Amazonia, Landscape and Species Evolution: A Look into the Past

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Hoorn_Frontispiece_Final.indd i

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MIDDLE TO LATE MIOCENE

Ceiba

POACEAE + Ludwigia

Phoberomys Gryposuchus

Anacardium Anhinga

Euterpe Amanoa Bactris Astrapotherium Terminalia Mauritia

Humiriastrum Cariniana Terminalia Mauritia Pachira

Barinasuchus Pseudoprepotherium

Purussaurus Anhinga + Pristobrycon

PAMPATHERIIDAE Calophyllum

Trigodon

Mourasuchus Bairdemys

PTERIDOPHYTA GASTROPODA

POACEAE + Ludwigia

Mauritia

MIDDLE TO LATE MIOCENE Charactosuchus

SERRASALMIDAE (Serrasalmus, Pygocentrus or Pristobrycon)

Plicodontinia

Carcharhinus

SERRASALMIDAE (Serrasalmus, Pygocentrus or Pristobrycon) Arapaima

Phractocephalus Ribodon Cariniana Pseudopimelodus

Colossoma Oxydoras Corydoras

Chelus

Terminalia

Hoplias

CHAROPHYTA

Potamotrygon

GASTROPODA

These plates show a reconstruction of the Middle to Late Miocene (16—7 million years ago) terrestrial and underwater landscape in Amazonia. Names for taxa are provided in latin and further explanations on the Miocene flora and fauna can be found in chapters 15 to 19 (Illustrations by Orlando Grillo).

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Amazonia: landscape and species evolution

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Companion website A companion website for this book is available at: www.wiley.com/go/hoorn/amazonia The website includes: • • •

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Figures from the book for downloading Color versions of selected figures Additional illustrations.

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Amazonia: landscape and species evolution A look into the past Edited by

C. Hoorn University of Amsterdam, The Netherlands F.P. Wesselingh Naturalis, Leiden, The Netherlands Editorial advisors

H.B. Vonhof Vrije Universiteit, Amsterdam, The Netherlands S.B. Kroonenberg Delft University of Technology, The Netherlands H. Hooghiemstra University of Amsterdam, The Netherlands

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This edition first published 2010, © 2010 by Blackwell Publishing Ltd Blackwell Publishing was acquired by John Wiley & Sons in February 2007. Blackwell’s publishing program has been merged with Wiley’s global Scientific, Technical and Medical business to form Wiley-Blackwell. Registered office: John Wiley & Sons Ltd, The Atrium, Southern Gate, Chichester, West Sussex, PO19 8SQ, UK Editorial offices: 9600 Garsington Road, Oxford, OX4 2DQ, UK The Atrium, Southern Gate, Chichester, West Sussex, PO19 8SQ, UK 111 River Street, Hoboken, NJ 07030–5774, USA For details of our global editorial offices, for customer services and for information about how to apply for permission to reuse the copyright material in this book please see our website at www.wiley.com/wiley-blackwell The right of the author to be identified as the author of this work has been asserted in accordance with the Copyright, Designs and Patents Act 1988. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, except as permitted by the UK Copyright, Designs and Patents Act 1988, without the prior permission of the publisher. Wiley also publishes its books in a variety of electronic formats. Some content that appears in print may not be available in electronic books. Designations used by companies to distinguish their products are often claimed as trademarks. All brand names and product names used in this book are trade names, service marks, trademarks or registered trademarks of their respective owners. The publisher is not associated with any product or vendor mentioned in this book. This publication is designed to provide accurate and authoritative information in regard to the subject matter covered. It is sold on the understanding that the publisher is not engaged in rendering professional services. If professional advice or other expert assistance is required, the services of a competent professional should be sought. Library of Congress Cataloguing-in-Publication Data Amazonia—landscape and species evolution : a look into the past / edited by C. Hoorn, F.P. Wesselingh ; editorial advisors, H.B. Vohnof, S.B. Kroonenberg, H. Hooghiemstra. p. cm. Includes bibliographical references and index. ISBN 978-1-4051-8113-6 (hardback : alk. paper) 1. Natural history—Amazon River Region. 2. Historical geology—Amazon River Region. 3. Paleontology—Amazon River Region. 4. Geology, Stratigraphic—Cenozoic. I. Hoorn, C. (Carina) II. Wesselingh, F. P. QH112.A435 2010 508.81´1—dc22 2009021979 A catalogue record for this book is available from the British Library. Set in 9/11.5 Minion by Newgen Imaging Systems Pvt. Ltd., Chennai, India Printed and bound in Malaysia 1

2010

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Contents

Dedication to Thomas van der Hammen List of contributors Prologue Thomas van der Hammen 1

Introduction: Amazonia, landscape and species evolution Carina Hoorn and Frank P. Wesselingh

Part I

vii ix xii

1

Tectonic processes as driving mechanisms for palaeogeographical and palaeoenvironmental evolution in Amazonia

2

Geological evolution of the Amazonian Craton Salomon B. Kroonenberg and Emond W.F. de Roever

9

3

The Paleozoic Solimões and Amazonas basins and the Acre foreland basin of Brazil Joaquim Ribeiro Wanderley-Filho, Jaime Fernandes Eiras, Paulo Roberto da Cruz Cunha and Paulus H. van der Ven

29

4

Tectonic history of the Andes and sub-Andean zones: implications for the development of the Amazon drainage basin Andres Mora, Patrice Baby, Martin Roddaz, Mauricio Parra, Stéphane Brusset, Wilber Hermoza and Nicolas Espurt

38

5

Cenozoic sedimentary evolution of the Amazonian foreland basin system Martin Roddaz, Wilber Hermoza, Andres Mora, Patrice Baby, Mauricio Parra, Frédéric Christophoul, Stéphane Brusset and Nicolas Espurt

61

6

The Nazca Ridge and uplift of the Fitzcarrald Arch: implications for regional geology in northern South America Nicolas Espurt, Patrice Baby, Stéphane Brusset, Martin Roddaz, Wilber Hermoza and Jocelyn Barbarand

89

Part II Cenozoic depositional systems in Amazonia 7

The Amazonian Craton and its influence on past fluvial systems (Mesozoic-Cenozoic, Amazonia) Carina Hoorn, Martin Roddaz, Rodolfo Dino, Emilio Soares, Cornelius Uba, Diana Ochoa-Lozano and Russell Mapes

103

8

The development of the Amazonian mega-wetland (Miocene; Brazil, Colombia, Peru, Bolivia) Carina Hoorn, Frank P. Wesselingh, Jussi Hovikoski and Javier Guerrero

123

9

Marine influence in Amazonia: evidence from the geological record Jussi Hovikoski, Frank P. Wesselingh, Matti Räsänen, Murray Gingras and Hubert B. Vonhof

143

10

Megafan environments in northern South America and their impact on Amazon Neogene aquatic ecosystems M. Justin Wilkinson, Larry G. Marshall, John G. Lundberg and Mikhail H. Kreslavsky

162

11

Long-term landscape development processes in Amazonia Georg Irion and Risto Kalliola

185

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vi

Contents

Part III Amazonian climate, past and present 12

Climate variation in Amazonia during the Neogene and the Quaternary Hubert B. Vonhof and Ron J.G. Kaandorp

201

13

Modelling the response of Amazonian climate to the uplift of the Andean mountain range Pierre Sepulchre, Lisa C. Sloan and Frédéric Fluteau

211

14

Modern Andean rainfall variation during ENSO cycles and its impact on the Amazon drainage basin Bodo Bookhagen and Manfred R. Strecker

223

Part IV Cenozoic development of terrestrial and aquatic biota: insights from the fossil record 15

A review of Tertiary mammal faunas and birds from western Amazonia Francisco Ricardo Negri, Jean Bocquentin-Villanueva, Jorge Ferigolo and Pierre-Olivier Antoine

245

16

Neogene crocodile and turtle fauna in northern South America Douglas Riff, Pedro Seyferth R. Romano, Gustavo Ribeiro Oliveira and Orangel A. Aguilera

259

17

The Amazonian Neogene fish fauna John G. Lundberg, Mark H. Sabaj Pérez, Wasila M. Dahdul and Orangel A. Aguilera

281

18

Amazonian aquatic invertebrate faunas (Mollusca, Ostracoda) and their development over the past 30 million years Frank P. Wesselingh and Maria-Inês F. Ramos

302

19

The origin of the modern Amazon rainforest: implications of the palynological and palaeobotanical record Carlos Jaramillo, Carina Hoorn, Silane A.F. Silva, Fatima Leite, Fabiany Herrera, Luis Quiroz, Rodolfo Dino and Luzia Antonioli

317

20

Biotic development of Quaternary Amazonia: a palynological perspective Hermann Behling, Mark Bush and Henry Hooghiemstra

335

Part V Modern perspectives on the origin of Amazonian biota 21

Contribution of current and historical processes to patterns of tree diversity and composition of the Amazon Hans ter Steege, ATDN (Amazon Tree Diversity Network: collective author) and RAINFOR (The Amazon Forest Inventory Network: collective author)

349

22

Composition and diversity of northwestern Amazonian rainforests in a geoecological context Joost F. Duivenvoorden and Alvaro J. Duque

360

23

Diversification of the Amazonian flora and its relation to key geological and environmental events: a molecular perspective R. Toby Pennington and Christopher W. Dick

373

24

Molecular studies and phylogeography of Amazonian tetrapods and their relation to geological and climatic models Alexandre Antonelli, Adrián Quijada-Mascareñas, Andrew J. Crawford, John M. Bates, Paúl M. Velazco and Wolfgang Wüster

386

25

Molecular signatures of Neogene biogeographical events in the Amazon fish fauna Nathan R. Lovejoy, Stuart C. Willis and James S. Albert

405

Part VI 26

Synthesis

On the origin of Amazonian landscapes and biodiversity: a synthesis Frank P. Wesselingh, Carina Hoorn, Salomon B. Kroonenberg, Alexandre Antonelli, John G. Lundberg, Hubert B. Vonhof and Henry Hooghiemstra

Index

421

433

Color plate section is found facing p. 210 Companion website for this book: www.wiley.com/go/hoorn/amazonia

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Dedication to Thomas van der Hammen

We dedicate this book to the life and work of Professor Thomas van der Hammen who is one of the most prominent Dutch geoscientists, making many links between geology, biology and archaeology. The study of altitudinal vegetation distributions in the northern Andes is a red line through his work and it has served studies of the Neogene uplift history of the northern Andes as well as studies of pollen-based Pleistocene climate change. During more than two decades he lectured at the University of Amsterdam and inspired generations of Dutch students. Since his retirement in 1989 he has lived in Colombia where, with his never-ending enthusiasm, he continues to motivate large numbers of Colombian students. Carina Hoorn, Frank P. Wesselingh (editors) Henry Hooghiemstra, Hubert Vonhof, Salomon Kroonenberg (editorial advisors)

Biography Thomas van der Hammen was born in The Netherlands in 1924 and had an innate interest and love for nature. After the Second World War he studied geology at Leiden University. He was trained as a palynologist by Professor F. Florschütz but also had regular contact with other founding fathers of this discipline such as J. Iversen and R. Potonié. His PhD dissertation was on ‘Late

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glacial flora and periglacial phenomena in the Netherlands’, a subject that would remain of interest to him for the rest of his life. In 1951 he started working for the Geological Survey in Colombia and did pioneering research on Cretaceous and Cenozoic sediments. Through his trademark multidisciplinary approach he unravelled the stages of tectonic uplift of the Andes. Later, he and his co-workers were able to make a link with the evolution of the montane forest and páramo vegetation of the Northern Andes. In 1959 Thomas returned to The Netherlands and worked at the Department of Geology of Leiden University. He developed a research line in palaeoecology and climate history in the eastern part of The Netherlands while continuing his research in tropical palynology, often in cooperation with the geologist Lex Wijmstra, and focusing on exploratory studies in Guyana, Suriname and the Amazon Basin. In 1966 Thomas moved to the University of Amsterdam where he was appointed as a Professor in Palynology. A suite of both Dutch and Colombian (PhD) students were trained in topics such as geology, archaeology, biostratigraphy, climate history and vegetation analysis, and conducted field work in areas located in Brazilian Amazonia, Colombian Amazonia, the Colombian Andes and Venezuela. During the late 1970s and early 1980s he designed the large ‘Ecoandes Project’ and the ‘Tropenbos Colombia Programme’ respectively. The Ecoandes Project focused on integrated palaeo/actuo-ecological research of transects across different sectors of the Colombian Andes. These unprecedented studies resulted in seven volumes in the series Studies of Tropical Andean Ecosystems, published at Cramer/Borntraeger in Germany. The Tropenbos Colombia Programme studies focused on a wide variety of subjects, ranging from fishery, plant systematics, floristic inventories, sociogeographical studies, anthropology, palaeoecology, geology and tropical vegetation ecology. These studies resulted in 20 volumes of the series Studies on Colombian Amazonia, published at Tropenbos-Colombia office in Bogotá. To promote distribution of scientific results among Colombian institutes and colleagues around the world in 1973 he started the series El Cuaternario de Colombia [The Quaternary of Colombia], which he edited up to volume 20 (1995). Perhaps his most valuable contribution to science was to increase our understanding of the history of Pleistocene climate change. His training in the climate history of Western Europe

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viii

Dedication to Thomas van der Hammen

enabled him to show us that the Neotropics also had a dynamic history of climate change. Thomas van der Hammen discovered the immense value of the pollen archives in the deep intraAndean sedimentary basins. He studied the first deep boreholes in the Bogotá Basin and the Fúquene Basin, and created a basis for later studies on long continental pollen records from Colombia. During the decades that Thomas lectured in The Netherlands he played an active role in Dutch nature conservation and in developing international structures for nature assessment studies. His contributions to the advancement of science were rewarded by her Majesty Queen Beatrix with a knighthood. After his retirement he implemented his valuable experience in Colombia and, in collaboration with national research institutes such as the Geographical Institute (IGAC), the Geological Institute (Ingeominas), the Archaeological Institute, and the Von

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Humboldt Biodiversity Institute, he helped to promote many collaborative studies. Thomas van der Hammen was the author of more than 100 international peer-reviewed publications and contributed much to our understanding of tropical ecology and tropical climate history. His contributions to the training of Colombian scientists, and to the development of nature conservation and awareness of infrastructural issues in Colombia are highly valued. For the latter Thomas received the Colombian Order of San Carlos, which he received out of the hands of the Colombian President. Thomas’s enthusiasm, charisma, vision and ability to make people work together made him a most inspiring person and a true leader. Henry Hooghiemstra and Carina Hoorn

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List of contributors

Aguilera, O.A. Universidad Nacional Experimental Francisco de Miranda, Centro de Investigaciones en Ciencias Básicas, Complejo Docente Los Perozos, Carretera Variante Sur, Coro, 4101, Falcón, Venezuela. e-mail: [email protected] Albert, J.S. Department of Biology, University of Louisiana Lafayette, PO Box 42451, Lafayette, LA 70504-2451 USA. e-mail: [email protected]

Bookhagen, B. Department of Geography, UC Santa Barbara, Santa Barbara, CA 93106, USA, e-mail: [email protected] Brusset, S. Laboratoire des Mécanismes et Transferts en Géologie, Université de Toulouse; UPS (SVT-OMP); CNRS/IRD; LMTG; 14 Av, Edouard Belin, F-31400 Toulouse, France. e-mail: [email protected]

Antoine, P-O. LMTG, University of Toulouse III-IRD-CNRS, France. e-mail: [email protected]

Bush, M. Department of Biological Sciences, Florida Institute of Technology, 150 W. University Boulevard, Melbourne, FL 32901, USA. e-mail: [email protected]

Antonelli, A. Institute of Systematic Botany, University of Zurich, Zollikerstrasse 107, CH 8008 Zurich, Switzerland. e-mail: [email protected]

Christophoul, F. Université de Toulouse; UPS (SVT-OMP); CNRS/IRD; LMTG; 14 Av, Edouard Belin, F-31400 Toulouse, France. e-mail: [email protected]

Antonioli, L. Universidade Estadual do Rio de Janeiro (UERJ), Faculdade de Geologia – DEPA. Campus Francisco Negrão de Lima Pavilhão João Lyra Filho R. São Francisco Xavier, 524, 4° andar Bloco A – Sala 2030 Maracanã – Rio de Janeiro – RJ – Cep 20550-900, Brazil. e-mail: [email protected]

Crawford, A.J. Naos Molecular Labs, Smithsonian Tropical Research Institute, Apartado 0843-03092, Balboa, Ancón, Republic of Panama. e-mail: [email protected]

ATDN (Amazon Tree Diversity Network) uu.nl/~herba/Guyana/ATDN/

http://www.bio.

Baby, P. Laboratoire des Mécanismes et Transferts en Géologie, Université de Toulouse; UPS (SVT-OMP); CNRS/IRD; LMTG; 14 Av, Edouard Belin, F-31400 Toulouse, France. e-mail: patrice. [email protected] Barbarand, J. Université Paris Sud, UMR CNRS 8148 IDES, Bâtiment 504, Orsay cedex, F-91405, France. e-mail: jocelyn. [email protected] Bates, J.M. Department of Zoology, The Field Museum of Natural History, 1400 S. Lake Shore Dr., Chicago, IL 60605-2496, USA. e-mail: [email protected] Behling, H. Department of Palynology and Climate Dynamics, Albrecht-von-Haller-Institute for Plant Sciences, University of Göttingen, Untere Karspüle 2, 37073 Göttingen, Germany. e-mail: [email protected] Bocquentin-Villanueva, J. Federal University of Acre, Rio Branco, AC, Brazil. e-mail: [email protected]

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Cunha, P.R. Petrobras Exploration and Production – Espírito Santos Basin, Avenida República do Chile 65, Rio de Janeiro, Brazil. e-mail: [email protected] Dahdul, W.M. Department of Biology, The University of South Dakota, 414 E. Clark St., Vermillion, SD 57069, USA. e-mail: [email protected] Dick, C.W. Department of Ecology and Evolutionary Biology, University of Michigan, 2011 Kraus Natural Science Bldg., 830 N. University, Ann Arbor, MI 48109-1048, USA. e-mail: cwdick@ umich.edu Dino, R. Petrobras, Rua Horácio Macedo, 950, Cidade Universitária – Ilha do Fundão, 21941-915 – Rio de Janeiro, RJ, Brazil. e-mail: [email protected] Duivenvoorden, J.F. Institute for Biodiversity and Ecosystem Dynamics, University of Amsterdam, Kruislaan 318, 1098 SM Amsterdam, The Netherlands. e-mail: [email protected] Duque, A.J. Universidad Nacional de Colombia, Departamento de Ciencias Forestales, Calle 59A No 63-20, A.A. 1027, Medellin, Colombia. e-mail: [email protected]

