred in southern Chile as a consequence of the 1960 earthquake. Gonza´lez Dı´az et al. (2000) identified a number of rock avalanches in northern Neuque´n Province, between 36150 , 36300 S and 70400 W. These avalanches would have caused natural dams in the Rı´o Varvarco valley (Fig. 1). Even though the age of the identified units has not been determined, these authors established a connection between these landslides and the gradual retreat of the glacier

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occupying the Rı´o Varvarco valley or after the main glaciation phase. They assumed a seismic origin for these processes. Gonza´lez Dı´az et al. (2003) described three large gravitational movements, at 36380 S and 70350 W, in Neuque´n Province. The two most important features are rock avalanches while the third one is smaller landslide. They rejected a glacial origin for these deposits, pointed out their large dimensions and their being part of the regional geomorphological evolution. They suggested seismic events as generators of these avalanches that would have taken place during the Quaternary, after the main Cordilleran glaciation. Near Volca´n Copahue, Gonza´lez Dı´az (2005) described rock avalanches and landslides that happened subsequently to the Pleistocene ice recession during the Holocene and related both to the seismotectonic conditions of the area, thus awarding them a seismic origin. The existence of rock avalanches in this region suggests the occurrence of earthquakes of at least a magnitude of 6 or higher during the Quaternary. However, for most cases it is very difficult to establish the true cause. Climatic conditions such as heavy rain or snowfall, together with lateral stream erosion may be alternative triggers for the occurrence of these phenomena.

5.2. Liquefaction Liquefaction processes are related to more severe damages caused by an earthquake, thus being commonly used to evaluate potential seismic hazard (Obermeier, 1994; Moretti et al., 1995; Paredes and Perucca, 2000). However, their observation is rare, mainly because of a poor preservation potential; therefore, compilation of historical material is essential. Liquefaction features have been described in most seismic region worldwide, but so far have been underestimated in Patagonia. After the Spanish conquest, at least 13 destructive earthquakes associated with liquefaction phenomena have been registered (1817, 1861, 1844, 1949, 1894, 1899, 1920, 1927, 1929, 1944, 1948, 1952 and 1977) in central-western Argentina, and these have affected the development and economy of the region during the last 200 yrs. Liquefaction is the disruption in situ of the mutual support among clastic particles, generally caused by a seismic shake, in which there is total or partial loss of the shear resistance of the affected materials. In poorly cohesive soils, the transformation from solid to a liquid state is the result of increased pore water pressures that decreases the friction coefficient during an earthquake. Completely saturated soils, with poorly cohesive sands, generally clean, which may include some gravel, may be liquefied during seismic shakes by the propagation of shear waves (Rodrı´guez Pascua, 1997). The poorly cohesive sand sediments and silts usually have a high shear resistance, supporting heavy loads without producing alterations in its internal structure. But for natural or artificial causes, the loss of resistance of these materials may be produced changing their state and making them behave as viscous liquids. The natural mechanism which affects this change of state, from solid to liquid, is liquefaction, which then generates

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liquefacted deposits (Allen, 1977). This loss of cohesion may produce downslope sliding or liquefacted material injections in response to pressure gradients. The more favorable sedimentary environments for liquefaction are beaches, sand bars in stream systems, and lacustrine and fluviolacustrine environments. Lacustrine and glaciolacustrine deposits with abundant sandy fraction are the ideal environment for the occurrence of liquefaction structures. Therefore, the study of areas that, because of their physical and lithological conditions, can suffer liquefaction during earthquake occurrence in populated areas of Patagonia is a high-priority investigation field for a real understanding of seismic hazard. A minimum 1 m thickness of medium to fine sand is generally required to form dykes or sills. Effects of liquefaction are most pronounced where the groundwater table lies within a few meters or less of the surface. A mineral composition of 95% silica in sands favors this phenomenon. The preservation of liquefaction structures in the geological record is a good paleoseismic evidence of undocumented earthquakes. They are very valuable tools for the study of seismicity in areas which possess a very short historical record of important earthquakes, as it is the case in Patagonia. Both Obermeier (1994) and Moretti et al. (1995) pointed out that an earthquake of a magnitude of 6 or more may generate liquefaction structures in a 40 km radius. Audemard and De Santis (1991) analyzed liquefaction structures 25 km from the epicenter for earthquakes of a magnitude of 5–5.7. For earthquakes of magnitude 7, Seed (1968) stated that the radius in which liquefactions may occur would be 70 km and for events of a magnitude 8 or higher, the radius would be 100 km (Moretti et al., 1995). However, in destructive earthquakes such as those that occurred in San Juan Province (1894, 1944, 1977), with a magnitude higher than 7, liquefaction was registered at distances of over 200 km from the epicenter. Liquefaction was one of the effects that characterized all the earthquakes which took place in the Central Western region of Argentina (Perucca and Bastias, 2005). At least nine destructive earthquakes (1861, 1894, 1903, 1917, 1920, 1927, 1929, 1944 and 1977) have adversely affected the development and economy of Mendoza and San Juan provinces over the last 150 yrs. During these earthquakes, liquefaction effects like dykes, sand volcanoes and lateral spreading were among the most widespread and most spectacular results of seismic shake; moreover a large part of the damage was the result of subsurface liquefaction. Near the boundary between the Mendoza and San Juan provinces, Paredes and Perucca (2000) and Bracco et al. (2005) identified Late Pleistocene–Holocene lacustrine deposits in an area of approximately 6 km2 and described numerous liquefaction structures such as sand dykes, sills and slump structures (Fig. 9). The study of these structures in this region has enabled the identification of two earthquakes and the likely presence of a third seismic event of a magnitude over 5 that has been registered in this sedimentary sequence. In this area several levels of seismites were recognized. The structures of deformation in all satisfy the approaches

DIAPIR AND SLUMP STRUCTURES (PHOTO 2*)

NODULES AND DISH STRUCTURES (PHOTO 3*)

Fig. 9. Sand dykes, sills and slump structures found near the boundary of the Mendoza and San Juan provinces (Paredes and Perucca, 2000; Bracco et al., 2005).

defined by Sims (1975) that allow their correlation with seismic events, because of the following reasons: (a) They are located in an active seismic area (b) Sediments showing deformation structures have a high potential of liquefaction (saturated sands). (c) The presence of sand volcanoes and dykes, when the feeding conduit is preserved, is another evidence for the occurrence of earthquakes. In the Patagonian region, there are some examples of sand volcanoes and dykes that could be of seismic origin. Borrello (1962, 1969) described clastic dykes in Miocene deposits along the Atlantic coast of Isla Grande de Tierra del Fuego. He concluded that they had an epigenetic origin and suggested a genesis associated with gas and/or water injection, mixed with subsurface sediments. Winslow (1983) studied the regional features, composition and field relations of clastic dykes in the Austral fold-and-thrust belt on the Chilean side of Isla Grande de Tierra del Fuego (53 S). He assigned these dykes to a synorogenic and synsedimentary origin. Schmitt (1991) made a study of the clastic dykes situated along the fold and thrust belt between 51 and 54 S and considered that some of them are a result of a system of transcurrent sinistral movements.

Neotectonics, Seismology and Paleoseismology Diraison et al. (1997) described sand dykes located in the fold and thrust belt and associated them to a moment of a generalized extension during the Neogene. Assuming that the age of the deposits affected by the clastic dykes represent a maximum age for deformation, there is good evidence for regional seismic activity after the Early Miocene. Van der Meer et al. (1992) recognized glaciolacustrine deposits cut off by clastic dykes, in the locality of San Martı´n de Los Andes (Fig. 1) at 40 S and 71100 W in Neuque´n Province. Sediments are fine grained and composed mainly of fine laminae of sand and silt beds with some fine gravel lenses. All of the laminae and beds are strongly disturbed by slumping and normal fault. These structural deformations and faults could indicate that these sediments were deposited under unstable conditions. The presence of rhythmites cut off by clastic dykes widening toward the top is also mentioned (Figs 10 and 11). Though glaciotectonic processes should not be ruled out, it is likely that the origin of these structures could be related to a seismic event in the northern Patagonian Andes during the Pleistocene. Near San Carlos de Bariloche, at the ‘‘Anfiteatro’’ road cut on National Route 237 (Fig. 1) Van der Meer et al.

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Fig. 12. Sand volcanoes in front of the Rı´o Manso Glacier snout, Nahuel Huapi National Park, province of Rı´o Negro. It is probably related to minor seismic shaking (Photo by J. Rabassa, 1983).

Fig. 13. Detailed view of a sand volcano. Rı´o Manso Glacier area (Photo by J. Rabassa, 1983).

Fig. 10. Clastic dyke in glaciolacustrine sediments, San Martı´n de los Andes, northern Patagonia (Photo by J. Rabassa, 1987).

Fig. 11. Deformation structures. Clastic dyke in glaciolacustrine deposits, San Martı´n de los Andes area (Photo by J. Rabassa, 1987).

(1992) described a glaciolacustrine sequence in a pre-Last Glaciation moraines. The sequence is heavily disturbed by folds of small and large scale, ball-and-pillow structures and faults. They inferred a glaciotectonic origin for the generation of these structures. However, it may also be possible that their origin is related to seismic activity during the Middle Pleistocene in nearby areas, possibly the adjacent Chilean territory. In an area near the Rı´o Manso Glacier on Monte Tronador (41100 S and 71500 W; Fig. 1), Meglioli (1984) described small sand volcanoes of a circular form and with a diameter between 15 and 25 cm and less than 15 cm high, with a well-formed central crater (Figs 12 and 13). He assigned an origin by an external physical disturbance like a recent glacier advance or the falling of a rock boulder nearby, upon freshly deposited sediments. However, it should not be ruled out that these structures could be related with an earthquake in the region. Finally, Obermeier et al. (1993) established several necessary conditions to attribute a seismic origin to structures like those recognized in Patagonian areas: •

Upward flow directions should be observed indicating an abrupt effect of hydraulic forces applied in a very short lapse.

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• •

Their sedimentary characteristics should be comparable with the liquefaction phenomena registered in historical earthquakes. The sedimentary environment should be saturated in water. To generate structures of sand injections such as those mentioned above, an earthquake of M = 7.5 is calculated.

Thus, it is necessary to make more detailed paleoseismological studies of the Patagonian Pleistocene–Holocene age lacustrine and glaciolacustrine sequences, in order to record possible liquefaction structures associated with earthquakes

6. Discussion Based upon the studies of seismic, tectonic and morphological characteristics that have been carried out throughout the Patagonian region, we conclude the following: 1. There is a marked lack of uniformity in the distribution of seismic activity and thus anomalous areas, either showing an excess or an absence of significant seismic events are found. For example, many earthquakes are located on Chilean territory, whereas toward the east, seismic activity is either low or null. 2. A close relationship exists between the larger structures and the site of seismic events, in particular with regard to areas with different levels of seismic activity. These larger structures would include volcanic environments, especially during the Quaternary. 3. Regional standards rule the larger morphostructural features, outlining four seismotectonic regions. The number and boundaries of these areas are to be modified in the future, as more information on the distribution of regional seismicity and Neogene volcanism becomes available. 4. Work done up to now on Patagonian neotectonics indicates an extensive research field to develop, particularly in Argentine territory, as much from the potential earthquake hazard point of view, as from the Neogene tectonic architectural point of view. 5. The findings of at least one historical event on the Magallanes–Fagnano fault, the last one taking place in 1949, offer the possibility of new natural laboratories. For instance, in the San Juan and Mendoza provinces, where destructive earthquakes have become most valuable tools for the study of neotectonics. The studies carried out in the Cuyo region are an example of the methodologies to be used for a detailed analysis of each area by means of trenching, calculation of seismic parameters associated to each structure and, finally, estimating seismic risk for any point of the Patagonian region. 6. The historical earthquake of M > 7.5 (Richter scale) on Isla Grande de Tierra del Fuego and the western border of Patagonia constitutes clear

evidence that an earthquake with similar characteristics is very likely to take place in the future. The earthquake recurrence with this magnitude, based on the distribution of crustal movements over time, is calculated in intervals of around 500 yrs for a magnitude 6.5 and more than 10,000 yrs for maximum earthquakes with M  7 (Slemmons, 1977). 7. One of the most difficult aspects of these evaluations is to establish when it is possible to make this analysis, necessitating carrying out of detailed studies and to obtain absolute ages of the affected deposits. 8. The seismic potential in the southwestern portion of South America and Isla Grande de Tierra del Fuego varies between moderate and high and must be considered seriously, since correct environmental and urban planning constitutes the best strategy to reduce the impact of a destructive earthquake. 9. Mitigating seismic risk by means of detailed studies should be taken into account by governments and planners, creating conscience that the occurrence of an earthquake in the area is most likely. This forces the awareness on big infrastructural works and expansion of cities of the southern portion of the continent. 10. Even though the examples of landsliding triggered by earthquakes are scarce in this region, it is possible that the reason may be the lack of studies on the true causes. It is important to study them profoundly in order to determine if sliding has been triggered by seismic events of different magnitudes or, instead, by climatic factors. 11. The presence of liquefaction structures in different places, such as Neuque´n Province and Isla Grande de Tierra del Fuego, associated with active faults and possible seismogenic sources, constitutes reliable evidence of the danger that these processes present to constructions. Lacustrine Holocene deposits, alluvial plains, paleochannels and beaches have been the main type of materials and landforms affected by these phenomena. For this reason, the study of sediments that are prone to liquefaction processes and seismic movement in particular areas of Patagonia is a priority in proper environmental and urban planning.

Acknowledgments The authors are grateful to Dr Jorge Rabassa for inviting us to contribute to this book and for helping us to improve a preliminary draft of the text. Thanks are due to E. Moyano for drafting the figures and to Dr J. Van der Meer for their invaluable comments on the content of this chapter.

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Neotectonics, Seismology and Paleoseismology Isla, F.I. and Bujalesky, G.G. (2004). El maremoto de los Yaganes. Nexos 9, 29–33. Universidad Nacional de Mar del Plata, Mar del Plata, Argentina. Jaschek, E., Sabbione, N. and Sierra, P. (1982). Reubicacio´n de sismos localizados en territorio argentino (1920–1963). Serie Geofı´sica 9, 1, Publicaciones Observatorio de la Universidad de La Plata, La Plata, 79 pp. Keefer, D. (1984). Landslides caused by earthquakes. Geological Society of America Bulletin 95, 406–425. Klepeis, K.A. (1994). The Magallanes and Deseado fault zones: Major segments of the South American – Scotia transform plate boundary in southernmost South America, Tierra del Fuego. Journal Geophysical Research 99, 22,001–22,014. Lagabrielle, Y., Sua´rez, M., Rossello, E., et al. (2004). Neogene to Quaternary tectonic evolution of the Patagonian Andes at the latitude of the Chile Triple Junction. Tectonophysics 385, 211–241. Lavenu, A. and Cembrano, J. (1994). Neotecto´nica de rumbo dextral en la Zonda de Falla Liquin˜e-Ofqui: geometrı´a, cinema´tica y tensor de esfuerzo. VII Congreso Geolo´gico Chileno, Actas 1, 81–85. Concepcio´n, Chile. Lavenu, A. and Cembrano, J. (1999a). Estados de esfuerzo compresivo plioceno y compresivo- transpresivo pleistoceno, Andes del sur, Chile (38–42300 S). Revista Geolo´gica de Chile 26, 1, 67–87. Santiago. Lavenu, A. and Cembrano, J. (1999b). Compressional and transpressional stress pattern for Pliocene and Quaternary brittle deformation in fore arc and intra arc zones (Andes of Central and Southern Chile). Journal of Structural Geology 21, 1669–1691. Lavenu, A., Cembrano, J., Herve´, F. et al. (1996). Neogene to Quaternary state of stress in the Central Depresio´n and along the Liquin˜e–Ofqui Fault Zone (Central and Southern Chile). Symposium International sur la Ge´odynamique Andine ISAG 195–198. St. Malo, France. Lavenu, A., Cembrano, J., Arancibia, G. et al. (1997). Neotecto´nica transpresiva dextral y volcanismo, Falla Liquin˜e-Ofqui, sur de Chile. VIII Congreso Geolo´gico Chileno, Actas 1, 129–133, Antofagasta, Chile. Lomnitz, C. (1970). Major earthquakes and tsunamis in Chile. Geologische Rundschau 59. Springer Verlag, Stuttgart, 951–955. Lo´pez, G., Hatzfeld, D., Madariaga, R. et al. (1997). Microsismicidad en la zona centro-sur de Chile. VIII Congreso Geolo´gico Chileno, Actas 3, 1771–1774. Antofagasta, Chile. Meglioli, A. (1984). Procesos actuales de acumulacio´n glacige´nica y geoformas asociadas en el Glaciar del Rı´o Manso. Parque Nacional Nahuel Huapi. Rı´o Negro. Unpublished Graduation Thesis in Geological Sciences, Universidad Nacional de San Juan, San Juan, Argentina, 176 pp. Melnick, D. (2000). Geometrı´a y estructuras de la parte norte de la zona de falla Liquin˜e-Ofqui (38 S): Interpretacio´n de Sensores Remotos. IX Congreso Geolo´gico Chileno, Actas 1, Sesio´n Tema´tica 5, 796–799. Puerto Varas, Chile. Moretti, M., Pieri, P., Tropeano, M. and Walsh, N. (1995). Tyrrhenian seismites in Bari area (MurgeApulian foreland). Atti dei Convegni Licenci, 122. Terremoti in Italia. Accademia Nazionale dei Lincei 211–216. Roma.

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Mun˜oz, J., Duhart, P., Huffman, L. et al. (1999). Geological and structural setting of Chiloe´ Island, Chile. XIV Congreso Geolo´gico Argentino, Actas 1, 182–184. Salta, Argentina. Nelson, E., Forsythe, R. and Arit, I. (1994). Ridge collision tectonics in terraine development. Journal of South American Earth Sciences 7, 3–4, 27–278. Obermeier, S. (1994). Using liquefaction-induced features for paleoseismic analysis. In: Obermeier, S. and Jibson, W. (eds), Using Ground-Failure Features for Paleoseismic Analysis, Geological Survey Open-File Report, 94–633: A1–A98. USA. Obermeier, S.F., Martı´n, J.R., Frankel, A.D. et al. (1993). Liquefaction evidence for one or more strong Holocene earthquakes in the Wabash Valley of southern Indiana and Illinois, which a preliminary estimate of magnitudes. U.S. Geological Survey, Professional Paper 1536. Paredes, J. and Perucca, L. (2000). Evidencias de paleolicuefaccio´n en la quebrada del rı´o Acequio´n, Sarmiento, San Juan. Asociacio´n Geolo´gica Argentina, Revista 55, 4, 394–397. Buenos Aires. Pelayo A. and Wiens, D. (1989). Seismotectonics and relative motions in the Scotia Sea Region. Journal Geophysical Research 94, B6, 7293–7320. Pereyra, F., Fauque´, L. and Gonza´lez Dı´az, E. (2002). Geomorfologı´a. Geologı´a y Recursos Naturales de Santa Cruz. XV Congreso Geolo´gico Argentino, Relatorio 1, 21, 325–352. El Calafate, Argentina. Perucca, L. (1995). Fallamiento Activo en la sierra de La Punilla. San Juan-La Rioja. Argentina. Unpublished Ph.D. Thesis, Facultad de Ciencias Exactas, Fı´sicas y Naturales. Universidad Nacional de San Juan, San Juan, Argentina, 135 pp. Perucca, L. and Bastias, H. (2005). El Terremoto Argentino de 1894: feno´menos de licuefaccio´n asociados a sismos. Libro Homenaje al Dr. Bodenbender, INSUGEO, Tucuma´n, Argentina. Perucca, L. and Moreiras, S. (2003). Avalanchas de rocas holocenas y feno´menos de licuefaccio´n asociados a paleoterremotos en el rı´o Acequio´n, provincia de San Juan, Argentina. II Congreso Argentino de Cuaternario y Geomorfologı´a, Actas 137–146. Tucuma´n, Argentina. Ramos, V. (1982). Descripcio´n Geolo´gico-Econo´mica de la Hoja 53a Cerro San Lorenzo y 53b Meseta Belgrano, provincia de Santa Cruz. Servicio Geolo´gico Nacional, unpublished report, 125 pp. Buenos Aires. Ramos, V. (2002). Evolucio´n Tecto´nica. Geologı´a y Recursos Naturales de Santa Cruz. XV Congreso Geolo´gico Argentino, Relatorio 1, 23, 365–387. El Calafate, Argentina. Ramos, V. and Folguera, A. (1998). Extensio´n cenozoica en la cordillera neuquina. 4 Congreso de exploracio´n y desarrollo de hidrocarburos, Actas 2, 661–664. Mar del Plata, Argentina. Ramos, V., Haller, M. and Butro´n, F. (1986). Geologı´a y evolucio´n tecto´nica de las islas Barnevelt: Atla´ntico Sur. Asociacio´n Geolo´gica Argentina, Revista 15, 3–4, 137–154. Buenos Aires. Rapela, C. (1997). El sistema de fallas de Gastre: e pur si muove. Asociacio´n Geolo´gica Argentina, Revista 52, 2, 219–222. Buenos Aires.

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Rapela, C., Dias, G., Franzese, J. et al. (1991). El batolito de la Patagonia central: Evidencias de un magmatismo tria´sico-jura´sico asociado a fallas transcurrentes. Revista Geolo´gica de Chile 18, 2, 121–138. Santiago. Rodrı´guez Pascua, M. (1997). Paleosismicidad en emplazamientos nucleares. Estudio en relacio´n con el ca´lculo de peligrosidad sı´smica. Consejo de Seguridad Nuclear, Coleccio´n ‘‘Otros documentos’’, 286 pp. Madrid. Russi, M., Febrer, J., Costa, G. and Panza, G. (1994). Analysis of digital waveforms recorded at the seismographic station Esperanza. Terra Antarctica 1, 162–166. Sabbione, N. (2004). Estaciones sismolo´gicas digitales de la Universidad Nacional de La Plata. Su puesta en funcionamiento. To´picos de Geociencias. Un volumen de Estudios Sismolo´gicos, Geode´sicos y Geolo´gicos en Homenaje al Ing. Fernando Se´ptimo Volponi, 95–138. Editorial Fundacio´n Universidad Nacional de San Juan. San Juan, Argentina. Schmitt, K. (1991). Sandstone Intrusions in the Andina Fold-Thrust Belt (51–54 S): Implications for the paleohydrogeologic Evolution of the Southernmost Andes. Unpublished Ph.D. dissertation, Graduate School of Arts and Science, Columbia University, 263 pp. Schwartz, D., Stenner, H., Costa, C. et al. (2001). Paleoseismology at the southern end of the world: Initial observations of the Fagnano fault, Tierra del Fuego, Argentina. Seismological Research Letters 72, 2, 265. Schwartz, D., Stenner, H., Costa, C. et al. (2002). Rupturas asociadas a los sismos Ms 7.8 de 1949 en Tierra del Fuego: Investigaciones Paleosismolo´gicas iniciales. XV Congreso Geolo´gico Argentino, Actas 1, 136–138. El Calafate, Argentina. Seed, H. (1968). Landslides during earthquakes due to solid liquefaction. Proceedings American Society Civil Engeneers, Soil Mechanics Foundations Division 94, 1053–1122. Servicio Geolo´gico Universidad de Chile. Sismicidad: Cata´logo de eventos. http:/ssn.dgf.uchile.cl. Sims, J.D. (1975). Determining earthquake recurrence intervals from deformational structures in young lacustrine sediments. Tectonophysics 29, 141–152. Slemmons, D. (1977). Faults and earthquake magnitude. Report 6 of State of the Art for Assessing Earthquake

Hazards in the United States: U.S. Corps of Engineers Miscellaneous Papers S–77–1, 129 pp. Washington, D.C. Steffen, H. (1944). Patagonia occidental. Las cordilleras patago´nicas y sus regiones circundantes. Ediciones de la Universidad de Chile, 333 pp. Santiago. Stern, C. (1989). Pliocene to present migration of the volcanic front, Andean Southern Volcanic Front. Revista Geolo´gica de Chile 16, 2,145–162. Santiago. Tunstall, C., Folguera, A. and Ramos, V. (2005). Absorcio´n del desplazamiento del Sistema de Fallas de Liquin˜e-Ofqui en el Retroarco Andino entre 37300 y 39 S. XVI Congreso Geolo´gico Argentino, Actas 2, 127–132. La Plata. USGS/NEIC (2002). National Earthquake Information Center, World Data Center A for Seismology. Global Earthquake Search. United States Geological Survey, National Earthquake Information Center, http://wwwneic. cr.usgs.gov/neis/epic/epic_global.html. Uyeda, S. and Kanamori, H. (1979). Back-arc opening and mode of subduction. Journal Geophysical Research 84, 1049–1061. Van der Meer, J., Rabassa, J. and Evenson, E. (1992). Micromorphological aspects of glaciolacustrine sediments in northern Patagonia, Argentina. Journal of Quaternary Science 7, 1, 31–44. Volponi, F. (1976). El riesgo sı´smico en territorio argentino. Anales de la Sociedad Cientı´fica Argentina, Serie II, Ciencias Aplicadas 42, 36–44. Buenos Aires. Von Gosen, W. and Loske, W. (2004). Tectonic history of the Calcatapul Formation, Chubut Province, Argentina, and the ‘‘Gastre fault system’’. Journal of South American Earth Sciences 18, 73–88. Winslow, M.A. (1981). The structural evolution of the Magallanes basin and neotectonics in the Southernmost Andes. Antarctic Geosciences 143–154. Madison, University of Wisconsin Press. Winslow, M.A. (1983). Clastic dike swarms and the structural evolution of the foreland fold and thrust belt of the southern Andes. Geological Society of America Bulletin 94, 1073–1080. Winslow, M.A. and Prieto, X. (1991). Evidence of active tectonics along the Strait of Magellan. VI Congreso Geolo´gico Chileno, Resu´menes Ampliados 654–655. Santiago.

6 Late Cenozoic Quaternary Volcanism in Patagonia and Tierra del Fuego Hugo Corbella1,2 and Luis E. Lara3 1

Museo Argentino de Ciencias Naturales, Buenos Aires, Argentina 2 CONICET 3 Servicio Nacional de Geologı´a y Minerı´a, Av. Santa Marı´a 104, Santiago, Chile

Quaternary volcanism. Thus, Andean volcanic provinces, as defined by Lo´pez-Escobar et al. (1995a and references therein), respond to these main driving factors. In addition, the architecture of the Late Cenozoic volcanic arc is related to the long-term geological evolution of Patagonia, which has built a heterogeneous continental crust as a substrate for volcanism. Tectonic evolution in Patagonia has been characterized by subsequent episodes of compression and transpression– transtension along the volcanic front with localized extension in the backarc region. Since the Middle Tertiary the convergence vector between the Nazca and South American plates has been oblique to the continental margin when a noncontractional orogen started to build in a transpressional (or transtensional) setting (Cembrano et al., 1996, 2000, 2002). After the main Andean orogenesis, whose strongest pulse occurred at the end of the Miocene (e.g. Thompson, 2002), an episode of east–west contraction has been reported in the Andean Cordillera between 39 and 46 S while the extra-Andean region remained undeformed (Lavenu and Cembrano, 1999a). Because of the oblique subduction, the northeast dextral transpression was resumed along the entire Quaternary arc in the southern Andes where the Liquin˜e–Ofqui fault system occurs (Lavenu and Cembrano, 1999a). A transtensional regime has been described at the northern end of this regional fault system (Folguera et al., 2004) and a similar setting was proposed for the southernmost portion (Forsythe and Diemer, 2006). In the extra-Andean area, the eastern foothills of the northern Patagonian Andes are characterized by structural blocks bounded by NNW- and NW-trending high-angle normal and reverse faults. These structures would have been acquired during the Paleozoic (Coira et al., 1975a) or the Late Triassic–Jurassic extensional events that generated grabens and half-grabens in Patagonia (Uliana et al., 1985; Barcat et al., 1989). The regional distribution of these faults, mostly oblique to the N–S direction of the Andes, can be seen in echelon arrangement along 1000 km throughout the cordilleran eastern foothills (Fig. 1). A large number of the Plio–Pleistocene volcanic emissions are spatially related to these NNW–NW fractures or other secondary faults that surround them. South of the Chile triple junction (46 S), arc tectonics is dominated by the Chile ridge subduction and the southernmost segment is influenced by the sinistral Magallanes fault system (Klepeis, 1994).

1. Introduction Patagonia, the southern region of the South American continent, extends from the Huincul Arch, which crosses the continent at ca. 39 S, to Cape Horn (56 S) in Tierra del Fuego (Baldis and Febrer, 1983; Ghidella et al., 1995; Chernicoff and Zapettini, 2004; Ramos et al., 2004a). In terms of volcanism, the northern boundary of the Patagonian Late Cenozoic domain could be established further north at the Cortaderas alignment near 37 S (Kay, 2005; Kay et al., 2006). Moreover, the Pleistocene–Holocene volcanic front shows a regional border at this latitude. Considering this, the geographical boundaries of Patagonia have been partially extended in this chapter to provide a synoptic view of magmatic provinces and volcanic episodes. Patagonia has both a vast and significant volcanic history, which started in the Triassic and developed mainly during the Jurassic with the emplacement of extended ignimbritic–rhyolitic plateaus covering the major part of the present area and of the Atlantic continental platform. With the break-up of Gondwana and the beginning of the migration of South America towards the west, during most of the Cretaceous, arc magmatism developed at the western margin of South America. To describe and better understand the evolution of the Late Cenozoic volcanic processes and their relationship with the geodynamic setting, a more extended period from the Late Miocene to the Holocene has been considered. However, mostly Pliocene to Holocene volcanic centers and sequences are described in detail as part of major volcanic provinces.

2. Late Cenozoic Tectonic Setting of Volcanism Quaternary volcanism in Patagonia has been strongly influenced by tectonic processes involved in both arc and backarc domains. The South American continental margin is in front of the Nazca and Antarctic plates, which are subducting underneath South America (Fig. 1). The Nazca–South America convergence occurs at ca. 8 cm/yr (De Mets et al., 1994), with a subduction angle of ca. 25 at this latitude (Cahill and Isacks, 1992; Bohm et al., 2002). Likewise, the Antarctic–South America and the Antarctic–Scotia convergence take place at ca. 2 cm/yr (Tebbens and Cande, 1997; Lagabrielle et al., 2004). These first-order features imprint geochemical signatures and exert a regional tectonic control on

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Fig. 1. Generalized map of Patagonia showing the present location of the principal geological and tectonic features; i.e. Pacific oceanic crust fracture zones, Chile seismic oceanic ridge, continental faults and lineaments, and the North Patagonian and Deseado Massifs. Data from De Barrio et al., 1994; Nullo et al., 1994; Delpino and Deza, 1995; Lizuaı´n et al., 1995; Caminos and Gonza´lez, 1996; Vivallo et al., 1999.

Late Cenozoic Quaternary Volcanism in Patagonia and Tierra del Fuego 0.5133

Chile Ridge

Nd / 144Nd

0.5131

AVZ Cook Island Transitional Plateau Lavas

0.5129

TSVZ

Pali-Aike

SSVZ

143

Since the Miocene, the plate convergence has caused the collision of the Chile ridge with the Chile–Peru Trench. The initial ridge collision started ca. 14 Ma at the southwestern tip of Tierra del Fuego and, given the angle of oblique convergence, the triple junction migrated northward to its present position at the Taitao Peninsula at ca. 46 S (Cande and Leslie, 1986; Forsythe et al., 1986). Ridge–trench interactions along continental destructive plate margins cause the development of slab windows or volcanic gaps during ridge subduction (Dickinson and Snyder, 1979; Forsythe and Nelson, 1985; Thorkelson and Taylor, 1989). Between 46300 and 49 S, above a subducted transform segment, arc magmatism ceases, thus creating the Patagonian magmatic gap (Ramos et al., 1982; Stern et al., 1984). In turn, the pass of an active ridge segment favors a mixing of Nazca and continental subarc mantle. In the backarc or foreland region, mafic, tholeiitic to alkaline volcanism with intraplate signatures seems to be also temporally and spatially related to slab windows (Johnson and O’Neil, 1984; Forsythe and Nelson, 1985; Hole et al., 1991). Adakitic magmas (Kay, 1978; Defant and Drummond, 1990) can be generated near the triple junction by partial melting of the young and buoyant oceanic plate.

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0.5127

0.5127 0.702

AVZ

Mio-Pleistocene Patagonian Plateau Lavas

0.703

0.704

0.705

0.706

87Sr / 86Sr

Fig. 2. Geochemical signatures of Neogene plateau and arc volcanic rocks of Patagonia in a 143Nd /144Nd versus 87 Sr/ 86Sr isotope plot. Data from Kay and Gorring, 1999; Kay et al., 2004b; D’Orazio et al., 2001, 2004, 2005; Stern, 2004. This diagram emphasizes the different geochemical signatures of Patagonian magmas, wich reflect heterogeneities in source, geodynamic settings and evolving process.

3. Late Cenozoic Magmatic Processes Along the central and southernmost Southern Volcanic Zone (CSVZ and SSVZ provinces after Lo´pez-Escobar et al., 1995a; Fig. 1), tholeiitic and high-alumina basalts and basaltic andesites are the dominant rock types from both Quaternary stratovolcanoes and monogenetic centers (Hickey-Vargas et al., 1984, 1986, 1989; Futa and Stern, 1988; Lo´pez-Escobar et al., 1993; Stern, 2004), although evolved compositions also occur (Gerlach et al., 1988; Stern, 2004). Geochemical signatures suggest mainly an asthenospheric mantle source, partially mixed with a small amount of Nazca subducted sediments and slab-derived fluids (Morris et al., 1990; Hickey-Vargas et al., 2002; Sigmarsson et al., 2002; Stern, 2004, for a complete overview). With minor exceptions (McMillan et al., 1989; Hickey-Vargas et al., 1995), crustal assimilation decreases southward or is virtually nonexistent (Fig. 2). Input of the slab-derived fluids to the subarc mantle decreases eastward as does the degree of mantle melting (Hickey-Vargas et al., 1989; Lara et al., 2004b; Mella et al., 2005). In extra-Andean Patagonia, along the backarc region, alkali basalts derived by even lower degrees of partial mantle melting exhibit little or no evidence of slabderived components (Stern et al., 1990). Magmatism can result from ridge–trench interaction along continental plate margins, causing asthenospheric upwelling above slab windows (Ramos and Kay, 1992; Gorring et al., 1997, 2002, 2003; D’Orazio et al., 2001, 2004, 2005; Gorring and Kay, 2001; Kay, 2002a; Kay et al., 2004b). Also, in the absence of significant extensional tectonics, some large volumes of Tertiary and Quaternary plateau lavas have been considered to be of an asthenospheric

plume-like mantle source (Gorring et al., 1997; Ntaflos et al., 2000; Kay, 2002a; Kay et al., 2004a). Finally, transient regional extension could have also caused adiabatic decompression of the as the nospheric mantle and basaltic outpours. In the Austral Volcanic Zone (AVZ 49–55 S), south of the Patagonian gap, adakitic hornblende andesites and dacites widely predominate and the usual magma compositions of the Andes are absent (Stern et al., 1984; Guivel et al., 2002; Kay, 2002a; Stern, 2004). Adakitic magmas are formed by partial melting of young subducting oceanic crust mixed with variable amounts of mantle material. They have also interacted to a greater extent with the overlying continental lithosphere (Sigmarsson et al., 1998), but this interaction decreases to the south (Stern, 2004). In the backarc area south of 49 S, Late Miocene adakitic lavas with age decreasing to the north are also known (Ramos et al., 1991, 1994, 2004b; Kay et al., 1993a, b). They seem to be formed in coincidence with the collision of discrete segments of the Chile ridge against the oceanic trench during the migration of the triple junction to the north.

4. Volcanism in the Patagonian Cordillera The Patagonian Cordillera is located on the western and active margin of the South American plate. Since the Early Cenozoic, its geotectonic and magmatic evolution has been closely related to the subduction of the Nazca and Antarctic plates beneath South America. The remarkable fact is

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that the magmatic arc front has been static since the Early Miocene and has preserved the same location along the Andean Cordillera. Cenozoic volcanism in western Patagonia includes remnants of Oligocene–Miocene volcanic–sedimentary sequences like Curamallin or Trapa Trapa formations (Sua´rez and Empara´n, 1997) near 39 S, and Estratos de Lago Ranco (Campos et al., 1998) at 40 S. These volcanic successions appear as roof pendants intruded by Miocene plutonic rocks. Further south, the higher exhumation rates facilitate the exposition of the roots of the magmatic arc along the North Patagonian Batholith (Cembrano et al., 2000; Thompson, 2002; Thompson and Herve´, 2002; Adriasola et al., 2005). Structural blocks inside the Liquin˜e–Ofqui fault domain expose Middle–Late Miocene to Early Pliocene granitoids between 39 and 46 S (Campos et al., 1998; Lara and Moreno, 2004). The present volcanic Andean region includes the volcanic front and the eastern orogenic volcanoes that are part of oblique chains or appear as isolated vents. A first group of Plio–Quaternary stratovolcanoes and volcanic edifices is recognized between 38 and 42 S as the eroded remnants of ancient volcanoes whose ages range from ca. 2.5 to 0.7 Ma (Lara and Folguera, 2005). The more recent history of volcanism in the northern Patagonian Cordillera can be explained in two evolutionary stages (Mun˜oz and Stern, 1988, 1989; Lara et al., 2001; Lara and Folguera, 2006). Recently published 40 Ar/39Ar ages together with a complete data set of K/Ar ages support the hypothesis of Miocene–Pliocene broadening of the volcanic arc and subsequent Pliocene or Early Pleistocene narrowing maintaining the front (Lara et al., 2001; Lara and Folguera, 2006). Thus, mostly eroded Pliocene volcanoes and volcanic sequences can be recognized at the Andean domain, with the best-preserved examples in the eastern Andean foothills. Overlying them, the present arc and backarc volcanic centers are located.

4.1. Pliocene–Quaternary Volcanoes Early Pliocene volcanic rocks related to heavily eroded volcanic centers commonly form the base of the Plio– Quaternary stratovolcanoes. South of 37 S, a prominent volcanic and sedimentary sequence, the Malleco Formation (Sua´rez and Empara´n, 1997), is composed of basaltic to low-silica andesitic rocks that have yielded K/Ar ages from ca. 4.4 to 2.3 Ma (Lara and Folguera, 2006, and references therein). To the south near 40 S, another thick succession composed of basaltic lavas, breccias and coarse gravels, informally known as Estratos de Pitren˜o (Campos et al., 1998), has been dated at ca. 5.8–2.4 Ma (Campos et al., 1998; Lara and Moreno, 2004). A few kilometers east of the Andean Range in the Lonquimay area (38 S) subhorizontal, mainly basaltic lavas, Llanque´n– Ranquil and Tuetue´ sequences (Sua´rez and Empara´n, 1997), have Early Pliocene K/Ar ages from ca. 5.2 to 3.2 Ma. All of these thick, basaltic to andesitic, subhorizontal sequences present morphological features of effusive eruptions not related to compound volcanic structures but to shield volcanism. South of 40 S, Early Pliocene magmatic

rocks in the Andean Cordillera are granitoids bounded by sin-plutonic mylonites from the North Patagonian Batholith, although the Yeli Formation (Levi et al., 1966) at 43300 S could be an equivalent eroded extrusive sequence. Estimated depths of granites emplacement are near 2–3 km and together with cooling ages describe the high exhumation rates of the Andean Cordillera in southern Patagonia (Cembrano et al., 2002; Thompson, 2002; Adriasola et al., 2005). More preserved yet eroded, Late Pliocene to Early Pleistocene shield volcanoes formed a wide volcanic arc that partially overlies the Early Pliocene volcanic sequences from the main Andean Range to the uplifted blocks (Copahue–Pino Hachado) in the east. Near the modern arc front, these volcanic rocks occur in lava flows, volcaniclastic sequences and as deeply eroded stratovolcanoes. Remnant subhorizontal or gently dipping thin flows that are mostly basaltic in composition have mainly effusive volcanic features. The eastern belt is formed by partially preserved volcanic structures. The upper member of the Malleco Formation (38 S) that can be up to ca. 500 m thick consists of basaltic-andesitic lavas associated with poorly preserved necks (Sua´rez and Empara´n, 1997). Near 39 S, Cerro Trautre´n (ca. 0.8 Ma), Cerro Maichı´n (ca. 0.9 Ma), Laguna Los Patos and Carirrin˜e (Lara et al., 2001; Lara and Moreno, 2004) are small volcanic accumulations, which can have poorly preserved vent facies and cover granitoids of the North Patagonian Batholith. Further south, a thick sequence of basalts and laharic breccias constitutes the Estratos de Chapuco (ca. 1.0–0.4 Ma) at the base of the present Osorno volcano (Moreno et al., 1985; Lara et al., 2001). Volcanic rocks in the Estratos de Huen˜u–Huen˜u (ca. 1.43 Ma), at the base of the Calbuco volcano (41180 S), seem to be comparable remnants (Moreno et al., 1985). The extended volcanic Garganta del Diablo sequence (Mella et al., 2005) at the base of the Tronador volcano (41060 S) has a K/Ar age of ca. 1.3 Ma. Better-preserved central volcanoes are also part of this group. For example, Nevados de Caburgua (39 S) is a ring structure whose pyroclastic beds and lavas surround an andesitic laccolith (ca. 2.4–0.8 Ma). Huanquihue´ at 39480 S, Pirihueico at 39540 S (ca. 1.5–0.6 Ma) and Quelguenco and Chihuı´o at 39540 S (ca. 0.7 Ma) are stratocones with well-preserved necks or radial dyke swarms and are located at the present Andean water divide (Lara and Folguera, 2005). Huanquihue´ volcano has a Holocene pyroclastic cone over the northern flank showing the persistence of magmatic activity. Other central vents like Mencheca at 40300 S (ca. 0.53 Ma), Cordo´n de Alvarez (40360 S), Fiucha´ (40480 S) and Sarnoso at 40480 S (ca. 0.9 Ma) can be recognized at the base of the active Puyehue and Casablanca volcanoes. Near 41 S, the La Picada stratocone is located between Osorno and Puntiagudo volcanoes. The best-preserved stratovolcanoes in this group can have Middle Pleistocene lavas overlapping the basal parts of the active stratovolcanoes. Thus, Middle Pleistocene volcanoes are nearly indistinguishable from Late Pliocene to Early Pleistocene centers with respect to morphology and extent of erosion. Many of these volcanoes

Late Cenozoic Quaternary Volcanism in Patagonia and Tierra del Fuego are also partially collapsed structures whose upper units are older than ca. 200 ka, and consequently older than the basal units of the Pleistocene to Holocene centers. Among these Middle Pleistocene centers are the Sierra Velluda (37180 S), Quinquilil or Colmillo del Diablo (39300 S) and Pantoja (40060 S) volcanoes, which are prominent necks surrounded by basaltic lava flows, as well as the Hawaiian-type calderas of Sierra de Quinchilca at 40 S (ca. 1.4–0.3 Ma) and Cordillera Nevada at 40300 S (ca. 1.2–0.1 Ma).

4.2. Active Volcanoes Active Patagonian volcanoes, i.e. those formed since the Middle–Late Pleistocene having Holocene eruptions, some of them historical (during the last 450 yrs that followed the Spanish conquest), have a wide range of morphologies (calderas, stratovolcanoes, fissure systems and isolated monogenetic cones) and compositions (basalts to rhyolites, with tholeiitic-calc-alkaline to alkaline signatures), and their own volcanic evolution. At least 46 volcanic centers can be considered active along the Patagonian Cordillera, Villarrica and Llaima being among the most active volcanoes in South America. The SVZ is characterized by the presence of several oblique volcanic chains and volcanism located inside the structural domain of the Liquin˜e–Ofqui fault system. Along the CSVZ (37–42 S), some transverse chains were built on pre-Andean structures. In turn, a frontal arc formed by isolated stratovolcanoes forms the AVZ. South of 37 S, main stratovolcanoes and clusters of monogenetic cones are mentioned and briefly described below. At the northern end of the CSVZ, the Antuco volcano (37120 S) is a compound Late Pleistocene–Holocene volcanic complex, basaltic to basaltic andesitic in composition (Lohmar et al., 1999; Lohmar, 2000). Partial collapse at ca. 6.5 ka formed a horseshoe-shaped amphitheater open to the west as well as a huge avalanche deposit that extends to the Central valley (Thiele et al., 1998). A postcollapse cone was built with some basaltic flank vents. At least 20 historical eruptions have been recorded (Gonza´lez-Ferra´n, 1995). On the border between Chile and Argentina, the Copahue volcano (37480 S) is an active basaltic-andesitic to andesitic complex built since the Early Pleistocene inside the Caldera del Agrio, itself a huge Pliocene depression (Pesce, 1989; Linares et al., 1999, 2001; Folguera and Ramos, 2000; Melnick and Folguera, 2001; Melnick et al., 2005). A first shield stage was followed by a minor Middle Pleistocene caldera collapse (Polanco, 2003; Melnick et al., 2005). The present Late Pleistocene to Holocene cone, partially built beneath ice, has nine aligned craters, the easternmost hosting an acid lake. Twelve mostly phreatic eruptions have been reported over the last three centuries (Naranjo et al., 2000; Naranjo and Polanco, 2004). Callaqui volcano (37540 S) is a fissure system that has emitted basalts and basaltic andesites from the Late Pleistocene to Holocene (Moreno et al., 1986). Basaltic NE-trending fissures and cone alignments are the most

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recent Holocene emissions although no historical eruptions have been reported (Naranjo et al., 2000). Active fumaroles can still be observed on the flanks. Tolhuaca (38120 S) is a Late Pleistocene–Holocene stratovolcano, of which the upper part is partially eroded. Basaltic andesites have been emitted from the central crater and some flank vents together with Holocene pyroclastic flows and ash fallouts (Naranjo et al., 2000). Fresh lava flows can be observed, but no historical eruptions have been recorded. Lonquimay volcano (38180 S) is part of a NE-aligned cluster of cones, mainly active during the Holocene (Moreno and Gardeweg, 1989). Basalts to dacites have erupted from the eastern fissures, but andesites widely predominate. Holocene pyroclastic deposits are abundant, but only five historical eruptions have been reported, the last one causing serious damage in 1989 (Moreno and Gardeweg, 1989; Barrientos and Acevedo, 1992; Naranjo et al., 1992, 2000). Further south, Llaima (39360 S) is a Pleistocene– Holocene compound volcano that has emitted mainly basalts and basaltic andesites from the central crater or flank vents. A caldera collapse event formed a depression, which is partially filled by the postglacial cone (Naranjo and Moreno, 2005). A thick succession of postglacial pyroclastic deposits is recognized nearby, which includes the Curacautin Ignimbrite (ca. 13.5 ka) at the base (Naranjo and Moreno, 1991). At least 30 historical eruptions have been reported, the largest in 1640, and active fumaroles are visible in the central crater. Sollipulli caldera (39 S) is a Middle Pleistocene– Holocene ice-filled volcanic complex that erupted basalts to dacites (Naranjo et al., 1993b). On the ancient caldera wall, several subglacial domes were emplaced (Gilbert et al., 1996) and explosion craters formed. The most recent one would be related to the Alpehue´ Ignimbrite eruption in ca. 2.9 ka (Naranjo et al., 1993b). Further south, Caburgua (39180 S) is a cluster of five Holocene pyroclastic cones that have erupted between 8 and 11 ka BP. They emitted basaltic lavas followed by Strombolian and phreatic eruptions that built the cones just above the Liquin˜e–Ofqui fault. Villarrica (39240 S) is a Pleistocene–Holocene compound volcano that has emitted mainly basaltic andesites (Moreno, 2000; Lara and Clavero, 2004; Moreno and Clavero, in press). Two nested calderas form a depression, which was filled by the present cone. A thick sequence of postglacial pyroclastic deposits starts with the Lica´n Ignimbrite (ca. 12.5 ka). A recent small caldera was probably related to the eruption of Puco´n Ignimbrite in ca. 3.5 ka (Clavero, 1996). More than 30 historical eruptions have been reported, the more recent in 1949, 1963 and 1971 causing severe damage in Puco´n village. Small Strombolian eruptions frequently occur due to the dynamics of the crater lava lake. Quetrupilla´n (39300 S) is a Pleistocene–Holocene volcanic complex formed by two nested calderas and several dome complexes and pyroclastic cones (Pavez, 1997). It is lying on top of the Liquin˜e–Ofqui fault and has emitted basalts to rhyolites. A thick postglacial pyroclastic succession has been recognized (Lara and Moreno, 2004). Some historical eruptions were reported, the most recent in 1872.

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Lanı´n (39420 S) is a Pleistocene–Holocene stratovolcano that has erupted basalts and siliceous andesites to dacites (Hickey-Vargas et al., 1989; Lara and Moreno, 2004; Lara et al., 2004a). Mafic magmas were mainly emitted from lateral vents or from an ancient crater rim. Instead, evolved lavas were evacuated through the central conduit. No historical eruptions have been recorded (Lara, 2004). Mocho–Choshuenco (39540 S) is a volcanic complex formed by a juxtaposed caldera and a coalescent Pleistocene stratocone (Choshuenco). Inside the caldera, a Holocene cone was built. Mocho–Choshuenco emitted mainly basaltic andesites to dacites with remarkable postglacial explosive eruptions (McMillan et al., 1989; Etchegaray et al., 1994; Rodrı´guez et al., 1999; Lara and Moreno, 2004). Further south, Carra´n–Los Venados (40180 S) is a cluster of mainly Holocene pyroclastic cones and maars, mostly basaltic in composition, which form a NE-trending alignment (Moreno, 1977; Rodrı´guez, 1999; Lara et al., 2005). Remarkable eruptions have been observed during the twentieth century in 1907, 1955 and 1979. Puyehue–Cordo´n Caulle (40300 S) is a Middle Pleistocene–Holocene volcanic complex formed by the Cordillera Nevada caldera, Cordo´n Caulle fissure system and the Puyehue volcano in a NW-trending alignment (Moreno, 1977; Lara et al., 2005, 2006a, b). After a basaltic shield stage, basalts to rhyolites were emitted together with large volumes of Holocene pyroclastic ejecta. Evolved compositions predominate in the youngest units of both Cordo´n Caulle and Puyehue volcanoes. A remarkable fissure eruption of rhyodacites followed the great magnitude earthquake (Mw: 9.5) in 1960 (Lara et al., 2004b). Casablanca (40420 S) is a Holocene cluster formed by the stratovolcano and several basaltic pyroclastic cones, which form a N- to NE-trending alignment. Late Holocene Strombolian and phreatic eruptions formed a pyroclastic succession nearby. Puntiagudo (41 S; Fig. 3) is a Pleistocene stratovolcano, which formed a NE-trending alignment with the

Fig. 3. Rising in the Patagonian Cordillera, Volca´n Puntiagudo (latitude 41 S), a Pleistocene stratovolcano of basaltic and basaltic-andesitic composition, partially eroded by Holocene and present glaciers (Photo by H. Corbella).

Fig. 4. At the foreground, Cerro Tronador (latitude 41.1 S) is a basaltic to dacitic Pleistocene stratovolcano eroded and covered by glaciers. Behind, the Pleistocene–Holocene Osorno stratovolcano stands out on the horizon (Photo by H. Corbella).

Cordo´n Los Cenizos fissure system and the Osorno volcano. Puntiagudo emitted mainly basalts and basaltic andesites and it is partially eroded by Holocene flank glaciers. Osorno (41060 S; Fig. 4) is a Pleistocene–Holocene stratovolcano that has erupted mainly basalts and basaltic andesites (Moreno et al., 1985). Holocene isolated dacitic domes and basaltic pyroclastic cones lie on the flanks. At least 10 historical eruptions have been reported, the last in 1835 when a NE-trending systems of fissures and cones erupted (Lo´pez-Escobar and Parada, 1991; Moreno, 1999a; Petit-Breuilh, 1999). Tronador (41060 S; Fig. 4) is a partially eroded Pleistocene stratovolcano that erupted basalts to dacites. It is widely covered by glaciers and no clear evidence of Holocene activity exists (Mella et al., 2005). Cayutue–La Viguerı´a (41120 S) is a cluster of Holocene pyroclastic cones and lavas that lie along the Liquin˜e–Ofqui fault (Moreno et al., 1985). Calbuco (41180 S) is a Pleistocene–Holocene compound volcano that emitted mainly andesitic magmas (Lo´pez-Escobar et al., 1992; 1995b). A sector collapse occurred at the early postglacial period when an avalanche flowed to the north (Moreno et al., 1985; Moreno, 1999b). An andesitic dome grew inside the collapse amphitheater. Eleven historical eruptions have been recorded, the last in 1961 (Petit-Breuilh, 1999). The SSVZ (42–46 S) is characterized by the presence of several stratovolcanoes and monogenetic cones located inside the structural duplex of the Liquin˜e–Ofqui fault system. The Yate volcano (41480 S) is a partially eroded Pleistocene–Holocene stratovolcano that erupted basalts to andesites. It is covered by glaciers and exhibits several scars of sector collapses. Two siliceous lava domes inside an amphitheater in the southern flank record the late activity. Yate volcano forms a NE-trending alignment with Hualaihue´–Cordo´n Cabrera volcanoes and it is sitting on top of the Liquin˜e–Ofqui fault. Hualaihue´–Cordo´n Cabrera (41540 S) is a NE-trending alignment formed by the Hualaihue´ (or Apagado) volcano

Late Cenozoic Quaternary Volcanism in Patagonia and Tierra del Fuego and the Cordo´n Cabrera fissure system. Hualaihue´ is a Holocene basaltic pyroclastic cone built inside a glacial amphitheatre. Cordo´n Cabrera is a Pleistocene–Holocene cluster formed by aligned necks, basaltic pyroclastic cones and a siliceous dome (Lo´pez-Escobar et al., 1993). Hornopire´n (41540 S) is a Holocene basaltic stratovolcano whose summit has three probably historic pyroclastic cones. To the north, it is aligned with a set of fissures and basaltic pyroclastic cones. Huequi–Calle–Porcelana (42240 S) is a cluster formed by the Huequi volcano, a small dome complex with historic eruptions, the Calle postglacial lava dome and the Porcelana Pleistocene volcano, which exhibits Holocene parasitic cones. Michinmahuida (42420 S) is a larger volcanic complex formed by an ice-filled Pleistocene caldera, a central stratovolcano and several parasitic Holocene fissure vents. Mostly basalts and some dacites have been erupted (Kilian and Lo´pez-Escobar, 1991; Lo´pez-Escobar et al., 1993). A thick postglacial pyroclastic succession is recognized nearby. At least two historical eruptions have been reported. Compositionally different, Chaite´n volcano (42480 S) is a complex formed by a Holocene dome grown inside a caldera complex. The basaltic caldera sequence appears deeply eroded while the inner rhyolitic dome and some other parasitic domes are probably related to historical eruptions (Kilian and Lo´pez-Escobar, 1991; Lo´pez-Escobar et al., 1993). Corcovado volcano (43120 S) is a partially eroded Pleistocene stratovolcano, the central neck of which is exposed. Mostly basaltic lavas erupted from the central vent while Holocene andesitic domes appear as isolated or parasitic centers. Palvitad is a group of eroded basaltic lavas and Holocene pyroclastic cones, siliceous domes and maars (Lo´pez-Escobar et al., 1993). Avalanchas–Cordo´n Yelcho (43180 S) is a NWtrending volcanic alignment formed by heavily eroded lava sequences, necks and stratocones partially icecovered. Yanteles volcano (43300 S) is a Pleistocene ice-filled caldera with an inner Holocene pyroclastic cone and a peripheral NE-trending fissure system. Two ancient necks, Nevado and Yeli, take part of the main volcanic alignment. Basalts and basaltic andesites have been erupted. Melimoyu (44 S) is a Pleistocene–Holocene complex formed by an ice-filled caldera. Basalts and basaltic andesites have erupted from this vent. Puyuhuapi (44180 S) is a cluster of Holocene monogenetic cones that lie along the Liquin˜e–Ofqui fault. Alkaline basalts have been emitted from this volcano (Lo´pez-Escobar et al., 1995b). Mentolat volcano (44420 S) is a Pleistocene–Holocene stratovolcano with a small ice-filled summit caldera. Basaltic andesites and andesite lava flows have been erupted together with Holocene pyroclastic deposits (Lo´pez-Escobar et al., 1993; Naranjo and Stern, 2004). Cay (45060 S) is a partially eroded stratovolcano that has erupted basalts to dacites (Lo´pez-Escobar et al., 1993; D’Orazio et al., 2003). Several parasitic cones occur at the flanks in a NE-trending alignment.

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Maca´ (45060 S) is a large partially eroded stratovolcano that emitted basalts and basaltic andesites. Some parasitic cones occur at the flanks in a NE-trending alignment (Lo´pezEscobar et al., 1993; D’Orazio et al., 2003). Late Holocene pyroclastic deposits have been recognized nearby (Naranjo and Stern, 2004). Finally, Hudson volcano (45540 S) is a large Early Pleistocene–Holocene ice-filled caldera complex that emitted basalts to basaltic andesites and minor dacites (Orihashi et al., 2004). Two Holocene flank cones and large pyroclastic deposits have been described (Naranjo and Stern, 1998). The last historical eruption occurred in 1991 (Naranjo, 1991; Naranjo et al., 1993a), with severe ashfall impacts all around Patagonia and Tierra del Fuego (Banks and Iven, 1991; Corbella and Paz, 1991; Hildreth and Drake, 1992; Scasso et al., 1994; Bitschene and Ferna´ndez, 1995). South of the Patagonian gap caused by the Chilean Rise subduction, the AVZ (49–56 S) is composed of six Pleistocene–Holocene stratovolcanoes. Lautaro (49 S) is a partially ice-capped Pleistocene– Holocene stratovolcano that lies on the Southern Patagonian Ice Field. Andesites and dacites together with pyroclastic deposits have been erupted (Orihashi et al., 2004; Motoki et al., 2006). Six possible historical eruptions, the most recent in 1959–1960 (Martinic, 1988), make Lautaro the most active volcano of the AVZ. Viedma (49180 S) is an ice-covered stratovolcano that emerges from the Southern Patagonian Ice Field where mainly andesites have been erupted. Possible historical eruptions would have been fed from the four nested summit craters. Aguilera (50180 S) is a stratovolcano that has erupted mainly dacites and pyroclastic ejecta (Futa and Stern, 1988). Reclus (50540 S) is an ice-covered stratovolcano that has erupted mainly dacites and pyroclastic ejecta. The upper part may be postglacial and even possible historical eruptions have been reported (Harambour, 1988). Mount Burney (52180 S) is a volcanic complex formed by a somma and an inner stratocone. Andesites to dacites have been erupted and possible historical eruptions were also reported. Finally, Cook (54540 S) is a cluster of postglacial pyroclastic cones and domes located in southwestern Tierra del Fuego that has mainly erupted calc-alkaline andesites. Possible historical eruptions have been reported (Sua´rez et al., 1985).

5. Volcanism in Extra-Andean Patagonia In extra-Andean Patagonia, south of 39 S and east of the present Andean magmatic arc, plateau basalts cover over 120,000 km2, twice the size of Ireland. The basalts crop out as remnants of multiple lava-flow sequences that built plateaus or conspicuous isolated ‘‘mesetas’’ which unconformably overlie Mesozoic and Cenozoic volcanic rocks and sediments. These near-horizontal lava sequences, frequently with slight eastern dips, present almost flat surfaces and many of them have been controlled by structural plains developed before the lava

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emissions (Panza, 2002). Occasionally, it is possible to observe several structural plains that have been laid out in steps. The high front scarps of the ‘‘mesetas’’ are mainly the product of mass-wasting favored by the sharp lithological contrast between the more resistant basalts and a substratum of friable sedimentary rocks. Unlike the varied calc-alkaline lithology of the Andean magmatic arc, olivine basalts (s.l.) are predominant in the extra-Andean region. Nevertheless, the chemistry of these basic rocks shows an important compositional variety in the alkaline and subalkaline fields. Alkaline olivine basalts, hawaiites, basanites, nephelinites, leucitites, tholeiitic basalts, basaltic andesites and mugearites have been mentioned so far. The ‘‘mesetas’’ culminate in monogenetic volcanic centers such as cinder, spatter and scoria cones, maars, and necks that once were feeders of the volcanic features. Caused by Strombolian low explosive eruptive activity, the spatter and scoria cones, generally reach moderate heights, ca. 100–200 m. The flat surface of the basaltic ‘‘mesetas’’, especially in southern Patagonia, is pockmarked by near-circular closed depressions occupied by ponds or lacustrine sediments called ‘‘bajos sin salida’’. They vary in diameter, from tens to up to 3000 m, with shallow depths occasionally reaching 100 m and over. As to their origin, Methol (1967) considered that subwash and subsequent transportation and removal of the underlying rocks formed a cavity beneath the basalts that, without mechanical support, collapsed causing the depression. Volkheimer (1972) proposed a tectonic control and Panza (1982, 1995a, 2001) suggested inverted relief due to differential erosion on initially emerged parts of a step-toe. Lastly, in some cases, an explosive phreatomagmatic mechanism must be considered (Corbella, 2002). Plio–Pleistocene magmatism along extra-Andean Patagonia took place in two distinct tectonic scenarios, north and south of 46300 S. From 39 to 46300 S alkali basalts, basanites and hawaiites erupted forming reduced outcrops linked with normal faulting. South of 46 S, the extra-Andean basaltic outpourings, covering a much bigger area and expanding hundreds of kilometers eastward from the Andean Cordillera, were assigned to a slab window (Ramos and Kay, 1992; Gorring et al., 1997, 2002, 2003; D’Orazio et al., 2001, 2004, 2005; Gorring and Kay, 2001; Kay, 2002a; Kay et al., 2004b). 5.1. Northern Extra-Andean Volcanism (39–46300 S) From the Early Pliocene, the Patagonian extra-Andean volcanism between 39 and 46300 S took place further east of the present Andean volcanic arc and largely to the west of the Somuncura´ volcanic complex. Prior to the Plio–Pleistocene volcanism, Late Oligocene–Early Miocene intraplate eruptions of the Somuncura´ complex – the largest plateau magmatic event in Patagonia – occurred (Corbella, 1984; Ardolino and Franchi, 1993). The origin of these voluminous outcrops was associated with a hotspot or mantle thermal anomaly (Kay et al., 1993a), and with a hot asthenospheric corner flow channelized into the mantle wedge by rollback

of the subducting plate (de Ignacio et al., 2001). The last eruptive manifestations of this notable magmatic event are scarce and occur as isolated Plio–Pleistocene outcrops: Cerro Trayen Niyeo, Meseta del Cuy, and postplateau cones, most of them located on the center and western side of the Somuncura´ complex. Northern extra-Andean Patagonia is crossed by NNWand NW-trending faults, which bound grabens along Bio ˜ irihuau and Mamil-Choique Bio-Alumine´; Collo´n Cura, N alignments; or host linear valleys of Cushamen, Arroyo ˜ orquinco, Chico-Genoa-Senguerr and Chubut Medio rivN ers (Gonza´lez Dı´az, 1978; Ramos, 1978; Ramos and Corte´s, 1984; Nullo et al., 1994; Panza and Nullo, 1994; Delpino and Deza, 1995; Lizuain et al., 1995). Comallo and Gastre are other oblique long-lived structural systems (Coira et al., 1975a) that intersect the Andean Cordillera (Fig. 1). Many Plio–Pleistocene volcanic outcrops are spatially related to these NNW–NW fractures or with secondary faults developed in proximity to the former. Most of Plio–Pleistocene extra-Andean lavas (39–46300 S) are silica-poor alkaline rocks, lack highly differentiated lithologies, and bear peridotitic xenoliths. The presence of these inclusions and the scarce geochemical evidence of crustal signatures have been interpreted as evidence of fast ascent through the crust, without ponding in intermediate magmatic chambers (Stern et al., 1990; Ntaflos et al., 2000; Stern, 2004; Kay et al., 2004b). The northern Pliocene extra-Andean basaltic eruptions are represented by the Coyocho, La Caban˜a and Epulef formations and Trayen Niyeu, Pereyra, Rumay and Huala´ Basalts. Basalts from the Coyocho Formation (Leanza and Leanza, 1979) crop out within a wide NNW-trending belt between 38400 and 40300 S at 70 W. They form extensive subhorizontal outcrops up to 70–100 m thick. Also known as Basalto II (Groeber, 1946a, b, 1947; Galli, 1969), Tipilihuque Formation (Turner, 1973, 1976) and Coyocho Formation (Rolleri et al., 1984; Cucchi, 1998), these basalts unconformably cover the Miocene Collo´n Cura Formation. K/Ar determinations indicate ages of ca. 4.9–4.6 Ma (Cortelezzi and Dirac, 1969). La Caban˜a Formation (Nullo, 1978) is composed of volcanic and pyroclastic rocks exposed mainly west of the Northern Patagonian Massif ca. 41 S–70 W. La Caban˜a Formation considered Pliocene in age (Cucchi, 1998, 1999) includes olivine basalts from the lower Loma Alta member, trachytic and trachyandesitic vitroclastic tuffs and lapillites from the Ojos de Agua member, and basalts from the upper Atraico´ member (Coira, 1979). In Sierra de Mesaniyeu this formation was also called Mesaniyeu Basalt (Cucchi, 1998, 1999), where it forms a large plateau. The Trayen Niyeu Basalt (Remesal et al., 2001) crops out in the northwest boundary of Meseta de Somuncura´ (41050 S–67500 W) far from the Andean Cordillera. The Cerro Trayen Niyeu is composed of alkaline olivine basalts with thick pyroclastic intercalations. These basalts have xenocrysts and peridotitic xenoliths. Radiometric dating indicates a Late Pliocene age of ca. 2 Ma (Cortelezzi and Dirac, 1969). The Pereyra Basalt (Getino, 1995) in the Meseta de Colitoro (41 S–69 W) and the basaltic lava flows south

Late Cenozoic Quaternary Volcanism in Patagonia and Tierra del Fuego of Sierra de Pire Mahuida (42100 S) are also assigned to this epoch (Salani and Pa´rica, 1990). The Rumay Basalt (Nullo, 1978) crops out at the Meseta Escorial de Lipetre´n (41500 S–69400 W). It is composed of Pliocene olivine basalts in a pile up to 40 m thick at the edges. The Epulef Formation (Turner, 1983) crops out at Sierra de Languineo (43120 S–70000 W), where it forms small ‘‘mesetas’’. It is composed of basaltic lavas with interbedded pyroclastic deposits and has a maximum thickness of 20 m. The Huala´ Basalt (Ploszkiewicz and Ramos, 1977) is located in the foothills of the Andean Cordillera next to Lago Fontana (45450 S). Lava piles of olivine basalts form ‘‘mesetas’’ with the feeder cones jutting out. The largest outcrop is observed at Cerro Huala´ (44520 S–71130 W). These basalts also appear at the headwaters of Rı´o Apeleg Chico and Arroyo Seco, at the northern edge of Sierra de Payaniyeu and upper Rı´o Apeleg (Ploszkiewicz, 1987; Lapido and Ma´rquez, 1999). K/Ar determinations yielded ages between ca. 6 and 4 Ma (Sinito, 1980; Ramos, 1981). Pleistocene volcanic rocks were recognized in Chenqueniyeu, Genoa-Sengerr, Pichi Huala´ Basalts and Mojo´n Formation. The Chenqueniyeu Basalt or Campana Formation (Gonza´lez Bonorino, 1944) crops out in a NNW-trending belt, between the Limay (40300 S–70450 W) and Chico rivers (41450 S). Composed of olivine basalts, it crowns the Chenqueniyeu, Las Bayas and Cerro Campana ‘‘mesetas’’, with maximum thickness of ca. 30 m at the rims (Feruglio, 1941, 1947; Volkheimer, 1964; Dessanti, 1972; Turner, 1973; Volkheimer, 1973; Rabassa, 1975; Ravazzoli and Sesana, 1977; Nullo, 1978; Gonza´lez, 1998; Giacosa and Heredia, 2002). On the Chenqueniyeu ‘‘meseta’’, the effusive center is a shield volcano. On the other plateaus, vents as scoria and spatter cones up to 300 m high have been reported. The chemical composition indicates a transitional nature, between cratonic and arc-like lithologies (Stern et al., 1990; Kay and Gorring, 1999; Ntaflos et al., 2000; Kay et al., 2004b). The Mojo´n Formation (Ravazzoli and Sesana, 1977) crops out in the same area as the Cra´ter Formation (ca. 41400 S–70120 W) lying on top of Quaternary sediments. The lavas fill a NS-trending valley, giving origin to the Escorial Mamil Choique 80–100 m thick. The Genoa-Senguerr basalts crop out along these rivers between 44300 and 46 S. Basaltic and basanitic centers appear in a NNW alignment at the Cerros Saiquen, de los Chenques, Pedrero, Grande, Mirador and Manantiales Grandes. In most of them, lava flows reach the Quaternary terraces of the Senguerr and Genoa rivers 70 W (Quartino, 1957; Ferello, 1969). The Pichi Huala´ Basalt (Ploszkiewicz and Ramos, 1977) crops out in Lago Fontana area (45 S). It is a Late Pleistocene and Holocene sequence of lavas lying on glaciofluvial terraces (Ramos, 1981; Ploszkiewicz, 1987). Between 39 and 46 S, the extra-Andean most recent Holocene volcanics belong to the Cra´ter Formation and the Aneco´n Chico Basalt. Basalts from the Cra´ter Formation (Ravazzoli and Sesana, 1977) appear as large and discontinuous outcrops

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emplaced in a NNW belt, from Arroyo Comallo (40580 S–70130 W) to Cerro Ventana (42150 S). They form ‘‘mesetas’’, small cones and intracanyon channeled lava flows that fill Quaternary valleys and cover postglacial deposits (Methol, 1968; Nullo, 1978, 1979; Proserpio, 1978; Coira, 1979; Volkheimer and Lage, 1981; Nun˜ez and Cuchi, 1997; Cucchi, 1998). The Cra´ter Formation is composed of alkaline olivine basalts and hawaiites, bearing xenocrysts, and peridotitic xenoliths (Gelos and Hayase, 1979; Massaferro et al., 2002). These lavas were originally assigned to the Holocene (Ravazzoli and Sesana, 1977) but dated ca. 0.8 and 1.9 Ma by Mena et al. (2005). The Aneco´n Chico Basalt (Cucchi, 1998), which lies over the Collo´n Cura Formation and the Mesaniyeu Basalt, forms small cones, such as the Cerro Aneco´n Chico (40560 S–69470 W) and small ‘‘mesetas’’ that cap the Sierra de Mesaniyeu. These basalts have been correlated with the Cra´ter Formation (Cucchi, 1998). 5.2. Southern Extra-Andean Volcanism (46300 –52 S) South of 46300 S the extra-Andean basaltic outpours extend hundreds of kilometers eastward from the Andean Cordillera forming huge lava fields. Alkaline and subalkaline volcanics were erupted following the end of subduction due to the Chile ridge–trench collision and the formation of a slab-free window (Ramos and Kay, 1992; Gorring et al., 1997). In this area, available geochronological data suggest that the eruptive activity started with Late Miocene to Early Pliocene (12–5 Ma) subalkaline or moderately alkaline voluminous effusions, named the ‘‘main-plateau sequence’’. It was followed by the Plio–Pleistocene (7–2 Ma), less voluminous, more alkaline ‘‘post-plateau sequence’’. Main-plateau lavas are considered to represent large magma volumes related to a high degree of partial melting, whereas post-plateau lavas can represent small volumes of low-degree partial melts within the garnet stability field in the asthenospheric mantle (Baker et al., 1981; Gorring et al., 1997, 2002, 2003; D’Orazio et al., 2004, 2005). The fissure systems feeding the extensive lava fields are frequently hard to distinguish because they are covered by large lava piles. In some cases, the alignment of vents allows to infer the fault planes that controlled the effusions. In the southernmost outcrops, active strike-slip faulting controls the onset of the extra-Andean basaltic flows. During the Pleistocene, extensional conditions gave place to the formation of grabens in Pali-Aike and Camusu-Aike volcanic fields (Corbella, 2002; Haller et al., 2002; Corbella, 2004). The Meseta Lago Buenos Aires is a basaltic plateau of ca. 5000 km2 that extends from 46 to 47 S and 70 to 72 W. The basaltic lava flows of the plateau sequence (Lago Buenos Aires Formation; Lapido, 1979) erupted between 11 and 7 Ma, unconformably covering subhorizontal Miocene sediments (Ugarte, 1956) with a thickness of up to 30 m. The best represented lithologies are quartz tholeiites, olivine tholeiites and, to a lesser extent, alkali basalts (Hashimoto et al., 1977; Niemeyer, 1978, 1979; Baker et al., 1981; Brown et al., 2004).

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Most of the post-plateau sequence lavas, for example, the El Sello Formation (Busteros and Lapido, 1983), are basanites and highly potassic leucite basanites, alkali basalts and trachybasalts of Early Pleistocene age (Baker et al., 1981; Gorring et al., 2002). The last erupted lavas, the so-called Cerro Volca´n Basalt (Escosteguy et al., 2003), were considered Middle Pleistocene in age. Several of the younger lava flows overlie moraines or till deposits and could be Late Pleistocene (Mercer and Sutter, 1982; Ton That et al., 1999; Singer et al., 2004a, b). Radiometric ages for the basaltic sequences were given by Charrier et al. (1977, 1979), Sinito (1980), Baker et al. (1981), Mercer and Sutter (1982), Guillou and Singer (1997) and Ton That et al. (1999). As pointed out by Brown et al. (2004), data covering the last 11 Myr show that basaltic volcanism was episodic rather than continuous. The ages define at least 10 volcanic pulses at ca. 11–10 Ma, 7.3–7.8 Ma, 3.2–3.0 Ma, 2.4 Ma, 1.7 Ma, 1.35 Ma, 1.0 Ma, 750 ka, 430–330 ka and 109–66 ka. At the southwestern end of the ‘‘meseta’’, Pliocene subvolcanic bodies, dismantled volcanoes and eroded lava flows of trachytes and trachyandesites have been surveyed. Collectively named as the Cerro Lapiz Trachyte (Giacosa and Franchi, 2001), they intrude or lie over the Meseta Lago Buenos Aires Formation and in turn they are covered by lavas of the El Sello Formation. In turn, the Meseta Chile Chico, immediately west of Meseta Lago Buenos Aires, comprises alkali basalts dated at 8–4 Ma, with two interbedded acid rhyolitic flows (Espinoza et al., 2003; Guivel et al., 2005). Spinel harzburgite xenoliths were described in some basaltic necks (Niemeyer, 1978), and subvolcanic rhyolitic bodies were dated at 3.6 Ma (Charrier et al., 1977, 1979). The Deseado Massif, within 46300 –49 S and 66– 70300 W, has been exposed to multiple basaltic spills since the Upper Cretaceous. After a substantial diminution of eruptivity, the volcanism was reactivated in the Late Miocene–Early Pliocene, with the emission of the Cerro Tejedor Basalt (Sacomani, 1984a, b; Panza and Marı´n, 1996) and the Cerro Mojo´n Basalt (Panza, 2001) dated at 4.8 and 5.6 Ma, respectively (Gorring et al., 1997; Panza and Franchi, 2002). During the Late Pliocene–Early Pleistocene, the basaltic eruptions once more reached another peak of activity. Large areas were covered by basaltic outpours (Marı´n, 1982, 1984; Sacomani, 1984a, b), which are named La Angelita Basalt, dated at 2.8 and 1.9 Ma (Panza, 1982, 1984, 1986, 1995a, b) and characterized by the great extent and thickness of the lava flows and lava fields. Peridotite xenoliths collected in Plio–Pleistocene ash cones and lavas of Gobernador Gregores area were described by Bjerg et al. (2002) and Aliani et al. (2004). South of Lago Posadas (47300 S–71460 W), the Belgrano, Aguila, Guitarra, Del Pobre and Del Olnie ‘‘mesetas’’ are crowned by the Belgrano Basalt (Riggi, 1957; Ramos, 1979). The Belgrano Basalt, mostly tholeiitic, with an age of ca. 10.1 Ma (Gorring et al., 1997) belongs to the main-plateau sequence, and precedes the Plio–Pleistocene glaciations. Between Lago Azul and Laguna Olnie (47450 S– 71300 W) over a deeply eroded relief lie the lava flows of the Olnie Basalt (Ramos, 1982a). Its alkaline

chemistry and Early Pliocene absolute ages of 4.0 and 3.8 Ma (Ramos, 1982a; Gorring et al., 1997) allow it to be correlated to other post-plateau basalts. North and northwest of Lago Cardiel (49 S–71140 W), the Del Strobel and De La Muerte ‘‘mesetas’’ are covered by 20–50 m thick basaltic lava flows (the Strobel Basalt; Ramos, 1982b) with radiometric ages of ca. 8.6 and 6.0 Ma, which are correlated with the main-plateau sequence. Between the Cascajosa and De La Muerte ‘‘mesetas’’, the Strobel Basalt is covered by olivine basaltic andesites – the Las Tunas Basalt – with radiometric ages of ca. 5–4 Ma (Ramos, 1982b; Gorring et al., 1997), which belong to the post-plateau sequence. The volcanic sequence culminates at Rı´o Cardiel with the La Cueva Basalt (Ramos, 1982b), which name refers to the lava caves up to 100 m long and 4 m high found there. These lavas lie unconformably above the Las Tunas Basalt and are ca. 4 Ma old. Another ‘‘meseta’’ between the Santa Cruz and Chalı´a rivers (50 S), extending over 2500 km2, is formed by sediments of the Miocene Monte Leo´n and Santa Cruz formations, capped unconformably by Pliocene lavas from the La Siberia Basalt and by Level I gravels, that were later covered by the Laguna Barrosa Basalt (Strelin et al., 1996, 1999; Cobos and Panza, 2003). The main vents of these younger lavas are Cerro Bi-Aike and other unnamed cones. The lavas flowed southeastward up to the Rı´o Santa Cruz valley, where they cover the Level II terraces (Panza, 2002) and the Early–Middle Miocene Condor Cliff Basalt. K/Ar data for the Laguna Barrosa Basalt indicate ages between 3.52 and 2.25 Ma (Schellmann, 1999; Wenzens, 2000). The Camusu-Aike Volcanic Field, with a surface of ca. 200 km2, crops out between 50170 –50370 S and 71000 – 71190 W. The volcanic rocks lie atop the Meseta Pampa Alta, a high proglacial plain located south of the upper Rı´o Santa Cruz valley (Strelin et al., 1999). It is composed by a sequence of lava flows emitted from several cones and eruptive fissures of two main NW and NE fracture systems. The area was affected by extensional tectonics, which generated a NW–SE graben in the northeastern sector (Haller et al., 2002; D’Orazio et al., 2005). The lithology includes tholeiitic basalts, hawaiites and quartz-normative basaltic andesites. Two absolute 40Ar/39Ar determinations yield ages of ca. 2.98 and 3.02 Ma (Mejia et al., 2004). Southeast of Lago Argentino and northeast of the Cordillera del Paine, the Meseta Las Vizcachas (50300 – 51000 S) rises up to 1500 m a.s.l. over 1400 km2. The ‘‘meseta’’ is topped by plateau and post-plateau basaltic sequences. The oldest unit is a thick and extended (ca. 200 km2) subhorizontal sequence of lavas, breccias, tuffs of tholeiitic basalt and hawaiitic composition with interbedded glacial and fluvial sediments and considered Late Pliocene–Early Pleistocene in age, deeply carved by glacial erosion (Mun˜oz, 1982; Strelin et al., 1996; D’Orazio et al., 2005). The youngest unit, ca. 10 km2 large and less than 100 m thick, consists of lava flows that partially fill glacial valleys, necks and basaltic dikes, sometimes bearing peridotitic xenoliths (Mun˜oz, 1981). These lavas, also affected by deep glacial erosion, were considered Late Pleistocene in age. By the Rı´o Turbio, the Glencross outcrops (51500 S– 71420 W) include three volcanic necks, Mt. Phillipi,

Late Cenozoic Quaternary Volcanism in Patagonia and Tierra del Fuego Mt. Domeyko and Mt. Cuadrado, rising ca. 200 m above the surrounding moraines in an EW-trending alignment. The necks are composed by subalkaline basalts and basaltic andesites (D’Orazio et al., 2001) that yield radiometric ages of ca. 8.5 and 8.0 Ma (Meglioli et al., 1990; Meglioli, 1992). In turn, the Pali-Aike volcanic field crops out north of the Magellan Strait (51400 –52200 S and 69100 –70500 W). Flanked to the south and west by various moraine systems, most of the emissions – scoria and spatter cones, maars and subvolcanic bodies – were controlled by two fracture systems: one with NW direction, the result of the rejuvenation of an ancient rift system, and the other with E–W direction (Chelotti and Trinchero, 1990, 1991; Corbella et al., 1991; Agostini et al., 1999; Corbella, 2004). The lithology comprises alkaline basalts and basanites (Skewes, 1978; Skewes and Stern, 1979; Stern, 1990; Corbella et al., 1991, 1996; D’Orazio et al., 2000) bearing a rich variety of peridotitic and granulitic xenoliths (Skewes and Stern, 1979; Selverstone, 1982; Selverstone and Stern, 1983; Stern et al., 1985, 1986a, b, c, 1989, 1999; Stern, 1989; Kempton et al., 1999a, b; Kilian et al., 2002; Vannucci et al., 2002). Most of the lava outpours took place between the Late Pliocene (3.82 Ma) and the Holocene, but there is also evidence of former Late Miocene basalt layers (Mercer, 1976; Meglioli, 1992; Corbella, 1999, 2002; Ton-That et al., 1999, ; Mejı´a et al., 2004). Finally, Cerro Pampa (47550 S), Puesto Nuevo (48590 S) and Chalte´n (49250 S) Miocene outcrops of hornblende-bearing dacites with adakite geochemical signatures are located east of the Andean axis (Ramos et al., 1991, 1994, 2004b; Kay et al., 1994; Kay, 2002b; Ramos, 2002a, b). Recent age determinations show that these adakites decrease in age northward. This is consistent with the melting of the trailing edge of the Nazca plate as the Chilean ridge–trench collision progressed northward (Kay et al., 2004a; Ramos et al., 2004b).

6. Quaternary Tephras Tephra is the name given to the solid materials ejected from the crater in an explosive volcanic eruption. The fine fraction of tephras, transported and dispersed by the predominant winds, can travel great distances and form widespread ash layers on different depositional environments. Because of the main direction of tropospheric winds, most of the fine-grained pyroclasts are dispersed to the east and southeast reaching extraAndean Patagonia, although proximal facies of both pyroclastic density currents and ash fallouts are also placed in the Andean Cordillera. In general, ash fallouts provide significant stratigraphic markers allowing correlations over huge distances and they also permit the study of the eruptive dynamics and frequency of volcanic eruptions confining a wide range of natural and human events. In southern South America, during the two major Plinian volcanic eruptions recorded in historical times, Quizapu in 1932 and Hudson in 1991, the fine-grained ashes reached places thousands of kilometers away from their sources (Banks and Iven, 1991; Corbella and Paz,

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1991; Hildreth and Drake, 1992; Scasso et al., 1994; Bitschene and Ferna´ndez, 1995). Proof of this are the Patagonian volcanic glass shards identified from dust deposited in east Antarctica during the last glacial periods (Basile et al., 1997). Auer (1946, 1948a, b, 1950, 1965), pioneer in tephrochronological studies in South America, was the first to emphasize the importance of tephras in lacustrine sediments, soils and peat bog to correlate glacial, climatic and flora events during the Holocene. His work, together with those of Sahlstein (1932) and Salmi (1942), outlined the first Patagonian tephrochronology studies. Later studies on tephras from Tierra del Fuego and the AVZ enlarged and improved the knowledge of nature, age and dispersion area of several tephra layers coming from the Hudson, Lautaro, Aguilera, Reclus and Burney volcanoes (Heusser et al., 1990; Stern, 1990, 1991, 1992, 2000; Naranjo et al., 2001; Kilian et al., 2003; Markgraf et al., 2003; Orihashi et al., 2004). Near the Magellan Strait, Paleoindian cremation burials were found in Pali-Aike cave (52 S) lying on white volcanic ashes (Bird, 1938, 1983) that were assigned to the ‘‘first cycle of Postglacial volcanism’’, the so-called Tephra I, and dated according to Auer at ca. 9000 yrs BP (Massone, 1981). In the Pali-Aike volcanic field, several tephra layers were recovered in lacustrine sediment cores in Laguna Azul and Laguna Potrok-Aike (Haberzettl et al., 2006; Zolitschka et al., 2006). Hudson volcano (45540 S) is the most active volcanic center in SSVZ. Tephra layers preserved in soil and sedimentary deposits record at least 12 explosive Holocene eruptions (Naranjo and Stern, 1998), including the 1971 and 1991 Plinian eruptions, the last one ejecting more than 4 km3 of pyroclastic material. Early Holocene Hudson tephras (ca. 11,910 and 9960 yrs BP) were found as thin layers in lake sediments on Taitao Peninsula, 150 km southwest of the volcano (Lumley and Switsur, 1993). Other young tephra layer (<10,000 yrs) occurs in Las Guanacas cave, Aise´n, 100 km southeast of Hudson volcano (Mena, 1983; Stern, 1990, 1991; Stern and Naranjo, 1995). In extra-Andean Patagonia, lacustrine cores and sediments of Lago Cardiel area (48500 S–71200 W) show two levels of tephras (Markgraf et al., 2003), one of them coming from Hudson Volcano. The major explosive events of Hudson volcano occurred in ca. 6700 yrs BP, 3600 yrs BP and 1991. The 6700 yrs BP deposit is the largest among them. It crops out as a gray-green andesitic tephra layer with a thickness decreasing southeastward, although a secondary maximum thickness occurs 900 km to the south in Tierra del Fuego, covering ca. 40,000 km2 (Stern, 1991, 1992). This eruption, which is considered to be the largest for any volcano in the southern Andes during the Holocene, may have created the 10 km diameter summit caldera (Naranjo and Stern, 1998). To the north, in the Andean Cordillera between latitude 42300 and 45 S, tephras from 11 Holocene explosive eruptions from 7 stratovolcanoes have been recognized (Naranjo and Stern, 1998, 2004). These eruptions of Chaite´n, Michimahuida, Corcovado, Yanteles, Melimoyu, Mentolat and Maca´ volcanoes occurred

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between 9370 and 1540 yrs BP. This implies a rate of one explosive eruption every 725 yrs in this segment of the Andean SVZ. Further north, in the Lago Nahuel Huapi area (41 S), along the road between Portezuelo Puyehue and Nahuel Huapi, seven Holocene tephra layers have been pointed out (Collini, 1943; Laya, 1977). The tephras recovered by piston coring from Lago Mascardi, Mallı´n Grande, Brazo Huemul, Villa La Angostura and Laguna de Carilaufquen were analyzed and described by Mazzoni (1983), del Valle et al. (1996) and Tatur et al. (2002). In Lago Mascardi (41200 S), tephra layers were deposited during postglacial times. More than 60 pyroclastic layers have been identified in an 11 m core of lacustrine sediments from the last 15,000 yrs. Rhyodacitic, dacitic, andesitic, basandesitic and basaltic tephras were dated and geochemically correlated with young rocks from Osorno, Puyehue and Calbuco volcanoes (Villarosa et al., 1999, 2001). At Villarrica volcano (39250 S), one of the historically most active volcanoes of the Patagonian arc, two ignimbritic events occurred at ca. 13,800 and 3635 yrs BP, leaving large pyroclastic deposits (Lohmar et al., 2005; Silva et al., 2005). Over the flank of the eroded Huanquihue´ volcano (39480 S), El Escorial cone emitted black tephras that underlie Holocene white lapilli tephras (Risso, 1977; Corbella and Alonso, 1989; Mazzoni and Stura, 1993) belonging to Rı´o Pireco Formation, which was dated at ca. 1400 yrs BP. Finally, on the Atlantic coast of Buenos Aires (38–39 S), distal deposits of rhyolitic cinerites have been described and dated. The 20,900 BP ashes could be correlated with present north Patagonian volcanoes distant 1000 km to the southwest (Osterrieth and Martı´nez, 1992; Corbella et al., 2000).

7. Volcanism and Glacial Processes The most extended Neogene glacial deposits in South America are found in the Patagonian Andes. Ice fields and mountain glaciers covered the land from 39 S to Cape Horn (55500 S). In the northern section the glaciers remained restricted to the mountains and piedmont. On the western slope of the Andean Cordillera, from Chiloe´ Island (42 S) to Cape Horn, the ice field outlet glaciers have repeatedly reached the Pacific Ocean. South of 46 S, toward the east, glaciers advanced over the extra-Andean plains reaching the present Atlantic shoreline from Rı´o Gallegos (51430 S) to the south (Rabassa, 1999). Because of the great extent of Late Cenozoic glacial processes in South America, which started during the Late Miocene and include several Plio–Pleistocene pulses (Mercer, 1969, 1976, 1983; Clapperton, 1993; Malagnino, 1995; Denton et al., 1999; Rabassa, 1999; Schellmann, 1999; Ton That et al., 1999; Rabassa and Coronato, 2002), the study of Neogene volcanism in Patagonia cannot be separated from the glacial history. Moreover, Pleistocene–Holocene volcanoes partially grew beneath the glaciers, which exerted also a physical influence on eruptive styles.

The first approach for a volcanic stratigraphy of active volcanoes is to determine the level of glacial erosion or the relation of volcanic units with some well-known glacial deposits. Present improvement of dating methods, such as 40 Ar/39Ar, allows an absolute chronostratigraphy of both glacial and volcanic processes. In the area of Copahue volcano (37450 S) two glacial periods were recognized, the principal one filling the caldera depression (Groeber, 1925). Subglacial eruptions cropping out as extrusive domes and flat-top outpourings linked to the glacial deposits were dated by 14C at 30,000 yrs BP (Bermu´dez and Delpino, 1999; Gonza´lez Dı´az, 2003). The Solipulli caldera (39 S) and the internal silicarich domes were described by Gilbert et al. (1996) as a constructional structure related to the influence of the thick ice-filling of an older depression. Eastward of Lanı´n volcano (39300 S), several drift deposits have been recognized whose minimum age is constrained by the Pino Santo andesite (Rabassa et al., 1990), dated at ca. 90 ka (Lara et al., 2004a). Both north and southern flanks of Lanı´n volcano present evidence of magma–ice interactions to a greater extent. Further south, in the Lago Nahuel Huapi area (41 S) two groups of moraines have been described (Feruglio, 1949–1950; Flint and Fidalgo, 1963; Schlieder, 1989; Rabassa et al., 1990; Rabassa and Evenson, 1996). The glacial deposits interbedded with volcanics at the foot of Cerro Tronador dated 1.32 and 1.39 Ma have been correlated with the oldest drift phases (Rabassa and Clapperton, 1990; Rabassa and Evenson, 1996). Geochronologic data suggest that volcanic activity in the Tronador Volcanic Complex started 1.0 Ma (Mella et al., 2005) and ended before the last two glacial cycles recognized in this region of the Andes, which occurred between ca. 262–132 and 70–14 Ka (Mercer, 1976; Porter, 1981; Clapperton, 1993). Phreatomagmatic deposits formed in a subglacial environment between 0.47 and 0.34 Ma are in part coincident with the last of these glacial cycles (Mella et al., 2003a, b, 2005). In the northwest corner of Meseta Lago Buenos Aires, a large till deposit 30–40 m thick lies between two basaltic lava flows. These flows, K/Ar dated at ca. 7.0–6.75 Ma and 5.0–4.43 Ma, respectively, indicate that a glacial advance took place in such early times as the latest Miocene. Three other basaltic lava flows overlying moraines or till deposits have been dated at 0.125, 0.764 and 1.016 Ma (Mercer and Sutter, 1982; Singer et al., 2004a, b). North of Lago Viedma, at Meseta Desocupada (49280 S–72250 W) and Meseta Chica (49320 S– 72150 W), basaltic lava flows are interbedded with till deposits. The ages of the lavas in these two ‘‘mesetas’’ are 3.48–3.55 and 3.50–3.68 Ma, respectively (Fleck et al., 1972; Mercer et al., 1975; Mercer, 1976). In an unnamed hill 1240 m a.s.l., (50270 S–72170 W), till deposits lie between two basaltic lava flows. Radiometric determinations indicate an age of 1.99 Ma (Fleck et al., 1972; Mercer et al., 1975; Mercer, 1976). Between Lago Argentino and Angostura Fortaleza Moraine sequences of at least five to six foreland glaciations have been noticed (Schellmann, 1999). In the upper Rı´o Santa Cruz valley (50150 S), the oldest Pleistocene glaciation reached the basaltic narrow

Late Cenozoic Quaternary Volcanism in Patagonia and Tierra del Fuego pass of Angostura Fortaleza, 75 km east of Lago Argentino. At Condor Cliff (50080 S–70480 W), basalts dated at ca. 2.66 and 2.95 Ma are interbedded with the Patagonian gravels (Mercer et al., 1975; Mercer, 1976). In the same valley, at Cerro Nunatak (50110 S–71150 W), glaciofluvial gravels interbedded with pyroclastic layers cover a thick glaciolacustrine deposit (Strelin and Malagnino, 1996; Strelin et al., 1999). South of Lago Argentino, Cerro del Fraile (50330 S– 72400 W) is composed of Cretaceous sediments unconformably capped on the western slope by ca. 180 m of interstratified till deposits and lava flows (Feruglio, 1944; Fleck et al., 1972; Mercer et al., 1975; Mercer, 1976; Rabassa et al., 1996). The sequence is composed of 10 lava flows interbedded with at least 7 till deposits. Absolute dating of the lava flows indicate that these eruptions occurred between 2.18 and 1.07 Ma (Singer et al., 2004a, b). Finally, the largest eastward-extending till deposits crop out in Pali-Aike, north of the Magellan Strait. The age of the Bella Vista Drift was estimated at ca. 1.2 Ma by dating the over- and underlying basaltic lava flows (Mercer, 1976; Meglioli, 1992; Mejia et al., 2004). Thus, the close relationship between glacial and volcanic processes shows a great extent in Patagonia with both genetic and stratigraphic consequences.

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Quaternary extra-Andean volcanism contrasts with the previous large backarc effusions from the Late Miocene and Early Pliocene because of its minor volume. During the Holocene, while the centers along the Quaternary volcanic front remained active, volcanic eruptions in the extra-Andean area were less frequent. Only the youngest outcrops of the Pichi Huala´ Basalt, the Cra´ter Formation and the Pali-Aike volcanic field – till now without radiometric age data – and the Cerro Volca´n Basalt have been considered Holocene. Nevertheless, along the Pacific but mainly in Atlantic Patagonia, tephras transported by the predominant western and northwestern winds are found as distal pyroclastic deposits of the Andean eruptions. Such is the case of several recognized tephra layers from the Hudson, Chaite´n, Michimahuida, Corcovado, Yanteles, Melimoyu, Mentolat, Maca´, Osorno, Puyehue and Calbuco volcanoes left there during the last 15,000 yrs. In southern South America, from Late Miocene to Holocene, extended glacial processes were coeval with volcanic eruptions. Modern dating methods applied on volcanic rocks interbedded with glacial deposits allow dating both glacial and volcanic processes along the Andean and extra-Andean Patagonia.

References 8. Concluding Remarks In Patagonia, Late Cenozoic volcanic activity is closely related to the subduction of the Nazca and Antarctic plates beneath South America. These plates form a triple junction at 46300 S, where the Chile ridge impinges on the Chile Trench. The asymmetric distribution of volcanoes north and south of the triple junction and their contrasting composition are related to different tectonic regimes of Nazca, Antarctic, and South American plates. Along the Andes, north of the triple junction, in the CSVZ and SSVZ, there are more than 34 active volcanic centers. Basalts and basaltic andesites and scarce andesites, dacites and rhyolites built them up. Oblique chains and volcanic clusters are controlled by both blind structures of the basement and first-order intraarc faults such as the Liquin˜e–Ofqui fault system. South of the Patagonian volcanic gap (49 S) in the AVZ, there are only six Quaternary volcanoes. They are formed by adakitic hornblende andesites and dacites, which are thought to be the result of partial melting of a young subducting oceanic crust mixed with variable amounts of a mantle source. In extra-Andean Patagonia, between 39 and 46300 S, volcanic outcrops of silica-poor alkaline rocks are spatially related to NNW–NW fractures. In the southern extraAndean area, Miocene to Early Pliocene voluminous volcanic fields of subalkaline and alkaline basalts cover thousands of square kilometers, followed during the Plio– Pleistocene by smaller and more alkaline spills. Some backarc plateau lavas have been considered to be of an asthenospheric plume-like mantle source. East of the Patagonian volcanic gap, plateau basalts seem to be linked to a slab window due to the Chile ridge–trench collision.

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7 Late Cenozoic Paleomagnetic Studies in Patagonia Guillermo H. Re1, Mario Mena1,2 and Juan Francisco Vilas1,2 1

INGEODAV (Instituto de Geofı´sica Daniel A. Valencio), Departamento de Ciencias Geolo´gicas, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires 2 CONICET (Consejo Nacional de Investigaciones Cientı´ficas y Te´cnicas)

Today, paleomagnetic studies have applications to a wide variety of geological problems. In paleogeographic studies, paleomagnetism is providing evidence about movement of large crustal blocks and localized rotation of smaller tectonic blocks. Also, paleomagnetism can be used for stratigraphic correlation and geochronological calibration of paleontological zonation. In addition, geomagnetic field directional changes due to PSV have been successfully used to date Late Cenozoic deposits and archeological artifacts. Because the patterns of secular variation are specific to subcontinental regions, these geochronological applications require the initial determination of the secular variation pattern in the regions of interest. Once this regional pattern of changes in declination and inclination has been established and calibrated in absolute age, patterns from other deposits of similar ages can be matched to the calibrated pattern thus dating those deposits. The first paleomagnetic studies in South America were carried out by D.A. Valencio in the 1960s. In those years, the scientific community discussed if the reverse magnetization was due to a self-reversal phenomenon, intrinsic to the rock, or changes of the polarity of the earth magnetic field. Among these pioneer works, the paleomagnetic study of the Miocene Barda Negra basalts and Uppermost Pliocene basalt of Pampa de Zapala, Neuque´n Province, Argentina, can be mentioned (Valencio, 1965a, b). Samples from these sites carried a stable primary remanent magnetization and a secondary viscous component, removable by 20–30 mT of AF (alternating field demagnetization) (Valencio, 1980; Butler, 1998; McEllinny and McFadden, 2000). The thermoremanent primary magnetization from Pampa de Zapala has reverse polarity. This was the first result from the Southern Hemisphere that supported the hypothesis of a polarity change of the geomagnetic field through geological times. Other paleomagnetic and radiometric (K/Ar) studies were carried out on volcanic rocks of Neuque´n and Mendoza provinces by Creer et al. (1969). These studies allowed defining two volcanic cycles developed in the Late Miocene–Middle Pliocene and in the Upper Pliocene– Latest Pleistocene. They could also establish two new boundaries between times of normal (N) and reversed (R) polarity: G1 = R/N to the 5.5+0.3 Ma and G2 = N/R to 6.6 + 0.3 Ma. Also, the obtained results showed a good correlation with the sea floor spreading theory which was in

1. Introduction Paleomagnetism is a relatively young scientific discipline. The first studies applying this methodology were carried out in the 1950s. In Argentina the first studies date from the mid-sixties. These pioneer studies were carried out on outcropping volcanic rocks in Neuque´n Province. From that moment, and in gradual form, paleomagnetic studies have been applied with many objectives. In this chapter, we present a broad (although possibly incomplete) summary of the studies that were carried out in Patagonia. Although the studies have been grouped according to the lithological types involved, it can be said that the main research lines are related to the following factors: (1) the understanding of the behavior of the terrestrial magnetic field through time; (2) to the contribution of new information that allows a better adjustment of the stratigraphic position of the studied units; or (3) studies centered on rock magnetism. Papers on (1) show the evolution of knowledge about the behavior of the terrestrial magnetic field, from the pioneer studies that take account of lithologies with reversed magnetization to the modern studies in which the paleosecular variation (PSV) is analyzed, from the distribution of the Virtual Geomagnetic Poles (VGPs) or studies in which excursions of the terrestrial magnetic field lasting a few thousands of years are analyzed. Papers on (2) deal with studies of volcanic sequences as well as lake sediments. Papers on (3) deal with studies of rock magnetism, essential to justify the results of any paleomagnetic study. However, it is noteworthy that anisotropy of magnetic susceptibility (AMS) has started to provide information on direction of flow (in the case of volcanic rocks) or of provenance (in the case of sedimentary or pyroclastic rocks). Paleomagnetism is a geoscience that incorporates aspects of geomagnetism, rock magnetism and geology. It became relevant in the twentieth century when paleomagnetism provided quantitative confirmation of continental drift and polar wandering by the interpretation of remanent magnetism observed in many different rocks. Also, most observations of geomagnetic polarity reversals and of ancient declination (dec.), inclination (inc.) and field intensity variations provided data that have contributed significantly to the knowledge of the geomagnetic field origin.

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discussion at that time. Lava flows of Neuque´n Province registering a normal polarity event occurred between 2.0 and 2.31 Ma (Creer and Valencio, 1969). This normal event occurred within the Matuyama Reversed Epoch and was defined as the Neuque´n Event by Valencio and Mendı´a (1974). Also, sea floor spreading supports the presence of a normal event for that time (Valencio et al., 1975). The works of Valencio were also pioneer studies for continental drift and plate tectonics. Valencio and Vilas (1969) using the available Mesozoic and Cenozoic paleomagnetic data from South America and Africa suggested that during the Cenozoic, South America and Africa drifted apart in a basically E–W direction. Also, on the basis of paleomagnetic and sea floor spreading data, they performed a tentative reconstruction of the relative positions of South America and Africa. Undoing the relative Cenozoic movement between South America and Africa from the current geographical position until the oldest symmetric magnetic anomalies fit on both sides of the mid-oceanic ridge, they obtained the configuration of both continents at 71 Ma. The adjustment was carried out taking into account that the paleolatitudes coincided with the available paleomagnetic data (Valencio, 1970, 1972; Vilas and Valencio, 1970; Vilas, 1974).

2. Paleomagnetic Studies from Lava Flows The study of the origin and behavior of the Earth’s magnetic field (EMF) is a branch of geophysics that has received much interest over the last few decades. Despite the extraordinary progress made in theoretical and simulation studies, the observational results of the field parameters remain a fundamental tool for its description, and they also provide constraints that every model of the field must account for (e.g. Carlut and Courtillot, 1998). Because direct observations only go back a few centuries at best (Merrill et al., 1996), reconstructions of the field direction and intensity in previous times must rely on paleomagnetic studies, both in archeological and in geological materials. The analysis of these different kinds of records has helped us to understand both the time-averaged field and its PSV. The data for PSV obtained from the study of lava flows Paleosecular variation from lavas (PSVL) from the past 5 Ma were compiled in databases (Lee, 1983; Quidelleur et al., 1994; Johnson and Constable, 1995, 1996; McElhinny and McFadden, 1997). Because of the low number (both in time and place) of published studies from the Southern Hemisphere, those carried out in areas like Patagonia become very important. Within the early paleomagnetic studies carried out in Cenozoic volcanic rocks of Patagonia, we can mention the work of Sinito (1980) who used radiometric (K/Ar) ages and paleomagnetic data to define and correlate the magnetic units of basalts cropping out near Lago Buenos Aires and in the Rı´o Pinturas valley, Santa Cruz Province (Fig. 1, localities 1 and 2), and near Lago Fontana (45 S, 71200 W; Fig. 1, locality 3)

and next to Colonia Sarmiento (45300 S, 69 W), Chubut Province (Fig. 1, locality 4). Paleomagnetic studies of Late Cenozoic volcanic rocks in high southern latitudes will certainly provide further constraints for a better modelling of the EMF behavior. In this sense, some important studies that have been developed are as follows: the PSV study in basaltic lavas, ranging from Miocene to Late Pleistocene in the Meseta del Lago Buenos Aires (Fig. 1, locality 2; Brown et al., 2004); the PSV studies of Miocene–Late Quaternary lava flows that include the Pali Aike volcanic field (Fig. 1, locality 5) and the Meseta Vizcachas Plateau lavas (Fig. 1, locality 6; Mejia et al., 2004); the paleomagnetic study on several recent dykes, lavas and pyroclastic flows from volcanic Deception Island (Antarctic Peninsula, Baraldo et al., 2003); and the geochronological and paleomagnetic studies of Plio–Pleistocene basaltic lava flows at Cerro del Fraile (20.5 S, 72.7 W; Fig. 1, locality 7; Mercer et al., 1975; Mercer, 1976; Singer et al., 2004a). The basaltic lavas from Meseta Lago Buenos Aires (47 S, 71 W) Range from Miocene to Late Pleistocene. Brown et al. (2004) studied 36 lava flows and shallow intrusions, determining paleomagnetic directions and 40Ar/39Ar ages. All lava flows have stable paleomagnetic directions. After stepwise demagnetization using either thermal or AF techniques, all sites yield characteristic directions held by magnetite and/or titanomagnetite. 40Ar/39Ar dating from paleomagnetic sites gave ages that range from 11.030 + 0.140 Ma to 0.067 + 0.004 Ma. The oldest age corresponds to thick plateau-forming lavas which are exposed along the southeast edge of the Meseta Lago Buenos Aires, and the youngest age for scoria cones and young lava flow outcrops in the Rı´o Pinturas valley. Most of the ages are younger than 3.3 Ma. Geochronology indicates that the volcanism was episodic with periods of more intense eruptive activity. The last 3.3 Ma are characterized by eight episodes of volcanism at 3.20–3.00 Ma, 2.40 Ma, 1.70 Ma, 1.35 Ma, 1.00 Ma, 750 ka, 430–330 ka and <110 ka. The bulk of lavas forming the surface of the Meseta erupted during the last 1.7 Ma. Ten sites have distinct transitional directions (defined by pole latitudes <55, see Fig. 2), and the associated ages could indicate possible reversals within the Matuyama Chron, including the onset of the Jaramillo Subchron (1.016 + 0.01 Ma), the Cobb Mountain Subchron (1.25 + 0.03 Ma), the Ontong-Java 1 Event (1.37 + 0.03 Ma) and the termination of the Olduvai Subchron (1.72 + 0.02 Ma) (Fig. 3). Remaining sites are divided into normal (14 sites) and reversed polarity (12 sites). After omitting sites with low latitude poles, the 26 remaining flows have a mean direction of inc. = –63.0, dec. = 3.4, a95 = 5.4, which is indistinguishable from the expected geocentric axial dipole. Paleosecular variation, measured by the angular dispersion of VGPs about the rotation axis is 20.0. This result is higher than the 17 dispersion predicted by Model G that uses data for the past 5 Ma (McFadden et al., 1991; Fig. 4). This discrepancy may be due to true dispersion in

Late Cenozoic Paleomagnetic Studies in Patagonia

123

Buenos Aires

74°

70°

66°

62°

58°

Figure 8 BUENOS AIRES 36°

LA PAMPA

36°

NEUQUÉN

11 40°

40°

RÍO NEGRO

13 21

9 8 CHUBUT

19 44° 44°

a Pla

te

3

References

Nazc

14 17

Sedimentary rocks

16 2

Volcanic rocks Lake sediments

15

48°

An

tar

tic

Pla

te

1

4

SANTA CRUZ

48°

20 ISLAS MALVINAS

76 10

12 5

52°

52°

18 TIERRA DEL FUEGO 0 74°

70°

66°

62°

100

200 km

58°

Fig. 1. Location of the sites. (1): Meseta del Lago Buenos Aires. (2): Rı´o Pinturas. (3): Lago Fontana. (4): Colonia Sarmiento. (5): Pali Aike. (6): Meseta Vizcachas. (7): Cerro del Fraile. (8): Gastre. (9): Mamuil Choique. (10): Mylodon Cave. (11): Piedra del Aguila. (12): Las Buitreras Cave. (13): Lago Nahuel Huapi. (14): Lago Blanco. (15): Lago Pueyrredo´n. (16): Lago Buenos Aires. (17): Alto Rı´o Mayo valley. (18): La Misio´n. (19): Rı´o Corintos. (20): Alto Rı´o Santa Cruz valley. (21): Lago Nahuel Huapi (Lago El Tre´bol, Brazo Campanario, Lago Moreno and Lago Escondido).

response to regional variation in the magnetic field in southern South America, or it may be an artifact of inadequate sampling (Brown et al., 2004). Another study of PSV from areas of southern Patagonia is presented in Mejia et al. (2004). Paleomagnetic directions were obtained from stepwise alternating field or thermal demagnetization of 53 lava flows from the Pali Aike volcanic field and the Meseta Vizcachas Plateau lavas (Fig. 1, localities 5 and 6). K/Ar and 40 Ar/39Ar radioisotopic ages of these flows (Mercer, 1976; Meglioli, 1992; Singer et al., 2004b) have been obtained to depict the glacial history of Patagonia and indicate primarily Pliocene–Pleistocene ages. These ages

agree with radioisotopic ages spanning from 0.1 to 15.4 Ma obtained by Mejia et al. (2004). Magnetic polarities for all samples, except one, are coincident with the expected polarities of the magnetic polarity timescale for these ages (Cande and Kent, 1995). The mean direction from sites with ages <4 Ma and a95  5 is dec. = 358.7, inc. = 68.2, a95 = 3.5, a value that is coincident within the statistical uncertainty with the direction of the geocentric axial dipole (GAD) for that area (inc. = –68.1). Likewise, the mean VGP coincides within the statistical uncertainty with the geographic pole. The secular variation described by the VGP angular standard deviation for these sites is 17.1 (Fig. 4), in

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Guillermo H. Re et al. SC

0,0

30

LBA

15

Gastre

MPS

0

BLAKE 0,2

45 0,4

60

0,8

90

270

BRUNHES

0,6

1,0 1,2 1,4 1,6 1,8

CF KAMIKATSURA SANTA ROSA JARAMILLO COBB MOUNTAIN ONTONG-JAVA 1a

MY CN

MATUYAMA

OLDUVAI

2,0

180 References Normal polarity

Transitional normal polarity

Reverse polarity

Transitional reverse polarity

Fig. 2. Virtual geomagnetic pole locations obtained from Meseta del Lago Buenos Aires. Rhombus show VGPs with latitudes <55 (modified from Brown et al., 2004).

2,2

REUNION NEUQUEN (1)

2,4 2,6 2,8

MJ

3,0 KAENA 3,2

GAUSS MAMMOTH

3,4 3,6

agreement with the value expected for that latitude according to Model G of paleosecular variation (McFadden et al., 1988). The International Geomagnetic Reference Field (IGRF) of the year 2000 for this area is dec. = 13.5 and inc. = –48.1, and the GAD for this same area is dec. = 0, inc. = –68.1. Therefore the present inclination anomaly in this area is 20. Such anomaly reflects the pronounced nondipole structure of the present field in South America. This value is greater than the inclination anomaly at any of the sites from Pali Aike and Meseta Vizcachas suggesting that the present inclination anomaly is among one of the greatest that has occurred in the area at least during nontransitional states of the magnetic field (Mejia et al., 2004). Paleomagnetic studies on several recent dykes, lavas and pyroclastic flows from volcanic Deception Island, at 62570 S, 60370 W, were presented in Baraldo et al. (2003). Deception Island is situated in the Bransfield marginal basin that separates the South Shetland Islands from the Antarctic Peninsula. The Cenozoic volcanic rocks from Deception Island had already been studied by Valencio and Fourcade (1969). They made paleomagnetic and petrographic studies of the lavas, and also of Late Miocene andesites and basalts from 25 de Mayo Island (Ardley Peninsula). Lavas from Deception Island carried on normal remanent magnetism, which led the authors to suggest an age of less than 0.78 Ma (Brunhes Chron). Lavas from 25 de Mayo Island show normal and reversed polarities of their remanence. In both localities, the magnetic directions suggest that the geomagnetic field was dipolar, axial and geocentric during the eruption of the lavas. With the directions of remanence, two

3,8

GILBERT

4,0 Ma

References

Normal polarity Reverse polarity

Transitional polarity (1) according to Valencio et al., 1979.

Fig. 3. Comparison of the polarities from the different sites with the geomagnetic polarity time scale (Cande and Kent, 1995). Gastre: Postglacial backarc basalts from Cerro Fermı´n (CF), Cerro Negro (CN), Gastre (MY) and Mamuil Choique (MJ) (Mena et al., 2005a). Late Cenozoic basalts from Cerro del Fraile (SC) (Singer et al., 2004b) and Lago Buenos Aires (LBA) (Brown et al., 2004). paleomagnetic poles (pp) were calculated for the Late Pleistocene (84 S, 170 E) and the Late Miocene (86 S, 126 W) (Valencio and Fourcade, 1969). In Baraldo et al. (2003), the sampling comprised all the stratigraphic units exposed on Deception Island that included basaltic, andesitic and trachytic lavas, basaltic dykes and pyroclastic flows. High quality paleomagnetic data were obtained. Following stepwise thermal and AF demagnetization procedures, consistent characteristic remanence directions were determined at 21 sites, using principal component analysis. A good correlation between bulk magnetic susceptibility and lithology was found in each stratigraphic unit (Baraldo and Rinaldi, 2000). Hysteresis cycles and forward and backfield isothermal remanent magnetization (IRM) curves were made using one

Late Cenozoic Paleomagnetic Studies in Patagonia

VGP scatter (degrees)

25

+

20

15

10

0

30

60

90

Latitude

Fig. 4. Least squares fit of Model G to both normal and reversed polarity scatter of VGP from lavas for the past 5 Ma (after McFadden et al., 1991) and secular variation described by the VGP angular standard deviation for •: Meseta del Lago Buenos Aires (modified from Brown et al., 2004); ¶: Pali Aike and Meseta Vizcachas (Mejia et al., 2004); ı : Deception Island (Baraldo et al., 2003). sample per site. A large majority of the domain size of the analysed samples fall in the pseudo-singledomain (PSD) field, suggesting that most rocks are potentially good recorders of the geomagnetic field direction. The overall mean remanence direction is dec. = 348.8, inc. = –73.7, a95 = 4.4, N = 21, and is consistent within error with the GAD direction at the study locality. All of the studied rocks show normal polarity, indicating a Brunhes Chron age. The only available radiometric date of 153 + 46 ka agrees with this and suggests a minimum chronostratigraphic span of 100 kyr for the sampled rocks. The mean directions show a Fisherian distribution and dispersion values compatible with current PSV models (Fig.4). No evidence of the far-sided or right-handed effect is

found in these data. The calculated mean paleomagnetic direction is 2 away from the expected GAD direction, well within its a95 (4.4). In any case, the inclination of the calculated mean direction is lower than the expected one, in opposition to the expected higher inclinations in the Southern Hemisphere owing to the far-sided effect (Baraldo et al., 2003). Other paleomagnetic studies on Late Cenozoic basalts were performed on the Cra´ter, Moreniyeu and Mojo´n formations outcropping in northwestern Patagonia (Fig. 1, localities 8 and 9; Mena et al., 2005a, b, 2006). The Holocene Cra´ter Formation is composed of basalt flows that fill Quaternary valleys and cover postglacial sediments forming a small volcanic field (Haller, 2000). The paleomagnetic study was made on oriented samples collected from eight effusive centers which belong to the Cra´ter Formation. After AF and thermal detailed demagnetization, primary remanent magnetization directions were defined. The related VGPs have normal polarities (Fig. 5a). The comparison between these polarities and the radiometric K/Ar ages performed several years ago on rocks from outcrops of the Cra´ter Formation, located at Cerro Fermı´n (0.8 + 0.1 Ma) and Cerro Negro (1.9 + 0.4 Ma), suggest that the Cerro Fermı´n vulcanite extruded during the Brunhes Chron (Fig. 3). Thus, their ages would most likely be between 0.78 (upper boundary of the Brunhes Chron) and 0.70 Ma. In the case of the basalts from Cerro Negro, the relationship between age data and their normal polarity suggests that these flows were extruded during the Olduvai Subchron. These data indicate ages between 1.95 and 1.78 Ma (Fig. 3). Pahoehoe lavas from the Cra´ter Formation volcanic plateau were also studied with AMS techniques (Singer et al., 2005). AMS describes the directional variation of magnetic susceptibility inside a material and represents the magnetic contribution of all minerals of the rock (diamagnetic, paramagnetic and ferromagnetic). The magnetic susceptibility can be described mathematically by a symmetric tensor of second order and be visualized by an ellipsoid. This ellipsoid is defined by the orientation and magnitude of the three principal axes: K1  K2  K3 (the eigenvectors and eigenvalues

0

0

Crater F. VGPs

270

125

VGPs

90

90

270 Mojón F. Moreniyeu F.

(a)

(b) 180

180

Fig. 5. South polar projection with VGPs from (a) Cra´ter Formation and; (b) Moreniyeu and Mojo´n formations (after Mena et al., 2005b).

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Guillermo H. Re et al.

of the magnetic susceptibility tensor). AMS is given by the orientation of anisotropic magnetic minerals and the magnetic fabric of the rock. The features of magnetic fabric associated with the petrofabric are magnetic lineation given generally by K1, magnetic foliation plane, defined by K1 and K2 axes and K3 is the foliation pole. The good preservation of the Cra´ter Formation outflows improves the AMS method. The average magnetic susceptibilities as well as studies in rock magnetism show that those lavas are poorly ferromagnetic and that the minerals carrying magnetic properties are titanomagnetites (Mena et al., 2005b, 2006). AMS studies were carried out at three sites. The samples of the first site were obtained in the lower part of a distal segment of a massive lava in Cerro Negro. The samples of the second site were obtained in an upper part with vesicles of the distal region of the same lava. The third site corresponds to a proximal segment of vesicular lava of Cerro Antitruz. Data from the first site show a well-defined, oblate fabric. The main feature of the fabric is the magnetic foliation plane that inclines toward northeast, coinciding with the sense of movement of the flow, whereas the minimum susceptibility axis inclines toward the source (Fig. 6, site 1). The magnetic fabric of site 2 (Fig. 6, site 2) is poorly

N

N

Site 1

Site 2

N

References Maximim Intermediate Minimum

Site 3

Fig. 6. Lower hemisphere projection of the AMS principal axes (maximum, intermediate and minimum) and AMS lineation (K1/K2) and foliation (K2/K3) ratios for the specimens from Gastre basalts. Site 1: Distal section, L = 0.2%, F = 2.6%. Site 2: Proximal section, L = 0.5% and F = 1.0%. Site 3: Proximal section, L = 0.4% and F = 0.6%. Where the flow was faster the fabric is well defined (site 1). The fabric is poorly defined, indicating degassing (subvertical magnetic foliation) for the proximal sections (sites 2 and 3) (Singer et al., 2005).

defined, because of degassing of the flow. The magnetic foliation plane is vertical and is defined by the orientation of the titanomagnetite grains. It is interpreted that the crystals of titanomagnetite are oriented with their foliation planes parallel to the upward moving gas. In this site, it is possible to determine the direction of fluid escape. The magnetic fabric of site 3 (Fig. 6, site 3) shows a great dispersion of the principal susceptibility axes. The random arrangement of crystals is attributed to the degassing process. When the results of this preliminary AMS study are compared with field data, it is possible to measure the fabric of the Cra´ter Formation lavas. Thus, it is possible to obtain paleodirections of lava flows to determine the orientation of their movement, to correlate the fabric with degassing pulses and to investigate the variation of flow behavior. The analysis of AMS data also indicates that the distal segments of pahoehoe flows show more consistent fabrics; on the other hand, proximal segments show the degassing process (Singer et al., 2005). Basalts of the Mojo´n Formation, considered to be of Pleistocene age (Ravazzoli and Sesana, 1977), conform an extensive lava flow located near Mamil Choique (41460 S, 70 080 W), Rı´o Negro Province (Fig. 1, locality 9). These basalts are petrographically very similar to the Cra´ter Formation basalts, but the latter are more vesicular and less altered. Radiometric K/Ar age obtained for rocks from outcrops of the Mojo´n Formation located at Gastre is 3.3 + 0.4 Ma (Mena et al., 2005a). The Moreniyeu Formation is assigned to the Early Holocene (Proserpio, 1978) and its outcroppings form an elongated basalt flow near Gastre, Chubut Province. Radiometric K/Ar age obtained for rocks from these outcrops is 1.6 + 0.2 Ma (Mena et al., 2005a). The paleomagnetic study of the Mojo´n and Moreniyeu formations was made on oriented samples collected from three sites for each unit. After AF and thermal detailed demagnetization, characteristic remanent magnetization (ChRM) directions were defined. The Moreniyeu and Mojo´n flows carry a ChRM component of natural remanent magnetism (NRM) with positive inclination and have reverse polarity VGPs with almost coincident positions (Fig. 5b). This result is interesting because the assigned ages for these formations are different. For the Mojo´n Formation, the relation between their reverse polarity and the radiometric age suggests that these flows extruded during the Mammoth or the Kaena Subchrons. Thus, their age is possibly bracketed between 3.33 and 3.22 Ma or 3.12 and 3.04 Ma (Fig. 3). The reverse polarity found for the Moreniyeu Formation and the polarity expected according to MPS (Magnetic Polarity Scale; Cande and Kent, 1995) for the range 1.8–1.4 Ma are both coincident (Mena et al., 2005a, b). The Plio–Pleistocene basaltic lava flows at Cerro del Fraile (Fig. 1, locality 7) form a classic lava-flow sequence that contributed to the nascent Geomagnetic Polarity Time Scale (GPTS). In this continuous radioisotopically dated section, three normal polarity events that occurred during the reversed Matuyama Chron were recorded (Fleck et al., 1972). New stratigraphic studies,

Late Cenozoic Paleomagnetic Studies in Patagonia paleomagnetic analysis, 40Ar/39Ar incremental heating ages and unspiked K/Ar dating of 10 lava flows were performed by Singer et al. (2004a) in this locality. Their results place these eruptions between 2.181 + 0.097 and 1.073 + 0.036 Ma. The Re´union Event is recorded by three lavas from Cerro del Fraile with transitional, normal and reversed polarity that yielded identical ages, with a mean age of 2.136 + 0.019 Ma. When combined with 40Ar/39Ar ages from lavas on Re´union Island and a normal tuff in the Massif Central (France), Singer et al. (2004a) found that the age of the Re´union Event is 2.137 + 0.016 Ma. This age is older by 50 kyr than the 2.086 + 0.016 Ma age of the Huckleberry ridge Event (Singer et al., 2004a). The 40Ar/39Ar ages of transitional, normal and reverse polarity flows 6, 7 and 8 at Cerro del Fraile, together with those from a transitional basalt on Moorea and a normal polarity tuff at Olduvai Gorge, constrain the Olduvai Subchron between 1.922 + 0.066 Ma and 1.775 + 0.015 Ma, in agreement with astrochronologic estimates (Fig. 7). The onset of the normal Jaramillo Subchron, defined from lavas at Punaruu valley (Tahiti), and the youngest normal polarity flow at Cerro del Fraile,

B R U N H E S M A

Ma Blake

0.12

Emperor

0.42

Kamikatsura Santa Rosa Jaramillo Punaruu Cobb Mountain

0.791 ± 0.004 0.899 ± 0.006 0.936 ± 0.008 1.001 ± 0.010 1.069 ± 0.011 1.122 ± 0.010 1.194 ± 0.014

T U

Gardar Gilsa

1.480–1.472 1.575–1.567

Y A M A

Olduvai

Hucleberry Ridge Réunion

Neuquén (1)

1.775 ± 0.015 1.922 ± 0.066 2.086 ± 0.016 2.137 ± 0.016

2.20–2.31

References Normal polarity

Reverse polarity

Transitional polarity

Fig. 7. Temporal extension of the transitional (gray), reverse (white) and normal (black) polarities of the Cerro del Fraile flows in relation with GMPS (modified from Singer et al., 2004a) [(1) according to Valencio et al., 1979].

127

occurred at 1.069 + 0.011 Ma. Flow 9 at Cerro del Fraile records a transitional paleomagnetic direction that is imprecisely constrained between the 40Ar/39Ar total fusion and the K/Ar ages of 1.61 and 1.43 Ma. (Singer et al., 2004a). In the region between 33 and 46 S, Cenozoic basaltic sequences, known as the South Volcanic Zone, are extant. Groeber (1929, 1933) classified those basaltic sequences by stratigraphic and geomorphological criteria and informally named them as ‘‘Basalto 0’’ (Oligocene) to ‘‘Basalto VII’’ (Holocene); later, this classification was replaced by another one that takes into account the type area of each volcanic sequence (Groeber, 1946, 1947). Some years later, following the stratigraphic code, these basaltic sequences received different formational names according to the region they belong to or the authors who studied the sequence; for example, the ‘‘Basalto I’’ rocks (‘‘Upper Palaocolitense’’) outcropping near Buta Ranquil were denominated Chapu´a Formation (Holmberg, 1976); the ‘‘Basalto I’’ lavas outcropping near Junı´n de los Andes were named as the Huechahue´ Formation (Turner, 1973). To ease the description of the studies, the Groeber classification (1929, 1933) will be used. These basaltic sequences have been very well studied from a regional geological point of view, during the regional mapping of the area (Lambert, 1956; Galli, 1969; Gonza´lez Dı´az, 1972; Turner, 1973, 1976; Zo¨llner and Amos, 1973; Holmberg, 1976; among others). These rocks have been the object of paleomagnetic and geochronological studies (Valencio and Creer, 1968; Cortelezzi and Dirac, 1969; Creer and Valencio, 1969; Valencio et al., 1969, 1970a, b; Nabel, 1970; among others). From paleomagnetic and geochronological studies, these basalts can be grouped into two main magmatic periods: (a) Pre-Miocene and (b) Plio–Holocene (Valencio et al., 1970a, b). The latter presents four peaks of activity: the oldest in the Early Pliocene, the next in the Late Pliocene–Early Pleistocene, the third in the Middle–Late Pliocene and the last one during the Holocene (Bermu´dez et al., 1993). The Cenozoic volcanic sequences of Neuque´n and southern Mendoza provinces are located east of the present volcanic front and show intraarc and backarc volcanic characteristics. In the first case, the volcanic centers are between Precordilleran blocks, which are separated by graben structures, suggesting the existence of an intraarc extension period during the Plio–Quaternary (Mun˜oz Bravo et al., 1987). The backarc volcanism can be described as consisting of large basaltic sheets, with volcanic cones aligned with fractures or randomly dispersed without any structural control (Bermu´dez et al., 1993; Mun˜oz Bravo et al., 1987). The first paleomagnetic studies made in Argentina were carried out by Valencio (1965a, b) in Cenozic basaltic sequences of Neuque´n Province (for location see Fig. 8). Most of the paleomagnetic studies were made to establish a chronology of different eruptive cycles (Fig. 9). The information on each basaltic unit is summarized below.

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Guillermo H. Re et al. Malargüe

Geomagnetic polarity interval

Age Era Epoch South central Neuquen my

er

Atuel riv

LLANCANELO AREA

Lag. 40

Q

Grand e rive

-

Pst

Jaramillo Matuyama

Llancanelo

T

36° 5

r

E

an

rr Ba

LOS VOLCANES AREA

N. Neuquen S. Mendoza

Brunhes

P L I O C E N E

Olduvai Kaena Mammoth

Gauss Gilbert

ca ive

sr r

C H I L E

10

MENDOZA province

Tricao Malal

Buta Ranquil 40

Colorado riv

er

R M

- 37° T

Andacollo

NEUQUEN province

Chos Malal

BUTA RANQUIL AREA

I

O

15

I C

uén

Neuq 40

river

- 38°

Loncopue

A

E

20 N S. Zapala

R

Ne

22

uqu

én

Las Lajas

rive

r

Cerro Bandera

Estancia Llamuco

Alumine lake

Rucachoroi

Zapala E

ZAPALA AREA

Covunco

Portada

Buta Ranquil Los volcanes

25

Y

Llancanello

Covunco

Lonco Primeros Luan Pinos

22

Zapala

- 39°

Puesto Quiroga

OL.

El Manzano

Q = Quaternary

Alumine 40

Pst = Pleistocene References

Normal polarity Catan Lil Lim

Junín de los Andes

NEUQUEN province

r

RÍO NEGRO province

ZAPALA SUR AREA

- 40°

71°

Fig. 9. Geochronological ages with estimated errors. Open symbols indicate normal magnetic polarity, black symbols indicate reversed magnetic polarity (from Valencio et al., 1970a).

70°

References Sampling localities by Valencio and Creer, 1968; Valencio et al., 1969;1970a, b. Sampling localities by Ré,1997a, b; Re et al. 1997a, b, and 2000a, b.

22

Reverse polarity

-

-

San Martín de los Andes

a

ive yr

OL = Oligocene

National road

Secundary road

Fig. 8. Map showing sample sites in Neuque´n and Mendoza provinces. Location of Fig. 8 is shown in Fig. 1.

Valencio (1965b), Valencio and Creer (1968), Creer and Valencio (1969), Valencio et al. (1969, 1970a, b, 1979), Re´ et al. (1997a, b, 2000a, b) and Re´ and Tomezzoli (1997) studied the regions where volcanic rocks of the ‘‘Basalto I’’ units are outcropping. Re´ and Tomezzoli (1997) studied these basalts near the Rı´o Ruca Choroy (Neuque´n Province), where the Rancahue´ Formation outcrops (Turner, 1976). This unit comprises alkaline effusive basalts with interbedded

breccias and agglomerates and tuffs which characterize the ‘‘Basalto I’’, also known as ‘‘Upper Palaocolitense’’ (Groeber, 1946). The Rancahue´ Formation is 500 m thick and it is considered as coeval with the Chimehuı´n Formation (Miocene); both of them have an unconformable relationship at the base except when it is interbedded with tuffs and tuffites of the Chimehuı´n Formation (Turner, 1976). The basalts of the Rancahue´ Formation erupted sometime during the accumulation of the upper part of the Chimehuı´n Formation and are observed as outflows interbedded in the profile of this formation, or comformably covering it (Turner, 1976). Because of these geologic characteristics, the Rancahue´ Formation is considered of Miocene age (Groeber, 1929). Re´ and Tomezzoli (1997) carried out a detailed paleomagnetic study, collecting 80 specimens (in eight sites) in outcrops near the Rı´o Ruca Choroy. This study was made using stepwise thermal demagnetization (until 670C) and AF (until 100 mT). The sequence has reversed polarity. Almost all the specimens have a single-component remanent magnetization;

Late Cenozoic Paleomagnetic Studies in Patagonia only few of them have two components and one can represent the present magnetic field (Re´ and Tomezzoli, 1997). From isothermal remanent magnetization and backfield studies, it was established that magnetite is the principal carrier of the magnetization and in some samples hematite is present in minor proportions (Fig. 10a; Re´ et al., 2000a, b). Comparing all these results with the previous ones of Neuque´n and southern Mendoza provinces (Creer and Valencio, 1969; Valencio et al., 1970a, b), there is a great discrepancy with the available geologic information between the seven radiometric ages (from 4.7 + 0.5 to 27.6 + 0.8 Ma) and

(a)

129

between these absolute datings and the geologic age according to the stratigraphy. The magnetic polarities of all the localities were supposedly of the same age, but in four localities they are reversed and three localities of the ‘‘Basalto I’’ would correspond to more than one volcanic and contemporary event. We can suggest at least two explanations: (1) an incorrect stratigraphic assignation of many localities of ‘‘Basalto I’’; (2) an incorrect radiometric age due to Ar loss in the K/Ar system. Thus, it might be recommended that more localities of this formation should be studied to refine the chronology of the volcanic activity during the Late Oligocene–Miocene.

IRM and Back-field for Basalto I samples (Ruca Choroy locality) 1.50E+00

Jr/Js 1.00E+00

5.00E–01

mtesla

0.00E+00

–2.00E–01

–1.00E–01

1.00E–01

2.00E–01

–5.00E–01

–1.00E+00 –1.50E+00

(b)

IRM and Back-field for Basalto II samples (Puesto Quiroga-Aluminé locality) 1.50E+00

Jr/Js 1.00E+00

5.00E–01

0.00E+00 –4.00E–01 –3.00E–01 –2.00E–01 –1.00E–01 0.00E+00 1.00E–01

mtesla 2.00E–01 3.00E–01

4.00E–01

–5.00E–01

–1.00E+00

–1.50E+00

Fig. 10. Isothermal remanent magnetization and backfield for (a) ‘‘Basalto I’’ samples of the Ruca Choroy locality; (b) ‘‘Basalto II’’ samples of the Puesto Quiroga (near Rı´o Alumine´) locality (modified from Re´ et al., 2000a).

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Guillermo H. Re et al.

The many effusions of the ‘‘Basalto II’’ (Groeber, 1929), also known as ‘‘Coyocholitense’’ (Groeber, 1946), are grouped in the Tipilihueque Formation (Turner, 1976). This group of eruptions is 150 m thick and has been considered as Pliocene in age (Turner, 1976). The first paleomagnetic studies were made in Neuque´n Province (Fig. 8): 27 km and 35 km of Catan-Lil, in Aguada del Rinco´n, 30 km north of Zapala and 5 km south of Zapala (Creer and Valencio, 1969; Valencio et al., 1979). Re´ et al. (1997b, 2000a) presented results of paleomagnetic and petrographic studies of the Tipilihuque Formation. The sampled localities crop out in the Rahue (near Puesto Quiroga, Alumine´) and Lonco Luan plains (20 km toward the north). Re´ and Tomezzoli (1997) carried out a paleomagnetic study, collecting 50 samples in 10 sites. Magnetite is the principal carrier of remanent magnetization using IRM and backfield studies and hematite is occasionally present (Fig. 10b). Ruffo (1996) studied hysteresis cycles at room temperature and low temperatures (–190C). This author observed a smaller increment in coercitivity values and no increase in remanence, which is characteristic of different grainsizes. This mixture would be composed of single domain (SD) grains (SD) and multiple domains (MD) (Ruffo, 1996). The remanent magnetization/saturation magnetization (Jr/Js) rate is 0.5; this value is typical of SD. As temperature increases, the behavior of the coercitivity field has a lower value than in pure SD. The same happens with the Jr/Js rate, which reaches a value of 0.5 at low temperature whereas pure SD has values near 0.8 (Table 1). Finally, if the mixture of domains were of equivalent concentrations, the hysteresis cycle would be thinner (wasp waist in shape); that behavior is typical of equivalent mixtures of domains, but it was not observed in this study. All these results suggest an equivalent concentration of SD instead of MD. The oriented specimens were demagnetized by the stepwise thermal method (until 680C) and AF (until 110 mT). The samples have a simple behavior, in general with only one component. In a few samples it was found a second component that coincides with the present magnetic field. Both localities have normal polarity which suggests a single volcanic event. Galli (1969) included the basalts of Lonco Luan plain within the ‘‘Basalto I’’ group. This is based upon the finding of olivine andesites (with andesine and hornblende almost without Fe) in the lower levels, which is

typical of ‘‘Basalto I’’. On the other hand, Turner (1976) considered those outcrops to belong to the ‘‘Basalto II’’ group because of (1) the absence of agglomerates, which are very common in the Rancahue´ Formation (‘‘Basalto I’’) in the Rı´o Alumine´ valley; (2) outflows of the Rancahue´ Formation dip smoothly to the east, whereas in the Lonco Luan plain they dip to the S–SE; (3) the Lonco Luan, Rahue´ and Leo´n plains are at the same altitude and have a different altitude from the Rancahue´ Formation outcrops. Both localities have a normal magnetic polarity. However, Creer and Valencio (1969) maintained that the ‘‘Basalto II’’ rocks in other localities (two near Catan-Lil and one near Zapala, 75 km southeast and 100 km northeast of Alumine´, respectively) possess reversed polarity. These three sections of ‘‘Basalto II’’ rocks are dated by K/Ar; however, the radiometric datings have a wide interval of ages (6.7 + 0.3; 16.0 + 0.6; 8.0 + 0.2 Ma; Valencio et al., 1970a). Those ages are different from the assigned geological age (Late Pliocene). Hence, those ages are older than the ones obtained by the same authors in ‘‘Basalto I’’ outcrops between Zapala and Las Lajas, where the K/Ar ages of the localities are 4.7 + 0.5 (normal magnetic polarity), 5.4 + 0.3 (reversed magnetic polarity), 5.7 + 0.4 Ma (normal magnetic polarity) and 20.6 + 0.8 (reversed magnetic polarity). By doing this, Re´ et al. (1997a, 2000a) concluded that the Tipilihueque eruptions had a huge areal distribution. However, the sequences in Lonco Luan and Rahue would not be coeval with the ‘‘Basalto II’’ rocks cropping out in Zapala. If the stratigraphy of the outcrops were connected, it would be considered that the group of eruptions (Lonco Luan, Rahue, Zapala and Catan Lil) was produced during two periods of different polarities (normal and reversed). Or, instead, the outcrops of Lonco Luan and Rahue would correspond to intermediate eruptions between the ‘‘Basalto I’’ and ‘‘Basalto II’’ units. However, an absolute age of this volcanic event can only be established by renewed dating. The first paleomagnetic studies in the ‘‘Basalto III’’ rocks (Groeber, 1929), also named as ‘‘Lower Chapualitense’’ by Groeber (1946), were made by Creer and Valencio (1969) and Valencio et al. (1970a, c, 1979). They were carried out in six localities of Neuque´n Province (Estancia Llamuco, south of Estancia Llamuco, Puesto Ramadilla, 30 km north of Zapala, north of Chos Malal, Cerros Chalo and Rodeo) and three in Mendoza

Table 1. Parameters of hysteresis cycle for samples of ‘‘Basalto III’’ rocks (modified from Ruffo, 1996). Parameters of hysteresis cycle at room temperature (ta) and low temperature (tb) Specimen CB5 CB2 EM41

ta tb ta tb ta tb

J saturation (Am2 104)

J remanence (Am2 104 )

Jrem/Jsat

80.5 72.5 109.8 102.5 31.1 43.9

18.3 29.3 22.0 22.5 11.0 27.5

0.2 0.4 0.2 0.3 0.4 0.6

B

coercitivity (mT)

11.2 29.9 15.0 18.7 11.2 74.1

Bcbt/Bcta

Bcr (mT)

Bcr/Bc

2.7

40.0

3.6

1.2

70.0

4.7

6.6

65.0

5.8

Late Cenozoic Paleomagnetic Studies in Patagonia Province (4 km north of El Manzano, Puesto Herrera and Loma Negra-Diamante). Re´ et al. (1997b) presented a paleomagnetic study of 11 sites in Cerro Bandera (near the Zapala-Las Lajas road), Arroyo Primeros Pinos and Can˜ado´n Santo Domingo (near Puesto Monti), Neuque´n Province. In this, 110 specimens were processed, two for each of the five samples obtained at each site. The IRM studies point out that the magnetization is carried by magnetite, and a little hematite in some specimens. From hysteresis cycle studies at room temperature and at low temperatures (–190C) done by Ruffo (1996), it could be seen that the magnetic carriers are a mixture of SD and superparamagnetic (SP) magnetite in 1% concentration. In few cases the magnetic carriers are a mixture of MD and SD or PSD particles. The oriented specimens were demagnetized by stepwise thermal method (until 680C) and AF (until 100 mT). The samples show a simple behavior, in general with only one component. In a few samples, a second component was found that coincides with the present-day magnetic field. The Cerro Bandera and Arroyo Primeros Pinos outcrops present reverse polarity, whereas the Can˜ado´n Santo Domingo outcrops depict normal polarity. Re´ et al. (1997a, b) pointed out that in all the information about the ‘‘Basalto III’’ unit there is a great discrepancy between magnetic polarities and the geochronology of previous studies. This was also noted by Valencio et al. (1970a). Following stratigraphic and geomorphological criteria, several authors (such as Lambert, 1956; Turner, 1973; Holmberg, 1976; Zo¨llner and Amos, 1973) assigned a Lower Pleistocene age to the unit. Creer and Valencio (1969) and Valencio et al. (1970a) determined that the basalts cropping at 30 km of Zapala and in El Manzano present reversed polarity, whereas those cropping out near Estancia Llamuco, in Puesto Ramadilla, north of Chos Malal and in Puesto Herrera present normal polarity. These data, plus the results of Re´ et al. (1997a), suggest that every locality of the ‘‘Basalto III’’ unit would not be related to a single volcanic event. On the other hand, K/Ar datings made by Creer and Valencio (1969) and Valencio et al. (1970a) show important age dispersion that varies between 1.8 and 8.6 Ma, with four ages around 2.3 Ma. Basalts cropping out at Cerro Bandera (between Zapala and Las Lajas) were K/Ar dated to 8.6 + 3.5 Ma (Valencio et al., 1970a). On the other hand, the geochronologic information presents the problem that the error interval (2s) of ages in many cases includes more than one interval of magnetic polarity; thus, it generates important uncertainties in the magnetostratigraphic position of the studied units. Regionally, and following the present stratigraphic code, the ‘‘Basalto IV’’ unit (which Groeber, 1946, called ‘‘Upper Chapualitense’’) has been named as the Chapua Formation in the Buta Ranquil region (Holmberg, 1976) or the Huechahue´ Formation in Junı´n de los Andes (Turner, 1973). These groups of eruptions, 50 m in thickness, are considered of Early Pleistocene age, though some datings by Valencio et al. (1969) considered this unit as of Pliocene–Early Pleistocene age. Paleomagnetic studies of the ‘‘Basalto IV’’ lavas were carried out by Creer and Valencio (1969), and Valencio

131

et al. (1969, 1970c, 1979) in Neuque´n Province, in Estancia Llamuco, at 35 km south of Las Lajas, Rı´o Barranca, Los Mallines de la Puntilla, Puntilla de Huinca´n, Cerro Chivato, Cerro La Menea and Aguada Lastra (Auca Mahuida), and in Mendoza Province, in Cerro Negro, Las Malvinas, Cerro San Rafael, Rı´o Seco – Los Toldos, Puesto Ortiz, 25 Isle de Mayo, Cerro Los Chanchos and Cerro Chato. Re´ (1997a) presented the results of a paleomagnetic study of samples of vulcanites of the ‘‘Basalto IV’’ unit from four sites in the Arroyo Covunco valley (National Route 40). From each of the five samples of each site, two specimens were obtained. Forty specimens were processed. The IRM studies and hysteresis cycles were carried out on these specimens (Ruffo, 1996). These studies show that magnetite is the principal magnetic carrier and hematite the secondary carrier in the same specimens (Ruffo, 1996; Re´, 1997a). Hysteresis cycle studies at room temperature and at low temperatures (–190C) show that the samples have constant values against temperature changes, which is typical of MD magnetite. Coercitivity of magnetic remanence/coercitivity field (Bcr/Bc) values near 4.0 are indicative of mixture of grainsizes and rather high Jr/Js values of 3.3 show a mixture of MD and PSD, more probable than SD presence (Ruffo, 1996). Re´ (1997a), using stepwise demagnetization techniques at high temperatures up to 700C and AF until 95 mT, found a two-component behavior in almost all the specimens with the second component assigned to present magnetic field; few specimens have a monocomponent behaviour. All the samples have a reversed magnetic polarity. Re´ (1997a) correlated the magnetic polarity obtained in this locality with geochronological and paleomagnetic data obtained by Creer and Valencio (1969) and Valencio et al. (1969). He found that the rocks from Arroyo Covunco and Estancia Llamuco present reversed polarity, whereas those from Rı´o Barranca, Los Mallines de la Puntilla and Puntilla de Huinca´n show normal polarity. In all cases the rocks were assigned, following geological criteria, to the Late Pleistocene (Groeber, 1929, 1946; Gonza´lez Dı´az, 1972; Zo¨llner and Amos, 1973; Holmberg, 1976, etc.) or, instead, to postglacial times (Lambert, 1956). Using paleomagnetic and geochronological studies (Valencio et al., 1969), the basalt of Estancia Llamuco was dated as Pleistocene, whereas the Los Mallines de la Puntilla outcrops were doubtfully assigned to the Pleistocene, the Puntilla del Huinca´n flow as Early Pleistocene–Late Pliocene, and that of Rı´o Barrancas as Late Pliocene. The group of ‘‘Basalto IV’’ rocks represents a continuous magmatic period between the Late Pliocene and the Pleistocene. In this case, the basalt outcrop in Arroyo Covunco can be correlated with those in Estancia Llamuco, and thus, it may be assigned to the Pleistocene (Re´, 1997a). The basaltic sequences cropping out in Mendoza Province correspond to a later volcanic event around the Pliocene– Pleistocene boundary. The volcanic centers of the ‘‘Basalto IV’’ or ‘‘Puentelitense’’ units (Groeber, 1946) are located between Precordilleran blocks which are separated by graben structures. These structures suggest a Plio–Quaternary

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Guillermo H. Re et al.

extensional period; this is the reason why the eruptions are limited by large normal faults of N–S orientation. In general, the basalts are alkaline, representing low-grade differentiation (Bermu´dez et al., 1993). Creer and Valencio (1969) and Valencio et al. (1970a, 1979) studied volcanic sequences in Cerro Michacheo (Zapala), Buta Ranquil, Tricao Malal, Portada de Covunco, Estancia Llamuco and Ranquil Norte, in Neuque´n Province, and 4 km south of Mallines de la Puntilla and south of Las Chacras, in Mendoza Province. Re´ (1997b) carried out paleomagnetic studies on seven sites of ‘‘Basalto V’’ rocks located at Cerro Michacheo (near Zapala) and Portada Covunco (20 km northwest of Zapala) (Lambert, 1956). These sequences are 50 m thick. The analysis of the stepwise demagnetization at high temperatures until 700C and AF until 90 mT within 40 specimens showed mono-component behavior in almost all samples and two-component behaviour in just a few samples. The secondary component coincides with the present-day magnetic field direction. The remanent characteristic magnetism directions of the Portada Covunco rocks correspond to reversed polarity and those of Cerro Michacheo flows belong to normal polarity. The IRM studies indicate that the magnetization is caused by magnetite and in few specimens by hematite (as secondary carrier) (Re´, 1997b). Some calcographic studies allowed the identification of grains of titanomagnetite altered to hematite or unmixing to ilmenite. In general, the grains of titanomagnetite have irregular shapes and, in some cases, they have a skeletal habit. It is also observed by the presence of hematite (Pucci, 1967). Re´ (1997b) correlated his paleomagnetic data with Creer and Valencio (1969) and Valencio et al. (1970a) data for Buta Ranquil, Tricao Malal, Estancia Llamuco (all in Neuque´n Province), and Ranquil Norte, Mallines de la Puntilla and Las Chacras (in Mendoza Province). He suggested that the outcropping of Portada Covunco flows and the localities sampled by Creer and Valencio (1969) and Valencio et al. (1970a) would correspond to a coeval volcanic event which occurred during a normal magnetic polarity epoch, whereas the Cerro Michacheo volcanic rocks with reversed magnetic polarity would indicate that the event had a longer duration or that they correspond to independent and separate events. Basaltic outflows of the ‘‘Basalto VI’’ unit crop out on the eastern bank of the Rı´o Grande in Puntilla del Huinca´n region, in southern Mendoza Province. Those outflows were K/Ar dated in 0.2 + 0.1 Ma by Valencio et al. (1970a, b). Their normal magnetic polarity is assigned to the Late Pleistocene–Holocene (Valencio and Creer, 1968) and the Brunhes magnetic interval (Valencio and Creer, 1968; Valencio et al., 1969, 1970a, b). Lavas corresponding to the ‘‘Basalto VII’’ unit were studied by Creer and Valencio (1969) and Valencio et al. (1970a, b) in outcrops near Estancia Llancanelo. These volcanic rocks were K/Ar dated and are at least 0.1 Ma old, although Valencio et al. (1970a, b) suggested that this age might be contaminated with atmospheric argon. From several studies, it has been established that they

have normal magnetic polarity and can be assigned to the Brunhes Chron (Late Pleistocene–Holocene), Valencio et al. (1970a, b).

3. Paleomagnetic Studies from Glaciolacustrine Sediments Strelin et al. (1999) analysed the chronology of glacial and basaltic deposits cropping out on margins of Alto Valle of Rı´o Santa Cruz (Fig. 1, locality 20), covering Pampa Alta and La Meseta plains. Those basaltic outflows should have developed at the same time as an important glacial episode in the Cordilleran region. This activity was developed during the Late Pliocene, when the outflows covered the tributary and main valleys, whereas extra-Andean valleys were affected by fluvial and lacustrine aggradations. Lavas of Middle Pleistocene eruptions covered the plains and were channelled in preexisting fluvial valleys. The age of those basalts was between 2.43 and 1.03 Ma (Fleck et al., 1972; Mercer et al., 1975), whereas the magnetic polarities of the same rocks should correspond to late Gilbert – early Gauss magnetic intervals. Early paleomagnetic studies from glaciolacustrine Pleistocene sequences in Patagonia have been carried out by Kodama et al. (1984, 1985a, b, 1986), studying the Pichileufu´ Drift (Flint and Fidalgo, 1964). The sequence is dominated by rhythmites with bands of silt and clay and it is the outermost drift identified in the San Carlos de Bariloche region (northern Patagonia, Fig. 1, locality 13). From these paleomagnetic studies, it is known that the El Condor glaciation (Flint and Fidalgo, 1964) and probably the Pichileufu´ glaciation would have occurred during the normal polarity Brunhes Epoch (<0.7 Ma). However, the Pichilefu´ sediments record an episode of extremely low inclination of the geomagnetic field during the Early to Middle Brunhes Epoch. This low inclination would correspond to a transition polarity or to a geomagnetic excursion; so it would be considered that those sediments were deposited during the Brunhes– Matuyama transition (0.7 Ma) or during one of the polarity transitions or excursions of the geomagnetic field of Brunhes age (Emperor or Biwa Event) (Kodama et al., 1986). Interpreting the rhythmites of first-order cyclicity as varves and the second-order cyclicity as interseasonal lamination, a number of 74 annual varves were counted and measured for a 10 m section outcropping east of Lago Nahuel Huapi (Fig. 1, locality 13) (Sylwan, 1989). The short duration of the deposit is consistent with the stability in the directional magnetic parameters found by Kodama et al. (1985a, b; 1986). The low magnetic inclination is interpreted as recording the upper part of the Brunhes/Matuyama polarity transition (Sylwan, 1989). An alternative explanation has been provided by Schlieder (1988), who suggested that these glaciolacustrine deposits are in fact representing an outer, ice-contact phase of the La Fragua glaciation, that is the earlier part of Flint and Fidalgo’s (1964) ‘‘El Co´ndor Drift’’. Thus, a Brunhes age for these materials

Late Cenozoic Paleomagnetic Studies in Patagonia –10 km

0

10

20

D

G

F

V

Va

30

40

50

+60 km

I

C III

IV

II

I

7

6 5

B 15 14 13 12 11 10 9

A

1

2

BRUNHES VGP – Latitude

N

S

8

4

3

?

4

1

3 2

0?

7 8

5, 6

MATUYAMA

?

60 30

60 30

0 –30 –60

0 –30 –60

N

S

BRUNHES

Fig. 12. Terminal moraines and moraine zones along three profiles to east of Lago Buenos Aires. (F = Finiglacial, G = Gotiglacial, D = Daniglacial, I = Initioglacial, according to Caldenius’ (1932) nomenclature), showing five main zones (I–IV) and 15 individual terminal moraines (1–15). 1–8 corresponding to sampled locality for paleomagnetism (from Sylwan, 1989).

FINIGLACIAL

Va Vb

14–15 >14 KA 12–13 <120 KA

GOTHIGLACIAL

IV

9–11 MID - B

DANIGLACIAL

III

5–8

~0.7

INITIOGLACIAL

II

2–4

~1.2

INITIOGLACIAL

I

1

~2.3

MATUYANA

0.73

2.48

GAUSS

would be undisputable, this transitional event being an intra-Brunhes Chron excursion. Sylwan (1987, 1989 and 1990) made paleomagnetic studies of glaciolacustrine sequences outcropping near Lago Blanco (Fig. 1, locality 14). Paleomagnetic measurements have been carried out from a sequence of 178 varves. The deposition of the glaciolacustrine sediments has been determined as associated with the last glaciation maximum (LGM), occurring either at around 20,000–19,000 yrs BP or at around 14,500–14,000 yrs BP. From paleomagnetic studies it could be stated that the low values of inclination and the easterly swing recorded in declination at the Lago Blanco sediments cannot be attributed to mechanical deformation or disturbance undergone by sediments. Mean declination and inclination values were used for the calculation of corresponding VGP positions (Fig. 11). In general, the VGP positions are located in northern Canada: during the first 40–50 yrs in the central region and during the last 90 yrs within a fairly stable zone in north-eastern Canada. The pattern is interrupted by a rapid swing down to central Africa lasting from varves 59 to 93, that is a period of 35 yrs. Because the values of declination and inclination cannot be explained in terms of sedimentological disturbance, this 35-year swing may represent a very short excursion of the geomagnetic field (Sylwan, 1989, 1990). In the Lago Pueyrredo´n zone (47200 S, 71 W; Fig. 1, locality 15), Sylwan et al. (1991) studied the four morainic systems proposed by Caldenius (1932). The samples representing Caldenius’ (1932) Finiglacial, Gotiglacial and Daniglacial intervals have normal polarity, which correspond to the Brunhes Epoch. The samples from the outermost Daniglacial have a reversed polarity which has been interpreted as belonging to the Matuyama ( > 0.7 Ma) Epoch (Fig. 12). The samples of Inicioglacial sediments have normal polarity corresponding to a normal event occurring during the Matuyama Epoch. Considering the normal polarity events that occurred during the Matuyama Epoch, the Olduvai Subchron (1.67–1.87 Ma) seems the more probable, because the Jaramillo Event (0.90–0.97 Ma) is too young taking into account that the age of the oldest Daniglacial is late Matuyama (Mo¨rner and Sylwan, 1987; Sylwan, 1989; Sylwan et al., 1991) (Fig. 13).

133

~3.0 MA

GENERAL CHANGE 3.40

180° W

41

52

References

0°

90° 43

80°

49

23 1 25

Normal polarity

Reverse polarity

88

37 101–172 78

84

Latitude

59

0° 68

80°

Longitude

Fig. 11. Positions of the VGPs calculated from mean declination and inclination values for the sequence of Lago Blanco (open triangle indicates the sampling place) (from Sylwan, 1989).

Fig. 13. Magnetostratigraphic position of the five main glaciations recorded along the Andean ranges in Patagonia (cross-hatched zones). I–V = moraine zones; 1–15 = individual terminal moraines in Lago Buenos Aires (from Sylwan, 1989).

In Lago Buenos Aires (Fig. 1, locality 16), 15 separate terminal moraines have been identified and grouped into five major zones representing ‘‘ice ages’’ or glaciation complexes. Paleomagnetism of proglacial sediments beneath the terminal moraines indicates that the eight innermost moraines (and two zones) have a normal (Brunhes) polarity and the five outermost moraines (and two zones) have a reversed

134

Guillermo H. Re et al.

(Matuyama) polarity. This indicates that the original chronology of Caldenius (1932), and hence correlations with the Northern Hemisphere, must be completely revised (Sylwan, 1989). Similar conclusions have been proposed by Ton-That et al. (1999), based on 40Ar/39Ar dating of associated lava flows. A sequence of 887 varves at Lago Buenos Aires (46350 S, 71 W) provides a 660 yrs paleomagnetic record. The age of the varves is in the range between 12,500 and 13,500 yrs BP (Sylwan, 1987). A continuous ice retreat is inferred through the gradual decrease in the thickness of the varves. Paleomagnetic studies show very strong intensities and stable polarity within single varves a well as from varve to varve. The general trend of declination is a smooth shift from 40–50 W to 0 at the top. The inclination shifts from – 70/–80 to –35 at the top of sequence. At the base of the sequence (13,500 yrs BP), the VGP is located in the ArcticSiberian region and swings later to W Greenland/NE Canada (13,000 yrs BP; Fig. 14). A similar swing is recorded in the Northern Hemisphere (Sylwan, 1987, 1989). In the Alto Rı´o Mayo valley (45350 S, 71300 W; Fig. 1, locality 17), a glaciolacustrine sequence containing 300 varves in between two tills was paleomagnetically studied, showing strong intensities, a gradual declination shifting to the west and an increasing negative inclination. The general declination trend shifts from 10 W at the base to 40 W at the top, while inclination shifts from –30 at the base to –60 at the top of the sequence (Fig. 15). The VGP positions reveal a wandering trend from NE Canada to SW Greenland to a final position around NW Siberia (Fig. 16). There are two possibilities when comparing the paleomagnetic record from Lago Buenos Aires with that of Alto Rı´o Mayo: the first is slightly older or slightly younger than the second one. The fact that the deposit of Alto Rı´o Mayo lies on the terminal moraine slope seems to indicate an age slightly older than that of Lago Buenos Aires, which lies a few kilometers inside the terminal moraine. This evidence supports an age for the Alto Rı´o Mayo sequence which is

180°

13.5

90° W 12.9

0°

Fig. 14. Positions of the VGPs for the sequence of Lago Buenos Aires (from Sylwan, 1989). slightly greater than 13,500 yrs BP. If so, a minor readvance might represent the final ice-marginal oscillation before the definitive ice retreat (Sylwan and Beraza, 1990). At the site of La Misio´n, northern Tierra del Fuego (53400 S, 67500 W; Fig. 1, locality 18), a 10 m oriented core of Holocene sediments has given the first paleomagnetic record. By microfossil analysis ages ranging between 8000 yrs BP for the basal sediments and less than 3000 yrs BP for the uppermost sediments are inferred. Paleomagnetic results show a very scattered record. Magnetic susceptibility and intensity of remanence are properties that depend on the kind and proportion of magnetic minerals in the sediment, whereas the mineralogy, concentration, magnetic grainsize and morphology depend on the environmental condition of

Dec. 3

2

1

cm

300 W 17 cm 10 cm

300

200

Varves

4

100

1

0

90° E

Inc. 60 E

–90

–60

–30

0

Fig. 15. Declination and inclination curves for the sequence of Alto Rı´o Mayo valley (open circles = NRM values, black circles = values after 300C demagnetization). The thickness of the varves is shown in the left portion of the figure (from Sylwan and Beraza, 1990).

Late Cenozoic Paleomagnetic Studies in Patagonia 180°

135

180°

1100

90° E

90° W

270°

740 980 300 60

80°

860

70°

60°

180 620

0°

90°

500

0°

Fig. 16. Positions of the VGPs for the varve sequence from Alto Rı´o Mayo valley. The shadowed arrow indicates the general shift (from Sylwan and Beraza, 1990). deposition. Changes of magnetic properties may provide information about environmental changes. In this way a sharp decrease of magnetic susceptibility and intensity is interpreted as corresponding to the regression of the sea at about 3000 yrs BP (Fig. 17) (Sylwan, 1989). A 78 m Lateglacial glaciolacustrine sequence was studied and sampled at Rı´o Corintos, Chubut Province (43100 S, 71150 W; Fig. 1, locality 19), by Beraza and Vilas (1990) and Beraza (1991). Approximately

Intensity Susceptibilty Q-factor (μ emu/cm3) (μ G/Oe) = Int./Susc. (Cgs) 1 10 100 10 100 0.1 1

1

50°

10

Fig. 18. Path of the VGPs calculated from mean declination and inclination values, using a stratigraphic window that spans 120 varves (from Beraza and Vilas, 1990). 1089 varves were counted; they have been deposited between 25,000 and 13,000 yrs BP. The samples showed strong and stable values of remanent magnetization, and the magnetite was identified as the mineral carrying the remanence. This sequence presents normal magnetic polarity (Brunhes Chron); the declination of the remanent magnetization oscillate between 30 W and 15 E and inclination between –3 and –56. This wandering trend was assigned to secular variations of the geomagnetic field. The VGPs show a 100% clockwise rotation, wandering around northern Canada (Fig. 18) (Beraza and Vilas, 1990; Beraza, 1991).