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List of contributors

Eiras, J.F. PetroGeo – Serviços Geológicos S/S Ltd, Manaus, Amazonas, Brazil. e-mail: [email protected] Espurt, N. Laboratoire des Mécanismes et Transferts en Géologie, Université de Toulouse; UPS (SVT-OMP); CNRS/IRD; LMTG; 14 Av, Edouard Belin, F-31400 Toulouse, France. Now: CEREGE UMR6635 Université Paul Cézanne CNRS BP80 13545 Aix en Provence, France. e-mail: [email protected] Ferigolo, J. MCN, Fundação Zoobotânica do Rio Grande do Sul, Porto Alegre, RS, Brazil. e-mail: [email protected] Fluteau, F. Institut de Physique du Globe de Paris, Place Jussieu, Paris, 75005 France. e-mail: [email protected] Gingras, M. Department of Earth and Atmospheric Sciences, 1–26 Earth Science Building, University of Alberta, Edmonton, T6G2E3, Canada. e-mail: [email protected] Grillo, O. Museu Nacional, Quinta da Boa Vista s/n, São Cristóvão, Rio de Janeiro, Brazil. e-mail: [email protected] Guerrero, J. Departamento de Geociencias, Universidad Nacional, A.A. 14490, Bogotá, Colombia. e-mail: geochron2002@ yahoo.com Hermoza, W. PeruPetro S.A., av. Luis Aldana 320, San Borja, Lima 41, Peru. Now: REPSOL-YPF, Paseo de la Castellana 280, 1ª Pl., 28046 Madrid, Spain. e-mail: [email protected] Herrera, F. Smithsonian Tropical Research Institute, Box 084303092, Balboa, Republic of Panama. e-mail: fabianyht@yahoo. com.mx Hooghiemstra, H. Institute for Biodiversity and Ecosystem Dynamics, University of Amsterdam, Kruislaan 318, 1098 SM Amsterdam, The Netherlands. e-mail: [email protected] Hoorn, C. Institute for Biodiversity and Ecosystem Analysis, University of Amsterdam, Kruislaan 318, 1098 SM Amsterdam, The Netherlands. e-mail: [email protected] Hovikoski, J. Department of Geology, University of Turku, 20014 Turku, Finland. Now at: Department of Stratigraphy, Geological Survey of Denmark and Greenland (GEUS), Øster Voldgade 10, 1350 Copenhagen K, Denmark. e-mail: [email protected] Irion, G. Research Institute Senckenberg, Marine Science Department, 26382 Wilhelmshaven, Germany. e-mail: g.irion@ gmx.de Jaramillo, C. Smithsonian Tropical Research Institute, Box 0843-03092, Balboa, Republic of Panama. e-mail: JaramilloC@ si.edu Kaandorp, R. Vrije Universiteit Amsterdam, Faculty of Earth and Life Sciences, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands. e-mail: [email protected] Kalliola, R. Department of Geography, University of Turku, FI-20014 Turku, Finland. e-mail: [email protected] Kreslavsky, M.H. Earth and Planetary Sciences, University of California-Santa Cruz, 1156 High St., Santa Cruz CA 95064, USA. e-mail: [email protected]

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Kroonenberg, S.B. Delft University of Technology, Department of Geotechnology, P.O. Box 5028, 2600 GA Delft, The Netherlands. e-mail: [email protected] Leite, F. Smithsonian Tropical Research Institute, Box 084303092, Balboa, Republic of Panama. Now: Institute of Geosciences, University of Brasília, 70910-900, Brasília, Brazil. e-mail: fprleite@ gmail.com Lovejoy, N.R. Department of Biological Sciences, University of Toronto Scarborough, 1265 Military Trail, Toronto, ON, M1C 1A4 Canada. e-mail: [email protected] Lundberg, J.G. Department of Ichthyology, The Academy of Natural Sciences, 1900 Benjamin Franklin Parkway, Philadelphia, PA 19103-1195, USA. e-mail: [email protected] Mapes, R. Department of Geological Sciences, University of North Carolina, CB#3315, Mitchell Hall, Chapel Hill, NC 275993315; now at: Exxon Mobil Corporation, Houston, TX, USA Marshall, L.G. Arizona Museum of Natural History, 53 North Macdonald St., Mesa AZ 85201, USA Mora, A. ECOPETROL, Instituto Colombiano del Petroleo. Piedecuesta, Santander, Colombia. e-mail: andres.mora@ ecopetrol.com.co Negri, F.R. Universidade Federal do Pará, Campus Universitario de Altamira, Rua Coronel Jose Porfirio, No 2515, Barrio São Sebastião, Altamira, PA Brasil, CEP 68372-040. e-mail: frnegri@ ufpa.br Ochoa-Lozano, D. Center for Tropical Paleoecology and Archeology, Smithsonian Tropical Research Institution, Apartado Postal 0843-03092, Balboa, Ancon, Panama. e-mail: dochoa1709@ yahoo.com Oliveira, G.R. Universidade Federal do Rio de Janeiro, Setor de Paleovertebrados, Departamento de Geologia e Paleontologia, Museu Nacional, Quinta da Boa Vista, 20940-040, Rio de Janeiro, RJ, Brazil. Fellow of Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq). e-mail: gustavoliveira@ gmail.com Parra, M. Institut für Geowissenschaften, Universität Potsdam, Potsdam, Germany; now at: The University of Texas at Austin, 1, University Station c1100 Austin, TX 78712-0254, USA. e-mail: [email protected] Pennington, R.T. Royal Botanic Garden Edinburgh, 20a Inverleith Row, Edinburgh EH3 5LR, UK. e-mail: t.pennington@ rbge.org.uk Quijada-Mascareñas, A. School of Natural Resources, 325 Biological Sciences East, The University of Arizona, Tucson, Arizona 85721, USA. e-mail: [email protected] Quiroz, L. Smithsonian Tropical Research Institute, Box 0843-03092, Balboa, Republic of Panama; and Department of Geological Sciences, University of Saskatchewan, Saskatoon, Saskatchewan S7N 5E2, Canada. e-mail: luisignacioquiroz@ gmail.com

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List of contributors RAINFOR (Amazon Forest Inventory Network) http://www. geog.leeds.ac.uk/projects/rainfor/ Ramos, M-.I.F. Museu Paraense Emílio Goeldi, Campus de Pesquisa, CCTE, Av. Perimetral, 1901 Caixa Postal 399, Bairro Terra Firme, CEP. 66077-530, Belém, Pará, Brasil. e-mail: [email protected] Räsänen, M. Department of Geology, University of Turku, 20014 Turku, Finland. e-mail: [email protected] Riff, D. Instituto de Biologia, Universidade Federal de Uberlândia, Campos Umuarama, Bloco 2D-sala 28, Rua Ceará s/n, Bairro Umuarama, Uberlândia, Minas Gerais, Brazil. e-mail: [email protected] Roddaz, M. Laboratoire des Mécanismes et Transferts en Géologie, Université de Toulouse; UPS (SVT-OMP); CNRS/IRD; LMTG; 14 Av, Edouard Belin, F-31400 Toulouse, France. e-mail: [email protected] de Roever, E.W.F. NALCO EUROPE, P.O. Box 627, 2300 AP Leiden, The Netherlands. e-mail: [email protected] Romano, P.S.R. Universidade Federal do Rio de Janeiro, Setor de Paleovertebrados, Departamento de Geologia e Paleontologia, Museu Nacional, Quinta da Boa Vista, 20940-040, Rio de Janeiro, RJ, Brazil. Fellow of Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq). e-mail: [email protected] Sabaj Pérez, M.H. Department of Ichthyology, The Academy of Natural Sciences, 1900 Benjamin Franklin Parkway, Philadelphia, PA 19103-1195, USA. e-mail: [email protected] Sepulchre, P. Department of Earth Sciences, University of California, Santa Cruz, USA. e-mail: [email protected] Silva, S.A.F. Smithsonian Tropical Research Institute, Box 0843-03092, Balboa, Republic of Panama. Now: Instituto Nacional de Pesquisas da Amazonia-INPA, Coordenação de pesquisas em Botânica, Laboratório de Palinologia, Av. André Araujo 2936. P.O Box-478, Manaus, AM, Brazil. e-mail: [email protected] Sloan, L.C. Department of Earth Sciences, University of California, Santa Cruz, USA, email: [email protected]

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Soares, E. Universidade Federal do Amazonas, Av. Gen. Rodrigo Octávio Jordão Ramos, 3000, Campus Universitário, Reitoria. Bairro Coroado I. CEP 69077-000. Manaus, AM, Brazil. easoares@ ufam.edu.br ter Steege, H. Institute of Environmental Biology, Section Ecology and Biodiversity, Utrecht University, Padualaan 8, 3584 CA Utrecht, The Netherlands. e-mail: [email protected] Strecker, M.R. Institut für Geowissenschaften, Universität Potsdam, Germany. e-mail: [email protected] Uba, C. Institut für Geowissenschaften, Universität Potsdam, 14476 Potsdam, Germany, e-mail: [email protected] Velazco, P.M. Department of Zoology, The Field Museum of Natural History, 1400 S. Lake Shore Dr., Chicago, IL 60605-2496, USA; Dept. of Biological Sciences, University of Illinois at Chicago, 845 W. Taylor St. Chicago IL 60607, USA. e-mail: pvelazco@ fieldmuseum.org van der Ven, P.H. Petrobras Exploration and Production – Equatorial Margin and Interior Basins, Avenida República do Chile 65, Rio de Janeiro, Brazil. e-mail: [email protected] Vonhof, H.B. Vrije Universiteit Amsterdam, Faculty of Earth and Life Sciences, Dept of Sedimentology and Marine Geology, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands. e-mail: [email protected] Wanderley-Filho, J.R. Petrobras Exploration and Production – Amazônia Business Unit, Manaus, Amazonas, Brazil. e-mail: [email protected] Wesselingh, F.P. Museum of Natural History, P.O. Box 9517, Darwinweg 2, 2300 RA Leiden, The Netherlands. e-mail: [email protected] Wilkinson, M.J. Jacobs Engineering, NASA-Johnson Space Center, 2224 Bay Area Blvd., Houston TX 77058, USA. e-mail: [email protected] Willis, S.C. School of Biological Resources, University of Nebraska-Lincoln, Lincoln, Nebraska, 68588 USA. e-mail: [email protected] Wüster, W. School of Biological Sciences, Bangor University, Bangor LL57 2UW, Wales, UK. e-mail: [email protected]

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Prologue

It is now almost 60 years since I arrived in Colombia for the first time to start investigations for a geological survey. I had one great desire: to work in Amazonia. Very soon afterwards, early in 1952, this desire was fulfilled; for one month I was able to work in one of the most remote and undisturbed areas of Western Amazonia, the middle to lower Apaporis River, to study the flora and the geology. This was possible because of the help of the (ethno)botanist Dick Schultes, who had good relations with the rubber trade company in Soratama. The company had a base there and collected rubber from the local Indian tribe. Once a month this rubber was sent to Bogotá with the Catalina (a small airplane) but – on request – it occasionally also transported researchers. An assistant, two local Indians and I set off in a tree-canoe equipped with two hammocks, a plant-press, sample bags and some food. We travelled several hundreds of kilometres along the Apaporis and Cananari Rivers to study the rainforest and the outcrops. We climbed the table mountains, measured the crossbedding in the old tepui sandstone formations and established that in early (Palaeozoic) times the rivers ran to the northwest, instead of to the modern southeasterly direction. We also encountered the younger Tertiary sediments, and concluded that the presence of iron oölite and manganite could only indicate one thing: that lacustrine and brackish-water conditions had once ruled in the heart of Amazonia. After a month of fieldwork in the area I came back to Soratama to wait for the plane; Schultes also arrived from another expedition at the same time, and so we had some days together. We were out of food and lived on what was available in Soratama. One day Schultes said to me: ‘I have still a tin with plum-pudding, let’s go into the forest and eat it together!’ And so we did: Christmas pudding in March, in the jungle. I was 27 years old then and at the beginning of a life lived in pursuit of understanding the composition and evolution of the forests through time in the Andes and in Amazonia. It was some 25 years after our first Amazonian survey that I again saw samples from this area. This time they came in the form of bagged clays that were collected during an extensive Colombian survey, the Proradam project (1974 to 1979). The question that came with the bags was whether the age and environment could be established through palynology. A Neogene

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age was soon evident, and the presence of abundant pollen of mangrove trees (Rhizophora) for the first time confirmed the presence of saline or brackish waters in ancestral Amazonia. Around this time two other major geological and geographical surveys were carried out in Brazil: the RadamBrasil survey and the coal exploration project by the Companhia de Pesquisa de Recursos Minerais (CPRM). The latter project drilled close to 50 cores in the subsurface of Brazilian Amazonia, and so far constitutes the best register of Neogene Amazonian history. In addition, the Brazilian oil company Petrobras drilled numerous cores though the Lower to Middle Cretaceous, which permitted the reconstruction of the floral history of that period. There were of course also groups of dedicated researchers who spent most of their life in Amazonia. One of them was Harald Sioli, who recently died but is much remembered through both his research papers and his autobiography. Another 10 years passed and in the 1980s Tropenbos International, an initiative of the Dutch government, established a large research project with the Amazonian ecosystem as its focal point. Within this project, Carina Hoorn carried out a much more extensive and profound geological, palynological and environmental study of the Miocene of western Amazonia. This coincided with a renewed interest in Amazonia by several other countries, which all greatly increased our knowledge of the Neogene history. Meanwhile Quaternary geologists and palynologists contributed to the knowledge of the younger Pleistocene-Holocene history of the area, indicating that Amazonia passed through periods of drier climate. Moreover, the first reconstruction by the international CLIMAP project (in 1976) of the Last Glacial Maximum indicated lower temperatures for Amazonia. It was Jürgen Haffer who in 1969 published his theory of speciation of Amazonian forest birds and his theory of glacial forest refugia. For many years his ideas had an enormous influence and caused deep controversies and forthright discussions, which, as more data become available, gradually became less extreme. The time necessary for the formation of subspecies or species may have been much longer than originally was assumed, but still the place and functions of the centres of endemism and their history continue to be a key point in the scientific debate.

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Prologue The first palynological data that showed the glacial time transition of rainforest to grass-savanna (in Rondondia, Brazil), were published in 1972. These were followed by data showing the more or less continuous presence of forest in other areas (Lake Pata, in the north of Brazilian Amazonia), the drying up of lakes during the Last Glacial Maximum and/or the replacement of forest by open vegetation (Carajas, Brazil). Other areas (Rio Branco) in the northern part of Brazil show a well-dated glacial time and early Holocene extension of dune fields. Vegetation maps of the possible – or probable – situation during the Last Glacial Maximum, based on the available data and the use of present rainfall patterns, have been published, and are open to corrections – if and when more data become available. Not all problems and discussions on Amazonia’s past have been resolved, and the cause of its enormous biodiversity is one of the great mysteries that still need an explanation. Nevertheless, our knowledge has advanced considerably since 60 years ago, and the time seems to be right for a major effort to gather all our present knowledge on Amazonia’s history and evaluate the problems and existing controversies, whilst reflecting on the gaps that still exist in our knowledge. Altogether this book will form a solid base to direct future research. One of the most promising avenues of future research that can resolve some of our current questions is the study of genetics and the use of the molecular clock as an indicator of the separation of subspecies and species. This could enable us not only to compare geological and climatic history with the present climatic pattern, but also to assess the differences within Amazonia and the earlier proposed centres of endemism, as suggested by Haffer, Prance and others. These centres of endemism are, at least in part, related to geographical and climatic patterns that existed since the Late Miocene, Pliocene and the Quaternary. In particular, during

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the glacial periods the differences may have become much more pronounced because of the resulting changes of vegetation. It now seems more than probable that new species and subspecies were formed over millions of years; there are even strong indications that biodiversity was greater during the Miocene than at Present. This suggests that the speciation– extinction balance may have become negative during the Pleistocene glaciations, when the lower temperatures and drier climate intervals led to higher extinction rates (but eventually to the appearance of certain new subspecies). The importance of the Amazonian rainforest and its enormous biodiversity for the conservation of the environmental equilibrium of the earth can only be underestimated. Moreover, the expected negative effect of the disappearance of a major part of the forest on both Amazonia and Earth as a whole, would affect us all. Therefore a better understanding of this sensitive ecosystem and its dynamics over a range of timescales is important to the global scientific and political community. The conservation of Amazonia, and a better understanding of its plant, animal and human life, is doubtlessly related to the future well-being of our planet. This book may therefore be considered as a very important contribution to the knowledge of Amazonia, but also to science in general. It concludes a period of intensive investigations but also might herald the beginning of a new era of investigations that will hopefully lead us to the answers of many of the questions that for long have remained unanswered, and to more definite guidelines that will ensure the future of our Earth and its living inhabitants. Thomas van der Hammen Chía (Colombia), July 2009

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ONE

Introduction: Amazonia, landscape and species evolution Carina Hoorn1 and Frank P. Wesselingh2 1

University of Amsterdam, The Netherlands Naturalis, Leiden, The Netherlands

2

Motivation The Amazon drainage basin covers over 8 million km2 and has the largest rainforest on earth (Sioli 1984). The Amazon River is 6400 km long, from its source in the Andes to its mouth in the Atlantic, and the drainage basin includes a variety of landscapes such as the enigmatic tepuis in the north, the forested slopes at the foot of the Andes in the west, and the wide tracts of rainforest in the central part of the basin. The region is renowned for its great biodiversity, both aquatic and terrestrial. Exact figures to quantify this diversity do not yet exist, and estimates of species numbers are still increasing. This incomplete understanding of species numbers makes any firm estimate impossible; nevertheless, the region is thought to harbour no less than 7500 butterfly species (possibly about 40% of the world butterfly species), 1500 species of birds (about onethird of the world total) and an estimated 11,200 tree species (Hubbell et al. 2008). The Amazon system plays a significant role in the world’s climate as it produces about 20% of the world’s oxygen supply. Nutrients delivered by the Amazon River to the Atlantic Ocean help to foster oceanic life that sequesters globally relevant amounts of carbon (Subramaniam 2008), and in the terrestrial realm the Amazon rainforest is responsible for 10% of the net primary productivity of the whole terrestrial biosphere (http://earthobservatory.nasa. gov). Therefore, Amazonia is of the greatest concern to us all. In spite of Amazonia’s importance the number of studies on species composition and their distribution is still limited. Diversity hotspots seemingly coincide with biological field stations and specific large-scale biological expeditions (Nelson et al. 1990), and indicate just how much basic research still is required. Even the classification of habitats in Amazonia is far from straightforward (e.g. Kalliola et al. 1993) as major parts of the region are hardly accessible and remote sensing techniques cannot grasp the variety without substantial ‘ground-truthing’. If our knowledge of Amazonia’s present is limited, this is even more so for its past. When did the Amazonian landscape and Amazonia, Landscape and Species Evolution: A Look into the Past, 1st edition. Edited by C. Hoorn and F.P. Wesselingh. © 2010 Blackwell Publishing

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jungles arise? What climatic, chemical, geological and other non-biological processes were involved in the development of these ecosystems and sustain them now, and what part did they play in the previous episodic demise of these ecosystems? In order to assess ecosystem resilience it is imperative to understand the historical (i.e. geological) processes that have shaped Amazonian landscapes and their biota. For decades scientists have speculated about the evolution of species and biodiversity. However, the scientific debate was mostly dominated by biologists and geomorphologists using species and geomorphology as a basis for their theories (Haffer 1969; Ab’Sabr 1982; Absy et al. 1991; Colinvaux et al. 2000, 2001; Haffer & Prance 2001; see also Chapter 26) and few geologists were involved in this discussion. Scientists are now increasingly aware that the geological substrate in Amazonia, and the relatively young age of the Andes and the Amazon River, were of paramount importance in species evolution and distribution of diversity hotspots (e.g. Salo et al. 1986; Hooghiemstra & Van der Hammen 1998; Lundberg et al. 1998; Lovejoy et al. 1998; Van der Hammen & Hooghiemstra 2000; Nores 2002; Wesselingh and Salo 2006; Tuomisto 2007; Antonelli 2008) yet an undisputed theory about the timing and context of Amazonian diversifications – in the light of geological evidence– still has to materialize. Geology only recently started playing a role in the debate on the origin of biodiversity as it was hampered by the same obstacles as the biological and geomorphological sciences – the lack of firm evidence due to the difficult access to the terrain. However, in the past two decades geological studies in Amazonia quickly followed one another. The sedimentary environments in Amazonia and their age (e.g. Räsänen et al. 1987; Hoorn 1993; Wesselingh et al. 2002; Hovikoski 2006), the ancient nature of rainforests (e.g. Morley 2000; Jaramillo et al. 2006), the importance of soil heterogeneity and distribution in relation to floristic biodiversity (e.g. Kalliola & Flores-Paitan 1998; Ruokolainen et al. 2007), past climate dynamics (Sugden 2000; Bush & Flenley 2006; Bush et al. 2007) and the exact age of the establishment of the Amazon River (Dobson et al. 2001; Figuereido et al. 2009) are but a few of the thrilling insights that were obtained. Simultaneously, a relatively young branch of science, DNA studies, increasingly suggested that the origin of extant biodiversity dates back well before the Quaternary (Antonelli 2008;

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Rull 2008) and may have coincided with regional geological events (see Chapters 23–25). Consequently, at the turn of the millennium, geology and biology were drawn to each other in a concerted effort to explain the origin of Amazonian biodiversity and landscapes.