Lithology Peat

1

1

2

2

3

3

4

4

5

5

6

6

Blue clay

4. Paleomagnetic Studies from Sedimentary Sequences

G r e y B

Clay l

7

7

8

8

9

9

10

10

11 m

11 m

a c k (FeS)

~~~~~~~~~~~~~~~~~~

Grey clay Boulder clay (Wellowish)

Fig. 17. Intensity, susceptibility and Q-factor curves of La Misio´n site, northern Tierra del Fuego, plotted parallel to a schematic sedimentological profile (from Sylwan, 1989).

The study of the behavior of the EMF during excursions and reversals is extremely complex due to many variables that affect the paleomagnetic recording process and the nature of the record (Merrill and McElhinny, 1983; Bogue and Merrill, 1992). However, though far from ideal, sedimentary sequences are suitable recording material because they allow continuous stratigraphic analysis, have a broad geographical distribution and may be datable by 14C or other techniques. In this sense, the archeological and paleontological sedimentary deposits located in southern South America have shown that they may contribute to the knowledge of the past geomagnetic field behavior. Paleomagnetic studies carried out in cave sediments and archeological sites from southern Argentina and Chile yielded evidence about PSV and a possible excursion of the geomagnetic field that occurred during the Holocene (Nami and Sinito, 1991, 1993; Nami, 1995; Nami et al., 1995; Sinito et al., 1997). Several paleomagnetic studies from sedimentary sequences, Late Pleistocene to Holocene in age, located at

136

Guillermo H. Re et al. presented by Nami (1999a) (Fig. 1, localities 11 and 12). The author found that a gentle but significant eastward shift in the declination (over 40) and a less conspicuous shallowing of the inclination can be observed on the upper part of the section, between 2.5 and 1.9 ka BP. The declination and inclination from Las Buitreras Cave show long pulses that suggest a possible excursion of the geomagnetic field during the Late Pleistocene to Early Holocene, similar to the one found in Mylodon Cave (Nami, 1999a). The positions of the corresponding VGPs are plotted in Fig. 19b and c. Many other archeological and paleontological sites in southern South America have yielded records of a probable excursion of the EMF at sometime in the Late Pleistocene and Early Holocene (Nami, 1999a). This suggests that the ‘‘Mylodon excursion’’ may have had a regional extent. This data was included in one analysis of VGPs distribution (Mena and Nami, 2002) made on the bases of spherical densitygrams (Love, 2000). The distribution suggests that not only the transitional VGPs have peculiar distribution, but also those of stable polarity or possible excursion fields are displayed in a geographic pattern where the longitudes have a nonuniform distribution (Mena and Nami, 2002) (Fig. 20a, b, c). The VGPs from this excursion coincide remarkably with those from the Red Rock archeological site, California (Fig. 20d; Mena and Nami, 2002), where paleomagnetic studies from three sedimentary sections attributed to Middle and Late Holocene, yielded normal, intermediate and reversed directions (Nami, 1999b). This suggests that the EMF probably underwent an excursion in southwestern North America during the Middle Holocene. Similar excursions were found in different sections and materials from Northern and Central Europe, Eastern Asia, South and North America. This may suggest that some

archeological and paleontological sites across southern Argentina and Chile, were presented by Nami (1995, 1999a). These studies were carried out with samples collected in cylinders in such a way that each sample overlaps the next one by about 50% and then they were consolidated with sodium silicate. Following stepwise thermal and AF demagnetization procedures, consistent characteristic remanence directions were determined. Paleomagnetic data from a section of a sedimentary sequence at Mylodon Cave (Seno de Ultima Esperanza, Chile) (Fig. 1, locality 10) are reported by Nami (1995). According to previous 14C data, the sediments have ages between 13,500 and 5360 ka BP. The unblocking temperature ranging from 450 to 550C suggested that the magnetic mineral belong to the titanomagnetite series. The changes in declination and inclination of the ChRM for the entire section show normal polarity directions with small deviation from the present axial dipole direction in the lower section, whereas samples from the top section show directions corresponding to an obliquely normal and obliquely reversed field. The corresponding VGPs are plotted in Fig. 19a. These suggest the existence of an excursion of the earth magnetic field found in southern South America during the Early to Middle Holocene, the ‘‘Mylodon excursion’’, younger than 10 ka BP according to the available radiocarbon ages. The inclination and declination found from the top part of Mylodon Cave agree with the data from Angostura Blanca (Nami and Sinito, 1993) which probably correspond to the last phase of the ‘‘Mylodon excursion’’ (Nami, 1995). Other paleomagnetic data from two sedimentary sequences corresponding to the Late Pleistocene to Holocene (11 + 0.5 ka BP), located in Piedra del Aguila (Neuque´n Province) and Las Buitreras Cave (Santa Cruz Province) are

Equator

90°

0°

90°

(a)

Equator

Equator 90°

0°

(b)

90°

90°

0°

90°

(c)

Fig. 19. VGPs positions for the excursion recorded in (a) Mylodon Cave; (b) Piedra del Aguila; (c) Las Buitreras (after Nami, 1995, 1999a).

Late Cenozoic Paleomagnetic Studies in Patagonia (b)

(a)

90

90

0.

02

0.

02

30

30

0

0

0.0

0.0

2

2

–30 –60

–30

0.02 0.02

Mylodon

–90 –180 –150 –120 –90 –60 –30

2 0.0

latitude

60

0.02

0.02

0.02

60

Piedra del Aguila

–60

0

30

60

90

–90 –180 –150 –120 –90 –60 –30 120 150 180

(c)

0

30

60

90

120 150 180

(d) 90

90 60

0.02

60 30

0.02

30

latitude

137

2

0

0.0

0

0.02 0.02

0.0

2

–60

0.02

–30

–30 –60

Las Buitreras

–90 –180 –150 –120 –90 –60 –30

0

30

60

90

0.02

Red Rock

–90 –180 –150 –120 –90 –60 –30 120 150 180

longitude

0

30

60

90

120 150 180

longitude

Fig. 20. Contour plot map of densities of VGP (units are probability/steradian) for the records from (a) Mylodon Cave; (b) Piedra del Aguila; (c) Las Buitreras; and (d) Red Rock (modified from Mena and Nami, 2002).

anomalous geomagnetic phenomenon might have occurred globally, although perhaps not simultaneously. A global reverse excursion (Merrill and McFadden, 1994) might have occurred sometime between Late Pleistocene and Late Holocene. According to Nowazick et al. (1994), during the last 170 ka and particularly between 50 ka and the latest Pleistocene, the normal polarity of the EMF has been interrupted by several short-lived reversed polarity events. The observed Holocene excursion might be another short-term manifestation of this process. 5. Paleomagnetic Studies on Lake Cores The remanent magnetization in lacustrine sediments has been widely studied since early paleomagnetic studies. Works by Mackereth (1971) in Windermere lake sediments (England) can be mentioned. This author found an earth magnetic field variation of 2000 yrs period studying a core from the Black Sea which covered the period 7000–25,000 yrs BP. Creer et al. (1972) also obtained new data in earth magnetic variation in the sediments of Lake Windermere. Thompson et al. (1975) checked the periodicity in secular changes of the magnetic declination registered in Lake Windermere and other lakes in NW Great Britain, but he did not find a correlation between these results and others in Europe, thus implying a complex model of secular changes (Valencio, 1980). The first studies of this kind in Argentina were made in Patagonia, in lake and pond sediments of Rı´o Negro Province. Several studies in lacustrine sediments can be mentioned: Laguna El

Tre´bol (Fig. 1, locality 21) (5780 + 100 yrs BP at a depth of 450 cm and 3040 + 180 yrs BP at a depth of 115 cm) (Valencio et al., 1982), Brazo Campanario of Lago Nahuel Huapi (Fig. 1, locality 21) (Sinito et al., 1983), and Lago Moreno (Fig. 1, locality 21) (7540 + 160 yrs BP at a depth of 335 cm and 4730 + 140 yrs BP at a depth of 225 cm) (Mazzoni and Sinito, 1982). Creer et al. (1983) fitted the correlations among lakes using 16 14C ages, between 540 + 50 and 11,710 + 120 yrs BP (see Table 2 and Figs 21 and 22). The first paleomagnetic and geochronologic results in Lago Escondido (Fig. 1, locality 21) were presented by Gogorza et al. (1998, 1999). These authors presented 12 14C ages, 11 of them from Lago Escondido and one from Lago Moreno; they also presented profiles of declination and inclination of remanent magnetism vs depth, which in these sequences have recorded the PSVs of the EMF. The representative profiles of variation intensity of NRM (Jo) and magnetic susceptibility (k) of the sediments allowed the authors to correlate the studied sequences. The characteristics of the Jo and k profiles would be related not only to the composition and amount of magnetic particles, but also to the dominant size of the mineral grains. In this way, the analysis of those parameters allows to delimite the same sections as defined by sedimentological–geological studies (Valencio et al., 1982; Figs 23 and 24). The interpretation of the profiles shows that during the deposition of the sequence sudden changes occurred in the clast composition in different lakes. These changes are recorded in both peaks of maximum Jo and k, suggesting a sudden increase in the number of magnetic minerals of volcanic origin (Sinito et al., 1981, 1983; Figs 25 and 26).

138

Guillermo H. Re et al.

Table 2. Radiocarbon dating for samples of El Tre´bol, Campanario and Moreno lake cores (modified from Creer et al., 1983). Core name

SRR no.

LT1 LT1 LT1 LT2 LT2 LT2 LT2 LM3 LM3 LM3 LM4 LM4 LM4 LC7 LC7 LC7

1816 1952 1817 1953 1954 1955 1956 1814 1815 1948 1949 1950 1951 1957 1958 1959

Average depth (cm)

Percentage organic carbon content (+0.05%)

115 170 450 70 130 170 350 240 340 455 100 300 370 70 150 330

C Age (yr BP + 1s)

3040 + 180 1410 + 60 5780 + 100 650 + 60 1320 + 50 1840 + 60 4210 + 60 4730 + 140 7540 + 160 11,710 + 120 1660 + 60 8600 + 70 10,820 + 90 540 + 50 1650 + 60 5190 + 70

2.80 1.50 2.40 4.00 6.60 1.10 2.20 0.70 0.90 0.90 0.75 1.15 1.50 1.05 1.60 0.50

El Trebol

1

Campanario –40°

–70°

1

–40°

28.4 26.8 28.4 27.0 28.2 27.8 26.6 22.2 20.9 23.0 23.2 23.1 23.5 23.1 23.6 23.3

Declination

Inclination –70°

d 13C + 0.10%

14

El Trebol

Moreno –70°

1

–30°

–40°

2 3

1

0°

Campanario 30° –30°

1

0°

Moreno 30°

–30°

1

4 2

6 7 8 9

2

Depth (m)

Depth (m)

2

2

2

3

3

3

4

4

4

4

5

5

5

5

5

6

6

6

6

6

3

3

3

4

4

5

6

30°

2 3 4

5 2

0°

5 6 7 8 9

Fig. 21. Mean declination and inclination log for El Tre´bol, Campanario and Moreno lake sediments (from Sinito et al., 1984). These results were completed with later studies of rock magnetism (Gogorza et al., 1999), in which it was determined that the peaks of Jo, specific susceptibility (h), saturation of the isothermal remanent magnetisation (SIRM) and anhysteric remanent magnetisation (ARM) observed values in the sequence are coincident with lithologic changes through the sequence in Lago Escondido. The high testvalues of low frequency susceptibility correspond to tephra layers, and the lowest values correspond to material with a high proportion of organic matter and fine to very fine grainsize (Fig. 27) (Gogorza et al., 1999). Taking into account the magnetic response, a lithostratigraphic correlation within the cores of the sediments has been made as well, enabling to observe a deepening of the lower sections and thickening upper sections in the center of the lake (Mazzoni and Sinito, 1982). The characteristics of Lago Moreno are similar to those of Lago El Tre´bol (Valencio et al., 1982). Taking into

consideration those features and other physiographic, geological and geographic analogies, it can be established that the general genetic conditions of both lakes were similar. However, the velocity of accumulation of the sediments in Lago Moreno was lower than that of Lago Nahuel Huapi (Brazo Campanario) and Lago El Tre´bol (Sinito et al., 1984a, b; Valencio et al., 1985). In this aspect, the accumulation is related to volcanic-pyroclastic eruptions from postglacial to recent times, as observed in the characteristics of sediment composition and texture, geochronologic absolute ages and the normal polarity of the remanent magnetism (Mazzoni and Sinito, 1982; Valencio et al., 1985). From the analysis of variation in inclination and declination vs time, a long trend in inclination was observed, in which period is similar to the length of the record. For the last 6500 yrs, the declination values are lower than the mean: on the other hand, the values

Inclination El Trebol

AGES C14 (ky)

–70°

Declination

Campanario

–40° –70°

El Trebol

Moreno

–40° –70°

–40°

–30°

1

1

2

2

3

3

4

4

5

5

6

6

7

7

8

8

9

9

10

10

11

11

12

12

13

13

14

14

0°

Campanario 30° –30°

0°

Moreno

30° –30°

0°

30°

Fig. 22. Mean declination and inclination log for El Tre´bol, Campanario and Moreno lake sediments in time scale (from Sinito et al., 1984). Lt1 1

4

5 m

1

G)

x

x x xx x x x xx x xx x x xx x xx x

x

1

xx x x x x x x x x x xx x xx x

x x x x

x xx x x x xx xx x x xx x x x xx x x x x x x x x xx x x xx x x x x x xx x x x x x x x xx x x x x x x x x x x xx x x x x x x xx x xx x x xx x x x x x xx xx x x x x x x x x x xxx x x xx x x x x x xx

2

xxx xx x x xx

x

3 x

4

10

x x x xx xxx xx x xx xx x x x xx xx x x x x xx x x xx x xxx xx xx xx xx x xx x

x

x xx x x

Lt3 2

10

x x x

x

3

–6

10 Jn(10

x

x x x x xx

2

3

10

x xx x x xx xx x

1

Lt2 2

10

x x

10 Jn(10

–6

G) 1

10

x

x

1

x x

xx

x x

x

2

x

x

x x x x x x x x x xx x x x xx x x xx x x xx xxx x x x xx x x x x x xx xx x x xx xx x x x x xxx xx

3

4

2

10

3

10 Jn(10

–6

G)

x

x xx x xx xx xx x x x x x x x x x x x x xx x x x x x xx x x x x x xx x x xx x x x x x xx x x x x x xx xx x x x x x xx x x x xx x xxx xx xx x x x x x x x x xx x x x x x x x x x xx xx x xx x x x x x xx xx xx x x x x xx x x x x x x x x x xx x x x x

Section I

Section II

Section III

Section IV

Section V

Reference Magnetic characteristics inside sections of fine grain. Magnetic characteristics inside sections of coarse grain. Correlations lines between sections of different grain.

5 m

5 m

Correlations lines inside a section.

Fig. 23. Correlation log intensity of natural remanent magnetization for the longer core of Lago El Tre´bol (from Valencio et al., 1982).

140

Guillermo H. Re et al. Lt1

Lt2 102

10

1

x xx xx x x x x x

x

3

G/Oe) 1

x

x

x x

x

x x x

x

1

x x x

1

x

x x x

x x x xx x xx x x x x

x

3

x

x

x x

4

3

x

xx

x

x x

x

x x

xx x x

x x

x x x x x x x

x

x

x

x

x

xx xx x

x x x x x x x

x

x

3 xx

x x x x x

xx

xxx xx

4 x x

x

x

x

x xx

x xx x x x x

4

xx x xx x x x x x x x xx x x x x x x

x

x

x x x x

x xx x x x x x xx x

x xx

x xx

x

Section III

x

x

4

Section IV

x

x x

Section II

4

x x x x x x x x x x x x x x xx x x xx x x x x x x x x x x x x xx x x x

x

x

x

x

5

x x

xx

x

4

x x x x x x x x x x

x x

x x x xx x x x x x x x

xx x x xx x x

Section I x x

x

x xx

2 x

4

x x

x

xxx

G/Oe)

5

x x

xx

–6

x

x

xx

x

x

x

x

x

x x

x x x x x

xx x xx x x x

3

10 Jn(10

x

x

1

x

2

x

x x x

x

x

x

10

x

x x x xx

x

x xx x

5 m

x xx

x x x x x xx x x x x x x x x x

x

x

x

x

10

x

x

x

G/Oe) 1

x

x x x x xx x x x

x

x

x

–6

x xx xx x x x x x

x

x x x x x xx x x xx x x

x x

x xx xx x x x xx x xx x

x

x x

2 x x

10 Jn(10

x x

x

x x x xxx x x x

x x x x x xx x x x x x x x xx x x xx x xx x x x xx

x x x x x x x x x x

x

2

10

x

x x x x

Lt3 2

10

xx

x xx

x x x x x x xx xx xx

–6

10 Jn(10

5 5

Section V

5

Reference

x x

Magnetic characteristics inside sections of fine grain. Magnetic characteristics inside sections of coarse grain.

5 m

x

5 m

Correlations lines between sections of different grain. Correlations lines inside a section.

Fig. 24. Correlation log of the magnetic susceptibility for the longer core of Lago El Tre´bol (from Valencio et al., 1982). Declination T1 310 330 0 0

30

Inclination T2

50(°)

310 0

0

T1 50(°)

–90 0

–60

T2 –30

0(°)

–90 0

0.5

0.5

0.5

0.5

1.0

1.0

1.0

1.0

m

m

m

m

(a)

60

30

0(°)

(b)

Fig. 25. Correlation log of declination and inclination of the natural remanent magnetization NRM for two cores obtained in Lago El Tre´bol (from Sinito et al., 1981).

Late Cenozoic Paleomagnetic Studies in Patagonia Sectors 4 and 5 levels

Brazo Campanario

Jn (10–6 G) 10

100

1000

0

50% 0

0

50%

100%

141

Fig. 26. Correlation log intensity of natural remanent magnetization, heavy minerals and vulcanite percentage in core LC3 obtained in Brazo Campanario, Lago Nahuel Huapi (from Sinito et al., 1983).

1

2

3

are higher than the mean from 12,000 to 6500 yrs BP. The inclination records show oscillating values around the mean in the last 2000 yrs: higher than the mean values are observed from 2000 to 6500 yrs BP and lower than the mean values from 6500 to 12,000 yrs BP. This behavior suggests an out-of-phase relationship (near 90 offset) between peaks and troughs of inclination and declination (Gogorza et al., 2000a, b). In the case of Lago Escondido, inclination data show two very well-defined periods: a long period (about 7700 yrs) and a short period (between 2900 and 2600 yrs according to the method). Declination data, although less reliable, show two intermediate periods of about 3700 and 2200 yrs, and a long wavelength trend (Gogorza et al., 1999). The analysed relative paleointensity records are located in distinct sedimentary environments representing four different places situated thousands of kilometres away from Lago Escondido. The good agreement is characteristic of a dominant global (i.e. dipolar) character of these records. They also reflect the general trend of the Earth’s dipole moment. The discrepancy between the preglacial records of Lago Escondido could be explained as a local characteristic due to a non-dipolar source but more data would be necessary to draw a conclusion about this behaviour (Gogorza et al., 2004). On other hand, Gogorza et al. (2002) indicate that ‘‘the results of D and I show anomalous directions in some cores, of Lago Escondido,

at about 18,000 calibrated years, which could be correlated with the excursion recorded on Lago El Tre´bol (from the same area), although further validation is necessary’’. Using the value obtained of declination and inclination, Creer et al. (1983) calculated VGPs for the sequence of Lago Moreno for 100 yrs and they drew the first secular variation path for South America. The results indicate that the velocity of westward wandering (0.19 + 0.07 grad/yr), obtained for inclination anomalies, is the same order as for the velocity of the nondipole magnetic earth wandering. These suggest that the most important contribution to the secular variation in inclination of Argentina and Australia is given by the nondipolar magnetic field, which would have had a constant velocity westward, at least in the last 6000 yrs (Sinito et al., 1984a, b). Gogorza et al. (2000a, b) made a precession analysis of the geomagnetic vectors for sediments of Lago Moreno of 0–12 ka BP. The average VGP was calculated using smoothed inclination and declination data with 250 yrs windows. The VGP path plots are shown in Fig. 28. Both clockwise and counterclockwise precession of the geomagnetic vector is evident, the first one being dominant. Relative changes in geomagnetic field intensity over the last 16,000 yrs BP were recovered from the study of four cores obtained from Lago Escondido. These studies have shown that

142

Guillermo H. Re et al. • (10–7 SI)

NRM (Am2/kg*10–6) 1

10

100

1

10

SIRM (Am2/kg) 0.01

0.1

ARM (Am2/kg) 1E-4 1E-3 0.01

10 ---

50 ---

100 ---

150 ---

Depth (cm)

200 ---

250 ---

300 ---

350 ---

400 --Empty Sandy clay Clayey silt Clay Tephra

(a)

(b)

(c)

(d)

Fig. 27. Sedimentology description (a) log intensity of natural remanent magnetization, (b) specific susceptibility, (c) saturation isothermal remanent magnetization, (d) anhysteric remanent magnetization for the ‘‘LES8’’ core (from Gogorza et al., 1999).

the MRN20mt is carried by magnetite PSD (grainsize 1–8 mm), and the mineral concentration varies between 0.01 and 0.1%. The normalised intensity record (MRN20mt/ ARM100mt) shows good agreement with Lake Pepin, St. Lawrence estuary, Lake Baikal, Larsen-A ice shelf records and an absolute paleointensity global record (Fig. 29) (Gogorza et al., 2004).

6. Final Remarks Paleomagnetic studies of Late Cenozoic rocks in Patagonia were made from the very early times of this discipline. So far an important number of studies was carried out in Late Cenozoic basaltic lava flows, glaciolacustrine

sediments, lake sediments and sediments from archeological and paleontological locations. Paleomagnetic studies carried out in basalts have allowed determining the secular variation of the geomagnetic field, measured by the angular dispersion of the VGP around the Earth’s rotation axis. In general, the determined value of PSV is concordant with the values of dispersion predicted by Model G for the last 5 Ma (McFadden et al., 1991), except for the basaltic plateau of the Lago Buenos Aires that present a higher value (Brown et al., 2004). These authors suggest that this discrepancy can be due either to a real variation of the geomagnetic field or to an artifact caused by inadequate sampling. Given the smaller dispersion found in other places, close in latitude, it is probable that this high

Late Cenozoic Paleomagnetic Studies in Patagonia W

E

W

E

0°

0° 80

°S

80

°S

85

85

°S

9125 yrs

°S

675 yrs 7125 yrs

4125 yrs

4125 yrs

270°

143

90° 270°

90° 6375 yrs

2125 yrs

5125 yrs

180°

180° W

E 0° 10875 yrs

80

°S

85

°S

9125 yrs

VGP Path 270°

90° 11875 yrs

625 c. Ages–11875c. Ages Western Argentina Windows: 250 yrs

180°

Fig. 28. VGPs path of the low-frequency field motion for Lago Moreno (from Gogorza et al., 2000a).

value of dispersion is due to sampling problems. These problems can be related to the presence, quite extended in Patagonia, of slumping process. These processes are many times difficult to identify and their effect is to increase the dispersion of the remanence vectors measured. The first studies of AMS done in basalts of Patagonia have given encouraging results. New and detailed paleomagnetic and AMS studies, supported by geochronological dating can contribute to recognize different volcanic events and to correlate them regionally as well as in time. The main directions of the AMS ellipsoid can help to locate the sources and to determine the provenance directions of lava flows. On the other hand, it would be convenient to carry out new paleomagnetic and geochronological studies in the lavas of Zapala to define the Neuque´n Event with more precision. Also, given the good magnetic behavior character of the basaltic lavas, it would be interesting to add paleointensity studies to contribute to the knowledge of the behavior of the Connection Management (CMT) during polarity reversals. The basalts of Patagonia provide an excellent opportunity to obtain detailed record of the changes of the magnetic field. It

is noted that the world distribution of these data is extremely irregular, since most of them come from Europe and North America. Thus, it would be of interest to have a higher number of studies in Patagonia to balance the asymmetric distribution of paleomagnetic information. As for the varves, new paleomagnetic and chronostratigraphic studies can contribute to determine the ages of different glaciations, as well as to contribute to the correlation between the different localities. The paleomagnetic studies on lake sediments have shown their utility to register the PSVs of the CMT. The high resolution records of South America are still not enough, since the whole SW of Argentina has good lakes for studies of sediment cores; these studies should be completed with radiocarbon ages. The correlation among the paleomagnetic records of lake sediments can provide a continuous curve of PSVs of the geomagnetic field for the south of South America. These paleomagnetic records of the Southern Hemisphere have great significance for the study of the behavior of the CMT. On the other hand, several archeological and paleontological sites in Patagonia have given records

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of a probable excursion of the geomagnetic field in the Late Pleistocene–Holocene. It will be necessary to increase the number of studies to confirm this excursion and to add new geochronological ages that fix it in time. This way it will be possible to correlate it with similar records from other continents, and to elucidate whether it is an excursion at the local or at the global scale.

The quality of the obtained results is good, and there is an increase in the quality and quantity of the results through time. However, further integration with other disciplines is necessary. For example, it is of crucial importance to have more geochronological data, which are scarce because of limited economic resources. In spite of this, it is clear that paleomagnetic studies contribute, and will keep doing so, to the knowledge of the Late Cenozoic in Patagonia.

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Negra, provincia de Neuque´n. Asociacio´n Geolo´gica Argentina, Revista 20, 1, 7–28. Buenos Aires. Valencio, D.A. (1965b). Estudio paleomagne´tico del Basalto II de edad suprapliocena, de la Pampa de Zapala, provincia de Neuque´n. Asociacio´n Geolo´gica Argentina, Revista 20, 2, 185–198. Buenos Aires. Valencio, D.A. (1970). The significance of the palaeomagnetic data from Mesozoic and Cenozoic rocks of South America. Proceedings and papers of the Second Gondwana Symposium, 169–174. Cape TownJohannesburg, South Africa. Valencio, D.A. (1972). La deriva continental cenozoica y mesozoica en el Atla´ntico Sur. Geoacta 6, 1, 81–92. Buenos Aires. Valencio, D.A. (1980). El magnetismo de las rocas. Buenos Aires, EUDEBA, 351 pp. Valencio, D.A. and Creer, K.M. (1968). El paleomagnetismo de algunas lavas cenozoicas de la Repu´blica Argentina. Asociacio´n Geolo´gica Argentina, Revista 23, 4, 255–278. Buenos Aires. Valencio, D.A., Creer, K., Sinito, A.M. et al. (1982). Estudio paleomagne´tico, sedimentolo´gico y palinolo´gico de ambientes lacustres. Parte I – Lago El Tre´bol. Asociacio´n Geolo´gica Argentina, Revista 37, 2, 183–204. Buenos Aires. Valencio, D.A. and Fourcade, N.H. (1969). Estudio paleomagne´tico y petrogra´fico de algunas formaciones Cenozoicas de las islas Shetland del Sur. Contribuciones del Instituto Anta´rtico Argentino 125, 1–25. Buenos Aires. Valencio, D.A., Linares, E. and Creer, K. (1969). Paleomagnetismo y edades geolo´gicas de algunos basaltos terciarios y cuartarios de Mendoza y Neuque´n. IV Jornadas Geolo´gicas Argentinas, Actas 2, 397–415. Buenos Aires. Valencio, D.A., Linares, E. and Creer, K. (1970a). Palaeomagnetic and K-Ar ages of Cenozoic basalts from Argentina. Geophysical Journal of Royal Astronomical Society 19, 147–164. Valencio, D.A., Linares, E. and Creer, K. (1970b). Paleomagnetismo y edades geolo´gicas de algunos basaltos terciarios y cuartarios de Mendoza y de Neuque´n. IV Jornadas Geolo´gicas Argentinas 2, 397–415. Buenos Aires. Valencio, D.A., Linares, E. and Vilas, J. (1970c). On the age of the Matuyama-Gauss transition. Earth and Planetary Science Letters 8, 2, 179–182. Valencio, D.A., Linares, E., Vilas, J.F. and Nabel, P. (1979). Edades magne´ticas y radime´tricas de algunas magmatitas cenozoicas de las provincias del Neuque´n y Mendoza. Asociacio´n Geolo´gica Argentina, Revista 34, 1, 36–41. Buenos Aires. Valencio, D.A. and Mendı´a, J.E. (1974). Paleomagnetism and K-Ar ages of some igneous rocks of the Trindade Complex and the Valado Formation from Trindade Island, Brazil. Revista Brasileira Geociencias 4, 124–132. Valencio, D.A., Sinito, A.M., Creer, K.M. et al. (1985). Palaeomagnetism, sedimentology, radiocarbon age determinations and palynology of the Llao-Llao area, southwestern Argentina (lat. 41 S, long. 71300 W). Palaeolimnological aspects. Quaternary of South

Late Cenozoic Paleomagnetic Studies in Patagonia America and Antarctic Peninsula 3. A.A. Balkema Publishers, Rotterdam, 109–147. Valencio, D.A. and Vilas, J.F. (1969). Age of the separation of South Ame´rica and Africa. Nature 223, 5213, 1353–1354. Valencio, D.A., Vilas, J.F. and Mendı´a, J. (1975). Palaeomagnetism of Quaternary rocks from South America. Anales de la Academia Brasileira de Ciencias 47, 21– 32 (supplement). Vilas, J.F. (1974). Las curvas de desplazamiento polar cenozoicas de Ame´rica del Sur, Ame´rica del Norte,

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Africa, India y Australia. Geoacta 6, 3, 113–134. Buenos Aires. Vilas, J.F. and Valencio, D.A. (1970). Palaeogeographic reconstruction of the Gondwanic continents based on palaeomagnetic and sea floor spreading data. Earth and Planetary Science Letters 7, 397. Zo¨llner, W. and Amos, A. (1973). Descripcio´n Geolo´gica de la Hoja 32b, Chos Malal (provincia del Neuque´n). Servicio Nacional Minero Geolo´gico, Boletı´n 143,1–98. Buenos Aires.

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8 Late Cenozoic Glaciations in Patagonia and Tierra del Fuego Jorge Rabassa Laboratorio de Geologı´a del Cuaternario, CADIC-CONICET, C.C.92, 9410 Ushuaia, Argentina and Universidad Nacional de la Patagonia at Ushuaia erosional surfaces (Kaplan et al., 2004; Singer et al., 2004a). In some cases, the magnetostratigraphy of glacial deposits is available, thus allowing the correlation with the Pampean (central eastern Argentina; Fig. 1a) continental sequences (mostly loess units) and with the global ocean record (Rabassa et al., 2005). Likewise, the stratigraphic and biostratigraphic units of the Pampean Region of Central Argentina have been chronologically linked by means of paleomagnetic dating techniques, thus providing a basis for regional and planetary correlation between the glacial events and the Pampean loess deposition (Cione and Tonni, 1999). The regions discussed in this chapter are shown in Fig. 1. Argentinian Patagonia extends southward of the Rı´o Colorado (Fig. 1b, Site 1), with a total length of almost 2500 km, between 36 and 55 S, on the eastern side of the Andean Cordillera, including Isla Grande de Tierra del Fuego (Fig. 1c). If a map of Patagonia is superimposed in an upside-down position on top of a map of Europe at the same scale, its extremes would be coincident with the latitudes of the island of Malta and Copenhagen, respectively, a very large distance that explains the great variety of climates and ecosystems of this region. This chapter includes also the most significant information available on the glaciations of the Chilean (western) side of the Andes, along the same latitudinal belt. It deals particularly with the Chilean Lake District (Fig. 1b, Sites 13, 14), where very important work has been done by several authors during more than three decades (Mercer, 1976; Porter, 1981; Denton et al., 1999a, b). Patagonia is formed by two main physiographical units: the Patagonian Andes (Fig. 1a), which extend in a N–S direction, except in Tierra del Fuego where they turn eastward to achieve a W–E arrangement, and extra-Andean Patagonia, mostly low-lying, semiarid flat terrains, volcanic tablelands and low ridges of varied geological composition. The localities cited in the text are found along the Patagonian and Fuegian Andes between 38 and 55 S, and the corresponding Patagonian plains, the Fuegian– Magellanic Basin and the adjacent Chilean areas (Fig. 1a).

1. Introduction Patagonia and Tierra del Fuego show one of the longest and most complete sequences of glacigenic deposits and landforms in the Southern Hemisphere outside of Antarctica and, perhaps, of the entire world. Starting in the latest Miocene, these units have been preserved, though sometimes rather in a fragmentary manner, thanks to their interbedding with volcanic flows that have protected the sediments from erosion, besides allowing their absolute dating. Similarly, the relative tectonic stability of the area, after the final emplacement of the southern Andes, and the dry climate that has dominated the region since the Late Miocene have contributed to keep the glacigenic deposits from denudation. The climate of Patagonia and Tierra del Fuego, following the general conditions on the Earth, has suffered significant variations during the Cenozoic, particularly since the Miocene. These climatic changes are related to various causes such as continental displacement due to plate tectonics, modification on greenhouse gases content in the lower atmosphere and changes in astronomical parameters, namely eccentricity of the Earth orbit, obliquity of the planetary axis and equinoccial precession. Though this process of climate deterioration was initiated possibly toward the end of the Mesozoic, but most likely, at the beginning of the Paleogene, it culminated with the recurrence of multiple cold-warm climatic cycles starting in the Miocene, which led to the development of global ice ages. The knowledge of the Late Cenozoic glaciations in Patagonia and Tierra del Fuego (Fig. 1a) has made significant progress in the last decade, thanks mainly to the application of absolute dating techniques, following the pioneer work of John Mercer (Mercer, 1976, among many other benchmark contributions; Meglioli, 1992; Clapperton, 1993; Ton-That et al., 1999; Singer et al., 2004a). The cited dating techniques have allowed to link the Patagonian records with other glaciated regions and with the global marine isotopic sequence (Shackleton, 1995). This chapter presents the status of our knowledge on the Patagonian and Fuegian glaciations, starting in the Late Miocene, when the junction of global, cooler climatic conditions and the final rise of the southern Andes enabled the formation of mountain glaciers in the area. The objective of this chapter is to present the absolute chronology of the Patagonian terrestrial glacial sequences, basically dated by means of 40Ar/39Ar dating techniques on volcanic rocks associated with glacial landforms and deposits, and more recently, cosmogenic isotope dating techniques on erratic boulders and glacial

2. Glaciers in Patagonia and Tierra del Fuego Patagonia and Tierra del Fuego are some of the regions of the world still largely covered by ice and snow. Three major mountain ice sheets can be observed along the Patagonian and Fuegian Andes, several smaller ones and countless cirque and niche glaciers and permanent snowfields of varied size. These three ice sheets are the Northern Patagonian  2008 ELSEVIER B.V. ALL RIGHTS RESERVED

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Ice Field (46300 –47300 S; 73–74 W), the Southern Patagonian Ice Field (48300 –51 S; 73–74 W) and the Darwin Cordillera Ice Field (54300 –55 S; 69–71 W). See Fig. 1a. These large ice bodies are, by far, the most important of the Southern Hemisphere outside Antarctica. They are the remnants of the Late Pleistocene mountain ice sheet that covered the southern Andes. This Pleistocene ice sheet had a total length of almost three times the size of the coeval European Alpine ice sheet, but elongated in a

N–S direction, allowing for significant changes in glacier type, size, volume, elevation, regime and climate. Local ice caps of much reduced dimensions are found usually at the summit of Tertiary and Quaternary volcanoes, that is, endogenetic, constructional features that have grown above the regional summit accordance surface. Examples of these local ice caps are those on Volca´n Lanı´n (39300 S; 71300 W, 3778 m a.s.l.; Fig. 1b, Site 17; Fig. 2), Monte Tronador (41300 S; 71500 W;

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Río Colorado Río Negro Río Limay Río Neuquén Río Chubut Río Deseado Río Collón Curá Río Aluminé Lago Nahuel Huapi and San Carlos de Bariloche 10. Lago Traful 11. Volcán Copahue 12. Monte Tronador

Fig. 1. Continued.

13. Lago Llanquihue and Chilean Lake District 14. Puerto Montt and Monteverde Archaeological Site 15. Lago Mascardi 16. Río Pichileufú 17. Volcán Lanín and Río Malleo 18. Lago Huechulaufquen and San Martín de los Andes 19. Epuyén and Cholila 20. El Maitén 21. El Bolsón and Lago Puelo 22. Esquel, Portezuelo de Apichig and Portezuelo de Leleque

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35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46.

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Fig. 1. Continued. 3556 m a.s.l.; Fig. 1b, Site 12; Fig. 3; see Rabassa et al., 1978, 1981), Monte San Lorenzo (47450 S; 72150 W; 3706 m a.s.l.; Fig. 1b, Site 32) and Isla Riesco (53140 S; 3000 W; 1183 m a.s.l.; Casassa et al., 2002a, 2002b; Fig. 1a), among many others, particularly on the Chilean side of the Andes. Hundreds of smaller cirque and short valley glaciers can be found elsewhere in the Patagonian and Fuegian Andes. Due to the impact of global warming (Rosenbluth et al., 1997), most of these mountain glaciers have been receding very intensively in the last two decades, and it is very likely that most of them will be totally gone by the middle of the present century (Casassa, 1995; Naruse et al., 1995; Aniya et al., 1997; Aniya, 1999; Rivera and Casassa, 2004; Rabassa, 2007). The loss of ice will have drastic effects on many environmental issues, such as water resources (Coudrian et al., 2005; Rabassa, 2007) and sea level rise (Rignot et al., 2003).

3. Snowline Position and Distribution of Past and Present Glaciers The permanent snowline or firnline is the line that connects the lowest topographical positions of snow fallen

during the previous winter on the surface of a glacier that has not melted away at the end of the Southern Hemisphere summer, that is, March and early April. The equilibrium line is an imaginary line that separates the accumulation area, with a net gain of mass, from the ablation area (net loss of mass) on the surface of a glacier. Permanent snowline and equilibrium line are coincident in most maritime and temperate glaciers (Clapperton, 1993). These lines differ only in polar or subpolar regions, where regelation takes place below the permanent snowline. Regional snowline, or equilibrium line altitude (ELA), is a very important geographical and climatic parameter in Patagonia, which tends to be very stable through time for a certain area as it is related to the position of the summer 0C isotherm. However, recent climatic change due to global warming has determined a significant rise in ELA in most of the studied area, with a rise of up to 200 m in only the last 20 yrs (Casassa et al., 2003; Rabassa, 2007). The snowline and ELA and the distribution of modern ice bodies have been discussed extensively by Clapperton (1993). The altitudinal position of snowline is highly dependent on local topographic and climatic conditions. The snowline decreases gradually from North to South, between around 2200 m a.s.l. in northern Patagonia and

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In northern Patagonia, between 36 and 44 S, the position of present and past snowline has been studied by Flint and Fidalgo (1964, 1969) and Rabassa et al. (1980). Present ELA has been estimated by means of a detailed glacier inventory, based on aerial and terrestrial photographs, completed in 1978 (Rabassa et al., 1980). Pleistocene ELA has been calculated using the cirque floor elevation, assuming that these cirques (presently with or without ice) have been reoccupied several times during the Quaternary (Flint and Fidalgo, 1964, 1969). In the region of temperate regime and dominant winter precipitation, the position of the ELA is roughly coincident with the atmospheric summer 0C isotherm, but further south, increased year-round precipitation brings ELA to much lower positions, reaching as low as 800 m a.s.l. in western Tierra del Fuego (Clapperton, 1993), around 1000 m a.s.l. in Ushuaia (Fig. 1b, Site 40; Coronato, 1995a, b) and possibly in between 500 and 900 m a.s.l. at Isla Riesco (Fig. 1a; Casassa et al., 2002b).

4. Glaciations in Patagonia and Tierra del Fuego

Fig. 2. Volca´n Lanı´n (39300 S; 71300 W, 3778 m a.s.l.; Fig. 1b, Site 17), southern slope, facing Lago Huechulauquen (Fig. 1b, Site 18), province of Neuque´n, Argentina. (Photo by J. Rabassa, 1983).

Fig. 3. Monte Tronador (41300 S; 71500 W; 3556 m a.s.l.; Fig. 1b, Site 12), western slope. Seen from Casa Pangue Glacier valley, western slope, Chile. (Photo by J. Rabassa, 1979).

less than 1000 m a.s.l. in the western Fuegian Andes. Its altitude increases sharply from West to East, as a consequence of the strong precipitation gradient in this direction, generated by the interference of the Andean mountain chains with the weather coming from the South Pacific anticyclonic center (Chapter 3).

Pliocene and Pleistocene glaciations were frequent in this region. Moreover, glacial tills of a latest Miocene glaciation have been found in southern Patagonia, and indirect evidence points at, at least, isolated mountain glaciers already in Late Miocene times, both in northern and southern Patagonia. Pliocene glaciations have been recorded in northern North America (northwest Canada, Alaska) as of Late Gauss paleomagnetic age (Barendregt and Duk-Rodkin, 2004; Duk-Rodkin et al., 2004; Harris, 2005), and as old as 2.5 Ma in central Missouri, USA (Balco et al., 2005). It is considered that valley and piedmont glaciers coming from the ice sheet or from local ice caps extended up to several hundred kilometers eastward during the most extensive glaciation, as well as to the deep Pacific Ocean waters in the west. The present Atlantic submarine platform was reached by the ice several times during this period, but only south of the present Rı´o Gallegos valley (Fig. 1b, Site 27). On most of the Argentinian side of the Andes, the glaciers only extended to the piedmont areas, not far beyond the mountain front. On the western side south of Isla Chiloe´ (Fig. 1a), the ice probably calved into the Pacific Ocean during glacial events. It should also be noted that the total length of the Pleistocene Patagonian Mountain Ice Sheet was almost three times the extent of the European Alps ice cap and more than five times that of the New Zealand Alps during the same period.

4.1. The History of Glacial Investigations in Patagonia and Tierra del Fuego The first scientific observations about Patagonian glaciations were presented by Charles Darwin who, during the famous ‘‘H.M.S. Beagle’’ voyage and together with Robert Fitz Roy, explored the Rı´o Santa Cruz valley (Fig. 1b, Site 23) in 1833. There, Darwin described erratic boulders at Co´ndor Cliff and several other sites along this valley (50 S; 71 W; Fig. 1b, Site 41), very far

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from the Andean ranges, to which he assigned a glacial origin though interpreting them as the product of iceberg deposition, as it was the paradigma of those times (Darwin, 1842; Imbrie and Imbrie, 1979). This was the first paper ever published on the Patagonian glaciations and probably one of the very first after the publication of Louis Agassiz’s glacial theory in 1840 (Imbrie and Imbrie, 1979), though Darwin’s actual observations preceded it by several years. Several decades later, the famous Swedish geologist and explorer Otto Nordenskjo¨ld (1899) made the first scientific study of the Patagonian and Fuegian glaciations, at both ends of the Pleistocene ice sheet, around San Carlos de Bariloche (41 S; Fig. 1b, Site 9) and in Tierra del Fuego. Nordenskjo¨ld followed the original work of Francisco P. Moreno (1897) who, during his exploratory work in the Patagonian Andes, made very early and significant observations about the nature and extent of the glaciations. Nordenskjo¨ld (1899) provided the first detailed map of the extension of the Quaternary glaciations in southernmost Patagonia and Tierra del Fuego (Fig. 4), and recognized different moraines, which he correctly interpreted as representing several glacial stages. He was the first to suggest that the ice had partly extended over the present submarine platform. Other important contributions of this pioneer epoch are those by Wehrli (1899), Rovereto (1912) and Willis (1914). In the first decades of the twentieth century, Patagonia was visited by many European scientists, who provided the bases of our knowledge of the region. Vaı¨no Auer, a Finnish geographer, over several decades explored extensive areas of Patagonia. His contributions (Auer, 1956,

1958, 1959, 1970, among many other papers) are an outstanding catalogue of field localities and sections. Unfortunately, his interpretations about the extent of the glaciations were biased by the influence of Czajka (1955) who proposed a total glacierization of Patagonia, a model that we now know to be incorrect. Auer (1970) partially revised these ideas of a totally ice-covered Patagonia, but he still insisted in local glaciations in the central Patagonian massifs, for which proof has never been found. The maximum extent of the different glacial advances was first presented full scale by Carl C:zon Caldenius, a Swedish geologist who, between 1928 and 1931, mapped the glacial deposits and landforms of Patagonia (Fig. 5). Caldenius’ academic advisor at Stockholm University was the famous glacial geologist Gerard De Geer. The latter had asked Dr Jose´ M. Sobral, an Argentinian member of Otto Nordenskjo¨ld’s 1901–1903 Antarctic Expedition and as his geology student graduated at Uppsala University, and who was by that time the Head of the Argentinian Geological Survey, to support the study of Patagonian glaciations, in the same way as it had been done on the Scandinavian Peninsula. Mostly, De Geer’s interest was to compare glacial varve sequences in both hemispheres, the early chronological tool that he had developed for the Scandinavian Peninsula. De Geer (1927) described in detail this binational arrangement and presented the first preliminary data of Caldenius’ expedition. A warm biography of Caldenius has been presented by Lundqvist (1983, 1991, 2001). In his paramount contribution, Caldenius (1932) presented a map that covered more than 1 M km2 (Fig. 6) extending from Lago Nahuel Huapi (41 S; Fig. 1a, Site 9) to Cape Horn (56 S; Fig. 1a). Caldenius (1932) identified

Fig. 4. Glacial map of Tierra del Fuego, by Otto Nordenskjo¨ld (1899).

Late Cenozoic Glaciations in Patagonia and Tierra del Fuego

Fig. 5. Carl C:zon Caldenius. (Photo by Jan Lundqvist; Lundqvist, 1983, 1991, 2001).

Fig. 6. Caldenius’ (1932) original glacial map of Patagonia.

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moraines corresponding to four glacial events which he named ‘‘Initioglacial,’’ ‘‘Daniglacial,’’ ‘‘Gotiglacial’’ and ‘‘Finiglacial,’’ assuming a direct correlation with the Scandinavian glacial model. He considered these units as succesive recessional phases of the Last Glaciation (LG) and observed, additionally, the existence of inner morainic belts, younger than the Last Glacial Maximum (LGM), which he named as ‘‘post-Finiglacial’’ advances. Although his stratigraphic scheme is quite sound and his glacial map is an outstanding work for its detail and precision, in spite of the lack of appropiate maps and reliable roads at the time, the chronostratigraphic scheme is unfortunately wrong. Caldenius (1932) underestimated the age of some of the morainic belts, most likely impressed by the excellent state of preservation of the landforms, even those occurring in some of the outermost (and older) arcs. This is due to the extremely dry climate of the Patagonian steppes. Such a high degree of preservation would never exist in the Scandinavian or Baltic regions, where no wellpreserved pre-LG moraines are known. Later authors have provided new data and evidence in support of Caldenius’ basic model, modifying only his original chronology. Although the currently identified boundaries of the different glacial advances are highly coincident with those mapped by Caldenius, the total number of glaciations and their chronological correlation has changed, based on absolute dating, new paradigms and interpretations. Nevertheless, his original terminology is still preserved, because it has a high value as unifying criteria for the different glacial events throughout the region. Caldenius (1932), following the methodology then imposed by De Geer, studied varves and other glaciolacustrine deposits, and used them to telecorrelate glacial events in Patagonia with those of Scandinavia. We know today that these attempts were unsound, and therefore, this methodology has been abandoned. Groeber (1936) recognized correctly that the glaciers in northern Patagonia never extended much beyond the Andean foothills. In later works, Groeber (1952) proposed a fourfold glacial model, and extended the glaciation not only over all of Patagonia, but even to the western Pampas, reconstructions that are today clearly unacceptable. Most likely, Groeber was strongly influenced by the works of Czajka and Auer, thus changing his original, correct points of view. Unfortunately, Groeber’s last works and his immense prestige among Argentinian geologists for a long time delayed a proper understanding of the real extent of glaciation. However, his ideas were soon firmly opposed by Polanski (1965), in his studies in the Andean piedmont of Mendoza, central Argentinian Andes (33–34 S, 300 km north of the Patagonian northern boundary), who had a great influence on the later work of his student, Francisco Fidalgo. Egidio Feruglio, an Italian geologist working for the Argentinian government, had a deep knowledge of the Patagonian regional geology and was, after Caldenius, the great innovator in the study of the Patagonian glaciations. Feruglio (1944) described with great precision a sequence of basaltic lava flows with interbedded tills at Cerro del Fraile, Santa Cruz Province (51 S, Fig. 1b, Site 28), just north of the Magellan Straits, recognizing the great antiquity of the glacial deposits and assigning them

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a Pliocene age, older than the maximum glacial extent (which was later known as the ‘‘Great Patagonian Glaciation’’ or GPG; Mercer, 1976). This was certainly an extraordinary, pioneer contribution to the knowledge of the Pre-Quaternary glaciations of Patagonia since absolute dating was then still unavailable, and at that time, speaking about ‘‘Pliocene glaciations’’ was certainly a revolutionary concept. Years later, and working at the full regional scale, Feruglio (1950) also recognized the existence of four major Pleistocene glacial events, which he named as ‘‘Pichileufuense inferior,’’ ‘‘Pichileufuense superior,’’ ‘‘Barilochense’’ and ‘‘Nahuelhuapense’’ (local names of northern Patagonia, see Fig. 1b for type localities), retaining Caldenius’ (1932) fourfold scheme, but linking each event to geomorphological positions that were indicators of clearly different (and older) ages. Thus, he recognized that the ‘‘Pichileufuense’’ landforms and sediments are found on the topographical divides, whereas the deposits of later glacial events are located within the valleys excavated in them. Therefore, Feruglio (1950) established the basic criteria that much later allowed to identify a Quaternary ‘‘Canyon Cutting Event’’ (Rabassa and Clapperton, 1990) in Patagonia. Likewise, he firstly established the possible correlation of the glacial deposits with (a) the ‘‘Rodados Patago´nicos’’ or ‘‘Rodados Tehuelches’’ (‘‘Patagonian Gravel Formation,’’ ‘‘Patagonian Shingle Formation’’; Darwin, 1842; Caldenius, 1940), which he considered to be of glaciofluvial origin, and (b) the loess acumulation events in those regions which he called ‘‘infraglacial’’, that is, the nonglaciated Pampas of eastern central Argentina (Feruglio, 1950). Richard F. Flint, the distinguished American Quaternary scientist at Yale University, was invited in the early 1960s by the Argentinian Geological Survey to work on the Patagonian glaciations. Flint, together with Francisco Fidalgo (Flint and Fidalgo, 1964, 1969), studied the glacial deposits in the northern Patagonian Andes (39–43 S; Fig. 1a), proposing a threefold glaciation model, based on what they named as the ‘‘Pichileufu,’’ ‘‘El Co´ndor’’ and ‘‘Nahuel Huapi’’ drifts, which they considered to be phases of the LG. However, they already suggested in their 1969 paper that the ‘‘Pichileufu Glaciation’’ might be older than the Late Pleistocene. Fidalgo and Riggi (1965) identified four main glacial drifts at Lago Buenos Aires (47 S; Fig. 1b, Site 24), as well as the glaciofluvial origin of at least a portion of the ‘‘Patagonian gravels,’’ but without assigning absolute ages to the studied units. John H. Mercer (Fig. 7), an English geographer working at Ohio State University, was a tireless explorer of the Patagonian mountains, and he combined his work in South America with simultaneous studies in Antarctica and New Zealand. His knowledge of the South American glaciations was unique for his times and his work was probably not appreciated as it deserved. Mercer brought new concepts and ideas to the problem of Patagonian glaciations since 1969 (Mercer, 1969, 1972) and he was the first to use modern techniques such as radiometric techniques (K/Ar and 14C dating) and paleomagnetic studies in glacial sequences. He put forward many original ideas, most of them confirmed by later work, and his

papers are a source of new research lines even today (e.g. Mercer, 1972, 1976, 1983). Fleck et al. (1972) and Mercer and Sutter (1981) studied many outcrops of glacial deposits interbedded with volcanic rocks, in which radiometric and paleomagnetic dating techniques were applicable, also restudying Feruglio’s (1944) Cerro del Fraile Locality (Fig. 1b, Site 28). It was Mercer (1976) who first chronologically established the existence of Patagonian glaciations throughout the entire Quaternary period, of frequent Pliocene glaciations and even of Late Miocene tills, also recognizing the correlation of these glacial episodes with global cold periods. He proposed a four-glaciation model for the Chilean Lake District (Fig.1b, Site 13) and demonstrated the ancient age of the older glaciations (Mercer, 1976). In this work, he gave the name of ‘‘Llanquihue Glaciation’’ to the last Pleistocene glaciation [18O marine isotope stages (MIS) 4–2], a term later extended by Clapperton (1993) for the entire South American continent, and coined the name ‘‘Great Patagonian Glaciation’’ (GPG) for the oldest and outermost morainic complex. Steve Porter (1981) identified also four major glaciations in the Chilean Lake District (39–41 S; Fig. 1b, Site 13) and defined their chronology throughout the Pleistocene, using radiometric dating and relative age techniques. Paul Ciesielski and colleagues (1982) were the first to present a correlation model for the Patagonian glaciations with the erosional and depositional history of the Maurice Ewing Bank (55 S), Southwestern Atlantic Ocean east of Tierra del Fuego (Fig. 1a), based on Mercer’s (1976) chronostratigraphic scheme. In this model, the great antiquity of the Patagonian glacial events and their relations with global paleoclimatic episodes are confirmed. The pioneer work of Edward Evenson and his colleagues from Lehigh University and other American universities since the mid-1980s brought for the first time a modern approach to the study of northern Patagonian glaciations on the Argentinian side of the Andes, combining detailed field mapping, radiometric dating and paleomagnetic studies (Kodama et al., 1985, 1986; Rabassa et al., 1986, 1990a; Schlieder, 1989; Rabassa and Evenson, 1996). Rabassa and Clapperton (1990) presented the first review of the Patagonian glaciations and a general

Fig. 7. John H. Mercer at an outcrop of the Llanquihue moraine with wood fragments, near Puerto Varas, southern Chile. (Photo by J. Rabassa, 1973).

Late Cenozoic Glaciations in Patagonia and Tierra del Fuego chronological correlation of all units known at that time. Recently, Mo¨rner and Sylwan (1989), Sylwan (1989), Meglioli (1992), Wenzens (1999a, 1999b, 2000), Wenzens et al. (1996), Schellmann (1998, 1999, 2003), Rabassa and Coronato (2002), Strelin et al. (1999), Malagnino (1995), Singer et al. (2004a, 2004b), and Sugden et al. (2005), among many others, have stressed the great antiquity and complexity of the Patagonian glacial sequence. Chalmers Clapperton (1993) presented the first continental summary of our knowledge of South American glaciations, including a complete description of those in Patagonia, for the first time showing great detail on either side of the Andes. His book is an outstanding compilation of all the available information at that time, with a global overview and correlation with other sectors of the Southern Hemisphere as well as with the Northern Hemisphere. Clapperton et al. (1995) expanded the investigations also in southern Patagonia, in the Magellan Straits area (Fig. 1a). Working in Patagonia since 1995, Bradley Singer (University of Wisconsin) has introduced powerful tools for the study of Patagonian glaciations. Detailed mapping of extensive areas, profound volcanological research, the wide use of 40Ar/39Ar dating on lava flows stratigraphically related with glacial deposits, careful paleomagnetic studies with Laurie Brown and, more recently, together with Robert Ackert and Michael Kaplan, the use of cosmogenic dating techniques on morainic boulders (Singer et al., 1998, 1999, 2004a, b; Kaplan et al., 2004) are significant contributions to the knowledge of Late Cenozoic glaciations in southern South America. Ton-That et al. (1999) proposed for the first time to correlate the glacial sequences of Lago Buenos Aires and Cerro del Fraile (Fig. 1b, Sites 24, 28) with the global marine isotopic sequence, as presented by Shackleton et al. (1990, 1995). A recent revision of the Patagonian glaciations has been presented by Coronato et al. (2004a, b), in which they indicated the development of the GPG around 1 Ma, and evidence of (a) several pre-GPG cold periods, between 7 and 2 Ma, (b) three post-GPGs during the Early and Middle Pleistocene, (c) the Last Pleistocene glaciation and (d) two main episodes of glacial stabilization during the Late Glacial (15–10 14C ka BP). A tentative correlation of glacial events with loess deposition in the Pampas has been recently presented by Rabassa et al. (2005). In the last years, the activity of large, multidisciplinary research groups focusing on certain geographical regions has provided excellent studies on the chronology of Pleistocene glaciations of the Chilean Lake District (Denton et al., 1999a, b; Fig. 1b, Site 13) and the LGM and younger, Late Glacial recessional events and ice readvances in the Magellan Straits region (Sugden et al., 2005, and other papers in the same volume; Fig. 1a). These studies are clearly the model to be followed in future studies of Late Cenozoic Patagonian glaciations.

4.2. The Late Tertiary Glaciations in Patagonia A significant amount of evidence suggests that the Patagonian Andes were already glaciated during Late Tertiary times. Based on the concentration of d18O and other isotopes

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found in Late Miocene Santa Cruz Formation carbonate concretions, Blisniuk et al. (2006) have suggested that the southern Patagonian Andes were uplifted >1 km, between ca. 17 and 14 Ma, significantly enhancing aridity. Such important uplift of the mountain belt would have brought a large landmass above the regional snowline, adding to a global cooling trend, forcing the development of at least local ice caps at the mountain summits or upon huge composite volcanic cones, which were rapidly growing at that time, as part of the same tectonic event. From a neotectonic point of view, the final push along the Liquin˜e–Ofqui fault zone in southern Chile (41–42150 S; Fig. 1a), associated with the arrival and subduction of the Chilean Rise beneath the Taitao Peninsula (Fig. 1a), generated a major denudation event immediately before 5 Ma (Adriasola et al., 2005) and probably the glacierization of the rising Andean summits. Similarly, Thomson (2002) has applied fission track thermochronology in the investigation of low-temperature cooling and denudation history of the Patagonian Andes along the southern part of the cited fault zone between 42 and 46 S. Enhanced cooling and denudation initiated in the earlier part of the Late Miocene, between ca. 16 and 10 Ma, but much faster rates of cooling and denudation took place after ca. 7 Ma and up to 2 Ma, being coeval with the collision of the Chile Rise with the Peru–Chile trench between 47 and 48 S and also with the initiation of significant Patagonian glaciations. Thus, Thomson (2002) stated that glacial and periglacial erosion processes would have been the main contributors to denudation already since ca. 7 Ma. A latest Miocene age for the first Patagonian glaciations is also supported by carbon isotopic data on tooth enamel (Cerling et al., 1997). These authors suggest that a global decrease in atmospheric CO2 took place between 8 and 6 Ma, enabling an expansion of C4 photosynthesis plants. This lowering of CO2 is compatible with global glaciation, as it has been demonstrated for the LGM. Supporting data from Tierra del Fuego have been published by Cerling and Harris (1999).

Glaciations of the Latest Miocene–Early Pliocene In the northern margin of the Meseta del Lago Buenos Aires (47 S, Fig. 1a, Site 24), which is entirely covered by volcanic rocks, till deposits over 30 m in thickness are found interbedded with basalt flows (Mercer, 1976; Clapperton, 1993; Fig. 8). Mercer (1976) and Mercer and Sutter (1981) obtained whole-rock K/Ar ages on the under- and overlying lavas of 7.34 + 0.11 to 6.75 + 0.08 and 5.05 + 0.07 to 4.43 + 0.09 Ma, respectively, which most likely assigns a Latest Miocene age for these glacial deposits (Busteros and Lapido, 1983; Ardolino et al., 1999). This allows these deposits as belonging to some of the oldest Late Cenozoic glacial events in Patagonia, and indicates that the Patagonian Andes in those times were bearing at least isolated ice caps with outlet glaciers that were clearly extending more than 30 km east of the mountain front. In the same locality, Ton-That et al. (1999) obtained 40Ar/39Ar (incremental-heating technique) ages of 7.38 + 0.05 Ma for the underlying

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(a)

(b)

(c)

Fig. 8. Latest Miocene–earliest Pliocene till at Meseta Lago Buenos Aires (Fig. 1b, Site 24), Santa Cruz Province, Argentina (Mercer, 1976; Ton-That et al., 1999). (a) Location of till in between lava flows; (b, c) till and striated glacial boulder (Photos by Bradley Singer, 1996). flow and of 5.04 + 0.04 Ma for the overlying flow, confirming in general terms the probable Latest Miocene (or, at most, Miocene–Pliocene boundary) age of this first preserved Patagonian glaciation. Four basalt flows dated by 40Ar/39Ar (incrementalheating) techniques between 10 and 6.7 Ma have overlying tills and three other ones with underlying tills have been dated between 4.9 and 4.3 Ma at the Lago Buenos Aires region. Additionally, no till was found below the Meseta Guanabara Basalt, Lago Buenos Aires (Fig. 1b, Site 24), dated at 9.87 Ma (Ton-That et al., 1999). Though the absence of evidence should never be considered as the

evidence of absence, the Meseta Guanabara is located in an area that should have been glaciated if the glaciers had extended away from the Patagonian Andes before that age. No precise, absolute ages may be yet assigned to those tills overlying the basalts, but their comparison with the global paleomagnetic sequence indicates that they could correspond to the C3 (a and b) Chron. During this period, the oceanic sequences (Opdyke, 1995; Shackleton, 1995) locate the strongest thermal lowering between 5.7 and 5.9 Ma, a period which is comprised between the limiting ages of these tills. This correlation allows to suggest that at least a major extra-Andean glaciation could have taken place in southern Patagonia between isotopic stages TG 20 and TG 22, during the Gilbert Chron. Schlieder (1989) had already recognized very coarse diamictons along the Rı´o Alumine´ valley, northern Patagonia (Fig. 1b, Site 8), and he assigned them to Late Miocene glacial events, based on whole-rock K/Ar ages of the limiting basalts. He additionally proposed that the Alicura´ Formation, originally assigned to the Lower Quaternary by Dessanti (1972) and later, as the Alicura´ Member of the Caleufu Formation, to the Miocene–Pliocene (Gonza´lez Dı´az et al., 1986), actually corresponds to the Late Miocene, its upper age limited by overlying basalts dated at 6.41 + 0.13 and 5.26 + 0.14 Ma, respectively. In this interpretation, the Alicura´ Formation would be the distal glaciofluvial unit of the Latest Miocene Patagonian Andean glaciations, whose water and sedimentary discharge would have been concentrated by the Rı´o Alumine´ and the Rı´o Collo´n Cura´ (Fig. 1a, Site 7), both tributaries of the paleoRı´o Limay, a main regional stream of the Atlantic slope already in those times (Rabassa, 1975; Fig. 1b, Site 3). The interpretation of a glaciofluvial origin for the Alicura´ Formation related to ancient glaciations was already proposed by Gracia (1958), though no absolute ages were then defined (in Gonza´lez Dı´az and Nullo, 1980). Recently, Wenzens (2006a) has indicated the existence of Late Miocene glacial deposits around Lago Cardiel, an area that had been considered unglaciated by all previous researchers (Fig. 1b, Site 44), with ages as old as 10.5 Ma, and nine glacial advances between 10.5 and 5.4 Ma. These units would correspond to ice advances of the Lago San Martı´n lobe of the Patagonian Ice Cap (Fig. 1b, Site 45) or even by local glaciers at Mount San Lorenzo (Fig. 1b, Site 32). As Wenzens (2006a) has stated, these glacier expansions would have been up to three times larger than their Pleistocene counterparts. This is quite difficult to explain, since the larger extent would require very cold and wetter (at the ice divide) environmental conditions as well as longer glaciation periods. This assumption does not agree with the oceanic record, which shows that the Late Miocene cold events are shorter and much less intense than the Quaternary glacial periods (Kenneth, 1995; Rabassa et al., 2005). The existence of these very early, extensive Late Miocene glaciations is extremely interesting, but intriguing and further studies are needed to confirm these interpretations and explain this apparently anomalous behavior. Some of these late Tertiary glacigenic deposits and landforms have been identified well beyond the outermost boundary of the most extended Pleistocene glaciation. Considering that the Patagonian Ice Sheet is assumed to have formed only in the Early Pleistocene, when the astronomical

Late Cenozoic Glaciations in Patagonia and Tierra del Fuego forcing cycles were dominated by the eccentricity period of 100 ka (Rabassa et al., 2005), it is difficult to understand why the ice margin reached such eastern position. It has been suggested that the ice expanded over a very flat original surface, with almost no incised drainage, which corresponded to the Late Miocene sedimentary accumulation plains. Thus, the glaciers would have extended as very low-gradient, wide ice fans over an almost reliefless surface, probably eastward-sloping, latest Miocene pediments.

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took place between the Middle and Late Pliocene in the Buenos Aires, Viedma and Argentino lake regions (Fig. 1b, Sites 24–26). The first event would have taken place around 3.5 Ma, during MIS MG6, Gauss normal polarity, the second one, during the MIS 100, 96, 92 and 88, Matuyama reversed polarity. Tills are found in overand underlying positions of the lava flows dated at 3.20 Ma (Lago Argentino) and 3.45 Ma (Lago Viedma; Mercer, 1976), respectively, and they are enclosing cold peaks found at MIS KM4, KM6, M2 and MG2.

Glaciations of the Middle Pliocene Evidence of Middle–Late Pliocene glaciations can be found in southern Patagonia as well. In the Lago Viedma region (Fig. 1b, Site 25), glacigenic deposits interbedded with basalt flows have been identified at Meseta Chica and Meseta Desocupada (49 S; Mercer et al., 1975; Mercer, 1976). At Rı´o Cangrejo valley, Meseta Chica, a till bed is found between two flows K/Ar dated at 3.55 + 0.19 and 3.68 + 0.03 Ma, respectively, and another till unit is overlying a lava flow dated at 3.46 + 0.22 Ma. At Meseta Desocupada, a till layer occurs in between lava flows dated at 3.48 + 0.09 and 3.55 + 0.07 Ma. Wenzens (2000) obtained limiting ages of 3.0 and 2.25 Ma for glacigenic deposits north and east of Lago Viedma. Sylwan (1989) indicated the presence of till at Lago Buenos Aires corresponding to MIS 88, during the Gilbert Geomagnetic Epoch, which is coincident with the limiting ages proposed by Wenzens (2000) and those of the basalt flows which underlie till at Cerro Fortaleza, Lago Argentino (Schellmann, 1998, 1999; Fig. 1b, Site 26). Mercer (1976) obtained an age of 2.79 + 0.15 Ma for a lava flow that buries till at Co´ndor Cliff, Rı´o Santa Cruz valley (50 S; Fig. 1b, Site 41). Younger glacigenic deposits appear over these flows, whereas the materials corresponding to the GPG are located at the base of these ‘‘mesetas’’ or tablelands. This clearly shows that even as early as the Middle Pliocene, in some regions the Patagonian glaciers expanded from the ice caps as far as or close to the extent of the outlet glaciers of the maximum Pleistocene expansion (GPG). However, these conditions are probably exclusive for southernmost Patagonia, since there is yet no conclusive evidence for a similar extension of the ice cap in Northern Patagonia, with the exception of Schlieder’s (1989) observations in the Alumine´ valley (Fig. 1b, Site 8). However, at Monte Tronador (41 S; Fig. 1b, Site 12), volcanics, lahars and pyroclastic flows of the Tronador Formation (long ago K/Ar dated at 3.2 and 2.0 Ma, though other much younger ages were obtained as well; Greco, 1975; Gonza´lez Dı´az and Nullo, 1980, p. 1131) appear in-filling deep valleys, possibly of glacial origin, carrying striated and faceted, volcanic boulders and cobble-sized clasts (Rabassa et al., 1986). These units should be redated with more modern techniques, but it is primarily acceptable that this part of the northern Patagonian Andes was already covered by at least local ice during the Middle Pliocene. The relative chronology of tills and basalt flows has been compared with the global climatic variability obtained from the oceanic isotopic sequences (Rabassa et al., 2005). This analysis indicates that several cold climatic events and their consequent glacier advances

Glaciations of the Late Pliocene and Earliest Pleistocene Feruglio (1944) described the glacigenic sequences at Cerro del Fraile (50330 S, Fig. 1b, Site 28), interbedded between volcanic flows, and considered them as of Pliocene age. These flows were K/Ar dated by Fleck et al. (1972), Mercer et al. (1975) and Mercer (1976) between 2.08 and 1.03 Ma, during the Matuyama Chron. Mercer (1976) identified six piedmont glaciations during this period. Recent studies by Rabassa et al. (1996), Guillou and Singer (1997), Singer et al. (1999, 2004b) and TonThat et al. (1999) have allowed to redate this sequence by 40 Ar/39Ar incremental-heating techniques and to provide a precision magnetostratigraphy (Figs 9 and 10). In these

(a)

(b)

Fig. 9. Cerro del Fraile, Santa Cruz Province, Argentina (Fig. 1b, Site 28). (a) Sequence of interbedded till and lava flows (Photo by Bradley Singer, 1996); (b) striated glacial boulder in till (Photo by J. Rabassa, 1996).

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Elevation 1200 m 7 6

Fleck et al. (1972) Flow Age (Ma) Polarity

Flow

H G

1.05 ± 0.06 1.51 ± 0.11

N R

(10) (9)

1.073 ± 0.036 1.61 – 1.43

N T

F

1.71 ± 0.04

N

(8) (7) (6)

1.787 ± 0.085 1.857 ± 0.050 1.922 ± 0.066

R N T

(5)

1.994 ± 0.040

R

5 E

This study Age (Ma) Polarity

4 D

1.91 ± 0.05

R

(4)

2.130 ± 0.038

R

C

2.11 ± 0.04

N

(3)

2.132 ± 0.031

N

B

2.12 ± 0.07

T

(2)

2.144 ± 0.030

T

A

cobbles 2.05 ± 0.04

R

(1)

2.181 ± 0.097

R

Till

3

2

1 sand 1020 m

cobbles Cretaceous sandstone

Fig. 10. Stratigraphic sequence at Cerro del Fraile (Fig. 1b, Site 28). From Singer et al., 2004b. Data from Fleck et al. (1972), compared with geochronological and paleomagnetic data in Singer et al., 2004b. A to H, basaltic flows, according to Fleck et al. (1972); (1) to (10), lava flows observed by Singer et al., 2004b; 1 to 7, tills; N, normal polarity, R, reversed, T, transitional.

studies, a minimum of seven glaciations have been recognized, and probably a glaciofluvial deposit at the base of the profile, all of which would have taken place between 2.16 and 1.43 Ma. These glaciations would have developed during MIS 82 to 48 (Matuyama Chron; Ton-That, 1997; Ton-That et al., 1999). Finally, a younger glaciation covered the uppermost lava flow, dated at 1.08 Ma, thus probably equivalent to the GPG (MIS 30–34). Strelin (1995) and Strelin et al. (1999) described moraines beyond the position of the GPG (Mercer, 1976) along the Rı´o Santa Cruz valley, overlying the Co´ndor Cliff basalts (2.66 Ma; Mercer et al., 1975; Fig. 1b, Sites 23, 41), which he considered to be possibly correlated with the glacial units at Cerro del Fraile. These moraines are older than a basalt flow dated by 40Ar/39Ar at 0.675 + 0.56 Ma. No information about the applied technique is given, nor about the meaning of the very large statistical error of this date; see, for example, Ton-That et al. (1999) and Singer et al. (2004a). Likewise, they suggested that the units known as Chipanque Moraines in Lago Buenos Aires by Malagnino (1995) could be correlated with the Santa Cruz valley units. However, Malagnino (1995, p. 80) suggested instead that the Chipanque Moraines could be older than 2.3 Ma and younger than 3.5 Ma, following Mercer’s (1976) chronology. Thus, in this interpretation,

the Chipanque Moraines would be older than even the basal glacigenic unit at Cerro del Fraile.

The Origin of the Earliest Patagonian Glaciations It is very important to consider that the definitive glacierization of Western Antarctica took place in the Early Miocene. The glacierization of eastern Antarctica had started in the Early Tertiary, when this continent achieved its present polar position (Kennett, 1995), but the glacierization of Western Antarctica and the Antarctic Peninsula did not occur until the Drake Passage opened (Fig. 1a). The Drake Passage is the consequence of the dismembering of both continents due to the continuous eastward movement of the Scotia plate since the Early Tertiary. This movement generated the huge bend of the Fuegian Andean axis from a N–S to an E–W position, the displacement of the southern Georgias Archipelago away from the South American continent and the formation of a volcanic, oceanic insular arc at the southern Sandwich Islands, where the Scotia plate subducts under the Atlantic oceanic plates. The environmental consequence of this new geographic configuration was the installation of the Antarctic Circumpolar Current in the Early Miocene, perhaps ca. 23 Ma (Mercer, 1983). This current isolated the Antarctic Peninsula from the temperate oceanic currents coming from lower latitudes and contributed to the lowering of the Antarctic oceanic water temperatures. This new environmental scenario allowed the rapid and definitive cooling of the polar and subpolar air masses, generating the glacierization of the Antarctic Peninsula (Ciesielski et al., 1982) and, subsequently, of the Fuegian and Patagonian Andes. In addition to the astronomical forcing (Shackleton, 1995), other causes of climatic deterioration and subsequent occurrence of Patagonian mountain glaciations should also be considered. The tectonic processes that slowly elevated the Patagonian Cordillera and originated the Scotia Arc (Ramos, 1999a and b) should not be moved aside in this analysis. The Patagonian Andes would have started its elevation process, at least partially, in the Late Oligocene or the Early Miocene (Gonza´lez Bonorino, 1973; Rabassa, 1975). The great pyroclastic eruptions that produced the tuffs and ignimbrites of the Collo´n Cura´ Formation in northern Patagonia (ca. 15 Ma; Rabassa, 1975) are indicators of such tectonic processes. An incremental-heating 40 Ar/39Ar dating on ignimbritic pumice overlying the Pilcaniyeu Ignimbritic Member of the Collo´n Cura´ Formation (Rabassa, 1975) has provided an age of 10.85 + 0.033 Ma (B. Singer, personal communication; Rabassa et al., 2005). This date may be interpreted as the age of the last pyroclastic episodes of the Miocene cycle, which would be representing the final emplacement of the Patagonian Andes at elevations comparable to its present position. The summit accordance line of the northern Patagonian Andes is located today around 2200 m a.s.l., whereas the regional, permanent snowline is placed at around 2000 m a.s.l., allowing the persistence of many small cirque glaciers and snow fields, even during the present Interglacial (Rabassa et al., 1980). It may be assumed

Late Cenozoic Glaciations in Patagonia and Tierra del Fuego that the regional snowline would have descended significatively during all Late Cenozoic cold episodes at least since the Late Miocene, thus favouring the formation of larger mountain glaciers, and even perhaps, extending beyond the mountain piedmont.

4.3. Quaternary Glaciations in Patagonia During the Early Pleistocene, the Patagonian Ice Sheet was fully developed, probably for the first time in the Late Cenozoic, when the orbital eccentricity forcing signal became dominant (Fig. 11). The lower time boundary of the Quaternary used in this chapter is the top of the Olduvai normal polarity event of the reversed Matuyama

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Chron, that is, ca. 1.8 Ma. Glacial climatic episodes became then long enough to allow the formation of a single, continuous mountain ice sheet that extended for almost 2500 km, at least between 36 and 56 S, that covered almost completely the Patagonian Andean ranges and extended over the piedmont areas to the east (and to the present submarine platform south of the Rı´o Gallegos; Fig. 1b, Site 27) and to sea level in the Pacific side.

Glaciations of the Early Pleistocene At the base of Monte Tronador (41 S, Fig. 1b, Site 12), northern Patagonia, Rabassa et al. (1986) and Rabassa and Clapperton (1990) identified glacigenic deposits Fig. 11. Map of Patagonia with the position and distribution of the Pleistocene Patagonian Ice Sheet and relevant tectonic features. From Singer et al., 2004a.

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Fig. 12. Glacial boulder within a tillite, interbedded in Early Pleistocene lava flows at Garganta del Diablo, Monte Tronador, northern Patagonia, Argentina (Fig. 1b, Site 12). (Photo by J. Rabassa, 1988). interbedded with volcanic flows. These rocks were originally K/Ar dated at 1.36 and 1.32 Ma, assigning them an Early Pleistocene age, previous to the GPG. However, the volcanic flow overlying both the Garganta del Diablo tillite and glacial surfaces eroded on the Cretaceous granites has been redated by 40Ar/39Ar incremental-heating techniques by B. Singer (personal communication; sample TR-01; Rabassa et al., 2005; Fig. 12) at 1.021 + 0.102 Ma. Therefore, at least part of these glacigenic deposits could be much younger and even equivalent to the GPG (Mercer, 1976). The GPG represents the maximum expansion of the ice in extra-Andean Patagonia. Its geographical distribution was correctly mapped by Caldenius (1932) and corresponds to his ‘‘Initioglacial’’ event, but considered by him as an initial phase of the last Pleistocene glaciation, as stated above. The morainic arcs pertaining to the GPG are well preserved, though somewhat lesser than the later sequences. In northern Patagonia, the GPG corresponds to the ‘‘Pichileufuense’’ (Feruglio, 1950) or Pichileufu Drift (Flint and Fidalgo, 1964, 1969), or at least to its outermost expansion. Most likely, the GPG represents more than one glacial advance and in the type area of this glacigenic unit, the Rı´o Pichileufu valley east of San Carlos de Bariloche (41 S; Fig. 1b, Site 16), at least three clearly defined morainic arcs have been observed. Flint and Fidalgo (1964) had considered this drift unit as corresponding to an earlier phase of the LG, following Caldenius’ model but ignoring Feruglio’s (1950) pioneer correlations, and only later (Flint and Fidalgo, 1969) they accepted the possibility that it could correspond to an earlier glaciation. Much later, Kodama et al. (1985, 1986) and Rabassa et al. (1986, 1990a) defined a preLG age for these deposits, and most likely, an Early– Middle Pleistocene age, based on 40Ar/39Ar (whole rock) dating and paleomagnetic studies. Rabassa and Evenson (1996) suggested that the Pichileufu Drift could be composed of at least the deposits of three different ice advances which may correspond to one or, perhaps,

several glaciations, all of which preceded a fluvial canyon cutting event during the Early Pleistocene (Rabassa and Clapperton, 1990). South of San Carlos de Bariloche, the region of Esquel is located (Fig. 1b, Site 22). This area was studied by Flint and Fidalgo (1969), where they extended their threefold glacial model from the Nahuel Huapi area. Caldenius (1932) described a four moraine sequence, plus several ‘‘postFiniglacial’’ (e.g. the age equivalent to the ‘‘Younger Dryas’’ (YD) moraines of Scandinavia) units inside the mountains. Miro´ (1967), Gonza´lez Dı´az (1993a, b) and Gonza´lez Dı´az and Andrada de Palomera (1995) basically followed Caldenius’ classical four-moraine system. The longitudinal valley of El Maite´n (42–42300 S; Fig. 1b, Site 20) is considered as of pre-Andean age (Martı´nez, 2002). This valley had been glaciated in several episodes by two major ice lobes, the Epuye´n and Cholila valley lobes (Fig. 1b, Site 19). Smaller transversal valleys, crossing the El Maite´n depressions, were occupied by the ice during ‘‘Initioglacial’’ times, reaching its maximum extent at ca. 70400 W. At Portezuelo de Apichig (Fig. 1b, Site 22), all cited authors have identified two or more morainic ridges, with abundant erratic boulders and faceted and striated cobbles, assigned to the GPG. Further south, Gonza´lez Dı´az (1993b) mapped a well-preserved moraine belt at Arroyo Pichico´, at 1090 m a.s.l. The same author has also identified another moraine belt of the same age at Can˜ado´n Blancura, 20 km farther to the SE. At Portezuelo de Leleque (71430 W; Fig. 1b, Site 22), Gonza´lez Dı´az (1993b) described two morainic arcs of ‘‘Initioglacial’’ age, at 700–800 m a.s.l. It is very important to mention that Gonza´lez Dı´az (1993a, b) and Gonza´lez Dı´az and Andrada de Palomera (1995) have proposed a glacifluvial origin for the Blancura Formation (previously considered as of piedmont origin by Volkheimer, 1963), one of the most important units of the Patagonian gravels in northern Patagonia. Feruglio (1950) advanced a similar opinion half a century before. In the Chilean Lake District, at Lago Llanquihue and neighboring basins (Fig. 1b, Site 13), Mercer (1976) described and mapped three drift units older than the Llanquihue Drift or LG, which he named Rı´o Frı´o, Colegual and Casma drifts. The intensely weathered nature of the Rı´o Frı´o Drift would suggest a GPG age for this unit (Mercer, 1976; Clapperton, 1993). Porter (1981) mapped the drift sequence in the same area, suggesting a new stratigraphy, composed of the Caracol, Rı´o Llico, Santa Marı´a and Llanquihue drifts. The drift units were identified in terms of their mappable features, including weathering rate, pedogenetic characteristics and landform preservation. Caracol, the oldest Drift, occurs along the bottom of the central valley and also in certain localities along the eastern slopes of the Cordillera de la Costa (Clapperton, 1993; Fig. 1a). However, this glaciation was probably less extensive than the following advance of the ice, indicated by the Rı´o Llico Drift. The Caracol Drift is fully weathered and its age is most likely corresponding to the GPG. This unit is not exposed at Isla Chiloe´ (Fig. 1a) and probably covered by the younger drifts (Clapperton, 1993). A precise, reliable correlation between the Early Pleistocene glacial events on both sides of the Andes at this latitude is still lacking, mostly due to the problems of

Late Cenozoic Glaciations in Patagonia and Tierra del Fuego accurate dating of these units. However, a glaciation model of four major events, the GPG and three younger episodes, the youngest being the LG of Late Pleistocene age, seems to be sustainable. Within the Lago Buenos Aires Basin (46300 S; Fig. 1b, Site 24), at least 19 terminal moraines, all of them of Pleistocene age, have been described by Mo¨rner and Sylwan (1989), Ton-That et al. (1999), Singer et al. (2004a) and Kaplan et al. (2004, 2005). These units were deposited by piedmont glaciers advancing eastward from the Patagonian ice cap during the last 1.2 Myr. 40Ar/39Ar incrementalheating and unspiked K/Ar experiments (Guillou and Singer, 1997; Singer et al., 2004a) on four basaltic lava flows interbedded with the moraines provide a chronologic framework for the entire glacial sequence. The 40Ar/39Ar isochron ages of three lavas that overlie till 90 km east of Lago Buenos Aires strongly suggest that the ice cap reached its greatest eastward extent ca. 1.1 Ma, during the GPG. At least six moraines were deposited within the 256 kyr period bracketed by basaltic eruptions at 1016 + 10 and 760 + 14 ka (Singer et al., 2004a; Fig. 13). Six other younger, more proximal moraines were deposited during a 651 kyr period bracketed by 760 + 14 and 109 + 3 ka basalt flows. Recently, Douglass and Bockheim (2006) have studied the relationships between the glacial landforms, particularly moraine belts, of the Lago Buenos Aires region with the soils developed on them. These authors used distinct parameters such as accumulation rates of organic matter, pedogenetic carbonate and clay, to show

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that they decreased with decreasing age of the moraines. A lack of changes in soil redness, and preservation of minerals that should have been weathered in the oldest soils indicates that chemical weathering is almost absent in these environments. According to Douglass and Bockheim (2006), measured dust input explained the accumulation of both clay and carbonate, and a carbonate cycling model describing potential sources and calcium mobility in Patagonia has been presented. These authors stated that calibration of rates of soil formation would provide a powerful correlation tool for soils developed on different Patagonian glacial deposits. A complementary point of view has been presented by Gaiero et al. (2004), who stated that fluvial- and windborne materials transferred from Patagonia to the SW Atlantic show a very homogeneous rare earth element (REE) signature. The REE composition is compatible with recent tephra from Volca´n Hudson (46 S; Fig. 1b, Site 35). This would imply a dominance of material supplied by this source and other similar Andean volcanoes. Due to the trapping effect of drainage basins, Patagonian streams deliver to the ocean a suspended load with a slightly modified Andean signature, showing an REE composition depleted in heavy REEs. These authors considered Patagonia a sedimentary source distinguishable from other sources in southern South America. Quaternary sediments deposited in the Scotia Sea, and most dust in ice cores of east Antarctica would have REE compositions very similar to Buenos Aires Province loess and to Patagonian eolian dust. The REE compositions of

Fig. 13. Map of the Pleistocene glaciations at Lago Buenos Aires, Santa Cruz Province, Argentina (Fig. 1b, Site 24). From Singer et al., 2004a.

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most sediment cores of the Scotia Sea and Antarctica would reflect a distal transport of dust with an admixed composition from two main sources: a major contribution from Patagonia, and a minor proportion from source areas containing sediments with a clear upper crustal signature (e.g. western Argentina) or from the Bolivian Altiplano. However, the evidence presented by these authors indicates that Patagonian materials were the undisputable predominant sediment source to the southern latitudes during the LGM only. During the GPG, the ice tongues reached the Atlantic coast in the area north of the Magellan Straits and south of the Rı´o Gallegos valley (Fig. 1b, Site 27) for the first time in the Cenozoic, and expanded deeply over the present submarine platform. It is not clear whether the ice margin was effectively calving into the Atlantic Ocean, perhaps as far as 200 km east of the present coast. The expansion of the ice over the present submarine platform was clearly mapped already by Caldenius (1932), as shown by his glacial map of Tierra del Fuego and the Magellan Straits (Fig. 14). Mercer (1976) estimated the age of the GPG, based on K/Ar dating of lava flows underlying glacigenic deposits in different localities south of the Rı´o Gallegos valley, between 1.47 + 0.1 and 1.17 + 0.05 Ma. Meglioli (1992) obtained total fusion, whole-rock 40 Ar/39Ar ages of 1.55 + 0.03 Ma at the Bella Vista Basalt, Rı´o Gallegos valley (Fig. 1b, Site 27), which is covered by glacial erratics, thus providing a basal limiting age for the GPG. Ton-That et al. (1999) and Singer et al. (2004a) redated the Bella Vista Basalt by incrementalheating 40Ar/39Ar techniques at 1.168 + 0.007 Ma,

Fig. 14. Glacial map of Tierra del Fuego and the Magellan Straits (Caldenius, 1932).

considering that the observed discrepancy with Meglioli’s (1992) date is given by the higher precision of the latter technique. Likewise, Ton-That et al. (1999) and Singer et al. (2004a) provided for the first time a reliable upper limit for the GPG by means of the incrementalheating 40Ar/39Ar date of 1.016 + 0.005 Ma for the Telken Basalt, which covers the ‘‘Initioglacial’’ ( = GPG) deposits at Lago Buenos Aires (Fig. 1b, Site 24). According to the morphological and chronostratigraphic evidence of the till deposits, the Fuegian Andes ice sheet would have had a different extent and thickness in each of the known glacier advances, reaching the present submarine platform and the Fuegian lowlands to

Fig. 15. Glacial map of the Magellan Straits, by Meglioli (1992). This is a portion of the still unpublished map of his renowned dissertation, showing the distribution of the different Pleistocene moraines. The outermost moraine corresponds to the GPG (Bella Vista Drift), the two following ones to the latest Early Pleistocene and earliest Middle Pleistocene (Cabo Vı´rgenes and Punta Delgada drifts, respectively), and the two innermost, to the Late Middle Pleistocene (Primera Angostura Drift) and the Late Pleistocene (Segunda Angostura Drift).

Late Cenozoic Glaciations in Patagonia and Tierra del Fuego the north and the Drake Passage waters to the south at several occasions, receding to the summits and highlands during the interglacial periods. The absolute number of glacier advances that took place in southernmost South America is still a matter of debate. North of the Magellan Straits at least six major glacier advances have been described on the basis of terminal moraines (Meglioli, 1992; Fig. 15), whereas in Tierra del Fuego the morphological evidence points to two ice advances in the southern part and five in the north. Enigmatic, isolated boulders, as well as small till remnants, have been found in the Rı´o Grande city area and the Rı´o Chico valley (Meglioli, 1992; Coronato et al., 2004b; Figs 1c and 16) at various elevations along the large triangular zone between the Bahı´a Inu´til–San Sebastia´n and Fagnano ice lobes and the ice margins along the high mountains of western Tierra del Fuego (Fig. 1c). This area had been considered unglaciated by Nordenskjo¨ld (1899) but implicitly totally covered by ice at the ‘‘Initioglacial’’ stage by Caldenius (1932). These boulders (of undoubtedly glacial origin, based on their size and shape) and the surviving till patches were named the Rı´o Grande Drift by Meglioli (1992), who estimated its age between 2.05 and 1.86 Ma, by stratigraphic and geomorphological correlation with drifts and radiometric dated basalts in southern Patagonia. Therefore, Meglioli (1992) interpreted these glacigenic remnants as older than the

Fig. 16. Large glacial erratic boulder, originated in the Darwin Cordillera (Fig. 1), and found today isolated on top of Tertiary marine sediments, together with small remnants of till. Estancia Marı´a Behety, 20 km west of Rı´o Grande, Tierra del Fuego, Argentina. This boulder corresponds to the elusive ‘‘Rı´o Grande Drift’’, as defined by Meglioli (1992). (Photo by J. Rabassa, 2004).

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GPG, of latest Pliocene or earliest Pleistocene age. Though the issue of full glaciation of the island is still open, these glacigenic remnants are strong evidence in that sense. Caldenius (1932) recognized the extension of his aforementioned four glacial events north of the Magellan Straits but only three on the southern coast. He located the eastern limit of the two oldest glaciations beyond the coast, onto the present Atlantic submarine platform. According to his interpretation, both of these glaciations covered the entire island. His field mapping was extremely detailed, and quite correct most of the times, in spite of the serious difficulties in doing fieldwork, which in some cases even prevented him of reaching some areas. The Quaternary glaciations of Tierra del Fuego were very extensive. Large outlet glaciers of the Darwin Cordillera (2000 m a.s.l.; 55 S–69 W; Figs 1c and 17) ice cap flowed north and eastward to reach the present Atlantic submarine platform (Porter 1989; Meglioli et al., 1990; Isla and Schnack, 1995) following large, deep valleys known today as the Magellan Straits, Bahı´a Inu´til–Bahı´a San Sebastia´n Depression, Lake Fagnano, Carbajal–Tierra Mayor valley and the Beagle Channel (Fig. 1c). Several glaciations have been recognized in the northern part of the island (Meglioli et al., 1990; Meglioli, 1992) and at least two along the Beagle Channel (Rabassa et al., 1992, 2000). Meglioli (1992; see also Coronato et al., 2004a) mapped in great detail a very large area (over 25,000 km2) of the Magellan Straits and surrounding areas. He identified five or six large glacial events that he gave local names for each major valley, and which he correlated with the GPG and subsequent glaciations. The GPG was named the Bella Vista Drift in the Rı´o Gallegos valley and Sierra de los Frailes Drift in the straits area and northern Tierra del Fuego. A drumlin and megaflute field of GPG or Bella Vista glaciation (Meglioli, 1992) age has been recently described by Ercolano et al. (2004), in the Rı´o Gallegos valley of southernmost Patagonia (52 S; Fig. 1b, Site 27; Fig. 18). Several tens of streamlined landforms, some of

Fig. 17. Landsat image (1996) of Cordillera Darwin, western Tierra del Fuego, Chile (Fig. 1), showing the Fuegian Ice Sheet, and large discharge glaciers flowing in all directions, including the Beagle Channel and the Magellan Straits.

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Jorge Rabassa and Rı´o Gallegos valley lobes. Its type locality is found in the marine cliffs between Rı´o Cullen and Cabo Espı´ritu Santo, along the Atlantic coast, where it can be observed that the till forms the high plains and mesetas. The GPG would thus have developed sometime between 1.168 and 1.016 Ma, during MIS 30–34, and even maybe MIS 36, most likely including more than one glacial advance.

Glaciations of the Latest Early–Middle Pleistocene Fig. 18. Drumlins and megaflutes of the Bella Vista Drift, Early Pleistocene (Meglioli, 1992; Ercolano et al., 2004), in the Rı´o Gallegos valley, Santa Cruz Province, Argentina (Fig. 1b, Site 27). The central megaflute is 2.5 km long. Oblique aerial photograph on a stormy day by Bettina Ercolano and Elizabeth Mazzoni, 2002.

them several kilometers long that clearly appear even in satellite images, have been identified in a 30 km long section. Uncovered since perhaps 1 Ma, these landforms are beautifully preserved, thanks to the very dry climate and the lack of surface runoff during most of the Quaternary, showing only the incision of meltwater channel related to the ice recession during the GPG. Likely, these features are some of the oldest, well-preserved glacial landforms in the world, outside Antarctica. The Magellan Strait lobe was the largest and most impressive glacier that covered this region. It originated in the Darwin Cordillera and flowed to the north and east toward the Atlantic Ocean. Several discharge tongues came out in several directions. The highest (and oldest) drift in the Magellan Strait lobe, in northern Tierra del Fuego (Sierra de los Frailes Glaciation; Meglioli, 1992), extends over the high plains (100 m a.s.l.). This is a wide flat surface, with poor fluvial drainage, but which underwent an intense deflation. Although the superficial morphology does not show clear glacial landforms in this area, the till, as seen along the marine cliffs, forms the sedimentary core of the high plains. On this surface, large volcanic clasts show a similar weathering degree to those of the corresponding till unit in the northern coast of the Straits. This drift unit occurs as remnants between the Magellan Straits and Bahı´a Inu´til–Bahı´a San Sebastia´n Depression lobes, probably representing a piedmont-type glaciation that would have covered the southern end of the continent and a large portion of Isla Grande de Tierra del Fuego. The Bahı´a Inu´til–Bahı´a San Sebastia´n Depression lobe, emerging from the main body of the Magellan Straits Glacier and the northern slope of the Darwin Cordillera, reached the Atlantic Ocean Platform and the inner portions of Isla Grande de Tierra del Fuego at various occasions (Fig. 1c). The oldest tills of this lobe are exposed on top of the flat and high surfaces that form the Pampa de Beta. There are no volcanic flows in this area to be dated. However, the Pampa de Beta Drift is thought to correspond to the GPG, by correlation with the Magellan Straits

After the GPG, the deposits corresponding to the following Patagonian glaciations (‘‘Daniglacial’’ and ‘‘Gotiglacial’’, according to Caldenius, 1932; or post-GPG 1, 2 and 3, in the sense of Coronato et al., 2004a and b) are located at lower elevations in the landscape, and sometimes nested inside the GPG limits but very far from them. This circumstance is different to what may be seen in the Northern Hemisphere Scandinavian and Laurentian ice sheets, where the younger ice expansions in most cases reached the outer positions of the older glaciations and even extended beyond them. These conditions could be due to (1) a smaller intensity of the Southern Hemisphere cold episodes after the MIS 30–34 or (2) local phenomena. Concerning the first hypothesis, the Southern Hemisphere oceanic isotopic sequences do not show significant deviations from their equivalents of the Northern Hemisphere and they suggest similar intensities and chronology. Therefore, the circumstances may be investigated through phenomena of local nature. The evidence suggests that episodes of valley deepening took place over most of the Pleistocene, particularly the Middle and Late Pleistocene. The most important would have taken place immediately after the GPG, forcing the later glaciations to develop a morphology of discharge glaciers entrenched in their valleys, whereas the dominant glacier morphology during the GPG would have been of large piedmont lobes, of great extension but relative reduced thickness. This characteristic would have been favored by the preexisting landscape, with little incision of the piedmont valleys. This event has been named the ‘‘canyon-cutting event’’ by Rabassa and Clapperton (1990) and Rabassa and Evenson (1996), in comparison with similar episodes that occurred in the Rocky Mountains. This valley deepening event may have been caused by (1) increased erosion related to a larger discharge during the interglacial periods, (2) increased erosion related to tectonic ascent of the Patagonian Andes, (3) different glacier behavior, with large areas of cold ice on the divides separated by temperate ice in the valleys, or (4) a combination of some or all of these processes. The much larger magnitude of the deepening between the GPG deposits and the later events, compared to that existing in between the latter, suggests that alternative (2) would probably be correct. The cited tectonic uplift would have taken place perhaps between 1.0 and ca. 0.8–0.7 Ma, since in the next post-GPG the glaciers were already entrenched (Ton-That et al., 1999). This event would have possibly contributed even more intensively to the development of ‘‘rain-shadow’’ conditions in

Late Cenozoic Glaciations in Patagonia and Tierra del Fuego extra-Andean Patagonia, but its effective influence in the extra-glaciated Pampean Region is still unknown. The glacial event immediately after the GPG is known as ‘‘Daniglacial’’, according to Caldenius (1932) or ‘‘post-GPG 1’’, in the sense of Coronato et al. (2004a, b). This unit is characterized by conspicuous and wellpreserved morainic arcs that are located in inner positions respective to the GPG and entrenched in the valleys younger than this glaciation. In northern Patagonia, this unit was complexively named as ‘‘El Co´ndor Drift’’ in the San Carlos de Bariloche type area (Fig. 1b, Site 9) by Flint and Fidalgo (1964, 1969), including also the ‘‘Gotiglacial’’, but considering it of Late Pleistocene age. Later, Schlieder (1989), Rabassa et al. (1990a) and Rabassa and Evenson (1996) proposed the subdivision of the ‘‘El Co´ndor Drift’’ in two allostratigraphic units, La Fragua Drift and Anfiteatro Drift, in the type area of the Rı´o Limay valley (41 S, Fig. 1b, Site 3), or their equivalents, the San Huberto and Criadero de Zorros drifts, further north, in the Rı´o Malleo valley (39 S, Fig. 1b, Site 17). This subdivision was based on detailed mapping in both valleys, where the aforementioned units occur clearly separated both in distance and in elevation. The La Fragua and Anfiteatro drifts appear also along the dirt road east of the San Carlos de Bariloche Airport (Fig. 1b, Site 9), where Flint and Fidalgo (1964) defined the ‘‘El Co´ndor Drift’’. Possibly, if these authors had concentrated their work in the Limay valley, their glaciation model would have been different, since the obvious drift elevation distribution in that valley is indicative of significant relative age differences. However, in the Estancia El Co´ndor area, the differentiation of the drift bodies is more difficult because of ice-contact glaciolacustrine sediments and several coastlines of proglacial lakes. The La Fragua Drift has been assigned to Caldenius’ (1932) ‘‘Daniglacial’’ event (Schlieder, 1989; Rabassa and Evenson, 1996; Rabassa et al., 2005). In the Esquel region (Fig. 1b, Site 22), the ‘‘Daniglacial’’ moraines are found immediately west of the outermost GPG terminal systems, as entrenched sedimentary bodies at lower topographical levels. These units have been named the ‘‘post-GPG 1’’ glaciations by Martı´nez (2002) and Coronato et al. (2004a). These ridges act today as local continental water divides, bounding the Pacific slope basins. At Portezuelo de Apichig, Caldenius (1932), Gonza´lez Bonorino (1944) and Gonza´lez Dı´az and Andrada de Palomera (1995) have identified a morainic arc of this age, which is physically related to the glaciofluvial gravels of the Fita Michi Formation (Volkheimer, 1963). Thus, the morainic ridges of ‘‘Daniglacial’’ times are clearly linked to the ‘‘Patagonian gravels’’ in northern Patagonia. At Portezuelo de Leleque, at least three frontal moraines highly degrade by massmovement processes have been mapped behind the ‘‘Initioglacial’’ ridges. Frontal moraines of assumed ‘‘Gotiglacial’’ age (post-GPG 3; Martı´nez, 2002; Coronato et al., 2004a) occur at Portezuelo de Apichig. The moraines have later been eroded by spillways from glacial lakes. At the latitude of Lago Epuye´n and the heads of Rı´o Chubut (Fig. 1b, Site 19), the best-preserved morainic arcs are found. These would correspond to the

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‘‘Gotiglacial’’ (post-GPG 3) and they merge eastward with glaciofluvial plains and toward the west with large, varved glaciolacustrine deposits. In the Lago Buenos Aires region (Fig. 1b, Site 24), Ton-That (1997), Ton-That et al. (1999) and Singer et al. (2004a) obtained limiting ages for the ‘‘Daniglacial’’ drift, by means of incremental-heating 40Ar/39Ar dating of two lava flows associated to glacial deposits. The alreadymentioned Telken Basalt is the first of them (1.016 + 0.005 Ma), which covers the ‘‘Initioglacial’’ deposits or GPG, and predate the ‘‘Daniglacial’’ or ‘‘postGPG 1’’ deposits. Moreover, the Telken Basalt presents a transitional paleomagnetic polarity, which corresponds to the upper portion of the Jaramillo Subchron. The second is the Arroyo Page Basalt, dated at 0.760 + 0.007 Ma, of normal magnetic polarity, which covers the recessional outwash deposits of the ‘‘Daniglacial’’ stage (see Fig. 13). Thus, the ‘‘post-GPG 1’’ or ‘‘Daniglacial’’ event would have taken place possibly around MIS 18–20, immediately before the Early–Middle Pleistocene, indicated by the Matuyama–Brunhes paleomagnetic transition, dated at 0.78 Ma (Singer and Pringle, 1996). In the Chilean Lake District (Fig. 1b, Site 13), Porter (1981) considered that at least one of the glaciations could have been developed in this period. The Rı´o Llico Drift is clearly older than the Santa Marı´a Drift, based on weathering criteria and other field evidence. Since the Santa Marı´a Drift is pre-Late Pleistocene in age (Porter, 1981; Clapperton, 1993), a Daniglacial age for the Rı´o Llico Drift is acceptable. On Isla Chiloe´, south of Puerto Montt and southwest of the lake district (42 S; Fig. 1a), Heusser and Flint (1977) recognized a three-glaciation sequence of which the oldest unit, the Fuerte San Antonio Drift, has been correlated with the Rı´o Llico Drift (Porter, 1981) and it is considered to be of early Middle Pleistocene age, overlying a lava flow dated at 0.75 Ma (K/Ar) (Clapperton, 1993). In the Magellan Straits area, Meglioli (1992; Coronato et al., 2004a) mapped two glacial units that can be correlated with Caldenius’ (1932) ‘‘Daniglacial’’ event: the Cabo Vı´rgenes and the Punta Delgada drifts. In northern Tierra del Fuego, these units are known as the Rı´o Cullen and Sierra de San Sebastia´n drifts, whereas only one unit, the Glencross Drift, has been mapped within the Rı´o Gallegos valley (Fig. 1b, Site 27). These glacial advances expanded within deeper valleys, following the ‘‘Canyon cutting event’’. Above their highest reach, the GPG deposits are forming almost relief lacking high plains. The oldest of these advances (the Cabo Vı´rgenes Glaciation; Meglioli, 1992) is represented by well-defined moraine arcs, though with subdued, planar summits (100 m a.s.l.), which reach the cape where the drift is defined. The moraines are represented on both sides of the straits; the terminal position is not visible, though it is inferred that it may be submerged on the present Atlantic platform in the eastern entrance of the Magellan Straits, or that its moraines have been eroded by the fluvioglacial streams of later glaciations. An inner moraine belt is developed on both margins of the Magellan Straits up to Bahı´a Posesio´n (northern margin) and Punta Catalina (southern margin, Fig. 1a, c).

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These moraines have been interpreted as a second advance of the ice lobe (the Punta Delgada Glaciation), which is separated from the Cabo Vı´rgenes Drift, due to differences in morphology, soil development, periglacial features and weathering rinds of the clasts incorporated in the till. Both the Cabo Vı´rgenes and the Punta Delgada drifts, and their Fuegian equivalents, are considered to be of Daniglacial age and probably pertaining to the latest Matuyama and the earliest Brunhes paleomagnetic chrons. In the Bahı´a Inu´til–Bahı´a San Sebastia´n depression (Fig. 1c), a post-GPG advance is evident on both its margins, represented by the Rı´o Cullen moraines, with SW–NE orientation, forming a wide, ample relief of planar summits and continuous landforms. On the southern margin, the Rı´o Cullen Drift covers the slopes and high plains of the Sierras de Carmen Sylva (350 m a.s.l.), in northern Tierra del Fuego, extending toward the Atlantic Ocean in the shape of a flat moraine, with elongated ridges formed by glaciofluvial deposits and an extensive erratic boulder field at Punta Sinaı´ (Fig. 1c; Coronato et al., 1999; Coronato et al., 2004b). Recent cosmogenic nuclide measurements and paleomagnetic studies have indicated a Middle Pleistocene age for these deposits (Kaplan et al., 2007; Walther et al., 2007; Fig. 19). On the basis of submarine morphology investigations (Isla and Schnack, 1995) the terminal position of the moraine arc is located 40 km into the sea. Toward the center of the cited depression and at both of its margins, the San Sebastia´n moraines are located, forming the core of the mountain ranges of this name on the northern margin, up to 60 m a.s.l. (Fig. 20). Its wellpreserved, kettle-hole topography represents disintegration ice stages along the highest plains. The type locality is Cabo Nombre, on the Atlantic coast (Fig. 1c), where a gray, compact till, with abundant fragments of fossil

Fig. 20. Giant erratic boulder on top of the San Sebastia´n Moraine, early Middle Pleistocene, Los Chorrillos, Bahı´a San Sebastia´n, Tierra del Fuego, Argentina (Fig. 1c). Calvin J. Heusser, the author and Nat Rutter for scale. (Photo by A. Meglioli, 1988). shells, wood and coal derived from the preexisting sedimentary rocks, has been identified. The frontal position of this moraine would be located below present sea level, at approximately 20 km into the sea (Isla and Schnack 1995). Finally, based on paleomagnetic and absolute dating, the Daniglacial event would have developed between 1.01 and 0.76 Ma, most of this unit being of latest Matuyama age, perhaps during MIS 21–25, perhaps even MIS 19 (Shackleton, 1995). These drifts are probably equivalent to the younger units of the ‘‘pre-Illinoian’’ glacial deposits of Midwestern United States (Stiff and Hansel, 2004). However, recent paleomagnetic work on the till units at Bahı´a San Sebastia´n have indicated a Brunhes age for all sampled deposits (Ana Walther, personal communication), showing that these deposits have an age  0.78 Ma (Singer and Pringle, 1996). These new data suggest that at least part of those stratigraphic units in Tierra del Fuego and the Magellan Straits corresponding to Caldenius’ ‘‘Daniglacial’’ stage are clearly Mid-Pleistocene in age.

Glaciations of the Middle Pleistocene

Fig. 19. Erratic boulder field on top of the Rı´o Cullen Moraine, latest Early–earliest Middle Pleistocene, Punta Sinaı´, Bahı´a San Sebastia´n, Tierra del Fuego, Argentina (Fig. 1c). The boulders are composed of one single lithology, granodiorites coming from the Darwin Cordillera, most likely a rock avalanche on top of the glacier. Some of the boulders have been exposed at the moraine surface at least for more than 200,000 years (Kaplan et al., 2007). (Photo by J. Rabassa, 2004).

The most important glacial event of the end of the Middle Pleistocene is the ‘‘Gotiglacial’’ period (Caldenius, 1932), though in more southern localities like Lagos Buenos Aires, Viedma and Argentino (Fig. 1b, Sites 24–26), in the Skyrring and Otway Sounds, in the Magellan Straits region and Tierra del Fuego (Fig. 1b, Sites 38, 39), a previous glaciation defined as post-GPG 2 has been recognized (Coronato et al., 2004a, b). The ‘‘Gotiglacial’’ event corresponds to the younger portion of the ‘‘El Co´ndor Drift’’ (Flint and Fidalgo, 1964), to the Anfiteatro Drift, of the Upper Rı´o Limay valley (Rabassa and Evenson, 1996; Fig. 1b, Site 3; Fig. 21) and to the Criadero de Zorros Drift, of the Rı´o Malleo valley (Fig. 1b, Site 17; Rabassa et al., 1990a), in Neuque´n, northern Patagonia. The ‘‘Gotiglacial’’ moraines or their stratigraphic equivalents appear in all studied localities as very

Late Cenozoic Glaciations in Patagonia and Tierra del Fuego

Fig. 21. Stratified drift, glaciotectonically deformed, Anfiteatro Moraine of Gotiglacial age (= Late Illinoian age), Rı´o Limay valley, Neuque´n Province, Argentina (Fig. 1b, Site 3). (Photo by J. Rabassa, 1988).

well-preserved morainic arcs, located on valley sides above the altitudinal range of the LG and at several tens of kilometers downvalley from its terminal moraines. Its state of preservation is excellent, which clearly explains why Caldenius (1932) and Flint and Fidalgo (1964) had mistaken them for deposits of a phase of the LG. The assignation of these deposits to this glaciation was possible, thanks to geomorphological studies and radiometric dating of associated volcanic rocks. At the Rı´o Malleo valley (39 S, Fig. 1b, Site 17), the Pino Santo Basandesite was originally dated by K/Ar at 0.207 Ma (Rabassa et al., 1990a). This flow is infilling a glacial valley excavated in post-Criadero de Zorros Drift times. This basandesite was redated later by B. Singer (Sample PSA-01; personal communication; Rabassa et al., 2005) at 0.089 + 0.004 Ma by incremental-heating 40 Ar/39Ar techniques. In both cases, these dates confirm the pre-Late Pleistocene age of the Criadero de Zorros Drift (= ‘‘Gotiglacial’’). In the Rı´o Limay valley (Fig. 1b, Site 3), the Anfiteatro Drift is correlated with the Criadero de Zorros Drift (Rabassa and Evenson, 1996; Rabassa, 1999) and, thus, to a pre-LG event, based on their surficial morphology and their respective altitudinal positions with respect to the LG deposits. However, a TL date performed on glaciofluvial sands incorporated in the Anfiteatro Moraine (Fig. 21) yielded an age of 0.065 Ma (Amos, 1998), thus implying that the Anfiteatro Drift would have formed during MIS 4 (Early Late Pleistocene). However, this TL date should be considered as a ‘‘minimum age’’, unless very local, unknown conditions have operated in the area, since in no other site in Patagonia MIS 4 morainic arcs are found so far downslope from and altitudinally above the LG moraines (Kaplan et al., 2004, 2005).

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In the Chilean Lake District (Fig. 1b, Site 13), Porter (1981) has identified a large glacial event, well beyond the boundaries of the LG, represented by the Santa Maria Drift. This unit is located around 7–14 km east of the Early Pleistocene glacial deposits and more than 20–30 km west of the LG Llanquihue moraines forming arcuate ridges interpreted as moraines by Laugenie and Mercer (1973). This drift has been 14C dated at 57.8 ka, which should be considered as a minimum age due to contamination with modern rootlets (Clapperton, 1993), and its degree of weathering, relative to older and younger moraines (Porter, 1981). In the Lago Buenos Aires region (47 S, Fig. 1b, Site 24), a lava flow of normal magnetic polarity, which erupted from the Cerro Volca´n, postdates the post-GPG 2 and post-GPG 3 (‘‘Gotiglacial’’) deposits and predates those of the LG (Coronato et al., 2004a). This flow was dated by whole-rock K/Ar by Mercer (1976) at 0.177 + 0.056 Ma. Ton-That et al. (1999) obtained a date by 40Ar/39Ar plateau age of 0.123 + 0.005 Ma and an unspiked K/Ar age of 0.128 + 0.002 Ma was presented by Guillou and Singer (1997) and Singer et al. (2004a). These dates were later confirmed by cosmogenic isotopes (3He) exposure dates from pyroxene concentrates, which provided an average age of 0.128 + 0.003 Ma (Ackert et al., 1998; Singer et al., 1998, 2004a; Fig. 13), as a weighed mean of four sites and two locations. These ages confirm also that the 3He production rates at 47 S are constant for the last 100 kyr. Later, in situ cosmogenic surface exposure ages of boulders in the Moreno moraines (Kaplan et al., 2005; Fig. 22) together with the 109 ka 40Ar/39Ar age of Cerro Volca´n (Singer et al., 2004a) imply that the moraines deposited during the penultimate local glaciation correspond to MIS 6. These ages have been challenged by Wenzens (2006b), who claimed that cosmogenic dates are useless in these environments and that the dated Cerro Volca´n flow is in fact redeposited basalt, suggesting instead MIS 2 ages for these units based on 14C dating. However, Kaplan et al. (2006) have rejected these objections, particularly those concerning the primary nature of the volcanic flows and confirmed their glacial chronology for the area. In the Magellan Straits area, Porter (1989) identified different drift units based on weathering criteria. Moraines older than the LG were described at Primera Angostura and marine shells included there were dated at >47 ka, confirming a pre-LG age for these deposits. The significantly higher degree of weathering of these moraines suggested a pre-LG age as well. Meglioli (1992) defined local glacial units corresponding to the Middle Pleistocene in the different ice lobes. In the Rı´o Gallegos valley (Fig. 1b, Site 27), the Rı´o Turbio Drift was defined, whereas in the Straits area the prominent Primera Angostura moraines have been assigned to this period. In northern Tierra del Fuego, Meglioli (1992) assigned the moraines in the middle portion of the Bahı´a Inu´til–Bahı´a San Sebastia´n depression (Fig. 1c) to the Lagunas Secas Drift, at an elevation of 180 m a.s.l., of Middle Pleistocene age. The Lagunas Secas Drift is composed of deeply dissected moraine arcs, with small E–W elongated lakes, probably ancient

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Fig. 22. Cosmogenic age of the Moreno Moraines, Lago Buenos Aires, Santa Cruz Province, Argentina (Kaplan et al., 2005; Fig. 1b, Sites 24, 42). The Moreno moraines represent two or more glaciations during the late Middle Pleistocene, including the Illinoian and perhaps, part of the pre-Illinoian glacial events.

outwash channels, strongly eroded and deepened by deflation. Both Porter (1989) and Meglioli (1992) have accepted an MIS 6 age for these moraines, but other previous glacial events (i.e. MIS 8–12) may be present as well. Clapperton (1993) suggested that the moraines at the Straits of Magellan are most likely of composite origin and thus presenting a large range of glacial events, not only equivalent to MIS 6, but throughout the entire Pleistocene. In central Tierra del Fuego, the Lago Fagnano lobe was built by many different glaciers merging to form a large, outlet valley glacier in the present Seno Almirantazgo, a branch of the Magellan Straits (Fig. 1c). The lake basin is in fact a tectonic depression crossed by the firstorder Magellan fault, the boundary between the South American and Scotia plates. An ice thickness of more than 1500 m favored its eastward spreading, with additional ice supply from local glaciers at Sierra de Beauvoir and Sierra Alvear, on both sides of Lago Fagnano.

Caldenius (1932), Auer (1956) and Meglioli (1992) suggested that the eastern limit of the ice lies between the Irigoyen and Noguera ranges, east of Lago Fagnano, or even along the Atlantic coast. However, Caldenius (1932) had indicated in his map the possibility that the entire island had been covered by the ‘‘Initioglacial’’ (= GPG) glaciers, with their terminal moraines lying somewhere on the submarine platform. Thus, Caldenius (1932) is clearly referring to post-GPG events. Between the Atlantic shore and the eastern end of the lake, a moraine belt is found, in which Laguna del Pescado (20 km east of Lago Fagnano) and a number of peat bogs are situated. This belt was mapped by Caldenius (1932) as the outermost extent of the ice in this area, corresponding to his ‘‘Gotiglacial’’ glaciation. In the Lago Fagnano lobe, the moraines at the eastern end of the lake are thought to correspond to glacier oscillations during the Middle Pleistocene. Meglioli

Late Cenozoic Glaciations in Patagonia and Tierra del Fuego (1992) identified two drift units predating the LG: (i) the Rı´o Valdez Drift, along the southern coast of Lago Fagnano, believed to be of Illionian/Riss or pre-Illinoian/Riss age; and (ii) the Lago Chepelmut Drift, beyond the northern lake coast, which is referred to Late Illinoian glaciation. These units are covered by LG deposits along the margins of the lake and several kilometers beyond its eastern end. A proglacial deltaic sequence assigned to the Rı´o Valdez Drift develops along the eastern lake margin, next to Hosterı´a Kaike´n, at the easternmost end of Lago Fagnano (Bujalesky et al., 1997; Fig. 23). The gravel and lacustrine sequence overlies basal till and other glacial deposits and underlies till and other glacigenic units related to the frontal moraines at the eastern margin of Lago Fagnano. Two peat beds, dated at 39,000 14 C yr BP and >53,000 14C yr BP (Bujalesky et al., 1997; Rabassa et al., 2000), interbedded in the upper lacustrine levels, suggest that the delta was formed by deglaciation processes during an Early Wisconsinan/ Wu¨rm or pre-Wisconsinan/Wu¨rm interstadial, when climate was colder and drier than today. Recent findings (November 2005) of thin organic layers in between till units much farther west along the lake cliff have confirmed pre-LGM radiocarbon ages for these units (unpublished data). The pollen content of these layers lacks arboreal (Nothofagus spp.) pollen, confirming an impoverished tundra environment associated to glacial conditions (J.F. Ponce, personal communication). Thus, the glacigenic materials outcropping along the southern margin of the lake were formed either during a very early phase of the LG or, most likely, during the final phase of the penultimate glaciation (MIS 6, Late Illinoian/Riss). The Beagle Channel (Fig. 1c) is a drowned glacial valley, formerly occupied by a large outlet glacier, the former ‘‘Beagle Glacier’’, from the Darwin Cordillera. This valley was repeatedly glaciated, at least in two major episodes. Caldenius (1932) described glacial deposits in the Beagle Channel and on the Nueva, Lennox and Navarino islands (Fig. 1c). These are the oldest

Fig. 23. Glacigenic sediments at the cliffs of Lago Fagnano, Hosterı´a Kaike´n (Fig. 1c), Tierra del Fuego. Glacial delta beds overlying basal till. Lacustrine and peaty deposits overlying the sequence have infinite radiocarbon ages, suggesting that these deposits may correspond to a glacial advance during MIS 4, or more likely, MIS 6. (Photo by J. Rabassa, 2004).

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known glacial deposits in the southern Fuegian Andes: the so-called Lennox Glaciation. Evidence from previous glaciations was certainly eroded by the ‘‘Beagle Glacier’’ during successive events. The oldest recognizable glaciation has been named the Sloggett Glaciation, which is considered of Illinoian/ Riss age, MIS 6 and older (Rabassa et al., 2004). During this event, the ice occupied the entire channel basin, as far away as Bahı´a Sloggett (Fig. 1c), depositing the Punta Jesse and Punta Argentina moraines (Rabassa et al., 2004), located east of the LG moraines or Moat Glaciation (see below). A thick sequence of glaciofluvial gravels along the bay head would represent ice melting episodes of perhaps both pre-Moat and Moat age. Inner moraines, closer to the mountain front, would represent the maximum development of a local glaciation of Moat age, with local cirque and valley glaciers, independent from the Fuegian mountain ice field. Fieldwork has established the existence of a drumlin field beyond the Moat moraines (David Serrat, pers. comm.). Whether these drumlins were formed during MIS 6 (corresponding to the Slogget Glaciation) or 4 (earlier phase of the LG) is still a task of future research. Based on the presented evidence, it is possible to confirm a pre-LG age for the ‘‘Gotiglacial’’ period and its equivalent units (post-GPG 3 and post-GPG 2; Coronato et al., 2004a). It is most likely that the glacial deposits included in this unit would have been formed during MIS 6, but they could also have been originated in other previous Middle Pleistocene cold periods, such as those between MIS 8 and 16. Thus, the ‘‘Gotiglacial’’ event are only partially coeval to the ‘‘Illinoian Stage’’ of Midwestern United States or the Riss Glaciation of the European Alps, since it includes this stage but most likely extends beyond MIS 10, perhaps comprising some of the so-called pre-Illinoian deposits in the United States.

Glaciations of the Late Pleistocene The glacigenic deposits of the LG in Patagonia and Tierra del Fuego are those that were formed after the Last Interglacial, that is, MIS 5e, 125 ka (Panhke et al., 2003). The LGM was reached during the last major glacial event of the Late Pleistocene, during MIS 2, after a relatively warmer period identified with MIS 3. The age of the glacigenic deposits of the LG may be estimated starting perhaps at a maximum of 85 ka, since the process of formation of the Patagonian Andes ice sheet was undoubtedly slow and took at least 30 ka after the maximum of the Last Interglacial. A maximum concentration of d18O has been identified in Atlantic marine records as well as a minimum of dD has been recognized at the Vostok ice core, both around 70–75 ka BP (Panhke et al., 2003), suggesting the most probable age of the largest temperature depression in the Southern Hemisphere during MIS 4. Therefore, the ice expansion could have taken place only at an advanced stage of MIS 4. In most available marine and ice isotopic records, MIS 4 temperature depression was significant, but not as large as that of MIS 2. The LG was named as ‘‘Finiglacial’’ by Caldenius (1932), and as Nahuel Huapi Drift by Flint and Fidalgo

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(1964). This denomination has been preserved by later authors. The LG in the Chilean Lake District is known as the Llanquihue Glaciation (Mercer, 1976; Porter, 1981; Clapperton, 1993). Clapperton (1993) proposed to extend the name of ‘‘Llanquihue Glaciation’’ to the LG in the whole South American continent, with its type area in the Lago Llanquihue, Chilean Lake District (40–41 S, Fig. 1b, Site 13; Lowell et al., 1995), being represented by the Nahuel Huapi Drift on the eastern slope of the Andes. Clapperton (2000) summarized the knowledge about the LG in the southern Andes, in the most significant areas, such as the Chilean Lake District, the Patagonian ice fields, the Magellan Straits and the Beagle Channel. This author recognized a minimum of five glacial advances during the LG in the southern Andes, around 30, 27, 22.5, 15 and 12–9.3 14C ka BP. The LG deposits form moraines of extremely wellpreserved morphology, very fresh appearance, abrupt slopes and abundant erratic boulders on their surface. The more reliable chronological dates for the LG are coming precisely from the Lago Llanquihue area. There, successive studies by Mercer (1976), Porter (1981), Lowell et al. (1995) and Denton et al. (1999a, b) have provided an adjusted chronology based on radiocarbon dates. According to these authors, there were ice expansions during the MIS 4 and recession during the MIS 3 (Laugenie, 1984; see Rabassa and Clapperton, 1990; Clapperton, 1993). Based on an extremely detailed radiocarbon chronology, Lowell et al. (1995) have identified later readvances during MIS 2, which peaked at 13,900–14,890, 21,000, 23,060, 26,940, 29,600 and more than 33,500 yrs BP. Revised chronology of these areas, including Isla Chiloe´ (Fig. 1a), by Denton et al. (1999a, b), indicated that full-glacial or similar environmental conditions were maintained between 29,400 and 14,550 14C yr BP, with major glacial advances at 29,400, 26,670, 22,295–22,570 and 14,550–14,805 14C yr BP. Cooling events, suggested by pollen data from Isla Chiloe´, would have taken place at 44,520–47,110, 32,105–35,762, 24,895–26,019, 21,430–22,774 and 13,040–15,200 14C yr BP. The maximum expansion of the ice in the northern part of the studied area occurred at 22,295–22,570 14C yr BP, whereas in the southern portion it took place at 14,805–14,869 14C yr BP (Denton et al., 1999b). This outstanding reconstruction of glacial events in a large, piedmont area, showing variable glacier behavior in different parts of the ice front, is very important to evaluate and understand apparent discrepancies in moraine chronology over extended areas. The facts exposed in the cited papers should be carefully taken into consideration when discussing chronology of terminal moraines, in relationship with global climate episodes. The external positions of MIS 4 ice were generally reached and even surpassed by MIS 2 readvances. There are perhaps exceptions at certain areas of Lago Llanquihue, where the outermost moraine, Llanquihue 1 (Porter, 1981) was formed more than 39,000 14C yr BP, and in Lago Ranco (north of Lago Llanquihue), where it was deposited more than 40,000 14C yr BP (Laugenie and Mercer, 1973). Mercer (1983) suggested that the Llanquihue I outer moraines would be ca. 73,000 yrs old, and correlated them with MIS 4.

Lowell et al. (1995) and Denton et al. (1999a, b) concluded that the glaciers of the Chilean Lake District finally collapsed ca. 14,000 14C yr BP and suggested that the ice advances in this region were coeval with ice-rafting pulses of the North Atlantic Ocean, and that the last termination was suddenly and simultaneously initiated in both hemispheres before the modern termohaline circulation was restarted. These authors concluded that interhemispheric coupling implied a global atmospheric signal forcing rather than regional climatic changes. The extent of the LG on the Argentinian slope of the Andes at this latitude, the San Carlos de Bariloche–Lago Nahuel Huapi area (Fig. 1b, Site 9; Fig. 24), has been studied by Feruglio (1950), Flint and Fidalgo (1964), Gonza´lez Bonorino (1973), Rabassa (1975), Schlieder (1989) and Rabassa and Evenson (1996), among many others. The LG is represented by the Bariloche moraines, wrapping around the eastern edge of the lake. Equivalent moraines in similar positions can be found near most lakes in the region. At least two well-defined moraines of this age (Nahuel Huapi I and II) have been identified, each of them around 1 km wide, though so far no absolute chronology is available (Rabassa and Clapperton, 1990). These two moraines are separated by a depression filled by outwash, tephra and eolian deposits. Radiocarbon dates on these moraines are lacking due to the absence of organic matter in the tills, probably because of the extreme aridity of the area during the maximum expansion of the ice. Contrary to what happened on the Chilean side, where the ice advanced into the northern Patagonian Nothofagus forest, on the Argentinian side the forest was displaced eastward and northward or just supressed, trapped in between the ice front and the 500 mm-yr isohyeth on the dry Patagonian tablelands. A few exceptions exist, as in the Rı´o Malleo valley (Fig. 1b, Site 17), where Rabassa et al. (1990a) found organic layers on top of the Criadero de Zorros Drift (Penultimate Glaciation) and covered by the LG outwash dated at more than 30 ka BP. Schlieder (1989) and Rabassa et al. (1990a) presented K/Ar data on volcanic flows in the Rı´o Malleo and neighboring valleys, which provided limiting ages for the LG, which is younger than 0.126 + 0.019 Ma. As cited above, Rabassa et al. (2005) quoted 40Ar/39Ar

Fig. 24. Lago Nahuel Huapi, northern Patagonia (Fig. 1b, Site 9), a glacial piedmont lake. (Photo by J. Rabassa, 1979).

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LGM in Lago Buenos Aires Area Moraine age (ka)1 Age (ka)2

Fig. 25. Clastic dykes in Late Pleistocene glaciolacustrine sediments, San Martı´n de los Andes (van der Meer et al., 1992; Fig. 1b, Site 18). (Photo by J. Rabassa, 1988). redating of the Pino Santo Andesite at Rı´o Malleo (B. Singer, personal communication; 0.089 + 0.004 Ma) providing a closer upper age for the LG in the area. Ongoing cosmogenic dating research may for the first time provide absolute dates for the northern Patagonian ice advances on the eastern side of the Andes (A. Hein and O. Martı´nez, personal communication, 2006). Glacial deposits of the LG have been studied by van der Meer et al. (1992) in San Martı´n de los Andes, in the northern Patagonian Andes (Fig. 1b, Site 18) from a sedimentological and glaciotectonic point of view (Fig. 25). In Volca´n Copahue, northern Neuque´n Province (37 S; Fig. 1b, Site 11), Gonza´lez Dı´az (2003) has recognized only one major glaciation on the eastern slopes of this active volcano. Previous works had identified two glaciations, but the older one is reinterpreted as a Pleistocene giant slide from the volcano slopes. The confirmed glacial event is considered of Late Pleistocene age, followed by a very rapid recession. The LG in the area south of San Carlos de Bariloche (Fig. 1b, Site 9) was studied by Caldenius (1932) who recognized extensive ‘‘Finiglacial’’ and ‘‘post-Finiglacial’’ moraines, the latter of assumed post-LGM age, i.e. Late Glacial. Flint and Fidalgo (1969) extended their respective four- and threefold glaciation model southward, being also unable to obtain an absolute chronology of their Nahuel Huapi Drift. Similar conditions were encountered by Gonza´lez Dı´az and Andrada de Palomera (1995) and Martı´nez (2002). Miro´ (1967) mapped two morainic arc of ‘‘Finiglacial’’ (LG) age in the Epuye´n valley (43 S; Fig. 1b, Site 19). Lapido et al. (1990) described the Mallı´n Grande Drift at 43 300 S, forming two well-preserved morainic arcs with their corresponding glaciofluvial plains, and adjacent glaciolacustrine deposits, assigning it to the LG. Martı´nez (2002) proposed to consider only the inner of the Mallı´n Grande moraines as of LG age (Coronato et al., 2004a). In the Lago Buenos Aires region (Fig. 1b, Site 24), recent work by Kaplan et al. (2004) has confirmed the age of the LGM by means of cosmogenic isotope dating, allowing the differentiation of five glacial episodes, of which the outermost corresponds to the LGM. The

Mean ± 1σ

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Fig. 26. Cosmogenic ages of the LGM moraine systems, Lago Buenos Aires area, Santa Cruz, Argentina (Fig. 1b, Site 24). From Kaplan et al., 2004. 1Means based on boulder 10Be/ 26Al ages that include propagation of all uncertainties except for production rate. 2Means based on boulder 10Be/ 26Al ages that include propagation of all uncertainties including production rate. For explanation see Kaplan et al., 2004.

respective ages, expressed in calendar years, extend between 25 ka for the outermost Fenix V Moraine and 16 ka for the innermost Fenix I Moraine (Fig. 26). An AMS radiocarbon age of 15.3 + 0.3 ka BP in post-LGM glaciolacustrine deposits confirms the validity of these exposure ages and provides an upper limiting age for the LGM in this region. On top of these glaciolacustrine deposits, the Menucos Moraine corresponds to a postLGM (early Late Glacial) advance, dated at 13.8 ka by cosmogenic isotopes. The whole set of moraines is younger than the Cerro Volca´n Basalt flow (0.109 + 0.003 Ma; Singer et al., 2004a). Surface exposure dating of boulders on these moraines, combined with the 14C age of overlying varved lacustrine sediment, indicates deposition during the LGM (23–16 ka). Although Antarctic dust records signal an important Patagonian glaciation as their most likely source at around 60–40 ka, moraines corresponding to MIS 4 are not preserved at Lago Buenos Aires, or elsewhere in southern Patagonia. Most likely, the MIS 4 moraines were overrun and obliterated by the younger (MIS 2) ice advance (Singer et al., 2004a). The LGM ages for the Fenix moraines have been recently discussed by Wenzens (2006b) and Kaplan et al. (2006). Wenzens (2000, 2002) suggested that the present separation of the northern and southern Patagonian ice sheets is not just a consequence of Holocene melting away of the Pleistocene Ice Sheet, but a feature that was already established during the LG. The depression between both ice sheets is related to a tectonic depression, which already in the Pleistocene oriented ice drainage toward the Pacific Ocean. The eastern margin of the ice at this latitude (4745–48150 S) would then be the result of moraine accumulation by local valley glaciers, isolated from the major ice cap. The expansion of the LGM glaciers south of this latitude was much reduced when compared with the Northern Patagonian Ice Sheet eastern margin. This has been interpreted as a result of the northward displacement of the Pacific precipitation belt during glacial times (Wenzens, 2000). The Patagonian glaciations are progressively smaller during successive glacial advances after the GPG. These circumstances were explained by Rabassa and

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Clapperton (1990) as the effect of tectonic uplift during the Early Pleistocene after the GPG, which induced the downcutting of fluvial valleys (the ‘‘canyon-cutting event’’), modifying the original pattern of piedmont lobes into subsequent development of discharge glaciers nested in deep valleys during later glaciations. However, the total volume of ice would have remained almost constant. Alternatively, Singer et al. (2004a) hypothesized that tectonically driven uplift of the Patagonian Andes, which began in the Pliocene, yet continued into the Quaternary, in part due to subduction of the Chile rise spreading center during the past 2 Myr, maximized the ice accumulation area and ice extent by 1.1 Ma. Subsequent deep glacial erosion has reduced the accumulation area, resulting in less extensive glaciers over time. Marden (1994) has discussed the volume and provenance of the glacigenic deposits in Torres del Paine (51 S, 73 W; Fig. 1b, Site 30) and other areas of the southernmost Andes, concluding that the sediment budget of the last ice sheet was low, with very little supraglacial debris input and limited older drift reworking, because most glaciers advanced over drift-free terrain, the deposits of earlier ice sheets being confined to areas beyond the extent of the Last Glaciation. For this author, glacial erosion in the southern Andes seemed to be decreasing with successive glaciations. Alternatively, in the opinion of the present author, once the upper portions of the Tertiary weathered surface (a subtropical planation surface) had been denudated, the lower, unweathered materials were not removed easily, thus modifying the total sediment budget. One of the interesting features of the glacigenic deposits of the southernmost part of this region is that gold particles, supposedly coming from the Darwin Cordillera, at the core of the ice sheet, and which were exploited intensively in Tierra del Fuego during the early twentieth century, occur only in the Early and Middle Pleistocene drifts, being almost absent in the youngest drifts. This fact may be related to total erosion of the original gold veins in the accumulation area of the ice sheet or a radical change in rock accesibility. The innermost moraine arc in the Magellan Straits region (Fig. 1a) is located at Segunda Angostura, a narrow pass in the Straits, representing the youngest glaciation. These moraines present little erosion, angular ridges, nonfilled depressions, and abundant, slightly weathered metamorphic clasts. Soils have very poor development. The regional glaciation model proposed by Meglioli (1992) includes the Segunda Angostura Drift as the local equivalent of the LG, composed of several moraines, in tightly packed belts. The Bahı´a Inu´til Drift is the local equivalent of the LG along the depression of this name. The heads of the bay and its margins are surrounded by these moraines, composed of a clayey–silty till, with scarce clast content, glaciolacustrine structures and abundant, large erratic boulders, aligned over the surface. This moraine is in fact a landform complex, representing different glacial advances. Clapperton (1989) described an LG drumlin field at Cabeza de Mar, along the northern shore of the Magellan Straits (Fig. 1c), in between the two older moraine ridges belonging to this glaciation.

Clapperton et al. (1995) mapped inner moraine arcs (in relation to the Segunda Angostura moraines) between Isla Santa Isabel and Penı´nsula Juan Mazı´a, central Magellan Strait (Fig. 1c). The five mapped moraine arcs have been interpreted as glacial advances that took place during the Last Glacial cycle and Late Glacial events. A similar model has been suggested by these authors for the Bahı´a Inu´til ice lobe. McCulloch and Davies (2001) discussed climatic events in the Magellan Straits, based on pollen and diatom sequences at Puerto del Hambre, south of the city of Punta Arenas (Fig. 1b, Site 31). They recognized that the ice receded from the site sometime before 14,470 14C yr BP (17,330 cal. yr) and that a significant glacier readvance took place between ca. 12,000 and 10,300 14C yr BP. After this date, a very dry period started, which they associated with a high rate of forest fires. A different approach had been presented by Heusser (2003), who considered the charcoal accumulation as an indication of human arrival at the area. Note that the original basal date at this site of 16,590 14C yr BP (Porter et al., 1984) had been recalculated to 14,455 + 155 14C yr BP, due to lignite contamination (Heusser et al., 2000; McCulloch and Davies, 2001). Sugden et al. (2005) presented extensive information about the paleoclimatic and paleoenvironmental evolution in the Magellan Straits area during the Late Glacial–Holocene transition. These authors have suggested that there is a ‘‘blend’’ of Northern Hemisphere (e.g. North Atlantic Ocean) and Southern Hemisphere (e.g. Antarctic) climatic signals during this period, such as ice advances at LGM times (ca. 25–23 ka) and again at 17.5 ka (both calendar years). They have also recognized a readvance of the ice during the ‘‘Antarctic Cold Reversal’’ (ACR), ca. 15.3–12.2 ka, with the beginning of the deglaciation in the middle of ‘‘YD’’ times. Sugden et al. (2005) estimated that these conditions implied that during the Last Glacial–interglacial transition the regional climate was determined by a strong Antarctic signal. They concluded that during deglaciation, the conditions are more related to oceanographic changes, such as thermohaline circulation, than to astronomical forcing. A careful geomorphological mapping of the Strait of Magellan and neighboring regions has been attempted by Bentley et al. (2005). These authors have stated that the LGM moraines and other landforms can be certainly separated from those of the older glaciations, on the basis of geomorphological features, mostly weathering and drainage development. Likewise, it was possibly to separate different ice margins during LGM times, based on discontinuous moraine belts and meltwater channels that run along their margins. These geomorphological units have been considered as a very important support to fully understand the radiocarbon chronology of the area. The chronology of the LG in the Magellan Straits has been presented in great detail by McCulloch et al. (2005). Several moraine belts, associated with individual glacial advances, have been recognized. The age of the outermost advance, named as ‘‘A’’, has not been clearly established. It could be related to a pre-LG

Late Cenozoic Glaciations in Patagonia and Tierra del Fuego advance (older than 90 ka, based on amino acid data) and synchronous with the Moreno moraines of the Lago Buenos Aires region (Kaplan et al., 2004) of Late Illinoian/ Riss age. The following stage B corresponds to the LGM in the area, with ages of ca. 25,200–23,100 cal. yr. Ice advanced again in stage C (sometime before ca. 21,700– 20,300 cal. yr) and in stage D (before ca. 17,500 cal. yr). Finally, the ice readvanced again in between ca. 15,507– 14,348 and 12,587–11,773 cal. yr (12,638 and 10,314 14C yr BP; see also McCulloch et al., 2005). This later advance is considered to be in phase with the ACR, as identified in the Vostok ice core record, though it also overlapped with the onset of the YD period of the Northern Hemisphere. The beginning of rapid warming and final retreat of the Magellan glaciers took place sometime before 10,315 14C yr BP (11,770–12,580 cal. yr; McCulloch and Davies, 2001), which seems to be coincident with the coolest portion of the YD event. These findings suggested to McCulloch et al. (2005) that there would be a clear antiphase behavior between the two hemispheres during the Late Glacial–Holocene transition. Benn and Clapperton (2000a) studied the glacial sediments and landforms preserved in the Strait of Magellan area. The available record showed repeated advances of outlet glaciers of the Patagonian Ice Field during and following the LGM (25,000–14,000 14C yr BP). The ice-marginal landform assemblages are composed of thrust moraine complexes, kame and kettle topography and lateral meltwater channels. When analyzed together with other forms of paleoenvironmental evidence, the landform complex showed that, during the LGM and Late Glacial time, permafrost occurred near sea level in southernmost South America, indicating that mean annual temperatures were ca. 7–8C lower than at present, somewhat lower than those reconstructed by current glacier–climatic models. In comparison with precipitation–temperature relations for modern glaciers, precipitation levels would have been lower than today. Precipitation during glacials would have been lower, forced by precipitation shadow conditions induced by the Patagonian Ice Field, as well as an equatorward migration of the average position of westerly cyclonic centers. The significant role of neotectonics in the development of local conditions and disturbances in the geological and geomorphological record has been discussed by Bentley and McCulloch (2005), particularly in reference to the classical site of Puerto del Hambre, Magellan Straits. Late Pleistocene and Holocene movements along regional faults have affected the sedimentary accumulation and generated drainage diversion, affecting glacial and sea level reconstructions. Several annomalies of the Puerto del Hambre record can be explained by postglacial neotectonic activity. Along the eastern margin of Lago Fagnano (Fig. 1c), Caldenius (1932) mapped ‘‘Finiglacial’’ moraines wrapping around the lake. Meglioli (1992) defined the Lago Cami Drift as represented by the moraines at the easternmost end of the lake. Further studies are under way in order to establish the latest glacial stades and glaciolacustrine sequences of the Lago Fagnano Basin, which extend toward the Atlantic coast along the valleys of

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the San Pablo, Lainez and Irigoyen rivers (Fig. 1c; Coronato et al., 2005). Although precise 14C dating is still lacking for the LGM in Argentinian Tierra del Fuego, the most extensive expansion of the ice in the eastern Beagle Channel, Tierra del Fuego, was probably attained between 18 and 20 14 C ka BP, but ice recession from its maximum position had already started before 14.7 14C ka BP (Heusser and Rabassa, 1987; Rabassa et al., 1990b). The Moat Glaciation is represented by a complex system of terminal moraines at Punta Moat (Fig. 1c). These deposits and landforms have been correlated to Meglioli’s (1992) Segunda Angostura and Bahı´a Inu´til drifts (Wisconsinan/Wu¨rm Glaciation; Rabassa et al., 1990b). The position and extent of the ice field during the LGM has been reconstructed from various lines of evidence (Coronato et al., 1999) and it is presented in Fig. 27. Coronato et al. (2004b) defined the Moat Glaciation as the maximum expansion of the ice in the Beagle Channel during the Late Pleistocene. Unfortunately, only minimum 14C ages at the base of peat bogs grown on top of the moraine have been obtained so far, which are clearly younger than the assumed ages for the LGM, and even younger than the basal date at the Harberton peat bog, located 50 km to the west (Fig. 1c). Further work is needed to attain a precise, absolute chronology of the LGM in the Beagle Channel. At least five, still undated moraine arcs have been recognized for the LGM, with very fresh morphology, extending between sea level and 150–200 m a.s.l. A striking feature is the development of a drumlin field on Isla Gable (Fig. 1c and Fig. 28; Rabassa et al., 1990c). Drumlins on Isla Gable are part of a larger field that extends along the Beagle Channel from Estancia Harberton to Bahı´a Brown, and Puerto Williams (Chile). Caldenius (1932) and Kranck (1932) misinterpreted these landforms as terminal moraines, but Halle (1910) had already suggested that these landforms could be drumlins or drumlinoid features. Sedimentary structures reveal that these landforms would have been formed during the final phases of the Moat Glaciation. No absolute dating has yet been obtained for the LGM in this area. A minimum radiocarbon age of 14,640 + 260 yrs BP for the glacier retreat from the Punta Moat moraines is given by a basal 14 C date at the Puerto Harberton peat bog (Heusser 1989a, b; Rabassa et al., 1990c; Heusser, 2003; Fig. 29a, b). The lateral complexes that extend to Estancia Moat, 100–150 m a.s.l., are considered to correspond to the LGM (Fig. 30). These moraines appear again on Isla Picton and then on Isla Navarino (Chile; Fig. 1c). At the same time, the upper surface of the Beagle Glacier at Ushuaia (110 km West of Punta Moat, Fig. 1c) reached over 1200 m a.s.l., as shown by glacially eroded surfaces and the occurrence of erratics inside the major cirques. According to abundant and relevant evidence, the LG in Patagonia and Tierra del Fuego is equivalent to the Wisconsin or Wu¨rm glaciations of the Northern Hemisphere, spanning over MIS 4, 3 and 2. Evidence for a significant expansion of the ice during MIS 4 is available probably only in the Chilean Lake District and it has been named as the Llanquihue 1 event. The LGM is represented by the Llanquihue 2 moraines in Chile and the

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Jorge Rabassa 71° O

GLACIOFLUVIAL PLAINS

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Fig. 27. Paleogeomorphological and paleoecological map of Tierra del Fuego during the LGM (from Coronato et al., 1999). Late Glacial Expansions of the Ice

Fig. 28. Last Glaciation drumlin field at Estancia Harberton, Beagle Channel, Tierra del Fuego, Argentina (Rabassa et al., 1990c; Fig. 1c). This drumlin is seen from its up-ice end, looking downslope, in a drowned portion of the drumlin field. Note the Middle Holocene marine terrace around the drumlin base, slightly above present sea level, and the wave cut erosional features on both sides of the drumlin. (Photo by J. Rabassa, 2004). Nahuel Huapi Drift in northern Patagonia, and the Fenix moraines in the Lago Buenos Aires Basin. The LGM was attained around 25 cal. ka and ended around 15 cal. ka, probably corresponding to the Heinrich 2 and 1 events of the North Atlantic Ocean, respectively.

The Patagonian glaciers reached their maximum expansion around 23 ka (calendar years), but several readvances took place until the definitive recession started around 18–17 ka, based upon basal radiocarbon ages in peat bogs and cosmogenic dates on recessional moraines (Kaplan et al., 2004), though other smaller advances occurred during the Late Glacial. The Late Glacial period conventionally extends between 15 and 10 14C ka BP, but ice fluctuations may have started before the older boundary. Caldenius (1932) was the first to clearly identify moraine systems younger than the LGM. He mapped these units along the bottom of the glacial valleys, in an intermediate position between the Finiglacial (= LGM) moraines and the present glacier margins or, instead, the source cirques. He labeled them ‘‘Post-Finiglacial moraines’’, implying also a timely concept. Most of these moraines have been found to have Llate Glacial ages. Porter (1981) described a recessional phase of the ice in the Lago Llanquihue lobe (Fig. 1b, Site 13), which he named the ‘‘Llanquihue III’’ event, indicating that there were no defined moraines assigned to this phase, but a general ice-disintegration terrain complex, due to stagnant ice, around between 14 and 12.2 14C ka BP. Mercer (1976) did not support the idea of Late Glacial advances of the Patagonian Andes. He concluded that the warming trend initiated around 13 14C ka BP had continued without interruption until Holocene climates were established. Clapperton (1983) challenged this point of view, following Caldenius (1932) mapping, and Rabassa

Late Cenozoic Glaciations in Patagonia and Tierra del Fuego

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(a)

(b)

Fig. 29. (a) Harberton Bog, Beagle Channel, Argentinian Tierra del Fuego (Fig. 1c; photo by J. Rabassa, 2004); (b) absolute pollen rain data. Note the basal age of 14,640 – 260 14C yr BP. From Heusser, 2003. (1983) proposed also that some of the inner moraines of Lago Nahuel Huapi (Fig. 1b, Site 9) could be of Late Glacial age, because they were younger than peat basins with basal ages of ca. 14 14C ka BP, but located far downslope from the Neoglacial moraines in the same valley. Glasser et al. (2004) presented evidence for Late Glacial glacier fluctuations of the Patagonian ice fields. These authors considered that glaciers still covered large areas of Patagonia at approximately 14,600 14C yr BP, but uniform and rapid warming took place after 13,000 14C yr BP. There has been no agreement about evidence for climate fluctuations equivalent to those of the Northern Hemisphere

YD cooling event (the YD Chronozone), dated to 11,000– 10,000 14C yr BP (12,700–11,500 cal. yr). Singer et al. (2004a) and Kaplan et al. (2004) have identified a significant advance of the Lago Buenos Aires ice lobe (Fig. 1b, Site 24), cosmogenic isotope dated at ca. 14.4 + 0.9 ka, which they have called the Menucos moraine, when the ice was overriding its own glaciolacustrine deposits. No other ice expansions have been recorded here until the early Holocene. Hajdas et al. (2003) have reported high-resolution AMS 14C chronologies from the San Carlos de Bariloche and Chilean Lake District areas (Fig. 1b, Sites 9, 13) that suggest the development of a cool episode between

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Fig. 30. LGM moraines at Punta Moat, Beagle Channel, Argentina (Fig. 1c). (Photo by J. Rabassa, 1989). 11,400 and 10,200 14C yr BP, which they named the ‘‘Huelmo/Mascardi Cold Reversal’’, that would have preceded the onset of the Northern Hemisphere YD cold event by at least 550 calendar years. However, these authors estimated that both events occurred during a radiocarbon-age plateau at ca. 10,200 14C yr BP. Thus, the Huelmo/Mascardi Cold Reversal and the YD would have been a couple of short-term cool-warm oscillations that immediately preceded the onset of the latter in the North Atlantic region. These observations partially agree with the discussion by Sugden et al. (2005) about the ACR, an Antarctic climatic signal affecting southern Patagonia during Late Glacial times. Bennett et al. (2000) had denied the existence of a YD cooling event in southern Chile, based on chronological, sedimentological and paleoecological records from sediments of small lakes in the coastal zone, which is controlled by a heavily oceanic climate. Thus, these authors have suggested that there was little or no cooling in the southern Pacific surface waters, and therefore, indicating that the YD cooling in the North Atlantic Ocean was a regional, rather than global, phenomenon. However, it should be noted that the climate of the studied region is very different from the rest of Patagonia. In this area, the available moisture brought from the ocean onto the continent would have probably been constant, regardless of temperature changes, thus not affecting the distribution of local species. Perhaps plants and insects do not react to 1–2C changes, whereas glaciers actually do so to ELA modifications at the same scale. It sounds very extreme to extend paleoenvironmental conclusions to all of Patagonia, based on findings of a rather unique area. Though working on a global scale, Blunier and Brook (2001) have found a close relationship between similar events in both hemispheres. They studied the methane and isotopic content in Greenland and West Antarctic ice cores, confirming that the onset of seven major millennial-scale warming events in Antarctica preceded the onset of equivalent periods in Greenland by 1500–3000 yrs. In general, Antarctic temperatures increased gradually, while Greenland temperatures were decreasing or constant, and the termination of Antarctic warming was apparently coincident with the onset of rapid warming in Greenland. This pattern provides further evidence for the

operation of a ‘‘bipolar see-saw’’ in air temperatures. However, an oceanic teleconnection between the hemispheres on millennial timescales can be proposed, thus linking the precedent ACR with the subsequent YD over such a delay. Ariztegui et al. (1997) stated that several highresolution continental records have been reported recently in sites in South America, but the extent to which climatic variations were synchronous between both hemispheres during the Late Glacial–Holocene transition, and the causes of the observed climatic changes have not been solved yet. According to these authors, east of the Andes, the middle and high latitudes of South America warmed uniformly and rapidly from 13,000 14 C yr BP, with no indication of subsequent climate fluctuations, equivalent, for example, to the YD cooling. They presented a multiproxy continuous record, 14C dated by accelerated mass spectroscopy, from proglacial Lago Mascardi (Fig. 1b, Site 15), which indicates that unstable climatic conditions, comparable to those described from records obtained in the Northern Hemisphere, dominated the Late Glacial–Holocene transition in Argentina at this latitude. They suggested that a significant advance of the Monte Tronador local ice cap (Fig. 1b, Site 12), which feeds Lago Mascardi through the Upper Rı´o Manso, occurred, however, during the YD Chronozone. These circumstances suggested a climatic history that reflected a global, rather than a regional, forcing mechanism. These authors indicated that the Lago Mascardi record provides strong support for the hypothesis that ocean–atmosphere interaction, rather than global ocean circulation alone, led interhemispheric climate teleconnections during the last termination. McCulloch et al. (2000) noted the uncertainty about the interhemispheric timing of climatic changes during the Last Glacial–interglacial transition. They discussed various hypotheses, according to different lines of evidence, which suggest either that the Northern Hemisphere climatic changes were leading the Southern Hemisphere ones, and vice versa, or alternatively that both hemispheres acted in synchrony. The location of southern South America is considered appropriate to test the various alternatives using both glacial and paleoecological evidence. These authors estimated that, from varied sources of evidence, there was a sudden rise in temperature that initiated deglaciation simultaneously over more than 16 of latitude at 14,600–14,300 14C yr BP (17,500–17,150 cal. yr). There was also a second warming episode in the Chilean Lake District at 13,000–12,700 14C yr BP (15,650–15,350 cal. yr), when temperatures almost achieved modern values. A third major warming step occurred at ca. 10,000 14C yr BP (11,400 cal. yr), reaching Holocene temperature levels. Following the initial warming, there was a lagged response in precipitation as the westerlies, after a delay of ca. 1.6 kyr, migrated from their northern glacial location to their present latitude, which took place ca. 12,300 14 C yr BP (14,300 cal. yr). According to these authors, the latitudinal contrasts in the timing of maximum precipitation are reflected in regional contrasts in vegetation change and in glacier behavior. A large, 80 km glacier advance in the Strait of Magellan at 12,700–10,300

Late Cenozoic Glaciations in Patagonia and Tierra del Fuego 14

C yr BP (15,350–12,250 cal. yr), a period that includes both the ACR and the earlier part of the YD, was influenced by the southward return of the westerlies. The delay in the migration of the westerlies would be perhaps coincident with the Heinrich 1 iceberg event in the North Atlantic. Thus, the suppressed global thermohaline circulation at that time may have also affected sea-surface temperatures in the South Pacific, modifying the position of the westerlies, which returned to their present southerly latitude only after oceanic conditions achieved their present interglacial mode. Marden (1997) presented evidence of two Late Glacial advances of the South Patagonian Ice Field at Torres del Paine (51 S, 73 W; Fig. 1b, Site 30), Chile, which challenge the concept raised by others that climate in southernmost South America was characterized by uninterrupted warming after LGM termination. These advances are marked by moraines and other ice-marginal deposits, 18–20 km and 10–16 km from the modern limits of two outlet glaciers, whereas older full glacial limits are indicated by other sets of moraines ca. 50 km from the modern glaciers. Pumice clasts included in the glacial deposits are related to an eruption of Volca´n Reclus (Fig. 1b, Site 34) at ca. 11,880 14C yr BP, which provided a close limiting age for the older Late Glacial event, whereas the younger advance occurred during the interval 11,880–9180 14C yr BP. This author supported the idea that deglaciation occurred slowly in the studied area because initial warming was accompanied by increased moisture as precipitation belts migrated southward. As the climate cooled, the outlet glaciers advanced. The temperature depression was estimated to have been not more than 2C below current values, since Late Glacial moraines at some local glaciers lie within 200 m of the modern ice margins. This idea of twofold, late glacial expansions had been previously supported by palynological evidence (e.g. Heusser, 1987; Heusser and Rabassa, 1987; Clapperton, 1993; Heusser, 1987, 1993, 2003). Fogwill and Kubik (2005) have presented preliminary cosmogenic 10Be data from a former ice limit in Torres del Paine. The offered data indicate a stillstand or a readvance of Patagonian glaciers culminating at around 12–15 ka with a mean age of 13.2 + 0.8 ka. The glacier extended some 40 km beyond the present ice margin and was within 15 km of the presumed LGM boundary in this area. This glacier stage is interpreted as partially coincident with the ACR (14.5–12.9 ka). According to these authors, the data implied that glaciers at these latitudes were out of phase with those in the Northern Hemisphere, but instead, followed an Antarctic climatic signal during Late Glacial times. The Puerto Banderas moraine at Lago Argentino (Fig. 1b, Site 26) was mapped by Caldenius (1932) as one of his ‘‘Post-Finiglacial’’ moraines, and described by Mercer (1976). Strelin and Malagnino (2000) proposed a Late Glacial age for a system of three moraine belts, with a maximum age of 15.5 + 2.4 cal. yr. Recently, Strelin and Denton (2005) have suggested a new chronology of these units, following the previous scheme of using radiocarbon ages of organic materials found at the marginal moraines. Becker et al. (2005) discussed the problem of the ACR and YD

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problem in relation to this unit. They have mapped and dated the Puerto Banderas I and II moraines, and 36Cl and 10 Be dated these moraines with an average age of 11.2 + 05 ka from 18 samples. Mercer (1976) had previously obtained a radiocarbon age of 11.7 + 0.3 ka BP. Becker et al. (2005) concluded that the Puerto Bandera Moraine is younger than previously thought, and it is not related to the termination of the LGM, neither deposited during the ACR. This ice advance would be consistent with the expansion of the South Patagonian Ice Field during, or shortly following, the YD period. Thus, these authors have stated that the proposal by Sugden et al. (2005) that the ACR is more prominent in southernmost Patagonia may be premature. Thus, not all of South America south of the Chilean Lake District seemed to be in phase with the Antarctic climate, and as the polar front and westerlies migrate, the boundary between ‘‘northern’’ and ‘‘Antarctic’’ response may be latitudinally displaced as well. Mercer (1976) found no evidence around the Patagonian ice fields that glaciers had advanced during the Late Glacial interval at 12–10 14C ka BP. But because peat older than 11 14C ka BP lies beneath Neoglacial moraines in some places, Mercer concluded that since the interval of deglaciation at ca. 13 14C ka BP, Patagonian glaciers had not been any more extensive than they are now until ca. 5000 14C yr BP. Consequently, Mercer (1976) suggested that the Holocene most probably began in southern South America at around 13 14C ka BP and that the late glacial cooling known as the Younger Dryas in the Northern Hemisphere had been restricted to north west Europe. This opinion was supported by studies of fossil beetles in the Chilean lake region by Hoganson and Ashworth (1992) and by pollen studies in Patagonia east of the Andes by Markgraf (1991, 1993). These authors also concluded that the so-called Hypsithermal warming trend had begun at about 13 14C ka BP and was not followed or interrupted by any significant cooling. These views are quite the reverse of those determined from palynological studies by Calvin Heusser who, in a number of articles (Heusser, 1974, 1984, 1987; Heusser and Streeter, 1980; Heusser et al., 1981; Heusser and Rabassa, 1987), had argued strongly that significant climatic cooling occurred during not only the last 5 kyr, but also at 11–10 14C ka BP. The high precipitation and low temperatures estimated by Heusser and Streeter (1980) for the Late Glacial interval, if valid, should have caused glaciers in the region to advance. Clapperton (1993) has argued, however, that in areas where the Andes are low and presently ice-free, as along much of the crest east of the Chilean Lake District, the Late Glacial temperature depression may have been inadequate to have caused glaciers to form again. He has also suggested (Clapperton, 1983) that if there are no Late Glacial moraines around the Patagonian ice fields, it is because this area had become almost icefree by 13 14C ka BP. Late Glacial cooling might have initiated the regrowth of glaciers, but these were restricted to the ranges now buried by the (Neoglacial) ice field systems. Thus, any Late Glacial moraines that were deposited lie beneath the present ice cover.

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Alternatively, John Mercer may have been wrong in his observation that Late Glacial moraines do not exist beyond the limits of those he dated to be part of the Neoglacial interval. It is particularly crucial to understand what happened around the Patagonian ice fields during this interval because evidence of late glacial advances has been found in other parts of the southern Andes. Also, in the San Carlos de Bariloche area, several moraines have been observed inside the limits of the LGM (Nahuel Huapi moraines). As they appear well down-valley from those of the Neoglacial interval in the Rı´o Manso valley, it is possible that they formed during the Late Glacial interval. They were mapped by Caldenius (1932) as ‘‘PostFiniglacial’’ moraines. Those observed by Rabassa (1983) at Lago Moreno, Puerto Blest and Divisoria de Aguas, near San Carlos de Bariloche (Fig. 1b, Site 9), are worthy of more detailed study as candidates for Late Glacial limits in this region. They are believed to be younger than ca. 14 14C ka BP, the age of basal peat in a bog lying between these moraines and Lago Nahuel Huapi. Moraines east of the South Patagonian Ice Field younger than those of the LGM were mapped by Caldenius (1932) and Feruglio (1950). Particularly striking are those at Punta Bandera, Punta Ciervo and Marı´a Antonia along the southern shore of Lago Argentino (Fig. 1b, Site 26). They are situated at distances of 22, 24 and 50 km beyond the present outlet glaciers. These distances suggest that the moraines are more likely to represent limits of a significant glacial stadial at ca. 15–1414C ka BP. Along the Beagle Channel, several glacial advances or stabilization periods took place during Late Glacial times (Rabassa et al., 1992, 2000). A first ice retreat phase probably took place before 14.7 14C ka BP. A model of a calving glacier front, in either the adjacent sea or a proglacial lake, has been favored. A 1–2 kyr long stabilization phase could have occurred when the ice front reached the Isla Gable rise (Fig. 1c). This is suggested by the basal 14C age of the Caleta Ro´balo peat bog (near Puerto Williams, Isla Navarino, Chile, Fig. 1c; 12,700 + 90 yrs BP), a minimum age for ice retreat from Isla Gable. During the initial recession period, the glacier thickness decreased at Ushuaia by a minimum of 550 m. Then, the main Beagle Glacier receded from the cirques, allowing their glaciers to expand downslope. Two large, extensive lateral moraines have been mapped around Ushuaia and named the Pista de Ski and Ushuaia moraines. Radiocarbon dating of basal peat at 12,060 + 60 yrs BP at Pista de Ski Moraine (300 m a.s.l.) suggested that this retreat phase probably peaked ca. 12 ka 14C yr BP, when a relative maximum of arboreal pollen is reached around 11,780 + 110 14C yr BP, at Puerto Harberton bog (Fig. 1c; Rabassa et al., 1990b). Morphological evidence of stabilization occurs also between Punta Segunda (35 km West of Isla Gable) and Arroyo Ferna´ndez (Fig. 1c), building up a four-stage frontal moraine complex that extends into the Beagle Channel, below present sea level. These moraines develop from 100 m a.s.l. at the mountain sides, in a discontinuous shape, as till pockets preserved against erosional bedrock remnants or as low moraines (<75 m a.s.l.) in the city of Ushuaia. Notwithstanding, the

basal ages of the Break Point (80 m a.s.l.; 12,430 + 80 14 C yr BP) and San Salvador (10 m a.s.l.; 12,100 + 50 14 C yr BP) peat bogs in Ushuaia show that the ice would have already disappeared from these sites allowing the formation of lacustrine environments (Heusser, 1998). The similarity of the peat bog basal ages between 300 and 10 m a.s.l. in Ushuaia suggested that the ice recession from Isla Gable to Ushuaia had taken place during a short period. Although the pollen profiles show evidence of cooling between 11 and 10 ka and subsequent vegetation changes (McCulloch et al., 1997), perhaps the climatic conditions had been not harsh enough so as to alter the Beagle Glacier dynamics and to allow ice stabilization and interrupt the general headward recession. Future studies will probably lead to a discussion of the chronostratigraphy of the lowest moraine arcs in Ushuaia, previously defined as Ushuaia Drift (Rabassa et al., 1990b). Radiocarbon ages of the terminal moraine complex at Punta Segunda are still needed to adjust the chronology in this region. The 10 ka glacial retreat was definitive: basal peat layers of Punta Pingu¨inos in Ushuaia (20 m a.s.l.) and Lapataia (20 km westward, 18 m a.s.l.; see Fig. 1c) showed ages of 10,080 14C yr BP (Rabassa et al., 1986; Heusser and Rabassa, 1987), a condition observed also for the glaciers that were tributaries to the glaciation axis located in the eastern end (66 W, Bahı´a Aguirre, Penı´nsula Mitre, Fig. 1c), where basal peat layers have yielded an age of 10,920 + 70 14C yr BP (UTC-5402). The rapid disappearance of the ice within the eastern portion of the Beagle Channel was probably due to the collapse of a floating ice snout, as sea level invaded the valley and rose to almost present positions around 8.7 14C ka BP (Gordillo et al., 1993). A glacierization model in mountain valleys of the Fuegian Andes, tributaries to the Beagle Channel valley, was proposed by Coronato (1995 a, b) and Coronato et al. (2004b). The transversal and longitudinal valleys of the Fuegian Andes show the effect of extensive Pleistocene glacier erosion. The tributary valleys were occupied by multiple valley glaciers, ranging from 20 to 30 km in length, though smaller, single-valley glaciers were also present. These valleys probably underwent the same sequence of glacial events as the rest of Tierra del Fuego, but such episodes are not represented in the existing geological record. This is probably due to erosion during the LGM. Moreover, the entire study area was mostly ice-covered and well above the ELA, impeding the formation of lateral moraines. As in all interdependent ice system, glacial activity in the tributary valleys was controlled by the behavior of the main ice stream and regional climatic variations. Several phases that took place between the Late Pleistocene and the Early Holocene in the Andean valley glaciers have been established: (i) the LGM, (20–18 14C ka BP); (ii) ‘‘Individualization’’, as the Moat Glaciation was decaying (18–14 14C ka BP); (iii) ‘‘Stabilization’’, when the ice bodies achieved their maximum positions during Late Glacial times (14–12 14C ka BP); and (iv) ‘‘Deglaciation’’ (10–9 14C ka BP), when glaciolacustrine environments were dominant in the mountain valleys.

Late Cenozoic Glaciations in Patagonia and Tierra del Fuego

Fig. 31. Kame and glaciolacustrine deposits at the mouth of the Rı´o Pipo valley, Beagle Channel, near Ushuaia, Tierra del Fuego, Argentina (Fig. 1c). These were formed as the valley glacier receded and sediments poured from the main Beagle Glacier, still occupying the valley during Late Glacial times. (Photo by J. Rabassa, 2003).

The erosive landforms are recognizable in the rocky areˆtes in which glacial peaks, horns and cirques abound, some of them still bearing mountain glaciers of significant magnitude. Glacial accumulation landforms are rare, although some depositional features have been modeled in subglacial, supraglacial and marginal ice environments. The presence of latero-frontal moraine arcs, basal moraines, kame terraces, glacial plains and peat bogs has been clearly defined and mapped in the valleys involved, as well as the remains of glaciolacustrine bodies (Coronato, 1990, 1995a, b). The frontal moraine arcs of the Andorra and Can˜ado´n del Toro valleys, near Ushuaia, are separated by glaciolacustrine deposits that are also present in the Pipo valley. In that valley, icemarginal landforms related to the general glacier recession prevailed (Coronato, 1993; Fig. 31). The Carbajal–Tierra Mayor valley (Fig. 1c) is another important glaciation axis in the Fuegian Andes, tributary of the Beagle Channel at Bahı´a Brown, 50 km east of Ushuaia (Fig. 1c and Fig. 32). During the maximum of the LG a trunk glacier established here, flowing from W to E along the tectonic alignment Carbajal–Tierra Mayor–Lasiparshak (Fig. 1c), with minor tributary glaciers, coming from lateral cirques. Due to glacial diversion, an overflowing ice tongue would have displaced southward along the Rı´o Olivia

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valley down to its confluence with the Beagle Glacier, E of Ushuaia (Coronato, 1995a, b). The Late Glacial–Early Holocene depositional sequence is presently under study concerning the palynological and paleoclimatic aspects. Geomorphological evidence found in the Fuegian Andes indicates that the definitive deglaciation process would have started after 10 14C ka BP. In the inner valleys, a lacustrine phase has been characterized, in lake sedimentary sequences or at the base of the present peat bogs (Coronato, 1991; Gordillo et al., 1993; Coronato, 1995a, b). The existence of paleolakes in different relative positions in between confluent glaciers has been dated ca. 10–9 14C ka BP (Coronato, 1993). Moraines situated close to the cirque basins above Ushuaia appear to be late glacial in age, but precise dating remains to be done. Planas et al. (2002) mapped the geomorphological units within the Martial Glacier cirque, near Ushuaia (Fig. 1c), with at least two moraine levels of Late Glacial age developed on both valley sides and Holocene moraines occurring next to the ice front, the latter still lacking plant colonization (Planas et al., 2002; Figs 33 and 34). Some support for a Late Glacial age cold interval came from the interpretation of pollen diagrams obtained for this area (Heusser, 2003). These records showed that a significant reduction in Nothofagus pollen occurred during the interval ca. 13–10 14C ka BP (Heusser, 1989a; Rabassa et al., 1990a). Such a decline has been associated with a significant climatic deterioration, perhaps coeval with glacier advance. The Late Glacial history of Patagonia and Tierra del Fuego is now much better known than only two decades ago, but much precise dating is still needed if definitive correlation with Northern Hemisphere climatic events is intended.