A journey through the geological history of Amazonia The scientific advances of the past two decades, and the newly gained perception that biotic and abiotic evolution might be intimately related, demanded an interdisciplinary, multinational effort to summarize the state of the art in Amazonian geological sciences. This book attempts to fulfil this role. It not only presents an outline of the geological history, but also assesses the implications of the geological past for landscape evolution and biotic diversity. The contributors show that the development of Amazonian diversity is intimately linked to landscape evolution, and that modern Amazonian ecosystems were formed during the geodynamic processes of the Cenozoic. The implication of this work is that before the Quaternary there were periods with even more diverse ecosystems. The contributions to this book are grouped into five themes, corresponding to the book’s five parts. The first of these themes discusses the origin, architecture and stratigraphic and tectonic relationships of the major geological units of the eastern Andes and Amazonia. The second theme focuses on the Amazonian sedimentary record from the Mesozoic era to the Quaternary period. This record is subdivided into cratonic and Andean-driven depositional systems although Neogene and Quaternary systems are a combination of both Andean and cratonic fluvial systems. In addition, megafan depositional systems in western Amazonia are also reviewed. Climatic evolution and the implications for the Amazonian region during the Miocene are assessed in the third part. The Amazonian palaeontological record of the aquatic and terrestrial realms constitutes the fourth part of the book. Despite the uneven concentration of fossiliferous deposits in western Amazonia and the adjacent Andes, the palaeontological chapters provide an in-depth insight into the development of Amazonian floras and faunas. The final, fifth, part of the book is concerned with modern perspectives on the origin of Amazonian biodiversity. The book concludes with a chapter by Wesselingh et al., who summarize the highlights of each chapter and provide a synopsis of the Cenozoic history of Amazonia. The best localities for observing the outcrops and fossils are shown in Fig. 1.1.

Main geological processes shaping Amazonia through time The geography of Amazonia was shaped during three principal geological phases. The first was a Proterozoic phase (3–1 Ga [gigayears]) of cratonic formation dominated by magmatism, continental accretion and tectonic processes (see Chapter 2 by Kroonenberg & de Roever). The craton forms most of eastern Amazonia and consists of ultrastable basement with landscapes that date back to the Cretaceous and Paleogene. In terms of biodiversity these areas are relatively poor compared to the nutrientrich, Andean-dominated western part of Amazonia (see Chapters 21 & 22). At the end of the Proterozoic a series of east–west

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orientated intracratonic sedimentary basins were formed, which acted as fluvial conduits. Throughout geological history basement reactivation formed ‘arches’ that, at different times, created drainage divides. Seismic data and new stratigraphic charts from the Brazilian oil company Petrobras illustrate the development of these sedimentary basins in Brazilian Amazonia (see Chapter 3 by Wanderley Filho et al.). The second major geological phase was characterized by rifting and break-up of the supercontinent Pangaea. This period also saw the opening of the Atlantic (Jurassic, c. 195 Ma) during which the Americas became fully separated from Europe and Africa. The separation was completed during the Cretaceous after which sedimentation of the intracratonic basins was resumed (c. 120 Ma). The third and final geological phase was determined by changes in plate configuration along the Pacific. This plate activity was an aftermath of the continental break-up and ultimately responsible for the uplift of the Andean Cordilleras that was initiated during the Cretaceous. Pulses of uplift continued throughout the Cenozoic; however, Andean tectonism only reached a climax during the Late Miocene and Pliocene (c. 10–4 Ma). This resulted in intense denudation, increased subsidence in the sub-Andean zone and progression of the sedimentary wedge into Amazonia, and ultimately connected the inland drainage system with the Atlantic Ocean creating the Amazon River (see Chapters 4 & 5 by Mora et al. and Roddaz et al.; Figuereido et al. 2009). Andean uplift remained high during the Pliocene while subduction of the Nazca Ridge caused tectonic uplift of the Fitzcarrald Arch (southeastern Peru and adjacent Brazil). As a consequence the western Amazonian lowlands, which during the Miocene formed continuous aquatic habitats, became fragmented and dissected (see Chapter 6 by Espurt et al.). A final marker event in the geological history of northern South America was the closure of the Panama isthmus around 3 Ma. Although tectonism is ongoing, this concluded the Present geographical configuration of the South American continent, its landscape and modern drainage systems (see also Chapter 26).

Cratonic and Andean-driven depositional systems River systems of cratonic descent or local lowland origin have dominated Amazonian landscapes throughout their history. In this book we review the Mesozoic and Cenozoic cratonic fluvial systems by comparing four different fluvial formations that range in age from Cretaceous to Late Neogene (see Chapter 7). From the Oligocene onwards Andean-driven depositional systems dominated the sub-Andean zone and western Amazonia. These systems extended to at least 1.5 million km2 during the Miocene and were characterized by very large lakes and wetlands and occasional marine influence. During the Early and Middle Miocene a lakeand wetland-dominated system occurs (Pebas phase) whereas in the Late Miocene the newly formed Amazon River introduces a fluvial element into this otherwise wetland-dominated system (Acre phase) (see Chapter 8). Andean drainages are crucial for the soil development and distribution of species-diverse vegetation on nutrient-rich Andean-derived substrate. Instead relatively species-poor vegetation develops on the craton-derived substrate. The presence and extent of marine influence in the history of Amazonia has been a hotly debated topic. In Chapter 9, Hovikoski

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Introduction

3

Fig. 1.1 This map represents the principal Amazonian rivers and outcrops of Cretaceous and Cenozoic origin in Amazonia that are referred to in the various chapters of this book. The locations are represented as numbers and either indicate the author or the common name of the locality. (1) Pebas/Solimões outcrops (Hovikoski et al., see Chapter 9). (2) Pebas/Solimões outcrops (Hoorn et al., see Chapter 8). (3) Fossil mammals (Negri et al., see Chapter 15). (4) Localities of both Negri et al. and Hovikoski et al. (the localities close to the city of Assis Brasil are situated on the margins of the Acre River, which is not represented here. (5) Cretaceous-Paleogene Alter do Chão (source Petrobras, in Hoorn et al., see Chapter 7). (6) Neogene fluvial deposits of cratonic origin (Hoorn et al., see Chapter 7). (7) Fitzcarrald mammal fauna (Negri et al., see Chapter 15). (8) Lower Miocene Castillo Formation – other important Venezuelan localities are placed close to/into the cities of Urumaco (Upper Miocene Urumaco Formation) or Barinas (Middle Miocene Parángula Formation) – see Riff et al., Chapter 16. (9) Middle Miocene Honda Group (La Venta Fauna), Magdalena Valley. (10) Atalaia Beach (Salinópolis city) and Ilha de Fortaleza (Sao João de Pirabas city), Lower Miocene Pirabas Formation (see Riff et al., Chapter 16). Map made by D. Riff and J. van Arkel.

et al. argue that in the past 30 Ma well-documented episodes of marine influence in Amazonia are limited to the Miocene. However, there is no evidence for fully established marine corridors (‘seaways’) throughout the South American continent in the Cenozoic. The Cenozoic Andean uplift and increased denudation rates further resulted in megafan systems along the Andean foothills (see Chapter 10 by Wilkinson et al.). Megafans are low-gradient river systems choked by sediments, which force them to continuously change their courses. Understanding their dynamic behaviour sheds light on the development and distribution of aquatic biota. The extent of megafan depositional systems in the history of Amazonia is greatly underestimated. Late Neogene and Quaternary fluvial systems are further explored in Chapter 11, by Irion & Kalliola. They outline the fluvial depositional environments and processes from the foreland basins in the west to the mouth of the Amazon in the east, and

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consider the resulting landforms, which dominate a major part of the surface of lowland Amazonia. Quaternary fluvial systems along the trunk Amazon River have been dominated by strong eustatic-driven base-level changes.

Amazonian climate Although palaeoclimatic data are hard to obtain, isotope data from fossil molluscs and cyclicity in the sediment beds indicate that the modern Amazonian hydrological cycle, which ensures the yearround wet conditions that sustain the rainforests, was in place in the Miocene (see Chapter 12). Experimental climate modelling for a low-elevation Andes and the effect on Amazonian climate is explored by Sepulchre et al. in Chapter 13. Based on their model, the role of the Andes in maintaining permanent wet conditions in the lowlands is seemingly less prominent than one would expect.

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The wet character of the Amazonian climate is mostly the result of the Amazonian hydrological cycle. However, a lower Andes would create different precipitation patterns than at Present, and the removal of the Andes would increase seasonality. Another climatic controlling mechanism that affects Amazonia is the El Niño Southern Oscillation (ENSO). In Chapter 14 Bookhagen & Strecker explore the influence of the negative ENSO climatic phenomenon (also known as La Niña) on sediment influx and aggradation in the fluvial systems. The extreme high water levels as a result of high precipitation during the negative ENSO years have a disproportionate effect on denudation and are thus extremely important to the Amazonian river dynamics.

The palaeontological record in Amazonia Amazonia has hosted a highly diverse mammal fauna at least since the Paleogene. Recently discovered Eocene-Oligocene faunas and Middle Miocene faunas from the Peruvian-Brazilian border area provide us with detailed information on the faunal composition. However, most noticeable is the rich Late Miocene fauna from Acre (Brazil), which includes species with remarkably large forms (see Chapter 15 by Negri et al.). The demise of the giants coincided with the arrival of North American immigrants associated with the emergence of the Panama land bridge (Stehli & Webb 1995). The Amazonian crocodile and turtle faunas indicate that during the Cenozoic diversification was slow, but culminated in the Miocene fauna with a diversity and disparity that remains unrivalled (see Chapter 16 by Riff et al.). This fauna contains the largest crocodile and turtle that ever lived, as well as a remarkable diversity of gharial species. The Pliocene and Quaternary faunas are clearly less diverse, a feature linked by the authors to global cooling and the disappearance of the large productive aquatic ecosystems of the Miocene. The diverse Amazonian fish fauna, too, has a long history of gradual diversification, as is shown by Lundberg et al. in Chapter 17. Already in the Miocene an essentially modern fauna inhabited the Amazonian aquatic ecosystems. The fishes have provided some of the best indications of the changing outline of Amazonian watersheds throughout their Cenozoic history. Especially well reflected in this fauna is the separation, during the Late Neogene, of northern coastal and Andean drainages from Amazonia. The Miocene invertebrate fauna developed through a large evolutionary radiation of endemic mollusc and ostracod species in the long-lived lakes of the Pebas megawetland (see Chapter 18 by Wesselingh & Ramos). In addition, species associations characteristic for restricted marine conditions occur in some intervals. Nevertheless, since the Late Miocene the Amazonian rivers and lakes have been the domain of a low-diversity fluvial mollusc fauna and a stunningly diverse decapod fauna. The palynological and palaeobotanical record of plants shows us that modern angiosperm-dominated rainforests existed in Amazonia throughout the Cenozoic (see Chapter 19 by Jaramillo et al.). Diversity culminated during the Eocene, and a major extinction occurred at the Eocene–Oligocene transition. Modern genera were already present during the Miocene, when the current rainforest biome developed and diversities were as high, or even

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higher, than at Present. In Chapter 20 Behling et al. further show that although the Quaternary glaciations affected the distribution of plant species in Amazonia, they did not seem to promote speciation in the Amazonian lowlands. During the Quaternary the fringes of the rainforest were affected at precessional timescales, but the core of lowland Amazonia remained covered by forest. Nevertheless, the composition of forests changed through different parts of the glacial cycle.

Modern perspectives on the origin of Amazonian biota In Chapter 21 ter Steege et al. present the region-wide diversity patterns and explore their relationships with a range of factors, such as edaphic variation and climate. Although the documentation of biodiversity is notoriously incomplete, the addition of niche modelling has substantially improved our insights, and will do so in future. The importance of edaphic heterogeneity for plant diversity is further illustrated by Duivenvoorden & Duque in Chapter 22, which investigates the relationships between the abiotic environment (geology, geomorphology, soils) and biodiversity. Recently, many important new insights into the origin of Amazonian biodiversity and biogeography have emerged from molecular studies. In Chapter 23 Pennington & Dick review evidence from plants, while Antonelli et al. in Chapter 24 review the development of tetrapods, and the fish are treated in Chapter 25 by Lovejoy et al. All contributors caution about hasty interpretation of age estimates from so-called molecular clock studies because of the underlying assumptions. Nevertheless, results clearly indicate that the origination of modern biota has been a steady process that mostly played in the Cenozoic.

Outlook New insights and data about the origin of Amazonian landscapes, ecosystems and biodiversity are accumulating even as we compile this book. Further integration of the various biological, geographical and geological disciplines, as well as further technical and conceptual developments within the different fields, will continue to bring new insights about the Amazonian biological system and its resilience, as well as the importance of Amazonia on global processes on a variety of time scales. As Amazonia is suffering badly from human activities, new and much more ambitious efforts to assess its biodiversity, mostly by time-consuming field-based taxonomic inventories combined with niche modelling, are paramount to get a better sense of the magnitude of species richness and to identify further priorities for conservation. Molecular studies have become an indispensable tool in identifying real species richness. Further processing of subsurface data, both seismic as well as borehole data and samples, will add to our knowledge of the development of the region and its landscapes. Study of the reaction of biodiversity to previous natural perturbations will bring more insights about ecosystem resilience, at a time when such insights are so badly needed. Raising awareness of the unique and amazing diversity of life in Amazonia is needed in order to achieve better protection

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Introduction for the region and its biota. With this book we hope to enhance appreciation of the vast timescales that were needed to create these great ecosystems, which we are challenging so profoundly at this moment in history.

Acknowledgements This book is the result of an intense cooperative effort of more than a hundred people from different continents who all share a scientific, and often a personal, interest in Amazonia. As editors we were overwhelmed by the enthusiasm of all our colleagues who participated in this project. The novel and original insights presented through the chapter authors and reviewers encouraged us at all times. It has been a long journey during which many things happened in our personal lives; however, the book project always joined us together. The compilation of this book has been a long process (2.5 years to reach publishable form) but during that process we have learnt a great deal about Amazonian history from colleagues old and new. However, this book would not have reached its present shape without the help of all the reviewers (sometimes in a dual role of authors) who shared their time with us and helped us improve the manuscripts. A big thanks to the following people (in alphabetical order): Frank Audemard, German Bayona, Bodo Bookhagen, Chris Brochu, Alberto Cione, Joost Duivenvoorden, Richard Field, Jorge Figueiredo, Paul Fine, Jose I. Guzman, Mathias Harzhauser, Ren Hirayama, Henry Hooghiemstra, Jussi Hovikoski, Teresa Jordan, Wolfgang Junk, Risto Kalliola, Salle Kroonenberg, John Lundberg, Bruce MacFadden, Richard Madden, Koen Martens, Jose Ignacio Martinez, Mark Maslin, Francis Mayle, Michael McClain, Bob Morley, Brice Noonan, Onno Oncken, Jim Patton, Toby Pennington, Sir Ghillean Prance, George Postma, Victor Ramos, Roberto Reiss, Colombo Tassinari, Eric Tohver, Graham Wallis, Wilfried Winkler, Martin Zuschin, and a number of colleagues who requested to remain anonymous. Our editorial advisors, Hubert Vonhof, Salomon Kroonenberg and Henry Hooghiemstra, helped us with many tasks related to the book. But above all we owe them special thanks for standing beside us and giving us all their support and advice during crucial moments in this project. Overseas, at the Petrobras offices, we warmly thank Paulus van der Ven for his continual support; Petrobras management are also thanked for the valuable subsurface information that the company made available to this book. None of this would have ever happened, though, if Ian Francis from Wiley-Blackwell Publishers had not believed in this project in the first place. We thank him for his trust and also thank Delia Sandford and Kelvin Matthews, also from the publishing house, and project manager, Nik Prowse, for their patience, help and advice in this project. The success of this project is also based on the unconditional support of our beloved ones (Alastair Milne and Maaike Wickardt, respectively) who enabled us to complete the process of the making of this book. Our families have born most of the brunt of our work. We are so happy they still love us and are in good health with us to enjoy this milestone after considerable periods of mental absence.

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5

Finally, we are immensely thankful that Thomas van der Hammen managed to complete the prologue of this book, this in spite of his deteriorating health. Thomas has been our mentor, who inspired us to take on the mighty Amazon. We dedicate this volume to him in admiration of his longstanding commitment to Amazonian research, and hope that this book will inspire young scientists to undertake further research, just as Thomas inspired us to engage in research in wonderful Amazonia. There is still so much to research out there in the virgin rainforest of South America . . .

References Ab’Sabr, A.N. (1982) The paleoclimate and palaeoecology of Brasilian Amazonia. In: Prance, C.T. (ed.) Biological Diversification in the Tropics. New York: Columbia University Press, pp. 41–59. Absy, M.L., Cleef, A.L., Fournier, M., Martin, L., Servant, M., Sifeddine, A., Da Silva, M.F., Soubies, F. Suguio, K., Turcq, B., Van der Hammen, T. (1991) Mise en évidence de quatre phases d’ouverture de la forêt dense dans le sud-est de l’Amazonie au cours des 60.000 dernières années. Première comparaison avec d’autres régions tropicales. C.R. Acad. Sci. Paris 312, 673–678. Antonelli, A. (2008) Spatiotemporal evolution of Neotropical organisms: new insights into an old riddle. Doctoral thesis. University of Gothenburg, Göteborg, Sweden, 84 pp. Bush, M.B., Crisci, J., Whittaker, R.J. (2007) Special issue: Conservation and Biogeography of Amazonia. J Biogeog 34, 1289. Bush, M., Flenley, J. (2006) Tropical Rainforest Responses to Climatic Change. Springer. Colinvaux, P.A., De Oliveira, Bush, M. (2000) Amazonian and Neotropical plant communities on glacial time-scales: The failure of the aridity and refuge hypothesis. Quaternary Sci Rev 19, 141–170. Colinvaux, P.A., Irion, G., Rasanen, M.A., Bush, M., de Mello, J. (2001) A paradigm to be discarded: Geological and paleoecological data falsify the Haffer & Prance Refuge Hypothesis of Amazonian sopeciation. Amazoniana 16, 609–646. Dobson, D.M., Dickens, G.R., Rea, D.K. (2001) Terrigenous sediment on Ceara Rise: a Cenozoic record of South American orogeny and erosion. Palaeogeogr Palaeocli 165, 215–229. Figuereido, J., Hoorn, C., van der Ven, P., Soares, E. (2009) Late Miocene onset of the Amazon River and the Amazon deep-sea fan: Evidence from the Foz do Amazonas Basin. Geology 37, 619–622. Haffer, J. (1969) Speciation in Amazonian forest birds. Science 165, 131–137. Haffer, J., Prance, G.T. (2001) Climatic forcing of evolution in Amazonia during the Cenozoic: On the refuge theory of biotic differentiation. Amazoniana 16, 579–608. Hooghiemstra, H., Van der Hammen, T. (1998) Neogene and Quaternary development of the neotropical rain forest: the forest refugia hypothesis, and a literature overview. Earth-Sci Rev 44, 147–183. Hoorn, C. (1993) Marine incursions and the influence of Andean tectonics on the Miocene depositional history of northwestern Amazonia: results of a palynostratigraphic study. Palaeogeogr Palaeocl 109, 1–55. Hovikoski, J. (2006) Miocene Western Amazonia in the light of sedimentological and ichnological data. PhD thesis, Annales Universitatis Turkuensis, AII, 1–138. Hubbell, S.P., He, F., Condit, R., Borda-de-Água, L., Kellner, J., ter Steege, H. (2008) How many tree species are there in the

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Amazon and how many of them will go extinct? PNAS 105, 11498–11504. Jaramillo, C., Rueda, M.J., Mora, G. (2006) Cenozoic plant diversity in the Neotropics. Science 311, 1893–1896. Kalliola, R., Flores-Paitan, S. (eds) (1998) Geoecologia y desarrollo Amazonico: estudio integrado en la zona de Iquitos, Peru. Annales Universitatis Turkuensis, A II, 114, 544 pp. Kalliola, R., Puhakka, M., Danjoy, M.W. (eds) (1993) Amazonia Peruana, Vegetación Humeda Tropical en el Llano Subandino. Turku: University of Turku. Lovejoy, N.R., Bermingham, R.E., Martin, P. (1998) Marine incursions into South America. Nature 396, 421–422. Lundberg, J.G., Marshall, L.G., Guerrero, J., Horton, B., Malabarba, M.C.S.L., Wesselingh, F. (1998) The stage for Neotropical fish diversification: A history of tropical South American rivers. In: Reis, R.E., Vari, R.P., Lucena, Z.M., Lucena, C.A.S. (eds) Phylogeny and Classification of Neotropical Fishes. Porto Alegre: Edipucrs, pp. 13–48. Morley, R.J. (2000) Origins and Evolution of Tropical Rainforests. New York: John Wiley & Sons. Nelson, B.W., Ferreira, C.A.C., da Silva, M.F., Kawasaki, M.L. (1990) Endemism centres, refugia and botanical collection density in Brazilian Amazonia. Nature 345, 714–716. Nores, M. (2002) An alternative hypothesis for the origin of Amazonian bird diversity. J Biogeogr 26, 475–485. Räsänen, M.E., Salo, J.S., Kalliola, R.J. (1987) Fluvial perturbance in the western Amazon Basin: Regulation by long-term Sub-Andean tectonics. Science 238, 1398–1401. Rull, V. (2008) Speciation, timing and neotropical biodiversity: The Tertiary-Quaternary debate in the light of molecular phylogenetic evidence. Mol Ecol 17, 2722–-2729.

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Ruokolainen, K., Tuomisto, H., Macía, M.J., Higgins, M.A., Yli-Halla, M. (2007) Are floristic and edaphic patterns in Amazonian rain forests congruent for trees, pteridophytes and Melastomataceae? J Trop Ecol 23, 13–25. Salo, J., Kalliola, R., Häkkinen, I., Mäkinen, Y., Niemelä P. et al. (1986) River dynamics and the diversity of Amazon lowland forest. Nature 322, 254–258. Sioli, H. (ed.) (1984) The Amazon: Limnology and Landscape Ecology of a Mighty Tropical River and its Basin. Dordrecht, Boston, Hingham, MA: W. Junk. Stehli, F.G., Webb, S.D. (1985) The Great American Biotic Interchange. Topics in Biogeology, vol. 4. New York, London: Plenum Press, 532 pp. Subramaniam, A. (2008) Amazon River enhances diazotrophy and carbon sequestration in the tropical North Atlantic Ocean. PNAS 105, 10460–10465. Sugden, D. (ed.) (2000) Quaternary climate change and South America. J Quaternary Sci (Special Issue) 15, 299–468. Tuomisto, H. (2007) Interpreting the biogeography of South America. J Biogeogr 34, 1294–1295. Van der Hammen, T., Hooghiemstra, H. (2000) Neogene and Quaternary history of vegetation, climate, and plant diversity in Amazonia. Quaternary Sci Rev 19, 725–742. Wesselingh, F.P., Salo, J. (2006) A Miocene perspective on the evolution of Amazonian biota. Scripta Geologica 133, 439–445. Wesselingh, F.P., Räsänen, M.E., Irion, G.E., Vonhof, H.B., Kaandorp, R., Renema, W., Romero Pittman, L., Gingras, M. (2002) Lake Pebas: a palaeoecological reconstruction of a Miocene, longlived lake complex in western Amazonia. Cainozoic Res 1, 35–81.

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PART I

Tectonic processes as driving mechanisms for palaeogeographical and palaeoenvironmental evolution in Amazonia

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TWO

Geological evolution of the Amazonian Craton Salomon B. Kroonenberg1 and Emond W.F. de Roever2 1

Delft University of Technology, Delft, The Netherlands NALCO EUROPE, Leiden, The Netherlands

2

Dedicated to the memory of Dr D.R. (Rob) de Vletter, deceased 24 May 2008.

Abstract The Amazonian Craton, the core of the South American continent, consists of: (i) Archean nuclei, including the Carajás-Amapá areas in the southeast and the Imataca area in the northwest (3.0–2.5 Ga); (ii) the 1500-km long Trans-Amazonian greenstone-tonalite belt with associated granulite belts along much of the northern coast of the Guianas and northern Brazil (2.2–2.0 Ga); (iii) a Grenvillian orogenic belt along the westernmost part in southwestern Brazil (1.3–1.0 Ga); and (iv) a vast central part in which Paleoproterozoic and Mesoproterozoic granitoid and low-grade metavolcanic rocks predominate. Granitoid magmatism continued here almost uninterrupted between 2.0 Ga and 1.0 Ga, although cratonization was largely completed by 1.75 Ga. We argue that previous continental accretion models based on progressively younger granite ages westwards have to be revised in view of new geochronological data. Little-deformed sandstone platforms unconformably overlying older basement are widespread, and have been deposited in different episodes of post-orogenic basin formation from the Archean down to the Phanerozoic. Mafic dykes of Proterozoic to Permo-Triassic age testify to various phases of extension, rifting and basin formation, including the formation of the Paleozoic basin system and the later Amazon drainage basin itself. Uplift and denudation since Gondwana break-up greatly increased sediment fluxes towards the surrounding basins from the Mesozoic onwards.

Introduction The Amazonian Craton forms the oldest nucleus of the South American continent, and is divided by the Amazon drainage basin into two parts, the Guiana Shield in the north, and the Guaporé or Central Brazilian Shield in the south (Fig. 2.1 & Plate 6). In all modern syntheses, the main geological units of the Guiana Shield continue below the Amazon drainage basin into the Guaporé Shield without offset (Tassinari & Macambira 1999, 2004; Almeida et al. 2000; Cordani et al. 2000; Santos et al. 2000; Tassinari et al. 2000; Santos 2003; Cordani & Teixeira 2007). The Amazonian Craton also continues westwards below the cover of the sub-Andean basins, as numerous blocks of Precambrian rocks have been incorporated into the cordilleras of the northern Andes during orogeny

Amazonia, Landscape and Species Evolution: A Look into the Past, 1st edition. Edited by C. Hoorn and F.P. Wesselingh. © 2010 Blackwell Publishing

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(Fig. 2.1), and seismic data and drilling by oil companies in the foreland basins invariably show the presence of Precambrian basement below (Kovach et al. 1976). This means that the western Amazon drainage basin, on which this book focuses, is underlain in its entirety by continental crust. As the oldest sediments in the deepest part of the Paleozoic basin system itself are of Ordovician age (Wanderley Filho et al. 2005; see also Chapter 3) the geological history of the Amazonian Craton is essentially restricted to the Precambrian. In predrift reconstructions the Amazonian Craton forms part of western Gondwana. It has its counterpart in western Africa, and southwards and eastwards it continues into other cratonic parts of the South American Platform (Fig. 2.1), but we will not consider these correlations further afield. Knowledge of the geology of the craton is of paramount importance for anyone studying the history of the basins that surround it. The origin and configuration of the basins themselves reflect tectonic patterns and processes, which often can be traced back to the early history of the craton (Brito Neves 2002). The craton is the source of most of the sediments in the intra- and pericratonic

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10

S.B. Kroonenberg & E.W.F. de Roever Legend

Phanerozoic Neoproterozoic Mesoproterozoic Paleoproterozoic Archean Archean

(Meta) sedimentary platform Felsic Felsic covers volcanics intrusives

a

Mafic and Highalkaline Greenstone grade intrusives belts belts

b

a: TTG and Uatumã-type granitoid rocks b: other granitoid rocks Tra ns

-Am

azo

nia

0

nb

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500 km

elt

Gre

lia nvil no

rog ic en

Archean

l be

lt

t

be

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an

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lt

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o ilian Bras

ge o ro

c ni

Fig. 2.1 Outline geology of the Amazonian Craton. Map details derived from Schobbenhaus & Bellizzia (2001), Gibbs & Barron (1993) and Tohver et al. (2004a). Compiled by S.B. Kroonenberg and E.W.F. de Roever (2009) - Design by GeoMedia (7242). See Plate 7 for a colour version of this figure.

sedimentary basins, and cratonic sediments often have a specific provenance fingerprint when compared with those from the other major source, the Andes (Franzinelli & Potter 1983; Potter 1994; see also Chapter 7). Past and present relief and drainage patterns in the craton not only reflect major uplift and subsidence events, but also minute differences in the susceptibility of specific lithologies to weathering and erosion. Understanding the history of the Amazonian Craton is hampered by many factors. In the first place, tropical rainforest and deep weathering adversely influence outcrop condition and accessibility. A serious survey of the geological features started only in the 1960s and 1970s, when detailed photogeological studies, such as in Suriname, were carried out, aerogeophysical surveys and extensive radar imagery was obtained through projects like Radambrasil in Brazil, Codesur in Venezuela and Proradam in Colombia; moreover, geochronological, structural and petrological data from numerous field surveys came pouring in. Since then, many excellent reviews of the geology of the craton have been made, including Amaral (1974), Mendoza (1974), Gibbs & Barron (1983, 1993), Teixeira et al. (1989), Cordani & Sato (1999), Cordani & Teixeira (2007), Tassinari & Macambira (1999, 2004), Santos et al. (2000), Almeida et al. (2000), Tassinari et al. (2000)

Hoorn_ch02_Final.indd 10

and Delor et al. (2003). This chapter is based on our own field experiences in the Guiana Shield, especially in Suriname and Colombia, and to a lesser extent in French Guiana, Venezuela and Brazil, and furthermore a large amount of literature in international and local journals and books. It is difficult to do justice to the enormous amount of data and analyses that have been obtained during the last decades, and this review does not pretend to do more than sketch the bare outlines of what we think are the most salient features of the craton. The Amazonian Craton consists essentially of three major groups of basement terranes: (i) greenstone belts, consisting of intensely folded, low-grade metasedimentary and metavolcanic rocks intruded by tonalitic plutons; (ii) high-grade metamorphic belts, consisting mainly of gneisses and granulites of varying parentage; and (iii) vast expanses of granitic and acid metavolcanic rocks, in which supracrustals are scarce. In several areas flat-lying Precambrian to Ordovician sandstones unconformably overlie the crystalline basement in impressive table mountains. Mafic dykes of greatly varying ages have intruded the basement as well as the sandstone plateaus. There is very little evidence of Precambrian fossil life in the Amazonian Craton, and therefore reconstructing its tectonic history hinges completely on isotope geochronology.

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Geological evolution of the Amazonian Craton

Archean > 2.5 Ga Paleoproterozoic 2.5–1.6 Ga Mesoproterozoic 1.6–1.0 Ga Neoproterozoic 1.0 Ga to 542 Ma Geochronology has made great technological advances in the past 40 years. On the one hand, this is an advantage, because now we know some parts of the history with much greater accuracy than in the past. On the other hand, it has also a disadvantage in that many earlier models, made on the basis of radiometric dating methods now considered obsolete or at least questionable, have to be reconsidered entirely. In the early years, for instance, 1.2 Ga (= giga annum, 109 years) potassium-argon (K-Ar) mica ages in the Colombian Amazonia were considered to reflect the age of the rocks themselves (Pinson et al. 1962), while later rubidiumstrontium (Rb-Sr) studies (Priem et al. 1982) demonstrated this age to represent a later metamorphic overprint over an older rock. Many Rb-Sr isochrons from the 1970s and 1980s, in their turn, now have been superseded by ages obtained by SHRIMP (sensitive highresolution ion microprobe) uranium-lead (U-Pb) dating on zircons. As a result some rocks are now known to be 100 to 400 million years older than originally thought, as for instance happened to the Roraima sandstones and their intruding mafic dykes (Santos et al. 2003b). Moreover, not all geochronological data reflect the same type of event: samarium-neodymium (Sm-Nd) model ages record the first differentiation of granitic rocks from the earth’s mantle (Cordani & Sato 1999; Sato & Siga 2000), an age that may considerably predate the date of actual emplacement of a granite pluton in an orogen. Therefore, a review of the history of the Amazonian Craton cannot simply lump together all existing data, but has to make choices in the light of newer insights and dating methods. A more fundamental problem is what weight to give to geochronological data when confronted with the major lithological and structural units of the Amazonian Craton. Age provinces and geotectonic provinces often do not correspond, and bitter controversies between geochronologists and more geodynamically oriented geologists continue up to the present day. A geochronological province maps all the rocks that show the same age in one province, irrespective of their origin. A geotectonic province, however, maps all the rocks with a common geodynamic origin together, for instance in one mountain belt, even if some radiometric ages in it do not match the general picture. A striking example of this lack of correspondence is given when comparing the geological map of the Guiana Shield by Delor et al. (2003) and of the geological sketch map of a large part of the craton by Dall’Agnol et al. (1994) with the geochronological maps by Tassinari & Macambira (1999) and Santos et al. (2000) and Santos (2003) (compare Plates 6 and 7, and see Fig. 2.4 and discussion below). This controversy is at the heart of an old question, the subject of the International Geological Correlation Project 204, about whether the Amazonian Craton is a large Archean platform reworked and reactivated during the Proterozoic (Almeida et al. 1981), or whether its evolution is punctuated by episodic crustal accretion during the Proterozoic (Tassinari 1981; see discussion

Hoorn_ch02_Final.indd 11

by Teixeira et al. 1989). In this chapter we will argue that the concept of geochronological provinces is no longer useful, and that many previous subdivisions of the Amazonian Craton based on geochronological provinces will have to be revised. We will also challenge the validity of the continental accretion model for the central granitoid part of the Amazonian Craton that has been in vogue in recent years on the basis of geochronological provinces.

Main subdivisions of the Amazonian Craton The Amazonian Craton contains three important and well-recognizable orogenic belts of different age: 1 Relatively small Archean cores with ages around 2.8 Ga, encompassing the Carajás range in the easternmost Guaporé Shield, the Amapá block in the easternmost Guiana Shield, and the Imataca Complex in the northwesternmost Guiana Shield; 2 a Trans-Amazonian greenstone belt, stretching along the northern coast of almost the whole Guiana Shield for over 1500 km, with ages around 2.2 to 2.0 Ga 3 a Grenvillian belt, along the southwestern and western margin of the Amazonian Craton, with ages around 1.3–1.0 Ga. Interestingly, detrital zircons from modern Orinoco and Amazonian sediments also show a predominance of these three age groups (Goldstein et al. 1997; Rino et al. 2004) (Fig. 2.2). The Amazonian Craton is bordered in the southeast by the Neoproterozoic Tocantins-Araguaia belt (Brasiliano cycle), which will not be discussed in this chapter. Granitoid and acid metavolcanic rocks occupy vast expanses between these three belts. The main geochronological provinces of the central part of the craton, as defined by Cordani & Sato (1999), Tassinari & Macambira (1999, 2004) and Tassinari et al. (2000), refer to these granitoid rocks. However, most of these ages

50

Oldest 238U-206Pb age 3401 ± 54 Ma

40 Frequency (number)

In this chapter we follow the internationally agreed subdivisions of the Precambrian based upon geochronological data (Gradstein et al. 2004):

11

30

20

10

0

0

500

1000 1500

2000 2500 3000 Age (Ma)

3500

Fig. 2.2 Histograms for U-Pb ages for 369 grains of detrital zircon collected from the mouth of the Amazon River outlet show peaks in the Archean, Trans-Amazonian and Grenvillian intervals. Pb-Pb ages give similar results. The error bars correspond to 1σ. (After Rino et al. 2004.)

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12

S.B. Kroonenberg & E.W.F. de Roever

are based on Rb-Sr isochrons, which are now no longer thought to reflect the age of crystallization. Santos et al. (2000), on the basis of new U-Pb zircon ages, make a different subdivision. Dall’Agnol et al. (1999) avoid the term geochronological provinces, and refer to them as blocks, using the same boundaries as Tassinari et al. (2000). All these subdivisions cannot hide the facts that there are great overlaps in ages between the different granitoid provinces (see tables 3, 4 and 5 in Tassinari & Macambira, 1999), and that there seems to be a continuum of granitoid magmatism between 2.0 and 1.75 Ga rather than a series of discrete events as suggested by both subdivisions. Furthermore, there is a series of better defined anorogenic Mesoproterozoic granitoid intrusions, concentrated along the northwestern and southwestern parts of the shield. We will therefore stick to a more descriptive subdivision of the craton.

Archean mobile belts What are the oldest nuclei around which the craton started to grow? There have to have been Archean cratons, if only to explain which plates caused the continental collision that gave rise to the 1500-km long Trans-Amazonian orogenic belt that stretches along the northern coast of most of the Guiana Shield and beyond; but where are they? Unfortunately there is no easy answer to that question. There are numerous Archean ages from restricted areas in the craton, as we will see below, but as Benjamim Bley de Brito Neves (1999) states in one of the seminal papers on the craton: ‘none of these Archean terranes/cores appear as autonomous units (full cratonic areas); they consist of not-autochthonous, reworked fragments, because they were structured and incorporated in orogenic movements and processes of the Paleoproterozoic (at least)’ (translation SBK).

Serra das Carajás terrane The only unquestionably pure Archean terrane in the Amazonian Craton is the Serra das Carajás area, which occupies a relatively restricted area in the easternmost part of the Guaporé Shield, 900 km south of Belém between the Tocantins and Xingú rivers. It is the most important mineral province of Brazil, hosting the largest iron mine in the world, and containing rich copper, gold, manganese, nickel and other ore deposits (Olszewski et al. 1989; Tassinari et al. 2000; Tassinari & Macambira 2004; Tallarico et al. 2005; Dall’Agnol et al. 2008). The area consists of two major tectonic domains: the older Rio Maria and Pau d’Arco or Inajá granite-greenstone terranes in the south, and the younger Carajás Basin proper in the north. Granulites from the Pium high-grade terranes in the southern part of the Carajás Basin proper show Pb-Pb whole rock protolith ages of around 3 Ga, and a SHRIMP zircon U-Pb age of 2.86 Ga for the granulite-facies metamorphism (Pidgeon et al. 2000). They may represent deep slabs caught in the suture zone between the older Rio Maria terrane and the younger Carajás Basin proper (Tallarico et al. 2005). The oldest rocks in the Rio Maria greenstone belt, the Andorinhas supergroup, give ages between 2.98 and 2.90 Ga

Hoorn_ch02_Final.indd 12

(Tassinari & Macambira 2004). They were affected by a shear event around 2.87 Ga. The Itacaiúnas Supergroup in the Carajás Basin itself consists of a lower-grade metamorphic greenstone sequence, the Grão Pará Group, and a higher-grade Salobo Group (Tallarico et al. 2005). The metavolcanic and metasedimentary sequences of the Grão Pará Group in the greenstone belt, which contain the Banded Iron Formation (BIF) ore bodies, were deposited between 2.75 and 2.74 Ga, according to SHRIMP zircon U-Pb datings (Trendall et al. 1998; Tallarico et al. 2005), and deformed and metamorphosed between 2.58 and 2.50 Ga. Detrital zircons in these series may be as old as 3.7 Ga. The Grão Pará Group shows the classic greenstone succession of a mainly metabasaltic unit, locally with conspicuous pillow structures, minor meta-andesites and metarhyolites, followed by the BIF and topped by intermediate to acid metavolcanics and metasediments. The greenstone sequences are intruded syntectonically by tonalite-trondhjemitegranodiorite (TTG) bodies dated around 2.87 Ga. Both domains are intruded by Archean granitic and mafic-ultramafic bodies bodies ranging in age between 2.74 and 2.53 Ga, and Paleoproterozoic granites dated around 1.88 Ga, according to U-Pb data on zircons (Tassinari et al. 2000; Tassinari & Macambira 2004). Neodymium isotope data from the Paleoproterozoic granites of the Carajás Basin favour an ensialic evolution (Dall’Agnol et al. 2008).