4.4. Holocene Glaciation Mercer (1965, 1968, 1970, 1976, 1982) published pioneer studies on Patagonian Holocene ice advances and termed them ‘‘Neoglaciations’’. Mercer (1976) found evidence that by ca. 13 14C ka BP deglaciation had cleared ice from the Rı´o Baker valley, which separates the two Patagonian ice fields (Fig. 1a), and concluded that glaciers in the region did not readvance again until about 5 14C ka BP. Investigations of moraines lying several kilometers from the present glaciers on the eastern and western sides of the two Patagonian ice caps led Mercer (1968, 1970, 1976) to conclude that three

Fig. 32. Panoramic view of the Carbajal valley, heads of the Rı´o Olivia, near Ushuaia, Tierra del Fuego, Argentina (Fig. 1c). Erosional glacial landscape, carved on metamorphic rocks during the LG. (Photo by J. Rabassa, 2004).

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Fig. 33. View of the lateral, cirque moraines of Late Glacial age, Martial Cirque, Fuegian Andes. The city below the cirque is Ushuaia, and an ample view of the Beagle Channel (Fig. 1c). The cirque moraines are actively covered by periglacial fans and talus, coming down from the summit areas. (Photo by J. Rabassa, 2004).

Fig. 34. Holocene moraines, Martial Cirque, Ushuaia (Fig. 1c). (Photo by J. Rabassa, 2004).

advances had occurred during an interval of Neoglacial cooling that spanned the last 5 kyr. The first was at 4700–4200 14C BP, the second at 2700–2000 14C BP and the third has taken place during the last three centuries (seventeenth to twentieth centuries). Most of the organic material from which radiocarbon dates were obtained gave only minimal ages for the advances, however, and none have been closely bracketed. Nevertheless, there is agreement with the ages of Neoglacial fluctuations determined in other parts of the Andes and in the Northern Hemisphere (Clapperton, 1993). Clapperton (1993) studied all of Mercer’s data on Neoglacial glacier fluctuations and noted that an interesting pattern exists. During the first advance at 4700–4000 14C BP glaciers in the west were more extensive than during the advance at 2700–2000 14 C BP; glaciers in the east were less extensive at 4700–4200 14C BP than at 2700–2000 14C BP. A preliminary hypothesis is that an eastward migration of the

iceshed occurred as the Patagonian ice fields built up over two mountain ridges separated by an intermontane depression. Clapperton (1993) has also suggested that the Patagonian ice fields may not have existed until the Neoglacial cooling and that only small glaciers restricted to the (now-buried) mountain ridges survived during the preceding interval of Hypsithermal warmth. Palynological studies of cores taken in the Chilean Lake District (Heusser, 1974; Heusser and Streeter, 1980; Heusser et al., 1981; Fig. 1b, Site 13) also indicate three cooling intervals during the last 5 kyr. Radiocarbon dating of major vegetational changes that indicate cooler conditions suggested that the climate reversals occurred at 4950–3160 14C BP, sometime between 3160 14C BP and 890 14C BP, and during the last 350 yrs. The intervals of relatively low temperature appear to have coincided with periods when precipitation was significantly higher than now, with total annual rainfall as much as 150% above the present mean (Heusser and Streeter, 1980). Bertani et al. (1986) recognized at least two Neoglacial advances in addition to that of the Little Ice Age (LIA) at the Castan˜o Overo Glacier (Monte Tronador, northern Patagonia; Figs 1b, Site 12; Figs 35 and 36), and Rabassa et al. (1984) and Brandani et al. (1986) noted that the Rı´o Manso Glacier had advanced at least once before the LIA (Fig. 37). However, none of these earlier events has been precisely dated yet. The LIA extended between middle seventeenth and middle nineteenth centuries, based on dendrochronological analysis of the trees colonizing the successive moraine ridges (Rabassa et al., 1984; Brandani et al., 1986). Rabassa et al. (1981) described in-transit moraines on the Casa Pangue Glacier (western slope of Monte Tronador, Chile), which supported forest colonization on the active glacier in those times (Fig. 38). Although the earliest Holocene has generally been considered an interval of ameliorating climatic conditions, Ro¨thlisberger (1987) and Ro¨thlisberger and Geyh (1985) concluded that glaciers advanced at least twice between 8600 and 8200 14C yr BP. These events were

Fig. 35. Castan˜o Overo Glacier, Monte Tronador, northern Patagonia, Argentina (Fig. 1b, Site 12; photo by J. Rabassa, 1975). This regeneration cone, reconstructed below a very high ice fall, melted away in recent years due to regional warming (see Bertani et al., 1986).

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Fig. 38. Casa Pangue Glacier, Monte Tronador, Chile (Fig. 1b, Site 12). This is the largest glacier in northern Patagonia. This is a debris covered, reconstructed glacier which supported in-transit moraines with soils. Vegetation was growing on top of the active ice (Rabassa et al., 1981). (Photo by J. Rabassa, 1979). In the 1990s, regional warming forced the collapse of the underlying ice, and with it, the soils and trees.

Fig. 36. Art Bloom (Cornell University) posing as scale on a striated glacial boulder in front of Castan˜o Overo Glacier, Monte Tronador, Argentina (Fig. 1b, Site 12; photo by J. Rabassa, 1982).

Fig. 37. Rı´o Manso Glacier, Monte Tronador, Argentina (Fig. 1b, Site 12). This is a debris covered glacier that has prominent Holocene and LIA moraines, which can be seen on both sides of the glacier (Rabassa et al., 1978). (Photo by J. Rabassa, 1983). This glacier is presently undergoing very rapid retreat due to regional warming.

suggested on the basis of volcanic ashes covering apparent Neoglacial moraines and should be considered as minimal ages only; the moraines could be much older, possibly even of Late Glacial age. Glacial advances in the earliest Holocene have been discussed for a long time. There has been a general agreement that ice readvances suggested for this interval either

have been wrongly dated or have been misinterpreted; for example, some were probably associated with glacier surges, kinematic waves or other oscillations related to internal glacier dynamics independent of climatic change and do not reflect global or regional cooling. Palynological interpretations in southern Chile by Heusser (1974) suggested that the warmest climate interval following the Late Glacial cooling occurred between ca. 8500 and 6500 14C yr BP, when temperatures averaged about 2C warmer than now. This corresponds well with data from other parts of the world indicating that Holocene Hypsithermal conditions had peaked before ca. 6000 14C yr BP; but a subsequent study by Heusser and Streeter (1980) suggested that the maximum warmth had occurred earlier, between 9410 and 8600 14C yr BP. Porter (2000) extensively discussed the nature and age of the Patagonian Holocene glaciations. Evidence of early Neoglacial expansion of glaciers in the Andes was primarily located within a belt extending between 46 and 52 S. The glaciers of this area included landterminating alpine glaciers as well as tidewater- and lakecalving glaciers that drain the north and south Patagonian ice fields (Fig. 1a). On the Chilean side of the southern Andes, the San Rafael Glacier is a large tidewater glacier, flowing from the northwestern sector of the North Patagonian Ice Field (Warren et al., 1995a). The Te´mpanos moraines (Muller, 1959; Heusser, 1960) border the Laguna de San Rafael beyond the glacier margin. A kettle (Lago 1) west of Laguna San Rafael on the outermost moraine was cored by Heusser (1960), who obtained an age of 3610 + 400 14C yr BP (later recalculated at 3740 + 400 yrs BP; Heusser, 1964) for peat at a depth of 2.1 m that was overlying laminated silt layers. Based on this date, Heusser inferred that the earliest (Te´mpanos I) advance culminated ca. 4000 14C yrs ago. Mercer (1982) subsequently suggested that the initial Te´mpanos advance likely dates to ca. 4700–4200 14C yr BP, consistent with then available evidence in the

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southern Andes. Clapperton and Sugden (1989) indicated that the date provides only an upper limiting age for a moraine that could be much older. A second limiting date was obtained from a site inside the limit of nineteenthcentury moraines, where 75 cm of unweathered till overlies sand and peat. Compressed wood in the peat has an age of 6850 + 200 14C yr BP. Heusser (1960) considered that this date was providing a lower limiting age for the initial Te´mpanos advance, the beginning of which he placed at ca. 5000 14C yr BP. Muller (1959) inferred that the overlying till at this locality dated to the nineteenth century, and that the date for the wood represented an upper limiting age for the Te´mpanos advance. He suggested that late glacial recession from the Te´mpanos moraines took place ca. 9000 14C yrs ago. Mercer (1982) did not agree with this chronology, noting that it did not explain why organic sedimentation would only have followed deglaciation 5000 yrs later. Porter (2000) suggested that further studies are needed on this glacier. However, as San Rafael is a calving glacier, a major advance of its terminus may not correlate with regional climatic events (Warren, 1993; Warren et al., 1995b). Porter (2000) summarized also the knowledge for other glaciers in the area. The Ofhidro Sur Glacier is an outlet glacier of the South Patagonian Ice Field (Fig. 1a) with a grounded snout. However, during Late Glacial times, its terminus calved into a fjord. Mercer (1970) stated that its snout was located at 2500 m from the fjord head and was bordered by a series of recent moraines, the innermost of which dated to the eighteenth century. At the time when the outermost moraine was constructed, the terminus was perhaps in contact with tidewaters. Basal peat from the crest of the second moraine was dated at 4060 + 110 14C yr BP; a similar sample from the sixth moraine had an age of 3740 + 110 14 C yr BP. Mercer (1970) estimated that the second moraine was built no later than ca. 4200 14C yr BP, but the outermost moraine was not dated. The Te´mpano Glacier is an outlet glacier of the South Patagonian Ice Field, ending at the Te´mpano fjord along a calving front. Mercer (1970) dated basal peat from a bog lying between the outer moraine and the adjacent hillside at 4120 + 105 14C yr BP, providing a minimum age for moraine construction. Probably, being a tidewater glacier, this advance might correspond to local conditions unrelated to regional climatic variations. At the Los Cipreses Glacier, Ro¨thlisberger (1987) described four lateral moraines lying inside deposits more than 6700 14 C yr BP old, also postdating a paleosol with a date of 5180 + 295 14C yr BP. Other outer moraines have not been dated. On the Argentinian side of the southern Andes, the San Lorenzo Este Glacier is a large land-terminating glacier on the eastern side of Cerro San Lorenzo (Fig. 1b, Site 32), ca. 100 km northeast of the South Patagonian Ice Field. Mercer (1968, 1976, 1982) described two end moraines bordering the glacier and three older ones. The outermost moraine dammed a small lake, where a rooted tree stump was dated at 4590 + 115 14C yr BP, the tree presumably having been drowned by a glacier advance. It is possible that local factors may have influenced the behavior of this glacier.

The Narva´ez Glacier is located about 50 km east of the South Patagonian Ice Field. It shows a proglacial lake and three moraines. Mercer (1968) inferred that the inner moraine dated to the nineteenth century and the outer to the seventeenth century. Basal peat on the outermost moraine had been dated at 4320 + 110 14C yr BP, providing a minimum age for that moraine. Wenzens (1999b) studied moraines of the Rı´o Manga Norte Glacier, on the eastern slope of the Precordillera between Lago Viedma and Lago Argentino (Fig. 1b, Sites 25, 26), inferred to date to the LG and postglacial times. Terminal Neoglacial moraines were dated as older than 4280 + 100 14C yr BP, but this may not be close to the real age. Dates of 8350 + 50 and 8694 + 45 14C yr BP were obtained from deposits downvalley from the late Neoglacial limit in nearby Arroyo Guanaco, and another date of 7370 + 70 14 C yr BP was obtained ca. 12.5 km downvalley from late Neoglacial moraines in the adjacent valley of Rı´o Guanaco. According to Porter (2000), further studies are needed to determine whether these moraines could be of pre-Neoglacial age, either Early Holocene or Late Glacial. The famous Moreno Glacier (Fig. 1b, Site 29) is a large outlet glacier of the South Patagonian Ice Cap, calving into Lago Argentino and Brazo Sur, and which has been permanently advancing during the last century (Fig. 39). Mercer (1968) obtained a date of 3830 + 115 14 C yr BP for basal peat indicating the end of a glacier advance. A similar age (3860 + 115 14C yr BP) suggested that the glacier had retreated by that time. Porter (2000) mentioned that the earliest lake-damming advance of Moreno Glacier occurred ca. 4850–5050 14C yr BP ago, and a date of 4640 + 40 14C yr BP for wood in a moraine probably provides a close age for the maximum of the most extensive Neoglacial advance. Warren (1994) pointed out that the unusual behavior of Moreno Glacier has been more closely controlled by calving dynamics and topography than by regional climate trends.

Fig. 39. Moreno Glacier (Fig. 1b, Site 29). A large outlet glacier of the Southern Patagonian Ice Field, Argentina, which has advanced almost constantly over the last two centuries across Lago Argentino (Fig. 1b, Site 26), blocking drainage and causing the lake level to rise. The pressure of the water finally breaks the ice wall, bursting the snout of the glacier in a remarkable, recurrent natural event. (Photo by J. Rabassa, 2004).

Late Cenozoic Glaciations in Patagonia and Tierra del Fuego The Frı´as Glacier, further south, shows Neoglacial moraines along the southwestern shore of Brazo Sur of Lago Argentino (Fig. 1b, Site 26). Mercer (1968, 1976) described three moraines dating to recent centuries, obtaining a date of 3465 + 130 14C yr BP from wood on top of the outermost moraine. He assigned the moraine to an early Neoglacial age (i.e. ca. 4600–4200 14C yr BP), but the moraine may be actually younger than this event. The O’Higgins Glacier is a lake-calving glacier flowing from the Southern Patagonian Ice Field (Fig. 1a). Ro¨thlisberger (1987) obtained radiocarbon dates for in situ wood fragments between 4675 + 120 and 6020 + 75 14C yr BP. Porter (2000) concluded that, due mostly to a lack of multiple and precise dating, the hypothesis that Southern Hemisphere glaciers advanced more or less synchronously in the Middle Holocene, and in concert with Northern Hemisphere glaciers, has not yet been rigorously proven. Glasser et al. (2004) presented evidence for Holocene glacier fluctuations of the Patagonian ice fields. These authors considered that during the early Holocene (10,000–5000 14C yr BP) atmospheric temperatures east of the Andes were about 2C above modern values in the period 8500–6500 14C yr BP. The period between 6000 and 3600 14C yr BP appears to have been colder and wetter than present, followed by an arid phase from 3600 to 3000 14C yr BP. From 3000 14C yr BP to the present, there is evidence of a cold phase, with relatively high precipitation. West of the Andes, the available evidence points to periods of drier than present conditions between 9400–6300 and 2400–1600 14C yr BP. Holocene glacier advances in Patagonia began around 5000 14C yr BP, coincident with a strong climatic cooling around this time (the Neoglacial interval). Glacier advances can be assigned to one of three time periods following a ‘‘Mercer-type’’ chronology, or instead, four time periods, following an ‘‘Aniya-type’’ chronology (Aniya, 1995). The ‘‘Mercer-type’’ chronology has glacier advances 4700–4200 14C yr BP; 2700–2000 14C yr BP and during the LIA (seventeenth to twentieth centuries). The ‘‘Aniyatype’’ chronology has glacier advances at 3600 14C yr BP, 2300 14C yr BP, 1600–1400 14C yr BP and during the LIA. These chronologies are best regarded as broad regional trends, since there are also dated examples of glacier advances outside these time periods. Possible explanations for the observed patterns of glacier fluctuations in Patagonia include changes related to internal characteristics of the ice fields, changes in the extent of Antarctic sea-ice cover, atmospheric/oceanic coupling-induced climatic variability, systematic changes in synoptic conditions and short-term variations in atmospheric temperature and precipitation. Douglass et al. (2005) used cosmogenic nuclide surface exposure dating to show that at least one glacier on the Chilean side of Lago Buenos Aires (46 S; Fig. 1b, Site 24) advanced ca. 8.5 and 6.2 14C ka BP. These data on the so-called Fachinal moraines suggest that the ice advanced most likely as a result of a northward migration of the southern westerlies, which caused an increase in precipitation and/or a decrease in temperature at this latitude. The older advance is 3000 yrs older than the accepted beginning of Holocene glacial advances in southern South America (Mercer, 1976). According to

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these authors, these events are temporally synchronous with Holocene climate oscillations that occurred in other parts of the world. If there are causal links between these events, then rapid climate changes appear to be either externally forced (e.g. solar variability) or are expanded shortly all over the surface of the Earth by atmospheric processes. After 10 14C ka BP, ice persisted only as cirque glaciers and small valley glaciers in the eastern Fuegian Andes, and as remnants of a mountain ice sheet in the Darwin Cordillera (Fig. 1a; Rabassa et al., 1992; McCulloch et al., 1997). In the Andorra and Can˜ado´n del Toro valleys, near Ushuaia (Fig. 1c), the cirques are dominantly oriented toward the S, SE and SW. The ice occurrence/orientation relationship shows concordant aspects with the hemispheric insolation and the regional climatic conditions, because ice relicts are still present facing toward the SE (45.1%) in the Andorra valley, to the S (18.5%) in the Can˜ado´n del Toro, to the SW (16.1% and 33.3%) in the Andorra valley and Can˜ado´n del Toro (Coronato, 1995a). Recession followed the late glacial maxima and evidence for several Neoglacial readvances are observed in the cirques. Three moraine arcs have been mapped at the Vinciguerra Glacier, near Ushuaia, the largest cirque glacier still existing in the Argentine Fuegian Andes (Fig. 1c). The oldest moraines reach 600–650 m a.s.l. and have been largely colonized by the Fuegian forest. The youngest one lies well above the timberline. This moraine was apparently formed during the LIA; older readvances are represented by complex moraine systems, but all of them remain undated. The occurrence of ice bodies within the glaciated valleys is restricted to a minimum elevation of 700–800 m a.s.l. The topography of the glacier valleys clearly shows the Holocene events. These were defined as (a) the Vinciguerra I phase (8.5–5.0 14C ka BP), when the glacier receded continuously without evidence of stabilization; (b) the Vinciguerra II phase (5.0 14C ka BP) represented the stabilization of the glacier, generating lateral moraines at 500–540 m a.s.l., and the erosional landforms on the first threshold at 480–600 m a.s.l., and (c) the last phase, or Vinciguerra III (LIA), corresponded to a second stabilization event with two well-developed pulsations, depicted by moraines formed within the present forefield (Rabassa et al., 1992).

4.5. Glaciation of Islas Malvinas/Falkland Islands Quaternary studies on the Islas Malvinas/Falkland Islands (Fig. 1a) started as early as the mid-nineteenth century, when Darwin (1846) described the presence of distinctive periglacial features like block streams or ‘‘stone rivers’’. The existence of Pleistocene glaciers in the archipelago was demonstrated by Clapperton (1971), Clapperton and Sugden (1976), Roberts (1984) and Clapperton and Roberts (1986), among others, and summarized by Clapperton (1990, 1993). During the Quaternary, only conditions for marginal glaciation had developed, whereas at the same latitude, very large ice fields existed in Patagonia. Roberts (1984) identified 76 nivoglacial

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features, made up of 8 nivation crests, 41 nivation hollows, 7 nivation cirques and 20 glacial cirques. Erosional and depositional characteristics identified on the islands suggest that there were at least two intervals of cirque glaciation. Three glacial cirques are larger than the rest, but they also show the development of cirque-in-cirque glaciation. A few glaciers expanded beyond the cirque basins and deposited till and terminal moraines in the valleys. Roberts (1984) suggested that the Pleistocene glacial history of the archipelago is restricted to only three events: an early cirque phase, a valley phase and late cirque phases. The latter probably represents the LGM. The entire archipelago is affected by periglacial mass wasting, suggesting that Quaternary cold periods are largely responsible for landscape development. According to Clapperton (1990), a radiocarbon date of 26,060 þ 400/–380 yrs BP, for a podsol buried by 1.5 m of solifluction debris, suggests that the last interval of solifluction probably coincided with the LGM.

4.6. Modeling the Late Pleistocene Ice Sheet and Glacier Behavior Sugden et al. (2002), Hulton et al. (2002) and Hubbard et al. (2005) have tried to model the expansion and recession of the Patagonian Ice Sheet during the LGM. Hulton et al. (2002) used a coupled ice sheet/climate numerical model with empirical evidence, simulating the ice sheet at the LGM and at different stages of deglaciation. Under LGM conditions, an ice sheet with a modeled volume slightly in excess of 500,000 km3 built up along the southern Andes. There is a marked contrast between the maritime and continental flanks of the modeled ice sheet. The model is most sensitive to variations in temperature and there is good agreement between modeled ice extent and empirical evidence. This was achieved by applying an estimate of a 6.1C temperature decrease with constant wind. Assuming a stepped start to deglaciation, modeled ice volumes declined sharply, contributing 1.2 m to global sea level, of which 80% occurred within only 2000 yrs. The empirical record suggested that such a stepped warming occurred around 17,500–17,150 cal. yrs ago. Hubbard et al. (2005) presented a time-dependent model to investigate the interaction between climate, extent and fluctuations of Patagonian ice sheets between 45 and 48 S during the LGM and the deglaciation that followed. The model was applied at 2 km resolution and enabled ice thickness, lithospheric response and ice deformation and sliding to interact freely. Relative changes in sea level and ELA were considered as well. Experiments implemented to identify an LGM configuration compatible with the available empirical record indicated that a stepped ELA lowering of 750–950 m was required over 15,000 yrs to fit the Fe´nix I–V moraines at Lago Buenos Aires (Fig. 1b, Site 24). However, 900 m of ELA lowering yielded an ice sheet that best matches the Fe´nix V moraine (ca. 23,000 14C yr BP) and Caldenius’ reconstructed LGM limit for the entire modeled area. According to these authors, this optimum LGM experiment yielded a highly dynamic, low aspect ice sheet, with a mean ice thickness of ca. 1130 m drained by numerous

large ice streams to the western, seaward margin and two large, fast-flowing outlet lobes to the east. Forcing this scenario into deglaciation using a rescaled Vostok ice core record resulted in a slowly shrinking ice sheet that was only 25% of the LGM volume by ca. 14,500 14C yr BP, after which it collapsed rapidly, with a loss of 85% of its volume in only 800 yrs. It is interesting to note that its margins stabilized during the ACR, after which it receded to near present-day limits by 11,000 14C yr BP. Wenzens (2004) has strongly criticized Sugden et al. (2002) models, and implicitly, his criticism applied also to Hubbard et al. (2005) later work. Wenzens (2004) estimated that the boundaries depicted in the models do not fit with any actual ice margin of comparable age, that the considerations did not apply to both Patagonian ice sheets and that the Andean topography had not been properly considered in the models. Another point of view is presented by Benn and Clapperton (2000b), who described proglacial and subglacial glaciotectonic sediments and landforms around the margins of the Strait of Magellan. These deposits recorded the advance and retreat of outlet glaciers of the Patagonian ice cap during the Last Glacial cycle. The glaciotectonic landforms in the area would have been the result of advancing ice lobes with cold-based margins due to permafrost regional conditions, but with wet-based inner portions. As the ice advanced, subglacial basins would have been dug underneath the glacier margins and the eroded material was pushed up into thrust moraines, probably because frozen-bed conditions formed a thermal dam against the free drainage of subglacial meltwaters. Later, these ice-marginal glaciotectonic landforms would have been overridden and streamlined into drumlins and flutes when thicker, wetbased ice advanced over these areas. Evidence for permafrost near sea level in Patagonia during the Last Glaciation suggests that mean annual temperatures were several degrees lower than indicated by recent modeling studies. The results indicate that future modeling experiments should incorporate more realistic basal boundary conditions, particularly the presence of a weak deforming layer at the glacier bed, to improve climatic reconstructions of southern South America. Concerning interhemispheric links, in the opinion of Blunier et al. (1998), a main aspect of climate dynamics is to understand if the Northern and Southern hemispheres are effectively coupled during climate events. The fast and strong temperature changes observed in Greenland (the Dansgaard–Oeschger events) during the last glaciation have a certain analogue in the temperature record from Antarctica. A comparison of the global atmospheric concentration of methane as recorded in ice cores from Antarctica and Greenland allowed establishing a phase relationship of these temperature variations. Greenland warming events between ca. 36 and 45 ka ago have a delay in relation to their Antarctic counterpart by more than 1 kyr. On average, Antarctic climate change precedes that of Greenland by 1–2.5 kyr over the period 47–23 ka. However, it should be considered that the observed delay is usually within the operational error of the dating techniques, and improved data are needed to confirm these opinions.

Late Cenozoic Glaciations in Patagonia and Tierra del Fuego

eustatic movements took place, with sea level lowering of at least several tens of meters during cold events, and up to 100–140 m during full glacial episodes. Climatic continentality of the surrounding areas increased, with rising extreme temperatures, precipitation diminution and lack of the sea moderating effect as the coastline moved eastward. This process occurred both in Pampa and Patagonia, with almost a doubling in size of the continental areas and subsequent strong continentalization. Note that for the Latest Pleistocene, this is an important fact regarding the environmental conditions and available pathways and space for human colonization in the Pampean and Patagonian Regions (Fig. 40). Sea-surface temperatures were lowered up to 4C in the tropical areas during MIS 2, with at least a lowering of 5–6C in southern South Africa (30–32 S; Tyson and Partridge, 2000), with increased lowering toward the poles. This lowering in mean sea-surface temperature (MSST) had certainly influence on the evaporation and mobility of marine currents, with a consequent diminution

5. Discussion 5.1. Environmental Changes in Southern South America Following the Glacial Events The Patagonian glacial sequence provides a reasonable framework to understand the environmental evolution of southernmost South America from the latest Miocene to the Pleistocene–Holocene boundary. Particularly, the relative lack of other long terrestrial records gives a paramount importance to the glacial evidence for preLate Pleistocene times. Clapperton (1993), Heusser (2003) and Rabassa et al. (2005) have discussed the climatic and environmental changes in southern South America that followed the establishment and development of the Late Cenozoic Patagonian glaciations, which may be summarized as follows. First, global sea level changes forced by glaciation partially exposed the Argentinian submarine platform, which enhanced the climatic continentality. Significant

Southamerican environments at Last Glaciation Maximum Sand desert

Savana? Rain Forest ion

Eros

Amazon Cone

Colder & Drier

Cooler & Greater Seasonality of Precipitation

ICE CAPS & VALLEY GLACIERS

g

loodin

Rain Forest

No F

Arid & Flash Floods & Melt

Savana, Coatingas & Campos Cerrados

ICE CAPS & VALLEY GLACIERS Colder & Wetter

Much less Vegetation Cover

Some forest

(Itatiaia)

Desert

Araucaria forest Dry & Windy Sand Dunes & Loess & ‘Bajos’

VALLEY GLACIERS Colder and northern westerlies –120 m

Colder and drier Pampero Patagonian Ice Cap

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Malvinas palaeocurrent?

Limit of Seasonally/Perennially Frozen Ground

A Cold desert TUNDRA POLYGONS ICE WEDGES

Sand dunes Loess

Partially modified form Clapperton, 1993 Sea Ice?

Coastline at –120 m

Bajos

Fig. 40. Map of South America during the LGM. Partly modified from Clapperton, 1993.

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of mean annual temperature in all continental areas, which in northern Patagonia would have been of at least 5–6C and perhaps much more in the southern regions (Heusser, 1989b; Clapperton, 1993). These conditions increased the influence of the Malvinas/Falkland Current (Fig. 40), which today reaches up to southern Brazil. Most likely, this current reached a much more northerly position during the glacial winters, and stayed there for longer portions of the year. As a consequence of the coastline mobility, the position of the littoral marine currents, both the Brazil and Malvinas/ Falkland currents were affected. During the glacial epochs their meeting front was displaced northward, modifying the Pampean winter storm pattern, and probably, diminishing the oceanic influence and increasing water deficit during these periods. Moreover, sea level lowering provoked a strong lowering of marine depth between the Patagonian coast and the Malvinas/Falkland Archipelago, forcing an eastward displacement of the Malvinas/Falkland Current, with a further increase in climatic continentality along the present littoral zones. The climatic conditions during glacial episodes had an influence on the displacement of the oceanic anticyclonic centers, both in the Pacific and in the Atlantic oceans. The South Pacific anticyclonic center was displaced northward during the glacial periods, increasing the effect of the ‘‘Pampero’’, cold-dry winds that dominate the weather and eolian sedimentation in the Pampas of eastern Argentina, Uruguay and southern Brazil. The northward movement of this anticyclonic area determined that those regions previously free of the cold and dry ‘‘westerlies’’ were progressively affected by these winds. The increasing eolian action leads to the development of intensive deflation processes, with the genesis of hydroeolian depressions, salt lakes and endorheic basins, and also the dune field formation in northern Patagonia and western Buenos Aires Province (Clapperton, 1993; Iriondo, 1999; Fig. 41). This eolian activity was also responsible for loess accumulation in the Pampean Region, Uruguay and southern Brazil, beyond the dune belts, where the Pampean vegetation, though thinner than in interglacial times, was capable of retaining the fine sand-coarse silt fractions. A similar role had the Rı´o Salado of Buenos Aires Province (35 S, Buenos Aires Pampean Region, Fig. 1a), which acted as a sand trap, originating the La Chumbiada (Dillon and Rabassa, 1985) and Guerrero (Fidalgo et al., 1975) members of the Luja´n Formation, during the Late Pleistocene (MIS 4 to 2). Similar conditions would have taken place in most, if not all, glacial events of the rest of the Pleistocene and before, since the Rı´o Salado has long been occupying a very ancient tectonic basin (Rabassa et al., 2005). Moreover, it is also probable that a northward displacement of the anticyclonic centers generated changes or at least a higher variability in the eolian sediment supply contributing to the Pampean loess formation, incorporating epiclastic products coming from western Argentina and the central Andes (Iriondo, 1999; Muhs and Za´rate, 2001). As a consequence, deflation was strongly dominant during all glacial events, with formation of sand dunes and loess mantles in the Pampas, excavation of endorheic

Fig. 41. Sand sea and loess accumulation in the Pampas and surrounding lowlands during the LGM. From Iriondo, 1999. The dotted area represents the sand sea, whereas the stippled area corresponds to the loess accumulation areas, marginal to the sand sea. hollows and depressions, and genesis of salt lakes in areas that are wetter today. Climatic changes forced changes in the plant cover, with large latitudinal displacements of the major ecosystems during glaciations. Tundra, which is restricted today to mountain summits above the treeline, developed all over southern Patagonia, and perhaps up to 42–44 S. Tundra conditions included permanent or transient frozen ground, at least around the ice margins, though its eastward expansion could have been larger (C. Heusser, in Bujalesky et al., 1997). Tundra paleoenvironment, inferred from palynological records of fossil peat at Lago Fagnano, near Tolhuin (Fig. 1c; Bujalesky et al., 1997), was characterized by the absolute lack of arboreal (Nothofagus spp.) pollen, while it was dominant during a glacial phase of the penultimate glaciation (MIS 6) and, most likely, was also present during the LG. Still unpublished palynological studies of fossil peat layers interbedded with tills in the area have confirmed these environmental conditions (J.F. Ponce, personal communication). See also Coronato et al. (2004c) and Trombotto (Chapter 12), for ice-wedge cast development in northern Tierra del Fuego during the Last Glaciation. In Late Glacial times as the glaciers receded, this tundra environment was probably rapidly replaced by a park vegetation, with isolated Nothofagus spp. forest patches in a grassy steppe environment. These conditions are particularly evident in the Harberton peat bog (Fig. 1c) pollen profile (Heusser, 1989a), where the recession of the ‘‘Beagle Glacier’’ from its outermost LGM positions allowed the partial recovery of the Fuegian forest as early as 14.8 14C ka BP. At that moment, and for several hundred years, the forest started its slow but steady

Late Cenozoic Glaciations in Patagonia and Tierra del Fuego recovery, advancing from (still) theoretical refuges located at the present submarine platform or perhaps at Isla de los Estados (Staaten Island; Fig. 1a), as suggested by the pollen record (Coronato et al., 1999). However, in at least two opportunities, around 13 and 11 14C ka BP, respectively, the arboreal pollen content practically disappears from the record, being entirely replaced by Gramineae and Empetrum, indicating the return to cold regional conditions, which forced perhaps a new east– northeastward recession of the Fuegian forest, toward its Pleistocene refugia. But toward 10.2 14C ka BP, the content of the pollen records indicates that the forest restarted its expansion on Isla Grande de Tierra del Fuego, reaching present-day conditions in the first millenium of the Holocene, though the present conformation of the forest was achieved only toward ca. 8 14C ka BP. These cold Late Glacial episodes (herein named as ‘‘late glacial I’’ and ‘‘late glacial II’’) may be comparable both in chronological and in intensity terms with their Northern Hemisphere equivalents, the ‘‘Oldest?/Older? Dryas’’ and ‘‘Younger Dryas’’. Alternatively, a strong influence of the ‘‘Antarctic Cold Reversal’’ event has been proposed (Sugden et al., 2005). Nevertheless, the pollen record undoubtedly indicates that the ‘‘late glacial II’’ event was more intense and extreme than the previous one, but its environmental consequences on the forest are still unknown. The Patagonian forest became isolated from other South American forest formations perhaps already in the Middle Miocene. On the Chilean side, as the ice reached the Pacific Ocean waters south of 44 S, the forest was probably completely suppressed, perhaps with isolated refuges on small, remote islands or uncovered coastal peaks. On the eastern slopes, the forest was concealed in between the glacier front and the 0C annual isotherm toward the west, and the shrubby steppe environments and the 300 mm annual isohyeth eastward, which would have bounded its eastern expansion. These ecosystems were severely damaged and the forest was disrupted in fragmented populations, in remote and restricted refuges, from 36 southward. In Tierra del Fuego, the forest was probably displaced toward the present submarine platform, northwest and north of Penı´nsula Mitre and Isla de los Estados (Fig. 1a, c; Coronato et al., 1999). The Pampean grassy prairies were spatially reduced and pushed north and northeast during glacial events. Thus, the Patagonian steppe expanded northward into the Pampean domain and perhaps, even into Uruguay and northeastern Argentina, thus becoming an important factor in loess accumulation. The Patagonian equivalents of the Pampean prairies, which had developed since the Early-Middle Miocene, disappeared as well, being replaced by the north- and eastward expanding Monte and steppe ecosystems (Rabassa et al., 2005). These ecosystem changes were closely followed by a significant terrestrial faunal replacement, with northward expansion of Patagonian faunas during cold events, reaching up to southern Brazil. Likewise, the Brazilian faunas invaded the Pampas and even northernmost Patagonia during interglacial periods. This has been proved for the Late Pleistocene in the Pampean vertebrate fossil records (Chapter 13) and it was probably in effect during each major climatic cycle.

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Under drier-colder climate, Patagonian faunas predominated in the Pampas during glacial epochs, and warmer-wetter Brazilian faunas during the interglacials. This faunal replacement, clearly observed during the Pleistocene–Holocene transition and, more recently, in the Late Holocene, would have probably taken place with similar characteristics during each glacial ‘‘termination’’, that is, for at least 15 times since the GPG, and perhaps even 50 or more times since the early Pliocene and even more than 100 times since the Late Miocene. The consequences that the high frequency of these displacements would have had on the Pampean faunas, both from a taxonomic and a biogeographical point of view, remain still in the hypothetical domain, but they should not be let aside in paleobiological, paleogeographical and paleoenvironmental reconstructions in southern South America (Rabassa et al., 2005). It is clear then that the climatic cycles identified in the global oceanic isotopic sequences have been confirmed by the terrestrial Patagonian glacial record. These changes have been very important and they should have had a significant influence in the development of the Pampean and other South American ecosystems, perhaps up to southern Brazil. These paleoenvironmental modifications would have had severe consequences in the entire studied region, although it is understandable that their characteristics and intensity would have not been identical over the huge Patagonian and Pampean geography. But, undoubtedly, they should have played an important role in the process of early peopling of Pampa and Patagonia. It is highly possible that the human expansion in southern South America would have started immediately after the LGM (ca. 25 cal. ka) and most certainly, after the last phase of morainic construction, ca. 16 calendar ka (Lago Buenos Aires, Fig. 1b, Site 24; Kaplan et al., 2004). The southheading human groups, probably looking for regions with a higher density of surviving Pleistocene megamammals, underwent not only the progressive environmental changes typical of the Last Termination, but they should have suffered as well the two Late Glacial cold episodes, which affected them and the regional biota in a similar manner (Rabassa et al., 2005). The Pleistocene–Holocene transition, the timing of the human occupation of Patagonia, was an epoch of high environmental instability. There was a varied environmental mosaic which, together with locations closer to the sea and under its influence, would have offered appropriate, though perhaps different, routes for human peopling. In those times, environments and thus, faunal resources would have been equivalent in both Pampa and Patagonia. These faunas are characteristic of grassland environments or, at least, grassy steppes of cold, dry to semiarid climates (Cione and Tonni, 1999). The changes leading to definitive Holocene environments took place only after 9 14C ka AP, toward a shrubby steppe, with the final disappearance of the Pleistocene faunas and increasing abundance of Lama guanicoe (Miotti and Salemme, 1999; Rabassa et al., 2005). When the Holocene environments were finally established, the glaciers were reduced to their present conditions, thus allowing for full occupation of most of the Patagonian lands, including the Andean piedmont and the

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Jorge Rabassa 5.2. The Buildup of the Patagonian Ice Sheet

Magellan Straits. Faunas changed from those of drier environments to others corresponding to higher relative moisture. The Brazilian faunas occupied the Pampas during the Late Holocene, and the Patagonian faunas have been similar to the extant ones throughout the Holocene (Cione and Tonni, 1999).

Magnetic Polarity

3.70 3.75 3.80 3.85 3.90 3.95 4 4.1 4.2 4.3 4.4 4.5

IOS Age

South American Stages Biozones

PLIOCENE

Ma

The variations in length and frequency of the cold-warm climatic cycles have determined that the intensity of the extreme isotopic content peaks of the global oceanic record became larger toward the Early Pleistocene. Thus,

EARLY

Locali- Source ties Sources 1: Mercer, 1983 2: Rabassa, 1999 3: Ton-That et al., 1999 4: Schlieder, 1989

Neocavia CHAPADMALALAN depressidens

LBA: Lago Buenos Aires PA: Pampa de Alicurá

C

N

LBA

1

LBA

1

4.6

Basalts layers Inferred glaciation

4.7 4.8

S

M O N T Trigodon gaudryi E H E R M O S A N

4.9

T

TG20? TG22?

C3a n1

6.2

LBA

2 Overlying basalt Underlying basalt

PA

4

PA

4

LBA

1

LBA

1

LBA

2

LBA

3

MIOCENE

5.2 5.3 5.4 5.5 5.6 5.7 5.8 5.9 6 6.1

C3

Gilbert

5 5.1

Glaciofluvial deposits

SI4?

6.3 6.4

? 6.5 6.6 6.7 6.8 6.9 7 7.05 7.1 7.15 7.2 7.25 8 9 10

C3b HUAYQUERIAN

C4 C4a

CHASICOAN

Fig. 42. Patagonian glaciations during the latest Miocene and Early Pliocene (from Rabassa et al., 2005). The dark bands correspond to radiometric dated lava flows; the dotted bands correspond to individual tills and the vertical line bands represent inferred glacial events. Black triangles depict whether the lava flow is used as an upper or lower limiting age for a certain glacial episode. The columns at the left of the figure represent the chronological global scale in million years, the global paleomagnetic scale and the global marine isotope stage sequences. The Pampean biostratigraphic units, their established biozones and stages and their time boundaries have been taken from Verzi et al. (2002).

Late Cenozoic Glaciations in Patagonia and Tierra del Fuego climate became more extreme and pleniglacial conditions were gradually achieved at lower latitudes since the Antarctic Peninsula was glaciated during the Middle Miocene and piedmont glaciers occurred in Patagonia at the Latest Miocene. This is due to the fact that, with shorter and milder cycles, the glacial conditions were not functional during such periods long enough so as to allow the building and persistence of extensive ice fields in the Patagonian Cordillera. Only in the Late Pliocene would appropriate conditions have been reached so as to develop a continuous mountain ice sheet, since latitudes from 36 S to Cape Horn (56 S; Fig. 1c), which would have recurrently grown in each subsequent glacial cycle during the whole Pleistocene up to the Last Glaciation. The high climatic variability recorded since the Late Miocene in Pampa and Patagonia was a consequence of changes in the astronomical, orbital parameters. These parameters would have been predominant in different times: (a) equinoctial precession from the Late Miocene to the Middle Pliocene, developing cycles of ca. 23–19 kyr during this period; (b) obliquity from the Late Pliocene to the Early Pleistocene, with cycles of ca. 41 kyr and (c) eccentricity from the Middle to Late Pleistocene, with cycles of 100 kyr (see, for example, Ruddiman et al., 1986; Berger and Loutre, 1991; deMenocal and Bloemendal, 1995; Opdyke, 1995). The shorter cycles would have impeded the formation of the Patagonian mountain ice sheet from the Late Miocene to the Middle Pliocene, favoring instead the development of local glaciers, of which the sedimentary record is still scarce (Rabassa et al., 2005). Ma

Magnetic Polarity

IOS

Age

5.3. The Correlation of the Patagonian Glaciations and the Pampean Land Mammal Stages A correlation of the Patagonian glaciations and paleoenvironmental conditions and the Pampean stratigraphy since the Late Miocene was proposed by Rabassa et al. (2005). The Pampean biostratigraphic units have been known since Ameghino (1889) and thoroughly described by Tonni and Cione (1995), Alberdi et al. (1995), Pascual et al. (1996), Tonni et al. (1999a, 1999b) and Verzi et al. (2002), among many others. The geomagnetic sequence of the continental Pampean sequences, as presented by Orgeira (1990) and Nabel et al. (2000), among others, has been the basic tool for the correlation with the Patagonian glacial sequences. The correlation results have been shown in Figs 42–44. The oldest Patagonian glacigenic deposit was formed during the Late Miocene, in the Montehermosan South American land mammal (SALMA) stage (Tonni and Cione, 1995), although it is not yet clear if this corresponds to a glacial event during the colder events MIS TG 20–22, in the Latest Miocene, or even somewhat later during MIS Si 4–Si 6 (Earliest Pliocene). In these periods the global temperatures would have been lower than during the Early Chapadmalalan (Early Pliocene) land mammal stage. In the Late Chapadmalalan, local glaciation would have taken place, at least in the Lago Viedma area (Fig. 1b, Site 25; Figs 42, 43). Colder than present conditions appeared only since ca. 2.6 Ma, in the Sanandresian land mammal stage. Before 3 Ma, the climatic conditions were always

South American Biozones Stages

Locali- Source ties

2 2.15 2.20 2.25 2.30 2.35 2.40 2.45

88

Gauss

2.80 2.85 2.90 2.95

Ctenomys chapadmalensis

P L I O C E N E

2.50 2.55 2.60 2.65 2.70 2.75

1

LV,CF LBA

1,2 3

A. (Akodon) lorenzinii

SANANDRESIAN

M A R P L A VOROHUEAN T A N

LV

2

LV,LA

2,4

2

BARRANCALOBIAN

C2a

CF: Cerro del Fraile LA: Lago Argentino LV: Lago Viedma LBA: Lago Buenos Aires

Inferred glaciation Overlying basalt

LA

5

LV LV

6 7

LV

6

KM4,KM6

M2 M MG2

1: Rabassa, 1999 2: Wenzens, 2000 3: Sylwan, 1989 4: Schellmann, 1998 5: Mercer, 1969 6: Mercer, 1976 7: Mercer et al., 1975

Basalts layers

LV Platygonus scagliai

K

Sources

Till

3

3.05 3.1 3.15 3.20 3.25 3.30 3.35 3.40 3.45 3.50 3.55 3.60

CF

C2

92,96 100

193

Underlying basalt

Paraglyptodon chapadmalense

LATE CHAPADMALALAN

MG6

Fig. 43. Patagonian glaciations during the Middle and Late Pliocene (from Rabassa et al., 2005). See Fig. 42 for explanation.

Jorge Rabassa South American Biozones Stages

Localities

Source

LUJANIAN

E.(Amerhippus) neogeus 6 8?

LBA VM,NH,LBA

1 2, 3

NH,LBA

4, 2

NH

2

LBA

5, 4

LA, SO,LBA,T CF CF RG LBA

6,7,8,4,9 9 9 6,4 5

LA, RG LA, RG RG

6,4 7 8

LA, CF LA CF CF LA LA, CF CF

6,9 10 9 9 5 6,7, 11,9 9

Megatherium americanum 12

BONAERIAN

16

C1 18,22

J

30 34 36

PLEISTOCENE

0.9 0.95 1 1.05 1.1 1.15 1.20 1.25 1.3 1.35 1.4 1.45 1.5 1.55 1.6 1.65 1.7 1.75 1.8 1.85 1.9 1.95

IOS Age

Brunhes

0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65 0.7 0.75 0.8 0.85

Magnetic Polarity

Matuyama

Ma

E N S E N A D A A N

Tolypeutes pampaeus

588

Olduvai

194

62 64 70 72

2 2.05 2.1

Sources 1: Guillou and Singer, 1997 2 : Rabassa and Evenson, 1996 3: Mercer, 1982 4: Ton-That et al., 1999 5: Sylwan, 1989 6: Mercer, 1976 7: Mercer, 1983 8: Meglioli, 1992 9: Rabassa, 1999 10: Mercer, 1969 11: Schellmann, 1998

78 82

LBA: Lago Buenos Aires VM: Malleo valley NH: Lago Nahuel Huapi LA: Lago Argentino SO: Otway Sound T: Tronador Mount CF: Cerro del Fraile RG: Río Gallegos valley

LA, CF

6,9

LA

6, 9

Basalts layers Till Overlying basalt Underlying basalt

Fig. 44. Patagonian glaciations during the latest Pliocene and the Pleistocene (from Rabassa et al., 2005). See Fig. 42 for explanation.

warmer than the last interglacials (Holocene, MIS 5 and MIS 7), according to the global isotopic record, with the exception of short events at 3.12, 3.3, 3.35, 4.8–4.9 and 5.7–5.8 Ma, during the Chapadmalalan and Montehermosan land mammal stages. Loess-like beds have been mentioned in the older Pampean units, at least since the Montehermosan land mammal stage and perhaps even before (see, for example, Zavala and Quattrocchio, 2001). The Pampean loess/ soil sequences are much more poorly developed than those that have been described in China (Rutter et al., 1991). This fact is probably due to either (a) the feeble

pedogenetic effect during the integlacials or (b) a powerful erosional action over the interglacial soils during the cold cycles (Rabassa et al., 2005). The Early Ensenadan land mammal stage is correlated by the recurrent glaciations at Cerro del Fraile (Fig. 43). The Late Ensenadan is characterized by the largest extension of the Patagonian glaciers, at the GPG, and the subsequent, still important ‘‘Daniglacial’’ events. The Ensenadan stage is the epoch of the astronomical shift from the 41 kyr to the 100 kyr cycle in the global record, and the establishment of full glacial conditions in temperate areas.

Late Cenozoic Glaciations in Patagonia and Tierra del Fuego During the Bonaerian stage (Fig. 44), several smaller glaciations and the ‘‘Gotiglacial 1’’ events (Early Illinoian; MIS 8–16) took place. Finally, the Lujanan stage hosted the ‘‘Gotiglacial 2’’ event (Late Illinoian, MIS 6) and the Last Glaciation (Wisconsinan, MIS 4 to 2). The analysis of the global isotopic record, the Pampean stratigraphy and the Patagonian terrestrial glacial evidence contradicts the Mid-Pliocene Antarctic full deglaciation hypothesis (following Bruno et al., 1997), because the climate would have stayed within the mean levels of the glacial/interglacial cycles in that epoch, and there is no regional evidence of such a very high sea level that could provide scientific support to this theory. It must be concluded that during the lapse comprised from the Montehermosan to the Lujanian stages, there would have existed at least 50 complete cold-warm climatic cycles, forcing the regional development of the Patagonian–Pampean ecosystems. These large-scale climatic variations should be taken into consideration when discussing paleoecological, paleobiogeographical and evolutionary characteristics of the Late Cenozoic Pampean faunas.

6. Final Remarks When the Antarctic Circumpolar Current became finally established and the Andean ranges reached elevations closer, or even higher, than their present positions, regional climatic conditions allowed the frequent development of glaciation, from the end of the Miocene. However, glacierization could have started even before, perhaps 11–12 Ma (Wenzens, 2006a) during the final phase of the Santacruzan–Friasan land mammal stage. However, these very early and supposedly extensive glaciations need further confirmation. The high climatic variability that began in the Late Miocene is due to Milankovitch cycles. Equinoctial precession would have been dominant during the Late Miocene and the Early Pliocene, with cycles of ca. 23 kyr. Axis obliquity would have been largely influential during the Late Pliocene and the earliest part of the Early Pleistocene, with cycles of 41 kyr. Orbital eccentricity would have prevailed during the final portion of the Early Pleistocene and later until today, with cycles of ca. 100 kyr. The shorter climatic cycles of earlier times would have impeded the formation of the Patagonian Ice Sheet as one single unit during the Late Miocene and the Pliocene. Thus, it is assumed that only local ice caps and mountain glaciers would have developed until the Early Pleistocene when the Patagonian Ice Sheet was finally established (Rabassa et al., 2005). Though Pampean loess layers from the Late Miocene are known, their occurrence is more frequent since the Late Pliocene, during the Marplatan land mammal stage. The Pampean loess/soil sequences are not as well preserved as their Chinese counterparts. This is probably due to either (a) a poor ‘‘Brazilian’’ (i.e. warmer climate) effect on the Pampa environments during the interglacial periods, when pedogenesis should have taken place, or (b) intense erosion (deflation) during the colder, arid glacial periods. The thicker loess units would have been formed only during the 100 kyr cycles,

195

that is, the last 1.2 Ma, when the Patagonian Ice Sheet was fully developed. The ecological, faunal and floral, mobility of the Patagonian and Pampean regions, as well as of other midlatitude areas in South America, would have been greater also in these periods. The glacioeustatic movements would have been smaller during the Pliocene, with a smaller exposure of the present submarine platform, a more reduced climatic continentalization and less extreme climatic events. Colder-than-today environmental conditions would have been frequent only after ca. 2.6 Ma ago. Before 3 Ma, climatic conditions would have been warmer than even the last interglacial periods, that is, the Holocene, MIS 5 and MIS 7, with possible exceptions at 3.12, 3.3, 3.35, 4.8–4.9 and 5.7–5.8 Ma, according to the Southern Ocean 18O record. The environmental and biogeographical changes that took place during the LG and the last Termination would have taken place at least 15 times during the last 1 Myr, since the GPG. Perhaps with smaller intensity, they could have occurred up to 100 times since the beginning of the Pliocene. The ecological consequences of such climatic changes are hard to quantify, but they must have been highly significant. They should not be ruled out when studying the Late Cenozoic biogeography, paleontology and paleoenvironments of the Pampas and similar areas of Uruguay and southern Brazil. Much more is known today compared with what had been proved only three decades ago. See, for instance, Mercer (1976), or the discussion during the INQUA ‘‘Till Commision’’ Patagonian Regional Meeting in March– April 1982 (Rabassa, 1983; Fig. 45), or even the summary in Rabassa and Clapperton (1990).

Fig. 45. Group photo of the participants of the ‘‘INQUA Till Commission South American Regional Meeting’’, San Carlos de Bariloche, Argentina, during postmeeting fieldtrip in front of Castan˜o Overo Glacier, Monte Tronador (Fig. 1b, Site 12; photo taken by a fieldtrip assistant in April 1982). The author is accompanied by several then graduate students (Andre´s Meglioli, Andrea Coronato and Elizabeth Mazzoni among them) and many distinguished visitors, Cal Heusser, Linda Heusser, Ernest H. Muller, Art Bloom, Gerry Richmond, Trevor Chinn, Dirk van Husen, Robert Vivian, Jan Lundqvist, Edward Evenson and Friedrich Ro¨thlisberger, among others.

196

Jorge Rabassa

Much has to be done yet, but the construction of the Patagonian glacial chronology, the best land-based glacial record of the temperate zones of the Southern Hemisphere and one of the most complete in the entire world has slowly but steadily developed. This has been possible, thanks to the efforts of many Argentinian and foreign scientists who have challenged the huge Patagonian distances, loneliness and emptyness, lack of logistics, roads or services, just as the great Carl C:zon Caldenius did 75 yrs ago. Much future work is needed to extend and consolidate this chronology, which will provide a firm base for correlation with glacial and paleoclimate records elsewhere in the world.

Acknowledgments The author wants to dedicate this chapter to the memory of Dr Carl C:zon Caldenius, on the 75th anniversary of the publication of his paramount work. The author is deeply grateful to Dr John Mercer, who kindly introduced him to the problem of ancient Patagonian tills in 1972, and Professor Francisco Fidalgo (Universidad de La Plata), who generously cosupervised his doctoral dissertation in 1974 and provided him with the basic concepts and methodology for the study and correlation of Patagonian glaciations. Professor Fe´lix Gonza´lez Bonorino (Fundacio´n Bariloche), who was the main advisor of his dissertation, oriented him in the study of past and present glacigenic sediments. The author also wants to thank all those colleagues and graduate students who over the last 30 years have generously educated him or kindly worked with him on the study of Patagonian glaciations: Calvin J. Heusser (deceased), Linda Heusser, Sigfrido Rubulis (deceased), Jorge Suarez (deceased), Edward B. Evenson, Stephen C. Porter, Andre´s Meglioli, Luis Bertani, Aldo Brandani, Daniel Cobos, Jose´ Boninsegna, Fidel Roig Junyent, Ricardo Villalba, Guida Aliotta, Gunnar Schlieder, George Stephens, Jim Clinch, David Serrat, Carles Martı´, Jaap van der Meer, Kenneth Kodama, Donald Easterbrook, Chalmers Clapperton, David Sugden, Nick Hulton, Bradley Singer, Thao Ton-That, Dave Mickelson, Mike Kaplan, James Bockheim, Matti Seppa¨la¨, Andrea Coronato, Claudio Roig, Juan Federico Ponce, Oscar Martı´nez, Mo´nica Salemme, Elizabeth Mazzoni and Bettina Ercolano, among many others. Dr Bradley Singer (Department of Geology and Geophysics, University of Wisconsin at Madison, USA), Dr Robert Ackert (Harvard University, USA) and Dr Michel Kaplan (Lamont-Doherty Geological Observatory, USA), generously provided published information and unpublished radiometric and cosmogenic data to support the interpretations of this work. The author is greatly indebted to Professor Jaap van der Meer for his careful and dedicated review of a first draft of the manuscript, thus certainly improving this chapter. This chapter is the result of more than 30 years of fieldwork in different Patagonian regions and lab work at CADIC-CONICET (Ushuaia), Fundacio´n Bariloche (San Carlos de Bariloche), Universidad Nacional del Comahue (Neuque´n) and other organizations. These investigations were funded by many grants from CONICET, Agencia

Nacional de Promocio´n Cientı´fica y Tecnolo´gica (ANPCYT, Argentina), Parques Nacionales (Argentina), National Geographic Society (USA) and other institutions. CONICET supported this work with Grant N 4305/97 and other more recent grants.

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Panhke, K., Zahn, R., Elderfield, H. and Schulz, M. (2003). 340,000-year centennial scale marine record of Southern Hemisphere climatic oscillations. Science 301, 948–952. Pascual, R., Ortiz Jaureguizar, E. and Prado, J. (1996). Land mammals: Paradigm for Cenozoic South American Geobiotic Evolution. In: Arratia, G., (ed.), Mu¨nchner Geowissenschaftliche Abhandlungen, A 30, Munich, Germany, 265–320. Planas, X., Ponsa, A., Coronato, A.M. and Rabassa, J. (2002). Geomorphological evidence of different glacial stages in the Martial Cirque, Fuegian Andes, Southernmost South America. Quaternary International 87, 19–27 Polanski, J. (1965). The maximum glaciation in the Argentine Cordillera. Geological Society of America, Special Paper 84, 444–472. Porter, S. (1981). Pleistocene glaciation in the southern Lake District of Chile. Quaternary Research 16, 263–292. Porter, S. (1989). Character and ages of Pleistocene drifts in a transect across the Strait of Magellan. Quaternary of South America & Antarctic Peninsula 7. A.A. Balkema Publishers, Rotterdam, 35–50. Porter, S. (2000). Onset of Neoglaciation in the Southern Hemisphere. Journal of Quaternary Science 15, 395–408. Porter, S., Stuiver, M. and Heusser, C. (1984). Holocene sea-level changes along the Strait of Magellan and Beagle Channel, southernmost South America. Quaternary Research 22, 59–67. Rabassa, J. (1975). Geologı´a de la Regio´n de PilcaniyeuComallo, provincia de Rı´o Negro, Argentina. Unpublished PhD Thesis, Universidad Nacional de La Plata, La Plata, and Fundacio´n Bariloche, Departamento de Recursos Naturales y Energı´a, Publicaciones 17, 1–128. San Carlos de Bariloche. Rabassa, J. (1983). INQUA Commission on lithology and genesis of Quaternary deposits: South American Regional Meeting, Argentina, 1982. In: Evenson, E.B., Schlu¨chter, C. and Rabassa, J. (eds), Tills and Related Deposits. A.A. Balkema Publishers, Rotterdam, 445–451. Rabassa, J. (1999). Cuaternario de la Cordillera Patago´nica y Tierra del Fuego. In: Haller, M.J. (ed.), ‘‘Geologı´a Argentina’’, Anales SEGEMAR. Buenos Aires. Rabassa, J. (2007). Global climate change and its impact on the glaciers and permafrost of Patagonia, Tierra del Fuego and the Antarctic Peninsula. Sa˜o Paulo, Regional Meeting on Global Climatic Change, Proceedings, University of Sa˜o Paulo, digital publication. Rabassa, J., Brandani, A., Boninsegna, J. and Cobos, D. (1984). Cronologı´a de la ‘‘Pequen˜a Edad del Hielo’’ en los glaciares Rı´o Manso y Castan˜o Overo, Cerro Tronador, Provincia de Rı´o Negro. IX Congreso Geolo´gico Argentino, Actas 3, 624–639. San Carlos de Bariloche. Rabassa, J., Bujalesky, G., Meglioli, A. et al. (1992). The Quaternary of Tierra del Fuego, Argentina: the status of our knowledge. Sveriges Geologiska Underso¨kning, Ser.Ca. 81, 249–256. Rabassa, J. and Clapperton, C.M. (1990). Quaternary Glaciations of the Southern Andes. Quaternary Science Reviews 9, 153–174. Rabassa, J. and Coronato, A.M.J. (2002). Glaciaciones del Cenozoico Tardı´o. In: Haller, M.J. (ed.), ‘‘Geologı´a y Recursos Naturales de Santa Cruz’’. XV Congreso

Geolo´gico Argentino, Relatorio 1, 19. El Calafate, Argentina, 303–315. Rabassa, J., Coronato, A. and Salemme, M. (2005). Chronology of the Late Cenozoic Patagonian glaciations and their correlation with biostratigraphic units of the Pampean region (Argentina). Journal of South American Earth Sciences 20, 81–104. Rabassa, J., Coronato. A.M., Bujalesky, G. et al. (2000). Quaternary of Tierra del Fuego, southernmost South America: an updated review. Quaternary International 68–71, 217–240. Rabassa, J., Coronato, A.M., Roig, C. et al. (2004). Un bosque sumergido en Bahı´a Sloggett, Tierra del Fuego, Argentina: evidencia de actividad neotecto´nica diferencial en el Holoceno tardı´o. In: Blanco Chao, R., Lo´pez Bedoya, J. and Pe´rez Alberti, A. (eds), Procesos Geomorfolo´gicos y evolucio´n costera, II Reunio´n Geomorfologı´a Litoral, Actas. Santiago de Compostela, Spain, 333–345. Rabassa, J. and Evenson, E.B. (1996). Reinterpretacio´n de la estratigrafı´a glaciaria de la regio´n de San Carlos de Bariloche. XIII Congreso Geolo´gico Argentino, Actas 4, 327. Buenos Aires. Rabassa, J., Evenson, E.B. and Stephens, G.C. (1986). Nuevas evidencias del englazamiento Plioceno-Pleistoceno inferior en los Andes Patago´nicos Septentrionales: Cerro Tronador, Rı´o Negro. Asociacio´n Geolo´gica Argentina, Revista 41, 405–409. Buenos Aires. Rabassa, J., Evenson, E.B., Clich, J.M. et al. (1990a). Geologı´a del Cuaternario del Valle del Rı´o Malleo, Provincia del Neuque´n. Asociacio´n Geolo´gica Argentina, Revista 45, 5–68. Buenos Aires. Rabassa, J., Heusser, C. and Rutter, N. (1990b). LateGlacial and Holocene of Argentine Tierra del Fuego. Quaternary of South America and Antarctic Peninsula 7, 327–351. A.A. Balkema Publishers, Rotterdam. Rabassa, J., Roig, C., Singer, B. et al. (1996). Bloques erra´ticos y rasgos periglaciales observados en el Cerro del Fraile, Lago Argentino (Santa Cruz, Argentina). XIII Congreso Geolo´gico Argentino and III Congreso Exploracio´n Hidrocarburos, Actas 4, 345. Buenos Aires. Rabassa, J., Rubulis, S. and Suarez, J. (1978). Los glaciares del Monte Tronador. Anales de Parques Nacionales 14, 261–316. Buenos Aires. Rabassa, J., Rubulis, S. and Brandani, A. (1980). Eastwest and north-south snow line gradients in the northern Patagonian Andes, Argentina. World Glacier Inventory. IAHS-AISH Publication, Cambridge, England, 126, 1–10. Rabassa, J., Rubulis, S. and Suarez, J. (1981). Moraine in-transit as parent material for soil development and the growth of Valdivian Rain Forest on moving ice: Casa Pangue Glacier, Mount Tronador (lat. 41100 S), Chile. Annals of Glaciology 2, 97–102. Rabassa, J., Serrat, D., Marti, C. and Coronato, A. (1990c). Internal structure of drumlins in Gable Island, Beagle Channel, Tierra del Fuego, Argentina. LUNDQUA Report 32, 3–5. Lund. Ramos, V. (1999a). Las provincias geolo´gicas del territorio argentino. In: Haller, M.J. (ed.), ‘‘Geologı´a Argentina’’, Anales SEGEMAR 29, 41–96. Buenos Aires.

Late Cenozoic Glaciations in Patagonia and Tierra del Fuego Ramos, V. (1999b). Evolucio´n tecto´nica de la Argentina. In: Haller, M.J. (ed.), ‘‘Geologı´a Argentina’’, Anales SEGEMAR 29, 715–759. Buenos Aires. Rignot, E., Rivera, A. and Casassa, G. (2003). Contribution of the Patagonian Ice Fields of South America to sealevel rise. Science 302, 434–437. Rivera, A. and Casassa, G. (2004). Ice elevation, areal and frontal changes of glaciers from National Park Torres del Paine, Southern Patagonian Icefield. Arctic, Antarctic and Alpine Research 36, 379–389. Roberts, D.E. (1984). Quaternary history of the Falkland Islands. Unpublished PhD Dissertation, University of Aberdeen, Scotland. Rosenbluth, B., Fuenzalida, H. and Aceituno, P. (1997). Recent temperature variations in Southern South America. International Journal of Climatology 17, 65–85. Ro¨thlisberger, F. (1987). 10,000 Jahre Gletschergeschichte der Erde. Verlag Sauerlander, Aarau, 416. Ro¨thlisberger, F. and Geyh, M. (1985). Gletscherswankungen der Nacheiszeit in der Cordillera Blanca (Peru) und den sudlichen Anden Chiles und Argentiniens. Zentralblatt Geologie und Pala¨ontologie 1, 11/12, 1611–1613. Rovereto, G. (1912). Studi di geomorfologia argentina. III, La valle del Rı´o Negro: 2. Il Lago Nahuel Huapi. Bolletino della Societa Geologica Italiana 31, 181–237. Ruddiman, W.F., McIntyre, A. and Raymo, M. (1986). Matuyama 41,000 years cycles in North Atlantic Ocean and Northern Hemisphere Ice Sheets. Earth & Planetary Science Letters 80, 117–129. Rutter, N., Zhongli Ding and Tungsheng Liu (1991). Comparison of isotope stages 1–61 with the Baojitype pedostratigraphic section of North-Central China. Canadian Journal of Earth Sciences 28, 985–990. Schellmann, G. (1998). Jungka¨nozoische Landschaftsgesschichte Patagoniens (Argentinien). Andine Vorlandvergletscherungen, Talentwicklung und marine Terrasen. Essener Geographische Arbeiten 29, 1–218. Schellmann, G. (1999). Landscape evolution and glacial history of Southern Patagonia (Argentina) since the Late Miocene – some general aspects. Zentralblatt Geologie und Pala¨ontologie 1, 7/8, 1013–1026. Schellmann, G. (2003). Su¨dpatagonien: Gletschergeschichte in einem Trockengebiet der su¨dhemispha¨rischen Mittelbreiten. Geographische Rundschau 55, 22–27. Schlieder, G. (1989). Glacial Geology of the Northern Patagonian Andes between lakes Alumine´ and La´car. Unpublished Ph.D. Dissertation, Lehigh University, Bethlehem, Pennsylvania, USA. Shackleton, N.J. (1995). New data on the evolution of Pliocene climatic variability. In: Vrba, E.S., Denton, G.H., Partridge, T.C. and Burckle, L.H. (eds), Paleoclimate and Evolution, with Emphasis on Human Origins. Yale University Press, New Haven and London, 242–248. Shackleton, N.J., Berger, A. and Peltier, W. (1990). An alternative astronomical calibration of the lower Pleistocene timescale based on OPD site 677. Transactions of the Royal Society Edinburgh, Earth Sciences 81, 251–261. Shackleton, N.J., Crowhurst, S., Hagalberg, T. et al. (1995). A new late Neogene time scale: application to

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LEG 138 sites. Proceedings of the Ocean Drilling Program, Scientific Results 138, 73–90. Singer, B., Ackert, R.P., Kurz, M. et al. (1998). Chronology of Pleistocene glaciations in Patagonia: a 3He, 40 Ar/39Ar & K-Ar study of lavas and moraines at Lago Buenos Aires, 46 S, Argentina. Geological Society of America 1998 Annual Meeting, Symposium 24, Abstracts 30, 299. Singer, B, Ackert, R.P. and Guillou, H. (2004a).40Ar/39Ar and K/Ar chronology of Pleistocene glaciations in Patagonia. Geological Society America Bulletin 116, 434–450. Singer, B., Brown, L., Guillou, H. et al. (1999). 40Ar/39Ar ages and paleomagnetic data from Cerro del Fraile, Argentina: further constraints on timing of reversals during the Matuyama Chron. IUGG Meeting, Abstracts Volume. Singer, B., Brown, L.L., Rabassa, J. and Guillou, H. (2004b). 40Ar/39Ar ages of Late Pliocene and Early Pleistocene Geomagnetic and Glacial Events in Southern Argentina. AGU Geophysical Monograph Timescales of the Internal Geomagnetic Field, dedicated to N.D.Opdyke, 176–190. Singer, B. and Pringle, M.S. (1996). Age and duration of the Matuyama-Brunhes geomagnetic polarity reversal from 40Ar/39Ar incremental heating analyses of lavas. Earth & Planetary Sciences Letters 139, 47–61. Stiff, B. and Hansel, A. (2004). Quaternary glaciations in Illinois. In: Ehlers, J. and Gibbard, P. (eds), Quaternary Glaciations –Extent and Chronology, Part II: North America. Elsevier, Amsterdam, Developments in Quaternary Science 2, 71–82. Strelin, J. (1995). New evidence on the relationships between the oldest extra-andean glaciations in the Rı´o Santa Cruz area. Quaternary of South America & Antarctic Peninsula 9. A.A. Balkema Publishers, Rotterdam, 105–116. Strelin, J. and Denton, G. (2005). Las morenas de Puerto Bandera. XVI Congreso Geolo´gico Argentino & IV Congreso de Exploracio´n de Hidrocarburos, Actas 4, 129–134. La Plata. Strelin, J. and Malagnino, E.C. (2000). late-glacial history of Lago Argentino, Argentina, and age of the Puerto Bandera moraines. Quaternary Research 54, 339–347. Strelin, J., Re, G., Keller, R. and Malagnino, E. (1999). New evidence concerning the Plio-Pleistocene landscape evolution of southern Santa Cruz region. Journal of South American Earth Sciences 12, 333–341. Sugden, D., Bentley, M., Fogwill, C. et al. (2005). lateglacial glacier events in Southernmost South America: a blend of ‘‘northern’’ and ‘‘southern’’ hemispheric climatic signals? Geografiska Annaler 87 A, 273–288. Sugden D., Hulton, N. and Purves, R. (2002). Modelling the inception of the Patagonian icesheet. Quaternary International 95–96, 55–64. Sylwan, C. (1989). Paleomagnetism, Paleoclimate and Chronology of Late Cenozoic Deposits in Southern Argentina. Meddelanden Stockholms Universitets Geologiska Institute 277, 1–110. Thomson, S.N. (2002). Late Cenozoic geomorphic and tectonic evolution of the Patagonian Andes between latitudes 42 S and 46 S: An appraisal based on

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fission-track results from the transpressional intra-arc Liquin˜e-Ofqui fault zone. Geological Society America Bulletin 114, 1159–1173. Tonni, E.P. and Cione, A.L. (1995). Los mamı´feros como indicadores de cambios clima´ticos en el Cuaternario de la Regio´n Pampeana de la Argentina. In: Argollo, J. and Mourguiart, P. (eds), Cambios Cuaternarios en Ame´rica del Sur, ORSTOM, Publicaciones. La Paz, Bolivia, 319–326. Tonni, E.P., Cione, A.L. and Figini, A.J. (1999a). Predominance of arid climates indicated by mammals in the Pampas of Argentina during the late Pleistocene and Holocene. Palaeogeography, Palaeoclimatology, Palaeoecology 147, 257–281. Tonni, E.P., Nabel, P., Cione, A.L. et al. (1999b). The Ensenada and Buenos Aires formations (Pleistocene) in a quarry near La Plata, Argentina. Journal of South American Earth Sciences 12, 273–291. Ton-That, T. (1997). 40Ar/ 39Ar dating of basaltic lava flows and the geology of the Lago Buenos Aires region, Santa Cruz province, Argentina. Unpublished Diploma Thesis, Universite´ de Gene`ve, Switzerland, 51 pp. Ton-That, T., Singer, B., Mo¨rner, N.A. and Rabassa, J. (1999). Datacio´n por el me´todo 40Ar/39Ar de lavas basa´lticas y geologı´a del Cenozoico Superior en la regio´n del Lago Buenos Aires, provincia de Santa Cruz, Argentina. Asociacio´n Geolo´gica Argentina, Revista 54, 333–352. Buenos Aires. Tyson, P.D. and Partridge, T.C. (2000). Evolution of Cenozoic climates. In: Partridge, T.C. and Maud, R. (eds), The Cenozoic of Southern Africa. Oxford University Press, Oxford, England, 371–386. van der Meer, J., Rabassa, J. and Evenson, E. (1992). Micromorphological aspects of glaciolacustrine sediments in northern Patagonia, Argentina. Journal of Quaternary Science 7, 31–44. Verzi, D.H., Tonni, E.P., Scaglia, O.A. and San Cristo´bal, J. (2002). The fossil record of the desert-adapted South American rodent Tympanoctomys (Rodentia, Octodontidae). Paleoenvironmental and biogeographic significance. Palaeogeography, Palaeoclimatology, Palaeoecology 179, 149–158. Volkheimer, W. (1963). El Cuartario pedemontano en el noroeste de Chubut (zona Cushamen). II Jornadas Geolo´gicas Argentinas, Actas 2, 439–457. Buenos Aires. Walther, A.M., Rabassa, J., Coronato, A. et al. (2007). Paleomagnetic studies on glacial sediments in northern Isla Grande de Tierra del Fuego, Argentina. GEOSUR Meeting, Abstracts, Santiago de Chile, November 2007. Warren, C.R. (1993). Rapid recent fluctuations of the calving San Rafael Glacier, Chilean Patagonia: Climatic or nonclimatic? Geografiska Annaler 75A, 111–125. Warren, C.R. (1994). Freshwater calving and anomalous glacier oscillations: recent behaviour of Moreno

and Ameghino glaciers, Patagonia. The Holocene 4, 422–429. Warren, C.R, Glasser, N.F., Harrison, S. et al. (1995a). Characteristics of tide-water calving at Glacier San Rafael, Chile. Journal of Glaciology 42, 279–291. Warren, C.R., Sugden, D.E. and Clapperton, C.M. (1995b). Asynchronous response of Patagonian glaciers to historic climate change. Quaternary of South America & Antarctic Peninsula 9. A.A. Balkema Publishers, Rotterdam, 89–108. Wehrli, L. (1899). Rapport preliminaire sur mon expe´dition ge´ologique dans la Cordille`re Argentine-Chilienne du 40 et 41 latitude sud (re´gion de Nahuel Huapi), Argentina. Revista del Museo de La Plata 9, 223–252. La Plata. Wenzens, G. (1999a). Evidences of Pliocene and early Quaternary glaciations east of Lago Viedma (Patagonia, Argentina). Zentralblatt Geologie und Pala¨ontologie 1, 1027–1049. Wenzens, G. (1999b). Fluctuations of outlet and valley glaciers in the Southern Andes (Argentina) during the past 13,000 years. Quaternary Research 51, 238–247. Wenzens, G. (2000). Pliocene Piedmont Glaciation in the Rı´o Shehuen Valley, Southwest Patagonia, Argentina. Arctic, Antarctic & Alpine Research 32, 1, 46–54. Wenzens, G. (2002). The influence of tectonically derived relief and climate on the extent of the last Glaciation east of the Patagonian Ice Fields (Argentina, Chile). Tectonophysics 345, 329–344. Wenzens, G. (2004). Comment on ‘‘modelling the inception of the Patagonian ice sheet’’. Quaternary International 112, 105–109. Wenzens, G. (2006a). Terminal moraines, outwash plains and lake terraces in the vicinity of Lago Cardiel (49 S, Patagonia, Argentina): evidence for Miocene Andean foreland glaciation. Arctic, Antarctic and Alpine Research 38, 276–291. Wenzens, G. (2006b). Comment on: Kaplan, M.R., Douglass, D., Singer, B.S., Ackert, R.P. and Caffee, M.W., 2005, Cosmogenic nuclide chronology of pre-glacial maximum moraines at Lago Buenos Aires, 46 S, Argentina. Quaternary Research 66, 2, 364–366. Wenzens, G., Wenzens, E. and Schellmann, G. (1996). Number and types of the piedmont glaciations east of the Central Southern Patagonian Icefield. Zentralblatt Geologie und Pala¨ontologie 1, 779–790. Willis, B. (1914). Physiography of the Cordillera de los Andes between latitudes 39 and 44 South. 12th International Geological Congress, Canada, ComptesRendu, 733–756. Zavala, C.A. and Quattrocchio, M. (2001). Estratigrafı´a y evolucio´n geolo´gica del Rı´o Sauce Grande (Cuaternario), provincia de Buenos Aires. Asociacio´n Geolo´gica Argentina, Revista 56, 25–37.

9 The Late Cenozoic Fluvial Deposits of Argentine Patagonia Oscar A. Martı´nez1 and Andrea M.J. Coronato2,3 1

Universidad Nacional de la Patagonia San Juan Bosco, Sede Esquel, Ruta 259, Km 4 (9200) Esquel, Argentina 2 CADIC-CONICET, C.C. 92, (9410) Ushuaia, Argentina 3 Universidad Nacional de la Patagonia San Juan Bosco, Sede Ushuaia, Darwin y Canga, (9410) Ushuaia, Argentina in base levels due to tectonic, epirogenic, eustatic and glacioisostatic movements during the Late Cenozoic (Strelin et al., 1999). Besides, even if a specific origin can be assigned to each of the studied units, the chronological relationship between them would be far from solved. In this sense, it seems pertinent to accept that the main piedmont aggradation events would have taken place and, in some cases, regionally even have been coinciding with glaciofluvial events, since at least the Middle Miocene. The general idea of assigning a greater age to the aggradation events in relation to glaciofluvial events (Fidalgo and Riggi, 1965, 1970) seems quite inconvenient, considering the complexity of the tectonic and climatic behavior of such an extensive region as Patagonia (Lapido and Pereyra, 1999). Patagonia has a particular drainage network (see Chapter 3). The superficial runoff is the result of a humid climate in the western mountains, with abundant precipitation (rain and snow) and intricate glacier, lake and mountain stream networks. These water courses form the head basins of the allochtonous streams that cross a vast and arid tableland landscape, until reaching the Atlantic Ocean or dying out in endorheic depressions. The rivers have eroded their valleys in the tablelands or ‘‘mesetas’’, developing terrace systems and wide floodplains. Discharge, imposed by glacial and interglacial climates, has greatly varied over the Pliocene–Pleistocene history of the region. Some southern Patagonia hydrological systems, such as the Deseado, Chubut, Shehuen, Coyle and Gallegos rivers have at present a very reduced discharge, or even become intermittent, as it is the case of the Rı´o Chico de Santa Cruz, although in the past, these streams were powerful glaciofluvial currents discharging the Pleistocene glacier basins when they stabilized their fronts in the piedmont areas, at approximately 70 W (Chapter 3). Nevertheless, this hydrological system does not explain by itself the presence of most of the gravel mantles that extend outside and above the present fluvial valleys. This lack of genetic relationship is even more evident in the Andean sector of northern Patagonia. There, in Rı´o Negro and Chubut provinces, the presence of the frontal moraines built up during the last Pleistocene glaciations implied an eastward displacement of the continental watershed. For this reason, it may be seen today that the heads of the Puelo, Futaleufu´, Carrenleufu´, Pico and Simpson stream basins, which discharge into the Pacific Ocean, are connected toward the east with extensive gravel plains of glaciofluvial origin, gently dipping eastward and in which one can still observe paleochannels that were active

1. Introduction The approach used in this chapter considers as fluvial deposits all of those that are the result of transportation and accumulation produced by moving superficial waters. This ample use of the term ‘‘fluvial’’ allows the incorporation of, in addition to the deposits associated to well-defined waterways and streams, a great variety of sand and gravel units, occasionally compacted and cemented, that characterize a large portion of the eastern Patagonian slopes. These units have a significant areal extent in Neuque´n, Rı´o Negro, Chubut, Santa Cruz and Tierra del Fuego provinces (Fig. 1). Thus, the present contribution includes those units generated by presentday streams by fluvial and glaciofluvial waters of the past, and by the ancient, recent and present piedmont accumulation processes. These clastic accumulations are usually forming horizontal to subhorizontal layers and sheets of varied extension, thickness, topographic position and age. Their genesis has been diversely interpreted in different times and they form various landforms such as floodplains, alluvial terraces, alluvial fans and bajadas, pediment covers, proximal and distal glaciofluvial plains, and covered structural plains, extending between the Andean Cordillera and the Atlantic Ocean coast. Most of them have been included under the denomination of ‘‘Rodados Patago´nicos’’ or ‘‘Rodados Tehuelches’’ or ‘‘Patagonian Shingle Formation’’ (as they were named by Charles Darwin in 1842; Darwin, 1848), a qualification that has been considered of little use in recent years, since this name does not refer to a unique geomorphological or geological unit which can be mapped all over Patagonia, but, instead, it has been used to refer to many different units corresponding to different origins and age (Panza, 2002). The purpose of this chapter is to discuss the present stage of our knowledge and to propose a classification based upon genetic, spatial and chronological criteria. In this last sense, as it has been pointed out by Lapido and Pereyra (1999), the lack of chronostratigraphic studies and the lack of absolute dates for the different Quaternary units of this region ensures that any age assignment or location of these deposits in a stratigraphic sequence can only be tentative. The gravel mantles that extend almost parallel to the present drainage lines may be genetically related to fluvial valley processes, whereas the rest of the units must be associated with climatic fluctuations, and particularly, the glacial and interglacial periods, and to modifications

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Fig. 1. Location of the areas mentioned in the text. Full lines are glaciofluvial gravels distribution; Dotted lines are nonglaciofluvial gravels. during the Pleistocene, or even perhaps during the Late Pliocene. These features are clearly marking a noted inversion in the drainage direction. Thus, the need to reconstruct the peculiar morphodynamic context becomes obvious, quite different to that which today characterizes the eastern slopes of Patagonia, in order to explain the development of many of these extensive gravel accumulations, which have intrigued the scientific community since the times of Charles Darwin.

2. Previous Work Of all the units mentioned, those that have been studied and discussed more extensively are undoubtedly the socalled ‘‘Rodados Patago´nicos’’, without yet reaching an agreement about their origin and chronology (Lapido and Pereyra, 1999). The existence of these characteristic and extensive sand and gravel sheets has been first documented by Darwin (1846, 1848) who recognized, during his trips in Patagonia, gravel deposits on the tablelands

Late Cenozoic Fluvial Deposits of Argentine Patagonia between the Andes and the sea. He described the outcrops of the ‘‘mesetas’’ that bound the Colorado, Negro and Santa Cruz rivers, naming them ‘‘Gravel Formation or Shingle Formation’’. He considered them to be alluvial fans at the foot of the Andean Cordillera, later redistributed by wave action during a marine transgression (Darwin, 1848). The latter interpretation was based on the finding of extant mollusc shells in the lower terraces. Later, Feruglio (1950) concluded that these organic remains had been accumulated by human action. Charles Darwin ignored then the extension and magnitude of the Andean Pleistocene glaciations because the ‘‘Glacial Theory’’ was still in its infancy. He therefore interpreted those gravels as having been deposited by ancient fluvial currents, very different to the extremely arid conditions existing at the moment of his observations. Doering (1882) considered these gravels as of glaciofluvial origin and correlated them with the lower stages of the Pampean sediments since they were also cemented by caliche duricrusts, locally known as ‘‘tosca’’, assigning them an Early Pliocene age, the ‘‘Piso Tehuelche’’ (‘‘Tehuelche Stage’’; Tehuelches were the dominant aboriginals in southern Patagonia). Carlos Ameghino (1890) distinguished between the marine deposits that form the higher and lower terraces, naming the first ‘‘Formacio´n Araucanense’’, deposited in successive episodes since the Early Miocene. In the southernmost region, between the Rı´o Santa Cruz and the Magellan Straits, Mercerat (1893) labeled them as ‘‘Rodados Tehuelches’’ and thought them to be of marine origin and Pre-Pliocene age, distinguishing them from certain coarse alluvial deposits with blackish, volcanic, fine-grained matrix of Pleistocene age. Hatcher (1897) , recovering the Darwinian term of ‘‘Shingle Formation’’ when referring to them, agreed with previous authors when assigning a marine origin to the Patagonian tableland gravels and considered them to be the result of a large marine transgression that would have affected the entire extra-Andean Patagonia during the Pliocene. The observations of Nordenskjo¨ld (1897) in southern Santa Cruz Province and the Magellan Straits allowed him to relate these deposits to glaciofluvial stream accumulations (outwash). Ameghino (1906) returned to this topic and indicated, correctly, that it was not appropriate to assign one unique origin to all the gravels and these units would have a different origin according to their geographical and topographical location. The first in correlating the ‘‘Rodados Patago´nicos’’ with the glacial events in the Patagonian Andes was Rovereto (1912), who identified four levels of terraced gravels and suggested a correlation with four hypothetical glaciations, following the European Alpine glacial model of those times. These glaciations would have been related to different marine terrace levels with mollusc faunas similar to present ones, as recognized along the Atlantic coast. Keidel (1917–1919) discussed the hypothesis of the previous authors suggesting that the gravels that cover many of the ‘‘mesetas’’ and valley terraces in northeastern Patagonia would correspond to bajadas that were built by fluvial systems coming from the Andean Cordillera, during the Pliocene and the Quaternary, as a response to regional uplift movements. He was the first to suggest the discordant relationship

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between the gravel and the Late Tertiary marine and continental sedimentary rocks. Later, Bonarelli and Na´gera (1922) returned to the concept of a marine origin when assuming that the ‘‘Rodados Tehuelches’’ of the highest ‘‘mesetas’’ were at least of Pliocene age, which were dispersed by marine waters of a transgression that would have reached the Andean foothills. These gravels would have been the source for the fluvial deposits of the lower terraces, carved by successive base-level lowering episodes. Windhausen (1931) suggested that the highest gravel mantles would have been deposited by alluvial processes over a rather flat, gentle sloping landscape, whereas the gravels of the lower terraces were the consequence of glaciofluvial deposition in different Quaternary uplifting events. Based on the ideas of Rovereto (1912), Frengu¨elli (1931) distinguished the ‘‘Tehuelchiano’’ stage, composed of three levels of marine and continental terraces, from the ‘‘Post-Tehuelchiano’’ stage, formed by gravels of the lower terraces of postglacial age. Caldenius (1932, 1940) attributed a fluvial and glaciofluvial origin to the ‘‘Tehuelche’’ gravels, originally deposited as piedmont glaciofluvial cones, and he suggested a certain reworking of these gravels by solifluction processes. He also recognized levels of higher gravels, older than the oldest glaciation of his fourfold glacial model, the ‘‘Initioglacial’’ stage. Groeber (1936) indicated an alluvial origin, whereas Feruglio (1950) recognized the existing correlation between the terraces of the different fluvial systems of the southernmost Patagonian tablelands, in the valleys of the Chubut, Deseado, Shehuen, Coyle, Santa Cruz and Gallegos rivers. The large extent of the terraces, the thickness of their alluvial mantles and the marked elevation that separate them justified their interpretation as related to glacial and interglacial periods which affected the extent of the Patagonian Andes ice sheet since the Pliocene and, to a lesser extent, to tectonic uplift phases. On these terraces, he distinguished morainic deposits and glaciofluvial gravels of diverse lithology, but mainly eruptive rocks. Frengu¨elli (1957) basically agreed with Feruglio’s (1950) ideas. Detailed studies were developed by Fidalgo and Riggi (1965, 1970) who, based upon geomorphological and sedimentological observations, classified these materials in two main groups: (a) those of piedmont, fluvial origin (the ‘‘Rodados Patago´nicos’’ sensu stricto), located at higher elevations and covering both ‘‘mesetas’’ and pediments and (b) those forming the glaciofluvial plains located in the valleys or depressions surrounding the ‘‘mesetas’’ and thus, of a younger age. These authors suggested that other deposits of smaller extent, such as alluvial fans and flanking pediments, should also be considered as ‘‘Rodados Patago´nicos’’, an inconvenient suggestion according to Clapperton (1993). The use of absolute, radiometric dating to confirm the existence of Pre-Pleistocene glaciations in the province of Santa Cruz allowed Mercer (1976) to propose the development of accumulations of glaciofluvial origin, assigned to the ‘‘Rodados Patago´nicos’’, with ages equivalent or even older than the oldest piedmont deposits indicated by other authors. Gonza´lez Dı´az and Malagnino (1984) and Malagnino (1989) centered their observations on northern Patagonia and agreed on the polygenetic

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characteristics of the ‘‘Rodados Patago´nicos’’ in those latitudes, suggesting essentially a glaciofluvial origin for the younger deposits and a piedmont origin, possibly also associated to tectonic pulses, for the older ones. Clapperton (1993) and, later, Lapido and Pereyra (1999) suggested to classify these deposits in (a) those located in northern Patagonia, between the Negro and Colorado rivers, to which they assigned a dominantly piedmont origin and (b) the gravels of southern Patagonia, in Chubut and Santa Cruz provinces, where the gravels have a dominant glaciofluvial nature. During the second half of the twentieth century, detailed geological surveys in extra-Andean Patagonia became more frequent, following the cartographic identification of the regional geology and mineral resources. Thus, many authors have proposed a series of stratigraphic units at the formation level that generally represent the fluvial deposits and the ‘‘Rodados Patago´nicos’’ in particular, for the entire Patagonian territory. Among many others, the contributions of Volkheimer (1963, 1964, 1965a, b, 1972, 1973), Cortelezzi et al. (1965), Gonza´lez (1971, 1978), Coira (1979), Page (1987), Corte´s (1987), Gonza´lez Dı´az (1993a, b), Panza (1994a, b), Panza and Yrigoyen (1994), Caminos (2001), Gonza´lez Dı´az and Tejedo (2002), Pereyra et al. (2002) and Leanza and Hugo (2005) must be cited. The work of Panza (2002) provided an integrated framework for the Cenozoic gravel deposits in Santa Cruz Province. This author has proposed to abandon the name of ‘‘Rodados Patago´nicos’’ due to the variety of unit types and origins included in this concept.

3. Criteria Adopted in this Work The approach adopted in this chapter is basically genetic, allowing the differentiation of (a) fluvial valley units, (b) glaciofluvial units and (c) piedmont units. Additionally, the use of other criteria of spatial, geomorphological and temporal nature has permitted a better characterization of the identified morphostructural units.

3.1. Criteria Based upon Surface Extent The superficial development of deposits and landforms, that is, the extent between the source area and the outermost facies was already considered by Fidalgo and Riggi (1970). These authors classified the provenance areas of the ‘‘Rodados Patago´nicos’’ sensu stricto (i.e. only those related to piedmont processes) in (i) an Andean or mainly distributive province and (ii) extra-Andean or secondary distributive provinces, assuming that some gravel surfaces, at least those more extensive, were connected with the Cordilleran front, whereas others were originated from extra-Andean mountain fronts and usually of more reduced extent. An adaptation of this scheme is applied here to the identified fluvial deposits. Thus, according to these criteria, it is possible to establish three categories: (1) local, when the deposits do not extend more than a few kilometers from their source area, which is either the mountain front (piedmont type) or a morainic front (glaciofluvial type); (2) regional,

accepting in this case three main regions, (a) Andean or western, (b) extra-Andean or central and (c) coastal or eastern; and (3) extra-regional, when the superficial expression of these units is so large that it affects more than one single geographical region. In the framework of this chapter, the regional and extra-regional scales are considered the most relevant, thus leaving the landforms of more reduced extent, such as alluvial fans, flanking pediments, talus cones and the like, out of this analysis, though some exceptions will be noted.

3.2. Criteria Based upon their Latitudinal Location The greater expansion of the ice with a southward latitudinal increase during each of the Patagonian glaciations as a result of a gradual lowering of the equilibrium line southward (Clapperton, 1993) had already been recognized by Caldenius (1932) during his mapping of the different glacial events, a paramount geological, cartographic, scientific product that still stands. This implies a more extensive presence of glaciofluvial sediments in the southernmost sectors, which is the reason why significant gravel deposits of this origin are located along the Atlantic coast of Santa Cruz Province, whereas equivalent deposits are absent in the coastal area of Chubut and Rı´o Negro provinces (Fig. 1). This criterion was already applied by Clapperton (1993), who recognized two groups of gravel units. Firstly, a group of northern ‘‘Rodados Patago´nicos’’ within the present basins of the Colorado and Negro rivers, between 36 and 39 S (Neuque´n and Rı´o Negro provinces, and the southern portion of La Pampa and Buenos Aires provinces, Fig. 1). This group is dominated by materials of piedmont fluvial and valley fluvial origin, according to what had been suggested by previous authors (Fidalgo and Riggi, 1970; Gonza´lez Dı´az and Malagnino, 1984; Malagnino, 1989). The second group, named as the southern ‘‘Rodados Patago´nicos’’, extends (according to Clapperton, 1993) south of the Rı´o Negro valley, between 39–56 S (center and southern portion of Rı´o Negro Province and Chubut and Santa Cruz provinces). These units are characterized by abundant glaciofluvial materials of Cordilleran origin and varied age.

3.3. Criteria Based upon their Chronological Position A close relationship exists between the age of the gravel accumulations and the type of processes that generated them. The glaciofluvial sediments correspond exclusively with the Neogene and Quaternary glacial pulses, whose regional chronology has been summarized by Clapperton (1993), Coronato et al. (1999, 2004a, b), Rabassa (1999; see also Chapter 3) and Rabassa et al. (2005). Besides, there is as yet no representation of the chronostratigraphic scheme for the extensive piedmont deposits for the whole of Patagonia. Fidalgo and Riggi (1965, 1970), as mentioned before, concluded that the oldest and highest grave mantles are of piedmont origin. Gonza´lez Dı´az and Malagnino (1984), in their observations in northern

Late Cenozoic Fluvial Deposits of Argentine Patagonia Patagonia, usually considered the piedmont deposits as older than the glaciofluvial units at lower elevations. Clapperton (1993) estimated that the oldest components of the Patagonian gravels are of piedmont and fluvial origin, generated as a consequence of an important Andean tectonic uplift in the Late Miocene. Schellmann et al. (2000) proposed that the dispersion of the ‘‘Rodados Patago´nicos’’ would already have been finished during the Late Miocene–Early Pliocene, before the epirogenetic uplift of the lowlands. However, many of the piedmont gravel sheets would have been generated during the Plio–Pleistocene interglacial or interstadial periods, probably as a product of tectonic pulses and/or glacioisostatic or epirogenetic adjustments, or as a response to favorable conditions for their development. Although it may be argued that alluvial fans, bajadas and covered pediments that characterize the flanks of most of the extra-Andean mountain ranges would have evolved both during the glacial and the interglacial periods, the latter seem to have been most favorable for the accumulation of this kind of fluvial deposits. The periglacial environment that affected the eastern-central Patagonian region during the glacial episodes (Auer, 1970; Haller, 1981; Galloway, 1985; Trombotto and Ahumada, 1993; Trombotto, 1994; see Chapter 12) would have significantly limited the fluvial action in these areas. Finally, the fluvial valley deposits corresponding to active floodplains or alluvial terraces not associated to glacial fronts would not be older than the Late Pleistocene or the Last Glaciation. The Pleistocene glaciofluvial aggradation and the consequent deglaciation have eroded or buried a significant portion of the older fluvial deposits nested in the valleys. Anyhow, several authors have mentioned relatively old terraces that laterally bound several of the most important Patagonian streams, assigning them a glaciofluvial origin (Feruglio, 1950; Panza, 2002). The increasing knowledge of Andean tectonics (see Chapter 4) and Patagonian glaciations (Rabassa et al., 2005; Chapter 3) reached during the last decade allows to conclude that the Cordilleran uplift, piedmont aggradation, climatic change and glaciation are intimately related phenomena and, also, that the piedmont or glaciofluvial accumulation processes alternated and/or were coincident in time, at least since the Late Miocene. This problem will be considered below.

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metamorphic and sedimentary rocks. They also noted the influence of the rocky substratum in the pediment formation, thus their genesis would have been favored in some Patagonian areas by the existence of easily erodable sedimentary rocks of Oligocene–Miocene age (i.e. the ‘‘Patagoniense’’, ‘‘Colloncurense’’ and ‘‘Santacrucense’’ formations of the classical regional stratigraphy). It may be accepted that the lithologies outcropping in the central, inner sectors of Cordillera would be better represented in the units of glaciofluvial origin. This is due to the extension and behavior of the Patagonian ice sheet. This immense, more than 3000 km long, temperate-based ice sheet occupied most of the Cordilleran surface during each glaciation allowing that only the highest peaks emerged as nunataks above the ice surface. The glacier source area included the inner sectors of the mountain range, and the materials incorporated by the ice were transported by the outlet glaciers, mostly accumulating in marginal moraines at the Andean foothills. Thus, the glaciofluvial streams, which pick up a lot of material while underneath the ice and then complete their sedimentary load when they leave the ice and cross these moraines, distributed and accumulated sediments which, according to their abundance and resistance to erosion, represented the existing Cordilleran lithology at those latitudes. The piedmont units of Andean origin would contain a smaller proportion of the inner sector lithologies and a larger proportion of rocks present in the eastern Cordilleran slope. The application of this criterion will be useful when those lithological types present in both environments (Andean core and Andean flanks) are, at least, partially different. In northern Patagonia, this criterion can be applied with reasonable efficiency because in these latitudes the Andean Batholith granitoids prevail in the central Cordilleran zone, whereas in the eastern flanks these plutonic outcrops disappear or are very scarce, with dominant early Tertiary, mesosilicic volcanic rocks (the Ventana Formation, Late Paleocene–Eocene, and equivalent units; Gonza´lez Bonorino, 1973). Gonza´lez Dı´az and Andrada de Palomera (1996) have used the clast composition to verify the origin of the gravel surfaces located in northwestern Chubut and southern Rı´o Negro, in areas near the Cordilleran ranges.

3.5. Criteria Based upon Surface Slope 3.4. Criteria Based upon Lithological Composition The lithological analysis of the gravels located on the extra-Andean tablelands may contribute to establish the provenance area and thus, their genesis. The pioneer studies of Cortelezzi et al. (1965, 1968) in the gravel mantles between the Colorado and Negro rivers indicated that the more abundant rock types are andesites, basalts and lamprophyres, with subordinate percentages of granites and pyroclastic rocks. Fidalgo and Riggi (1965, 1970) marked the unanimity of the different authors in the identification of a high percentage of volcanic components. They concluded, based upon their own observations, that the pediments showed, in general, 90% of volcanic and 10% of plutonic and sedimentary rocks, whereas the glaciofluvial plains held roughly 70% of volcanic and 30% of plutonic,

These criteria are easily applied although their usefulness is limited, and they directly relate to the results of other type of observations. In tectonically undisturbed terrain, the slope values may indicate the proximity to the source area, be it a mountain front or a moraine. Gradients larger than 1 would be indicating proximal facies of piedmont deposits. The direction of surface inclinations could also be useful to determine their origin, since both glaciofluvial and Cordilleran piedmont surfaces present in all cases a dominant inclination toward the east. For the case in which the surface dips strongly toward the west, the glaciofluvial origin should be overruled and it should be considered as a piedmont landform developed from an extra-Andean range.

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3.6. Criteria Based upon Altitude Since the earliest pioneer works on this subject, it has been assumed that the gravel levels located at higher elevations are the oldest. This criterion is absolutely valid when morphologies of a same environment and region are considered. However, it seems risky to apply this rule to establish a time or space relationship between very distant sites, for instance, between the Andean Cordillera and the Atlantic coastal zone. From a genetic point of view, Fidalgo and Riggi (1970) assumed that the highest terraces were of piedmont origin and older than the Quaternary glaciations. This was due, basically, to their acceptance of Caldenius’ (1932) glacial model, which was incorporating four glacial advances as stadials only of the Last Glaciation. Other authors, such as Clapperton (1993), Schellman et al. (2000) and Panza (2002), have demonstrated that the most extensive piedmont gravel mantles in Santa Cruz Province correspond to the highest levels that predate the Pleistocene glaciations. However, the presently accepted glaciostratigraphic model, based on absolute, radiometric dates, has shown that there have been glaciations in this area at least since the latest Miocene (Mercer, 1976; Ton-That et al., 1999; Rabassa et al., 2005), that these glaciations have alternated with interglacial periods in which the fluvial processes were dominant, and that in these periods pediments, bajadas, alluvial fans and fluvial terraces were also developed. Thus, it should not be ruled out that at least in certain localities (see Section on ‘‘Discussion’’) some of the highest gravel levels could be of glaciofluvial origin.

4. Fluvial Valley Deposits This category applies to the detritic materials which appear confined to the valleys, forming alluvial plains and fluvial terraces, usually of local extent only, although occasionally being regional. These deposits have been formed mainly during the Late Pleistocene and Holocene, after the Last Glaciation, though some deposits are considered to be older. The terraces cited in this paragraph are laterally bounding the main streams of the region, which flow largely from west to east throughout Patagonia and which are essentially coincident with the Neuque´n, Negro, Colorado, Chubut, Senguerr, Deseado, Chico, Santa Cruz and Gallegos rivers, among others of smaller size (Fig. 1). The deposits found in these terraces are composed of sediments of varied grainsize, practically not lithified and with scarce or no cementation. The Atlantic Ocean functioned then as it does now as the regional base level to which most of these surfaces are adjusted, although some streams are controlled by local base levels, as for instance the Rı´o Senguerr in Chubut Province. The fluvial deposits that form the most extensive terraces are the product of eastward transportation of material generated during erosive processes in the Cordilleran environment. Besides, the carving of each terrace would be related to rejuvenation as a product of extraAndean uplifting, climatic changes or sea level lowering. Therefore, the knowledge of the origin and chronology of the extra-Andean Patagonian fluvial terraces becomes

particularly significant in the regional Neogene history from the Andean Cordillera across to the Atlantic Ocean. Some of the most representative examples of deposits and landforms are presented below.

4.1. Ancient Fluvial Deposits in the Rı´o Colorado Basin (Rı´o Negro and Chubut Provinces) The Rı´o Colorado Basin is an elongated transversal depression generated during the Cretaceous–Tertiary periods and marks the northern border of Patagonia. There, a thick sequence of continental, marine and mixed sediments is covered by alluvial deposits corresponding to the Bele´n Formation (Zambrano, 1972) or Rı´o Negro Formation. They are mainly composed of medium to coarse bluish gray sandstones, generally unconsolidated, interbedded with gray or yellowish tuffs, which tend to grow north and eastward. The sands are rich in heavy minerals and basaltic rock fragments. Conglomeratic layers occur frequently, which are thicker toward the west. The thickness varies between 100 m and 480 m, and they are overlain by Quaternary eolian silts and loess (Pampa Group, Yrigoyen, 1999). A few mammal remains have been found in the lower section of this formation. A Late Miocene–Pliocene age is accepted for these sediments which would have been accumulated during an Atlantic Ocean regression (Malumia´n, 1999).

4.2. Fluvial Terraces at the Heads of the Rı´o Neuque´n (Neuque´n Province) In central Neuque´n Province, in areas close to the Andean Cordillera, at least seven fluvial terraces are found within the valleys of the Agrio, Covunco and Neuque´n rivers (Ardolino and Franchi, 1996; Leanza and Hugo, 2005). Developed between the Middle Pleistocene and the Holocene, they are composed basically of poorly consolidated gravels of mesosilicic to basic volcanic rocks, which outcrop near the Cordillera. The thickness of the sediments covering these terraces is variable; they may reach up to 10 m. Their presence and quantity would be indicating a very complex history for this area during practically all Quaternary times. Although it may be interpreted that some of them would be the result of tectonic uplift (those of strictly fluvial origin), it is possible that some of the oldest ones would have been generated as glaciofluvial units during the Cordilleran glaciations.

4.3. Fluvial Terraces of the Rı´o Limay (Neuque´n and Rı´o Negro Provinces) The Rı´o Limay is the natural boundary between the Neuque´n and Rı´o Negro provinces and flows in a northeast direction, and within its valley at least four terrace levels have been identified and considered to be of Pleistocene age (Leanza and Hugo, 1997). The oldest one, named as ‘‘Nivel I’’, is widely exposed along the 500 m contour line and is composed largely of bluish sandstones. The lower levels, II, III and IV, occupy a more restricted valley area,

Late Cenozoic Fluvial Deposits of Argentine Patagonia stepwise graded below the 450 m contour line until just a few meters above the present river bed. They are composed of conglomerates, gravels, sands and silts. The lithological differences and the larger areal extent of ‘‘Nivel I’’ with respect to the other terraces implies different origins and age for both groups of landforms.

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up to three step-like fluvial terraces, separated by low scarps < 2 m high. They are composed of gravels of varied thickness. These landforms are younger than the Pliocene gravel accumulations, thus being generally assigned to the Quaternary (Gonza´lez, 1971). The Rı´o Senguerr ends in Lago Musters in an alluvial fan with a length of more than 45 km, being this one of the largest, single fluvial landforms in extra-Andean Patagonia.

4.4. Ancient Deposits of the Rı´o Chubut (Chubut Province) In northeastern Chubut Province, an elongated N–S depression extends 6 to 8 km in width and tens of km long, completely filled by fluvial sediments. This depression corresponds to a former course of the Rı´o Chubut, which today runs a few km to the southeast. This ancient channel is filled with unconsolidated sands and gravels (the Morgan gravel; Lapido and Page, 1979) with an estimated thickness of 2.5 m and which has been carved into a wide tableland with older gravels. Lateral leve´es, meander landforms and coalescent channels are still visible (Page, 1987). Their components are essentially acid volcanic rocks, of varied color, poorly consolidated, partly cemented with calcium carbonate and crudely stratified. A Pleistocene age in a wide sense has been proposed, while the areal extent is only local. Their origin would have been linked to a modification of the Rı´o Chubut main channel, due to a change in base level (Lapido, 1981).

4.5. Lower Rı´o Chubut Valley Terraces (Chubut Province) In the lower Rı´o Chubut valley, between the Florentino Ameghino Dam and the Atlantic Ocean, four fluvial terraces have been described. The highest and oldest one is overlying the Meseta de Montemayor gravels (see Section on ‘‘Piedmont Deposits’’). Their elevation varies between 210 and 190 m a.s.l. and the thickness of their deposits has been estimated at 6 m (Lapido, 1981). At lower elevations, there are three more terrace levels, with altitudes varying between 190 and 150 m a.s.l. for the highest landform, between 150 and 100 m a.s.l. for the middle landform and between 100 and 85 m a.s.l. for the younger landform, the sedimentary cover of the latter being only 1 m thick. This lower terrace correlates with a level described by Franchi (1983) at the mouth of this stream. The composition of these deposits is very similar, mostly volcanic pebbles between 2 and 8 cm in diameter, corresponding to the underlying Marifil Formation, with outcrops in the neighboring area, west and southward. All these terraces are of the cyclic type and were interpreted as the result of regional uplift of the Rı´o Chubut Basin during the Quaternary (Lapido, 1981).

4.6. Fluvial Terraces of the Mayo and Senguerr Rivers (Chubut Province) The valleys of the Mayo and Senguerr rivers, located in southwestern Chubut Province and running between gravel covered, volcanic and structural ‘‘mesetas’’, show

4.7. Fluvial Terraces of the Chico and Santa Cruz Rivers (Central Eastern Santa Cruz Province) In the lower reaches of the Chico and Santa Cruz rivers, streams which discharge in the same estuary (which has been named as the ‘‘Rı´a de Santa Cruz’’, as a comparison with similar features in Galicia, northwestern Spain) in the Atlantic Ocean (Fig. 1), a minimum of five terraces have been identified (Panza and Yrigoyen, 1994). The highest ones (I and II), located at mean elevations of 75–100 m a.s.l., are of Late Pleistocene age, connecting westward to a large alluvial cone, perhaps of glaciofluvial origin, which appears at the scale of satellite imagery. They are composed of poorly sorted, sandy matrix conglomerates, with a thickness of less than 4 m. The III, IV and V terraces are assigned to the Holocene and developed between 60 and 5 m a.s.l. The lithology of their deposits is very similar to that of terraces I and II and the thickness of each is around 2.5 m. The accumulation of these fluvial, and perhaps also glaciofluvial deposits and the carving of the terraces were correlated by Panza and Yrigoyen (1994) and Panza (1994a) to landscape rejuvenation during the Fourth Andean Movements.

5. Glaciofluvial Deposits The Patagonian glaciofluvial units correspond to the remains on the ancient proglacial plains, both in their proximal and distal facies, composed of sand and gravels of varied size and composition. They have been transported eastward from the eastern slope of the Andean Cordillera by recurrent glacial meltwaters at least since the Late Miocene to the Late Pleistocene. Their sedimentological characteristics, particularly in their distal facies, do not usually allow the differentiation between these deposits and those of fluvial, nonglacial origin, considered in other sections of this chapter. Thus, only those units to which a reliable glacial genesis has been assigned are included in this paragraph, generally based upon the utilization of lithological and geomorphological criteria. The glaciofluvial deposits are more widespread in southern Patagonia, in Santa Cruz and Tierra del Fuego provinces, due to the larger size of the Patagonian ice sheet in these regions during the successive Cenozoic glaciations. There, the glaciofluvial deposits extend into the present Atlantic Ocean submarine platform (in Tierra del Fuego at least up to 65 W) and some of them would correlate with the oldest known drifts, of latest Miocene– earliest Pliocene age. Distal facies of glaciofluvial deposits of Late Miocene age (older than 6 Ma) have been

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indicated by Schlieder (1989) and more recently by Rabassa et al. (2005), in central western Neuque´n Province. These deposits have been labeled the Alicura´ Formation (Dessanti, 1972) or the Alicura´ Member of the Collo´n Cura Formation (Gonza´lez Dı´az et al., 1986) and assigned by these authors to a piedmont genesis. In all cases, the outer boundaries of the glaciofluvial units in northern Patagonia are located closer to the Andean Cordillera axis than their coeval deposits at higher latitudes. The allochtonous character (extra-regional) of these deposits in the extra-Andean environments is exposed by the lithology of their components. Although volcanic rocks are dominant, plutonic pebbles and cobbles, coming from the inner sections of the Patagonian Andes, are common (Gonza´lez Dı´az and Andrada de Palomera, 1996), a feature that, together with a relative lower clast roundness index (Fidalgo and Riggi, 1970), may contribute to differentiate them from the piedmont units. These criteria are generally more pertinent when those facies closer to the source area in the Andean Cordillera are considered. The surface of these gravel mantles present a smooth but definite eastward slope, because the Atlantic Ocean acted as the regional base level, though with significant variations due to glacioeustatic changes. However, some of these accumulations would have been related to local base levels, particularly those which did not extend beyond the Andean front. Besides, modifications in the regional slope may have been introduced by seismotectonic, glacioisostatic or epirogenetic adjustments, particularly when considering the oldest units. The thickness of these deposits is larger at the proximal facies and usually very small in the distal portions, particularly in the extraAndean and coastal outcrops. In Santa Cruz Province, these units present significant gravel levels which grade eastward into entrenched fluvial terraces along the principal streams of the region, thus reaching the Atlantic Ocean coastal area. Applying the aforementioned criteria and the present state of knowledge, the following glaciofluvial units are described, which have been grouped following a geographical ordering:

5.1. Glaciofluvial Plains of Southwestern Rı´o Negro and Northwestern Chubut Provinces (41–44 S; Sector 1 in Fig. 1) Rabassa and Evenson (1996) confirmed the existence of at least five Pleistocene glaciations at 41 S (San Carlos de Bariloche, Rı´o Negro Province), mostly based on the work of Schlieder (1989). Farther south, between 43 and 44 S (Chubut Province), up to 5 or 6 full-glacial cycles and at least 11 stadials or moments of second order ice stabilization have been identified (Martı´nez, 2002, 2005). Each of these episodes implied the development of glaciofluvial plains that covered the bottom of practically all valleys connected to the eastern Cordilleran slope, extending several tens of kilometers eastward. These plains merge laterally or are superposed, which makes it difficult to correlate between different valleys or between regions. They are directly connected to their corresponding moraines. The oldest glaciations identified in the region [the Great Patagonian Glaciation (GPG) and

equivalent units; Mercer, 1976; Ton-That et al., 1994] would have been developed over a landscape of smoother topography than today, probably extending as relatively thin piedmont ice lobes (Rabassa et al., 1990; Rabassa, 1999). The regional relief would have been dramatically modified following what Rabassa and Clapperton (1990) called the ‘‘canyon cutting event’’. This over-deepening of the valleys controlled the nature of later glaciations, when the Andean glaciers advanced eastward as outlet glaciers. Most, if not all, remains of proglacial plains in these latitudes are the product of the advance, stabilization and decay of more modern glaciers. Three unit subgroups, corresponding to different areas, ordered from north to south, are described here. Probably, these units are only partially representing the entire set of formations of the same origin developed along these latitudes. The Upper Rı´o Chubut Basin (41–42300 S) In the extra-Andean belt, close to the Andean Cordillera of Rı´o Negro and Chubut provinces and largely following the Rı´o Chubut valley, relicts of five subhorizontal gravel levels are located at successive lower elevations, corresponding to the following formations: Martı´n, Blancura, Fita Michi, Caban˜a and Confluencia (Fig. 2), ordered according to their decreasing age. All these units are dipping eastward, usually less than 1%. The upper surfaces of these stratigraphic units (locally called ‘‘pampas’’) have been interpreted as the product of piedmont fluvial accumulation (Volkheimer, 1963, 1964, 1965a, b; Volkheimer and Lage, 1981; Lage, 1982), which would have taken place between the Late Pliocene and the Middle Pleistocene. However, more recent studies (Gonza´lez Dı´az, 1993a, b; Gonza´lez Dı´az and Andrada de Palomera, 1996) assigned them, with the exception of the Martı´n Formation, a glaciofluvial origin and considered them of Pleistocene age only. The Martı´n Formation is located at higher altitudes, above 1000 m a.s.l. at least in its proximal facies, and it is therefore considered as the oldest of the series. Its original morphology very frequently appears modified by fluvial and mass-movement activity (Fig. 2). This formation is correlated with other units in the region to which several authors have also assigned a piedmont genesis (Gonza´lez Bonorino, 1944; Flint and Fidalgo, 1968; Rabassa, 1975; Coira, 1979; Turner, 1982; Fidalgo and Rabassa, 1984), and thus it has been considered in the corresponding section. The remaining levels extend stepwise downwards and eastward, separated by erosive scarps which may be tens of meters high (Fig. 2). They reach their ˜ orquinco best surface development in the localities of N (southern Rı´o Negro Province) and Cushamen and Gualjaina (northwestern Chubut Province), forming the gravelly ‘‘pampas’’ that characterize this Patagonian region. Some of these levels are morphologically and altitudinally connected to moraines (Gonza´lez Dı´az, 1993a, b; Gonza´lez Dı´az and Andrada de Palomera, 1996). The Blancura Formation (originally defined by Volkheimer, 1963) is connected to the till hills corresponding to the ‘‘Initioglacial’’ phase of Caldenius (1932) which, according to Martı´nez (2002), would represent the GPG (Mercer, 1976). The deposits of this formation are located around 100 m

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Fig. 2. Location of the gravel units in the upper Rı´o Chubut Basin. White full lines are the piedmont units (1000 m a.s.l.); white dashed lines and white dotted lines are the younger glaciofluvial units. Black lines are the morainic deposits correlated to the Greatest Patagonian Glaciation.

below the Martı´n Formation gravels, between 700 and 980 m a.s.l., depending on the distance separating them from the source area. The following glaciation, labeled ‘‘Daniglacial’’ by Caldenius (1932) and ‘‘Post-GPG 1’’ by Coronato et al. (1999) would have generated the gravel beds of the Fita Michi Formation (Volkheimer (1963). This level extends several tens of meters below the precedent Blancura Formation. The younger Caban˜a and Confluencia formations are forming extensive ‘‘pampas’’, although it is frequent to observe them entrenched in the outcrops of the aforementioned older units or as terraces in the present fluvial valleys (Fig. 2). Although the physical, spatial connection between these units and the corresponding ancient

glacial fronts is not exposed, it should not be ruled out that the Confluencia Formation would correlate with the landforms and deposits of the Last Glaciation (Late Pleistocene) and more precisely, the Last Glacial Maximum (LGM) (ca. 24 ka B.P.). This seems to have been confirmed by mapping (Gonza´lez Dı´az, 1993c), where a spatial correlation between the Caban˜a and Confluencia formations and the glaciofluvial beds west of Cordo´n El Maite´n (Fig. 3) and corresponding to the last glaciation of the region has been observed (Miro´, 1967). All these deposits are composed, basically, of medium to coarse gravels in a coarse sand matrix, with minimum silt content and practically no clay (Fig. 4). The lithology

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Fig. 3. Extent of the glaciofluvial gravel deposits located on the western side of the Rı´o Chubut. They were deposited during the two last glaciations that have occurred in the region (full white lines). The present floodplain is pointed out in dotted lines.

of the gravels indicates its allochtonous (glacial) or local (piedmont) nature. According to Lage (1982), volcanic rocks (andesites and basalts) are dominant, derived from the neighboring outcrops corresponding to the Paleocene– Eocene Ventana Formation, whereas granitic rocks are practically absent. Gonza´lez Dı´az and Andrada de Palomera (1996) have noted the abundance of grayish, hornblende-biotite granites, which have no local outcrops, thus being considered as of an allochtonous nature, justifying a glacial origin. The presence of these granitic rocks would thus be indicating a plutonic provenance from the ‘‘farther Cordillera’’ (as stated by Gonza´lez Dı´az, 1993a). The outcrops of the Patagonian batholith at these latitudes (41–43 S) are located 70 km westward, a distance large enough so as to rule out a piedmont origin for some of the formations under consideration. The application of lithological criteria seems to clearly favor a glacial origin, even when the large compositional variation of the Patagonian batholith at these latitudes (Lizuaı´n, 1983; Gordon and Ort, 1993; Haller, 2002) and the existence of smaller outcrops of older granitic rocks in the sub-Andean belt (Aleusco Formation, Turner, 1982) are considered. Accepting a glacial origin for these gravel units seems to

be thus confirmed, on the basis of their lithology and the close spatial relations between several of these units and the neighboring moraines.

The Pre-Andean Esquel Valley (43 S) The bottom of the wide valley developed at 43 S on the eastern edge of the northern Patagonian Andes, known as the ‘‘pre-Andean Esquel valley’’, is characterized by many levels of terraced, glaciofluvial gravels, which extend from the moraines located in the west (Fig. 5). The more modern proglacial levels show a significant lateral continuity, have a maximum elevation close to 800 m a.s.l. in contact with the moraines and descend down to 650 m a.s.l. when they reach the Rı´o Tecka valley, 45 km to the east, defining a mean slope of 0.35%. In Fig. 3, the terminal area of an ice lobe which advanced from the south (the Laguna Su´nica moraines) significantly contributed to the accumulation of these materials. Northward, a marked connection exists between these units and the ‘‘pampas’’ of the same origin already described for the Rı´o Chubut upper basin. The

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gravels vary greatly both in size and shape according to their proximity to the related moraines. The deposits are well stratified, with sometimes a good grainsize sorting, with dominating gravels and sands. The more common colors are light brown and ochre. The andesitic (breccias, lavas and porphyritic varieties) pebbles and cobbles are predominant although basaltic and acid volcanic rocks are also important. The percentage of granite rocks may be important in relation to the age of the sediments, sometimes exceeding 20% (Martı´nez, 2002). The sequence represents an important number of glaciations including at least the last five that occurred in the region (Martı´nez, 2002, 2005). The correlation of these units with the glacial sequence established at 41 S in San Carlos de Bariloche (Rabassa and Evenson, 1996) and with that at Lago Buenos Aires, at latitude 46 S (Ton-That et al., 1999) have allowed to suggest (Martı´nez, 2005) that the period in which the moraines and their corresponding proglacial plains were formed is posterior to the largest fluvial incision found in the region. This happened after the GPG, thus temporally limiting the sequence between the Middle and Late Pleistocene (1.0 Ma to 10 ka BP). Heads of the Rı´o Tecka (43300 S) Fig. 4. Glaciofluvial gravel outcrop eroded on the Confluencia Formation, southwest of Chubut Province.

In the valley that joins the towns of Corcovado and Tecka, very close to the heads of the Rı´o Tecka, moraines corresponding to almost all those identified farther north, at Esquel, Gualjaina and Cushamen, are found. This NW–SE depression merges eastward with the N–S

Fig. 5. Gravel mantles widely extended over the Rı´o Esquel pre-Andean valley. The rounded gravels were deposited by glaciofluvial processes during the last four glaciations that have taken place in the region. Dotted lines indicate morainic deposits and full white lines indicate outwash plains.

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trending Rı´o Tecka valley, which was one of the main water discharge routes toward the Atlantic Ocean during the last glaciations of the region (Caldenius, 1932). For this reason and in spite of intense later fluvial action, extensive, terraced glaciofluvial deposits, clearly correlated to terminal moraines are preserved at the bottom of these elongated depressions. The stratigraphy for this locality was proposed by Lapido et al. (1989) who, although they mapped the main glacigenic units (moraines and proglacial plains), also noted the presence of a gravel sheet on the higher parts of the neighboring hills and dipping eastward on the western slope of the Rı´o Tecka valley. These authors assigned a piedmont origin to these gravels, having been deposited during an interglacial period preceding the valley glaciations. This proposal seems to contradict the scheme suggested by Turner (1982), who assumed that all terraced levels in this valley were of glacial origin. These are, at least spatially, coincident with the unit cited by Lapido et al. (1989). At present, a better knowledge of the shape, extension and behavior of the glaciers of this zone suggests that the Rı´o Tecka valley was not glaciated during the Pleistocene, thus suggesting a piedmont origin for the aforementioned levels.

5.2. Glaciofluvial Plains of Southwestern Chubut and Northwestern Santa Cruz Provinces (4430–47 S; Sector 2 in Fig. 1) Large surfaces covered by glaciofluvial deposits appear south of 44 S in Chubut Province and beyond 47 S in Santa Cruz Province. In this area but also farther south, the glaciers extended beyond the Andes, even during the last glaciations, as extensive, large glaciers, sometimes as real piedmont glacier lobes. The Plio–Pleistocene glacial system is summarized in a series of lobes that, from north

to south, have been named (Dal Molin and Gonza´lez Dı´az, 2002): (i) El Coyte, (ii) Rı´o Mayo and (iii) Lago Blanco (Fig. 6). Additionally (iv), a fourth large lobe, already in Santa Cruz Province, advanced in the present Lago Buenos Aires Basin (Fig. 7). This lobe built one of the more complete and older Patagonian glacial sequences (Ton-That et al., 1999). In the southernmost area, the extra-Andean sector close to the mountain is characterized by an open landscape dominated by tablelands of volcanic origin (Meseta Lago Buenos Aires, Meseta Cuadrada, etc.). Only at 100 km eastward, the volcano-sedimentary basement outcrops at Sierra de San Bernardo. This favored the development of wide and continuous proglacial plains, represented by the Meseta del Rı´o Senguerr and the ‘‘pampas’’ of Guenguel, Loma Redonda and Loma Cuadrada (Figs 6 and 7). These units extend eastward along the valleys of the main streams, thus reaching the proximities of the Atlantic coast. These units have been described and temporally ordered by Panza (2002). This author identified five to six terraces of gravels and sandy gravels that follow the Senguerr and Deseado river valleys and their tributaries (Can˜ado´n Salado and Can˜ado´n El Pluma, among others), correlating the older ones (‘‘Nivel I’’, ‘‘Nivel II’’ and ‘‘Nivel III’’) with the latest Miocene glaciations or, at least, those of the Early to Late Pliocene. The youngest ones (known as ‘‘Nivel IV’’, ‘‘Nivel V’’ and ‘‘Nivel VI’’) would be assigned a Late Pliocene–Pleistocene age. Farther north, in Chubut Province, some of these gravel sheets are equivalent to the levels defined as ‘‘II’’ and ‘‘III’’ by Gonza´lez (1978) and with the deposits recently identified by Dal Molin and Gonza´lez Dı´az (2002). In general terms, the gravels are composed of volcanic and pyroclastic rocks and a few granitic pebbles, usually rounded to subrounded, with a diameter from 1 to 5 cm (Panza, 2002). Thickness is variable, between 10–15 m.

Fig. 6. Maximum ice-lobes extent (dashed black lines) belonging to the Last Glaciation and glaciofluvial gravel deposits location (dashed white lines) along the border between Santa Cruz and Chubut provinces.

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Fig. 7. Gravel plains (‘‘pampas’’) remains in the Lago Buenos Aires surroundings (northwest of Santa Cruz Province). Black dashed lines indicate the terminal position of the glacial lobes and white dashed lines indicate the Meseta Lago Buenos Aires tableland limits.

6. Piedmont Deposits Piedmont deposits have a large distribution in northern Patagonia, in the provinces of Neuque´n and Rı´o Negro, although important surfaces and thicknesses exist also in Chubut and Santa Cruz. Different landforms are included in this category, such as alluvial fans, bajadas, pediments and pediment and strath terrace covers, of variable extent and that cannot be correlated with the former glaciers. This group of units would encompass an extended period, between the Miocene and, probable, the Early Holocene. According to Schellman et al. (2000), the dispersion of the ‘‘Rodados Patago´nicos’’, considered by these authors as only those of strict piedmont nature, would have terminated toward the Late Miocene–Early Pliocene, before the epeirogenic uplift of extra-Andean Patagonia and after the deposition of the Santa Cruz Formation (Middle Miocene). According to Marshall et al. (1986) and Marshall and Salinas (1990), the upper section of the Santa Cruz Formation has been dated around 15 Ma (see Chapter 4). The incision of the present valleys would be the result of intense erosion generated by base-level changes after such uplift. Later, after 2.5 Ma, deepening and widening of these valleys started as a product of frequent glacial events. This is the reason why relicts of some of the oldest units are preserved near the Andes, as high remnants, usually above 1000 m a.s.l. These belong to much more extensive surfaces whose distal facies would correspond to some of the terraced levels in extra-Andean Patagonia and also with other accumulations near the Atlantic coast. Besides, more recently, similar landforms were developed along the flanks of the Sierras or in the large endorheic basins that characterize the eastern-central portion of Patagonia, acting as local base levels.

It is assumed that the construction of the extraregional piedmont surfaces, those that would have extended from the Andean slopes toward the easternmost extraAndean sectors, required a very long tectonically stable period as well as relatively dry and uniform climatic conditions, but which would imply enough seasonal precipitation to allow fluvial transport. During glaciations, the existence of ice masses in the Patagonian Andes and the acting climatic conditions (reduced precipitation and lower temperatures) would have limited the development of piedmont deposits. These units would have been basically developed during the interglacial periods, or as it has been stated above, before the first Cenozoic Patagonian glaciation, in the latest Miocene (Mercer, 1976), when the absence of large glaciers and the predominant pluvial precipitation favored fluvial erosion and transport from the Andean front. Likewise, piedmont deposits like those found in pediments, bajadas and alluvial fans, of local or regional extent, originated in the extra-Andean sierras but were limited in their expansion to the flanks of the mountains. These morphological features are very well exposed along the margins of the Meseta de Somuncura´ and Sierra Grande in Rı´o Negro Province (Fidalgo and Riggi, 1970), the Can˜ado´n Salado and Guenguel pediments (Fidalgo and Riggi, 1965) in Santa Cruz Province, and the Rı´o Tecka valley terraces in the Precordillera of Chubut Province. The smaller extension of these landforms and their adjustment to local base levels associate them to slopes steeper than those of extraregional extent, which can be a useful criterion in their classification. Besides, it is expected that, in the same manner as for the glaciofluvial deposits, those units generated in the Andean Cordillera are composed of Andean lithological types.

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According to the criteria listed above and the available descriptions from other authors, the following groups of representative piedmont units have been defined:

6.1. Extra-Andean Gravels of Neuque´n, Rı´o Negro and Northern Chubut Provinces (Sector 3 in Fig. 1) The units included in this group are not necessarily coeval although they form the oldest group of all considered here. They are located immediately east of the Patagonian Andes, between 38 S in southern Neuque´n Province and 44 S in Chubut Province. Their easternmost relicts are found 200–250 km out in the extra-Andean sectors. They have been considered as remnants of extensive Cordilleran bajadas developed during a period close to the Plio– Pleistocene boundary, although some of the outcrops herein mentioned, those corresponding to the Martı´n Formation (southwestern Rı´o Negro and northwestern Chubut provinces) have been genetically related to ancient glacier fronts located a few kilometer westward (Gonza´lez Dı´az and Andrada de Palomera, 1996). They are composed of various levels of superficially non cemented or partially cemented gravels (Fidalgo and Rabassa, 1984), with maximum diameter around 10–12 cm and massive to crudely stratified. The mantle-like aspect of these units is usually strongly modified by fluvial and mass-movement action; there are very few sites where their original morphology may be fully appreciated (Fig. 2). The thickness of these deposits varies between 70 and 50 m in the proximal area [Leanza and Hugo (2005) mentioned more than 100 m in thickness in Neuque´n Province] and 10–5 m in the distal facies. These beds are different from the other piedmont units mentioned in this chapter because of their topographic location. They have been preserved above 1000 m a.s.l., being this the main reason for assigning them a very ancient age. In Neuque´n Province, at this latitude, granite, gneiss and porphyritic rocks are present (Leanza and Hugo, 2005), which correspond to an Andean lithological composition, whereas correlating units to the south are essentially composed of volcanic rocks, such as andesites, dacites and rhyolites, of grayish to purple colors. The latter come from Early Tertiary units (the Ventana Formation and equivalent entities; Gonza´lez Bonorino, 1973) that characterize the eastern flank of the Patagonian Andes. In these areas, the basaltic cobbles and pebbles are less frequent; however, very scarce granite, schist and gneiss boulders and cobbles have been mentioned (Ravazzoli and Sesana, 1977). The accumulation of these deposits would have been coincident with Cordilleran uplift sometime between the Late Pliocene and the Early Pleistocene. The formational units included in this group, as it has been stated, were certainly not formed during one single period. Regional correlation has been proposed by Fidalgo and Rabassa (1984), including (a) the ‘‘Primer Nivel de Pie de Monte’’ (i.e. ‘‘First Piedmont Level’’, Gonza´lez Bonorino, 1944) in southern Rı´o Negro Province, (b) the ‘‘Gravas Pedemontanas’’ (i.e. ‘‘Piedmont Gravels’’, Flint and Fidalgo, 1968) at 43 S, (c) the Martı´n Formation (Volkheimer, 1964) in the region of Cushamen in Chubut Province, (d) the reduced gravel outcrops named Huayqui Formation by Turner (1982) and found at the summit of Sierra de

Tecka, while presenting important similarities with those of the Martı´n Formation in neighboring areas; (e) the Jacobacci Formation in Rı´o Negro Province (Coira, 1979) and the Pampa Encima Formation (Leanza and Hugo, 2005) in central Neuque´n Province. Although the Alicura Formation (Nullo, 1979) outcropping in extra-Andean Neuque´n (renamed the Alicura Member of the Caleufu´ Formation by Gonza´lez Dı´az and Ferrer, 1986) had been considered as a possible equivalent of the aforementioned piedmont units (Fidalgo and Rabassa, 1984), this unit has been more recently considered to be of a glaciofluvial nature and Late Miocene in age (Schlieder, 1989; Rabassa et al., 2005).

6.2. Northeastern Chubut Province Gravels These gravels are a group of beds of regional and local extension, not always coeval but probably limited to Late Pliocene–Late Pleistocene times. They form alluvial fans, bajadas and, partially, alluvial plains. They are essentially sandy gravels, massive, crudely stratified or cross-bedded. The presence of basaltic clasts is characteristic, related to the lithological types of Meseta de Somuncura´, located immediately westward. The thickness of these deposits varies between 2 and 10 m (Corte´s, 1987). The stratigraphic units included in this group are the El Porvenir Formation (Corte´s, 1981), the Ranquil Huau Conglomerate (Page, 1987) and the Eizaguirre Formation (Corte´s, 1981). All these units can be correlated on the base of field relations and sedimentological characteristics with the Morgan gravels, which outcrop in the region but are most likely related to fluvial valley deposits (Lapido and Page, 1979).

6.3. Gravel-covered Strath Terraces of Eastern Chubut and Northeastern Santa Cruz Provinces (Sector 4 in Fig. 1) The strath terraces, together with those of a volcanic nature (the latter are not considered in this contribution), are some of the most characteristic landforms of Patagonia. The strath terraces are generally subhorizontal surfaces, elevated with respect to the valley floor and the neighboring depressions, and they have a variable though constant gravel cover. They have a large extent not only in Rı´o Negro Province but also along the eastern sector of Chubut and northeastern and central Santa Cruz provinces. In this paragraph, those extending along the coastal area of Chubut and northern Santa Cruz provinces, approximately between 42 and 46300 S, are considered because of their marked continuity. These units include (a) the surfaces north of the Rı´o Chubut valley (extending into Penı´nsula Valde´s as well), (b) those forming the Meseta de Montemayor (between the Rı´o Chubut valley and Bahı´a Camarones), (c) Pampa de Salamanca (Fig. 8) and (d) Pampa del Castillo (Fig. 8). A total of three different levels have been proposed by Lizuaı´n et al. (1995) on the Chubut Province geological sheet, a number that should be considered a minimum. Lapido (1981) analyzed the deposits north of Rı´o Chubut, which may correspond to those located along the right margin of the Meseta de Montemayor

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Fig. 8. Main landforms and deposits located in northeastern Chubut Province. The tablelands (‘‘pampas’’) covered by gravel mantles are the main feature here, characterizing the flat landscape of Patagonia. (Montemayor Formation; Lizuaı´n et al., 1995) and south of the cited stream. This author described the sediments as open-work gravels, composed of cobbles and pebbles with a dominant size between 3 and 10 cm, with a larger frequency of rounded and egg-shaped subspherical clasts. From a compositional point of view, they are mainly related to the significant outcrops of Jurassic acid volcanic rocks (the Marifil Formation), which are located immediately west of the terraces considered here. According to this same author, the unit varies between 1.5 and 9 m in thickness, with a mean of 6 m. It has a coarse to fine sand matrix and the carbonate cement is whitish (Fig. 9), which is easily weathered and enables the headwater erosion of the ‘‘meseta’’ front. The materials forming the more distal facies are found east and northward, near Puerto Madryn and all over the surface of Penı´nsula Valde´s. Haller (1981) described them as having up to 6 m in thickness and clast size ranging between 3 and 5 cm. According to Fidalgo and Riggi (1965), these deposits are forming true pediments, formed around the Plio–Pleistocene boundary. The depositional conditions represent an arid to semiarid climate, assuming a very cold (periglacial) climate in the previous period and, perhaps, during the accumulation of these gravels. Evidence would allow the inference (according to Page, 1987) that aggradation took place due to laminar flow produced by fast flood sheets, probably favored by large water volumes released during interglacial periods. Suggesting a dominantly fluvial origin, Haller (1979) identified

paleochannels in the northern plains, which formed a fluvial network draining the hills located toward the northeast. An observation that highlights the importance of fluvial processes as depositional phenomena in these regions has been provided by Gonza´lez Dı´az and Tejedo (2002) in their analysis of the strath plains forming the del Castillo, Pelada and Vaca ‘‘pampas’’ (Fig. 8) in southeastern Chubut Province (see Section on ‘‘Final Comments’’). These authors

Fig. 9. Rounded gravel deposit of the ‘‘Pampa del Castillo’’ strath terrace located in southeastern Chubut Province. Note the calcium carbonate precipitation on the gravels.

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concluded that the existence of the gravel sheet intensively cemented by CaCO3 and covering friable, Tertiary sedimentary rocks, enabled the genesis as well as the preservation of these landforms. From a stratigraphical point of view, the units included in this group are the Dos Naciones Formation (Corte´s, 1981) and the Montemayor Formation. (Lizuaı´n et al., 1995) (Fig. 8).

6.4. Gravels of Central and Eastern Santa Cruz Province (Sector 5 in Fig. 1) The four geological formations considered in this group form a set of sandy gravel terrace levels dipping toward the Atlantic Ocean, near the mouth of the Chico and Santa Cruz rivers (approximately between 48 and 51 S) and which are successively younger as their elevation decreases. These are the La Avenida Formation (Marı´n, 1982; Panza, 2002), Cordo´n Alto Formation (Panza and de Barrio, 1987), Pampa de la Compan˜´ıa Formation (Panza and de Barrio, 1987) and Mata Grande Formation (Panza and Yrigoyen, 1994). The Cordo´n Alto Formation is the highest and thus the oldest; it may correspond to the distal deposits of the uppermost level of piedmont aggradation, formed by coalescent alluvial fans after the main phase of the Miocene Patagonian Cordillera uplift. Their outcrops close to the Atlantic coast have thicknesses varying between 2 and 10 m, and they show apparent physical

continuity with some Cordilleran levels (La Ensenada Formation, near Lago Cardiel; Ramos, 1982). This formation has been assigned a Late Miocene age. The level corresponding to the Pampa de la Compan˜´ıa Formation is located a few meters below the preceding unit, presents a steeper slope and has a thickness between 3 and 5 m. From the geomorphological point of view, it would correspond with a flanking pediment although its distribution in elongated fringes does not allow ruling out a sedimentary origin in fluvial valleys. An Early Pliocene age has been tentatively assigned to this unit. The extensive levels corresponding to the Mata Grande and La Avenida formations occupy ever lower topographical positions and maintain an eastward gradient. The first of these units is assigned to the Late Pliocene–Early Pleistocene and the second is considered to be of Middle to Late Pleistocene age. Both of them would correspond to flanking pediments (Fig. 10).

6.5. Other Piedmont Gravel Units There are several formational names proposed by different authors to define piedmont deposits of small extent, usually forming bajadas and flanking pediments. Due to their nature, there are great difficulties correlating them with larger units. A good example of this is the Choiquepal Formation (Volkheimer, 1963), widely distributed in the extra-Andean sector of southwestern Rı´o Negro and

Fig. 10. Gravel formations, apparently of piedmont origin, located in eastern Santa Cruz Province. The unit limits are drawn in dashed white lines, meanwhile the Cordo´n Alto Formation, probably of Miocene age, is drawn in white dotted line.

Late Cenozoic Fluvial Deposits of Argentine Patagonia northwestern Chubut provinces, which corresponds to a set of bajadas of local extent and composed of gravel, sands and silts with boulders in the proximal sectors. In the most distal parts, this unit has a mean grainsize of 5–10 cm. The maximum thickness has been estimated as 20 m. Clast lithology is exclusively of nearby provenance. Thus, in some areas granitic cobbles are dominant (associated to the locally outcropping Paleozoic plutonic rocks of the Lipetre´n Formation) whereas Mesozoic and Tertiary volcanic rocks, also very abundant in the region, prevail in others. The age of these deposits, assigned by different authors (Ravazzoli and Sesana, 1977; Proserpio, 1978; Nullo, 1979; Coira, 1979; Volkheimer and Lage, 1981; Lage, 1982) for various localities within the area, is restricted in all cases to the Middle to Late Pleistocene (Fig. 2). In positions closer to the Andean Cordillera, in Chubut Province, Ploszkiewicz (1987) gave the name of Rı´o Senguerr Formation to gravel deposits forming the highest plain along the right margin of Rı´o Senguerr (45 S), which would be representing the distal facies of a larger unit of Pliocene–Early Pleistocene age, which he named as ‘‘Depo´sitos Pedemontanos’’. In southeastern Rı´o Negro Province, covering the flanking terraces of the large endorheic depressions (‘‘bajos sin salida’’) that characterize this region, poorly consolidated, crudely stratified, with a sandy-silty matrix, polymictic conglomerates and agglomerates appear, which have been named by Sepu´lveda (1983) as the Indio Muerto Formation. These deposits show gentle slopes, as in flanking pediments, with thicknesses that do not exceed 6 m and being assigned a Holocene age.

7. Final Remarks The study of the Patagonian fluvial deposits comprises the analysis of a very extensive group of units that have historically been known as ‘‘Rodados Patago´nicos’’ or ‘‘Patagonian gravels’’. In agreement with Panza (2002), the present authors suggest to abandon, whenever possible, the use of this denomination, since it applies to neither a unique genetic process, nor a specific formation period and in a sense, from the sedimentological and petrological point of view, it does not group homogeneous materials other than all of them being gravels. Moreover, there are also accumulations cited as ‘‘Rodados Patago´nicos’’ where fluvial processes did not play a relevant role. That is the case of the deposits that cover some structural terraces, which according to different authors, are the product of physical weathering (and scarce or no transport) of the sedimentary bedrock. These landforms would correspond to denudation surfaces in the sense of Gonza´lez Dı´az and Malagnino (1984), Gonza´lez Dı´az and Ferrer (1986) and Fauque´ (1996). As examples of the latter, the pebble and cobble sheets that partially cover the Pampa de Chalı´a, in southwestern Chubut Province (Gonza´lez, 1978), the cemented gravel sheet of the del Castillo, Pelada and Vaca ‘‘pampas’’ (Fig. 8) in the southeastern region of this same province (Gonza´lez Dı´az and Tejedo, 2002) and the ‘‘Dorso de los Chihuidos’’ morphostructural unit in Neuque´n Province (Leanza and Hugo, 2005) can be cited. Thus, the ‘‘Rodados Patago´nicos’’ can be classified

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as ‘‘fluvial’’ and ‘‘non fluvial’’ gravels, which clearly shows the confusion and ambiguity implied by this denomination. Sometimes, the sedimentary bedrock enabled the development of pediments (Fidalgo and Riggi, 1970), landforms that even though they are present in different Patagonian regions (as flanking pediments of local nature in extra-Andean sectors and the Atlantic coast area), the higher remnants tend to concentrate and have a larger development in the Cordilleran region. These erosion and transport surfaces may have an alluvial–colluvial cover (Gonza´lez Dı´az and Ferrer, 1986) and they have been considered as the oldest gravel units (Fidalgo and Riggi, 1970; Schellman et al., 2000). According to Ton-That et al. (1999), piedmont plains partly confined by Tertiary lava flows developed in the Lago Buenos Aires region (46 S) in Late Miocene times, between 7.4 and 5 Ma. Surfaces of piedmont aggradation such as bajadas are landforms terraced by fluvial or glaciofluvial erosion, which by their nature may have the larger gravel thickness. Another group of fluvial accumulation landforms are the pediment gravels of Fidalgo and Riggi (1965, 1970), generated by the sedimentation in bajada environments, possibly over older pediment surfaces. These landforms are of special interest because they would indicate reactivation with a relative lowering of the ancient pediment surfaces, then aggradation and finally tectonic uplift and/or incision produced by younger fluvial or glaciofluvial streams. Thus, it is clear that, at least in some areas of Patagonia, geology has been favorable for the development and preservation of these piedmont nature deposits. These important outcrops of subhorizontal, soft and erodable Tertiary sedimentary rocks (Fidalgo and Riggi, 1965, 1970) correspond totally or partially with the Huincul, Santa Cruz, Collo´n Cura´, Pedregoso, Rı´o Mayo, Chalı´a and Patagonia formations, among many others, and they have acted, at least since the Oligocene–Miocene, as a significant source of debris. However, the presence of a rocky substratum and convenient structures is not enough to explain the occurrence of most gravel units, especially if it is considered that many of them are of glaciofluvial origin. At least from the Miocene, close space-time relations developed between the Andean cycle of tectonic/epeirogenic adjustments, the glacioisostatic response and the profound climatic changes. Mercer (1976) established that the uplift of the Patagonian Cordillera during the Miocene, associated to climatic modifications (Rabassa et al., 2005), resulted in emplacement of the highest areas of the mountain ranges above the regional climatic snow line allowing glaciers to advance to extra-Andean positions already in those ancient times. Glaciation in Santa Cruz Province during the latest Miocene has been documented by absolute dating (Mercer, 1976; Ton-That et al., 1999) and gravel deposits, of possible glaciofluvial nature of equivalent age, have been identified much farther north in Neuque´n Province (Schlieder, 1989). According to Strelin et al. (1999), at least in southern Santa Cruz Province, the Early Miocene uplift corresponding to the ‘‘Pehu´enchica’’ Andean phase (Ramos and Ramos, 1979) would have been responsible for the glaciation that

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accumulated the ‘‘Proglacial Cerro Cuadrado’’ gravel beds. For Schellman et al. (2000), the dispersal of the ‘‘Rodados Patago´nicos’’, which they interpreted as of a strict piedmont nature, would have stopped around the Late Miocene–Early Pliocene, before the epeirogenic uplift of the extra-Andean areas. The incision resulting from this uplift put the valley floors tens of meters below these ancient surfaces, thus controlling glacial and glaciofluvial discharge that characterize this area during Pliocene and Quaternary times. Resulting from the Andean uplift, the aridification of eastern Patagonia would have started around 16.5 Ma (following Stern and Blisniuk, 2002) and would have become even more pronounced around 14 Ma (Blisniuk et al., 2003) when a new tectonic pulse increased the efficiency of the orographic effect on the incoming humid winds from the South Pacific Ocean (see Chapter 4). After this time glaciofluvial and piedmont sedimentation predominated over any other geomorphic process east of the Cordilleran ranges. Although the cited tectonic events (mostly coincident with the ‘‘Pehue´nchicos’’ and/or ‘‘Que´chuicos’’ movements; Ramos and Ramos, 1979) reactivated the fluvial processes along the entire Patagonian environments, these would have been strongly influenced during each of the glaciations. Moreover, these glaciofluvial gravels would have been superposed and interfingered with the local fluvial and piedmont deposits as they reached the extra-Andean sectors. Within this scenario, an aspect of most interest that even today is highly controversial is the great extension of these sediments. Even if the aforementioned ‘‘Rodados Patago´nicos’’ which were not generated by fluvial currents are left aside, those deposits generated by surface runoff form the largest geological unit in Patagonia. The gravel sediments that occur in sea cliffs carved along the Atlantic coast, usually more than 600 km east of the Andean Cordillera, containing cobbles of more than 1 decimeter in diameter, and which are undoubtedly non marine terraces, are a clear evidence that the distal facies of these landforms are extending below sea level, on the present submarine platform. Moreover, the highest gravel plain located between the present Rı´o Colorado and Rı´o Negro valleys in northern Patagonia and that starts in the Andean foothills and ends at the Atlantic Ocean cliffs more than 500 km away without relevant slope changes is perhaps one of the longest single accumulation landforms in the whole world. All efforts to explain the dispersal mechanisms of these gravels have failed because of the impossibility to identify any current geomorphic environment on the globe where such conditions are extant and thus, the processes capable of generating such accumulations. The very existence of these sediments and landforms implies a serious challenge to the principles of Actualism. So far it has also been impossible to establish the absolute age of these accumulations or the landforms associated with them. With the exception of the absolute dating obtained in glacial sequences and particularly on terminal moraine systems, which laterally grade into glaciofluvial units, the remaining non-glacigenic gravel accumulations are still lacking acceptable methods to establish their chronological location. Careful, extensive mapping projects, regional

geomorphological correlation, relative soil dating and Ar/39Ar dating of underlying and overlying lava flows would perhaps be the most appropriate tools to tackle this still unsolved scientific problem. The marked climatic, tectonic and geological differences existing throughout Patagonia do not encourage the use of a single genetic scheme for all these units. Evidence seems to indicate that it is necessary to develop specific models that would explain the origin and occurrence of these gravel sheets. These models must be formulated from a regional or ‘‘Patagonian’’ perspective, but addressing also the different local conditions for each latitude. An alternative methodology would be to increase the studies related to their petrology and sedimentology. It is possible that, in this manner, models of dispersal for different lithologies can be suggested so as to establish the morphodynamic features of these units and relate them to the corresponding source areas. Likewise and directly associated to these products, it would be necessary to elaborate paleogeomorphological schemes for certain periods and for different regions and elevations. Taking into account that a complete spatial, temporal and genetic reconstruction for this characteristic Patagonian detritic cover is still far from being achieved, new, creative and audacious approaches, incorporating the idea that essentially different processes and materials may generate very similar or even identical accumulations and landforms, should be put forward. In this sense, these units should be studied accepting that the acting phenomena and variables during the last half of the Neogene may have had distinctive scales and intensities in relation to preceding geological periods and even present times. More than 160 yrs after Charles Darwin’s observations, it is appropriate to put forward new ideas on this fascinating scientific problem. 40

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Late Cenozoic Fluvial Deposits of Argentine Patagonia Thesis, Universidad Nacional de la Patagonia-San Juan Bosco, Comodoro Rivadavia, Argentina. Martı´nez, O. (2005). Incisio´n fluvial y glaciaciones durante el Pleistoceno a los 43 l.s., noroeste de la provincia de Chubut. In: Cabalero, N., Cingolani, C.A., Linares, E., Lo´pez de Luchi, M.G., Ostera, H.A. and Panarello, H.O. (eds), XV Congreso Geolo´gico Argentino, Actas, CD-ROM edition. El Calafate, Argentina, 125, 1–7. Mercer, J.H. (1976). Glacial history of southernmost South America. Quaternary Research 6, 125–166. Mercerat, A. (1893). Contribuciones a la geologı´a de la Patagonia. Anales de la Sociedad Cientı´fica Argentina 36, 65–103. Buenos Aires. Miro´, R. (1967). Geologı´a glaciaria y preglaciaria del Valle de Epuye´n. Asociacio´n Geolo´gica Argentina, Revista 22, 3, 177–202. Buenos Aires. Nordenskjo¨ld, O. (1897). Algunos datos sobre la naturaleza de la regio´n Magalla´nica. Anales de la Sociedad Cientı´fica Argentina 44, 190–240. Buenos Aires. Nullo, F.E. (1979). Descripcio´n Geolo´gica de la Hoja 39c, Paso Flores. Provincia de Rı´o Negro. Servicio Geolo´gico Nacional, Boletı´n 88. Buenos Aires. Page, R. (1987). Descripcio´n Geolo´gica de la Hoja 43g, Bajo de la Tierra Colorada, Provincia de Chubut. Direccio´n Nacional de Minerı´a y Geologı´a, 81 pp. Buenos Aires. Panza, J.L. (1994a). Hoja Geolo´gica 4969 – II, Tres Cerros, Provincia de Santa Cruz. Direccio´n Nacional del Servicio Geolo´gico, Boletı´n 213, 1–103. Buenos Aires. Panza, J.L. (1994b). Hoja Geolo´gica 4966 – I y II, Bahı´a Laura, Provincia de Santa Cruz. Direccio´n Nacional del Servicio Geolo´gico, Boletı´n 214. Buenos Aires. Panza, J.L. (2002). La cubierta detrı´tica del Cenozoico superior. In: Haller, M.J. (ed.), Geologı´a y Recursos Naturales de Santa Cruz. XV Congreso Geolo´gico Argentino, Relatorio. El Calafate, Argentina, 1, 17, 259–284. Panza, J.L. and de Barrio, R. (1987). Informe preliminar del levantamiento geolo´gico de las Hojas 55f Cordo´n Alto y 55g Puerto San Julia´n, Provincia de Santa Cruz. Direccio´n Nacional de Minerı´a y Geologı´a, unpublished report, 122 pp. Buenos Aires. Panza, J.L. and Yrigoyen, M.V. (1994). Hoja Geolo´gica 4969-IV, Puerto San Julia´n, Provincia de Santa Cruz. Direccio´n Nacional del Servicio Geolo´gico, Boletı´n 211, 1–77. Buenos Aires. Pereyra, F., Fauque´, L. and Gonza´lez Dı´az, E.F. (2002). Geomorfologı´a. In: Haller, M.J. (ed.), Geologı´a y Recursos Naturales de Santa Cruz. XV Congreso Geolo´gico Argentino, Relatorio. El Calafate, Argentina, 1, 21, 325–352. Ploszkiewicz, J.V. (1987). Descripcio´n Geolo´gica de la Hoja 47c, Apeleg, Provincia de Chubut. Direccio´n Nacional de Minerı´a y Geologı´a, Boletı´n 204, 1–101. Buenos Aires. Proserpio, C.A. (1978). Descripcio´n geolo´gica de la Hoja 42d, Gastre, Provincia de Chubut. Servicio Geolo´gico Nacional, Boletı´n 159, 1–75. Buenos Aires. Rabassa, J. (1975). Geologı´a de la regio´n de PilcaniyeuComallo, Provincia de Rı´o Negro, Argentina.

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Fundacio´n Bariloche, Departamento de Recursos Naturales y Energı´a, Publicaciones 17, 1–128, San Carlos de Bariloche. Rabassa, J. (1999). Cuaternario de la Cordillera Patago´nica y Tierra del Fuego. In: Caminos, R. (ed.), Geologı´a Argentina, Instituto de Geologı´a y Recursos Minerales, Anales. Buenos Aires, 29, 23, 710–714. Rabassa, J. and Clapperton, C.M. (1990). Quaternary Glaciations of the Southern Andes. Quaternary Science Reviews 9, 153–174. Rabassa, J. and Evenson, E.B. (1996). Reinterpretacio´n de la estratigrafı´a glaciaria de la regio´n de San Carlos de Bariloche (Prov. de Rı´o Negro, Argentina). XIII Congreso Geolo´gico Argentino & III Congreso de Exploracio´n de Hidrocarburos, Actas 4, 237. Buenos Aires. Rabassa, J., Evenson, E.B., Clinch, J.M., et al. (1990). Geologı´a del Cuaternario del valle del rı´o Malleo, Provincia del Neuque´n. Asociacio´n Geolo´gica Argentina, Revista 45, 1–2, 55–68. Buenos Aires. Rabassa, J., Coronato, A. and Salemme, M. (2005). Chronology of the Late Cenozoic Patagonian glaciations and their correlation with biostratigraphic units of the Pampean region (Argentina). Journal of South American Earth Sciences 20, 1–2, 81–104. Ramos, E.D. and Ramos, V. (1979). Los ciclos magma´ticos de la Repu´blica Argentina. VII Congreso Geolo´gico Argentino, Actas 1, 771–786. Neuque´n, Argentina. Ramos, V. (1982). Geologı´a de la regio´n del Lago Cardiel, Provincia de Santa Cruz. Asociacio´n Geolo´gica Argentina, Revista 37, 1, 23–49. Buenos Aires. Ravazzoli, I. and Sesana, F. (1977). Descripcio´n Geolo´gica de la Hoja 41c, Rı´o Chico, Provincia de Rı´o Negro. Servicio Geolo´gico Nacional, Boletı´n 148, 1–101. Buenos Aires. Rovereto, G. (1912). Studi di geomorfologia Argentina. III. La valle del Rı´o Negro: 2. Il lago Nahuel Huapi. Societta´ Geologica Italiana Bollettino 31, 181–237. Schellman, G., Wenzens, G., Radtke, U., et al. (2000). Landscape evolution of Southern Patagonia. Geodesy, Geomorphology and Soil Science, SonderHeft 1, 63–68. Schlieder, G. (1989). Glacial stratigraphy and chronology in the province of Neuque´n, between Alumine´ and San Martı´n de los Andes. Unpublished Ph.D. Dissertation, Lehigh University, Bethlehem, U.S.A. Sepu´lveda, E.G. (1983). Descripcio´n geolo´gica de la Hoja 38 I, Gran Bajo del Gualicho. Provincia de Rı´o Negro. Servicio Geolo´gico Nacional, Boletı´n 71. Buenos Aires. Stern, L.A. and Blisniuk, P.M. (2002). Stable isotope composition of precipitation across the southern Patagonian Andes. Journal Geophysical Research 107, D23, 4667. Strelin, J.A., Re´, G., Keller, R. and Malagnino, E. (1999). New evidence concerning the Plio-Pleistocene landscape evolution of southern Santa Cruz region. Journal of South American Earth Sciences 12, 333–341. Ton-That, T., Singer, B., Mo¨rner, N. and Rabassa, J. (1999). Datacio´n de lavas basa´lticas por 40Ar/39Ar y geologı´a glacial de la regio´n del Lago Buenos Aires, provincia de Santa Cruz, Argentina. Asociacio´n Geolo´gica Argentina, Revista 54, 4, 333–352. Buenos Aires.

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Trombotto, D. (1994). El permafrost patago´nico pasado. Revista del Museo de Historia Natural de San Rafael 12, 4, 229–249. San Rafael, Argentina. Trombotto, D. and Ahumada, A.L. (1993). Sı´ntesis del ana´lisis de estructuras sedimentarias en los ‘‘Rodados Patago´nicos’’ causadas por la presencia de permafrost en el crio´mero Penfordd, Puerto Madryn, Nordpatagonia. XII Congreso Geolo´gico Argentino, Actas 6, 97–105. Buenos Aires. Turner, J.C. (1982). Descripcio´n Geolo´gica de la Hoja 44c, Tecka. Servicio Geolo´gico Nacional. Boletı´n 180. Buenos Aires. Volkheimer, W. (1963). El Cuartario Pedemontano en el noroeste de Chubut (zona Cushamen). Segundas Jornadas Geolo´gicas Argentinas, Actas 2, 439–457. Buenos Aires. Volkheimer, W. (1964). Estratigrafı´a de la regio´n extrandina del Departamento de Cushamen (Chubut) entre los paralelos 42 y 42300 y los meridianos 70 y 71. Asociacio´n Geolo´gica Argentina, Revista 20, 2, 85–107. Buenos Aires. Volkheimer, W. (1965a). El Cuaternario pedemontano en el noroeste del Chubut (zona Cushamen). Segundas Jornadas Geolo´gicas Argentinas, Actas 2, 439–451. Buenos Aires. Volkheimer, W. (1965b). Bosquejo geolo´gico del noroeste del Chubut extraandino (zona Gastre-Gualjaina).

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10 Coastal Geology and Morphology of Patagonia and the Fuegian Archipelago Federico I. Isla1 and Gustavo G. Bujalesky2 1

Centro de Geologı´a de Costas y del Cuaternario, CONICET-UNMDP, Funes 3350, 7600 Mar del Plata, [email protected] 2 Centro Austral de Investigaciones Cientı´ficas, CADIC, CONICET, Bernardo Houssay 200, 9410 Ushuaia, [email protected]

1. Introduction

3.1. Early Middle Pleistocene

The Earth is not a perfect sphere and continents are not proportionally distributed in it. The Northern Hemisphere is dominated by land and the Southern Hemisphere by oceans. Patagonia and Tierra del Fuego are the only continental regions to test the windblown belt effects at high latitudes of the Oceanic Hemisphere. Interannual effects (El Nin˜o–Southern Oscillation cycles) may significantly affect the areas of recharge of their watersheds. Quaternary marine terraces are witness of the interaction between plates, and the climatic changes that caused different distribution of oceans and glaciers (Fig. 1).

The sea-level highstands that occurred during the Middle Pleistocene are difficult to distinguish (Fig. 5) since it is necessary to consider the precision of the dating method and if the uplifting rate permitted a vertical displacement between two consecutive highstands (Schellmann, 1997, 1998). Schellmann (1997) also stressed the reliability of dating performed on paired shells, which are assumed not to have been much reworked. In northern Tierra del Fuego, two formations with coastal deposits were defined: the Laguna Arcillosa and the Las Vueltas formations, both of Middle Pleistocene age (Bujalesky et al., 2001). Although the deposits from where mollusk shells were dated were not completely isolated from weathering processes, an age between 600,000 and 400,000 yrs was estimated by the U/Th method, suggesting an MIS 11 age for the Laguna Arcillosa Formation (Table 1). For the Las Vueltas Formation, the same method yielded an age of less than 300,000 yrs. Therefore, these beach deposits could be considered as belonging to MIS 9.

2. Coastal Settings Patagonian climate does not respond to latitude because of the topographic effects of the Andes: precipitation diminishes toward the Atlantic Ocean. However, in Tierra del Fuego, the trend follows latitudinal bands: precipitation diminishes from south to north, with the effect of the Darwin Cordillera (Tuhkanen, 1992). If the Buenos Aires coastline is microtidal and subject to storm effects, the Patagonian coastline is dominated instead by tides increasing their effects inside gulfs and embayments (Fig. 2). Regarding the tectonic setting, Atlantic Patagonia occupies the trailing-edge coast of the South American plate. The coastline of Pacific Patagonia is subject to tectonic collision between that plate and the rapidly moving southern Pacific and Nazca plates (Fig. 3). More difficult to explain are the interrelations between the South American plate and the Scotia microplate that affect the tectonic behavior of Tierra del Fuego.

3.2. The Middle Pleistocene (MIS 7) South of Bahı´a Camarones, Schellmann (1997) obtained U/Th dates between 178 + 16 and 231 + 20 ka, thus suggesting an MIS 7 age. This author (1997) also presented dates between 196 + 33 and 225 + 25 ka from shells found (two shells attached) at his northernmost profile from Bahı´a Bustamante (Can˜ado´n Restinga). North of Caleta Olivia (Santa Cruz) and close to National Route 3, shells from a road section yielded U/ Th ages between 266 + 36 and 420 + 45 ka (Schellmann, 1997). In Tierra del Fuego, an uplifted beach has been described south of Estancia Viamonte and named as the Shaiwaal Formation and tentatively assigned to this age (Bujalesky and Isla, 2006; Fig. 6).

3. Pliocene–Pleistocene Interglacial Episodes

3.3. The Sangamonian Highstand (MIS 5e)

Feruglio (1950) gave us a detailed description of the marine terraces laid on the coast of Patagonia, and conducted an essay about their ages, before the development of radiometric dating techniques. Those Quaternary highstands are today clearly related to the marine oxygen isotope stages (MIS), as recorded also on the abyssal plains (Fig. 4). Regarding their content in mollusks, the different assemblages respond to latitude (temperature), habitat (rocky or soft bottoms) and salinity (Pastorino, 2000).

Schellmann (1998) has redated most of the coastal terraces and uplifted beaches described by Feruglio (1950) and previously dated by Rutter and colleagues (1989, 1990). The Sangamonian highstand (Late Pleistocene, MIS 5e) is commonly extending along the coast at an altitude of 16–20 m a.s.l. In Tierra del Fuego, it is located at 8.6 m above the stormlevel, or 14.33 m a.m.s.l. (Bujalesky et al., 2001).  2008 ELSEVIER B.V. ALL RIGHTS RESERVED

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South of Caleta Olivia, close to National Route 3, Sangamonian uplifted beaches gave U/Th dates between 111 + 20 and 157 + 21 ka (Schellmann, 1997). At Puerto Mazarredo, a paleobeach yielding shells was U/Th dated between 93 + 10 and 150 + 19 ka (Schellmann, 1997). Southwest of Puerto San Julia´n, a profile of 2 m was sampled (Schellmann, 1997). Beach ridge facies gave U/ Th ages between 104 + 8 and 119 + 13 ka. Deposits of Pleistocene age have also been recognized to the south of the city of Rı´o Gallegos (Codignotto and Ercolano, 2002). Between Bahı´a San Sebastia´n and the city of Rı´o Grande, a beach deposit composed of oxidized gravel

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60°S

Fig. 3. Patagonia and Tierra del Fuego in their tectonic setting related to the South American, Antarctic, Nazca and Scotia plates (modified from Diraisson et al., 2000).

230

Federico I. Isla and Gustavo G. Bujalesky

Fig. 4. Although some oxygen isotopic stages are clearly related to sea surface temperature (MIS 1 or 5e), in other stages the isotopic response is not so clear (MIS 7; record obtained from core MD 94-101, 42300 S, 79250 W, 2920 m; Waelbroek, 1999).

Fig. 5. Quaternary highstand elevations recorded along the coast of Patagonia and Tierra del Fuego (modified from Schellmann, 1997, and Bujalesky et al., 2001). was described originally by Codignotto and Malumia´n (1981) as La Sara Formation, of Pleistocene age. Radiocarbon ages gave minimum ages until it was dated by U/Th series at 82,000 yrs BP (Bujalesky et al., 2001), and indicating positively that it corresponds to the Sangamonian highstand (MIS 5). The Malvinas (Falklands) Islands have not been subject to a detail study of Pleistocene sea-level highstands,

although they would be very appropriate to this type of studies since there were no mountain glaciers that could have eroded the Pleistocene beach deposits. At Bertha’s Beach, eastern Soledad Island, a former shoreline has been surveyed (Greenway, 1972). There has been much controversy about those coastal terraces that have yielded radiocarbon ages between 25 and 35 ka BP. Today, these dates are usually discarded

Coastal Geology and Morphology

231

Table 1. Stratigraphic table of interglacials of northern Tierra del Fuego (modified from Bujalesky et al., 2001; Bujalesky and Isla 2006). Absolute age

San Sebastia´n Fm (Codignotto, 1969) La Sara Fm (Codignotto, 1969) Shaiwaal Fm Las Vueltas Fm Laguna Arcillosa Fm Viamonte Fm Najmishk Fm Hanging beach at Cerro La Arcillosa

4.6 ka 14C 82 U ka ? ?

300? U ka 400–600? U ka

3–4 Ma

Altitude m a.h.s.l 0 6–8 12 19 23 38 53 79

Stage

18

O

1 5 7 9 11 13–15 25–31 K1–G15? ?

Stratigraphic unit

Age Holocene Late Pleistocene Middle Pleistocene

Early Pleistocene Pliocene

The 18O isotopic stages are in correspondence to Shackleton (1995). Altitude m a.h.s.l.: m above present highest (storm) sea level. Fm: Formation.

Fig. 7. In Caleta Valde´s, Quaternary and Holocene highstands have developed connected to each other (Rutter et al., 1989).

Fig. 6. Setting of the Shaiwaal Formation between the Estancia Viamonte and the Rı´o Ewan inlet (Bujalesky and Isla, 2006).

because they are very close to the limit of this dating method, and because they gave older ages when other methods (U/Th, amino acid racemization) were used (Rutter et al., 1989).

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Federico I. Isla and Gustavo G. Bujalesky

Fig. 8. Bahı´a Bustamante recorded several Quaternary highstands developed in relation to volcaniclastic outcrops constituting capes (Photo by Toma´s Boski).

4. Holocene Transgression Witte (1918) distinguished six systems corresponding to sea-level highstands. Systems I–III are of Late Pleistocene age (Rutter et al., 1989). Systems IV and V are of Holocene age and yielded radiocarbon dates from 5370 to 4100 yrs BP at altitudes of 7 m a.m.s.l. More recent datings fluctuate between 3450 + 110 and 2170 + 110 14 C yr BP at altitudes of 3 m a.s.l. (Trebino 1987). In Tierra del Fuego, evidence of a sea level higher than present during the Holocene is known since the Swedish Expedition of 1907–1909 (Nordenskjo¨ld 1898; Halle, 1910; Urien, 1966). Today, it is possible to reconstruct this fluctuation at the Atlantic coast of Tierra del Fuego combining transgressive dates recorded at the La Misio´n bog (11 km north of the city of Rı´o Grande) and the complete regressive sequence lying to the soutwest of Bahı´a San Sebastia´n (Fig. 9). At La Misio´n, Markgraf (1980) drilled down to the basal contact of the profile described originally by Auer (1959). A Holocene lake (at a present level of 5.7 m below high tide level) was flooded by the marine transgression. This basal contact gave a radiocarbon age