Amapá and Imataca terranes A large area with predominantly Archean U-Pb zircon ages in high-grade meta-igneous gneisses, intruded by Paleoproterozoic granitoid bodies, is found in the 400-km long Amapá block (JaríGuaribas Complex), whereas smaller terranes with such ages are found nearby in Parú, Cupixi and Tartarugal Grande areas in the extreme southeastern corner of the Guiana Shield (see Fig. 2.1) (Santos et al. 2000; Delor et al. 2003; Avelar et al. 2003; Da RosaCosta et al. 2006). The Imataca Complex, a 500-km long ENE–WSW-stretching high-grade metamorphic complex in the northwesternmost part of the Guiana Shield in Venezuela consists of granulites, gneisses and amphibolites with important occurrences of BIF, and has long been considered to be of Archean age as well (Montgomery & Hurley 1978). New SHRIMP U-Pb data on oscillatorily zoned zircon cores confirm that the protoliths, the parent rocks from which the metamorphic rocks originated, are of Archean age, but the 2.2–2.0-Ga metamorphic overgrowths of the zircons suggest that the high-grade metamorphism is of Trans-Amazonian age, unlike the Carajás granulites (Tassinari et al. 2004a,b). The Carajás, Amapá and Imataca terranes are far apart from each other, and there is no physical connection between them; all rocks in the intervening part show Trans-Amazonian or younger ages, although occasionally Archean zircons have been found in some rocks (Delor et al. 2003).

Trans-Amazonian orogenic belt Trans-Amazonian greenstone belts The term ‘Trans-Amazonian Orogenic Cycle’ was coined by Hurley et al. (1967) to designate an orogenic event around 2.0 Ga

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Geological evolution of the Amazonian Craton

lithological similarity, apart from the absence of an upper detrital unit in the western part. Both belts are closely associated with tonalitic bodies (tonalite/trondhjemite/granodiorite suite, TTG). A large central TTG complex separates the belt in French Guiana into southern and northern branches, which merge westwards in Suriname into a single belt (Delor et al. 2003). The western belt in Venezuela and Guyana consists of three greenstone branches separated by TTG bodies (Gibbs & Barron 1993).

that affected the eastern half of the South American continent. A large greenstone belt extends for about 1500 km along the northern part of the Guiana Shield, from the Orinoco River (and Imataca Complex) in the west to the Amazon delta in the east. The belt is not continuous: a western Venezuela–Guyana belt and an eastern Brazil–French Guiana–Suriname belt are separated by the Bakhuis horst in western Suriname (Fig. 2.3). A clear continuation between the two parts is lacking. The two belts have a large 65º

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"Kayser" dolerite (1.5 Ga) Rio Negro province

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Granulite (2.07 - 2.05 Ga) : a) Banded granulite; b) Charnockite, gabbro; c) Charnockite intrusion among granite; d) Reworking of Archean protolith

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Metaluminous monzogranite (2.07 - 2.06 Ga)

Main Transamazonian event (2.26 - 2.08 Ga) Neorhyacian granitic plutonism 10 in part Mg-potassic (2.11 - 2.08 Ga) a

PALEOPROTEROZOIC

12

11

13

5a "Avanavero"/"Comté" dolerite (1.8 Ga)

5b

a) Biotite granite, two-mica granite b) Acid volcanism and hypovolcanic plutonism, c) conglomerate

Granulite, charnockite and upper amphibolite gneiss ( 2.00 - 1.93 Ga) a

8

MESOPROTEROZOIC

rn

51°

Late Transamazonian tectonometamorphic and magmatic event (Late Rhyacian to Orosirian : 2.07 - 1.93 Ga)

NEOPROTEROZOIC

4

2

53°

Late Transamazonian acid plutono-volcanic event (Orosirian : 2.01 - 1.96 Ga, pro parte "Uatumã")

"Apatoe" dolerite (200-195 Ma)

2

55°

PALEOPROTEROZOIC Undifferentiated Paleoproterozoic

Continental cover

1

57°

14

"Roraima" sandstone

Pull-apart detrital basin synchronous with the sinistral transcurrent tectonism

15

Mesorhyacian TTG plutonism Central complex (2.15 - 2.13 Ga) a) Gabbro N and S complexes (2.18 - 2.16 Ga) 17

b

c 16 Eorhyacian tholeiitic magmatism a 18 in an oceanic domain ( 2.26 - 2.20 Ga)

ARCHEAN Mesoarchean basement (3.30 - 2.90 Ga) 19 reactivated by Transamazonian event

Greenstone belt a) low-to intermediate pressure thermal aureoles b) intermediate-pressure metamorphism c) Archean derived pelitic sediments

Transamazonian structural event (2.18 - 1.93 Ga) D1 synform axis

D2 synform axis

D1 foliation trend

D2 foliation trend

Nickerie/K'Mudku event ( 1.20 Ga) brittle/ductile cataclasis

D2 sinistral shear zone

D2 normal fault

D2 dextral shear zone

Zone of D2 low dip foliation Uatumã plutonovolcanic trend

Fig. 2.3 Geological subdivision of the Trans-Amazonian orogenic belt in the Guiana Shield. From Delor et al. (2003), reproduced with permission.

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S.B. Kroonenberg & E.W.F. de Roever

The greenstone belts comprise volcanic and sedimentary members, with low- or medium-grade metamorphism. The names for the greenstone successions vary from country to country: ‘Pastora’ in Venezuela, ‘Barama-Mazaruni’ in Guyana, ‘Marowijne’ in Suriname, ‘Maroni’ in French Guiana and ‘Vila Nova’ in Brazil. The volcanics consist of tholeiitic (meta)basalt and minor komatiite, with interstratified andesite, dacite and rhyolite. The lower units comprise primitive, low-K, Fe-rich tholeiitic basalts with a strong affinity to island-arc tholeiite, overlain by or partly alternating with calc-alkaline andesitic rocks with a characteristic island-arc signature (Veenstra 1978; Gibbs 1980; Gibbs & Barron 1983, 1993; Elliott 1992; Voicu et al. 1997) in the higher units. (Meta)greywacke, shale and some chemical sediments are interbedded with the volcanics. They were derived predominantly from erosion of the associated volcanics, and show widespread graded bedding indicative of deposition by turbidity currents, probably in an arc-trench environment (Bosma et al. 1983). The sequence is up to 10 km thick (Bosma et al. 1978, 1984; De Vletter 1984; Gibbs & Barron, 1993; De Vletter et al. 1998; Delor et al. 2003). The eastern belt was formed between 2.18 and 2.13 Ga. In French Guiana two TTG suites were found, an older suite of 2.18–2.16 Ga and a younger one of about 2.15–2.13 Ga. Zircon Pb-Pb ages on acid to intermediate volcanics ranged from 2.16 to 2.14 Ga (Delor et al. 2003). The western belt shows a similar age. An upper detrital unit unconformably overlies the greenstone successions and TTG suite in north Suriname (Rosebel Formation; Bosma et al. 1984) and French Guiana (Orapu Formation). The unit consists of cross-bedded sandstones and polymict conglomerates, including monogenic gold-bearing conglomerate, and is of the order of 5000 m thick. Milesi et al. (1995) inferred the maximum age of deposition to be 2.12 Ga on the basis of detrital zircon dating. The greenstone belt is considered to represent a Paleoproterozoic belt accreted to an Archean block in the south, during a collisional orogeny with the West-African Archean Shield at 2.2–1.95 Ga, thus forming a major period of crustal growth (Cordani et al. 2000; Santos et al. 2000; Tassinari et al. 2000; Delor et al. 2003). The positive neodymium isotope ratio ε(Nd)t values of acid metavolcanics and metagreywacke from the greenstone belt and from associated TTG (Lafrance et al. 1999; Nogueira et al. 2000; Delor et al. 2001) point to a juvenile character and preclude the involvement of significant pre-Trans-Amazonian crust. Palaeomagnetic studies by Nomade et al. (2003) suggest that the West African Craton and the Guiana Shield were separate blocks until they joined at about 2.02–1.99 Ga. Several stages were distinguished in French Guiana (Ledru et al. 1994; Vanderhaeghe et al. 1998; Delor et al. 2003). A remnant of an oceanic stage with juvenile oceanic crust was found in the Ile de Cayenne gabbro and trondhjemite along the northern margin of the belt, with zircon ages as old as 2.21–2.22 Ga. A 2.18–2.13-Ga north–south convergence stage with southward subduction followed, with two pulses of TTG batholiths and with a major tectonic phase D1 in metavolcanics and metasediments. When the island-arc basins were closed, convergence continued at an oblique angle, 2.11–2.08 Ga, with sinistral sliding and

Hoorn_ch02_Final.indd 14

the formation of ‘pull-apart’ basins in which the upper detrital unit was deposited. Syntectonic granitic batholiths were emplaced during this stage and a major tectonic phase D2 developed in metavolcanics and metasediments, including the upper detrital unit. Further crustal stretching occurred as late as 2.07–2.06 Ga, with coeval leucogranite emplacement. The problem of the plate tectonic interpretation is that the Amazonian Craton to the south shows only limited amounts of older (Archean) material. However, metapelite from the southern and northern branches of the greenstone belt in French Guiana shows markedly negative ε(Nd)t values, pointing to pre-TransAmazonian crustal sources for the metapelite. Also Archean inherited zircons were found locally in granites, and locally in metasediments in southeastern French Guiana (Delor et al. 2003). The Vila Nova greenstones in Amapá and northern Pará in northeastern Brazil are separated from the greenstone belt in French Guiana and differ in lithology (prominent banded iron formation, BIF). They occur in part next to or mixed with a large Archean zone mentioned above. Their relation to the main greenstone belt is not clear. The same holds for enclaves of medium-grade metasediments (such as BIF) in the southern parts of French Guiana and Suriname.

Trans-Amazonian high-grade terranes The Central Guiana Granulite belt (Kroonenberg 1976) is a 1000-km long SW–NE belt of mainly granulitic rocks stretching from western Suriname across southern Guyana into northwestern Brazil, State of Roraima. It transects the east– west trending greenstone belt and separates the Guiana Shield into western and eastern parts. The belt is defined in part by prominent faults of younger age along the Kanuku and Bakhuis horsts. The belt splits in Suriname into a northeastern branch, the Bakhuis horst, and a southeastern branch, the Coeroeni Group. The high-grade terranes have a conspicuous feature in common: two metamorphic phases, with an older cordierite-producing phase and a static younger phase of cordierite replacement. Gibbs & Barron (1993) did not include the Coeroeni Group in the belt. However, according to Delor et al. (2003), aeromagnetic evidence and lack of relief east of the Kanuku horst as well as geochronological data do not support a continuation towards the Bakhuis horst in northwest Suriname, but an arcuate continuation to the southeast, to the Coeroeni area in southwestern Suriname. Recent work in Roraima State in Brazil supports the latter concept, and includes the Coeroeni Group into a sinuous high-grade Cauarane-Coeroeni belt forming a key tectonic feature of the Guiana Shield that marks the approximate limit between two entirely distinct domains: the 2.22–2.08 Ga granite-greenstone domain with remnants of older crust recycled at round 2.05 Ga in the north, and a southern domain in which extensive 1.89–1.81 Ga volcano-plutonic magmatism is largely dominant (Fraga et al. 2008). The high-grade metamorphic rocks of the Falawatra Group occur in a northeast-trending horst in the Bakhuis Mountains (Dahlberg 1973; de Roever 1973, 1975a, 1975b). The horst is bounded by mylonitized NE–SW faults attributed to the 1.2 Ga

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Geological evolution of the Amazonian Craton Nickerie Metamorphic Event (Priem et al. 1971). The central part of the horst consists almost entirely of granulite-facies rocks with compositional banding on a centimetre to metre scale of basic and intermediate layers with intercalations of pelitic gneiss and spessartine quartzite suggesting a predominantly metasedimentary and/or metavolcanic nature. Charnockitic intrusives occur in particular at the southwestern side of the horst. The metamorphic rocks show local ultrahigh-temperature (UHT) metamorphism (de Roever et al. 2003a). Single zircon Pb-evaporation dating (de Roever et al. 2003a) and U-Pb data (Priem et al. 1978; J.-M. Lafon, unpublished SHRIMP data, 2003) indicate that the UHT metamorphism and charnockitic magmatism occurred at 2.07–2.05 Ga. Sm-Nd data (Ben Othman et al. 1984; de Roever et al. 2003a) point to a short crustal residence time, without Archean inheritance, suggesting a basin/trough filled from Paleoproterozoic greenstoneTTG areas. Delor et al. (2003) relate the UHT metamorphism to the late stage of continued crustal stretching postulated for French Guiana around 2.07–2.06 Ga. The supracrustal Coeroeni Group in southwest Suriname comprises mainly quartzofeldspathic gneiss and sillimanite gneiss, with minor amphibolite, metamorphosed in the amphibolite or granulite facies (Kroonenberg 1976). Rb-Sr dating of drill cores and river samples gave an age of 2.00 Ga (Priem et al. 1977). Reworked Archean zircons were found in a sample of Coeroeni gneiss (unpublished data, Delor et al. 2003). The Coeroeni gneiss dome is surrounded by younger acid metavolcanics, leucogranites and biotite granites of the Uatumã plutono-volcanic acid suite (see below). The Kanuku horst in southwest Guyana comprises granulites and migmatites (Berrangé 1977). The granulites may have been derived from magmatic rocks (Gibbs & Barron 1993). The migmatites include mainly paragneiss, with subordinate basic rocks and rare quartzites, in which the metamorphic grade ranges from granulite facies in the west to amphibolite facies in the southeast and east (Berrangé 1977). Nd-Sm data (Ben Othman et al. 1984) point to a short crustal residence time, without Archean inheritance. The westernmost branch of the Central Guiana Granulite belt in Roraima State, Brazil, comprises high-grade gneisses of the Urariqüera Complex, including pelitic gneisses and magmatic charnockites. Pb-Pb zircon ages of charnockite range from 1.97 to 1.93 Ga (Fraga 2002; Santos et al. 2004b), which is still younger than the age of the Bakhuis high-grade metamorphism and contemporaneous charnockitic magmatism (2.07–2.05 Ga). The intracontinental surrounding of the Coeroeni – and other – supracrustals and the presence of Archean zircons would be in line with an older craton south of the greenstone belt, in which the Uatumã suite intruded. A number of medium- and low-grade metamorphic enclaves of supracrustal character occur in Guyana and northwest Brazil, State of Roraima (Gibbs & Barron 1993). Large enclaves of mainly pelitic rocks, schists and metasiltstones occur in southern Guyana, surrounded by granite (Kwitaro Group: Berrangé 1977). Crossbedding and conglomerates would point to a shallow-water depositional environment. The Parima and Cauarane belts in Roraima comprise mainly schists (in part derived from intermediate volcanics), quartzites and amphibolites, and show Trans-Amazonian ages (Santos et al. 2004b; Fraga et al. 2008).

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Granitoid terranes Paleoproterozoic granitoid terranes Central Amazonian Province (CAP) In its original definition (Teixeira et al. 1989; Tassinari & Macambira 1999; Tassinari et al. 2000; Fig. 2.4, Plate 5), the Central Amazonian Province encompasses both the Archean Carajás terrane and a vast area further west in the Xingú-Iricoumé area underlain mainly by granitic and acid metavolcanic rocks, which show Pb-Pb zircon ages between 1.96 and 1.87 Ga (Lamarão et al. 2002, 2005). These rocks show Nd model ages between 2.5 and 2.6 Ga (Sato & Tassinari 1997, cited by Tassinari & Macambira 1999; Lamarão et al. 2005) implying a large gap (> 0.5 Ga) between the origin of the magma from the mantle, and its emplacement in the crust. The granitoid-metavolcanic terrane of the CAP continues northwards across the Amazon drainage basin into the Roraima territory in the Guiana Shield, bordering with Venezuela and Guyana. Tassinari & Macambira (1999) also include these rocks in the northern block of the Archean CAP, because ‘they are covered by ancient unmetamorphosed volcanic rocks (1.95 Ga), which have the same age as the neighbouring Maroni-Itacaiúnas Province. Thus it is possible to assume that the Roraima block acted as a stable foreland for the marginal Paleoproterozoic belt’. However, the Roraima area shows no vestiges of any Archean ancestry (Basei & Teixeira 1975; Dall’Agnol et al. 1994). The highgrade Urariqüera Complex, part of the Central Guiana Granulite Belt (Kroonenberg 1976), and the Parima Group intermediate to acid metavolcanics and quartzites in this area invariably show Trans-Amazonian or younger SHRIMP U-Pb zircon ages and Sm-Nd TDM model ages of 2.18–2.02 Ga, suggesting a short crustal residence interval (Santos et al. 2004b). In a later paper, Tassinari & Macambira (2004) no longer include the Roraima part in the CAP, but place it in the Trans-Amazonian Maroni-Itacaiúnas province. Acid metavolcanic rocks and associated hypabyssal granites cover vast areas in the central part of the Amazonian Craton, often loosely grouped together as the Uatumã Supergroup (Gibbs & Barron 1993; Schobbenhaus et al. 1994; Brito Neves 2002; Delor et al. 2003). They are known in Brazil as Surumu volcanics (Santos 1984; Reis et al. 2000), in Venezuela as Cuchivero Group (Mendoza 1974), Dalbana Formation in Suriname (Bosma et al. 1983) and Iwokrama Formation in Guyana (Gibbs & Barron 1993). From these acid metavolcanics in the Roraima area Santos et al. (2004b) obtained SHRIMP U-Pb zircon ages of around 2.03–2.00 Ga, and Cuchivero volcanics in Venezuela were dated at 1.98 Ga (Brooks et al. 1995). Dalbana acid metavolcanics and leucogranites in the Sipaliwini area in Suriname also give 1.99–1.98 Ga Pb-Pb zircon ages (J.-M. Lafon, personal communication). These metavolcanics are in turn intruded by essentially comagmatic granitoid rocks dated by the same methods between 1.99 and 1.92 Ga (Dall’Agnol et al. 1994; Santos et al. 2004a,b; Almeida et al. 2007). Also in the Colombian part of the shield granitoids are often associated with acid metavolcanic rocks, as along the Atabapo River and in the Araracuara area (Kroonenberg 1985; Rivas 1985). Acid metavolcanic rocks and granites from the Roraima area also show Nd model ages between 2.19 and 2.05 Ga (Lamarão et al. 2005).

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70

54

70

0

54

0

1

16 S

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2

16 S

54

70

0

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0

Front Sunsas

Front Sunsas 3

16 S

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PROVINCES

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Phanerozoic Sunsás

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Rondoniano and Rondoniano-Rio Negro Rio Negro and Rio Negro-Juruena Tapajós-Parima and Tapajós-Ventuari Trans-Amazonian (Maroni-Itacaiúnas) Central Amazonian Carajás 16 S

5

Fig. 2.4 Changes in the delimitation of geochronological provinces in the Brazilian part of the Amazonian Craton (modified after Santos 2003): 1: Amaral (1974); 2: Cordani et al. (1979); 3: Teixeira et al. (1989); 4: Tassinari (1996); 5: Santos et al. (2000) (all cited by Santos 2003).

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Geological evolution of the Amazonian Craton So, the CAP consists of three types of terranes: unambiguous Archean ones (Carajás); Paleoproterozoic granitoid-volcanic ones generated from Archean sources according to Nd model ages (Xingú-Iricoumé); and unambiguous Paleoproterozoic ones with juvenile magmas (Roraima territory). This situation raises two issues. Firstly it shows that the old question of whether the Amazonian Craton was formed by reworking of the Archean crust or by continental accretion of juvenile magmatic arcs (Teixeira et al. 1989) is not dead at all. Both answers are correct, though the terrane underlain by reworked Archean crust is much smaller than envisaged by Almeida et al. (1981), and also much smaller than suggested on the maps published by Tassinari and Macambira (1999), Tassinari et al. (2000) and others. It is no bigger than the XingúIricoumé terrane and possibly other areas that show wide gaps between Archean Nd model ages and Proterozoic U-Pb zircon emplacement ages. Therefore, the solution proposed by Santos et al. (2000) of Carajás (and Imataca) as a separate geochronological province has some logic in it. But this question has deeper implications. What is a geochronological province? Should the Xingú-Iricoumé area be called an Archean province because of its old Nd model ages, or should it be called a Paleoproterozoic province because of its Paleoproterozoic emplacement ages? In our view, mapping a granitic-acid volcanic area with U-Pb zircon emplacement ages overwhelmingly around 2.0–1.8 Ga as an Archean geochronological province does not do justice to plain surface geology, even though these rocks may have developed from Archean crust. One might want to call them crustal age provinces, but geochronological province is a misnomer: the many Proterozoic ages are as ‘geochronological’ as the few Archean ones. Central-Western granitoid provinces (Tapajos-Ventuari/Parima, Rio Negro-Juruena) The question about the nature of geochronological provinces becomes more pressing when considering the vast expanses of granitoid and gneissic rocks that constitute the major part of the western Amazonian Craton. The older schemes only distinguish the Proterozoic Rio Negro-Juruena Province west of the CAP (Tassinari 1981; Teixeira et al. 1989; see Fig. 2.4), but, as Almeida et al. (2000) state, slightly tongue-in-cheek: ‘the concept of the Rio Negro-Juruena belt is derived from geochronological reconnaissance rather than from geological fieldwork’. More recent work squeezes a Ventuari-Tapajós province between them (Tassinari & Macambira 1999). A still newer scheme was designed by Santos et al. (2000) on the basis of a whole series of new U-Pb zircon and Sm-Nd data. They split the CAP into three parts along NW–SE boundaries (see Fig. 2.4): an Archean part in the east now named Carajás; a central part that maintains the name Central Amazonian Province but consists mainly of the Paleoproterozoic comagmatic granitoidacid metavolcanic series of the Uatumã Supergroup discussed above, and a westernmost part merged with the Ventuari-Tapajós province and renamed Tapajós-Parima province. The Rio Negro province is added to the remains of the Ventuari province but split from the Juruena part. The Rondonia granitoid provinces, a separate unit in older schemes, is merged with the Juruena remains (see

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17

Fig. 2.4). Dall’Agnol et al. (1999) dismiss the idea of geochronological provinces altogether and divide the Amazonian Craton in a few loosely defined blocks that nevertheless coincide roughly with previously defined geochronological provinces. A few facts strike the eye in this confusing jigsaw puzzle. Firstly, virtually all granitoid provinces show greatly overlapping U-Pb zircon emplacement ages, except for some anorogenic rapakivitype granites of 1.4–1.5 Ga and the 1.3–1.0 Ga Sunsás belt in the southwest, which will be discussed in the next paragraph. Secondly, there seems to be no gap in emplacement ages from 1.90 Ga down to at least 1.70 Ga. Furthermore, although older reconstructions emphasize that basement ages become younger towards the west; this trend is hardly discernible in the granitoid provinces discussed here, especially in the northern, Guiana Shield part of the craton. The Tapajós-Parima province of Santos et al. (2000) is older than their rebaptised Central Amazon Province but is situated further to the west, thus inverting the normal order for westward continental accretion. This is largely due to the 2.10 Ga ages obtained from the gold-bearing 150-km long Jacareacanga greenstone belt with associated Cuiú-Cuiú 2.01 Ga tonalites in the Tapajós area. This series is indistinguishable in age, lithology and geochemistry from the large Trans-Amazonian greenstone belt described above, and therefore seems to form a far equivalent. If these supracrustal belt ages are left out of consideration, we see that granitoid magmatism between 1.9 and 1.75 Ga is ubiquitous in major parts of the Amazonian Craton, including: the 1.88 Ga Paleoproterozoic intrusions into the Carajás area in the easternmost part of the craton (Tassinari et al. 2000); the 1.88 Ga Parauari intrusions in Tapajós (Santos et al. 2000); the magmatism of Uatumã affinity of around 1.87 Ga in the Tapajós area in the centre and also in Roraima State (Santos et al. 2004b); the 1.86–1.78 Ga granites and gneisses in the Rio Negro-Ventuari area of Venezuela in the west (Barrios et al. 1985; Gaudette & Olszewski 1985; Tassinari et al. 1996; Santos et al. 2000); the 1.75– 1.73 Ga protolith age of the tonalite gneisses in the basement of the Rondônia Tin Province (Bettencourt et al. 1999; Payolla et al. 2002) and the 1.79–1.74 Ga gneisses in the Mato Grosso State (Geraldes et al. 2001, 2004). Almost all younger ages of around 1.6 Ga, such as those of the granitoid-metavolcanic rocks in the Colombian part of the Guiana Shield (Galvis et al. 1979; Priem et al. 1982; Kroonenberg 1985; cf. Fig. 2.1) are based essentially on Rb-Sr data, a method now known generally to give ages that are too young (Santos et al. 2000). Therefore, continuous granitoid magmatism and associated acid volcanism occurred between 1.9 and 1.75 Ga throughout the basement of the Amazonian Craton, just as in many other cratons in the world. The associated acid metavolcanic rocks are generally undeformed and only slightly metamorphosed. There are very few supracrustal series or metamorphic belts that would enable separation of its western part into a progressively younger westward series of clear orogenic belts. The plate tectonic setting of this magmatism is unclear: Tassinari (1981) considers the Rio Negro-Juruena province to be a partially preserved subductionrelated magmatic arc; later he redefines it as two successively accreted island arcs (Tassinari et al. 1996), whereas Dall’Agnol et al. (1987; cited by Bettencourt et al. 1999) consider the province to be a continental collision belt. Dall’Agnol et al. (1999) consider

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S.B. Kroonenberg & E.W.F. de Roever

a large part of these granites to be rapakivi granites, partly on the basis of the A-type geochemical characteristics, and suggest they originated in an extensional, anorogenic or intracratonic setting. All this leads us to the conclusion that the similarities in age and lithology between these provinces are larger than their differences, and that the strict subdivisions of the western Amazonian Craton into geochronological provinces as attempted by previous authors is still very much debatable, let alone a reconstruction in plate tectonics-related orogenic phases or magmatic pulses. Clear evidence of continental accretion cannot be derived from the data from these granitoid provinces alone. One must conclude that the major part of the Amazonian Craton had consolidated no later than 1.75 Ga.

rocks of this age are most probably only a side-effect of the widespread felsic magmatism. Tassinari et al. (2000) distinguish various ‘orogens’ in the southwestern part of the craton, such as the Cachoeirinha Orogen (1.57–1.53 Ga) and the Santa Helena Orogen (1.48–1.42 Ga), but all of these consist essentially of granitoid plutonic rocks, and the only greenstone belt around, the Alto Jauru belt, is late Paleoproterozoic (1.79–1.74 Ga; Tassinari et al. 2000). The designation of these Mesoproterozoic granites as anorogenic, following Dall’Agnol et al. (1999), seems therefore more logical, although the tectonic mechanism for their genesis and emplacement remains obscure, or at best, conjectural.

Granitic magmatism: continental accretion or anorogenic magmatism? Mesoproterozoic anorogenic rapakivi and tin granites In the extreme northwestern and southwestern parts of the Amazonian Craton the situation changes. Here we find a wellindividualized belt of clearly anorogenic granites stretching from the Parguaza rapakivi granite straddling the border between Venezuela and Colombia, to the Rondônia tin granite province in the southwest of the Guaporé Shield, continuing into eastern Bolivia. These granites intruded in various pulses between 1.6 and 0.9 Ga at a shallow level into older basement, and show often well-defined discordant contacts (Dall’Agnol et al. 1994, 1999; Bettencourt et al. 1999). However, they are not restricted to this area, as similar granites are found much further east, such as the Surucucus (1.58 Ga) and Mucajaí (1.54 Ga) granites in the Roraima territory (Dall’Agnol et al. 1999). Therefore they do not constitute a geochronological province and their rocks and ages should not be used to define geochronological provinces. The most typical representatives, such as the Parguaza granite (1.54 Ga U-Pb; Gaudette et al. 1978) are undeformed and characterized by ovoid alkali feldspar crystals surrounded by a plagioclase rim (wyborgite, pyterlite types; Bangerter 1985), but less characteristic granites with similar textures are also included in the designation rapakivi granite (Dall’Agnol et al. 1994). Furthermore, they show specific geochemical features such as metaluminous to peraluminous character and an A-type (A stands for ‘anorogenic’) granite signature (Dall’Agnol et al. 1994). A biotite granite from the Vaupés River in the Colombian Amazonas also gave a U-Pb zircon age of 1.55 Ga (Priem et al. 1982). In the Rondônia Tin Province, Bettencourt et al. (1999) recognised seven U-Pb age groups: one between 1.61 and 1.53 Ga, four between 1.41 and 1.31 Ga, and two between 1.08 and 0.97 Ga, of which only the last ones are tin-mineralized. For the first five groups a relation with orogenic events is difficult to establish because of the absence of associated greenstone or high-grade supracrustal belts, and the close spacing of the magmatic pulses invites consideration of these episodes as a continuum rather than as a series of discrete events (cf. Tohver et al. 2004a). Litherland et al. (1985, 1989) coin a San Ignacio orogenic event around 1.33 Ga in the adjacent Bolivian part of the craton. However, the only indications for a high-grade event in the adjacent part of Rondônia are 1.33–1.30 Ga Sm-Nd isochrons on garnets; all other ages in this range are derived from plutonic rocks (Payolla et al. 2002). Tohver et al. (2004b, 2005) suggest that associated metamorphic

Hoorn_ch02_Final.indd 18

In light of the above, there appear to be two major associations: an older Paleoproterozoic series (1.9–1.75 Ga), which is often spatially and comagmatically associated with extensive acid metavolcanic rocks (Uatumã-affinity), and a younger Mesoproterozoic one (1.55–1.30 Ga), probably truly anorogenic in character, including typical rapakivi and tin granites concentrated in the west. Numerous attempts have been made to put these granitoid complexes into preconceived plate-tectonic pigeonholes using geochemical and isotopic criteria. But even though ‘reworked’ and ‘juvenile’ magmas can be distinguished on the basis of Nd and Sr systematics (Cordani & Sato, 1999; Sato & Siga 2000), these methods fall short when it comes to the reconstruction of plausible geodynamic scenarios. This holds particularly for the recognition of continental accretion. Now that more and more SHRIMP U-Pb data are becoming available, the progressively younger westward trend formerly recognized in the granitoid terranes on the basis of Rb-Sr datings is gradually losing ground, because all Paleoproterozoic intrusives throughout the craton from east to west show U-Pb ages between 2.0 and 1.75 Ga, as discussed above. Also Mesoproterozoic rapakivi-like intrusives are widespread, though more concentrated in the west (Rondônia, Parguaza) than in the east (Surucucus, Mucajaí). We believe that a fresh look at these granites is needed. We believe it is much more useful to discuss Proterozoic granite genesis as a problem sui generis, as a specific and unique episode in the development of the Amazonian Craton, spanning almost 1 Ga between the Trans-Amazonian orogeny around 2.0 Ga and the Grenvillian orogeny around 1.0 Ga. The suggestion of Dall’Agnol et al. (1999) that the major part of both Paleoproterozoic and Mesoproterozoic granites are rapakivi-type granites deserves more attention. Vigneresse (2005) wrote: ‘Rapakivi granites and the associated suite (anorthosite, mangerite and charnockite) that developed during the Mid-Proterozoic escape the global rules that control usual granitic magma formation. About 40 points that characterise the Proterozoic magmatism confirm the originality of the magmatism. Rapakivi granites developed without a global orogenic context within the supercontinent Columbia, a Paleoproterozoic plate assembly. They show no definite trend in age or chemistry that would explain large-scale (mantle plume) effects.’ This might well apply specifically to the Amazonian Craton as well.

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Geological evolution of the Amazonian Craton

Grenvillian mobile belts The Grenvillian Orogen, originally defined after the Grenville Province in the eastern Canadian Shield, is now used to designate a mobile belt on a supracontinental scale, recognized in all major shields, marking the Rodinia plate assembly 1.3–1.0 Ga (Hoffman 1991). There is ample evidence that the western margin of the Amazonian Craton was affected extensively by the 1.3–1.0 Ga Grenvillian collision of Gondwana with Laurentia that led eventually to the formation of this supercontinent (Kroonenberg 1982; Litherland et al. 1985, 1989; Hoffman 1991; Sadowski & Bettencourt 1996; Restrepo-Pace et al. 1997; Sadowski 2002; Tohver et al. 2002, 2004a,b, 2005, 2006; Cordani et al. 2005; Fuck et al. 2008). The evidence comes from two widely separated highgrade metamorphic belts, the Sunsás-Nova Brasilândia belt in the Brazil/Bolivia border area in the southwestern corner of the Amazonian Craton, and the Garzón-Santa Marta belt in the Colombian Andes, a Precambrian fragment of the Guiana Shield incorporated in the Andean Orogen (see Fig. 2.1). Their continuity is suggested by the presence of Grenvillian-aged basement rocks in drill holes through the sediments of the sub-Andean basins (Kovach et al. 1976). There is also a Grenvillian fragment in the Coastal Cordillera of Peru. Furthermore, Grenvillian shearing and mica rejuvenation by collisional indentation tectonics is evidenced in large parts of the Amazonian Craton far beyond the high-grade Grenvillian belts themselves.

Sunsás and Nova Brasilândia high-grade metamorphic belts In the extreme southwestern part of the Amazonian Craton in eastern Bolivia Litherland et al. (1985, 1989) recognized an important 1.3–1.0 Ga mobile belt, parallel to the trends of the present Andean chain, the Sunsás belt. The Sunsás Group is a continental sedimentary unit consisting mainly of conglomerates and quartzites, correlatable to the Aguapeí Group in adjacent Brazil. In deformation zones the Sunsás Group occurs as metamorphosed synformal infolds. It is this deformation and subsequent felsic magmatism that led Litherland et al. (1985, 1989) to recognize this as a separate orogenic event at the southwestern border of the Amazon Craton, although the rocks themselves are not at all characteristic for Grenvillian terranes elsewhere. The Aguapeí conglomerates and quartzites are only locally deformed (Tohver et al. 2004a). The Sunsás Group unconformably overlies an older high-grade basement of the Lomas Maneches granulite group. Litherland et al. (1989) obtained a Trans-Amazonian whole-rock regional Rb-Sr reference isochron of around 1.96 Ga, but with a 1.34 Ga overprint that led these authors to make the correlation with the Grenvillian Garzón-Santa Marta belt. Later, Sm-Nd mantledepleted model ages of 2.0–1.9 Ga were reported from these granulites (Darbyshire, in Tassinari et al. 2000), and new zircon U-Pb data by Boger et al. (2005) show ages between 1.66 and 1.69 Ga for the basement, so the high-grade metamorphism recorded here is apparently not Grenvillian, but older. Granitoid intrusions of around 1.3 Ga were taken by Litherland et al. (1985, 1989) as evidence for the existence of an earlier San Ignacio event, although there are no clear associated supracrustals. Younger granites in the

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19

Sunsás area range in age between 1.05 and 0.85 Ga and are correlated with the youngest group of the Rondônian tin granites rather than with the Grenville event. This all means that the evidence for a Grenvillian signature of the Sunsás belt is now less clear. However, recently a new Grenvillian candidate in the southwestern craton has been identified by Tohver et al. (2004a, 2004b, 2005, 2006) further north, across the Brazilian border. The 1.1–1.0 Ga high-grade metamorphic E–W-trending Nova Brasilândia belt consists of calc-silicates, metabasites, metapelites and quartzites of at least partly turbiditic origin. Aeromagnetic data suggest that the belt continues over 1000 km eastwards below Phanerozoic sedimentary covers across the southern part of the Amazonian Craton, until it is truncated by the Neoproterozoic AraguaiaTocantins-(Paraguai) belt. Metamorphic grade increases southwards in the belt from higher amphibolite to granulite facies. The clockwise PT (pressure-temperature)-path during metamorphism suggests crustal thickening during orogeny, in harmony with an origin through continental collision. The maximum age of sedimentation is given by a 1215 ± 20 Ma detrital zircon age from the metapelites; zircon from leucosomes in the same unit give an age of 1122 ± 12 Ma; and cooling from peak metamorphic conditions is recorded by U-Pb monazite ages of 1.09 Ga and titanite ages of ~ 1.06 Ga. There is no evidence of the involvement of any older basement, and therefore the Nova Brasilândia rocks can be regarded as juvenile additions to the southern margin of the Amazonian Craton. In its lithology, metamorphic history and geochronology this belt matches the Grenvillian rocks exposed along the eastern margin of the North American Craton much better than the Sunsás rocks. Tohver et al. (2004a, 2004b, 2005, 2006) consider the Nova Brasilândia belt as a Grenvillian suture zone along which a separate microcontinent, the Paragua Craton, encompassing the whole Bolivian Precambrian terrane including the Sunsás belt, collided with the Amazonian Craton. The large strike-slip deformation patterns in the Nova Brasilândia belt are inferred by Tohver et al. (2004a) to indicate that initial basin formation and the subsequent Grenvillian orogeny took place in a transpressional setting when Amazonia collided obliquely with Laurentia and shifted northwards along the suture.