of 9300 + 180 yrs BP; somewhat higher in the section another sample gave an age of 8490 + 400 14C yr BP (Porter et al., 1984). Another date obtained 4.2–4.3 below mean high tide level gave a radiocarbon age of 7850 + 110 yrs BP (Auer, 1959; Porter et al., 1984). The top of the transgression is represented by a tidal flat developed at an altitude of 0.9 m above high tide level (a.h.t.l.) at 4000–2000 14C yr BP (Mo¨rner, 1991). At the Rı´o San Martı´n beach ridge plains (Bahı´a San Sebastia´n), two regressive sequences are indicating a Mid-Holocene sea-level drop, whereas storm-triggered deposits are depicting the offshore progradation. The western profile extended between 5616 + 282 and 509 + 41 14C yr BP (Vilas et al., 1999); north of the river, the beach ridge plain extended between 4070 + 90 and 975 + 120 14C yr BP (Isla and Bujalesky, 2000). The Rı´o Chico beach ridge plain prograded alongshore southward. Considering the radiocarbon dates performed here this progradation took place at least between 4620 + 70 and 2880 + 80 14C yr BP (Isla and Bujalesky, 2000; Bujalesky et al., 2001). The Beagle Channel is a special place to analyze the effects of the glacioeustatic, isostatic and tectonic effects. In Playa Larga, there are well-developed terraces located close to the Rı´o Olivia inlet and also near to the city of Ushuaia. Five superimposed raised beaches developed at 1.6 m (405 + 55 14C yr BP), 3.8 m (3095 + 60 14C yr BP), 5.2 m (4335 + 60 14C yr BP), 7.5 m (5615 + 60 14C yr BP) and 10 m a.m.s.l. (Gordillo et al., 1992). The Bahı´a Lapataia–Lago Roca valley (20 km west of Ushuaia) is a paleofjord that was occupied by a lateral and tributary glacier system during the last glacial maximum (about 18–20 ka BP; Gordillo et al., 1990). Holocene marine deposits are scattered along Bahı´a Lapataia, Archipie´lago Cormoranes, Rı´o Ovando, Rı´o Lapataia and the eastern shoreline of Lago Roca, overlying glacial landforms and reaching a maximum altitude of at least 8.4 m a.s.l. (Table 2; Gordillo et al., 1990). The cold and shallowwater mollusk assemblages associated to the Beagle Channel raised beaches have not shown significant climatic changes during the Holocene (Gordillo et al., 1992). These deposits of Mid-Holocene age contained remains of the activity of the earlier inhabitants of the coast of Patagonia and Tierra del Fuego (Go´mez Otero et al., 2000; Orquera and Piana, 1998; Salemme and Bujalesky, 2000). Some archeological remains that could be related to the maximum Holocene highstand are located in Golfo San Jorge (Cabo Tres Puntas: 6060 + 70 14C yr BP; Go´mez Otero et al., 2000), Beagle Channel (Tu´nel site: 6980 + 110 14C yr BP; Orquera and Piana, 1998), Penı´nsula Mitre (Bahı´a Valentı´n: 5900 + 80 14C yr BP; Go´mez Otero et al., 2000) and Isla de los Estados (Caleta Crossley: 2730 + 90 14C yr BP; Horwitz, 1990).

5. Holocene Sea-level Curves Fig. 9. At the headlands of the El Pa´ramo Spit, the Holocene beach deposits are locally overlain by washover deposits (Isla et al., 1991).

A minimum sea level of 105 m below present is assumed for the Last Glacial Maximum (Guilderson et al., 2000). Both‘‘jumps’’ in the rate of sea level rise described in the island of Barbados have been recognized at the

Coastal Geology and Morphology Patagonian continental shelf. Although there are still some doubts, the Quaternary beach terraces described above served to get a mean long-term tectonic uplift of 8–9 cm/kyr (Guilderson et al., 2000). Codignotto and colleagues (1992) proposed that maximum transgression occurred between 6500 and 4000 14C yr BP for the Patagonian coast, while maximum ages of 9500–8000 have been obtained from the Colorado Basin. Schellmann and Radtke (2003) perfectly understood the different criteria to apply in relation to deposits originated during storm effects (beach), and those originated simply when the high tide reworked the gravel transported to the coast by an episodic fluvial input (valley mouth terraces). The former are higher and reached its maximum height about 7000–8000 14C yr BP (Schellmann and Radtke, 2003). Sea-level curves have been also proposed for the Fuegian Archipelago. Several radiocarbon dates from the Magellan Strait, the Atlantic coast and the Beagle Channel were used to distinguish the tectonic behavior of the region. The transgressive phase of the Flandrian transgression is very well preserved at Caleta La Misio´n (Auer, 1959), while the regressive phase is better recorded in the beach gravel plain of Bahı´a San Sebastia´n (Vilas et al., 1999). The area of the Magellan

233

Strait was assumed to experience an exponentially decreasing rate of isostatic uplift. For the Fuegian Archipielago, a maximum relative sea level of 3.5 m was proposed for 7000 yrs ago (Porter et al., 1984); but within the Beagle Channel, maximum heights of 8 m (Rabassa et al., 1986) and 12 m (Rabassa et al., 1992) have been indicated.

6. Cliffs and Beach Ridge Plains Many coastal areas of Patagonia and northern Tierra del Fuego are known by high cliffs composed of Oligocene, Miocene or Pliocene sedimentary rocks. As these cliffs are very conspicuous, some of them more than 80 m high, the region is assumed to be still under the erosive phase of the Flandrian transgression. However, at the foot of those cliffs there are accumulations of gravel and sand subject to the reworking action of waves, tides and littoral currents. These littoral currents could transport sediment from where it was eroded to produce the progradation of the coast as plains of gravel beaches from Rı´o Negro Province to Tierra del Fuego. When the input of sediment is small or the depth

Table 2. Radiometric dates from Holocene littoral deposits along the Beagle Channel. Locality Bahı´a Lapataia Bahı´a Lapataia Bahı´a Lapataia Lago Roca Lago Roca Isla El Salmo´n Alakush Nacientes Rı´o Ovando Rı´o Ovando Rı´o Ovando Camping Bahı´a Ensenada Bahı´a Golondrina Punta Pingu¨inos Punta Pingu¨inos Ushuaia Playa Larga Playa Larga Playa Larga Playa Larga Punta Parana´ Bahı´a Brown Bahı´a Brown Cutalataca Rı´o Varela Punta Piedra Buena (Isla Navarino-Chile) Penı´nsula Gusano (Isla Navarino-Chile)

Altitude (m a.s.l.) 1.65 1.80 1.95 8.40 3.95 4.30 5.00 3.10 2.20 10.00 8.50 2.50 8.00 1.60 3.80 5.70 8.00 6.00 1.80 3.30 2.30 1.26 0.65 3.55

Lab. #

Reference

8240 + 60 7260 + 70 5800 + 65 5920 + 90 7518 + 58 3860 + 75 4400 + 120 4160 + 45 4425 + 55 7500 + 80 2120 + 45 5460 + 110 5430 + 270 1400 + 300 5160 + 130 405 + 55 3095 + 60 4335 + 60 5615 + 120 4370 + 70 985 + 135 2970 + 70 2770 + 50 6290 + 70 1470 + 30

SI-6737 SI-6738 SI-6739 AC-1060 NZ-7730 SI-6734 AC-0937 Pta-7573 SI-6735 Pta-7691 Pa-1012 AECV-877C L-1016C L-1016B AECV-876C Pa-1017 Pa-1016 Pa-1015 Pa-1018 Pta-7686 Pa-1011 Pa-1010 Pa-1009 Pta-7581 QL-1653

Rabassa et al., 1986 Rabassa et al., 1986 Rabassa et al., 1986 Rabassa et al., 1986 Gordillo et al., 1990 Rabassa et al., 1986 Figuerero and Mengoni, 1986 Coronato et al., 1999 Rabassa et al., 1986 Coronato et al., 1999 Gordillo et al., 1992 Gordillo et al., 1992 Urien, 1966 Urien, 1966 Gordillo et al., 1990 Gordillo et al., 1992 Gordillo et al., 1992 Gordillo et al., 1992 Gordillo et al., 1992 Coronato et al., 1999 Gordillo et al., 1992 Gordillo et al., 1992 Gordillo et al., 1992 Coronato et al., 1999 Porter et al., 1984

4600 + 30

QL-1652

Porter et al., 1984

Age

14

C yr (BP)

234

Federico I. Isla and Gustavo G. Bujalesky

of the bay is significant, cuspate forelands or flying spits can be formed. When there is a large input of sediment or the depression is shallow, it can be filled by a beach plain. In Caleta Valde´s, a Holocene plain of gravel beach ridges are attached to the Pleistocene coastal ridges at a lower altitude. They ranged in age from 5720 + 105 to 1330 + 80 14C yr BP (Codignotto, 1983; Rutter et al., 1989). Bahı´a Engan˜o is another gravel beach plain at the inlet of the Rı´o Chubut (Fig. 10). From the southwestern limit of the plain, radiocarbon ages spanned 4987 + 106 to 1009 + 88 yrs BP (Monti, 2000). In Bahı´a Camarones, the Mid-Holocene highstand persisted between 6708 + 46 and 6663 + 59 14C yr BP (Schellmann, 1998; Schellmann and Radtke, 2000). In Bahı´a Bustamante, beach ridges span younger ages, between 5424 + 40 and 4420 + 80 14C yr BP (Schellmann and Radtke, 2000). South of the city of Comodoro Rivadavia, at Punta Marque´s, beach deposits were dated between 5381 + 60 and 5240 + 50 14C yr BP (Schellmann and Radtke, 2000). To the north and south of Punta Delgada, beach plains evolved during the same Mid-Holocene times but with significant differences in wave and storm actions. The southern beach plain reached heights of 8 m above highest tide water, whereas the northern plain only reached altitudes of 5 m above the same datum (Schellmann, 2003). North of the inlet of Bahı´a San Julia´n, a rocky embayment was filled in three phases (systems) but mostly during the recent Holocene. The beach plain of Playa

de los Caracoles (system II) was deposited between 1779 + 82 and 570 + 80 14C yr BP (Schellman, 2003). At the northern portion of the inlet of the Rı´o Gallegos, a cuspate foreland, named Punta Bustamante, grew southward during the Middle Holocene. A radiocarbon date gave an age of 6300 yrs BP (Gonza´lez Bonorino et al., 1999). At the southern shore of the inlet there is another beach plain narrowing from the north (Punta Loyola) to the south (Zanja Grande; Codignotto, 1990). The coast between Cabo Vı´rgenes and Punta Dungeness is a cuspate foreland (Fig. 11) developed at the Atlantic inlet of the Magellan Strait (Uribe and Zamora, 1981; Codignotto, 1990; Gonza´lez Bonorino et al., 1999). In the Chilean side a sample gave an age of 900 BP 14C yr (Uribe and Zamora, 1981). On the northern coast of Tierra del Fuego, it the relationships are clearer between glacial deposits and beach ridge plains. The El Pa´ ramo spit grew from the glacial deposits of Cabo Nombre (Bujalesky, 1998) and other glacial deposits today submerged or eroded (Isla and Schnack, 1995) and recognized as a (partially) submerged boulder field (Vernengo and Zarpello´n, 2002). The Rı´o Chico beach ridge plain also grew from the deposits of Punta Sinaı´ (Isla and Bujalesky, 2000; Bujalesky et al., 2001). Toward the south, beach ridge plains as Ensenada La Colonia and Rı´o Fuego are related to capes (Cabo Pen˜ as and Punta Marı´a, respectively) but without any evident relation to glacial deposits (Bujalesky and Isla, 2006).

Fig. 10. Bahı´a Engan˜o is a beach ridge plain related to the evolution of the Rı´o Chubut.

Fig. 11. Punta Dungeness is a cuspate foreland developed from glacigenic deposits toward the Magellan Strait during the Middle Holocene (Uribe and Zamora, 1981; Gonza´lez Bonorino et al., 1999).

Coastal Geology and Morphology

235

7. Estuarine Environments

Ushuaia

Playa Larga Bahía Ushuaia

B. Golondrina Ensenada Península Ushuaia Bahía Lapataia

68°00’W Punta Segunda

1999; Fig. 12). During the Middle Holocene there was a significant drift of gravel and sand within the bay, and the regression produced the progradation of the gravel beach ridge plain at a rate of 2.35 m/yr at the beginning and 0.6 m/yr for the last 1000 yrs (Vilas et al., 1999). The complex spit of the Rı´o Chico grew about 20 km from north to south (Codignotto, 1990). It is composed predominantly of gravel delivered by two glaciofluvial fans. During its complex formation there were intervals of erosion or cannibalization over the last 3000 yrs (Isla and Bujalesky, 2000; Bujalesky et al., 2001). Along the Beagle Channel, which joins the Atlantic and Pacific oceans, there are many Alpine glaciers entering it (Rabassa et al., 1988; 1989; 1990; Coronato et al., 1999), and several small estuaries are delivering freshwater to the channel (Fig. 13). A core obtained from Bahı´a Lapataia gave radiocarbon ages between 8550 + 120 and 7260 + 70 14C yr BP indicating sea level higher than present; the palynological study of this core indicates that, during the Early–Middle Holocene, the climate was more humid and warmer than in the Late Pleistocene (Borromei and Quattrocchio, 2001).

Isla Grande de Tierra del Fuego ARGENTINA Pta. Remolino Pta. Paraná

Brown

67°10’ W 54°50’ S

Canal Beagle

Isla Gable

54°55’S

Paso Mac Kinley u rr lM na Ca

Isla Hoste CHILE

N

67°30’ W

Río Varela

68°30’W Lago Roca

Fig. 12. El Pa´ramo complex spit originated from the reworking of submerged glacial deposits and gave origin to the mudflats and tidal channels of northern Bahı´a San Sebastia´n (Isla et al., 1991; Vilas et al., 1999).

Cutalataca

At the estuarine complex developed at Bahı´a San Blas (Buenos Aires Province), at the northeasternmost end of Patagonia, a vibracore was drilled at Arroyo Jabalı´. Mollusk shells (Heleobia australis) yielded a radiocarbon age of 4310 + 40 yrs BP (Isla and Espinosa, 2005). Bahı´a San Antonio (Rı´o Negro Province) is a coastal lagoon with a macrotidal re´gime. Two barriers delimit the estuarine environments. Pleistocene beach deposits are confirming that this depression was also flooded during former sea-level highstands (Rutter et al., 1989). Both barriers, Villarino and Delgada, have been formed as a complex of beach ridges during the Holocene highstand. Caleta Valde´s (Chubut Province) was formed as a coastal lagoon, though without any mixing of waters as it is located in a semiarid region. The gravel spit grew from north to south enclosing the sound and leaving mudflats and sandflats at the northern portion of the coastal lagoon. Also the Rı´o Deseado inlet (Santa Cruz Province) has been flooded during the Sangamonian and the Holocene highstands. Late Pleistocene beach deposits are aligned along the coast of the estuary very close to Puerto Deseado (Iantanos, 2004). Two Holocene beach deposits were surveyed within the estuary: at the inlet of the gorge called Can˜ado´n del Puerto, a beach was dated at 1040 + 40 14C yr BP. This beach is covered by a shell midden composed of gravel and mollusk shells (Brachidontes rodriguezi). Another deposit, composed of sand and mud, is found at the southern shore of the estuary, on Penı´nsula Barrancas, overlying the volcanic rocks of the Bahı´a Laura Group (Iantanos, 2004). Bahı´a San Sebastia´n (Tierra del Fuego) is a depression excavated by piedmont glaciers coming from the southern Fuegian Cordillera (Caldenius 1932; McCulloch and Bentley 1998). When glaciers receded, the area drowned by the Flandrian transgression that reworked former glacial deposits (Isla and Schnack, 1995). Maximum levels are located in the Chilean part of Isla Grande of Tierra del Fuego, but the area has never been a marine corridor as was believed by the Spanish sailors. The bay is located completely in the Argentine part of the island. A gravel spit 15 km long has progressively limited the embayment environments: tidal flats, marshes and cheniers (Isla et al., 1991; Bujalesky, 1998; Vilas et al.,

Punta Península Piedra Buena Gusano

Isla Navarino CHILE 0

10 km

ay

Fig. 13. Location of coastal features within the Beagle Channel.

54°55’S

236

Federico I. Isla and Gustavo G. Bujalesky Sediment scarcity

No sediment

Sediment availability 7

7 5

5 1

Uplifting New Guinea

Bustamante Bay Chubut

Rio Chico plain Tierra del Fuego 7

5

1

1

Stability Rio Grande do Sul Brazil

7

1

5

1

Submergence 7

5

Fig. 14. Stratigraphic models for Quaternary highstands in relation to tectonic behavior and sediment availability (modified from Aharon, 1983; Villwock et al., 1986; Scholz, 1986). 8. Tectonics

9. Conclusions

The altitudinal position of the Quaternary marine terraces is related to the tectonic behavior and the sediment supply to the coast (Fig. 14). Coastal classifications (models) are difficult to apply to Patagonia as there are different scales of analysis: in a large scale, Patagonian cliffs are evidence of a transgressive erosive phase. At the small scale, gravel beach plains laid at the foot of those cliffs indicate a regressive phase temporarily spanning the last 6000 yrs (Middle Holocene to Present). In Patagonia, the different tectonic behavior between the Argentine, Chilean and Fuegian coasts should be recognized. Codignotto et al. (1992) used sea-level indicators from the Atlantic coast, to evaluate tectonic uplift and subsidence rates. According to these authors, horsts uplifted while sedimentary basins remained stable. They calculated uplift rates between 0.21 and 1.63 m/kyr while the general trend is on the order of 0.7 m/kyr. Schellmann (1998) accepted a slow rate of less than 0.12 m/kyr uniform uplift; however, he calculated that, north of San Julia´n (Deseado Massif), the area has been subsiding relatively since the Middle Pleistocene. Within the Magellan Strait, at Peninsula Brunswick, some morphological features are indicating subsidence (bays of San Nicola´s, del Indio and Brookes), while other places are suggesting uplifting (Bahı´a Snug; Fuenzalida and Harambour, 1984). Within the Beagle Channel, and considering a regional uplifting rate of 1.5–2.0 m/kyr (Rabassa et al., 1986), the very recent dates obtained at Playa Larga (405 + 55 14 C yr BP, at þ1.7 m) and Bahı´a Brown (985 + 135 14C yr BP, at þ2.6 m) are indicating coseismic uplift above the regional trend (Gordillo et al., 1992). These coseismic uplifts are difficult to discern on the sequence of storm events described for Bahı´a San Sebastia´n, on the southern end of the South American plate.

Patagonia and Tierra del Fuego show a wide and varied record of coastal evolution for the Quaternary period, comprising the complicated relationships between glacial deposits, interglacial highstands and different uplift trends. In detail, the following can be concluded: 1. From several sea-level highstands distinguished in Patagonia and Tierra del Fuego, the oldest has been surveyed in northern Tierra del Fuego and would correspond to an MIS older than stage 11. 2. Gravel supply to the northern Patagonian coast was provided mainly by the Patagonian Shingle Formation. The highstands of Tierra del Fuego derived their material from glacial or glaciofluvial deposits outcropping at the coast and others nowadays submerged on the inner shelf. 3. Comparatively, the tectonic uplift of northern Tierra del Fuego was lower than that recorded in Chubut or Santa Cruz Province. 4. In this comparison, the Patagonian highstands developed in relation to volcaniclastic rocks (Camarones, Bahı´a Bustamante, Puerto Deseado). The highstands of Tierra del Fuego (and southern Santa Cruz) developed on a glacially eroded landscape. 5. In northern Patagonia, Holocene beach plains originated due to gravel abundance (Valde´s Spit), between capes (Bahı´a Bustamante, Bahı´a Solano) or related to river inlets (Rı´o Chubut beach plain). On the Atlantic coast of Tierra del Fuego there are regressive-like sequences of beach ridge plains in protected areas, and transgressive-like beach plains developed at exposed areas subject to fluctuations in gravel availability. 6. On the northern Atlantic coast, evidence indicates a very mature stage of evolution: in-filled shallow

Coastal Geology and Morphology paleo-embayments (Rı´o Chico), progressive thinning of spits (El Pa´ramo), cannibalization of spits (El Pa´ramo, Rı´o Chico). During the Holocene, the growth of the gravel beach ridge plains of the northern Atlantic coast took place under limited sediment supply. The progressive elongation was sustained by erosion and sediment recycling (cannibalization) at the seaward flank, resulting in a significant landward retreat. The younger and distal beach ridges show evidence of sediment starving pulses. Toward the central Atlantic littoral region, the littoral gravel ridge plains developed regressive facies, thus indicating here that equilibrium existed between the sediment supply and the littoral drifting in the system. 7. The inner estuaries of the paleo-embayments of the northern Atlantic coast of Tierra del Fuego were plugged approximately 5000 14C yr BP in coincidence with the highest Holocene sea level. The altitude of the fossil barrier that plugged former lagoons indicates a relative sea level fall of 0.214 m/kyr. Part of this gradient could be also attributed to wave dynamics processes, such as a higher storm wave setup. Littoral forms have developed under relatively stable eustatic conditions since 5000 14C yr BP. 8. The tectonic uplift during the last 8000 yrs was greatest at the western Beagle Channel (approximately 1.2 + 0.2 mm/yr), diminishing northward and eastward. It seems to be negligible toward the northern coast of Isla Grande. The glacioisostatic rebound at the Beagle Channel seems to have operated only during deglaciation or in 1–2 millennia after the final ice recession.

References Auer, V. (1956). The Pleistocene of Fuego-Patagonia. Part I: The Ice and Interglacial Ages. Annales Academiae Scientarium Fennicae, A III Geologica-Geographica 48, 1–226. Auer, V. (1959). The Pleistocene of Fuego-Patagonia. Part III: Shoreline displacements. Annales Academiae Scientarium Fennicae, A III Geologica-Geographica 60, 1–247. Bonarelli, G. (1917). Tierra del Fuego y sus turberas. Anales del Ministerio de Agricultura de la Nacio´n, Seccio´n Geologı´a, Mineralogı´a y Minerı´a 12, 1–119. Buenos Aires. Borromei, A.M. and Quattrocchio, M. (2001). Palynological study of Holocene marine sediments from Bahı´a Lapataia, Beagle Channel, Tierra del Fuego, Argentina. Revista Espan˜ola de Micropaleontologı´a 33, 1, 61–70. Madrid. Bujalesky, G.G. (1998). Holocene coastal evolution of Tierra del Fuego. Quaternary of South America & Antarctic Peninsula 11, A.A. Balkema Publishers, Rotterdam, 247–242. Bujalesky, G.G., Aliotta, S. and Isla, F.I. (2004). Facies del subfondo del Canal Beagle, Tierra del Fuego. Asociacio´n Geolo´gica Argentina, Revista 59, 1, 29–37. Buenos Aires. Bujalesky, G.G., Coronato, A. and Isla, F. (2001). Ambientes glacifluviales y litorales cuaternarios de la

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regio´n de Rı´o Chico, Tierra del Fuego, Argentina. Asociacio´n Geolo´gica Argentina, Revista 56, 1, 73–90. Buenos Aires. Bujalesky, G.G. and Isla, F. (2006). Depo´sitos Cuaternarios de la costa atla´ntica fueguina, entre los cabos Pen˜as y Ewan, Argentina. Revista de la Asociacio´n Geolo´gica Argentina 61, 1, 81–92. Caldenius, C.C. (1932). Las glaciaciones cuaternarias en la Patagonia y Tierra del Fuego. Geografiska Annaler 14, 1–164. Stockholm. Codignotto, J.O. (1969). Nota acerca de algunos aspectos geolo´gicos de la costa patago´nica comprendida entre Punta Loyola y el Cabo Vı´rgenes. Servicio de Hidrografı´a Naval, Boletı´n 6, 3, 257–263. Buenos Aires. Codignotto, J.O. (1983). Depo´sitos elevados y/o acrecio´n Pleistocene-Holocena en la costa fueguino-patago´nica. Simposio Oscilaciones del nivel del mar durante el u´ltimo hemiciclo deglacial en la Argentina. CONICET, CAPICG, IGCP 61, Mar del Plata, 12–26. Codignotto, J.O. (1990). Evolucio´n en el Cuaternario alto del sector de costa y plataforma submarina entre Rı´o Coig, Santa Cruz y Punta Marı´a, Tierra del Fuego. Asociacio´n Geolo´gica Argentina, Revista 45, 1–2, 9–16. Buenos Aires. Codignotto, J.O. and Ercolano, B. (2002). Cordones litorales pleistocenos al sureste de Rı´o Gallegos, Santa Cruz. XV Congreso Geolo´gico Argentino, Actas 2, 525–527. Buenos Aires. Codignotto, J.O., Kokot, R.R. and Marcomini, S.C. (1992). Neotectonism and sealevel changes in the coastal zone of Argentina. Journal of Coastal Research 8, 125–133. Codignotto, J.O. and Malumia´n, N. (1981). Geologı´a de la regio´n al norte del paralelo 54 Sur de la Isla Grande de la Tierra del Fuego. Asociacio´n Geolo´gica Argentina, Revista 36, 1, 44–88. Buenos Aires. Coronato, A., Rabassa, J., Borromei, A. et al. (1999). Nuevos datos sobre el nivel relativo del mar durante el Holoceno en el Canal Beagle, Tierra del Fuego,Argentina. I Congreso Argentino de Geomorfologı´a y Cuaternario, Actas 27–28, Santa Rosa, Argentina. Diraisson, M., Cobbold, P.R., Gapais, D. et al. (2000). Cenozoic crustal thickening, wrenching and rifting in the foothills of southernmost Andes. Tectonophysics 316, 91–119. Feruglio, E. (1950). Descripcio´n Geolo´gica de la Patagonia. Direccio´n General de Yacimientos Petrolı´feros Fiscales 3, 1–431. Buenos Aires. Figuerero, M. and Mengoni, G. (1986). Excavaciones arqueolo´gicas en la Isla Salmo´n, Parque Nacional Tierra del Fuego. Informes de Investigacio´n 4, 1–95. Buenos Aires. Fuenzalida, R. and Harambour, S. (1984). Evidencias de subsidencia y solevantamiento en la Penı´nsula Brunswick, Magallanes. Universidad de Chile, Comunicaciones 34, 117–120. Santiago. Gelo´s, E.M., Spagnuolo, J.O. & Isla, F.I. (2000). Caracterı´sticas tecto´nicas de a´reas de aporte para arenas de playas de Tierra del Fuego y Penı´nsula Anta´rtica, Argentina. Revista Pesquisas en Geociencias 27, 1, 69–76. Universidade Federal do Rı´o Grande do Sul, Porto Alegre, Brazil.

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Go´mez Otero, J., Lanata, J.L. and Prieto, A. (2000). Arqueologı´a de la costa atla´ntica patago´nica. Revista de Arqueologı´a Americana 15, 107–185. Gonza´lez Bonorino, G. and Bujalesky, G.G. (1990). Spit growth under high-energy wave climate on bay and ocean flanks, Tierra del Fuego, Southernmost Argentina. International Symposium on Quaternary Shorelines: Evolution, Processes and Future Changes’ (IGCP 274) 35. La Plata, Argentina. Gonza´lez Bonorino, G., Bujalesky, G.G., Colombo, F. and Ferrero, M. (1999). Holocene coastal paleoenvironments in Atlantic Patagonia, Argentina. Journal of South American Earth Sciences 12, 325–331. Gordillo, S., Bujalesky, G.G., Pirazzoli, P.A. et al. (1992). Holocene raised beaches along the northern coast of the Beagle Channel, Tierra del Fuego. Palaeogeography, Palaeoclimatology, Palaeoecology 99, 41–54. Gordillo, S., Coronato, A. and Rabassa, J. (1990). Palaeocology and geomorphological evolution of a fjord during the late Glacial and early-Middle Holocene of Tierra del Fuego. In: Bujalesky, G.G. (ed.), Field Guide: The Quaternary of Tierra del Fuego (Argentina). IGCP 274, INQUA, CIC, 23-30. CADIC, Ushuaia, Argentina. Greenway, M. (1972). The geology of the Falkland Islands. Scientific Report 76, British Antarctic Survey, 30, Gilligan Street, London, UK, 42 pp, 8 plates. Guilderson T.P., Burckle, L., Hemming, S. and Peltier, W.R. (2000). Late Pleistocene sealevel variations derived from the Argentine Shelf. Geochemistry, Geophysics, Geosystems v1, December 15, paper # 2000GC000098. Halle, T. (1910). On quaternary deposits and changes of level in Patagonia and Tierra del Fuego. Bulletin Geological Institute Uppsala, 9, 17–18, 93–117. Horwitz, V. (1990). Maritime Settlement Patterns in Southern Tierra del Fuego (Argentina). Unpublished PhD Dissertation, University of Kentucky at Lexington, Kentucky, USA. Iantanos, N. (2004). Dina´mica sedimentaria de la Rı´o de Puerto Deseado, Provincia de Santa Cruz. Unpublished Ph.D. Thesis, Facultad de Ciencias Naturales, Universidad de la Patagonia San Juan Bosco, Comodoro Rivadavia, Argentina. Isla, F.I. (1988). Where was the sea level 30–40,000 years ago? The Patagonian point of view. Quaternary of South America and Antarctic Peninsula 6. A.A. Balkema Publishers, Rotterdam, 33–64. Isla, F.I. (1989). The Southern Hemisphere sea level fluctuation. Quaternary Science Reviews 8, 359–368. Isla, F.I. (1994). Evolucio´n comparada de bahı´as de la Penı´nsula Mitre, Tierra del Fuego. Asociacio´n Geolo´gica Argentina, Revista 49, 3–4, 197–205. Buenos Aires. Isla, F.I. and Bujalesky, G.G. (1995). Tendencias evolutivas y disponibilidad de sedimento en la interpretacio´n de formas costeras: Casos de estudio de la costa argentina. Revista Asociacio´n Argentina de Sedimentologı´a 1–2, 75–89. Buenos Aires. Isla, F.I. and Bujalesky, G.G. (2000). Cannibalization of Holocene gravel beach plains, northern Tierra del Fuego, Argentina. Marine Geology 170, 1–2, 105–122.

Isla, F.I and Bujalesky, G.G. (2004). Morphodynamics of a gravel-dominated macrotidal estuary: Rı´o Grande, Tierra del Fuego. Asociacio´n Geolo´gica Argentina, Revista 59, 2, 220–228. Buenos Aires. Isla, F.I. and Espinosa, M.A. (2005). Holocene and historical evolution of an estuarine complex: the gravel spit of the Walker creek, Southern Buenos Aires. XVI Congreso Geolo´gico Argentino, Actas, La Plata. Isla, F.I. and Schnack, E.J. (1995). Submerged moraines offshore Tierra del Fuego, Argentina. Quaternary of South America and Antarctic Peninsula 9. A.A. Balkema Publishers, Rotterdam, 205–222. Isla, F.I., Spagnuolo, J.O. and Gelo´s, E.M. (2000). Sedimentologı´a y mineralogı´a de playas de Tierra del Fuego y sector Anta´rtico Argentino (Arco de Scotia e islas asociadas). Asociacio´n Geolo´gica Argentina, Revista 55, 3, 216–228. Buenos Aires. Isla, F.I., Vilas, F.E., Bujalesky, G.G. et al. (1991). Gravel drift and wind effects over the macrotidal San Sebastia´n Bay, Tierra del Fuego. Marine Geology 97, 211–224. Markgraf, V. (1980). Nuevos datos para la historia vegetacional del Tardiglacial y Postglacial de ‘‘La Misio´n’’, Tierra del Fuego. 3rd. Coloq. Paleobot. Palinol., Mexico City, Memoria Instituto Nacional de Antropologı´a 86, 75–81. Mexico. McCulloch, R.D. and Bentley, M.J. (1998). Late Glacial ice advances in the Strait of Magellan, Southern Chile. Quaternary Science Reviews 17, 775–787. Monti, A.J.A. (2000). Edades 14C y ciclicidad de la acrecio´n en depo´sitos costeros elevados, Bahı´a Engan˜o, Chubut. Asociacio´n Geolo´gica Argentina, Revista 55, 4, 403–406. Buenos Aires. Mo¨rner, N.A. (1991). Holocene sea level changes in the Tierra del Fuego region. Boletin IG-USP, Special Publication 8, 133–151. Sa˜o Paulo. Nordenskjo¨ld, O. (1898). Notes on Tierra del Fuego. An account of the Swedish Expedition of 1895–1897. Scottish Geographical Magazine 12, 393–399. Edinburgh. Orquera, L.A. and Piana, E.L. (1998). Human littoral adaptation in the Beagle Channel region: the maximum possible age. Quaternary of South America and Antarctic Peninsula 5. A.A. Balkema Publishers, Rotterdam, 133–165. Pastorino, G. (2000). Asociaciones de moluscos de las terrazas marinas cuaternarias de Rı´o Negro y Chubut, Argentina. Ameghiniana 37, 2, 131–156. Buenos Aires. Porter, S.C., Stuiver, M. and Heusser, C.J. (1984). Holocene sea level changes along the Strait of Magellan and Beagle Channel, southernmost South America. Quaternary Research 22, 59–67. Rabassa, J., Bujalesky, G., Meglioli, A. et al. (1992). The Quaternary of Tierra del Fuego, Argentina: the status of our knowledge. Sveriges Geologiska Underso¨kning, Ser. Ca 81, 241–256. Rabassa, J., Heusser, C.J. & Stuckenrath, R. (1986). New data on Holocene sea transgression in the Beagle Channel: Tierra del Fuego, Argentina. Quaternary of South America and Antarctic Peninsula 4. A.A. Balkema Publishers, Rotterdam, 291–309. Rabassa, J., Heusser, C.J. and Rutter, N. (1989). Late Glacial and Holocene of Argentina, Tierra del Fuego.

Coastal Geology and Morphology Quaternary of South America and Antarctic Peninsula 7, A.A. Balkema Publishers, Rotterdam, 327–351. Rabassa, J., Serrat, D., Martı´, C. and Coronato, A. (1988). Estructura interna de drumlins, Isla Gable, Canal Beagle, Tierra del Fuego. II Reunio´n Argentina de Sedimentologı´a 222–226. Buenos Aires. Rabassa, J., Serrat, D., Marti, C. & Coronato, A.M. (1990). El Tardiglacial en el Canal Beagle, Tierra del Fuego, Argentina. XI Congreso Geolo´gico Argentino, Actas 1, 290–293. San Juan, Argentina. Rutter, N., Radtke, U. and Schnack, E.J. (1990). Comparison of ESR and amino acid data in correlating and dating Quaternary littoral zones along the Patagonian coast. Journal of Coastal Research 6, 391–411. Rutter, N., Schnack, E.J., Fasano, J.L. et al. (1989). Correlation and dating of Quaternary littoral zones along the Patagonian coast, Argentina. Quaternary Science Reviews 8, 213–234. Salemme, M. and Bujalesky, G., 2000. Condiciones para el asentamiento humano litoral entre Cabo San Sebastia´n y Cabo Pen˜as (Tierra del Fuego) durante el Holoceno medio. ‘‘Desde el paı´s de los gigantes: una perspectiva arqueolo´gica de Patagonia’’, Rı´o Gallegos, 12 pp. Schellmann, G. (1997). Jungka¨nozoische Landschaftsgeschichte Patagoniens (Argentinien). Essener Geographische Arbeiten 29, 216 pp. Schellmann, G. (1998). Coastal development in Southern South America (Patagonia and Chile) since the Younger Middle Pleistocene—sea-level changes and neotectonics. In: Kelletat, D.H. (ed.), German Geographical Coastal Research, The Last Decade, Institute for Scientific Co-operation Tu¨bingen, IGU Sonderband, 289–304. Tubingen. Schellmann, G. and Radtke, U. (2000). ESR dating stratigraphically well-constrained marine terraces along the Patagonian Atlantic coast (Argentina). Quaternary International 68–71, 261–273. Schellmann, G. and Radtke, U. (2003). Coastal terraces and Holocene sea level changes along the Patagonian Atlantic coast. Journal of Coastal Research 19, 4, 983–996.

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Scholz, J. (1986). Sea level data from the Quaternary fringing reefs and barrier reefs of Cebu (Phillipines). The Phillipine Scientist 23, 50–57. Shackleton, N. (1995). New data on the evolution of Pliocene climatic variability. In: Vrba, S., Denton, G., Partridge, T. and Burckle, L. (eds), Paleoclimate and Evolution, with emphasis on Human Origins. Yale University Press, New Have, 242–248. Trebino, L.G. (1987). Geomorfologı´a y evolucio´n de la costa en los alrededores del pueblo de San Blas, Provincia de Buenos Aires. Asociacio´n Geolo´gica Argentina, Revista 42, 1–2, 9–22. Buenos Aires. Tuhkanen, S. (1992). The climate of Tierra del Fuego from a vegetation geographical point of view and its ecoclimatic counterparts elsewhere. Acta Botanica Fennica 145, 1–64. Uribe, P. and Zamora, E. (1981). Origen y geomorfologı´a de la Punta Dungeness, Patagonia. Anales del Instituto de la Patagonia 12, 143–158, Punta Arenas, Chile. Urien, C.M. (1966). Edad de algunas playas elevadas en la Penı´nsula de Ushuaia y su relacio´n con el ascenso costero postglaciario. III Jornadas Geolo´gicas Argentinas, Actas 2, 35–41. Comodoro Rivadavia, Argentina. Vernengo, L.A. and Zarpello´n, C.J. (2002). Tierra del Fuego-San Sebastia´n: adquisicio´n de sı´smica 3D marina en agua somera. V Congreso de Exploracio´n y Desarrollo de Hidrocarburos, Mar del Plata, 82, 12 pp. Vilas, F., Arche, A., Ferrero, M. and Isla, F. (1999). Subantarctic macrotidal flats, cheniers and beaches in San Sebastia´n Bay, Tierra del Fuego, Argentina. Marine Geology 160, 301–326. Villwock, J.A., Tomazelli, L.A., Loss, E.L. et al. (1986). Geology of the Rı´o Grande do Sul coastal province. Quaternary of South America and Antarctic Peninsula 4. A.A. Balkema Publishers, Rotterdam, 79–97. Waelbroek, C. (1999). Southern Ocean MD 94–101. Pages 7, 3, 9. Witte, L. (1918). Estudio geolo´gico de la regio´n de San Blas (Partido de Patagones). Revista Museo de La Plata 24, La Plata.

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11 Late Pleistocene Environmental Change in Eastern Patagonia and Tierra del Fuego – A Limnogeological Approach Daniel Ariztegui1, Flavio S. Anselmetti2,3, Adrian Gilli2 and Nicola´s Waldmann1 1

Earth Sciences Section, University of Geneva, Switzerland 2 Geological Institute, ETH-Zu¨rich, Switzerland 3 Eawag, Du¨bendorf, Switzerland

change – a very poetic and early version of the Milankovitch orbital theory. One of the audacious De Geer’s students was Carl Caldenius, who went to Patagonia and – as mentioned in several chapters of this book – set up the grounds of our present knowledge about glaciations in the region. He published in the Swedish journal Geografiska Annaler in the 1930s, the first classic papers regarding glaciations in the Southern Hemisphere (Caldenius, 1932). He further compared varved lacustrine records from Sweden with varve series from proglacial lake sediments outcropping at the Corintos River, Argentina (43100 S, 71200 W). Many marine and continental records have been obtained at both boreal and austral latitudes since this pioneer attempt of interhemispheric climate correlation. Limnogeology, as defined by Kerry Kelts in the early 1980s, refers to a broad approach to study lake systems driven by the progress in ocean research in the context of marine geology (Kelts, 1987). Thus, it includes the study of complex systems and their interactions and is interdisciplinary by nature. At present, this approach has become widely accepted and a large number of lake studies are performed all over the world including South America (e.g. Baker et al., 2001; Bradbury et al., 2001; Piovano et al., 2002; Brenner et al., 2003 among others). There is, however, a very limited number of publications dealing with both modern lakes (e.g. Baigunand and Marinone, 1995; Cielak, 1995) and ancient lacustrine sediments in Patagonia (e.g. Iriondo, 1989; Ariztegui et al., 2001; Zolitschka et al., 2004). In this contribution, we would like to summarize some of the results of our research of the last 10 yrs in eastern Patagonia (Fig. 1). These examples illustrate the use of a multiproxy limnogeological approach to tackle some of the standing questions dealing with the Late Quaternary environmental evolution of southernmost South America.

1. Introduction: Lakes and Limnogeology Modern lakes and lacustrine sediments are ideal sites to study both ongoing and past environmental changes. Limnogeology refers to a broad approach to study lake sediments, investigating complex systems and their interactions and, thus, it is interdisciplinary by nature. This chapter summarizes geophysical, sedimentological and geochemical results from several lacustrine basins in eastern Patagonia and Tierra del Fuego. These examples illustrate the potential of a multiproxy limnogeological approach to tackle some of the standing questions dealing with the Late Quaternary environmental evolution of southernmost South America. Lacustrine basins are ideal sites to study a wide variety of geological processes and their sediments can be used as excellent archives of past environmental changes. Hence, modern lakes and their sediments have always fascinated earth scientists and naturalists in general. Early on, lakes have been compared with oceans. In 1705 AD, an Italian naturalist, Count Marsili, in his wandering with his Swiss colleague J.J. Scheuchzer described some Swiss lakes as ‘‘small seas’’ with all attributes of their larger counterparts such as the Mediterranean (Kelts, 1987). At the end of the nineteenth century, F.-A. Forel was the first to recognize the enormous erosional power of the turbidity currents generated by the turbid Rhoˆne River when entering Lake Geneva and set up the basis of the new field of limnology (Forel, 1892). Almost contemporaneously with this Swiss scientist, Gerard De Geer started to explore the potential of lacustrine sediments in Scandinavia to archive environmental information with a high temporal resolution (De Geer, 1912, 1927a). In his pioneer study of several Swedish proglacial lakes, he defined the concept of varves or annually laminated sediments. He further proposed that long- and short-term responses of glaciers to globally concurrent climatic changes could be inferred using the sedimentary record of proglacial lakes. Several of his doctoral students went to distant areas in the world searching for varved records that could then be compared to the freshly established Swedish varve chronology. Years later, De Geer concluded that glacial growth in both hemispheres inferred from these records occurred in response to a global, astronomically controlled or ‘‘cosmic’’, forcing mechanism (De Geer, 1927a, b). He referred to a Cosmic Melody that dictated climate

2. Main Components of the Patagonia Climate System The study of present climates indicate that only a few places on the globe are dominated by a single meteorological element as southernmost South America with the persistence and strength of westerly winds (Prohaska, 1976). As backbone of the continent, the Andean Cordillera is a major geographical barrier that generates a sharp  2008 ELSEVIER B.V. ALL RIGHTS RESERVED

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(b)

Fig. 1. (a) Map of South America showing the location of Patagonia as defined in this publication. Average annual precipitation in the region displays a sharp latitudinal gradient with a strong seasonal imprint as shown on the left panel (after Lawford, 1996). Paleoenvironmental records from Patagonia are particularly suited to be compared with the growing dataset emerging from Antarctica ice cores (PAGES News, 2006); (b) Map displaying the lake sites discussed in this article (gray dots) and other records used for comparison (black dots).

longitudinal trend in precipitation. While most of the moisture is precipitated on the western (Chilean) side of the Andes, a striking gradient in precipitation characterizes the eastern (Argentinean) side that can decrease from 4000 to 200 mmyr–1 in less than 200 km within a west–east transect. Meteorological data from western Patagonia show an additional sharp latitudinal gradient in precipitation with a clear seasonal behavior (Lawford, 1996; Fig. 1). This gradient mimics seasonal variations in wind intensity (i.e. westerly winds). While the westerlies migrate poleward during Austral summer (December–March), the associated storm tracks are centered around 45 S. Its northern migration during winter (July–September) to latitude of  40 S generates a welldefined rainy season. Recent instrumental data indicate that the southern westerly belt intensity and associated storm tracks are related to the strength and latitudinal position of the subtropical anticyclone in the southeastern Pacific and the circum-Antarctic low pressure belt (Pittock, 1978; Aceituno et al., 1993). Due to its particular geographical location, Tierra del Fuego is affected directly not only by the westerly winds but also by the Southern Ocean circumpolar flow and the South Pacific Gyre. Markgraf (1993) suggested that the onset of the modern behavior with a seasonal latitudinal shift of the westerly winds occurred sometimes during the Middle Holocene. The past location of the southern westerlies during the Last Glacial Maximum (LGM), however, is still controversial (e.g. Heusser, 1989; Markgraf, 1989; Lamy et al., 1999; Jenny et al., 2001). This uncertainty is partially related to the paucity of multiproxy records with enough latitudinal coverage and comparable high time resolution. In this chapter, we briefly review a series of case studies that combine seismic stratigraphy and multiproxy results of seismically targeted sediment cores (Fig. 1b).

The combination of these results provides information essential to the interpretation of the paleoclimate evolution of southernmost eastern Patagonia for the Late Quaternary. The following examples have been selected to illustrate a range of environments: •

• •

Northernmost Patagonia: Proglacial open lakes, Mascardi and Frias; and Laguna Cari-Laufquen (41 S), a closed-basin system. Central Patagonia: Lago Cardiel (49 S) illustrates a relatively large closed basin. Southernmost Patagonia: Lago Fagnano (54 S) is an open lake system representing the largest and southernmost non-ice covered lake in the world.

All these examples will be cross-correlated and confronted with the existing limnogeological datasets from the Patagonian region of Argentina and Chile. The main goals pursued with these case studies are (1) to check on the timing and magnitude of the observed stepwise climatic evolution of the Lateglacial–Holocene transition; (2) to identify latitudinal variations during the Early Holocene; (3) to spot changes in El Nin˜o Southern Oscillation (ENSO) activity during the second part of the Holocene; and (4) to highlight new evidence for the Little Ice Age (LIA) at different latitudes.

3. Methodology The different case studies discussed in this chapter utilized various types of seismic data acquisition and processing as well as diverse coring equipments. All the investigations, however, were conducted using the same limnogeological approach that includes a seismic survey

Late Pleistocene Limnogeological Records prior to coring followed by a true multiproxy laboratory study. More than 150 km of seismic profiles were collected in lakes Frı´as, Mascardi (1993/94) and Cari-Laufquen Grande (1998) using an ORE-geopulse 3.5 kHz singlechannel pinger system with a vertical seismic resolution of 10–20 cm (Ariztegui et al., 2001). The system achieved a maximum of 40–50 m of penetration, and navigation was accomplished through conventional Global Positioning System (GPS) system. Long sediment cores were retrieved in Lago Mascardi using a Kullenberg system from a self-propelled coring platform (Ariztegui et al., 1997), whereas only short gravity cores were obtained in both lakes, Frı´as and Cari-Laufquen (Ariztegui et al., 2007). Additional vibrocores were taken in Laguna Cari-Laufquen Grande in April 2000 (Gilli, 1999). Over 240 km of seismic profiles and 90 m of sediment cores were recovered in the Lago Cardiel Basin during two-field campaigns in 1999 and 2002 in the framework of the multidisciplinary, international Patagonian coring project (Ariztegui et al., 1998; Gilli et al., 2001). Seismic surveys were conducted using three different systems (3.5 kHz pinger, 1–12 kHz boomer and 1 in3 airgun) to optimize resolution and acoustic penetration of the imaged sections (Gilli et al., 2005a, Beres et al., 2008). In March 2005, >800 km of geophysical data were acquired in Lago Fagnano, combining simultaneous single-channel 3.5 kHz (pinger) with 1 in3 (airgun) multichannel systems. A preliminary set of short gravity cores were recovered at selected locations (Waldmann et al., 2008). All retrieved cores were stored in a dark cold room at 4C and scanned prior to opening at the ETH-Zu¨rich with a GEOTEKTM multisensor core logger to obtain the petrophysical properties (P-wave velocity, gamma-ray attenuation bulk density and magnetic susceptibility). After opening, the cores were photographed and described in detail. Element mapping in selected samples at ca. 50 mm resolution was carried out at the University of Geneva with a Ro¨ntgenanalytik Eagle II Micro X-ray Fluorescence system using a Rh tube at 40 kV and 800 mA. Chronology was resolved through accelerator mass spectrometry (AMS) radiocarbon dating and further combined with 137Cs fallout and tephrochronology for lakes Frı´as and Cardiel, respectively. 4. Limnogeological Case Studies from Northeastern Patagonia 4.1. Lago Mascardi Proglacial Lago Mascardi is located 15 km east of the Tronador ice cap (41100 S, 71530 W; 3554 m a.s.l.), at an altitude of 800 m a.s.l. This horseshoe-shaped lake has a surface area of 38 km2 and a maximum water depth of 200 m (Fig. 2). The western branch of the lake is directly fed by glacial meltwater through the Upper Manso River. Previous work showed that the extent of

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Fig. 2. Bathymetric map of Lago Mascardi showing the location of seismic profile AB close to the lake’s outlet and the position of core PMAS 93.4 discussed in this chapter.

the Tronador ice cap is sensitive to both winter precipitation, derived from the Southern Pacific Westerlies, and mean summer temperatures (Villalba et al., 1990). Thus, Lago Mascardi sediments record fluctuations in glacial meltwater activity providing evidence of the Southern Hemisphere postglacial climate variability (Ariztegui et al., 1997 and references therein). The bathymetry of this lacustrine basin was reconstructed using 60 km of 3.5 kHz seismic profiles that further allowed the identification of the dominant sedimentary geometries as well as the effects of climate and neotectonics on lake sedimentation. The latter is critical since the lake is located in an area of significant Holocene volcanic activity associated with earthquakes of variable magnitude (Chapron et al., 2006). Seismic profiles image the sediments up to 50 m below the lake floor, representing approximately the last 15,000 yrs of infill history. Sedimentation is characterized by a relatively simple stratigraphy with sporadic thin, up to a few centimeter thick packages of chaotic debris (Fig. 3). Bedrock surface and overlying thick proglacial sediments reflect glacial erosion and the impact of proglacial meltwater influxes to the basin. Although the predominant pattern of sedimentation comprises simple and continuous basin infilling, variable sedimentation rates as well as hiatuses were identified in certain areas of the lake (Ariztegui et al., 2001). The combination of seismic profiles with results of multiproxy analyses from a set of sediment cores allowed to establish a well-dated lithostratigraphy (Ariztegui et al., 1997) that was further refined for the last glacial transition using a high-resolution AMS 14C dating approach (Hajdas et al., 2003). A non-interpreted seismic profile at 30 m water depth is shown in Fig. 3 (see Fig. 2 for profile location). Different gray shades in the zoomed rectangle indicate the interpreted seismic sequences mapped by tracing reflection terminations abutting unconformities. Radiocarbon ages in core PMAS 93.4 were assigned to sediment layers equivalent to prominent seismic

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Fig. 3. Seismic profile showing distinctive seismic facies for both Lateglacial and Holocene sediments. The light gray zone indicates the Huelmo-Mascardi Cold Event that is visualized in core PMAS 93.4 (located in this seismic section) by both increasing quality of the lamination and total organic carbon content. push moraines (Rabassa et al., 1979; Villalba et al., 1990). We carried out the first bathymetric survey in 1994 documenting a maximum water depth of 75 m for this 4.1 km long and 1.1 km wide lacustrine basin (Fig. 4).

Fri

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Like the Lago Mascardi system, Lago Frı´as (40 S, 71 N, 790 m a.s.l.) is a proglacial lake located 7.5 km north of the Frı´as Glacier that is one of the seven Argentinean tongues of the Tronador ice cap with well-identified glacial advances between 1800 and 1850 AD and recent

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reflections (Ariztegui et al., 1997; Hajdas et al., 2003). Coherent high-amplitude reflections characterize the Holocene, whereas low amplitude to transparent facies distinguishes lateglacial deposits. These different seismic facies are caused by differences in physical properties. Mapping of reflection patterns that define seismic unconformities indicate two major environmental events probably representing lake-level changes (Fig. 3). The first of these unconformities corresponds to the Lateglacial–Holocene transition, also noticed by a distinct onlap surface. The sedimentation at this relatively shallow site is very sensitive to changes in water depth recorded in the sediments not only as a change in lithology, but also as a change in sediment geometry as documented by the onlapping reflections. The multiproxy analyses of a sediment core retrieved in this part of the lake show further changes in the percentage of total organic carbon throughout time. Hence, combined acoustic, lithological and geochemical features indicate an abrupt rather than a smooth change in the depositional environment related to climate change (Ariztegui et al., 1997). The excellent chronology of these changes and their synchronism with the equivalent Huelmo site on the western side of the Andean Cordillera (Moreno et al., 2001) allowed to define the HuelmoMascardi Cold Event (HMCE) interpreted as a cool event encompassing the European Younger Dryas chronozone, the Gerzensee/Killarney Oscillation and intervening warm spell (Hajdas et al., 2003). The second and youngest unconformity seems to be associated with an event in the Middle Holocene ( 6.0 ka) showing a less prominent impedance contrast than the Holocene boundary and, thus, may reflect a less abrupt change in environmental conditions.

71° 48’

Fig. 4. Bathymetric map of Lago Frı´as displaying the location of sediment core F94.2 discussed in this chapter. This proglacial lake is located much closer to the Tronador ice cap than Lago Mascardi.

Late Pleistocene Limnogeological Records During the twentieth century, the ENSO and ENSOlike phenomena have dominated climatic variations in the Americas on interannual as well as decadal time scales (Dettinger et al., 2001). The ENSO impact on local climate has been well determined using meteorological, historical and dendrochronological approaches at the Frı´as valley (Villalba et al., 1990, 1998). More recently, a sediment trap study in Lago Mascardi covering the 1992–1998 interval combined with meteorological data has shown changes in sedimentation rates that can be linked to ENSO climatic events (Villarosa et al., 1999). Since proglacial Lago Frı´as, like Lago Mascardi, is fed by the Tronador ice cap, the laminated sequence of Lago Frı´as can provide a continuous record of ENSO and ENSO-like variations through time. Similar to Lago Mascardi, Lago Frias is located in a tectonic and volcanic sensitive region. The latter combined with the abrupt margins of the basin are both propitious features to generate mass wasting events, precluding the accumulation of undisturbed sediments in all areas. Non-perturbated areas of the lake bottom were spotted in the field using high-resolution seismic profiling that further allowed to target core sites containing well-laminated sequences (Ariztegui et al., 2007). The combination of instrumental and historical information as well as event stratigraphy and radioisotopic data permitted the calibration of the sedimentary model in core F94-2 (Fig. 4), generating a robust chronology that confirms the annual character of the lamination (Fig. 5). Variations in the thickness of the clay lamina of a continuous laminated sequence have been related to changes in winter precipitation covering the last 200 yrs. Statistical analyses of this dataset indicate a dominant ENSO signal that has been previously identified in the Frı´as valley using tree rings (Villalba et al., 1990; 1998), and more recently in the sedimentary sequence of Lago Puyehue on the Chilean side of the Andes (Boe¨s and Fagel, 2008; see Fig. 1 for lake location). Conversely, these lacustrine records indicative of variable cold and rainy conditions during the LIA seem to be out of phase with the contemporary record of Laguna

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Mar Chiquita in subtropical Argentina (Piovano et al., 2002, 2004; refer to Fig. 1 for site location). Furthermore, the Lago Frı´as laminated sequence shows additional frequencies superimposed on the decadal ENSO variations that can be related to both the 11 yrs solar cycles and the Tropical Atlantic Dipole (TAD) (Ariztegui et al., 2007).

4.3. Laguna Cari-Laufquen Laguna Cari-Laufquen Grande and its small tributary Laguna Cari-Laufquen Chica (Fig. 6) are two closed basins located in a tectonic depression surrounded by basalt plateaus or ‘‘mesetas’’ of Mesozoic to Tertiary age (Coira, 1979). In contrast to lakes Mascardi and Frı´as, this region was not affected by the last glaciation of the Andes Cordillera. At an elevation of 800 m a.s.l., lakes CariLaufquen Grande and Chica are ephemeral, brackish water bodies with an average water depth of 3 m during the rainy season. Although high precipitation rates characterize the Andean region at the same latitude (e.g. lakes Frı´as and Mascardi), the mean annual precipitation in Laguna Cari-Laufquen is only about 200 mm/yr, occurring primarily in the winter months (May–August). Mean annual temperature is 4C with prevailing winds from the west. Paleoshorelines have been observed and mapped at elevations up to 68 m above the present lake level (Coira, 1979), and even older shorelines up to 100 m above today’s level have been previously described (Galloway et al., 1988). During these lake-level highstands, both lakes merged forming a large paleolake. Today, they are two separate basins but connected through the Rı´o Maquinchao (Fig. 6). Dated paleoshorelines indicate higher lake levels than nowadays occurring ca. 19 ka (Galloway et al., 1988) and also between 14 ka and 10–8 ka (Bradbury et al., 2001). Finegrained lacustrine deposits underlying the uppermost two shorelines contain high amount of diatoms and ostracods suggesting deposition in a deeper, saline and alkaline lake (Cusminsky and Whatley, 1996). Frequency (1/yr.) 700

1 mm

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Fig. 5. SEM backscattering microphotograph of a representative section of Lago Frı´as varves as shown in the interpreted sketch on its right. The statistical analyses of these annually deposited sediments indicate a dominant but not exclusive ENSO signal (see text for discussion).

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Fig. 6. Location map for lakes Cari-Laufquen Grande and Cari-Laufquen Chica presently joined by Rı´o Maquinchao. The black dot in Laguna Cari-Laufquen Grande indicates the location of sediment core CLG99-5b.

A seismic survey using a 3.5 kHz pinger system was undertaken in both lakes (Anselmetti et al., 1998). The dominant organic-rich sediments of Laguna CariLaufquen Chica prevent the acquisition of any subsurface images. Conversely, seismic sections obtained in Laguna Cari-Laufquen Grande yielded good acoustic stratigraphy in the central area of the lake. A very weak waterbottom multiple allowed imaging subsurface geometries in great detail to a depth of over 15 m documenting major structures related to paleo-lakelevel variations (Ariztegui et al., 2001). These geophysical data were used for choosing optimal coring sites. Sediment cores were retrieved using a short core and a vibrocoring system (see Fig. 6 for core location). Figure 7 shows results from multiproxy analyses of core CLG99-5b. Sharp differences in sediment color and fabric are accompanied by concurrent changes in both physical properties and geochemical character of the sediments. Increasing values of density and magnetic susceptibility may indicate lowstand and even desiccation intervals. Conversely, sediments holding relatively higher organic carbon and carbonate contents have been most probably deposited during periods of comparatively higher lake levels than today. Two range finding AMS 14C dates indicate very variable sedimentation rates that support the hypothesis of alternating erosional or constructional processes at or near desiccation levels in the lake during the Late Quaternary (Ariztegui et al., 2001). As a result, the sedimentary record is fragmented containing several hiatuses that, if well-dated, can provide a unique record of dramatic changes in the hydrological balance. A dataset combining modern ostracod assemblages and stable isotopes from the region indicates that biological remains

Fig. 7. Sedimentological, petrophysical and geochemical data for core CLG99-5b from Laguna Cari-Laufquen Grande. Concurrent changes in these various parameters are indicative of lake-level changes (see text for explanation).

Late Pleistocene Limnogeological Records may provide an additional approach to identify and calibrate these former variations in moisture budgets using lake cores (Schwalb et al., 2002).

5. Limnogeological Case Studies from Central Eastern Patagonia 5.1. Lago Cardiel Lago Cardiel (49 S) is a closed lake system on the Patagonian plateau about 200 km east of the Andes mountain chain. The lake is situated in a tectonic depression covering a modern surface area of about 370 km2 and has a maximum water depth of 76 m (Fig. 8). Mean

Fig. 8. Bathymetric map of Lago Cardiel in Santa Cruz Province (Argentina).

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annual precipitation in the area of the lake is relatively low ( 150 mm) due to the orographic rain shadow effect on the southern westerlies (Stern and Blisniuk, 2002). But as the lake’s catchment area is mostly located to the west, it is characterized by a steep precipitation gradient receiving an annual precipitation of up to 500 mm. This geographical setting within a precipitation gradient makes the lake a sensitive recorder of past changes in the regional climate as shown by a series of paleoshorelines indicating past lake-level changes. Galloway et al. (1988) and, in greater detail, Stine and Stine (1990) dated these paleoshorelines using bulk radiocarbon analysis and proposed a major lake-level highstand of þ55 m above the modern level around 10.8 ka BP, an intermediate highstand (þ21.5 m) around 5.9 ka BP and four minor lakelevel fluctuations in the last 2500 yrs. But a complete record of lake-level fluctuations can only be acquired in the deepest part of the lake, where the sedimentation can be expected to be continuous. A combined approach of seismic surveying and analyzing sediment cores permitted to identify, map the extent and date past lakelevel fluctuations. The excellent acoustic subsurface penetration up to 70 m allowed the mapping of the acoustic basement and the subsequent sedimentary infill almost throughout the entire basin. By applying the concept of seismic sequence stratigraphy, the imaged subsurface geology was divided into six major seismic sequences, which are labeled in roman numbers (Fig. 9). Sequence VI represents the acoustic basement of the basin consisting of Cretaceous–Tertiary claystones that make up the bedrock surrounding the basin (Beres et al., 2008). Sequence V overlays Sequence VI in a restricted area on the western side of the basin and is interpreted as a former alluvial fan unit. The Pleistocene and Holocene climate history is recorded in the four youngest seismic sequences (Fig. 10). The restricted occurrence of Sequence IV in the central basin indicates a low lake level during the Late Pleistocene that is out of phase with the tropical South America record from Lago Titicaca in the Bolivian altiplano (Baker et al., 2001; see Fig. 1 for lake location). On the basis of the onlap geometries of the seismic reflections in Sequence IV, the lake’s water depth was only a few meters. A desiccation period of a few

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6. Limnogeological Case Studies from the Fuegian Archipelago 6.1. Lago Fagnano Located at 54 S in the southern part of Isla Grande de Tierra del Fuego, Lago Fagnano is the southernmost and largest ice-free water body close to Antarctica. It is a latitudinally elongated lake of more than 110 km length and approximately 15 km width (Fig. 11). The lake occupies the deepest continental pull-apart basin in a series of graben-shaped sinks along the Magallanes–Fagnano transform (MFT) fault system that separates the South American plate from the Scotia plate (Lodolo et al., 2003). It comprises two subbasins: a smaller and deeper basin toward the east reaching a maximum depth of 210 m, and an elongated shallower basin toward the west with 110 m maximum water depth (Lodolo et al., 2002). The lake is located between the Cordillera Darwin in the south reaching a maximum altitude of more than 2400 m a.s.l., and the foothills of the cordillera in the north with a relatively low altitude mountainous belt (Olivero and Martinioni, 2001). The Claro, Milna, Tuerto, Valdez and Turbio rivers are the main feeders of this lake system, whereas Rı´o Azopardo at the western extreme of the lake is the only outlet toward the Pacific Ocean through the Magallanes Strait. With a total area of more than 1650 km2, this oligothropic lake (Mariazzi et al., 1987) evolved within a glaciotectonic basin after the retreat of the glaciers at the end of the Late Pleistocene. Modern evidence of neotectonic activity along the MFT can be found on outcrops along the Turbio River in the eastern part of the lake. In 1949 AD, a 7.7 magnitude earthquake caused the subsidence of a large area close to the lake shore forming a series of connected lagoons to the main Fagnano lacustrine system (Menichetti et al., 2001). Hence, the sedimentary infilling of the lake allows to reconstruct both the paleoclimatic and the paleoseismic histories of the region. Previous work showed that glaciolacustrine sediments cover the entire Holocene and probably date back even to the LGM (Lodolo et al., 2003; Tassone et al., 2005). A recent seismic survey (March, 2005) revealed a more than 100 m thick sedimentary package for the eastern basin. The seismic images indicate a relatively even sedimentation often interrupted by chaotic and transparent seismic facies that can be interpreted as mass-wasting

Fig. 10. Reconstructed lake-level curve for Lago Cardiel using a limnogeological approach. hundred years occurred after 13.16 ka BP resulting in a peaty, gravelly layer deposited at the Sequence IV–III boundary. This was followed by a large change in the hydrological balance at the base of Sequence III. This sequence is found throughout the entire basin implying a large lake-level rise after 12.6 ka BP up to at least the modern lake level. This fast transgression of almost 80 m occurred within a few hundred years but it was not constant. The occurrence of buried beach ridges during this transgression points toward a stepwise character of the lake-level rise, because only such a mechanism would allow their preservation preventing their erosion by wave action. This transgression exceeded modern lake level and reached an Early Holocene highstand of 55 m (Stine and Stine, 1990). Subsequently, the lake level receded but never dropped significantly below modern level (Markgraf et al., 2003; Gilli et al., 2005a). Seismically depicted sediment geometries revealed the presence of a large drift mound in the central part of the basin deposited during the last 6800 yrs. This sedimentation pattern is related to the existence of a persistent gyral lake current leading to a strong concentration of sediment in the central basin. The driving force for this lake current is likely the strong westerly winds affecting the area of Lago Cardiel inducing a movement of the water mass by surface shear stress. The presence of the drift deposition is therefore interpreted as an intensification of the 68° 30’

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Late Pleistocene Limnogeological Records

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Fig. 12. Original and interpreted seismic profile of Lago Fagnano showing well-defined mass-wasting events. The petrophysical properties of the well-laminated core displayed on the right confirm the potential of these lake sediments to study both the environmental history and the tectonic events.

events (Fig. 12). These episodic facies are most probably triggered by paleoearthquakes along the MFT fault zone. Physical properties analysis of the sediments clearly show density and magnetic susceptibility peaks correlating with sedimentary features indicative of single mass flow events (Waldmann et al., 2008). A detailed inspection of the sediments shows an excellent laminated sequence mostly composed of diatoms, amorphous organic matter and clays with very low carbonate content. Further, chemical and isotopical analyses of these sediments are in progress. Additionally, in March 2006 more than 90 m of sediment cores were retrieved in both subbasins at selected locations using the results of the seismic profiling. They will allow to further calibrate the seismic dataset and to constraint the timing of both climatic and tectonic events.

7. Outlook Coordinated seismic surveys and sediment coring allowed obtaining optimal paleoclimate records from several lake sites in Patagonia. The presented examples illustrate the use of a limnogeological approach integrating a large number of proxies to obtain a comprehensive picture of each lake system at various temporal and spatial scales. Amalgamating the petrophysical, sedimentological and geochemical data, this broad approach allowed the

calibration of seismic reflection sequences for both lake level and environmental change reconstructions. Several general conclusions can be assessed despite the existing differences among datasets for each individual record. A clear stepwise evolution for the Lateglacial–Holocene transition emerges from all the presented examples covering this time interval independently of their latitude. These truly multiproxy datasets are challenging early views, indicating a smooth warming east of the Andes during this transition (e.g. Mercer, 1983, 1984). Furthermore, intrahemispheric correlations of comparable datasets like Lago Mascardi (Argentina) and Lago Huelmo (Chile) show the same behavior and timing of key events during the deglaciation at both sides of the Andes. This comparison is a good example of the use of a limnogeological approach to tackle previous disagreements between records most often based on one proxy only. The exact timing of the observed environmental changes during the deglaciation, however, seems to differ at various latitudes, and a better and more detailed dating of the records is still necessary. This is similar to the present situation in Antarctica (PAGES News, 2006) and more efforts are needed to elucidate the regional pattern of changes. The early Holocene in Lago Mascardi ( 41 S) is marked by warming temperatures and further retreating ice. A clear latitudinal variation in precipitation is

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observed during this interval. Paleoshoreline data from Lago Cardiel ( 49 S) indicate that lake levels at this time slice were the highest during the past 20,000 yrs ( 55 m higher than today). Seismic and core data document that these paleoshorelines developed after a rapid lake-level rise of 80 m in a few hundred years. Pollen records from farther south in southern Tierra del Fuego, however, provide evidence for relatively dry conditions with an increase in fire frequency that has been related to fluctuations in the latitudinal position of the westerlies (Markgraf, 1993). Ongoing research in Lago Fagnano and Laguna Potrok-Aike will provide new multiproxy data to clarify this issue. Advancing glaciers and cooling conditions in Patagonia have been proposed during the Mid to Late Holocene (e.g. Porter et al., 1984; Ariztegui et al., 2000). Neoglacial ice advances have been reconstructed from moraine sequences in the Cordillera Darwin at 6.5 ka BP, and increasing in frequency and extent toward the late Holocene (Strelin et al., 2002). Isotopic studies of mosses recovered from peat bogs indicate an overall increase in moisture during this period with no significant changes in temperature (Pendall et al., 2001). An increase in the wind stress after 6.8 ka BP has been shown for Lago Cardiel through wind-driven current deposition (Ariztegui et al., 2004; Gilli et al., 2005b;). All these data would indicate that perhaps changes in precipitation more than temperature may have dominated the Late Holocene, producing the observed glacial advances in southernmost Patagonia. Evidence of the LIA is clear in Lago Frı´as sediments allowing good dating and comparison with recent moraines and instrumental data. Further to the south, Lago Cardiel evidence from both shoreline outcrops (Stine, 1994) and sediment cores (Gilli, 2003) is less clear. Stine (1994) has indicated drought conditions coinciding with at least part of the Medieval Warm Period (MWP) that precedes the LIA. It has been suggested that these droughts may have been caused by redirecting the mid-latitude storm tracks either by a general contraction of the circumpolar vortices or by a change in the position of their waves. Based on oxygen isotope ratios on authigenic carbonates, Gilli (2003) further concluded that an opposite precipitation signal dominates both the MWP and LIA intervals. This is in agreement with other Patagonian limnogeological records from Chile and Argentina showing warm-dry and cool-moist conditions during the MWP and LIA, respectively (Piovano and Ariztegui, 2006). Hence, the often underestimated role of precipitation during LIA is now emerging from the multiproxy analyses of these lacustrine records in Patagonia. As compiled in this book, numerous different proxy records have been used to reconstruct the Late Cenozoic in Patagonia. Many of these records, however, are discontinuous with a low time resolution preventing to register environmental variability at decadal or centennial time scales. Pollen profiles have so far had the best distribution to secure regional reconstructions of environmental change. They alone can often have multiple interpretations regarding paleoclimate. Paleoenvironmental reconstructions derived from lacustrine sediments in Patagonia combined with pollen and other proxies can provide critical

evidence to obtain more realistic reconstructions of environmental changes at different time and regional scales. The challenging issue of retrieving lacustrine sequences covering several glacial–interglacial cycles in Patagonia may soon be achieved through a multidisciplinary ICDP (International Continental Scientific Drilling Program) initiative (Zolitschka et al., 2006). Further comparisons of these lake records with other lacustrine, geomorphological and tree-ring evidence from the Southern Hemisphere as well as with high latitude marine and Antarctic ice core records will improve our understanding of both hemispheric and interhemispheric climate linkages.

Acknowledgments We are grateful for our collaborators who assisted the seismic and coring campaigns. The late A. Amos, M.M. Bianchi, J. Masaferro, G. Villarosa and the former PROGEBA team from CONICET, Argentina, were instrumental to the starting of our research in Patagonia. For lakes Mascardi and Frı´as, we thank in particular F. Niessen, A. Lehmann, C. Chondrogianni and K. Ghilardi. The study of Laguna Cari-Laufquen and Lago Cardiel was supported by the PATO Team headed by V. Markgraf and the late K. Kelts. J.P. Bradbury, J.A. McKenzie, A. Schwalb, A. Lu¨din, R. Hofmann, M. Beres, J. Captain, J.B. Belardi, R. Gon˜i, M.A. Gonzalez, B. Ercolano, S. Stine and J. Lezaun assisted the field campaign in these lakes. The tireless efforts of J.D. Moreteau and his crew and the infrastructural support of the Kusanovic family from Estancias La Angostura and La Siberia have been instrumental to the success of the Lago Cardiel project. J.A. Austin Jr, S. Saustrup and M. Wiederspahn (University of Texas, Austin, USA); R. Dunbar, C. Moy and D. Mucciarone (Stanford University, USA), G. Gonza´lez Bonorino and G.G. Bujalesky from CADIC-CONICET (Argentina), and C. Recasens (University of Geneva, Switzerland) contributed to the logistics to accomplish seismic acquisition as well as coring operations in Lago Fagnano. We acknowledge financial support from the Swiss National Foundation (projects NF21-37689.93, NF2100-050862.97/1, NF2000211006668/1 and NF200020-111928/1 to D. Ariztegui).

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Moreno, P.I., Jacobson Jr., G.L., Lowell, T.V. and Denton, G.H. (2001). Interhemispheric climate links revealed by a late-glacial cooling episode in southern Chile. Nature 409, 804–808. Olivero, E.B. and Martinioni, D.R. (2001). A review of the geology of the Argentinian Fuegian Andes. Journal of South American Earth Sciences 14, 175–188. PAGES News (2006). Ice Core Science. Fischer, H., Kull, C. and Kiefer, T. (eds), 14, 1, 45 p. Pendall, E., Markgraf, V., White, J. W. C. and Dreier, M. (2001). Multiproxy record of late Pleistocene-Holocene climate and vegetation changes from a peat bog in Patagonia. Quaternary Research 55, 168–178. Piovano, E.L. and Ariztegui, D. (2006). Cold-dry vs warmwet events in the South American extratropics since the Last Glacial Maximum. ICDP-Workshhop PASADO. Terra Nostra – Schriften der Alfred-Wegener – Stiftung, Berlin (Germany), 14–15. Piovano, E., Ariztegui, D. and Damatto Moreira, S. (2002). Recent environmental changes in Laguna Mar Chiquita (Central Argentina): a sedimentary model for a highly variable saline lake. Sedimentology 49, 1371–1384. Piovano, E., Ariztegui, D., Bernasconi, S.M. and McKenzie, J.A. (2004). Stable isotopic record of hydrological changes in subtropical Laguna Mar Chiquita (Argentina) over the last 230 years. The Holocene 14, 4, 525–535. Pittock, A.B. (1978). Climatic change and variability – A southern perspective. Cambridge, Cambridge University Press, 455 pp. Porter, S.C., Stuiver, M. and Heusser, C.J. (1984). Holocene sea level changes along the Strait of Magellan and Beagle Channel, southernmost South America. Quaternary Research 22, 59–67. Prohaska, F. (1976). The climate of Argentina, Paraguay and Uruguay. In: Schwerdfeger, W. (ed.), Climates of Central and South America. Elsevier, Amsterdam, 13–112. Rabassa, J., Rubulis, S. and Sua´rez, J. (1979). Rate of formation and sedimentology of (1976–1978) pushmoraines, Frı´as Glacier, Mount Tronador (41100 S; 71530 W), Argentina. In: Schlu¨chter, Ch. (ed.), Moraines and Varves. A.A. Balkema Publishers, Rotterdam, 65–79. Schwalb, A., Burns, S.J., Cusminsky, G. et al. (2002). Assemblage diversity and isotopic signals of modern ostracodes and host waters from Patagonia, Argentina. Palaeogeography, Palaeoclimatology, Palaeoecology 187, 323–339. Stern, L.A. and Blisniuk, P.M. (2002). Stable isotope composition of precipitation across the southern Patagonian Andes. Journal of Geophysical ResearchAtmospheres, 107 (D23), Art. No. 4667, December 3, 2002. Stine, S. (1994). Extreme and persistent drought in California and Patagonia during medieval time. Nature 369, 546–549. Stine, S. and Stine, M. (1990). A record from Lake Cardiel of climate in southern South America. Nature 345, 6277, 705–708. Strelin, J., Casassa, G., Rosqvist G. and Holmlund, P. (2001) Holocene glaciations at Glaciar Ema Valley, Monte Sarmiento, Tierra del Fuego. In: Moreno, P. (ed.), Abstracts of the International Symposium:

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12 Geocryology of Southern South America Darı´o Trombotto Liaudat IANIGLA, CONICET, 5500 Mendoza, Argentina, and Guest Researcher, Humboldt-Universita¨t zu Berlin, Germany Cordillera despite the fact that they appear very discontinuous, or restricted only to certain localities. What is important to note here, regardless of the number of glaciations, is that there are a large number of cold episodes. These events date from the Neogene to the Younger Dryas, or even from the Holocene–Pleistocene transition, and possibly from some time in the Holocene. Many of these are documented as minor events, the Holocene ones being defined as Neoglacial (Mercer 1976, 1985; Garleff and Stingl, 1984; Stingl and Garleff, 1985; Villalba, 1990; Rabassa et al., 1992, 2000). During these cold episodes the paleoenvironmental conditions were favorable for the formation of permafrost in Patagonia and the southern Andes. The hypothesis glacial climate – glaciation – permafrost allows us to calculate an approximate age but humidity, being a determinating factor for glacial expansion, may not necessarily correspond to a glacial stage, that is to say that the increase in ice volume occurs without a considerable drop in the mean annual air temperature (MAAT), as it may be presently observed in some regions of the Earth. The second parameter to be considered is the probability that the Andean Patagonian environments may have been climatically very different from present-day climate and highly variable as well. This chapter covers the most important periglacial or cryogenic traces of the Patagonian region and gives a chronological account of those traces during at least two important cold episodes, with considerable falls in MAAT, over a long period of time, which would actually correspond to glaciations. One of these episodes in Patagonia corresponds to the GPG, as mentioned above. Another cold episode is related to the latest glaciation, which affected more recent sediments of the Patagonian stratigraphy. The ice limit of Caldenius (1932), as indicated in Fig. 1, is a historic maximum. The relict system is reconstructed mainly in accordance with characteristics and morphodynamics of present-day cold environments. Equating glaciation with cryogenics is only a preliminary approach, which requires further thorough research. Subdivision of the cold episodes sensu stricto into different categories and ages may also be carried out with the help of cryostratigraphy. As a result, major changes may be expected both vertically in the profiles and horizontally along their exposition of the ground. Many aspects of the geocryology of Patagonia have been summarized separately by Arturo Corte (1997). His book was written in the early eighties but, after numerous setbacks, it took until 1997 for it being finally published. Arturo Corte first attempted a cryostratigraphical model

There were no voices here. There was this, what I saw; and though beyond it were mountains and glaciers [. . .], there was nothing to speak of, nothing to delay me further. Only the Patagonian paradox: tiny blossoms in vast space; to be here, it helped to be a miniaturist, or else interested in enormous empty spaces. There was no intermediate zone of study. Either the enormity of the desert or the sight of a tiny flower [. . .] you had to choose between the tiny or the vast. – Paul Theroux, 1992 – To Bronmai and Meyriona Lewis

1. Introduction Geological evidence indicates that during the ‘‘Ice Ages’’ the ground remained frozen for long periods of time. Cryogenic phenomena may be observed both in the mountainous or Andean region and in the Patagonian steppe. According to Mercer (1976, 1985) glaciation during the Plio–Pleistocene left traces at Cerro del Fraile, in the Patagonian Andes. Other, even older, traces occur at the Meseta of Lago Buenos Aires, which Mercer linked to the time of the Messinian Stage (Tertiary). In his publication (Mercer, 1976), he refers to the most important glacial advance as the ‘‘Greatest Patagonian Glaciation’’ (GPG; see also Clapperton, 1983, 1993; Sylwan, 1989; Rabassa and Clapperton, 1990) with an approximate age of 1–1.2 Ma. This glaciation reached the present coast of the Atlantic Ocean and advanced into the present submarine platform in southernmost Patagonia and Tierra del Fuego. Other authors (see Schellmann, 1998; Schellmann et al., 2000), however, have found glacial deposits in the Patagonian lowlands or steppe, where glaciers extended beyond the southern Andes also during the Neogene. This is a long mountain range, which is divided here into the Lakes Region between 35 and 45300 S, the Patagonian Andes between 45300 and 53300 S and the Fuegian Andes between 53300 and 55 S, approximately. According to such findings the possible occurrence of even older cold episodes cannot be ruled out. The cold episodes must have taken place during different glaciations of diverse activity, concerning their expansion and duration. All these episodes must have been decisive for the formation of permafrost, in order to shape the Patagonian landscape. Even the Neoglacial events of the Holocene demonstrate reactivation of periglacial Andean processes at numerous sites in the

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DEVELOPMENTS IN QUATERNARY SCIENCES VOLUME 11 ISSN 1571-0866 255

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Darı´o Trombotto Liaudat

Fig. 1. Cryogenic structures and landforms in Patagonia assigned to the Last Glaciation Maximum (approx. 18–20 ka), according to various authors. The interrupted line indicates the Initioglacial ice limit of Caldenius (1932). Romberg Cold episode threshold. Sealevel, –120 and –200 m below present sea level. The numbered sites on this and the following map are (1) Malvinas/Falkland Islands, (3) Pampa del Castillo, Holdich, (6) Rı´o Santa Cruz, (7) Tres Lagos, Rı´o Chalı´a, (10) Sierras Australes or Ventana Range, (11) Lago Cardiel, (12) Chimen Aike, Rı´o Gallegos, (13) Rı´o Deseado, (27) Las Heras (North), (28) Cerro Kensel, (29) Somoncura, El Dinosaurio, (30) Rı´o Santa Cruz, (31) Lago Fagnano or Cami, (32) Meseta El Pedrero, (33) Romberg, area of the Meseta de las Lagunas Sin Fondo, (35) Puerto San Julia´n, (36) Telken, (39) Valley of Haichol, (41) Cabo Pen˜as, Tierra del Fuego.

Geocryology of Southern South America for Argentina as early as 1991. Two years later, in 1993, Clapperton briefly summarized the Patagonian periglacial features. Trombotto (1994, 1998, 2002) presented the first inventories of cryogenic landforms with fossil cryogenic evidence and established a new chronological order of cold episodes. For the present work bibliographical data have been assembled and field data have been collected using standard geocryological and geological procedures both in the field and in the laboratory (Trombotto, 1991, Trombotto and Stein 1993a, b, Trombotto and Ahumada 1995a, b; Scha¨bitz et al., 1999).

2. Present Periglaciation The southern Andes can be divided into the Lakes Region (‘‘Comarca Andina’’), Patagonian Andes and Fuegian Andes (Table 1). A closer look at the ice-covered areas and ice-capped mountains (see Lliboutry, 1998), however, reveals the importance of the presence of perennial ice. Approximately an area of 20,400 km2 is still glaciated from 35 S southward. The most important

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ice-covered surfaces (Fig. 1) are the Northern Patagonian Ice Field located between 46300 and 47300 S (4300 km2), the Southern Patagonian Ice Field between 48150 and 51300 S (13,000 km2), the Mun˜oz Gamero Peninsula (200 km2), Santa Ine´s Island between 53 and 54 S (300 km2) and the Darwin Cordillera between 54 and 55 S (2300 km2). It has to be taken into account that these surfaces have suffered a considerable reduction caused by global warming over the last decades. The analysis of the climatic map of South America (Hoffmann, 1975) indicates an even larger surface with a MAAT between –5 and 0C, without visible surface ice but probably with permafrost. On the contrary, an approximate surface of 170,000 km2 (where temperature is between 0 and 5C on the map), which does reveal important glaciers (see Lliboutry, 1998), must coexist with some form of permafrost at high altitude – for example, Volca´n Lanı´n, Cerro Tronador, the Queulat ice cap (80 km2), the ice-capped Volca´n Hudson, Cerro San Lorenzo or Cochrane, Cordillera Sarmiento and Cerro Hatcher. Other areas without significant, major glaciation reach a MAAT of 0C simply because of their altitude, as for instance the area of Monte

Table 1. Present-day periglacial environments of the southern Andes Area/Altitudinal profile

Latitude S (approx.)

Periglacial level: most important landforms and microforms (descending in elevation)

Probable lower mountain permafrost limit (m a.s.l.)

Andes at Lago Alumine´

39 S

2600

Volca´n Lanı´n

39390 S

Cerro Tronador

41090 S

Cerro Nevado, Cerro Aguja Sur (‘‘Valle de Avalo´n")

42 S

Cerro Katterfeld–Cerro Mineral Cerro Ap Iwan–Cerro Rojo

45 S 46100 S

Cerro San Lorenzo

47360 S

Sierra de Sangra–Cerro Hatcher–Meseta de la Muerte Cerro Fitz Roy

48300 S

Slopes covered by cryogenic sediments, patterned ground and garlands >50 cm Ø, stone lobes, tors, miniature patterns Solifluction lobes, patterned ground and garlands >50 cm Ø, ploughing blocks, miniature patterns Slopes covered by cryogenic sediments, protalus ramparts, gelifluction, frost mounds, patterned ground and garlands >50 cm Ø, miniature patterns Slopes covered by cryogenic sediments, stone-banked solifluction lobes, patterned ground and garlands >50 cm Ø Stone-banked solifluction lobes, solifluction terraces, patterned ground and garlands Slopes covered by cryogenic sediments, patterned ground and garlands >50 cm Ø, tors Slopes covered by cryogenic sediments, solifluction sheets, rock streams, rockglaciers, patterned ground, thufur Slopes covered by cryogenic sediments, patterned ground and garlands >50 cm Ø, tors

Pico Reichert–Lago Argentino Fuegian Andes: Tierra del Fuego–Darwin Cordillera

49150 S 50300 S 54–55 S

Slopes covered by cryogenic sediments, stone-banked solifluction lobes, patterned ground and garlands >50 cm Ø, tors Stone lobes, solifluction terraces patterned ground and garlands >50 cm Ø, tors Rockglaciers, gelifluction, patterned ground >50 cm Ø, solifluction lobes, garlands

2500?

2300–2200

2000

1800–1900 1700 1600–1700

1600

1500

1300–1400 1000–1300

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Darı´o Trombotto Liaudat

Zeballos–Meseta del Lago Buenos Aires and Cerro Tetris–Meseta de la Muerte. Although for a simultaneous cooling effect the icecovered areas enhance freezing of the surrounding ground or rise in the permafrost table, the humid areas of southern South America tend to display landforms associated with the presence of glaciers and perennial snow. There is a considerable disparity between the lower limit of the snowline and the lower permafrost level in the Dry Andes, which is known as the depression of the lower permafrost limit. From 35 S southward, this limit coincides with the perennial snowline (Garleff and Stingl, 1986). This means that in these areas glacial, nival and periglacial landforms appear in combination. This is a humid to subhumid periglacial environment with possible permafrost (Garleff and Stingl, 1988). Garleff (1975, 1978) defined a periglacial zone of superficial frost, characterized by the removal of sediments which affects an Andean altitudinally lower level and also certain heights of the extra-Andean Patagonian mesetas and sierras. It is a level of scarce precipitation (200–400 mm/yr) where seasonal needle ice formation and deflation play an important role. The latter would correspond to the parageocryogenic level as defined by Corte (1983a) for the Andean subregion of Cuyo. Garleff and Stingl (1985, 1986, 1988) characterized cryogenic phenomena and permafrost in the humid Andes, as being dependent mainly on temperature. In the Dry Central Andes however, the most decisive factor is precipitation. Present-day cryogenic activity in the southern Andes is characterized by (1) fresh extrusions of fine sediments, (2) damage of vegetation cover, (3) creation of needle ice, (4) forms of movement, solifluction, vertical and horizontal sorting, and (5) thermal contraction and dessiccation cracking. According to temperature calculations, superficial drillings and geomorphological inventories (Garleff, 1977, 1978; Garleff and Stingl, 1985, 1986; Corte 1997; Trombotto, 2000), permafrost can be expected above specific heights listed in Table 1. According to Garleff and Stingl (1988), the lower limit of the mountain ‘‘continuous’’ permafrost is associated with a MAAT of –2 to –3C. The altitudinal

levels of minor periglacial processes (see Table 1) continue to descend much more depending on latitude (see Garleff, 1978), but they correspond in height with periglacial levels without permafrost and are the result of extraordinary or seasonal frost. MAAT and mean annual precipitations for Patagonia and Tierra del Fuego (Table 2) offer a most varied spectrum due to the vast extent and the diversity of the terrain (appr. 900,000 km2). The highest seasonal freezing index is registered east of the Andes between Chubut and Santa Cruz provinces, in the western part with a strongly continental influence (Buk, 1992). In the same area the highest depth of seasonal freezing was 70 cm at Alto Rı´o Sengerr in 1982 (Garce´s, 1992, oral communications). Nothofagus krummholz can be interpreted as an indicator of a periglacial zone and its frost dynamics. Above treeline, most miniature patterns (sorted nets, polygons and stripes particularly) are observed in an Andean ‘‘tundra type’’ environment. Landforms related to the possible occurrence of permafrost are limited because the periglacial zone is much narrower than in the Dry Andes. In the Lakes Region a humid periglacial environment prevails. Cryogenic mesoforms in the humid or Southern Andes, such as rockglaciers, do not attain the same importance and development as in the Dry Andes, but their presence has an environmental key role, because they indicate an arid (or semiarid) periglacial environment. Barsch (1996) stated that rockglaciers represent a relatively dry, continental climate. According to the present Andean conditions, active rockglaciers appear at MAAT around the freezing point (0C) with precipitation between 200 and 600 mm (Trombotto, 2000). They appear discontinuously, leaving the zone of Andean continuous permafrost behind, and reaching lower altitudes. Their presence indicates the so-called phase of rockglaciation at a subregional scale (Garleff and Stingl, 1988). This has to be emphasized, because it implies a strong periglacial involvement in the reconstruction of the paleoenvironment. Approaching approximately 35 S, the occurrence of active stone runs or rockglaciers (?) is reported (Dessanti, 1978) up to heights of 2000–2300 m a.s.l. at the Sierra de las Aguadas. Rockglaciers are also mentioned (Trombotto, 2000) from the southernmost southern Andes. A detailed report on

Table 2. Patagonian registers of MAAT and mean annual precipitations Locality

Latitude (S)

Longitude (W)

Height (m a.s.l.)

MAAT (C)

Precipitation (mm/yr)

Period

Viedma San Carlos de Bariloche Puerto Madryn Estancia Valle Huemules Las Heras Rı´o Gallegos Ushuaia

40 41090

63 71100

6 840

14.1 7.9

345.2 907

1971–1990 1971–1980

42

65

7

13,5

154

ca. 1902–1982

46

71

650

5

345

1933–1947

46330 51620 54

68570 6920 68

332 20 16

9.9 7.1 5.5 (1931–1990)

167 243.8 555.9 (1876–1990)

1941–1950 1931–1990 – –

Geocryology of Southern South America the rockglacier at Cerro Krund, Fuegian Andes, has been recently presented (Redondo Vega, 2004), with considerations on other periglacial features in the area. Permafrost has been reported in the Lakes Region close to Lago Vintter (at ca. 44 S) at an altitude of 2060 m a.s.l. and with an active layer of 20 cm thickness (C. Bianchi, oral communications) and at 51300 S between 1977 and 1978 at an altitude of 980–1100 m ´ ltima Esperanza, Meseta Latorre and Cordillera a.s.l. in U Chica (Roig et al., 1985; Roig, 1986), with an active layer of approximately 30 cm in the first two areas. In Tierra del Fuego periglacial processes not associated with permafrost such as small solifluction forms and sorted patterned ground have been registered as low as 800 m a.s.l. at Cerro Atukoyak and 650 m a.s.l. at Monte Martial (Garleff, 1978). The Andes between 35 and 39 S constitute a transitional region from a climatic point of view. Garleff and Stingl (1985) hold that Pleistocene-age permafrost extended down to 2500–3000 m, and that the depression of the MAAT was 15–18C. At the same time, the permanent snowline was estimated to reach down to 1500 m a.s.l. Other landforms closely linked to nival processes and also to mountain, or Andean, permafrost are cryoplanation surfaces or cryopediments. Today these mesoforms are often considered to be of polygenetic origin. Outside the Andes, between 42 and 46 S, in the Patagonian Sierras or Patago´nides (up to 1000 m a.s.l.), as well as in the Sierra de Tecka and Sierra de San Bernardo, paraperiglacial forms according to (Corte, 1983a) and patterned ground with small diameters have been detected, but no permafrost has been found (Trombotto, 2000). At the Meseta de la Muerte the presence of sorted stone-banked solifluction lobes and patterned ground >50 cm diameter suggest cryogenic activity at an elevation of 1600 m upward (Garleff, 1977). In the Andes of Tierra del Fuego, Valca´rcel-Dı´az et al. (2005, 2006a, b) have recently described permafrost conditions at elevations above 1100 m, with the development of cryoclastic processes, nivation hollows, subnival ‘‘boulder’’ (clast) pavements, debris lobes, patterned grounds, protalus ramparts and cryoejected clasts.

259

the LGM at 18–20 ka (MIS 2) (see Trombotto, 2002) (Fig. 1). The most reliable indicators of the former existence of permafrost in Patagonia, and in this particular case a permafrost type considered as continuous and characteristic of lowlands, are ice wedge casts and their polygonal and orthogonal patterns, which correspond to contraction and frost cracking. It has to be said, however, that the appearance of such patterns does not automatically imply the existence of permafrost (Washburn, 1979; Ballantyne and Harris, 1994). Active contraction cracks are frequently observed in the Andes (see Garleff, 1978; Trombotto, 2000), but they have also been found in southern Santa Cruz in March 1984 (Bustos and Corte, 1984) caused by abrupt falls in temperature. For the formation of active ice wedges, a MAAT ranging from –2.5 to –8C is necessary in Arctic and Antarctic regions, whereas those formed in gravels require the lowest temperatures (<–5C) (Pe´we´, 1969; Romanovskiy, 1973; French, 1996). Ice wedge pseudomorphs and polygons are frequently observed along Patagonian roads or deposits where the Patagonian gravels are quarried. They are identified by field observation and sedimentological analyses. These structures correspond mainly to epigenetic wedges containing secondary filling and without clear stratification (Fig. 2). Syngenetic ice wedge pseudomorphs belong to an intraformational type and localizing them is crucial because they enable a specific paleoform to be linked to certain episodes or sedimentary cycles.

3. Patagonian Pleistocene Permafrost Many experts agree that Patagonia and the southernmost regions of South America experienced cryogenic processes associated with the episodic occurrence of permafrost (Garleff and Stingl, 1986, 1988; Corte, 1991, 1997; Clapperton, 1993; Trombotto, 1996a, 1998, 2002). Permafrost covered all of Patagonia and reached the Atlantic Ocean during different cold episodes. Its extension reached northward penetrating the district of Payunia (southern Mendoza Province), a region that shares many characteristics with northern Patagonia. The most extensive event was the GPG dated to ca. 1–1.2 Ma (more recently dated in between 1.01 and 1.16 Ma; Ton-That et al., 1999); the second event was

Fig. 2. Epigenetic ice-wedge cast in Miocene sandstone with visible CaCO3 and Patagonian Gravel infill (Photo by Sabine Herfert, 1990). Site 5, Puerto Madryn.

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The vertical pattern of numerous Patagonian profiles, however, still raises many questions, for example, concerning the presence of forms that could just as well be remnant sand wedges. The simultaneous existence of ice and sand wedges could be accepted because of various characteristics accompanying the pseudomorphs, i.e. filling with autochtonous or allochtonous material, content of medium to fine sand, contacts to hosting sediments, internal laminations, interfingering edges and relatively plain tops coincident with a former permafrost table. The patterns are reminescent of presentday polar environments. Despite the fact that these patterns are not always a condition sine qua non, they strongly support the idea of former Patagonian cryoarid steppe environments. Other forms that characterize Patagonian permafrost are cryoturbation structures corresponding to type 2 of Vandenberghe (1988), involutions >0.50 cm amplitude, droplike structures, sand pockets and folds, the latter corresponding to type 5 of Vandenberghe (1988). 3.1. The Oldest Permafrost in Patagonia Cryogenic paleoforms enabled Czajka (1955, 1957) to infer that permafrost reached as far north as the Rı´o Negro. His studies were essential for the development of geocryology in Argentina, as they formed the stratigraphical basis as well as the paleoclimatic theories of Auer (1956, 1970) and Corte (1991, 1997). The latter extended the limit of Pleistocene permafrost to the Rı´o Colorado (Gonza´lez and Corte, 1976; Corte, 1991, 1997). Some paleoforms remained questionable, due to various phenomena that may create pseudoforms and the existing difficulties to date paleoforms (Clapperton, 1993). The resulting cryostratigraphy usually leaves many questions unanswered (Galloway, 1985a, b). The cryogenic structures of Puerto Madryn are possibly some of the most expressive examples to be attributed to the oldest periglacial environment in northern Patagonia (Trombotto, 1996b). They saylong to the Penfordd cold episode, after the name of the hills Penfordd or Penffordd, close to Puerto Madryn. Ice wedge casts reach a maximum development at a depth of 2.8 m and are combined with superficial orthogonal structures that are separated from each other by up to 2 m. Structures associated to ice veins (Katasonov, 1973) and to suffusion (Trombotto, 1996b) have also been observed. During the evolution of the cryogenic structures, different types of reliefs were probably present (see Trombotto and Ahumada 1993, 1995a, b). On a lower level of fluvial Patagonian gravels, a fluvial conglomerate (‘‘Rodados Patago´nicos’’ or ‘‘Patagonian gravels’’) does not display cryogenic influence but has been deposited consistently in an E–NE direction. In other cases, however, with thin layers of ‘‘Rodados Patago´nicos’’, the ice wedge casts and cryoturbations affected the upper part of the Puerto Madryn Formation (Miocene; Haller, 1981), which is composed of sandstone, siltstone, gypsum, sometimes with limestone levels or vegetal remains (Araucaria) deposited in a shallow marine and continental environment (Fig. 2, Trombotto and Ahumada, 1995b). The ‘‘Rodados Patago´nicos’’ are a typical Plio– Pleistocene sedimentary deposit composed mainly of

acid igneous rocks (rhyolite, porphyry, pumice) of variable size (pebbles and cobbles), which were already studied by Caldenius (1940), who also presumed that the dispersion of the Patagonian Gravel was caused by solifluction. The ‘‘Rodados Patago´nicos’’ are of fluvial, fluvioglacial or polygenetic origin forming the different geomorphological units (Fidalgo and Riggi, 1970) and they are found in many areas of Patagonia. They are not present on top of the highest basaltic ‘‘mesetas’’. They lie uncomformably on the aforementioned Tertiary formations. The cryoturbations associated with the Tertiary rocks were dated (ESR, Th/U and AAR) to the Last Glaciation caused by polygonal network development (approx. 2 m diameter) and are interpreted as indicators of continuous permafrost, which occurred probably in the Early Pleistocene (Trombotto, 2002). They were discovered and studied by Auer (1956), who supported the idea of glacioblastos (Groeber, 1950), that is local glaciation on the ‘‘mesetas’’. He suggested that the forms were molded by glacier ice. Auer (1956) made an important contribution because he added phenomena of solifluction to the scientific discussion, as a parallel participating agent. Work by Liss (1969) supports the search for a periglacial framework in the enviromental settings of extra-Andean glaciations. Liss (1969) added valuable information on ice wedge casts and deduced a cryogenic epigenesis. The area, however, even reveals types of syngenetic ice wedge casts between the ‘‘Rodados Patago´nicos’’ layers, which help to date the forms stratigraphically, as well as polygonal cryoturbations, certifying the presence of permafrost. CaCO3 networks with different types of calcretes (Trombotto, 1996b) in various stages of evolution are very closely linked with the ‘‘Rodados Patago´nicos’’ but not always with visible cryogenic structures (ice wedge casts, foldings). Trombotto (1992, 1996a, b) presented changes in the paleoactive layer, different types of ice wedge casts and a model of eolian CaCO3 deposition correlated with active volcanism. The model is based on physical laws and cryogenic criteria as also applied in some cases in Siberia (Dostovalov and Popov, 1966; Romanovskiy, 1973). Vogt (1992) tried to explain the origin of CaCO3 deposits of sedimentary structures assigned to periglacial phenomena comparing the area of Puerto Madryn with slopes between 2000 and 3000 m a.s.l. in the Cordillera Frontal of Mendoza, north of Patagonia. However, she did not consider the stratigraphical position of the cryogenic structures. On the one hand, forms and mesoforms at such heights and latitudes of the Cordillera Frontal were active during the Holocene and even until recently. On the other hand, the CaCO3 structures of the Plio–Pleistocene ‘‘Rodados Patago´nicos’’ levels do not always correspond to an ‘‘ice wedge infilling’’, as described in the example. Vogt and Del Valle (1994) concluded that the carbonate is of eolian origin, derived from a submarine platform that was exposed during the last glacial period. CaCO3 structures in the ‘‘Rodados Patago´nicos’’ (Trombotto, 1992, 1996a, b) do not always coincide with the position of ice wedge casts (Trombotto and Ahumada, 1993). Vogt and Larque´ (1998) presented the hypothesis that the opal of the silica concretions, which appear together

Geocryology of Southern South America with CaCO3 in the profile described in Puerto Madryn in 1992 at levels showing periglacial phenomena, would have originated on the basis of clay minerals (smectite and sepiolite) near and even within permafrost. The problem of precipitation and neomineralization in periglacial environments is far from being a novelty, because it has been widely known in Russian literature for a long time. They are found within the active layer, in the horizon of ‘‘active cryohypergenesis’’ (Trombotto, 1985, 1991) or the layer identified as Eisrinde by Bu¨del (1981). Other forms with similar characteristics and probably of corresponding age are the ice wedge casts in San Antonio Oeste at 40500 S and 65300 W. These were already described by Czajka (1955) and Auer (1956). Other forms considered to be cryogenic in northernmost Patagonia are folds found in Stroeder (40150 S and 62400 W), Buenos Aires Province. These were associated with a very old glaciation by Auer (1956). Auer (1956) also mentioned traces of cryoturbation in connection with the ‘‘Patagonian gravels’’, from Stroeder (40 S) southward to 50300 S and quoted cases from the Ventania ranges at 38 S. Cryoturbation affected not only Tertiary rocks, mainly sandstones, as in Salinas Chicas (southern part of Buenos Aires Province) and Puerto Madryn (Chubut Province, Northern Patagonia), but also the lower, upper and intermediate levels of the ‘‘Patagonian gravels’’. 3.2. Periglacial Structures of the Last Glaciation

261

road to Laguna Sirven (Las Heras Sur). The first three examples were studied in detail by Trombotto and Stein (1993a, b) and later by Scha¨bitz et al. (1999). Other examples of ice wedge casts (see Fig. 1) assumed of Weichselian age are reported from Romberg (46400 S and 68500 W, approx. 325 m a.s.l.), Telken (46480 S and 71460 W; Fig. 4), Puerto Deseado (47520 S), Puerto San Julia´n (approx. 49150 S) and Tres Lagos – Rı´o Chalı´a (49350 S) (Auer, 1956, 1970; Schellmann, 1998). Casts from the Puerto San Julia´n area were dated to the Last Glaciation on the basis of cryoturbated coastal deposits bearing mollusks (Schellmann, 1998). The most recent ones are allocated to the Romberg cold episode after the name of a nearby estancia (ranch). This profile contains two generations of well-preserved ice wedge casts. They are located on the penultimate fluvial terrace of Rı´o Deseado (Feruglio, 1950), just like those at Las Heras (Scha¨bitz et al., 1999). Cryoturbations (foldings, subhorizontal droplike structures) indicating permafrost of that age occur frequently (Trombotto, 2002) and appear as well preserved from 46 S southward. The forms can be grouped together with solifluction and ice wedge casts (Figs 3 and 5). Good examples of remnant patterned ground, polygonal networks with uplifted borders indicating ice pressure, are found at site 27 ‘‘Las Heras N’’ (Trombotto, 2002). They are hexagons combined with ice wedge casts (Scha¨bitz et al., 1999). Moreover, Corte (1997) found

Small ice wedge pseudomorphs, with a maximum depth of 90 cm and a width of 30 cm, are only present in southern Patagonia. Typical sites are Holdich–Pampa del Castillo (45500 S and 68 W, 700–720 m a.s.l.), Cerro Kensel (46080 S and 70350 W, 720 m a.s.l.) and Las Heras Norte (46300 S and 68550 W, 330 m a.s.l.) (Figs 1 and 3). Corte (1997) mentioned casts along the

Fig. 3. Ice-wedge casts and cryoturbation from the Last Glacial Maximum. On top of the ice wedge casts a paleosol indicates a warmer episode (>15 ka?). The pseudomorphs penetrate up to 90 cm into the marine Oligocene and are filled with a head (13–15 ka?). The filling has more than 80% sandy material (quartz grains also show eolian characteristics). Length of hammer: 31.8 cm (Trombotto, 2002). Site 3, Holdich-Pampa del Castillo, Chubut.

Fig. 4. Ice-wedge casts from the Last Glacial Maximum. Length of hammer: 31.8 cm. The dark polyhedral horizon outlines the cryogenic structures, indicating a paleosol or warm episode. Site 36, Telken, Santa Cruz.

262

Darı´o Trombotto Liaudat Fuego have been assigned to the Last Glaciation, because they are intruding the marine La Sara Formation, which is dated from the Last Interglaciation (marine isotope stage 5e), with minimum radiocarbon ages of 43 ka BP and Th/U ages of 82 ka + 2.5 (Coronato et al., 2004).

3.3. Remnant Periglacial Structures in Rı´o Gallegos

Fig. 5. Cryoturbation, irregular deformation structures probably created by cryohydrostatic pressure (Trombotto, 1998). Site 36, Telken, Santa Cruz.

vegetation patterns N of Rı´o Gallegos and on the Rı´o Deseado planation level. Here, cryophilous vegetation alignments were structured following a pattern left by ice wedge pseudomorphs, which reflect a polygonal tundra pattern (Roig et al., 1985). Dating of CaCO3 structures from Puerto Madryn by C14, resulted in the following ages: (1) within an ice wedge cast, 22,700 + 500 yr BP, and (2) outside the cast, 27,200 + 800 yr BP. For this reason the casts were attributed to a cold episode around 25 ka (Corte and Beltramone, 1984). Later on, Del Valle and Beltramone (1987) and Beltramone (1989) dated a second generation of ice wedge casts to approximately 40 ka. Based on these datings, Corte (1991) assigned two generations of ice wedge casts in Puerto Madryn to the Last Glaciation. The older of the two is of a similar age as the structures found in El Diamante (Mendoza Province; Grosso and Corte, 1989), the younger would be of an age similar to that of the casts found in Rı´o Gallegos–Chimen Aike. The ages of the Last Glaciation are also accepted by Vogt and Del Valle (1994), although they prefer the age of the carbonates outside the ice wedge cast above the more recent dating (27.2 + 0.8 ka BP). They contradict the idea of Corte and Beltramone (1984) who supposed that at 27 ka permafrost was nonexistent in that area and that the date (22.7 + 0.5 ka BP) of the carbonates within the filling were related to the final permafrost degradation. Other ice wedge pseudomorphs are reported from the Malvinas/Falkland Islands near the mouth of San Carlos (51300 S and 59 W) though they are of unknown age (Clark, 1972). Finally, ice wedge pseudomorphs found in the area of Cabo Pen˜as (53500 and 67350 ) in northern Tierra del

The Rı´o Gallegos–Chimen Aike (51400 S and 69200 W) site is complex. Cryogenic phenomena were first presented by Arturo Corte in 1968 (Corte, 1968, 1983a, b, 1997). Galloway (1985a, b) recognized two generations of ice wedge pseudomorphs: the one in the upper part of the profile is associated with the Last Glacial Maximum. It cross-cuts a till and passes through deformed carbonate layers. The second belongs to ‘‘an earlier glacial phase’’ because the casts are glacially deformed. These two sets of casts were recognized by Corte in 1983 (1997), but he added another generation of casts associated with those from the upper till and which he called ‘‘the third cryogenic period’’. Galloway (1985a, b) compared this site with Australia and figured a change of the MAAT of 10–12C, similar to the Snowy Mountains. Clapperton (1993) proposed that casts in the lower till, which also show massive carbonate structures similar to those in Puerto Madryn, might have an age comparable to that of the till itself, or else have formed during the next glaciation, which should be close to the age of the upper till (1–1.2 Ma following Mercer, 1976; 1.01–1.16 Ma according to Ton-That et al., 1999). Similar structures in the region were reported by Ercolano et al. (1997). Corte (1991, 1997) presented a more complex model. He distinguished two different situations, based on several profiles, with different cold episodes and various generations of casts. Trombotto (1998) summarized Corte’s chronology presenting Chimen Aike I (24 ka) and II (40 ka) for the more recent cold episodes of the study area and Rı´o Gallegos I (3.5 Ma) and II (1.2 Ma) for the older cold episodes. Thus, the chronology included seven cryogenic events although not all are well documented. Corte (1991) also reported on casts in Tierra del Fuego (Meglioli and Evenson, 1990, in fide Corte, 1991; Meglioli, 1992) of 1–1.2 Ma.

4. Other Indicators of Former Permafrost Other important landforms to infer the existence of former permafrost would be the confirmation of pingo scars (Garleff, 1977; Corte, 1991) and relict rockglaciers (Czajka, 1957; Corte, 1976, 1983b, 1997; Trombotto, 1996a). Some bajos sin salida (endorheic basins, without outlet; Methol, 1967; Clapperton 1983) found in Patagonia and Tierra del Fuego may eventually be linked to cryogenic activity. Garleff (1977) named them kaven (Garleff, 1968), which include possible pingo scars. He used this to indicate a possible origin of many Patagonian kaven (Santa Cruz Province). In general, data on

Geocryology of Southern South America remnants of frost mounds are very scarce. The size and the polygenetic geomorphological characteristics complicate the analysis of the kaven, and many of them may be small lakes or lagoons. Corte (1997) mentioned remnant palsas near Lago Fagnano (Cami) in Tierra del Fuego and reported one pingo remnant on basaltic rock at La Leonora in Southern Patagonia (Corte, 1991). Clark (1972) also mentioned pingo scars on the Doyle River (Malvinas/Falkland Islands) but in an environment somewhat climatically different than the Patagonian periglacial conditions (see Clapperton, 1993). There is also information on relict rockglaciers. The first author to mention them is Czajka (1957) who identified them in the Ventania Range (Buenos Aires Province, 38 S, 62 W) on quartzites and sandstone. Corte (1976, 1983b, 1997) supported Czajka’s ideas and supposed their lower limit to be at 500–700 m a.s.l. The Ventania Range appears to have been a Pleistocene ‘‘island’’ with permafrost and Trombotto (1991) studied sediments of lobes considered to be part of rockglaciers. The presence of former cryoplanation surfaces was recognized in a pioneering paper by Keidel (1922). He described them for northwestArgentina and gave them the name of penillanuras cuspidales, upland cryoplanation surfaces, referring to one of the types known from presentday periglacial environments. These landforms appear everywhere in the Andean summits and are closely linked with variations in the lower limit of the permanent snowline. In the particular case of the southern Andes, these upland cryoplanation surfaces are of great interest because, following Sukhodrovskiy (1967; in Czudek, 1989), it is assumed that they reach their final stage by cryogenic downwearing processes. Garleff (1977) used former cryopedimentation and cryoplanation to calculate the depression of the lower snowline and the lower, humid periglacial level. He estimated that these values vary from 1000 m in the north and center of the wet Andes to 500–800 m in the south, possibly because of precipitation variations in a drier Pleistocene environment. Reports of a past periglacial maritime climate come from the Malvinas/Falkland Islands (Clapperton, 1993). Clark (1972) mentioned fossil cryoplanation surfaces and relicts of tors at Mt Kent (ca. 480 m a.s.l., Mt. Challenger and Wichham Heights). He also identified relict nivation hollows at mounts Usborne, Adam and Maria (50 S). Elsewhere in southern Patagonia Magnani (1962) registered relict nivation hollows at 48 S near Lago Cardiel, whereas in the basaltic region of the ‘‘Meseta de las Lagunas sin Fondo’’ (Meseta del Pedrero; approx. 47 S) relict nivation hollows have been identified on southward exposed slopes close to 1000 m a.s.l. (Trombotto, 2002). Other two, supposedly fossil, cryogenic sedimentary structures have to be considered. First, there are relict debris slopes, a type of solifluction deposit. Second, there are gre`zes lite´es, sedimentary layers of supra- or intranival origin of alternating granulometric deposition (Trombotto, 2000). In terms of solifluction processes, the pioneer study of Andersson (1906) in the Malvinas/Falkland Islands was

263

an important early contribution to the international literature, while felsenmeer and stone runs were studied by various authors on the islands (Clark, 1972; Bellosi and Jalfı´n, 1984). A comprehensive study of the topic is presented by Clapperton (1993). Solifluction deposits are observed near the Andean Cordillera (Garleff, 1977) and close to or surrounding the great Patagonian mesetas, predominantly on south-facing slopes like the Meseta de Somoncura (El Dinosaurio, Fig. 1) and the Meseta El Pedrero (Trombotto and Stein, 1993b; Trombotto, 1996a, 2002; Scha¨bitz et al., 1999) or at the Meseta del Castillo. Solifluction layers related to the mesetas are of limited extent, particularly in the north of Patagonia, whereas the thickness of these layers is more pronounced in southern Patagonia. At approximately 46 S, there are several representative levels that are related to ice wedge casts and associated with the Last Glaciation (Trombotto and Stein, 1993a; Trombotto, 1996a, 2002; Corte, 1997). A solifluction layer on top of ice wedge casts and an undated, interstadial (?) paleosol at the Meseta del Castillo could even be linked to a last cryogenic event 14–15 ka ago (Clapperton, 1983; Mercer, 1983, 1984; Mercer and Ager, 1983), which would certainly have left traces in the Patagonian steppe (Trombotto and Stein, 1993a, b; Scha¨bitz et al., 1999). In 1997, Corte also reported on fossil solifluction on the slopes of the southern Andes and Lago Buenos Aires. Outside the Andean contact zone, Auer (1956) mentioned solifluction in the Ventania Range or Sierras Australes at 38 S (Buenos Aires Province). He even mentioned carbonate crusts affected by cryogenic processes, but did not agree with the importance of solifluction in Patagonia, because of limited occurrence and insufficient precipitation. Corte (1983b) extended the Pleistocene lower limit to 500 m a.s.l. in the Ventania Range. In the Malvinas/Falkland Islands, considering their Pre-Holocene beaches, Clapperton and Roberts (1986) and Clapperton (1993) analyzed solifluction layers with cryoturbation, that were assigned to the Last Glaciation Maximum (MIS 4 and MIS 2), based on a paleosol dated at approximately 26 ka in between two solifluction layers. In southern Chile, Veit and Garleff (1993) described solifluction layers with a thickness of 50–70 cm south of 42 S (72200 W); north of this latitude such layers appear less frequently and are restricted to the southeast part of the Valle Longitudinal and Cordillera de la Costa (41 S, 72200 W). The latter were also assigned to the Llanquihue Glaciation (equivalent to the Last Glaciation). Solifluction phenomena contribute to the genesis of asymmetrical valleys and dells. Such valleys have been reported by Magnani (1962), Garleff (1977) and Corte (1997), but only for southern Patagonia.

5. Conclusions, Discussion and Recommendations For at least the last million years, cold arid environments seem to have prevailed in southern South America. The nonglaciated landscape of Patagonia and the southern

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Andes was shaped in periglacial environments. The formation of fluvial terraces was influenced not only by fluvial dynamics and tectonics, but also by cold climate and glaciations (Garleff et al., 1983; Garleff and Stingl, 1983; Stingl and Garleff, 1983). Large former rivers on the steppe occasionaly modified these characteristics (Gonza´lez and Trombotto, 1990). This picture is completed by strong volcanic activity, not only in the Andean Cordillera but also in the Patagonian steppe. Processes and landforms are interpreted in the context of two (hypothetical) dominant cold episodes: the GPG and the Last Glaciation Maximum. There may have been other episodes, possibly associated with frequent Pleistocene Andean glaciations. Two different scenarios seem to emerge during the two episodes: one for northern Patagonia and one for southern and central Patagonia. The situation in northern Patagonia is of particular interest because it leaves more questions unanswered since it is related to very old cryogenic forms. It might be linked with repeated intensification of the Arid Diagonal extension in South America, the arid region between ca. 25 and 45 S (Garleff et al., 1991), as suggested by Garleff and Stingl (1985). Changes of the MAAT during different cold episodes are an important parameter for regional and temporal differentiation. In northern Patagonia, at 42 S, the MAAT differences, based on ice wedge casts, were approximately 18C. They are related to the GPG when all of Patagonia was occupied by continuous permafrost. As far as the data assigned to the LGM are concerned, the most expressive information is found in southern Patagonia, south of 46 S. It is unlikely that the northern littoral zone of Patagonia had continuous permafrost, although the environment was predominantly cold and arid; hence moisture pulses were of key importance for the generation of periglacial landforms (Trombotto, 1998, 2002). In northern Patagonia, the Puerto Madryn site leaves unanswered questions as well. Profiles are complicated by the network structure (‘‘windows’’ and ‘‘columns’’), marked by CaCO3 precipitation within the fluvial nature of the Patagonian gravels (Trombotto, 1992, 1996b). These forms may be confused with ice wedge casts (Trombotto, 1996b). The CaCO3 structures seem to have formed in different steps and at different times because there are evidently different types of concretions and carbonates presumably of eolian origin, given their great dispersion. It is thought that the carbonates are very old, because of their quantity and consolidation. On the contrary, the lithological composition of the modern coast makes it unlikely that marine wind would be responsible for the accumulation of such quantity of carbonates, with a large dispersion and in a short time (Schellmann, 1998). Another probable eolian origin would be related to W and SW winds, interfering with the Andean Cordillera and related to volcanic events. This theory is validated by various paleoclimatic models (Heusser, 1991; Markgraf, 1991, among others). Finally, the massive infilling structure (Fig. 2; Trombotto, 1996b) of the ice wedge casts sometimes contains gypsum, indicating an environment with high evaporation, and it always displays ‘‘Rodados Patago´nicos’’ and

quartz coming by collapse from the matrix of these gravels. In the work presented by Vogt and Del Valle (1994), it remains unclear how the carbonate could have penetrated the ice wedge (supposedly filled by ice) when at the same time the frost table prevented carbonate penetration. These authors doubt whether the structures are indeed ice wedges or possibly frost wedges. Their argument for an origin by thermal contraction cracking does not sufficiently explain forms generated by cryostatic pressure. Problems of dissolution and mobilization of carbonates with changes in their fabric still require detailed analysis in a regional context. An interesting theory by Salomon and Pomel (1997) about the origin of the carbonates and possibly also of chlorine, typifying a littoral origin (Vogt and Del Valle, 1994), proposes that the carbonates may have an eolian volcanic origin while the chlorine was already part of the sediments, which would favor precipitation of carbonates. It should be noted that gypsum is associated with Late Tertiary surfaces and involved in the CaCO3 deposits and ice wedge casts. The age of the pseudomorphs is based on two hypotheses. One assigns them to the Last Glaciation Maximum and the other considers them to be much older, possibly from the GPG. Trombotto (1992) considers that some of the syngenetic wedges be related to the age of the ‘‘Rodados Patago´nicos’’. Clapperton (1993) also supposed that the casts were old and possibly belonged to the GPG. Following the principles of carbonatation cycles (Mayewski, 1994) the carbonates may have rejuvenated during the cold episodes incorporating younger additions using the system of cracks and fissures already described. In doing so, they affect the ages and interpretation of cryogenic structures filled with carbonates. This might apply to the ca. 40 ka date by Del Valle and Beltramone (1987). Beltramone’s (1989) interpretation of the dating of the two cold episodes between 36.5 + 2.3 and 24.3 + 0.5 yr BP as coincident with the Last Glacial Maximum is not satisfactory, because these ages are pretty much outside the range considered as the LGM, as MIS 2 in the Vostok curves. These ages are most likely correlated with the Last Glaciation and with the ‘‘columns’’ but, definitely, not with the casts. But the line of reasoning of Vogt and Del Valle (1994) that the ice wedge casts prove the fluvioglacial origin of the Patagonian gravels is not correct either, because it is not a necessary condition. Fluvial and fluvioglacial deposits are notoriously hard to distinguish, but in case the Patagonian Gravel is supposed to be of glacigenic origin, where would these glaciers be? It would be much more reasonable to assume that the deposits respond to deglaciation processes when the transportation system had much more transportation capacity and that they alternate with cold episodes. There are many possible explanations for the origin of the Patagonian gravels (see Martı´nez and Coronato, this volume). In any case, it is very likely that they have been modified by exogenic processes and that they are of polygenetic origin. The absence of relict rockglaciers can be explained either by high aridity during the Last Glaciation or by inappropriate relief and lithology. On the contrary, the

Geocryology of Southern South America possibility of relict rockglaciers in the Ventania Range opens an interesting perspective for the presence of discontinuous permafrost during the Last Glaciation Maximum in Buenos Aires Province at an elevation above 500 m. The limited presence of solifluction in Patagonian profiles can be explained by a paleoclimatic model with very low precipitation, though lithology may have played a key role. Their ages remain still uncertain. The potential for cryogenic indicators further north leaves many problems to be solved: these include maximum extent of continuous, discontinuous or sporadic permafrost during the Cenozoic and correct dating. In Salinas Chicas (approximately 38450 S, 63 W; 60 m a.s.l.) there is a structure resembling cryoturbation (Trombotto, 1991); is this one of the northernmost cryogenic sites? It is also necessary to distinguish cryogenic processes from other processes. For example, ‘‘carbonateturbation’’ by dissolution, retransport and precipitation ought to be analyzed in detail to avoid possible confusion. Finally, it would be helpful to study the interaction of periglacial environments and life in the past, thus allowing for a better paleoenvironmental reconstruction.

Acknowledgments I would like to thank Sabine Herfert and Pippa Genner for their help in translating the manuscript. I am very thankful to Prof. Dr Karsten Garleff, Prof. Dr Helmut Stingl and Prof. Dr Gerhard Schellmann (Bamberg University) for their revision and critical comments on the manuscript. Special thanks to Michael Remmers (KomRegis). I thank editors and reviewers for improvement of the manuscript.

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Valca´rcel Dı´az, M., Blanco Chao, R., Pe´rez Alberti, A. et al. (2005). Terrazas de crioplanacio´n en la Sierra de Alvear, Andes Fueguinos, Tierra del Fuego, Argentina. XVI Congreso Geolo´gico Argentino, Actas 262. La Plata. Valca´rcel Dı´az, M., Carrera Go´mez, P., Coronato, A. et al. (2006a). Caracterizacio´n y primeros datos sobre geoformas y depo´sitos de origen crioge´nico en las Sierras de Alvear, Andes Fueguinos, Argentina. VI Jornadas Nacionales de Geografı´a Fı´sica, Actas, Resu´menes 21. Rı´o Gallegos, Argentina. Valca´rcel Dı´az, M., Carrera Go´mez, P., Coronato, A. et al. (2006b). Cryogenic landforms in the Sierras de Alvear, Fuegian Andes, Subantarctic Argentina. Permafrost and Periglacial Processes 17, 371–376. Vandenberghe, J. (1988). Cryoturbations. In: Clark, M.J. (ed.), Advances in Periglacial Geomorphology, 8, 179–198. John Wiley & Sons Ltd., Chichester. Veit, H. and Garleff, K. (1993). El Cuaternario de la Regio´n de los Lagos del sur de Chile. ‘‘Guı´a de excursio´n’’, compiled by Carolina Villagra´n, Santiago. Villalba, R. (1990). Climatic fluctuations in Northern Patagonia during the last 1000 years as inferred from tree-ring records. Quaternary Research 34, 346–360. Vogt, T. (1992). Western Anti-Atlas (Morocco) and Central Patagonia (Argentina) calcretes: the calcium carbonate origin. Zeitschrift fu¨r Geomorphologie, N.F., Suppl.Bd. 84, 115–127. Vogt, T. and Del Valle, H. (1994). Calcretes and cryogenic structures in the area of Puerto Madryn (Chubut, Patagonia, Argentina). Geografiska Annaler 76 A, 1–2, 57–75. Vogt, T. and Larque´, P. (1998). Transformations and neoformations of clay in the cryogenic environment: examples from Transbaikalia (Siberia) and Patagonia (Argentina). European Journal of Soil Science 49, 367–376. Washburn, A.L. (1979). Geocryology. E. Arnold, London, 406 pp.

13 Neogene Vertebrates from Argentine Patagonia: Their Relationship with the Most Significant Climatic Changes Eduardo P. Tonni1,2 and Alfredo A. Carlini1,3 1

Departamento Cientı´fico Paleontologı´a de Vertebrados, Museo de La Plata, Paseo del Bosque s/n, (1900) La Plata, Argentina. 2 Comisio´n de Investigaciones Cientı´ficas de la provincia de Buenos Aires, CIC. 3 Consejo Nacional de Investigaciones Cientı´ficas y Te´cnicas (CONICET)

San Julia´n, Monte Leo´n and Gaiman formations, see Zinsmeister, 1981; Cione and Cozzuol, 1990; Cione, 2002) ranges from the Late Eocene to the Early Miocene; in northeast Patagonia, it is represented by the Puerto Madryn Formation which is Middle–Late Miocene in age (Arratia and Cione, 1996; Cione et al., 1996). In the Patagonian region, no mammals that may be clearly correlated with those characterizing the uppermost Miocene–Pliocene units of the extra-Patagonian area (Montehermosan, Chapadmalalan and Marplatan stages; Cione and Tonni, 1999, 2005) have been recorded. The single exception are the few reports of the Puerto Madryn Formation (Dozo et al., 1999, 2002) or those of the Cerro Azul Formation at the northern boundary of the studied area (Montalvo, 2000, 2001, 2003; Urrutia and Scillato-Yane´, 2003; Montalvo and Verzi, 2004; among others). Those of the Early Pleistocene (Ensenadan stage) are not present either. The remains of the Late Pleistocene are relatively frequent, but almost restricted to those representing approximately the last 15,000 yrs, many of which are directly or indirectly associated with archeological sites. Tonni et al. (1982: 149) pointed out that during the Pleistocene ‘‘. . . gran parte del territorio patago´nico estuvo habitado por megamamı´feros de las mismas especies o muy cercanamente emparentadas a las que habitaron el a´rea pampeana’’ [‘‘. . . a large portion of the Patagonian territory was inhabited by megamammals of the same species or very close related to those inhabiting the Pampean region’’]. The successive faunas recognized since the Neogene in Patagonia will be described, as well as their contribution to the chronology of the host sediments and their relationships to climatic and environmental changes. Given that the vertebrate record in Patagonia is very scarce since the Late Miocene, it was necessary to base the analysis on the northern extra-Patagonian faunas (Pampasian), which are highly diversified and better known, to interpret the changes of the Patagonian region more precisely.

1. Introduction Pascual and Odreman Rivas (1973) made an excellent update of the mammal-bearing units of the Patagonian Cenozoic and their relationships with diastrophic processes. In what these authors called ‘‘Patagonic area’’, they distinguished two regions with mammal-bearing sediments, belonging to two different basins limited southward by the Patagonian massifs: (1) the region between the Macizo de Somun Cura or North Patagonian Massif and the Macizo del Deseado (Deseadan Massif), and (2) south of the Deseadan Massif. Both regions correspond mainly to the San Jorge Gulf Basin and the Austral Basin, respectively (Fig. 1). In the San Jorge Gulf Basin, sediments with faunas pertaining to the Paleogene prevail, while in the Austral Basin the beginning of the Neogene is best represented with the outstanding exposures of the Santa Cruz Formation, already known since the end of the twentieth century through the fieldwork of Carlos Ameghino and the paleontological descriptions of his brother Florentino (see Ameghino, 1889, 1906). According to Pascual et al. (2002), the oldest Cenozoic mammals recorded in southern Patagonia (Santa Cruz Province) belong to the Paleogene, the Casamayoran (Late Eocene) and Deseadan (Late Oligocene) stages. In turn, the Neogene is represented by the Santacrucian (the latest Early Miocene) stage, followed by younger units up to the Mayoan stage of the ‘‘Estratos del Guenguel’’ (late Middle Miocene) in the northwest of the province (Dal Molin and Franchi, 1996). The Santacrucian stage is undoubtedly the richest of these Neogene units in mammal remains within the Argentine territory, and probably the entire South American continent (Pascual et al., 2002), whereas in Chubut Province there are also older sediments assigned to the Peligran and Riochican (Paleocene) stages. The continental vertebrate-bearing units are interfingered with marine sedimentary rocks rich in paleontological content, both invertebrate and vertebrate remains. Among the latter, abundant and diverse remains of condrichthyan fish with significant biostratigraphic value (Arratia and Cione, 1996) and spheniscid birds (Simpson, 1972; Cione and Tonni, 1981; Acosta et al., 2004) are recorded. For the Late Paleogene and the Neogene, several marine units have been recognized. The ‘‘Patagoniano’’, ‘‘Patagoniense’’ or ‘‘Patagonia Formation’’ (including the

2. Climate The entire Patagonian territory was essentially an oceanic peninsula during the Tertiary and Quaternary periods, since a short distance has always separated the two  2008 ELSEVIER B.V. ALL RIGHTS RESERVED

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Fig. 1. Map of Patagonia, Argentina, showing main localities and local names used in the text. large oceanic masses that border it east and west, the Pacific and Atlantic oceans. Moreover, after the complete opening of the Drake Passage, farther south of insular Patagonia, Patagonia became completely surrounded by the sea. In addition, immediately after the climatic optimum of the late Early Miocene (Pagani et al., 1999; Zachos et al. 2001), the Antarctic continent became permanently covered by ice (Flower and Kennett, 1994; Shevenell et al., 2004), and the Circum-Antarctic current certainly acted decisively on the climatic ruling of Patagonia. We are considering here the main climatic changes reflected on the fauna; notwithstanding, if we consider changes on partial groups of vertebrates that are better known for local fauna assemblages (i.e. rodents), we would be able to give a more accurate view. It is impossible to do it in Patagonia as a whole, but in a few localities (see Kramarz and Bellosi, 2005). The whole period considered in this contribution, from the Middle Miocene to the end of the Pleistocene (or the beginning of the Holocene), is characterized by a continuous decrease of mean marine temperatures (Zachos et al., 2001) (Fig. 2). This constant temperature decrease certainly defined the climate in most of the Patagonian peninsula, which suffered, in addition to the changes of temperature, its own changes derived from the rising of the Andean Cordillera, all along the western part of its

territory, with the consequent modification of the moisture supply from the Pacific Ocean. Hence, the mountain rise that affected the path of wetter winds from the Pacific eastward forced the precipitation of most of the discharge almost at the beginning of its way over Patagonian territory, and led the extra-Andean area to progressive desiccation. Changes toward lower mean temperature and humidity were gradual and largely determined the faunal changes in different mammal groups and at different moments according to their specific sensitivity (Ortiz Jaureguizar et al., 1993; Pascual et al., 1996). Quattrocchio et al. (1988), on the basis of palynomorphs and vertebrate remains, stated that a marked climatic deterioration occurred by the end of the Miocene in the Colorado Basin (northern Patagonia). Scillato-Yane´ et al. (1993) analyzed the variation of the xenarthran diversity during the Friasian sst–Mayoan lapse and determined that the shift toward colder and drier conditions was a gradual process that occurred during this lapse, resulting in the conditions that prevailed since the Chasicoan stage. This change of climatic conditions influenced the xenarthran diversity selectively and progressively, while tardigrades were the first group showing a taxonomical change, followed by the cingulates. The new environmental conditions are compatible with open areas of grasslands developed during dry

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Fig. 2. Miocene to Recent variation of sea temperature and the approximate chronology of the main stages analyzed in the text (temperature curve modified from Zachos et al., 2001).

seasons. Montalvo and Verzi (2004) arrived to similar conclusions based on octodontoid rodents for the following lapse, the Huayquerian–Montehermosan stages, reinforcing the idea that the trend of the climaticenvironmental change had a defined directionality since the Middle Miocene. From the latest Pliocene to the Early Pleistocene (ca. 2.5–1 Ma), frequent glaciations were recorded in southern Patagonia, with a remarkable increase of the continental ice sheet between 1.5 and 1.2 Ma (Singer et al., 2005). In the latest Pleistocene (ca. 13–11 14C ka BP), a new glacial advance in southern Patagonia under humid conditions (McCulloch et al., 2000; see also Strelin and Denton, 2005) generated a favorable environment for herbivores, including megaherbivores and their predators (Tonni et al., 2003); these conditions seem to have favored also the southern expansion of the running bird Rhea americana (see Tambussi and Tonni, 1984). In summary, from the Santacrucian to the Late Lujanian, within the territory of the present Patagonian region, the faunal associations that were developed correspond to a variety of climatic changes. These changes may be summarized in four successive climaxic scenarios (see Fig. 3 for explanations; Figs 4–7): 1. the climatic optimum of the late Early Miocene (Fig. 4), with a well-developed tree vegetation, with full- and semi- tree-dwelling forms (e.g. Primates, Eretizontidae and various Xenarthra, Phyllophaga); 2. later, during the ‘‘Friasian’’ stage (Friasense sst., ‘‘Colloncurense’’ and Mayoense stages), more open environments become predominant, allowing the occurrence of more cursorial and larger forms. The forested areas would have been restricted to the valleys of the rising cordillera, hosting a few

tree-dwelling species (e.g. the last record of Primates and Eretizontidae) (Fig. 5). This changes occurs progressively along the ‘‘Friasian’’ stage, and affected selectively different mammalian lineages (i.e. among the Xenarthra, the Pansantacrucian Tardigrada were more sensitive than Cingulates, and were replaced by the beginning of the Friasian sst. for the lineages that became dominant during the Panaraucanian period); 3. partially in coincidence with the tectonic Quechua phase, neatly open environments with at least one dry season developed, including extensive savannas with Attini (Formicidae) mounds (see Laza, 1982) (Fig. 6). In several mammalian lines, a tendency toward size increase and the beginning of the Pampean lineages are observed. For the first time, Holarctic taxa are recorded (e.g. Procionidae); 4. during the last glacial advance in Late Glacial times (13 to 11 14C ka BP), favorable conditions (higher moisture) were recorded in southern Patagonia to support a high diversity of large mammals corresponding to Pampean lineages that dispersed toward the south (e.g. Milodontidae, Tremartidae bears, Machrauquenidae, smilodons) (Fig. 7).

3. The Santacrucian Stage As it has been pointed out in the Introduction, the bestknown land-mammal faunas south of the Rı´o Chico of Santa Cruz Province were found in lithostratigraphic units included in the Santa Cruz Formation, the Santacrucian stage. This formation was originally recognized for its exposures at the southern end of the Atlantic coast (see Tauber, 1997) whose radiometric dating yielded a

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Fig. 3. A, Explanation Figure 4: 1, scansorial Megatheriidae; 2, medium size ground sloth; 3, Armadillo; 4, Glyptodonts; 5, Protherotherid native ungulate; 6, Platirhyne monkey; 7, Phororacoid ground bird; 8, Toxodontid native ungulate. B, Explanation Figure 5: 1, Megatheriinae; 2, Armadillo; 3, Glyptodonts; 4, Toxodontid; 5, Protherotherid native ungulate; 6, Rodent; 7, Platirhyne monkey; 8, Thylacosmiliid marsupial. C, Explanation Figure 6: 1, Mylodontid ground sloth; 2, Sclidotheriine ground sloth; 3, Armadillo; 4, Glyptodonts; 5, Macrauchenid native ungulate; 6, small rodent; 7, large rodent; 8, Inmigrant racoon carnivore; 9, Small Phororacoid ground bird; 10, Giant flying bird, Argentavis. D, Explanation Figure 7: 1, Mylodontid; 2, Camelids; 3, Sabertooth carnivore; 4, Large felid Panthera; 5, Large bear Arctotherium; 6, Glacier.

mean of 16.53 Ma (Fleagle et al., 1995), that is the latest Early Miocene. This formation extends all along the southern territory of Santa Cruz Province, often with quite scattered outcrops, up to the foot of the southern Patagonian Andes, especially along the larger rives such as the Gallegos, Santa Cruz, Shehuen or Chalı´a and Chico. For many years, the outcropping sediments at the foot of the southern Patagonian Andes were identified as the Santa Cruz Formation; an example are those units surrounding Lago Argentino that were referred to the Santa Cruz Formation, more because of their fossil mammals than for their lithological features. These sediments included a high number of taxa apparently similar to those of the typical Santa Cruz Formation, from the coastal locality between the Coyle and Gallegos rivers. However, Ameghino (1900–1902) had already proposed that this fauna (collected by his brother Carlos in 1889) was older than previously assumed, and identified it as representative of a chronologically different unit that he named as E`tage Notohippidien (see Marshall and Pascual, 1977). In addition, he outlined that his brother collected the most representative fossils in Karaike´n, near Lago Argentino and a little farther north of the sources of the Rı´o Santa

Cruz. In 1930, L. Kraglievich recognized this unit as horizonte Karaikense of the ‘‘Formacio´n Santacrucen˜a’’, stating its greater antiquity within the ‘‘Formacio´n Santacrucen˜a’’ of Ameghino (Kraglievich, 1934). Another locality bearing ‘‘Santacrucian’’ fossils and known for more than a century is located along the streams of the Rı´o Pinturas Basin, at the northwestern sector of Santa Cruz Province. Following the tradition, these sediments were also assigned to the Santa Cruz Formation. The first mammals were also collected by Carlos Ameghino, and described by his brother Florentino in 1906, but the precise locality was never published. However, after the study of these fossils, Ameghino considered them as intermediate between those somewhat older from the Colhuehuapian and those somewhat younger from the Santacrucian of the coast. He considered them as representing a different biozone, which he named as ‘‘Astrapothericulense Zone’’ (sic). In 1931, J. Frenguelli made additional collections in Arroyo Feo and agreed with Ameghino that they were intermediate between the Colhuehuapian and Santacrucian faunas; this argument was followed more recently by De Barrio et al. (1984). In turn, Pascual and Odreman Rivas (1971) considered that the mammals from the Rı´o Pinturas should not be separated in a

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Fig. 4. Climaxic scenario of the climatic optimum of the upper Early Miocene, well-developed tree vegetation, with full- and semi- tree-dwelling forms (e.g. Primates, Eretizontidae and various Xenarthra, Phyllophaga).

Fig. 5. Climaxic scenario during the ‘‘Friasian’’ stage (Friasian sst., ‘‘Colloncuran’’ and Mayoan stages), more open environments become predominant, allowing the occurrence of more cursorial and larger forms. The forested areas would have been restricted to the valleys of the rising cordillera, hosting a few tree-dwelling species (e.g. the last record of Primates and Eretizontidae).

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Fig. 6. Climaxic scenario partially coincident with the tectonic Quechua phase, neatly open environments, including extensive savannas with Attini (Formicidae) mounds. Several mammal lineages show a tendency toward an increasing body size. For the first time, Holarctic taxa are recorded (e.g. Procyonidae).

Fig. 7. More favorable conditions (higher moisture) were recorded in southern Patagonia which supported a high diversity of large mammals corresponding to Pampean lineages which dispersed southward (e.g. Mylodontidae, Tremarctinae bears, Machrauchenidae, Smilodons) during the last glacial advance in Late Glacial times (13 to 11 14C ka BP).

Neogene Vertebrates from Argentine Patagonia different ‘‘Land-mammal Age’’ from the Santacrucian, but later Fleagle et al. (1995: 129) did not share this argument and concluded that the ‘‘Pinturan’’ fauna should be considered as ‘‘. . . a separate faunal zone’’. Probably both the Pinturan mammals and the Notohippidian mammals from Lago Argentino have not such a taxonomic and ecological differentiation to justify a clear separation from the Santacrucian; however, the span of time between both deposits is quite short (see also Kramarz and Bellosi, 2005). This was supported by Patterson and Wood (1982) and Marshall (1976). Notwithstanding, Kramarz and Bellosi (2005) considered that in the Pinturas Formation there are two different assemblages of rodents, the lower and middle with affinities with both Colhuehuapian and Santacrucian genera, and the upper bearing the typical Santacrucian species. Multivariate similarity analyses among different mammal units, taking them as units, and considering the families of Cenozoic mammals as ‘‘characters’’, revealed a hierarchical organization of the ‘‘Land-mammal Ages’’ (see Pascual et al., 1996). According to this criterion, the Patagonian Faunistic Cycle includes the Deseadan Subcycle (older), and the PanSantacrucian Subcycle (younger). This latter includes the Colhuehuapian and Santacrucian ‘‘Land-mammal Ages’’ being the ‘‘Notohippidian’’ and the ‘‘Pinturan’’ part of the Santacrucian. According to the evolutionary degree of their mammals, they represent a succession of very close communities which immediately precede the mammal communities of the Santa Cruz Formation of the coast. The well-known degree of taxonomical identity between the communities of the ‘‘Notohippidian’’ with those typically Santacrucian of the coast could be explained because ‘‘. . . el perı´odo de subsidencia del mar ‘‘Patagoniano’’ (= Formacio´n Cerro Centinela en la regio´n del Lago Argentino y Formacio´n Monte Leo´n en la costa Atla´ntica) haya sido ma´s corto que en la costa actual del Atla´ntico, y que el movimiento regresivo haya comenzado en el Oeste para extenderse gradualmente hacia el Este’’ [‘‘. . . the subsidence period of the ‘‘Patagonian’’ Sea (= Cerro Centinela Fm. in the Lago Argentino region and Monte Leo´n Fm. at the Atlantic coast) had been shorter along the present Atlantic coast and that the regressive movement had begun toward the west to move gradually eastwards’’; Feruglio, 1944: 99; 1949: 181]. The ‘‘Notohippidense’’ of Lago Argentino (Santa Cruz Formation auct.) and the Santa Cruz Formation of the Atlantic coast represent the continental facies after the regression of the ‘‘Patagonian’’ sea; and probably this is the reason of the minor difference between the evolutionary degrees of their communities. Besides, the mammal communities characterizing the Colhuehuapian present an evolutionary degree immediately preceding that of the ‘‘Notohippidian’’ and ‘‘Pinturan’’ of the cordilleran foot. Radiometric dating approach the age of the sediments bearing Colhuehuapian fauna of the Sarmiento Formation at Gran Barranca, south of Lago Colhue´ Huapi, Chubut (ca. 19 Ma; Kay et al., 1999; Madden et al., 2005), to that of the Monte Leo´n Formation of the Atlantic coast (ca. 19.35 Ma; Fleagle et al., 1995). Although radiometric dating of the ‘‘Notohippidian’’ sediments are available, the mean dates of the Santa Cruz Formation at the Atlantic coast (16.53 Ma) and the ‘‘Pinturan’’ of Rı´o Pinturas (17.08 Ma) explain the degree of taxonomic identity and

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compositional similarity of the Colhuehuapian communities (ca. 19 Ma fide Kay et al., 1999), ‘‘Pinturan’’ (ca. 17 Ma) and Santacrucian (ca. 16.5 Ma). These two were estimated from those obtained by Fleagle et al. (1995). The most recent contributions on vertebrates, especially mammals, recorded in the Santa Cruz Formation, include papers by Tauber (1997, 2000 a, b), Tauber et al. (2003, 2004 a, b, c, d), Kramarz (1998, 2001), Ribeiro and Bond (1999), Candela (2003) and Vizcaı´no et al. (2004), among others. For the Pinturas Formation (the latest Early Miocene) of Santa Cruz Province, see Kramarz (1999, 2004) and Tejedor (2003). In turn, Tauber et al. (1999) described the first group of Santacrucian continental vertebrates for Neuque´n Province; in this association, characteristic taxa of this age such as Astrapotherium, Protypotherium, Adinotherium and Hegetotherium may be found. Forasiepi et al. (2001) analyzed the carnivorous marsupial Arctodictis, recorded in the Santacrucian (Middle Miocene) of Patagonia and La Venta (Colombia), and concluded that quite probably the specimen of Colombia has to be referred to another larger taxon related to Prothylacynus or Dukecynus.

4. The Friasian and Colloncuran Stages In the Middle Miocene, the diastrophic movements that uplifted the Patagonian Cordillera (especially the early subphases of the Quechua phase) caused the sediments of this age to be restricted to structural valleys located along the piedmont of the already defined Patagonian Cordillera (Pascual and Odreman Rivas, 1973). Such lithostratigraphic units as the Chimehuı´n and Rancahue formations ˜ orquinco (Turner, 1965), and the Collon Cura and N formations (Cazau, 1972) are assigned to these ages. During the ‘‘Friasian’’ s.l., typical mammals of warm, tropical or subtropical environments that were present during the last climatic optimum of the Santacrucian, even at more southern latitudes, disappeared from the record. This event marked a sharp turnover in the composition of the mammal faunas of the southern tip of South America (Pascual, 1984; Pascual et al., 2002), pointing out the beginning of a new faunistic cycle – the Panaraucanian which replaced the preceding Patagonian cycle (Pascual and Ortiz Jaureguizar, 1990). On the other hand, the withdrawal of marine environments is followed by open environments, named by Pascual and Bondesio (1982) as Edad de las planicies australes (‘‘Age of the Southern Plains’’), which mainly corresponds to the development of depocenters north of the Patagonian region, in the Chaco-Pampean plains. This Edad de las planicies australes is represented by mammal-bearing sediments of the Chasicoan, Huayquerian and Montehermosan stages (Pascual and Ortiz Jaureguizar, 1990). At the beginning of the Panaraucanian cycle (represented by the transitional ‘‘Friasian’’ s.l. stage (included the Colloncuran, Mayoan and Chasicoan stages), a few members of the Patagonian cycle are still recorded. This is the case of the ungulates Homalodotheriidae and Nesodontinae, and the cingulate xenarthrans

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Pelthephilidae (Bondesio et al., 1980a). These forms disappeared from the record in later units in which cursorial grazers prevailed. However, among the xenarthrans, the changes in the beginning of the Panaraucanian are slow and by groups, the tardigrades being the first group to show a taxonomical change, and the cingulates the last one (Scillato-Yane´ et al., 1993; Scillato-Yane´ and Carlini, 1998). In Rı´o Negro Province, mammals found in the Collo´n Cura Formation, near Pilcaniyeu Viejo, were assigned to the Friasian stage (Bondesio et al., 1980a, b; Pascual et al., 1984). In addition to the mammals that characterize the sediments of this age, a few specimens of fish, amphibians, anurans, reptiles and birds are present (Pascual et al., 1984). Later, the contributions of Vucetich et al. (1993, and literature therein; Scillato-Yane´ and Carlini, 1998) showed that the Collo´n Cura Formation, present in Neuque´n and Rı´o Negro provinces, bears a transitional fauna between the older one from the Friasian sst., and the younger from the Mayoan stage. In this way, these authors updated the sequence that had been supported originally by L. Kraglievich (1934). The Collo´n Cura Formation has an absolute age of 15.4 Ma, dated on the Pilcaniyeu Ignimbrite which occurs within this formation (Rabassa, 1975; Bondesio et al., 1980a).

5. The Mayoan Stage Sediments of the Rı´o Mayo Formation (‘‘Estratos del Guenguel’’), exposed in the northwest of Santa Cruz Province, are 40Ar/39Ar dated in ca. 11.5 Ma (see Dal Molı´n and Franchi, 1996). Above the tuffs with this dating, a megatheriine xenarthran assigned tentatively to Eomegatherium (Brandoni and Carlini, 2004) was found, in addition to remains of Megathericulus and new xenarthrans described by Scillato-Yane´ and Carlini (1998). In the El Pedregoso Formation and other coeval units (‘‘Estratos del Guenguel’’; Franchi et al., ms) of this province, Pascual et al. (2002) found transitional mammals between those of the Patagonian and Panaraucanian cycles (Ortiz Jaureguizar, 1986), which means ‘‘. . . un cambio ecolo´gico sustancial que hemos tomado como efecto de las primeras subfases de la compleja Fase Quechua’’ (‘‘. . . a substantial ecological change that we have taken as the effect of the first sub-phases of the complex Quechua Phase’’; Pascual et al., 2002: 540). The Rı´o Mayo Formation is also exposed at the margins of the Rı´o Chico in Rı´o Negro Province and yielded a primitive Thilacosmyliidae (Goin and Carlini, 1993) probably related to the genus described by Goin (1997) for the ‘‘Laventan’’ SALMA, La Venta, Colombia (Kay et al., 1997).

6. The Huayquerian Stage The sediments of the Puerto Madryn Formation, exposed in Penı´nsula Valde´s (Chubut Province) between Punta Delgada and Pico Lobo, include remains of continental vertebrates described by Dozo et al. (1999; 2002). The

presence of the pampatherid Kraglievichia, as well as certain dolichotine and cardiatherine rodents, certifies the Huayquerian age of the sediments, deposited under temperate-warm climatic conditions (Dozo et al., 2002). From a paleobiogeographic point of view, the southernmost distributions of several mammal taxa are recorded here (e.g. Kraglievichia sp.), showing a southern displacement of climatic conditions, at least near the coast. In sediments of the Cerro Azul Formation exposed in Salinas Grandes de Hidalgo, La Pampa Province, Urrutia and Scillato-Yane´ (2003) recorded the dasypodid Macroeuphractus retusus, which represents also a western expansion of its known geographic distribution. This species had been previously recorded in the Montehermosan stage of Buenos Aires Province (Farola Monte Hermoso) and the Ituzaingo´ Formation (in the conglomerado osı´fero [‘‘bone bearing conglomerate’’]), Entre Rı´os Province, in NE Argentina. The sediments of the Cerro Azul Formation are referred by these authors also to the Huayquerian stage, being another case of southern (and western) extension of fauna. For 11 sites of La Pampa Province, 10 of which correspond to the Cerro Azul Formation and 1 to niveles coeta´neos de la Formacio´n Rı´o Negro (‘‘coeval levels of the Rı´o Negro Formation’’), Montalvo and Cerden˜o (2002) recorded the hegetotherid notoungulate Hemihegetotherium achathaleptum, also present in sediments referred to the Huayquerian of Catamarca, San Juan and Mendoza provinces. Montalvo (2000) also assigned a Huayquerian age for the Cerro Azul Formation in Tele´n, La Pampa Province.

7. The Huayquerian–Montehermosan Stages In Caleufu´, La Pampa Province, Verzi et al. (2003) described a new octodontid rodent of the genus Xenodontomys, based on more than 200 specimens. They conclude that it is the most derived chronomorph of the phyletic sequence previously known of Xenodontomys and hence the site of Caleufu´ would represent the youngest levels of the Cerro Azul Formation (probably Montehermosan), perhaps coeval with those of the Irene ‘‘Formation’’ of southern Buenos Aires Province. Montalvo et al. (2000a, b) and Montalvo (2001) assigned the Caleufu´ exposures to the Late Miocene–Early Pliocene. In the first paper, the presence of the rodents Phtoramys and Neophanomys biplicatus suggested to the authors an Early Pliocene age (Montehermosan, see Montalvo et al., 2000a, b). For this same site of the Cerro Azul Formation (Caleufu´), Montalvo and Rocha (2003) cited the presence of the cavid rodent Neocavia which would confirm the assignment to the Montehermosan stage (Late Miocene-Early Pliocene). Similar conclusions were reported by Esteban et al. (2003) based on the study of dasypodids found in the Estancia El Recado, located 10 km farther southwest of Caleufu´, and by Abello et al. (2002), with the study of marsupials. For them, the age of the site would be somewhat older than that of Caleufu´ with Xenodontomys, and assigned it to the Huayquerian stage (Late Miocene).

Neogene Vertebrates from Argentine Patagonia 8. The Lujanian Stage Among the first findings of Pleistocene mammals are those recovered by Charles Darwin in 1834 in Puerto San Julia´n, belonging to a large native South American ungulate, described by Owen (1838–1840) as Macrauchenia patachonica. Although this author recognized the similarity with the camelids in the length of their cervical vertebrae – hence its generic name – he pointed out that it was an ungulate different from any other known before. In 1889, Ameghino proposed to include this curious genus, together with others from the Neogene, in the Order Litopterna. Mercerat (1897) mentioned Typotherium (= Mesotherium), a guide taxon from the Ensenadan, as collected in Shang Aiken, Rı´o Coig. However, later authors such as Ameghino and Feruglio considered this reference as highly dubious (see Tonni et al., 1982; Pascual et al., 2002). In Bahı´a Sanguinetto, Santa Cruz Province, Parodi (1930) cited the record of Megatherium australis, Mylodon ‘‘darwini’’ and Glyptodon clavipes (?). Likewise, Tonni et al. (1982) cited several unpublished records on materials housed in the collections of the Divisio´n Paleontologı´a Vertebrados of the Museo de La Plata. This is the case of Antifer sp. (MLP 57-III-7-1) from the confluence of the Limay and Neuque´n rivers; Macrauchenia sp. (MLP 80-IX-5-1) in central Chubut Province (‘‘Cerro Guacho’’) and Equidae indet. (MLP 67-XI-7-1) from Can˜ado´n Seco, Santa Cruz Province. Also in Santa Cruz Province (Puerto Deseado), the geologist C.A. Ferrari found remains of Megatherium sp. (see Tonni et al., 1982: 149). In Sierra de Portezuelo (Neuque´n Province), Garrido ´ lvarez (2004) reported remains of Equus in sediand A ments that have been assigned to the Late Pleistocene. The gomphotherid proboscidean Stegomastodon has been found in Rı´o Negro Province, in the Huahuel Niyeu valley, near the city of Ingeniero Jacobacci (Pascual et al., 1984). During the Late Pleistocene and beginning of the Holocene, there were favorable climatic-environmental conditions in southern Patagonia for the peopling of the territory (Miotti, 1998; Miotti et al., 2003; Massone, 2003). Humans arrived accompanied by a relatively high diversity of mammals, among which there were representatives of the large, later extinct, South American mammals. Open archeological sites, refuges and caves are testimonies of the coexistence of men with many of those large mammals, which certainly were part of their diet. Men have been considered as responsible of the socalled ‘‘Megafaunal Extinction’’ (Martin, 1986). The most representative taxa of Santa Cruz are Megatherium sp., Mylodon sp., Lama gracilis, Hippidion sp. From an ecological point of view, the presence in such high latitudes of the Mylodontidae Mylodon is quite surprising, since living xenarthrans in general and particularly the Pilosa (e.g. sloths and anteaters) have endothermic mechanisms which led them to having a variable body temperature. It has been assumed that they could not have inhabited cold or temperate-cold environments. Scillato-Yane´ (1976: 310) proposed the following argument: . . . los Mylodontidae de la subfamilia Mylodontinae vivieron durante el Perı´odo Cuaternario hasta latitudes

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relativamente elevadas, tanto en Norteame´rica como en Sudame´rica (Patagonia Austral); por lo tanto, tuvieron que soportar temperaturas que, en ese tiempo y lugares, fueron sin duda bastante bajas. Evidentemente pudieron de algu´n modo adaptarse a condiciones ambientales mucho ma´s rı´gidas que aque´llas en las que prosperaron los Mylodontidae terciarios. Esta adaptacio´n so´lo resulta explicable si consideramos la posibilidad de que los Mylodontinae cuaternarios hayan sido mejores termorreguladores que sus predecesores. Tal hipo´tesis esta´ avalada por la siguiente circunstancia: el estudio anato´mico de las inserciones craneanas y mandibulares revela que se ha verificado un paulatino perfeccionamiento de la musculatura masticatoria de estos tardı´grados en el transcurso del Cenozoico; dicho perfeccionamiento se ha de haber visto reflejado en una ma´s adecuada masticacio´n de los alimentos, que a su vez permitio´ un mejor aprovechamiento energe´tico de los mismos. El metabolismo ma´s intenso resulta imprescindible para el mantenimiento de una temperatura corporal ma´s elevada [‘‘. . . the Mylodontidae of the Mylodontinae subfamily lived during the Quaternary period up to relatively high latitudes, both in North America and in South America (southern Patagonia); therefore, they had to bear temperatures which in those times and places were undoubtedly quite low. Evidently, they could in some way adapt to environmental conditions much more rigid than those in which the Tertiary Mylodontidae had prospered. This adaptation is only explained if we consider the possibility that the Quaternary Mylodontinae were better thermo-regulators than their predecessors. Such hypothesis is supported by the following circumstance: the anatomical study of the cranial and mandibular insertions reveals a gradual improvement of the masticatory muscles of these tardigrads during the Cenozoic; such improvement has been reflected in a much better chewing of the food, which in turn allowed a better energetic utilization. A more intense metabolism was necessary for the maintenance of a higher body temperature’’]. This adaptive possibility could have been favorable when coincident with some of the frequent, more benign climatic pulses that characterized the Finiglacial and Postglacial (Tonni et al., 2003). This seems to be ratified because many other animals lived with mylodonts, most of which still inhabit those latitudes of Patagonia, and others are currently restricted to intertropical regions, like the jaguar (Martin et al., 2005). Numerous radiocarbon dating of remains of Mylodon cf. M. listai in one of the caves (‘‘Cueva del Milodo´n’’) yielded a mean age of 11,200 + 170 14C yr BP (see also Borrero, 1997; Tonni et al., 2003). In this same ‘‘Cueva del Milodo´n’’, remains of a felid, Panthera onca mesembrina, has been recorded (see Tonni et al., 2003). Prevosti et al. (2003) reported the southernmost record of a bear, Pararctotherium (= Arctotherium tarijense, see Soibelzon et al., 2005), found in a cave of the Pali-Aike National Park, Magallanes, Chile. In this cave, remains were also found of the extinct equiid Hippidion sp., on which a radiocarbon dating yielded 11,210 + 50 14C yr BP. Hippidion

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is frequent in several archeological and paleontological sites referred to the Uppermost Pleistocene of the southernmost tip of Patagonia (Alberdi et al., 1987). Smilodon populator seems to be also present in the ‘‘Cueva del Milodo´n’’ (southern Chile) on the basis of recently published materials from old collections housed in the Zoological Museum of Amsterdam (Barnett et al., 2005); another reference from the same region (Cueva del Medio) was published by Massone (1996). In central Chile, remains of the proboscidean Cuvieronius humbolti were found in the archeological site of Tagua Tagua, south of Santiago (Casamiquela, 1999), as well as somewhat farther south in the site Monte Verde (Casamiquela et al., 1996).

9. Discussion The Late Miocene and Pliocene vertebrates come from continental and marine formations related to the last geotectonic processes that affected the Colorado Basin and led to the differentiation of the present basins of the Negro and Colorado rivers. In several mammal lineages, chronomorphs grading from west to east can be observed, which display more primitive features in those closer to the modern Cordilleran region. According to Pascual et al. (1984), this is related to the displacement of continental environments in this direction. The remains of the most recent Tertiary mammals of the study area are those from the Rı´o Negro Formation, in eastern Rı´o Negro Province, and equivalent levels of the Cerro Azul Formation in La Pampa Province. Remains of rodents such as Cardiatherium (including Kiyutherium, see Vucetich et al. 2005) recorded there suggest a Huayquerian age, which does not contradict the absolute dating (Alberdi et al., 1997). As it can be seen in Fig. 2, after the Mi-1 glaciation in Antarctica (Zachos et al., 2001), there was a trend toward increasing temperature that reaches the highest point with the climatic optimum of the latest Early Miocene, coinciding with part of the Santacrucian. Notwithstanding, Shevenell et al. (2004) clearly stated, ‘‘. . . southwest Pacific sea-surface temperatures (SSTs) cooled 6 to 7 C during the late Early Miocene climate transition (14.2 to 13.8 Myr ago).’’ (p. 1766); so, part of the Santacrucian could have had a climate cooler than during the climatic optimum. All the Colhuehuapian to Colloncuran mammals of Patagonia have among their components Platyrhine primates, associated with other modern intertropical species. Although the proterotheriid litopterns (convergent with equids) are characteristic of open areas, they coexisted with several species ambulatoryscansorial as megatherioid sloths (ancestors of living tree-sloths) and with fossorial rodents. As Webb (1978) pointed out, the extraordinary diversity of middle- to large-sized mammals suggests an optimum balance between grasslands and forests, as those of the savannapark. Especially in the west, other mammals (rodents and digging marsupials) are evidence of drier climate events, or a complex of environments represented by dune deposits alternating with higher areas of wet forests (Bown and Larriestra, 1990).

Approximately 11 Ma ago, the development of the ice sheet in Antarctica seems to have caused a remarkable climatic change, seen both in a lithogenesis change (certainly associated with an Andean orogenic phase), and in a mammal-communities turnover since the Mayoan stage of southern Patagonia. With this turnover starts a new faunistic cycle, named by Pascual and Ortiz Jaureguizar (1990) as the Panaraucanian, which had the northern extra-Patagonian area as main setting (Pascual et al., 1996: 290–294). The mean values of @ 18O went on rising gradually in the Late Miocene. In this lapse, the Great American Biotic Interchange (the ‘‘New Island Hoppers’’ of Simpson, 1950, 1980) began. In the Early Pliocene or Uppermost Miocene (6 Ma), there was an additional cooling and a little expansion of the ice sheets of Western Antarctica and the Arctic. A little after, the closing of the Panama Isthmus occurred and, consequently, the connection between both Americas started, triggering a massive interchange of biotas. The Early Pliocene is marked by a soft trend of increasing temperature up to 3.2 Ma, when the @ 18O increased again, evidenced by the establishment of a glaciation in the Northern Hemisphere, which seems to have intensified the mammal interchange between both Americas. The scarce mammal remains known of the first part of the Pleistocene belong to taxa that were current inhabitants of the Pampas and the rest of the South American continent. Hence, the climatic-environmental conditions prevailing by those times in the southern end of Patagonia cannot be recognized through them.

Acknowledgments We wish to thank Dr M.G. Vucetich for critical reviewing of the manuscript, Dr C. Deschamps for the English translation. Illustrations are the work of J. Gonza´lez. Finally, we wish especially to thank Dr Jorge Rabassa for inviting us to contribute to this book. The work was partially supported by the PICT 38171 (to EPT) and PICT-R 0074 G3, and UNLP N514 (to AAC) projects.

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14 Late Cenozoic Invertebrate Paleontology of Patagonia and Tierra del Fuego, with Emphasis on Molluscs Marina L. Aguirre1,2, Julio C. Hlebszevitsch Savalscky3 and Florencia Dellatorre2 1

2

CONICET Universidad Nacional de La Plata, Facultad de Ciencias Naturales y Museo, INGEA, Laboratorio 6, calle 64 N 3 entre 119 y 120, 1900 La Plata, Argentina 3 Pluspetrol, Lima 339, C1073AAG Buenos Aires, Argentina

from Quaternary gastropods and bivalves from marine or marginal marine sediments, both from the Northern and Southern hemispheres (among others, Peacock, 1989; Aguirre, 1990, 1993b; Ortlieb et al., 1990; Gordillo, 1991; Cohen et al., 1992; Gordillo et al., 1993; Lutaenko, 1993; Guzma´n et al., 1995; Ortlieb et al., 1996a, b; Aguirre et al., 1998, 2002, 2005a, b, 2006a, b; Bonadonna et al., 1999; De Francesco and Za´rate, 1999; MurrayWallace et al., 2000; Pastorino, 2000; Aliotta et al., 2001; Aguirre, 2001, 2002, 2003; Farinati et al., 2002; Lutaenko et al., 2002; Razjigaeva et al., 2002; Martin et al., 2003; Carre´ et al., 2005; Martı´nez et al., 2006). The aim of this contribution is to provide a revised synthesis of the taxonomy, distribution, paleoecology and paleoenvironmental implications of the macroinvertebrate fauna preserved along the Patagonian coast, with emphasis on the molluscan assemblages of Quaternary age. They are the most evenly distributed and best preserved within the so-called Terrazas Marinas (marine terraces, MT) named MT III, MT IV, MT V and MT VI (Feruglio, 1950) (Table 1). In turn, they present a major gap in our knowledge of paleoenvironmental and paleoclimate variations in Patagonia linked to changes in sea level and oceanic–atmospheric circulation patterns during the Quaternary. By contrast, the marine Late Miocene Patagonian molluscan faunas and their paleoclimatic information have been dealt with in detail (Del Rı´o, 1991, 1992, 1994, 1997; Del Rı´o and Martı´nez, 1998) and been recently reviewed by Martı´nez and Del Rı´o (2002, other references therein). On the contrary, records of the marine Pliocene are scarce (Camacho, 1967; Feruglio, 1950; Farinati et al., 1981) and lack precise chronological control. For marine terraces preserved at þ40 to >150 m a.m.s.l. in a few localities of Rı´o Negro, Chubut and Santa Cruz provinces (Table 1) there are no modern dates available, except for isolated localities, which only suggest a Pre-Quaternary age (Rutter et al., 1990) The marine Pliocene is therefore not well known and has been little studied in terms of the molluscan content. Illustrations of the main invertebrate groups preserved together with an update of the molluscs recorded, including the paleoecological requirements and a synthesis of the stratigraphical and geographical ranges for the most characteristic taxa, provide the basic framework to carry out further systematic and

1. Introduction Late Cenozoic marine transgressions covered extensive areas along the southwestern Atlantic margin (Fig. 1). In Argentina there are abundant records of those during the Neogene (23–1.6 Ma; Cowie and Bassett, 1989) and Quaternary, when they reached furthest inland around the Chaco-Paranense Basin (Entre Rı´os Province) and, mainly, the Salado Basin and Colorado Basin in Buenos Aires Province and parts of the Valde´s, San Jorge and Austral basins in Chubut and Santa Cruz provinces (Patagonia) (Fig. 2). In eastern Patagonia the records of subsequent sealevel highstands include shell concentrations of Late Miocene (ca. 10 Ma), Pliocene (undated) and Quaternary age (since ca. 400 ka during the Pleistocene and until ca. 3 ka BP during the Holocene). The latter correspond to at least three Pleistocene highstands, which have been correlated with marine oxygen isotope stages (MIS 11/9 (?), 7, 5; and the last one during the Mid-Holocene (MIS 1) (Table 1). These deposits form shell ridges, marine terraces or marginal marine sediments as testimonies of the last marine transgressions in the area. Studies on Late Cenozoic marine invertebrates recorded in Patagonia have been performed since the nineteenth century (D’Orbigny, 1834–1847; Darwin, 1846; Sowerby, 1846; Philippi, 1887, 1893; Ihering, 1897, 1899, 1907; Pilsbry, 1899; Ortmann, 1900, 1902; Frenguelli, 1926; Feruglio, 1933a, b; 1936–1937; 1950; Zinsmeister, 1981; Camacho, 1997). Mollusc shells (mostly gastropods and bivalves) represent the dominant biogenic elements within the fossil assemblages. Some bivalves, like oysters, Mytilidae, Pectinidae and Veneridae and, among the gastropods, the Patellacea, Muricidae, and Volutidae are among the most outstanding taxa in terms of abundance, size, thickness and preservation. On the contrary, the associated macroinvertebrate fauna is represented by skeletons of cnidarians, bryozoans, brachiopods, polyplacophors, scaphopods, polychaets, echinoderms and cirripeds (balanids) and crabs. It is widely known that molluscs are useful tools as indicators of former sea level and of environmental and climatic changes. Past conditions regarding substrate nature, water energy levels, sea-surface temperature (SST) and oceanic–atmospheric circulation have been deduced

 2008 ELSEVIER B.V. ALL RIGHTS RESERVED

DEVELOPMENTS IN QUATERNARY SCIENCES VOLUME 11 ISSN 1571-0866 285

286

Marina L. Aguirre et al.

Late Miocene

Pleistocene

Mid-Holocene

(ca. 10 Ma)

(MIS 11/9, 7, 5)

(MIS 1)

Fig. 1. Neogene and Quaternary marine transgressions as recorded along the southwestern Atlantic coast (Argentina, Uruguay and southern Brazil). (a) Late Miocene (‘‘Entrerriense’’) transgression modified from Del Rı´o (1990) and Martı´nez and Del Rı´o (2002); scarce imprecise Pliocene records; (b) Pleistocene marine transgressions (several highstands from MIS 11 [?]/9 to MIS 5); (c) Mid-Holocene transgression. Modified from Aguirre and Farinati (1999).

Pliocene (ca. 5 Ma)

(a)

(b)

(c) 0

68°

64°

60°

56°

52°

48°

I

32°

32° Ri

od

eL

aP

II

36°

40°

V 44°

O

CE

Deltas Estuaries and marshes

40° Sandy shorelines Pebble shoreline Cliffs Glaciated coastline

Basins I, Chaco-Paranense II, Salado III, Colorado IV, Valdes V, San Jorge VI, Austral

48°

VI

36°

a

AN

SO UT HA

IV

TL AN TI C

III

lat

44°

Antarctic Sector

48°

52°

52° 0 72°

68°

64°

200 400 km 60°

56°

52°

48°

44°

Fig. 2. Sedimentary basins and littoral landforms along the coast of Patagonia and other areas of Argentina. Modified from Schnack (1985, in Clapperton, 1993) and Codignotto et al. (1992; other references therein). paleobiogeographical studies, to select the most reliable and useful species to be dated with modern techniques and stable isotope analyses of autochthonous shells, and to search for more detailed conclusions regarding the age and paleoclimate scenarios of the Quaternary littoral deposits of Patagonia.

2. Area of Study and Environmental Conditions of the Present-Day Littoral Zone The Patagonian region analyzed here comprises the coastal area along the Rı´o Negro, Chubut, Santa Cruz

250

500 km

and Tierra del Fuego provinces, which extends between the Golfo San Matı´as in the north and the Beagle Channel in the south (Figs 1 and 3). This Argentine coastal sector, located at a ‘‘passive’’ margin with an extensive continental shelf, has been affected by dynamic processes associated with waves, littoral currents and tides, which resulted in differential erosional and accretional features. Neotectonic effects were acknowledged early in the literature (Feruglio, 1950) and more recently recognized all along the Argentine littoral (Codignotto et al. 1990b, 1992; Rostami et al., 2000; Peltier and Rostami, 2000; Guilderson et al., 2000). Modern work with estimations of neotectonic deformation and uplift rates has been shown to be minor within sedimentary basin areas like Golfo San Jorge (Codignotto et al., 1992). The scale of coastal erosive and accretional processes responds to sea-level oscillations during the Quaternary. Along the Patagonian littoral zone, formed on volcanic Jurassic rocks and Tertiary and Quaternary sediments, the coasts developed over Tertiary sediments and, only in the southernmost portion, over Pleistocene glacial deposits. The coast consists of active cliffs reaching over 50–60 m high in Rı´o Negro, more than 100 m high in Santa Cruz and ca. 70 m high in Tierra del Fuego. Less often gravel and sandy beaches or embayments do occur. Linked to the magnitude of the erosive processes, the re´gime and amplitude of the tidal range are mostly macrotidal ( > 4 m) in many areas of Patagonia. The littoral drift is southward south of 42 S (Codignotto, 1996; references therein). Atmospheric circulation along the Argentine littoral is primarily zonal. Air masses moving from west to east are controlled by semipermanent pressure fields (high pressure in the subtropical and low pressure in temperate latitudes), which control climatic variations. The Patagonian cool-warm temperate (semidesert) climate is characterized by very low mean precipitation (Clapperton, 1993). Two shallow oceanic currents influence molluscs and the marine invertebrates and biotas and other littoral biotas of Patagonia: the warm Brazilian Current (BC; 19–27C and 35–37ø) and, especially, the cool Malvinas (Falkland) Current (MC; 5–19C and 33.5ø)

Table 1. Stratigraphical synthesis of the marine units of Neogene and Quaternary age described for Patagonia and other coastal areas in southern South America (Southwestern Atlantic margin).

Sprechmann, 1978; Martínez et al., 2001

VIZCAIN O Fm = V.Soriano Fm

A E NT R E RÍOS P R O VI NC E

Fidalgo, 1979 Fasano et al., 1982; Aguirre and Farinati, 2000

HOLOCENE TRANSGR. (+3–4 m)

P

G A

E T

N A

T G

I

N

O

N

A I

A

BUE NO S AI R E S P R O VI NC E

Fidalgo, 1979 Fasano et al ., 1982; Aguirre & Farinati 2000

HOLOCENE TRANSGR. Las Escobas Fm Mar Chiquita Fm

“TERRAZAS MARINAS” (Marine Terraces) F e ru g lio , 1 9 5 0

VI Comodoro Rivadavia (+6–12 m)

(+3 m)

R

AGES (AAR - ESR) R u tte r e t a l., R 1 9ut8te9r, e1t9a9l0., 1989, 1990 & Schellmann and 00 Radtke, 2000

RIO NE G R O SAN ANTONIO OESTE Feruglio, 1950; Angulo et al., 1970; Rutter et al ., 1990

YOUNG MIS 5e

SAN ANTONIO FM

(+6.5–3.5 m)

BAHIA BUSTAMANTE Feruglio, 1950; Cionchi, 1987; Rutter et al., 1990

ZANJON EL PINTER FM + 8–10 m D/L 0.21–0.29 14 C (2,030–8,950)

T. DE L F UE G O

SANTA CRUZ

CHUBUT BAHIA SOLANO

COMODORO RIVADAVIA

CALETA OLIVIA

Codignotto, 19 83

Feruglio, 1950; Rutter et al., 1989 1990

Feruglio, 1950; Codignotto, various

Littoral Ridges

Marine T VI

Marine T VI

PUERTO DESEADO

PUERTO MAZAREDO

CENTRALSOUTH

Feruglio, 1950; Rutter et al., 1989–1990

Feruglio, 1950; Codignotto et al., 1987; Rutter et al., 1989–1990

Feruglio, 1950;

Marine T VI

Marine T VI

Codignotto, 1983 Rabassa, 1987; Gordillo et al., 1993

HOLOCENE TRANSGR. 1.3–2.8 ka (+2.5 m)

(ca.2–ka)

(+ 8–10 m) (6,940)

(+ 8–12 m)

(+8–10 m) (6,940)

+8–10 m

7.5 ka (+3.95 m) 5.9 ka (+8.5 m)

(500–5,850; 9,520)

5.46 ka (+10)

Ma rin e T V (+15–

V Puerto Mazaredo

(+ 1–4 m)

PLEISTOCENE TRANSGR. (+ 5–9.8 m) 32–39 ka

PLEISTOCENE TRANSGRESSION (123 ka+ 8–10m)

CHUY FM

PLEISTOCENE TRANSGR.

La s t In te rg la c ia l

IV

MIS 5 e

Puerto Deseado

INTERMEDIATE Ult. Interglacial MIS 5e

BALIZA SAN MATÍAS FM

(+30–40m)

(29.5–35.5 ka) MIS5e PLEISTOCENE TRANSGRESSION

PLEISTOCENE

(+15–30m)

MIOCENE PLIOCENE

QUATERNARY

U R U G U A Y

III Camarones (+ 40–95m)

OLD Pre-Last / Penultimate Interglacial MIS 7 or 9 ?

CALETA MALASPINA FM

Marine T V?

Marine T V?

(+25–29 m) 36–37 Ka D/L 0.74 ESR 116–195

(+ 20–25 - m) min 30–35 Ka MIS 5 e ?

(+ 20-25m) D/L 0.57 MIS 5 e ?

(+ 34–41 m) D/L 0.73–0.81; ESR 219->356 BP MIS 7 or 9 ?

Marine T IV (+30–40 m) ESR > 249 BP MIS 7 or 9 ?

II

PLEISTOCENE

TRANSGR.

31–32 ka >43 ka (+20 m)

Marine T IV (+30–40 m) D/L 0.66 ESR > 249 BP MIS 7 or 9 ?

Cabo Tres Puntas

Ca b o Tre s P u n ta s

+ 65–70 m

+ 65–70 m

I

Cerro Laziar

Ce rro La z ia r

> 170 m

(>170 m)

References: Ka:1000 yrs B P LA LOBERÍA

PELOTAS BASIN

CAMACHO FM El Chuy

PARANÁ FM "Entrerriense"

SALADO BASIN

GRAN BAJO DEL GUALICHO FM

PUERTO MADRYN FM

ESR: Electron spin resonance dates MIS: Marine oxygen isotope stage or substage

287

Complete source of references in Feruglio (1950), Closs and Madeira (1968), Malumia´n (1970), Fidalgo (1979), Fasano et al. (1982), Codignotto et al. (1987), Aguirre and Farinati (1999), Martı´nez and Del Rı´o (2002), Martin et al. (2003) and Aguirre (2003)

Late Pleistocene Cenozoic Invertebrate Paleontology

NEOGENE

Martin and Suguio, 1992; Martin et al., 1993

HOLOCENE TRANSGRESSION

U N I T S

HOLOCENE

M A R I N E

B R A Z I L

288

Marina L. Aguirre et al.

Fig. 3. Localities analyzed in Patagonia and other Argentine areas. Provinces from north to south: Entre Rı´os, Buenos Aires, Rı´o Negro, Chubut, Santa Cruz. Typical coastal deposits at Bahı´a Samborombo´n (shell ridges) and Patagonia (marine terraces). Details for Tierra del Fuego can be obtained from previous geomorphological and paleoecological work performed by Rabassa and Clapperton (1990) and Gordillo et al. (1993).

(Boltovskoy, 1981; Guerrero and Piola, 1997; Boltovskoy et al., 1999, 2005). The MC, a northward-running branch of the Subantarctic Cabo de Hornos Current formed when the West Wind Drift or Circumpolar Antarctic Current became active after the opening of the Drake Passage (Oligocene–Miocene boundary; Beu et al., 1997), moves northward up to about latitude 28 S (Santa Catarina, Brazil) in winter or to Rı´o de La Plata in summer. Its mean temperature ranges yearly from 4 to 11C and salinity ranges yearly from 33.8 to 34.4ø. The minor Patagonian Current, of Subantarctic origin, influences the coast and moves northward up to 38 S, with mean temperature ranging yearly from 5 to 16C and salinity from 33 to 33.5ø (Boltovskoy, 1979; Bastida et al., 1992). The BC, a branch of the South Equatorial current, moves from north to south (mainly subtropical

shallow water masses of 34.5–38.85ø and 18–24C) (Bastida et al., 1992). The modern littoral northward of ca. 42–43 S belongs to the southwestern Atlantic sector known as the Argentine Zoogeographical Province and southward to the Magellanean Province (Fig. 3). The latter extends from Golfo Nuevo (Chubut) to Cabo Hornos (55 S), highly influenced by the MC. The Argentine Province (north of Golfo Nuevo to 28 S, southern Brazil) is a transitional area influenced by the cool MC and the warm BC. Most of the Patagonian modern littoral is characterized by open marine conditions in temperate and Subantarctic cold temperate waters, salinity of ca. 34–34.5ø and temperatures reaching 3–11C in winter and 5.5–14.5C or 18C in summer (Knox, 1960; Boltovskoy, 1979). The substrates consist of scarce coarse sandy beaches in the

Late Pleistocene Cenozoic Invertebrate Paleontology north and gravel–sandy sediments in most of central and southern Patagonia where colonial organisms are among the dominant benthic species (i.e. algae; Porifera; Sertulariidae, Hydrozoa; Bowerbankia, Bryozoa), and sandy– mud sediments are distributed along the Golfo San Jorge excluding the extremes (with the bivalves Nuculana sulculata and Nucula puelcha as dominant fauna; Roux and Ferna´ndez, 1997; Roux et al., 1995).

3. Previous Studies 3.1. Late Miocene The Late Miocene transgression covered wide areas of Argentina (Entre Rı´os, Patagonia) and of eastern South America (Uruguay, Brazil). Marine facies along the eastern margin of Argentina and marginal and lacustrine facies in the western and northwestern areas have been recognized (Del Rı´o, 2000; Martı´nez and Del Rı´o, 2002). The deepest sections were studied in the ChacoParanense Basin while sequences of less than 200 m thick outcrop in northeastern Patagonia (Puerto Madryn Formation) and along the eastern Entre Rı´os Province (Parana´ Formation). For Penı´nsula Valde´s (Golfo Nuevo, Chubut) and the Rı´o Negro outlet (Rı´o Negro Province) different stratigraphic interpretations and correlations have been proposed (Darwin, 1846; Doering, 1882; Ameghino, 1898, 1908; Roveretto, 1913, 1921; Windhausen, 1921a, b, 1931; Frenguelli, 1926; Feruglio, 1949; Haller, 1978; Erdmann, 1984; Scasso and Del Rı´o, 1987; a synthesis in Del Rı´o, 2000 and references therein). Both in the Entre Rı´os Province (Parana´ Formation; Yrigoyen, 1969) and in Penı´nsula Valde´s and northern Patagonia (Puerto Madryn Formation; Haller, 1978) most authors have used the local denominations ‘‘Entrerriense’’ or ‘‘Paranense’’ Sea. These deposits correlate with sediments from the Salado Basin in Buenos Aires Province (Wahnish, 1942; Malumia´n, 1970; Garcı´a, 1970), the Camacho Formation (Goso and Bossi, 1966; Figueiras and Broggi, 1976) of Uruguay and the Rı´o Grande do Sul area (Brazil) (Closs and Madeira, 1968; Closs, 1970). The molluscs and other invertebrates have been known since the nineteenth century, mainly from the classical contributions by Darwin (1846), Sowerby (1846), D’Orbigny (1842), Ihering (1897, 1899, 1907), Ortmann (1900, 1902), Philippi (1893) and Pilsbry (1899), among others. In the twentieth century, different invertebrates, microfossils and paleoclimatic evidence linked to the Late Miocene transgression were analyzed by Camacho and Ferna´ndez (1956), Camacho (1966), Bertels and Madeira-Falcetta (1977), Bertels (1979, 1984), Boltovskoy (1979), Erdmann (1984), Mancen˜ido and Griffin (1988) and Beu et al. (1997). Studies by Del Rı´o (2000) and Del Rı´o et al. (2001, other references therein) yielded the most modern and complete set of geological, taxonomic and paleoenvironmental data available. These studies include the analysis of bathymetric depositional environments, paleoclimates and dating (Scasso and Del Rı´o, 1987; Del Rı´o, 2000; Martı´nez

289

and Del Rı´o, 2002). Similar molluscan assemblages are known from other areas along eastern South America, mainly from records in Brazil and Uruguay (Closs and Madeira, 1968; Closs and Forti, 1971; Figueiras and Broggi, 1976; Sprechmann, 1978; Martı´nez, 1994; Del Rı´o, 1998). Although the age of these deposits has been a subject of debate (Eocene to Pliocene; Zinsmeister et al., 1981; Del Rı´o, 1988; Martı´nez, 1994; Scasso et al., 2001), based on K/Ar and 86Sr dates Martı´nez and Del Rı´o (2002) accepted a Late Miocene (Tortonian) age (ca. 10 Ma) for the deposits of the Puerto Madryn Formation. In these assemblages, molluscs (29 gastropods and 42 bivalve species) represent the dominant invertebrate content, and the associated macroinvertebrate fauna consists of bryozoans, brachiopods, barnacles, crabs and echinoids.

3.2. Pliocene The marine Pliocene is restricted to the highest marine terraces, labeled MT II and MT I by Feruglio (1950), for example terraces at þ40 to > 150 m a.m.s.l. in Santa Cruz Province (i.e. Cabo Tres Puntas, Cerro Laciar) and in localities of Rı´o Negro and Chubut provinces (Table 1) (La Ernesta Formation, Camacho, 1967). It is still in doubt whether MT III at Camarones (Chubut) is of Pliocene or of Middle Pleistocene (MIS 11 [?]) age. Among the few paleontological studies performed, Farinati et al. (1981) suggested that a marine fossiliferous level containing molluscs in the province of Rı´o Negro is probably Pliocene in age. Further sampling, dating, paleoecological and paleobiogeographical studies of the molluscs from these rare deposits are urgently needed.

3.3. Quaternary Pleistocene and Holocene marine sediments are preserved along the entire Argentine coast (Fig. 3). However, the number of Quaternary marine highstands preserved along the Patagonian coast is still disputed. A stratigraphical synthesis of the terraces sampled and correlation with those from other coastal areas in Argentina and along the southwestern Atlantic is given in Table 1. Studies of the marine terraces V and VI from Patagonia and comparison with the Late Pleistocene and Holocene marine deposits from the Buenos Aires coastal area were made for different regions by Feruglio (1932, 1933a, b, 1950), Codignotto (1983, 1984, 1987), Cionchi (1985, 1987, 1988), Codignotto et al. (1988, 1990a, b, 1992, 2003), Gordillo et al. (1992, 1993), Rutter et al. (1990), Aguirre and Whatley (1995), Aguirre and Codignotto (1998), Pastorino (2000), Aguirre (1990, 1993a, 2001, 2002, 2003) and Aguirre et al. (2005a, b, 2006a, b) among others. The available dating undertaken for these terraces includes conventional radiocarbon (Codignotto, 1983, 1984, 1987; Codignotto et al., 1988), amino acid

290

Marina L. Aguirre et al.

racemization (AAR) and electron spin resonance (ESR) techniques (Rutter et al. 1990; Schellmann and Radtke, 1997, 2000; Rostami et al., 2000), allowing correlation of Feruglio’s marine terraces and identification of the corresponding sea-level highstands. In some cases, however, the ages for the Pleistocene are limited or inaccurate and thus are unable to resolve the number of Pleistocene sea-level highstands and to which MI substages they belong (for example within MIS 5). Geochronological results at certain terraces suggest a Last Interglacial (MIS 5) and a previous or even older interglacial (MIS 7 or MIS 9 or both) at similar and/or different altitudes (Schellmann and Radtke, 2000). The information now available indicates that Feruglio’s marine terraces V, IV and III are Pleistocene and that marine terrace VI is Holocene. The MIS recognized are MIS 7/9 (?) (MT IV), MIS 5 (MT V) and MIS 1 (MT VI). In the area of Camarones MIS 11 is probably preserved very restrictively (Table 2). The remaining terraces, I and II, are now regarded as Pre-Pleistocene (Pliocene [?]) (Rutter et al., 1990; Schellmann and Radtke, 1997, 2000). Some typical fossiliferous sites sampled are shown in Figs 4–6. More complete data on the geology, altimetry, dating and correlation of the MT can be obtained from Codignotto et al. (1988), Rutter et al. (1990), Schellmann (1998), Schellmann and Radtke (2000), Schellmann et al. (2000) and Rostami et al. (2000). The molluscan assemblages from the Patagonian marine Quaternary are known from the classical monographs (see above) and mainly by the work of Feruglio (1933a, 1950). Different aspects (taxonomy, paleoecology, distribution, taphonomy) were more recently studied in localities of Rı´o Negro and Chubut provinces by Pastorino (1994a, 2000), in Rı´o Negro, Chubut and Santa Cruz provinces by Aguirre and Codignotto (1998), Aguirre (2003), Aguirre et al. (2005a, b, 2006a, b) and in Tierra del Fuego and Beagle Channel by Gordillo (1991, 2006) and Gordillo et al. (1990, 1992, 1993). On the contrary, the modern molluscan fauna from Patagonia is dealt with by Carcelles (1950), Carcelles and Williamson (1951), Castellanos and Landoni (1993), Bastida et al. (1992), Pastorino (1994b), Roux et al. (1995), Roux and Ferna´ndez (1997), Lasta et al. (1998), Gordillo (1998), Nu´n˜ez Corte´s and Narosky (1997), Forcelli (2000), and in Tierra del Fuego by Gordillo (1990, 1995). For some taxa illustrations of modern specimens living along the southwestern Atlantic Ocean can be found in Rı´os (1994).

In general all along the coastline the outermost Holocene terrace is located at lower elevation than the inner series of Pleistocene terraces (Fig. 5). Rostami et al. (2000) have shown that Pleistocene deposits at different altitudes can be of the same or of different ages. As example, molluscan assemblages preserved in coastal deposits along Bahı´a Vera–Bahı´a–Camarones (Chubut Province, central Patagonia) were assigned to four main marine terraces, MT III (Camarones, higher than þ30 m after Feruglio, 1950, older than 300 ka and probably ca. 400 ka; Rostami et al., 2000; Schellmann and Radtke, 2000), MT IV (Punta Pescadero and Camarones, þ22–29 m, 178–239 ka), MT V (Bahı´a Vera, Punta Pescadero, Camarones, þ16–18 m, 92–135 ka) and MT VI (Punta Loberı´a, Punta Pescadero, Camarones, þ6–12 m, 2.5–8 ka). Tables 1 and 2 synthesize data on the altitude above mean sea level and dating available for MT III–VI. Additional detailed information on the morphostratigraphy and sedimentology can be obtained from Codignotto (1983), Codignotto et al. (1988), Rutter et al. (1990) and Schellmann and Radtke (2000). The fossil material recovered consists of 80–95% of gastropod and bivalve skeletons. The remaining faunal content is composed of an associated invertebrate macrofauna. In general the shells show good preservation and are appropriate for paleoenvironmental comparisons. Of all the taxa identified (Table 3a, Table 3b), bivalves are in general the better preserved. In only a few cases have complete bivalve shells in living position be found, most of Protothaca antiqua (Fig. 5). Gastropod shells show in general more abraded surfaces with loss of their original color and luster, except for the big shells of Tegula spp., for example of the biggest Tegula atra (Fig. 5) in spite of their older age (Mid- to Late Pleistocene). The material studied is part of personal collections along the coast between Rı´o Negro and Santa Cruz Province or was examined from museum collections (Museo Argentino de Ciencias Naturales (MACN), Buenos Aires; Museo de La Plata (MLP), La Plata; British Museum of Natural History (BMNH), London). Comparisons were made with modern material sampled along the adjacent littoral regarding taxonomic composition, diversity and biogeographic ranges for each taxon. The set of data includes results published elsewhere (Aguirre, 1993a, 2002, 2003; Aguirre et al., 2005a, b, 2006a, b; in press).

5. Quaternary Molluscs and Associated Macroinvertebrates 4. Fossiliferous Localities and Fossil Content Along the entire Patagonian coast studied, a series of regressive, pebbly, raised elongated beach ridge systems rich in molluscan shells are preserved. The Pleistocene and Holocene deposits studied were sampled at selected localities (Figs 3–5), in outcrops atop ridges and marine terraces or in abandoned quarries.

Within the Patagonian marine terraces of Quaternary age (MT III–VI), molluscs (51 gastropod and 36 bivalve species) are the most abundant and best-preserved invertebrate groups, together with the associated macrofauna (cnidarians, bryozoans, brachiopods, polyplacophores, scaphopods, serpulids, balanids and echinoids), and are illustrated in Plates 1–8.

Late Pleistocene Cenozoic Invertebrate Paleontology

291

Table 2. Dates available for the deposits sampled (Bahı´a Camarones area). Marine Terraces (F)

Altitude + m a.m.s.l

LOCALITIES S Bahía Vera– Camaronesa samples studied by previous authors

MODERN LITTORAL HOLOCENE MT VI + 10–12 (F) + 8–10 (C) + 6–12 (1, 2, 3) H1,H2, Pa31, Pa32, Pa33 + 4–10 (6) 5 km North of + 5–6 (7) Camarones

MIS

b

1

PLEISTOCENE

c

DATING

2880 ± 90; 2880 + 85; 3860 ± 95; 4370 ± 95; 7520 ± 120 (1, 2, 3) 2618 ± 92 (6), 5380± 70–6708 ± 46 (6) 7 + 2 (7)

14

30,900 ± 1100; 31,800 ± 1400 as minimum C (1)

MT V + 22–26 (F) + 16–19 (6)

5e

+ 12–13 (6)

Pa47c Pa47a

+ 16–17 (7)

12 km South of Camarones

92 ± 9 ka; 99 ± 12 ka;115 ± 9 ka; 117 ± 13 ka; 131 ± 17 ka; 133 ± 15 ka; 135 ± 18 ka (6) 112 ± 13; 115 ± 9; 117 ± 5; 117 ± 6 (Th/U) 110 ± 8; 114 ± 9 ESR) (7)

MT IV

36,000 ± 2000; 37,000 ± 2400; > 43,0000 as minimum 14C (1) + 28–40 (F) + 20–26 (6) + 25–29 (C)

Pa31

7

178 ± 16 ka; 180 ± 22 ka; 196 ± 33 ka; 199 ± 27 ka; 200 + 40 ka; 209 + 18 ka; 231 + 20 ka (6) 1 58–239 (7)

Pa35

9/11

342 ± 29 ka; 372 ± 30 ka; 378 ± 62 ka; 380 ± 92 ka; 383 ± 38 ka (6)

6–7km South of Camarones

9/11

309 ± 50,000/35,000; 354 ± 45; 338 ± 34 (7)

MT III + 35–40 (C) + 33–35 (6)

+ 33–34 (7)

References for geomorphology, sedimentology and dating available (Foot note c) from previous work performed in the area: F: Feruglio (1950); C: Cionchi (1988). 1: Codignotto, 1983 (14C, most in Protothaca antiqua). 2: Codignotto et al., 1988 (14C). 3: Codignotto et al., 1992 (14C). 4: Rutter et al., 1989, 1990 (D/L, ESR, Th/U; different species). 5: Schellmann and Radtke, 1997 (ESR; no details about the species dated). 6: Schellmann and Radtke, 2000 (14C, Th/U, ESR; mostly probably on P. antiqua and on unidentified taxa of unknown paleoecological value or taphonomic history; samples H, Pa). 7: Rostami et al., 2000 (Th/U and ESR; mostly probably on P. antiqua, not Mercenaria, several in Mytilus sp. or in unidentified species). aFrom North to South: Bahı´a Vera, Punta Loberı´a, Punta Pescadero, Cabo Raso; Bahı´a Cruz; Bahı´a Camarones; Bahı´a Bustamante, Caleta Malaspina (see also Fig. 3). bMIS = marine oxygen isotope stage/substage (see Fig. 6b). Modified from Aguirre et al. (2005a). cEstimations for equivalent samples.

The most common gastropod species belong to the genera Nacella (Patinigera), Tegula, Siphonaria, muricids (Trophon spp.), Crepidula and buccinids (Buccinanops spp.). Among the bivalves, Protothaca, Brachidontes spp., Aulacomya and, less commonly, Eurhomalea, Clausinella and Mulinia are those most frequently preserved.

A complete list of the most characteristic and relevant gastropod and bivalve taxa recovered, their distribution and the ecological requirements of modern representatives are synthesized as follows and in Tables 3a, b and 4a, b. Comprehensive systematic references are available in Aguirre and Farinati (2000).

292

Marina L. Aguirre et al. Fig. 4. Main coastal sectors of Patagonia where Late Pleistocene and Holocene ridges and marine terraces were sampled for molluscan studies. Areas of study not in scale: (a) Puerto Lobos area (b) Cabo Raso–Camarones area (c) Bahı´a Bustamante area (d) Golfo San Jorge area. Modified from Aguirre (2003) and Aguirre et al. (2005a, b, 2006a, b).

+ 28–40 m

Quaternary marine terraces

P

S.L. T

MT VI (Holocene)

Modern beach

MT IV, V (Pleistocene) m a.m.s.l. 30 20 10 0

S.L.

Approx.:1450 m

Fig. 5. Marine terraces after the traditional local pattern at Bahı´a Bustamante area. Schematic profile of the MT IV–VI sensu Feruglio (1950). Photographs of the modern beach at B. Bustamante and the MT sampled. In MT IV, complete shells of P. antiqua (P) with joined valves, some in living position, and large shells of T. atra (T). From Aguirre et al. (2005a, b).

Late Pleistocene Cenozoic Invertebrate Paleontology

293

(a) A

B

TMIII > + 30 (MOI11?)

C

D

E

+ 9–14

+ 17–26

+ 28–30

1000 m

pmsl TMIV (MIS7?)

TMV (MIS5?)

TMVI (MIS1)

(b) Palaeomagnetic scale Age (years)

0

–1.0

Oxygen isotope stages –2.0

1 Wisconsin

122,000 130,000

Sangamon Illinoisan

300,000

5

7 9

Depth

11

Fig. 6. (a) Most characteristic molluscs from Late Pleistocene (MT IV and V) and Holocene (MT VI) coastal deposits studied at Bahı´a Camarones area. Cross section modified from Feruglio (1950, p. 89). A (MTIII, is not in this section, samples taken from exposures southwards of Camarones): Ostrea cf. tehuelcha (H = 113), Mactra aff. isabelleana (L = 37), Pitar rostratus (L = 30), Corbula patagonica (L = 11), Diplodonta vilardeboana (L =14); B (MTIV): Tegula atra (Wm = 23, Protothaca antiqua (L = 55), Clausinella gayi (L = 28), Crepidula protea (Hm = 11.8); C (MTV): Protothaca antiqua (L = 49), Tegula atra (Wm = 25) (maximum size), Crepidula dilatata (Hm = 19), Mytilus edulis (L = 48); D (MTVI): Nacella deaurata (Wm = 35), Nacella magellanica (Wm = 37), Trophon geversianus (Hm = 49), Brachidontes purpuratus (L = 18), Aulacomya atra (L = 72); E (modern): Aequipecten tehuelchus (Hm = 47), Aulacomya atra (L = 72), Nacella delicatissima (Wm = 29), *Lyonsia sp. (L = 16), *Panopea abbreviata (L = 35) (*not collected, but mentioned in the literature). Dimensions in mm. L = length, H = height; Wm = maximum width; Hm = maximum height. Modified from Aguirre et al. (2006a). (b) MIS = marine oxygen isotope stage. Modified from Aguirre (2006a) and previous references therein.

294

Marina L. Aguirre et al.

Table 3. General molluscan fauna typical of Quaternary marine terraces in Patagonia.

Late Pleistocene Cenozoic Invertebrate Paleontology

295

Table 3. (Continued)

List of taxa illustrated. Geographical and stratigraphical distribution of the taxa: (a) gastropods; (b) bivalves. Records within marine Quaternary deposits from Buenos Aires Province coastal zone (Argentina), Surinam, Brazil and Uruguay (South America) and their occurrence within Late Miocene sediments from Patagonia are indicated for each taxon. Information of the biogeographical distribution in the modern malacological provinces, temperature affinity and absence along the adjacent littoral zone are also indicated for each taxon. Groups I–IV refer to groups of species arranged according to their modern distribution. Modern zoogeographical provinces: ANT = Antillean Province, BRA = Brazilian Province, ARG = Argentine Province, MAG = Magellanean Province. Group I: pandemic; II: tropical or subtropical; III: warm-temperate to temperate waters; IV: cold waters. W: warm or warm-temperate affinity; typical of warm shallow water masses. C: cold water affinity. Note that the fauna from Tierra del Fuego is not analyzed (see Gordillo et al., 1992, 1993, for information).

Marina L. Aguirre et al.

Phylum MOLLUSCA Linne´, 1758 Class GASTROPODA Cuvier, 1797 Subclass PROSOBRANCHIA Milne Edwards, 1848 Order ARCHAEOGASTROPODA Thiele, 1925 Superfamily FISSURELIDACEAE Fleming, 1822 Family FISSURELLIDAE Fleming, 1822 Subfamily FISSURELLINAE Fleming, 1822 Genus FISSURELLA Bruguie`re, 1789 Subgenus FISSURELLA s.s.

Fossil records in Patagonia: Pleistocene: Cabo Raso, Camarones, Bahı´a Bustamante; Holocene: Cabo Raso, Camarones, Bahı´a Bustamante, Comodoro Rivadavia, Puerto Mazarredo, Puerto Deseado, San Julia´n, Tierra del Fuego Geographic range: Valparaı´so, Chile (32520 S), Tierra del Fuego, Estrecho de Magallanes, Cape Horn, Isla de los Estados, Islas Malvinas (Falkland Islands); probably south of Santa Cruz Province

Fissurella picta (Gmelin, 1791) (Plate 1, 1, 2) Dimensions: 28–80 mm Stratigraphic range: Pleistocene–Recent

Fissurella oriens G.B. Sowerby I, 1835 (Plate 1, 3) Dimensions: 22.5–70 mm Stratigraphic range: Pleistocene–Recent

Table 4. Ecological requirements compiled from modern representatives of the taxa studied: (a) bivalves; (b) gastropods.

Glycymeris longior (Sowerby) Mytilus edulis Linné Brachidontes rodriguezi (d'Orbigny) Brachidontes purpuratus (Lamarck) Brachidontes cf. purpuratus (Lamk.) Aulacomya atra Aequipecten tehuelchus (d'Orbigny) Zygochlam ys patagonicus (King & Brod.) Chlam ys lishkei (Dunker) Pododesmus rudis (Brod.) Plicatula gibbosa Lamarck Ostrea equestris Say Ostrea tehuelcha Feruglio Diplodonta patagonica (d'Orbigny) Diplodonta vilardeboana (d'Orbigny) Mactra isabelleana d'Orbigny Mulinia lateralis (Say) Mulinia edulis (King & Broderip) Darina solenoides (King) Solen tehuelchus d'Orbigny Solen sp. Ensis macha (Molina) Macoma uruguayensis (Smith) Abra sp. Tivella isabelleana (d'Orbigny) Pitar rostratus (Koch) Eurhomalea exalbida (Dillwyn) Protothaca antiqua (King) Venericardia procera Gould Clausinella gayi (Hupé) Petricola patagonica d´Orbigny Corbula patagonica d'Orbigny Hiatella arctica (Linné) Panopea abbreviata Valenciennes

OTHERS

DETRIT.

SUSPEN.

Trophic type DEEP INF.

SH. INF.

EPIBYSS.

HARD.

CEMENT.

Life mode

Substrate SUBLITT.

INTERT.

BIVALVIA

Zonation SUPRAL.

ECOLOGICAL DATA

SOFT.

296

Late Pleistocene Cenozoic Invertebrate Paleontology Table 4. (Continued)

F ILTE R -F .

C AR NIV.

S E S S ILE

HE R BIV.

Trophic type

Life mode

FR EE

HAR D

S O FT

Substrate S UBLITT.

GASTROPODA

INTE R T.

Zonation S UP R AL.

ECOLOGICAL DATA

Fissurella picta (Gmelin) Fissurella oriens Sowerby Fissurella radiosa Lesson Lucapinella henseli (Martens) Nacella (P.) delicatissima

(Strebel)

Nacella (P.) magellanica (Gmelin) Nacella (P.) deaurata (Gmelin) Scurria ceciliana (d´Orb.) Tegula (A.)patagonica (d'Orbigny) Tegula (A.)blakei ( Clench & Aguayo) Tegula (C.) atra (Lesson) Photinula caerulescens

(King & Brod.)

Ataxocerithium pullum (Phil.) Calliostoma tehelchum

Ihering

Calliostoma nordenskjoldi Strebel Littoridina australis (d'Orbigny)

? ?

Crepidula aculeata ( Gmelin) Crepidula protea d'Orbigny Crepidula onyx Sowerby Crepidula dilatata Lamarck Crepidula cf. unguiformis Lamarck Trochita pileus (Lamarck) Natica isabelleana d'Orbigny Natica magellanicum Hombron & Jacq. Falsilunatia patagonica (Philippi) Epitonium (E.) georgettinum (Kiener) Epitonium (B.) magellanicum (Philppi) Trophon varians (d'Orbigny) Trophon geversianus

(Pallas)

Trophon necocheanum

Ihering

Trophon elongatus Strebel Fuegotrophon pallidus (Broderip) Ximenopsis muriciformis Urosalpinx sp.

(King & Brod.)

Acanthina monodon (Pallas) Zidona dufresnei (Donovan) Adelomelon ancilla (Lightfoot) Adelomelon beckii (Broderip) Adelomelon (P.) brasiliana (Lamarck) Adelomelon ferussaci

(Donovan)

Odontocymbiola magellanica

(Gmelin)

Olivella (O.) tehuelcha (Duclos) Olivancillaria urceus (Roding) Olivancillaria auricularia (Lamk.) Olivancillaria carcellesi Klapp. Pareuthria plumbea (Philippi) Pareuthria cerealis Rochebr. & Mab. Buccinanops cochlidium Buccinanops globulosus Buccinanops paytensis

(Dillwyn) (Kiener) (Kiener)

Siphonaria lessoni (Blainville)

Zonation: Supratidal, Intertidal, Sublittoral; Bivalve Life Modes: Epibyssate, Shallow Infaunal, Deep Infaunal; Trophic Types: Carnivorous, Herbivorous, Filter-Feeding, Suspensivorous, Detritivorous.

297

298

Marina L. Aguirre et al.

1 a

3

2

b

a

4

5

b

a

b

6

a

b

c 7 a a 7´

9

b

8

b

a

10

12

11

a

13 a b

a 14

b

b 15

Plate 1. Most characteristic molluscs collected in Late Pleistocene (MT III, IV, V) and Holocene (MT VI) coastal deposits studied along the modern Patagonian littoral. Dimensions (in mm) for gastropods: H = height and W = maximum width; a, exterior, apical or apertural view, b, interior or abapertural view, c, lateral view. 1a, b, Fissurella picta (H = 28 mm), Camarones, Holocene (PA02Hol7) (DCG-MLP0007-424); 2, Fisurella picta (H = 59), Pleistocene, Bahı´a Bustamante (reillustrated from Feruglio, 1933a, Pl.IX, fig. 43); 3a, b, Fisurella oriens (H = 22.5), Holocene, Bahı´a Camarones (PA02Hol6) (DCG-MLP0007-427); 4a, b, Fisurella radiosa (H = 20.5), Modern, Bahı´a Camarones (PA02M5) (DCG-MLP0007-428); 5, Lucapinella henseli (H = 34), Holocene, south of Bahı´a Blanca (PI-UNS 1980); 6a, b, Nacella delicatissima (H = 29), modern, Bahı´a Camarones (PA02M4) (DCGMLP0007-430); 7a, c, Nacella delicatissima (H = 5), Holocene, Bahı´a Bustamante (reillustrated from Feruglio, 1933a, Pl.IX, fig. 25a, b); 70 , Puncturella conica (H = 17), Holocene, Mazarredo (PA04A9) (DCG-MLP0007-467); 8a, b, Nacella magellanica (H = 32), Pleistocene, Ba. Camarones (PA02Q11) (DCG-MLP0007-435); 9a, b, Nacella deaurata (H = 18.5), Holocene, Ba. Camarones (PA02Hol7) (DCG-MLP0007-429); 10, Nacella deaurata (H = 31), Holocene, Puerto Deseado (reillustrated from Feruglio, 1933a, Pl.IX, fig. 9); 11, Scurria ceciliana (H = 8), Holocene, Comodoro Rivadavia (reillustrated from Feruglio, 1933a, Pl.IX, fig. 27); 12, Tegula blakei (H = 7), Holocene, San Antonio Oeste (MLP 26575); 13a, b, Tegula patagonica (H = 11), Pleistocene, Bahı´a Camarones (PA02Q15) (DCG-MLP0007-437); 14 a, b, Tegula atra (H = 28), Pleistocene, Bahı´a Camarones (PA02Q15) (DCG-MLP0007-436); 15a, b, Tegula atra (H = 31) (reillustrated from Feruglio, 1933a, Pl.XI, fig. 12a, b).

Late Pleistocene Cenozoic Invertebrate Paleontology Fossil records in Patagonia: Pleistocene: Puerto Lobos, Camarones, Bahı´a Bustamante; Holocene: Camarones, Bahı´a Bustamante, Comodoro Rivadavia, Puerto Mazarredo, Puerto Deseado, San Julia´n, Tierra del Fuego Geographic range: Valparaı´so, Chile, Cape Horn, Isla de los Estados, Islas Malvinas (Falkland Islands) Fissurella radiosa Lesson, 1831 (Plate 1, 4) Dimensions: 20.5–50 mm Stratigraphic range: Pleistocene–Recent Fossil records in Patagonia: Pleistocene: Cabo Raso, Camarones, Bahı´a Bustamante; Holocene: San Antonio Oeste, Puerto Lobos, Puerto Pira´mides, Puerto Madryn, Cabo Raso, Camarones, Bahı´a Bustamante, Comodoro Rivadavia, Rada Tilly, Puerto Mazarredo, San Julia´n, Tierra del Fuego Geographic range: Chiloe´, Chile (42420 S), Tierra del Fuego, Islas Malvinas (Falkland Islands), Golfo San Matı´as (Argentina) Genus LUCAPINELLA Pilsbry, 1890 Lucapinella henseli (Martens, 1900) (La´m. 1, 5) Dimensions: 20–30 mm Stratigraphic range: Holocene–Recent Fossil records in Patagonia: Holocene of Bahı´a Blanca and Bahı´a Solano Geographic range: Rio Grande do Sul (Brazil), Uruguay, Argentina down south to Estrecho de Magallanes Superfamily PATELLACEA Rafinesque, 1815 Family PATELLIDAE Rafinesque, 1815 Subfamily NACELLINAE Thiele, 1929 Genus NACELLA Schumacher, 1817 Subgenus PATINIGERA Dall, 1905 Nacella (Patinigera) delicatissima (Strebel, 1907) (Plate 1, 6, 7) Dimensions: 11–50 mm Stratigraphic range: Pleistocene–Recent Fossil records in Patagonia: Pleistocene at Camarones; Holocene: Bahı´a Bustamante, Bahı´a Solano, Comodoro Rivadavia, Caleta Olivia, Puerto Mazarredo, Puerto Deseado, San Julia´n, Tierra del Fuego Geographic range: southern Patagonia, Puerto Deseado, Estrecho de Magallanes, Islas Malvinas (Falkland Islands) Nacella (Patinigera) magellanica (Gmelin, 1791) (Plate 1, 8) Dimensions: 32–60 mm Stratigraphic range: Pleistocene–Recent Fossil records in Patagonia: Pleistocene: Puerto Lobos, Cabo Raso, Camarones, Bahı´a Bustamante; south of Caleta Olivia, Puerto Deseado; Holocene: Puerto Lobos, Puerto Pira´mides, Puerto Madryn, Cabo Raso, Camarones, Bahı´a Bustamante, Bahı´a Solano, Comodoro Rivadavia, Rada Tilly, Caleta Olivia, Bahı´a La´ngara, Puerto Mazarredo, Puerto Deseado, Bahı´a Laura, San Julia´n, Tierra del Fuego Geographic range: Valdivia, Chile, down south to Isla de los Estados, Argentina, Estrecho de Magallanes,

299

Islas Malvinas (Falkland Islands), Patagonian coast northward to Rı´o Negro Province (deeper in Buenos Aires Province northward to the Rı´o de la Plata) Nacella (Patinigera) deaurata (Gmelin, 1791) (Plate 1, 9, 10) Dimensions: 35–70 mm Stratigraphic range: Pleistocene–Recent Fossil records in Patagonia: Pleistocene: Camarones, Bahı´a Bustamante, Caleta Olivia; Holocene: Camarones, Bahı´a Bustamante, Comodoro Rivadavia, Rada Tilly, Bahı´a La´ngara, Puerto Deseado, Tierra del Fuego Geographic range: Buenos Aires Province coasts from 38 S down south to Tierra del Fuego, Isla de los Estados and Islas Malvinas (Falkland Islands) Family LOTTIDAE Gray, 1840 Subfamily LOTTIINAE Gray, 1840 Tribu SCURRIINI Lindberg, 1988 Genus SCURRIA Gray, 1847 Scurria ceciliana (d0 Orbigny, 1841) (Plate 1, 11) Dimensions: 20–26 mm Stratigraphic range: Holocene–Recent Fossil records in Patagonia: Holocene of Bahı´a Bustamante, Comodoro Rivadavia, Puerto Deseado Geographic range: Peru´ and Chile, southern Patagonia Superfamily TROCHACEA Rafinesque, 1815 Family TROCHIDAE Rafinesque, 1815 Subfamily TEGULINAE Kuroda, Habe & Oyama, 1971 Genus TEGULA Lesson, 1835 Subgenus AGATHISTOMA Olsson & Harbison, 1953 Tegula (A.) patagonica (d0 Orbigny, 1835) (Plate 1, 13) Dimensions: 10–23 mm Stratigraphic range: Miocene–Recent Fossil records in Patagonia: Miocene: Puerto Madryn; Pleistocene: San Antonio Oeste, Puerto Lobos, Camarones, Bahı´a Bustamante; Holocene: San Antonio Oeste, Puerto Pira´mides, Puerto Madryn, Puerto Lobos, Cabo Raso, Camarones, Bahı´a Bustamante, Comodoro Rivadavia, Puerto Mazarredo Geographic range: southern Brazil to Golfo Nuevo (Argentina); scarce in Camarones–Bahı´a Bustamante area Tegula (Agathistoma) blakei (Clench and Aguayo, 1938) (Plate 1, 12) Dimensions: 5–8 mm Stratigraphic range: Pleistocene–Recent Fossil records in Patagonia: Pleistocene at Bahı´a Bustamante; Holocene at San Antonio Oeste Geographic range: Rio Grande do Sul (Brazil) to Golfo Nuevo (Argentina) Subgenus CHLOROSTOMA Swainson, 1840 Tegula (Chlorostoma) atra (Lesson, 1830) (Plate 1, 14, 15) Dimensions: 25–60 mm Stratigraphic range: Pleistocene–Recent Fossil records in Patagonia: Pleistocene: Puerto Lobos, Cabo Raso, Camarones, Bahı´a Bustamante, Puerto

300

Marina L. Aguirre et al.

Deseado; Holocene: San Antonio Oeste, Puerto Lobos, Cabo Raso, Camarones, Bahı´a Bustamante, Comodoro Rivadavia, Caleta Olivia, Puerto Mazarredo, Puerto Deseado, San Julia´n Geographic range: southern Peru´, Chile and Estrecho de Magallanes Remarks: T. atra can be regarded as a biostratigraphical indicator of the Patagonian marine Quaternary, dominant and reaching largest sizes within the Pleistocene marine terraces MT IV and V sensu Feruglio (1950). Nowadays, it is extinct along the Southwestern Atlantic Ocean. Family TROCHIDAE Rafinesque, 1815 Genus PHOTINULA Adams y Adams, 1854 Photinula caerulescens (King and Broderip, 1831) (Plate 2, 16, 17) Dimensions: 15–23 mm Stratigraphic range: Pleistocene–Recent Fossil records in Patagonia: Pleistocene: Camarones, Bahı´a Bustamante, Puerto Deseado; Holocene: Bahı´a Bustamante, Puerto Deseado, San Julia´n Geographic range: Rı´o de la Plata to Patagonia and Islas Malvinas (Falkland Islands) down south to 53 S Family CERITHIOPSIDAE H & A. Adams, 1854 Genus ATAXOCERITHIUM Tate, 1894 Ataxocerithium pullum (Philippi) (Plate 2, 18) Dimensions: 6–19 mm Stratigraphic range: Pleistocene–Recent Fossil records in Patagonia: Pleistocene: Punta Pescadero, Cabo Raso; Holocene: Bahı´a Bustamante, Comodoro Rivadavia, Puerto Deseado Geographic range: Buenos Aires Province down south to Tierra del Fuego, Islas Malvinas (Falkland Islands) and Burwood Bank (ca. 41–56.33 S; 74–57 W) Genus CALLIOSTOMA Swaison, 1840 Calliostoma nordenskjoldi Strebel, 1908 (Plate 2, 19, 21) Dimensions: 14 mm Stratigraphic range: Holocene–Recent Fossil records in Patagonia: Pleistocene at Puerto Deseado Geographic range: 37–46 S (Buenos Aires Province to northern Santa Cruz Province, Argentina) Remarks: Calliostoma tehuelchum Ihering, 1907 as illustrated by Feruglio (1950) is considered a synonym. Order MESOGASTROPODA Thiele, 1925 Superfamily RISSOACEA H. y A. Adams, 1854 Family HYDROBIIDAE Stimpson, 1865 Subfamily LITTORIDINAE Thiele, 1929 Genus LITTORIDINA Souleyet, 1852 Littoridina australis (d0 Orbigny, 1835) (Plate 2, 22) Dimensions: 0.25–10 mm Stratigraphic range: Miocene (?)-Pleistocene–Recent Fossil records in Patagonia: Holocene at San Antonio Oeste

Geographic range: southern Brazil to Golfo San Matı´as (Argentina) Superfamily CALYPTRAEACEA Blainville, 1824 Family CALYPTRAEIDAE Blainville, 1824 Subfamily CREPIDULINAE Fleming, 1822 Genus CREPIDULA Lamarck, 1799 Crepidula aculeata(Gmelin, 1791) (Plate 2, 25) Dimensions: 8–36 mm Stratigraphic range: Pleistocene–Recent Fossil records in Patagonia: Holocene: San Antonio Oeste, Camarones Geographic range: from Florida (USA) and Antilles to Brazil and Patagonia; Chile Crepidula protea d0 Orbigny, 1841 (Plate 2, 26) Dimensions: 4–16 mm Stratigraphic range: Miocene–Holocene Fossil records in Patagonia: Pleistocene: Camarones, Bahı´a Bustamante, south of Caleta Olivia; Holocene: San Antonio Oeste, Puerto Lobos, Puerto Pira´mides, Camarones Geographic range: Antilles to Golfo San Matı´as (Argentina) Crepidula onyx Sowerby, 1824 (Plate 2, 27) Dimensions: 5–40.5 mm Stratigraphic range: Miocene–Recent Fossil records in Patagonia: Pleistocene at Camarones; Holocene at San Antonio Oeste Geographic range: Pacific coast from California to Chile; southwestern Atlantic from 42 to 47 S Crepidula dilatata Lamarck, 1822 (Plate 2, 28) Dimensions: 11–65 mm Stratigraphic range: Miocene–Holocene Fossil records in Patagonia: Pleistocene: Puerto Lobos, Cabo Raso, Camarones, Bahı´a Bustamante, south of Caleta Olivia; Holocene: San Antonio Oeste, Puerto Lobos, Puerto Pira´mides, Puerto Madryn, Cabo Raso, Camarones, Bahı´a Bustamante, Bahı´a Solano, Comodoro Rivadavia, Rada Tilly, Caleta Olivia, Puerto Mazarredo, Puerto Deseado, Bahı´a Laura, San Julia´n, Tierra del Fuego Geographic range: pandemic; very common in South America from Brazil to Tierra del Fuego, Islas Malvinas (Falkland Islands), and Chile to Ecuador Crepidula cf. unguiformis Lamarck, 1822 Dimensions: 20–26 mm Stratigraphic range: Pleistocene–Recent Fossil records in Patagonia: Pleistocene at Cabo Raso Geographic range: pandemic; western Atlantic coast from North America to Uruguay, Pacific from California to Peru´; Mediterranean; Australia, New Zealand Genus TROCHITA Schumacher, 1817 Trochita pileus (Lamarck, 1822) (Plate 2, 23, 24, 240 ) Dimensions: 20–30 mm Stratigraphic range: Holocene–Recent

Late Pleistocene Cenozoic Invertebrate Paleontology

301

17 18 a

16

b

a a b

19

a

b 20

21

b a

23 c b

22

a

a

a

25

b

26

27

a

28

24

24′ b

a b

b

b 29 30

31

32 34

a

b

a

b

33

a

b

Plate 2. Most characteristic molluscs collected in Late Pleistocene (MT III, IV, V) and Holocene (MT VI) coastal deposits studied along the modern Patagonian littoral. Dimensions (in mm) for gastropods: H = height and W = maximum width; a, exterior, apical or apertural view, b, interior or abapertural view, c, lateral view. 16, Photinula caerulescens (H = 17), Holocene, Santa Cruz (MACN 5176); 17a, b, Photinula caerulescens (H = 11), Holocene, Bahı´a Bustamante (reillustrated from Feruglio, 1933a, Pl.IX, fig. 22a, b); 18a, b, Ataxocerithium pullum (H = 27), Holocene, Bahı´a Bustamante (reillustrated from Feruglio, 1933a, Pl.IX, fig. 20a, b); 19a, b, 20a, b, Calliostoma tehuelchum, Pleistocene, Puerto Deseado (reillustrated from Feruglio, 1933a, Pl.IX, fig. 23a, b, H = 9; fig. 24a, b, H = 17); 21, Calliostoma nordenskjoldi (H = 13), Holocene south of Bahı´a Blanca (PI-UNS 1561); 22, Litttoridina australis (H = 10), Holocene Mar Chiquita (MLP 25947); 23a–c, Trochita pileus (H = 20), Holocene, Bahı´a Solano (MLP 26579); 24a, b, Trochita pileus (H = 25), Holocene, Puerto Deseado (reillustrated from Feruglio, 1933a, Pl.IX, fig. 37, 39); 240 a–b, Trochita pileus (H = 11), Holocene?, south of Cabo Tres Puntas (PA04A16) (DCG-MLP0007-458); 25a, b, Crepidula aculeata (H = 36), modern, Bahı´a Camarones (PA02M5) (DCG-MLP0007-426); 26a, b, Crepidula protea (H = 16), Pleistocene, Bahı´a Camarones (PA02Q15) (DCG-MLP0007-434); 27a, b, Crepidula onyx (H = 33.5), Pleistocene, Bahı´a Camarones (PA02Q11) (DCG-MLP0007-431); 28, Crepidula dilatata (H = 31.5), Pleistocene, Bahı´a Camarones (PA02Q11) (DCG-MLP0007-432); 29, Natica isabelleana (H = 18), Pleistocene, south of Puerto Lobos (PA04Q5a) (DCG-MLP0007-415); 30, Falsilunatia patagonica (H = 18); Holocene, Puerto Madryn (MLP 26587); 31a, b, Epitonium georgettinum (H = 14), Holocene, Puerto Madryn (MLP 26590); 32a, b, Epitonium orbignyi (H = 18), Holocene, Bahı´a Camarones (reillustrated from Feruglio, 1933a, Pl.IX, fig. 19a, b); 33, Epitonium magellanicum (H = 17), Holocene, Estrecho de Magallanes (M.H.N.S.s/n); 34a, b, Epitonium magellanicum (H = 20), Pliocene, Cerro Laziar (reillustrated from Feruglio, 1933a, Pl.IX, fig. 3a, b).

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Fossil records in Patagonia: Holocene: Bahı´a Bustamante, Bahı´a Solano, south of Caleta Olivia, Tierra del Fuego Geographic range: Islas Malvinas (Falkland Islands); Tierra del Fuego, Estrecho de Magallanes to Cabo San Antonio; Chile (?) Superfamily NATICACEA Forbes, 1838 Family NATICIDAE Forbes, 1828 Subfamily NATICINAE Swainson, 1840 Genus NATICA Scopoli, 1777 Natica isabelleana d0 Orbigny, 1835 (Plate 2, 29) Dimensions: 4–30 mm Stratigraphic range: Miocene–Recent Fossil records in Patagonia: Pleistocene: Puerto Lobos, Camarones; Holocene: San Antonio Oeste, Puerto Lobos, Puerto Pira´mides, Puerto Madryn, Cabo Raso, Camarones, Bahı´a Bustamante Geographic range: southern Brazil to Golfo Nuevo (Patagonia) Natica magellanica Hombron & Jacquinot, 1848 Dimensions: 23 mm Stratigraphic range: Pleistocene–Recent Fossil records in Patagonia: Pleistocene at Puerto Deseado Geographic range: Atlantic coasts from 52 to 54 S Genus FALSILUNATIA Powell, 1951 Falsilunatia patagonica (Philippi, 1845) (Plate 2, 30) Dimensions: 25 mm Stratigraphic range: Holocene–Recent Fossil records in Patagonia: Holocene: Puerto Madryn, Bahı´a Bustamante, San Julia´n Geographic range: Atlantic coast from 37 S down to Tierra del Fuego; Antarctica Family EPITONIIDAE Berry, 1910 Subfamily EPITONIACEA Berry, 1910 Genus EPITONIUM Ro¨ding, 1798 Subgenus EPITONIUM s.s. Epitonium (Epitonium) georgettinum (Kiener, 1839) (Plate 2, 31, 32) Dimensions: 4–35 mm Stratigraphic range: Miocene–Recent Fossil records in Patagonia: Pleistocene at San Antonio Oeste, Camarones; Holocene: San Antonio Oeste, Puerto Madryn, Camarones, Puerto Mazarredo Geographic range: Rio Grande do Sul to Golfo Nuevo (Argentina) Subgenus BOREOSCALA Kobelt, 1902 Epitonium (Boreoscala) magellanicum (Philippi, 1845) (Plate 2, 33, 34) Dimensions: 23 mm Stratigraphic range: Pliocene–Recent Fossil records in Patagonia: Pliocene: Cerro Laciar (Santa Cruz); Pleistocene San Julia´n; Holocene: Bahı´a Solano, Tierra del Fuego Geographic range: Rio Grande do Sul to Tierra del Fuego; Chile

Order NEOGASTROPODA Wenz, 1938 Superfamily MURICACEA da Costa, 1776 Family MURICIDAE Rafinesque, 1815 Subfamily TROPHONINAE Cossmann, 1903 Genus TROPHON Montfort, 1810 Trophon varians (d0 Orbigny, 1841) (Plate 3, 35) Dimensions: 26–42 mm Stratigraphic range: Miocene–Recent Fossil records in Patagonia: Pleistocene: San Antonio Oeste, Puerto Lobos, Cabo Raso, Camarones, Bahı´a Bustamante; Holocene: Camarones, Bahı´a Solano, Rada Tilly Remarks: this taxon has recently been considered a posterior synonym of Trophon geversianus (Pallas). Until the results are completed by a thorough morphometric analysis including several Trophon species and on account of eventual paleoenvironmental and biostratigraphic implications, we here maintain this name Geographic range: Rı´o de La Plata to Golfo San Matı´as (Argentina); scarce in Camarones–Bahı´a Bustamante area. Trophon geversianus (Pallas, 1774) (Plate 3, 36) Dimensions: 49–70 mm Stratigraphic range: Miocene–Holocene Fossil records in Patagonia: Pleistocene: Puerto Lobos, Cabo Raso, Camarones, Bahı´a Bustamante, south of Caleta Olivia; Holocene: San Antonio Oeste, Puerto Lobos, Puerto Pira´mides, Puerto Madryn, Cabo Raso, Camarones, Bahı´a Bustamante, Bahı´a Solano, Comodoro Rivadavia, Rada Tilly, Caleta Olivia, Bahı´a La´ngara, Puerto Mazarredo, Puerto Deseado, Bahı´a Laura, San Julia´n, Tierra del Fuego Geographic range: Atlantic coast from 36 S to Tierra del Fuego, Islas Malvinas (Falkland Islands); Chile Remarks: T. geversianus exhibits a large intraspecific variability, mainly regarding ornamentation (exterior nearly smooth or with ribs, lamellae, spines) and shell shape (more globose or typically muriciform). The morphological variations could have a biostratigraphic value (smooth specimens dominate within the Holocene) and we constrain this taxonomic name for the specimens ornamented with typical growth (collabral) sculpture generally made of ribs of variable form and thickness. Remarks: Trophon necocheanum Ihering (Plate 3, 37) is probably a posterior synonym. Trophon elongatus Strebel (Plate 3, 38, 39) Dimensions: 50 mm Stratigraphic range: Pleistocene–Recent Fossil records in Patagonia: Pleistocene at Estancia San Miguel (Bahı´a Bustamante); Holocene: Cabo Raso; Estancia La Ibe´rica and Zanjo´n El Pinter (Bahı´a Bustamante area) Geographic range: Magellanean Province Genus FUEGOTROPHON Powell, 1951 Fuegotrophon pallidus (Broderip, 1833) (Plate 3, 40, 41) Dimensions: 17–30 mm Stratigraphic range: Pleistocene–Recent

Late Pleistocene Cenozoic Invertebrate Paleontology

35

36

b

a 37

a

303

b

38 42

39

40

43

41

46′ b a

48

46

45

44

b

a

47

47′

51 50 49

50′

51′

52

53 54

Plate 3. Most characteristic molluscs collected in Late Pleistocene (MT III, IV, V) and Holocene (MT VI) coastal deposits studied along the modern Patagonian littoral. Dimensions (in mm) for gastropods: H = height and W = maximum width; a, exterior, apical or apertural view, b, interior or abapertural view, c, lateral view. 35, Trophon varians (H = 35), Holocene, Cabo Raso (PA02Hol6) (DCG-MLP0007-439); 36, Trophon geversianus (H = 49), Cabo Raso, Holoceno (PA02Hol6) (DCG-MLP0007-440); 37a, b, Trophon necocheanus (H = 32), Holocene, Bahı´a Bustamante (reillustrated from Feruglio, 1933a, Pl.IX, fig. 42a, b); 38a, b, Trophon elongatus (H = 32), Holocene, Comodoro Rivadavia (reillustrated from Feruglio, 1933a, Pl.IX, fig. 33a, b); 39, Trophon elongatus (H = 35), Pleistocene, Puerto Lobos (PA04Q5a) (DCG-MLP0007-423); 40, Trophon cf pallidus (H = 29), Pleistocene, Puerto Lobos (PA04Q1) (DCG-MLP0007-416); 41, Fuegotrophon pallidus (H = 34), Holocene, Bahı´a Solano (MLP 26591); 42, Ximenopsis muriciformis (H = ca. 30), Modern, patagonia (reillustrated from Forcelli, 2000); 43, Urosalpinx sp. (H = 24), Holocene, Punta Indio area (MLP1392/2); 44a, b, Urosalpinx p.(H = 17), Pliocene, Cerro Laziar (reillustrated from Feruglio, 1933a, Pl.IX, fig. 12a, b); 45, Acanthina monodon (H = 29.5), Pleistocene, south of Bahı´a Vera (Cabo Raso) (PA02Q10); 46, Acanthina calcar (H = 39), Holocene, Mazarredo (reillustrated from Feruglio, 1933a, Pl.XI, fig. 7b); 460 , Acanthina monodon (H = 22), Holocene, south of Cabo Tres Puntas, (PA04A17) (DCG-MLP0007-464); 47, Adelomelon ferrusacii (H = 95), Holocene, Tierra del Fuego (RR-86-9) (DCG-MLP0007-441); 470 ,Adelomelon ferrusacii (H = 53), Holocene, P. Mazarredo, (PA04A14) (DCG-MLP0007-459); 48, Zidona dufresnei (H = 56), Pleistocene, south of Puerto Lobos (PA04Q6) (DCG-MLP0007-417); 49, Adelomelon ancilla (H = 42), Holocene, Bahı´a Bustamante (MLP 26608); 50, Adelomelon beckii (H = 120), Holocene, Bahı´a Blanca (PI-UNS 2797); 500 , Adelomelon beckii (H = 126), Holocene, south of Cabo Tres Puntas (PA04A14) (DCG-MLP0007-460); 51a, b, Adelomelon brasiliana (H = 86), Holocene, Bahı´a Blanca (PI-UNS 2798); 510 , Adelomelon brasiliana (H = 74), Pleistocene, south Punta Nava (PA04A14) (DCG-MLP0007461); 52, Odontocymbiola magellanica (H = 81 mm), Holocene, Bahı´a Bustamante (PA02Hol3) (DCG-MLP0007-452); 53, Odontocymbiola magellanica (H = 63), Holocene, Puerto Madryn (MLP 26610); 54, Odontocymbiola magellanica (H = 132), Pleistocene, Puerto Lobos (PA04Q3) (DCG-MLP0007-418).

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Fossil records in Patagonia: Pleistocene at Puerto Lobos, Holocene at Bahı´a La´ngara, Puerto Deseado, Bahı´a Laura, San Julia´n Geographic range: Tierra del Fuego, Beagle Channel, Estrecho de Magallanes; Islas Malvinas (Falkland Islands)

Stratigraphic range: Pleistocene–Recent Fossil records in Patagonia: Pleistocene at Camarones; Holocene: San Antonio Oeste, Puerto Madryn, Camarones, Bahı´a Bustamante, Puerto Mazarredo, Tierra del Fuego Geographic range: southern Brazil to Estrecho de Magallanes; Islas Malvinas (Falkland Islands)

Genus XYMENOPSIS Powell, 1951

Adelomelon beckii (Broderip, 1836) (Plate 3, 50, 500 ) Dimensions: 200–400 mm Stratigraphic range: Miocene–Holocene Fossil records in Patagonia: Miocene of Chubut; Holocene at Bahı´a Solano Geographic range: southern Brazil to Tierra del Fuego

Ximenopsis muriciformis (King & Broderip, 1832) (Plate 3, 42) Dimensions: 20–25 mm Stratigraphic range: Holocene–Recent Fossil records in Patagonia: Holocene at Bahı´a La´ngara, Puerto Deseado, Bahı´a Laura, San Julia´n Geographic range: Patagonia, Tierra del Fuego, Isla de los Estados, Islas Malvinas (Falkland Islands), Islas Georgias del Sur (South Georgia) Subfamily OCENEBRINAE Cossmann, 1903 Genus UROSALPINX Stimpson, 1865 Urosalpinx sp. (Plate 3, 43, 44) Dimensions: 10–20 mm Stratigraphic range: Pliocene–Recent Fossil records in Patagonia: Pleistocene of Cerro Laciar (Santa Cruz) Geographic range: Antillean and Brazilian Provinces Family THAIDIDAE Roding, 1798 Genus ACANTHINA Fischer von Waldheim, 1807 Acanthina monodon (Pallas, 1774) (Plate 3, 45, 460 ) Dimensions: 30–65 mm Stratigraphic range: Pleistocene–Recent Fossil records in Patagonia: Pleistocene at Camarones; Holocene: Cabo Raso, Camarones, Bahı´a Bustamante, Bahı´a Solano, Comodoro Rivadavia, Caleta Olivia, Puerto Mazarredo, Puerto Deseado Geographic range: Chilean coast from 40 S to Estrecho de Magallanes; Atlantic Patagonia, Tierra del Fuego; Islas Malvinas (Falkland Islands) Remarks: Acanthina calcar (Plate 3, 46) is here considered as a posterior synonym of Acanthina monodon. Superfamily VOLUTACEA Rafinesque, 1815 Family VOLUTIDAE Rafinesque, 1815 Subfamily ZIDONINAE H. & Adams, 1853 Genus ZIDONA H. & A. Adams, 1853 Zidona dufresnei (Donovan, 1823) (Plate 3, 48) Dimensions: 40–250 mm Stratigraphic range: Pleistocene–Recent Fossil records in Patagonia: Pleistocene: San Antonio Oeste, Penı´nsula Valde´s, Puerto Lobos, Holocene at San Antonio Oeste Geographic range: Rio de Janeiro to Golfo San Matı´as (Argentina) Genus ADELOMELON Dall, 1906 (non Pilsbry & Olsson, 1954) Adelomelon ancilla (Lightfoot, 1786) (Plate 3, 49) Dimensions: 150–161 mm

Adelomelon (Pachycymbiola) brasiliana (Lamarck, 1811) (Plate 3, 51, 510 ) Dimensions: 36–160 mm Stratigraphic range: Pleistocene–Recent Fossil records in Patagonia: Pleistocene and Holocene at San Antonio Oeste Geographic range: Rio de Janeiro to Golfo San Matı´as (Argentina) Adelomelon ferussaci (Donovan, 1824) (Plate 3, 47, 470 ) Dimensions: 54.5–110 mm Stratigraphic range: Holocene–Recent Fossil records in Patagonia: Pleistocene at Puerto Lobos; Holocene: Puerto Lobos, Camarones, Bahı´a Bustamante, Caleta Olivia, Bahı´a La´ngara, Puerto Deseado, Bahı´a Laura, San Julia´n, Tierra del Fuego Geographic range: with doubts in Patagonian deep waters Subfamily ODONTOCYMBIOLINAE Clench y Turner, 1964 Genus ODONTOCYMBIOLA Clench y Turner, 1964 Odontocymbiola magellanica (Gmelin, 1791) (Plate 3, 52–54) Dimensions: 50–300 mm Stratigraphic range: Pleistocene–Recent Fossil records in Patagonia: Pleistocene: Puerto Lobos, Cabo Raso, Camarones; Holocene: San Antonio Oeste, Puerto Lobos, Puerto Pira´mides, Puerto Madryn, Cabo Raso, Camarones, Bahı´a Bustamante, Bahı´a Solano, Comodoro Rivadavia, Rada Tilly, Puerto Mazarredo, Puerto Deseado, San Julia´n, Tierra del Fuego Geographic range: Rı´o de La Plata to Estrecho de Magallanes; Islas Malvinas (Falkland Islands); Chile up to Chiloe´ Family OLIVIDAE Latreille, 1825 Subfamily OLIVELLINAE Troschel, 1869 Genus OLIVELLA Swainson, 1831 Subgenus OLIVINA d0 Orbigny, 1840 Olivella (Olivina) tehuelcha (Duclo´s, 1835) (Plate 4, 55) Dimensions: 5–12 mm Stratigraphic range: Miocene–Recent Fossil records in Patagonia: Pleistocene at Bahı´a Bustamante; Holocene: San Antonio Oeste, Puerto Pira´mides

Late Pleistocene Cenozoic Invertebrate Paleontology

55

a

56

b

57

a 59

305

58

a

b 60

61

b 62

59′

64

65

66

63

a 67

68

70 69

b

71

Plate 4. Most characteristic molluscs collected in Late Pleistocene (MT III, IV, V) and Holocene (MT VI) coastal deposits studied along the modern Patagonian littoral. Dimensions (in mm) for gastropods: H = height and W = maximum width; a, exterior, apical or apertural view, b, interior or abapertural view, c, lateral view. 55, Olivella tehuelcha (H = 12), Holocene, Mar Chiquita (MLP 25955); 56, Olivancillaria urceus (H = 44), Pleistocene, Puerto Lobos (PA04Q1) (DCG-MLP0007-450), 57a, b, Olivancillaria auricularia (H = 25) Pleistocene, Bahı´a Camarones (reillustrated from Feruglio, 1933a, Pl.IX, fig. 21a, b); 58, Olivancillaria carcellesi (H = 34), Pleistocene, Puerto Lobos (PA04Q4) (DCG-MLP0007-419); 590 , Pareuthria plumbea (H = 19), Holocene?, south of Cabo Tres Puntas (PA04A18) (DCG-MLP0007-462); 60a, b, Pareuthria plumbea (H = 36), Pleistocene, Bahı´a La´ngara (Golfo San Jorge) (MLP 28551); 61, Pareuthria plumbea (H = 19.5), Holocene, Camarones (PA02Hol7) (DCG-MLP0007-438); 62a, b, Pareuthria cerealis (H = 21), (reillustrated from Feruglio, 1933a, Pl.IX, fig. 45a, b); 63, Buccinanops cochlidium (H = 65), Holotype (ZMD); 64, Buccinanops globulosus (H = 27), Puerto Lobos (PA04Hol1) (DCG-MLP0007-420); 65, Buccinanops globulosus (H = 24), Pleistocene, Bahı´a Camarones (reillustrated from Feruglio, 1933a, Pl.IX, fig. 30a, b); 66, Buccinanops paytensis (H = 55), Holocene, Bahı´a Bustamante (MLP 26601); 67, Siphonaria lessoni (H = 20), Holocene, Puerto Lobos (PA04Hol1) (DCG-MLP0007-421); 68, Siphonaria lessoni (H = 19), Modern, Puerto Lobos (PA04M1b) (DCG-MLP0007-422); 69a b, Siphonaria lessoni (H = 14), Holocene, Puerto Madryn (reillustrated from Feruglio, 1933a, Pl.IX, fig. 36a, b); 70, Siphonaria lessoni (H = 18), Pleistocene, north of Camarones (PA02Q15) (DCG-MLP0007-433); 71 Siphonaria lessoni (H = 19), Holocene, Bahı´a Camarones (PA02Hol6) (DCG-MLP0007-425).

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Geographic range: Rio de Janeiro to Golfo Nuevo (Argentina) Subfamily AGARONIINAE Olsson, 1956 Genus OLIVANCILLARIA d0 Orbigny, 1842 Subgenus OLIVANCILLARIA s.s. Olivancillaria urceus (Ro¨ding, 1798) (Plate 4, 56) Dimensions: 31–55 mm Stratigraphic range: Miocene–Recent Fossil records in Patagonia: Pleistocene at Puerto Lobos, Holocene at San Antonio Oeste Geographic range: Espı´ritu Santo (Brazil) to Golfo San Matı´as (Argentina) Olivancillaria auricularia (Lamarck, 1810) (Plate 4, 57) Dimensions: 48 mm Stratigraphic range: Pleistocene–Recent Fossil records in Patagonia: Pleistocene at Camarones Geographic range: Rio de Janeiro to Golfo San Matı´as (Argentina) Olivancillaria carcellesi Klappenbach, 1965 (Plate 4, 58) Dimensions: 35–45 mm Stratigraphic range: Pleistocene–Recent Fossil records in Patagonia: Pleistocene at Puerto Lobos, Holocene at San Antonio Oeste, Puerto Lobos, Camarones, Tierra del Fuego Geographic range: Rio de Janeiro to Golfo San Matı´as (Argentina) Superfamily BUCCINACEA Rafinesque, 1815 Family BUCCINIDAE Rafinesque, 1815 Genus PAREUTHRIA Strebel, 1905 Pareuthria plumbea (Philippi, 1844) (Plate 4, 59–61) Dimensions: 19–30 mm Stratigraphic range: Pleistocene–Recent Fossil records in Patagonia: Pleistocene: Cabo Raso, Camarones, Bahı´a Bustamante, south of Caleta Olivia, Tierra del Fuego; Holocene: Puerto Pira´mides, Cabo Raso, Camarones, Bahı´a Bustamante, Bahı´a Solano, Comodoro Rivadavia, Rada Tilly, Caleta Olivia, Bahı´a La´ngara, Puerto Mazarredo, Puerto Deseado, Bahı´a Laura, San Julia´n, Tierra del Fuego Geographic range: Along the Atlantic coast from Bahı´a Camarones to Islas Malvinas (Falkland Islands) and Estrecho de Magallanes Pareuthria cerealis Rochebrune & Mabille, 1891 (Plate 4, 62) Dimensions: 9 mm Stratigraphic range: Pleistocene–Recent Fossil records in Patagonia: Pleistocene at Bahı´a Bustamante area Geographic range: Patagonia Family NASSARIIDAE Iredale, 1916 Subfamily BULLINAE Allmon, 1990 Genus BUCCINANOPS d0 Orbigny, 1841 Buccinanops cochlidium (Dillwyn, 1817) (Plate 4, 63) Dimensions: 6–41 mm Stratigraphic range: Miocene–Recent

Fossil records in Patagonia: Pleistocene: Cabo Raso, Camarones; Holocene: San Antonio Oeste, Puerto Lobos, Cabo Raso, Camarones, Bahı´a Bustamante, Bahı´a Solano, Comodoro Rivadavia, Rada Tilly, San Julia´n Geographic range: Rio de Janeiro to Golfo San Matı´as; probably coasts of Santa Cruz Province; New Zealand Remarks: the material recovered is very similar to the type material (Plate 4, 63). Buccinanops globulosus (Kiener, 1834) (Plate 4, 64, 65) Dimensions: 9–70 mm Stratigraphic range: Pleistocene–Recent Fossil records in Patagonia: Pleistocene: San Antonio Oeste, Puerto Lobos, Cabo Raso, Camarones, Bustamante, Caleta Olivia, south of Caleta Olivia Geographic range: Uruguay down to Cabo Buen Tiempo (Santa Cruz) Buccinanops paytensis (Kiener, 1834) (Plate 4, 66) Dimensions: 26–58 mm Stratigraphic range: Pleistocene–Recent Fossil records in Patagonia: Pleistocene: Bahı´a Bustamante, Punta Medanosa; Holocene: Bahı´a Bustamante Geographic range: south of Santa Cruz Province, Estrecho de Magallanes, Chile, Peru´ Subclass PULMONATA Cuvier, 1817 Order BASOMMATOPHORA A. Schmidt, 1855 Family SIPHONARIIDAE Gray, 1840 Genus SIPHONARIA G.B. Sowerby I, 1824 Subgenus TALISIPHON Iredale, 1940 Siphonaria lessoni (Blainville, 1824) (Plate 4, 67–71) Dimensions: 3–20 mm Stratigraphic range: Pleistocene–Recent Fossil records in Patagonia: Pleistocene at Puerto Lobos; Pleistocene: Puerto Lobos, Cabo Raso, Camarones; Holocene: San Antonio Oeste, Puerto Lobos, Puerto Pira´mides, Puerto Madryn, Cabo Raso, Camarones, Bahı´a Bustamante, Bahı´a Solano, Comodoro Rivadavia, Rada Tilly, Caleta Olivia, Bahı´a La´ngara, Puerto Mazarredo, Bahı´a Laura, San Julia´n, Tierra del Fuego Geographic range: southwestern Atlantic coast from 12 S to Estrecho de Magallanes; Pacific coast from Chile to Central America Class BIVALVIA (Buonanni, 1681) Linne´, 1758 Subclass PTERIOMORPHIA Beurlen, 1944 Order ARCOIDA Stoliczka, 1871 Superfamily LIMOPSACEA Dall, 1895 Family GLYCYMERIDIDAE Newton, 1922 Subfamily GLYCYMERIDINAE Newton, 1922 Genus GLYCYMERIS Da Costa, 1778 [non Glycymeris Lamarck, 1799 (= Panopea Me´nard, 1807)] Subgenus GLYCYMERIS (GLYCYMERIS) Da Costa, 1778 (see Cox et al., 1969) Glycymeris (Glycymeris) longior (Sowerby, 1832) (Plate 5, 1, 2)

Late Pleistocene Cenozoic Invertebrate Paleontology Dimensions: 23–40 mm Stratigraphic range: Miocene–Holocene Fossil records in Patagonia: Pleistocene: San Antonio Oeste, Puerto Lobos, Camarones; Holocene: San Antonio Oeste, Puerto Lobos, Cabo Raso Geographic range: Espiritu Santo (Brazil) to Golfo San Matı´as (Argentina) Order MYTILOIDA Fe´rrussac, 1822 Superfamily MYTILACEA Rafinesque, 1815 Family MYTILIDAE Rafinesque, 1815 Genus MYTILUS Linne´, 1758 Subgenus MYTILUS (MYTILUS) Linne´, 1758 Mytilus (Mytilus) edulis Linne´, 1758 (Plate 5, 3–50 ), Dimensions: 20–85 mm Stratigraphic range: Miocene–Recent Fossil records in Patagonia: Miocene from Puerto Madryn; Pleistocene: Penı´nsula Valde´s, Camarones, Bahı´a Bustamante, Comodoro Rivadavia, south of Caleta Olivia, Puerto Deseado; Holocene: San Antonio Oeste, Puerto Lobos, Cabo Raso, Camarones Geographic range: pandemic Remarks: Mytilus edulis chilensis (Hupe´) (Plate 5, 5) has been recognized as a subspecies typical of the Magellanean Province (Patagonian coast and Chile). Genus BRACHIDONTES Swainson, 1840 Subgenus BRACHIDONTES s.s. Brachidontes (Brachidontes) rodriguezi (d0 Orbigny, 1846) (Plate 5, 6) Dimensions: 1050 mm Stratigraphic range: Miocene–Recent Fossil records in Patagonia: Miocene at Puerto Madryn; Pleistocene: San Antonio Oeste, Puerto Lobos, Camarones, Bahı´a Bustamante; Holocene: San Antonio Oeste, Puerto Lobos, Puerto Madryn, Camarones, Bahı´a Bustamante, Comodoro Rivadavia, Puerto Mazarredo Geographic range: southern Brazil to Golfo San Matı´as; scarce in Camarones–Bahı´a Bustamante area Brachidontes (B.) purpuratus (Lamarck, 1797) (Plate 5, 7–10) Dimensions: 13–30 mm Stratigraphic range: Miocene–Recent Fossil records in Patagonia: Pleistocene: Puerto Lobos, Camarones, Bahı´a Bustamante, Comodoro Rivadavia; Holocene: San Antonio Oeste, Puerto Lobos, Puerto Madryn, Cabo Raso, Camarones, Bahı´a Bustamante, Bahı´a Solano, Comodoro Rivadavia, Rada Tilly, Caleta Olivia, Bahı´a La´ngara, Puerto Mazarredo, Puerto Deseado, Bahı´a Laura, San Julia´n, Tierra del Fuego Geographic range: Pacific coasts from Ecuador to Estrecho de Magallanes; Patagonian coast between Golfo San Matı´as and Tierra del Fuego Remarks: Brachidontes purpuratus has often been placed under the generic name of Perumytilus Olsson, 1961. However, considering the remarkable phenotypic plasticity and that the shell characters proposed by Olsson (1961) to differentiate Perumytilus from Brachidontes are unstable and ambiguous, we maintain this ‘‘taxon’’ under Brachidontes Swainson (Aguirre et al., 2006b).

307

On the contrary, the specimens referred to Brachidontes (B.) purpuratus in the traditional literature are all very similar to B. rodriguezi, B. darwinianus d0 Orb., B. citrinus Ro¨ding and B. exustus Linn. They seem to represent transitional (latitudinal) morphological variants linked to environmental parameters (sea surface temperature, substrate nature, water energy, salinity) and to biotic factors (space available between specimens within the clusters, availability of nutrients). B. purpuratus Lam. could be a senior synonym of B. rodriguezi and the rest (see below). However, its type material is lost (former at the Muse´um Nationale d0 Histoire Naturelle at Parı´s) and proper comparisons were not possible Brachidontes (B.) cf. purpuratus (Lamarck) Dimensions: 10–30 mm Stratigraphic range: Pleistocene–Recent Fossil records in Patagonia: Pleistocene and Holocene of Patagonia (the same localities mentioned for B. purpuratus) Remarks: the specimens referred to Brachidontes (B.) cf. purpuratus are transitional forms (ecomorphs) between B. purpuratus (dominant in coarse sand and gravel bottoms in cold and highly energetic waters of the Magellanean Province) and B. rodriguezi (dominant on less hard bottoms of temperate to warm-temperate waters of the Argentine Province). On the contrary, B. purpuratus seems to represent an extreme within the large variability range of a single polymorphic species. A recent paper dealing with the morphological variability of Brachidontes spp. along the SW Atlantic has shown that there are no clear differences between shell morphs assigned to different ‘‘species’’. At the same time, they are practically identical to the type specimen of B. exustus Linne´ from Jamaica (Antillean Province), which could probably represent a senior synonym (Aguirre et al., 2006b). All these taxa could therefore be considered as synonyms which represent an example of geographical morphological variation. The names for both rodriguezi and purpuratus have here been maintained in order to allow their use as a tool for palaeoenvironmental interpretations of the ancient littoral scenarios. The taxonomic status of the Brachidontes species in the Atlantic must be confirmed by means of multivariate studies of the soft parts and genetic analyses on modern representatives Genus AULACOMYA Mo¨rch, 1853 Aulacomya atra (Molina, 1782) (Plate 5, 11–15) Dimensions: 70–120 mm Stratigraphic range: Pleistocene–Recent Fossil records in Patagonia: Pleistocene: Puerto Lobos, Camarones, Bahı´a Bustamante, Comodoro Rivadavia, south of Caleta Olivia; Holocene: Puerto Lobos, Puerto Madryn, Cabo Raso, Camarones, Bahı´a Bustamante, Bahı´a Solano, Comodoro Rivadavia, Rada Tilly, Caleta Olivia, Puerto Mazarredo, Puerto Deseado, Bahı´a Laura, San Julia´n, Tierra del Fuego

308

Marina L. Aguirre et al.

3 a

1

b

a

a

b

5 a

b

b 6

5′

a

4

2

b

8

9

a

7

b

10 b

a

11

13 12

a

16

14

b

a

b

a

15 b

19

18 17

Plate 5. Most characteristic molluscs collected in Late Pleistocene (MT III, IV, V) and Holocene (MT VI) coastal deposits studied along the modern Patagonian littoral. Dimensions (in mm) for bivalves: L = length, H = height; a, exterior view, b, interior view. When whole specimens ideal for illustration were not available, material from other coastal areas of Argentina were supplied. Some are re-illustrations from Aguirre and Farinati (2000) and Feruglio (1933a). 1a, b. Glycymeris longior (L = 31), Pleistocene, Puerto Lobos (PA04Q1) (DCG-MLP0007-444); 2, Glycymeris longior (L = 32), Pleistocene, Puerto Lobos (PA04Q2) (DCG-MLP0007-451); 3a, b, Mytilus edulis (L = 65), Holocene, North of Camarones (PA02Hol7) (DCG-MLP0007-407); 4a, b, Mytilus edulis (L = 42), Pleistocene, Bahı´a Bustamante (PA02Q15) (DCG-MLP0007-404); 5a, b. Mytilus chilensis (L = 55), Holocene, Bahı´a Solano (reillustrated from Feruglio, 1933a, Pl. XI, fig. 2a, b); 50 a–b. Mytilus edulis (L = 52), Holocene?, south of Cabo Tres Puntas, (PA04A18) (DCG-MLP0007-463); 6, Brachidontes rodriguezi (L = 16.5) Holocene, Puerto Lobos (PA04Hol1) (DCG-MLP0007-445); 7, Brachidontes purpuratus (L = 24), Holocene, Bahı´a La´ngara (Golfo San Jorge) (MLP 26507); 8a, b, Brachidontes purpuratus (L = 25), Holocene, Comodoro Rivadavia (reillustrated from Feruglio, 1933a, Pl. XI, fig. 14a, b); 9a, b, Brachidontes purpuratus (L = 22), Holocene, Comodoro Rivadavia (reillustrated from Feruglio, 1933a, Pl. XI, fig. 15a, b); 10a, b, Brachidontes purpuratus (L = 18), Holocene, Cabo Raso (Punta Pescadero) (PA02Hol6) (DCG-MLP0007-405); 11, Aulacomya atra (L = 70), Holocene, Tierra del Fuego (PI-UNS 2254); 12, Aulacomya atra (L = 72), Holocene, Golfo San Jorge (reillustrated from Feruglio, 1933a, Pl.IX, fig. 13); 13a, b, Aulacomya atra (L = 75), Pleistocene, North of Caleta Olivia, Golfo San Jorge (MLP26508); 14a, b, Aulacomya atra (L = 72), Holocene, Pa. Pescadero (PA02Hol5) (DCG-MLP0007-453); 15, Aulacomya atra (L = 36), Modern, Puerto Lobos (PA04M1b) (DCG-MLP0007-442); 16, Aequipecten tehuelchus (H = 33), Pleistocene, Puerto Lobos (PA04Q2) (DCG-MLP0007-443); 17, Aequipecten tehuelchus (H = 30), Pleistocene, San Antonio Oeste (RR-86-71); 18a, b, Aequipecten tehuelchus (H = 43), Pleistocene, Bahı´a Vera (north of Camarones) (PA02Q10) (DCG-MLP0007-456); 19, Aequipecten tehuelchus (H = 50), Pleistocene, Bahı´a Vera (north of Camarones) (PA02Q10) (DCG-MLP0007-457).

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Geographic range: southern Brazil to Estrecho de Magallanes, Islas Malvinas (Falkalnd Islands); Pacific coasts of Chile and Peru´

Family OSTREIDAE Rafinesque, 1815 Subfamily OSTREINAE Rafinesque, 1815 Genus OSTREA Linne´, 1758 Subgenus OSTREA (OSTREA) Linne´, 1758

Order PTERIOIDA Newell, 1965 Suborder PTERIINA Newell, 1965 Superfamily PECTINACEA Gray, 1847 Subfamily CHLAMYDINAE Korobkov, 1960 Genus AEQUIPECTEN Fischer, 1887

Ostrea (Ostrea) puelchana d0 Orbigny, 1841 (Plate 6, 24) Dimensions: 3–62 mm Stratigraphic range: Miocene–Recent Fossil records in Patagonia: Pleistocene from Puerto Lobos; Holocene at San Antonio Oeste Geographic range: southern Brazil to Golfo San Matı´as (Argentina)

Aequipecten tehuelchus (d0 Orbigny, 1846) (Plate 5, 16–19) Dimensions: 40–77 mm Stratigraphic range: Miocene–Recent Fossil records in Patagonia: Miocene from Puerto Madryn; Pleistocene: Puerto Lobos, Camarones, Comodoro Rivadavia; Holocene: San Antonio Oeste, Puerto Lobos, Puerto Madryn, Cabo Raso, Camarones, Bahı´a Bustamante, Bahı´a Solano, Puerto Mazarredo, San Julia´n Geographic range: Rio de Janeiro (Brasil) to Golfo Nuevo (Argentina); scarce in Camarones–Bahı´a Bustamante area Remarks: Chlamys patagonicus (King & Broderip, 1831), typical from colder Patagonian waters, generally differentiated by a finer radial sculpture and minor auriculae. Genus ZYGOCHLAMYS Ihering, 1907 Zygochlamys patagonicus (King, 1831) (Plate 6, 20, 21) Dimensions: 50–65 mm Stratigraphic range: Pleistocene–Recent Fossil records in Patagonia: Pleistocene at Bahı´a Bustamante; Holocene: San Antonio Oeste, Comodoro Rivadavia, Caleta Olivia, Puerto Mazarredo, San Julia´n Geographic range: Atlantic coasts from Rı´o de La Plata to Tierra del Fuego; Chile up to Puerto Montt; typical of the Magellanean Province Remarks: Chlamys lishkei (Dunker, 1850) (73 mm) (Plate 6, 21) is thought to be a synonym. Family ANOMIDAE Rafinesque, 1815 Genus PODODESMUS Philippi, 1837 Pododesmus rudis(Broderip, 1834) (Plate 6, 27) Dimensions: 73 mm Stratigraphic range: Pleistocene–Recent Fossil records in Patagonia: Miocene from Puerto Madryn; Pleistocene from Puerto Lobos Geographic range: Caribbean coasts and SW Atlantic down to Golfo San Jose´ (Chubut, Argentina) Family PLICATULIDAE Watson, 1930 Genus PLICATULA Lamarck, 1801 Plicatula gibbosa Lamarck, 1801 (Plate 6, 22) Dimensions: 28 mm Stratigraphic range: Miocene–Recent Fossil records in Patagonia: Pleistocene from Puerto Lobos Geographic range: North Carolina, Antilles, Brazil to Golfo San Matı´as (Argentina) Suborder OSTREINA Fe´rrussac, 1822 Superfamily OSTREACEA Rafinesque, 1815

Ostrea tehuelcha Feruglio (Plate 6, 25, 26) Dimensions: 70–80 mm Stratigraphic range: Pliocene–Middle Pleistocene Fossil records in Patagonia: Pleistocene from Bahı´a Bustamante area Geographic range: extinct Ostrea sp. (Plate 6, 23) Dimensions: 30–50 mm Stratigraphic range: Pleistocene–Recent Fossil records in Patagonia: Pleistocene at Puerto Lobos; Holocene from Cabo Raso, Camarones, Bahı´a La´ngara Geographic range: Argentinian and Magellanian malacological Provinces Subclass HETERODONTA Neumayr, 1884 Order VENEROIDA H. Adams & A. Adams, 1856 Superfamily LUCINACEA Fleming, 1828 Family UNGULINIDAE Adams & Adams, 1857 [= DIPLODONTIDAE Dall, 1895] Genus DIPLODONTA Bronn, 1831 Subgenus DIPLODONTA (DIPLODONTA) Bronn, 1831 Diplodonta (Diplodonta) patagonica (d0 Orbigny, 1842) (Plate 6, 28, 29) Dimensions: 6–30 mm Stratigraphic range: Pliocene–Recent Fossil records in Patagonia: Pleistocene from Bahı´a Bustamante area Geographic range: Rio de Janeiro to Golfo Nuevo (Argentina) Superfamily MACTRACEA Lamarck, 1809 Family MACTRIDAE Lamarck, 1809 Subfamily MACTRINAE Lamarck, 1809 Genus MACTRA Linne´, 1767 Subgenus MACTRA (MACTRA) Linne´, 1767 Mactra (Mactra) isabelleana d0 Orbigny, 1846 (Plate 6, 30, 31) Dimensions: 10–52 mm Stratigraphic range: Pleistocene–Recent Fossil records in Patagonia: Pleistocene: Camarones, Bahı´a Bustamante, Comodoro Rivadavia; Holocene: Cabo Raso, Camarones, Bahı´a Bustamante Geographic range: southern Brazil to Golfo San Matı´as Mactra aff. isabelleana d0 Orbigny, 1846 (Plate 6, 32) Dimensions: 30–40 mm

310

Marina L. Aguirre et al.

20

a

21

b

23

a

22

a

b

b

24

25

a

b

26

b a

28

27

31

30

32 29

a

a

b 33

34

b

Plate 6. Most characteristic molluscs collected in Late Pleistocene (MT III, IV, V) and Holocene (MT VI) coastal deposits studied along the modern Patagonian littoral. Dimensions (in mm) for bivalves: L = length, H = height; a, exterior view, b, interior view. When whole specimens ideal for illustration were not available, material from other coastal areas of Argentina were supplied. Some are re-illustrations from Aguirre and Farinati (2000) and Feruglio (1933a). 20, Zygochlamys patagonicus (H = 63), Modern, Patagonia (reillustrated from Forcelli, 2000); 21, Chlamys lishkei (H = 65), Modern, Patagonia (reillustrated from Forcelli, 2000; 22a, b, Plicatula gibbosa (H = 32), Pleistocene, Puerto Lobos (PA04Q1) (DCG-MLP0007-446); 23a, b, Ostrea sp.(H = 49), Pleistocene, Puerto Lobos (PA04Q1) (DCGMLP0007-448); 24a, b, Ostrea puelchana (H = 100), Pleistocene, Bahı´a Camarones (reillustrated from Feruglio, 1933a, Pl.X, fig. 1a, b); 25, Ostrea tehuelcha (H = 85), Pleistocene, Bahı´a Camarones (reillustrated from Feruglio, 1933a, Pl.X, fig. 2b); 26a, b, Ostrea tehuelcha (H = 90), Pleistocene, Northwest of Bahı´a Camarones (PA02Q16bis) (DCG-MLP0007-396); 27a, b, Pododesmis rudis (H = 45), Pleistocene, Puerto Lobos (PA04Q2) ((DCG-MLP0007447); 28, Diplodonta patagonica (L = 14), Holocene, Mar Chiquita (MLP25966); 29,Diplodonta patagonica (L = 23), Holocene, Puerto Lobos (PA04Hol1) (DCG-MLP0007-449); 30, Mactra isabelleana (L = 39), Pleistocene, Northwest of Camarones (PA02Q16) (DCG-MLP0007-454); 31, Mactra cf. isabelleana (L = 30), Pleistocene, Northwest of Camarones (PA02Q16) (DCG-MLP0007-397); 32, Mactra aff. isabelleana (L = 35), Holocene, Tierra del Fuego (RR86-12); 33a, b, Mulinia edulis (L = 96), Pleistocene, North of Caleta Olivia (MLP26510); 34a, b, Darina solenoides (L = 34), Holocene, Bahı´a Solano (Golfo San Jorge)(reillustrated from Feruglio, 1933a, Pl. XI, fig. 8a).

Late Pleistocene Cenozoic Invertebrate Paleontology

311

Stratigraphic range: Miocene–Recent Fossil records in Patagonia: Miocene from Puerto Madryn; Pleistocene: Bahı´a Bustamante, Holocene: Tierra del Fuego Geographic range: southern Brazil to Tierra del Fuego

Geographic range: eastern North America, Texas, Antilles to Bahı´a San Blas (Argentina) Remarks: the specimens recovered are very similar to A. aequalis Say (Plate 7, 38) also identified along the Buenos Aires Province marine Quaternary.

Genus MULINIA Gray, 1837

Superfamily VENERACEA Rafinesque, 1815 Family VENERIDAE Rafinesque, 1815 Subfamily MERETRICINAE Gray, 1847 Genus TIVELA Link, 1807 Subgenus EUTIVELA Dall, 1891

Mulinia edulis (King & Broderip, 1831) (Plate 6, 33) Dimensions: 70 mm Stratigraphic range: Miocene–Recent Fossil records in Patagonia: Pleistocene from Penı´nsula Valde´s; Holocene: Bahı´a La´ngara, Bahı´a Laura, San Julia´n Geographic range: Patagonia and Tierra del Fuego, Chile and Peru´ Subfamily ZENATIINAE Dall, 1895 Genus DARINA Gray, 1853 Darina solenoides (King, 1831) (Plate 6, 34) Dimensions: 40 mm Stratigraphic range: Pleistocene–Recent Fossil records in Patagonia: Pleistocene from Penı´nsula Valde´s, Holocene from Puerto Lobos, Bahı´a La´ngara, Bahı´a Laura, Tierra del Fuego Geographic range: Buenos Aires coast, Patagonia, Tierra del Fuego, Islas Malvinas (Falkland Islands) Superfamily SOLENACEA Lamarck, 1809 Family SOLENIDAE Lamarck, 1809 Genus SOLEN Linne´, 1758 Solen tehuelchus d0 Orbigny, 1843 (Plate 6, 35, 350 ) Dimensions: 70–80 mm Stratigraphic range: Pleistocene–Recent Fossil records in Patagonia: Holocene from San Antonio Oeste; Bahı´a Bustamante, Bahı´a Solano Geographic range: Rio de Janeiro to Golfo San Matı´as (Argentina) Superfamily TELLINACEA De Blainville, 1814 Family TELLINIDAE De Blainville, 1814 Subfamily MACOMINAE Olsson, 1961 Genus MACOMA Leach, 1819 Subgenus MACOMA (PSAMMACOMA) Dall, 1900 Macoma (Psammacoma) cf. uruguayensis (Smith, 1885) (Plate 7, 36, 360 ) Dimensions: 12–42 mm Stratigraphic range: Pleistocene–Recent Fossil records in Patagonia: Holocene from Cabo Tres Puntas (north of Puerto Deseado) Geographic range: southern Brazil to Golfo San Matı´as (Argentina) Genus ABRA Lamarck, 1818 Abra aequalis (Say, 1822) (Plate 7, 37) Dimensions: 3–22 mm Stratigraphic range: Miocene–Recent Fossil records in Patagonia: Miocene from Puerto Madryn, Pleistocene: Bahı´a Bustamante, south of Comodoro Rivadavia, Caleta Olivia; Holocene from Cabo Tres Puntas (north of Puerto Deseado)

Tivela (Eutivela) isabelleana (d0 Orbigny, 1846) (Plate 7, 38) Dimensions: 50 mm Stratigraphic range: MioceneRecent Fossil records in Patagonia: Holocene from San Antonio Oeste Geographic range: Espiritu Santo (Brazil) to Golfo San Matı´as (Argentina) Subfamily PITARINAE Stewart, 1930 Genus PITAR Ro¨mer, 1857 Subgenus PITAR (PITAR) Ro¨mer, 1857 Pitar (Pitar) rostratus (Koch in Philippi, 1844) (Plate 7, 39, 40) Dimensions: 55 mm Stratigraphic range: EoceneRecent Fossil records in Patagonia: Pleistocene: Camarones, Bahı´a Bustamante, Comodoro Rivadavia; Holocene: San Antonio Oeste, Puerto Lobos, Camarones, Bahı´a Bustamante, Puerto Mazarredo, San Julia´n Geographic range: Brazil to Golfo San Matı´as (Argentina) Remarks: Aminatis purpuratus (Plate 7, 41) is a close species very abundant northward along the Buenos Aires coast and in Uruguay and Brazil. Subfamily TAPETINAE Adams y Adams, 1857 Genus EURHOMALEA Cossmann, 1920 Eurhomalea exalbida (Dillwyn, 1817) (Plate 7, 42) Dimensions: 60–90 mm Stratigraphic range: Pleistocene–Holocene Fossil records in Patagonia: Pleistocene at San Antonio Oeste, Puerto Lobos; Holocene Puerto Lobos Geographic range: Chile from Chiloe´ to Magallanes; Estrecho de Magallanes, Patagonia, Buenos Aires Province, Uruguay to Rı´o Grande do Sul (Brazil) Subfamily CHIONINAE Frizzell, 1936 Genus PROTOTHACA Dall, 1902 Protothaca antiqua (King, 1832) (Plate 7, 43, 45) Dimensions: 70–80 mm Stratigraphic range: Miocene–Recent Fossil records in Patagonia: Pleistocene: San Antonio Oeste, Peninsula Valde´s, Cabo Raso, Camarones, Bahı´a Bustamante, south of Caleta Olivia; Holocene: Puerto Lobos, Puerto Madryn, Cabo Raso, Camarones, Bahı´a Bustamante, Bahı´a Solano, Comodoro Rivadavia, Rada Tilly, Puerto Mazarredo, Puerto Deseado, Bahı´a Laura, San Julia´n, Tierra del Fuego

312

Marina L. Aguirre et al.

36′

35 36 35′

a

38

a

37 b

a

b

39

40

b

a

b

42

41

45

44 43

a 46

a

47

a 48

a

b 50 b

b

a 49

b

b a

a

51

b

a

52

b

53

b

Plate 7. Most characteristic molluscs collected in Late Pleistocene (MT III, IV, V) and Holocene (MT VI) coastal deposits studied along the modern Patagonian littoral. Dimensions (in mm) for bivalves: L = length, H = height; a, exterior view, b, interior view. When whole specimens ideal for illustration were not available, material from other coastal areas of Argentina were supplied. Some are reillustrations from Aguirre and Farinati (2000) and Feruglio (1933a). 35, Solen tehuelchus (L = 49), Pleistocene, Bahı´a La´ngara (PA04A7) (DCG-MLP0007465); 36, Macoma uruguayensis (L = 52), (PA04A18) (DCG-MLP0007-463); 37a, b, Abra aequalis (L = 10), Modern USA, Lectotype (ANSP53227); 38a, b, Tivella isabelleana (L = 27), Holocene, Bahı´a Blanca (PI-UNS 1363); 39a, b, Pitar rostratus (L = 40), Pleistocene, south of Las Brusquitas (RR-86-52B); 40, Pitar rostratus (L = 49), North of Camarones, Pleistocene (PA02Q14) (DCG-MLP0007406); 41, Amiantis purpuratus (L = 30), Holocene, Bahı´a Blanca area (PI-UNS2814); 42a, b, Eurhomalaea exalbida (L = 77 H = 72), Modern, Bahı´a Bustamante (PA2M5) (DCG-MLP0007-408); 43, Protothaca antiqua (L = 75), PA02Hol5 (DCG MLP 0007-402; 44, Protothaca antiqua (L = 74), Holocene, Bahı´a Bustamante (PA02Hol5) (DCG-MLP0007-402); 45, Protothaca antiqua (L = 72), Holocene, Bahı´a Bustamante (PA02Hol5) (DCG-MLP0007-456); 46a, b, Clausinella gayi (L = 30), Pleistocene S of Bahı´a Vera, (North of Camarones) (PA02Q11) (DCG-MLP0007-399); 47a, b, Clausinella gayi (L = 22), Pleistocene S of Bahı´a Vera, (North of Camarones) (PA02Q11) (DCG-MLP0007-455); 48a, b, Venericardia procera (L = 14), Pleistocene, Puerto Deseado (reillustrated from Feruglio, 1933a, Pl. IX, Fig. 6a, b); 49a, b, Corbula patagonica (L = 11), Pleistocene, Bahı´a Camarones (reillustrated from Feruglio, 1933a, Pl. IX, fig. 18a, b); 50a, b, Hiatella arctica (L = 30), Holocene, Bahı´a Blanca (PI-UNS 2818); 51a, b, Hiatella arctica (L = 30), Pleistocene, Puerto Deseado (reillustrated from Feruglio, 1933a, Pl. IX, fig. 9a, b); 52a, b, Panopea abbreviata (L = 70), Holocene, San Antonio Oeste (PIUNS 2819); 53a, b, Petricola patagonica (L = 45), Modern, Patagonia (reillustrated from Forcelli, 2000).

Late Pleistocene Cenozoic Invertebrate Paleontology Geographic range: Peru´ and Chile to Magallanes; Atlantic Tierra del Fuego to southern Brazil Genus CLAUSINELLA Gray, 1851 Clausinella gayi (Hupe´, 1854) (Plate 7, 46, 47) Dimensions: 35 mm Stratigraphic range: Pleistocene–Recent Fossil records in Patagonia: Pleistocene San Antonio Oeste, Cabo Raso, Comodoro Rivadavia; Holocene: Puerto Lobos, Puerto Madryn, Cabo Raso, Bahı´a Bustamante, Bahı´a Solano, Comodoro Rivadavia, Rada Tilly, Bahı´a La´ngara, Puerto Mazarredo, Puerto Deseado, Bahı´a Laura, San Julia´n, Tierra del Fuego Geographic range: Rio Grande do Sul to Patagonia, Estrecho de Magallanes; Chile up north to Valparaı´so Genus VENERICARDIA Lamarck, 1801 Venericardia procera Gould (Plate 7, 48) Dimensions: 10–16 mm Stratigraphic range: Pleistocene Fossil records in Patagonia: Pleistocene: Bahı´a Bustamante, Puerto Deseado Geographic range: Caribbean and Brazilian malacological Provinces Family PETRICOLIDAE Deshayes, 1839 Genus PETRICOLA Lamarck, 1801 Subgenus PETRICOLA (PETRICOLA) Lamarck, 1801 Petricola patagonica d0 Orbigny (Plate 7, 53) Dimensions: 45 mm Stratigraphic range: Holocene–Recent Fossil records in Patagonia: Holocene from Puerto Madryn, Bahı´a Solano, Comodoro Rivadavia, Rada Tilly, Puerto Mazarredo Geographic range: Patagonia Order MYOIDA Stoliczka, 1870 Suborder MYINA Stoliczka, 1870 Superfamily MYACEA Lamarck, 1809 Family CORBULIDAE Lamarck, 1818 Subfamily CORBULINAE Gray, 1823 Genus CORBULA Bruguie`re, 1797 Subgenus CORBULA (CORBULA) Bruguie`re, 1797 Corbula (Corbula) patagonica d0 Orbigny, 1846 (Plate 7, 49) Dimensions: 5–14 mm Stratigraphic range: Miocene–Holocene Fossil records in Patagonia: Pleistocene from Bahı´a Bustamante; Holocene: Camarones, Bahı´a Bustamante, Comodoro Rivadavia, Caleta Olivia Geographic range: southern Brazil to Golfo San Matı´as (Argentina); western Africa; southeastern Australia Superfamily HIATELLACEA Gray, 1824 Family HIATELLIDAE Gray, 1824 Genus HIATELLA Bosc (ex Daudin, 1801) Hiatella arctica (Linne´, 1767) (Plate 7, 50, 51) Dimensions: 15–40 mm Stratigraphic range: Pleistocene–Recent

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Fossil records in Patagonia: Pleistocene from Bahı´a Bustamante, south of Caleta Olivia; Holocene: Golfo San Jorge (Bahı´a Solano, Rada Tilly, Bahı´a La´ngara), Tierra del Fuego Geographic range: pandemic; typical of cold water masses Genus PANOPEA Me´nard, 1807 Panopea abbreviata Valenciennes, 1839 (Plate 7, 52) Dimensions: 90 mm Stratigraphic range: Miocene–Recent Fossil records in Patagonia: Holocene from Tierra del Fuego Geographic range: Rio de Janeiro (Brazil) to Patagonia 6. Other Invertebrate Groups Recovered or Mentioned by Previous Authors Phylum CNIDARIA Hatscheck, 1888 Class ANTHOZOA Ehrenberg, 1834 Order SCLERACTINIA Bourne, 1900 Family RHIZANGIDAE d0 Orbigny, 1851 Genus ASTRANGIA Milne-Edwards & Haime, 1848 Astrangia rathbuni Vaugham, 1906 (Plate 8, 54) Astrangia patagonica Squires, 1963 (Plate 8, 55) Genus STYLATULA Verrill, 1864 Stylatula darwini Verrill, 1864 (Plate 8, 56) Remarks: scleractinids have only been mentioned in the old traditional literature and need a detailed review. Phylum BRYOZOA Ehrenberg, 1831 Class GYMNOLAEMATA Allmann, 1856 Order CHEILOSTOMATIDA Busk, 1852 Family MEMBRANIPORIIDAE Busk, 1854 Genus MEMBRANIPORA Blainville, 1830 Membranipora puelcha (d0 Orbigny) (Plate 8, 57) Membranipora tenuı´sima Canu´ (Plate 8, 58) Membranipora flabellate Canu´ Membranipora ameghinoi Canu´ Lunulites cuvieri Defranca (Plate 8, 59) Cellepora tenella Canu´, 1909 (Plate 8, 60) Conopeum reticulum (Linne´) Electra monostachys (Busk) Discoporella umbellata (Conrad) Cellaria ornata d0 Orbigny Remarks: as in the previous group, bryozoans have only been mentioned in the old traditional literature and need a detailed review. Phylum BRACHIOPODA Dumerill, 1806 Class RHYNCONELLATA Williams et al., 1997 Order TEREBRATULIDA Waagen, 1883 Family TEREBRATELLIDAE King, 1850 Terebratellidae indet. (Plate 8, 61) Genus TEREBRATELLA d0 Orbigny, 1847 Terebratella gigante (Plate 8, 62) Terebratella dorsata Gmelin, 1791 (Plate 8, 63)

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Remarks: these taxa are only mentioned in the old literature and are under current revision. Phylum MOLLUSCA Linne´, 1758 Class POLYPLACOPHORA de Blainville, 1816 Family ISCHNOCHITONIDAE Dall, 1889 Genus CHAETOPLEURA Schuttleworth, 1853 Chaetopleura isabellei (d0 Orb., 1841) (Plate 8, 64, 65) Dimensions: 30 mm Stratigraphic range: Pleistocene–Recent Fossil records in Patagonia: Pleistocene at Bahı´a Bustamante, Puerto Mazarredo Geographic range: Tierra del Fuego, Orcadas – Southern Orkney Islands (Antarctica) Habitat and life mode: Littoral, hard substrate Chaetopleura angulata (Spengler, 1797) (Plate 8, 66) Dimensions: 50 mm Stratigraphic range: Pleistocene–Recent Fossil records in Patagonia: Pleistocene at Bahı´a Bustamante, Puerto Mazarredo Geographic range: Patagonia Habitat and life mode: Littoral, hard substrate Class SCAPHOPODA H.G. Bronn, 1862 Order GADILIDA Starobogatov, 1982 Suborder GADILIMORPHA Steiner, 1992 Family GADILIDAE Stoliczka, 1868 Subfamily GADILINAE Stoliczka, 1868 Genus CADULUS Philippi, 1844 Subgenus CADULUS (POLYSCHIDES) Pilsbry & Sharp, 1897 Cadulus (Polyschide) tetraschistus (Watson, 1879) (Plate 8, 67) Dimensions: 10 mm Stratigraphic range: Pleistocene–Recent Fossil records in Patagonia: Pleistocene at Bahı´a Bustamante, Puerto Mazarredo Geographic range: SW Atlantic from North Carolina to Golfo San Matı´as (Argentina) Habitat and life mode: commonly offshore in sandy bottoms; can occasionally be found onshore after severe storms Phylum ANNELLIDA Lamarck, 1802 Class POLYCHAETA Grube, 1859 Order SABELLIDA Levinsen, 1883? Family SERPULIDAE Johnston, 1865 Genus SERPULA Linnaeus, 1758 Serpula vermicularis Linnaeus, 1767 (Plate 8, 68) Dimensions: 50–70 mm (length of living tube dwelling) Stratigraphic range: Pleistocene–Recent Fossil records in Patagonia: Pliocene and Pleistocene of Santa Cruz Province Geographic range: pandemic Serpula patagonica Ortmann, 1900 (Plate 8, 68) Dimensions: 3 mm (diameter of the tube) Stratigraphic range: Miocene–Recent Fossil records in Patagonia: Miocene at San Julia´n; Pleistocene at Puerto Deseado Geographic range: Patagonia

Phylum ECHINODERMATA Bruguiere, 1791 [ex Klein, 1734] Subphylum ECHINOZOA Haeckel, 1985 Class ECHINOIDEA Leske, 1778 Subclass EUECHINOIDEA Bronn, 1860 Superorder ECHINACEA Claus, 1876 Order TEMNOPLEUROIDA Mortensen, 1942 Family TEMNOPLEURIDAE A.Agassiz, 1872 Genus PSEUDOCHINUS Mortensen, 1903 Pseudochinus magellanicus (Phillipi, 1857) (Plate 8, 73) Dimensions: 65 mm Stratigraphic range: Holocene–Recent Dimensions: 43 mm (diameter of the test) Stratigraphic range: Holocene–Recent Fossil records in Patagonia: Bahı´a Bustamente, Comodoro Rivadavia Geographic range: Southern Chile, Argentina, Antarctica, Islas Malvinas (Falkland Islands) Habitat and life mode: Littoral Order ECHINOIDA Claus, 1876 Family ECHINIDAE A.Agassiz, 1872 Genus LOXECHINUS Desor, 1856 Loxechinus albus Molina, 1782 (Plate 8, 72) Dimensions: 65 mm (diameter of the test) Stratigraphic range: Holocene–Recent Fossil records in Patagonia: Pleistocene at Bahı´a Bustamante; Holocene south of Comodoro Rivadavia Geographic range: Peru´, Chile, Tierra del Fuego, Isla de los Estados Habitat and life mode: Littoral Phylum ARTHROPODA Subphylum CRUSTACEA Bru¨nnich, 1771 Clase MAXILLOPODA Dahl, 1956 Subclass THECOSTRACA Gruvel, 1905 Infraclass CIRRIPEDIA Burmeister, 1834 Order SESSILIA Lamarck, 1818 Suborder BALANOMORPHA Pilsbry, 1976 Superfamily BALANOIDEA Leach, 1917 Family ARCHAEOBALANIDAE Newman & Ross, 1976 Genus AUSTROMEGABALANUS Newman, 1979 Austromegabalanus psittacus (Molina, 1782) (Plate 8, 71) Dimensions: 48 mm (height) Stratigraphic range: Miocene–Recent Fossil records in Patagonia: Miocene at San Julia´n; Pliocene at Camarones; Pleistocene: Camarones, Bahı´a Bustamante, Puerto Mazarredo; Holocene: Bahı´a Solano; Caleta Co´rdova–Comodoro Rivadavia, Puerto Coig (Santa Cruz) Geographic range: Peru´, Chile, Patagonia Habitat and life mode: Intertidal, hard substrate Family BALANIDAE Leach, 1817 Genus BALANUS Da Costa, 1778 Balanus laevis Bruguiere, 1789 (Plate 8, 57, 70) Dimensions: 14 mm (height) Stratigraphic range: Miocene–Recent

Late Pleistocene Cenozoic Invertebrate Paleontology

54

56

55

57

58 2

68

59

b

61

315

7

a

60

63

62

69 a

64

b

c

65

66

67 70

72 a

71

73 b

a

b

Plate 8. Miscellanea (minor molluscs and other invertebrate groups). Dimensions refer to maximum with (diameter) or height. PI-UNS refer to the collection of E. Farinati (Universidad Nacional del Sur, Bahı´a Blanca, Argentina). 54, Astrangia rathbuni (each corallite 3–4 mm in cross section), Holocene, Bahı´a Blanca (PI-UNSi); 55, Astrangia patagonica (each corallite 3–4 mm in cross section), Holocene, Bahı´a Blanca (PI-UNS); 56, Stylatula darwini (fragments), Holocene, Bahı´a Blanca (PI-UNS); 57, Membranipora puelcha, Holocene, Bahı´a Blanca (PI-UNS); 58, Membranipora tenuissima, Holocene, Bahı´a Blanca (PI-UNS); 59, Lunulites cuvieri Holocene, Bahı´a Blanca (PI-UNS); 60, Cellepora tenella, Holocene, Bahı´a Blanca (PI-UNS); 61, Terebratellidae indet (H = 40 mm), Pleistocene, Golfo San Jorge (reillustrated from Aguirre, 2003); 62, Terebratella gigante, high 44 mm, Pliocene, Cape Fairweather beds, Santa Cruz (from Ortmann, 1902); 63, Terebratella dorsata, high 41 mm, Miocene Lake Pueyrredo´n, Patagonian beds, (from Ortmann, 1902); 64–65, Chaetopleura isabellei (a, b, c are anterior, intermediate and terminal plates, respectively), 66, Chaetopleura angulata, 50 mm , Modern, Patagonia (from Forcelli, 2000);Chaetopleura angulata, 50 mm, Modern, Patagonia (from Forcelli, 2000); 67, Polyschides tetraschistus (10 mm), Holocene, Bahı´a Blanca (PI-UNS); 68, Serpula vermicularis Holocene, Bahı´a Blanca (PI-UNS); 69, Serpula patagonica Miocene, Patagonian beds, San Julian, Santa Cruz (diameter of tubes 4 mm); Oven Point, Santa Cruz (reillustrated from Ortmann, 1902); 70, Balanus laevis (high 10 mm) Pliocene, Cape Fairweather beds (reillustrated from Ortmann, 1902); 71, Megabalanus psittacus (hight 47 mm), Miocene, Patagonian beds, Lago Pueyrredon (Santa Cruz) (from Ortmann, 1902); 30 mm, Modern, Patagonia (from Forcelli, 2000); 72a, b, Loxoechinus albus (maximum diameter = 70 mm), Modern, Ushuaia (Tierra del Fuego) (private collection); 73a, b, Pseudechinus magellanicus (maximum diameter = 65 mm), Modern, Ushuaia (Tierra del Fuego) (private collection, Julio C. Hlebszevitsch Savalscky).

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Fossil records in Patagonia: Miocene: Lago Pueyrredo´n (Santa Cruz); Pliocene at Camarones, Puerto Deseado, San Julia´n; Pleistocene: Camarones, Bahı´a Bustamante, Peninsula Aristizabal, Caleta Olivia, Puerto Mazarredo; Holocene: Camarones, Comodoro Rivadavia, Santa Cruz Geographic range: Atlantic Ocean: from Tierra del Fuego to Brazil; Pacific Ocean: from Tierra del Fuego to California Habitat and life mode: Intertidal, hard substrate.

7. Paleoenvironments and Paleoclimate Nearly all the macroinvertebrate taxa (scleractinids, bryozoans, brachiopods, polyplacophores, scaphopods, serpulid polychaetes, echinoderms, cirripeds and, mostly, gastropods and bivalves) recorded in the Patagonian marine terraces III–VI have modern representatives, allowing some kind of paleoecological interpretation. However, the potential preservation of some groups (polyplacophores, scaphopods, serpulid polychaetes, echinoderms) is very low, these are therefore not useful for stratigraphic correlations or reliable paleoenvironmental interpretations. Brachiopods need a detailed systematic revision, while cnidarians, bryozoans and echinoderms have not as yet been thoroughly studied. On the contrary, gastropods and bivalves show the best preservation potential, generally exhibiting almost complete taphonomic signatures. They are the most abundant biogenic elements within the studied assemblages and they are in general better known and more useful in terms of paleoenvironmental and paleoclimate proxies. However, it must be acknowledged that the reliability of mollusc shells as paleoenvironmental signals depends mostly on the final depositional environment where they have been accumulated. The degree of postmortem transport influences the occurrence, taphonomic signatures, diversity, quantity and spatial distribution of the shells within skeletal concentrations sampled at different localities. In turn, the amount of taphonomic loss suffered by these shells (i.e. fragmentation, abrasion, lack of original color and luster, dissolution, surface borings) indicates the degree of reworking of the material, especially during periods of exposure along an extensive beach zone (Aguirre and Farinati, 1999; Farinati et al., 2002). All these factors were taken into account, considering that they have implicancies for a precise interpretation of paleoenvironmental and paleoclimate conditions of the littoral area. Special attention was paid to the dominant taxa with good preservation for each main region of the Patagonian coastal area (i.e. Puerto Lobos, Cabo RasoCamarones, Bahı´a Bustamante and Golfo San Jorge). On the contrary, the main utility of Quaternary gastropods and bivalves as proxy of paleoclimate conditions is revealed, mostly, by changes or shifts of their geographic ranges in comparison with the present (Ortlieb et al., 1990; Clarke, 1993; Fields et al., 1993; Roy et al., 1995; Gaylord and Gaines, 2000; Hellberg et al., 2001; Cintra-Buenrostro et al., 2002; Warwick and Turk, 2002; Kitamura and Ubukata., 2003; Zachert et al., 2003). For the studied Patagonian coastal area they allow general

interpretations, based on the assemblages formed during the last high sea level stands after MIS 11 ([?], MT III). The results of this review and update on 51 gastropods and 36 bivalves (Table 3a, b), focused on material from localities with the best available chronological control (Figs 3, 4; Table 2), show that among all the molluscan skeletons, shells of infaunal bivalve taxa (i.e. Protothaca, Clusinella, Mulinia, Eurhomalea) are generally better preserved (as whole complete or odd valves) and can be considered more reliable in terms of dating material and of paleoecological indicators (substrate type, depth, temperature requirements). More exposed to unstable littoral conditions and taphonomic processes, epifaunal taxa are commonly represented by higher numbers of incomplete bivalve shells (i.e. Mytilus, Brachidontes, Aulacomya) or broken gastropod shells (Crepidula, Fissurella). As expected, in most localities shells from Holocene sediments are better preserved than from Pleistocene terraces, while shells from the modern samples exhibit minimum or no taphonomic loss. According to the ecological data synthesized in Tables 4a and 4b, the original habitat of the molluscs recovered was characterized by varied hard substrates, high-energy, shallow euhaline waters, similar to the modern littoral that today is strongly influenced by the cool Malvinas/Falkland current. Comparisons with modern associations reveal minor differences: (1) qualitative (taxonomic differences); (2) quantitative (different percentages of the taxa in common); (3) size variation of individual taxa (Protothaca, Tegula, Trophon, generally bigger in the Quaternary); (4) shape variation of individual taxa (Crepidula, Tegula, Buccinanops, Glycymeris, Brachidontes, Pitar; variability linked to local environmental conditions); and (5) geographical shifts (latitudinal changes) (Tegula, Trophon, Brachidontes, among others). Although the Patagonian molluscan assemblages show differences in composition, distribution, taxonomic diversity and size or shape variability between MT of different areas and in comparison with the modern littoral (see for example Fig. 6), no dramatic taxonomic appearances or disappearances seem to have occurred since the Mid-Late Pleistocene. This suggests that the Malvinas/ Falkland Current may have been active in the Magellanean Province at least since then. Similarities (34%) with the taxonomic composition of the Late Cenozoic molluscan assemblages (‘‘Entrerriense’’) suggest that this current could have been active since the Miocene–Pliocene. Paleobiodiversity seems not to have been uniform through time. As expected, in general terms the highest diversity corresponds to the modern samples, which represent complete populations and where taphonomic loss is disregarded, followed by diversity of the Holocene MT VI, higher than for the Late Pleistocene samples. Several species can be recognized as exclusive or most common for the marine Quaternary of Patagonia: Mytilus edulis chilensis, Brachidontes cf. purpuratus, Aulacomya atra, Zygochlamys patagonicus, Mulinia edulis, Eurhomalea exalbida, P. antiqua, Clausinella gayi, Hiatella arctica, Petricola patagonica, Fissurella picta, Fissurella oriens, Fissurella radiosa, Nacella magellanica, Nacella deaurata, Nacella delicatissima, Tegula

Late Pleistocene Cenozoic Invertebrate Paleontology blakei, T. atra, Photinula coerulescens, Trochita pileus, Epitonium magellanicum, Trophon geversianus, Fuegotrophon pallidus, Acanthina monodon, Odontocymbiola magellanica, Adelomelon ferussaci, Pareuthria plumbea, Buccinanops paytensis. A small number of the taxa (i.e. T. atra, Littoridina australis, Urosalpinx, Adelomelon spp., Olivancillaria urceus; Pododesmus rudis, Plicatula gibbosa, Ostrea tehuelcha, Venericardia procera) are presently absent or uncommon in the adjacent littoral and have shifted northward. In addition, specimens of Chama spp. (Bivalvia) (not illustrated here) were collected by Pastorino (1991) in Pleistocene units of Rı´o Negro Province. The biogeographical shifts of these taxa suggest responses to climate changes (SST). Regarding a comparison with other areas, the most relevant taxonomic differences with the northern coastal area of the Buenos Aires Province are the absence or scarcity of L. australis, Calliostoma spp., Urosalpinx spp., Adelomelon brasiliana, Zidona dufresnei, Olivancillaria, Olivella spp., Noetia bisulcata, Glycymeris longior Sow., Amiantis purpuratus, Raeta plicatella, Tagelus plebeius, Corbula lyoni (Aguirre, 1993a; Fucks et al., 2005). Along southern Patagonia the molluscan assemblages contain taxa in common with the Pacific Quaternary terraces of Chile (Guzma´n et al., 1995; Ortlieb et al., 1996c) (i.e. T. atra, Fissurella spp., Siphonaria lessoni, M. edulis, B. purpuratus, A. atra, M. edulis chilensis). Most likely, this similarity is due to a composed origin or biogeographic history of the cold Magellanean fauna, including some taxa of a Pacific origin. Dispersion and mixing of a more recent Pacific group with the dominant Atlantic group by the cool current system through the Chilean coast (Humboldt Current and West Wind Drift) could be an explanation. Similar interpretations were postulated earlier by Lo´pez Gappa (2000) and Martı´nez and Del Rı´o (2002) based on different invertebrate taxa. Regarding their distribution in modern seas, all the species recovered have modern representatives in subtropical, warm-temperate, cool-temperate or exclusively in cold water masses, mainly along malacological Provinces of the southwestern Atlantic Ocean (Brazilian, Argentine or Magellanean). All the taxa recorded can be gathered in four groups (I–IV) according to their occurrence in certain water masses (Table 3). The occurrence and abundance of these groups in the deposits sampled provide a measure of SST conditions and climate variability. Based on the global isotope curve (Haq et al., 1987) (Fig. 6b), it is widely accepted that a higher SST than present characterized both MIS 1 (Mid-Holocene transgressive maximum, ca. 5–8 14C ka BP) and, especially, MIS 5e (Last Interglacial maximum; ca. 125 ka) as documented in several regions worldwide (Burckle, 1993; Zazo, 1999b). Levels of extinctions or migrations of warm and cold water taxa, changes in diversity patterns and quantitative taxonomic variations through time are among the most common responses to the effect of climate change. However, our lack of precision about the number and age of MI substages preserved within the Late Pleistocene Patagonian terraces prevents from establishing

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paleotemperature changes per unit of time, a very difficult task until further research, dating and stable isotope analyses are carried out. At this stage it is only possible to put special emphasis on the Mid-Holocene (MIS 1) assemblages, which offer more precise dating (uncorrected 14C ages). Our data show that at present there are a greater proportion of species group IV and of other taxa typical or exclusive to cold water masses that are absent in the Quaternary MT. No marked abundance of species group II (Antillean) or IV (Magellanean) is apparent during the Quaternary in Patagonia. Species group III (Argentinian Province) is predominant in all Pleistocene and MidHolocene samples. But during the Mid-Holocene (MT VI), species groups II and III were more abundant than in the Late Pleistocene and at present, their percentages showing a latitudinal (southward) decreasing trend. Conversely, typical cold water elements common at present along the Magellanean Province are scarcer or absent (Aguirre, 2001) in Mid-Holocene samples. Slightly warmer waters during the Mid-Holocene ‘‘Climatic Optimum’’ (Hypsithermal) recognized worldwide probably caused this biogeographical response and are likely to have influenced the littoral biota downward at least to ca. 43–44 S (Aguirre, 2002). The Climatic Optimum (ca. 9/8–6 14C ka BP interval) is expected to have been synchronous with atmospheric and oceanographical changes, such as a southward shift of the warm Brazilian Current and the intensification and southward displacement of the South Atlantic Anticyclonic Center. This effect on the littoral molluscan fauna from Patagonia, however, is less evident than northward along the Buenos Aires Province coastal area, where apart from quantitative differences with the modern molluscan associations, geographical shifts were linked to SST differences of ca. 1–2C (Aguirre, 1993b, 2002). It has been assumed that the MIS 5e highstand was the warmest since the Mid-Pleistocene (global isotope curve; Haq et al., 1987; Zazo 1999a, b; Winnograd et al., 1997). However, MIS 11 (ca. 423–362 ka; Zazo, 1999b) in South America was hypothesized to be the longest and warmest of the Pleistocene interglacial highstands preserved (Ortlieb et al., 1996a, b; Chilean coast, instead of; Burckle, 1993, Southern Ocean, longer and warmer than the following interglacials). One problem to address is the eventual preservation and climatic scenario of this stage along the southwestern Atlantic margin. In addition, the traditional interpretation of the Patagonian MT V representing MIS 5e (warmer SST) is not clear and needs further investigations. Only at Puerto Lobos (Figs 3, 4) the Pleistocene molluscan assemblage supports a typical MIS 5e paleoclimate scenario (higher SST than present, revealed by warm water taxa absent in this area at present (Chama, Pododesmus, Plicatula)). Of the remaining Pleistocene MT (IV and III) only some levels sampled at Camarones (MT III [?], MIS 11 [?]) suggest warmer SST than at present (Fig. 6a). On the contrary, the Holocene Climatic Optimum was probably responsible for the greater percentages of species groups II and III within MT VI in comparison with the present, when a majority of cold water elements (group IV) occur, including species that are absent or uncommon within the terraces.

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It must be acknowledged that high-resolution Quaternary records of molluscs, with stable isotope analyses performed on well-dated shells by modern techniques, have not yet been performed for the Patagonian marine terraces, another task for future studies in the area. Along the Beagle Channel (Tierra del Fuego), Panarello (1987) and Obelic et al. (1998) have carried out isotopic studies on molluscs. At Golfo San Jorge stable isotope analysis of some modern shells of marine gastropods and bivalves has been initiated (Aguirre et al., 2002). Good results from analyses of the isotopic composition (C, O and/or Sr) of modern and Holocene molluscs were recently obtained for the northeastern Buenos Aires Province (Aguirre et al., 1998, 2002). Another unsolved topic is that the paleoclimate marine evidence still needs to be compared to continental climate proxy records. Meanwhile, several observations and general paleoenvironmental interpretations can be accounted for each main Patagonian sector sampled (complete data in Aguirre, 2003; Aguirre et al., 2005a, b, 2006a, b), as follows.

7.1 Puerto Lobos Area – characteristic taxa, absent or uncommon in deposits located southward along Patagonia, are O. urceus, Z. dufresnei, G. longior, P. gibbosa, P. rudis, Diplodonta patagonica, Chama sp. – these taxa occur in the Buenos Aires area, except for P. rudis, which is recorded for the first time in the marine Quaternary of Argentina – this is the only area where molluscs from Pleistocene deposits assigned to MIS 5e (MT IV) suggest higher SST than at present based on the occurrence of Plicatula, Glycymeris, Pododesmus (northward displaced) and Chama (extinct) from the modern adjacent littoral in the southwestern Atlantic Ocean (Argentine Sea).

7.2 Cabo Raso–Bahı´a Camarones – Natica delicatissima, Chlamys sp., Panopea abbreviata characterize the modern nearshore. – MT VI (MIS 1) is characterized by B. cf. purpuratus, Natica (Patinigera) magellanica, Natica (P.) deaurata, T. geversianus, B. purpuratus and A. atra. – slightly higher SST (1–2C) than present during the Mid-Holocene Climatic Optimum (5–8 ka) can be inferred from greater percentages of groups II and III. – MT V (MIS 5c [?], 5e [?]), characterized by T. atra, Tegula patagonica, Crepidula dilatata, M. edulis, B. purpuratus, P. antiqua and Pitar rostratus, is not indicative of a climatic optimum such as the Last Interglacial maximum highstand (MIS 5e). – MT IV (MIS 7) contain typically shells of T. atra, biggest P. antiqua, and Veneroida indet., indicative of water temperatures similar to the present cold waters, – in MT III (MIS 11) several groups occur exclusively, like Pectinidae, O. tehuelcha and Mactra cf. patagonica d0 Orbigny. These and other taxa (i.e.

Corbula patagonica, Diplodonta vilardeboana) suggest SST warmer than today. – the MT III assemblage (MIS 11?, ca. 400 ka BP) indicates warmer conditions than for MIS 5e. It can be correlated with the oyster assemblages from the Belgranense deposits of the Buenos Aires Province littoral (in its type locality, Barrancas de Belgrano, Buenos Aires City).

7.3 Bahı´a Bustamante Area – the main examples for considerable migration are T. atra (cold species) and V. procera (warm species), living far from the area of study. – general conditions of the littoral environment (substrate, depth, water energy conditions) during the late Pleistocene (MIS 7, 5) and Mid-Holocene (MIS 1) highstands were similar to the modern nearshore, only SST probably changed, though not dramatically, since at least MIS 7. – dominance of the association T. atra–P. antiqua can be considered a valid biostratigraphical tool for deposits of MT IV (MIS 7?) where both taxa reach maximum abundance and size. – the molluscan assemblage from MT V (MIS 5c or 5a [?]), with P. antiqua in scarce quantities and lowest diversity, is not indicative of warmer SST typical of an interglacial climatic optimum, but rather colder than the Mid-Holocene and similar to the present, probably not during MIS 5e (substages 5c or 5a [?]). – assemblages of MT VI (Mid-Holocene, MIS 1) support the influence of the Climatic Optimum during the ca. 6–9 14C ka BP interval in the area.

7.4 Golfo San Jorge Area – the molluscs from MT V with large shells of M. edulis and the occurrence of cold water taxa, is not indicative of a climatic optimum with higher SST within MIS 5e. On the contrary, they suggest colder SST and softer substrates than at present. – the absence of cold water pateliform taxa (F. picta, F. oriens, Patinigera delicatissima) within the MidHolocene MT VI suggests warmer SST than at present during MIS 1, and consequently, atmospheric and palaeoceanographic changes: a southward displacement of the dominant warm (Brazilian) and cool (Malvinas/Falkland) currents (and of the Argentinian and Magellanean provinces).

8. Concluding Remarks The molluscs reviewed provide the basic data set for preliminary paleoenvironmental and paleoclimatic scenarios of sea-level highstands since the Mid-Late Pleistocene (MIS 11 [?]). Considering that the molluscs are preserved in littoral deposits, significant water energy and postmortem

Late Pleistocene Cenozoic Invertebrate Paleontology transport of the shells cannot be ruled out, except for a few levels where whole or complete shells appear in scarce quantities. Infaunal bivalve taxa (Protothaca, Eurhomalea, Mulinia, Clausinella) showed most resistant shells to taphonomic processes and are the most adequate for future dating and isotope analyses. Only gastropods with thick shells (Tegula, Trophon, some Patinigera, Buccinanops) are more resistant and adequate for similar studies. However, infaunal bivalve taxa are more reliable and should be selected for dating and isotope studies. Some general final remarks are as follows: 1. According to the state of our knowledge the molluscs from MT III to VI belong to nearshore environments similar to the modern euhaline waters of Patagonia in the southwestern Atlantic Ocean. 2. Approximately 34% of the molluscs range back to the Miocene (they belong to taxa of warm water affinity). 3. The variations between the assemblages of the Late Pleistocene (MT I II, IV, V), Mid-Holocene (MT VI) and modern (Magellanean Province) littoral area are a result of environmental changes, of which SST of the shallow water masses seem to have been most important. 4. Extinctions in the adjacent modern littoral are represented by T. atra and Urosalpinx (Gastropoda) and O. tehuelcha, P. rudis and V. procera, dominant or exclusive of the Pleistocene. 5. Within the marine Quaternary of Patagonia T. atra is the best indicator of cold SST, while Chama, Pododesmus, Plicatula, V. procera and O. tehuelcha of warm SST. 6. The most plausible hypothesis is to consider that paleotemperature change was the main factor controlling the qualitative, quantitative, morphological or biogeographic differences observed between and within different MT and in comparison with the present. This must be tested by future isotope studies in this area. Further work is needed to address: (a) the climatic scenario for MT V (MIS 5e [?]), which according to the molluscan evidence was not characterized by warmer waters (with the exception of the Punta Lobos area); (b) new records to support or reject the effect of the Holocene Hypsithermal (Climatic Optimum) on the Patagonian littoral molluscs; (c) modern dating of Holocene molluscan assemblages from different areas of Patagonia for a better control of this event; (d) further dating of the Pleistocene molluscan assemblages for a better precision of the paleoenvironmental changes observed; (e) undertake stable isotope analyses for selected taxa of MT III–VI.

Acknowledgments We are grateful to Prof. J.O. Codignotto and R. Kokot (Universidad de Buenos Aires, Buenos Aires), to Prof. Alicia Castro (Universidad Nacional de La Plata, La Plata), Javier Sanagua and Germa´n Merletti (Repsol YPF) for their help in the field. To E. Fucks (Universidad

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Nacional de La Plata), M.I. Trucco (Universidad Nacional de Mar del Plata), S. Martı´nez (Facultad de Ciencias, Universidad de la Repu´blica, Montevideo, Uruguay) and C. Del Rı´o (Museo Argentino de Ciencias Naturales, Buenos Aires) for comments or discussions on several ideas. Special thanks are due to Jorge Chiesa, Ester Farinati, Guido Pastorino, who helped to gather bibliographic references or made specimens from their own private collections and photographs available. To Alejandro Tablado and Mo´nica Longobucco (Museo Argentino de Ciencias Naturales, Buenos Aires) for their permission and assistance in examining fossil and modern molluscs from Argentina. To Ms Kathie Way (Natural History Museum London) and Olle Israelsson (Museum of Evolution, Uppsala University) for providing photographs of some type material compared. This study was partially supported by grants from Programa de Incentivos to Project 11N459 (Facultad de Ciencias Naturales y Museo, Universidad Nacional de La Plata), Agencia Nacional de Promocio´n Cientı´fica (PICT 07-10967 and PICT2006-00468) and CONICET, (PIP-CONICET5077/4). This is a contribution to IGCP 437 and to IGCP 495 and IGCP526.

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15 Calcareous Microfossils (Foraminifera and Ostracoda) of the Late Cenozoic from Patagonia and Tierra del Fuego: A Review Gabriela C. Cusminsky1 and Robin C. Whatley2 1

Centro Regional Universitario Bariloche, Universidad Nacional del Comahue, Quintral 1250 San Carlos de Bariloche 8400, Rı´o Negro, Argentina, and CONICET 2 Micropalaeontology Research, Institute of Earth Studies, University of Wales, Aberystwyth, Cardiganshire SY23 3DB, United Kingdom

The class Ostracoda belongs to the Phylum Crustacea. Post-Cretaceous ostracods range in size from very small (some being <0.3 mm) to the large-living nektonic oceanic genus Gigantocypris which is almost 2.5 cm long. Most of them posses a bivalved carapace of calcareous material, which given the appropriate diagenetic circumstances is readily fossilized. Ostracods are known from the Cambrian onwards (Whatley et al., 1993) and today they still are among the commonest organisms found in practically every aquatic environment (van Morkhoven, 1963; Whatley, 1983a, b, 1988a). The spatial distribution of benthic ostracods in marine and nonmarine environments is controlled by a plexus of parameters such as temperature, oxygen concentration, salinity, depth, predation pressure, food availability, pH, turbidity, energy level and type of substrate. At intervals in the geological succession, often during the absence of planktonic microorganisms, ostracods can be of invaluable use as biostratigraphical indices. They are almost invariably of supreme importance in paleoenvironmental analysis (Ware and Whatley, 1983; Whatley, 1983a, b, 1988a, b, 1992; 1993; Whatley and Coles, 1991; Yassini and Jones, 1995; Whatley et al., 2003.). The first studies on Foraminifera and Ostracoda date back to the nineteenth century with the work of d’Orbigny (1839, in Boltovskoy, 1976) made on littoral material from the coast and from the Malvinas/Falkland Islands. G.S. Brady (1880) studied the Ostracoda from the oceanographical voyage of H.M.S. Challenger (1873–1876), while his brother, H. B. Brady (1884), investigated the Foraminifera. G.S. Brady (1868) also described many species from Tierra del Fuego and the Subantarctic region. The study of the Ostracoda continued with the German South Polar Expedition (Mu¨ller, 1908) and the very important work by Skogsberg (1928) in the southern South Atlantic and the Subantarctic area. Cushman and Parker (1931) and Heron-Allen and Earland (1932) studied various foramiferal species from the Patagonian coast, the Malvinas/Falkland Islands and Antarctica. The present chapter comprises a bibliographical synopsis of the most significant works on the Order Foraminiferida and the Class Ostracoda from the Late Miocene to present-day in Patagonia and Tierra del Fuego, in order to better understand the major events that took place during that interval. It is divided into two sections: the first one referred to continental sequences which includes all the lacustrine records systems studied; the second concerns

1. Introduction Micropaleontology is that part of paleontology devoted to the study of small fossils, mainly microscopic. The abundance of microfossils in certain rocks is of great value to those who wish to study them for biostratigraphical and evolutionary research by statistical analysis and since their small size enables many of them to escape destruction by the drilling process, they are of particular significance in hydrocarbon exploration and in the analysis of sedimentary basins in sequence stratigraphy. They are widely employed by commercial and academic micropaleontologists not only in biostratigraphy, but also very importantly in paleoenvironmental, paleotemperature, paleosalinity and paleogeographical reconstructions, by making use of their inferred paleoecology (Ware and Whatley, 1983; Whatley, 1983a, b, 1988a, b; Bignot, 1988; Whatley and Coles, 1991; Dowsett et al., 1992; Whatley, 1992, 1993; Whatley et al., 2003). Among important groups of microfossils are the Ostracoda, Foraminifera, Radiolaria and Conodonta, all animals and such plant groups as diatoms, carophytes, calcareous nannoplankton, spores and pollen. The Foraminiferida are unicellular organisms commonly classified within the Phylum Protista. They range in size commonly from 100 mm to 1000 mm. Most foraminifers secrete a mineralized shell or test, which may be preserved in various sedimentary environments ranging back to the Cambrian (ca. 550 Ma). They are predominantly marine organisms, living on the sea floor, attached to other objects such as algae, or floating in the water column, mainly near the surface. They are found from the shoreline down to 5000 m depth, and studies of Recent foraminiferal distribution patterns show that species diversity and population density are controlled by the physical and chemical variables in the surrounding environment. Among the physical factors, temperature is the most important parameter affecting benthic foraminifera. Other factors are circulation of the water mass and current velocity and turbidity. Salinity and dissolved oxygen concentration are the major chemical variables, whereas alkalinity is another important chemical parameter, especially in shallow, marginal marine conditions (Boltovskoy et al., 1980; Ware and Whatley, 1983; Whatley, 1983a, b, 1988a, b, 1992, 1993; Whatley and Coles, 1991; Yassini and Jones, 1995; Whatley et al., 2003).

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DEVELOPMENTS IN QUATERNARY SCIENCES VOLUME 11 ISSN 1571-0866 327

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Gabriela C. Cusminsky and Robin C. Whatley

the marine sequences referred to outcrops and core analysis of marine sediments in the analysed area. Figure 1 shows all the localities studied by different authors. The illustrated specimens of this area are deposited in the collections of the Facultad de Ciencias Exactas, Laboratorio de Micropaleontologı´a (FCEN-LM) and Museo de La Plata, Micropaleontologı´a (MLP-Mi), respectively.

2. Continental Sequences Before 1900, very little information was available on Late Miocene to Recent ostracods from nonmarine environments in Patagonia. The only work in existence was that of Daday (1902), who described a number of freshwater ostracods from Patagonia.

Calcareous Microfossils of the Late Cenozoic

329

Continental Sequences

Pliocene

Pleistocene/Holocene

13. Boltovskoy, 1973. 14. Cusminsky, 1991, 1992, in press. 15. Cusminsky and Whatley, 1998, 2000. 16. Whatley and Cusminsky, 2002. 17. Malumia´n et al., 1984. 18. Echeverrı´a, 1988.

1. Cusminsky, 1994a, 1995. 2. Cusminsky and Whatley, 1995, 1996 a, b. 3. Whatley and Cusminsky, 1995a, b, 1999, 2000. 4. Pe´rez et al., 2002. Recent 5. Daday, 1902. 6. Schwalb et al., 1998, 2000, 2001, 2002. 7. Cusminsky et al., 2000, 2005; Pe´rez et al., 2001.

Pleistocene 19. Rossi de Garcı´a, 1967. 20. Cionchi, 1987. 21. Cusminsky, 1994a. 22. Boltovskoy and Watanabe, 1980. 23. Boltovskoy, 1973.

Marine Sequences

Holocene

Miocene

24. Boltovskoy, 1973. 25. Bernasconi and Cusminsky, 2004.

8. Bertels, 1975. 9. Bertels, 1979; Bertels-Psotka, 1995. 10. Malumia´n, 1978, 1982. 11. Malumia´n et al., 1998. 12. Carame´s et al., 2004.

Fig. 1. Location map. The numbers indicate the papers that refer to each locality on the map.

The systematic study of nonmarine Patagonian ostracods began in 1992 with the initiation of the Lagos Comahue project, within which the present authors studied late Quaternary lacustrine ostracods from northern Patagonia, based on the analysis of numerous samples from lake bed cores and from outcrops along an approximately E–W transect from near Mount Tronador (41000 S; 71500 W) to Lago Cari-Laufquen (41120 S; 69250 W). This study was based on the analysis of ostracods recovered from the following cores: El Tre´bol II, Mallı´n Aguado; Lago Seco, Los Juncos, La Salina and from the profile of the Rı´o Maquinchao during 1991 and 1992 (Table 1). The study of the ostracods revealed the existence along this transect, although their extension has not been established, of two probably nonsynchronous lacustrine paleoenvironments. In the western sector (El Tre´bol II, Mallı´n Aguado; Lago Seco) of the transect in the area of the large ‘‘Elpalafquen’’ palaeolake (Del Valle et al., 1993, 1996), factors such us oligotrophy, high acidity, and taphonomy combined to ensure the complete or partial absence of Ostracoda from the cores examined. In the eastern sector (Los Juncos, La Salina, and profile of the Rı´o Maquinchao) of this transect, conditions in the Late Quaternary were more favourable for the existence of ostracods (Cusminsky, 1994a, 1995; Cusminsky and Whatley, 1995, 1996a, b; Whatley and Cusminsky, 1995a, b, 1999, 2000). This can be seen clearly in the Maquinchao Basin on the Patagonian Steppe, where a large saline paleolake, Lake ‘‘Maquinchao’’ (Del Valle et al., 1996), was developed during a pluvial period at around 13,200 14C yr BP. This confirms the model proposed by Garleff et al. (1994) on the basis of pollen data, which showed that at around 13,000 14C yr BP there was a more humid interval with heightened lake levels. These authors also indicate that, later in the Holocene, there was a change in conditions toward greater aridity that led to a shrinking of the paleolake, the precipitation of evaporite minerals, and the formations of dunes around the lakes shores (see also Whatley and Cusminsky, 2000; Pe´rez et al., 2002). Many of the ostracod species in the study are of widespread occurrence across this transect but it is notable that Limnocythere rionegroensis Cusminsky & Whatley is absent from the Los Juncos core. This absence

is possible evidence that, in the Late Quaternary as at present-day, the area around Los Juncos enjoyed a climate of greater humidity than that of the Maquinchao Basin; therefore, the presence of L. rionegroensis may be used as an index species of more saline water and drier environments in this sector of South America (Whatley and Cusminsky, 2000). Schwalb et al. (1998, 2000, 2001, 2002), Cusminsky et al. (2000, 2005) and Pe´rez et al. (2001) analysed modern lacustrine ostracod species assemblages and stable oxygen and carbon isotope ratios of living and recent ostracods together with d18O and d13C (dic) (= dissolved inorganic carbon) values of host water samples. They provided a first dataset that characterizes a wide range of modern aquatic environments from the Lago Cardiel (Santa Cruz Province) and Lago Cari-Laufquen (Rı´o Negro Province) areas. Species assemblages and isotope values can be assigned to three groups: 1. Springs, seeps and streams characterized by Darwinula sp; Heterocypris incongruens (Ramdohr), Eucypris fontana (Graf), Amphicypris nobilis Sars and Ilyocypris ramirezi Cusminsky & Whatley. Ostracod and water isotope values range between –13 and –5ø for oxygen and between –15 and –3ø for carbon. They are the most negative of the entire sample set, reflecting ground water input with little or no evaporative enrichments. 2. Permanent ponds and lakes with Limnocythere patagonica Cusminsky & Whatley, E. labyrinthica Cusminsky & Whatley, Limnocythere sp. and E. fontana as typical species. Isotope values indicate a high degree of evaporation of lake waters relative to feeder springs and streams, and range between –7 and þ5ø for oxygen and –5 and þ 4ø for carbon. 3. Ephemeral ponds and lakes with L. rionegroensis as the dominant species. These systems display the most enriched isotope values in both ostracodes and host waters, ranging from –5 to þ 7ø for oxygen and from –5 to þ 6ø for carbon. Cusminsky et al. (2005) confirm these data with the study of living ostracods with isotopic signal that tend to agree with the original study.

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Table 1. Locality and date of samples (from Whatley and Cusminsky, 2000). Locality Laguna El Tre´bol

Mallı´n Aguado

Lago Seco Los Juncos La Salina Rı´o Maquinchao Section

Coordinates (Lat. Long) 41000 S; 71500 W

41000 S; 71290 W

41010 S; 71280 W 41030 S; 71020 W 41160 S; 69320 W 41120 S; 69250 W

Core length (m)

Age and depth

11

10,480 + 130 at 525 cm

14

9 2 1

3. Marine Sequences During the Cenozoic, the Valdez, San Jorge and Austral basins were subjected to several marine transgressions (Bertels-Psotka, 1995). Camacho (1967) mentioned one transgression during the Late Miocene. This transgression covered northeastern Patagonia and extended through the western part of Uruguay, to the Argentine provinces of Entre Rı´os, Corrientes and Santiago del Estero, and the southern part of Paraguay. He recognized various facies in this transgression: (a) a relatively deep water facies (the Parana´ or ‘‘Paranaense’’ Formation), (b) a deltaic facies of transition (Mesopotamia or ‘‘Mesopotamense’’ Formation) and (c) a coastal environment facies (Entre Rı´os or ‘‘Entrerriense’’ Formation). From the point of view of Foraminifera and Ostracoda, this transgression has been recognized by various authors. Bertels (1975) mentioned that Rossi de Garcı´a described for the first time species of the ostracod genus Bensonia from Puerto Pira´mides, Penı´nsula Valdez Chubut, for the ‘‘Entrerriense’’ ingression (Rossi de Garcı´a, in Bertels, 1975). Similarly, Bertels (1975) described a new species of Bensonia from its type locality at Puerto Pira´mides and showed that its age corresponded to the ‘‘Entrerriano’’ stage, Late Miocene to Early Pliocene. Bertels (1979) Bertels-Psotka, (1995) and Malumia´n (1978, 1982) identified the Entre Rı´os Formation in Penı´nsula Valde´s. Malumia´n (1978, 1982) showed that the dominant foraminiferid was Protelphidium accompanied by Buccella, Cibicidoides, Rotalia becarii (Linne´) and Nonionella. He suggested that the

14

Laboratory number C yrs BP

AMS ETH N 13,227

13,120 + 220 14C yrs BP at 650 cm 10,160 + 80 14C yrs BP at 1125 cm 13,850 + 100 14C yrs BP at 1285 cm 14,635 + 130 14C yrs BP at 1300 cm 12,500 + 220 14C yrs BP at 870–875 cm 12,050 + 240 14C yrs BP at 150–160 cm In process

Cambridge University C14 Q-2951 AMS ETH N 16,982

AMS ETH N 13,226

13,200 TL yrs BP base of the paleolake

Proszynsky laboratory Wa 26/93

AMS ETH N 16,983

Cambridge University C14 Q-2953 Cambridge University C14Q-2952

environment of deposition was shallow and somewhat warmer than at the same latitude at the present day. He also showed that the foraminiferal fauna, in the sequences corresponding to a transgression, was very similar to that of the modern Argentine coastline (Malumia´n, 1982; Malumia´n and Na´n˜ez, 1996). Malumia´n et al. (1998) analysed the type section of the Barranca Final Formation in the Golfo San Matı´as sector. These authors encountered a low diversity of benthonic foraminifera with scarce miliolids and aglutinating species and a predominance of such rotalids as Protelphidium turberculatum (d’Orbigny) and Criboelphidium discoidale (d’Orbigny), indicating hiposaline marginal marine environments such as an estuary, with fluvial influences, which the authors consider that characterize the ‘‘Entrerriense’’. Similar conclusions were drawn by BertelsPsotka (1995) for deposits corresponding with the marine Miocene of the Entre Rı´os Formation. Carame´s et al. (2004) analysed a borehole in the Valdez Basin in which they recognized the ‘‘Entrerriense’’ transgression, which is equivalent to the Puerto Madryn Formation and itself correlatable with the Barranca Final Formation of Malumia´n et al. (1998). Based on the low diversity of the foraminifera, these authors considered the environment of deposition to be marginal marine and of abnormal salinity; the foraminiferal genus Protelphidiun being indicative of hiposaline, very shallow environments, is typical of an estuary with fluvial influence. They also noted the absence of species indicative of cold water and suggested that marine conditions in the Entrerriensian seas were warmer, due to the proximity to the coast and consequent greater

Calcareous Microfossils of the Late Cenozoic distance from the influence of the colder waters of the Malvinas/Falkland Current. With respect to the temperature obtained during the Late Miocene, Boltovskoy (1979) and Sprechmann (1978) considered it to be higher at the present day. For this reason, the benthonic foraminiferal biogeographical provinces recognized by Boltovskoy (1970, 1976, 1979) and Boltovskoy et al. (1980) are situated approximately 10 further south (Boltovskoy, 1979), and this also explains the presence of Brazilian species that penetrated south during the Miocene and have remained there up to now. Boltovskoy (1979) has shown that the fauna of the Late Miocene is very similar to that of today. Indeed, the benthonic foraminifera on the continental shelf have shown great constancy during the past 20 Myr, thus confirming the great paleoecological value of the group as well as their relatively limited biostratigraphical significance (Boltovskoy, 1979). Paleogeographically, at this time, the Drake Passage did not exist. The surface ocean currents formed an anticyclonic gyre with the Brazilian Current extending a little further south than it does at the present day. However, a probable connection existed between the Pacific and Atlantic oceans in the extreme south of South America via what is termed the Southern Straits, allowing the colder waters of the Pacific to penetrate into the Atlantic (Boltovskoy, 1979, 1980). During the very Late Miocene–Early Pliocene, the orogenic movements that elevated the Andean Cordillera brought about the closure of the Southern Straits and the opening of the Drake Passage. This permitted greater access of cold Pacific water that brought about a rapid decline in temperature. It also produced a displacement northward of the Antarctic Convergence (Kennett, 1977, 1978) and the initiation of the Antarctic Circumpolar Current (Boltovskoy, 1973, 1979, 1980). Boltovskoy (1973) analysed a number of cores from the South Atlantic which he was able to date as Pliocene. He showed that during this interval, the water temperature was some 5 C lower than at the present day. The study of Foraminifera and Ostracoda from a Late Pliocene core from the Burwood Bank (Cusminsky, 1991, 1992, in press; Cusminsky and Whatley, 1998, 2000; Whatley and Cusminsky, 2002) indicated that the fauna is characteristic of the continental shelf. The ostracods studies show that modern geographical distributions extend from Antarctica northward to the Argentine province of Buenos Aires, although Antarctic forms predominate. This suggests a considerable influence of Antarctic waters during the Late Pliocene which may be related to changes of the position of the polar front (Cusminsky and Whatley, 1998, 2000; Whatley and Cusminsky, 2002). These observations confirm the studies of Boltovskoy (1979), Mercer (1976, 1978) and Kennett (1977, 1978), which indicate that the first great glaciation of southernmost South America occurred in the Mid and Late Pliocene. With respect to micropaleontological studies on Pliocene outcrops in Patagonia, it is important to mention those of Malumia´n et al. (1984) and Echevarrı´a (1988). These authors studied a section located at a site called Playa Bonita, on the northern coast of Golfo San Matı´as. According to Angulo and Casamiquela (in Echevarrı´a, 1988), these authors proposed the name ‘‘Balneario Las

331

Loberı´as’’, for a unit which is a marine intercalation occurring within the Rı´o Negro Formation, of continental origin. According to Franchi et al. (in Echevarrı´a, 1988) this marine facies has a ‘‘Huayqueriense’’ (Middle Pliocene) to ‘‘Montehermosense’’ (Late Pliocene) or ‘‘Montehermosense’’ age (Franchi et al., 1984). The Ostracoda are less numerous but are very well preserved, despite some specimens being recrystallised. Among the fauna the following forms appear: Callistocythere litoralensis (Rossi de Garcı´a); Cushmanidea sp.; Cytheretta punctata Sanguinetti; Hemicytherura sp.; Soudanella? sp.; Cytherelloidea viedmaensis Echevarrı´a; Hemicytherura playabonitaensis Echevarrı´a and so on, which, taken together, are typical of marginal marine environments, with fluctuating salinity and a paleotemperature higher than that presently observed. According to Echevarrı´a (1988), the ostracod fauna is related to ostracod assemblage from the Middle Oligocene to the lower part of the Middle Miocene of the south of Santa Cruz Province; the Lower Miocene of Tierra del Fuego; the Late Miocene of the Penı´nsula Valdez, Entre Rı´os Province and the subsurface of Santa Fe Province and also the Miocene, Pleistocene and Early Holocene of southern Brazil. This seems to indicate a progressive chronological displacement northward of species because of the gradual climatic deterioration conditions (Echevarrı´a, 1988). The study of Foraminifera from the same sequence undertaken by Malumia´n et al. (1984), that is, from the marine Balneario las Loberı´as facies, are indicative of species that at present occur in shallow water, estuarine environments in the post ‘‘Entrerriense’’ times, with the presence of Elphidium galvestonense Kornfeld; Buccella frigida (Cushman); Elphidium advenun depressulum Cushman; ‘‘Protelphidium’’ cf tuberculatum (d’Orbigny) and Buliminella elegantı´sima (d’Orbigny). Records from the Pleistocene of various sectors of the continental shelf are to be found in Boltovskoy (1973) and Boltovskoy and Watanabe (1980). According to Boltovskoy (1973, 1979), the occurrence of Truncorotalia truncatulinoides (d’Orbigny) is synchronous with the start of the Pleistocene and remains abundant in modern oceans. In the South Atlantic, this species is typical of Subantarctic waters and the mixed waters of the Subtropical/Subantarctic zone of convergence (Boltovskoy, 1979). On the other hand, Boltovskoy (1973), based on the relation of warm and cool water species, the relationship between Globorotalia menardi (d’Orbigny) and Turborotalia inflata (d’Orbigny) and the coiling directions of Neogloboquadrina pachyderma (Ehrenberg) and Globigerina bulloides d’Orbigny found two cool and two warm periods in the Pleistocene, which were related to the early Wu¨rm-main Wu¨rm and Riss–Wu¨rm/interstadial Wu¨rm respectively (Boltovskoy, 1973, 1979; Bertels and Madeira-Falcetta, 1977). Similarly, isotopic studies of 18 O and 13C made on the basis of foraminifera, from a Quaternary core situated in the Malvinas/Falklands Basin adjacent to the continental shelf, pointed out the presence of colder and more corrosive water in sediments corresponding to the Pleistocene (Cusminsky, 1994b). With reference to outcrops of the Pleistocene age, Rossi de Garcı´a (1967) in the area of Bahı´a Bustamante,

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Chubut Province, mentioned the presence of ostracods corresponding to a littoral epineritic environment of mesohaline to euryhaline salinity and to level IV of the terraces of Feruglio (1950). On the other hand, subsequent studies of the foraminifera and ostracods by L. Ferrero (in Cionchi, 1987) in the Bahı´a Bustamente area indicated shallow waters, mixohaline environment, probably marine littoral. According to Cionchi (1987), the age of this deposit, referred to as the Caleta Malaspina Formation, is Pleistocene and corresponds to the Riss-Wu¨rm interglacial, that is, around 120,000 14C yr BP. For the Holocene, the studies of Boltovskoy (1973, 1979) demonstrate that the temperature at that time was a little lower than at present. Studies of foraminifera have been made from cores of probable Holocene age from Golfo Nuevo (Bernasconi and Cusminsky, 2004). These authors reveal the benthonic fauna to be very similar to that of the present day and that the faunal abundance is closely related to the type of sediment sampled, the most abundant being yielded by finegrained facies deposited under low energy conditions. As mentioned before, the first studies of forams and ostracods began with the analysis of recent fauna. The beginning of the second half of the twentieth century saw a great development in the study of recent Foraminifera and Ostracoda in the area. With respect to the former group, the works of E. Boltovskoy (who has published more than 50 papers in this subject) and of other workers dealing with both planktonic and benthonic taxa, their ecology and distribution, should be mentioned. Among these, the following may be cited: Boltovskoy, 1954a, b, 1965, 1971; Boltovskoy and Watanabe, 1979, 1980; Boltovskoy et al., 1980; Boltovskoy and Giussani de Khan, 1982; Malumia´n et al., 1991; Bernasconi et al., 2001, 2002; Cusminsky and Bernasconi, 2004, and Bernasconi and Cusminsky, 2004, 2005. Boltovskoy (1970, 1976, 1979) and Kahn and Watanabe (1980) have shown that the recent benthonic foraminifera in their biogeographical distribution reflect the surface hydrological scheme on the continental shelf. Boltovskoy (1970, 1976, 1979) and Boltovskoy et al. (1980) have determined distinct biogeographical provinces and subprovinces for this group: (a) India Province: this unit extends from latitude 4 N to 33 S, but it is not discussed here in any detail since it is outside our area of interest. (b) Argentine Province: This province extends from latitude 32 or 33 to 56 S and is characterized by the presence of Buccella peruviana (d’Orbigny) s.l. Other species that occur commonly in this province are as follows: Bolivina compacta Sidebotton, B. striatula Cushman, Bulimina patagonica d’Orbigny, Buliminella elegantissima (d’Orbigny), Cibicides dispars (d’Orbigny), Elphidium depressulum Cushman, Epistominella exigua (Brady), Miliolinella subrotunda (Montagu), Nonion pauperatum (Balkwill & Wright), Nonionella auris (d’Orbigny), Quinqueloculina seminula (Linne´) and so on. The province is subdivided into two: Subprovince B1, the northern Patagonian Subprovince, which extends latitudinally from about 32–33 S to some 42–43 S, is characterized by the presence

of Elphidium discoidale (d’Orbigny) and also contains certain species attesting to the influence of the Southern Brazilian Subprovince, such as Massilina secans (d’Orbigny), Poroeponides lateralis (Terquen) and Spiroloculina planulata (Lamarck), and Subprovince B2, the southern Patagonian Subprovince, that extends from latitude 42–43 to 56 S and is marked by the absence of E. discoidale or any Southern Brazilian species, which are replaced by E. macellum (Fichtel & Moll). Further south, the Malvinas Province is marked by larger species typical of colder waters, such as Discorbis williamsoni (Chapman & Parr), various species of Cibicides, Q. seminula, and so on. They are accompanied by species that also occur along the Patagonian coast, albeit as smaller individuals. These are Angulogerina angulosa (Williamson), Globocassidulina crassa d’Orbigny, Cassidulinoides parkerianus (Brady), Discorbis isabelleanus (d’Orbigny), Ehrengergina pupa (d’Orbigny), Heronallenia kempii (Heron-Allen & Earland) and Uvigerina bifurcata d’Orbigny (Kahn and Watanabe, 1980). The planktonic foraminifera show a considerable similarity to the benthonic forams in their distribution in shallow water (Boltovskoy, 1976, 1979). The planktonics can be divided into (a) those representative of warm waters associated with the Brazilian Current, such as Globorotalia hirsuta (d’Orbigny), Orbulina universa d’Orbigny, G. menardii, among others, and (b) those representative of cold waters of the Oceanic Drift that can be divided into the Patagonian Current with few planktonic species and the Malvinas/Falklands Current with rather more. The species concerned are N. pachyderma (both sinistral and dextral), Turborotalita quinqueloba Natland, G. bulloides, Globigerinita uvula (Ehrenberg), Turborotalia inflata, G. scitula (Brady) and T. truncatulinoides (Boltovskoy, 1979). There are numerous modern studies devoted to the taxonomy, ecology and zoogeography of the recent marine benthonic Ostracoda of the area. Among the most important are those by Hartmann-Schro¨der and Hartmann (1962, 1965; the first author working on the polychaete worms) and Hartmann (1997) and especially a series of studies on the Ostracoda of the littoral and the continental shelf by one of us (R. Whatley and his various colleagues at Aberystwyth; Whatley and Moguilevsky, 1975, Whatley et al., 1987, 1988, 1995, 1996b, 1997a, b, and 1998; McKenzie et al., 1995; Wood et al., 1999). Whatley et al. (1996a), on the basis of the distribution of recent littoral and shelf species between Rio de Janeiro and Tierra del Fuego, recognized four zoogeographical provinces. They relate the geographical limits of these provinces to major oceanographical conditions, particularly to the interaction of cold and warm waters, since these were evidently the major factors influencing the

Calcareous Microfossils of the Late Cenozoic

333

Table 2. Forams subprovince boundaries Forams

Southern Patagonian Subprovince

Northern Patagonian Subprovince

Southern Brazilian Subprovince

Northern Brazilian Subprovince

Boundaries

42/43–56

32/33–42/43

23–32/33

4 N–23 S

Table 3. Ostracods province and subprovince boundaries Ostracods

Fuegian– Magellanic Subprovince

Southern PatagonianFalkland Subprovince

Northern Patagonian Subprovince

Bonaerensian Province

Platensian Uruguayan Pelotensian Subprovince

Southern Brazilian Subprovince

Boundaries

52–56

48/47–52

43/42–47/48

36–43/42

31/32–36

22/21–31/30

distribution of the Ostracoda. The provinces they recognized are as follows: 1. Brazilian Province, extending from latitude 22–21 to 36 S; 2. Bonaerense Province, between latitudes 36 and 42–43 S. The characteristic species of this province are, among others, Semicytherura sp.1; Loxochoncha bullata Hartmann; Cytherois minor Mu¨ller and Semicytherura clandestina Whatley, Chadwick, Coxill & Toy; 3. Subantarctic Province, which is divided into the northern Patagonian Subprovince between latitudes 42–43 S and 47–48 S which is still under the influence of the Brazilian Current and to which only two species (Semicytherura sp. 2 and Krithe sp. 1) are confined, although Perissocytheridea sanantoniensis (Whatley, Moguilevsky, Toy, Chadwick & Feijo´ Ramos) does not extend further south than 48–47 S and seven species do not occur further north than 42–43 S (Cytheretta sp., Propontocypris sp. 1, Paradoxostoma dolichoforma (Whatley, Moguilevsky, Toy, Chadwick & Feijo´ Ramos), Paradoxostoma fuegensis (Whatley, Moguilevsky, Toy, Chadwick & Feijo´ Ramos), Procythereis iganderssoni (Skogsberg), Propontocypris simplex (Brady), Loxoreticulatum cacothemon Whatley, Chadwick, Coxill & Toy, and the southern Patagonian/Malvinas/Falklands Subprovince which extends from Rı´o Gallegos to Cabo Blanco, between latitudes 47–48 and 52 S, and is defined by the preponderance of colder water species of which 10 are restricted to the Subprovince, 18 species do not extend further south than 52 S and 15 species do not extend north of 47–48. The limit between these two subprovinces in the area of Puerto Deseado, approximately where the cold Malvinas/Falklands Current begins to lose some of its potency and becomes directed further offshore;

4. Antarctic Province. This is represented by the Fuegian/Magellanic Subprovince (since the Antarctic Subprovince to the south is beyond the scope of this contribution), which extends between latitudes 52 and 56 S. The Subprovince is of cold water aspect, being strongly influenced by the Antarctic Surface Waters. Eleven species are restricted to this subprovince (Paradoxostoma sp., Propontocypris sp. 2, Cytherois spp. 1, 2, & 3, Kuiperiana sp. cf K. meridionalis (Mu¨ller), Semicytherura sp. 3, Xestoleberis ventribullata Hartmann, Heterocythereis chilensis (Hartmann), Australicythere devexa (Mu¨ller), and Bythocythere sp. 1). Four species do not occur north of this subprovince. Wood et al. (1999) consider the paleozoogeography of Late Paleogene to Recent Ostracoda from the Neotropics to the Antarctic and provide an interesting framework to our understanding of modern distribution patterns. It is interesting to observe that both modern biogeographical subdivisions of the southwest Atlantic area based on Foraminifera and Ostracoda, exhibit considerable correlation between the geographical limits of their respective provinces and subprovinces (Tables 2 and 3).

4. Conclusions Throughout the Late Cenozoic in Patagonia and Tierra del Fuego, various authors have studied Foraminifera and Ostracoda. These studies demonstrate that the distribution of both, ostracods and foraminifers, would response to different paleogeographic and paleoclimatic events occurring in that area from the Late Miocene to the present day confirming the usefulness of microfossils in the contribution of the overall scientific knowledge of the area. Future micropaleontological studies are required to seek more detailed information of the area as contribution to our understanding of global change.

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Acknowledgments This study was funded through the Universidad Nacional del Comahue (UNC) (B940 and B4/001), the Agencia Nacional de Promocio´n Cientı´fica y Tecnolo´gica (ANPCYT) (PICTs 01-13550, 03-14653, 07-09659 and 26057) and CONICET (PIP’s 02135 and 2040 and 6416) projects. G.C.C. wishes to acknowledge Cecilia Ezcurra for her suggestions, Victoria Amos for the artwork and the late Dr Alwine Bertels for introducing her to Micropalaeontology. R.C.W. wishes to acknowledge the late Professor Dr Arturo Amos, good friend and colleague, who made his initial three-year most enjoyable sojourn in Argentina possible and was equally responsible for the many subsequent visits. To Irma Gamundi, Alberto Riccardi, Susana Damborenea, Miguel Mancen˜ido, Sara Ballent and her family, Gabriela Cusminsky and family, Magda Bertels, Mario and Mrs Teruggi, the D’Allesandro family, Marina Aguirre and family, Alicia Echevarrı´a, Luis Dalla Salda, members of the Club Garza Mora, and all his many friends in La Plata, San Carlos de Bariloche, Buenos Aires, Santa Fe and Corrientes, heartfelt thanks are extended for overwhelming help and kindness.

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Whatley, R.C., Chadwick, J., Coxill, D. and Toy, N. (1988). The Ostracod family Cytheruridae from the Antarctic and South-West Atlantic. Revista Espan˜ola de Micropaleontologı´a 20, 171–203. Whatley, R.C., Siveter, D.J. and Boomer, I. (1993). The Ostracoda. In: Benton, M.J. (ed.), The Fossil Record 2, 343–356. Chapman & Hall. Whatley, R.C., Toy, N., Moguilevsky, A. and Coxill, D. (1995). Ostracoda from the South West Atlantic Part I, the Falklands Islands. Revista Espan˜ola de Micropaleontologı´a 27, 17–38. Whatley, R.C., Feijo´ Ramos, M.I., Moguilevsky, A. and Chadwick, J. (1996a). The provincial distribution of Recent littoral and shelf ostracoda in the SW Atlantic. In: Crasquin-Soleau, S., Braccini, E. and Lethiers, F. (eds), What about Ostracoda? 3rd. European Ostracodologist Meeting, Paris, 318–327. Whatley, R.C., Staunton, M., Kaesler, R. and Moguilevsky, A. (1996b). The taxonomy of recent Ostracoda from the southern part of the Magellan Straits. Revista Espan˜ola de Micropaleontologı´a 28, 919–939. Whatley, R.C., Kaesler, R. and Staunton, M. (1997a). The depth distribution of recent marine Ostracoda from the southern Strait of Magellan. Journal of Micropalaeontology 16, 121–130. Whatley, R.C., Moguilevsky, A., Toy, N. et al. (1997b). Ostracoda from the South west Atlantic. Part II. The littoral fauna from between Tierra del Fuego and the Rı´o de La Plata. Revista Espan˜ola de Micropaleontologı´a 29, 5–83. Whatley, R.C., Moguilevsky, A., Chadwick, J. et al. (1998). Ostracoda from the South West Atlantic, Part. III, The Argentinian, Uruguayan and Southern Brazilian continental shelf. Revista Espan˜ola de Micropaleontologı´a 30, 89–116. Whatley, R.C., Pyne, R.S. and Wilkinson, I.P. (2003). Ostracoda and palaeo-oxygen levels in the Upper Cretaceous of East Anglia. Palaeogeography, Palaeoclimatology, Palaeoecology, 194, 355–386. Wood, A., Ramos, M.I. and Whatley, R.C. (1999). The palaeozoogeograpy of Oligocene to Recent marine Ostracoda from the Neotropics (mid and South America) and Antarctica. Marine Micropaleontology 37, 197–223. Yassini, I. and Jones, B.G. (1995). Foraminifera and Ostracoda from estuarine and shelf environments on the southeastern coast of Australia. The University of Wollongong Press, Wollongong, Australia, pp. 484.

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1. 2. 3. 4. 5. 6. 7. 8. 9.

Amphicypris nobilis Sars MLP-Mi 1374 Candona sp. cf. C. neglecta Sars MLP-Mi 1372 Cypridopsis intermedia Sars MLP 217 Darwinula sp. MLP-Mi 1410 Eucypris fontana Graf MLP-Mi 1377 Eucypris labyrinthica Cusminsky and Whatley MLP-Mi 1381 Eucypris virgata Cusminsky and Whatley MLP-Mi 1384 Eucypris cecryphalium Cusminsky et al MLP-Mi 1391 Heterocypris incongruens (Ramdorh) MLP-Mi 1394

10. Ilyocypris ramirezi Cusminsky and Whatley MLP-Mi 1404 11. Kapcypridopsis megapodus Cusminsky et al MLP-Mi 1395 12. Limnocythere patagonica Cusminsky and Whatley MLP-Mi 1422 13. Limnocythere rionegroensis Cusminsky and Whatley MLP-Mi 1365 14. Newnhamia patagonica (Vavra) MLP-Mi 1413 15. Potamocypris smaradigma Vavra MLP-Mi 1403 16. Sarscypridopsis aculeata (Costa) MPL-Mi 216

Plate 1. Ostracods from Pleistocene–Recent lacustrine sequences

Calcareous Microfossils of the Late Cenozoic

1. 2. 3. 4. 5. 6. 7. 8. 9.

Anomalina vermiculata (d’Orbigny) FCEN-LM 1996 Globocassidulina rorrensis (Kennett) FCEN-LM 1985 Cassidulinoides parkerianus (Brady) FCEN-LM 1939 Cibicides dispars (d’Orbigny) FCEN-LM 1976 Discorbis floridanus Cushman FCEN-LM 1949 Discorbis williamsoni Chapman FCEN-LM 1951 Ehrenbergina pupa (d’Orbigny) FCEN-LM 1987 Globocassidulina subglobosa (Brady) FCEN-LM 1988 Globorotalia oscitans Todd FCEN-LM 1959

Plate 2. Forams from Pliocene marine sequences

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10. Lagena caudata (d’Orbigny) FCEN-LM 1904 11. Lagena substriata Williamson FCEN-LM 1909 12. Neogloboquadrina pachyderma f. typica (Ehrenberg) FCEN-LM 1972 13. Neogloboquadrina pachyderma f. superficiaria (Ehrenberg) FCEN-LM 1971 14. Notorotalia clathrata (Brady) FCEN-LM 1957 15. Quinqueloculina seminula (Linne´) FCEN-LM 1891 16. Angulogerina carinata Cushman FCEN-LM 1948

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1. 2. 3. 4. 5. 6. 7. 8. 9.

Ameghinocythere reticulata Whatley et al MLP-Mi 1151. Australicythere devexa (Mu¨ller) MLP-Mi 1127 Austroaurila impluta Brady MLP-Mi 1136 Aurila radiata (Skogsberg) MLP-Mi 1134 Austroaurila theelli (Skogsberg) MLP-Mi 1142 Procythereis tourquata Skogsberg MLP-Mi 1156 Cativella bensoni Neale MLP-Mi 1160 Copitus caligula Skogsberg MLP-Mi 1099 Hemicytherura splendifera Whatley et al MLP-Mi 1107

Plate 3. Ostracods from Pliocene marine sequences

10. Hemingwayella pumillo (Brady) MLP-Mi 1114 11. Henryhowella heros (Whatley et al) MLP-Mi 1163 12. Oculocytheropterom burdwoodbankensis Whatley and Cusminsky MLP-Mi 1121 13. Oculocytheropteron gaussi (Mu¨ller) MLP-Mi 1124 14. Pectocythere magellanensis Whatley et al MLP-Mi 1149 15. Austroaurila recurvirostrata (Skogsberg) MLP-Mi 1141 16. Stethocythere acuticaudata Whatley and Cusminsky MLP-Mi 1096.

Calcareous Microfossils of the Late Cenozoic

1. 2. 3. 4. 5. 6. 7. 8. 9.

Bulimina aculeata d’Orbigny FCEN-LM 1940 Bulimina affinis d’Orbigny FCEN-LM 1941 Bulimina inflata Sequenza FCEN-LM 1942 Buccella peruviana f. campsi (Boltovskoy) FCEN-LM 1953 Cassidulina carinata Silvestri FCEN-LM 1983 Globigerina bulloides f. trilocularis d’Orbigny FCEN-LM 1966 Globigerina bulloides f. typica d’Orbigny FCEN-LM 1967 Turborotalia inflata (d’Orbigny) FCEN-LM 1958 Turborotalita quinqueloba Natland FCEN-LM 1968

Plate 4. Forams from Quaternary marine sequences

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10. Truncorotalia truncatulinoides malvinensis (Boltovskoy and Watanabe) FCEN-LM 1961 11. Globigerinita glutinata (Egger) FCEN-LM 1973 12. Globigerinita uvula (Ehrenberg) FCEN-LM 2005 13. Melonis affine (Reuss) FCEN-LM 1997 14. Pullemia bulloides (d’Orbigny) FCEN-LM 1995 15. Angulogerina angulosa (Williamson) FCEN-LM 1947 16. Uvigerina bifurcata d’Orbigny FCEN-LM 1944 17. Uvigerina peregrina Cushman FCEN-LM 1945

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16 Late Miocene Continental and Marine Palynological Assemblages from Patagonia Viviana Barreda1, Vero´nica Guler2 and Luis Palazzesi1 1

CONICET-Museo Argentino de Ciencias Naturales ‘‘B. Rivadavia’’, Divisio´n Paleobota´nica, Av. A. Gallardo 470 C1405DJR, Buenos Aires, Argentina 2 CONICET-Departamento de Geologı´a, Universidad Nacional del Sur, San Juan 670 (8000), Bahı´a Blanca, Argentina

Zamaloa, 2000; Guerstein and Junciel, 2001; Barreda et al., 2003; Guerstein et al., 2004). There is also information from the Argentine continental shelf (Gamerro and Archangelsky, 1981; Malumia´n et al., 1999; Palamarczuk and Barreda, 2000; Guerstein and Junciel, 2001; Guler and Guerstein, 2002). The spore–pollen assemblages recovered from these areas indicate that forests were widespread in extra-Andean Patagonia, with patches of open vegetation. Forests were dominated by Nothofagaceae, Podocarpaceae and Araucariaceae with abundant ferns. Lowland regions were occupied by coastal zone families such as Chenopodiaceae, Convolvulaceae and Ephedraceae. Megathermal Arecaceae, Sapindaceae, Euphorbiaceae and Salicaceae were also documented. Some of these elements, coupled with the first Fabaceae Mimosoideae, may have grown in gallery forests. On the contrary, palynological records from the Patagonian Late Miocene times are limited, and no confirmed Pliocene material is available so far. Despite the fact that Late Neogene paleoenvironments were not particularly suitable for pollen preservation, some Late Miocene data have been reported from estuarine and shallow marine deposits of northeastern Patagonia. The data here analyzed come from outcrop samples from Rı´o Negro (Barranca Final and El Espigo´n sections, Barranca Final and Rı´o Negro formations) and Chubut (Puerto Pira´mide section, Puerto Madryn Formation) Provinces in northeastern Patagonia (Fig. 1), and represent the southernmost deposits of the so-called ‘‘Entrerriense’’ sea (Guler et al., 2002; Guler, 2003; Palazzesi and Barreda, 2004). The Puerto Madryn Formation was deposited about 10 Ma ago considering 87Sr/86Sr ages of pectinid shells (Scasso et al., 2001). These dates correspond with a previous 40K/40Ar age for a tuff level from the upper part of the marine sequence (Zinsmeister et al., 1981). The Barranca Final Formation was considered as of Middle–Late Miocene age on the basis of foraminifera (Malumia´n et al., 1998). Vertebrate studies from higher levels of the Upper Member of the Rı´o Negro Formation suggested an Early Pliocene, ‘‘Montehermosense’’ South American age for these terrestrial deposits. Other data came from boreholes drilled in the Colorado basin, penetrating into the Barranca Final Formation (Gamerro and Archangelsky, 1981; Quattrocchio and Guerstein, 1988; Guerstein and Quattrocchio, 1988; Guerstein, 1990a–c; Guerstein and Quattrocchio, 1991; Guerstein et al., 1995;

1. Introduction During the Miocene (23.03–5.33 Ma; Lourens et al., 2004), the Patagonian vegetation structure started to strongly resemble that of today. Early Miocene times (23.03–15.97 Ma) were marked by the initial emergence in Patagonia of modern families belonging to Asteraceae (sunflower family) and Poaceae (grasses), in a general context characteristic of the southern forests (Nothofagaceae, Podocarpaceae) (Barreda, 1993, 1996). By Middle–Late Miocene times (15.97– 5.33 Ma) shrubby–herbaceous groups increased consistently in abundance and diversity with widespread arid-adapted communities by the latest Miocene (Guler et al., 2001; Palazzesi and Barreda, 2004). The last demises of some megatherm elements were also recorded at this time. Several forcing factors may have influenced Miocene vegetation trends to their current establishment. The development of the Antarctic Circumpolar Current was of particular importance, which prevented equatorial currents from penetrating into the southern polar regions. The result was the thermal insulation of the Antarctic continent and the development of the major ice sheet on western Antarctica by the Late Miocene (11.61–5.33 Ma). This process caused an abrupt sea-level drop, and a general decrease in moisture availability. Based on global deep-sea oxygen isotope record a long cooling trend was established after the Miocene climatic optimum (17–15 Ma) (Zachos et al., 2001). The mountain building episodes (diastrophic phases) are also thought to have had significant impact on the global climate, having contributed to general increasing aridity and the development of arid-adapted communities. The pattern of changes in continental configuration and ocean circulation coupled with reductions in atmospheric CO2 concentrations led to a decline in global temperatures all through the Neogene (Berner, 1991; McElwain, 1998). Thus, inland areas became progressively arid and wide shelf regions were exposed due to sea-level fall. Pollen is particularly susceptible to oxidation and arid landscapes tend to leave poorer records. Early–Middle Miocene palynological data from Patagonia are relatively abundant. Records come from Rı´o Negro, Chubut, Santa Cruz and Tierra del Fuego provinces (Guerstein and Quattrocchio, 1988; Guerstein, 1990a–c; Barreda, 1993, 1996, 1997a–d; Palamarczuk and Barreda, 1998; Barreda and Palamarczuk, 2000a–c; Guerstein and Guler, 2000;

 2008 ELSEVIER B.V. ALL RIGHTS RESERVED

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Values of 87Sr/86Sr from the Puerto Madryn Formation were determined with a VG-354 mass spectrometer using the multidynamic routine SrSLL (Thirlwall, 1991, in Scasso et al., 2001). These isotopic measurements were made in the Radiogenic Isotope Laboratory at the Royal Holloway Hospital (Scasso et al., 2001). The 40 K/40Ar dates from the Rı´o Negro Formation were based on glass concentrates of a waterlaid tuff containing abundant ripple marks (Zinsmeister et al., 1981).

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3. Palynological Assemblage Composition Palynological assemblages from the Barranca Final, Rı´o Negro and Puerto Madryn formations are integrated by marine (dinoflagellates, prasinophycean algae and foraminiferal linings) and continental (pollen, spores and freshwater algae) palynomorphs. Marine elements show a decreasing trend toward upper levels, while continental palynomorphs prevail.

52°

Tierra del Fuego 0

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300 km

Fig. 1. Location map. Guerstein and Guler, 2000; Guerstein and Junciel, 2001; Guler et al., 2001). The recorded palynofloras are difficult to date with precision, due to the type of material recovered (cutting samples). However, they are adequate for comparison with other palynological data. In spite of the scarcity of suitable pollen data, the main trends of the Late Miocene Patagonian vegetation are sketched in this chapter. 2. Materials and Methods The geological timescale presented by Gradstein et al. (2004) has been followed. The Miocene Epoch, the oldest of the Neogene Period, ranges from 23.03 to 5.33 Ma, with three subseries: Lower (23.03–15.97 Ma), Middle (15.97– 11.61 Ma) and Upper (11.61–5.33) (Lourens et al., 2004). The megatherm (>24C), mesotherm (>14C, <20C) and microtherm (<12C) groups were used to refer plants response to major environmental variables such as light, temperature and precipitation (Nix, 1982). They were adopted instead of tropical, subtropical, warm temperate and cool-cold temperate groups, since these have a geographical connotation as well. Patagonian vegetation trends are sketched in analogy with modern taxa and their habitat preferences. Classification of fossil pollen was based on morphological characteristics (International Code of Botanical Nomenclature, Art. 1.2; Greuter et al., 2000). They may be in part, but not entirely, the reflections of evolution within the groups of plants that produced the pollen. In this report, fossil pollen species are included between brackets in order to be separated from the extant ones.

3.1. Continental Palynomorphs (Fig. 2) Spore pollen assemblages are angiosperm dominated. The most abundant families are Chenopodiaceae, Convolvulaceae Cressa and Asteraceae almost reaching 50% of the continental spectrum. Other families are Malvaceae, Anacardiaceae, Fabaceae and Poaceae. Cyperaceae, Sparganiaceae/Typhaceae and Restionaceae are well represented. Algal content is abundant, particularly Pediastrum colonies that are locally dominant. Botryococcus is subordinate. Gymnosperms are mainly represented by Ephedraceae. On the contrary, Podocarpaceae and Araucariaceae are scarce. Floating ferns belonging to Azollaceae are abundant in some levels. Other pteridophytes and bryophytes are uncommon, being poorly represented by a few species only. Trace amounts of Malphigiaceae (Perisyncolporites pokornyi), Escalloniaceae (Quintiniapollis sp.) and Goodeniaceae (Poluspissusites sp.) are also recognized. According to these data, at least three main vegetation types developed in extra-Andean Patagonia: 1. coastal zone vegetation, characterized by an open landscape occupied by plants adapted to extreme environments; halophytic and xerophytic taxa such as Chenopodiaceae, Convolvulaceae Cressa (Tricolpites trioblatus) and Asteraceae Nassauviinae being specially frequent; 2. freshwater and marginal swamp communities, represented by the presence, and local abundance, of Sparganiaceae, Restionaceae and Cyperaceae. Azollaceae Azolla and Haloragaceae Myriophyllum type (Haloragacidites harrisii) would have also occurred; 3. hinterland vegetation, mainly occupied by shrubby herbaceous taxa; Anacardiaceae, Malvaceae, Ephedraceae and Asteraceae being the major groups. On the contrary, Poaceae are scarce. Asteraceae are well documented in the Patagonian fossil record, and became abundant and diverse during

Late Miocene Continental and Marine Palynological Assemblages (a)

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(b)

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(e)

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(h)

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Fig. 2. Main families developed during the Late Miocene in the extra-Andean Patagonian pollen record. Scale in all figure parts = 5 mm, except in (h) where scale = 1 mm. (a) Asteraceae; (b) Malvaceae; (c) Asteraceae (Nassauviinae); (d) Anacardiaceae; (e) Convolvulaceae (Cressa type); (f) Chenopodiaceae; (g, h) Sparganiaceae – (g) general view, (h) detail of specimen in (g), showing pore and reticulum; (i) Ephedraceae.

the Late Miocene. Among the recorded taxa, several morphotypes identified are related to Mutisieae (Mutisiinae, Nassauviinae), Heliantheae (Ambrosia type) and Barnadesioideae. Some of these taxa, together with Fabaceae, would have led to the development of the sclerophyllous– woodland vegetation as nowadays found in the Chaco region (Mene´ndez, 1971; Romero, 1993). The low frequencies of Podocarpaceae and Nothofagaceae pollen types indicate that these taxa would have grown to considerable distance from the coast. These arboreal pollen grains may have been transported long distances from the west (e.g. the Andean region). Evidence for Late Miocene forests was only recorded from central Chile, at the Boca Pupuya (lat. 33570 S) locality. These floras are characterized by high frequencies of megatherm–mesotherm elements such as Myrtaceae and Lauraceae coupled with lower frequencies of austral–Antarctic components (Villagra´n and Hinojosa, 1997; Hinojosa and Villagra´n, 2005). No direct Late Miocene palynological data from the southern Andean region have been recorded so far, but mesotherm to microtherm forests would have prevailed according to pollen records from the eastern areas in which Nothofagus fusca is the major element.

3.2. Marine Palynomorphs (Fig. 3) Marine palynomorphs are mainly represented by organicwalled dinoflagellate cysts (dinocysts) and subordinate phrasinophycean algae. The Barranca Final Formation (type section) and the Puerto Madryn Formation (Puerto Pira´mide section) bear well-preserved dinocyst assemblages, with dominance of gonyaulacalean cysts, such as Lingulodinium hemicystum, Operculodinium centrocarpum, O. israelianum and Spiniferites spp. Peridinialean cysts are represented by several species of Protoperidinaceae including Selenopemphix dionaeacysta, S. quanta, Brigantedinium cariacoense, B. simplex and Lejeunecysta spp. High diversity of these species characterizes inner neritic environments (Head et al., 1989; Head and Westphal, 1999). Outer neritic and oceanic species of Impagidinium and Nematosphaeropsis are scarce. Assemblages from the Lower Member of the Rı´o Negro Formation (Espigo´n section) are dominated by protoperidiniacean dinoflagellate cysts including protoperidinioid and diplopsalioid species, the latter represented by the genera Dubridinium. Late Miocene–Early Pliocene deposits from the Barranca Final Formation were also documented in different

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Fig. 3. Dinoflagellate cysts from Barranca Final and Rı´o Negro formations. All digital images were taken using interference contrast. Scale in all figure parts = 10 mm. (a) Brigantedinium sp., dorsal view, high focus; (b, c) Dubridinium sp.; apical view, high focus; (d) Labyrinthodinium truncatum subsp. truncatum Piasecki, lateral view, intermediate focus; (e, f) Lejeunecysta spp., dorsal view, high focus; (g–i) Lingulodinium hemicystum McMinn, (g, h) antapical view, intermediate focus and cross section, respectively. (i) ventral view, high focus, showing the sulcal tab; (j) Operculodinium centrocarpum (Deflandre and Cookson) Wall, ventral view, intermediate focus; (k) Reticulatosphaera actinocoronata (Benedek) Bujak and Matsuoka, general view; (l) Selenopemphix quanta (Bradford) Matsuoka, antapical view, high focus; (m–o) S. dionaeacysta Head et al., (m) antapical view, high focus, (n, o) antapical view, high and intermediate focus, respectively; (p–r) S. brevispinosa subsp. brevispinosa Head et al., (p) antapical view, high focus, (q, r) apical view, high and cross section, respectively; (s) S. nephroides (Benedek) Bujak in Bujak et al., general view, (t) Tuberculodinium vancampoae Wall, antapical view, high focus.

wells in the Colorado basin. Dinoflagellate cysts from this interval are scarce and poorly diversified, mainly represented by Spiniferites species and the O. centrocarpum/ israelianum complex. The Peridiniales order is only represented by Lejeunecysta sp. and Barssidinium sp. The base of this interval is indicated by a last maximum of dinocysts, and an abrupt change in the palynological assemblage composition. This bioevent, widely documented in the YPF Ranquel (Ra x-1), DNGM Puerto

Belgrano 20, YPF Ombucta x-1 and Cx-1 wells was correlated with the base of the Palinozona A of Gamerro and Archangelsky (1981) (Guerstein, 1990b; Guerstein and Guler, 2000; Guler et al., 2001). Although most of the identified dinocyst species are long-ranging, assemblages are consistent with a Late Neogene age. In outcrop sections Labyrinthodinium truncatum subsp. truncatum, Selenopemphix dionaeacysta, S. brevispinosa subsp. bresvispinosa, Brigantedinium

Late Miocene Continental and Marine Palynological Assemblages cariacoense and B. simplex constitute biostratigraphical markers, which constrained the deposits to the Mid/Late Miocene to Early Pliocene age interval, according to Williams et al. (1998) and Rochon et al. (1999). Most of the data from the Colorado Basin derive from subsurface material (cutting samples) and the biostratigraphical control of wells is based on last occurrences of selected dinoflagellate cysts. Following the confirmed stratigraphical ranges by Williams et al. (1998), the last occurrences of Reticulatosphaera actinocoronata, Labyrinthodinium truncatum subsp. truncatum, Hystrichosphaeropsis obscura, Dapsillidinium pseudocolligerum and Cleistosphaeridium (as Systematophora) placacanthum together with a significant change in the palynological composition, contributed to assign a possible Late Miocene–Early Pliocene age range for the uppermost deposits in Colorado Basin (Guerstein and Guler, 2000; Guerstein and Junciel, 2001).

4. Discussion and Conclusions The Neogene was a time of development of the modern vegetation in southern South America. Early Miocene floras were modified by the addition of some taxa from northern South America [Euphorbiaceae Alchornea, Fabaceae Caesalpinoideae Caesalpinea, Malphigiaceae (Perisyncolporites pokornyi)], mixing with those of Australasian affinity [Escalloniaceae Quintinia, Fabaceae Mimosoideae Acacia, Casuarinaceae Casuarina, Goodeniaceae (Poluspissusites sp.)]. Available data indicate that a marked contrast between coastal and inland communities was established by the Late Miocene. The pattern of changes of the Patagonian landscape was related to the restriction of forests to the west and the wide spread of herbaceous and shrubby vegetation to extra-Andean Patagonia. This physiognomic shift may have been influenced by several forces; the final uplift of the Patagonian Andes, which took place ca. 10 Ma ago (Rabassa et al., 2005), would have been the most determinant factor. As the palynological record is incomplete (scarcity, poor age control and quality of the data available), the accurate time when the vegetation structure became close to the modern one is still uncertain. Nevertheless, it was during the Late Miocene when nonarboreal vegetation went through progressive adaptations to the new environmental conditions. In this context, xerophytic shrubby–herbaceous taxa increase in abundance and diversity (Chenopodiaceae, Convolvulaceae Cressa, Ephedraceae Ephedra, Asteraceae, Anacardiaceae). However, local environments would have still permitted the development of some megatherm elements of northern South America (Malphigiaceae (P. pokornyi), Fabaceae Caesalpinoideae Caesalpinea), which no longer occur in extra-Andean Patagonia. Other taxa currently restricted to Australasian regions [Escalloniaceae Quintinia, Fabaceae Mimosoideae (Acaciapollenites myriosporites type)] were also reported. The regional extinction of these floristic elements would have surely been induced by both global and local climatic changes. The significant diversity of the Asteraceae recognized in these assemblages certainly suggests that relatively

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modern communities were developed in these southern latitudes by Late Miocene times. The occurrence of Mutisieae (Mutisiinae, Nassauviinae), and even species from Heliantheae (Ambrosia type) and Barnadesioideae, along with other related families (Goodeniaceae, Lythraceae – previously reported from Patagonian Early Miocene sediments), support a major diversification of the Asteralean group in southern South America. Poaceae form a stenopalynous family and, therefore, its diversity has been difficult to measure considering pollen evidence. The low grass pollen occurrences reported from the analyzed sequences do probably not mean that the family was underrepresented. Even in recent grassland environments, grass pollen percentages are limited (Leopold et al., 1992). The C4 metabolism concentration is another means for identifying grassdominated communities (Jacobs et al., 1999). C4 photosynthetic pathway is the most common among the Poaceae. Available data from carbon isotope ratios of fossil tooth enamel indicate that during Late Miocene there was a global expansion of the C4 photosynthetic pathway in South America (Latorre et al., 1997), representing widespread grass-dominated communities. The palynological evidence of the Late Miocene vegetation outlined in this chapter is preliminary. It is likely to be modified and improved by future research, providing new data about the floristic patterns of change during Neogene times in these southern latitudes. Spores and pollen content reflects the type of vegetation growing in the vicinity of the depositional basin, whereas dinocyst assemblages contribute to determine changes in superficial water conditions. The dominance of O. centrocarpum/israelianum complex, species of Spiniferites, Lingulodinium machaerophorum, L. hemicystum and Tuberculodinium vancampoae in dinocyst assemblages, together with the presence of freshwater algae and the low representation or absence of oceanic species, indicates that Late Miocene deposits accumulated in marine shallow waters near the shoreline. High relative frequencies and diverse representation of protoperidiniaceans, which characterize assemblages from northeastern Patagonian outcrops, are related to rich-nutrient shallow waters. The round brown protoperidiniacean cysts are linked to the modern genus Protoperidinium, heterotrophic dinoflagellates that are abundant in environments associated with high nutrient levels like coastal regions (Head, 1993, 1996, 1998). The most distinctive bioevent recorded by the end of the Miocene in different wells in the Colorado Basin is defined by the simultaneous disappearance of numerous dinocyst species along with changes in the palynological composition, mainly evidenced by a marked decrease in the dinocyst/sporomorphs ratio. This paleoenvironmental change was considered a response to the drop of sea level occurring during the Late Miocene, and correlated to the last stage of the ‘‘Entrerriense’’ transgression (Guerstein and Guler, 2000; Guerstein and Junciel, 2001; Guler et al., 2001). The common and consistent presence of S. quanta, L. machaerophorum and T. vancampoe suggests warm temperate to warm surface water conditions, in accordance with the distribution of these species in modern

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sediments (Wall et al., 1977; Edwards and Andrle, 1992; Head, 1996, 1997; Londeix et al., 1999). According to Edwards and Andrle (1992), the highest relative abundance of T. vancampoae and L. machaerophorum indicates winter sea-surface temperature above 15–16C and summer sea-surface temperature above 27C.