Garzón-Santa Marta granulite belt Precambrian rocks crop out in several areas in the Andean mountain belt (see Fig. 2.1). The most important of these is the Garzón-Santa Marta belt in the Colombian Andes (Kroonenberg 1982, 1983, 1985, 1990; Restrepo-Pace et al. 1997; Ruiz et al. 1999; Jiménez-Mejía et al. 2006). The major outcrop areas are the Garzón Massif, the Santander Massif and the Santa Marta Massif. The former consists of two units: the Garzón Complex, a mainly supracrustal series of banded quartzofeldspathic, metapelitic, mafic, ultramafic and calc-silicate rocks metamorphosed under granulite facies conditions; and the syntectonic GuapotónMancagua granite-orthogneiss body. Grenvillian Rb-Sr ages were first obtained from the Garzón Massif by Alvarez and Cordani (1980), Alvarez (1981) and Priem et al. (1989). Extensive U-Pb zircon and argon-argon (Ar-Ar) dating by Restrepo-Pace et al. (1997) confirmed that peak metamorphism took place around 1.09 Ga, related to the Orinoqüense orogeny. Cordani et al. (2005)

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S.B. Kroonenberg & E.W.F. de Roever

show that the protolith magmatic rocks were emplaced around 1.15 Ga, followed by two high-grade metamorphic events at 1.1 and 1.04 Ga. The other two massifs show similar patterns. Apart from these, there are numerous smaller Precambrian outcrops in the Colombian Andes, some considered by Cediel et al. (2003) and Ordóñez-Carmona et al. (2006) to belong to other tectonic blocks, but their age characteristics are similar to those of the Garzón, Santander and Santa Marta massifs. Another indication that Grenvillian crust underlies larger parts of the Central Cordillera of Colombia is the 1031 Ma Sm-Nd isochron in garnet that Rodríguez-Vargas et al. (2005) obtained from xenoliths in the Cenozoic Granatífera tuff in Mercaderes, Cauca.

The K’Mudku/Nickerie/Orinoqüense/Madeira tectonometamorphic event Important shearing events accompanied by 1.2–1.0 Ga mica rejuvenation deep into the adjacent parts of the western Amazonian Craton have been recognized for a long time as constituting a separate tectonic episode far beyond the Grenvillian belts themselves. In the Guiana Shield large NE–SW mylonite zones occur along the Imataca Complex (Guri Fault), along the Kanuku Complex and along the Bakhuis horst. In Guyana the event is known as the K’Mudku event (Barron 1969; term adopted also in Brazil by Santos et al. 2000), in Suriname and Colombia as the Nickerie Tectonometamorphic Episode (Priem et al. 1971, 1982), in Venezuela as the Orinoqüense event (Martín Bellizzia 1974), and elsewhere in Brazil as the Jari-Falsino event (Almeida et al. 1981). In the southwestern part of the Amazonian Craton, adjacent to the Nova Brasilândia belt, sinistral strike-slip shear zones such as the Ji-Paraná shear zone network (Tohver et al. 2006), already described by Almeida et al. (1981) as the Madeira event, show rejuvenated micas with Grenvillian ages of around 1.2–1.12 Ga as well (Tohver et al. 2005). Because of their ages and their restriction to the western part of the craton these shear zones are interpreted as being due to indentation tectonics caused by the Grenvillian Gondwana–Laurentia collision (Kroonenberg 1982; Tohver et al. 2005).

Platform covers In vast areas of the Amazonian Craton extensive covers of largely undeformed sandstone and conglomerates, sometimes with finer intercalations, are found, most of which apparently have been deposited in intracratonic basins following the major stages of orogeny (Brito Neves 2002). The oldest sedimentary unit is the Águas Claras Formation on top of the Archean Carajás province, which has been dated at between 2.68 Ga (intercalated volcanics, Trendall et al. 1998) and 2.64–2.70 Ga (intruded mafic sills; Galarza et al. 2008, and papers cited herein). The Roraima Group or Supergroup in the Guiana Shield consists of thick sequences of gently dipping to flat-lying sandstones, conglomerates and shales, with horizons of pyroclastic volcanics. The sequences form spectacular table mountains covering Paleoproterozoic formations in Venezuela, Guyana and northwest Brazil (Roraima State, Plate 2a). In the Pakaraima Mountains

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alone they occupy over 73,000 km2. Outliers occur at the Tafelberg in central Suriname, near Pitinga in north Brazil, and at many scattered sites throughout the Amazonian Craton over an area of 1,350,000 km2. Thicknesses range from 200 m in some outliers to ~3000 m in the Pakaraima Plateau. Not all of these occurrences show the same characteristics, and especially in the western part of the Guiana Shield in Brazil, Venezuela and Colombia some of these plateaus, such as the Moriche, Tunuí/La Pedrera, Naquén and Guainía sandstones are slightly folded and metamorphosed (Galvis et al. 1979) (Fig. 2.5). The Roraima sediments sensu stricto represent braided fluvial sediments deposited in an alluvial plain to subaerial braided delta, and possibly shallow marine environments. They are derived mainly from the Trans-Amazonian orogenic belt to the north and northeast, and filled a vast foreland basin (Santos et al. 2003b). The age of the Roraima sandstones has been the subject of considerable controversies, as the ages of the underlying basement, of intercalated rhyolitic tuff horizons, and cross-cutting dolerite dykes sometimes gave conflicting results. Basement and tuff Rb-Sr ages were often too young, while intruding dolerite dykes suffered from excess argon accumulation (Priem et al. 1973). Also, Precambrian palaeosols at the base of the formation were earlier erroneously identified as intrusive contacts (Kroonenberg 1978). A recent series of new U-Pb datings on zircon and baddeleyite by Santos et al. (2003b) solved the previous discrepancies, showing that in the type area they overlie rocks of 1.99–1.92 Ga and older, intercalated tuffs are 1.87 Ga, while intruding dolerite dykes gave ages around 1.78 Ga. The 1.87 Ga age of the main sandstone formation sets back the previous Rb-Sr ages by as much as 130–300 Ma. Detrital zircons in the sediments had an age of 1.95 Ga and older, confirming the Trans-Amazonian provenance of the sediments. Roraima-like outliers elsewhere in the Amazonian Craton (Cox et al. 1995) show ages up to 0.3 Ga younger than the main sandstone formations, and therefore represent a separate phase of sedimentation. Ar-Ar dating on muscovites from these formations by Santos et al. (2003b) shows that their deformation and lowgrade metamorphism took place around 1.3 Ga and is related to the Grenvillian event. Even further west in the Colombian Amazonia, impressive sandstone plateaus overlying Paleoproterozoic Uatumãlike acid metavolcanic rocks in the Araracuara area, are of Ordovician age (Galvis et al. 1979; see Plates 2b, 2c, 3a). Other covers of Proterozoic sandstones and conglomerates in the Guaporé Shield are the Gorotire and Beneficente Formation in the Xingú, Iriri and Tapajós areas, all overlying unconformably Uatumã-type acid metavolcanics and intruded by Avanaverotype (‘Crepori’) dolerites (Brito Neves 2002). The 200–300 m thick Mesoproterozoic Palmeiral Formation in the Pacaás NovasUopione basin in western Rondônia and Mato Grosso unconformably overlies a 1.20 Ga gabbro sill and is linked to the break-up of the Rodinia supercontinent (Pedreira & Bahia 2000; Tohver et al. 2002, 2004a).

Mafic dykes and sills Large dolerite sills and dykes of the Avanavero type occur in Venezuela, Brazil (Roraima State), Guyana, Suriname and probably also French Guiana. They have been dated at c. 1.79–1.78 Ga

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Geological evolution of the Amazonian Craton

21

Fig. 2.5 Roraima-like Moriche conglomerates, slightly metamorphosed during Grenvillian Orogeny, lie unconformably on Cuchivero (Uatumã-type) subvolcanic granites. Las Carmelitas, Ventuari River, Venezuela. Excursion Primer Simposium Amazónico, Puerto Ayacucho, Venezuela, 1981.

by zircon and baddeleyite dating (Norcross et al. 2000; Santos et al. 2003b), much older than previously thought. A swarm of narrow alkaline dolerite dykes in western Suriname runs along a graben structure (de Roever et al. 2003b). Ar-Ar dating of biotite suggested an age of about 1.5 Ga. The Nova Floresta gabbro and dolerite sills in Rondônia give an age of around 1.2 Ga (Tohver et al. 2002). Some narrow dolerite dykes in French Guiana were dated at 809 ± 29 Ma by K-Ar (Delor et al. 2003); dykes of comparable (K-Ar) age were found in Guyana and northwest Brazil. The Piranhas dike swarm in the Tapajós region in Brazil is Middle Cambrian in age (0.51 Ga), and might mark the opening of the Paleozoic basin system. The break-up of Pangaea is evidenced by large dolerite dyke swarms transecting all previously mentioned lithologies in Venezuela, Guyana, Suriname, French Guiana and Brazil, forming part of the Central Atlantic Magmatic Province (CAMP, Deckart et al. 2005; Fig. 2.6). These Apatoe-type dolerites range in age from Late Triassic to Early Jurassic (Priem et al. 1968; Deckart et al. 2005). More or less contemporaneous outpourings are the Apoteri basalt in the North Savannas/Takutu graben in central Guyana (Berrangé 1977), and the Penatecaua magmatism in the Amazon drainage basin itself (Milani & Zalán 1999; Deckart et al. 2005).

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Origin of Paleozoic intracratonic basins The origin of the Amazon drainage basin has long intrigued scientists. It struck earlier observers that the basin is situated in the prolongation of one of the major breaks in the Andean chain: the NW–SE-trending Central Andes, with only ensialic orogeny, and the SW–NE-trending Northern Andes, characterized by at least three episodes of Phanerozoic westward continental accretion through obduction of oceanic crust. The E–W line separating these two domains is also called the Huancabamba deflection or Amotape lineament (Gansser 1973) and further eastwards follows the axis of the present Amazon drainage basin. Furthermore, the correspondence with major E–W features on the ocean floor has been noted: the Carnegie ridge in the Pacific ocean, and the Romanche transform zone in the Atlantic (De Loczy 1977; Grabert 1983). At present, an origin by Late Neoproterozoic to Early Paleozoic rifting is generally accepted (Milani & Zalán 1999, Santos et al. 2002; da Silva et al. 2003). There is a pronounced positive gravity anomaly along the axis of the basin, suggesting shallow ultrabasic bodies. Moreover, Proterozoic or Early Paleozoic rift successions of alluvial, fluvial and lacustrine sedimentary rocks occur in its

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S.B. Kroonenberg & E.W.F. de Roever Dyke

IBERIA

Sill and lava-flow Archean Craton/shield Continental shelf MOROCCO

ALGERIA 500 km

NORTH AMERICA MAURITANIA WEST AFRICA

Fouta Djalon

MALI West African Craton

GUINEA Kakoullma LIBERIA SURINAME French GUIANA Guiana

Shield SOUTH AMERICA f it o aton Lim é Cr r po ua

G

BRAZIL

Fig. 2.6 The Central Atlantic Magmatic Province (CAMP). Reproduced from Deckart et al. (2005), with permission from Elsevier. western portion, composing the Purús group (Milani & Zalán 1999). The earliest sedimentary successions in the Paleozoic basin system are Ordovician (Wanderley Filho et al. 2005), and they continue up to the Lower Triassic. Further Mesozoic sediments are virtually absent. An important phase of mafic magmatism took place in the Triassic and Early Jurassic, the Penatecaua magmatism (Milani & Zalán 1999), which evidently is related to reactivation of older rift structures during the separation of South America and Africa. Sedimentation starts again in the uppermost Cretaceous and Cenozoic (Wanderley Filho et al. 2005; see also Chapter 3).

Relief development and sediment generation The fact that platform covers of varying age already occur in the Precambrian shows that throughout the history of the craton basin formation, uplift and differential erosion have been going on continuously. Brito Neves (2002) distinguishes 12 sedimentary phases

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in South America, one in the Archean, seven in the Proterozoic (including marine basin formation preceding the major orogenies) and four in the Phanerozoic. The rate at which these processes have operated must have varied strongly both temporally and spatially. The oldest landscapes preserved in the Amazonian Craton are those buried below Precambrian platform covers, and evidenced by unmetamorphosed palaeosols such as below the Tafelberg in Suriname (Kroonenberg 1978) and Santa Elena de Uairén in the Gran Sabana area, Venezuela. The lack of metamorphism in these rocks shows that they have never been buried very deep throughout their history. However, there are rocks now exposed at the surface, such as the UHT metamorphic rocks in the Bakhuis mountains, that have been buried at least 30 km deep in the earth’s crust (de Roever et al. 2003a), and from which, therefore, 30 km of rock have been removed since Trans-Amazonian times. The distribution of the platform covers in the craton suggests that the palaeogeography of eroded uplands and sedimentary basins in the Precambrian was very different from the present. The history of the present relief of the Amazonian Craton as a source of sediments in the surrounding basins, therefore, started only after cratonization and after the development of the Paleozoic basin system. A major role is played by the Mesozoic break-up of Gondwana. Opening of the Atlantic in the present-day northeastern part of South America took place along transform faults, and therefore, unlike the eastern Brazilian Atlantic coast range, no updomed rift shoulders have been formed along the Atlantic margins of the Amazonian Craton (Peulvast et al. 2008). Early ideas about the development of Amazonian relief were based upon the concept that after continental break-up and due to intermittent uplift, several Mesozoic and Cenozoic planation surfaces were being formed by parallel slope retreat from the coast inland. Recognition of planation surfaces was based upon topographical profiles, evidence of long-term stasis in the landscape such as laterite caps, and depositional hiatuses in adjacent sedimentary basins (King 1956; McConnell 1966; Zonneveld 1969; see reviews by Gibbs & Barron 1993, and recently for the Brazilian Northeast: Peulvast & Claudino Sales 2004). Since then many new data have been acquired and new ideas put forward. Apatite fission track data have enabled dating of uplift and cooling history; the detailed chronology of correlated basin fills allows for volumetric assessment of past denudation rates; and present-day denudation rates can be derived from measured sediment yields of major drainage basins. Apatite fission track dating of rocks from the eastern Guaporé Shield shows that the corresponding rock masses cooled beyond the 120° isotherm between 137 and 309 Ma, and samples closer to the northern margin exhibit younger ages (137–159 Ma) than those from further inland (144–309 Ma) (Harman & Gallagher 1998). This implies considerable uplift in Mesozoic times, especially in the northern part of the Shield, and leads to denudation rates of 25–50 m/million years in the north and 15–30 m/million years in the interior. The age distribution suggests a clear relation with continental break-up around 130 Ma. Peulvast et al. (2008), studying correlative Mesozoic basins in the Brazilian northeast (outside the Amazonian Craton), claim that denudation rates in the last 90 Ma never exceeded mean rates of 10 m/ million years. A similar rate of denudation was inferred from water chemistry in the Orinoco drainage basin, where highlands show rates

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Geological evolution of the Amazonian Craton of 15–20 m/Ma and lowland areas of around 5 m/Ma (Edmond et al. 1995). Santos et al. (2003b) suppose that much of the exhumation of the Roraima-like and related sandstone plateaus also took place from the Mesozoic onwards. All these data suggest that Mesozoic and Cenozoic tectonic and climatic history largely determined the present-day relief of the Amazonian Craton. This is in harmony with the greatly increased sediment flux towards the surrounding basins from the late Mesozoic onwards (Potter 1997; see Chapter 7). Differential uplift and subsidence, often along preexisting faults, play an important role on a regional scale. For instance, the 6000 m deep Jurassic failed rift of the Takutu graben in Guyana follows reactivated Grenvillian shear zones, and lines up with the Bakhuis horst in Suriname, which can be followed as an uplifted basement high offshore in the Guiana sedimentary basin. Differential weathering is the most powerful mechanism in sculpturing the major landforms of the Amazonian Craton. This is conspicuous in the most impressive relief features of the Guiana Shield, the tepui: flat sandstone plateaus, the highest one of which, the Pico de Neblina at the border between Brazil and Venezuela, reaches to almost 3000 m. As the sandstones must have been deposited originally in basins surrounded by basement rocks, the tepuis underwent relief inversion due to the differential resistance to weathering of the sandstones in comparison with the surrounding basement rocks. Here, along the sandstone scarps, some planation due to slope retreat (backwearing), as envisaged by King (1956), McConnell (1966) and others, may have reduced the size of the plateaus and uncovered the preexisting relief. However, in crystalline rocks slope retreat is not a likely mechanism. Planation surfaces produced by backwearing are supposed to truncate lithological boundaries, but in the Amazonian Craton lithological boundaries are surprisingly well expressed in the relief. Subtle differences in weathering susceptibility of specific lithologies cause them to stand out by small differences in morphology and degree of dissection, and hence enable detailed mapping of lithological boundaries in remotely sensed imagery, especially when the original rainforest cover is still intact (see examples in Kroonenberg & Melitz 1983). Specific lithologies are prone to formation of laterite and bauxite caps, such as metabasalts (Brownsberg, Suriname), gabbros (De Goeje, Suriname), rapakivi granite (Pijiguaos bauxite on Parguaza granite, Soler & Lasaga, 2000), Bakhuis bauxites on charnockitic granulites and anorthosites (Krook & de Roever, 1975). Inselbergs and tors are usually underlain by granitoid rocks. The excellent expression of lithology in morphology explains the success of earlier airborne and space-borne geological mapping campaigns such as Radambrasil. Laterite caps have often been taken to indicate specific planation levels in the development of the basement relief. Although the presence of a laterite cap indicates a period of stability in the landscape, their altitude is not only a function of age but also of the specific weathering rate of the specific underlying rock. Therefore, the concept of the development of planation surfaces by backwearing across lithologies seems incorrect. We consider etchplanation to be a more logical process to explain the present-day features of the Amazonian relief (Fig. 2.7). Etching in this respect means deep chemical weathering of the crystalline rocks during humid periods with a dense vegetation cover. Differences in susceptibility of specific rocks to chemical weathering cause differences in the depth of the weathering front.

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1. Deep weathering: etching

2. Erosion: stripping

3. Etching

4. Stripping

Fig. 2.7 Etchplanation through alternate deep weathering and stripping of saprolite explains the excellent morphological expression of lithology. Modified after Kroonenberg & Melitz (1983). During more arid phases with a less effective vegetation cover the saprolites are stripped by erosion, and the bedrock becomes exposed, at the level of the erstwhile weathering front. The differences in depth of the weathering front in different lithologies are now translated into altitudinal steps in the morphology. Repeated cycles enhance the topographic differences. Laterite caps, sandstone covers and granite monoliths are no longer affected during subsequent cycles, and come to stand out above the surrounding lowlands as plateaus and inselbergs (Kroonenberg & Melitz 1983; Dohrenwend et al. 1995; Vitte 2001). This means that the altitude of such plateaus can no longer be used to ‘date’ planation surfaces, and the denudation history of the Amazonian Craton therefore has to be reconsidered using modern techniques such as apatite fission track dating.

Conclusions In summary, the Amazonian Craton is a cratonic area surrounded on all sides by mobile belts of different ages: an Archean nucleus in the east, a Trans-Amazonian belt in the north, a Grenvillian belt in the west, and a Neoproterozoic one in the southeast (the Brasiliano TocantinsAraguaia belt, not discussed further in this chapter because it is not

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pertinent to the Amazon drainage basin). In each of these belts metamorphosed supracrustals predominate, and although associated tonalitic bodies are widespread in greenstone belts, their distribution is essentially restricted to the mobile belts themselves. The central part of the craton is occupied by vast expanses of Proterozoic granitoid and acid metavolcanic rocks, which neither spatially nor temporally are easily connected with any of these mobile belts. Although there are small outcrops of greenstone and high-grade units, the sheer volume of the granites precludes considering them as side-products of these minor units. In our view, the geological evolution of the Amazonian Craton is not so much the product of westward continental accretion around an Archean nucleus as envisaged in most recent reviews, but as the product of two major orogenic events, one in the Archean and a second one in the Paleoproterozoic, the Trans-Amazonian Orogeny, followed by a period of intense granitoid magmatism between 2.0 and 1.75 Ga, of which both the sources and the plate-tectonic significance remain obscure. This granitic magmatism was the major phase of consolidation of the craton. A Mesoproterozoic pulse of anorogenic granitic magmatism took place between 1.55 and 1.30 Ga. A third orogenic event along the western border of the craton, the 1.3–1.0 Ga Grenvillian orogeny, was the result of the Gondwana-Laurentia collision, leading to the formation of the Rodinia supercontinent. The collision led to extensive shear zones in both the Guiana and the Guaporé shield, which locally were reactivated during the Phanerozoic. Important molassic sedimentation took place after each orogenic phase, now preserved in impressive platform covers. Several episodes of tensional stresses led to intrusion of mafic dykes, from Paleoproterozoic times onwards. An Early Paleozoic one probably led to the formation of the E–W intracratonic basin now occupied by the Amazon River, and a Mesozoic phase of basaltic magmatism expressed in dolerite dyke swarms and basaltic rift valley fills evidences the break-up of Pangaea. From Mesozoic times onwards the Amazonian Craton has been mainly a source of sediment for the surrounding sedimentary basins.