Acknowledgements Support was provided by Consejo Nacional de Investigaciones Cientı´ficas y Te´cnicas, Argentina (PIP 5001), and Agencia Nacional de Promocio´n Cientı´fica y Tecnolo´gica, Argentina (PICT 32344).

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Leopold, E.B., Liu, G. and Clay-Poole, S. (1992). Lowbiomass vegetation in the Oligocene? In: Prothero, D.R. and Berggren, W.A. (eds), Eocene-Oligocene Climatic and Biotic Evolution, Princeton Univ. Press, New Jersey, 399–420. Londeix, L., Benzakour, M., deVernal, A. et al., (1999). Late Neogene dinoflagellate cyst assemblages from the Strait of Sicily, Central Mediterranean sea: Paleoecological and biostratigraphical implications. In: Wrenn, J.H., Suc, J.P. and Leroy, S.A.G. (eds), The Pliocene: Time of change; American Association of Stratigraphic Palynologists Foundation, 65–91. Lourens, L., Hilgen, F., Shackleton, N.J. et al., (2004). The Neogene Period. In: Gradstein, F.M., Ogg, J.G. and Smith, A.G. (eds). A Geologic Time Scale, Cambridge University Press, 409–440. Malumia´n, N., Suriano, J.S. and Cobos, J.C. (1998). La Formacio´n Barranca Final en su localidad tipo, Mioceno, Cuenca del Colorado. Actas del 10 Congreso Latinoamericano de Geologı´a y 6 Congreso Nacional de Geologı´a Econo´mica, Actas 1, 125–130. Buenos Aires. Malumia´n, N., Palamarczuk, S., Alonso, S. et al., (1999). Micropaleontologı´a, palinologı´a y sedimentologı´a del Eoceno-Mioceno del pozo Aries x-1. Plataforma continental de Tierra del Fuego. 14 Congreso Geolo´gico Argentino, Actas 1, 369–371. Salta. McElwain, J.C. (1998). Do fossil plants signal palaeoatmospheric CO2 concentration in the geological past? Philosophical Transactions of the Royal Society of London, Series B 353, 1–15. Mene´ndez, C.A. (1971). Floras Terciarias de la Argentina. Ameghiniana 8, 357–371. Buenos Aires. Nix, H. (1982). Environmental determinants of biogeography and evolution in Terra Australis. In: Barker, W.R. and Greenslade, P.J.M. (eds), Evolution of the Flora and Fauna of Arid Australia, 47–66. Frewville. Palamarczuk, S. and Barreda, V.D. (1998). Bioestratigrafı´a de dinoflagelados de la Formacio´n Chenque (Mioceno), provincia del Chubut. Ameghiniana 35, 415–426. Buenos Aires. Palamarczuk, S. and Barreda, V.D. (2000). Palinologı´a del Paleo´geno tardı´o-Neo´geno temprano, Pozo Aries x-1, plataforma continental argentina, Tierra del Fuego. Ameghiniana 37, 221–234. Buenos Aires. Palazzesi, L. and Barreda, V. (2004). Primer registro palinolo´gico de la Formacio´n Puerto Madryn, Mioceno de la provincia del Chubut, Argentina. Ameghiniana 41, 355–362. Buenos Aires. Quattrocchio, M. and Guerstein, G.R. (1988). Evaluacio´n paleoambiental y paleoclima´tica del Terciario de Cuenca del Colorado, Repu´blica Argentina. Palinofloras. Asociacio´n Geolo´gica Argentina, Revista 43, 375–387. Buenos Aires. Rabassa, J., Coronato, A.M.J. and Salemme, M. (2005). Chronology of the Late Cenozoic Patagonian glaciations and their correlation with biostratigraphic units of the Pampean region (Argentina). Journal of South American Earth Sciences 20, 1–2, 81–104. Rochon, A., de Vernal, A., Turon, J.L. et al., (1999). Distribution of recent dinflagellate cysts in surface sediments from the North Atlantic Ocean and adjacent

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17 Late Quaternary Vegetation and Climate of Patagonia Marı´a Virginia Manzini1, Aldo R. Prieto1,2, Marta Mercedes Paez1 and Frank Scha¨bitz3 1

Laboratorio de Paleoecologı´a y Palinologı´a. Departamento de Biologı´a, Facultad de Ciencias Exactas y Naturales, Universidad Nacional de Mar del Plata, Funes 3250, 7600 Mar del Plata, Argentina. 2 CONICET. 3 Seminar fu¨r Geographie und ihre Didaktik, Universita¨t zu Ko¨ln, Gronewaldstr. 2, D-50931 Ko¨ln, Germany.

west (5– 6 C). Winter (July) mean temperature distribution is much more regular than summer (January) temperature. Latitudinal and altitudinal distribution of vegetation in Patagonia is mainly related to these climatic conditions and associated with geomorphologic and edaphic characteristics. The main plant formations in north–south direction are (Fig. 1): xerophytic woodlands of the Espinal, Monte xerophytic shrublands and the Patagonian steppe. Westward, south of 37 S, the grass steppe and Subantarctic forests are present (Cabrera, 1976; Soriano et al., 1983; Leo´n et al., 1998; Roig, 1998). Northeastward the mosaic of xerophytic woodlands of the Espinal extends, with Prosopis caldenia, P. flexuosa, Prosopidastrum angusticarpum, P. striatum, Monttea aphylla, Atamisquea emarginata, Discaria longispina, Schinus longifolia, Condalia microphylla and Geoffrea decorticans. In the Monte Formation, south of the Rı´o Colorado, vegetation is characterized by Larrea shrubs associated with open thickets of Prosopis alpataco. Azonal vegetation of these arid and semiarid formations is represented by Chenopodiaceae (Atriplex lampa, Suaeda divaricata and Sarcocornia perennis) that dominate on saline soils of endorheic basins (‘‘bajos’’), some of them below sea level. Patagonian steppe extends over the plateau with (1) the semidesert of Nassauvia shrubs (N. glomerulosa, N. axillaris), Chuquiraga aurea, Ephedra frustillata, with dominant Stipa humilis among grasses; (2) in the southernmost area, the semidesert is dominated by tussock grasses or ‘‘coirones’’ (Stipa speciosa, S. humilis and S. chrysophylla) with a low-shrub physiognomy (N. glomerulosa, Ephedra frustillata, Azorella caespitosa and patches of Junellia tridens) on the highest parts of the plateau; and (3) on naturally disturbed areas, such as alluvial plains, flat or rolling, rocky slopes, valley bottoms and ‘‘can˜adones’’ (dryland, wadi-like valleys), a shrub steppe composed of open thickets of Asteraceae subf. Asteroideae (Senecio filaginoides and Nardophyllum obtusifoluim), Junellia tridens, Lycium chilense and Berberis heterophylla develops. In the westernmost parts of Patagonia, from 37 to 51 S grass steppe and Subantarctic forests extend. The grass steppe covers a narrow and discontinuous strip along the Andes and the plateau, and is dominated by Festuca pallescens, cushions plants and isolated Asteraceae subf. Asteroideae, Anarthrophyllum rigidum and Berberis heterophylla shrubs. In the southern strip of Patagonia, the grass steppe is dominated by Festuca

1. Introduction General knowledge of the Late Cenozoic landscape evolution of southern Patagonia has recently been increased by detailed morphological and chronostratigraphical investigations (e.g. Wenzens et al., 1996, 1997; Wenzens, 2002; Glasser et al., 2004). Over the last years, reconstructions of quantitative climatic parameters use multivariate statistics on the basis of correlations between modern pollen spectra and recent climate data (Paez et al., 2001; Markgraf et al., 2002; Scha¨bitz, 2003; Prieto et al., 2004), with the aim to quantitatively reconstruct climatic conditions in Patagonia during the Late Quaternary. There are numerous pollen records from southern South America showing different vegetation and climatic histories for different latitudes (Markgraf, 1991, 1993; Heusser, 2003; Mancini, 2003; Mancini et al., 2005, among others). In Patagonia, most pollen records are from postglacial (Holocene age) and only a few sequences represent the Middle and Late Pleistocene. This chapter is a compilation of existing pollen datasets from Patagonia, between 37 and 52 S covering the Late Quaternary and a comparison with selected pollen data from Chile (Table 1).

2. Modern Climate and Vegetation The basins of Rı´o Colorado and Rı´o Negro (38–39 S) mark the limit between subtropical summer rains and mid latitude winter rains (Prohaska, 1976). North of 40 S, the topographic barrier of the Andes disrupts this dominant zonal flow and the circulation over subtropical South America is under the influence of two large semi-permanent anticyclones. The South Pacific anticyclone dominates the climate of the coastal region of western South America. On the Atlantic side, the southeast trade wind circulation, associated with the Atlantic Subtropical anticyclone, carries abundant moisture to the central region of the continent east of the Andes. Prevailing westerlies, associated with a strong meridional pressure gradient are the dominant circulation systems of South America south of 40 S resulting in high wind speeds throughout the year. South of 40 S, rainfall decreases from west to east, showing one of the strongest precipitation gradients in the world. Mean annual temperature on the Patagonian plateaus is 8–10 C and decreases toward the

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DEVELOPMENTS IN QUATERNARY SCIENCES VOLUME 11 ISSN 1571-0866 351

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Table 1. Late Quaternary pollen records of Patagonia and selected pollen records from Chile (37– 52 S) mentioned in the text. Site No.

Site name

Latitude (S)

Longitude (W)

References

1 2 3 4 5 6 7 8 9

Vaca Lauque´n Salinas Chicas 1 Salinas Chicas 4 Paso del Arco Salina Anzoa´tegui Bajada de Rahue Rucan˜ancu Rı´o Malleo Mallı´n Aguado 1 (core 2)

36500 38440 38450 38520 39000 39220 39330 39360 40000

71050 62560 62550 71040 63460 70560 71180 71240 71290

10 11 12 13 14 15 16 17 18 19 20 21

Epulla´n Grande Salina Gualicho 2 Laguna Indio Muerto Salina Gualicho 1 Salina Piedra Salina Ingle´s Cueva Traful I Canal de la Puntilla El Tre´bol Lago Moreno (Morenito) Lago Escondido Laguna Cari Laufquen Chica Fundo Llanquihue Fundo Nueva Braunau Mallin Book Lago Mascardi-Gutie´rrez Alerce Huelmo Lago Condorito Rı´o Negro Taiquemo´ Dalcahue Campo Moncada 2 Mayol RC12-241 V15-142 Penı´nsula Taitao Alero Ca´rdenas Los Toldos Piedra Museo Parque Nacional Perito Moreno La Martita Te´mpano Sur Lago Cardiel Puerto Ede´n El Sosiego Can˜ado´n El Mosquito Charles Fuhr Cerro Frı´as Chorrillo Malo 2 Cerro Verlika 1 Torres del Paine Cueva Las Buitreras

40230 40240 40250 40260 40350 40410 40430 40560 41000 41030 41050 41130

70120 65110 66040 65110 62400 62300 71070 72540 71000 71310 72340 69250

41130 41170 41200 41200 41230 41310 41450 42030 42100 42200 42300 42380 43280 44530 46250 47180 47220 47530 47530

73030 73040 71350 71300 72520 73000 73070 73490 73350 73390 69300 73450 57400 51320 74240 70260 68580 67520 72510

Markgraf, 1987 Scha¨bitz, 1999 Scha¨bitz, 1999 Heusser et al., 1988 Scha¨bitz, 1994 Markgraf et al., 1986 Heusser, 1984, 2003 Heusser et al., 1988 Markgraf and Bianchi, 1999; Markgraf et al., 2002 Prieto and Stutz, 1996 Scha¨bitz, 1999 Scha¨bitz, 1991, 1999 Scha¨bitz, 1999 Scha¨bitz, 1994 Scha¨bitz, 1999 Heusser, 1993a Moreno et al., 2001 Bianchi, 1999, Bianchi et al., 1999 Markgraf, 1984 Bianchi, 2000 Scha¨bitz and Liebricht, 1998; Scha¨bitz, 1999 Heusser et al., 1999a Heusser et al., 1999a, 2000 Markgraf, 1983 Markgraf, 1983; Bianchi, 1999 Heusser et al., 1999a Moreno et al., 2001 Moreno 2000, 2004 Villagra´n, 1988 Heusser et al., 1999a,b Heusser et al., 1999a Paez, 1991 Heusser et al., 1999a Heusser and Wingenroth, 1984 Groot et al., 1967 Lumley and Switsur, 1993 Mancini, 1998 Paez et al., 1999; Prieto et al., 2002 Borromei, 2003 Mancini et al., 2002

48240 48440 48480 49080 50090 50110 50160 50260 50300 50360 50590 51070

69150 74020 71130 74250 72330 71200 71520 72430 72400 72170 72400 70160

Mancini, 1998 Ashworth and Markgraf, 1989 Markgraf et al., 2003 Ashworth et al., 1991 Mancini, 2002 Scha¨bitz and Schellmann, 1999 Mancini, 2002 Franco et al., 2004 Mancini, 2002 Mancini, 2001, 2002 Heusser, 1995 Prieto et al., 1998

22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52

Late Quaternary Vegetation and Climate of Patagonia

353

Table 1. (Continued) Site No.

Site name

Latitude (S)

53 54 55 56 57

Meseta Latorre 1- 2 Cueva Lago Sofı´a 1 Cueva Mylodon Cueva Markatch Aike Laguna Potrok Aike

58 59 60 61 62 63

Cueva Don Ariel Cueva Fell Laguna Azul Magallanes maar Rı´o Rubens Cabo Vı´rgenes

Longitude (W)

References

51310 51320 51350 51530 51580

72030 72350 72380 69370 70230

52000 52040 52050 52070 52080 52190

70090 69070 69350 69160 71520 68230

Scha¨bitz, 1991 Prieto, 1991 Markgraf, 1993; Heusser et al., 1994 Borromei and Nami, 2000 Zolitschka et al., 2004; Haberzettl et al., 2005 Borromei and Nami, 2000 Markgraf, 1988 Scha¨bitz et al., 2003; Corbella et al., 2000 Huber et al., 2004 Franco et al., 2004; Mancini, 2006

300 N

600

50

0

Rio Colorado Lim

Rio

Ne

gro

O

ce

an

200

Pa c i f i c

ic

Rio Chubut

At

la

nt

42°

Chil

Ocean

e

ay

Rio

Lago Cardiel

200

References

600

Espinal (xerophytic woodlands) Monte (xerophytic shrublands)

52°

30 0

Patagonian steppe Grass steppe Subantarctic forests Upper Andean (grass and shrub steppe) 300 isohyet (mm)

200

0

200 km 72°

62°

Fig. 1. Map of major Patagonian plant formations (modified from Cabrera, 1976; Soriano et al., 1983; Roig, 1998) and isohyets (mm). gracillima (Boelcke et al., 1985). Nothofagus is the main component of the Subantarctic forest formation that extends along the eastern fringe of the Andes, from ca. 37 S to Tierra del Fuego in a narrow strip which is interrupted south of 45 S. In the northern

sector, the forest constitutes open and discontinuous stands composed of N. pumilio, N. antarctica and N. obliqua. Between 40 and 41 S, a xerophytic forest dominated by Austrocedrus chilensis in the east of the foothills is associated with the evergreen N. dombeyi

354

Marı´a Virginia Manzini et al.

more to the west. South of 47 S, the forest is impoverished due to the low temperatures of the growing season. Deciduous forest is represented by N. pumilio which defines the upper timberline, and by N. antarctica that forms the lower limit in the forest steppe ecotone. Large-scale human impact on the vegetation began with major settlement of European colonists only ca. 150 yrs ago. Intensive overgrazing led to severe changes in floristic composition, physiognomy and plant cover in some areas of the extra-Andean regions, especially in the grass steppe. At the eastern foot of the Andes settlers extensively burned forest areas to create grazing land as shown by an increase in grass steppe species. Strong anthropogenic disturbance in different plant formations of Patagonia is also evident by overrepresentation of certain taxa (e.g. Brassicaceae, Asteraceae subf. Cichorioideae, Rumex and Plantago) in modern pollen spectra (Mancini, 1993, 1998; Scha¨bitz, 1999; Paez et al., 2001).

3. Vegetational History and Climate Reconstruction Results and interpretation presented here are based on palynological sequences (Table 1, Figs 2 and 3) from

different deposits such as lakes, peat, caves and archeological sites. The genus Nothofagus is represented by the N. dombeyi type (Heusser, 1971). Dates are presented in uncalibrated radiocarbon years (14C yr BP), except where indicated. Up to now, no continuous pollen records exist from the Patagonian region that expand throughout the Holocene and back to the penultimate interglacial. The only records that illustrate cyclical fluctuations between glacial and interglacial environments at the latitude of Patagonia are from deep sea sediments from the Argentine Basin (e.g. sites 34, 35, Groot et al., 1967; Heusser and Wingenroth, 1984). The interpretation of the vegetation and climatic change from these pollen records is speculative, because of low age control. The records show essentially two types of zones, which alternate with depth in the cores. One type is characterized by low percentages of Chenopodiaceae and Ephedra pollen and, high percentages of Pteridophytes spores (up to 50%) and somewhat higher than average percentages of arboreal pollen. This zone is attributed to interglacial periods. The second type is correlated with glacial periods and is characterized by relatively higher percentages of Chenopodiaceae and Ephedra taxa, and low percentages of spores and arboreal pollen.

Rio Colorado

6 9

17 18 22/23 24 26/27/28 30/31 29 33

Rio

ay

Rio

Lim

20 25

Ne

gro

34

At

la

Rio Chubut

ic

O

ce

an

21

nt

42°

Pa c if ic O

cean

Chile

N

35

36 39

38

Lago Cardiel

42/44 46

51 54/55

61

52°

62

59

1

Reference Fossil pollen record

200

0

200 km

72°

Fig. 2. Locations of Pleistocene fossil pollen records (see Table 1).

62°

Late Quaternary Vegetation and Climate of Patagonia 1

Chile

N

RioRio Colorado Colorado

4 ay

2/3

5

Rio

N 12 11 /13 egro 14

Lim 9 Rio 10 16 18/19 17 20 21 24 27/28 25

ce

an

15

ic

O

32

nt

Rio Chubut

At

la

42°

Pa c if ic O

cean

7/8

355

36 37

40

Lago Cardiel Lago Cardiel

44

52°

38

41

43

45 51

39

47 49 50 57/60/61 53 56/58 54 52 63

48

1

200

0

Reference Fossil pollen record

200 km

72°

62°

Fig. 3. Locations of Holocene fossil pollen records (see Table 1).

3.1. Middle Pleistocene

3.2. Late Pleistocene

At 50 S in the Patagonian semidesert of the upper Rı´o Santa Cruz valley, about 60 km east of today’s dense Subantarctic forest, a fossil bog sequence (Can˜ado´n El Mosquito, site 46) above a moraine has been studied by Scha¨bitz and Schellmann (1999) and it very possibly belongs to the interglacial (ca. 500 ka) as based on stratigraphical and geomorphological evidence (Schellmann, 1998). The pollen record is dominated by arboreal pollen taxa, mainly Podocarpus, and a lesser amount of Nothofagus, N. obliqua-type and Cupressaceae. Nonarboreal taxa do not exceed 10% of the total. Pollen percentages and concentration values suggest a dense forest. A forest of such composition does not exist in Patagonia today and indicates a more humid and warmer climate than the one that predominates in the recent semidesert. At present N. obliqua develops only between 38 and 40 S, Podocarpus nubigenus is found between 39 and 50 S in Chile and at 41 S in Argentina, accompanied by N. betuloides, N. dombeyi and Fitzroya cupressoides. P. nubigenus grows best in humid areas with high rainfall, mild winters and cool summers. In Chile, this conifer can nowadays be found in an area with an annual precipitation range from less than 1000 mm in the north (Mediterranean climate type) to more than 4000 mm in the southern Magellanic rainforests.

On the Argentine side, two pollen sites in Patagonia at 39 S (Bajada de Rahue, site 6) and 52 S (Magallanes maar, site 61) record pre-full glacial times. Bajada de Rahue is a discontinuous lacustrine section located near the Nothofagus forest-steppe ecotone and encompasses the interval 27,900 +1200–32,600 + 1500 yrs BP (Markgraf et al., 1986). The fossil pollen assemblages reflect a local sedge-marsh/shallow pond environment surrounded by steppe-scrub and suggest a precipitation and temperature pattern similar to the modern one, but precipitation was higher than during the full-glacial (Marine Oxygen Isotope Stage – MIS 2) at this latitude. The Magallanes maar, in the southern part of Patagonia, is a core drilled at a dry maar located in the Patagonian grass steppe and 14 C dated between 31,560 + 480 yrs BP at 37 m and > 51,700 yrs BP at 47 m (Corbella, 2002). Only 12 samples between 29 and 56 m core depth contain pollen. The pollen assemblages are dominated by Poaceae, Asteraceae and Empetrum reflecting a grass steppe environment (Corbella et al., 2000). The climate conditions during the interstadial (?) of MIS 3 are interpreted as colder and more humid than today. On the Chilean side of the Andes, vegetation reconstructed on Isla Grande de Chiloe´, during Middle–Late Llanquihue glaciations and dated between > 50,000 and

356

Marı´a Virginia Manzini et al.

10,000 14C yr BP at Taiquemo´ (site 30) consisted initially of Subantarctic evergreen forest under cool, humid interstadial climate. The forest developed optimally at > 47,000 yrs BP (MIS 4), but was later stepwise reduced in extent, so that by 26,000 yrs BP (early MIS 3) under increasingly cold and humid conditions, it became replaced by Subantarctic parkland through a series of stadials and interstadials. With no modern analog, Subantarctic parkland appears to be driven by conditions not unlike those prevailing in present-day Magellanic moorland, where the Valdivian evergreen forest regionally exists today (Heusser et al., 1999a, b, 2000; Heusser and Heusser, 2006). Three warm interstadial episodes are recorded between 57,000–>49,000 yrs BP, 50,000–>47,000 yrs BP and 45,000–35,000 yrs BP (Denton et al., 1999). At >35,000 yrs BP, mean summer temperatures were estimated as 2–3 C lower than at present, while during stadials between 25,000 and 21,000 yrs BP temperatures were as much as 8 C lower than today. After 30,000 and before 17,500 yrs BP, pollen assemblages are indicative of a cold and wet climate (Heusser et al., 2000; Heusser, 2003). Interstadial pollen records are comparable to those from deposits with conifer trunks of Isla Tenglo (41300 S, 72580 W), Punta Pirque´n (42120 S, 73210 W) and Molulco (42510 S, 73440 W) as well as other pollen records from the Chilean Lake District and Isla Grande de Chiloe´ from the same time interval (Villagra´n et al., 2004). During the stadials, between 30,000 and 14,000 yrs BP (MIS 3–2), at Taiquemo´ (site 30) as at Fundo Nueva Braunau (site 23) and Rı´o Negro (site 29) a mosaic of Magellanic tundra with Nothofagus and conifer thickets developed, suggesting cold conditions (Heusser et al., 1999a, b, 2000; Villagra´n et al., 2004). Isla Grande de Chiloe´ apparently was a refuge for a number of tree species under conditions that match the present forest environment at 48–54 S in southern Chile. In general, glacial pollen spectra represent a mosaic of north Patagonian forest and Magellanic moorland which is found today at the highest parts of the mountains in the Chilean Lake District and Channels Region. These pollen records suggest altitudinal and latitudinal forest displacements during glaciation and show a decrease in temperature and an increase in precipitation in these regions (Villagra´n et al., 1995).

Full and Late Glacial East of the Andes the only pollen record that provides evidence of full-glacial conditions is Mallı´n Aguado (site 9, Fig. 4) at 40 S (Markgraf and Bianchi, 1999; Markgraf et al., 2002). This record that dates back to ca. 17,000 yrs BP is located in the present N. dombeyi and A. chilensis mixed forest. The pollen record represents a grass–shrub steppe with disturbance indicators before 16,000 yrs BP suggesting climatic conditions drier and cooler than today. From 16,000 yrs BP Poaceae dominance and mire vegetation indicate climatic conditions milder than the previous period. After 14,000 yrs BP, a mesic forest indicates an increase in precipitation

(Markgraf and Bianchi, 1999). Pollen assemblages of similar composition were recorded at 41 S (El Tre´bol, Lago Escondido, Lago Mascardi–Gutie´rrez; sites 18, 20, 25) associated with rapid forest expansion (Bianchi et al., 1999; Bianchi, 2000). Other pollen records from the eastern part of the Andes only represent Late Glacial times. In the Patagonian shrub steppe, the sediment record of Laguna Cari Laufquen Chica (site 21) (Scha¨bitz and Liebricht, 1998; Scha¨bitz, 1999) indicates that the lake never dried out for long periods before 12,700 yrs BP. Sedimentation occurred during the Late Glacial, indicating slowly rising but still cooler temperatures than today. Possibly during the end of the Late Glacial (after 12,700 yrs BP), high amounts of clay and the appearance of Myriophyllum pollen seem to indicate a huge lake area. According to regression pollen models (Scha¨bitz, 2003), slightly wetter conditions occurred with annual precipitation values between 210 and 340 mm. The regional vegetation was dominated by Poaceae and typical elements of the Patagonian steppe, especially Nassauvia. This suggests the presence of still low temperatures. At ca. 9600 yrs BP, the characteristic Patagonian steppe vegetation was present. Chenopodiineae pollen values increased and aquatics were reduced, very possibly in relation to lower lakelevel (Scha¨bitz and Liebricht, 1998; Scha¨bitz, 1999). In the Patagonian steppe, the pollen sequence at Los Toldos (site 38, Fig. 5) is showing a steppe with Ephedra and indicates that environmental conditions were extremely arid with precipitation lower than 200 mm between 12,600 and 11,000 yrs BP (Paez et al., 1999). At the nearby rockshelter of Piedra Museo (site 39), a shrub steppe with Asteraceae subf. Asteroideae and halophytic vegetation developed (Borromei, 2003). A desiccation phase was dated prior to 11,200 yrs BP probably covering the period of the Last Glacial Maximum (LGM) in the Lago Cardiel record with the lakelevel below 75 m (Gilli et al., 2001). Between 11,000 and 10,000 yrs BP, a grass steppe with Ephedra and dwarf shrubs (sites 38, 39) resembles the patches of grass in the semidesert situated on plateaus above 700 m. It suggests that effective moisture increase was probably related to an increase in precipitation under cold and semiarid conditions (Prieto et al., 2002). Starting at ca. 10,000 yrs BP with the Asteraceae shrub steppe expansion, temperature increased while water availability decreased (Paez et al., 1999). With only minor local differences, the paleoenvironmental sequence derived from pollen data is similar for all records from southernmost Patagonia at latitudes south of 50 S (Moore, 1978; Markgraf, 1993; Heusser et al., 2000; Pendall et al., 2001). Before 11,000–10,000 yrs BP, Poaceae are dominant indicating a humid grass steppe between 51 and 52 S (Torres del Paine, Cueva Lago Sofı´a 1, Cueva Mylodon, Cueva Fell and Rı´o Rubens; sites 51, 54, 55, 59, 62). According to Markgraf (1993), this suggests a decrease in wind intensity, an increase in moisture availability and temperature in relation to conditions prevailing before 12,500 yrs BP. At the western slope of the Andes, the Fundo Llanquihue pollen record (site 22) shows short-term pulses of Nothofagus at ca. 20,000 yrs BP, which

Late Quaternary Vegetation and Climate of Patagonia

357

Fig. 4. Fossil pollen diagram (%) of Mallı´n Aguado (cores 1 and 2, 40000 S; 71290 W, from Markgraf and Bianchi (1999)). Reprinted from Paleoenvironmental changes during the last 17,000 years in western Patagonia: Mallı´n Aguado, province of Neuque´n, Argentina, Bamberger Geographische Schriften 19: 175–193, with permission from Fach Geographie an der Universita¨t Bamberg.

ultimately increased to dominance without interruption until high Poaceae frequencies at 16,700 yrs BP. Climate was moderate but cool during this period (Heusser et al., 1999a). The Dalcahue pollen record (site 31) provides data for the glacial history of Isla Grande de Chiloe´. At ca. 21,000 yrs BP, Poaceae is codominant with Nothofagus under apparently colder conditions of developing Subantarctic parkland (Heusser et al., 1999a). Temperate evergreen rainforest has dominated the lowlands of the Chilean Lake District over the past 15,000 yrs. Discrepancies about the timing, frequency and direction of climate changes during the last termination in the Chilean Lake District and Isla Grande de Chiloe´ have led to proposals for a single-step warming at 14,000 yrs BP (Hoganson and Ashworth, 1992), a twostep model with a warming event at about 12,500 yrs BP, followed by climate cooling starting at 11,000 yrs BP

(Heusser, 1981) and a multi-step model with a warming event at about 13,900 yrs BP and as many as three cooling events starting 11,000–13,000 yrs BP (Heusser, 1993b; Heusser et al., 1995, 1996; Moreno, 2000). Pollen records from Canal de la Puntilla, Huelmo and Lago Condorito (sites 17, 27, 28) indicate the predominance of closed-canopy North Patagonian forest under a temperate and humid climate, suggesting conditions approaching modern climate in the foothills of the mountain ranges at about 41 S between ca. 13,000 and 12,200 yrs BP. The expansion of Podocarpus at ca. 12,200 yrs BP and the persistence of rainforest vegetation are consistent with an onset of cooler conditions, as suggested by the latitudinal and altitudinal distribution of conifers in the modern North Patagonian forest. This was followed by a general reversal trend with cooling events at ca. 12,200 and ca. 11,400 yrs BP, and then by subsequent warming at

Marı´a Virginia Manzini et al. excluded

ae de

ub

oi

rs hr

f. As te r

ac ea

ot he

po

ja

C

C

ol

he

lig

no

ua

ra ed

uv ia sa

Ep h

as N

di

+

su b e As te ra ce a

ae ce Po a

yr .B P C

14

e

Los Toldos (site 38)

s

358

1,410 ± 40 volcanic ash

ca. 3600

7260 ± 350 8730 ± 480 ca. 10,000

ca. 11,000 12,600 ± 600 20

40

60

20

40

60

80

20

20

40

60

80

20

40

60

20

40

60

Fig. 5. Diagram (%) of major pollen types of Los Toldos (47220 S; 68580 W). Integrated single sequence from caves 3 and 13 pollen samples arranged in chronostratigraphic order (see Fig. 1).

9800 yrs BP. The total temperature depression between ca. 11,400 and 9800 yrs BP was relatively small (£ 3 C), as indicated by the persistence of rainforest vegetation (Moreno et al., 2001). At Mayol (site 33) (Isla Grande de Chiloe´) between 15,000 and 10,000 yrs BP, parkland was rapidly replaced to a great extent by Nothofagus as an apparent result of Late Glacial warming and subsequent cooling. This sequence is similar to both Fundo Llanquihue (site 22) and Alerce (site 26) pollen sequences and is considered to be of high regional climatic significance (Heusser, 1984; Heusser et al., 1996, 1999 a, b). Three successive periods of climate cooling, the first and second generated at and before termination of LGM, and the third at the time of the Late Glacial, are recognizable by pollen data at Lago Condorito (site 28), Alerce (site 26), Fundo Llanquihue (site 22) and other sites spanning the same time interval (Heusser et al., 1999a, b; Heusser, 2003; Moreno, 2004). These cooling periods led to the expansion of cold-resistant rainforest trees represented by Nothofagus and Podocarpus. The expansion of lowland thermophilous trees in pulses centered at 11,000 and 10,000 cal yr BP suggest a warming trend (Moreno, 2004). At ca. 12,500–13,000 yrs BP on the Isla Grande de Chiloe´ the Nothofagus forest was already established (Villagra´n, 1988; Villagra´n et al., 1995). The similar and synchronic forest transformation on both flanks of the Andes supports the hypothesis that forest recolonization occurred soon after 14,000 yrs BP from a sheltered population located in Andean valleys (Bianchi et al., 1999). The paleoclimatic inferences from

Lago Mascardi-Gutie´rrez (site 25) suggest four deglaciation phases between 15,000 and 8,000 yrs BP including the Younger Dryas episode between 11,400 and 10,200 yrs BP, which has been called the ‘‘Huelmo–Lago Mascardi Event’’ (Ariztegui et al., 1997). The onset of the final cool episode of the last glacial in the southern mid-latitudes, that is the Huelmo–Mascardi Cold Reversal, preceded the onset of the Younger Dryas cold event by about 550 cal yrs. Both events ended during a radiocarbon-age plateau at about 10,200 yrs BP (Hajdas et al., 2003). On the west side of the Andes between 46 and 49 S, pollen records are located at Penı´nsula Taitao (site 36), Te´mpano Sur (site 42) and Puerto Ede´n (site 44). Records begin during the Late Glacial at ca. 14,000 yrs BP and show a sparsely distributed Empetrum-dominated heath and grassland, that was prevalent prior to the expansion of Nothofagus at ca. 13,000 yrs BP, without having modern analogues. The climate inferred was windier and probably drier than that of the present day. During the predominantly treeless Late Glacial interval, Nothofagus pollen in small amounts and beetle assemblages associated with Nothofagus woodlands indicate that the extensive growth of glaciers in full-glacial times did not completely eliminate the Nothofagus woodland biota (Ashworth et al., 1991). Nothofagus-dominated forest became afterwards mixed with Pilgerodendron by 11,000 yrs BP and with Podocarpaceae by 10,200 yrs BP at Penı´nsula Taitao (Lumley and Switsur, 1993). Instead, at Puerto Ede´n (Ashworth et al., 1991) and

ae

Late Quaternary Vegetation and Climate of Patagonia

d) de no po di yr in io ph ea e yl lu m (e xc lu

Fig. 6. Diagram (%) of major pollen types of Laguna Cari Laufquen (41130 S; 69250 W) (see Fig. 1).

M

e ce a

C he

0

Po a

14

C

yr B D ep P N t as h ( sa cm uv ) La i rre a Pr a o As sop te is ra ce ae su b

f. As te ro i

de

Laguna Cari Laufquen (site 21)

359

50

4020 ± 65 100 150 200

9575 ± 105

250 300 350 400 450

12,720 ± 100 500

20

20

40

20

40

60

Te´mpano Sur (Ashworth and Markgraf, 1989), only Nothofagus forest expanded and the other taxa were not members of the forests. The spread of Nothofagus appears to have been more or less simultaneous over the range of sites in the Penı´nsula Taitao–Chilean Channels sector, possibly in response to less windiness and more moisture. Neither pollen nor stratigraphical evidence shows a cooling during the Younger Dryas episode in these latitudes.

3.3. Early Holocene On the eastern slope of the Andes, at 37 S (Vaca Lauque´n, site 1), there are no known modern pollen assemblages that closely resemble the pollen composition between ca. 11,000 and 10,000 yrs BP, mostly because of the presence of Prumnopitys andina. During this period precipitation could have been similar to present day, but probably summers had a longer period of moisture stress, that is they were either warmer or longer than today. Between ca. 10,000 and 8500 yrs BP, the abundance of wetland taxa coupled with low proportions of arboreal taxa (Prumnopitys, Nothofagus and Myrtaceae) suggests that higher temperatures prevailed, resulting in the reduction of the forest island and especially the disappearance of P. andina, but that running water was abundant (Markgraf, 1987). Between 12,000 and 8500 yrs BP at 41 S (Lago Moreno and Mallı´n Book; sites 19, 24), precipitation levels must have approached the modern 1500 mm annually with winter as well as summer rain. This comes from the increased presence of Podocarpus and N. pumilio, both growing today under 2000 mm (Markgraf, 1984). Between 11,000

20

50

100

150

and 9000 yrs BP, ash levels without pollen in the Lago Escondido record (site 20) suggest environmental disturbance; after this period Nothofagus increase indicates dense forest at this site (Bianchi, 2000) as well as at 40 S (Cueva Traful 1, site 16) (Heusser, 1993a). On the Patagonian plateau, at 40 S (Epulla´n Grande, site 10), a grass steppe developed with Patagonian shrubs, which suggest conditions related to higher moisture availability than today (Prieto and Stutz, 1996). At 41 S, conditions of higher moisture availability ended about 8000 yrs BP with an inundation phase of Laguna Cari Laufquen Chica (site 21; Fig. 6) after which, the climate slowly became drier. Poaceae and typical elements of the Patagonian steppe dominated the regional vegetation. The appearance of the first elements of Monte vegetation (Prosopis) suggests slightly rising temperature (Scha¨bitz and Liebricht, 1998; Scha¨bitz, 1999). In the middle reaches of Rı´o Chubut (Campo Moncada 2, site 32), a shrub steppe of Asteraceae subf. Asteroideae and Nassauvia, with high values of Poaceae, suggests low temperature during the Early Holocene (Paez, 1991). At northeasternmost Patagonia (Salinas Chicas 1 and 4, Salina Anzoa´tegui, Laguna Indio Muerto, Salina Gualicho I, Salina Piedra; sites 2, 3, 5, 12, 13, 14) characteristic Monte vegetation indicates warm-arid conditions whereas, toward the east, Monte–Espinal vegetation is related to warm and more humid conditions (Scha¨bitz, 1991, 1994, 1999). At 47–48 S (Alero Ca´rdenas, Los Toldos, Piedra Museo, La Martita; sites 37, 38 – Fig. 5 – 39, 41), the shrub taxa increased and a shrub steppe of Asteraceae subf. Asteroideae developed (Mancini, 1998; Paez et al., 1999; Borromei, 2003) until ca. 7500 yrs BP. These shrub

360

Marı´a Virginia Manzini et al. temperature, probably seasonal, and precipitation lower than modern levels (Mancini, 2002; Franco et al., 2004). West of the Andes before 10,000 yrs BP, the drop in frequency of Prumnopitys and rise of Myrtaceae in the Rucan˜ancu record (site 7) establishes the onset of Holocene warming that culminated when Poaceae reached maximum frequency at 8350 yrs BP. The landscape had become drier, more steppe-like and ecotone-type, among a reduced cover of arboreal communities in the northern part of the Chilean Lake District (Heusser, 2003). Between 11,000 and 9500 yrs BP, a change in forest composition representing a Valdivian forest expansion is observed in Isla Grande de Chiloe´ and the Chilean Lake District (Lago Condorito, site 28) probably from refugia located in the Coastal Cordillera north of 41 S (Villagra´n et al., 1995). Forest diversification after 9800 yrs BP included the expansion of thermophilous rain forest species and the decline/disappearance of Podocarpus (Canal de la Puntilla, Huelmo and Lago Condorito; sites 17, 27, 28). The character of the vegetation change at 9000 yrs BP suggests a warming trend that led to interglacial climate conditions (Moreno, 2000; Moreno et al., 2001). At 46 S, the pollen record of Penı´nsula Taitao (site 36) after ca. 10,000–9500 yrs BP indicates a rise in Tepualia stipularis in response to warming and thereafter the Nothofagus forest remained constant until the Late Holocene (Lumley and Switsur, 1993). At 49 S, the

physiognomies suggest an increase of temperature linked to a decrease of water availability. After about 9500 yrs BP shoreline dates from Lago Cardiel (site 43), as well as diatom records and magnetic susceptibility indicate a lake relatively deep with levels up to 55 m above the present ones, suggesting an effective moisture increase until 7700 yrs BP (Stine and Stine, 1990; Bradbury et al., 2001; Gilli et al., 2001; Markgraf et al., 2003). Eastern pollen sequences from high latitude (51–52 S) show the transition from a humid grass steppe to xeric grass steppe (Cueva Las Buitreras, Cueva Fell; sites 52, 59) (Prieto et al., 1998; Markgraf, 1988). Instead, western pollen records from Rı´o Rubens (site 62) and Cueva Lago Sofı´a 1 (site 54) after 11,000 yrs BP show a replacement of steppe by open forests (Prieto, 1991; Huber et al., 2004). This suggests a change from moister to arid climatic conditions during the Early Holocene. After 9000 yrs BP between 50 (Cerro Frı´as, site 48; Fig. 7) and 51 S (Meseta Latorre, site 53), an expansion of Nothofagus forests occurred (Scha¨bitz, 1991; Franco et al., 2004). These forests were more open than today, as it is indicated by high pollen values of grass steppe elements. The association of Poaceae and Nothofagus forest was extended from the valley floors to the lower and medium parts of Meseta Latorre slopes, but probably not reaching the highest elevations (Scha¨bitz, 1991). Open Nothofagus forest at Cerro Frı´as and shrub steppes in Chorrillo Malo 2 (site 49) indicate a trend of increasing

Laguna Anzoategui (site 5)

rr Pr ea os R op ha is C mn as ac s e C ia ae hu Po qui ac rag ea a e

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th

La

As

ep

yr

D

C

14

te

BP

ea

e

Monte-Espinal

0 20 40 60 80

3640 ± 55

100

120 140 160 180 200 220

4615 ± 60

240 260

280 300 320 340

9065 ± 70

360 380

20

20

40

60

20

40

60

Fig. 7. Diagram (%) of major pollen types of Salina Anzoa´tegui (39000 S; 63460 W) (see Fig. 1).

Late Quaternary Vegetation and Climate of Patagonia Puerto Ede´n record (site 44) suggests that, between 9500 and 5500 yrs BP, precipitation was similar to today, based on an increased representation of moorland plants, pollen and aquatic beetle species (Ashworth et al., 1991). Calcareous limnic sediments, accumulated with little interruption over three millennia of the Early Holocene, suggest a wetter period and that the winds brought greater moisture producing higher lakelevels at 50 S in Torres del Paine (site 51). Early Holocene arboreal communities that spread on the landscape formed a patchwork of Nothofagus in the steppe, which was chiefly of grass and composites, as climate became warmer with low humidity (Heusser, 1995).

3.4. Middle Holocene

ac ea e Po

C y D rB ep P t N h (c ot ho m) fa gu s

Cerro Frias (site 48)

14

after 6000 yrs BP (Cueva Traful 1, site 16) (Heusser, 1993a). At the northwest margin of the Patagonian plateau (Epulla´n Grande, site 10) a grass–shrub Patagonian steppe developed (Prieto and Stutz, 1996). In the Laguna Cari Laufquen Chica record (site 21; Fig. 6) the sediment and pollen assemblage suggest the retreat of the lake and a drier period in Mid-Holocene times. Decreasing values of Nassauvia pollen and the occasional appearance of Monte elements, such as Larrea and Prosopis, are indicators of higher temperatures than before and possibly decreased precipitation (between 190 and 300 mm; Scha¨bitz and Liebricht, 1998; Scha¨bitz, 1999, 2003). In the middle reaches of Rı´o Chubut (Campo Moncada 2, site 32) a Patagonian shrub steppe (Asteraceae subf. Asteroideae, Nassauvia, Ephedra and Mulinum) developed (Paez, 1991). Toward the northeast, the Monte–Espinal communities (Salinas Gualicho 2 and 1, Salina Ingle´s; sites 11, 13, 15), Espinal (Salina Anzoa´tegui, site 5; Fig. 8) and grass steppes with Espinal and Patagonian taxa (Salinas Chicas 1 and 4; sites 2, 3), suggest arid– semiarid conditions with low frequency of rainfall and increase in temperature (Scha¨bitz, 1999). To the west, at 47–48 S (Parque Nacional Perito Moreno, site 40), the forest–shrub steppe ecotone suggests the increase of summer temperature and higher moisture availability after 6500 yrs BP (Mancini et al., 2002). In the Patagonian plateau (Alero Ca´rdenas, Los Toldos and La Martita, sites 37, 38, 41; Fig. 5), grass– shrub steppes of Asteraceae subf. Asteroideae developed under semiarid conditions and, shrub steppes with predominance of Asteraceae subf. Asteroideae associated with other shrubs (Ephedra, Nassauvia and Lycium) and

As te ra Em ce p ae C etru su yp m bf .A er ac st er ea oi e de (e ae xc lu de d)

At the eastern slopes of the Andean region, at 37 S (Vaca Lauque´n, site 1), between 8500 and 6000 yrs BP open Nothofagus forest developed with high values of Poaceae reflecting temperatures higher than today suggesting an upward shift of the upper treeline and an increase in aridity at the lower treeline. Between 6000 and 4500 yrs BP total Nothofagus pollen is lower than before showing a descent of the upper treeline. The regional climate probably became cooler and drier than before but locally with more abundant moisture due to increased runoff near the lower treeline (Markgraf, 1987). At 41 S (Lago Moreno and Mallin Book; sites 19, 24), Nothofagus deciduous forest and some Austrocedrus indicate rainfall increase toward present levels of 1500 mm annual precipitation after 7000 yrs BP (Markgraf, 1984) while, fluctuating steppe seems to have gained dominance

0 50

990 ± 40

100 150 200 250 300

5590 ± 40

350 400 450

8480 ± 50

500 550

10,390 ± 470

600

20 40

60 80

20 40

60 80

20

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20 40

60

80 100

Fig. 8. Diagram (%) of major pollen types of Cerro Frı´as (50260 S; 72430 W) (see Fig. 1).

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Poaceae (Mancini, 1998; Prieto et al., 2002), suggest increase in temperature and arid conditions. After 6100 yrs BP Lago Cardiel levels (site 43) fell, judging by diatom, ostracode and pollen assemblages. Regional vegetation experienced no major changes (Markgraf et al., 2003). At 50 S (Chorrillo Malo 2, site 49) the increase of grasses associated with Mulinum reflects the forest – steppe ecotone, while the development of an open Nothofagus forest (Cerro Frı´as, site 48; Fig. 7) suggest slightly moister conditions than at 8000 yrs BP (Mancini, 2002; Franco et al., 2004). At this latitude, the beginning of the Neoglacial cooling at ca. 6000 yrs BP (Mercer, 1976; Clapperton, 1993) coincides with peat formation eastward of the present moisture-controlled tree limit at the Lago Argentino area (Strelin and Malagnino, 2000). On Meseta Latorre (site 53), increasing Nothofagus pollen suggest an upward displacement of the upper treeline and a slight increase in temperature (Scha¨bitz, 1991) whereas on the Patagonian plateau, at 51 S (Cueva Las Buitreras, site 52), a humid grass steppe indicates more water availability (Prieto et al., 1998). On the western slope of the Andes, at 39 S, the Rucan˜ancu pollen record (site 7) suggests marked expansion of N. obliqua type over an interval lasting until 6900 yrs BP indicating the spread of Lowland Deciduous Nothofagus Forest in a paleoenvironment characterized by an increase in moisture, reduced evaporation and cooling (Heusser, 2003). At 41 S, in the lowlands of the Chilean Lake District (Lago Condorito, site 28), an abrupt reversal in trend characterized the Early to Mid-Holocene transition, with cooling events at 7600, 6900 and 5700 cal yr BP. This shift led to the re-expansion of cold-resistant rainforest trees between ca. 6900 and 2900 cal yr BP, following a transitional period between 7600 and 6900 cal yr BP. These results strongly suggest the onset of cool-temperate conditions, concomitant with an increase in humidity brought on by an equatorward shift and/or intensification of the westerlies (Moreno, 2004). At 49 S, the Puerto Ede´n pollen record (site 44) show the heath expansion and drying up of the bog between 5500 and 3000 yrs BP indicating a substantial decrease in precipitation to levels comparable to those during Late Glacial times (Ashworth et al., 1991). At 50 S (Torres del Paine, site 51), Nothofagus forest communities were already at the same distance as today, except an increase possibly at about 6000 yrs BP (Heusser, 1995).

3.5. Late Holocene In western Patagonia, at 37 S (Vaca Lauque´n, site 1), the pollen frequencies of Nothofagus suggest that this tree was actually growing in the region, with climatic conditions similar to the modern regime, that is with cold, moist winters and moisture stress during some months in summer (Markgraf, 1987). The past 3000 yrs of vegetation history of the northwestern part of Patagonia (39 S) are recorded at Paso del Arco (site 4) and Rı´o Malleo (site 8). These sequences

indicate that Nothofagus and grass generally codominate, indicating mostly open vegetation while varying proportions of these taxa imply community changes in the forest-steppe ecotone on the east side of the Andes (Heusser et al., 1988). At 40– 41 S (Mallı´n Aguado, El Tre´bol, Lago Moreno, Lago Escondido, Mallı´n Book, Lago Mascardi-Gutie´rrez; sites 9 [Fig. 4], 18, 19, 20, 24, 25) the forest composition is comparable to the modern mixed Nothofagus – A. chilensis forest with annual precipitation over 1500 mm and higher summer rains (Markgraf, 1983, 1984; Bianchi, 1999; Markgraf and Bianchi, 1999; Bianchi et al., 1999; Bianchi, 2000; Markgraf et al., 2002). On the Patagonian plateau, the Patagonian-Monte shrub steppe with affinity to modern La Payenia vegetation established at 40 S (Epulla´n Grande, site 10) (Mancini et al., 2005) whereas a grass–shrub steppe with high values of Chenopodiaceae predominated at 42 S (Campo Moncada 2, site 32). The first appearance in this area of Monte elements (such as Larrea and Chuquiraga) indicate higher temperatures than before and/or decrease of precipitation (Paez, 1991). These mixed formations are all indicators of semiarid climate. The sedimentary sequence of Laguna Cari Laufquen Chica (site 21, Fig. 6) also indicates increasing aridity (Scha¨bitz and Liebricht, 1998). At northeastern Patagonia both sedimentological and pollen sequences from endorheic basins (Salina Anzoa´tegui, Salinas Gualicho 2 and 1, Salina Ingle´s; sites 5 [Fig. 6], 11, 13, 15) indicate that Monte, Monte–Espinal and Espinal developed under arid to semiarid conditions (Scha¨bitz, 1994, 1999). In the western region at 47 S (Parque Nacional Perito Moreno, site 40), high pollen values of Asteraceae subf. Asteroideae together with Nothofagus show the continuity of the forest-shrub steppe ecotone and climatic conditions similar to 6000 yrs BP (Mancini et al., 2002). After 5000 yrs BP, modern environmental conditions characterized by increased variability, with ENSO influence in this intermediate latitude, are illustrated by repeated lakelevel fluctuations in Lago Cardiel (Markgraf et al., 2003). On the Patagonian plateau (Alero Ca´rdenas, La Martita; sites 37, 41) Poaceae and characteristic semidesert taxa (Nassauvia, Ephedra and Junellia) are found with shrubs of Asteraceae subf. Asteroideae, which represent semiarid conditions similar to modern ones (Mancini, 1998). In southern Patagonia, at 50 S (Cerro Frı´as, site 48; Fig. 7), high values of Nothofagus and low values of herbaceous and shrubby taxa suggest the development of a dense forest and higher effective moisture related to higher precipitation and lower temperatures before 3000 yrs BP (Franco et al., 2004). Toward the southeast (Chorrillo Malo 2, site 49), the grass–shrub steppe continued until ca. 3500 yrs BP. In the higher mountains (1100 m a.s.l., Cerro Verlika 1, site 50), the Upper Andean vegetation is represented by grass–shrub steppe with Empetrum associated with cold conditions (Mancini, 2001, 2002). During this time, Neoglacial advances occurred in the southern Andes (Mercer, 1976; Glasser et al., 2004). However, the development of the forest-shrub steppe ecotone at 47 (Parque Nacional Perito Moreno, site 40) and 50 S (site 50) indicates that the

Late Quaternary Vegetation and Climate of Patagonia effective humidity did not reach present levels (Mancini, 2002; Mancini et al., 2002). At 51 S (Meseta Latorre, site 53), Nothofagus stands have probably grown on the uppermost parts of the plateau slopes, but did not spread onto the elevated plateau. This evidence indicates that temperature rose up to present-day values and sometimes seems to reach even higher temperatures than today between 3000 and 1300 yrs BP with a cold period between 2700 and 2100 yrs BP (Scha¨bitz, 1991). During the last period lower treeline expansion is recorded at 47 S (site 40) in the forest-steppe ecotone, suggesting episodic precipitation increase and a decrease in temperature. After 2000 yrs BP lower Nothofagus values and higher values of grass and shrub taxa (Parque Nacional Perito Moreno, El Sosiego, Charles Fuhr, Cerro Frı´as and Chorrillo Malo 2; sites 40, 45, 47, 48, 49) suggest open forest and grass and shrub steppes similar to today indicating low moisture availability related to temperature increase and probably, precipitation decrease (Mancini, 2002, 2003; Mancini et al., 2002). At 52 S (Cueva Markatch Aike and Cueva Don Ariel; sites 56, 58), pollen data show steppe communities with Ephedra after 3000 yrs BP suggesting drier conditions (Borromei and Nami, 2000). High resolution paleoclimatic and paleoenvironmental archives from crater lakes in the Pali Aike volcanic field (Laguna Potrok Aike, Laguna Azul; sites 57, 60) suggest relatively frequent moist/dry fluctuations between the fifth and eleventh centuries, a dry period between the thirteenth and fifteenth centuries (Medieval climate anomaly), relatively moist conditions during the fifteenth to nineteenth centuries (Little Ice Age) and the warming of the twentieth century (Scha¨bitz et al., 2003; Zolitschka et al., 2004; Haberzettl et al., 2005). Similar climatic trends were observed in Cabo Vı´rgenes pollen sequences (site 63), with drier conditions between ca. 1300 and 700 yrs BP and moister during the last 600 yrs (Franco et al., 2004; Mancini, 2006). West of the Andes, at 39 S, high frequencies of Nothofagus in the Rucan˜ancu pollen record (site 7) reflect the expansion of Valdivian evergreen forest by 3900 yrs BP (Heusser, 2003). At 41 S (Lago Condorito, site 28), the Mid-Holocene cooling trend was interrupted by a brief warm and dry event between 4100 and 3800 C yr BP. Subsequent warming at ca. 2900 C yr BP led to a decline in coldresistant trees, followed by a rise in precipitation at 1800 C yr BP and establishment of modern vegetation and climate (Moreno, 2004). Between 46 and 49 S, after 3000 yrs BP higher levels of precipitation combined with somewhat warmer temperature characterized the Late Holocene interval in the Penı´nsula Taitao (site 36; Lumley and Switsur, 1993) and the Chilean channels (site 44; Ashworth et al., 1991).

4. Final Remarks No continuous pollen records exist in Patagonia that link the LGM with previous interglacial–glacial periods. The only pollen record that probably represents interglacial conditions about 430 ka (MIS 11) is that from Can˜ado´n

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El Mosquito, in southern Patagonia. This interglacial has been identified as unique and exceptionally long – 28,000 yrs – and this prolonged warmth is recorded elsewhere (EPICA, 2004). Interstadial (?) conditions are pointed out at 39 and 52 S in Patagonia but resolution of the pollen records and the usually finite radiocarbon dates make the interpretation speculative. It is difficult to find modern analogues for pollen assemblages older than 10,000 yrs BP, possibly due to the fact that climate patterns were markedly different from today. One explanation for these vegetational conditions was that the westerlies and its associated precipitation peak shifted to the north during the LGM (Markgraf, 1993; Bradbury et al., 2001; Heusser, 2003). In northern Patagonia, the differentiation in humidity range between semiarid and arid during the last 13,000 yrs may primarily be caused by variations of the southern westerlies storm tracks. This region was under greater influence by the westerlies during Late Glacial times resulting in higher humidity for the western and central parts than today (Scha¨bitz and Liebricht, 1998). During the Early Holocene (10,000–8500 yrs BP), the location of the westerlies centered between 45 and 50 S probably the whole year around, with reduced seasonality (Markgraf, 1993; Grimm et al., 2001) and a consequent reduction of its influence north and south of this latitudinal range. After 8500 yrs BP, moisture increased both in the northern and high southern temperate latitudes whereas intermediate latitudes became drier and warmer (Markgraf et al., 2003). During the Mid Holocene (ca. 6000 yrs BP) the westerlies brought higher humidity to both the Andean zone and the southern extreme of Patagonia. Semiarid conditions in extra-Andean region suggest that the westerlies were less intense than present. An increase of aridity and temperature at the northeast Patagonia led to an arid climate and to the establishment of the Arid Diagonal during this time (Mancini et al., 2005). The increase in magnetic susceptibility in the Lago Cardiel records at 47 S documents an intensification of the westerlies storm tracks after 6300 yrs BP that can be regarded as the beginning of the intensification of southern westerlies at 49 S (Gilli et al., 2005). Atmospheric circulation patterns and the seasonal amplitude of the latitudinal shifts of storm tracks, which produce maximum precipitation at 50 S, seem to be established in the Late Holocene (van Geel et al., 2000). In northwest Patagonia, arid conditions were caused by a reduced influence of the west wind belt (Scha¨bitz and Liebricht, 1998). Evidence from paleodata suggests a transition toward present-day vegetation and climate patterns during this time. However, changes at a smaller scale and more environmental heterogeneity in the regional vegetation patterns occurred in the different environments (Mancini, 2002; Mancini et al., 2002, Heusser, 2003; Scha¨bitz et al., 2003). In Patagonia, postglacial times were characterized by high fire frequency. Human impact, volcanism and climate have been proposed to explain spatial and temporal variations of this fire frequency throughout different intervals (e.g. Heusser, 1987, 1994; Bianchi, 2000; Huber et al., 2004). Independent of the type of ignition sources, fire

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conductive climate conditions appear to have been a prerequisite for the spread of fire (Huber et al., 2004).

Acknowledgments Different research projects were funded by the Agencia Nacional de Promocio´n Cientı´fica y Tecnolo´gica (FONCYT PICT 07-6477), Consejo Nacional de Investigaciones Cientı´ficas y Te´cnicas (CONICET PIP 418/98), and Universidad Nacional de Mar del Plata (Exa 275/03) the German Science Foundation (DFG), the German Federal Ministry for Education and Research (BMBF) and the Volkswagen Foundation.

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Markgraf, V. (1991). Late Pleistocene environmental and climatic evolution in southern South America. Bamberger Geographische Schriften 11, 271–282. Markgraf, V. (1993). Paleoenvironments and paleoclimates in Tierra del Fuego and southernmost Patagonia, South America. Palaeogeography, Palaeoclimatology, Palaeoecology 102, 53–68. Markgraf, V. and Bianchi, M.M. (1999). Paleoenvironmental changes during the last 17,000 years in the western Patagonia: Mallı´n Aguado, Province of Neuque´n, Argentina. Bamberger Geographische Schriften 19, 175–193. Markgraf, V., Bradbury, J.P. and Ferna´ndez, J. (1986). Bajada de Rahue, Province of Neuque´n, Argentina: an interstadial deposit in Northern Patagonia. Palaeogeography, Palaeoclimatology, Palaeoecology 56, 251–258. Markgraf, V., Webb, R., Anderson, K. and Anderson, L. (2002). Modern pollen/climate calibration for southern South America. Palaeogeography, Palaeoclimatology, Palaeoecology 181, 375–397. Markgraf, V., Bradbury, J.P., Schwalb, A. et al. (2003). Holocene paleoclimates of southern Patagonia: limnological and environmental history of Lago Cardiel, Argentina. The Holocene 13, 581–591. Mercer, J.H. (1976). Glacial History of Southernmost South America. Quaternary Research 6, 126–166. Moore, D.M. (1978). Post.glacial vegetation in the South Patagonian territory of the giant ground sloth, Mylodon. Botanical Journal Linnean Society 77, 177–202. Moreno, P.I. (2000). Climate, fire and vegetation between about 13,000 and 9200 14C yr BP. in the Chilean Lake District. Quaternary Research 54, 81–89. Moreno, P.I. (2004). Millennial-scale climate variability in northwest Patagonia over the last 15,000 yr. Journal of Quaternary Science 19, 35–47. Moreno, P.I., Jacobson Jr., G.L., Lowel, T.V. and Denton, G.H. (2001). Interhemispheric climate links revealed by a Late-glacial cooling episode in southern Chile. Nature 409, 804–808. Paez, M.M. (1991). Palinologı´a de Campo Moncada 2 (Chubut): Interpretacio´n paleoecolo´gica para el Holoceno. Unpublished Ph.D. Thesis, Universidad Nacional de La Plata, La Plata, Argentina. Paez, M.M., Prieto, A.R. and Mancini, M.V. (1999). Fossil pollen from Los Toldos locality: A record of the Late-glacial transition in the Extra-Andean Patagonia. Quaternary International 53/54, 69–75. Paez, M.M., Scha¨bitz, F. and Stutz, S. (2001). Modern pollen-vegetation and isopoll maps in southern Argentina. Journal of Biogeography 28, 997–1021. Pendall, E., Markgraf, V., White, J.W.C. et al. (2001). Multiproxy record of Late Pleistocene-Holocene climate and vegetation changes from a peat bog in Patagonia. Quaternary Research 55, 168–178. Prieto, A. (1991). Cazadores tempranos y tardı´os en Cueva Lago Sofı´a I. Anales del Instituto de la Patagonia (Serie Ciencias Sociales) 20. Punta Arenas, Chile, 75–99. Prieto, A.R. and Stutz, S. (1996). Vegetacio´n del Holoceno en el norte de la estepa patago´nica: Palinologı´a de la Cueva Epulla´n Grande. Praehistoria 2, 267–277. Buenos Aires.

Prieto, A.R., Stutz, S. and Pastorino, S. (1998). Vegetacio´n del Holoceno en la Cueva Las Buitreras, Santa Cruz, Argentina. Revista Chilena de Historia Natural 71, 277–290. Santiago. Prieto, A.R., Mancini, M.V. and Paez, M.M. (2002). Ana´lisis polı´nico de la localidad de Los Toldos: armando rompecabezas. V Jornadas de Arqueologı´a de Patagonia, Resu´menes 27. Buenos Aires. Prieto, A.R., Mancini, M.V., Paez, M.M. et al. (2004). The Argentinean modern Pollen Database. Polen 14, 288. Prohaska, F. (1976). The climate of Argentina, Paraguay and Uruguay. In: Schwertfeger, W. (ed.), Climates of Central and South America. World Survey. Climatology. Elsevier, Amsterdam, 13–112. Roig, F.A. (1998). La vegetacio´n de la Patagonia. In: Correa, M.N. (ed.), Flora Patago´nica. Coleccio´n Cientı´fica INTA 8, 47–166. Buenos Aires. Scha¨bitz, F. (1991). Holocene vegetation and climate in southern Santa Cruz, Argentina. Bamberger Geographische Schriften 11, 235–244. Scha¨bitz, F. (1994). Holocene climatic variations in northern Patagonia, Argentina. Palaeogeography, Palaeoclimatology, Palaeoecology 109, 287–294. Scha¨bitz, F. (1999). Pala¨oo¨kologische Untersuchungen an geschlossenen Hohlformen in den Trockengebieten Patagoniens. Bamberger Geographische Schriften 17, 1–239. Scha¨bitz, F. (2003). Estudios polı´nicos del Cuaternario en las regiones a´ridas del sur de Argentina. Revista del Museo Argentino de Ciencias Naturales 5, 291–299. Buenos Aires. Scha¨bitz, F. and Liebricht, H. (1998). Landscape and climate development in the south-eastern part of the ‘‘Arid Diagonal’’ during the last 13,000 years. Bamberger Geographische Schriften 15, 371–388. Scha¨bitz, F. and Schellmann, G. (1999). Ein bewaldetes Interglazial im Can˜ado´n El Mosquito – oberes Rı´o Santa Cruz-Tal (Argentinien). Bamberger Geographische Schriften 19, 195–210. Scha¨bitz, F., Paez, M.M., Mancini, M.V. et al. (2003). Estudios paleoambientales en lagos volca´nicos en la Regio´n Volca´nica de Pali Aike, sur de Patagonia (Argentina): palinologı´a. Revista del Museo Argentino de Ciencias Naturales 5, 301–316. Buenos Aires. Schellmann, G. (1998). Jungka¨nozoische Landschaftsgeschichte Patagoniens (Argentinien). Essener Geographische Arbeiten 29, 1–207. Soriano, A., Movia, C.P. and Leo´n, R.J.C. (1983). Deserts and semi-deserts of Patagonia (Vegetation). In: West, N.E. (ed.), Temperate deserts and semi-deserts. Elsevier, Amsterdam, 440–454. Stine, S. and Stine, M. (1990). A record from Lake Cardiel of climate in southern South America. Nature 345, 705–708. Strelin, J.A. and Malagnino, E.C. (2000). Late – Glacial history of Lago Argentino, Argentina, and age of the Puerto Banderas moraines. Quaternary Research 54, 339–347. van Geel, B., Heusser, C.J., Renssen, H. and Schuurmans, C.J.E. (2000). Climatic change in Chile at around 2700 BP and global evidence for solar forcing: a hypothesis. The Holocene 10, 659–664.

Late Quaternary Vegetation and Climate of Patagonia Villagra´n, C. (1988). Expansion of Magellanic Moorland during the Late Pleistocene: palynologycal evidence from Northern Isla Grande de Chiloe´, Chile. Quaternary Research 30, 304–314. Villagra´n, C., Leo´n, A. and Roig, F. (2004). Paleodistribucio´n del alerce y cipre´s de las Guaytecas durante perı´odos interestadiales de la Glaciacio´n Llanquihue: provincia de Llanquihue y Chiloe´, Regio´n de los Lagos, Chile. Revista Geolo´gica de Chile 31, 133–151. Santiago. Villagra´n, C., Moreno, P. and Villa, R. (1995). Antecedentes palinolo´gicos acerca de la historia cuaternaria de los bosques chilenos. In: Armesto, J.J., Villagra´n, C. and Arroyo, M.K. (eds), Ecologı´a de los bosques nativos de Chile. Editorial Universitaria, Santiago de Chile, 51–69. Wenzens, G. (2002). The influence of tectonically derived relief and climate on the extent of the last

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glaciations east of the Patagonia Ice fields (Argentina, Chile). Tectonophysics 345, 329–344. Wenzens, G., Wenzens, E. and Schellmann, G. (1996). Number and types of the piedmont glaciations east of the Central Southern Patagonian Icefield. Zentralblatt fu¨r Geologie und Pala¨ontologie 7/8, 779–790. Wenzens, G., Wenzens, E. and Schellmann, G. (1997). Early Quaternary genesis of glacial and Aeolian forms in semi-arid Patagonia (Argentina). Zeitschrift fu¨r Geomorphologie, Neue Folge 111, 131–144. Zolitschka, B., Scha¨bitz, F., Lu¨cke, A. et al. (2004). Climate changes in Southern Patagonia (Santa Cruz, Argentina) inferred from lake sediments: the multi-proxy approach of SALSA. PAGES News 12, 9–11.

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18 Late and Postglacial Paleoenvironments of Tierra del Fuego: Terrestrial and Marine Palynological Evidence Ana M. Borromei1,2 and Mirta Quattrocchio1,2 1

2

INGEOSUR-CONICET Departamento de Geologı´a, Universidad Nacional del Sur, San Juan 670, B8000ICN Bahı´a Blanca, Argentina

The climate is determined by the belt of prevailing humid and cold westerlies. It is highly oceanic in the western and southern parts of the archipelago and it becomes increasingly continental eastward and northward. Mean summer isotherms increase northeastward from 9 to 12C. Precipitation decreases to the north and east. Mean annual rainfall in Ushuaia is 570 mm, but less than 300 mm in Rı´o Grande in the northern part of the island (Heusser, 2003). The present vegetation corresponds to the Fuego– Patagonian steppe in the north, followed southward successively by the Subantarctic Deciduous Beech Forest and the Evergreen Beech Forest (Fig. 1). They are characterized by three species of southern beech, Nothofagus pumilio (lenga), N. betuloides (guindo) and N. antarctica (n˜ire), which grow from seashore to an average altitudinal limit of 550–600 m a.s.l. The forest predominates where precipitation reaches between 400 and 800 mm/year. Magellanic Moorland occurs beyond the forest boundaries along the exposed outermost coast under conditions of increased precipitation, wind and poor drainage. High Andean Desert vegetation develops above treeline (600 m a.s.l.) in the Fuegian Cordillera until the snowline is reached (Heusser, 2003).

1. Introduction The vegetation and paleoclimate of southernmost part of South America, Tierra del Fuego, during Late Pleistocene– Holocene times and following the wastage of iceage glaciers, were subjects of intensive study mainly by Heusser (1987, 1989a, b, c, 1990, 1993a, b, 1994a, b, 1995a, 1997, 1998, 2003), Heusser and Rabassa (1987, 1995), Markgraf (1980a, b, 1983, 1991a, b, 1993a, b), Markgraf et al. (1992, 2002), Markgraf and Anderson (1994), Borromei (1995), Markgraf and Kenny (1997), Quattrocchio and Borromei (1998), Borromei and Quattrocchio (2001), Pendall et al. (2001), Grill et al. (2002), Mauquoy et al. (2004), following the pioneering work earlier in this century of Va¨ino¨ Auer (1933, 1956, 1958, 1970, 1974). Data from the southernmost part of Chile (south of 53 S), by comparison, are relatively limited with few records, mainly along the Estrecho de Magallanes and the Bahı´a Inu´til depressions (Heusser, 1987, 1995b, 2003; Heusser et al., 2000), on the northern side of the Fuegian Andes axis (the Onamonte site) (Heusser, 1993a, 2003) and Caleta Ro´balo, at Puerto Williams, Isla Navarino (Heusser, 1987, 1989a). This contribution primarily includes the published data available to the authors on the palynofloristic evolution of the Late Cenozoic of southern Tierra del Fuego (Beagle Channel area). Paleovegetational communities and paleoenvironments of the Late Pleistocene and Holocene in southern Tierra del Fuego are interpreted from pollen assemblages, microplankton and palynofacies in radiocarbon-dated peat bogs, glaciolacustrine sediments and marine deposits.

3. Terrestrial and Marine Paleoenvironmental and Paleoclimatic Reconstruction of Tierra del Fuego The available evidence of the pollen sequences from the interior Fuegian valleys close to the Beagle Channel area, such as Valle de Andorra (Borromei, 1995; Quattrocchio and Borromei, 1998), Valle Carbajal and the Route 3 exposure site (Borromei et al., 2007) (Fig. 2), allows a comparison with the pollen record from the Beagle Channel peat bog sections (mainly the Puerto Harberton site) to confirm and refine the previously established Late Pleistocene and Holocene paleoecology and chronostratigraphy. The pollen sequence from the Puerto Harberton site (Heusser, 1989b; Fig. 2) is taken as a proxy record at the Beagle Channel area due to its time extension (the oldest record in the area) and detailed studies developed there in the last two decades. The palynological analysis based on pollen and spores have been intensively developed in Tierra del Fuego to reconstruct Quaternary environments (see Section on Introduction). However, little attention has been paid to the study of marine microplankton, mainly dinoflagellate

2. Geographical Setting The Isla Grande de Tierra del Fuego is the largest of the Fuegian Archipelago islands, located at the southernmost end of South America, between latitude 53–55 S and longitude 66–74 W (Fig. 1). It is the highest latitude landmass in the Southern Hemisphere outside Antarctica and is strongly influenced by climatic conditions in the Southern Ocean and the Antarctic Peninsula. Tierra del Fuego can be subdivided in two major regions: the rather gently sloping northern steppe plains and the southern, forest-covered Fuegian Andes, and the adjacent islands to Cape Horn.

 2008 ELSEVIER B.V. ALL RIGHTS RESERVED

DEVELOPMENTS IN QUATERNARY SCIENCES VOLUME 11 ISSN 1571-0866 369

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Fig. 1. Location of Late Quaternary pollen sites in Tierra del Fuego and southern Patagonia.

cysts and acritarchs, for the characterization and documentation of the marine Quaternary environments in Tierra del Fuego. In marine sediments, the most common palynomorphs are generally organic-walled cysts of dinoflagellates, one of the main primary producers in marine environments. Their abundance and distribution depend on the primary production and the physico-chemical conditions in surface water of the photic zone. The dinoflagellate cysts are not only important for biostratigraphical and paleoceanographical studies, but they are also the most suitable proxies for use in quantitative modelling of past changes in sea surface temperature and salinity conditions (Mudie et al., 2001). In addition to pollen, spores and cysts of dinoflagellate, several other biogenic organic products are fossilizable palynomorphs, notably copepod egg-envelopes, foraminifera test-linings, prasinophyte phycoma, freshwater algae such as Botryoccocus, Pediastrum and Zygnemataceae. Absolute and relative counts of the palynomorphs in sedimentary records may therefore be used to quantify and characterize organic fluxes and paleofluxes (de Vernal and Giroux, 1991). Evidence of a Holocene marine incursion on the coast of southern Tierra del Fuego is given by several discontinuous raised terraces along the Beagle Channel. These deposits are the objective of recent studies by the authors. The palynological records from Bahı´a Lapataia sites (Borromei and Quattrocchio, 2007; in press) and the Rı´o Varela locality (Grill et al., 2002) (Fig. 2) are the first account of dinoflagellates and other microplanktonic organisms and palynofacies from the Quaternary of Tierra del Fuego. Table 1 shows the Late Quaternary pollen records of Tierra del Fuego (53–55 S) cited in the text.

3.1. Pleistocene Middle–Late Pleistocene So far the oldest dates from pollen records are on interstadial peat layers interbedded with glacial sediments at the southeastern end of Lago Fagnano (54330 S, 67190 W) (Fig. 1). A Pleistocene glaciolacustrine sequence of a proglacial delta facies contains peaty lacustrine deposits interlayered between gravels. The peat layers have been dated at 39,560 and >58,000 14C yr BP (Bujalesky et al., 1997). Pollen analysis indicates regional steppe/tundra conditions, represented by Poaceae and Empetrum with a very low content of tree pollen grains (Nothofagus spp.), instead of the present closed Nothofagus forest. The pollen evidence supports the interpretation of a much colder and drier paleoenvironmental setting than today in eastern Tierra del Fuego during Early Wisconsinan times (Marine Oxygen Isotope Stage – MIS 4) or even an older glacial (MIS 6?) (Bujalesky et al., 1997).

Latest Pleistocene The latest Pleistocene pollen records are mainly from several sites along the Beagle Channel: Puerto Harberton (54870 S, 67880 W), Caleta Ro´balo (Isla Navarino, Chile) (54930 S, 67630 W), Ushuaia 1 (54470 S, 68230 W), Ushuaia 2 (54 470 S, 68180 W) and Ushuaia 3 (54 480 S, 68 230 W) (Heusser, 1989a, c, 1990, 1998). There is one section from the interior Fuegian valleys (the Route 3 site, 54430 S, 6890 W) (Borromei et al., 2007) (Fig. 2). Two records are located in central Tierra del Fuego: the Onamonte (53540 S, 68570 W) (Chile) (Fig. 1) and

Late and Postglacial Paleoenvironments of Tierra del Fuego 66º45′W

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Fig. 2. Location map of Late Quaternary pollen sites in the Beagle Channel and the Lago Roca-Bahı´a Lapataia area, southern Tierra del Fuego.

Lago Fagnano sites (54570 S, 67620 W) (Heusser, 1993a, 2003) (Fig. 2). Other pollen sequences from the Estrecho de Magallanes-Bahı´a Inu´til area are the Punta Arenas (53090 S, 70570 W), Puerto del Hambre (53360 S, 70550 W), Bahı´a Inu´til (53450 S, 70100 W) (Heusser, 1995b; Heusser et al., 2000, Heusser, 2003) and Isla Capita´n Aracena sections (54120 S, 71140 W) (Auer, 1974; the original site designation is Isla Clarence; Markgraf, 1983; Heusser, 1987) (Fig. 1).

New palynological and geomorphological studies have been carried out by Juan Federico Ponce (CADIC, Ushuaia; personal communication, 2005) in Bahı´a Franklin, southwestern Isla de los Estados (Staaten Island), as part of his doctoral dissertation. A basal radiocarbon date of 10,679 + 62 14C yr BP (AA62509) from a peat bog developed on top of Pleistocene sand dunes provided a minimum age for dune stabilization and initial peat formation. These studies will be very useful to correlate the

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Table 1. Late Quaternary pollen records of Tierra del Fuego (53–55 S). Site No.

Latitude (S)

Longitude (W)

53.09 53.30 53.36 53.45 53.54 54.11 54.12

70.57 67.50 70.55 70.10 68.57 66.21 71.14

8 9 10 11

Punta Arenas La Misio´n Puerto del Hambre Bahı´a Inu´til Onamonte Viamonte Isla Capita´n Aracena (Isla Clarence) Cabo San Pablo Lago Yehuin Lago Fagnano Valle de Andorra

54.18 54.20 54.33 54.40

66.45 67.45 67.19 68.25

12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

Paso Garibaldi Route 3 Valle de Carbajal Valle de Andorra Ushuaia 2 Ushuaia 1 Ushuaia 3 Bahı´a Lapataia 1 Bahı´a Lapataia 2 Rı´o Ovando Rı´o Varela Lago Fagnano Puerto Harberton Bahı´a Moat Caleta Ro´balo

54.43 54.43 54.44 54.45 54.47 54.47 54.48 54.50 54.50 54.51 54.52 54.57 54.87 54.90 54.93

67.50 68.90 68.12 68.18 68.18 68.23 68.23 68.34 68.34 68.35 67.11 67.62 67.88 66.73 67.63

1 2 3 4 5 6 7

Site name

island information with the palynological data previously collected at Tierra del Fuego.

The Beagle Channel Area (Southern Tierra del Fuego) The Beagle Channel, a drowned glacial valley, was formerly occupied by the ancient ‘‘Beagle Glacier’’, a large outlet glacier from the Cordillera Darwin during the Last Glacial Maximum (LGM). Although precise 14C dating is still lacking, the LGM in Tierra del Fuego was probably attained between 18,000 and 20,000 14C yr BP (Rabassa et al., 2000). Ice recession of the ‘‘Beagle Glacier’’ from its outermost position centered on Isla Picton, at the eastern mouth of the Beagle Channel, had already started before 14,640 + 260 14C yr BP, as indicated by a basal radiocarbon date at the Puerto Harberton peat bog (Heusser, 1989c, 1990). Later, the ice front had retreated from Caleta Ro´balo (with a basal age of 12,730 + 90 14C yr BP, located 25 km west of Puerto Harberton) on Isla Navarino, Chile (Heusser, 1989a), and it had also withdrawn from the present location of Ushuaia (another 50 km farther west) approximately at 12,100 14C yr BP (Heusser, 1998, 2003). The latest Pleistocene vegetation, according to the pollen stratigraphy along the Beagle Channel at Ushuaia

References Heusser, 1995b, 2003 Markgraf, 1983, 1993b Heusser et al., 2000, 2003 Heusser, 2003 Heusser, 1993a, 2003 Auer, 1956 Auer, 1974; Markgraf, 1983; Heusser, 1987 Heusser and Rabassa, 1995, 2003 Markgraf, 1983, 1993b Bujalesky et al., 1997 Borromei, 1995; Quattrocchio and Borromei, 1998 Markgraf, 1993b Borromei et al., 2007 Borromei et al., 2007 Mauquoy et al., 2004 Heusser, 1998 Heusser, 1998 Heusser, 1998 Borromei and Quattrocchio, 2001 Borromei and Quattrocchio, 2007 Candel et al., in preparation Grill et al., 2002 Heusser, 2003 Heusser, 1989c, 1990, 2003 Heusser, 1995a, 2003 Heusser, 1989a, 2003

(Ushuaia 1, Ushuaia 2 and Ushuaia 3 sections; Heusser, 1998), Puerto Harberton (Heusser, 1989c, 1990) and Caleta Ro´balo (Heusser, 1989a) profiles, was composed of communities of dwarf shrub heath and of grass and sedge containing a partial cover of southern beech forest showing an impoverished steppe-tundra vegetational environment. According to Heusser (1998), fluctuations in the influx of Nothofagus pollen in these records indicate variable temperature settings for Late Glacial times. Emphasis is placed on Nothofagus influx because it is the exclusive arboreal component of Late Glacial records at the Beagle Channel. Climate implied by the pollen sequence at the Puerto Harberton site (Fig. 3) was apparently warmer when peak Nothofagus and total influx increased between 14.6 and ca. 14.0–13.5 14C ka BP (Pollen Subzone PH-3d), and later become cooler between ca. 13.2 until about 11.8 14C ka BP (Pollen Subzone PH-3c) when reversal toward relatively warmer conditions occurred until about 11.2 14C ka BP (Pollen Subzone PH-3b). The pollen sequence shows a marked decrease in arboreal pollen after 11.1 14C ka BP with a minimum level with no Nothofagus pollen at 10.2 14C ka BP (Pollen Subzone PH-3a). There is a lack of charcoal particles in this part of the section, and thus there is no indication that fire has disturbed the record. At about

Late and Postglacial Paleoenvironments of Tierra del Fuego Zones PH-

m 14

C yr BP × 103 10.00

Rabassa, 1995) sections and even from sites as far as Bahı´a Moat (54900 S, 66730 W) on the Beagle Channel shore (Heusser, 1995a) (Figs 1 and 2).

2

8

3a

10.20 11.16

Early to Middle Holocene 3b

11.78

373

9 3c

13.00 3d

10

3e

14.67 0

1

2

3

Nothofagus dombeyi type cm–2 yr–1 × 10–2

Fig. 3. Influx of Nothofagus during Late Glacial–Early Holocene in a core at Puerto Harberton. From Heusser (1998).

10.1–10.0 14C ka BP (Pollen Subzone PH-2), a significant recovery of the Nothofagus forest suggests the spread of arboreal communities under increasingly warmer Early Holocene climate (Heusser, 1993b). The last cooling event, indicated by the Pollen Subzone PH-3a, suggests the development of a cool period during the Younger Dryas Chron of the Northern Hemisphere (11,000–10,000 14C yr BP) in this part of the Southern Hemisphere, where the estimated summer temperature was <3C lower than the present conditions at Ushuaia (Heusser and Rabassa, 1987; Heusser, 1998). Noteworthy is the positive evidence for a colder climatic episode, associated with the Younger Dryas event not only along the Beagle Channel area but also in the interior valleys. At the Route 3 exposure site (Fig. 4), a minimum of Nothofagus pollen frequency has been recorded at 10,310 + 100 14C yr BP (Pta 7990) (Unit 2), accompanied by communities of grass, low scrub and dwarf shrub heath (Borromei et al., 2007).

3.2. Holocene The published pollen sequences recording the Holocene vegetation changes of Tierra del Fuego are by far more abundant than the latest Pleistocene pollen records. There are several Holocene pollen records from the Beagle Channel, like those cited before (Heusser, 1989a, 1990, 1998), from the interior valleys (Borromei, 1995; Borromei et al., 2007), as well as from central, northeastern and eastern Tierra del Fuego: the La Misio´n (53300 S, 67500 W; Markgraf, 1983, 1993b), Onamonte (53540 S, 68570 W, Chile; Heusser, 1993a), Lago Fagnano (54570 S, 67620 W; Heusser, 2003), Lago Yehuin (54200 S, 67450 W; Markgraf, 1983, 1993b), Paso Garibaldi (54430 S, 67500 W; Markgraf, 1993b), Cabo San Pablo (54180 S, 66450 W; Heusser and

Emphasis is placed on the interior valleys of the Fuegian Andes, such as the Valle de Andorra peat bog (54400 S, 68250 W; Borromei, 1995; Quattrocchio and Borromei, 1998), the Valle de Carbajal site (Oyarzu´n peat bog, 54440 S, 68120 W ; Borromei et al., 2007) and the Route 3 exposure (54430 S, 6890 W; Borromei et al., 2007) (Fig. 2). As in all interdependent ice system, the glacial activity in the interior valleys has been controlled by the glaciological behavior of the main ice stream and regional climatic variation. The 10,310 + 130 14C yr BP absolute age obtained for the base of the Route 3 exposure location allows to infer that the inner valley glaciers had already receded by this time and that the valley slopes (240 m a.s.l.) were free of ice. The Alpine interior valley glaciers were fed by local cirques, independent of the Darwin Cordillera mountain ice sheet, from where the main Beagle Glacier was nourished. Most likely, these local glaciers were more severely affected by the abrupt climate change, and thus receded faster (Coronato, 1990; Rabassa et al., 2000). This new date refines by 230 yrs the definitive ice disappearance in the interior valleys of the region, previously considered to be around 10,080 14 C yr BP for the ‘‘Beagle Glacier’’ at Ushuaia and Bahı´a Lapataia (Heusser and Rabassa, 1987). According to the available radiocarbon date at the Oyarzu´n peat bog (7640 + 80 14C yr BP, Pta 7607; Borromei et al., 2007), lacustrine sediments were being deposited at the bottom of the postglacial lakes around 9.6 14C ka BP taking into account that in the surrounding valleys the top of glaciolacustrine sedimentation was estimated at 9.1–9.3 and 9.7 14C ka BP. A minimum age of 9310 + 180 14C yr BP for the Andorra Glacier retreat is given by a basal radiocarbon age at the Valle de Andorra peat bog (Coronato, 1990; Borromei, 1995) and 9780 + 70 14C yr BP for the Rı´o Pipo Glacier recession (Heusser, 1998). The interrupted trend toward steadily milder climate, indicated by a continuous expansion of the Nothofagus forest during the latest Pleistocene in the Beagle Channel at Lapataia, Ushuaia, Caleta Ro´balo (Chile), Puerto Harberton, and Punta Moat, followed during the Early Holocene after 10,000 14C yr BP. At the same time, an Early Holocene vegetation of open-grown communities spread into the interior valley floors (Route 3 exposure: Unit 1, Oyarzun peat bog: Pollen Zone O-3, and Valle de Andorra peat bog: Pollen Subzones VA-2c and VA-2d, Figs 4, 5 and 8) (Borromei, 1995; Borromei et al., 2007), which were fully occupied by ponds and braided rivers, whereas the forest-steppe ecotone was dominant along the Beagle Channel, under warmer and drier conditions in conjunction with repeated fires, as shown by abundant charcoal particles found in the Early and Late Holocene pollen records (Heusser, 2003).

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Fig. 5. Pollen and spore frequency (%) diagram at Oyarzu´n peat bog, Valle de Carbajal. From Borromei et al. (2007). By about 7600 14C yr BP, the landscape displayed a forest-steppe vegetational pattern in southern Tierra del Fuego, at the Beagle Channel and interior valleys (Valle de Andorra peat bog: Pollen Subzones VA-2b and VA-2a, Oyarzun peat bog: Pollen Zone O-2, Fig. 8). Estimated summer temperatures of 10–11C and 400–500 mm annual precipitation resemble modern forest-steppe ecotone of the central part of Tierra del Fuego.

Holocene Marine Incursion into the Beagle Channel The Early Holocene climatic amelioration is coincident with a transgressive event into the Beagle Channel which took place around 8000 14C yr BP. Probably, the marine advance occurred through the Murray Channel (Rabassa et al., 1986). The northern Beagle Channel coast is characterized by a terrace system and at least three levels which have been identified at 8–10 m, 4–6 m and 1.5–3 m (Gordillo et al., 1992). These deposits are mostly sandy and gravelly in grainsize, although clay-like sediments are found mainly in the westernmost sector of the Beagle Channel. Several authors have debated the processes concerning the depositional characteristics of this marine system and the problem of sea-level changes along the coast of the Beagle Channel (Porter et al., 1984; Rabassa et al., 1986; Mo¨rner, 1991; Gordillo et al., 1992). Taking into account the local glacial history, the oldest Holocene coastal deposits may only partially be the result of glacioisostatic recovery and the younger levels would have been fully due to recent tectonic uplift (Rabassa et al., 2000). Rabassa et al. (2004) and Bujalesky et al. (2004) suggested that the differential seismotectonic movements since the Middle Holocene would have forced the

uplifting of the western portion of the Argentine sector of the Beagle Channel and the downwarping of its eastern section. The transcurrent fault system with NW–SE and NE–SW orientations delimits sectors in which the marine deposits and landforms do not present a direct chronologicalaltitudinal correlation. The oldest marine radiocarbon dates in the Beagle Channel are located in the Lago Roca-Bahı´a Lapataia area (Fig. 2 Table 2). It is a typical glacial landscape that was submerged below sea level, thus generating a deep and narrow fjord and intricate archipelagos during the Holocene transgression. The transgressive-regressive cycle has generated several levels of marine terraces. These radiocarbon-dated marine deposits are scattered along Bahı´a Lapataia up to the eastern shore of Lago Roca, including the Archipie´lago Cormoranes area and both margins of Rı´o Ovando and Rı´o Lapataia (Gordillo et al., 1993) (Fig. 2 Table 2). At the Bahı´a Lapataia site (54500 S, 68340 W) (Fig. 2), palynological analysis of two sequences, Lapataia 1 and Lapataia 2 (LP1 and LP2, Borromei and Quattrocchio, 2001; 2007, Figs 6 and 7), corroborates the existence of two relatively high sea levels, one of them between 8240 + 60 14C yr BP and 7260 + 70 14C yr BP (Pollen Subzone LP-2f) and the other at ca. 6000 14C yr BP (Pollen Subzone LP-2g, 5800 + 65 14C yr BP at the Lapataia 2 section), with a great abundance of zoomorphs, mainly foraminiferal test-linings accompanied by copepod eggenvelopes, the acritarch Halodinium sp. and dinoflagellate cysts. Among the dinoflagellate cysts recovered from the marine level (Unit C) are Brigantedinium spp., Selenopemphix spp., Spiniferites spp. and Operculodinium centrocarpum sensu Wall and Dale 1966. The microplankton assemblages at LP1 and LP2 display characteristics of nearshore environments with a

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Table 2. Radiometric dates from Holocene marine deposits in the Lago Roca-Bahı´a Lapataia area.

5920 + 90 7518 + 58 7260 + 70 8240 + 60 5800 + 65 4160 + 45 4425 + 55 7500 + 80 3860 + 75 4440 + 120

Lago Roca

Bahı´a Lapataia

Rı´o Ovando

Source

AC-1060 NZ-7730 SI-6738 SI-6737 SI-6739 PTA-7573 SI-6735 PTA-7691 SI-6734 AC-0937

Rabassa et al., 1986 Gordillo et al., 1990 Rabassa et al., 1986 Rabassa et al., 1986 Rabassa et al., 1986 Rabassa et al., 2000 Rabassa et al., 1986 Rabassa et al., 2000 Rabassa et al., 1986 Figuerero and Mengoni Gon˜alons, 1986

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Fig. 6. Palynological frequency (%) diagram at Bahı´a Lapataia 1 site, Beagle Channel. great abundance of terrestrial palynomorphs, high nutrient influxes, low species diversity and abundance of dinocysts, cool-temperate seawater temperature and low salinity with significant dilution of surface waters from meltwater discharge (Borromei and Quattrocchio, 2007). At the Rı´o Ovando Locality (54510 S, 68350 W; Figuerero and Mengoni Gon˜alons, 1986; Rabassa et al., 1986) (Fig. 2), northward of the Bahı´a Lapataia site, marine sediments dated at 4160 + 45 14C yr BP represent the transgressive-regressive phase of the coastline. The dinoflagellate assemblages, similar to those of the Lapataia site, contain cysts of dinoflagellate of Echinidinium/ Islandinium complex and Polykrikos spp., characteristics

of modern Arctic fjords (S. Candel, personal communication, 2005). The littoral vegetation at the time of the marine incursion in the Beagle Channel was mainly arboreal, as can be seen in the pollen records by a significant increase in Nothofagus pollen, while a forest-steppe vegetational pattern spread at the regional level. A similar vegetational pattern was observed in the Rı´o Varela Locality (54520 S, 67110 W) (Fig. 2) at marine levels dated at 6240 + 70 14C yr BP and 6060 + 70 14C yr BP (Grill et al., 2002). In this Holocene sedimentary sequence, the palynofacies and sedimentological analyses allowed to identify a succession of seven different

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paleoenvironments from bottom to the top, namely (a) marine, nearshore and low energy environment, high nutrient supply, low salinity and cooler sea water temperature (Unit 1 and 2), (b) continental, lacustrine (Unit 3), (c) marshy (Unit 4), (d) fluvial to estuarine (Unit 7), (e) fluvial environment of meandering streams (Unit 6) and (f) the present soil (Unit 7). At the La Misio´n section, northeastern Tierra del Fuego, the pollen record from marine clays dated 8490 + 400 14C yr BP shows a forest expansion during the transgressional phase indicated by the appearance of marine diatom assemblages and increasing halophytes (Chenopodiaceae) (Markgraf, 1983, 1993b). At the Puerto del Hambre site, in the Estrecho de Magallanes (Chile), the pollen spectrum also shows an increase of the Nothofagus pollen frequency at the time of the marine incursion, as shown by the deposition of estuarine silty clays dated 7980 + 50 14C yr BP containing fossils of microforaminifer tests and dinoflagellate cysts (Heusser, 1995b).

Pollen Zone PH-1, Figs 5 and 8). The paleoclimatic conditions resemble those of the modern forest with annual precipitation varying between 500 and 800 mm, and summer temperature averaging 8–9C. Climatic fluctuations are registered during the last ca. 1400 yrs. which correspond to the Medieval Warm Period, although evidence for the Little Ice Age is less clear (Mauquoy et al., 2004). Changes in temperature and/or precipitation were inferred from multiproxy analyses of a raised bog at Valle de Andorra (54450 S, 68180 W). The chronology of the record was established using a 14 C wiggle-matching technique. The study of plant macrofossils, pollen, fungal spores, testate amoebae and peat humification showed two periods of cooler and/or wetter conditions estimated at ca. cal AD 1800–1930 and between ca. AD 1030 and 1100 and, drier and warmer conditions with a period of low local water tables between AD 960–1020.

Middle to Late Holocene

4. Charcoal and Human Ocupation of Tierra del Fuego

About 5000 14C yr BP and onwards, a regressive phase is documented until reaching present-day sea level. Raised beaches are recognized in this regressive phase along the eastern Beagle Channel coast (Gordillo et al., 1992). The paleoenvironmental conditions in southern Tierra del Fuego became more rigourous, the temperature decreased and the precipitation increased, and as a consequence, the Nothofagus woodland started to expand developing a closed forest together with the spreading of Empetrum mires (Oyarzu´n peat bog: Pollen Zone O-1; Valle de Andorra: Pollen Zone VA-1; Puerto Harberton peat bog:

Ancient fires are in evidence from charcoal particles recorded in pollen sequences in the Quaternary deposits in Tierra del Fuego (Heusser, 1994a, 1998, 2003). Forest fires are believed to be triggered by Paleoindian activities in those regions where other causal agents for fire, lightning from thunderstorms and volcanic eruptions, can logically be discounted. In this way, charcoal offers a possible means of tracing human presence and migrations in Tierra del Fuego (Heusser, 1994a). The charcoal record, in this context, suggests that human ocupation

378

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in Tierra del Fuego is dated at 13,280 14C yr BP at Bahı´a Inu´til, northwest of Tierra del Fuego. Archeological evidence of human occupation at Tres Arroyos site (53230 S, 68470 W), dated between 11,880 and 10,280 14 C yr BP, confirm their presence in the northern part of the island during the last millennium of the Pleistocene (Rabassa et al., 1992). The Paleoindian hunters would have reached Tierra del Fuego from southernmost Patagonia, when the island was yet part of the continent and a land connection existed along the present Magellan Straits valleys, particularly in the area of Segunda Angostura (Fig. 1) (Heusser, 2003). Toward the end of the Pleistocene, they had expanded to the southern sector of Tierra del Fuego. Incidence of fire increased and reached widespread frequency in the Early–Middle Holocene as can be inferred from the charcoal records (Lago Fagnano,

Lapataia, Ushuaia, Caleta Ro´balo, Puerto Harberton and Bahı´a Moat sites) (Heusser, 1994a, 2003). The charcoal evidence predates considerably the arrival of canoeing people at the Beagle Channel coast around 7000 14C yr BP (Tu´nel site, east of Ushuaia, Fig. 1) (Orquera and Piana, 1987 in: Heusser, 1994a). Peak amounts of charcoal in the records from the central (Onamonte section) and northeast (Cabo San Pablo section) sector of Tierra del Fuego allow to infer human presence much later, in the Late Holocene. Although archeological evidence of human occupation at Bahı´a Valentı´n site, dated 5900 14C yr BP (Fig. 1), confirm their presence in the southeastern area of Tierra del Fuego during the Middle Holocene (Acede de Reinoso et al., 1988 in: Rabassa et al., 2000).

Late and Postglacial Paleoenvironments of Tierra del Fuego 5. Conclusions The more relevant features of the Quaternary palynological studies of Tierra del Fuego exposed above lead to the following conclusions: 1. The oldest pollen stratigraphies located in eastern Tierra del Fuego (Lago Fagnano site) record steppetundra vegetational communities developed during an Early Wisconsinan or an older glacial event (MIS 6?, MIS 4?). 2. The controversial cold interval which most probably took place during the Younger Dryas Chron, with a duration on the order of a millennium, has been recorded in southern Tierra del Fuego (Beagle Channel area) based on pollen stratigraphy of peat bog sections (Heusser and Rabassa, 1987; Heusser, 1989c, 1990, 1998; Borromei et al., 2007). The assumption that changes in pollen accumulated during the Younger Dryas Chron were caused by fire (Markgraf, 1993b) is not supported by the charcoal record in Tierra del Fuego (Heusser, 2003). However, Markgraf et al. (2002) have suggested that the Late Glacial environmental variability appears to be related to increased fire frequency, interpreted from high levels of charcoal during that interval. Thus, a decrease in rain forest taxa and an increase in woodland or steppe elements would be the vegetation response to increased fire frequency. 3. The treeless vegetation environment installed prior to 10.3 ka BP indicates that the inner valley glaciers had already receded to higher positions in the landscape by this time and that the Younger Dryas cooling event would have affected the cirque and valley glaciers located in the Alpine landscape. This assumption, opposite to Heusser´s (1998) inference that ice in interior valleys north of Ushuaia apparently wasted slowly, favours the hypothesis thatarapid deglaciation process had taken place along the Fuegian Andes and, later, in the Beagle Channel region. 4. On the southern slope of the Fuegian Andes, along the Beagle Channel, the forest-steppe ecotone established toward 10,000 14C yr BP, while into the interior valleys an open-grown vegetation developed on the valley floors. It was not until ca. 7600 14C yr BP that the landscape displayed a forest-steppe regional pattern along the Beagle Channel area, under warmer and drier conditions in an environmental setting subject to high fire incidence. Closedcanopy forest of Nothofagus dominated the landscape under wetter and cooler conditions by about 5000 14C yr BP due to increasing humidity coming from the southern Pacific and the weakening of the subtropical anticyclonic cells (Heusser, 2003). The northern side of the Fuegian Andes axis presents an Early Holocene prevalence of grass steppe, coincident with warming and low humidity in the Fuego–Patagonian steppe (Heusser, 2003). The Nothofagus forest gained space into the steppe after 8000 14C yr BP at Lago Fagnano (Heusser, 2003), Lago Yehuin (Markgraf, 1983) and Paso Garibaldi

379

(Markgraf, 1993b) localities and after 5000 14C yr BP at the Onamonte site (Heusser, 1993a). Definitive expansion of closed Nothofagus forest occurred toward 5000 14C yr BP north of the Beagle Channel area at Lago Fagnano, Lago Yehuin and Paso Garibaldi and, after 1570 14C yr BP, toward the center at Onamonte (Heusser, 2003). In northeastern and eastern Tierra del Fuego, at Cabo San Pablo and La Misio´n, the forest-steppe ecotone prevailed until about 300 14C yr BP when the forest increased and gained supremacy at Cabo San Pablo and the steppe environments prevailed at La Misio´n (Markgraf, 1993b; Heusser and Rabassa, 1995). 5. At the time of the Holocene marine transgression into the Beagle Channel by about 8000 14C yr BP, the Nothofagus forest was already installed at the adjacent lands, whereas the forest-steppe ecotone spread inlandward at the regional level. 6. The beginning of the transgressive event into the Beagle Channel, ca. 7500 14C yr BP, flooded the west sector of the channel at Lago Roca–Bahı´a Lapataia area. The palynological assemblages reveal nearshore environments, where the great abundance of pollen and spores indicates large fluvial inputs. The marine environment was characterized by sparse assemblages of dinoflagellate cysts dominated by Peridiniales taxa (Brigantedinium spp. and Selenopemphix spp.), in addition to Gonyaulacales taxa (Spiniferites spp. and Operculodinium centrocarpum). The occurrence of Halodinium sp. together with copepod egg-envelopes and foraminiferal testlinings is abundant. The overall palynological assemblages reflect inner estuarine environments related to low and variable salinities and/or turbulence, cooltemperate seawater temperature and abundant dissolved nutrients associated with significant dilution of surface waters from freshwater runoff. 7. Wetter and cooler climatic conditions about 5000 14C yr BP and onwards, appear not to be uniform in southern Tierra del Fuego. This variability is shown along the pollen records where the Nothofagus and Empetrum pollen frequencies are highly fluctuating. The cause of this behavior may be climatic, or it may simply be a constructional feature characteristic of regional raised mires. Pollen evidence points to peak influx, cooler, more humid climate, and Late Holocene glacier advances (Heusser, 1998). Mercer’s chronology infers a culmination of neoglacial advances at 4500–4000 14C yr BP, 2700–2000 14C yr BP and during recent centuries for southern Patagonian glaciers (Heusser, 2003). Although no absolute dates are yet available for these cool periods in the cirques of the hanging lateral valleys in the studied area, some inferences can be made from diverse paleoclimatic studies carried out at Ushuaia bay, in the Canal Beagle. According to pollen (Heusser, 1998), dendrochronology (Villalba, 1989, 1994) and marine waters isotopic (Obelic et al., 1998) studies, climatic reversal episodes would have occurred in the region between 4400–3400, 2800–2000, 1800–1400 and 500 14C yr BP in southern Tierra del Fuego (Borromei et al., 2007).

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8. During the last ca. 1400 yrs climatic shifts are established from multiproxy analyses (plant macrofossil, pollen, fungal spores, testate amoebae and humification) of a raised bog. The 14C wiggle-match dating suggests that a period of low local water tables in a raised peat bog occurred between AD 890 and 1020, which may correspond to the Medieval Warm Period date range of AD 950–1045 derived from the Northern Hemisphere tree-ring data. Evidence for ‘Little Ice Age’ climatic deteriorations is less clear, although a period of cooler and/or wetter conditions was detected between ca. AD 1010–1110 and a later period of cooler/wetter conditions estimated at ca. cal. AD 1790–1930 (Mauquoy et al., 2004).

References Acede de Reinoso, T., Ca´mera, P. and Vidal, H. (1988). Bahı´a Valentı´n: Encuentros en la costa. IX Congreso Nacional de Arqueologı´a Argentina, Resu´menes 115. Buenos Aires. Auer, V. (1933). Verschiebungen der Wald- und Steppengebiete Feuerlands in postglazialer Zeit. Acta Geographica 5, 1–313. Helsinki. Auer, V. (1956). The Pleistocene of Fuego-Patagonia. Part I: The ice and interglacial ages. Annales Academiae Scientarum Fennicae. Series A. III. Geologica-Geographica 45, 1–226. Auer, V. (1958). The Pleistocene of Fuego-Patagonia. Part II: The history of flora and vegetation. Annales Academiae Scientarum Fennicae. Series A. III. GeologicaGeographica 50, 1–239. Auer, V. (1970). The Pleistocene of Fuego-Patagonia. Part V: Quaternary problems of Southern South America. Annales Academiae Scientarum Fennicae. Series A. III. Geologica-Geographica 100, 1–194. Auer, V. (1974). The isorhythmicity subsequent to the Fuego-Patagonian and Fennoscandian ocean level transgressions and regressions of the last glaciation. Annales Academiae Scientarum Fennicae. Series A. III. Geologica-Geographica 115, 1–88. Borromei, A.M. (1995). Ana´lisis polı´nico de una turbera holoce´nica en el Valle de Andorra, Tierra del Fuego, Argentina. Revista Chilena de Historia Natural 68, 311–319. Santiago. Borromei, A.M. and Quattrocchio, M. (2001). Palynological study of Holocene marine sediments from Bahı´a Lapataia, Beagle Channel, Tierra del Fuego, Argentina. Revista Espan˜ola de Micropaleontologı´a 33, 61–70. Madrid. Borromei, A.M. and Quattrocchio, M., (2007). Palynology of Holocene marine deposits at Beagle Channel, southern Tierra del Fuego, Argentina. Ameghiniana 41(1), 161–171. Buenos Aires. Borromei, A.M., Coronato, A., Quattrocchio, M. et al. (2007). Late Pleistocene – Holocene environments in Valle Carbajal, Fuegian Andes valley, Southern South America. Journal of South American Earth Sciences 23, 321–335. Bujalesky, G., Heusser, C., Coronato, A. et al. (1997). Pleistocene glaciolacustrine sedimentation at Lago Fagnano, Andes of Tierra del Fuego, Southernmost South America. Quaternary Science Reviews 16, 767–778.

Bujalesky, G., Coronato, A., Roig, C. and Rabassa, J. (2004). Holocene differential tectonic movements along the Argentine sector of the Beagle Channel (Tierra del Fuego) inferred from marine palaeoenvironments. In: Carcione, J., Donda, F. and Lodolo, E. (eds), Geosur 2004, International Symposium on the Geology and Geophysics of the Southernmost Andes, the Scotia Arc and the Antarctic Peninsula, Extended Abstracts, Bolletino di Geofisica teorica ed applicata 45, 235–238. Trieste. Coronato, A. (1990). Definicio´n y alcance de la u´ltima glaciacio´n pleistocena (Glaciacio´n Moat) en el Valle de Andorra, Tierra del Fuego. XI Congreso Geolo´gico Argentino 1, 286–289. Buenos Aires. de Vernal, A. and Giroux, L., (1991). Distribution of organic walled microfossils in Recent sediments from the estuary and Gulf of St. Lawrence: some aspects of the organic matter fluxes. In: Therriault, J.-C. (ed.), The Gulf of St. Lawrence: small ocean or big estuary? Canadian Special Publication of Fisheries and Aquatic Sciences 113, 189–199. Figuerero, M. and Mengoni Gon˜alons, G. (1986). Excavaciones arqueolo´gicas en la Isla El Salmo´n, Parque Nacional de Tierra del Fuego. PREP, Informes de Investigacio´n 4, 1–95. Buenos Aires. Gordillo, S., Bujalesky, G., Pirazzoli, P. et al. (1992). Holocene raised beaches along the northern coast of the Beagle Channel, Tierra del Fuego, Argentina. Palaeogeography, Palaeoclimatology, Palaeoecology 99, 41–54. Gordillo, S., Coronato, A. and Rabassa, J. (1993). Late Quaternary evolution of subantarctic paleofjord, Tierra del Fuego. Quaternary Science Reviews 12, 889–897. Grill, S., Borromei, A.M., Quattrocchio, M. et al. (2002). Palynological and sedimentological analysis of Recent sediments from Rı´o Varela, Beagle Channel, Tierra del Fuego, Argentina. Revista Espan˜ola de Micropaleontologı´a 34, 2, 145–161. Madrid. Heusser, C.J. (1987). Quaternary vegetation of southern South America. Quaternary of South America and Antarctic Peninsula A.A. Balkema Publishers, Rotterdam, 5, 197–221. Heusser, C.J. (1989a). Late quaternary vegetation and climate of Southern Tierra del Fuego. Quaternary Research 31, 396–406. Heusser, C.J. (1989b). Polar perspective of late quaternary climates in the Southern Hemisphere. Quaternary Research 32, 60–71. Heusser, C.J. (1989c). Climate and chronology of Antarctica and adjacent South America over the past 30,000 yr. Palaeogeography, Palaeoclimatology, Palaeoecology 76, 31–37. Heusser, C.J. (1990). Late-glacial and Holocene vegetation and climate of subantarctic South America. Review of Palaeobotany and Palynology 65, 9–15. Heusser, C.J. (1993a). Late Quaternary forest-steppe contact zone, Isla Grande de Tierra del Fuego, subantarctic South America. Quaternary Science Reviews 12, 169–177. Heusser, C.J. (1993b). Late-glacial of southern South America. Quaternary Science Reviews 12, 345–350. Heusser, C.J. (1994a). Paleoindians and fire during the late Quaternary in southern South America. Revista Chilena de Historia Natural 67, 435–443. Santiago.

Late and Postglacial Paleoenvironments of Tierra del Fuego Heusser, C.J. (1994b). Quaternary paleoecology of Fuego-Patagonia. Revista do Instituto Geolo´gico 15, 1/2, 7–26. Sa˜o Paulo, Brazil. Heusser, C.J. (1995a). Palaeoecology of a Donatia-Astelia cushion bog, Magellanic Moorland-Subantarctic Evergreen Forest transition, Southern Tierra del Fuego, Argentina. Review of Palaeobotany and Palynology 89, 429–440. Heusser, C.J. (1995b). Three Late Quaternary pollen diagrams from Southern Patagonia and their palaeoecological implications. Palaeogeography, Palaeoclimatology, Palaeoecology 118, 1–24. Heusser, C.J. (1997). Deglacial setting of the Southern Andes following the last glacial maximum: a short review. Anales del Instituto de la Patagonia 25, 89–103. Punta Arenas, Chile. Heusser, C.J. (1998). Deglacial paleoclimate of the American sector of the Southern Ocean: Late GlacialHolocene records from the latitude of Beagle Channel (55 S), Argentine Tierra del Fuego. Palaeogeography, Palaeoclimatology, Palaeoecology 141, 277–301. Heusser, C.J. (2003). Ice age Southern Andes – A chronicle of paleoecological events. Developments in Quaternary Science 3, 240 pp. Elsevier, Amsterdam.. Heusser, C.J. and Rabassa J. (1987). Cold climatic episode of Younger Dryas Age in Tierra del Fuego. Nature 328, 6131, 609–611. Heusser, C.J. and Rabassa J. (1995). Late Holocene forest-steppe interaction at Cabo San Pablo, Isla Grande de Tierra del Fuego, Argentina. Quaternary of South America and Antarctic Peninsula 9, 179–188. A.A.Balkema Publishers, Rotterdam. Heusser, C.J., Heusser, L.E., Lowell, T.V. et al. (2000). Deglacial palaeoclimate at Puerto del Hambre, subantarctic Patagonia, Chile. Journal of Quaternary Science 15, 2, 101–114. Markgraf, V. (1980a). New data on the late and post glacial vegetational history of La Misio´n, Tierra del Fuego, Argentina. In: Proceedings of the IV International Palynological Congress (1976–1977, Lucknow, India) 3, 68–74. Markgraf, V. (1980b). Paleoclimatic changes during the last 15,000 years in subantarctic and arid environments in Argentina (South America). V International Conference of Palynology 33. Cambridge, England. Markgraf, V. (1983). Late and postglacial vegetational and paleoclimatic changes in subantarctic, temperate and arid environments in Argentina. Palynology 7, 43–70. Markgraf, V. (1991a). Late Pleistocene environmental and climatic evolution in southern South America. Bamberger Geographische Schriften 11, 271–281. Markgraf, V. (1991b). Younger Dryas in southern South America? Boreas 20, 63–69. Markgraf, V. (1993a). Paleoenvironments and paleoclimates in Tierra del Fuego and southernmost Patagonia, South America. Palaeogeography, Palaeoclimatology, Palaeoecology 102, 53–68. Markgraf, V. (1993b). Younger Dryas in southernmost South America – an update. Quaternary Science Reviews 12, 351–355. Markgraf, V. and Anderson, L. (1994). Fire history of Patagonia: Climate versus human cause. Revista do

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Instituto Geolo´gico 15, 1–2, 35–47. Sa˜o Paulo, Brazil. Markgraf, V. and Kenny, R. (1997). Character of rapid vegetation and climate change during the late-glacial in southernmost South America. In: Huntley, B. (ed.), Past and Future Rapic Climatic Changes: The Spatial and Evolutionary Responses of Terrestrial Biota Berlin, Springer-Verlag, 81–90. Markgraf, V., Dodson, J.R., Kershaw, A.P. et al. (1992). Evolution of late Pleistocene and Holocene climates in the circum-South Pacific land areas. Climate Dynamics 6, 193–211. Markgraf, V., Webb, R.S., Anderson, K.H. and Anderson, L. (2002). Modern pollen/climate calibration for southern South America. Palaeogeography, Palaeoclimatology, Palaeoecology 181, 375–397. Mauquoy, D., Blaauw, M., van Geel, B. et al. (2004). Late Holocene climatic changes in Tierra del Fuego based on multiproxy analyses of peat deposits. Quaternary Research 61, 148–158. Mo¨rner, N.A. (1991). Holocene sea level changes in the Tierra del Fuego region. Revista do Instituto Geolo´gico 8, 133–151. Sa˜o Paulo, Brazil. Mudie, P.J., Harland, R., Matthiessen, J. and de Vernal, A. (2001). Marine dinoflagellate cysts and high latitude Quaternary paleoenvironmental reconstructions: an introduction. Journal of Quaternary Science 16, 7, 595–602. Obelic, B., Alvarez, A., Argullo´s, J. and Piana, E.L. (1998). Determination of water palaeotemperature in the Beagle Channel (Argentina) during the last 6000 yr through stable isotope composition of Mytilus edulis shells. Quaternary of South America and Antarctic Peninsula 11, 47–71. A.A. Balkema Publishers, Rotterdam. Orquera, L. and Piana, E. (1987). Human littoral adaptation in the Beagle Channel region: the maximum possible age. Quaternary of South America and Antarctic Peninsula 5, 133–165. A.A. Balkema Publishers, Rotterdam, 5, 133–165. Pendall, E., Markgraf, V., White, J.W.C. et al. (2001). Multiproxy record of late Pleistocene – Holocene climate and vegetation change in Patagonia. Quaternary Research 55, 168–178. Porter, S., Stuiver, M. and Heusser, C.J. (1984). Holocene sea level changes along the Strait of Magellan and Beagle Channel, southernmost of South America.Quaternary Research 22, 59–67. Quattrocchio, M. and Borromei, A.M. (1998). Paleovegetational and paleoclimatic changes during the late Quaternary in southwestern Buenos Aires Province and southern Tierra del Fuego (Argentina). Palynology 22, 67–82. Rabassa, J., Heusser, C.J. and Stuckenrath, R. (1986). New data on Holocene sea transgression in the Beagle Channel: Tierra del Fuego, Argentina. Quaternary of South America and Antarctic Peninsula 4, 291–309. A.A.Balkema Publishers, Rotterdam. Rabassa, J., Bujalesky, G., Meglioli, A. et al. (1992). The Quaternary of Tierra del Fuego, Argentina: The status of our knowledge. Sveriges Geologiska Underso¨kning, Ser. Ca. 81, 249–256. Stockholm.

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Rabassa, J., Coronato, A., Bujalesky, G. et al. (2000). Quaternary of Tierra del Fuego, Southernmost South America: an updated review. Quaternary International 68–71, 217–240. Rabassa, J., Coronato, A.M.J., Roig, C. et al. (2004). Un bosque sumergido en Bahı´a Sloggett, Tierra del Fuego, Argentina: evidencia de actividad neotecto´nica diferencial en el Holoceno tardı´o. In: Blanco Chao, R., Lo´pez-Bedoya, J. and Pe´rez Alberti, A. (eds), Procesos geomorfolo´gicos y evolucio´n costera, Cursos

e Congresos N145, Publicacio´ns. Universidad de Santiago de Compostela, Spain. 3331–346. Villalba, R. (1989). Latitude of the surface high-pressure belt over western South America during the last 500 years as inferred from tree-ring analysis. Quaternary of South America and Antarctic Peninsula 7, 273–303. A.A.Balkema Publishers, Rotterdam. Villalba, R. (1994). Tree-rings and glacial evidence from the Medieval Warm Epoch and the Little Ice Age in southern South America. Climatic Change 30, 1–15.

19 Diatoms from Patagonia and Tierra del Fuego Marcela A. Espinosa Centro de Geologı´a de Costas y del Cuaternario Universidad Nacional de Mar del Plata CC 722, 7600 Mar del Plata, Argentina. compile the most important contributions in order to know the importance of diatoms for characterizing aquatic paleoenvironments in high latitudes of the Southern Hemisphere. This review is divided into lacustrine and coastal-marine records. Some sequences like Laguna Los Juncos (Rı´o Negro Province) and Bahı´a San Blas (Buenos Aires Province) were studied by the author, but are still unpublished.

1. Introduction Diatoms are microscopic algae with a siliceous exoskeleton (frustule) resistant to decay. There are 285 genera (according to Round et al., 1990) encompassing 10,000–12,000 recognized species. They are ubiquitous, occurring in almost all aquatic habitats, where they may be planktonic, benthic, periphytic (growing on plant or seaweed surfaces), epizoic (on animals) or endozoic (within animals). Because of their siliceous composition, they are often very well preserved. Once incorporated in sediments, diatom frustules remain forming a record of the populations from the benthos or the water column. Marine floras are rarely as well preserved as freshwater ones. Many have pointed out that diatoms are almost ideal biological monitors (Dixit et al., 1992). There is a very large number of ecologically sensitive species, which are abundant in nearly all habitats where water is at least occasionally present, leaving their remains preserved in the sediments of most lakes and many areas of the oceans, as well as in other environments (Stoermer and Smol, 1999). Diatoms are useful in biostratigraphy and paleoecology. The study of assemblages from lake sediments can be used to identify water-level changes and eutrophication processes (Haberzettl et al., 2005) and climatic changes (Markgraf, 1993). They can be used also as indicators of coastal paleoenvironments and relative sea-level changes (Espinosa, 1988, 1994, 1998, 2001; McCulloch and Davies, 2001; Espinosa et al. 2003). In addition, the species composition of marine-littoral diatom communities is fairly similar throughout the world (Denys and de Wolf, 1999) enabling spatial and temporal correlations. In marine environments, they are used to determine water chemistry, paleosalinities, paleodepth, paleotemperatures, nutrient concentration and currents. Joaquı´n Frenguelli was a pioneer on diatom studies in Argentina. He wrote more than 50 papers about continental and marine diatoms of different sites of the country describing and drafting thousand of taxa. Most of the papers were about fossil diatoms collected in Buenos Aires Province. His reference work constitutes the base of diatom taxonomy and the application of diatoms in stratigraphic studies in Argentina. The information about fossil diatoms from Patagonia and Tierra del Fuego is scarce. The first papers are merely lists of taxa without a paleoecological interpretation. Recently, multiproxy studies include diatoms to interpret paleoenvironmental and paleoclimatic changes during the Cenozoic. The aim of this chapter is to

2. Lacustrine Records 2.1. Late Tertiary Collo´n Cura´ Formation, Rı´o Negro Province Tertiary diatomite deposits of the Collo´n Cura´ Formation were studied in Rı´o Negro Province (41150 S, 71 W, Fig. 1) by Martinez Macchiavello (1984). Three assemblages have been described: 1. Lower Oligocene: Anomoeoneis sphaerophora var. sculpta, Pinnularia spp., Aulacoseira (= Melosira) distans and Aulacoseira (= Melosira) granulata. 2. Lower Miocene: Aulacoseira (= Melosira) distans and Aulacoseira (= Melosira) granulata, Gomphoneis olivacea (= Gomphonema olivaceum) and Cymbella cymbiformis. 3. Lower to Middle Miocene: Aulacoseira (= Melosira) distans and Aulacoseira (= Melosira) granulata, Cymbella cymbiformis and A. sphaerophora var. sculpta. Martinez Macchiavello (1984) characterized these assemblages as belonging to continental environments with important climatic changes.

2.2. Quaternary Lago Fagnano, Andes of Tierra del Fuego A Pleistocene glaciolacustrine sequence exposed at the southeastern end of Lago Fagnano, Tierra del Fuego (54330 S, 67190 –68480 W; 80 m a.s.l., Fig. 1), was described by Bujalesky et al. (1994, 1997; Fig. 2). The diatom content of the lacustrine sediments of Section 6 Unit E was analyzed (Table 1). The base of this level was dated at 39,560 + 3980 14C yr BP. This radiocarbon date should be considered as a minimum age, since another  2008 ELSEVIER B.V. ALL RIGHTS RESERVED

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Fig. 1. Location of Patagonian and Fuegian sites with diatom records.

m.a.l.l 30

Silt

layer in the sequence has provided an infinite age ( >58,000 14C yr BP; Bujalesky et al., 1997, p. 773). The diatom bearing layers containing a dominance of Epithemia adnata (70%) reveal a freshwater, cool, temperate, low energy environment, with alkaline pH and relative stable trophic conditions. This assemblage may be related to climatic warming during the termination of a glacial stade or, otherwise, to local warming from hydrothermal sources in hot springs related to a major fault zone in this region at relatively short distance (20 km).

Sand Gravel

20

Bajada de Rahue, Neuque´n Province 40 14C ka. B.P.

diatoms

E

10

References Silty bottomset Parallel lamination Upper gravels Lacustrine deposits – Fossil peat beds Foreset Wavy bedding Flaser bedding

0

Section 6

Fig. 2. Stratigraphic profile of Section 6 of Lago Fagnano, Tierra del Fuego (modified from Bujalesky et al., 1997).

Markgraf et al. (1986) performed a semiquantitative evaluation of diatom concentration on a section of 450 cm from Bajada de Rahue (39220 S, 70560 W, 1000 m a.s.l., Fig. 1). This sequence was deposited between 33,500 and 27,900 14C yr BP. The results showed that Staurosirella (= Fragilaria) pinnata is the dominant taxon associated with E. adnata and Cymbella cymbiformis indicating shallow waters, probably not exceeding 1–2 m in depth (Fig. 3). This is particularly valid for the top and base of the section. Aulacoseira (= Melosira) distans and Eunotia spp. in the middle part of the profile indicate that during this period of deposition a small lake or marsh was receiving an abundant supply of terrestrial organic material. Diatoms, pollen and dates suggest that the

Diatoms from Patagonia and Tierra del Fuego

385

Table 1. Diatom taxa and ecological preferences from Lago Fagnano section (modified from Bujalesky et al., 1994). Taxon

Relative abundance

Life form

Salinity and Temperature

pH

Nutrients content

Epithemia adnata (Ku¨tz.) Bre´b. Staurosira construens var. binodis (Ehr.) Ham. Cymbella cymbiformis Hustedt Planothidium haukianum (Grun.) Round & Burkht Cocconeis placentula Ehr.

High (70%)

Epiphyte

Medium Low

Tychoplankton Epiphyte Epiphyte

Low

Epiphyte

Low

Tychoplankton Benthos epipelon Tychoplankton Epiphyte

Alkalibiontic Alkaliphilous Alkaliphilous Alkaliphilous Alkaliphilous Alkaliphilous Alkaliphilous Alkaliphilous Alkaliphilous

Mesooligotrophic Mesoeutrophic Eutrophic

Low

Freshwater Temperate Freshwater Temperate Freshwater Temperate Brackish Temperate Freshwater Temperate Freshwater Temperate Freshwater Temperate Freshwater Cold

Staurosira construens var. venter (Ehr.) Hamilton Sellaphora bacillum (Ehr.) D. Mann Staurosirella pinnata (Ehr.) Williams & Round Gomphoneis olivacea (Hornemann) Dawson

Very low Very low Very low

Freshwater Temperate

Eutrophic Eutrophic Mesoeutrophic Mesotrophic Eutrophic Mesoeutrophic

present from lacustrine sediments of Maar Magallanes (52070 S; 69160 W, Fig. 1) has been presented by Maidana and Corbella (1997). Cluster analysis shows six diatom groups. Some of them are dominated by planktonic forms and others by shallow water taxa indicating fluctuations of the water level. The dominance of Staurosira (= Fragilaria) construens at 32 m depth was interpreted as a peak of colder conditions.

whole lacustrine section was deposited during a relative short time interval under an environmental regime resembling the modern, local environments.

Maar Magallanes, Santa Cruz Province

bi fo rm ia Di is ad pl na Au one t a l i Eu aco s sp n se . Pi otia ira nn ( d ul cu ista ar rv n ia at s sp a/p p. ec tin a em

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References

Very abundant (diatomite) Abundant Common Occasional Rare Barren

0

100

200

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A preliminary study of the diatom content of a core spanning between 31,560 + 480 14C yr BP and the

28.8 ± 1.1 29 ± 1.4

300

Diatomite 27.9 ± 1.2 33.5 ± 1.5

400

Gravel Peat Sand Ash Clay

Fig. 3. Diatom relative abundances from Rahue section (modified from Markgraf et al., 1986).

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Marcela A. Espinosa

Laguna Cari Laufquen, Rı´o Negro Province

Lago Cardiel, Santa Cruz Province

The diatom flora from Pleistocene sediments between 18,400 and ca. 13,000 14C yr BP was studied by Bradbury et al. (2001) at Laguna Cari Laufquen (41130 S, 69420 W, 800 m a.s.l., Fig. 1). According to the lakelevel curve from S. Stine (in Bradbury et al., 2001), the highest level of the lake occurred during this interval. Rare diatoms and ostracodes indicate that the late glacial lacustrine system was turbid and saline, but not especially productive. Similar conditions are indicated by the second highstand at 10,000 14C yr BP. The high lakelevels at Laguna Cari Laufquen document greater precipitation at this latitude as indicated by Galloway et al. (1988) who postulated lakes larger than today between 19,500 and 15,000 14C yr BP.

Two cores spanning 10,230 + 65 to 1,630 + 40 14C yr BP were analysed by Markgraf et al. (2003) in Lago Cardiel (49 S, 71 W, 276 m a.s.l., Fig. 1). These authors integrated the information from grainsize, ostracode, pollen, green algae, diatoms and stable isotopes studies to interpret the Holocene environmental history of Lago Cardiel. The diatom stratigraphy of both lake margin cores begins about 10,000 14C yr BP with high percentages of Epithemia argus, an epiphyte taxon indicative of shallow water suggesting fresh, if alkaline, water with a significant magnesium and sulphate concentration (Fig. 4). From 9600 to 6800 14C yr BP, the section is characterized by a small species of Hyalodiscus with small numbers of planktonic diatoms (Cyclostephanos sp. and Cyclotella

Fig. 4. Diatom diagram from Lago Cardiel cores (modified from Markgraf et al., 2003).

Diatoms from Patagonia and Tierra del Fuego meneghiniana) indicating high water levels. The fluctuating dominance of Diploneis smithii after 6000 14C yr BP with other species may track rapid changes in lakelevel. Markgraf et al. (2003) concluded that after the desiccation phase (11,200 14C yr BP) the lake rose rapidly and reached the present-day shoreline shortly after 10,000 14C yr BP. After about 9500 14C yr BP, the lake was relatively deep (55 m above present water levels) and then, after 6100 BP the lakelevels fell to less than 15 m (Bradbury et al., 2001). Since that time, the lake did not fall for extensive periods by more than a few meters. After 4900 BP all the indicators show generally more variable, almost cyclic changes.

387

sixteenth centuries moisture increased and higher lakelevels explain the supply of more diatoms from the littoral zone of the lake due to an enlarged habitat. In between AD 1770–1940, the lakelevel remained high. Eutrophication trends are reflected by all the proxies. Staurosira (= Fragilaria) construens var. venter, which is indicative of meso-eutrophic water conditions, appears in high relative abundance. In the course of the twentieth century, Laguna Potrok Aike reacted like many other Patagonian lakes with a lakelevel lowering after 1940, culminating around 1990, and followed by a subsequent rise and recession (Haberzettl et al., 2005). Laguna Azul, Santa Cruz Province

Laguna Potrok Aike, Santa Cruz Province A continuous high resolution climatic record from AD 400 to the present for the volcanogenic lake Laguna Potrok Aike (51580 S, 70230 W; 113 m a.s.l., Fig. 1) was analyzed applying multi-proxy data by Haberzettl et al. (2005). Between AD 400 and AD 1120, rapid fluctuations in almost all proxies indicate unstable conditions. Planktonic diatoms decrease and epiphytic taxa increase. A high lakelevel and wet climate conditions occur in the interval AD 1120–1240 where epiphytic and littoral diatoms increase probably in response to an enlarged habitat caused by a higher lakelevel. From the mid thirteenth century until the early fifteenth century (AD 1240–1410), low values of epiphytic and littoral taxa point to a reduction of shallow water environments and the beginning of drought conditions. During the fifteenth and

A multi-proxy study of sediment cores from the crater lake Laguna Azul (52050 S, 69350 W, Fig. 1) was carried out by Mayr et al., 2005. One of these cores spanning AD 900 to the present was studied for diatoms, pollen, charcoal, isotopes and geochemistry. Between AD 900 and AD 1850, diatom assemblages were dominated by Staurosira construens var. venter and Staurosirella pinnata indicating meso-eutrophic water conditions. From AD 1700 to AD 1900, low values of d13Corg indicate that the lakelevel rose markedly. High charcoal concentrations recorded at the end of that period reflect strong fire events. During the time when minerogenic input reached its maximum the diatom assemblage changed from S. construens var. venter dominated assemblage to a predominance of Stephanodiscus parvus (Fig. 5) indicating a change in the

Fig. 5. Diatom diagram from Laguna Azul (modified from Mayr et al., 2005).

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Marcela A. Espinosa

nutritional budget of the lake or changes in the lacustrine habitats. Mayr et al. (2005) postulated that lower lakelevels during the period AD 1450–1670 offered enlarged habitats for S. construens var. venter. With an increase in effective moisture in the eighteenth century, the lakelevel rose with the consequence of rapidly shrinking shallow water areas. Enhanced nutrient supply by fire-induced erosion and the land use of farmers led the change in the diatom assemblage with the dominance of S. parvus.

pH 100 80 60

% 40 20 0 Central zone

Laguna Los Juncos, Rı´o Negro Province Modern lacustrine diatoms from Laguna Los Juncos were analyzed as part of the project ‘‘Recent ostracods and chironomids of North Patagonia’’ (a project sponsored by CONICET, the National Research Council of Argentina, 1998). The studied transect was divided in four zones: wetland (outer zone), closed grasses, grasses and center (inner zone). The vegetation around the pond consists of herbaceous vegetation represented by Juncus balticus, J. stipularis, Carex aculatta, C. gayana, Veronica anagallis, V. serpillifolia and Trifolium repnes, and the grasses Poa pratensis, Holcus lanatus and Hordeum comosum (Acevedo et al., 1995). The four zones are dominated by freshwater and alcalophilous diatoms. Benthic taxa dominating wetland and central zone indicate shallow waters. Tychoplankton is more important on grasses and closed grasses sediments characterizing higher water levels. Epiphytes rise from the central zone to wetland showing the increasing vegetation cover. Aerophilous taxa are present in all the sites indicating periods of desiccation (Fig. 6). The presence of E. adnata, Epithemia argus, Staurosira construens and S. construens var. venter, Staurosirella pinnata, Cymbella spp. and Pinnularia spp. along the transect reveal a similarity between modern assemblages and those occurring in Pleistocene deposits of Bajada de Rahue, Neuque´n (Markgraf et al., 1986), ca. 200 km north of this pond, and the Pleistocene deposits of Lago Fagnano (Bujalesky et al., 1994, 1997), much more distant, ca. 2000 km south of this site.

3. Coastal-Marine Environments

Grasses

Closed grasses

Wetland

Alkaliphilous Acidophilous

Alkalibiontic Indifferent

Salinity 100 80 60

% 40 20 0 Central zone

Grasses

Closed grasses

Mesohalobous O.Indifferent

Wetland

O.Halophilous Halophobous

Life Form 60

40

% 20 0

Central zone

Grasses zone

Closed grasses

Plankton

Tychoplankton

Benthos

Epiphytes

Wetland

Aerophilous

Fig. 6. Distribution of diatom ecological groups from Laguna Los Juncos transect.

3.1. Late Tertiary Gran Bajo del Gualicho, Rı´o Negro Province Rada Tilly, Golfo San Jorge, Chubut Province A sequence extending in age from the Late Miocene to the Early Pliocene was analyzed in the Gran Bajo del Gualicho (Martinez Macchiavello, 1984). Some of the taxa mentioned here are Actinocyclus octonarius (= A. ehrenbergii), Actinoptychus splendens, Coscinodiscus asteromphalus, C. marginatus, C. jonesianus, C. oculus iridis, Amphora ovalis, A. sphaerophora var. sculpta, Aulacoseira (= Aulacosira) italica, Auliscus sculptus, Cymatosira sp., Fragilaria fonticola and Martyana (= Opephora) martyi. These assemblages have been assumed by the author to represent a marine environment with continental influence.

A qualitative study of bottom sediment samples from Rada Tilly (Fig. 1) was performed by Frenguelli (1939a). These sediments are overlying Middle Tertiary deposits. The diatom record is composed of 62 neritic forms. The assemblages are dominated by cold waters taxa: Pinnularia quadratarea, Campylopyxis garkeana (= Rhoikoneis garkeana), Nitzschia socialis var. kariana, Chaetoceros socialis and Nitzschia sigma, all of them are characteristic forms from the littoral zone of southern Patagonia (Frenguelli, 1939a, 1939b).

Diatoms from Patagonia and Tierra del Fuego

389

Table 2. Diatoms from northern Tierra del Fuego (modified from Frenguelli 1923, 1924). Marine taxa

Freshwater taxa

Actinoptychus senarius (= A. undulatus) (Ehr.) Ehr. Actinoptychus vulgaris Schumann

Epithemia turgida Ku¨tz. Epithemia adnata (= E. zebra) (Ku¨tz.) Bre´b. Eunotia praerupta Ehr. Frustulia rhomboides Ehr. Pinnularia borealis Ehr. Mastogloia imperfecta Cleve Pinnularia divergens W. Sm. Amphipleura sp.

Arachnoidiscus ehrenbergii Bailey Odontella (= Biddulphia) rhombus (Ehr.) Ku¨tz. Endictya minor A. Schmidt Grammatophora marina Lyngb. Gyrosigma wansbeckii (Donkin) Cleve Hyalodiscus scoticus Ku¨tz. Navicula magellanica Cleve Nitzschia panduriformis Greg. Nitzschia sigma Ku¨tz. Paralia sulcata Ehr.

Northern Tierra del Fuego

Argentine Basin

Frenguelli (1923, 1924) listed the diatom taxa described by Cleve (1900) in Late Tertiary clays at the Rı´o Cullen inlet within the clay. These Tertiary strata contain the diatoms listed in Table 2: The marine and freshwater taxa included in Table 2 comprise living forms of the Magellan region, and characterize an estuarine environment.

Several cores from the Argentine Basin were studied by Groot et al. (1965). They were investigated for their plant microfossil content, diatoms and lithology recognizing two diatom assemblages. One of these assemblages represented a high latitude Antarctic flora and the other indicated a Subantarctic flora similar to that described for the southern Indian Ocean (Groot et al., 1965) for the southern Indian Ocean. Eucampia balaustium and Charcotia actinochilus represent glacial or Antarctic conditions and Coscinodiscus lentiginosus and Fragilariopsis antarctica are characterizing interglacial or Subantarctic conditions. The Malvinas (Falkland) Current is assumed to carry cold water to the north. The percentage of interglacial and glacial diatoms decreases with depth. This suggests that the Malvinas (Falkland) Current was less vigorous during the Early and Middle Pleistocene (Groot et al., 1965). The pollen and diatoms from core V17–121 (43580 S, 52090 W, 875 km offshore, Fig. 1) confirm the glacial and interglacial stages and indicated an age of 12,000 14C yr BP for the last glacial–postglacial boundary (Groot and Groot, 1966).

3.2. Quaternary Puerto del Hambre, Central Region of the Strait of Magellan, southern Chile Paleoecological and lithostratigraphical data for the Late Glacial to Early Holocene were recorded by McCulloch and Davies (2001). Diatom analysis was carried out on samples from Puerto del Hambre (53360 S, 70550 W; 6.25 + 0.5 m a.s.l., Fig. 1) and provide detailed evidence for a marine incursion. Between 14,500 and 12,200 14C yr BP, diatom assemblages reflect a local succession change from a shallow lake to a peat bog. The continued succession of wetland to drier peat bog taxa indicates a shift to drier climate, which culminated in an intense arid phase between 10,650 and 8100 14C yr BP, where diatoms were not preserved. The diatom record between 8265 and 3970 14C yr BP represents a period of marine influence at Puerto del Hambre dominated by species of the genus Fallacia. A peak of Actinoptychus senarius at 6450 14C yr BP is interpreted as the maximum of the marine transgression. After 3970 14C yr BP, the basin became isolated from the sea, and reflected by the dominance of Staurosira (= Fragilaria) construens var. subsalina and var. venter, Staurosirella (= Fragilaria) pinnata and Pseudostaurosira (= Fragilaria) brevistriata. A temperate and more humid climate during the Late Holocene is inferred from the pollen assemblages between 8100 14C yr BP and the present (McCulloch and Davies, 2001).

La Misio´n, Tierra del Fuego Markgraf (1993) reviewed pollen records of Auer (1933, 1956, 1958, 1959) from southern Patagonia south of latitude 50 S. Among these records, the La Misio´n site (53300 S, 67500 W; 3 m a.s.l., Fig. 1) was studied in detail. The diatom analysis of Auer’s cores studied by Frenguelli (1951, 1953) were updated and plotted by J.P. Bradbury (in Markgraf, 1993; Fig. 7). The multiproxy results (pollen, diatoms and ostracodes) provide a sealevel history. Before than 8000 and after 1000 14C yr BP, freshwater diatoms, such as Staurosira construens (= Fragilaria construens), E. adnata (= E. zebra) and Cocconeis placentula, dominate the sequence, indicating that nonmarine conditions prevailed. Coastal-marine taxa are abundant in the

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Marcela A. Espinosa

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References Dominant Abundant Common Few Rare

0 100

200

300

400

500

600

700

800

900

7,850 ± 110

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Fig. 7. Diatom data of the La Misio´n core updated and plotted by J.P. Bradbury (modified from Markgraf, 1993). intermediate levels: Paralia sulcata (= Melosira sulcata), Actinoptychus senarius (= A. undulatus) and Hyalodiscus spp. A similar assemblage was found between 8265 and 3970 14C yr BP at Puerto del Hambre (see Fig. 1), and was interpreted by McCulloch and Davies (2001) as a period of marine influence with a maximum level of the sea at 6450 14 C yr BP. However, a quantitative diatom analysis of the La Misio´n section would provide a detailed record of the transgressive phase (Markgraf, 1993).

Jabalı´ Creek, Bahı´a San Blas, southern Buenos Aires Province Frenguelli (1938) studied the diatom composition of surface sediments collected from the complex estuary of

Fig. 8. Estuarine complex of Bahı´a San Blas.

Bahı´a San Blas (Fig. 8). This author described the dominance of marine taxa accompanied by some brackish and freshwater species. Isla and Espinosa (2005) analyzed the evolution of this estuarine complex from vibracores obtained from different areas. One of them was drilled at the Jabalı´ creek (1.73 m in length), on the access to Bahı´a San Blas (40340 S; 62150 W; Fig. 1). A radiocarbon age of 4310 + 40 14C yr BP was obtained for the basal layer. The preliminary analysis of this core stressed the abundance of marine plankton and marine-brackish epiphytic diatoms. The high diversity at this tidal channel is recorded by more than 60 taxa. The frequencies of dominant diatom species (over 1%) are illustrated in Fig. 9. Cymatosira belgica is the most frequent taxon (usually more than 40%) accompanied by Thalassiosira spp., Cyclotella stylorum, Paralia sulcata and Raphoneis amphiceros. The dominance of tide transported planktonic diatoms is often found in these tidal channels and tidal inlets (the conditions of these environments are unfavorable for the development of autochthonous diatoms, benthos or epiphytes).The most abundant taxon Cymatosira belgica lives in the littoral zone at a water depth of 3–10 m, characterizing tidal inlets and large tidal channels (Vos and de Wolf, 1988a). However, marine/brackish epiphytes and benthic taxa are important in the PSBLAS core; these assemblages and marine plankton together are also characteristic of tidal mudflats. The fluctuations of the assemblages indicated by the three diatom zones (Fig. 9) point at changes in depth, which has been decreasing from 4310 + 40 yrs BP to the present. The hypersaline regime of the Walker-Jabalı´ system persisted during the Late Holocene according to the diatom content along the core of Jabalı´ creek (Isla and Espinosa, 2005).

Diatoms from Patagonia and Tierra del Fuego Marine

Brackish/Freshwater

14

C D ye ep ar th s B (c .P A . s m) . A ple ct n A ino de c p n C tino tyc s o p h C sci ty us yc n ch s lo od u en te is s v a lla cu u riu C st s r lga s ym yl ad ri or ia s at um tu os s ira be lg O ic do a n Pa tell ra a r h l Po ia s om u b d Th os lca us a ira ta Th lass st a i e Th las osi llig al si ra era Th ass osi dec a io ra ip Th las sir de ien al sio a e ns s R ass sir cc ann ap io a en u h s na tr la R one ira no ica ap is oe lin am str ea A ho ch n u ta na eis ph pii A nt s ice ch he ur r na s ire os b l C oc nth rev la e i C co s l pes oc ne a c C con is g us yc e u -v u H lote is s ttat lc an ya l c a l u i R lod a s te ho is tr llu i R pal cus ata m v ho o a s r. pa dia co pa lo g tic A rv ch dia ibb us a na m eru u n l D im the scu a s e l us O re pe g del r i N pho am cat av r m ul a N icu m a m a itz la a in r N sch py ina or itz ia gm C sch co ae yc ia m a lo tr p C tel ybli res yc la o s l o n a Ps ote ce ella ll v e ll St ud a m ata ar. au o le e vi St ro sta neg de au sir ur ns h is C ros a c osi inia oc i on ra n r a R con ella stru bre ho e p e v i n i A pal s d inn s v stri ch o is a a a na dia cu ta r. ta ve nt g lu nt he ibb s er s a la nc eo la ta

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4,310 ± 40

140 160 20 40 60

20

20

20

20

20

0.5

1.0 1.5 2.0

2.5

Total sum of squares

Fig. 9. Fluctuations in the relative abundance of important diatoms species of the Bahı´a San Blas profile (modified from Isla and Espinosa, 2005). Although the assemblages found by Frenguelli (1938) are similar, the proportions are different from the PSBLAS core. 4. Conclusions The present review reveals that information about Cenozoic diatoms from Patagonia and Tierra del Fuego is scarce. Although there are meritorious lists of taxa prepared in the early twentieth century, the first detailed papers with rigorous sampling and quantitative diatom analysis began at the end of twentieth century. Therefore, high resolution studies should be encouraged. In order to analyse the evolution of any water body in relation to climatic change, spatial and temporal distributions, water quality changes, acidification and eutrophication more studies are needed. In relation to sea-level changes, salinity variations and interaction between oceanic currents, the Quaternary highstands of Patagonia and Tierra del Fuego may be the subject of detailed diatom research. Knowledge of paleoenvironmental conditions derived from fossil diatom assemblages is very important to infer changes induced by man in coastal areas and can be used as reference levels for the assessment of recent changes. The use of diatoms in oil and gas exploration is another application in the future. Diatoms may be recovered from Tertiary marine and nonmarine sediments of basins characterized by ‘‘cold water’’ Neogene sediments where calcareous microfossils are rare or have long chronologic ranges. The ecological significance of the fossil assemblages of the region will be better understood when more comparative diatom studies have been carried out. Acknowledgments The author wishes to acknowledge G. Hassan and M. Farenga for the draft of figures and F. Isla for his useful comments on the manuscript.

References Acevedo S., Iglesias, G., Peralta, C. et al. (1995). Evaluacio´n de impacto ambiental excursio´n Expreso Caravana. Sociedad Naturalista Andino Patago´nica, Seccio´n Ecoandina 1–84. San Carlos de Bariloche. Auer, V. (1933). Verschiebungen der Wald-und Steppengebiete Feuerlands in postglazialer. Zeit. Acta Geographica Helsinki 5, 1–313. Auer, V. (1956). The Pleistocene of Fuego-Patagonia. Part I: The ice and interglacial ages. Annales Academiae Scientiarum Fennicae, A III 45, 1–226. Helsinki. Auer, V. (1958). The Pleistocene of Fuego-Patagonia, Part II: The history of the flora and vegetation. Annales Academiae Scientiarum Fennicae, A III 50, 1–239. Helsinki. Auer, V. (1959). The Pleistocene of Fuego-Patagonia. Part III: Shoreline Displacements, Annales Academiae Scientiarum Fennicae, A III 1–247. Helsinki. Bradbury, J.P., Grosjean, M., Stine, S. and Sylvestre, F. (2001). Full and late glacial lake records along the PEP 1 transect: their role in developing interhemispheric paleoclimate interactions. In: Markgraf, V. (ed.), Interhemispheric climate linkages. Academic Press, San Diego, 265–289. Bujalesky, G.G., Coronato, A.M., Roig, C.E. et al. (1994). Facies deltaicas proglaciales pleistocenas del Lago Fagnano, Tierra del Fuego, Argentina. IV Reunio´n Argentina de Sedimentologı´a, Actas 235–242. La Plata. Bujalesky, G.G., Heusser, C.J., Coronato, A.M. et al. (1997). Pleistocene glaciolacustrine sedimentation at Lago Fagnano, Andes of Tierra del Fuego, southernmost South America. Quaternary Science Reviews 16, 767–778. Cleve, P.T. (1900). Report on the diatoms of the magellan territories. Svenska Exped. Till Magellanslanderna 3, 273–282. Denys, L. and de Wolf, H. (1999). Diatoms as indicators of coastal paleo-environments and relative sea-level change. In: Stoermer, E.F. and Smol, J.P. (eds), The Diatoms.

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Applications for the Environmental and Earth Sciences. Cambridge University Press, London, 277–297. Dixit, S.S., Smol, J.P., Kingston, J.C. and Charles, D.F. (1992). Diatoms: Powerful indicators of environmental change. Environmental Science and Technology 26, 22–33. Espinosa, M.A. (1988). Paleoecologı´a de diatomeas del estuario del Rı´o Queque´n (Provincia de Buenos Aires, Argentina). Thalassas 6, 33–44. Espinosa, M.A. (1994). Diatom paleoecology of the Mar Chiquita lagoon delta, Argentina. Journal of Paleolimnology 10, 17–23. Espinosa, M.A. (1998). Paleoecologı´a de diatomeas en sedimentos cuaternarios del sudeste bonaerense. Unpublished Ph.D. Thesis. Universidad de Mar del Plata, Mar del Plata, Argentina. Espinosa, M.A. (2001). Reconstruccio´n de paleoambientes holocenos de la costa de Miramar (provincia de Buenos Aires, Argentina) basada en diatomeas. Ameghiniana 38, 27–34. Buenos Aires. Espinosa, M.A., De Francesco, C.G. and Isla, F.I. (2003). Paleoenvironmental reconstruction of Holocene coastal deposits from the southeastern Buenos Aires province, Argentina. Journal of Paleolimnology 29, 1, 49–60. Frenguelli, J. (1923). Diatomeas de Tierra del Fuego. Anales Sociedad Cientı´fica Argentina 96, 225–263. Buenos Aires. Frenguelli, J. (1924). Diatomeas de Tierra del Fuego. Anales Sociedad Cientı´fica Argentina 98, 5–89. Buenos Aires. Frenguelli, J. (1938). Diatomeas de la Bahı´a San Blas (Provincia de Buenos Aires). Revista Museo de La Plata, I, Bota´nica 5, 251–337. La Plata. Frenguelli, J. (1939a). Diatomeas de Rada Tilly en el Golfo de San Jorge (Chubut). Revista del Museo de La Plata, II, Bota´nica 9, 179–199. La Plata. Frenguelli, J. (1939b). Diatomeas del Golfo de San Matı´as (Rı´o Negro). Revista del Museo de La Plata, II. Bota´nica 10, 201–226. La Plata. Frenguelli, J. (1951). Ana´lisis microsco´pico de las muestras de la turbera del Rı´o de la Misio´n, Rı´o Grande, Tierra del Fuego. Ann. Acad. Scient. Fennicae, Ser. A 3, Geol.-Geogr. 26, 1–60. Helsinki. Frenguelli, J. (1953). Ana´lisis microsco´pico de una segunda serie de muestras de la turbera del Rı´o de la Misio´n, rı´o Grande, Tierra del Fuego. Annales Academiae Scientiarum Fennicae, Ser. A, 3 Geol.-Geogr. 34, 1–52. Helsinki. Galloway, R.W., Markgraf, V. and Bradbury, J.P. (1988). Dating shorelines of lakes in Patagonia, Argentina. Journal of South American Earth Sciences 1, 195–198. Groot, J.J. and Groot, C.R. (1966). Pollen spectra from deep-sea sediments as indicators of climatic changes in Southern South America. Marine Geology 4, 6, 525–537.

Groot, J.J., Groot, C.R., Ewing, M. et al. (1965). Spores, pollen, diatoms and provenance of the Argentine basin sediments. Progress in Oceanography 4, 179–217. Haberzettl, T., Fey, M., Lu¨cke, A. et al. (2005). Climatically induced lake level changes during the last two millennia as reflected in sediments of Laguna Potrok Aike, southern Patagonia (Santa Cruz, Argentina). Journal of Paleolimnology 33, 283–302. Isla, F.I. and Espinosa, M.A. (2005). Holocene and historical evolution of an estuarine complex: the gravel spit of the Walker creek, southern Buenos Aires. XV Congreso Geolo´gico Argentino, Actas 1–6. La Plata. Maidana, N. and Corbella, H. (1997). Ana´lisis preliminar de las asociaciones de diatomeas cuaternarias en un paleolago volca´nico, Santa Cruz austral, Argentina. VI Congresso da Associac˛a˜o Brasileira de Estudos do Quaterna´rio da Ame´rica do Sul 336–340. Curitiba, Brazil. Markgraf, V. (1993). Paleoenvironments and paleoclimates in Tierra del Fuego and southernmost Patagonia, South America. Palaeogeography, Palaeoclimatology, Palaeoecology 102, 53–68. Markgraf, V., Bradbury, J.P. and Ferna´ndez, J. (1986). Bajada de Rahue, Province of Neuque´n, Argentina: an interstadial deposit in northern Patagonia. Palaeogeography, Palaeoclimatology, Palaeoecology 56, 251–258. Markgraf, V., Bradbury, J.P., Schwalb, A. et al. (2003). Holocene palaeoclimates of southern Patagonia: limnological and environmental history of Lago Cardiel, Argentina. The Holocene 13, 4, 581–591. Martinez Macchiavello, J.C. (1984). Diatomeas. IX Congreso Geolo´gico Argentino, Relatorio 518–525. San Carlos de Bariloche, Argentina. Mayr, C., Fey, M., Haberzettl, T. et al. (2005). Paleoenvironmental changes in southern Patagonia during the last millennium recorded in lake sediments from Laguna Azul (Argentina). Palaeogeography, Palaeoclimatology, Palaeoecology 228, 203–227. McCulloch, R.D. and Davies, S.J. (2001). Late glacial and Holocene palaeoenvironmental change in the central Strait of Magellan, southern Patagonia. Palaeogeography, Palaeoclimatology, Palaeoecology 173, 143–173. Round, F.E., Crawford, R.M. and Mann, D.G. (1990). The diatoms. London, Cambridge University Press, 747pp. Stoermer, E.F. and Smol, J.P. (1999). Applications and uses of diatoms: prologue. In: Stoermer, E.F. and Smol, J.P. (eds), The diatoms: Applications for the Environmental and Earth Sciences. Cambridge University Press, London, 3–8. Vos, P. and de Wolf, H. (1988a). Methodological aspects of paleo-ecological diatom research in coastal areas of the Netherlands. Geologie en Mijnbouw 67, 31–40.

20 Quaternary Fossil Insects from Patagonia Julieta Massaferro1, Allan Ashworth2 and Stephen Brooks3 1

CONICET – Laboratorio de Ecotono, CRUB/Universidad del Comahue, 8400 – Bariloche, Argentina 2 Department of Geosciences, North Dakota State University, Fargo, 58105-5517 North Dakota, US 3 Department of Entomology, Natural History Museum, SW7 5BD London, UK

sediments, is that they offer a continuous record of climate change. The study of these records combined with physical and chemical parameters can provide a comprehensive array of information for climate change studies. Recent palynological studies from southern South America have provided a thorough overview of past vegetation and climate change, specially those changes associated with shifts in the latitudinal position of the westerlies in southern temperate ecosystems (Markgraf, 1987; Heusser, 1989, 2003; Markgraf et al., 1995; Moreno, 1997; Moreno et al., 1999, 2001; Moreno and Leo´n, 2004). Fossil insects are often abundant in a wide range of Quaternary deposits. Several orders of insects can be found in fluvial and lacustrine sediments such as species from the Order Hemiptera including Gerridae, Corixidae and Notonectidae. The larvae of caddisflies (Trichoptera) are aquatic and their sclerites can be found in lacustrine sediments. Caddisfly larvae provide valuable information about the water quality they inhabit, as many species are stenothermic and are sensitive to changes in pH and trophy (Elias, 1994). Another group that has recently proved to be useful in paleolimnology, especially in fluvial sediments, is the Family Simuliidae. Simuliids have been studied by Currie and Walker (1992) in North America who demonstrated they were useful indicators of precipitation changes. However, to date, most of the paleoenvironmental studies available have focused on the remains of beetles (Order Coleoptera, Families Carabidae, Scarabaeidae, Cydnidae, Chrysomelidae, Coccinellidae) and midges (Order Diptera, Family Chironomidae). Some of the earliest evidence for beetle species comes from late Tertiary and early Quaternary assemblages in Alaska. However, those records are generally poorly preserved, laid down in bedrock, full of spatial and temporal gaps and lacking in continuity (Elias, 1994). For this reason, this chapter focuses on midges and beetles from the Quaternary period, especially the Late Pleistocene and Holocene, with which, to date, most of the fossil insect studies have dealt. The use of fossil insects in Quaternary studies at midlatitude South America is relatively limited, and the major reason is the lack of taxonomical information available from these remote areas. Many of these studies have been conducted in the southern part of South America, in Argentina and Chile (Ashworth and Hoganson, 1987, 1993; Hoganson et al., 1989; Ashworth et al., 1991; Hoganson and Ashworth, 1992; Massaferro and Brooks, 2002; Massaferro et al., 2004).

1. Introduction: The Importance of Patagonia for Climatic Studies Patagonia is the region of Argentina and Chile that extends from 39 to 55 S. In the last 20 yrs Patagonia has become increasingly important in paleoclimatic research due to its exceptional geographic location between the South Pacific, Atlantic and Antarctic oceans and the abundance of lakes and bogs from which climate indicators can be easily obtained. The Andean region of Patagonia is ideal for monitoring Late Quaternary climate at mid-latitudes because it is one of the few areas sustaining a suite of rainforest communities along altitudinal and latitudinal gradients within the belt of the southern westerlies (Moreno, 1997; Whitlock et al., 2001). Such a location is a key for investigations related to the reorganization of climate during the Late Pleistocene and Holocene, especially for testing the synchroneity of climate changes in the Northern and Southern Hemispheres. Patagonia is also a key place for the study of interannual and decadal climate variations such as the El Nin˜o Southern Oscillation (ENSO) that affect the Pacific Ocean and leaves paleoecological evidence in lake records. Furthermore, Patagonia also provides evidence of past climate changes from regions located in the same latitude and climate as those in the Northern Hemisphere where climate has been extensively studied (Denton, 1999). Finally, neo-ecological work developed in this part of South America has emphasized the strong need for the study of catastrophic disturbances such as earthquakes, volcanic activity, insect outbreaks, windstorms and fires that affected flora and fauna in Patagonian rainforest in the past (Szeicz et al., 2003).

2. Paleoclimatic Proxies So far, most of the evidence for climate change in southern South America has been derived from abiotic climate proxies and a few biotic proxies, especially pollen records. Extensive mapping and dating of glacial and fluvio-glacial features has produced a detailed history of glacial behavior in the Chilean Lake District (Denton et al., 1999) and at other latitudes in Argentinean and Chilean Patagonia (Clapperton et al., 1995; Marden and Clapperton, 1995; Strelin and Malagnino, 2000; Kaplan et al., 2004). The advantage of biological proxies, which can be retrieved from long lake-sediment cores, bogs or marine

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3. Chironomids Chironomidae belong to the Order Diptera: Nematocera and they are colloquially known as non-biting midges. The larvae of the majority of these species are aquatic and constitute one of the most abundant bottom-dwelling macroinvertebrates of freshwaters (Cranston, 1995) (Fig. 1a). The distribution of the different chironomid taxa is restricted by environmental conditions. Most species are stenotopic (i.e., able to adapt only to a narrow range of environmental conditions) and respond rapidly to environmental change. The sensitivity of chironomids to different environmental variables such as dissolved oxygen, nutrient and organic content, pH and salinity has led to their use as indicators of lake quality and in other ecological studies (Porrinchu and MacDonald, 2003). Midges began to be used in paleoecological studies during the 1980s and 1990s. Excellent reviews of chironomids as paleoindicators can be found in Frey (1964, 1988), Hofmann (1971, 1988) and Walker (1990, 1995, 2001). Their remains are of special interest in paleolimnology because their strongly sclerotized larval head capsules are preserved in sediment deposits (Fig. 1b). There are several reasons why chironomids are considered important in paleolimnology: (i) they are sensitive to

(a) Adult Adult swarms

Egg masses

Lake Sediment core

Head capsule

Pupae

Larval instars

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environmental variables such as temperature, pH, trophic conditions, dissolved oxygen; (ii) they have relatively short life cycles; (iii) the adult are mobile; (iv) the larvae possess chitinous head capsules that are well preserved in lake sediments and (v) they are abundant, diverse and readily identifiable to generic and species-group level, enabling high resolution studies. Past chironomid stratigraphies can be reconstructed and readily used to infer environmental conditions at the time of deposition. Chironomids are now well established as paleoecological proxy indicators and have been used effectively to document changes in salinity (Walker et al., 1995), lake productivity (Lotter et al., 1997, 1998; Brodersen and Lindegaard, 1999; Brodersen and Anderson, 2000) and hypolimnetic anoxia (Quinlan et al., 1998). However, perhaps the most successful use of chironomids is to quantify past climate changes during the Late Glacial and Holocene in the Northern Hemisphere (Levesque et al., 1996; Lotter et al., 1999; Brooks, 2000; Korhola et al., 2000; Brooks and Birks, 2001).

3.1. Methodology The volume of sediment required to recover a sufficient number (50–150) of chironomid head capsules varies from lake to lake, but usually 1–3 g of wet sediment is plenty. However, in Late Glacial inorganic clayish sediments or volcanic sediments sometimes this amount of sediment is not enough. The methodology involved in processing and identification of chironomid remains generally follows standard procedures (Warwick, 1980; Hofmann, 1986; Walker, 1995). Although the approaches in individual laboratories vary slightly, the concept remains the same: a careful separation of the subfossil head capsules from the sediment matrix using mild chemical treatment and sieving (Porrinchu and MacDonald, 2003). The procedure involves deflocculating the sediment sample using 10% KOH solution at 50–70C for 10 minutes. The warm KOH serves to break up colloidal matter without damaging the remains. The next step requires sieving the sediment and a mesh size of 95 mm should retain most of the heads. Sorting the sample requires a grooved counting tray. The chironomid heads are then handpicked with forceps or micropipettes into 80% ethanol using a stereo dissection microscope (25–50). Finally, chironomid remains are dehydrated by transfer from 80% ethanol to 100% ethanol to Euparal essence and finally to Euparal, which is a permanent mounting medium for permanent slides.

3.2. Applications of Chironomids in Paleoenvironmental Reconstructions Mentum

Fig. 1. (a) Chironomidae life cycle (modified from Porrinchu and MacDonald, 2003). (b) Fossil chironomid head capsules from Laguna Stibnite, Chile, showing taxonomical useful features (Massaferro and Brooks, 2002).

Subfossil chironomids have been used extensively as qualitative indicators of past environmental conditions. Since the earliest days of modern limnology, chironomids have been used in lake typologies to indicate trophic status of waters (Thienemann, 1918; Brundin, 1949; Saether, 1979; Wiederholm, 1984). Late Glacial sediments from littoral and profundal lake zones in the Northern Hemisphere

Quaternary Fossil Insects from Patagonia include cold stenothermic, ultraoligotrophic taxa such as Heterotrissocladius spp. and Tanytarsus lugens (Brundin, 1949). In mesotrophic lakes, those assemblages are replaced by taxa more tolerant to oxygen depletion such as Sergentia coracina and Stictochironomus. Finally, warm-adapted Chironomus spp. are characteristic of profundal zones in eutrophic lakes. This faunistic system applies in the Northern Hemisphere and has been used extensively to develop transfer functions (TF), which relate the modern chironomid distribution to a particular environmental variable such as temperature, oxygen concentration or total phosphorous. These TF allow quantitative reconstructions of particular environmental variables (see ‘‘Quantitative temperature reconstructions using fossil insects’’ in this chapter). In an expedition to the southern Andes in South America, Brundin (1958) collected and described much of the chironomid fauna of the area demonstrating that the fauna had a great resemblance to the Holarctic fauna. He recognized taxa with similar ecological requirements in both hemispheres. For example, Tanytarsus rothicommunity replaces T. lugens; Lenzia (Sergentia) coracina instead of Sergentia coracina and Parachironomus species replacing the northern Paracladopelma species. Later on, Brundin (1966) published an extensive study of the midge subfamilies Podonominae and Aphroteniinae. This invaluable work also shows evidence of chironomid biogeographical relationships between the separated Gondwana land masses of Australia and southern South America. Little has been done on larval chironomid taxonomy in Patagonia since Brundin’s work (Gonser and Spies, 1997; Andersen and Contreras-Ramos, 1999; Cranston and Edwards, 1999; Cranston, 2000). Recently, Massaferro and Brooks (2002) and Massaferro et al. (2005) described specimens from the subfossil chironomid fauna and identified additional taxonomic groups that could be ecologically related to European taxa. However, more ecological work is needed especially regarding the distribution and habitat requirements of this group of insects in South America.

3.3. Eutrophication Increase in lake productivity is accompanied by oxygen depletion in the hypolimnium. Certain chironomid larvae such as Chironomus spp. can tolerate very low oxygen levels that commonly exist in eutrophic and hypereutrophic lakes. They contain invertebrate haemoglobin enabling respiration in sites with low oxygen concentrations. There are many studies that have used chironomids as indicators of the trophic status of lakes (Hofmann, 1978; Warwick, 1980; Wiederholm, 1983) and there have also been several quantitative reconstructions estimating past changes in productivity based on chironomids (Lotter et al., 1998; Quinlan et al., 1998; Brooks et al., 2001; Quinlan and Smol, 2001). In Patagonia a few qualitative studies have been done on this subject, especially related to human activities, the impact of building developments, fish introduction and increase of nutrients in lakes (Bianchi et al., 1997, 2000; Massaferro et al., 2004).

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3.4. Acidification The effects of acid rain on ecosystems have had an important impact in North America and Europe during the 19th and 20th centuries. The immediate consequence of acid rain is lowered pH that affects the function of ecosystems. In aquatic systems, there are some species of chironomids that can tolerate low pH such as species of Chironomus, Zavrelia, Zalutschia and Psectrocladius. In the Northern Hemisphere there are a number of studies dealing with pH and chironomids (e.g., Wiederholm and Eriksson, 1979; Henrikson and Oscarson, 1985; Brodin and Gransberg, 1993). Lake acidification can also be caused by natural factors, usually induced by development of soils related to vegetational succession. However, in southern South America there have been no studies related to acidification.

3.5. Lake-level Changes Changes in lake level can influence the proportion and volume of the littoral and profundal zones causing changes in the composition and distribution of the chironomid assemblages. Strongest impacts may be detected in a littoral core where deepening may cause an increase in profundal taxa. When lake level falls, the proportion of profundal taxa is likely to decline. The influence of depth on the distribution of chironomids has long been known (Hofmann, 1998; Korhola et al., 2000; Massaferro and Brooks, 2002; Marchetto et al., 2004) and there is a clear differentiation between littoral taxa such as Dicrotendipes, Glyptotendipes and Polypedilum, which are associated with macrophytes, semi-terrestrial taxa such as Limnophyes, Smittia and Gymnometriocnemus and profundal taxa such as Chironomus, T. lugens and Procladius (Armitage et al., 1995).

3.6. Climate Chironomid distribution is significantly affected by temperature, albeit in different ways. For instance, egg and larval development are influenced directly by water temperature (Tokeshi, 1995), but also water and air temperatures affect midges in an indirect way. In general, an increase in water temperature leads to an increase in productivity, which, in turn, increases food supply and decreases oxygen availability. Since the 1990s the use of fossil midges as indicators of climate change has proliferated and there is plenty of evidence concerning the significance of temperature in controlling chironomid distribution and abundance, especially during Late Glacial times. These studies were first developed in arctic and alpine lakes (Walker and Mathewes, 1987; Walker et al., 1991) sediments and showed that cold-stenothermic taxa such as Heterotrissocladius spp. and T. lugens dominate the Late Glacial period and that these taxa disappear with climate warming at the beginning of the Holocene, when they are replaced by a diverse, thermophilic chironomid assemblage dominated by Chironomini.

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Julieta Massaferro et al. stored products. Their most important beneficial roles, as pollinators and as recyclers of nutrients, are activities that ensure the health of ecosystems. Species in many families of beetles are susceptible to environmental change. In general, predators and scavengers receive the most attention in paleoenvironmental reconstructions because they are able to respond more rapidly to climate change, since they are not tied to specific types of vegetation (Elias, 1994). Beetles respond to Quaternary climate change mostly by dispersal to track a favourable climate envelope, which ultimately led to large-scale changes in geographical distribution. Quaternary beetle fossils consist mostly of disarticulated exoskeletons; setae and scales are frequently preserved in unconsolidated sediments from shallow lacustrine, paludal and fluvial environments. Internal structures such as the male genitalia also occur as fossils. However, for practical purposes, the parts most studied by paleoentomologists are heads, pronota (thoraces) and elytra (wing cases) (Ashworth, 2001) (Fig. 2a, b).

4. Beetles Coleoptera or beetles are a diverse (more than 300,000 known species) and abundant order of insects. This high diversity makes them an important group in the fossil record. They are ubiquitous occurring from arctic polar deserts to the Subantarctic islands and at elevations as high as 5600 m in the Himalayas. There are species with physiologies adapted to survive periodic below freezing temperatures and other littoral species adapted to survive daily tidal inundations by burrowing in sand. Like other insects, they are ectotherms and are dependent on environmental temperatures during all phases of their life cycle. Most beetles are small organisms, ranging in length from 0.25 mm to several centimetres. Average length is estimated to be in the range of 4–5 mm. Because of their abundance, beetles are important food items for numerous species of reptiles, birds, small mammals and fish. Beetles are also important agricultural and forestry pests, with numerous species being injurious to crops, trees, and (a)

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Cerroglossus sp. (Coleoptera: Carabidae) Pair of elytra (wing covers) in peat. Rio Caunahue Site, southern Chile. Ca. 10,000 yr BP

Anisosticta bitriangularis (Say) (Coleoptera: Coccinellidae) Left elytron Winter Gulf Site, North Collins, New York, U.S.A. Ca. 12,700 yr BP

Fig. 2. (a) Generalized drawings of coleopteran sclerites frequently preserved as Quaternary fossils, showing a range of diagnostic features used in fossil identification (modified from Elias, 1994). (b) SEM and light microscope photographs of fossil beetles, showing structural and pigmented patterns (from http://www.ndsu.nodak.edu/instruct/schwert/qel/ images.htm).

Quaternary Fossil Insects from Patagonia 4.1. Methodology Fossils of beetles for Quaternary studies are usually obtained from stratigraphic sections exposed in cut-banks on rivers, in road cuts and by excavating pits in bogs. The quantities of sediment used in the studies vary but typically are 10 kg for each 5 cm stratigraphic interval. Organic sediments are wet sieved and the fraction larger than 300 mm is further processed using a kerosene flotation technique (Elias, 1994). This technique concentrates large numbers of insect skeletal parts, mostly the heads, pronota and elytra of beetles. The fragments tend to be very well preserved, with setae and scales and structural colours intact. The details preserved on the fossils facilitate their identification. Data from fossil beetle analysis are usually presented in the form of species abundance lists showing the number of individuals occurring within a particular sample. Occasionally, further information about specific parts of the body such as elytra or pronotum is provided (Lowe and Walker, 1997). As with their counterparts in North America and Europe, the fossils of Patagonian Coleoptera are identical to extant species. The beetle fauna of South America is not as well known as that of Europe or North America but even so large numbers of species are identifiable from their fossils.

4.2. Applications of Beetles for Paleoenvironmental Reconstructions Coleoptera are very useful in paleoecological reconstructions because they are such an important element in terrestrial ecosystems. They are relatively abundant and highly diverse in a wide range of deposits. Speciation and extinction is extremely rare, at least during the Pleistocene (Elias, 1994). The common response is for species to survive by dispersal. In consequence, beetle geographic distributions shrink and expand constantly. In this respect, the South American fauna at temperate latitudes is no different from that in the northern Hemisphere. Therefore, beetle species constancy over the last million years allows us to make use of ecological and distributional data drawn from modern populations (Elias, 1994). In addition to that, beetles are stenotopic, which means that they show a marked preference for a very restricted number of environments. A large number of beetles are associated with aquatic habitats, for example flowing water is indicated by Esolus, Limnius volckmari and Ochthebius pedicularis whereas Potamonectes and Halyplus live in still waters with sandy or silty bottoms. Other beetles indicate the presence of particular plants or animals. A profusion of dung beetles, for example, would indicate the presence of mammals (Lowe and Walker, 1997). Beetle assemblages can therefore provide valuable information on a diverse range of contemporaneous habitats with differences in vegetation, soils, water quality, forest composition and health, and may provide environmental insights that are difficult to obtain from other lines of evidence (Elias, 1994).

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5. Quantitative Temperature Reconstructions Using Fossil Insects The analysis of fossil insects can be used for both qualitative and quantitative environmental reconstructions. Although the qualitative approach is long established, recent advances in statistical techniques have allowed paleoecological research to undergo a quantitative revolution. Temperature reconstructions using fossil insect assemblages have played an important role in NW Europe in recent attempts to reconstruct timing, magnitude and rates of climate changes during the Late Glacial–Interglacial transition and Holocene (Brooks, 2003). The close relationship between temperature and the distribution and abundance of fossil assemblages has been used to develop temperature inference models that can produce quantitative environmental reconstructions.

5.1. Chironomids One of the advantages of temperature reconstruction using chironomids is that many hundred of head capsules can be obtained from as little as 1 cm3 of sediment giving a higher resolution record than other methods such as for beetles, where large amounts of sediments are required. Pioneering work using chironomids for quantitative reconstructions was done in Canada by Levesque et al. (1993) and Walker et al. (1997) who produced temperature models that have been used successfully numerous times in eastern and western Canada. Further chironomid temperature quantitative models were developed in Northern and Central Europe (Lotter et al., 1997; Olander et al., 1999; Brooks and Birks, 2000, 2001; Larocque et al., 2001). Based on a calibration data set of 111 lakes from Norway, Brooks and Birks (2001) applied the inference model to Late Glacial chironomid assemblages from Whitrig Bog in Scotland for a high-resolution quantitative climate reconstruction. The correlation between the chironomid-inferred temperature and the GRIP oxygen isotope stratigraphy shows striking similarities (Fig. 3). Relatively few quantitative reconstructions have been carried out in the Southern Hemisphere. In Australia, Dimidiatris and Cranston (2001) developed a chironomid temperature model based on the MCR method (see below) that has been applied to a Holocene sequence from a maar lake in Queensland. In an attempt to explore the potential of chironomids as quantitative indicators of past temperatures in Patagonia, Gilchrist (2005) developed a statistical model to infer paleoenvironmental changes in two lakes in southern Chile: Laguna Leta (41 S, 73 W) and Laguna Boal (44 S, 73 W). Records from both lakes indicated that climate changes occurred during the Late Glacial–Holocene transition and give evidence of the existence of a reversal event at the time of the Younger Dryas (YD) between 13,300 and 12,000 C yr BP. Concomitant with this cooling event, there is evidence of an increase of moisture that potentially caused a rise in lake levels. These promising results highlight the importance of continuing this kind of investigations.

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Julieta Massaferro et al. This approach is yet to be fully exploited in investigations at mid-latitudes in the southern Hemisphere. The power of chironomids to infer past temperatures can be validated against other climatic reconstructions such as ice core records, marine micropaleontological records, isotopic records and evidence from other biotic proxies such as pollen, beetles, cladocera and diatoms. Good examples are the comparison of the Late Glacial isotopic and pollen records from Hawes Water, ostracods from Ammersee, the chironomid temperature curve from Whitrig Bog to GRIP ice core isotope records (Jones and Marshall, 2002) and the multiproxy study developed in Krakenes (Norway) that present a comparison of temperature reconstructions from different groups of fossil organisms including beetles and midges (Birks et al., 1999) (Fig. 4).

5.2. Beetles

Fig. 3. (a) Chironomid reconstruction of mean July air temperature at Whitrig Bog, Scotland. (b) The GRIP oxygen isotope record. Vedde Ash was included in both plots to show synchronicity between both reconstructions (modified from Brooks and Birks, 2001).

The most important factor driving beetle distribution during the Quaternary has been climate (Coope, 1990). In Europe, modern distribution maps show that the geographical range of many species corresponds with welldefined climate zones. Coope (1977) demonstrated that during the Quaternary, extremely rapid climate changes may have allowed Coleoptera to colonize new habitats relatively quickly and in most of the cases where the range limit of the coleopteran species coincides with a climatic boundary, this relationship has been used to apply quantitative paleotemperature reconstructions (Lowe and Walker, 1997). In the Holarctic region, beetles have been widely used for quantitative temperature reconstructions (Coope, 1977; Elias, 1994; Ashworth, 2001). The Mutual Climate Range method (MCR) (Atkinson et al., 1986, 1987) is used to infer temperatures from fossil beetle assemblages. In this method, the geographic ranges of species are plotted within a climatic framework, usually

Fig. 4. Comparison of mean July air temperature reconstruction for different groups of organisms during the Late Glacial and Holocene in western Norway (Birks et al., 1999).

Quaternary Fossil Insects from Patagonia

only two studies dealing with fossil insects available at present. In Venezuela, an investigation of fossil chironomids from a sediment core from Lake Valencia indicated lake-level changes and trophic changes during the last 12,000 14C yrs (Binford, 1982). The other study was developed in Peru (Churcher, 1966) using insect remains from Talara. However, despite there being good information about the ecology of the different groups of Coleoptera in Peru, no specific identifications, essential for paleoenvironmental reconstructions, were made.

mean January (T min) and mean July temperature (T max). In a fossil assemblage the overlapping climate envelopes of each species provide the mutual climate range, or the climate space between T max and T min that the assemblage was most likely to inhabit. The method depends on detailed modern distributional information for species. The MCR method has been used to reconstruct climatic conditions in Europe during the Late Glacial–Interglacial transition (Ponel and Coope, 1990; Lemdahl, 1991; Walker et al., 1993). MCR beetle curves are strongly supported by other climatic reconstructions such as Greenland ice core records and marine evidence (Lowe and Walker, 1997). Due to little knowledge of modern species composition and distribution in South America the MCR method and the climatic summary it provides cannot be used at present.

6.1. Fossil Midges There are currently few chironomids paleoecological studies from Patagonia. There have been some investigations carried out in Argentine Patagonia, within the limits of the Nahuel Huapi National Park (41 S, 71300 W) (Ariztegui et al., 1997; Bianchi et al., 1997, 2000; Corley and Massaferro, 1998; Massaferro and Corley, 1998; Massaferro et al., 2004) (Fig. 5).

6. Fossil Insects and Climate Studies in South America The current knowledge of insect paleoecology in Patagonia is relatively limited. In the Neotropics, there are

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Julieta Massaferro et al. show the response of chironomids to natural (volcanic) and non-natural (human) environmental disturbances during the last 200 yrs. The study also demonstrated the importance of the multiproxy approach for paleoenvironmental reconstructions. There have not been other detailed studies of the response of chironomid assemblages to Late Quaternary climate change in other areas of Argentina. The reason for this is the lack of information on chironomid taxonomy due to the small number of researchers working on the subject. The first high-resolution environmental reconstruction using chironomids in Chile was published recently by Massaferro and Brooks (2002) (Fig. 6). Changes in the chironomid assemblages at Laguna Stibnite (46 S) in the Taitao Peninsula in Chile suggest that the climate in southern Chile was at it coolest during the YD (Fig. 7). In addition, during this period, chironomid head capsule concentrations fall suggesting low lake productivity, which would be consistent with low temperatures during this event. The specific chironomid assemblage during this period also indicates that the climate was cooler and drier. Between 11,300 and 9,400 14C yr BP (13,000–11,200 cal. yr BP), the coldstenothermic Podonominae were consistently present in every sample attaining a peak abundance during this period. This suggests that the lake waters were cool and oligotrophic during the YD. The low concentration of head capsules also reflects low lake productivity. During the Holocene, chironomids from Laguna Stibnite show

The research conducted by Ariztegui et al. (1997) is a multiproxy study in which changes in pollen, chironomid and geochemistry in Lago Mascardi during the Late Glacial were interpreted as a response to a reversal coincident with the timing of the YD. Between 11,400 and 10,200 14C yr BP, there was a decrease in the total pollen influx as a consequence of an increase in inorganic sedimentation. A sharp decrease in the hydrogen index was also recorded. These results were interpreted as a cooling accompanied by an increase in subglacial erosion due to an advance of the Tronador icecap that feeds proglacial Lago Mascardi. The disappearance of the warm-adapted Chironomus and the decline in the total chironomid abundance at this time also suggests a climatic deterioration. Bianchi et al. (1997, 2000) show the results from a multiproxy study of a sediment core from Lago El Tre´bol. Although the site was suitable for paleolimnological studies, the sampling resolution was not enough to discern climatic changes during the Late Glacial period. Corley and Massaferro (1998) and Massaferro and Corley (1998) also studied subfossil chironomid assemblages from Lago Mascardi. However, the results focused mostly on the importance of paleolimnological studies in understanding the role of natural disturbance and diversity patterns of biological communities in the past. Massaferro et al. (2004) studied geochemical and chironomid records from a short core from Lago Morenito, near San Carlos de Bariloche. The results

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a response to cyclical precipitation patterns. Results from beetles from Puerto Ede´n (Ashworth et al., 1991) located 300 km south of Taitao Peninsula also suggest that changes in precipitation occurred during the Holocene; however, they are almost out-of-phase with those inferred at Laguna Stibnite. If these interpretations are both correct, changes in precipitation patterns may be caused by a latitudinal shift of the westerlies (Massaferro and Brooks, 2002). Pollen records from Laguna Stibnite (Lumley and Switsur, 1993; McCulloch et al., 2000) could find no evidence of cooling during the Late Glacial–Interglacial transition. At other sites in Patagonia, changes in pollen assemblages at the end of the Late Glacial have been interpreted as a response to climatic cooling (Heusser and Rabassa, 1987; Heusser, 1989, 1993; Rabassa et al., 1990; Heusser et al., 1996; Moreno, 1997). On the contrary, palynological studies performed in seven other lakes located in this area of Chile bear no evidence of a reversal during the Late Glacial period (Bennett et al., 2000; Haberle and Bennett, 2004). In addition, Bennett et al. (2000) found no changes in lithology or magnetic susceptibility that might indicate periods of cooling in southern Chile. Recently, Massaferro et al. (2005) carried out a study on pollen and fossil chironomids in Laguna Fa´cil, located in the Chonos Archipelago in Chile. The results showed that no response to a cooling event coinciding with the

YD is apparent at this lake. Instead, chironomids seem to be responding to local rather than regional environmental changes, perhaps in response to the gradual migration and colonization of trees in the lake catchment (Fig. 8). Pollen records from Laguna Fa´cil and Laguna Oprasa (separated by 50 km) also showed no cooling during the YD (Haberle and Bennett, 2004). Despite the similarities of fauna and the proximity between them, the more northerly location of Laguna Fa´cil means that it is less likely than Laguna Stibnite to have been influenced by any resurgence of Andean glaciers during the Younger Dryas Chronozone (YDC). Glacial resurgence could be related to changing patterns of atmospheric moisture, from latitudinal movement of the southern westerlies, which resulted in a highly variable glacier system (Heusser, 2002; Glasser et al., 2004). Geographic variability in glacial activity would indicate that climatic conditions were possibly insufficiently intense and/or of insufficient duration to effect uniform regional paleobiotic changes; therefore a response during the YD may be recognizable at some sites but not at others. Results from a chironomid study (Massaferro, unpublished) carried out on samples from Lago Mascardi not previously examined by Ariztegui et al. (1997) provide new evidence of climatic cooling during Late Glacial times. The improved radiocarbon chronology spans the interval between 12,000 and 9,500 14C yr BP and allows changes in chironomids in response to

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Fig. 8. Late Quaternary chironomid stratigraphy from Laguna Facil. Only the dominant taxa are shown. Solid horizontal lines indicate the position of chironomid zone boundaries identified by optimal partitioning and a brokenstick model. For comparison, pollen zone boundaries are indicated by dash-lines and show how some elements of the chironomid fauna show synchronous change with vegetation change (Massaferro et al., 2005).

climatic variability to be pinpointed, and also a comparison with the well-established Late Glacial chronology from Greenland and Antarctic ice core records. Chironomids indicate a cooling during the YD but of a longer duration than in the Northern Hemisphere (Fig. 9). These results are in agreement with Hajdas et al. (2003) and Massaferro et al. (submitted) who demonstrated the occurrence of a cold event in the mid-latitudes of South America known as the Huelmo Mascardi Cold Reversal (HMCR) that encompasses the North Atlantic YD and the Gerzensee/Killarney Oscillation. Similar results have been found in Kaipo Bog in New Zealand from pollen records. These results indicate a cooling of ca. 600 14C yr BP before the YD (Newnham and Lowe, 2000).

6.2. Fossil Beetles Fossil beetle faunas have been investigated from several lowland sites in Chile, in the Chilean Lake District (see Fig. 5) (Hoganson et al., 1989; Hoganson and Ashworth, 1992; Ashworth and Hoganson, 1993) and further south, in the Chilean Channels (Fig. 10) (Ashworth and Markgraf, 1989; Ashworth et al., 1991). To develop reliable information about the modern distribution and ecological requirements of the coleopteran fauna, Ashworth and Hoganson (1987) carried out an extensive collecting programme for several years. They identified 462 species of beetles belonging to 48 families in 41 locations of the Puyehue National Park, Chile (40–41 S, 71–72 W). Their collections were

the basis of a multivariate ordination study that demonstrates significant differences between the low and high elevation fauna, especially between those of the forested and treeless Andean tundra habitats. The study clearly demonstrated a relationship between beetles and climate within the region. Hoganson and Ashworth (1992) and Ashworth and Hoganson (1993) reported on several fossil beetle assemblages from Puerto Octay, Puerto Varas and Rı´o Canahue in the Chilean Lake Region (40–41 S, 72–73 W) that spans the interval from the Last Glacial maximum to the Holocene. They showed that full glacial assemblages were species-poor, containing only about 20% of the species of the Holocene assemblage. They inferred that the full-glacial beetle assemblages represented a Magellanic moorland environment which existed in a climate with mean January temperatures 4–5C cooler than present (Fig. 11a, b). What was particularly striking was the rapidity and timing of the change to the postglacial fauna. The change started before 14,000 14C yr BP and was completed by 12,500 14 C yr BP. Moreno (1997), based on a high resolution study of pollen, confirmed that the full glacial flora was Magellanic moorland and that the transition to forest occurred between 15,000 and 14,000 14C yr BP. These changes are similar in timing to changes marking the end of the Pleistocene in the Pacific Northwest of North America. These results also show that the time of the transition from glacial to interglacial conditions in the Southern Hemisphere occurred earlier than in the North Atlantic Ocean.

δ 18 O

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Fig. 9. Chironomid stratigraphic diagram from Lago Mascardi showing Younger Dryas (YD), Huelmo-Mascardi Cold Reversal (HMCR) and Antarctic Cold Reversal (ACR). Dark profiles represent cold species (Parakiefferiella include mixed cold and warm species). EPICA (Antarctica) and GISP2 (Greenland) ice core records are shown to allow comparisons (original data from NOAA web page) (Massaferro, unpublished).

Quaternary Fossil Insects from Patagonia

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Julieta Massaferro et al. Glaciar Témpano

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Fig. 10. Map of the Chilean Channels and Tierra del Fuego, Argentina, showing fossil beetle sampling sites. Stars indicate sampling sites for the chironomid training set by Gilchrist (2005). On the right side, the map of Patagonia shows the location of the Patagonian Ice Fields, as in Fig. 5 (modified from Haberle and Bennett, 2004).

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Fig. 11. (a) Change in composition of the beetle fauna in the Chilean Lake District during the interval between 18,000 and 10,000 14C yr BP. The dashed line represents the total number of taxa. The solid line represents the ratio of the three dependent taxa to total taxa. The zigzag dotted line marks the transition between moorland and forest assemblages. (b) Compositional and diversity changes in the beetle fauna of the Chilean Lake region during the last glacial to interglacial transition (modified from Hoganson and Ashworth, 1992; Ashworth and Hoganson, 1993).

However, the lack of a response of the beetle fauna to cooling at the time of the YD contradicted the results of Heusser (1974, 1997) and Heusser and Streeter (1980) which showed a change in the vegetation that was interpreted as a response to a temperate depression up to 6C lower than present. Moreno (1997) has also subsequently reported climatic cooling in the Lake District at the time of the YD.

Further south in the Chilean Channels, Ashworth and Markgraf (1989) and Ashworth et al. (1991) reported on fossil beetle and pollen assemblages from Glaciar Te´mpano (48 S, 72 W) and from Puerto Ede´n (49 S, 74 W) that showed an excellent correlation indicating that regional and local biotic changes were in phase (Fig. 12). The interpretation of the fossil beetle assemblages was aided by studies of

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the modern beetle fauna reported in Ashworth et al. (1991). The results suggest that in Puerto Ede´n, prior to 13,000 14C yr BP, the climate was probably windier than today; between 13,000 and 9,500 14C yr BP the Nothofagus forest expansion and the presence of aquatic beetles were in response to an increase in the precipitation that lasted till 5,500 14C yr BP. From 5,500 to 3,000 14C yr BP the precipitation declined and, after 3,000 14C yr BP, precipitation increased again to the present-day levels. The authors conclude that the cyclical wet/dry periods were in response to latitudinal shifts of the westerlies. These results show no evidence of cold reversal in the area at the time of the YD. These results also demonstrate that in a heavily glaciated environment adjacent to the South Patagonian Ice Cap, deglaciation had begun by 13,000 14C yr BP. One of the surprising results of this study was the discovery, in the basal deposits of the bog at Puerto Ede´n, of the remains of relatively large flightless beetles. If this discovery can be substantiated, it will require a major revision of our current understanding of the extent of the Patagonian Ice Sheet. For a flightless beetle to occur in the basal deposits it would require that some parts of the archipelago were unglaciated and the climatic conditions sufficiently moderate to support a refuge for the biota. The standard biogeographic interpretation is that the current biota of the archipelago was derived by dispersal from both the north and the south during postglacial times. Fossil beetles and pollen from Glaciar Te´mpano come from an exposed section in the walls of a meltwater channel located 2 km from the margin of the South Patagonian Ice Cap. Ashworth and Markgraf (1989) reported a flora and beetle fauna for the time of the YD that was very similar to that of present day. The implication was that at the time of the YD, the margin of the South Patagonian Ice Cap was in similar position as it is today and that the climate was similar. This site was revisited in April 2005 and the basal deposits will be re-dated at the Climate Change Institute of the University of Maine. If the radiocarbon dates obtained in 1989 are confirmed, then the evidence from certain areas of westernmost southern South America will indicate that there was no glacier advance at the time of the YD in this portion of the continent. These studies indicate that the transition

from glacial to postglacial conditions was the major regional climate change in the last 15,000 yrs and that coleopteran evidence from Chilean Patagonia does not support the occurrence of climate reversals during the Late Glacial period. Although pollen and beetle records show no evidence for temperature changes during the Late Glacial–interglacial transition, they all indicate alternating dry and wet periods due to changes in the precipitation pattern during this period. They interpret these changes as being caused by latitudinal shifts in the position of storm tracks in the belt of southern westerlies. Markgraf (1989, 1991, 1993a, b) indicated that vegetation changes in this area were successional, edaphic or in response to disturbance by fire. Throughout the Holocene, the beetle fauna of midlatitude South America was not affected by any major climate change (Hoganson and Ashworth, 1992). For the period 9,500–5,500 14C yr BP Ashworth et al. (1991) concluded that the climate was as wet as present day, but an expansion in Empetrum heath between 5,500 and 3,000 14C yr BP suggests that conditions then became drier than today.

6.3. Inter-Hemispheric linkages Synchronicity of climate change and comparability of climate signals, both temporal and spatial, are the principal parameters for evaluating inter-hemispheric linkages. Today, due to the many uncertainties in the absolute timing and magnitude of the events recorded in the two hemispheres, the relationship between Late Glacial and Interglacial transition climate change in the Northern and Southern Hemispheres remains clouded. Different hypotheses, relying on different lines of evidence, point variously to the Northern Hemisphere leading the Southern Hemisphere and vice versa, or to synchrony between hemispheres (Glasser et al., 2004). A large body of data from ice core, sedimentary, geomorphological and paleoecological investigations supports the argument that during the Late Glacial climate change was globally synchronous (Denton and Hendy, 1994, 1995; Lowell et al., 1995; Denton et al., 1999;

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Moreno et al., 1999; Steig et al., 1998). To investigate the causes for the inter-hemispheric synchroneity, Whitlock et al. (2001) apply a Community Climate Model version (NCAR CCMI) using time-series of insolation and glacial, ice core, and ocean records from the Northern and Southern Hemispheres. The paleoclimatic simulations were then compared to pollen and beetle records from both hemispheres and showed that Glacial–Interglacial climatic oscillations during the Quaternary affected both regions synchronously but the extent of ice cover was different. However, the mechanisms that link glacial cycles in the two hemispheres were not completely explained. Chironomid assemblages from Lago Mascardi in Argentina (Massaferro, unpublished) show excellent agreement with both EPICA Antarctica and GISP2 Greenland ice core records giving evidence of out-ofphase Late Glacial climatic events in both hemispheres (see Fig. 9). These results support the anti-phased north– south deglacial patterns and the relationship to the THC (thermohaline circulation) changes that produces a ‘‘seesaw’’ transfer of ocean heat between the hemispheres (Broecker, 1998). Recent studies of Antarctic ice cores added to the discussion but did not resolve the controversy (Lamy et al., 2004). The discrepancies in the different chronologies in the various Antarctic ice core records make it difficult to arrive at any conclusion. Summarizing, the existence of the cold reversals during the Late Glacial–Interglacial transition in the southern Hemisphere is still controversial (Ashworth et al., 1991; Markgraf, 1993a, b; Heusser et al., 1996; Ariztegui et al., 1997; Bennett et al., 2000; McCulloch et al., 2000; Massaferro and Brooks, 2002). Some studies indicate a cooling in South America that is more coincident with the Antarctic Cold Reversal (ACR) than the YD (Newnham and Lowe, 2000; Turney et al., 2003) whereas Bennett et al. (2000) looking at pollen studies suggest gradual warming with no reversals in the Southern Hemisphere. In New Zealand, the evidence of climate reversal during the YD is also problematic. Many pollen studies indicate progressive forest development implying that deglacial temperatures and precipitation increased gradually without significant reversal (McGlone, 1995; Vandergoes and Fitzsimons, 2003). In the South Island of New Zealand, Shakau (1986, 1990) developed a modern training set using chironomids; nevertheless, these results did not focus on climatic reconstructions. New attempts to infer temperature using chironomid-temperature models have been recently carried out at the University of Maine (USA) with the aim to produce a reliable dataset for quantitative reconstruction of past temperatures.

7. Future Investigations in Patagonia This chapter has provided evidence that the location of Patagonia in combination with the study of insects is ideal for testing hypotheses related to climate change during the Quaternary. It highlights the importance of southern South America as one of the most important regions in the world for testing whether climate changes are global or not. The studies presented in this chapter clearly show the potential of chironomids and beetles as proxy indicators of

climate change. However, there is still a lack of information about insect taxonomy, ecology and modern distribution in the Southern Hemisphere that is indispensable for accurate inferences. In addition, a better knowledge of insect taxonomy would allow development of transfer functions and quantitative reconstructions of past temperature which, in turn, would provide new approaches for understanding the synchronicity of climatic events in both hemispheres. Summarizing, the use of fossil insects as independent quantitative indicators could be key to a new approach for future paleoclimatic work. Although there have been an increasing number of paleoecological studies in Patagonia there is also much work to be done to fully understand and interpret the regional Quaternary scenarios, involving changes in biodiversity, tree refugia, species dispersion, successional changes and migration patterns since the last ice age.

Acknowledgments The authors want to thank Andrea Rizzo for her assistance during the fieldtrips in Patagonia and Sarah Gilchrist for providing them access to her unpublished PhD Thesis. Thanks also to Pat Haynes for helping in the preparation of samples.

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21 Understanding Climate from Patagonian Tree Rings Fidel A. Roig and Ricardo Villalba Laboratorio de Dendrocronologı´a e Historia Ambiental, IANIGLA-CONICET, CC 330, Mendoza (5500), Argentina

represented by the subantarctic forest, an ecosystem that extends along thesouthern Andes and the Coastal Cordillera from 37 S to Cape Horn. This huge latitudinal distribution provides unique opportunities for paleoenvironmental interpretations through a variety of ecological and temporal scales. Below, some recent dendroclimatic research in southwestern South America by using tree rings from subantarctic species as proxy for climate are reviewed. The paleoclimate background for different time-window scales are deduced from new tree ring data sets interpreted in a context of large-scale climate dynamics or forcings.

1. Introduction Efforts to improve our understanding of significant environmental variations occurring in recent geologic times should be directed toward obtaining a worldwide chronology of climatic fluctuations that emphasizes the sequence of events during periods of rapid change. This will certainly contribute to evaluating numerical models of climate variability, testing current theories of interactions of climate-forcing factors by comparision with previous behavior and quantifying the past frequency of extreme events. The study of climatic variations cannot depend entirely on recorded meteorological observations since these records are very brief and provide only a limited perspective on the present. The lack of data on timescales longer than the short instrumental record is a major obstacle, which stands in the way of producing reliable predictions of climate change and its ecological impacts. For this reason, it is important to extend the instrumental records through ‘‘proxy data’’, improving the resolution and spatial cover of paleoclimate records (IPCC, 2001). Many natural biological and geochemical archives can now be deciphered in order to provide data for these types of studies. In paleoclimatic sciences, tree rings are recognized as a model tool. Although limited to terrestrial contexts, their annual resolution provides more detail compared to any other biological (e.g. corals, pollen, ostracods) or chemical (e.g. ice layers) sources of paleoenvironmental information. By close examination of the fine cell structure and chemical composition of the wood from both temperate and tropical trees, it has been possible to uncover information about historical growing conditions (Bradley, 1992). The application of dendrochronology, or the analysis of growth ring records of trees, is relatively new in South America, but its development has grown during the last few decades. The first systematic tree ring studies in South America where done for temperate regions in Patagonia. Many scientists were first attracted by the possibility of developing long tree ring chronologies in areas where strong seasonality in temperature induces winter dormancy and annual ring formation in many tree species. Since the 1950s, when Edmund Schulman from the University of Arizona developed one of the first tree ring chronologies in Argentina, an enormous amount of information has been derived from hundreds of tree ring chronologies from different sites in southern South America. The temperate forests of the Patagonia region are primarily

2. Climate of Southwestern South America The climate characteristics in the southern extreme of South America (33–55 S) show patterns of spatial and temporal variation, partially explained by atmospheric, topographic and oceanic influences. The predominant meteorological elements determining the climate are the prevailing winds from the Pacific. They occur between the subtropical high and the subpolar low, expressed in an east–west component – on average between 35 and 65 S (Miller, 1976). The atmospheric circulation is closely linked to the north–south distribution of the higher values of atmospheric pressure at sea level of the eastern South Pacific. These maximum pressure values delimit the latitudinal range of the South Pacific subtropical anticyclone throughout the year (Taljaard, 1967; Saavedra and Foppiano, 1992). During the austral winter, the anticyclone is generally centered between 25 S and 90 W, and expands its range upto around 35 S (Fig. 1a). During the summer it moves to more austral positions, with its center around 39–40 S, and has an area of influence that reaches the region of Chiloe´ Island (Fig. 1b; Aceituno, 1988). Therefore, the seasonal activity of the anticyclone diminishes or blocks the flow of the westerlies toward minor latitudes. When surpassing 40 S, the westerlies have greater influence on the general atmospheric circulation, with a maximum intensity about 50 S (Taljaard, 1967). The westerlies, or the Pacific winds, transport the water that will then precipitate over this portion of the continent. Nevertheless, this precipitation does not have a uniform distribution due to the wind pattern explained previously. Nor does it have a uniform meridional distribution because of the massive presence of the Andes Range. Toward the south, the annual precipitation substantially increases in lower altitudes and even more in  2008 ELSEVIER B.V. ALL RIGHTS RESERVED

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Fig. 1. Spatial distribution of sea-level pressure, mean temperatures and net precipitation for the southern portion of South America: (a) sea-surface pressure fields for winter (June–August); (b) sea-surface pressure fields for summer (December–February), both adapted from Schwerdtfeger (1976); (c) annual isohyets (mm); (d) summer (January) isotherms; (e) winter (July) isotherms. All plots show regional climate patterns related to atmospheric circulation and frequency of humid air masses of oceanic provenance. Adapted from Hoffmann (1975). the mountain summits (diCastri and Hajek, 1976). This precipitation increase is related to the major frequency of frontal systems, as there is nothing to block the anticyclone from reaching land. Thus, the fjords region to the south is considered one of the most rainy extratropical zones of the world, with precipitation records in excess of 7000 mm/yr (Carrasco et al., 1998; Miller, 1976). In this region, seasonal rain cycles are moderate or lacking. Toward northern Patagonia the situation slowly changes due to the presence and seasonal migration of the anticyclone. Hence, the passage of fronts occurs more frequently during the winter than during the summer, because of an anticyclone positioned at lower latitudes.

Mediterranean climates are therefore present in this region, with prolonged summer droughts and lower volumes of annual precipitation (Kalin Arroyo et al., 1994). As such, a zone between 42 and 55 S has a similar distribution of rain over the year, while another between 36 and 42 S has strongly seasonal rains. This climatic zonation clearly correlates with vegetation types and their distribution. The height of the Andes and their meridian orientation is a determining factor in the distribution of precipitation on its eastern slope. The westerlies are intercepted by the mountains and, subsequently, rain is mostly discharged on the windward slopes. As a result, air masses

Understanding Climate from Patagonian Tree Rings with low humidity pass through toward the Patagonian steppe. This explains the enormous disparity in the distribution of precipitation moving from west to east in this South American region. Figure 1c shows the decrease in annual precipitation from south to north and from west to east. Precipitation goes from over 2000 mm/yr at the base of the Andes to under 200 mm/yr some 70–80 km on the Patagonian steppe to the east. Temperatures in southern South America show less dramatic changes than those observed in precipitation. Air temperature has a poleward decrease along more than 2000 km, but the absolute differences between south and north Patagonia are somewhat moderate (Kalin Arroyo et al., 1994). For example, on the Pacific side of the Andes, the mean air temperature decreases from 12C at latitudes of 36 S to 6C at ca. 55 S (diCastri and Hajek, 1976). The regulatory effect of the cold waters transported by the Humboldt Current (Miller, 1976) contributes to maintain these thermal conditions. Further inland, on the eastern side of the Andes, the horizontal temperature gradient becomes increasingly continental when compared with littoral areas exposed to ocean winds (Burgos, 1985; Tuhkanen, 1992). This has a significant biological effect on the distribution of species and forest.

3. Broad Climate Controls Maritime polar and tropical air interacts to produce the major climatic components in southern South America. Most of the climate features described above for the higher latitudes of Patagonia are linked to atmospheric circulation patterns originated in Antarctica (Burgos, 1985). The westerlies are a consequence of the meridional temperature and pressure gradients exerted between 55 and 60 S, a gradient that appears to be intensified during winter months. Polar fronts (the contact between maritime polar and tropical air) are sources of low temperatures and humidity conditions over the continent. During winter, these fronts are particularly active and reach more equatorward positions than during the summer (Taljaard, 1967; Saavedra and Foppiano, 1992). At lower latitudes, tropical air has a more pronounced influence (Taljaard, 1972). The high-pressure anticyclonic cell in the South Pacific is responsible for most of the seasonal shifts in precipitation, as was previously explained. Among other important factors influencing climate characteristics in the region is the El Nin˜o–Southern Oscillation (ENSO) (Diaz and Pulwarty, 1994). During an El Nin˜o year, heavy winter rainfalls are produced along the South American Pacific coasts, especially between Ecuador/Peru and central Chile. These high rainfalls are the result of greater evaporation through exceptionally warmer ocean conditions. Moreover, these Pacific Sea surface warmings (corresponding to negative phases of the Southern Oscillation) may precede rising summer air temperatures in the northern sector of Patagonia (Aceituno, 1988; Kiladis and Diaz, 1989). The ENSO anomalies appear within a preferred band between 3- and 7-year cycles

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and have been detected from many tree ring chronologies from the Patagonian region (Aceituno, 1988).

4. The Southern Andes Forests The temperate forests of southern South America (hereafter referred to as the subantarctic forests) occupy a narrow and long band, with the Pacific Ocean to the west and the Antarctic Ocean to the south. Their northern boundary is defined by a transition to the Chilean sclerophyllous Mediterranean woodlands, slightly north of 35 S (Schmithu¨sen, 1956; Oberdorfer, 1960; Cabrera and Willink, 1980; Gajardo, 1993). To the east, in the oriental spurs of the Andes Range, the subantarctic forests extend to the Patagonian steppe (Donoso, 1993; Roig, 1998). These forests are geographically isolated from Neotropical forests of South America, and even more distant forested regions in the Southern Hemisphere, such as New Zealand and Tasmania. Nevertheless, the subantarctic forests maintain important floristic affinities with all these other forests, having indicated ancestral links with floras of Neotropical and Gondwanic origin (Kalin Arroyo et al., 1995). The biogeographic isolation of the subantarctic forests explains the large number of endemisms (Armesto et al., 1995a), many of which are essential species for dendroclimatic research. There are geographic variations in species diversity and richness of the subantarctic forests, resulting from N–S and E–W climatic gradients, as well as topographic changes. For example, on the southwestern coastline there are less than 10 tree species. In contrast, between 36 and 38 S (the Bio Bio region in Chile), the temperate forests reach their maximum heterogeneity, with more than 70 tree species (see Fig. 2; Gajardo, 1993; Kalin Arroyo et al., 1995). Although the subantarctic forests are dominated by Nothofagus species from sea level to the treeline in the Andes, the forestal diversity in the E–W gradient is conditioned by topography. As a result of the rain shadow on the eastern slope of the Andes, the forests are progressively poorer in species, until they are abruptly replaced by steppe vegetation (Roig, 1998). This dramatic change occurs within an area of only a few kilometers, between the water divide of the Andes and the Patagonian steppe (see Fig. 2). Cooler and humid conditions progresively prevail moving south, as the sclerophylous woodlands in Chile intergrade with the temperate forests into the rain forests (sensu Schmithu¨sen, 1956). Although the rain forests are more extensive on the Chilean side, they extend across the Andes into neighboring Argentina, and include both evergreen and deciduous forest types. These forests intergrade, to the north and east, into drier areas of conifers (Austrocedrus, Araucaria) and deciduous Nothofagus (N. pumilio, N. antarctica) forests. The low- to mid-elevation closed-canopy rain forests develop on well-drained soils and include tree latifoliate species such as Weinmannia trichosperma, Nothofagus dombeyi, Laureliopsis philippiana, Drimys winteri as well as conifers such as Saxegothaea conspicua and Podocarpus nubigena. In permanently flooded areas, the conifer Pilgerodendron uviferum and the broad-leaved Tepualia stipularis are the prominent species.

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Fidel A. Roig and Ricardo Villalba Fig. 2. North-to-south geographic diversity of tree species in the temperate austral forests according to the following E–W transects: (a) to the north, the temperate forests gradually intergrades into sclerophyllous Mediterranean formations, (b) transition between the North Patagonian and valdivian rainforests and (c) the Fuegian-Patagonian forests. The southward and eastward reduction in plant species richness (with additionally simpler structural complexities) is a consequence of the strong climate gradients mentioned in the text. Data from Gajardo (1993), Roig (1998), Roig et al. (1985), Pisano (1977).

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The distribution of the swamp forests continues to the southernmost boundary of the subantarctic forests. Approximately between 40 and 43300 S, the Valdivian rain forest subtype occurs on both sides of the Andes and Coastal ranges. Here, the gymnosperm-dominated forest principally contains the emblematic Fitzroya cupressoides trees. Between important variants, the Fitzroya forest includes both pure stands and mixed angiosperm–gymnosperm forests with a predominance of Nothofagus species. This conifer can be found on upland slopes with winter snow or on gentle slopes with over 4000 mm annual precipitation. On swampy sites the Fitzroya–Pilgerodendron forest becomes the dominant variant (Schmithu¨sen, 1960; Donoso et al., 1993; Armesto et al., 1995b). Small areas of this forest type appear on the eastern side of the Andes. From the latitude of southern Chiloe´ to the last portions of

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the forested lands at 56 S, the Valdivian rain forest becomes structurally simpler and floristically impoverished as harsh climatic conditions increase. Nothofagus betuloides becomes dominant southward of 48 S, together with other tree species such as Maytenus magellanica, Pseudopanax laetevirens and P. nubigena. Throughout much of the Patagonian Andes, the evergreen angiosperm and mixed angiosperm–gymnosperm forests are eventually succeeded at higher altitudes by predominantly deciduous forests comprised of N. pumilio and N. antarctica. In northeastern Patagonia and below the deciduous forest belt, the rain forest becomes floristically impoverished, a fact dictated by the strong climate gradients produced by the Andes. These deciduous forests characterize much of the landscape on the leeward slopes down to south-central Tierra del Fuego.

Understanding Climate from Patagonian Tree Rings 5. Key Tree Species for Dendrochronological Research of Patagonia Only a limited number of tree species inhabiting these temperate forests have been the subject of dendrochronological studies. The lack of or poor clarity of periodical growth cycles in wood limits the value of a particular species for dendrochronological purposes. Some important genus in the subantarctic forests for which the application of dendrochronological techniques has been somewhat difficult include Laurelia, Weinmannia, Drimys, Podocarpus, Laureliopsis, Aextoxicon, Prumnopitis, Maytenus and the Mirtaceae. After several years of dendroclimatic research in Patagonia, certain species stand out as being more interesting for the study of growth rings. Relevant aspects to consider in tree ring research are the sensitivity of the growth rings to seasonal variations in climate, the tree longevity and the ecological distribution of the taxa. Compared to the large diversity of tree species and even shrubby plants in the Patagonian region, there is only a short list of species that are appropriate for growth ring studies. Future studies in the fields of dendroecology and dendroclimatology will allow us to include other species for research. Below, the ecological, distributional, morphological and anatomical requirements of those Patagonian species that are currently used for dendrochronological studies are described. 5.1. Nothofagus Nothofagus is the most diverse and widespread tree genus in the Patagonian Andes forests (Rodriquez et al., 1983).

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Many of the nine southern beech species (in addition to the several hybrids produced between them) can be listed as valuable sources of paleoenvironmental information derived from tree ring data. However, dendrochronological studies have been, up to now, centered on a few of these taxa, such as the deciduous species N. pumilio and the evergreen N. betuloides and N. dombeyi trees. These tree species are major constituents of the beech forests in Chile and Argentina in a belt of more than 3000 km, between 33 and 56 S (Moore, 1983; Donoso, 1993; Roig, 1998). Nothofagus pumilio [(Poepp. et Endl.) Krasser], commonly called ‘‘lenga’’, forms part of the coterminous Andean Patagonian woodland, which is a more than 2000 km stretch of forest on both sides of the Andes between 35300 and 56 S (Tortorelli, 1956; Donoso, 1993; see Fig. 3a). At any given latitude, lenga is usually the primary tree species colonizing drained slopes at intermediate elevations up to the treeline, which is at about 1700 m altitude in northern Patagonia and about 600 m altitude in Tierra del Fuego (Kalela, 1941; Rodriquez et al., 1983; Barrera et al., 2000). This species commonly forms dense and pure uneven-aged stands, or even-aged stands developed as a consequence of recently opened areas; but it may also occur in mixed stands with a variety of other Nothofagus species (Armesto et al., 1992; Veblen et al., 1996). However, as a result of its long meridional distribution, mountain beech stands demonstrate a variety of ecological responses in relation to strong relief and altitudinal gradients. The stands may show gradients of change, from the lowland up to the timberland, in growth forms, tree size, stand structure, seedling characteristics and modifications in the length of the physiological

Fig. 3. Geographical distribution and tree ring anatomical characteristics of the principal tree species used in dendroclimatic research in South America. See text for more details.

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growth cycle. These changes correspond to a decrease of both air and soil temperatures with altitude, and an increase in different stress factors such as frost frequencies and snow cover. These stresses can result in deep modifications of tree structure and size, including development of prostrated trees with twisted and gnarled branches of the so-called krumholz zone, which occurs just before the treeline (Eskuche, 1973; Puigdefabregas et al., 1988; Barrera et al., 2000; Cuevas, 2000; Holtmeier, 2000; Roig et al., 2003; Premoli, 2004). With increasing altitude, trees become more sensitive to the changing environment, thus they turn into a promising tool to monitor climaterelated altitudinal shifts as a result of changes in population regeneration and growth patterns (Daniels, 2000; Daniels and Veblen, 2003, 2004). Essentially deciduous by cold, its longitudinal distribution is determined by rainfall. Nothofagus pumilio is found in areas with as much as 5000 mm of annual rainfall, but its tolerance to dry soils allows this species to grow in areas with only 500 mm annual precipitation (Veblen et al., 1996). This is particularly evident in the vicinity of the Patagonian steppe where this species is progressively replaced by other xeric tree species, including N. antarctica. As the altitude increases, the length of the growth period shortens. At different altitudes, N. pumilio makes maximum use of the shortening growing season to produce the complete annual layer of wood. Data from Tierra del Fuego estimated a shortening of the growth period of more than 1 month between the lowest elevations in the forests and treeline (Roig et al., 2003). Tree rings formed in N. pumilio have proved to be high-quality material for dendrochronological research (Boninsegna et al., 1989). Irrefutable proof of the strong effects of the physical environment on the development of the annual wood increments is the high interseries correlation at any latitudinal site considered (Fritts, 1976). Which factor influences growth more, temperature or precipitation, appears to depend on the ecological characteristics of the forest environment. Tree ring chronologies from trees growing in more xeric environments indicate that precipitation is the main limiting factor, while chronologies from trees growing in more humid or altitudinal sites indicate that temperature is the more determining factor for growth. These growth/climate models have been the baseline for ecological and dendroclimatic reconstructions in the southern Andes (Boninsegna et al., 1989). Two other species used in dendroecological and dendroclimatological studies are the evergreen trees N. dombeyi, commonly called ‘‘coihue’’ (Mirb.) Oerst. and N. betuloides, commonly called ‘‘coihue de Magallanes’’ or ‘‘guindo’’ (Mirb.) Oerst. Nothofagus dombeyi is dominant and widespread in the Andes and Coastal ranges, and the moist lowlands of central and southern Chile and Argentina from 35 to 47 S (Rodriquez et al., 1983; see Fig. 3b). This species is relatively less tolerant of both drought and freezing than N. pumilio (Veblen et al., 1996; Donoso, 2004). Severe droughts caused high N. dombeyi mortality in the areas around the Nahuel Huapi National Park in Argentina and in the Chilean Lake District (Veblen et al., 1996; Bran et al., 2001; Suarez et al., 2004). Drought episodes in northern

Patagonia can be reconstructed from the mortality rates of Nothofagus tree species (Suarez et al., 2004), providing a history of droughts induced by La Nin˜a–Southern Oscillation events (Kiladis and Diaz, 1989). In the south, between 47 and 50 S, N. dombeyi is entirely replaced by N. betuloides, as more humid and cold conditions prevail. Nothofagus betuloides also appears as a small tree south of 40300 S in the summits of the coastal range in Chile and continues at high elevations, mainly along the western flanks of the Andes, and then extends along the archipelagos and coastal territories of Tierra del Fuego and Isla de los Estados (Moore, 1983). The first tree ring climatic reconstruction developed for Tierra del Fuego was based on N. betuloides tree rings and covered the past 250 years (Boninsegna et al., 1989). Its longevity and its long geographic distribution with many special conditions conducive to long-term wood conservation (Roig et al., 1996) makes this species a valuable tool for climatic reconstruction in the austral environments.

5.2. Austrocedrus chilensis (D. Don) Florin et Bouletje Austrocedrus chilensis, commonly named ‘‘cipre´s de la cordillera’’, is a monospecific, evergreen and dioecious tree with a narrow crown and short trunk surrounded by a reddish brown bark that peels in long strips (Bernath, 1937; Rodriquez et al., 1983; Castor et al., 1996). Austrocedrus is the most northerly distributed conifer tree species of the Andean Patagonian forest. The natural range of the species is a narrow strip that runs parallel to the Andes Cordillera, in eastern Chile, between ca. 35 and 38 S, and across the Andes, in western Argentina between ca. 39300 and 43400 S (Schmithu¨sen, 1956; Oberdorfer, 1960; Schlegel, 1962; Dezzotti and Sancholuz, 1991; Roig, 1998; Gallo et al., 2004; see Fig. 3c). Isolated groups of Austrocedrus, at the northern extent of its distribution in both Argentina and Chile, provide interesting populations for dendroclimatic research (Boninsegna and Holmes, 1978; LaMarche et al., 1979a, b). Populations of this taxa are found mainly in mountain environments, in low densities, sparsely distributed stands, and occupying an exceptionally broad gradient of soil moisture contents (Hu¨ck, 1978; Seibert, 1982). In the rain shadow on the Argentine side of the Andes, in marginal relict forest patches or as dispersed individuals, these are probably the driest conditions in which this species survives. This ecological behavior is linked, in many cases, to tree morphology and longevity. In deep and moist soils, Austrocedrus can reach 15–20 m, with a trunk of up to 1 m dbh (= diameter at breast height), and a lifespan of 200–300 years. However, on rocky and dry substrata, trees develop narrower size bodies, and even as dwarf trees in extreme cases (Pastorino and Gallo, 2002). However, these trees have a lifespan of 700–800 years. Moreover, in these particularly dry areas, dead logs can be preserved for centuries, a factor that allows us to extend modern tree ring chronologies for long periods into the past. Wood samples from Austrocedrus stands at El Asiento (Chile, 32400 S; 70490 W) and Huinganco (Argentina, 36300 S; 70360 W) have provided exceptionally long chronologies of 1200

Understanding Climate from Patagonian Tree Rings (Schlegel, 1962) and 1800 (Morales, personal communication) years, respectively. Paleobiogeographical interpretations from genetic information of these marginal and isolated forest patches indicate that they may be relict units of the continuous forest to the east (Pastorino and Gallo, 2002; Gallo et al., 2004). The structure and extent of the A. chilensis forest stands are deeply linked to climate variability and landscape management. In many areas, the forest structure has been strongly influenced by changes in fire frequency and severity, particularly after European settlement (Rothkugel, 1913, 1916; Kitzberger and Veblen, 1999). Additionally, naturally occurring fires, such as those produced by the climatic conditions originating from ENSO cycles, contributed to modifications of the original stand structures, disappearance of old tree relicts and inhibition of stand recruitments (Veblen et al., 1999). Livestock and deer browsing activities have also intensified these disturbance factors (Relva and Veblen, 1998). Reforestation with exotic tree species has rapidly increased in the northern Patagonian region of Argentina, principally with ponderosa pine (Pinus ponderosa). The agressive expansion of these introduced tree species is endangering the remaining natural populations of A. chilensis. However, despite these many disturbance factors, scattered areas still remain unaffected in both Argentina and Chile. Particularly those relict forest patches on rocky outcrops escape widespread fires because of the lack of understory that is necessary to fuel fire. Austrocedrus chilensis was one of the first species used for dendrochronological studies in Argentina. Since the pioneer studies by Schulman during the 1950s, this species has been the subject of a variety of ecological and climatological studies (e.g. LaMarche, 1975; Baccala et al., 1998; Buamscha et al., 1998; Filip and Rosso, 1999; Pastorino and Gallo, 2002). Despite the fact that the active growth of Austrocedrus takes place during the warm season, tree ring chronologies previously developed from Austrocedrus reveal that annual ring width varies depending on precipitation and moisture stored in the soil during the winter and spring (Schulman, 1956; Villalba and Veblen, 1998; Roig et al., 2006). This strong dependence on cool season rains prior to growth is explained by the Mediterranean character of the precipitation regime, where ca. 50% of the total annual precipitation is during the winter, and only 10% during the summer (Prohaska, 1976). As this precipitation is highest in the mountains and lowest along the steppe boundary, the growth/climate association probably becomes more pronounced as the forests moves from wetter to drier conditions. Moreover, the d 18O composition of A. chilensis tree rings reveals strong correlations with the Southern Oscillation Index, creating unparallel opportunities for reconstructing ENSO behavior in sensitive areas to this phenomenon, such as the northwestern Patagonia (Roig et al., 2006).

5.3. Pilgerodendron uviferum (D. Don) Florin Pilgerodendron uviferum, commonly called ‘‘cipre´s de las Guaitecas’’, is a 1.5–10 m tall, narrow, pyramidal tree with a trunk up to ca. 0.40–1 m dbh and covered with a dark

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brown bark that comes off in long strips (Moore, 1983). Its transversal wood surfaces have easily detectable and uniform growth ring structures (Roig, 1992). It is mainly distributed from Cape Horn and the fjords of the southwest to around 40 S in Chile (Reiche, 1934; Moore, 1983; Gajardo, 1993; Kalin Arroyo et al., 1995; see Fig. 3d). In western Argentina, there are scarce stands between about 47 and 41 S latitude (Roig, 1998, 1991; Rovere et al., 2002). These environments correspond to the temperate rain forests, a vegetation type more adapted to wetter and cooler climates than those in the northern regions of Patagonia. This species generally forms coastal forests, precursors of climax forests dominated by evergreen Nothofagus species, or open stands in sheltered lowland bogs further inland, at 0–150 m above sea level (Moore, 1983; Roig et al., 1985). Also, smaller populations can be found at higher altitudes (ca. 1000 m) in both Chile and Argentina (Roig, 1991; Szeicz et al., 2000). Pilgerodendron is an important natural resource; its wood is widely used for building and roof shakes, fences and for other construction, because of the ease with which it is worked and its resistance to decay. For this reason, large-scale human disturbance of the forest during colonial times to the present has led to the disappearance of the species from most of its original distribution, thus limiting its availability for climatic or ecological reconstructions. Nevertheless, because it is one of the conifer species with the southernmost distribution and reaches several hundred years old (Szeicz et al., 2003), Pilgerodendron is an interesting taxa for paleoenvironmental reconstructions (Roig and Boninsegna, 1990, 1991; Roig, 1991; Roig et al., 1992; Szeicz et al., 2000). Occurring in areas strongly influenced by oceanic climates, Pilgerodendron tree rings reflect a more uniform annual temperature and high precipitation throughout the year. The climatic conditions under which Pilgerodendron grows slowly change along a latitudinal distribution that spans more than 1500 km, with a northward increase in mean annual temperature from ca. 6 to 10C (diCastri and Hajek, 1976). In the northern part of its range, variations in annual precipitation mostly seem to affect growth, while in the south, annual temperature is the main factor affecting growth (Roig and Boninsegna, 1990; Rivera et al., 1997; Szeicz et al., 2000). A larger matrix of tree ring chronologies along the entire distributional range should be studied to better understand this dual geographical response.

5.4. Fitzroya cupressoides (Molina) Johnston Fitzroya cupressoides, commonly known as ‘‘lahuan’’ (Mapuche) or ‘‘alerce’’ (Spanish), is probably one of the tallest trees in South America. Its reduced crown is up to 50 m high on the tree. The trunk can reach 3 m dbh, is covered by a reddish bark, furrowed and comes off in long strips. Tree rings are clearly discernible and almost always very narrow (Roig, 1992). For example, wood samples containing 100 annual rings per centimeter in radial stripes have been detected for some trees growing on leached soils at the Chaite´n region, Chile. Fitzroya occurs discontinuously in the evergreen forest of the coastal range south of Valdivia, on Chiloe´ Island, and

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on both sides of the Andes, between 41 and 43 S latitudes, although the area in Argentina is significantly smaller (Oberdorfer, 1960; Dimitri, 1972; Fraver et al., 1999; Kitzberger et al., 2000; see Fig. 3e). Fitzroya is one of the representative species of the subantarctic temperate rain forest. It mainly grows on mountain slopes between 300 and 900 m altitude, over shallow, acidic and volcanic soils (Veit, 1994). It is also found in lowlands, but is restricted to poorly drained soils, where there is less competition (Peralta, 1975; Veblen et al., 1976, 1982). This species regenerates in open and sunny gaps resulting from episodes of windblow or fire disturbance (Parker and Donoso, 1993; Lara et al., 1999). The climate in its distribution range is cool-temperate and rainy, receiving between 2000 and 5000 mm of annual precipitation (diCastri and Hajek, 1976; Donoso, 1993). There is not a wide disparity in mean seasonal temperatures, with winter average between 2 and 4C and summer average between 13 and 16C (diCastri and Hajek, 1976). Once extensively exploited in both Chile and Argentina, today this species is protected by law. Fitzroya is one of the oldest tree species in the world. Lifespans of 2500 years have been recorded in early references by Reiche (1934), and another reference indicates lifespans surpassing 3500 years (Lara and Villalba, 1993). Because of its extraordinary resistance to fungal and insect attacks, scientists, using subfossil woods, have been able to push the modern records back as far as 5666 years (Wolodarsky-Franke et al., 2003), which is the longest continuous tree ring record yet obtained for the Southern Hemisphere. Logs preserved in sediments for more than 50 14C ka BP are other outstanding examples of this species resistance to decay (Roig et al., 2001; Villagra´n and Roig, 2004). Pioneer dendrochronological studies with Fitzroya were carried out by Edmund Schulman during the 1950s. Extensive collecting was done at this time in relatively inaccessible forest stands on the Chilean side of the Andes and foothills of Argentina (Schulman, 1956). However, Schulman was disappointed with these samples due to cross-dating weakness and did not continue to work with this species. Later, Boninsegna and Holmes (1985) developed the first tree ring chronology with Fitzroya. Successive work with Fitzroya demonstrates its usefulness for ecological and climatic studies. Fitzroya tree rings and their annual densities are indicators of periods of extended warm season temperatures, as well as an integrator of annual and decadal temperature trends. This particular quality of the tree rings has allowed scientists to extend temperature estimates for northern Patagonia back several thousand years (Villalba, 1990; Roig, 1996).

5.5. Araucaria araucana (Molina) K. Koch Araucaria, commonly called ‘‘pehue´n’’ [in Mapuche language], is a coniferous evergreen with evenly spaced nodes of horizontal-spreading branches that are arranged in regular whorls around straight and cylindrical boles. At maturity, A. araucana can reach 30–40 m tall and develop trunks of 1–1.5 m dbh. The tree has a hard and thick bark organized in polyhedral-shaped plates that protect living

tissues against fire (Aagesen, 2004). The crown eventually develops a rounded or flattish top that gives the tree a characteristic silhouette in the landscape, recognizable from great distances. A shade-tolerant species, A. araucana prefers well-drained, volcanic soils, but is surprisingly tolerant of many soil types (Veblen, 1982). Araucaria araucana occurs between 900 m and 1200 m altitude in southwestern Argentina and southcentral Chile in two disjunct areas in the coastal range (Nahuelbuta) and the Andean Range (Fig. 3f). This includes areas between 37200 and 40200 S. Even though A. araucana has been protected by law, its modern distribution has been reduced from a wider historical range by anthropic disturbances (Veblen, 1982; Bekessy et al., 2004). The populations in the coastal range are more influenced by the proximity of the Pacific Ocean than those distributed along both sides of the Andes (Veblen, 1982). The Araucaria region on the Chilean side of the Andes is characterized by a Mediterranean climate with large amounts of annual (mostly winter) precipitation (ca. 4000 mm per year) that gives way to the development of mixed and dense humid forests. In contrast, only sparse A. araucana stands are found on the Argentine side, as the summer drought conditions increase toward the steppe (Veblen, 1982; Roig, 1998). The area of Araucaria distribution is subject to frequent disturbance, such as active volcanism, as well as anthropic and lightning fires. The affected A. araucana forests form postfire stands, demonstrating a dynamic cycle adapted to survive fire (Burns, 1993; Roig and Roig, 1995; Veblen et al., 1995; Aagesen, 2004). Stand disturbance histories have been developed from tree ring analysis to reconstruct past fire activities and their link to associated climatic anomalies in northern Patagonia (Kitzberger and Veblen, 1999; Kitzberger et al., 2001). Araucaria araucana trees are known to have long lifespans. Trees of ca. 900 years old have been recorded from marginal forest patches on rocky outcrops surrounded by the xeric steppe in Argentina (LaMarche et al., 1979a). This indicates that it may be possible to find trees of millennial ages from similar environments. Although it is a long-lived tree species, few environmental paleoreconstructions have been done with A. araucana until now, probably because of the difficulty in reading the growth rings. Sometimes, undifferentiated early-to-latewood tracheid morphologies and sharply contrasting lignin contents in ring-by-ring entities result in ill-defined annual rings. Fortunately, wood samples with sharply bounded rings can be used in tree ring analysis. Tree ring chronologies derived from A. araucana growing in the foothills of the Patagonian Andes of Argentina were used to reconstruct streamflow back to the 1600s. This was possible because of a marked response to climatic variability and hence, to a strong correlation between tree growth and total yearly runoff in the corresponding river basins (Holmes, 1982; Holmes et al., 1979).

6. Growth Seasonality, Climatic Signals in Growth Rings and Longevity of Patagonian Trees The annual growth cycle of temperate woody plants is closely linked with seasonal climate changes (Fritts,

Understanding Climate from Patagonian Tree Rings 1976; Schweingruber, 1996). This involves adaptive mechanisms related to vegetative dormancy and active shoot growth. As such, cambial activity follows this seasonal cycle of growth and dormancy. This is a general rule for hardwood and softwood species for both the Northern and Southern hemispheres. As it would be expected, favorable environmental conditions trigger xylogenesis development, which is controlled by a complex network that regulates its opportunity, intensity and anatomical properties. According to dendrochronological statistical model estimations, the annual cycle of growth and dormancy of Patagonian trees closely follows the cycle of the two primary growth promoters, water and temperature, depending on the tree species and site characteristics. As environmental conditions change, cambial activity reacts to that change. The spring and fall thresholds of growth activity/dormancy are thus geographically dependent. A good example is N. pumilio that forms closed stands from the foothill up to a sharply demarcated timberline at about 1700 m altitude in northern Patagonia and about 600 m in Tierra del Fuego. This altitudinal gradient results in modifications of the stem growth forms, the length of the physiological growth cycle, the timing of the ring formation and the tree ring sensitivity (Rusch, 1993; Barrera et al., 2000; Roig et al., 2003). These complex environmental interactions vary in the influence that climate has on growth formation (Massaccesi et al., 2007). As wood is formed by the successive addition of secondary xylem, each seasonal growth expression, namely the growth ring, represents to some extent the intensity that the environmental controls exert on the dividing cell regulation in the so-called cambial zone (Han, 2001). This cambial zone is where the vascular cambium tissue is contained, and its function in tree growth and development interest us for two reasons: (1) it promotes stem diameter by producing new wood portions and (2) it acts as a communication center for the transmission of developmental signals from plant growth regulators. Putting aside the contribution of hormonal regulation in controlling diameter enlargement of trees (wood production), what interests us is that the size and anatomical expression of the growth rings can adjust according to the needs for water transport and temperature thresholds, both necessary to activate cell divisions and consequent biomass production. Many of the interactions between seasonal climate variations and growth rings of trees have been statistically inferred through regression and correlation models (Fritts, 1976). Through this dendroclimatological approach, it is possible to identify the type and intensity of the climatic signal contained in tree rings. However, considering the complex regulatory networks controlling the biology of the annual growth cycle, our current understanding of tree biology is still limited. New insights into the relationship between environmental variations and xylem development are immerging as a result of an increase in wood ecological studies (Sperry and Sullivan, 1992; Martı´nez-Vilalta et al., 2002; Martı´nez-Vilalta and Pin˜ol, 2002). These studies will provide new opportunities to overcome many inherent tree-related questions that are essential to the development of paleoenvironmental estimations.

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The major limiting climatic factors influencing wood formation of Patagonian trees vary according to regional climates, topography, and species physiology and distribution. To establish the influence of macroclimatic factors on tree growth, we usually compare tree ring chronologies and monthly climatic (temperature, precipitation) records using correlation functions (Blasing et al., 1984). The following is the state of the art concerning dendroclimatic knowledge of the previously described Patagonian species. The Andean stands of A. araucana usually occur on rocky ground and in xeric environment. Thus, the logical assumption is that this is a species sensitive to precipitation variability. This has been partially demonstrated by observations of the close relationship between the annual growth of this tree and the total yearly runoff of the principal rivers in norwestern Patagonia (Holmes et al., 1979). Moreover, high temperature values during summer increase evapotranspiration rates and reduce moisture availability, forcing trees to produce narrow growth rings. Significant but inverse correlations among the ring-width index series and November–April air temperatures (warmer temperatures result in less growth) have been detected for several Araucaria sites (Villalba et al., 1989), suggesting a complex interaction between temperature and soil humidity on annual growth regulation. Because of this particular growth response to climate, A. araucana have been recognized as a valuable tool for riverflow reconstructions (Holmes, 1982). Austrocedrus chilensis show similar regulations regarding annual growth and climatic variables, but shifted for some months. At its northernmost extent in Chilean territory, Austrocedrus growth is controlled mainly by water precipitated and stored in soils during winter–spring (May to November) months, while air temperature during the same period appears to be inversely correlated with growth (LaMarche, 1975; LeQuesne et al., 2000). On the eastern side of the Andes, Austrocedrus growth seems to be more strongly controlled by late spring and early summer (November–December) precipitation, and inversely related to mean air temperatures during the same period. A similar response to climate has been observed at the southern distribution of Austrocedrus on the Chilean side of the Andes (LeQuesne et al., 2000). Growth–climate response models of F. cupressoides indicate that ring width formation is largely dependent on summer air temperature (Villalba, 1990; Lara and Villalba, 1993; Roig, 1996). Below normal temperatures between December and March are associated with large ring widths and enhanced ring density (Roig, 1996). Increased growth during cold summers in Fitzroya may be related to a decrease in physiological stress due to lower rates of evapotranspiration (Golte, 1974). Statistical temperature–growth correlations show, however, that the annual development of Fitzroya tree rings is controlled exclusively not only by the temperatures of the current growing season, but also by those of the previous summer. This lag effect in the climate–tree growth relationship is slightly stronger than the current-year correlation for most of the chronologies from both Argentina and Chile. The Fitzroya’s relative P. uviferum shows more complex responses to climate variations than those observed

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for the aforementioned conifers. Topographic position seems to be a determinant factor in the response of Pilgerodendron to climate. Trees at low to high altitude forested or bog sites show a strong negative response to regional summer temperatures in the summer preceding growth, and a complex but generally positive relationship to precipitation (Roig and Boninsegna, 1990; Roig, 1991; Szeicz et al., 2000). These results suggest a strong regional moisture stress signal in Pilgerodendron tree rings, indicating that these trees grow best when the preceeding summer is cold and wet. However, trees at or near treeline, at high elevation open forest to forest–tundra sites, are characterized by a positive response to warmer summer temperatures in the year of growth while increased precipitation often negatively affects growth (Roig, 1991; Szeicz et al., 2000). The growth of the southern beech tree is particularly affected by temperature restrictions imposed by altitudinal gradients over its more than 2000 km latitudinal distribution. Nothofagus pumilio is positively correlated with warmer spring to early summer temperatures (Schmelter, 2000; Massaccesi et al., 2007). This direct response of N. pumilio to warmer temperatures prior to and during the active vegetation period is a common pattern for trees growing near or at the upper treelines, where increasing temperatures promote physiological growth mechanisms (Holtmeier, 2000). The influence of altitudinal gradients on the growth of the southern beech is evidenced through fine-scale monitoring networks. At Tierra del Fuego, for example, a five-year observational period confirms sequential changes in tree growth between the valley bottom and the timberline zone (Massaccesi, personal communication). A significant delay of the onset of the initial stages of leaf swelling and the reactivation of cambial divisions, as well as a shortening of the vegetative growth period occurs with increasing altitude. This is a clear demonstration that rapid changes in temperature, part of the harsher environmental conditions at higher altitudes, regulate the start

and conclusion of the leaf phenological phases and tree ring morphological changes. Toward the lower treelines on the eastern slope of the Andes, that is in the zone of transition to the steppe and on the northernmost sector of the species distribution west of the Andes, the annual growth of N. pumilio appears to be mainly limited by precipitation. Thus, the lack of water availability becomes the stress factor. Associated high summer temperatures may intensify the soil water loss and hence produce even more pronounced negative growth effects, as evidenced by inverse correlations between tree growth and summer temperatures along the whole length of the Patagonian region (Boninsegna et al., 1989; Lara et al., 2005; Schmelter, 2000).

7. Tree Longevity and Chronology Extension of Patagonian Species Long chronologies are essential to construct a detailed history of past climate. Many tree ring chronologies in Patagonia do not exceed 300 years, which is a restriction inherent to the typical lifespan of trees. The southern beeches, for example, rarely surpass 300–500 years old, as with its relatives Quercus and Fagus of the Northern Hemisphere. The lifespan of the trees referenced in Table 1 probably represents those of individuals of exceptional longevity but of scarce representation in the Patagonian forests. The older trees in Patagonia are conifer species, which have been the source of very longclimatic reconstructions in South America. As given in Table 1, many conifer taxa can reach or surpass the millennial age, but only the long-lived conifer F. cupressoides has been the subject of the several millennial long chronologies that have been constructed in the southern Andes. Although the longevity of a species is mainly dictated by its specific genetic makeup, other factors can play a key role in increasing tree longevity and wood

Table 1. Maximum ages and wood conservation for some subantarctic tree species. Species

Age

Age determination

Source

Subfossil conservation

Nothofagus alpina

500–650

Ring counting

Unknown

Nothofagus pumilio

ca. 600

Ring counting

Veblen et al., 1996; Pollmann, 2004 Pisano, pers. comm.

Nothofagus betuloides

500

Ring counting

Boninsegna et al., 1989

Nothofagus dombeyi

500–700

Laurelia philippiana

400

Veblen et al., 1996; Pollmann, 2004 Pollmann, 2004

Araucaria araucana Austrocedrus chilensis Pilgerodendron uviferum Fitzroya cupressoides

865 1000 700

Age–diameter ratio Age–diameter ratio Ring counting Ring counting Ring counting

LaMarche et al., 1979a LaMarche et al., 1979b Szeicz et al., 2000; Dı´az, 1997

3600

Ring counting

Lara et al., 2000

Peat bogs and river sediments Peat bogs and river sediments Unknown Unknown Unknown Dry soils Buried in peat and soil Buried in peat and soil

Understanding Climate from Patagonian Tree Rings conservation. As we move to more ecologically marginal areas, the probability of obtaining longer tree ring chronologies increases. This has two main explanations: first, the relative growth rate of trees diminishes and consequently their longevity increases; second, the natural preservation of dead wood increases as the environment becomes dry or cold enough to prevent wood biological decay. For example, an 800-year-old living tree ring chronology of A. chilensis from northern Patagonia was extended to a 1800-year-old record by overlapping dead pieces of wood preserved for centuries in a very dry environment (Morales, personal communication). Other times, a combination of climate and physicochemical soil conditions can contribute to long-term preservation. Subfossil logs of F. cupressoides and P. uviferum have been shown to have some of the most decayresistant woods, facilitated by the wet anoxic conditions and low pH values of the sediments where samples of these species have been found (Roig et al., 1996; Roig et al., 2001; Castro and Roig, 2007). Long-term preserved wood is the essential material for understanding climatic characteristics beyond the limits of the typical lifespan of trees.

421

8. Paleoclimatic Settings in Patagonia Progress in southern South America dendroclimatology permitted the expansion of tree ring chronology coverage, to refine paleoclimatic tree ring-based interpretations and to explore new wood materials from specimens that are still living or those that have been long dead. These specimens provided important new information about aspects of the general climate circulation dynamics during Holocene/Pleistocene times. Below, some recent dendroclimatic research relevant to understanding detailed aspects of past climatic variability of Patagonia is reviewed.

8.1. Evidence for Climate Variability over Hundreds of Years (0–1000 BP) In this section, the use of tree rings from subantarctic species as proxy for climate reconstructions for the past few centuries to millennia is focused. Table 2 summarizes the dendroclimatic research conducted in southern South America during the past 50 years, intended to gain insight into past climatic variations

Table 2. Tree ring-based reconstructions of temperature, precipitation and sea-level pressure in southern South America. Area

Climatic/environmental variable reconstructed

Period (length)

Tree species utilized

Source

Central Andes

Winter precipitation

AD 1220–1971 AD 1310–2000

Austrocedrus chilensis

Annual precipitation Summer temperature

AD 1600–1950 AD 864–1985 1634BC-AD1987 AD 1750–1989

Austrocedrus chilensis Fitzroya cupressoides Nothofagus pumilio

Lamarche, 1974; Boninsegna, 1988; LeQuesne et al., 2006 Schulman, 1956 Villalba, 1990; Roig, 1996; Lara and Villalba, 1993 Villalba et al., 1997a

AD 1640–1998

Nothofagus pumilio

Villalba et al., 2003

AD 1556–1986

Roig and Boninsegna, 1990

AD 1600–1988

Pilgerodendron uviferum Austrocedrus chilensis

Villalba et al., 1998

AD 1837–1996 AD 1750–1984

Nothofagus pumilio Nothofagus pumilio

Lara et al., 2001 Villalba et al., 1997a

AD 1601–1966

Holmes et al., 1982

Northern Patagonian Andes

Mean annual temperature Mean annual temperature Summer precipitation Summer and annual precipitation Summer precipitation Spring snow cover duration Total annual river discharge

Southern Patagonian Andes

Summer temperature

AD 1750–1984

Minimum annual temperature Mean annual temperature Summer transpolar sealevel pressure Summer mean sealevel pressure

AD 1829–1996

Austrocedrus chilensis, Araucaria araucana N. pumilio, N. betuloides Nothofagus pumilio

AD 1640–1998

Nothofagus pumilio

Villalba et al., 2003

AD 1700–1995

Nothofagus pumilio

Villalba et al., 1997b

AD 1746–1984

N. pumilio, N. betuloides

D’Arrigo and Villalba, 2000

Boninsegna et al., 1989 Aravena et al., 2002

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in the region. Selected records will be discussed below, mainly those related to the most relevant issues associated with long-term climate variations.

Temperature Changes in Northern Patagonia The work of Villalba et al. (1989) was the first attempt to provide a quantitative estimate of temperature variations in northern Patagonia. Sixty years (1914–1973) of a regional temperature record were regressed against seven A. araucana chronologies developed by LaMarche et al. (1979a) to provide a summer temperature reconstruction that covers the period of AD 1500–1973. The reconstruction accounts for 49% of the total variance in the observed temperature record; however, the complexity of A. araucana tree growth response to climate makes its interpretation problematic. Recent ecological studies of Araucaria (Burns, 1993; Gonza´lez, 2003) will provide the basis for a better interpretation of dendrochronological studies of this species. Millenium-long chronologies were developed from F. cupressoides during the 1980s (Boninsegna and Holmes, 1985; Villalba, 1990) and the 1990s (Lara and Villalba, 1993; Lara et al., 2000). To date, two millennial-long temperature reconstructions have been made from single-site chronologies on the eastern (Villalba, 1990) and western sides of the northern Patagonian Andes (Lara and Villalba, 1993). Both reconstructions are well correlated during the years AD 869–1984, a period for which they have common records. However, important differences, particularly in low-frequency oscillations, are observed. The cold phases related to the Little Ice Age events from AD 1300 to 1380, 1520 to 1660 and 1820 to 1850 are well marked in the records for the Argentine side of the Andes, but more subdued in the records for the Chilean side. Recent comparison of instrumental records shows different trends in temperature variations between Puerto Montt and Bariloche during the twentieth century, suggesting some difference in temperature patterns for each side of the Andes in Northern Patagonia at 41 S (Villalba et al., 2003). Both reconstructions show highly variable temperatures in the high-frequency range, which also may contribute to the differences observed in long-term oscillations between the Argentinean and Chilean record. The temperature reconstruction for Lenca, Chile, extends back to 3620 years and represents the longest temperature tree ring-based reconstruction for South America to date (Lara and Villalba, 1993). The need to reconcile temperature reconstructions between the two sides of the Andes favored the rapid increase in the number of tree ring chronologies in the late 1990s. The accepted opinion in dendroclimatology is that trees growing at the upper treeline have a greater sensitivity to temperature variations. This has motivated the use of N. pumilio for developing a network of highelevation chronologies (Roig et al., 2003; Lara et al., 2005; Villalba et al., 2005). Indeed, more than 90 chronologies from N. pumilio were developed on both sides of the Andes in a relatively short period of 5–6 years. The availability of a relatively large, uniformly

distributed set of high-elevation chronologies across the northern and southern Patagonian Andes inspired the development of local and regional reconstructions of temperature. Fifteen tree ring chronologies from N. pumilio were developed along three elevational gradients on the Argentinean side of the northern Patagonian Andes. Trends in tree ring characteristics as well as variations in relationships between tree growth and climatic fluctuations were examined along these elevational gradients (Villalba et al., 1997a). Annual variations in the growth of subalpine N. pumilio were compared to variations in mean annual temperature and duration of snow cover. Based on these relationships, multiple regression models were developed to reconstruct the duration of snow cover and mean annual temperature fluctuations in the subalpine zone of northern Patagonia since AD 1750. In a similar attempt, Aravena et al. (2002) reconstructed the minimum annual temperature for Punta Arenas to the year AD 1829 (see below), while Schmelter (2000) and Massaccesi et al. (2007) described the influence of altitudinal gradients of temperature on tree growth. More recently, temperature records from 13 stations across Patagonia were analyzed to identify the dominant patterns of temperature variations in southern South America. For the period of AD 1930–1990, three major patterns in temperature trends were identified (Fig. 4). Records from stations along the Pacific coast between 37 and 43 S show negative trends in mean annual temperature with marked cooling during the period of AD 1950 to the mid-1970s. A clear warming trend is observed in the southern stations (south of 46 S), which intensifies at higher latitudes. No temperature trends were detected for the stations on the Atlantic coast north of 45 S (Villalba et al., 2003). Two composite tree ring records from upper-treeline (1410–1500 m elevation) in Mount Tronador (41100 S) sector of the Nahuel Huapi National Park, Rı´o Negro, Argentina, were used to reconstruct past temperature fluctuations for northern Patagonia. Tree ring series were standardized using the regional curve standardization method, which allows for the preservation of the low-frequency variance in excess of the length of the individual segments used in creating the tree ring chronologies (Briffa et al., 1996). The temperature reconstructions for northern Patagonia extend back to 1640 and explain 55% of the temperature variance during the period AD 1930–1989. Cross-spectral analysis of actual and reconstructed temperatures over the period AD 1930–1989, which these records have in common, indicates that most of the explained variance is for periods of >10 years. For periods of >15 years, the squared coherency between actual and reconstructed temperatures ranges between 0.6 and 0.95 for both reconstructions. Consequently, this reconstruction is useful for studying multidecennial temperature variations in the northern Patagonian sector of the southern Andes over the past 360 years. Despite relatively cool years in the late 1960s and 1970s, temperatures during the twentieth century were the warmest of the past four

Understanding Climate from Patagonian Tree Rings

423

Fig. 4. Spatial (upper) and temporal (lower) patterns of annual temperature variations across southern South America. The variance explained by each pattern is indicated in the upper left corner. Circles indicate station locations. The PC1, PC2 and PC3 amplitudes are associated with the meteorological stations located in the northern sector of the southern Andes along the Pacific coast (Temuco and Puerto Montt), in the southern tip of South America (Rı´o Gallegos, Punta Arenas and Ushuaia), and on the Atlantic coast, north of 45 S (Bahı´a Blanca, San Antonio Oeste and Trelew).

Fig. 5. Tree ring-based reconstruction of mean annual temperature departures for northern Patagonia from AD 1640 to 1989. The reconstruction is also shown in smoothed version (thick line) resulting from filtering the original data with a cubic spline designed to reduce 50% of the variance with periodicity of 25 years. Both actual and reconstructed means are shown.

centuries. The mean annual temperature for northern Patagonia during the period 1900–1990 was 0.53C above the longer period mean of AD 1640–1899 (Fig. 5). Significant cold events are centered on the period of AD 1650–1660, 1700 and the period of 1820–1870. The rate of temperature increase from AD 1850 to 1920 was the highest of the past 360 years, a common feature observed in several proxy records from higher latitudes in the Northern Hemisphere.

Temperature Changes in Southern Patagonia The first attempt to reconstruct temperature variations in southern Patagonia was conducted by Boninsegna et al. (1989). Four chronologies from Tierra del Fuego were used to reconstruct the temperature at Ushuaia for the period AD 1901–1984. This reconstruction, which covers the period AD 1750–1984, is a preliminary effort in the use of tree rings as proxy for temperature variations in southern South America.

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Fidel A. Roig and Ricardo Villalba

Fig. 6. Tree ring-based reconstruction of mean annual temperature departures for southern Patagonia and Tierra del Fuego from AD 1640 to 1989. The reconstruction is also shown in smoothed version (thick line) resulting from filtering the original data with a cubic spline designed to reduce 50% of the variance with periodicity of 25 years. Both actual and reconstructed means are shown.

Based on the positive correlation between tree growth and temperature in seven study sites in the southern sector of Chilean Patagonia, Aravena et al. (2002) developed a reconstruction of the Punta Arenas minimum annual temperatures, covering the period AD 1829–1996. The reconstruction shows that during most of the nineteenth century, minimum annual temperatures remained below average and increased to values fluctuating around the mean during the period of AD 1900–1960, followed by a clear trend of above-average values after AD 1963. This warming trend in the recent decades coincides with the observed pattern in temperature records described for southernmost South America (45–55 S) (Rosenblu¨th et al., 1995; Villalba et al., 2003). Following a similar procedure as the study from northern Patagonia, Villalba et al. (2003) have recently reconstructed the dominant pattern of temperature variations in Punta Arenas, Rı´o Gallegos and Ushuaia in southern South America. Three dendrochronological records were used to reconstruct the temperature variations in southern Patagonia: composite chronology results from two nearby N. pumilio sites between 1060 and 1100 m elevation around Lago Cochrane (47100 S), Chile, and composite chronology results from the combination of three individual records located within the Rı´o Narva´ez catchment (48 S), ranging between 930 and 990 m elevation, and the Piedras Blancas Glacier (49210 S) record at 650 m elevation. The temperature reconstruction for southern Patagonia shows an extended period of cold years lasting from AD 1640 to 1850, followed by a strong increase in temperature, which peaked in the 1980s (Fig. 6). Notable cold events were recorded in the 1650s, 1660s, 1690s, 1700s, 1740s, 1810s and the 1850s. Mean annual temperatures during the twentieth century were 0.89C above the mean of the period of 1640–1899. These findings place the current warming trend within a longer historical perspective, and add new support for the existence of unprecedented twentieth-century warming over much of the globe.

Precipitation Changes in Patagonia The Central Andes. Several dendrochronological studies have been conducted to identify precipitation variations during the past centuries in central Chile. Valmore C. LaMarche, who was in charge of developing the first network of tree ring chronologies in the Southern Hemisphere, provided the first estimation of winter precipitation variations from Santiago, Chile, which covered the past seven centuries (LaMarche, 1975). This early reconstruction was based on a single chronology from A. chilensis at El Asiento in Chile. Through careful examination of the relationships between precipitation variations in Santiago and tree growth for each individual core in El Asiento, Boninsegna (1988) selected a subset of the samples (those strongly correlated with climate) used by LaMarche. Based on this subset, a new reconstruction of winter rainfall for Santiago back to the year AD 1220 was developed. An expanded network of moisture-sensitive tree ring chronologies has recently been developed for central Chile. A composite record consisting of El Asiento and El Baule (34290 S) chronologies has been used by Le-Quesne et al. (2006) to develop a new estimate of June through December precipitation extending from the year AD 1280 to 2000 (Fig. 7). The reconstruction suggests that the decadal variability of precipitation in central Chile was greater before the twentieth century, with more intense and prolonged dry and wet episodes. Multiyear drought episodes in the eighteenth, seventeenth, sixteenth and fourteenth centuries exceed the estimates of decadal drought during the 20th century. The reconstruction also indicates an increase in interannual variability after AD 1850. In fact, the risk of drought exceeding all thresholds increases dramatically in the reconstructed precipitation series after 1850, which is consistent with the drought trends indicated by selected long-term instrumental precipitation records. Precipitation Changes in Northern Patagonia. Precipitation variations in northern Patagonia have been

Understanding Climate from Patagonian Tree Rings

425

Annual precipitation (mm)

(a) Observed versus reconstructed precipitation

Annual precipitation (mm)

800

1000 r

800

2

adj = 0.51

Reconstructed Observed

600 400 200 0 1850

1900

1950

2000

(b) Reconstructed precipitation of Santiago de Chile

600

400 200

1200

1300

1400

1500

1600

1700

1800

1900

2000

Year

Fig. 7. The reconstructed winter–spring precipitation for central Chile for the last 700 years. The upper inbox shows the parallel affinity between reconstructed and observed precipitation. The full precipitation reconstruction shown below points out the cyclicity of dry conditions experienced during the last centuries.

200

Schulman (1956)

(a)

Tucson (1974)

(b)

Mendoza (1991)

(c)

Precip. Recons.

(d)

150 100 50

Tree ring index

1.5 1.0 0.5 1.5 1.0 0.5

0 –1 1600

1650

1700

1750

1800

1850

1900

Precipitation departure (°C)

1

1950

Year

Fig. 8. Comparison of three Austrocedrus chilensis records from Cerro Leo´n, Northern Patagonia, developed by (a) Schulman (1956), (b) LaMarche et al. (1979) and (c) Villalba and Veblen (1997). For comparison, a regional precipitation reconstruction based on 16 Austrocedrus chronologies is shown in (d) (Villalba et al., 1998). reconstructed for both sides of the Andes Range. The first estimate of past precipitation variations for the eastern slopes of the Andes was conducted by Schulman (1956) near San Carlos de Bariloche. Based on four A. chilensis trees growing on Cerro Los Leones (41 S), he developed a 378-year chronology covering

the period AD 1572–1949 (Fig. 8a). Since Schulman’s visit to northern Patagonia in 1949, the Cerro Los Leones site has been revisited many times. The site was first resampled by Richard Holmes in the austral summer of 1976 as part of an initiative of the Laboratory of TreeRing Research, University of Arizona (LaMarche et al.,

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Fidel A. Roig and Ricardo Villalba

1979b), and again in 1991 by members of the Laboratory of Dendrochronology from Mendoza, Argentina (Villalba and Veblen, 1998). Although the new chronologies resulting from these later samplings were based on a larger number of samples and developed using modern computational facilities, the major changes in tree growth that were recorded in Schulman’s original chronology were remarkably similar to those recorded in the newer chronologies (Fig. 8b, c). Quantitative reconstructions of precipitation during the past 400 years were developed for east of the Andes in northern Patagonia, using a new set of 16 tree ringwidth chronologies from A. chilensis (Villalba et al., 1998). Reconstructions of seasonal (November–December and October–March) and annual precipitation departures capture between 41% and 50% of the precipitation variance observed in the instrumental records. It is apparent from these reconstructions that the twentieth century has the most variable and extreme intervals of drought and wetness since the seventeenth century (Fig. 8d). For the past 400 years, the three precipitation reconstructions show that the driest and wettest 25-year periods are from 1895 to 1919 and from 1925 to 1949, respectively. Average departures for these two 25-year intervals are at least two standard errors from the long-term means. These results demonstrate that climatic records of only a few decades do not, in all respects, yield representative statistics of long-term climatic variability. The precipitation reconstructions for northern Patagonia east of the Andes provide clear evidence of fluctuations in the variability of precipitation. The eighteenth century was characterized by low variability, after which precipitation variability rose to remain at middle levels throughout much of the nineteenth century. Variability rose again at the beginning of the twentieth century, to reach the highest level for the entire record. During the past 100 years there has been a striking concentration of historic extremes for dry and wet events. Notably, the most extreme wet years concentrated during the 1940s (1940, 1941 and 1945). There is no apparent relationship between increased variability in precipitation and dry periods, as has been noted elsewhere (Sontakke et al., 1993). On the west side of the Andes, eight tree ring chronologies from N. pumilio at the limit of its northern range in the central Andes of Chile (36 to 39 S) have recently been used to develop a reconstruction of November– December (summer) precipitation for the period AD 1837–1996 (Lara et al., 2001). This is the first precipitation reconstruction from N. pumilio chronologies along the Patagonian Andes. The observed sensitivity of the N. pumilio chronologies to precipitation in the northernmost sector of its distribution is consistent with recent, large-scale studies, indicating that the dominant patterns of sensitivity of N. pumilio to climate varies with latitude, from precipitation to temperature sensitivity with increasing latitudes (Lara et al., 2005). The reconstruction, which accounts for 37% of the instrumentally recorded precipitation variance, indicates that the driest and wettest 25-year periods within the past 160 years are 1890–1914 and 1917–1941, respectively. The November–December precipitation reconstruction from Austrocedrus on the eastern side of the Andes (Villalba

et al., 1998) shares several common features with this reconstruction. For example, both reconstructions indicate that 1943 was the driest year since the 1830s, and the years 1868 and 1941 are among the five wettest years since 1837. Remarkable coincidences are also observed for the low-frequency variations in the reconstructions. The Austrocedrus-based reconstruction indicates that the driest and wettest 25-year periods are 1893–1917 and 1925–1949, respectively, almost exactly coinciding with the 1890–1914 and 1917–1941 intervals identified in the Nothofagus reconstruction (Lara et al., 2001). On the insular sector of northern Patagonia, past variations in summer precipitation for Chiloe´ Island (43 S) were inferred from two chronologies of P. uviferum back to the year AD 1557 (Roig and Boninsegna, 1990). The reconstruction, which explains 32% of the summer precipitation variability, points out the existence of a marked reduction in rainfall during the period 1815–1829, a common feature of most precipitation reconstructions in northern Patagonia. Precipitation Changes in Southern Patagonia. In contrast to northern Patagonia, estimates of historical precipitation variability in southern Patagonia are scarce. The absence of precipitation-sensitive conifers in the forest– steppe ecotone, such as the A. chilensis in northern Patagonia, and the lack of long, homogeneous instrumental records have hampered the development of precipitation reconstructions for this region. However, some promising results suggest that this situation will be overcome in the near future. A recent study has explored the potential of N. pumilio records from relatively dry sites in southern Patagonia as proxies for precipitation variations (Masiokas and Villalba, 2004). On the moisture-limited north-facing slope of the Ameghino valley (50 S), tree growth of N. pumilio is positively correlated with precipitation for most spring and summer months but negatively correlated with temperatures during the current growing season. A chronology based on 41 samples from 20 N. pumilio trees is a first estimate of summer precipitation in the Lago Argentino region for the period of AD 1750–1997 (Fig. 9). The records indicate extended summer dry periods from 1807 to 1825, from 1842 to 1854 and more recently from 1956 to 1971. The tree ring records show two more conspicuous wet periods from 1776 to 1807, and from 1895 to 1916. Interestingly, the 1895–1916 wet period in northern Patagonia is concurrent with the most severe drought of the past 400 years (Villalba et al., 1998). Recent instrumental records show contrasting patterns of precipitation variation between northern and southern latitudes along the Chilean coast (Quintana, 2004). These preliminary estimates from tree ring chronologies of precipitation variations in the Lago Argentino region suggest that atmospheric circulation patterns similar to those described by Quintana have also been operating in the recent past centuries. Masiokas and Villalba (2004) also reported the presence of intra-annual bands (or false rings) in the wood of N. pumilio trees from the Ameghino valley chronology. Twenty-seven bands were recognized for the period of AD 1760–1997. Intra-annual bands in relatively xeric

Tree ring index

Understanding Climate from Patagonian Tree Rings (a) 1.5 1.0 0.5 100 60 40 20 1750

1800

1850

1900

1950

% Samples

80

(b)

427

Fig. 9. The precipitation-sensitive Nothofagus pumilio chronology for Lago Argentino and the record of associated intra-annual bands for the period AD 1760–1997. (a) Standard tree ring width chronology. (b) Percentage of samples with intraannual bands. Note the increase in the occurrence of intra-annual bands during the past 100 years.

0 2000

Year

environments in southern Patagonia are associated with a distinctive pattern of meteorological conditions consisting of dry springs (October–December) followed by warm summers (January–March). The percentage of trees with intra-annual bands in a given year was used as an estimator of the strength of the weather event. Bands were particularly conspicuous and found in a greater proportion of the trees during the periods 1910– 1918 and 1969–1979 (see Fig. 9). This increased frequency of intra-annual band formation during the twentieth century is probably related to the combined effect of increasing temperatures and decreasing rainfall observed in the region (Rosemblu¨th et al., 1995; Villalba et al., 2003). More frequent and severe high temperatures in summer combined with drier springs are thought to have increased the occurrence of intra-annual bands in stands of N. pumilio located at relatively xeric sites, where moisture availability is already a limiting factor. These observations have important implications with respect to our knowledge of interactions between lowand high-frequency modes of climatic variability and their impacts on tree growth. The increased number of intra-annual bands in the wood of N. pumilio during the past 100 years coincides with drier and hotter conditions across southern Patagonia. This suggests that climate variability lasting for more than an extreme event, but for decades or centuries, may also produce climaterelated signals in the wood anatomy of this species (Masiokas and Villalba, 2004).

9. Evidence for Climate Variability over Tens of Thousands of Years (Mid-Pleistocene Ages Between 60,000 and 30,000 BP)

conditions that imposed contrasting environmental conditions with clear influences on the global distribution of biota (Heusser, 2003). The major interestade warm condition climatic scenario within the last global cooling context probably corresponds to the marine isotope stage (MIS) 3, during which the Mid-Llanquihue (and its North American Mid-Wisconsinan counterpart) glacial sequence developed in southern South American (Clapperton, 1993; Denton et al., 1999). This period of reduced glaciation has been assigned an approximate age of between 59 and 29 ka BP (Voelker, 2002), corresponding to a time period that yielded rich deposits for paleoenvironmental studies and radiocarbon age control for selected areas of the world (Clapperton, 1993; Denton et al., 1999; Anderson and Lozhkin, 2001). Glacial, marine and terrestrial evidence shows an MIS 3 interstadial climate that was warmer and more humid than during the stadials (Voelker, 2002). At this interstadial, the Chilotan coastal slopes in southern Chile were forested, as indicated by pollen stratigraphic records spanning portions of this interstadial period (Denton et al., 1999; Heusser et al., 1999; Heusser, 2003; Villagra´n and Roig, 2004). The presence of this vegetation type is currently being validated by the discovery of well-preserved subfossil tree stumps at several locations in the Chiloe´ region (Klonn, 1975; Roig et al., 2001; Villagra´n et al., 2004), containing wood which oldest AMS radiocarbon date indicates ages of ca. 50 ka BP (Roig et al., 2001). Consequently, this forest development, mostly from conifer woods, generally corresponds to the early part of MIS 3. In this section, we concentrate more specifically on describing tree growth patterns during late Quaternary climatic scenarios and examining their possible linkages to climate variations as well as the mechanisms responsible for these variations.

9.1. The Ice Age Background In this section, the discussion moves from estimates of climate conditions during the Late Holocene to the analysis of Late Pleistocene environmental conditions for the Patagonia region. This is possible because of the availability of ancient preserved wood, as is described below. The Last Glacial cycle has experienced substantial millennial-scale climate fluctuations (Shackleton, 2000; Watanabe et al., 2003). This instability has alternated between cool ‘‘glacial’’ and warm ‘‘interglacial’’ periods,

9.2. Regional Pattern of Forest Distribution in Southern South America Full-glacial climate conditions in the middle latitudes of southwestern South America, during the Late Pleistocene (particularly during the extensive glaciation from about 29.4 to 14.5 14C ka BP) forced latitudinal and elevational changes in life zones and habitats, which were very different than the present-day vegetation belts (Villagra´n, 2001).

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Fig. 10. Wood remains as testimony of ancient forests developed at times of lesser minimum temperatures during interstadials of the Last Glaciation in southern Lake District–Isla Grande de Chiloe´. Two first lines correspond to the Punta Pirque´n site, while the lower lines are figures of the Punta Pelluco and Punta Detico sites. The lowering of the treelines was a response to new temperature limits defined by the snowline depressions and glacier advances (Veit and Garleff, 1995; Denton et al., 1999). Thus, the consequence in the southern Lake District–Isla Grande de Chiloe´ region was a displacement of the austral and mountain floras into the lowlands along the longitudinal valley and near the marine shores. Reversal tracks were experienced by the forests during decisive warmings at the Late glacial (14,600–10,000 yrs BP) and the Holocene, as has been amply documented from pollen and glaciological records (e.g. Denton et al., 1999; Villagra´n, 2001; Heusser, 2003). Behind the Last Glacial Maximum (LGM), paleotemperature readings, reconstructed from pollen assemblages, indicate the occurrence of three interstadial phases within the mid-Llanquihue glacial stage for the southern sector of Chile at >49,000–57,000, 50,000– >47,000 and 45,000–35,000 BP (Denton et al., 1999; Heusser et al., 1999). These extended warm intervals were resumed in a stage 3 interstadial complex (Anderson and Lozhkin, 2001). The occurrence of a warm interstadial is suggested by dominance of thermophilous tree species, such as Nothofagus dombeyi, and lower frequencies of Fitzroya/Pilgerodendron, P. nubigena, D. winteri, P. laetevirens and Myrtaceae, among others. In addition to information

recovered from the regional pollen assemblages, the subfossil conifer logs exhumated from many coastal sites of the Chiloe´ region are substantial proof of propitious climatic conditions for the expansion of conifers within the northern Patagonian forests (Roig et al., 2001; Villagra´n and Roig, 2004). Moreover, the pollen evidence recovered from the interdrift organic layers containing these subfossil wood remnants reproduces the forest structure mentioned by Heusser et al. (1999) for periods coincident with the warm interstadial MIS 3 period. Figure 10 shows the Pleistocene forest remnants located at different sites of continental and insular Chiloe´. Many stumps still remain in living position, suggesting a rapid forest extincion probably by transgressive episodes linked to stadium changes in glacier growth (Roig et al., in preparation). These stumps are the basis of our paleoclimatic inferences from tree rings.

9.3. Climatic Inferences for Glacial Ages The climatic parameters derived from subfossil wood should be identical to the information recovered from living trees. This general agreement, based on the uniformitarian principle, provides insights into the climate conditions of the Pleistocene interstadials. As it has been

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(a) (b) (c) (d) (e) (f) >100

90 0.010

80

70

60

50

40

30

20

0.019 Period (years)/frequency in cycles per year

10

5

0

0.10

Fig. 11. (a) Modern Fitzroya tree ring chronology; (b) subfossil Fitzroya tree ring chronology from Punta Pelluco site; (c, d, e) subfossil tree ring chronologies from Punta Pirque´n site developed with wood records from Pilgerodendron, Fitzroya and a combination of both records, respectively; (f) subfossil Fitzroya tree ring chronology from Punta Detico site. Black circles indicate significant frequencies at 95% CL, while white, smaller circles indicate the existence of the frequency peak but not significant at the 95% CL.

mentioned before, F. cupressoides and P. uviferum are climate-sensitive species and for this reason have been widely used for tree ring-based climatic reconstructions (e.g. Villalba, 1990; Roig, 1996; Szeicz, 1997). Both conifers show interannual tree ring growth with complex climatic responses associated with temperature and precipitation variability. Fitzroya has yielded continuous tree ring records for the recent millennium that has served as a basis for temperature reconstructions for northwestern Patagonia (e.g. Lara and Villalba, 1993). Although Pilgerodendron produces shorter records than Fitzroya, this species offers the advantage of a much wider geographical distribution (Szeicz et al., 2000). Both taxa are found in the aforementioned subfossil sites, and hence, tree growth pattern characteristics and associated climatic inferences can be obtained from the resulting tree ring chronologies. Signal attributes in the subfossil tree ring records were examined in order to characterize the general structure of cycles detected in the spectral power. For this purpose, the Blackman–Tukey time–frequency techniques (Jenkins and Watts, 1968) have been used. For comparison, the same approach was applied to modern tree ring chronologies developed with wood of the same taxa (see Fig. 11). The power spectra of the subfossil records revealed a rich spectral complexity with important powers between f = 0.0 and 0.01 cycles/yr. Some significant peaks at this power region are roughly aligned at the same frequencies, and on the whole, they concentrate an important fraction of the total power of variance. However, other sectors of this power distribution show little consistence in terms of shared frequencies. The visual intercomparison revealed coincidences through significant spectral lines. Important oscillatory components at a low frequency are shown in our chronologies for time windows around the 90–80 year period band and frequencies >100 years. Periodicities related to long wavelengths have been described in spectra for modern millennial-length F. cupressoides chronologies (Villalba et al., 1996; Roig et al., 2001). Solar modulations may be responsible for much of the oscillations detected at the low-frequency window. For example, a near 220-year 14 C cycle is linked to sunspot activity (Stuiver and Braziunas, 1989), as well as periodicities close to 90 years, which may be related to the Gleissberg global solar changes.

Significant peaks around the 20-year period are apparent in nearly all living and subfossil chronologies. Moreover, the significant peaks between 10 and 11 years are perhaps a representation of an harmonic of the previous cycle. These cycles are coincident with those detected from solar inputs such as the 11-year sunspot cycle and the magnetic 22-year (Hale) cycle, according to Stuiver and Braziunas (1989). At powers between f = 0.14 and 0.40 cycles/yr, which means approximately between 7 and 2.5 years, we can identify several phasecoherent frequency components. Diaz and Pulwarty (1994) found that summer temperature indices of north Patagonia obtained by tree rings of living trees showed a high percentage of total variance contained in the highfrequency bands. They attributed this particular behavior to the modulation of climate by ENSO activity. These authors also proved that coherency-squared spectra exhibit major peaks at 3.3, 4 and 6 yrs when the temperature reconstruction for northern Patagonia was compared with the Quinn El Nin˜o historical record. This result suggests teleconnection processes between ENSO and short-term air temperature variations in latitudes between 39 and 43 S of the Andes region. An interesting discussion may be derived from the finding of high-frequency signals in subfossil wood. But new additional information should be developed to more deeply explore these particular signals in the context of isochronous modulation of tree growth through time.

10. Concluding Remarks Important progress has been made during recent years in the identification of plant–climate relationships in Patagonia, and how these relationships have been expressed as growth variables during part of the Late Holocene and Pleistocene. Among other things, this knowledge has provided the basis for recuperating climate signs contained in growth rings, and consequently the derivation of functions for the reconstruction of past climate changes. These climatic reconstructions are the basis for extending instrumental climate records further into the past.

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Although the variability of tree rings has been explored with some success and will continue to provide valuable information, there are still some aspects of tree ring analysis that are not yet fully resolved. First, the measurement of the width of the growth ring has been until now a widely used data. However, the chronologies of tree ring width only explain one part of the variability, a part that rarely is greater than 50%. One of the ways to optimize the quality of the climatic information derived from tree rings is to incorporate new measurement methods, such as those based on anatomic (wood structure, density) and chemical (stable isotopes, chemical contents) properties, which could open unexpected results and new areas of work. Second, studies related to temperature and precipitation changes during the twentieth century provide interesting evidence for parts of South America regarding climate modifications that are being observed globally. If trees are our climate indicators, why are not all the thermosensitive chronologies showing the same patterns of sustained temperature increase as in Patagonia? The answer to this question is in the continuation and deepening of dendroclimatic studies. New methods, areas and species should be incorporated in our studies to increase the quality of these observations. Ecological studies of great detail such as those being carried out with N. pumilio in southern Patagonia will provide elements for an improved dendroclimatic interpretation. In addition, the semiarid regions of northwest Patagonia, which are highly sensitive to interannual variability of the ENSO with very long-lived and climatically sensitive species such as A. araucana, could be the next area of expansion for dendroclimatic research. Interpretation of glacial climate that is based on the study of growth in subfossil conifers has allowed us to distinguish diverse components of the climatic system with an unprecedented year-by-year level of detail. These subfossil records are a step toward more complete multiproxy archives of climate that are necessary for evaluating global climate models for the past and into the future.

Acknowledgments The authors thank Rafael Bottero and Judy Boshoven for their assistance during the preparation of the manuscript. Our gratitude also goes to members from many institutions in Patagonia, but particularly from the Laboratorio de Dendrocronologı´a e Historia Ambiental, Mendoza, for assisting in field data collections and for laboratory support.

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22 Archeological Hunter-Gatherer Landscapes Since the Latest Pleistocene in Fuego-Patagonia Mo´nica C. Salemme1 and Laura L. Miotti2 1

CADIC-CONICET, C.C. 92, and Universidad Nacional de la Patagonia San Juan Bosco, 9410 Ushuaia, Tierra del Fuego, Argentina 2 CONICET and Departamento de Arqueologı´a, Museo de La Plata, Paseo del Bosque, 1900 La Plata, Argentina

challenging of the standard models. In this sense, the region has been identified since the sixteenth century as a landscape where the social and natural changes were extremely slow or did not even happen. Early chroniclers, as well as nineteenth-century naturalists – Charles Darwin among them – described the hunter-gatherers of Patagonia as ‘‘the last living exponents of Stone Age’’ or as ‘‘the missing link’’. Cave sites such as Fell Cave (Bird, 1938, 1988) and Los Toldos (de Aparicio, 1933/1935; Menghin, 1952; Cardich et al., 1973) have been frequently taken as providing strong arguments for supporting a great antiquity for the peopling of South America. However, radiocarbon dates coming from the lower levels in Los Toldos Cave 3 have been considered at least as doubtful (Borrero, 1999; Miotti and Salemme, 2003; Waters and Stafford Jr., 2007). New evidence from Cerro Tres Tetas, La Marı´a and Piedra Museo localities has lately questioned the Clovis migration model from the great North American plains (Martin, 1973; Haynes, 1984; Lynch, 1990; Dincauze, 1993; Fiedel, 2006). A reanalysis of the age range for this Clovis model ‘‘overlaps non-Clovis sites in North and South America’’ (Miotti, 2004; Waters and Stafford Jr., 2007), confirming that people would have been already settled elsewhere in the Americas before the arrival of Clovis hunters. The peopling of the Americas, as a noteworthy issue in American archeology, has always caused a strange interest and fascination to specialists and nonspecialists around the world. Since the classical controversy between Ameghino (1880, 1910) and Hrdlicka (1912) about the antiquity of man in America up to the present discussion on the first Americans in southern South America, the circulation of knowledge from peripheral countries has notably increased as a result of globalization (Miotti and Salemme, 2001; Podgorny, 2002; Miotti, 2003a, b, 2006a; Politis et al., 2004). At present, the available information generated from excavations all around Patagonia is trustworthy and a crucial matter of debate in American archeology (Waters and Stafford Jr., 2007, and references quoted therein). The southern tip of South America, the Patagonian landscape, was the scenario of people who made their living from hunting and gathering techniques over at least 12,000 yrs; they had always been nomads until small villages began to develop certain areas as recently as 150 yrs ago. As stated above, Patagonia was the last

1. Introduction Patagonia reminds the reader of a sort of fascination. It evokes without doubt a very far place on the Earth where time seems to have stopped in glaciers, volcanoes, arid plateaus and living beings (a ‘‘finisterre’’ in the sense of De Agostini, 1929; Darwin, 1983; Herrero Pe´rez and Salemme, 2005). Patagonia has been of vast and special interest for naturalists, explorers, missionaries, adventurers as well as anthropologists since the sixteenth century. In the last decades, the development of adventure tourism has generated a renewed interest in the search of this far and ‘‘mysterious’’ region. Sub-Antarctic environments, dinosaur bones and nests filled with eggs, fossil wood from huge, ancient trees and archeological sites containing histories about the earliest Americans hold a special attraction for voyagers. Patagonia is thought of as a pure or pristine landscape where scarce population has generated minor changes through time. Nowadays, many research projects inquire into the past and present of this extended territory of approximately 1 million km2. As fast as the scientific knowledge on these attractive geological, biological and human landscapes grows, the interest for new findings increases as well and it enhances an inspiration source for arts and tourism. Quite a few recent books and films have had their source in this landscape and their inhabitants. As it has been mentioned elsewhere, Patagonia – as well as southern South America – appears as a paradox and, at the same time, as a very well-equipped laboratory for building images of the New World’s human colonization (Miotti, 2003a, 147). A paradox, because according to the available models of the peopling of the Americas, it was the last segment of the continent to be occupied by humans and, in spite of this supposition, sites with the same antiquity or even older than those in North America have been discovered during the last two decades. A laboratory, because although there are many Pleistocene archeological sites in the southern Cone, the last 10 yrs have placed this southern end in an advantaged situation to rearrange previous ideas about the first inhabitants – who they were, when and how they came, what aims and ideas they brought with them to accomplish this colonization and how their ways of using space and resources of different Patagonian environments are displayed in the archeological record. Patagonia displays, as does South America, this dual characteristic, with archeological sites both critical and

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sector of this continent to be explored and colonized (Miotti, 1998, 2006b). Nevertheless, and even with the feeling that time had stopped there, social and environmental changes that took place were plentiful and diverse. In this sense, this chapter deals with the available archeological knowledge, building a history of the peopling, the mobility of human groups and their relationship with the environment until the arrival of the Europeans. Then, since the sixteenth century, the hunter-gatherer societies were shocked by the global free market rules. Their breakdown had started. In this chapter, we have focused the attention on Patagonia (both east and west of the Andean ranges) and we will examine the archeological contexts and available radiocarbon dates for three chronological intervals: 1. the Pleistocene–Holocene transition and the Early Holocene: a hazardous life at the uttermost end of the continent; 2. the Middle Holocene: the most significant socioeconomic and environmental changes for mobile hunter-gatherer societies occurred during this interval (Borrero, 1989–1990, 2001a; Miotti, 1998, 2001, 2003a, 2006c; Miotti and Salemme, 1999, 2003, 2004); 3. the Late Holocene: when the major population dispersal occurred, until the contact with Europeans took place. Five subareas will be considered following a N–S direction (Fig. 1). The Andean Cordillera must be taken into account as a barrier, permeable only through a few paths and only after 10 ka BP when the last glaciation

finished, thus allowing connection between the Pacific and Atlantic oceans. This mountain range – that runs from north to south across the continent but that changes into a W–E direction in the Fuegian Archipelago – is considered as a regional divide not only for most drainage basins and different landscapes but also as a filter for peopling processes since the earliest occupations during the latest Pleistocene. The criteria used to subdivide this region into subareas have been the main river basins on both slopes (as it has been already considered by Miotti, 2003b, and Miotti and Salemme, 2003). In the following description, the eastern basins are highlighted since they are larger and longer than their western counterparts: 1. Northern Patagonia: from the Rı´o Colorado south to the Rı´o Chubut 2. Central Patagonia: from the Rı´o Chubut south to the Rı´o Deseado 3. Southern Patagonia: from the Rı´o Deseado to the Rı´o Santa Cruz 4. The Magellan Basin: from the Rı´o Santa Cruz to the southern margin of the Straits of Magellan 5. The Fuegian Archipelago This latter sector, presently separated from the continent by the Straits of Magellan, became an insular area after 8 14C ka BP (Rabassa and Clapperton, 1990; Clapperton, 1992; Rabassa et al., 2000). Note that all absolute dates herein presented are uncorrected radiocarbon ages, unless otherwise explicitly stated. This region was the southernmost end of the South American continent until the Pleistocene glacial valley occupied by the ‘‘Magellan Glacier’’ (Bentley et al.,

Fig. 1. Location map. The main areas and subareas considered are generally outlined. a) Location of Patagonia in Southernmost South America 17; b) Detail of Patagonia Region; c) Detail of Tierra del Fuego Archipelago.

Archeological Hunter-Gatherer Landscapes in Fuego 2005; McCulloch et al., 2005) was flooded by marine waters, thus becoming separated from it. Today the archipelago is formed by a major island, Isla Grande de Tierra del Fuego, surrounded by hundreds of minor isles that formed a unique territory together with continental Patagonia during the last glaciation, including also a large extension of the present submarine platform, which was then exposed eastward. This geographic isolation is not a minor question since it affected not only the human population but also the regional ecosystems. Several plant communities are still living both north and south of the Magellan Straits; however, the evolution of fauna and people was different. The history of cultural divergence will be discussed below. It must be taken into account that the environmental bands show a longitudinal exposure in continental Patagonia, whereas in the main island they are transversal, following the mountain range orientation, with the

439

exception of the coastal zones that fully enclose them (Table 1, see Fig. 1). Three geomorphological/ecological areas have been considered for the continent; from west to east they are (a) the Andean Cordillera, which separates the Pacific Rim from the eastern piedmont, (b) the Patagonian plateaus and (c) the Atlantic coast. In the Fuegian Archipelago, these landscape stripes run from west to east (see Fig. 1 and Table 1). The western side of the Andes is narrower; due to the shorter length of the streams that drain right away into the Pacific Ocean, narrow fluvial valleys and a few open plains were available landscapes for colonizers. The southern ice sheet was a permanent barrier but low passageways are found again south of 51 S. The northern ice cap has been another barrier for the occupation of the western side, but it was separated from the southern ice sheet since very early Late Glacial times (Rabassa, Chapter 8).

2. Central

1. North

Stripes

Subareas

Table 1. Main archeological localities and sites arranged in environmental stripes divided into five latitudinal subareas. Andean range and piedmont

Plateaus

Atlantic coast

Main archeological localities and/or sites Alero Marifilo Monte Verde Cueva Epulla´n Cueva Haichol El Tre´bol Cueva del Manzano Traful Cuyı´n Manzano Alero Nestares Aleros Las Coloradas Alero Los Cipreses El Manantial 1/88 CPO – Corralito Alero Cicuta Pilcaniyeu Cueva Sarita Alero La Figura Alero Larivie`re Valle Encantado 1 Los Sauces ´ lamos Los A

Casa de Piedra Tapera Moreira Arroyo Quetrequile Angostura Colorada, de Calcatreu Negro Muerto Angostura 1 La Lomita La Petrona La Primavera Laguna Ganso Azul Vacalaufken Ham Ham Plan Luan Yamnago-Anekenk microrregion Sierra de Apas La Rural–Cerro Castillo Gastre 1

San Blas, Isla Jabalı´ San Matı´as Gulf, Northern coast

Alero Sendero de Interpretacio´n Rı´o Iba´n˜ez Cueva Las Guanacas

Piedra Parada Cerro Pintado

El Riacho La Azucena 1 El Golfito 1 Playa del Pozo Calle Tehuelches Punta Leo´n El Elsa Calle Villarino Rawson Bahı´a Solano 13 Bahı´a Solano 16 Cabo Tres Puntas 1 Sitio Moreno Cabo Blanco

(Continued)

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Mo´nica C. Salemme and Laura L. Miotti

Plateaus

Arroyo Feo Cueva de las Manos Charcamata Alero del Bu´ho Alero Ca´rdenas Alero Rosamel Meseta Lago Bs As Pto. El Rodeo Cerro Casa de Piedra Cerro de Los Indios Rio Robles Lago Salitroso Ban˜o Nuevo Chorrillo Malo Chan Chan Guaitecas Quille´n Morhuilla Lebu Alero Manuk Alero del Leo´n A. D. Guardaparque A Direccio´n Obligatoria A. Gorra de Vasco

Los Toldos Piedra Museo Aguada del Cuero Tito del Valle La Primavera Las Mercedes Cerro Tres Tetas La Reconquista La Marı´a El Ceibo Casa del Minero La Martita El Verano Lago Cardiel Guer Aike Las Buitreras Potrok Aike

Punta Bonita-Wualicho Campo del Lago El Sosiego Cerro Verlika Alice-Charles Fuhr Lago Roca Rinco´n Amigo Sitio Marchand Alero Dos Herraduras Lago Sofı´a Cueva del Medio Cueva del Mylodon Englefield Ponsonby Punta Colorada Bahı´a Buena Punta Santa Ana Km 44 Rı´o Verde Camdem Alero Los Chilcos Pizzulic Los Noruegos

El Volca´n 4 Cueva Fell Cueva PalliAike Cerro Sota Can˜ado´n Leona Lago Thomas Gould

Strait of Magellan Canal Maule Punta Baxa Marazzi

Steppe/Forest Tres Arroyos Bloque Erra´tico Cabeza de Leo´n Cerro Bandurrias San Genaro La Arcillosa Rı´o Chico Avile´s Las Vueltas Chacra Pafoy Marina 1 Rı´o Ewan

5. Fuegian archipelago

Atlantic coast

Main archeological localities and/or sites

3. South

Stripes

Andean range and piedmont

4. Magellan Basin

Subareas

Table 1. (Continued)

Me´dano (1, 3, 4) Laguna del Tele´grafo Punta Guanaco Me´danos Canal 196 Can˜ado´n del Puerto Cabo Curioso Monte Leo´n Punta Bustamante Punta Loyola

Beagle channel Lomada Alta del Olivia Tu´nel Imiwaia Lancha Packewaia Shamakush Isla Salmo´n Tolkeye´n Ajej Mischiuen Seno Lauta Grandi ´ ridos de Guerrico A Caleta Segura Ro´balo

Atlantic coast Espı´ritu Santo Punta Marı´a Cabo Pen˜as C. San Pablo Marı´a Luisa Rancho Donata Bahı´a Valentı´n

Archeological Hunter-Gatherer Landscapes in Fuego 2. Paleoenvironments The earliest peopling in the region took place during the Pleistocene–Holocene transition, a critical time for the dispersal of hunter-gatherer societies. Pioneers that explored and populated the southernmost end of the continent had to face great environmental instability, as a consequence of extreme climatic changes that thoroughly affected the abiotic and biotic structures in different regions; thus, the beginning of the colonization at higher latitudes had to be a long and complex process, as humans were trying to find shelter and successfully flourish in those new landscapes (Borrero et al., 1998; Miotti, 1998, 2003a, 2006b; Borrero, 1999, 2004; Miotti and Salemme, 1999, 2003; among others). The ecosystem instability, which affected not only Patagonia but also other regions around the globe, was ameliorating during the Early Holocene (between 8500 and 7500 14C yrs BP); however, during the Middle Holocene, i.e. between 7500 and 3500/3000 14C yrs BP when temperature started to increase, sea level raised and changes in continentality as well as in the Patagonian biota occurred, societies had already begun to establish firm territorial and social networks in the higher latitudes of South America. This process would have continued up to historical times when hunter-gatherers’ major dispersal and higher mobility took place, together with environmental trends that developed toward the present configuration of Patagonian landscapes. The acquaintance of the explored territory regarding water availability, raw materials, shelter and other resources, as well as the potential communication network (exchange of raw materials, technological and ideological items or extraregional goods) is shown by the high archeological and temporal variability (Miotti, 1995, 2003b; Flegenheimer et al., 2003; Gnecco, 2003, 2006). However, there is imbalanced quality and quantity of information relating to human population during each period of the Holocene; in fact, such knowledge is very rich for the first and the last interval, but there are less data corresponding to Middle Holocene occupation. A feasible hypothesis to explain this shortage could be the record of deep environmental changes that would have caused the population migration of at least the southernmost area of the American continent (Neme and Gil, 2001; Za´rate et al., 2005). Nonetheless, an increase in information during the last years allows to verbalize other hypotheses, bearing in mind not really a removal of people but the introduction of new strategies to get resources or changes in the differential use of the space during the Middle Holocene, a tendency that continued to increase during the Late Holocene (Orquera and Piana, 1999; Salemme and Bujalesky, 2000; Aguerre, 2003; Miotti, 2006b; Salemme et al., 2007a). It seems that a potential decrease of human occupation during the Middle Holocene has to be examined from a regional point of view, according to not only paleoenvironmental and paleoclimate differentiation as well as local vegetation patterns (Pa´ez et al., 1999, 2003; Borromei, 2003; Mancini et al., 2005), but also cultural knowledge. Paleoecological and climatic oscillations occurred all over Patagonia since the end of the Pleistocene and they had deep consequences on the dispersal and distribution

441

of the people during the Holocene (Coronato et al., 1999). In spite of this, the main focus has been directed to the earliest populations because they explored territories under more difficult and changing conditions during the Late Pleistocene–Early Holocene transition. However, weaker archeological indicators and/or sampling bias hide the changes that took place during the Middle Holocene, though Late Holocene times are better documented. In this sense, many research publications on the first and the last stages of human colonization in Patagonia are available, but human dispersal during the Middle Holocene has been treated in a different way (Tables 2–4; Miotti and Salemme, 2003, 2004; Miotti, 2006c). Some recently published papers (Gil et al., 2005; Za´rate et al., 2005) as a result of discussions during a symposium (Neme and Gil, 2001) have coupled the scattered and new archeological and paleoenvironmental evidence assigned to the interval 8–4 ka BP. Some reports added significant and differential patterns to the record for this period, compared to previous and later ones, as it will be presented herein. In Table 5, we have summarized the available information generated through multidisciplinary joint projects, developed along continental and insular Patagonia during recent years. There, pollen analyses, geological data (based upon glaciological, sedimentological, geomorphological, sea level change and volcanological studies), and paleontological and zooarcheological information are integrated showing the main characteristics and processes involved in Patagonia (Heusser and Rabassa, 1987; Rabassa, 1987; Mancini, 1993; Markgraf, 1993; Ariztegui et al., 1997; Pa´ez et al., 1999, 2003; Bennett et al., 2000; Borromei, 2003; Hajzda et al., 2003; Heusser, 2003). Basically, the scheme suggests the existence of cold and dry conditions at the end of the Pleistocene (ca. 13.0 ka BP), mainly in the Andean and piedmont areas. That was the time of first exploration by humans in the southernmost end of Patagonia, which was apparently different on the western slopes compared to the eastern side and in northern Patagonia in relation to southern Patagonia (see Miotti and Salemme, 2004). Glacier readvance and retreat had a strong influence on the displacement of the Nothofagus forest along the Andean Range, but also in the development of shrubby or gramineous steppes during the Pleistocene–Holocene transition in extra-Andean areas. Also, a typical Pleistocene grazer fauna is shown in the record of several sites of the Rı´o Deseado Basin (Miotti and Salemme, 1999, 2005; Paunero, 2003a, b; Paunero et al., 2005); this faunal stock of Pleistocene megamammals or large flightless birds disappeared or was replaced by extant fauna in the Early Holocene, by ca. 9.0 ka BP (Miotti and Salemme, 1999, 2005), when temperature and aridity were increasing. After a period of climatic amelioration during the Pleistocene–Holocene transition, the trend during the Holocene was toward rising aridity for the extra-Andean steppes, whereas the forests were definitively and progressively established along the Cordillera (Heusser, 2003). The only disruption with higher temperature and humidity took place in conjunction with sea level rise by ca. 6.5 ka BP. This worldwide event

442

Mo´nica C. Salemme and Laura L. Miotti

Table 2. Pleistocene/Holocene Transition and Early Holocene radiocarbon dates (ca. 13.0–8.5 ka BP). Localities/Sites Alero Marifilo-1

Monte Verde II MV-6 Layer MV-5 Layer El Tre´bol Cave, lower levels Cueva del Manzano, Arroyo Corral Cuyı´n Manzano Traful 1 Cueva Epulla´n

14

C years BP

Lab. Code

Northern Patagonia 10,410 – 70 Beta-164473 10,190 – 120 Beta-164475 8420 – 40 Beta-138919

Mera and Garcı´a, 2004

Dillehay, 1997; see Table 3.1, pp. 43–44, in which lab codes are shown Hajduk et al., 2004

12,780 – 240/11,920 – 120 11,800 – 80/10,860 – 130 ca. 8000–10,000 ca. 10,000 9,920 – 85 9,285 – 105 9,430 – 230 9,970 – 100

References

KN-1432 GX-1711G INGEIS 2676-571 LP213

Hajduk, 1998 Ceballos, 1982: 31 Crivelli Montero et al., 1993 Crivelli Montero et al., 1996

Central Patagonia No archaeological data Rio Deseado Basin Los Toldos Nivel 11 (Level 11b) Toldense (Level 9) Piedra Museo AEP-1 1st Occupation U6 Transition U6/U5 Base U5 2nd Occupation U5/U4 Top U4 Cerro Tres Tetas

Casa del Minero 1 Cave La Mesada Cave El Ceibo El Verano Cave 1, IVb La Martita

Cueva de Las Manos Arroyo Feo

CCP7

Cardich et al., 1973 12,600 – 650 8750 – 480 12,890 – 90 11,000 – 65 10,925 – 65 10,390 – 70 10,470 – 60 10,470 – 65 10,400 – 80 9710 – 105 9230 – 105 11,560 – 140 11,100 – 150 11,015 – 66 10,915 – 65 10,853 – 70 10,850 – 150 10,260 – 110 10,999 – 55 10,967 – 55 9090 – 40 ca. 9500 8960 – 140 7500 – 250 8050 – 90 7940 – 260

FRA 98 (doubtful) FRA 97 (doubtful) AA-20125 AA-27950 OxA8528 OxA8527 OxA9249 GRA9837 AA8428 LP 859 LP 949 LP525 OxA9244 AA39368 AA22233 AA39366 LP781 LP800 AA37207 AA37208 Beta 135963 I.13,797-1 INGEIS 2854 CSIC-506 CSIC-506

Piedmont, Eastern and Western Andean Basins 9320 – 90 CSIC-138 9300 – 90 CSIC-385 9410 – 70 CSIC-514 9330 – 80 CSIC-396 8610 – 70 CSIC-515 9730 – 100 n/d 9100 – 150 n/d 8300 – 115 n/d

Miotti et al., 1999 Miotti et al., 2003

Paunero, 2003a

Paunero, 2003b Paunero, 2003b A. Cardich, pers. comm. Dura´n, 1986/87 Aguerre, 1987

Gradı´n and Aguerre, 1994 Gradı´n and Aguerre, 1994

Aschero, 1996

Archeological Hunter-Gatherer Landscapes in Fuego

443

Table 2. (Continued) Localities/Sites

14

Chorrillo Malo 2

9740 – 50 9690 – 80 13,480 – 35 12,510 – 30 12,400 – 30 12,325 – 30 12,320 – 30 12,000 – 35 11,480 – 50* 11,410 – 25 11,265 – 35 11,255 – 30 11,250 – 50 11,240 – 40 9530 – 25 9435 – 25 9260 – 25 9245 – 25 9200 – 80 9155 – 25 9070 – 50 9070 – 25 8990 – 30 8975 – 20 8950 – 50 8950 – 60 8945 – 45 8890 – 90 8850 – 50 8880 – 50 8695 – 25 8530 – 160

Ban˜o Nuevo 1 Only Mylodon dermal bones without human association

Cueva Lago Sofı´a 1 Fell I

Cueva del Medio

Alero Marazzi

C years BP

Lab. Code

GX-25279 CAMS 71152 UCIAMS-10100 UCIAMS-10107 UCIAMS-10111 CAMS-32685 UCIAMS-10109 UCIAMS-10110 CAMS-72356 UCIAMS-10104 UCIAMS-10106 UCIAMS-10105 CAMS-71702 UCIAMS-10094 UCIAMS-10093 UCIAMS-10097 UCIAMS-10103 Beta 90888 CAMS-80532 UCIAMS-10087 CAMS-80532 UCIAMS-10091 UCIAMS-10098 UCIAMS-10095 CAMS-79933 CAMS-101893 CAMS-101894 Beta 90889 CAMS-36633 CAMS-36634 UCIAMS-10099 Beta 90892 Magellan Basin 11,570 – 60 PITT-0684 11,000 – 170 I-3988 10,080 – 160 I-5146 10,720 – 300 W-915 12,390 – 180** PITT 0343 11,120 – 130 NUTA 1737 11,040 – 250 NUTA 2197 10,960 – 150 NUTA 2330 10,930 – 230 Beta-39081 10,860 – 160 NUTA 2331 10,850 – 130 NUTA 1812 10.710 – 100 NUTA 1811 10,710 – 190 NUTA 2332 10,550 – 120 GrN-14911 10,450 – 100 NUTA 1735 10,430 – 80 Beta 52522 10,430 – 100 NUTA 1734 10,350 – 130 Beta 58105 10,310 – 70 GrN-14913 9770 – 70 Beta-40281 9595 – 115 PITT-0344 9590 – 210 GIF-1034

References Franco and Borrero, 2003 Mena et al. 2000 Mena et al., 2003 Mena and Stafford, 2006

Borrero, 1999 Borrero, 1999

Nami and Nakamura, 1995 Borrero, 1999

Morello et al., 1999; Morello, 2000 (Continued)

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Mo´nica C. Salemme and Laura L. Miotti

Table 2. (Continued) Localities/Sites

14

Tres Arroyos

11,880 11,085 10,685 10,630 10,600 10,580 10,575 10,280 10,420 10,130

C years BP – – – – – – – – – –

250 70 70 70 90 50 65 110 100 210

Lab. Code

References

Beta 20219 OxA-9248 OxA-9247 OxA-9246 Beta 101023 Beta 113171 OxA-9245 Dic. 2732 Dic. 2733 OxA-9666

Massone, 1987, 2003

Notes: Localities and sites listed by subareas. *Dating performed on Mylodon dermal bones. They come from the underlying level of the burial context (Mena et al., 2000); they are probably in a secondary association within human burials from the upper level. **Dating considered equivocal by the authors

Table 3. Middle Holocene radiocarbon dates (ca. 8.5–3.5 ka BP). Localities/Sites

14

Alero Marifilo-1

7000 – 40 5940 – 40 4870 – 40

Monte Verde II MV-3 Upper MV-3 Middle Cueva Epulla´n

Lab. Code #

Northern Patagonia Beta 164476 Beta 164474 Beta 138918

References Mera and Garcı´a, 2004

Dillehay, 1997 6530 4750 7900 7550 7060 5140 7020 6775 8620 7560 6080 4590 3900

Cueva Haichol Casa de Piedra

– – – – – – – – – – – – –

110 90 70 70 90 70 120 75 190 230 190 60 60

Beta Beta Beta Beta Beta Beta n/d

52012 6753 44412 47401 41622 611470

I-12,067 I-12,159 I-12,067 Beta 91937 Beta 82556 Central Patagonia 5080 – 100 AC 666 4885 – 135 AC 1110 4770 – 90 AC 671 Rı´o Deseado Basin

Tapera Moreira, lower levels Campo Moncada 2 Early occupation

Central Plateau Piedra Museo AE UE 2 La Ventana Cave

C years BP

P-1,

Los Toldos, Cave 3 ‘‘Casapedrense’’ Maripe Cave Los Toldos, Cave 13, layer 9 Cerro Tres Tetas Los Toldos Cave 3

7670 7470 7665 7970 7260

– – – – –

110 90 75 40 350

LP-450 LP-850 AA35237 Beta 135965 FRA 96

5084 – 49 ca. 5500

AA65173 n/d

5220 – 70 4850 – 90

LP 538 LP 136

Crivelli Montero et al., 1996

Ferna´ndez, 1988-1990 Gradı´n, 1984 (pp: 42)

Bero´n and Curtoni, 2002; Politis and Madrid, 2001 Bellelli, 1988 Pe´rez de Micou et al., 1992 Bellelli, 1991

Miotti, 1996; Miotti et al., 1999 Paunero, 2003b Cardich et al., 1973 Miotti et al., 2007b Miotti, 1998; A. Cardich, 1990 (pers. comm.) Paunero 1994, 2003a Cardich, 1984–85

Archeological Hunter-Gatherer Landscapes in Fuego

445

Table 3. (Continued) Localities/Sites

14

La Martita Cave 4 Lower Component La Mesada cave Atlantic Littoral Me´dano 1 Me´dano Alto Sitio 2, 3, 4 y 5, Cabo Tres Puntas Sitio 1, Cabo Tres Puntas CCH 1, lower level Chan Chan Uch 22 Uch18 GUA 010 Quille´n 1 Morhuilla Lebu LE-2 Level II Morhuilla Lebu LE-2 level IV Rı´o Iba´n˜ez RI-16 Rı´o Iba´n˜ez RI-22 Ban˜o Nuevo 1

Las Guanacas Cave Alero Ca´rdenas (7b) De las Manos Cave Arroyo Feo

Alero Charcamata Puesto El Rodeo CCP 7

CCP 5

C years BP

Lab. Code #

References

4520 – 50 4475 – 95 4500 – 40

CSIC-505 I-11904 Beta 135964

Aguerre, 1987

6300 – 90 5790 – 80 5480 – 80

LP 1544 LP 1579 LP 1647

Castro et al., 2007 Castro et al., 2007 Castro et al., 2007

6060 – 70 AA1363 5420 – 80 LP 1692 5550 – 90 LP 1539 Piedmont, Eastern and Western Andean Basin 5340 – 80 n/d 5320 – 150 5000 – 70 5020 – 90 n/d 4750 – 150 n/d 4690 – 50 Beta 110334 4900 – 60 5340 4720 7990 7450 7165 4180 3925 4830 7750 7300 7280 6000 5500 5500 4900 4480 4050 5290 5040 4860 8300 7060 6130 5610 5320 5310 5120 4270 6780 6540 5170

– – – – – – – – – – – – – – – – – – – – – – – – – – – – – – –

180 60 20 20 25 25 25 60 125 200 60 60 50 60 50 60 50 60 60 150 115 105 90 100 90 110 80 90 110 110 70

Paunero, 2000

Castro and Moreno, 1998 Castro et al., 2007 Caratcoche et al., 2005 Navarro Harris and Pino, 1999

Rivas et al., 1999 Navarro Harris and Pino,1999 Quiroz et al., 2000

Beta 110335

Quiroz et al., 2000

n/d n/d UCIAMS-10090 UCIAMS-10092 UCIAMS-10089 UCIAMS-10086 UCIAMS-10088 n/d AC 497 AC 499 NOVA-117 CSIC-518 CSIC-519 CSIC-800 CSIC-397 CSIC-521 CSIC-520 CSIC-800 CSIC-801 AC 1075 LP 384 LP 397 LP 286 LP 374 LP 300 LP 282 n/d n/d n/d Beta 27796 Beta 59924

Mena, 1983 Lucero and Mena, 2000 Mena and Stafford, 2006

Lucero and Mena, 2000 Alonso et al., 1984–1985 Gradı´n et al., 1976 Alonso et al., 1984–1985 Gradı´n and Aguerre, 1994 Alonso et al., 1984–1985 Gradı´n and Aguerre, 1994 Gradı´n and Aguerre, 1994 Aschero, 1996 Aschero et al., 1992

Rindel, 2004 Rindel, 2004 Aschero et al., 1992 Aschero, 1996 (Continued)

446

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Table 3. (Continued) Localities/Sites

Alero Manuk I (level 12) Alero del Leo´n (great crevice) Chorrillo Malo 2 Alero Destacamento Guardaparque Cerro de los Indios 1 Layer 17

Las Buitreras Layer V Cueva del Medio Fell III Layer 8

Pizzulic Los Noruegos Ponsonby layer D layer D layer B layer B layer B layer B layer B Bahı´a Colorada Punta Santa Ana 1

Bahı´a Buena

Potrok Aike Laguna Thomas Gould III (equivalent to Fell IV) Cerro Sota Lago Sofı´a 1, human cremation Beagle Channel ´ ridos Guerrico 136 A Seno Grandi 1 Caleta Segura, 169B

14

C years BP

Lab. Code #

5120 4930 4850 4735 4590 4330 6790 6550

– – – – – – – –

80 160 110 110 60 120 40 440

Beta 59926 AC 1102 Beta 27797 AC 11101 Beta 59923 AC 1103 UGA 10011 UGA 8714

6170 6700 5570 4900 3860

– – – – –

50 70 70 70 90

CAMS 71153 n/d n/d n/d LP 455

7670 4290 6740 6560 6485 6225 5585 7450 6690 4605 4580 4430 4150 4130 5500 6810 6020 5620 5895 5770 5210 4480 4050 4560

– 70 – 13 – 130 – 115 – 115 – 70 – 65 – 80 – 130 – 55 – 70 – 50 – 100 – 75 – 70 – 70 – 120 – 120 – 65 – 110 – 110 – 60 – 50 to 4280

Magellan basin n/d Beta 37167 I-5138 I-5141 I-5140 n/d n/d Gif-10139 Gif-10140 Gif-10138 GifA-93233 Gif-9567 Gif-10141 Gif-10142 n/d GrN 7612 Gif 2928 n/d GrN 7614 GrN 7613 Gif 2927 n/d n/d n/d

3900 n/d 3950 – 60 PITT-0526 3915 – 60 PITT-0527 Tierra del Fuego archipelago 6495 6160 6120 5635

– – – –

60 110 80 70

A10918 Gif-8851 Gif-9314 A10913

References Aschero et al., 1992

Aschero, 1996 Aschero et al., 1992 Gon˜i et al., 2004 Gon˜i, 2000–2002 Franco and Borrero, 2003 Rindel, 2004

Aschero et al., 1992; Aschero, 1996; Aschero et al., 1999; Figuerero Torres, 2000; Aguerre, 2003 Sanguinetti, 1976 Nami and Nakamura, 1995 Bird, 1988

San Roma´n and Prieto, 2004 San Roma´n and Prieto, 2004 Legoupil and Fontugne, 1997

Legoupil, 1997 (in Manzi, 2004) Legoupil and Fontugne, 1997 Ortiz Troncoso (in Manzi, 2004) Legoupil and Fontugne, 1997

Go´mez Otero, 1996 Massone, 1989–1990; Borrero et al., 1991 Borrero, 1993 Prieto, 1991

Ocampo and Rivas, 2000 Legoupil 1993–1994 Ocampo and Rivas, 2000

Archeological Hunter-Gatherer Landscapes in Fuego

447

Table 3. (Continued) Localities/Sites

14

Caleta Segura, 171 Bahı´a Honda Tu´nel I 1st Comp.

4895 – 60

2st Comp.

3rd Comp. Lancha Packewaia Older Component Mischiuen Imiwaia I

Lomada Alta del Mte. Olivia Peninsula Mitre Bahı´a Valentı´n 11

Northern Area Marazzi Rockshelter Cerro Bandurrias Rı´o Chico 1 La Arcillosa 1 La Arcillosa 2

La Arcillosa 3

C years BP

6980 6680 6470 6200 6150 6140 6070 6020 5960 5950 5850 5840 5700 5690 5630 5050 4590 4300 4900 4215 4890 4330 6490 6274 6048 5872 5600 5410

– – – – – – – – – – – – – – – – – – – – – – – – – – – –

110 210 110 100 220 130 100 120 70 170 70 185 170 180 130 520 130 80 70 305 210 180 120 119 111 147 125 160

5900 – 80 4939 – 43 4359 – 42 5570 5440 5700 5856 5918 5410 5508 5205 4440 3690 5353

– – – – – – – – – – –

400 30 180 44 44 70 48 58 60 70 53

Lab. Code #

References

A10914

Ocampo and Rivas, 2000

Beta 2517 AC 674 Beta 21969 Beta 3270 AC 883 Beta 2819 CSIC 310 AC 1028 CSIC 309 AC 838 CSIC 308 AC 845 AC 236 AC 238 AC 683 AC 844 AC 833 Beta 4385 CSIC-307 MC-1068 AC 1626 AC 1648 AC 1400 AC 1398 AC 1399 AC 1397

Orquera and Piana, 1999

Orquera and Piana, 1995; Orquera and Piana, 1999 Piana et al., 2004 Orquera and Piana, 2000

Orquera and Piana, 1999

Beta 23138 AA66713 AA66714

Vidal, 1988 Va´zquez et al., 2007

n/d

Laming Emperaire, 1968, Morello et al., 1998 Favier Dubois and Borrero, 2005 Santiago et al., 2007 Salemme et al., 2007a Salemme and Bujalesky, 2000 Salemme et al., 2007a

AC 1599 AA65165 AA65166* CSIR-7685 AA60934 AA60935 LP 994 CSIR-7682 AA65164

Salemme and Bujalesky, 2000; Bujalesky, 1998 Salemme et al., 2007a

Note: Localities and sites listed by subareas. *This dating comes from a paleobeach underlying de archeological shell midden.

brought several changes in the distribution of people along Patagonia; it matched also with different volcanic events and the collapse of rockshelter roofs, which are mirrored in the abandonment of some archeological localities in Patagonia and/or in the improvement of

landscape use in different ways (Orquera, 2005; Za´rate et al., 2005; Miotti, 2006c). Thus, in Table 5, temporal disturbance of some climatic or environmental events is observed. It could be considered whether data come from the Andean

448

Mo´nica C. Salemme and Laura L. Miotti

Table 4. Late Holocene radiocarbon dates (ca. 3.4 ka BP–AD 1500). Localities/Sites

14

Alero Marifilo-1 Cueva Epulla´n

590 – 50 2740 – 50 2360 – 50 1720 – 50 320 – 60 1550 – 50 1525 – 80 3490 – 80 2890 – 100 1510 – 90 840 – 90 2440 – 100 2130 – 110 2230 – 40 1360 – 90 1310 – 70 1040 – 70 540 – 60 2840 – 80 2710 – 100

Alero Nestares Alero Las Coloradas Alero Los Cipreses

Cueva Haichol Cueva Traful I El Manantial 1/88, Sondeo 16

CPO, Corralito subarea (lower layers) Alero Cicuta Layer 3 lower level Layer 3 top Tapera Moreira – La Lomita Burial Tapera Moreira – Site 3 Burial Tapera Moreira site 1 1st Occupation 2nd Occupation

3rd Occupation Tapera Moreira – Site 5 – 3rd Occupation La Petrona

La Primavera Campo Moncada 2

C years BP

Lab. Code # Northern Patagonia UCTL-1278 n/d n/d Beta 61145 Beta 54769 LP 1157 I 11308 LP 159 INGEIS 2814-936 LP 145 INGEIS 2813-954

References Mera and Garcı´a, 2004 Crivelli Montero et al., 1996, Crivelli Montero and Ferna´ndez, 2004

Chauvin, 2000 Crivelli Montero and Ferna´ndez, 2004 Silveira, 1996

Ferna´ndez, 1988–1990 LJ 5130 Beta 92642 Beta 92641 Beta 92640 LP 590 n/d n/d

Crivelli Montero et al., 1993 Sanguinetti et al., 1999

Sanguinetti et al., 1999 Silveira, 1999

1370 – 55 1080 – 50 2960 – 90

LP 637 LP 650 Beta 91934

2630 – 60

Beta 82558

3470 – 80 3010 – 80 2320 – 70 2200 – 40 2110 – 70 1940 – 90 1870 – 70 1830 – 100 1800 – 80 1190 – 60 1710 – 90 740 – 50 730 – 40 770 – 49 481 – 37 462 – 39 411 – 39 352 – 51 314 – 45 2800 – 60

Beta 91936 LP 264 Beta 82557 Beta 91935 LP 275 LP 358 Beta 81695 LP 352 LP 343 LP 265 LP 340 Beta 81698 Beta 91938 AA43125 AA43124 AA43123 AA43112 AA43126 AA43127 GX28772 Central Patagonia AC 670 U.G 7621 AC 669

3350 – 90 3210 – 50 1750 – 80

Bero´n and Curtoni, 2002 Politis and Madrid, 2001 Bero´n and Curtoni, 2002 Politis and Madrid, 2001 Bero´n and Curtoni, 2002 Politis and Madrid, 2001

Bero´n and Curtoni, 2002 Politis and Madrid, 2001 Martı´nez, 2004

Martı´nez, 2004 Bellelli, 1988 Nacuzzi, 1987 Bellelli, 1998

Archeological Hunter-Gatherer Landscapes in Fuego

449

Table 4. (Continued) Localities/Sites Late Occupation La Rural (Cerro Castillo)

Cerro Pintado

Los Alerces Alero Sendero Interpretacio´n, Comp.II Aceramic, Comp. IIb Ceramic, Comp. El Riacho Level 2 Level 1 Las Lisas Conchero 2 Calle Tehuelches El Elsa (Rawson) Playa del Pozo Punta Leo´n (open sea coast) La Azucena I El Golfito 2 Calle Villarino La Armonı´a (2) Rawson Los Abanicos 1 Gastre 1 (Central Plateau of Chubut) Bahı´a Solano 16 Bahı´a Solano 13 Central Plateau Los Toldos, Cave 3 Industries 1,2,3 Los Toldos, Cave 13, layer 6 La Martita Cerro Tres Tetas layer 3b bed Strwa Layer 3a, bottom Layer 3a, top Cueva Maripe Cueva Moreno Las Cuevas 2 Cueva de La Hacienda Chenque El Sargento

14

C years BP

860 – 80 780 – 80 3470 – 70 2240 – 90 1740 – 90 1870 – 80 1120 – 60 1100 – 60 680 – 60 1670 – 80 1450 – 70 740 – 70 400 – 40 3220 – 70 2640 – 70 2600 – 60 2410 – 50 1990 – 50 1540 – 50 1050 – 50 880 – 50 770 – 50 550 – 60 470 – 45 460 – 40 440 – 50 380 350 – 50 2954 – 195 205 – 95

Lab. Code #

References

AC 668 AC 667 LP 514 LP 359 LP 371 LP 1313 LP 1427 LP 1439 LP 1333 LP456 LP556 LP1118 LP1119 n/d n/d LP 868 LP 692 LP 712 LP 651 LP 678 LP 633 LP 685 LP 839 LP 969 LP 1001 LP 697 LP 889

Pe´rez de Micou, 2002 Carballido Calatayud, 2004 Stern et al., 2000 Belardi, 1996

I-11-794 n/d Deseado river basin

Bellelli et al., 2003

Arrigoni and Ferna´ndez, 2004>

Go´mez Otero et al., 1999 Go´mez Go´mez Go´mez Go´mez Go´mez Go´mez Go´mez Go´mez Go´mez

Otero Otero Otero Otero Otero Otero Otero Otero Otero

and Stern, 2005 and Dahinten, 1998 and Dahinten, 1998 and Dahinten, 1998 and Dahinten, 1998 and Dahinten, 1998 and Dahinten, 1998 and Dahinten, 1998 et al., 2002

Go´mez Otero et al., 1998 Go´mez Otero and Sua´rez, 1999 Go´mez Otero et al., 2000 Caviglia et al., 1982 Go´mez Otero et al., 1998

ca. 2500

n/d

Cardich et al., 1973

ca. 2500 2190 – 115 1620 – 90 2190 – 70

n/d AC 604 AC 603 LP-541

Miotti 1998; A. Cardich, pers. comm. 1990 Aguerre,1987

1740 – 60 1340 – 50 830 – 60 3210 – 60 1078 – 40 3000 – 110 2940 – 90 2510 – 110 2250 – 70 727 – 48

LP-1117 LP-1180 LP-770 LP-1497 AA65176 OS-23754 S/d

Paunero, 2003a

Miotti et al., 2005 Mengoni Gon˜alons, 1987

OS-23753 AA65180

Miotti et al., 2005 Miotti, 2006d

Paunero, 1994

Miotti et al., 2007b

(Continued)

450

Mo´nica C. Salemme and Laura L. Miotti

Table 4. (Continued) Localities/Sites Atlantic Littoral Cabo Blanco 2

Sitio Moreno Comp.3 Comp. 1 Me´dano 1, hearth 1 Laguna del Tele´grafo (M-259) Punta Guanaco 91 Me´danos DEL Canal 196 Me´dano 3 Punta Guanaco 97 Cabo Blanco 1 Can˜ado´n del Puerto Me´dano 4, bouy Rı´o Iba´n˜ez RI-50b Ban˜o Nuevo 1 CCP 5

CCP7 Alero Ca´rdenas De las Manos Cave

Arroyo Feo

Alero Rosamel Alero del Buho

Alero Direccio´n Obligatoria

Puesto El Rodeo Alero Gorra de Vasco

14

C years BP

Lab. Code #

References

3310 – 60 1700 – 30 960 – 60 3290 – 90 2720 – 50 2390 – 90 2380 – 60

LP-992 Beta 134598 Beta 134599 1063cSM LP 206 LP-1536 LP-1677

Castro et al., 2005 Moreno, 2003 Moreno, 2003 Moreno and Castro, 1995

2280 – 60 2280 – 70 2240 – 80 1480 – 70 1420 – 50 1040 – 40 920 – 40

LP-1694 LP-1522 LP-1532 LP-1648 Beta 134597 LP-1298 LP-1344

Castro et al., 2007 Zubimendi et al., 2005 Zubimendi et al., 2005 Castro et al., 2007 Moreno, 2003 Iantanos, 2003 Zubimendi et al., 2005

Zubimendi et al., 2005 Castro et al., 2007

Piedmont, Eastern and Western Andean basins 2290 – 90 n/d Lucero and Mena, 2000 2830 – 70 Beta 90894 Mena et al., 2000 2805 – 105 AC-1107 Aschero et al., 1992; Aguerre, 2003 2740 – 105 AC-1104 2550 – 50 Beta-27798 3480 – 70 LP-294 Aschero et al., 1992 3460 – 70 LP-279 3450 – 110 AC-498 Alonso et al., 1984–1985 1180 – 85 AC-500 3380 – 90 Nova-116 Gradı´n et al., 1976 1610 – 60 Nova-115 430 – 50 CSIC-137 3330 – 50 CSIC-398 Gradı´n et al., 1979 3260 – 50 CSIC-522 Alonso et al., 1984–1985 1885 – 36 CSIC-523 1660 – 50 CSIC-399 1170 – 50 CSIC-524 1590 – 70 CSIC-799 Gradı´n and Aguerre, 1994 1570 – 50 CSIC-798 1520 – 50 CSIC-511 Alonso et al., 1984–1985 1450 – 50 CSIC-512 1170 – 50 CSIC-513 960 – 50 CSIC-584 900 – 50 CSIC-545 1510 – 50 Beta-27800 Aschero et al., 1992 1200 – 70 LP-281 770 – 60 LP-301 390 – 110 LP-283 240 – 50 LP-277 1380 – 90 AC-943 Gradı´n and Aguerre, 1994 1360 – 60 LP-289 Aschero et al., 1992 490 – 60 LP-276 360 – 60 LP-293

Archeological Hunter-Gatherer Landscapes in Fuego

451

Table 4. (Continued) Localities/Sites Alero Destacamento Guardaparque layer 5 (5a)

Cerro de los Indios 1 Layer 3-4 Layer 3c2 Layer 3e Layer 2 Layer 3d Area 3 layer 6 Area 2 layer 11 layer 7b3f layer 7bf layer 6a layer 6a layer 5b layer 4 Area 1 layer 3b layer 3b CCP7 SAC4N2 (niche) SAC4N1 (niche) SAC1-7-1 chenque SAC1-7-2 SAC4-1-1 chenque SAC1-6-1 chenque SAC1-6-2 SAC1-6-3 SAC10-1-1 chenque SAC10-1-4 SAC1-1-3 SAC1-1-8 SAC1-2-1 SAC1-2-2 SAC20-3-2 Chorrillo Malo 2 Alero del Bosque 3 Bloque erra´tico Alero 2 Punta Bonita Campo del Lago 2 El Sosiego 2 El Sosiego 4 Cerro Verlika 1 Alice 1 Charles Fuhr Lago Roca 3

14

C years BP

3.440 – 70 2830 – 60 1200 – 70 890 – 70 200 – 50

Lab. Code #

References

n/d LP-290 LP-281 LP-288 Beta-27799

Rindel, 2004 Aschero et al., 1992

3400 – 90 3350 – 110 3320 – 50 3230 – 120 3150 – 90 1790 – 50 1420 – 50 1630 – 50 1170 – 50 1810 – 50 1660 – 60 1290 – 50 1250 – 50 1420 – 50 990 – 110 3480 – 70 3460 – 70 2607 – 41 2520 – 40 1147 – 37 1142 – 42 794 – 63 756 – 32 690 – 40 539 – 46 662 – 43 687 – 43 352 – 40 622 – 57 418 – 40 389 – 40 380 – 40 3790 – 80 1950 – 60 3110 – 50

LP 480 LP 378 CSIC 395 LP 369 AC1098 LP 493 CSIC-394 n/d n/d LP 708 LP 679 LP 687 LP 689 CSIC 394 AC-1099 LP-294 LP-279 AA38568 AA38567 AA38559 AA385561 AA38565 AA38556 AA38557 AA38558 AA38569 AA38570 AA38555 AA38560 AA38553 AA38552 UGA10623 Beta 148743 LP 502 Beta 91301

2540 – 70 2940 – 90 1920 – 40 1640 – 90 1685 – 70 1420 – 70 1480 – 70 1120 – 110 170 – 30

LP-402 LP-235 GX-25278 LP-420 GX-25277-G Beta 112231 Beta 112232 LP-406 Beta 91302

Aschero et al., 1992; Aschero, 1996; Aschero et al., 1999; Figuerero Torres, 2000; Aguerre, 2003

Aschero et al., 1992 Gon˜i and Barrientos, 2004; Zangrando et al., 2004

Franco and Borrero, 2003 Franco, 2002 Borrero and Franco, 2000 Borrero and Franco, 2000 Borrero and Franco, 2000 Borrero and Franco, 2000 Borrero and Franco, 2000 Borrero and Franco, 2000 Borrero and Franco, 2000 (Continued)

452

Mo´nica C. Salemme and Laura L. Miotti

Table 4. (Continued) Localities/Sites

14

Fell V layer 4 Cueva del Medio (unidentif.component) Alero Dos Herraduras, IV Cerro Sota

685 – 90 2100 – 60

Can˜ado´n Leona

El Volca´n Cave, layer 3 Lago Sofı´a 1, human cremation Pta. Bustamante RUD01BK RUD01BK RUD01BK RUD01BK RUD02FOI CEM02LPM HST01AM HST01AM CEM04CAN Cueva del Mylodon Camden 2-10 KM44 Puesto Leo´n 1 Alero Los Chilcos 1 Canal Maule Angostura Titus o SK14 Rı´o Verde I Caleta 2-14 Beagle Channel Seno Lauta

Caleta Segura, 170C Punta Baxa, 7 Eugenia, 52 Caleta Segura, 169A Bahı´a Virginia, 282 Ro´balo, 113 Guerrico Alto, 138 Ens. Villarino, 296 C. Segura/B. Honda 172B Cal. Santa Rosa 390

C years BP

Lab. Code # Magellan Basin I-5139 Beta 52521

2.870 – 65 3755 – 65 3645 – 65 2280 – 60 2270 – 50 2130 – 80 1740 – 70 3600 – 100 3950 – 60 3915 – 60

References Bird, 1988 Nami and Nakamura, 1995

DIC-2622 n/d

Borrero et al., 1991. Bird, 1988; Guicho´n et al., 2001

GIF 10791 GIF 10790 GIF 10236 GIF 10789 n/d PITT-0526 PITT-0527

Prieto et al., 1998

Massone, 1981 Prieto, 1991 Mansur, 1988, Mansur, 2007

3690 – 80 3400 – 60 3200 – 80 3050 – 60 2550 – 50 1060 – 50 890 – 90 750 – 70 710 – 40 2566 – 45 3030 – 80 2960 – 60 1570 – 50 1100 – 60 920 – 55 860 – 30 280 – 60 110 – 40

LP 533 LP 521 LP 160 LP 192 LP 499 LP 187 LP 454 LP 479 LP 201 BM-1202 Beta 153514 Beta 153516 Beta 123470 Beta 151873 Ua 17351 n/d Beta 152793 Beta 153515 Tierra Del Fuego Archipelago

2780 – 110 1080 – 60 280 – 90 1895 – 60 1820 – 10 1590 – 70 1540 – 70 1490 – 60 1275 – 50 945 – 30 870 – 60 880 – 60 920 – 20 810 – 60 590 – 70

n/d n/d n/d A10917 Beta 149813 Beta 127304 A10912 Beta 127306 A10910 RT-3214 Beta 127300 Beta 127301 TR-3215 Beta 127303 Beta 127308

Borrero et al., 1991 Morello et al., 2001 Morello et al., 2001 San Roma´n and Morello, 1999 Morello et al., 2001 San Roma´n and Morello, 2001 San Roma´n and Morello, 2001 Morello et al., 2001 Morello et al., 2001

Ortiz Troncoso, 1978

Ocampo and Rivas, 2000 In: Martı´n, 2004 Ocampo and Rivas, 1999 Ocampo and Rivas, 2000 Ocampo and Rivas, 1999 Ocampo and Rivas, 2000 Ocampo and Rivas, 1999 Ocampo and Rivas, 1999 Ocampo and Rivas, 1999 Ocampo and Rivas, 1999

Archeological Hunter-Gatherer Landscapes in Fuego

453

Table 4. (Continued) Localities/Sites

14

Puerto Eugenia 27B Ro´balo, 113 Bosque Isla Martı´nez, 198 Tu´nel I

4th Comp.

5th Comp. 6th Comp.

Tu´nel II Lancha Packewaia Late Component

Shamakush I

Shamakush VIII

Shamakush X Shamakush Enterratorio Mischiuen I layer D/E layer C layer C Mischiwen III Ajej I Isla El Salmo´n 5

Rı´o Pipo 17 Tolkeyen Mitre Peninsula Bahı´a Valentı´n S 11

Bahı´a Valentı´n S13 Bahı´a Valentı´n S1

C years BP

Lab. Code #

References

630 – 40 365 – 40 220 – 90 3530 – 90 2930 – 100 2880 – 60 2690 – 80 2660 – 100 2520 – 135 2000 – 110 1990 – 110 1920 – 80 670 – 80 450 – 60 1140 – 90 1120 – 90 1590 – 50 1120 – 50 1080 – 100 470 – 50 455 – 85 410 – 75 280 – 85 280 – 85 1927 – 120 1020 – 100 940 – 110 890 – 10 1490 – 90 1380 – 115 730 – 55 1450 – 100 500 – 100 620 – 60 1970 – 190 1060 – 85 890 – 90 625 – 25 1400 – 90 1820 – 120 1765 – 25 1560 – 90 1080 – 85

A10915 A10911 A10916 AC 702 AC 856 Beta 4387 Beta 2516 AC 1030 AC 854 AC 852 AC 851 AC 850 AC 701 Beta 4388 AC 1031 AC 824 CSIC 312 CSIC 311 MC 870 CSIC 314 MC 1063 MC 1066 MC 1062 MC 1064 AC 1291 AC 1293 AC 1047 AC 1029 AC 1678 AC 1681 AC 1679 AC 831 AC 832 AC 1680 AC 1625 AC 1624 AC 1623 KIA 19492 AC 1584 AC 939 GrN 12430 AC 938 GX 14317

Ocampo and Rivas, 2000 Ocampo and Rivas, 2000 Ocampo and Rivas, 2000 Orquera and Piana, 1999

760 – 80 490 – 80

GX 14315 GX 14316

1870 – 50 1350 – 60 550 – 50 modern 370 – 120 335 – 85

Beta 23139 Beta 23140 Beta 23141 AC 0964 AC 0966 AC 0965

Orquera and Piana, 1999 Orquera and Piana, 1999

Orquera and Piana, 1999

Piana and Va´zquez, 2007

Orquera and Piana, 1999 Piana et al., 2006 Piana et al., 2004

Piana et al., 2006 Piana et al. 2001 Figuerero Torres and Mengoni Gon˜alons, 1986; Figuerero Torres, 1988 Figuerero Torres and Mengoni Gon˜alons, 1986 Figuerero Torres and Mengoni Gon˜alons, 1986 Vidal, 1988

Va´zquez et al., 2007 Vidal, m.s. unpublished Vidal m.s. unpublished (Continued)

454

Mo´nica C. Salemme and Laura L. Miotti

Table 4. (Continued) Localities/Sites

14

Bahı´a Valentı´n S1 Bahı´a Valentı´n S42 Fagnano Lake Area Marina 1 Northern Area Marazzi 2 Marazzi 38 Bloque Erra´tico Espı´ritu Santo 1 Cabeza de Leo´n 1 Cabeza de Leo´n 4 Puesto Pescador San Genaro 1

San Genaro 2

San Genaro 3 Avile´s 1 Las Vueltas 1 Chacra Pafoy 1 Chacra Pafoy 3 Cantera Rhasa Margen Sur 1 Cabo Pen˜as Punta Marı´a 2 (upper levels)

San Pablo Marı´a Luisa

C years BP

Lab. Code #

References

modern 984 – 36

AC 0968 AA 66715

Va´zquez et al., 2007 Va´zquez et al., 2007

1800 – 250 900 – 170

AC 1471 AC 1470

Mansur et al., 2000

Beta 113690 UA 21182 UA 21183 n/d

Morello et al., 1998 In: Martı´n, 2004

910 795 785 785

– – – –

70 35 35 120

960 – 80 1100 – 95 230 – 60* 3780 – 70* 1600 – 60* 335 – 35 1070 – 80 1479 – 90 1620 – 140 1190 – 90 610 – 45 1483 – 80 1420 90 520 – 80 440 – 70 380 – 70 250 – 80 600 – 90 1609 – 38 949 – 41 320 – 60 804 – 33 332 – 39 1314 – 36 1295 – 50 897 – 38 620 – 45 1230 300 – 100 250 290 – 70 1020 – 80 360 – 50

LP 453 MC 1069 LP 604 LP 607 LP 413 AA69652 Beta 51997 INGEIS 1403 Moscow Ac.Sc. Moscow Ac.Sc. LP 661 AC 1404 AC 1484 LP 785 Beta 82291 LP 1291 AC 1600 AA69653 AA69656 LP 1069 AA65162 AA65163 AA69654 AA69657 AA69655 CSIR 7684 n/d n/d n/d n/d n/d n/d

Borrero and Casiraghi, 1980; Borrero et al., 1985 Horwitz, 1995, 2004 Saxon, 1979 Borrero, 2000 Favier Dubois, 1998; Borrero, 2000 Salemme et al., in press b Horwitz, 1995; Isla and Selivanov, 1993; Favier Dubois and Borella, 1999

Horwitz, 1995; Favier Dubois and Borella, 1999; Favier Dubois 2001; Martı´n 2004

Santiago and Orı´a, 2007 Salemme and Santiago, unpubl. Salemme and Bujalesky, 2000 Santiago et al., 2007 Salemme et al., 2007b Salemme et al., 2007b Salemme and Bujalesky, 2000 Borrero, 1985

Borrero, 1985 Lanata, 1985; Borrero, 1985 Yesner et al., 1991

Note: Localities and sites listed by subareas. *Datings coming from the archeological sites but unrelated to the cultural context.

Range or from the plateaus (or steppe areas). Based on pollen results, this disruption is also related to latitudinal effects; besides, the palynological sections usually come from sites along the Andean Range piedmont and fewer of them do so from the steppe (most of them from caves).

Peat bogs, mires and caves are the best places for sampling pollen; while the first two are excellent, the arid conditions in extra-Andean caves are usually less appropriate (Pa´ez et al., 1999; Borromei, 2003; Mancini et al., Chapter 17).

Table 5. Environmental conditions since Late Pleistocene, based on pollen analysis, faunal presence/absence, glacial and volcanic evidence. 14

C Yrs BP

Vegetation

Late

Grassy steppe advance

Present Holocene

configuration

Middle

Forest and grass steppe advance Schrubby steppe increase

Holocene

Fauna

Sea level

Temperature

Pampean fauna, Rhea sp.

Present sea level

Increasing temperatures

Lama guanicoe Pterocnemia pennata Rhea sp. Lama guanicoe Pterocnemia pennata Canis familiaris

Little Ice Age Medieval Optimum Second cooling pulse Arid and colder First cooling pulse

Flandrian transgression

Fisurella patagonica Opening of the Straits of Magellan

Holocene

Forest retreat – steppe

9500 10,000 10,500 11,000 12,000 13,000 14,000 15,000

---------------- advance Pleistocene/ Shrubby steppe advance Holocene Grassy steppe Transition Forest advance Higher moisture Late Pleistocene

Optimum climaticum Wetter and warmer Increased aridity and gradual warming

Aguilera volcano

North, central and Southern Plateau coast, Andean basins Magellan Basin and Tierra del Fuego

Hudson volcano

Lama guanicoe Pterocnemia pennata Lama gracilis Hippidion saldiasi Rhea americana Hemiauchenia paradoxa Xenarthra, Glyptodontidae, Mylodontidae

Lower global sea levels

Mored arid and colder Llanquihue 3 readvance Colder and wetter Glacier readvance

Northern Chilean Basin Northern Patagonia Deseado Plateau Magellan Basin Reclus volcano Reclus volcano Reclus volcano

Varas interstadial

North, South Plateau Magellan, east and West Andean basins

Burney Mt. volcano

Lutra sp.

End of LGM Source: Modified after Miotti (2006d).

Human occupation

Hudson volcano Mytilus sp.

Early

9000

Volcanic eruptions

No evidence of human occupation

Archeological Hunter-Gatherer Landscapes in Fuego

100 200 500 1000 2000 3000 3500 4000 5000 6000 7000 7500 8000

Period

455

456

Mo´nica C. Salemme and Laura L. Miotti

Based on the archeological information provided in Tables 2–4 and the paleoenvironmental data in Table 5, we may infer the following: 1. During the Late Pleistocene and Late Pleistocene– Holocene transition, dramatic changes in the environment occurred that, in a sense, reduced the landscape availability to the human groups who were then colonizing Patagonia. Last major readvance and retreat of glaciers (ca. 15.0 ka BP), tundra environment close to the foothills, shrubby to grassy steppes where Pleistocene megamammals were plentiful resources for people, frequent lakes providing continuous water availability and a very cold climate were the existing conditions for the earliest inhabitants. They mainly occupied, at the beginning, the Rı´o Deseado Basin (ca. 12.0 ka BP) on the eastern side of the Andes and the Magellan Basin further south (ca. 11.0–10.0 ka BP). In northern Patagonia, the ecotone and the steppes close to the Andean foothills were probably under a more unfriendly climate by the end of the Late Pleistocene; thus, the area was probably still uninhabited until ca. 9.0 ka BP (see Table 2 and references cited there). On the western side of the Andes – the Pacific fac¸ade – the Chinchihuapi valley was peopled ca. 13.0 ka BP, the Valdivia Basin ca. 10.0 ka BP and the Aysen region was occupied ca. 9.0 ka BP or maybe even earlier (see Mena and Stafford, 2006). 2. Between 15.0 and 8.5 ka BP, the Pleistocene megafauna (like American horse, Mylodon, Lama gracilis, Hemiauchenia paradoxa, among others) was slowly becoming extinguished, depending on the adaptive strategies of the different species to those changing environmental conditions. During the Late Pleistocene–Holocene transition, the Atlantic coast was located up to 200 km eastward from today; thus, as the coastal sites of those times are submerged today, the use of resources and space in those environments is still unknown. 3. The Middle Holocene (8.5–3.5 ka BP) was also a critical time, with a tendency toward aridization that lasted throughout the Holocene. Sea level rise was the main consequence as climate became warmer; the maximum transgression took place then and several remains coming from marine environments were found in archeological contexts of the steppe, indicating certain knowledge of such resources either by exchange or by taking advantage of new environments. Several volcanic eruptions during the Holocene that accumulated tephra layers in caves (Stern, 1992, 2007) and seismic events that may have forced the collapse of rockshelter roofs were probably the cause of the abandonment of caves, as well as the reorganization and distribution of human groups in other landscapes (neighboring areas) but returning later to the abandoned places. 4. Around 3.5 ka BP, Patagonia achieved its present environmental configuration (climate, fauna and vegetation); smaller glacier readvances, confined

only to the higher mountain valleys, triggered short colder and drier episodes, which probably had no strong influence upon human life conditions; in fact, the high archeological variability all over Patagonia during the Late Holocene indicates a very accurate knowledge of the environment plus very well-established social networks. The dispersal of human population, especially during the Middle Holocene, can be analyzed from the distribution of the available radiocarbon dates. Sometimes, microenvironmental discontinuities identified based on the lack of archeological record may have been coincident with chronological discontinuities in human occupation and/or changes in mobility patterns, but the cultural knowledge and communication networks allowed the hunter-gatherer societies to spread and occupy different landscapes and environments following diverse routes, until a wider proliferation all over Patagonia by the Late Holocene.

3. The Archeological Data 3.1. The Beginnings of Human Colonization in Patagonia: Late Glacial and Late Pleistocene– Early Holocene Transition Times Several papers published during the last 10 yrs synthesize our knowledge on human colonization of Patagonia (Borrero et al., 1998; Borrero, 2001b, 2004; Miotti, 2003a, 2006b; Miotti and Salemme, 2003, 2004; among others). All of these papers claim that this process was very slow, lacking a linear evolution. This history shows continuities and discontinuities in the occupation, sometimes under fast displacement of people with widespread occupation followed by long periods of stasis. What were the reasons for this? Probably environmental conditions – frozen soils, deserts, adverse climatic conditions – combined with human attitudes – fission and fusion of bands, social stress, decision making. But it has been recognized in all cases that at least three main subareas (Table 1: 3, 4 and 5) have provided abundant data about the earliest human occupations of Patagonia, as it is shown by the cited references in Table 2 (see also Miotti and Salemme, 1999, 2003, 2004; Borrero, 2001a; Miotti, 2003a, b, 2006b, c; Miotti et al., 2004; Carden et al., in press). In fact, the Deseado central Plateau and the Magellan Basin area, including Tierra del Fuego before it became isolated from the continent (Clapperton, 1992; Clapperton et al., 1995; Rabassa et al., 2000), show the highest density of sites chronologically allocated to the first interval (Fig. 2a, b; see also Fig. 1 in Miotti and Salemme, 2003). In this sense, several ways of space and resources exploitation indicate an ample microenvironmental diversity, with the exception of the coastal area: natural refugia like caves, rockshelters, boulders, sheltered wetland dells and deep canyons, as well as quarries of lithic raw material and pigments, campsites (residential and logistic task bases), walls and caves for rock art representation, sites for gathering roots and fruits, or mussels along the marine coast, use of wood for heating, etc.

Archeological Hunter-Gatherer Landscapes in Fuego Deseado basin

(a) 13,000

Foothill, eastern and western andean basins

(c) 13,000

12,000

12,000 11,000

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Fig. 2. Span and density of radiocarbon dates in each subarea during the Pleistocene–Holocene transition and Early Holocene: (a) Deseado Basin; (b) Magellan Basin; (c) Foothill and Andean basins and (d) northern Patagonia. These figures intend to show the chronological distribution of available uncallibrated radiocarbon dates, revealing their continuity and/or existing gaps for each subarea. The numbers on the x-axis corresponds to each one of the total number of available radiocarbon dates for the indicated subarea, which are depicted in growing order of age. The vertical lines for each radiocarbon date illustrate the statistical error of every date, but only in those cases when it is larger than 200 yrs. Plateaus, valleys or basins, rockshelters and caves with rock art (paintings and/or engravings) clearly show evidence of human activities or occupation both in stratified and in superficial material deposits. Several sites have been assigned to the latest Pleistocene and Early Holocene in the Deseado Central Plateau through three main chronological clusters (using AEP-1 and Maripe sites as markers: 12.9–10.5 ka BP, 10.45–9.0 ka BP and 9.0–8.0 ka BP; Figs 2a, 3–5; see Miotti et al., 2003); that is also the case of Cerro Tres Tetas, Casa del Minero and La Mesada Cave sites (Paunero, 2003a, b; Paunero et al., 2005). In the western piedmont, the Monte Verde site (Dillehay, 1997) shows recurrent occupation at least since 12.5 ka BP and probably even earlier, and the Marifilo 1 site since 10.5 ka BP (Fig. 2c; Mera and Garcı´a, 2004). The Magellan Basin seems to have been inhabited around 11.5 ka BP (Fig. 2b; Borrero, 1999; Morello, 2000; Massone, 2003). On the contrary, along the eastern Andean foothill, the earliest sites are not older than 9.7 ka BP (Civalero and Aschero, 2003; Civalero and Franco, 2003; Franco and Borrero, 2003), showing a higher concentration between 9.4 and 7.0 ka BP (Fig. 2c; Miotti and Salemme, 2004, and references cited therein). On the western Andean slope, the Ban˜o Nuevo 1 site is very interesting because of the finding of several human burials in the cave, with differential conservation but within a reliable and well-documented archeological context dated ca. 9.5–8.0 ka BP (Table 2; Mena and

Stafford Jr., 2006: 148). Some earliest dates show the evolution of the cave after the retreat of a proglacial lake, but the association of the earliest layers containing megafaunal bones with cultural remains is still doubtful (Mena and Stafford Jr., 2006: 150). The authors argue that the area of Ban˜o Nuevo, as a marginal territory, could have been explored from the extra-Andean steppe during the effective colonization stage. Regarding the rock art on both sides of the Andean ranges, great differences are observed. But, although paintings and engravings in eastern rockshelters and caves are abundant (Figs 3b, d, 4a, b, 5c), rock art has not been reported yet from the western basins (Carden, 2004; Carden et al., in press; Fiore, 2006; Miotti et al., 2007a). The Andean foothills, in the Rı´o Pinturas (Fig. 6a, b) and the Rı´o Belgrano–Lago Posadas basins, contain early colonization sites, but of a later age than those of the Deseado Central Plateau. Evidently, as the first occupation took place in the Deseado Basin toward the end of the Pleistocene (Table 2; Fig. 2a), and this Andean region has no signs of human occupation by that time, the expansion of human population seems to be addressed from plateau to Andean basins in the Early Holocene. Probably the foothills would have been still under the influence of glacial climate within the valleys; therefore, the first effective exploration and occupation would have started ca. 9.3 ka BP (Gradı´n and Aguerre, 1994; Civalero and Franco, 2003), being concurrent to the second

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Fig. 3. Piedra Museo locality: (a) AEP-1 site, view of the rockshelter; (b) boulder below the roof, showing petroglyphs and (c) details of the engravings on the boulder shown in 3b, depicting American horse footprints (Photos by Laura Miotti, 2005).

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Fig. 4. Piedra Museo locality: (a) Cueva Grande and (b) details of petroglyphs on boulders (Photos by Laura Miotti, 2005). pulse of colonization and territorial consolidation of the Rı´o Deseado Basin. This fact may likely be indicating the expansion of this sociocultural system toward western habitats at a higher altitude, closer to the Andean ice fields (Fig. 2c). The raw materials from this region also reveal management of local resources. Thus, it is possible to support the hypothesis that this first occupation is the

result of the expansion and consolidation of groups coming from the central Plateau; they shared the same technologies, unifacial and bifacial techniques for different purposes, and the choice of best quality raw materials for highly conservative tools (Fig. 7; Miotti, 1995; Catta´neo, 2002). But the intersite variability seems to be higher here than in the other region, which turns

Archeological Hunter-Gatherer Landscapes in Fuego (a)

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Fig. 5. La Primavera locality: (a) view of Maripe Cave and the mire environment in the valley; (b) excavation in the southern chamber of Maripe Cave and (c) rock paintings on the walls of the southern chamber (Photos by Bruno Pianzola, 2006).

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Fig. 6. (a) Rı´o Pinturas environment and (b) caves with paintings (Photos by Jorge Rabassa, 2007). more reliable the assumption that the trend of the settlement system goes toward the logistic forager strategy, with multiple activity and aggregation sites, linked to locus of special activities. Good examples of special

activities are AEP-1 at Piedra Museo locality (Fig. 3; Miotti, 1995; Miotti and Catta´neo, 1997, 2003; Miotti et al., 1999; Miotti and Salemme, 2005) and El Ceibo Cave 6 (Mansur, 1983; Cardich, 1987; Miotti, 1998).

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Fig. 7. Fishtail projectile point and unifacial artifacts from AEP-1 lower units (Photo by Laura Miotti, 2003).

Examples of multiple activities are Los Toldos Cave 3 (Fig. 8; Cardich and Miotti, 1983; Miotti, 1998), Maripe Cave (Fig. 5; Miotti et al., 2004; 2007b), Cerro 3 Tetas Cave 1 (Paunero, 2003a) and various sites at La Marı´a Locality (Paunero et al., 2005).

However, in some cases the archeological resolution of intrasite activity areas is low, due to a coarse-grained taphonomical integrity. This hypothesis can be tested in the La Mesada, Tu´nel, Casa del Minero and Cerro Tres Tetas caves (Paunero et al., 2005), and in Los Toldos Cave 3 (Cardich et al., 1973). All of them need to intensify taphonomical analysis to reduce the grain and the kinds of agents and processes that contributed to the accumulation. In the latter caves, inner spaces were interpreted as multiple activity areas. Records from these contexts indicate that the structure of the inner space was lower at the earliest human occupation, or the archeological integrity is coarse-grained and an enhanced reading is difficult or even impossible. Zooarcheological studies were carried out only for Los Toldos Cave 3 (Miotti, 1998) and AEP-1 (Miotti, 1996; Miotti et al., 1999; Miotti and Salemme, 2005); zooarcheological and taphonomical analyses were performed on the Cueva Maripe bone record (Miotti et al., 2007b; Salemme et al., 2006), and in Tres Arroyos (Borrero, 2003) and Lago Sofı´a (Prieto, 1991), as well. Nonetheless, faunal and taphonomical analyses are still absent in other key sites for the peopling of Patagonia. A deeper study of

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Fig. 8. Los Toldos locality: (a) view of dell and some caves; (b) view of Cave 2; (c) rock paintings of Cave 2, showing one hand and a horse and (d) rock paintings of Cave 3 (Photos by Laura Miotti, 2004).

Archeological Hunter-Gatherer Landscapes in Fuego these aspects is needed shortly, just to be able to improve the grain analysis and to produce more and betteradjusted hypotheses about the use of space by the first inhabitants. The main point is that all of these sites (special and multiple activity places) were recurrently occupied during shorter or longer terms; in each different time of occupation the use could or could not have been the same (Miotti, 2006b, d; Salemme et al., 2006). This evidence is coherent with a peopling exploration and dispersion strategy from a region where effective colonization had already started. It must also be noticed that in the piedmont area (the Andean foothills), neither fishtail projectile points (FTPP) nor associations with extinct fauna exist, which strengthens the idea of an area that is explored and selected later than the central Plateau, once the Pleistocene megamammal fauna had already vanished. Although the resources from this region are good and varied by the time of the first occupations, altitude and mountain effect could have generated in these places a more noticeable seasonal occupation than in the eastern lowlands. Probably, something similar occurred in northern Patagonia (Cuyı´n Manzano, Traful Cave, Epulla´n Cave; Table 1, Fig. 2a, b, c, d).

3.2. Territorial Consolidation as a Continuous Process: Disruptions and Discontinuities into a Mobile World of Hunter-Gatherers During the Middle Holocene The time boundaries of the Middle Holocene may be variable according to the region analyzed, but essentially most authors accept that it was a warmer period with a variable effective humidity depending on the area considered (Tonni et al., 1999). In this sense, Barrientos and Pe´rez (2005: 96) acknowledged that ‘‘ . . . from a global perspective, it can be said that data from several sources suggest an increased regionalization of climate from the early to the late Holocene’’. However, and in terms of regional peopling, many examples of southern South America illustrate a contradictory picture about human occupation during the Middle Holocene: archeological ‘‘silence’’ in Atacama (Nu´n˜ez et al., 1996), in the higher Andes at 35–36 S (Gil et al., 2005) or in the Puna, where ‘‘the aridity increase and perhaps higher temperatures promoted people change mobility patterns and social strategies’’ (Yacobaccio and Morales, 2005: 12), whereas in other sectors of the Andean Range, at 32–34 S, an increase in human occupation has been recorded (Garcı´a, 2005). From an archeological viewpoint, the analysis of proxy data for paleoenvironmental reconstruction is more or less coincident with studies on the Pampean and Patagonian sites by different authors (see Politis and Madrid, 2001; Barrientos and Pe´rez, 2005: Fig. 2; Miotti, 2006c). Table 5 lists some disparity regarding proxy information, but in all cases the paleoenvironmental conditions during the Middle Holocene could be interpreted as the consequence of events indicating regional asynchronicity.

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The most relevant differences are found between the Andean Range zones and extra-Andean Patagonia at middle and high latitudes. However, beyond the latitudinal stripes, two main climatic ‘‘pulses’’ were detected in the southern extra-Andean region (see Table 5). During the first period (8.0–6.5 ka BP) the climatic conditions show characteristics similar to those of the previous period: cooler temperatures but increase in effective moisture considering that, during the transition from glacial to postglacial periods, environmental conditions went from drier to wetter. As in the mountainous areas temperature was decreasing (probably as snow precipitation increased), these regions became hostile for human settlement. This fact should at least partly explain the depopulation process detected at those times in nearby areas like the southern ecotone in the Cuyo Region (southern Mendoza Province) and in northern Patagonia (Gil and Neme, 2001). On the other side, while population was diminishing in this Cordilleran area, the occupation of several environments by hunter-gatherers expanded in some subareas of the Patagonian and Pampean Regions (Table 3, Fig. 9a, b, c, d, e, f, g; Politis and Madrid, 2001; Barrientos and Pe´rez, 2005). At the local level, the exploitation strategies changed, showing higher occupation variability. In Patagonia, three sites that developed at the very beginning of the Middle Holocene (or the latest Early Holocene) indicate that climate amelioration allowed human settlement in the southern portion of the Deseado Plateau (El Verano Cave, ca. 8.9 ka BP; Dura´n, 1986/1987) and closer to the Andean foothills, in the northwestern sector of the Deseado Basin (CCP 7, ca. 8.3 ka BP; Aschero, 1996). Immediately after that, the more easily achieved occupation of La Martita at 8.0 ka BP (Aguerre, 1987) demonstrates again the excellent conditions for people and goods circulation (probably a warmer and wetter climate) in this subarea (see Tables 2 and 3, Fig. 9a, b). A second pulse during the Middle Holocene (between 6.0 and 3.5/3.0 ka BP) is characterized by a decrease in effective humidity. Mean temperature had remained stable (Table 5), but it also started to decrease. At a regional scale, the lowest mean annual temperatures occurred between 3.3 and 3.0 ka BP and diminished to – 1C, returning later to temperatures closer to those achieved at 8.0 ka BP, but under moisture conditions effectively lower, with accordingly higher evaporation (Bonadonna et al., 1995; Carlini and Tonni, 2000). The most dramatic event recorded in southern Patagonia by 8.0 ka BP is the flood of the Magellan glacial valley, turning into the present Strait of Magellan and separating the Fuegian Archipelago from the continent. This transgression probably played a special role among the Indian populations of the southern Cone tip. Mobility and communications must have changed because the groups of pedestrian people that had colonized this area became isolated. Since then and up to reorganization in social communication the archeological record is very scarce or even zero. Perhaps the definitive settlement and development of canoeing people would have linked people from both sides of the straits.

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At a regional scale, the environmental conditions might be different during the Middle Holocene in higher latitudes, when compared to middle or lower latitudes. At least, for the Deseado Central Plateau (Miotti, 2006b) and the Beagle Channel (Orquera, 2005) reliable evidence allows to give a different interpretation to what has been verbalized in several papers of the volume edited by Za´rate et al. (2005), about dispersal and mobility of human bands during the Middle Holocene. The strong littoral maritime adaptation is supported by uninterrupted occupation since 6.0 ka BP with a specific technology and special use of such macroenvironment (channel, coast, forest and open prairies beside the coast) along

the Beagle Channel (Fig. 10) but also in the western channels (Table 3; Fig. 9d, e). The conditions would have been favorable for a different use of the space among hunter-gatherers, particularly geomorphological landscapes, like the Deseado Central Plateau, where microenvironments with higher water concentration and a larger variability in the frequency of wind, rainfall, moisture and temperature could be found. In the Deseado Basin, human occupation would have reached a major differentiation in the use of microenvironments during the Middle Holocene, compared to those of the Colonization Phase, during the Pleistocene–Holocene

Archeological Hunter-Gatherer Landscapes in Fuego

Fig. 10. View of the Beagle Channel environment at Playa Larga Locality (Photo by Mo´nica Salemme, 2003). transition and the Early Holocene. This is shown by the increase and/or redundancy in occupations in the same sites (Miotti, 2006b). It is possible that this sector of extra-Andean Patagonia would have been a preferential area for human occupation, even better than the Patagonian coast (Fig. 9a, d). Or, effectively, an unequal distribution in the human occupation occurred in other regions of midlatitudes, like the Pampa or Cuyo regions (see Politis and Madrid, 2001; Barrientos and Pe´rez, 2005; Gil et al., 2005). Going back to the hypothesis of the Andean Range as a geographical barrier for peopling (Miotti, 2003a, b; Miotti and Salemme, 2003, 2004; Borrero, 2004), toward the Middle Holocene the southern Patagonian Cordillera became partly permeable to human communication through certain paths near the ice fields. There, the fluvial and lacustrine basins considered as ‘‘dead ends’’ (sensu Borrero, 2004) were related to both still existing ice fields, the Northern Patagonian Ice Cap (Ayse´n Region, Chile) and the Southern Patagonian Ice Cap (from Perito Moreno National Park to southern Lago Argentino, Argentina); they were occupied in a discontinued and marginal manner, sometimes as satellite groups from those located in the steppe (Miotti and Salemme, 2003, 2004; Borrero, 2004). The peripheral occupation of Rı´o Iba´n˜ez, like Las Guanacas on the western slope of the Andes (Table 3; Mena, 1999) and eastern occupations in the Perito Moreno National Park (Table 3, Fig. 9b; Aschero et al., 1992;Espinosa and Gon˜i, 1999; Borrero, 2004) can be considered as adequate examples of these conditions. If deep climatic changes occurred in other subareas, the Deseado Central Plateau could have been alternatively a better place for concentration of resources that gathered human groups from those regions. The Atlantic coast seems to have been an environment that was occupied sporadically or at least for shorter periods, both on the continent and in Tierra del Fuego (Fig. 9c, d, e) unless a sampling bias may be masking the evidence. Likewise, the archeological results from the Magellan Basin and the fluvial and lacustrine Andean basins show a significant increase for the sites occupied since the Middle Holocene, especially close to the heads of Rı´o Santa Cruz (Fig. 9a, b; Borrero, 2005; Carballo

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Marina, 2007). The archeological contexts and radiocarbon dates are clear evidence of occupation in northern and central Patagonia areas. A few scarce records are found in the Curaco´ and Colorado basins (Casa de Piedra, Tapera Moreira; Table 3) and Chubut Basin (Campo Moncada), respectively. The Somuncura´ Plateau is another sector of central Patagonia that shows occupation during this time, as well as the effective colonization along the foothills of the Andes (Ayse´n sector, Traful caves, etc). Some of the sites had been previously known by the human groups, but others were occupied for the first time (Tables 2 and 3). Under these conditions, during Middle Holocene times, Patagonian hunter-gatherers seem to have consolidated and extended their territories to neighboring regions, such as the southern Pampas. The archeological evidence from some lake basins located in zones defined as ‘‘dead ends’’ in the vicinity of the Andean Cordillera (Borrero, 2004), between 46 and 47550 S, indicates that the definitive initial colonization of this area took place during the Middle Holocene. Earlier occupations such as Ban˜o Nuevo 1, Rı´o Iba´n˜ez (46 S) and CCP 7 (on the eastern side of the mountains) were interrupted and discontinuous (see Tables 2 and 3, Figs 2b and 9b). ‘‘Dead ends’’ are considered here as biogeographical marginal areas, influenced by high altitudes and in the case of the main Patagonian Cordillera, by extensive ice fields (Borrero, 2004). East of the Andes, at the headwaters and near the highest peaks, the piedmont basins and in the lowlands (like the La Payunia region) around 35–36 S, local population history shows a significant gap between 5960 and 5060 14C yr BP (Gil et al., 2005), whereas Barrientos and Pe´rez (2005) have argued that the new population that occupied the southeastern Pampas could have arrived from Patagonia by the Middle Holocene as well. This model is in agreement with the replacement theory; morphometric evidence supports such hypothesis (Barrientos and Pe´rez, 2005). However, the multiproxy data (Table 5), the archeological record and radiocarbon dates (Table 3 and Fig. 9a, b, c, d) indicate noteworthy paleoenvironmental fluctuations at supraregional and subregional scales. Sea level achieved then its maximum elevation along the Atlantic coastline, woodlands advanced toward the steppe on the eastern slope of the Andean Range, available records indicate dry conditions from 7700 to 4000 cal yr BP along the Pacific coastal zone (Za´rate et al., 2005) and temperature and aridity became higher over the Patagonian plateaus. All these dramatic changes should be reliable causes to assume a trend toward resource specialization in different areas. For instance, and for the first time the most noticeable, intense and long-lasting human adaptive explorations during the Middle Holocene became visible. This happened in the archipelagos of southern Chile and the Beagle Channel, due to specialization in marine resources exploitation. There, rainy and cool forest environments were available for strengthening in the use of marine littoral resources, which was noticeably different from what happened along the Atlantic coast of extra-Andean Patagonia (Fig. 9c; Orquera, 2005) and northern Tierra del Fuego (Figs 9e and 11; Salemme et al., 2007a). Nonetheless, in these latter areas, the increasingly intense

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Fig. 11. A Middle Holocene human burial in La Arcillosa 2, northern plains of Tierra del Fuego (Photo by M. Salemme, 2003). use of plateaus and basins triggered a specialization in ‘‘guanaco’’ (Lama guanicoe) hunting. Radiocarbon dating shows substantial occupation continuity in all the studied subareas in Patagonia, including those which were considered marginal areas such as the higher basins of the Andes (Borrero, 2004), very close to the large ice fields (see Fig. 9b). The expansion and diversification of resources and land use were possible by alliances among logistics and/ or family groups, which would have stimulated geographical mobility. Cultural items like beads made of marine shells and sizable increase of obsidian artifacts in archeological contexts of the central plateaus (Los Toldos, Piedra Museo, La Primavera, La Martita and La Marı´a localities) allow us to suggest that goods and raw material circulation reached long distances. In this sense, different bands of hunter-gatherers would have been moving across Patagonia, probably with higher mobility than during the Late Pleistocene. Thus, the hypothesis of fission and fusion of bands is trustworthy to reassess peopling dynamics, taking advantage of new spaces with better habitability conditions, at least seasonally – during spring and summer – both closer to Cordillera and the marine coastline (Fig. 9b, c, d). Treatment of death became an important marker by the Middle Holocene. There is a record of burials in the eastern foothills like Puesto El Rodeo (Table 3: 4860 yrs BP, Gradı´n and Aguerre, 1994), as well as cremation evidence in Lago Sofı´a 1 by 3950 yrs BP (Prieto, 1991) and in Cerro Sota by 3900 yrs BP (Borrero, 1993), or a burial at La Arcillosa 2 (Atlantic coast of northern Tierra del Fuego, 5200 yrs BP; Fig. 12; Salemme et al., 2007a, b), in this case using an open air site between a lagoon and the ancient coastline (Fig. 12). This latter burial shows reddish sediments surrounding the pelvic area of a woman (Salemme et al., 2007a). Another important topic that confirms the intense occupation of Patagonia, at least in the plateaus and the Cordillera basins, is the increase of sites with rock art (Carden, 2004; Carden et al., in press; Miotti et al., 2007a, and references cited there). Rock art increases not only in spatial distribution, but also in the variability of motives and production techniques. Considering the chronological span of human occupation in different subareas of Patagonia, a main hiatus

Fig. 12. View of the northern plains in Tierra del Fuego – Rı´o Chico (Photo by Mo´nica Salemme, 2005). between 6.0 and 5.0 ka BP has been noticed in the Deseado Massif and the Magellan Basin (Fig. 9a, c), but it appears that movements of the hunter-gatherers toward the Atlantic coast and Cordillera took place. Table 5 summarizes at least a catastrophic event by this time: intensive and frequent volcanic eruptions. The evidence found in several caves is a record of tephra layers that translates into archeological silence or a mirror of site abandonment. In the Fuegian Archipelago, a brief occupation lapse took place between 5.5 and 5.0 and between 4.9 and 4.7 ka BP (Fig. 9e). On the other side, the Atlantic coast and central and northern Patagonia show a stable occupation during this period (Fig. 9d, f, g). Two particular occupation disruptions have been identified in the Andean foothills and lacustrine basins, the first one at 6.0–5.5 ka BP and the second between 4.9 and 4.7 ka BP (Fig. 9b). How these human occupation continuities and discontinuities (or spatial movements) can be interpreted for the different subareas in southern Patagonia? A correlation of these discontinuities with volcanic events along the Andean Range is observed, if the archeological radiocarbon dates are compared to environmental data (Table 5). Those events are coincident both with the collapse of rockshelter roofs as well and cave depopulation, suggesting that the following: (a) This fact could be indicative of human mobilization to other inhabitable places, abandoning those more risky areas affected by such catastrophic events. (b) It could also be due to strategic fission of bands that probably spread over already known neighboring areas. (c) This last strategy may or may not be concurrent with environmental effects; instead, it could probably be an answer to social necessities, but available data point out that this relationship is sealed by volcanic and other environmental events. In the case of the Fuegian Archipelago, the isolation of the area by the end of the Early Holocene and the beginning of the Middle Holocene is documented in the islands (Orquera and Piana, 1999, 2000, 2005; Ocampo and Rivas, 2000; Salemme and Bujalesky, 2000; Favier Dubois and Borrero, 2005; Salemme et al., 2007a)

Archeological Hunter-Gatherer Landscapes in Fuego but also in the northern margin of the Strait of Magellan (Bahı´a Buena, Punta Santa Ana, Ponsonby; Legoupil and Fontugne, 1997). The presence along the Beagle Channel and other waterways seems to portray the beginning of a maritime littoral adaptation, whereas in the steppes it appears to keep supporting a terrestrial way of life, though taking profit of marine environments as well. This information demonstrates the occurrence of certain mobility and territoriality patterns that, together with the presence of extraregional objects, should be indicative of an increase in social identity, which should denote a certain complexity of these societies.

3.3. Late Holocene: Daily and Sacred Landscapes The first absolute dates from open air sites in continental Patagonia are known only for the Late Holocene. Before that, dates come usually from contexts located in caves or rockshelters, at least in the Deseado and Magellan Basin subareas. In Tierra del Fuego, on the contrary, there are just a few sites located in rockshelters or next to glacial erratic boulders. In addition to the Tres Arroyos site (which was occupied recurrently since the Late Pleistocene–Holocene transition), the Marazzi site was also settled during the Early Holocene and then later in the Middle Holocene, but this locality is a huge erratic boulder. Bloque Erra´tico (another erratic boulder) and Cabeza de Leo´n are both localities close to Bahı´a San Sebastia´n; this last one, together with San Julio 1 and 2 (Saxon, 1979) in the northwestern area of Argentine Tierra del Fuego, are examples of Late Holocene or postcontact archeological sites located under rockshelters. Some restricted sectors acted like territorial markers; they could have been natural pigment exposures for painting (Aguerre, 2000; Miotti, 2006d and references cited there), burial areas (locally named as ‘‘chenques’’), places with high concentration of rock art but few tools, and also other peculiar topographic features of the landscape which in general would coincide with the presence of specific mineral resources that could be added to human burials. Forests could also be used for certain social practices, what is shown in the archeological record. Other areas could be used by logistic groups in a discontinuous manner, like coastal sectors, foothills or the central plateaus themselves, where human burials are recorded for the first time separated from daily life. During the Late Holocene (Table 4), the spread of human population seems to have saturated all habitats. However, ‘‘dead ends’’, i.e. nearly the ice fields in the Andean ranges and their foothills, were used always as special loci of activities and, in this sense, they have been considered as seasonal hunting sites and/or collecting logistical stations, i.e. the area of the Perito Moreno National Park (Aschero et al., 1992; Espinosa, 2002; Aschero et al., 2005), Casa de Piedra (CCP 5 and 7; Aschero, 1983) and, farther south, Chorrillo Malo (Franco, 2002; Franco and Borrero, 2003) (Table 4, Fig. 13a). Toward the Late Holocene most of the areas would have been occupied (Table 4) for settlement in new spaces not only due to environmental changes that pushed human displacement but also due to demographic pressure. This

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fact probably provoked fission and/or fusion of bands as well as other intrasite and intersite structures, maybe at a territorial scale. The regional archeological evidence indicates a higher recurrence in the occupation of the same places or of new sites that were unexplored before or marginally located from a biogeographical view point. Sometimes, these new areas could be assigned to rarely visited, sacred or taboo sites, or perhaps accessible only to special people (elite). These are special places, probably sacred sites where the archeological record is different from those in areas dedicated to domestic or daily activities (Miotti, 2006c). Around 3.5 ka BP, Patagonia achieved its present environmental configuration, in terms of climate, fauna and vegetation; smaller glacier readvances, only confined to the higher valleys, forced short colder and drier episodes in the lowlands (Table 5), although they probably had little influence on human life. In fact, the high archeological variability all over Patagonia during the Late Holocene indicates a very accurate knowledge of the environment plus very well-established social networks. By this time, the record of human burial fields (‘‘chenques’’) becomes frequent in extra-Andean Patagonia, like the area of Lago Salitroso (Gon˜i and Barrientos, 2000; Table 4) and other places along the plateaus. On the Deseado Plateau, data come mainly from unicomponent sites; they are dated between 3.2 and 0.7 ka BP (Fig. 13b), as in Aguada del Cuero, La Huella, Moyano, Bajo Pantano, El Sargento and the Monumento Natural Bosques Petrificados (MNBP; Hermo and Va´zquez, 1999; Miotti et al., 2002, 2005; Hermo and Miotti, 2003). In the headwaters of the Rı´o Deseado and in higher latitudes it is important to note microenvironmental discontinuities that seem to indicate a chronological interruption in the occupation regarding the colonization of the area, such as it was demonstrated above for the Late Pleistocene–Holocene transition. A paradigmatic example is the Aguada del Cuero Locality, only 30 km south from Los Toldos canyon, 65 km west of Piedra Museo and 57 km north-northwest of Cueva Maripe, at La Primavera locality. In these three latter localities, the earliest occupations were assigned to the Late Pleistocene–Early Holocene transition. However, the earliest occupation in Aguada del Cuero came from two caves situated at higher altitude and dating from the Late Holocene (Cueva Moreno and Cueva de La Hacienda, see Table 4). The question is why this sector of the Deseado Plateau – similar to others in quality and quantity of resources – seems to have been occupied so lately. Moreover, in this locality, in addition to the open air sites and caves, other sites were recorded at the base of basaltic exposures nearby lagoons and springs, like in Laguna Cerro Bonete and La Leonera, both of them with a great number of petroglyphs, engravings (Fig. 14; Miotti et al., 1999, 2002; Carden, 2004; Carden et al., in press) and lithic tools indicating a later chronology. Regarding different ways of burial, which may be used as direct evidence to interpret certain spaces as sacred in the subareas of the Andean foothills – like Salitroso and Cardiel lakes – and the Atlantic coast, the abundance of burials in ‘‘niches’’ and ‘‘chenques’’ (Fig. 13a; Gon˜i and Barrientos, 2000; Gon˜i et al.,

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2004) is directly related to the available data up to this time. On the contrary, in the Deseado Plateau area there is still not much relevant information; the only dated ‘‘chenque’’ is El Sargento site (ca. 0.7 ka BP, Table 4 and Fig. 15), in the area of Piedra Museo locality. Lately, more than 30 structures built with stones and considered as ‘‘chenques’’ have been located in several Foothills, eastern and western andean basins

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Fig. 14. Aguada del Cuero locality: (a) Cerro Bonete, cliffs and lake and (b) basaltic exposures with engravings (Photos by Laura Miotti and Mo´nica Salemme, 2001). the Deseado Plateau was also used for burial purposes, without considering seasonal displacements of groups for such immense long distances with the only goal of burying their dead. Other ‘‘chenques’’ discovered in Cerro Madre Hija (also known as Cerro Horqueta) show two different kinds of structures: (1) five ‘‘chenques’’ grouped on the southern slope of the hill, one of them with the floor and

walls covered by flat stones to arrange the dead bodies; this previous conditioning in a certain sector of the space points toward a major definition of these places for special practices; and (2) a secondary burial was found in a small structure, covered with a pile of stones on top of columnar basalts, at the ancient crater of the Cerro Madre e Hija, an inactive volcano. At least one individual and some long bones of other humans were buried there. The

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Fig. 15. Piedra Museo locality: El Sargento burial (‘‘chenque’’) (Photo by Laura Miotti, 2005). burial was already open by the time of discovery and most of the bones were spread in the surroundings and among the columnar basalts. Neither the skull nor the vertebrae were found, only highly weathered folded bones were still inside the structure. Besides the ‘‘chenque’’ and close to the top of the hill, two ochre lenses (yellow and red) were found; some long bones and the basement of the structure were impregnated with such pigments. Other ‘‘chenques’’ were found at the border of El Cuadro Plateau and in a low hill at its base, in this lower sector of the basin. Some materials usually distributed nearby the burial structures suggest activities of stone chipping in those places. Usually, this activity coincides with the manufacturing of projectile points probably used as offerings, as well as beads. The technology used in the manufacture of points and other artifacts found in several ‘‘chenques’’ from Aguada del Cuero (Miotti et al., 2002) and La Suerte localities (Miotti, 1998, 2006d) is assignable to the Middle and Late Holocene. In some cases, European objects have been found that are taken as evidence of a burial style practiced up to very late hunter-gatherer occupation. In those caves and places defined as campsites for limited activities (logistics) or for multiple activities (residential base camps), none of the burial styles from the foothills or the marine coast have been found yet. In all cases, its presence is associated with the margins of basaltic plateaus or low hills from where the panoramic view of valleys and basins could be managed, whereas spaces for domestic activities are restricted to areas where shelter, resources and especially drinking water were available. Places for burials could be coincident or not with sectors where special raw materials, such as pigments, were available. This discontinuous spatial relationship between such sites that are very different according to their function is relevant, since the extractive activity of ochre and its social circulation should be related to sacred activities directed to special groups, as Aguerre (2000) has documented for ethnographical cases. In archeological contexts, the use of ochre has been recorded by Miotti (2006d) for the Deseado Basin, Martı´nez (2004), Bero´n and Baffi (2004) and Prates (2006) for northern Patagonia, and Messineo and Politis (2005) for the Interserrana Area in the Pampean region.

Finds of ochre exposures are other examples of territoriality markers. Between the ‘‘chenque’’ at the top of Cerro Madre e Hija and others on the slopes, an exposure of yellow ochre is highly visible; it has a thickness of 2.5 m and a width of 15.0 m. At Piedra Museo, the El Sargento ‘‘chenque’’ was covered by red ignimbrite boulders from the same hill and nodules, cores and waste debris of an allochthonous greenish silex. The emplacement of this burial in the landscape is very interesting. From the top of the ignimbrites and basalts covering the tuff hill and from the burial itself, the Cerro Madre e Hija (20 km to the north) is highly visible; the exposures where the rockshelters with petroglyphs were excavated are located to the east and the red silex quarry on the slope of El Sargento Plateau. At the bottom, the Los Algarrobitos open air campsite is easy to identify. Southward, the southern margin of the paleolake at Piedra Museo is observed (Fig. 4a). Although GPS positioning, taphonomic observations, topography and landforms of emplacement have been studied over the last 20 yrs, available information is still unsatisfactory (Miotti, 1998; Miotti et al., 2002). Rock art confirms, on the continent, the complexity of this picture and the increase of stylistic regionalization of images represented on rocks and mobile artifacts (see Carden et al., in press; Belardi and Gon˜i, 2006; Miotti et al., 2007a; Fiore and Podesta´, 2006, and references cited there). In summary, during the Late Holocene people were well organized, with strong communication networks, living under a changing climate but then with less environmental stress and changes, occupying all of Patagonia on both sides of the Andean Range, which had been previously thought of as a topographic filter. Likewise, the archeological results from the Magellan Basin (Fig. 13b) and the fluvial and lacustrine Andean basins show a strong increase for those sites occupied during the Middle and Late Holocene (Fig. 13a), especially close to the heads of Rı´o Santa Cruz, and for the Late Holocene in the mouth of the Chico and Gallegos rivers (Table 4; Fig. 13c; Gradı´n et al., 1979; Gon˜i, 2000–2002; Gon˜i and Barrientos, 2004; Gon˜i et al., 2004; Borrero, 2005; Carballo Marina, 2007, among many others). The Atlantic coast seems to have been occupied recurrently during this interval, as verified in the record of materials in open air sites, but the number of radiocarbon dates is still very low (Fig. 13d); instead, the large chronological information existing for northern and central Patagonia (Fig. 13e, f) demonstrates more stable occupation by this time and/or higher demography than in previous periods. The record along the central coast in Patagonia is very interesting, as is the increase of radiocarbon data coming from Tierra del Fuego (Fig. 13g), particularly in the Beagle Channel (Fig. 13h), but there may be a sampling bias (E. Piana, personal communication). In the northern plains of Tierra del Fuego, the archeological record is particularly concentrated close or very close to the Atlantic coast and the radiocarbon dates indicate a large population in this area since 2000 yrs ago (Fig. 13i). For the time being, the low frequency of radiocarbon dated sites in Bahı´a Valentı´n at Penı´nsula Mitre is still related to the few investigations done there, although

Archeological Hunter-Gatherer Landscapes in Fuego research activities have recently been reactivated in this remote area (M. Va´zquez, personal communication.).

3.4. The Breakdown of the Aboriginal Society After the European Contact The end of the expansion of the aboriginal society (considering it as a whole) has been considered herein partly as the end of the Holocene, but in this case not as a consequence of climatic or environmental changes. Moreover, another kind of impact took place upon the Patagonian populations with the entrance of the ‘‘white’’ people, the Europeans who started their expansion toward the Patagonian territories southward from the Pampas. Such expansion took place since the last part of the seventeenth century. This event very strongly changed the aboriginal habits and resources, with the introduction of a new large mammal species such as the European horse, which created a whole complex of tools and life strategies, known as the ‘‘horse complex’’. The crash between these two societies with very different behavior, tools, economy and social organization was too strong to allow the preservation of the aboriginal lifestyle much longer. Natives were unable to replace or accommodate their social organizations when these changes became effective. Thus, less than 200 yrs were enough for the Europeans to weaken and destroy these aboriginal societies (even when considering different ethnographic groups) and to dominate the entire extension of Patagonia. Many archeological sites belonging to the so called ‘‘postcontact period’’ have been studied all over Patagonia from different points of view. The extremely long list of these sites makes it impossible to include them in this brief study, but they are proving the very high population density distributed all over Patagonia during the latest part of the Late Holocene.

4. Discussion: Stability, Discontinuities and Radiocarbon Chronology No radiocarbon chronology has yet been established for the Pleistocene–Holocene transition along the Atlantic coast and the extra-Andean area of central and northern Patagonia. Human occupation has been recorded only along the piedmont of the northern Patagonian Andes (Table 2, Fig. 2). The earliest and more concentrated human occupation known so far occurred in the Deseado Massif and in the Magellan Basin area. The region of the Pali Aike volcanic field and Seno ´ ltima Esperanza extended by that time into the present Isla U Grande de Tierra del Fuego (Fig. 1, Table 2 and Fig. 2c); the Strait of Magellan was then occupied by a land bridge composed of moraines and glaciofluvial plains and a large proglacial lake. Tierra del Fuego was still the continental southern tip of Patagonia by the end of the Pleistocene. In the area of the Deseado Massif, the earliest known occupations are at least 500 yrs earlier than in the southern margin of this Magellan paleolake. The oldest records are

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concentrated in sites within the Deseado Massif and the age of the surrounding localities decreases southward and westward. Northern Patagonia shows a few records for the earliest occupations, which are located in the eastern Andean foothills or in the ecotone area (see Fig. 1, Tables 1 and 2). Along the Pacific fac¸ade, the site of Monte Verde is the only one known with human occupation during the Pleistocene–Holocene transition. This site ´ ltima was occupied ca. 1.5 kyr before the sites in Seno U Esperanza and the Pali Aike volcanic field (Fig. 1, Table 2). The Marifilo-1 site displays the human settlement at a rockshelter in the Rı´o Valdivia basin, ca. 10.0 ka BP; in the Ayse´n area, the earliest occupation at Ban˜o Nuevo occurred during the Early Holocene, but only around 3000 yrs later. However, this occupation seems to be the result of an effective colonization phase, not just an exploration event (Mena and Stafford Jr., 2006). New research in this site could perhaps solve the question of an earlier occupational event within the cave, according to the available radiocarbon dates and contexts (Table 2). During the last period of the transition and the Early Holocene (ca 9.7 and 8.0 ka BP), population seems to have spanned and colonized other zones such as basins in the Eastern Andean foothills of northern and southern Patagonia, as well as occupying new localities in the already known and colonized regions. The earliest occupations in Tierra del Fuego seem to have been interrupted during these times (Tres Arroyos and Marazzi sites, see Table 2, Fig. 2). Is it possible to believe that the exploration of the southern edge of the Magellan Basin had failed or, instead, that the population had moved back northward of the proglacial lake? Whatsoever the answer is, any theory should explain the causes of site abandonment. However, if the paleoenvironmental conditions are analyzed (Table 5), a few answers may be found. About ca. 9.0 ka BP, volcanic activity started in the area with several eruptions; within the caves, roofs frequently collapsed and several caverns and rockshelters were abandoned perhaps due to a dramatic reduction of the available space for human occupation, i.e. CCP 7, AEP-1, Chorrillo Malo, Las Buitreras and Tres Arroyos (Table 1, Fig. 2). Probably, tectonic and/or volcanic events had an influence upon the discontinuity of peopling in certain areas as well as on the beginning of colonization of new places. A second and great event that took place at ca. 8.0 ka BP, which could have had effect upon human mobility and certainly increased the isolation of the Fuegian population, was the opening of the Strait of Magellan that firstly connected the Atlantic and the Pacific oceans since the arrival of humans at this latitude. The occupation recorded in the Ayse´n zone, on the western side of the Andes, would be a result of different populations arriving along the Pacific fac¸ade from the north or, alternatively, of human groups (maybe mongoloid people sensu Mena et al., 2003) from the east of the Andes who found a cordilleran path and colonized the Rı´o Iba´n˜ez Basin. Likewise, the archeological contexts of the western side of the Andean Range show several differences in the use of the space and resources. Human burials recorded in Ban˜o Nuevo 1 indicate some aspects of social complexity for this time that are

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reproduced in those entombments specially prepared (probably wrapped in a sort of skin, see Mena et al., 2003) or arranged in singular places that became common practice in the Andean area ca. 9.0 ka BP. During the Early Holocene, human occupation has not been recorded neither in central Patagonia nor along the Atlantic coast, as it can be observed in Table 2 and Fig. 2; clearly, the earliest records as well as their highest frequency came from the Deseado Plateau; younger and less frequent sites are recorded in the Magellan Basin and the foothill and lacustrine Andean basins. Only one record for the earliest colonization times is known so far for Tierra del Fuego (the Tres Arroyos site, Table 2). At present, this site is located 20 km away from the Atlantic coast; coastal areas were probably as far as 100 km eastward by the time of human peopling. This site is found within the Rı´o San Martı´n Basin, which drains to Bahı´a San Sebastia´n at the Atlantic coast. This fact could be reliable evidence to support that this first peopling wave into the Fuegian lands came from the eastern or northeastern steppes, instead of coming from the west, where the receding ice front was still occupying the bottom of the valley and blocking the proglacial lake (McCulloch et al., 2005). The other very early site, Marazzi, is a rockshelter probably occupied by humans just before the opening of the strait (Table 2). In this sense, it can be suggested that the southward exploration in the western side of the Andes, alongside the glaciated area with extant glaciers, deep fjords, abrupt channels and bare rocky islands, was slower and more limited than on the eastern side of the mountain ranges and extra-Andean Patagonia, mostly due to severe spatial and environmental constraints. Archeological, environmental and radiocarbon data indicate that population west and east of the Andes would not have been in contact until 9.0 ka BP (Tables 2 and 5). Then, it can be concluded that toward the end of the Pleistocene, a slow but continued peopling expanded from east to west coming from the Atlantic fac¸ade, reaching the eastern Andean foothills by the Early Holocene. Another arrival event would have happened along the Pacific fac¸ade, but this migration would have been discontinuous and probably with failing exploration expeditions other than the one aforementioned or, sometimes, short trips intentionally organized with very precise, logistic goals like searching for new resources or as elite excursions. But also the special conditions of the area (glaciated landscape, ice fields) may have required special technologies for littoral adaptation that were even not sufficiently developed in those exploration times. On the eastern side, though with more evidence of occupation, human dispersal was discontinuous, probably due to hostile environmental conditions when getting closer to the Andean Range. There should be places used by eastward incoming people during times of climatic amelioration, but this must have occurred briefly and/or seasonally due to the higher latitudes where they are located. Then, it is suggested that groups from both sides of the Cordillera would have gotten in touch only 2 kyr after the earliest exploration west of the Cordillera, while the process of expansive colonization was taking place in the Rı´o Deseado Basin. Within the Magellan Basin, instead, more

intensive contact probably increased during the Middle Holocene between people from both sides of the Andean Range, though there was probably still only a small population in both the Ayse´n and Puerto Montt areas. As the Tierra del Fuego Archipelago became isolated, people adapted to two very different environments became established there, i.e. the maritime littoral of the Beagle Channel and western waterways and isles and the steppe plains north of the Fuegian Andes. The appropriation of the landscape was different for these two groups, but the pedestrian people also used the coastal landscape, not only for economic purposes but also as burial places (Salemme et al., 2007b). The fission and fusion mechanisms of human bands were probably effective since the exploration times, but it intensified throughout the Holocene as shown by increasing colonized spaces all over Patagonia. The evidence of allochthonous raw materials in different areas (rocks, marine shells, pigments, etc), the chronological gaps of certain settlements in some subareas, whereas complete sequences are found in others, the paleoenvironmental data based on changes in fauna and vegetation, and the transition from generalist to specialist economy are some of the topics that allow to sustain that the initial peopling of Patagonia took place alongside the main fluvial basins, from the Atlantic coast toward the central mainland.

5. Final Remarks The southernmost end of South America was one of the last territories to become populated on the Earth. The Andean Range and its ice caps were the cause for the asymmetrical pattern of human distribution: almost all of the archeological sites are concentrated on the eastern side of the cordillera (Miotti and Salemme, 2003, 2004; Borrero, 2004), as there was a glacier barrier that would have been explored at different times until several paths were found, opening the communication between both sides. It seems clear so far that the earliest arrivals occurred independently on both sides. Six main issues have been considered in the analysis of the peopling of Patagonia’s landscapes: (a) the most important environmental conditions that could have affected human occupation during each interval; (b) the chronological gaps, clearly visible through the analysis of radiocarbon data; (c) the generalist or specialist adaptations in the use of faunal resources; (d) the rock art; (e) the human burials; and (f) the allochthonous raw materials. Extreme environmental conditions during the latest Pleistocene would have been the scenario under which the first human groups entered the continent. But these harsh circumstances did not threaten them to begin the exploration and colonization of the new available lands. Several of the best and earliest known archeological localities are today under desert climate conditions, with

Archeological Hunter-Gatherer Landscapes in Fuego very scarce water sources, most of them salty areas today, like Piedra Museo. However, between 13.0 and 10.0 ka BP, there were basins partially related to ancient snow and permafrost melting into the Rı´o Deseado Basin, special environments for grassy plains hosting the then existing fauna. In fact, the Rı´o Deseado would have flown toward the Atlantic Ocean during the Late Pleistocene, with as much discharge as the present Rı´o Santa Cruz, with glaciers still occupying the Lago Buenos Aires Basin. But when its drainage was interrupted at its heads and deglaciation opened westward routes into the Pacific Ocean, desertification processes were intensified on the central Deseado Plateau. The colonizers of the Piedra Museo area lived through an environmental impoverishment, with changes and degradation of faunal and vegetation resources, as shown by palynological and faunal records. Similar conditions are found at Cueva del Minero Cave 1 in La Marı´a Locality and Los Toldos Cave 3. In this latter locality, caves would have been located along the margins of an old tributary of Rı´o Deseado and close to two paleolakes. Piedra Museo, a locality with a high archeological integrity and good radiocarbon chronology and paleoenvironmental resolution shares these characteristics with other localities during the Colonization Phase, such as Cueva del Medio, Cueva Lago Sofı´a, Cueva Fell and Tres Arroyos Rockshelter, all of them within the Magellan Basin, though environments were different. All of them had direct relationship with water: the two first are located close to the Skyring and Otway fjords, which were at least temporarily occupied by proglacial, freshwater lakes during the latest Pleistocene, in an open forest environment, and the other two were related to the Chico and San Martı´n fluvial basins. Note that the Pleistocene megafauna was equally exploited in all these sites. The case of Monte Verde site is an example of ecological and adaptive variability; its economy seems to have been related to forest resources and, less likely, with megamammals; though remains of Cuvieronius are associated to this site, its synchronicity with the occupational context is still doubtful. Monte Verde has been considered a site of human interaction with the valdivian forest, where the main attractor for this residential camp was the basin of Arroyo Chinchihuapi, which was relatively close to the Pacific Ocean (approximately 50 km westward) by 13.0 ka BP. In the Ayse´n Region, Ban˜o Nuevo 1 site provided an interesting record of eight human skeletons dated ca. 9.0 ka BP, especially taking into account the type of burial. However, earlier occupations (related to extinct fauna like Mylodon sp.) are still doubtful since the disturbance provoked by the burials may have masked or blended them. During the 15.0–8.5 ka BP interval, the extinction of Pleistocene megafauna occurred stepwise, considering not only space pattern but also species type, depending on their degree of adaptability and/or resistance to the changing environmental conditions. The Atlantic coast was as much as 200 km further east than today in some places; thus, if this centripetal model of peopling following large fluvial basins along the Atlantic slope is accepted, the existing oldest archeological sites should

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be found at least several tens of meters below present sea level on the present submarine platform. The change in the faunal component triggered by the extinction of megamammals forced people to turn from generalists into specialists. Since the end of the Early Holocene, camelids such as ‘‘guanaco’’ (L. guanicoe) became the most frequent and well-represented mammal in the hunter-gatherer’s economy. The Atlantic Ocean fac¸ade seems to indicate not only the highest antiquity but also the highest archeological variability, with more varied cultural development and segregation: the Piedra Museo, Cerro Tres Tetas Cave, Cueva del Minero, Cueva Maripe, Arroyo Feo, CCP 7, Rı´o Robles, Chorrillo Malo and Fell sites, among others, should be considered in this sense. Nonetheless, the Pacific Ocean fac¸ade shows different geographical and topographical scenery in those high latitudes: shorter valleys that reach the marine coast and less availability of settling places were probably unstable circumstances for human occupation. Anyway, sites like Monte Verde, Ban˜o Nuevo, Marifilo in northern Patagonia and Cueva Sofı´a and Cueva del Medio in the south are showing that by the latest Pleistocene and the Early Holocene human groups were exploring and colonizing the western side of the Andes, maybe with little or without contact with the eastern side of the mountain ranges. The case of the Tres Arroyos site, in the Magellan Basin, shows a privileged place because it dominates both fac¸ades. It is close to the main basin of Rı´o San Martı´n – which flows into the Atlantic Ocean – but it was in the neighborhood of the Magellan proglacial lake before the opening of the straits. Thus, access to this area would have been possible also from the Pacific Ocean side; however, the still receding glacial front was likely blocking the passage. Therefore, it is more likely that people would have been moving southward from the northern steppe areas. In summary, the exploration stage was lengthy, gradual and discontinuous, with advance and retreat related to the environmental availability, leaving some areas and recolonizing others, with culturally defined territorialities, as well as to those communication and mobility strategies accumulated through time and known as landscape appropriation. Certainly, ca. 12.0 ka BP was the time when both sides of the Andes would have been explored using different strategies: this moment has been defined as the Initial Colonization stage, sensu Miotti and Salemme (1999), or Exploration Stage, sensu Borrero (1990) and Franco and Borrero (2003). Between 10.5 and 9.0 ka BP, the bonds of human groups in landscape knowledge and communicational networks are widely demonstrated up to the eastern Andean piedmont. With the available evidence, the Rı´o Deseado Basin becomes the most likely way of entering the continent at the earliest times; it is also possible that the Rı´o Santa Cruz Basin was another equivalent route at somewhat later times. Unfortunately, the lack of information for the other subareas, which could be due to sampling bias, obscures the interpretation. In fact, the record of allochthonous raw materials such as obsidian flakes (coming from the central–northwestern area) and shells of Fisurella and Aulacomya

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(coming from the Atlantic Ocean) in the middle Deseado Basin sites are excellent examples that communication networks worked intensively during the Early Holocene. Those gaps detected in the occupation for a certain area but the available record in others show the mobility of hunter-gatherer societies that were searching for better living conditions, at least temporally and/or seasonally. Between 12.8 and 11.5 ka BP, the central Plateau was populated, though it seems that people moved to the eastern piedmont (Chorrillo Malo, CCP 7, Cuyin Manzano, Epulla´n sites), returning to the area around 8.0 ka BP (El Verano Cave, La Martita Cave and the AEP-1 upper component). Idiosyncratic communication between human groups has been through rock art. This cultural expression is very common along the eastern side of the Andes, particularly represented in the central Plateau for the beginning of colonization and very frequent in different scenarios such as the Rı´o Pinturas area and La Marı´a locality since the end of the Early Holocene. On the western side, the Ayse´n area is the only known area where this representation is available for the Late Holocene. Death practices are still uncertain for this period, except for those burials found in Ban˜o Nuevo Cave 1, which offered not only physical information due to their excellent degree of preservation, but also about treatment of the death. The record of human skeletons for the Middle Holocene is very scarce; only a few examples are known for the Magellan Basin and northern Tierra del Fuego (Table 3). But this indicator provides exciting information for the Late Holocene, when special attention was devoted to death rituals, not only considering the body treatment but also bearing in mind the special places used for burial (selectivity of space, grave building, body preparation, etc). On the eastern Andean slopes, pulses of human occupation were more frequent during the Late Holocene. Sites CCP 7 and 5 are examples of occupation and abandonment, with subsequent repopulation again during the Late Holocene, but sites closer to the Cordillera like those from the Perito Moreno National Park could be interpreted as a model of marginal use, probably in response to seasonal environmental variations (sensu Miotti and Salemme, 2003, 2004; Borrero, 2004). Similar conditions would have taken place farther south, within the Magellan Basin, in intermittently occupied sites like ´ ltima EsperChorrillo Malo 2, or in the area of Seno U anza, where low-altitude paths and ice-free corridors were practicable and the connection between the Pacific and Atlantic basins was feasible. The Andean foothills were intensively used during the Late Holocene since the hunter-gatherer bands settled there, using the area under logistic or residential camps. Communication networks with the central areas were operational as regional demography was increasing. Very likely, the differential use of space in central Patagonia, the Deseado Basin, the Magellan Basin and the Andean piedmont would have began during the Middle Holocene (Miotti, 2006c), an episode named the ‘‘Territorial Consolidation Interval’’ by Miotti and Salemme (1999, 2003, 2004), and later greatly increased during the Late Holocene. This could be perhaps a cause

for the ‘‘depopulation’’ of southern Patagonia between 7.5 and 4.0 ka BP (Miotti, 2006c). Changes in spatial structures are visible in increase in sites and variability, as well as the regular and recurring occupations in certain types of landscapes. In the case of the Fuegian Archipelago, shell middens along the channels show a systematic reoccupation of the same sites, particularly during the Late Holocene, but this is much less frequent in the shell middens along the open plains of northern Tierra del Fuego, besides the higher variability observed there. Microenvironmental discontinuities are sometimes coincident with chronological colonization discontinuities, which may be related to catastrophic events or more local environmental modifications. From an environmental and chronological point of view (at the regional scale), different, complementary movements among the defined stripes are observed. While the Initial Colonization Phase occurred on the plateaus (very well represented in caves), the Consolidation Phase took place along the piedmont of the Andes, northern Patagonia or along the coast, which had been bypassed until then. Later on, in the ‘‘dead ends’’ of the foothills, Holocene Glacier fluctuations (neoglaciations) and volcanic events probably displaced people to other places, thus forcing the exploitation of new landscapes in other regions, which we suggest to call the Definitive Settlement Phase, during the Late Holocene. Some open air campsites from the Consolidation Phase show the occupation of northern Patagonia (see Table 3, Fig. 10), but the main bias is the scarcity of properly radiocarbon dated open air sites until the Late Holocene, with the exception of the Atlantic coast, where the most common open air sites correspond to burials (‘‘chenques’’). In the isolated Fuegian Archipelago, after the opening of the Strait of Magellan, the main island is the area best known archeologically. Two main adaptive strategies developed then, evolving in different ways of appropriation of the landscape. A long sequence of occupation has been recognized along the Beagle Channel since 6.0 ka BP (Orquera, 2005; Orquera and Piana, 2006), while just a few sites in the steppes reach this antiquity (Salemme et al., 2007a; Favier Dubois and Borrero, 2005). Nonetheless, the expansion of huntergatherer groups in both areas is very well documented for the Late Holocene (Orquera and Piana, 1999; Borrero and Barberena, 2004). The cultural divergence between the present continental area and the Archipelago area occurred after 8.0 ka BP. Such discrepancy is clear in the archeological record by means of three topics: (a) Rock art increased in variability and expanded over the continental territories, but it is absolutely absent in the Tierra del Fuego Archipelago. (b) Burials were performed as rocky monuments, particularly during the Late Holocene, in the three longitudinal environmental stripes of the continent, but they are absent in the archipelago, where the open air sites in the steppe or some rockshelters along the channels were used for burials.

Archeological Hunter-Gatherer Landscapes in Fuego (c) Circulation of exotic raw materials – Obsidian was used in places very far away from the Cordilleran source, like in the plateaus and Atlantic coast. Marine shells have been found in archeological contexts from the plateaus and the foothill Andean basins; however, they were not commonly used by people of the northern Tierra del Fuego inland steppe. These differences are clear evidence for latest occupation by hunter-gatherers of Fuego-Patagonia. They might be the representation of only one ethnic materiality or territoriality patterns, which could have began after 8.0 ka BP. But the main distinctive feature was given by the introduction of European horse (ca. AD 1725 in Punta Arenas, Chile, sensu Bulkeley and Cummins, 1744), bringing in further divergence between the Fuegian Archipelago and the continental populations. Up to extinction, natives from Tierra del Fuego never adopted the horse, whereas those from the continent fully did so. Thus, the continental aborigines expanded their mobility tracks though with unexpected additional consequences. In both areas, the European takeover drove them to extinction. It was a slow but constant and brutally aggressive process that in 200–300 yrs on the continent and in less than hundred years on the islands destroyed a huntergatherer life way more than 12,000 yrs old.

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Acknowledgments This chapter is the result of many years of work on the Archeology of Patagonia, especially in Santa Cruz Province and northern Tierra del Fuego. CADIC (Ushuaia) and Departamento de Arqueologı´a at the Museo de La Plata hosted our research projects during more than 20 yrs. Several grants helped the development of these research projects, coming from CONICET (the National Research Council of Argentina), ANPCyT (the Argentine National Promoting Agency for Science and Technology), National Geographic Society and the SECYT– ECOS Program (between the Science Secretariats of Argentina and France). Many pre- and postgraduate students have participated in fieldwork and their assistance in the laboratory has been very helpful. Fernando Santiago helped us with the drawing of Fig. 1. Radiocarbon dating of our projects was performed in the laboratories of Arizona University (USA) and LATYR (Universidad de La Plata, Argentina). The authors are very grateful to all of them, as well as to the reviewers that helped to improve a first version of this manuscript. However, we would like to emphasize that we are solely responsible for the concepts and ideas as well as the errors and inaccuracies included in this contribution.

References 6. Agenda Both the list of references and the exhaustive (but probably incomplete) inventory of radiocarbon dated sites are intended to give the reader a global idea of the state of archeological knowledge of Patagonia and Tierra del Fuego. Likewise, the information of nonradiocarbon dated archeological and historical sites (which have not been included in this study) is colossal. Nonetheless, we are aware of the gaps we still have to achieve for a comprehensive interpretation of the native cultures before the European arrival. Particularly, northern and central Patagonia and the Atlantic coast (with the exception of the areas actually mentioned in this chapter) show a true paucity of reliable radiocarbon dates. Areas like the Somuncura´ Plateau and the Rı´o Colorado, Rı´o Negro and Rı´o Chubut basins appear as exciting places where to look for different stages of human peopling. In Tierra del Fuego there are three main gaps in knowledge to be investigated: (a) that one of about 2000 yrs between the only two sites related to the earliest population (Tres Arroyos and Marazzi sites); (b) the absence of any archeological record between those early dates and the Middle Holocene record; and (c) the archeological record of the ecotonal area of Lago Fagnano, in central Tierra del Fuego. Some new interdisciplinary projects that are just starting will undoubtedly bring enlightenment on those open questions and also to many others still to be formulated.

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23 Late Cenozoic Mineral Resources of Argentine Patagonia Isidoro B. Schalamuk1,2, Rau´l E. de Barrio1 and Miguel A. Del Blanco1 1

Instituto de Recursos Minerales (INREMI), Universidad Nacional de La Plata, La Plata, Argentina 2 Consejo Nacional de Investigaciones Cientı´ficas y Te´cnicas (CONICET), Argentina

generally narrow and of reduced thickness. From a grainsize point of view, boulders and gravels are dominant, showing very poor sorting. The yield of these placers was up to 16 gms of gold per person per day, according to Stoll (1957). Castro (1999) also cited the continuous presence of gold, although of very low content (traces of 256 mg/m3), along the alluvial deposits of the main channel of the Rı´o Neuque´n between the localities of Huinganco and Balsa Huitrı´n, located more than 150 km apart. From a mineralogical standpoint, Castro (1994) mentioned magnetite and baryte as the main heavy mineral species frequently associated with gold. There is also information about occasional finds of cinnabar in the alluvial deposits of the Manzano and the Milla Michico´ creeks (Pichetti, 1943). Mercury was found in deposits of the Rı´o Neuque´n (Gamba and Castro, 1994).

1. Introduction The Late Cenozoic mineral resources of Argentine Patagonia (Fig. 1) represent a group of deposits of varied characteristics, intimately related to the geology of the existing morphostructural units and to the sequence of geological, geomorphological and paleoclimatic processes that took place during the Late Cenozoic times. Besides, the prevailing climatic conditions during the last 10 Ma in this region have played a significant role in the genesis of some of the mineral concentrations herein discussed. The mineral deposits have been subdivided into three main groups according to the geological environment in which they have been formed: (1) detritic, (2) evaporitic and (3) volcanic. 2. Detritic Deposits 2.1. Fluvial Environments

Alluvial Deposits in the Andean Cordillera of the Rı´o Negro Province

Gold-bearing Alluvial Deposits of the Rı´o Neuque´n

There are only a few bibliographic data about limited and rudimentary exploitation in some creeks of this region. Angelelli (1984) cited mining activities that took place in small creeks at the headwaters of the Rı´o Chubut as, for example, the Los Mineros, Klondike and Seso creeks. Giacosa et al. (2001) mentioned also an unsuccessful attempt of gold exploitation in the sandy bed of the Rı´o Azul, only a few kilometers west of El Bolso´n.

Gold-bearing detritic deposits were discovered already in 1890 before the primary metalliferous deposits in the Andacollo district, from which they are derived, were identified. Since their discovery until today, these deposits have been only temporarily and irregularly exploited, on a reduced scale, through panning by single small-scale miners or by small companies. These placers have been investigated by many authors. The work of Stoll (1957) and, more recently, Castro (1999) should be mentioned. Alluvial gold-bearing deposits were recognized along many creeks reaching the left margin of the Rı´o Neuque´n, between Huinganco and the Chos Malal region. Castro (1999) mentions the presence of various types of exogenetic concentrations. Those most important are of eluvial and glaciofluvial character or related to permanent and ephemeral present-day streambed channels. Among the eluvial deposits, those of Cerro Minas on the western slope of the Cordillera del Viento (the Cajo´n de los Caballos, Los Maitenes and El Durazno creeks, among others), and those on the slopes of the Yeguas, Caycaye´n and Mayal hills have been exploited. Glaciofluvial deposits in the La Primavera and Pampa de Malal Caballo areas have also been mentioned. The more important channel deposits are those of the Huemules, Huinganco, Huaraco´, Malal Caballo, El Durazno, Milla Michico´, Colo, Mayal creeks, and many others. All these deposits are of limited extent, displaying irregular shapes. Those found in stream channels are

Alluvial Deposits in the Andean Cordillera of the Chubut Province The existence of several areas in which gold-bearing sands have been sporadically washed is well known. Among others, areas located in Rı´o Pedregoso (El Ciclo´n, Claudia I and others), which is a tributary to the southern margin of Lago Epuye´n, should be mentioned. Native gold nuggets, 2–6 mm in diameter and of variable weight between 2.5 and 6 gms, have been found in this creek. Likewise, other minerals such as lollingite, allemontite, pyrite, native silver, electrum, hematite, ilmenite, pyrrothite, leucoxene and native copper have been identified. Similar works have been developed along the Agua Escondida and Planicie Alta creeks, near Rı´o Pedregoso and the Las Piedras creek on the western slope of Cordo´n de Cholila (Cardo´ et al., 2004).  2008 ELSEVIER B.V. ALL RIGHTS RESERVED

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Fig. 1. Location map of the main Late Cenozoic mineral resources of Argentine Patagonia. Toward the end of the 1980s, there were unsuccessful attempts at semi-industrial exploitation of gold-bearing alluvial deposits located 20 km northwest of Cushamen, ˜ orquinco and Fita along the basins of the Cushamen, N Michi creeks, all tributaries of the Rı´o Chubut. The presence of gold in different Quaternary aggradational levels and in the recent alluvium of the creeks in this region was recorded here in the form of minute nuggets and/or flakes, accompanied by abundant magnetite. Southwards, near the city of Esquel, potentially goldbearing deposits in the basins of the Percey and Corintos creeks have only been cursory investigated. In the upper reaches of these creeks, which drain the area of the polimetallic Huemules ore deposit, the presence of gold traces has been detected (Prez and Parisi, 1989; Viera, 2005), but this resource has not yet been assessed. Likewise, in the Rı´o Corintos, Haller (1995) verified the existence of gold particles, mostly planar and of variable size between 30 mm and 2.5 mm. In one of the exploratory trenches excavated in the alluvial plain, recovery was 0.5 g/m3. Other areas with available historical information on this kind of deposit are located in the middle and upper reaches of the Caquel and Cuche creeks, some 30 km west of Tecka.

Valvano (1949) made reference to the first exploitation, both of primary mineral (veins) and alluvial materials, by a British company during 1893/1894. Such venture was rapidly abandoned for economic reasons. The same author also mentioned discontinuous works on these alluvial deposits by local miners, always resulting in very poor yields. However, he stressed the exceptional find of 10–12 gms nuggets. The width of the alluvial deposits rarely exceeds 50 m and they are also rather thin. Genini and Zubia (1989) have also cited findings of gold and silver particles during washings in the current course of the Cascada creek, a tributary of Cuche stream, downstream of the primary vein mineralization. Currently, the alluvial deposits of Cuche and Caquel hills are being explored again by private companies.

2.2. Coastal Marine Environments Iron-titanium and Zircon Deposits of the Atlantic Coast (Bahı´a San Blas Region, Buenos Aires Province) Accumulations of iron, titanium and zircon of detritic origin are locally present along the southeastern coastal region of

Late Cenozoic Mineral Resources of Argentine Patagonia Buenos Aires Province. The metalliferous concentrations are located along 30 km of the coastal zone, both within beach sands and dune ridges of Bahı´a San Blas, in Carmen de Patagones county (Angelelli, 1973; del Rı´o, 1988). The San Blas zone is located in the so-called ‘‘Meridional’’ or ‘‘Southern Area’’ of Buenos Aires Province (Fidalgo et al., 1975), in which sedimentary stratigraphical units known as ‘‘Mesopotamiense’’, Arroyo Chasico´ Formation, Rı´o Negro Formation, ‘‘Patagonian’’ or ‘‘Tehuelche gravels’’ and Recent marine and eolian sediments are found. The metalliferous minerals sandy deposits forming the beaches and the dune ridges overlie the Rı´o Negro Formation. and the ‘‘Patagonian gravels’’. The iron-titaniferous and zircon-bearing accumulations are part of the Holocene deposits. The source of these heavy minerals is assigned to felsic and basic volcanic rocks (Teruggi et al., 1959, 1964). The presence of magnetite, ilmenite and hematite is due, according to these authors, to the weathering and erosion of basic volcanic rocks, while the occurrence of rutile, monazite and zircon is related to acid rocks of the rhyolitic type. Teruggi et al. (1959, 1964) considered that the largest contribution to the detritic materials would be derived from the coastal outcrops of Pliocene–Pleistocene terrains, while a minor proportion would originate in Patagonia, as the result of northward drift along the inner shelf. Consequently, it has been established that the sandy minerals belong at least to two sedimentary cycles. It has also been stated that the accumulation of heavy minerals is due to the joint action of marine currents and drift; likewise, it has been suggested that the Malvinas (Falkland) Current transported, by traction processes, the heavy minerals which, then, were concentrated by selective wave action. The estimation of reserves for both the dune and the beach deposits (Angelelli and Chaar, 1964; in Ca´bana and Mykietiuk, 1999) reached 1,300,000 tons of magnetic minerals with 57.3% Fe and 14.5% TiO2 and 650,000 tons of nonmagnetic minerals with 44.9% Fe, 22.1% TiO2 and 10,400 tons of zircon. These deposits were temporarily exploited during 1960–1961 for titanium. Likewise, during 1969 and 1970, a production of 1250 tons of concentrated minerals was recorded, intended to the preparation of a dense media for coal washing in the Rı´o Turbio Coal District (Santa Cruz Province) and for the cement industry.

Coastal Gold-bearing Alluvium of Santa Cruz Province These deposits are located along the shore of this province between Punta Loyola to the north and Punta Dungeness to the south. The most important sector, however, occurs between Punta Dungeness and Cabo Vı´rgenes, some 10 km to the northeast. There are no systematic records of gold production in these marine beach placers. However, based upon the scarce data from the Argentine mining statistics, it is estimated that at least 13% of the accumulated gold production between 1900 and 1996 has come from this area. These placers have been sporadically exploited since 1876 (Angelelli, 1984) by individual small-scale miners that usually increased their activity after large coastal storms or

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exceptional tides. The beaches with alluvial deposits are generally bounded by cliffs and are only appropriate for nonmechanical working during low water periods. Because of beach morphology and action, the gravel and sand deposits, which are typically thin, are constantly varying in shape, size and position. Prez (1985) indicated that, in spite of the high tidal amplitude in this area (10–12 m in normal tides), the exposed intertidal environment (the working alluvial width) rarely exceeds 100 m due to the steep slope of the abrasion platform. Within the alluvial units, the deposition of thin sandy layers with a gold-magnetite association is repeated along the sequence. Once the black sands are collected during low tide, they are gravimetrically processed and the gold is amalgamated in nearby sites. Prez (1985) cited mean values of 5.5 g/m3 Au on selected samples from these ‘‘renewable’’ natural concentrates of heavy minerals, where the gold occurs in the shape of small flakes. This author suggested that the source of detritic material in these deposits, including the gold, is the glacial sediments forming the constantly receding high cliffs.

Coastal Gold-bearing Alluvial Deposits of Tierra del Fuego Province Gold-bearing sands have been recorded in many sites along the Fuegian coasts. The most important area occurs in the northern sector of the Argentine portion of the Isla Grande, between Cabo Espı´ritu Santo and Bahı´a San Sebastia´n. There, among others, the localities at the mouths of the Alfa, Beta and Tortugas creeks, and at Rı´o Cullen, El Pa´ramo beach and Cabo San Sebastia´n are located. Methol and Sister (1949) described different types of alluvial deposits: glacial, glaciofluvial, marine beach and fluviomarine. Gold is included in the abundant glacial sediments and, later, it is concentrated by fluvial or wave and marine current action. Thus, the alluvial deposits of higher interest are those of the marine beaches, which have undergone strong reworking. It was basically on these deposits that, toward the late 1800s, the semimechanized exploitations led by the Rumanian miner Julius Popper (1887) took place, with a total annual production of almost 600 kg of gold. Methol and Sister (1949) observed the presence of up to 50% of heavy minerals in the sandy fraction of the alluvial beach deposits, in which, in addition to flakes and scarce gold nuggets, magnetite, hematite, ilmenite, hornblende, turmaline, garnet, zircon and pyrite were present. These authors estimated gold contents close to 1 g/ m3 for these sands, indicating also the very rare presence of platinum. The intensive exploitation, mostly located in the El Pa´ramo zone, was abandoned in the early 1900s. Later, and in a discontinuous manner, the coastal sands have been individually mined in the Mina Marı´a area. Along the coast of the Beagle Channel, in Bahı´a Sloggett, at the mouth of the Rı´o Lucio Lo´pez, a temporary exploitation of gold-bearing sands was also set up, with the installation of a steel dredge at the beach. There is today only one active mining establishment along the coastal zone between Rı´o Cullen and the area

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of Rı´o Grande (57 km long, 80 m wide). The most important sector is Mina Delia, located at the mouth of the Gamma creek. The heavy mineral accumulation is found along the coastal littoral ridge between the high water sea mark and the present streambeds. It has been determined that these alluvial deposits would yield approximately 1 g Au/m3. In addition to the presence of gold, the presence of titanium (basically as rutile), rare earth and platinum minerals is significant (M. Zubia, personal communication).

Fluvial, Glaciofluvial and Coastal Marine, Clayey-Silty Sand and Gravel Deposits These materials, which are used in the construction industry, are exploited in different areas of the Patagonian region, particularly near towns and cities or where road works are undertaken. The largest sand production comes from fluvial accumulations in the stream channel of the Limay and the Negro rivers, as well as from many other streams, and also from the Atlantic coast. The material is composed of fine, medium and coarse sand, grainsize between 1/16 and 2 mm. Its composition is related to primary rock mineral constitution. Exogenic processes rework these sands, resulting in silica-rich material. Gravel deposits are exploited at different localities where outcrops of the ‘‘Patagonian gravels’’ (or ‘‘Rodados Tehuelches’’, of Pliocene and Pleistocene age) occur as a thin blanket over extensive portions of this region. Gravels are also obtained from stream channels, and from the Atlantic shores. Clayey deposits are exploited by the brick industry. They are plastic clays formed by alteration of volcanic material (mainly tuff, rhyolite and ash). They have a widespread distribution and are exploited close to the main cities, as in Rı´o Gallegos (Santa Cruz Province); Bariloche, Viedma and General Roca (Rı´o Negro Province); Trelew, Madryn and Esquel (Chubut Province) and Zapala and Neuque´n (Neuque´n Province).

2.3. Eolian Environments The Great Salt Lake of Cabo Curioso (Santa Cruz Province) These eolian deposits are composed of gypsum material and are located inside the salt lake of Cabo Curioso, 30 km north of the town of San Julia´n (Angelelli et al., 1976; Iglesias, 2002). They cover a rectangular, east-west area, some 11 km long and 5 km wide. The geology of this zone includes volcaniclastic rocks of the Bahı´a Laura Group, over which sedimentary rocks of the San Julia´n and Monte Leo´n formations (locally known as ‘‘Patagoniano’’) outcrop. The thickness of the gypsum-bearing layers varies between 0.35 and 1.35 m. The upper portions are friable, while the lower sections are formed by compact layers, overlying clayey sediments with development of an incipient soil on top. These deposits correspond to evaporitic rocks, probably of marine origin, later reworked by eolian action. Codignotto (1983) indicated that 30,000 yrs ago, the area of the Great Salt Lake of Cabo Curioso was a marine bay, gradually isolated from the open sea by accretionary processes, forming littoral ridges.

3. Evaporitic Environments The evaporitic bodies of extra-Andean Patagonia (Figs 1 and 2) are associated with a complex set of geological, geomorphological and climatic processes, which have been active since the Pleistocene. In several cases, tectonic factors have contributed to the genesis of the depressions in which they are located. Among the most important deposits, the Salina del Gualicho should be mentioned as one of the largest NaCl reserves in Argentina and South America.

Fig. 2. General schematic model of the Quaternary evaporitic NaCl deposits of Argentine Patagonia (modified from Del Blanco et al., 2005).

Late Cenozoic Mineral Resources of Argentine Patagonia 3.1. Sodium Chloride Deposits Del Gualicho Salt Lake, Rı´o Negro Province This large saline body is located 53 km west of San Antonio Oeste, Rı´o Negro Province. It is situated in a very large and deep topographic depression, whose central coordinates are 40230 S and 65130 W. This region is characterized by a very arid climate, with annual precipitation of 200–250 mm, and evapotranspiration values exceeding ten times that of precipitation. During the summer, the salt lake is usually totally dry. The depression of the Gran Bajo del Gualicho, with a minimum elevation of –78 m below sea level, is fed by surface water of ephemeral character, active only during the scarce rainy periods in the area and by highly saline groundwater sources (Brodtkorb, 1999a). The origin of this and other depressions in the area seems to have been controlled by the main fracture systems in the region, trending E–W, NW–SE and NNW–SSE, which would have resulted in subvertical movements of the basement blocks, and a later removal of the sedimentary cover by eolian action. The geology of this area is represented by Precambrian, low-grade metamorphic rocks (El Jagu¨elito Formation), cropping out usually at the bottom of the depressions, which are covered by ignimbritic Jurassic layers of the Marifil Formation. Overlying these units, Tertiary marine sedimentary rocks of the Arroyo Barbudo Formation (Maastrichtian–Danian), and the Gran Bajo del Gualicho Formation (Late Oligocene to Early Miocene), as well as continental sandstones of the Rı´o Negro Formation (Pliocene) are found. These lithological units underwent differential erosion and compaction processes, thus preserving the preexisting landscape, which was partially covered by the Patagonian gravels of Pliocene–Pleistocene age, which acted as a protective blanket. The eolian action exhumed a Jurassic landscape, as it happened in the Bajo del Gualicho and the Bajo Piedra Coloradas, located immediately toward the southeast (Fig. 3). The saline deposit itself is composed of a muddy beach section (distal beach in the sense of Lowenstein and Hardie, 1985), with a weaker development in the southern sector of the salt lake, due to the influence of northern winds which push the brine and allow larger salt precipitation in this area (Brodtkorb, 1999a). Shallow excavations reaching a depth of 5.20 m (Re´ and Brodtkorb, 1960; Lombardi et al., 1994) enabled the identification of several salt layers, with interbedded dark silt, and a thin volcanic ash bed at a depth of 2.70 m. Recently, Garleff et al. (1996) indicated a continuity of salt layers down to a depth of 3.50 m. From here to a depth of 5.50 m, the salt content in the different halocycles decreases (Fig. 5b). The dominant mineral in all levels is halite. Glauberite appears as a common species, whereas gypsum is found only in the marginal sectors. Thenardite has been only occasionally identified. Isotopic studies of 2H/H and 17O, as well as of 87 Sr/86Sr, on phreatic waters up to a distance of 200 km from the saline basin (Angelucci et al., 1996), have suggested that the upper levels of the salt lake may have been derived from continental water evaporation.

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These high salinity waters (12 g/litre) come from aquifers located westward in Andean sectors, which were recharged with ancient water infiltrated during colder periods. The strontium isotopic relationship enabled also to reject the hypothesis of a marine water supply to this salt lake. The total reserves of this saline body are 582,819,300 tons (Brodtkorb, 1999a) of which 15,800,000 tons correspond to the ‘‘temporary’’ layer. Grande and Chica Salt Lakes (Chubut Province) The Grande and Chica salt lakes are located at Penı´nsula Valde´s, Chubut Province (Fig. 4). The depression in which they are found (42400 S, 64 W) covers an area of 230 km2 (Brodtkorb, 1999b). The geology of this area is composed of Tertiary marine sediments which form the floor of the depression. During the Pliocene and the Early Pleistocene, the ‘‘Patagonian gravels’’ were deposited as glaciofluvial coarse gravels (Fig. 4). The Grande salt lake is surrounded by silty sediments, with a high content of organic matter and a gradual increase in grainsize with depth, and isolated halite crystals, gypsum and glauberite. In the saline basin, a maximum thickness of the permanent or main halite layer of up to 5.70 m (Re´ and Brodtkorb, 1960) has been recorded. At the Chica salt lake, a thickness of 1.20 m was found. In both cases, the halite levels are interbedded with clastic layers. The NaCl content is 90% in the upper layers, diminishing to 70–75% in the lower ones. The NaCl total reserves are 41,900,000 tons at Grande salt lake and 4,900,000 tons at Chica salt lake (Angelelli et al., 1976). Although the production has not been very significant in the regional context, the proximity of these sites to large and active sea harbors made them some of the first producers of NaCl in Argentina. De Piedra Salt Lake, Buenos Aires Province This salt lake is located at 14 km northwest of the town of Cardenal Cagliero, in an WNW–ESE elongated depression, with a maximum length of 12.5 km and a width of 3 km (Fig. 5). Along the cliffs surrounding the saline body, the Rı´o Negro Formation and the Patagonian gravels crop out. Finally, covering the entire area, there are loess-like sediments (Franchi, 1977). The neighbouring plain has elevations of 30 m a.s.l., whereas the bottom of the salt lake is 5 m below sea level. The surrounding beaches are composed of salty silts and sands. The main mineral species is halite, with a percentage that varies between 95 and 98, with small quantities of gypsum, and glauberite, the proportion of which increases in the deeper halocycles (Del Blanco and Schalamuk, 1992). Scha¨bitz and Liebricht (1998) indicated that the salt content has been preserved up to 2 m from the surface, with a marked decrease below this level, coinciding with an increase in coarser (sandy) grainsize (Fig. 6A). Such evidence, the pollen content and the plant remains have suggested the existence of an arid climate during the Middle Holocene for this region, with a subsequent more

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Fig. 3. Geomorphological map of the Del Gualicho salt lake area (simplified from Brodtkorb, 1999a).

humid phase since 4500 14C yrs BP and the beginning of the currently dominant semiarid climate 2500 yrs ago. The total salt reserves (Cordini, 1967) reach 8,500,000 m3 with a temporary reserve of 741,500 tons. Other 500,000 tons dissolved in the brines should be added to these figures (Del Blanco and Schalamuk, 1992; Schalamuk et al., 1999). According to Kaaschieter (1965), Zambrano (1972), Kostadinoff and Font (1984), and Franchi (1977), this saline depression has a deep, tectonic control corresponding to minor fractures of WSW–ESE, and E–W directions, related to the development of the Colorado basin. The basin that includes the De Piedra salt lake in particular was modified later by fluvial and eolian processes. The main salt supply would correspond to dissolved materials in continental groundwater outcropping in this depression.

Del Ingle´s Salt Lake (Buenos Aires Province) This salt lake is located in Patagones county, 20 km away from the Atlantic coast, in the southernmost portion of Buenos Aires Province. It shows an oval contour, with a longest axis of 5.5 km in an E–W direction and a width of 4.8 km (Fig. 5). The landscape is smoothly undulating, with alternating hills and depressions, some of them connected to the sea by narrow tidal channels. The cliff zone surrounding this depression has heights of up to 10 m above the salt horizons that occur at sea level. The oldest outcropping geological unit consists of continental sands (Rı´o Negro Formation), with the ‘‘Patagonian gravels’’ overlying them. In higher areas, several terrace levels are recognized (Trebino, 1987) composed of reworked gravels, corresponding to ancient tidal plains. Their ages range between 3000 yrs BP and ca. 30,000 yrs BP.

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Fig. 4. Location map of the salt lakes of Peninsula Valde´s (from Brodtkorb, 1999b, simplified).

Fig. 5. Geological map of the Del Ingle´s and De Piedras salt lakes area (modified from Del Blanco et al., 2005).

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(a) 20 40 50 60 100 % Clay

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Fig. 6. Sedimentological sections of Cagliero salt lake (a) and Del Gualicho salt lake (b). Simplified from Scha¨bitz and Liebricht (1998) and Garleff et al., (1996). Finally, light brownish sandy silts, poorly consolidated and lacking visible stratification, are forming the youngest unit. At the saline basin, salt layers are present, composed almost exclusively of NaCl, with massive fabric of in situ grown crystals (crystals of the Hopper type; Ortı´ Cabo, 1988). Underneath, brownish to greenish silts occur, with gypsum and/or crystals of displacement growth and rare mollusk remains (Del Blanco and Schalamuk, 1993). Del Blanco and Schalamuk (1992) cited an average NaCl content of 252 g/l in the brines, and total mineral resources of 1,400,000 tons of NaCl. The existence of an ancient tidal channel, which linked the eastern end of the Del Ingle´s salt lake to the Salitral Grande, and through this to the Atlantic coast and a mollusk fauna composed of, among other species, Brachidontes rodriguezi (D’Orbigny), indicating eurihaline environments, both point toward a genesis related to an initial marine supply, to which highly saline, continental groundwater contribution may have added later (Del Blanco and Schalamuk, 1993). In this sense, the high Br concentration in the saline mud (up to 200 ppm) should be noted, which together with the 87Sr/86Sr isotopic data in the saline brine and mud (relationships of 0.70864 and 0.70862, respectively), very similar to present seawater (r = 0.7092), are pointing toward a marine origin.

Cabo Blanco Salt Lake (Santa Cruz Province) This salt lake is located 90 km north of Puerto Deseado. This lake occupies a depression of 9 km2, excavated in Tertiary marine sedimentary rocks of the San Julia´n Formation. A general profile of the salt lake (Domı´nguez, 1997; in Iglesias, 2002) indicates  0.03–0.06 m of NaCl (exploitation level)  0.40–0.75 m of clayey silts with high organic matter content  0.10–10.0 m of NaCl The main salt level (with a mean thickness of 8.00 m) contains 95% NaCl. Its texture is massive to banded (Iglesias, 2002) the latter given by interbedding of salt and clayey silt layers. Although reserves here are important (15,000,000 tons measured, Domı´nguez, 1997; probably 35–40 million tons, Cordini, 1967) the lack of a firm surface below the exploitation level, which impedes extraction by mechanized tools, suggests that this salt lake should be considered only as a potential resource.

Other NaCl Deposits La Espuma Salt Lake (Buenos Aires Province) This small saline body is located at 30 km west-northwest of Carmen de Patagones. It measures 2 km in length, in N–S orientation, by 1 km in width. The temporary saline crust does not exceed 5 cm; underlain by silty-clayey sediments with high organic matter content. The normative composition of the brines is 95% NaCl, 1.5% Na2SO4, 1.5% CaSO4 and 3.5% of insoluble residues (Del Blanco and Schalamuk, 1993; Del Blanco, 2004).

Other smaller deposits are those of Can˜ado´n Grande and Voluntad, in northeast Santa Cruz Province, about which very little information is available. Can˜ado´n Grande, 6 km north of Caleta Olivia, with a surface area of 0.6 square kilometer, is located within a depression eroded in Tertiary marine sediments. Angelelli et al. (1976) indicated a ‘‘harvesting’’ level of 2–4 cm, underlain by up to 7 m of salts, with 93% NaCl. The Voluntad deposit is located at 25 km of Fitz Roy, with a surface area of 0.6 km2. It has a harvesting level

Late Cenozoic Mineral Resources of Argentine Patagonia of 3–4 cm (Angelelli et al., 1976) and great purity (98% NaCl).

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The more important reserves presently defined correspond to the Paso Roballos sector, which have a total of crystallized mineral of 175,000 tons of anhydrous sodium carbonate.

3.2. Sodium Sulphate Deposits Florentino Ameghino (Santa Cruz Province) This deposit is situated at 6 km east of the town of Pico Truncado, in Deseado County. The salt lake occupies an area of 30 ha, in a depression that may be flooded during winter times. The geology is composed of volcanic tuffs of the Koluel Kaike Formation, continental sediments of the Rı´o Chico Formation, Pliocene and Quaternary basalts and the ‘‘Patagonian gravels’’. This deposit has a superficial layer of NaCl (0.10 m thick), fine-grained sodium sulphate (0.30 m) and, separated by a clastic level of 0.10 m, a thenardite layer of 0.70 m occurs. The mean content of the saline levels (Sabio, 1987) reaches 33% Na2SO4. The estimated reserves, according to the Provincial Mining Department, are 300,000 tons of mirabilite and 75,000 of thenardite.

Other Na2SO4 Deposits The Fa´tima accumulation is located at 45 km north of Caleta Olivia, Santa Cruz Province, and it is placed within a depression excavated in Tertiary marine sedimentary rocks, with a surface area of 13 ha. The 3 m thick salt layer is impure. The sodium sulphate content reaches up to 45%. The estimated reserves are 30,000 tons, according to the Provincial Mining Department. Other sodium sulphate evaporitic bodies of smaller sizes are San Eduardo (Valcheta county), Los Menucos (25 de Mayo county) and Laguna Chacay (9 de Julio county), all in the Rı´o Negro Province.

3.3. Sodium Carbonate Deposits

4. Volcanic Deposits 4.1. Sulphur Deposits These deposits are found in the Andean Cordillera Principal, in the Chos Malal sector, northwest portion of Neuque´n Province, in the Bayo, Huayle and Tromen volcanoes. They are situated within a zone of 50–100 km east of the international border between Argentina and Chile. These accumulations have only modest reserves. The deposit having had a larger extraction activity is Mina Hilda Mary (Cerro Bayo, Fig. 7), with a total refined sulphur production of 1136 tons produced between 1957 and 1963. Cerro Huayle was also subject to smallscale exploitation during the 1950s and Cerro Tromen produced 72 tons of mineral between 1939 and 1942. These deposits are located in volcanic environments composed of Late Quaternary basalts, andesites and their respective tuffs, known as the ‘‘Tilhuelitense’’ stage, and have mainly been affected by glaciofluvial erosive processes. The volcanic rocks are associated with sandstones and limestones belonging to the Tithonian–Neocomian stages. The sulphur deposits are the product of volcanic activity, as a consequence of the oxidation of gas venting (H2S) and the reaction of these gases with sulphur dioxide, with the corresponding precipitation of native sulphur which generally occurs as irregular veins, ‘‘nests’’ or bowls preferably located in volcanic tuff materials. They are also found disseminated as impregnations in the already mentioned volcanic tuffs. The mineral is lemon yellow to reddish yellow in colour. The sulphur manifestations occupy reduced areas in the different volcanic centers.

Pueyrredo´n, Buenos Aires and Ghı´o lakes (Santa Cruz Province) This is an extensive area located in the northwest sector of Santa Cruz Province. These deposits are found in small depressions excavated in the Upper Jurassic volcano clastic rocks of the El Quemado Group. These depressions are sometimes generated by tectonic processes and later modelled by Quaternary glacial action. This latter process determined the formation of lakes and ponds, enclosed by ancient valley moraines (Caldenius, 1932; Pereyra et al., 2002). The salts accumulated there are the product of leaching of Tertiary marine and continental sedimentary rocks (Angelelli et al., 1976). These deposits are composed of natron (CO3Na2O.H2O) and trona (CO3HNa.NaCO3.2H2O), with halite and thenardite traces. Basically, they appear as salty crusts along the lake margins. The salt contents are 2570 g/l of sodium carbonate, to which contents of sodium sulphate of 28 g/l must be added.

Fig. 7. General view of Hilda Mary Mine at Cerro Bayo, Neuque´n Province.

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Hilda Mary Mine This mine is located in the western side of Cerro Bayo, at an elevation of 3000 m a.s.l., west of Chos Malal. The lithological units of this area correspond to a lower level composed of stratified volcanic tuffs, cinder layers, brecciated materials and a compact andesitic tuff. The latter unit is where most of the mineralization is contained, which is exposed either in NE–SW trending faults, or in several other forms, like infilling joints or hollows, as impregnations, or as replacement nodules. Likewise, the upper section is composed of an andesitic flow extending from northwest to southeast. The whole complex is covered by a thick layer of modern gravels. The reserve estimation reaches 35,000 tons of mineralized tuff (positive plus probable), with mean content of 23% sulphur (Angelelli et al., 1976), and with 150,000 tons of geological resources (Me´ndez et al., 1995).

Cerro Tromen This deposit is found in various parts of the northern slope and the margin of the main crater of the Tromen (or Pun-Mahuida) volcano, over a NE–SW zone, 300 m long and 120 m wide. A layer of mineralized tuff has been recorded, of variable thickness between 0.50 m and 3 m, over a surface area or 600 m2. It occurs in the shape of crusts, nests and irregular veins. Sulphur impregnations in andesitic rocks are also observed. Druses with greenish yellow, bi-pyramidal sulphur crystals, up to 3–4 cm long, may be observed. Exploitation of these deposits was attempted between 1939 and 1942, but resulted only in the small production of 72 tons of crude mineral with 45% sulphur content.

Cerro Hayle Mineralization in this volcano occurs in the northern portion of the Tromen volcano, at an elevation of 3130 m a.s.l. The sulphur concentration, identified by several tunnels, is present as a blanket-shaped body 20 m long and an approximate thickness of 2 m. The mineral, with a sulphur content of 30%, is included in a very porous and widely kaolinized and silicified andesitic volcanic rock.

Other Sulphur Deposits of Neuque´n Province La Emperatriz mine is located in an area near the Blanco stream, 30 km northeast of Chos Malal. It is a mineralization composed of andesites and andesitic tuffs, impregnated with native sulphur over an area 150 m by 150 m. The sulphur was deposited by the action of thermal water springs. Sulphur content reaches up to 35%. Up to 9000 tons of probable mineral have been determined. Along the margins of the Cochico´ Grande stream, in the southeast slope of the Palao Mahuida mountain range, 120 km from Zapala, sulphur manifestations, infilling cracks preferably in basaltic rocks, represent concentrations

of no economic interest. The origin of the sulphur is related to sulphurous waters coming from several springs in the area. These springs occur along the contact between granodiorite and basalts, in an area known as ‘‘Ban˜o del Azufre’’.

4.2. Basaltic Materials Several basaltic materials have been exploited during recent years. Pleistocene olivine basalts appear as individual flows, about 4–5 m thick. The overall extraction of these materials, although there are many Quaternary basaltic outcrops in Patagonia, represents small amounts. In general, their use is restricted to hydroelectric dams, public roads and civil constructions. The main production localities are situated near Zapala, Neuque´n Province, where compact and impermeable basalts were used for the construction of the Cerros Colorados hydroelectric dam. On the other hand, brownish volcanic granulated materials are exploited near Las Lajas, Picunches county, Neuque´n Province. This material is used for brick manufacturing. Recently, some Holocene basaltic materials have been tested for their potential use in glass wool production.

5. Final Remarks The Late Cenozoic mineral resources and industrial rocks of Argentine Patagonia are potentially important materials widely exposed in this region. However, detailed geological knowledge and technical specifications on these resources have only been poorly studied. The Quaternary gold-bearing detritic deposits are found both along the Atlantic coast and in the fluvial drainage systems at the Andean foothills. Their importance is related to their probable economic use and as a prospective tool in the search for older primary mineralization areas. The detritic iron-titaniferous deposits in the southern coast of Buenos Aires Province are the sole titanium resources of Argentina. However, their actual economic importance is still unknown. Evaporitic deposits are the most important economic Holocene mineral resources of Argentine Patagonia and show a wide regional distribution. The saline concentrations of main interest are represented by sodium chloride deposits. However, there is no detailed geological knowledge, so far, especially for the deeper layers. Because of the development of new industrial applications, and favourable economic conditions, the exploitation of the evaporitic resources has increased over the last years.

Acknowledgments The authors thank Mario Zubia for their suggestions and comments. Also we would like to thank Jorge Rabassa for his encouragement to publish this chapter.

Late Cenozoic Mineral Resources of Argentine Patagonia References Angelelli, V. (1973). Recursos minerales y rocas de aplicacio´n de la provincia de Buenos Aires. LEMIT, Anales 2, 204. La Plata. Angelelli, V. (1984). Yacimientos metalı´feros de la Repu´blica Argentina. Comisio´n de Investigaciones Cientı´ficas de la Provincia de Buenos Aires 5, 1–2, 1–704. La Plata. Angelelli, V. and Chaar, E. (1967). Los depo´sitos de titanomagnetita, ilmenita y circo´n de Bahı´a San Blas (Tramo Baliza La Ballena – Faro Segunda Barranca), Partido de Carmen de Patagones, provincia de Buenos Aires. Comisio´n Nacional de Energı´a Ato´mica, Report 210, 1–25. Buenos Aires. Angelelli, V., Schalamuk, I. and Arrospide, A. (1976). Los yacimientos no metalı´feros y rocas de aplicacio´n de la regio´n Patagonia Comahue. Secretarı´a de Estado de Minerı´a, Anales 17, 1–142. Buenos Aires. Angelucci, A., Barbieri, M., Brodtkorb, A. et al. (1996). Morphology, Geology and Geochemistry of the Salar del Gran Bajo del Gualicho (Rı´o Negro, Argentina). Geologica Romana 32, 109–139. Brodtkorb, A. (1999a). La Salina del Gualicho, Rı´o Negro. In: Zappettini, E., (ed)., Recursos Minerales de la Repu´blica Argentina. Instituto de Geologı´a y Recursos Minerales SEGEMAR, Anales. Buenos Aires, 35, 1963–1970. Brodtkorb, A. (1999b). Salinas Grande y Chica de la Penı´nsula de Valde´s, Chubut. In: Zappettini, E. (ed.), Recursos Minerales de la Repu´blica Argentina. Instituto de Geologı´a y Recursos Minerales SEGEMAR, Anales. Buenos Aires, 35, 1971–1976. Ca´bana, M.C. and Mykietiuk, K. (1999). Arenas ferrotitanı´feras y circonı´feras del litoral de la provincia de Buenos Aires. In: Zappettini, E. (ed.), Recursos Minerales de la Repu´blica Argentina. Instituto de Geologı´a y Recursos Minerales SEGEMAR, Anales. Buenos Aires, 35, 1899–1903. Caldenius, C. (1932). Las glaciaciones cuaternarias en la Patagonia y Tierra del Fuego. Geografiska Annaler 22, 160–181. Cardo´, R., Segal, S. and Zubia, M. (2004). Descripcio´n del Mapa Metalogene´tico del Oro de la Repu´blica Argentina. Instituto de Geologı´a y Recursos Minerales, Servicio Geolo´gico Minero Argentino, Anales 38, 1–140. Buenos Aires. Castro, L. (1994). Los aluviones del Rı´o Neuque´n, aguas abajo del distrito aurı´fero Andacollo, provincia del Neuque´n. International Mining Meeting 1, 141–145. Buenos Aires. Castro, L. (1999). Aluviones aurı´feros del rı´o Neuque´n, Neuque´n. In: Zappettini, E. (ed.), Recursos Minerales de la Repu´blica Argentina, Instituto de Geologı´a y Recursos Minerales, SEGEMAR. Anales. Buenos Aires, 35, 1875–1881. Codignotto, J.O. (1983). Depo´sitos elevados y/o de acrecio´n Pleistoceno-Holoceno en la costa FueguinoPatago´nica. Simposio Oscilaciones del nivel del mar durante el u´ltimo hemiciclo deglacial en la Argentina, Actas 1, 12–26. Mar del Plata. Cordini, R. (1967). Reservas salinas de Argentina. Secretarı´a de Estado de Energı´a y Minerı´a, Anales 13, 1–106. Buenos Aires.

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Del Blanco, M. (2004). Industria salinera en la regio´n pampeana. In: Lavandaio, E. (ed.), Historia de la Minerı´a Argentina. SEGEMAR, Anales. Buenos Aires, 40, 2, 329–332. Del Blanco, M. and Schalamuk, I. (1992). Caracterı´sticas geoquı´micas y econo´micas de las salinas del sudoeste de la provincia de Buenos Aires. IV Congreso Nacional de Geologı´a Econo´mica 1, 293–304, Co´rdoba, Argentina. Del Blanco, M. and Schalamuk, I. (1993). Contribucio´n al conocimiento de los cuerpos salinos del partido de Patagones, provincia de Buenos Aires. Situacio´n Ambiental de la provincia de Buenos Aires. A. Recursos y rasgos naturales en la evaluacio´n ambiental. CIC 3, 23, 1–35. La Plata. Del Blanco, M., Marchionni, D., Romero, S. and Ca´bana, C. (2005). Depo´sitos evaporı´ticos de la provincia de Buenos Aires. In: de Barrio, R., Etcheverry, R., Caballe´, M. and Llambı´as, E. (eds), Relatorio del XVI Congreso Geolo´gico Argentino, Geologı´a y Recursos naturales de la provincia de Buenos Aires. La Plata, 27, 417–434. del Rı´o, J.L. (1988). Dina´mica de las acumulaciones de minerales pesados en playas del litoral atla´ntico bonaerense entre Mar Chiquita y Necochea. Unpublished Ph.D. thesis, Facultad de Ciencias Naturales y Museo, Universidad Nacional de La Plata. 190 pp. La Plata. Domı´nguez, E. (1997). Informe exploracio´n salinas del Cabo Blanco, provincia de Santa Cruz. Compan˜´ıa Minera Aurora Agropecuaria, unpublished report, 35 pp. Mar del Plata. Fidalgo, F., De Francesco, F. and Pascual, R. (1975). Geologı´a superficial de la Llanura Bonaerense. 6 Congreso Geolo´gico Bonaerense, Relatorio 103–138. Buenos Aires. Franchi, M. (1977). Informe final de las hojas geolo´gicas 39m, 40m y 39n, provincias de Buenos Aires y Rı´o Negro. Servicio Geolo´gico Nacional, unpublished report, 90 pp. Buenos Aires. Gamba, M.T. and Castro, L. (1994). Estudio econo´micominero del Prospecto Placeres aurı´feros Cerro Mayal y su a´rea de influencia sobre el Rı´o Neuque´n, provincia del Neuque´n, Argentina. 2 Congreso Cubano de Geologı´a y Minerı´a 1, 92–93. La Habana. Garleff, K., Reichert, T., Sage, M. et al. (1996). Perı´odos morfoclima´ticos y el paleoclima en el norte de la Patagonia durante los u´ltimos 13000 an˜os. Bamberg University, Germany, unpublished report, 15 pp. Bamberg. Genini, A. and Zubia, M. (1989). Informe expeditivo proyecto Arroyo Cascada. Servicio Econo´mico Minero, Plan Patagonia Comahue, unpublished report, 10 pp. Buenos Aires. Giacosa, R., Heredia, N., Ce´sari, O. and Zubia, M. (2001). Hoja Geolo´gica 4172-IV, San Carlos de Bariloche (provincias de Rı´o Negro y Neuque´n). Instituto de Geologı´a y Recursos Minerales, Servicio Geolo´gico Minero Argentino, Boletı´n 279, 1–67. Buenos Aires. Haller, M. (1995). Prospeccio´n de oro en los aluviones del rı´o Corintos. Consejo de Ciencia y Te´cnica de la Provincia del Chubut. Centro Nacional Patago´nico, CENPAT-CONICET, unpublished report, 15 pp. Puerto Madryn.

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Iglesias, J. (2002). Recursos Minerales no metalı´feros y rocas de aplicacio´n. In: Haller, M.J. (ed.), Geologı´a y Recursos Naturales de Santa Cruz. Relatorio del XV Congreso Geolo´gico Argentino: Parte IV, Recursos Minerales. El Calafate, Argentina, 4, 4, 729–742. Kaaschieter, J. (1965). Geologı´a de la cuenca del Colorado. II Jornadas Geolo´gicas Argentinas, Actas 3, 251–269. Buenos Aires. Kostadinoff, J. and Font, G. (1984). La cuenca del Colorado en el litoral sur de la provincia de Buenos Aires. IX Congreso Geolo´gico Argentino 3, 7–26. San Carlos de Bariloche. Lombardi, G., Brodtkorb, A., Romero, S. et al. (1994). The salt body of the Salina del Gualicho (Rı´o Negro, Argentina). Bolletino della Societa´ Geologica Italiana 112, 1037–1057. Roma. Lowenstein, T. and Hardie, L. (1985). Criteria for the recognition of salt-pan evaporites. Sedimentology 32, 5, 627–644. Me´ndez V., Zanettini, J.C. and Zappettini, E.O. (1995). Geologı´a y metaloge´nesis del Oro´geno Andino Central. Direccio´n Nacional del Servicio Geolo´gico, Anales 23, 1–190. Buenos Aires. Methol, E.J. and Sister, R.G. (1949). Informe sobre los aluviones aurı´feros del Departamento de San Sebastia´n entre Rı´o Gamma y Cabo Espı´ritu Santo, Tierra del Fuego. Direccio´n Nacional de Geologı´a y Minerı´a, Carpeta 132, unpublished report, 102 pp. Buenos Aires. Ortı´ Cabo, F.B. (1988). Introduccio´n a las formaciones evaporı´ticas de origen marino. Curso de postgrado, Universidad Nacional de Salta, 90 pp. Salta. Pereyra, F., Fauque´, L. and Gonza´lez Dı´az, E. (2002). Geomorfologı´a. In: Haller, M.J. (ed.), Geologı´a y Recursos Naturales de Santa Cruz. Relatorio del XV Congreso Geolo´gico Argentino: Parte I, Geologı´a. El Calafate, Argentina, 1, 21, 325–352. Pichetti, J. (1943). Descubrimiento de mercurio en los aluviones de los arroyos del Manzano y Milla Michico´. Departamento Minas, Neuque´n. SEGEMAR, unpublished report. Buenos Aires. Popper, J. (1887). Exploracio´n de la Tierra del Fuego. Conferencia. Boletı´n del Instituto Geogra´fico Argentino 8. Buenos Aires. Prez, H. (1985). Proyecto Punta Norte-Punta Du´ngenes, Santa Cruz. Secretarı´a de Minerı´a, Direccio´n Nacional de Minerı´a y Geologı´a, Centro de Exploracio´n Patagonia Sur, unpublished report. Buenos Aires.

Prez, H. and Parisi, C. (1989). Informe Preliminar de campan˜a Aluviones Rı´o Percey. SEGEMAR, Delegacio´n Comodoro Rivadavia, unpublished report. Buenos Aires. Re´, N. and Brodtkorb, A. (1960). Los depo´sitos salinos del Bajo del Gualicho y de la Penı´nsula de Valde´z, Provincias de Rı´o Negro y Chubut. I Jornadas Geolo´gicas Argentinas, Actas 3, 307–328. Buenos Aires. Sabio, D. (1987). Sulfatera Florentino Ameghino, Departamento Deseado, provincia de Santa Cruz. Sulfargentina Samic, unpublished report, 43 pp. Buenos Aires. Scha¨bitz, F. and Liebricht, H. (1998). Landscape and climate development in the south eastern part of the ¨ rid Diagonal’’ during the last 13,000 years. BamberA ger Geographische Schriften 15, S, 371–388. Bamberg. Schalamuk, I., Del Blanco, M., Marchionni, D. et al. (1999). Salinas y sulfateras de la regio´n Pampeana. Buenos Aires y La Pampa. In: Zappettini, E. (ed.), Recursos Minerales de la Repu´blica Argentina, Instituto de Geologı´a y Recursos Minerales, SEGEMAR 35, 1947–1962. Buenos Aires. Stoll, W. (1957). Geologı´a y depo´sitos minerales de Andacollo, provincia del Neuque´n. Direccio´n de Minerı´a, Boletı´n 6. Buenos Aires. Teruggi, M.E., Chaar, E., Remiro, J.R. and Limousin, T.A. (1959). Las arenas de la costa de la provincia de Buenos Aires entre Cabo San Antonio y Bahı´a Blanca. L.E.M.I.T. 77, Serie II. La Plata. Teruggi, M.E., Etchichury, M.C. and Remiro, J.R. (1964). Las arenas de la costa de la provincia de Buenos Aires entre Bahı´a Blanca y Rı´o Negro. L.E.M.I.T. 81, Serie II, 88 pp. La Plata. Trebino, G. (1987). Geomorfologı´a y evolucio´n de la costa en los alrededores del pueblo de San Blas, provincia de Buenos Aires. Asociacio´n Geolo´gica Argentina, Revista 42, 1–2, 9–22. Buenos Aires. Valvano, J. (1949). Informacio´n sobre los depo´sitos aurı´feros de la zona de Tecka, Territorio Nacional del Chubut. Direccio´n Nacional de Geologı´a y Minerı´a, Carpeta 280, unpublished report. Buenos Aires. Viera, R.L.M. (2005). Hoja Geolo´gica 4372-I/II, Esquel. Capı´tulo Recursos Minerales. SEGEMAR, unpublished report. Buenos Aires. Zambrano, J. (1972). Comarca de la cuenca Creta´cica del Colorado. Geologı´a Regional Argentina, Academia Nacional de Ciencias 2, 1033–1070. Co´rdoba, Argentina.

24 Late Cenozoic Geohydrology of Extra-Andean Patagonia, Argentina Mario A. Herna´ndez1, Nilda Gonza´lez1 and Lisandro Herna´ndez2 1

2

Ca´tedra de Hidrogeologı´a, Universidad Nacional de La Plata, 1900 La Plata Ca´tedra de Fundamentos de Geologı´a, Universidad Nacional de La Plata, 1900 La Plata

Ocean and its continental boundary with Chile, with a total area of approximately 700,000 km2, roughly the size of Texas, or France and England together. These huge dimensions are highly contrasted by the scarcity and the poor quality of the available geological information. In particular the hydrogeological data of low and irregular density and basically based on the exploitation of underground resources instead of on scientific research. Thus, the most recent advances in its knowledge are due to the oil and gas and mining industries. The regional geology and stratigraphy studies are nowadays largely supported by the work of the Argentine Geological Survey (SEGEMAR), national universities and research centers, although in the first half of the twentieth century, the information was mostly provided by pioneer oil and coal explorers. It is then understood that in the field of hydrogeology, the information is dispersed, with different levels of scale and resolution. This situation undoubtedly complicates the efforts of synthesis like the present one, forcing the use of case studies and spot examples as a means to approach comprehension.

1. Introduction Among the relevant characteristics of Argentine extraAndean Patagonia, its arid to semiarid present climate should be mentioned. The scarcity of hydrological resources is one of the most significant consequences of these climates, particularly when compared with the Late Cenozoic paleoclimate of this region. Contrarily to the situation along the Andean margin, where superficial and underground water is frequent, mostly related to the dominant snow re´gime, such lack of water resources in extra-Andean Patagonia has historically been a strong limitation to growth and development. It is a territory crossed by allochtonous streams with Andean sources, such as the Neuque´n, Limay, Negro, Colorado, Chubut, Senguerr, Deseado, Santa Cruz, Coyle and Gallegos rivers. In this territory, when a location was away from these rivers, all intents of economic enterprise were forced to look for an underground water supply to make them viable. The Native American itineraries across Patagonia were necessarily related to the distribution of springs or underground water outcrops, which would serve as milestones to travel and migration along large distances away from the streams. Even during the historical trip of Hernando de Magallanes, the first reference of the Spanish conquest of Patagonia, underground waters were already mentioned. Around March 1520, his expedition made a stopover at Puerto San Julia´n, where they were able to get a good water supply from local springs. These springs would never be found again, though the site was visited by Garcı´a Jofre´ de Loaisa in 1526, the legendary Francis Drake in 1577, Toma´s Cavendish in 1591 and the Dutch explorer Narborough in 1670, among others. Most likely, these expeditions disembarked in winter or in the late fall, when these springs usually cease their flow (Herna´ndez, 2000). This chapter will focus on the part played by Late Cenozoic rocks and sediments of continental extraAndean Patagonia in the geohydrological processes. They act not only as carriers of usable underground water, but mainly as participant in recharge mechanisms to other formations of older age as well as support for highly fragile ecosystems, such as the regional wetlands known as ‘‘mallines’’. The geographical framework of this Argentine territory is given between the Andean Cordillera and its adjacent piedmont lands, the Rı´o Colorado, the Atlantic

2. Methodology The general methodological concepts are basically deductive, making use of evidence provided by indicators to define the most accurate hypothesis. Consequently, it is a model in which the hypothesis that explains most of the evidence is the more plausible. From a particular point of view, the work was oriented to identify the geohydrological conditions to ascertain the dominant regional characteristics. It comprised critical reviewing of existing information, pointing out the hydrolithological properties of the geological materials and the outcrop characteristics, in a geomorphological approach. It is herein intended to define their capacity of receiving, storing and transmitting water, as the underground circuit of the hydrological cycle. Different physical conditions of the geohydrological system have thus been determined and, in this Late Cenozoic rocks and sediments, they play a significant role. As stated before, the territorial dimensions in relation to information density forced to use representative case studies for the principal systems. However, the difficulty in extrapolating this information to the regional level is herewith acknowledged for most areas.  2008 ELSEVIER B.V. ALL RIGHTS RESERVED

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3. Limiting Geohydrological Factors The limiting factors are, first of all, those related to the regional characteristics, which affect the availability, quantity and quality of underground water resources.

3.1. Climate Two main characteristics are climatically highly significant: latitude, related to solar insulation and mean annual temperature, and the presence of the Andean Cordillera, with its influence on the precipitation re´gime. The against-trade winds coming from the Pacific Ocean ascend on reaching the Andes, while cooling

adiabatically and generating high precipitation on the western slopes, whereas the eastern side of the Andes is in the rain shadow, which generates true desert conditions. Exceptions are found in the lower portions of the ranges, mostly lacustrine basins of glacial origin. These depressions are gateways for moisture carrying winds, extending eastward the location of the 300 mm/yr isohyet, enabling the penetration of the Valdivian rain forest biotope. While on Cordilleran summits, precipitation may reach up to 3000 mm/yr, in the extra-Andean regions it is reduced to 150 and 300 mm/yr, with a slight increase along certain areas of the Atlantic coast and in the southernmost sector through by direct oceanic influence (Fig. 1).

References

Fig. 1. Rainfall and potential evapotranspiration map.

Late Cenozoic Geohydrology of Extra-Andean Patagonia Snowfall is very important in the eastern ranges of the Cordilleras and the adjacent piedmont area, as well as on the tablelands, due to its influence on the hydrological and geohydrological re´gime. When comparing the mean annual precipitation with the average amount of mean annual potential evapotranspiration (600–900 mm/yr), a hydrological deficit of over 500–600 mm/yr becomes apparent. For this reason, in case the precipitation would be the exclusive source, aquifer recharge would be nonexistent, a condition which is not real, as there are other mechanisms which will be presented below. The climate is cold-wet in the Andean zone, grading into temperate and cold-temperate eastward, from subhumid in the Andean slopes to arid and semiarid in most of the territory. The mean annual temperatures range from 12–15C in northern Patagonia to 6–11C in the southern and western portions of the region, with frequent drops below 0C in winter.

3.2. Vegetation Within the Patagonian Phytogeographical Province, as defined by Soriano (in Paruelo et al., 1992), four districts make up the continental environment: Sub-Andean, Central, Western and Golfo San Jorge (Paruelo et al., 1992). These districts are shown in Fig. 1. The first one is characterized by a high cover, grassy steppe, extending along a narrow fringe following the Andean Cordillera between Futaleufu´ and Lago Buenos Aires and south of Lago Belgrano (though with interruptions), with its southern end south of Rı´o Coyle. The Central district is the most extended in this region, represented by a low shrubby steppe or a very low cover, following the areas with lower precipitation. North of Lago Buenos Aires, the Western district follows as a very open, 50% coverage, shrubby steppe. The Golfo San Jorge district surrounds the gulf of that name, from Cabo Raso to Bahı´a La´ngara, with dominant grasslands in the higher plains, locally known as ‘‘pampas’’ and shrubs on the slopes to the ocean and in the tributary canyons. The ecological characteristic of relevant hydrogeological significance is the xerophytic nature of all plants, in which the stomas are replaced by certain specializations such as aphilia, thorns, hairs or coriaceous derma. This morphology reduces to a minimum the release of water vapor to the atmosphere, enabling infiltration, even with very low rain events. An exception to this are the wetlands, locally known as ‘‘mallines’’, with a Juncaceae, Cyperaceae and Gramineae vegetation, associated with valley bottoms and/or ponds with underground discharge and with generally clayey to silty–clayey channel sediments (Paruelo et al., 1992). These are microhabitats used as stopovers for migratory birds, residence of other aquatic birds and drinking sites for the autochtonous fauna and sheep and cattle.

3.3. Hydrology The rivers crossing extra-Andean Patagonia are of allochtonous character, with their heads in the Andean

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Cordillera and snow or rain/snow re´gime, the first one with a main flood peak generated by spring melt and the latter characterized by winter rains. The most important water courses are the Colorado (mean discharge of 148 m3/sec), Neuque´n and Limay (forming the Rı´o Negro, 858 m3/sec), Chubut (48 m3/sec), Senguerr (48.6 m3/sec), Deseado (transitory), Chico (transitory), Santa Cruz (790 m3/sec), Coyle (also known as Coig, 5 m3/sec) and Gallegos (34 m3/sec) rivers. Most of these streams are permanent and all of them show a discharge deficit (with respect to the underground flow), which is very important with respect to underground water reserve recharge (Lerner et al., 1990).

3.4. Geomorphology Though there is a vast diversity of landforms and scales in such an enormous territory, from the hydrogeological (or hydrogeomorphological) point of view the positive ones must be noted, such as the eastern Cordilleran slopes (known as ‘‘Precordillera’’), the high northern Patagonian and Deseado tablelands, the frequent hilly ranges (Sierras de Huantriaco´, Auca Mahuida, Lipetre´n, Pire´ Mahuida, Lonco Trapial, Taquetre´n, Olte, Canquel, Chaira, San Bernardo, Can˜ada Grande, Cordo´n Alto, etc.), high erosion remnants, the tablelands and their terraced levels, pediments, volcanic structures and cones, dune fields and the glacial landscape. The negative landforms are also important, such as stream valleys and associated features, glacial valleys, large lakes and ponds, endorheic basins (of structural, eolian or volcanic origin) and the ‘‘can˜adones’’ or ‘‘uadis’’, ephemeral valleys, usually dissecting alluvial plains. The role of these landforms in the terrestrial phases of the hydrological cycle will be described later, when discussing the geohydrological phenomena and the system configuration.

3.5. Soils Aridisols are dominant, particularly in the two eastern thirds of the region, with the presence of Entisols in many of the fluvial valleys and ‘‘can˜adones’’. Inceptisols and Mollisols are present in the western transitional areas and Alfisols in the northern sector of Neuque´n and Rı´o Negro provinces. They are mostly high permeability soils with low specific retention, a circumstance that should be taken into account when analyzing the recharge process.

3.6. Geology The geological aspects, such as lithology, stratigraphy, structure and geomorphology, are considered elsewhere in this volume. It is preferred to use a mainly hydrogeological approach to differentiate the two main media in which underground water circulates (porous and fissured) and their relationship with the material capacity to receive, store and transmit water (hydrolithology).

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The porous media (primary porosity) is mostly represented by Late Tertiary and Quaternary deposits, among them the widely distributed marine deposits in the eastern sector, terraced aggradational levels, glacial and periglacial deposits, present and past alluvial accumulations (paleochannels), and colluvial, lacustrine, fluviolacustrine and eolian sediments. Interconnected joint systems, columnar jointing and hollows in lava flows, open bedding planes in tuffs, schistosity planes and faults are the features characterizing the fissured media (secondary porosity) of this region, with a reduced share of spaces generated by solution (karst and pseudo-karst). With respect to their hydrolithological specifically relative permeability characteristics, the following types are identified: (a) aquifer – materials that easily receive, store and transmit water; (b) aquiclude – those that receive and store water, but do not transmit it; (c) aquitard – those that receive and store water but transmit it with difficulty and only under certain conditions; and (d) aquifuge – those that do not receive, store or transmit water. The fissured media is essentially aquifer, differentiating rocks according to their higher or lower permeability. In this context, the Late Cenozoic deposits, although they are of wide extent in extra-Andean Patagonia (up to 600,000 km2; Lapido and Pereyra, 1999), they lack a significant thickness, thus making their role in the geohydrological cycle mostly that of a water transmitting element rather than that of an aquifer.

4. Porous Media For the identification of those hydrogeological units acting like an aquifer, the geological units that fulfill this role have been grouped, independently of their stratigraphic assignation or nomenclature. The hydrogeological concepts of Struckmeier and Margat (1995) are used in this chapter, which means that the identification of hydrogeological units is mainly based on hydrolithological criteria. Under this denomination, units of varied genesis or different stratigraphy and age are often included or, inversely, different geohydrological characteristics are attributed to segments of the same formation. Under these premises, units of porous media (primary or intergranular) are briefly identified as active parts of the geohydrological systems described below. 4.1. Late Cenozoic Gravel Deposits The gravel and coarse sand deposits included in the ‘‘Patagonian gravels’’ are herein taken as a single unit, in spite of the recommendations of Panza (2002), who used geological and geomorphological concepts. This unit includes the terraced deposits of the tablelands, the

more recent fluvial terraces as well as the piedmont aggradation levels (Fidalgo and Riggi, 1970; Lapido and Pereyra, 1999). These deposits are very frequent in the western sector of the region. Although with some variations, all of these units possess very important hydrodynamic features and offer moderate to high permeability, a mostly subhorizontal position and high continuity in the field. Apart from the mentioned differences, including those related to grainsize, sorting and mineralogical composition, the consolidation and cementation degree is highly significant (sometimes lowering rock permeability by one order of magnitude), as is the anisotropy of the deposit with respect to water transference. These units are very common in extra-Andean Patagonia (Fidalgo and Riggi, 1970) and their participation in the geohydrological system is very important. In most cases, they are direct receptors of atmospheric recharge, are overlain by soils with very low specific retention or, eventually, they are not overlain by soil at all. These facts make them act as deep recharge generators toward underlying porous or fissured layers and providing springs or superficial sources along their basal stratigraphic contact with formations of lower permeability.

4.2. Glacigenic Deposits These units are recognized along the western and southwestern margin of the region, and essentially composed of till, proglacial and glaciofluvial sediments of the Andean Cordillera and the piedmont areas, from Neuque´n and Rı´o Negro (Gonza´lez Dı´az and Malagnino, 1984) to the southernmost end of Santa Cruz and Tierra del Fuego (Rabassa and Clapperton, 1990; Pereyra et al., 2002; Rabassa and Coronato, 2002;). These sediments are basically aquifers, with a very important lateral relationship with glaciers and streams of glacial origin which, due to their nature, provide huge volumes of water to the neighboring aquifer bodies. That is the case of the Santa Cruz, Coyle (Coig) and Gallegos rivers. The glacigenic deposits are also usually watertransmitting units to the aquifers nearby, both in porous and in fissured media.

4.3. Aeolian Deposits These units are widely distributed in the region with many morphological types such as sand blankets, barkhans, longitudinal dunes, coastal dunes, cover sand bodies on fluvial terraces and so on. Their occurrence has been favored by the arid climatic conditions dominating during most of the Quaternary and the availability of mobile material of fluvial, pyroclastic and glaciofluvial origin (Lapido and Pereyra, 1999). They are very little known from a geological point of view, particularly due to their physical instability. They have strong permeability due to the high degree of sand sorting, even though silt and clay dunes (‘‘lunettes’’) or volcanic ash dunes may also be present. They do not have a large thickness, with perhaps the exception of a few sites, such as Ricardo

Late Cenozoic Geohydrology of Extra-Andean Patagonia Rojas at Penı´nsula Valde´s. Their recharge is analyzed as a mechanism of fast infiltration in Section 6.2. 4.4. Modern Alluvial Sediments These deposits are located in the present alluvial plains of the streams of the region, even in those of an ephemeral or transitory nature. They have a very high hydrogeological significance since they host enormous amounts of subbed water. These deposits show a large variety of grainsize types, from boulders and coarse gravels to silts and clays, but predominantly medium to fine gravels and sands. The hydrolithological behavior is naturally aquifer, generally overlying sediments of the same kind or aquiclude, aquitard or aquifuge, in the latter case referring to rocks without secondary porosity. Their development is highly variable both spatially, several kilometers wide in the lower Colorado, lower Chubut and middle Deseado rivers, down to a few meters width and thickness in narrow valley canyons. The exploitation of the subbed river aquifer is of great importance in the northern areas (valleys of the Negro and Colorado rivers) both for drinking water for rural population, as it is in the Chubut and Santa Cruz rivers for secondary recovery in oil production (Senguerr and Deseado rivers). 4.5. Mass-Movement Deposits, Colluvial Sediments and Slumping at the Edge of Volcanic Plateaus These sediments and rocks are of an obvious varied origin. However, they are included in this section due to their similar hydrogeological characteristics: lack of sorting, large clast size, high permeability, reduced thickness and geomorphological position. They are located at the edge of structural plains, mountain ranges and the eastern Cordilleran slopes, allowing a fast concentration recharge mechanism (see Section 6.2). 4.6. Coastal Marine Deposits These deposits are of very little regional significance, restricted to a 10 km fringe parallel to the present coastline (Lapido and Pereyra, 1999). These units are sandy string-like alignments, gravel accumulations, terraced ridges and shelly accumulations, whose larger influence on local hydrogeology is to form real lower salinity water lenses, bounded by fresh/salty water interphase toward both the sea and the arid continent. The latter salty waters are the product of the hydrochemical evolution in the zone of regional discharge. They form high permeability aquifers and their recharge is produced by the fast infiltration mechanism described in Section 6.2. 5. Fissured Media This condition is achieved mostly by joints affecting different rock types of aquifer significance such as the extensive Jurassic volcanic and pyroclastic (ignimbrites and tuffs) rocks, open bedding planes (tuffs), columnar

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joints in basalts of varied age and, to a lesser degree, solution cracks and channels in calcareous and gypsum rocks (Herna´ndez and Gonza´lez, 2003). The fissured Pre-Cenozoic rocks are also indirectly important in this aspect, because they form the basement of several geohydrological systems. The Cenozoic basalts are directly related because of their part in the recharge processes through their cracks and joints. These basalts are occupying, according to Lapido and Pereyra (1999), more than 120,000 km2 along extra-Andean Patagonia, with larger surfaces in Neuque´n and Santa Cruz provinces. Alkaline, olivine basalts, basandesites and leucocitic basalts are dominant. The most relevant hydrogeological unit is the La Angelita Basalt and its corresponding units, Laguna del Guadal, Cerro Piedra and Laguna Barrosa basalts, all of them located in Santa Cruz (Panza and Franchi, 2002). They are important both for the extension of their flows as well as for their subhorizontal position and the high density of columnar jointing, with up to 5 cm separation at Pampa de Cerro Rubio. The cracks and joints are, as discussed below, a basic element in the process of water transference toward the underlying, porous media aquifer beds (gravel units, Patagonia Formation sedimentary rocks) or fissured media (Bahı´a Laura Group, Jurassic) participating in the fast infiltration and delayed recharge processes. Less frequent and important is the secondary porosity due to hollows in the lava flows, particularly because they are not always interconnected by water-bearing channels. The importance of these jointed lava flows is highlighted since they contribute to the definition of three of the models presented in Section 6.1.

6. The Geohydrological Systems The distribution of the geohydrological systems in Patagonia presents a wide variety of elements and different acting mechanisms in the hydrodynamic cycle of Recharge-Circulation-Discharge. For this reason, case studies must be used. Nevertheless, it seems appropriate to start with a general characterization based upon the conditions sketched in previous sections, together with the relationship between units, to present a physical scheme which may explain the geohydrological phenomena. The Non-Saturated Zone (NSZ) in a strict sense (its definition is usually offered for porous media) has a varied development, from more than 70 m in the tablelands between the Colorado and Neuque´n rivers, lacking a capillary zone, down to a few meters nearby the waterloosing streams and the southern Santa Cruz lower coastal plains, and even zero thickness in the wetlands or ‘‘mallines’’. Logically, the thinner the NSZ is, the more favored the aquifer recharge processes are, but there are lesser opportunities for natural mitigation of pollution. From a physical point of view, practically all materials mentioned in Section 4 are part of the NSZ. A remarkable condition is the frozen state of the upper portion of the NSZ during winter in many areas of the southern and southwestern parts of the region, particularly because of the delayed recharge processes.

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In many sites, the porous media NSZ underlies a fissured media cover, particularly in the case of the more recent basalt flows (La Angelita, Lipetre´n, Trayen Niyeu and other basaltic units) affected by columnar jointing or high cavity density. It must be explained that the NSZ thus described does not extend to the topographic surface itself, but starts instead at the base of the flow. Concerning the Saturated Zone (SZ) in geohydrological schemes, the phreatic term of the systems is the most relevant for this study, since it is better connected, directly or indirectly, to the Pleistocene rocks and sediments. The semiconfined units, and even more so the confined ones, are generally lying in Pre-Quaternary terrains. The physical configuration of different models of systems identified in extra-Andean Patagonia is presented below, with representative case studies exposed in Section 7. Naturally, the occurrence of Late Cenozoic rocks or sedimentary deposits is closely related to the uppermost part of the schemes.

6.1. Geohydrological System Models Different geohydrological models are herein proposed to understand the nature and distribution of groundwater resources. A1 Model: fissured rocks (columnar jointed basalts) overlying aquifuge or fissured rocks. The aquifer unit is included in the gravel beds (sometimes only transitorily) and in bedrock fissures. A2 Model: fissured rocks (columnar jointed basalts) directly overlying aquifuge or fissured rocks. In this case, the aquifer unit is the underlying fissured rock. A3 Model: fissured rocks (columnar jointed basalts) overlying thick clastic beds. The aquifer is in the porous media. B1 Model: gravel layers overlying fissured rocks. The aquifer bed may be located in the gravels and/or in the rocks with secondary porosity. B2 Model: gravel layers overlying aquifuge rocks. The aquifer is obviously located in the gravel beds, generally with transitory re´gime (seasonal in the Cordilleran areas). B3 Model: gravel layers overlying clastic deposits of medium to large thickness. The aquifer condition is generally located in both units, except where the gravelly cover is very thin or when the local climate imposes a deep phreatic surface. C1 Model: alluvial sediments in permanent, active stream valleys, with well-developed flood plains, overlying fissured or aquifuge bedrock. In these cases, the phreatic aquifer is located in the fluvial sediments, due to permanent nourishment from the feeding streams and depending upon the bedrock behavior, even in their fissures. C2 Model: alluvial sediments in permanent, active stream valleys, with well-developed flood plains, overlying clastic sediments. In these cases, a hydraulic continuum is observed in the phreatic layer, including both types of materials.

D1 Model: alluvial sediments in inactive streams, overlying aquifuge or fissured rocks. These are aquifiers located in the alluvial beds, of a transitory character, and in fissured rocks with a very deep phreatic surface. D2 Model: alluvial sediments in inactive streams, overlying porous deposits. The aquifer beds are generally contained in the underlying sediments, and in the alluvial deposits with a climate-regulated, transitory re´gime. E1 Model: very thick, glacigenic deposits. The phreatic aquifer is of a non permanent, perennial re´gime, and basically regional flow, generating deep recharge. E2 Model: glacigenic sediments overlying aquifuge of fissured rocks. The glacigenic deposits permanently contain the phreatic aquifer and are transmitting the recharge into the fissured media. F1 Model: eolian formations (dunes or sand blankets) overlying gravel beds which are lying on sandy or pelitic clastic materials, or fissured rocks. These elements may or may not integrate a hydraulic continuum. It is possible that the dune formation would be only NSZ or become a transitory re´gime. F2 Model: eolian formations (dunes) overlying sandy or pelitic, porous materials. The main aquifer is generally in the dune formation. G Model: heterogeneous colluvial deposits, frothy lava tablelands (‘‘escoriales’’), tillites or proximal apex of debris cones, overlying aquifuge or fissured rocks. According to their stratigraphic position, they may be only water transference or aquifer container elements. H Model: sandy, gravelly or shelly ridges of the present coast, transmitting and containing lens-like aquifers of water relatively less saline than the surroundings. This systematic classification, based upon local information and trying to achieve a regional overview, is based on physical systems. But, because of the restricted hydrological analysis, their behavior within the geohydrological circuit may be largely significant whenever the role of Late Cenozoic materials is highly relevant.

6.2. Hydrodynamics As it has been stated above, the role of the Late Cenozoic rocks and sedimentary deposits is perhaps more valuable because of their participation in the Recharge-Circulation-Discharge cycle than for being aquifers of strategic or utility interest. Within this cycle, the more relevant episodes are those related to the recharge and the discharge, since circulation is frequently limited by the reduced extent of the deposits and their peculiar hydrogeological characteristic. Concerning the recharge, it should be noted that whatever has been the methodology for establishing the hydrological budget, it clearly emerges that there would be no possibility of water surplus charging into the underground, because the results are clearly showing a permanent deficit. It has already been explained that the order of magnitude of the hydrological deficit is of 500–600 mm/yr (see Section 3.1). But the presence of underground

Late Cenozoic Geohydrology of Extra-Andean Patagonia water reserves is confirmed, both in terms of quality and quantity, sometimes to unexpected levels. There are particular mechanisms (Herna´ndez et al., 2002, 2004) that may explain this apparent contradiction between climate and recharge, a synthesis of which is presented below, following a plausibility analysis, as explained above in the methodological presentation. (a) Fast infiltration. This process takes place thanks to the relatively high permeability (sandy, gravelly sediments and jointed lavas) and the low specific retention capacity of the local soils. The largest portion of the net rainfall becomes gravity water, with up to 33.7% of infiltration of 172 mm/yr net rainfall for northern-central Santa Cruz and 58% of only >5 mm effective rain according to Herna´ndez et al. (2002). It was also possible to establish the lack of or poor development of a capillary fringe, mainly because of supercapillary porosity. Recharge in dune formations belongs to this type. (b) Presence of highly specialized vegetation lacking stomas. These are succulent plants with different adaptations, such as thorns and hairs, with coriaceous derma or ephemeral types (Paruelo et al., 1992). Among the most frequent species, Azorella trifurcata, Azorella nucamentacea (‘‘piedra len˜a’’ or ‘‘llareta’’), Trevoa patagonica (‘‘malaspina’’), Chilliotrichum diffussum (‘‘mata negra’’), Berberis buxifolia (‘‘calafate’’), Accantholippia sp. (‘‘tomillo’’), Chuquiraga sp. (‘‘un˜a de gato’’), Nassauvia ulicima (‘‘colapiche’’), Poa linguaris (‘‘coiro´n’’) and Stipa humilis (‘‘coiro´n’’) may be cited. Only the vegetation established in the wetlands (‘‘mallines’’), such as Poa pratensis (‘‘pasto mallı´n’’), Trifolium repens (‘‘tre´bol blanco’’), Taraxacum officinale (‘‘achicoria’’), among other species, does not need these specializations. The specialized vegetation together with fast infiltration eliminates or minimizes consumptive losses. (c) Delayed recharge. This is due to snow accumulation, frost or water freezing in the soil, particularly in the tablelands and sub-Andean regions, which melt during spring or on very sunny days. The resulting volumes of water are important, because the supply is almost instantaneous. (d) Fast concentration: Where sediments are exposed close to the mountain ranges in the Andean piedmont or in erosive landscape along the edge of the ‘‘pampas’’, superficial runoff is rapidly incorporated in the subsoil when flowing in a concentrated way over a steeper relief of a mostly aquifuge character. (e) Supply by stream loss: Rivers, creeks and even irrigation canals in extra-Andean Patagonia (valleys of the Colorado, Negro and Chubut rivers; CORFO irrigation systems, Rı´o Colorado) show this behavior, providing important discharge to the river bed within the alluvial plain, and also toward the fluvial terraces. For example, the subbed flow of the Senguerr and Deseado streams are under strong groundwater exploitation for secondary oil recovery operations.

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It is necessary to note that the aforementioned mechanisms are generally convergent, as it happens in wide sectors of the region, such as the central zone of Chubut and Santa Cruz provinces. Because it corresponds to recent recharge, it should be mentioned that apart from quantity, these are low salinity waters, frequently less than 2 g/l, thus differentiating them from the surrounding water resources, where underground waters are salty or saline. The discharge of these systems takes place toward the ocean, or inland in endorheic depressions, generally occupied by salt lakes and lacustrine fine sediments. The underground supply, particularly in salt and dry lakes whose bed is below sea level, frequently merges with superficial runoff during high precipitation events to maintain the brines in salt lakes and other depressions. The high evaporation rate makes that, to a large extent, these bodies have a transitory re´gime. A very frequent, particular way of underground discharge is by means of springs, very important to the regional economy, both for cattle and sometimes even for domestic use or as supply for the wetlands (‘‘mallines’’) and grasslands fed on by cattle, sheep or the local fauna. Although they occur in Tertiary formations, the springs that supplied the city of Comodoro Rivadavia (named as Manantiales Bher and Manantial Rosales) or those that still do so in Puerto Deseado and Puerto San Julia´n should be noted. Most of the Patagonian ‘‘mallines’’ receives their supply from springs, thanks to which they maintain their ecological role hosting an important local and migratory bird fauna. Springs of varied genesis are recognized (Herna´ndez, 2000; Herna´ndez et al., 2004): (a) structural, (b) located at thalwegs, (c) of stratigraphic nature (the most frequent ones), (d) placed in fissures or in lava flows and finally (e) a few of karstic origin (northwestern Neuque´n and central Santa Cruz provinces). According to their re´gime, they are in general either of transitory or perennial, subvariable or variable nature. It is common that many springs which nourish the wetlands (‘‘mallines’’) or small lakes are not observed directly because they are sub-aqueous, thus being identified only during extreme drought conditions. The circulation of underground water, as the time between recharge and discharge processes, is constrained by the extent of the aquifers. In these particular cases, as it has been noted before, circulation corresponds to relatively short distances due to interfering local discharge (springs, discharge in wetlands, endorheic basins and micro-basins), thus implying a predominant active or local flow over passive or regional conditions. The more extensive circulation cases are related instead to older aquifers, because the Late Cenozoic ones are of a smaller, thinner or shorter type.

7. Case Studies Though it should be noted that it is inconvenient to assume regional behavior in such an extensive territory while lacking appropriate hydrogeological information, a series

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References –

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Fig. 2. Location of study cases. of case studies is presented below, with the aim of demonstrating representative examples in a relatively wide area. The location of the basins or sites cited in this section is presented in Fig. 2 and the graphic expression of system configuration in Fig. 3a, b and c.

7.1. Cerro Rubio–Cerro Vanguardia Sector (Santa Cruz Province) This basin, located in the central part of Santa Cruz Province, is very significant (Fig. 2) because many of the models presented in Section 6.1 (A1, A2, A3, B1, B3) and most of the recharge mechanisms are found here (Herna´ndez, 2000; Herna´ndez et al., 2002, 2004; Herna´ndez and Gonza´lez, 2003).

Starting with the La Angelita Basalt flows, which are highly jointed and of subhorizontal position, they encompass models A1 to A3. When these flows directly overlie the gravel cover (La Avenida Formation or Pampa de la Compan˜´ıa Formation), they act as quasi-instantaneous transmitters of the reduced rainfall supply to the gravels. Together they form an aquifer of medium thickness (3–7 m), which discharges through stratigraphic springs in the local channels and canyons. If the gravelly cover overlies fissured rocks, such as the Bahı´a Laura Group (Jurassic ignimbrites and tuffs), it partly discharges into this aquifer, which is of regional significance. If it does instead overlie aquifuge rocks (such as the Bajo Pobre Basalt), it discharges directly in canyons or wetlands. In both cases, it corresponds to the

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References Fissured basalt

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References Fissured basalt

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References Clean gravel

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Fig. 3. Case studies (a) Cerro Rubio–Cerro Vanguardia sector. (b) Auca Mahuida basin. (c) Lower valley of Rı´o Chubut.

A1 model. The mechanism of fast infiltration is dominant, though delayed discharge occurs as well. It may happen as well that the basalt covers the fissured Jurassic rocks directly, thus being responsible for their recharge (A2 model), or if it overlies the marine sedimentary rocks of the Patagonian Formation in the eastern sector of this region, it has the identical function of recharging into the porous media (A3 model). In those cases where the lavas are absent, the B1 and B3 models may be observed. The first one corresponds to a moderate thickness (3 a 5 m) of the ‘‘Patagonian gravels’’ (‘‘Rodados Patago´nicos’’) on ignimbrites of the Chon Aike Formation (Jurassic), which has secondary porosity due to jointing, whereas the B3 model occurs on sedimentary deposits of the Patagonia Formation, which is a low yield and saline water aquifer.

The aquifer contained in the gravels is thin and contains little but relatively good quality water. In these systems we find several of the recharge mechanisms described in Section 6.2, such as fast infiltration, reduction of consumption losses by specialized vegetation and delayed recharge.

7.2. Lower Basin of the Rı´o Coyle (Santa Cruz Province) This case study is focused in the zone of Las Horquetas, close to the merging of the southern branch with the main Rı´o Coyle, in southern Santa Cruz Province (Fig. 2).

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It corresponds to Late Pleistocene sand and gravel deposits covering the youngest terrace of the Rı´o Coyle valley (Fasano et al., 2003), with a 6–7 m thickness, overlying a 26 m thick layer of green clays, and an even greater underlying thickness of silts, sandy silts and very fine sands of undifferentiated Tertiary age. This would correspond to a B3 model, but the clayey bed has an aquitard behavior and separates a transitory phreatic aquifer (mostly dry at the end of the summer) from another semiconfined one, both discharging into the alluvial plain of the Rı´o Coyle. This stream also drains into this landform as a looser stream, thus corresponding to a C2 model. 7.3. Lower Valley of the Rı´o Chubut at 28 de Julio (Chubut Province) This example is located in the uppermost part of the lower valley of the Rı´o Chubut, between the sites of 28 de Julio and La Angostura. The C1 model is recognized here since medium gravel, alluvial deposits are overlying Jurassic volcanic rocks of the Lonco Trapial Formation. The clastic cover is bearing low salinity phreatic water, supplied by the influent stream, thus making this the dominant recharge mechanism. Discharge is consumptive due to agricultural activities (fruit crops and implanted grasslands) while underground loss downstream into the valley (Herna´ndez et al., 1983) occurs as well. It is still unknown if the substratum is truly aquifuge or if it has secondary porosity.

7.4. Lower Valley of Rı´o Chubut at Gaiman – Rawson (Chubut Province) Downstream from the previous locality, the gravels of the alluvial plain are replaced by sandy to silty-sandy sediments, with a shallow phreatic aquifer supplied by the influent Rı´o Chubut, with Tertiary sediments at the base (Patagonia Formation and Sarmiento Group). The latter also forms the higher terrace of the valley and their redeposited materials, locally known as ‘‘terraza intermedia’’. Below the phreatic level, there is a semiconfined aquifer which it recharges, the finer facies of the Patagonia Formation or the tuffs of the Sarmiento Group acting as aquitard. It is a case of local recharge by direct input from the river, which was subjected to seasonal bank storage release before the regulation of the system by an upstream dam (Florentino Ameghino Dam) eliminated its effects. The Rı´o Chubut presents a ‘‘yazoo’’ type morphology within the valley (Herna´ndez et al., 1983). The physical configuration of the geohydrological systems clearly belongs to the C2 model. 7.5. Gastre–Gan Gan Basin (Chubut Province) This basin is localized in the Arroyo Sacanana valley, its tributary creeks and the neighboring tablelands, where

several models apply, which although having a reduced extent, they all merge to the supply of a large number of ‘‘mallines’’ on both sides of the Provincial Route N 4. The zone is bounded by the Pire´ Mahuida and Talagapa ranges to the north and the Rosada ranges to the south. These are gravel and sandy gravel deposits overlying aquifuge rocks, like the Permo-Triassic granites of the Lipetre´n Formation, corresponding to the B2 model, which is very thin (up to 3 m). Thinner, poorly sorted gravel beds are found in the ‘‘pampas’’ bounding the Sacanana valley, over pyroclastic rocks and jointed basalts of the Jurassic Can˜ado´n Asfalto Formation, with a B1 model, and terraced gravel beds that cover or are interbedded with modern, undifferentiated basalts on the lower ‘‘pampas’’ of Gastre and Gan Gan. These sediments do not have any aquifer value, but they are essential for the rapid collection of rainfall that flow along the bedrock contact or through its secondary pores. They merge with small alluvial and colluvial fans (G model) to form the supply that feeds the wetlands and salt lakes of Pampa de Gastre, like the Grande, del Molle and del Pito saline depressions. Though superficially restricted, eolian accumulation over granular materials (F1 model) and supply from ephemeral creeks to a poorly developed subbed river, sometimes releasing water into fissured bedrock (D1 model), is found as well. 7.6. Catriel Valley (Rı´o Negro and La Pampa Provinces) In this area (Fig. 2), two typical schemes can be found: the phreatic aquifer of the Rı´o Colorado valley and water storage in the terraced gravel deposits. The first case represents a typical condition of the Patagonian streams, extending from the river to the edge of the present alluvial plain. These are sands and gravels fed by river supply following a C2 model. The second one is similar to that presented in other examples: gravel beds that may be grouped as ‘‘Patagonian gravels’’ (Fidalgo and Rabassa, 1984), overlying the fluvial terraces and the clayey sediments of the Malargu¨e Group (Allen Formation), which act as aquiclude to the underlying confined aquifer (Neuque´n Group). These gravel deposits have a thickness of up to 10–12 m and the included phreatic aquifer has a mean saturated thickness of 5–6 m, with a flow direction toward the alluvial plain. The dominant recharge mechanisms (direct authochtonous) are those of fast infiltration and reduction of consumption loss due to xerophytic vegetation.

7.7. Zapala (Neuque´n Province) Carrica et al. (1997) described the hydrogeology of this zone, where an F1 model can be found, with a sequence starting with thin eolian deposits overlying an up to 1 m thick gravels and jointed, vesicular basalts (Zapala Basalt, Pliocene), a set that covers the main aquifer under exploitation, the sediments of the La Bardita Formation of Late Miocene age.

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The most important characteristics are that the eolian and gravel deposits and the lava flows are under unsaturated conditions, have high primary and secondary porosity and their role in the transmitting system is bringing the water to the first aquifer contained in the La Bardita Formation (gravels, coarse and fine sands and silts). As presented by these authors, the main recharge mechanism is fast infiltration supported by xerophytic vegetation that minimizes the evapotranspiration effects.

confining conditions toward the east and southeast, due to the aquiclude characteristics of the Malargu¨e Group (Late Cretaceous–Paleocene), continental and marine sedimentary rocks that overlie it. This aquifer is perhaps the most important in the extra-Andean region of Neuque´n Province, both for its high yield and for the low salinity of its waters. It is widely used for water supply to secondary recovery operations of oil and gas.

7.8. Southwestern Cordilleran Sector (Santa Cruz Province)

7.10. Can˜ado´n Esther–Can˜ado´n Quintar (Santa Cruz Province)

The hydrogeology of this area is poorly known, mostly because surface waters are easily available. Some of our current knowledge derives from hydroelectricity projects (Turazzini, 2002) and oil and gas activities, though the latter are frequently subject to confidentiality agreements. Glacial and glaciofluvial deposits of the upper basins of the Santa Cruz, Coyle and Gallegos rivers are part of the hydrological systems (E1 and E2 models), in which a phreatic aquifer is contained in glacigenic deposits of high permeability, with permanent recharge thanks to meltwater and supply by stream loss. Downstream, deeper, semiconfined (recharged by vertical infiltration) and confined (with allochtonous recharge coming from the upper portions of the basins) aquifers may be located. These deposits may be filling the valleys or are found in higher topographic positions, on the terraces or their slopes (La Leona). In the first case, it corresponds to heterogeneous anisotropic systems with recharge by fluvial influence and lateral supply from the Andean ranges and piedmont areas. In the second one, the anisotropy is more strongly indicated by the convergence of massmovement deposits, lacustrine clays, marine sediments, ancient alluvial beds and volcanic material (volcanic and pyroclastic rocks), where the phreatic surface has typical convex profiles of variable gradient.

This area, next to the city of Caleta Olivia, is taken as the prototype for a hydrogeological phenomenon in many of the ephemeral streams of the eastern portion of extraAndean Patagonia, with their sources in the tablelands and discharging into the ocean (Hidroar, 2003). Included in the D2 model, they offer the singular characteristic of receiving the discharge of the springs originated along the contact between the terraced gravel mantle (‘‘Rodados Patago´nicos’’) that cover the tablelands and the shelly, calcareous beds included in the Patagonia Formation (Tertiary). These stratigraphic springs outcrop along the dry valley slopes and their discharge (together with the episodic activation of superficial flow) provides the recharge of the underlying multiaquifer set (Grizinik and Fronza, 1996). Occasionally, the F1 model is present when dune or sandy bodies occur on the tablelands.

7.9. Auca Mahuida Basin (Neuque´n Province) This area has a variety of the identified systems for the extra-Andean territory: the A3 model applies to fissured Pleistocene basalts, covering gravel deposits (Gonza´lez Dı´az, 1978), which overlie Tertiary clastic sedimentary rocks; the A2 model corresponds to fissured volcanic rocks that overlie other units of similar characteristics; the F1 model is valid for sandy accumulations overlying basaltic flows with secondary porosity or overlying clastic deposits. There are also endorheic depressions and salt lakes that represent the active portion of shallower underground flow. But the most important hydrogeological role of these systems in this basin is their participation in the recharge of an important aquifer contained in the Neuque´n Group (Cretaceous) which, radiating from a virtual epicenter located in the Auca Mahuida volcano zone, achieves

8. Human Impact Until the second half of the twentieth century, the main human intervention in the natural, regional ecosystems was by sheep farming, with significant soil damage due to overgrazing and with sediment mobilization enabling deflation. Since 1940 and because of the oil production development in the Golfo San Jorge and Neuque´n basins, a significant impact on the underground water resources started to be noticeable, due to the usual practice of releasing saline water with salt concentrations higher than 90 g/l onto the fields. In later decades, the development of the Austral basin, the northward expansion in the Neuque´n basin, as well as in the Rı´o Colorado and Golfo San Jorge basins, this impact became more noted and increasingly worrying, including the hydrocarbon pollution of the underground stores. Only as recently as 1992, environmental protection laws concerning the oil and gas activities were passed at the national level. The exploration phase operations, particularly those related to geophysical studies, have introduced significant changes in the landscape, with frequent alterations of the superficial flow during the flood season, which although not usually beyond the alluvial plains of the presently active channels may reach an abundant size during exceptional meteorological events.

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More recently, and favored by an increase in the international price of precious minerals (mostly gold), an important mining activity has developed in the region, which may potentially introduce significant changes in shallow aquifer water quality due to pollution and a strong water demand. Nonmetallic and metallic mineral mining in open pits may generate, if the environmental aspects are not efficiently protected, important negative effects if part of the NSZ is eliminated. This zone acts as a natural buffer against pollution, because oxidation, precipitation and co-precipitation, complex formation and aerobic degradation processes take place in it. In many cases, the sand and gravel resources under exploitation are included in formations with aquifers of good quality or local recharge. In addition to the qualitative effects, there is a quantitative damage to the already scarce underground water resources, for the competition and subsequent conflict between several water uses (domestic, rural, urban, cattle and sheep raising, agricultural, mining, industrial, etc.) and the true impact on the wetland (‘‘mallines’’) sustainability, as supply is diminishing as a consequence of intensive (for this specific region) aquifer exploitation. Further discussion about environmental damage generated by wetland destruction on the local and regional ecosystems seems to be obvious. Though they are reduced in comparison with the physical size of the region, the fast expansion of urban areas, generally unplanned, should be mentioned, as it is the case of the cities of Neuque´n, Viedma, General Roca, Comodoro Rivadavia, Trelew, Esquel, Caleta Olivia or Rı´o Gallegos, as well as the inadequate agricultural land treatment in the upper and lower valley of the Rı´o Negro and the lower valley of the Rı´o Chubut, or sheep overgrazing at the Rı´o Genoa valley. The climatically imposed fragility of these territories results in a very significant environmental impact, as for example, the very slow, natural recovery of vegetation affected by mining or oil activities or the slowing down of the natural mitigation processes in oil-polluted soils and aquifers.

9. Conclusions It is clear that the present-day characteristics of the region with respect to hydrogeological resources are important limiting factors to its growth, development and life and environmental quality. Even though the hydrological budgets may numerically demonstrate the impossibility of natural infiltration processes, several mechanisms that explain the presence of high quality, underground water resources have been recognized. The role played by Late Cenozoic rocks and sediments in these processes is fundamental and, actually, it is much larger than the importance as aquifers themselves. The recharge mechanisms defined here are fast infiltration, xerophytic vegetation suppression of consumption losses, delayed recharge, rapid concentration and supply from feeder streams.

The physical settings of the different geohydrological systems have been defined on the basis of the hydrolithological characteristics of the Late Cenozoic (and underlying) geological materials. Due to the difficulties in extrapolating hydrogeological behavior to such a vast territory (approximately 700,000 km2), mainly because of scarcity of specific information, case studies have been presented for the Santa Cruz, Chubut, Rı´o Negro and Neuque´n provinces, each with differently acting geohydrological systems. The human influence in the region, limited only to sheep farming until the first half of the twentieth century, is increasing with the on-going expansion of mining, forestry, intensive agriculture, urban development and most importantly, both present day and future oil and gas exploration and exploitation activities. Then, its impact on underground water resources will become enormous and of serious, mostly unpredictable, environmental consequences.

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Late Cenozoic Geohydrology of Extra-Andean Patagonia Herna´ndez, M.A., Fidalgo, F. and Ruiz de Galarreta, V.A. (1983). Diagnosis geohidrolo´gica aplicada en el Valle Inferior del Rı´o Chubut. Revista Ciencia del Suelo 1, 2, 83–91. Buenos Aires. Herna´ndez, M.A., Gonza´lez, N. and Sa´nchez, R. (2002). Mecanismos de recarga de acuı´feros en regiones a´ridas. Cuenca del Rı´o Seco, Provincia de Santa Cruz, Argentina. XXXII International Association of Hydrology (IAH) Congress and VI Congreso ALHSUD, CD Rom edition. Mar del Plata. Herna´ndez, M.A., Gonza´lez, N., Trovatto, M.M. and Herna´ndez, L. (2004). Flujo regional y local en una regio´n a´rida (Patagonia Argentina Extraandina). Implicaciones ambientales. XXXIII International Association of Hydrology (IAH) Congress and VII Congr. ALHSUD, Actas, CD Rom Edition. Zacatecas (Me´xico). Hidroar S.A. (2003). Estudio hidrogeolo´gico ambiental en la zona de chacras. Ciudad de Caleta Olivia. Subsecretarı´a de Medio Ambiente, Provincia de Santa Cruz. Unpublished report. La Plata & Rı´o Gallegos. Lapido, O. and Pereyra, F.X. (1999). Cuaternario de la Patagonia extraandina. In: Geologı´a Argentina, SEGEMAR, Anales 29, 23, 704–709. Buenos Aires. Lerner, D.N., Issar, A.S. and Simmers, I. (1990). Groundwater recharge. International Association of Hydrology (IAH), 8. Verlag Heinz Heise, Hannover. Panza, J.L. (2002). La cubierta detrı´tica del Cenozoico superior. In: Haller, M.J. (ed.), Geologı´a y Recursos Naturales de Santa Cruz. XV Congreso Geolo´gico Argentino, Relatorio 259–284. Buenos Aires (ISBN 987-20190-0-2).

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Index

Andean orogenic phase, 278 Anisotropy of magnetic, 121 Argentine Patagonia, 3–4, 13, 36, 38, 73, 76, 205–22, 269–78, 399, 485–94 Arid Diagonal, 264, 363 Astrapothericulense Zone, 272 Bajos sin salida, 30, 102, 221 Biostratigraphic (units), 151, 192–3 Cabo Pen˜as, 234, 256, 262, 454 Calcareous microfossils, 4, 327–34, 391 Carbonatation cycles, 264 Carbonate, 17, 34, 62, 159, 165, 211, 219, 246, 249–50, 260, 262–5, 493 Casamayoran, 269 Cenozoic, Patagonia, 159, 189, 217 Cerro Ap Iwan, 257 Cerro Atukoyak, 259 Cerro Fitz Roy, 17–18, 60, 257 Cerro Hatcher, 257 Cerro Katterfeld, 257 Cerro Kensel, 256, 261 Cerro Krund, 259 Cerro San Lorenzo or Cochrane, 60, 186, 257 Cerro Tronador, 100, 106, 257 Chapadmalalan, 193–4, 269 Chasicoan, 270, 275–6 Chimen Aike, 256, 262 Chronomorphs, 276, 278 Cingulates, 270–1, 275–6 Climate, 2, 4, 7, 13, 20–2, 23–4, 30–1, 33–8, 41–2, 46, 48–50, 57, 63, 67, 86, 151–2, 157, 168, 173–4, 176, 178–81, 184–8, 191, 193, 195–6, 205, 219, 227, 235, 241–5, 247, 249–50, 255, 258, 263–4, 269–71, 278, 285–6, 289, 316–18, 329, 343, 351–63, 369, 372–3, 379, 387, 389, 393–400, 402, 405–6, 411–30, 441, 456–7, 461, 465, 468, 470, 489–90, 497–9, 502–3 Climate change, 2, 187–8, 241, 244, 317, 357, 373, 393–400, 405–6, 411, 418–19, 429 Climatic reconstructions, 188, 354–63, 369–77, 398–9, 406, 416, 420, 429 Coastal geology, 3, 227–37 Cold episode, 3, 163, 168, 191, 255–7, 259–62, 264 Colonization, 75, 85–6, 183–4, 189, 358, 401, 437, 441, 456–61, 462–3, 465, 469–72 Comarca Andina, 257 Condrichthyan, 269 Cryogenic landforms, 257 Cryogenic processes, 259, 263, 265 Cryogenic sediments, 257, 263 Cryogenic structures, 256, 260–1, 264 Cryohypergenesis, 261 Cryophilous vegetation, 262 Cryoplanation, 31, 34, 259, 263 Cryostratigraphy, 255, 260 Cryoturbation, 260–3, 265

Darwin Cordillera, 18, 33, 152, 167–8, 170, 173, 176, 187, 227, 257, 273 Dasypodid, 276 Dell, 460 Dendroclimatology, 415, 421–2 Depression of the lower permafrost limit, 258 Deseadan, 269, 275 Diatoms, 4, 17, 176, 245, 249, 327, 360, 362, 377, 383–91 Dry Andes, 258 Dry-land vegetation, 38–9, 43 Earthquakes, 74–5, 82–90, 100, 248 Eisrinde, 261 Ensenadan, 194, 269, 277 Environmental change, 3, 241–50, 394, 396 Epigenetic ice wedge, 259 Eretizontidae, 271, 273 E`tage Notohippidien, 272 Extra-Andean Patagonia, 4, 15, 17–19, 22, 30, 33, 38, 42, 46, 65, 85, 97, 101–5, 107, 164, 169, 207–8, 210–11, 217, 258, 343, 344–5, 347, 461, 463, 465, 470, 488, 497–508 Felsenmeer, 263 Fluvial proccess, 33–4, 205 Foraminifera Ostracoda Review, 327–34 Formicidae, Attini, 271, 274 Fossil insects, 4, 393–406 Freezing index, 258 Friasian, 270–1, 273, 275–6 Fuegian Andes, 18, 20, 23, 26–7, 33, 36, 42, 45, 77, 151, 154–5, 166, 173, 182–4, 187, 255, 257, 259 Garlands, 257 Geohydrology, 497–508 Glacial processes, 33, 35, 106–7, 258–9 Glaciations, 3, 7, 9, 15–17, 23, 33, 35, 42, 48, 51, 57, 62–3, 66–7, 87, 89, 104, 106, 132–3, 143, 151–96, 205, 207–17, 221–2, 241, 245, 249, 255–6, 257–9, 260–5, 271, 278, 331, 355–6, 358, 405, 427–8, 438–9, 471–2 Greatest Patagonian Glaciation, 213, 255 Gre`zes lite´es, 263 Groundwater, 4, 48, 88, 489–90, 492, 502–3 Guide taxon, 277 Hegetotherid notoungulate, 276 Heusser, C. J., 2, 7, 170, 181 Holdich, 256, 261 Holocene transgression, 232, 286–7, 375 Huayquerian, 271, 275, 276, 278 Hunter gatherers, 1, 4, 51, 437–73 Ice wedge casts, 190, 259, 260–4 Incision, 15, 168, 215, 217, 221–2 Krummholz, 258

512

Index

Lago Vintter, 259 Lakes, 1, 3–4, 8–9, 15–16, 22–3, 25, 27–31, 33–5, 38, 43–5, 47–9, 51, 80–1, 85, 87, 99, 121, 137–42, 143, 151, 158–9, 161, 164, 167, 169, 171–4, 177, 179–87, 190, 205, 232, 241, 242–50, 255, 257–9, 263, 315, 329, 356–7, 360–3, 373, 383–4, 386–9, 393–5, 397, 399, 400–2, 404, 416, 428, 456–7, 463, 465, 467–71, 487–93, 499, 503, 506, 507 Lake sediments, 105, 121, 137–9, 142–3, 183, 241, 243, 249, 383, 393 Landscape, 2–4, 14–15, 18–20, 25, 30, 31, 33–6, 40, 43–51, 57, 73, 168, 183, 188, 205, 207, 211–12, 216, 219, 236, 255, 343–4, 347, 351, 360–1, 375, 379, 414, 417–18, 437–73, 489–90, 499, 503, 507 Landslides, 33, 85, 86–7 Las Heras, 83, 256, 258, 261 Last Glacial Maximum, 15, 57, 157, 213, 232–3, 242, 261–2, 264, 356, 372, 402, 428 Late Cenozoic, 1–4, 14, 51, 57, 59, 66, 76, 87, 95–107, 121–44, 151–96, 205–22, 285–319, 327–34, 351, 369, 485–94, 497–508 Late Holocene, 100–1, 136–7, 191–2, 250, 360, 362–3, 373, 377, 378–9, 389–90, 427, 429, 438, 441, 448, 456, 461, 465–9, 472 Late Miocene, 1–2, 4, 66–7, 80, 95, 97, 98, 103, 104–7, 121, 124, 151, 155, 158–61, 163, 191, 193, 195, 209, 210–11, 217–18, 220–2, 269, 276, 278, 285, 289, 295, 327, 330–1, 343–8, 388, 506 Limnogeology, 241 Liquefaction, 86, 87–90 Lithostratigraphic (units), 271–2, 275 Ma, 17, 20, 58–9, 61–2, 103, 168, 193, 195, 221, 276, 343, 485 Machrauquenidae, 271 Malvinas/Falkland Islands, 20, 35, 42, 50–1, 187–8, 230, 256, 262–3, 299–300, 304, 306, 309, 311, 314, 327 Marine, 1–4, 14–15, 17–20, 35, 39, 58, 190, 207, 227, 250, 261, 264, 285, 289, 318, 327, 329, 330–3, 343–8, 369–80, 383, 388–91, 399, 439, 456, 464, 486–8, 501, 505, 507 Marine terraces, 3, 35, 178, 207, 222, 227, 236, 285, 288–90, 292, 294, 300, 316, 318, 375 Marplatan, 195, 269 Mayoan, 269–70, 273, 275–6, 278 Megafaunal Extinction, 277 Megaherbivores, 271 Meseta de la Muerte, 257–8, 259 Meseta de las Lagunas Sin Fondo, 256, 263 Mineral resources, 4, 208, 465, 485–94 Molluscs, 207, 285–319 Montehermosan, 193–5, 269, 271, 275–6 Monte Martial, 259 Multiproxy, 180, 241–4, 246, 249–50, 377, 383, 389, 400, 430, 463 Native South American ungulate, 277 Neogene, 3–4, 59, 61, 67, 73, 76, 78, 80–1, 83, 89, 97, 106, 210, 222, 255, 269–78, 285–6, 343, 347, 391 Nivation hollows, 31, 188, 259, 263 Northern Patagonian Ice Field, 152, 153, 257 Octodontid, Octodontoid, 271, 276 Paleoclimates, 2, 10, 35, 196, 242, 249–50, 285–6, 289, 316–18, 411, 441, 497 Paleoenvironments, 2, 4, 9, 51, 176, 191, 242, 255, 258, 285, 290, 316–18, 343, 356, 362–3, 369–80, 383, 394–5, 397, 400, 411, 415, 417, 427, 441–56, 461, 471 Paleogene, 16–18, 57–60, 67, 80, 151, 269, 333

Paleomagnetic poles, 124 Paleomagnetism, 121, 133 Paleosecular variations of CMT, 143 Palynology, 4, 7–10 Palynomorphs, 270, 344, 345, 370, 376 Pampa del Castillo, 218, 219, 256, 261 Pampatherid, 276 Panaraucanian, 271, 275–6, 278 Paraperiglacial, 259 Patagonia, 1, 4, 7–11, 13–51, 57–67, 73–83, 85–90, 95–107, 121–44, 151–96, 205–22, 227–37, 241–50, 255–65, 269–78, 285–319, 327–33, 343–8, 351–64, 369–80, 383–91, 393–406, 411–30, 437–73, 485–94, 497–508 Patagonian Andes, 1, 3, 15, 17–18, 31, 35–6, 42, 44, 47–9, 57–67, 78–80, 106, 151, 156, 159, 168, 214, 255, 272, 347, 414–15, 418, 421–2 Patagonian Archeology, 4, 153, 269, 277, 378, 437–73 Patagonian Cordillera, 41, 57–63, 65–7, 97–101, 162, 193, 220–1, 275, 463 Patagonian gravel, 3, 17, 107, 158, 164, 169, 209, 221, 259–61, 264, 487–90, 493, 500, 505–6 Patagoniano, Patagoniense, 269 Patagonian paleopermafrost, 3, 177, 188, 255, 257–8, 259–63, 264–5, 271 Patterned ground, 31, 35, 257, 259, 261 Peat, 4, 10–11, 15, 20, 27, 38, 41–3, 45–7, 49, 51, 105, 172–3, 177–8, 179, 181–3, 185–6, 190, 248, 250, 354, 362, 369–73, 375, 389, 405, 454 Peligran, 269 Penfordd cold episode, 260 Penillanura cuspidales, 263 Periglacial level, 257–8, 263 Permafrost, 3, 177, 188, 255, 257–9, 259–63, 264–5, 471 Phyllophaga, 271, 273 Physical geography, 3, 13–51 Pilosa, 277 Pingo scars, 262–3 Pinturan, 275 Pliocene, 3, 16–17, 19–20, 29, 48, 60, 79–80, 95, 98–9, 102–5, 107, 121, 123, 130–2, 155, 158, 160–1, 193–4, 206, 210, 216–18, 276, 286, 289, 331, 340, 493, 506 Pliocene, Quaternary, 35, 51, 60, 98–9 Pollen analysis, 370, 441, 455 Polygons, 258–62 Present periglaciation, 257–9 Primates, Platyrhine: New World monkeys, 278 Proboscidean, gomphotherid, 277 Procyonidae, 274 Proterotheriid litopterns, 278 Pseudomorphs, 259–62, 264 Puerto Madryn, kaven, 262–3 Puerto San Julia´n, 288 Quartz grain, 261 Quaternary, 1–4, 7–11, 51, 87, 95–107, 125, 156, 163–83, 208, 227, 230, 269, 285, 289–313, 351–64, 370, 372, 383–8, 393–406, 493, 497 Quaternary models, 488 Quaternary Palynology, 4, 9 Quaternary tectonics, 80–1 Quechua Phase, 271, 274–6 Rain shadow, 20–1, 57, 63, 65–7, 168, 247, 413, 416, 498 Ridge subduction, 57, 61–2, 66, 67, 95, 97

Index Riochican, 269 Rı´o Gallegos, 21–2, 35, 83, 85, 105, 155, 163, 166–9, 228, 234, 256, 262, 333, 424 Rockglacier, 257–9, 262–4 Rock magnetism, 121, 126, 138 Rock streams, 257 Rodados Patago´nicos, 158, 205–9, 217, 221–2, 260, 264, 505, 507 Romberg, 256, 261 Romberg cold episode, 256, 261 Sand wedges, 35, 260 Santacrucian, 269, 271–5, 278 Scansorial, 272, 278 Seismotectonic, 73–4, 76, 77–83, 84, 87, 90, 212, 375 Smilodons, 271, 274, 278 Solifluction, 35, 188, 207, 257–61, 263, 265 Solifluction layers, 263 Solifluction lobes, 257, 259 South America, 1–3, 7, 9–10, 13–14, 20–1, 29, 33, 35, 38, 60, 62–3, 66–7, 73, 75, 95, 97–8, 105–7, 124, 135–6, 141, 143, 158–9, 162, 165, 167, 172, 177, 180–1, 187–8, 191, 195, 227, 236, 241–2, 247–8, 255–65, 269, 275, 277–8, 289, 317, 329, 331, 343, 347, 351, 369, 393, 395, 397, 399–406, 411–13, 415, 417, 420–4, 427–8, 430, 437–8, 441, 461, 488 South American Land-Mammal Age (SALMA), 193, 276 Southern Patagonian Ice Field, 45, 101, 152, 186, 187, 257, 399 Southern water lines, 13, 208 Stage, 41, 98–100, 156, 158, 160, 170, 173, 181, 192–5, 227, 230, 269, 271–8, 355, 370, 389, 427–8, 473, 493 Stone-banked solifluction, 257, 259 Subantarctic forest, 351, 353, 355, 411, 413–15 Sub-fossil woods, 4 Susceptibility, 124–6, 134, 135, 137–8, 140, 142, 243, 246, 249, 360, 363, 401 Syngenetic ice wedge, 259–60

513

Tardigrades, 270, 276 Taxon, 275, 290, 295, 302, 307, 384–6, 390 Tectonics, 1–3, 13, 15, 30, 57–67, 73–90, 95–7, 121, 151, 159, 175, 205, 209, 221–2, 229, 233, 236, 245, 249, 274, 375, 469, 493 Tectonic setting, 60–1, 66, 95–7, 227, 229 Tectonic uplift, 66, 168, 176, 207, 209–10, 221, 233, 236–7, 375 Telken, 166, 169, 256, 261–2 Tephrochronology, 105, 243 Terrestrial and marine paleoenvironments, 369–77 Thilacosmyliidae, 276 Tierra del Fuego, 1–4, 7–11, 13–51, 73, 75–6, 81–2, 85, 88, 95–107, 134–5, 151–96, 205, 211, 227, 230–6, 241–50, 255–9, 262–3, 285–319, 327–33, 343, 353, 369–80, 383–91, 399, 404, 414–16, 419–20, 423–4, 439, 456, 462–70, 472–3, 487, 500 Tors, 31, 257, 263 Tremarctidae, 271 Tundra, 35, 43, 49–50, 173, 190, 258, 262, 356, 370, 402, 456 Uplift, 15, 17, 29, 36, 51, 57, 60, 61–3, 67, 98, 176, 207, 209, 222, 233, 236, 261, 347, 375 Valle de Avalo´n, 257 Vegetational trends, 343–4 Ventania Range, 261, 263, 265 Volcanism, 1, 3, 19, 35, 51, 76–80, 90, 95–107, 122, 127, 260, 363, 418 Westerlies, 1, 13, 20–1, 35, 180–1, 187, 190, 242–3, 247–8, 250, 351, 362–3, 369, 393, 401, 405, 411–13 Windows and columns, 264 Xenarthra, Xenarthran, 270–1, 273, 275–7, 455

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