Acknowledgements Ton Markus and Margot Stoete of GeoMedia are thanked for professionally preparing Fig. 2.1. Claude Delor and João Orestes Schneider Santos are thanked for permission to reproduce Figs 2.3 and 2.4, respectively. We also thank Colombo Celso Tassinari and Eric Tohver for their thorough reviews and valuable suggestions for improvement. This paper is dedicated to the memory of Dr. D.R. de Vletter. His indefatigable efforts to document both major characteristics and minor details of Suriname geology and to understand their regional significance have significantly contributed to our knowledge of the Amazonian Craton.

References Almeida, F.F.M. de, Hasui, Y., de Brito Neves, B.B., Fuck, R.A. (1981) Brazilian structural provinces: An introduction. Earth Sci Rev 17, 1–29.

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Almeida, F.F.M. de, de Brito Neves, B.B. dal Ré Carneiro, C. (2000) The origin and evolution of the South American Platform. Earth Sci Rev 50, 77–111. Almeida, M.E., Macambira, M.J.B., Oliveira, E.C. (2007) Geochemistry and zircon geochronology of the I-type high-K calc-alkaline and S-type granitoid rocks from southeastern Roraima, Brazil: Orosirian collisional magmatism evidence (1.97–1.96 Ga) in central portion of Guyana Shield. Precambrian Res 155, 69–97. Alvarez, J. (1981) Determinación de la edad Rb-Sr en rocas del Macizo de Garzón. Geología norandina 4, 31–38. Alvarez, J., Cordani, U.G. (1980) Precambrian basement within the septentrional Andes: Age and geological evolution. 26th International Geological Congress, Paris, Abstract 1, 10. Amaral, G. (1974) Geologia Pré-Cambriana da Região Amazônica. PhD thesis, Universidade de São Paulo, Instituto de Geociências. Avelar, V.A., Lafon, J.M., Delor, C., Guerrot, C., Lahondère, D. (2003) Archean crustal remnants in the easternmost part of the Guiana Shield: Pb-Pb and Sm-Nd geochronological evidence for Mesoarchean versus Neoarchean signatures. Géologie de la France 2003(2–4), 83–99. Bangerter, G. (1985) Estudio sobre la petrogénesis de las mineralizaciones de niobio, tántalo y estaño en el granito Rapakivi de Parguaza y sus diferenciaciones. Memoria I simposium Amazónico, Puerto Ayacucho, Venezuela. Boletín de Geología (Publicación Especial) 10, 175–185. Barrios, F., Rivas, D., Cordani, U., Kawashita, K. (1985) Geocronología del Territorio Federal Amazonas. Memoria I simposium Amazónico, Puerto Ayacucho, Venezuela. Boletín de Geología (Publicación Especial) 10, 22–31. Barron, C.N. (1969) Notes on the stratigraphy of Guyana. Geol Surv Brit Guyana Records 6(II), 1–28. Basei, M.A.S., Teixeira, W. (1975) Geocronologia do Território de Roraima. Anais 10ma Conferência Interguianas, Departamento Nacional de Produção Mineral, Belém, 453–473. Ben Othman, D., Polvé, M., Allègre, C.J. (1984) Nd-Sr isotopic composition of granulites and constraints on the evolution of the lower continental crust. Nature 307, 510–515. Berrangé J.P. (1977) The Geology of Southern Guyana, South America. London: Institute of Geological Sciences, Overseas Division, Memoir 4, 112. Bettencourt, J.S., Tosdal, R.M., Leite, W.B., Payolla, B.L. (1999) Mesoproterozoic rapakivi granites of the Rondônia Tin Province, southwestern border of the Amazonian craton, Brazil – I. Reconnaissance U-Pb geochronology and regional implications. Precambrian Res 95, 41–67. Boger, S.D., Raetz, M., Giles, D., Etchart, E., Fanning, C.M. (2005) U-Pb age data from the Sunsás region of Eastern Bolivia, evidence for the allochthonous origin of the Paragua Block. Precambrian Res 139, 121–146. Bosma, W., Kroonenberg, S.B., van Lissa, R.V., Maas, K., de Roever E.W.F. (1978) Geological Map of Suriname. Geological and Mining Service, Paramaribo. Bosma, W., Kroonenberg, S.B., Maas, K., de Roever, E.W.F. (1983) Igneous and metamorphic complexes of the Guiana Shield in Suriname. Geologie en Mijnbouw 62, 241–254. Bosma, W., Kroonenberg, S.B., van Lissa, R.V., Maas, K., de Roever, E.W.F. (1984) An explanation to the geology of Suriname. Geologisch Mijnbouwkundige Dienst Suriname 27, 31–82. Brito Neves, B.B. de (1999) America do Sul: Quatro fusões, quatro fissões e o processo acrescionario andino. Revista Brasileira Geociências 29, 379–392.

10/24/2009 10:41:06 Shobha

Geological evolution of the Amazonian Craton Brito Neves, B.B. de (2002) Main stages of the development of the sedimentary basins of South America and their relationships with the tectonics of supercontinents. Gondwana Res 5, 175–196. Brooks, W.E., Tosdal, R.M., Nuñez, F.J. (1995) Gold and diamond resources of the Icabaru Sur area, Estado Bolivar, Venezuela. In: Sidder, G.B., Garcia, A.E., Stoeser, J.W. (eds). Geology and Mineral Deposits of the Venezuelan Guyana. Washington: U.S. Geological Survey Bulletin 2124, pp. L1–L2. Cediel, F., Shaw, R.P., Cáceres, C. (2003) Tectonic assembly of the Northern Andean Block. In: Bartolini C. et al. (eds) The circum Gulf-of-Mexico and the Caribbean: Hydrocarbon Habitats, Basin Formation and Plate Tectonics. AAPG Memoir 79, pp. 815–848. Cordani, U.G., Sato, K. (1999) Crustal evolution of the South American Platform, based on Nd isotopic systematics on granitoid rocks. Episodes 22, 167–173. Cordani, U.G., Teixeira, W. (2007) Proterozoic accretionary belts in the Amazonian Craton. In: Hatcher, R.D. et al. (eds) 4D Framework of Continental Crust. GSA Memoir 200, pp. 297–320. Cordani, U.G., Sato, K., Teixeira, W., Tassinari, C.C.G., Basei, M.A.S. (2000) Crustal evolution of the South American Platform. In: Cordani U.G. et al. (eds) Tectonic Evolution of South America. Rio de Janeiro: 31st International Geological Congress, pp. 19–40. Cordani, U.G., Cardona, A., Jiménez, J.M., Liu, D., Nutman, A.P. (2005) Geochronology of Proterozoic basement inliers in the Colombian Andes: tectonic history of remnants of a fragmented Grenville belt. In: Vaughan, A.M.P. et al. (eds) Terrane Processes at the Margins of Gondwana. Geological Society of London Special Publication 246, pp. 329–346. Cox, D.C., Wynn, J.C., Skidder, G.B., Page, N.J. (1995) Geology of the Venezuelan Guayana Shield. In: Geology and Mineral Resource Assessment of the Venezuelan Guayana Shield by USGS and CVG Técnica Minera CA. USGS Bulletin 2062, pp. 9–15. Dahlberg, E.H. (1973) Lithostratigraphical correlation of granulitefacies rocks of the Guiana Shield. Segundo Congreso LatinoAmericano, Caracas, Venezuela. Geologisch Mijnbouwkundige Dienst Suriname Mededelingen 23, 26–33. Dall’Agnol, R., Lafon, J.-M., Macambira, M.J.B. (1994) Proterozoic anorogenic magmatism in the Central Amazonian Province, Amazonian Craton: geochronological petrological and geochemical aspects. Miner Petrol 50, 113–138. Dall’Agnol, R., Costi, H.T., Leite, A.A. da S., de Magalhães, M.S., Teixeira, N.P. (1999) Rapakivi granites from Brazil and adjacent areas. Precambrian Res 95, 9–39. Dall’Agnol, R., Oliveira, M., Almeida, J., Oliveira, D., Althoff, F., Barros, C., Leite, A. (2008) Granitoid magmatism and crustal evolution of the Carajás Province, Amazonian Craton. Abstract 33rd IGC, Oslo, Norway, Symposium AMS-07 paper 1342553. Da Rosa-Costa, L., Lafon, J.M., Delor, C. (2006) Zircon geochronology and Sm-Nd isotopic study: further constraints for the Archean and Paleoproterozoic geodynamical evolution of the southeastern Guiana Shield, north of Amazonian Craton, Brazil. Gondwana Res 10, 277–300. Deckart, K., Bertrand, H., Liégeois, J.-P. (2005) Geochemistry and Sr, Nd, Pb isotopic composition of the Central Atlantic Magmatic Province (CAMP) in Guyana and Guinea. Lithos 82, 289–314. De Loczy, L. (1977) Genetic relationships between the transcurrent faulting of Equatorial South America and the Oceanic Fracture zones. Boletín Geológico (Caracas), Publicacion Especial 7 III, 2053–2069. Delor, C., Lafon, J.M., Lahondère, D., de Roever, E.W.F., Fraga, M.L., Rossi, P. (2001) Paleoproterozoic framework of the Guiana Shield II – continental scale boudinage and ultra-high temperature granulite belt exhumation at 2.07–2.06 Ga. In: Simpósio de Geologia da Amazônia, 7, Belém, SBG-NO. Resumos Expandidos (CD-ROM).

Hoorn_ch02_Final.indd 25

25

Delor, C., de Roever, E.W.F., Lafon, J.M., Lahondère, D., Rossi, Ph., Cocherie, A. et al. (2003) The Bakhuis ultra-high temperature granulite belt (Suriname): II. Implications for late Trans-Amazonian crustal stretching in a revised Guiana Shield framework. Géologie de la France 2003(2–4), 207–230. Dohrenwend, G., Yánez, G., Lowry, G. (1995) Cenozoic landscape evolution of the southern part of the Gran Sabana, southeastern Venezuela – Implications for the occurrence of gold and diamond placers. In: Sidder, G.B., García, A.E., Stoeser, J.W. (eds) Geology and Mineral Deposits of the Venezuelan Guayana Shield. US Geological Survey Bulletin 2124, K1–K17. Edmond, J.M., Palmer, M.R., Measures, C.I., Grant, B., Stallard, R.F. (1995) The fluvial geochemistry and denudation rate of the Guyana Shield in Venezuela, Colombia and Brazil. Geoch Cosmochim Acta 59, 3301–3325. Elliott, R.G. (1992) The geology and geochemistry of the Omai Goldfield, Guyana. Thesis, Oxford Brookes University, UK. Fraga, L.M.B. (2002) Associação anortosito-mangerito-granito rapakivi (AMG) do Cinturão Guiana Central, Roraima, e suas encaixantes paleoproterozóicas: evolução estrutural, geocronologia e petrologia. Tese de doutorado, CPGG-UFPA, Belém, 351 pp. Fraga, L.M., Reis, N.J., Dall’Agnol., R., Armstrong, R. (2008) The Cauarane-Coeroene belt, the tectonic southern limit of the preserved Rhyacian crustal domain in the Guyana Shield, northern Amazonian Craton. Abstract of the 33th IGC, Oslo, symposium AMS-07, paper 1344505. Franzinelli, E., Potter, P.E. (1983) Petrology, chemistry, and texture of modern river sands, Amazon river system. J Geol 91, 23–39. Fuck, R.A., Brito Neves, B.B., Schobbenhaus, C. (2008) Rodinia descendants in South America. Precambrian Res 160, 108–122. Galarza, M.A., Macambira, M.J.B., Villas, R.N. (2007) Dating and isotopic characteristics (Pb and S) of the Fe oxide-Cu-Au-U-REE Igarapé Bahia ore deposit, Carajás mineral province, Pará state, Brazil. J S Am Earth Sci 25, 377–397. Galvis, J., Huguett, A., Ruge, P. (1979) Geología de la Amazonia Colombiana. Boletín Geológico (Bogotá) 22, 1–86. Gansser, A. (1973) Facts and theories on the Andes. J Geol Soc London 129, 93–131. Gaudette, H., Olszewski, W.J., Hurley, P.M., Fairbairn, H.W. (1978) Geology and age of the Parguaza Rapakivi granite, Venezuela. Geol Soc Am Bull 89, 1335–1340. Gaudette, H.E., Olszewski W.J. Jr, (1985) Geochronology of the basement rocks, Amazonas Territory, Venezuela, and the tectonic evolution of the western Guiana Shield. Geologie en Mijnbouw 64, 131–143. Geraldes, M.C., Von Schmus, W.R., Condie, K.C., Bell, S., Teixeira, W., Babinski, M. (2001) Proterozoic geologic evolution of the SW part of the Amazonian Craton in Mato Grosso state, Brazil. Precambrian Res 111, 91–128. Geraldes, M.C., Bettencourt, J.S., Teixeira, W., Matos, J.B. (2004) Geochemistry and isotopic constraints on the origin of the Mesoproterozoic Rio Branco ‘anorogenic’ plutonic suite, SW of Amazonian craton, Brazil: high heat flow and crustal extension behind the Santa Helena arc? J S Am Earth Sci 17, 195–208, Gibbs, A.K. (1980) Geology of the Barama-Mazaruni Supergroup of Guyana. PhD thesis, Harvard University, Cambridge, MA, 385 pp. Gibbs, A.K., Barron C.N. (1983) The Guiana Shield reviewed. Episodes June, 7–14. Gibbs, A.K., Barron, C.N. (1993) Geology of the Guiana Shield. Oxford: Oxford University Press, 246 pp. Goldstein, S.L., Arndt, N.T., Stallard, R.F. (1997) The history of a continent from U-Pb ages of zircons from Orinoco River sand and

10/24/2009 10:41:06 Shobha

26

S.B. Kroonenberg & E.W.F. de Roever

Sm-Nd isotopes in Orinoco basin river sediments. Chem Geol 139, 271–286. Grabert, H. (1983) Das Amazonas-Entwässerungssystem in Zeit und Raum. Geologische Rundschau 72, 671–683. Gradstein, F., Ogg, J., Smith, A. (2004) A Geologic Time Scale. Cambridge: Cambridge University Press, 589 pp. Harman, R., Gallagher, K. (1998) Accelerated denudation and tectonic/geomorphic reactivation of the cratons of northeastern Brazil during the Late Cretaceous. J Geophys Res 103, 27091–27105. Hoffman, P.F. (1991) Did the breakout of Laurentia turn Gondwanaland inside-out? Science 252, 1409–1412. Hurley, P.M., de Almeida, F.F.M., Melcher, G.C., Cordani, U.G., Rand, J.R., Kawashita, K. et al. (1967) Test of continental drift by comparison of radiometric ages. Science 157, 495–550. Jiménez-Mejía, D.M., Juliani, C., Cordani, U.G. (2006) P-T-t conditions of high-grade metamorphic rocks of the Garzón Massif, Andean basement, SE Colombia. J S Am Earth Sci 21, 322–336. King, L.C. (1956) A geomorphological comparison between eastern Brazil and Africa. Q J Geol Soc 112, 445–474. Kovach, A., Fairbairn, H.W., Hurley, P.M., Basei, M.A.S., Cordani, U.G. (1976) Reconnaissance geochronology of basement rocks from the Amazonas and Maranhão basins in Brazil. Precambrian Res 3, 471–480. Krook, L., de Roever, E.W.F. (1975) Some aspects of bauxite formation in the Bakhuis Mountains, western Suriname. Anais Decima Conferencia Geologia Interguianas, Belém, Brasil, 1, pp. 686–695. Kroonenberg, S.B. (1976) Amphibolite facies and granulite-facies metamorphism in the Coeroeni-Lucie area, southwestern Surinam. Geologisch Mijnbouwkundige Dienst Suriname, Mededelingen 25, 101–289. Kroonenberg, S.B. (1978) Precambrian paleosols at the base of the Roraima Formation in Surinam. Geologie en Mijnbouw 57, 445–450. Kroonenberg, S.B. (1982) A Grenvillian granulite belt in the Colombian Andes and its relation to the Guiana Shield. Geologie en Mijnbouw 61, 325–333. Kroonenberg, S.B. (1983) Litología, metamorfismo y origen de las granulitas del Macizo de Garzón, Cordillera Oriental (Colombia). Geología norandina 6, 39–46. Kroonenberg, S.B., Melitz, P.J. (1983) Summit levels, bedrock control and the etchplain concept in the basement of Suriname. Geologie en Mijnbouw 62, 389–399. Kroonenberg, S.B. (1985) El borde occidental del Escudo de Guayana en Colombia. Memoria I Simposium Amazónico, Puerto Ayacucho, Venezuela. Boletín de Geología (Caracas) Publicación Especial 10, 51–63. Kroonenberg, S.B. (1990) Geochemistry of the Garzón granulites in the Colombian Andes: Evidence for Proterozoic calc-alkaline magmatism. Symposium International. ‘Géodynamique Andine’, 15–17 Mai 1990, Grenoble, France, pp. 383–385. Lafrance, J., Bardoux, M., Voicu, G., Stevenson, R., Machado, N. (1999) Geological and metallogenic environments of gold deposits of the Guiana Shield: A comparative study between St-Elie (French Guiana) and Omai (Guyana). Explor Min Geol 8, 117–135. Lamarão, C.N., Dall’Agnol, R., Lafon, J.-M., Lima, E.F. (2002) Geology, geochemistry, and Pb-/Pb zircon geochronology of the Paleoproterozoic magmatism of Vila Riozinho, Tapajós Gold Province, Amazonian craton, Brazil. Precambrian Res 119, 189–223. Lamarão, C.N., Dall’Agnol, R., Pimentel, M.M. (2005) Nd isotopic composition of Paleoproterozoic volcanic and granitoid rocks of Vila Riozinho: implications for the crustal evolution of the Tapajo´s gold province, Amazon craton. J S Am Earth Sci 18, 277–292. Ledru, P., Johan, V., Milési, J.P., Tegyey, M. (1994) Markers of the last stages of the Paleoproterozoic collision: evidence for a

Hoorn_ch02_Final.indd 26

2 Ga continent involving circum-South Atlantic provinces. In: Onstott, T. (ed.) Proterozoic paleomagnetism and paleogeography. Precambrian Res 69, 169–191. Litherland, M., Klinck, B.A., O’Connor, E.A., Pitfield, P.E.J. (1985) Andean-trending mobile belts in the Brazilian Shield. Nature 314, 345–348. Litherland, M., Annells, R.N., Darbyshire, D.P.F., Fletcher, C.J.N., Hawkins, M.P., Klinck, B.A. et al. (1989) The Proterozoic of Eastern Bolivia and its relationship to the Andean Mobile Belt. Precambrian Res 43, 157–174. McConnell, R.B. (1966) Planation surfaces in Guyana. Geogr J 134, 506–520. Mendoza, V. (1974) Evolución tectónica del Escudo de Guayana. Segundo Congreso Latinoamericano de Geologia. Boletín Geológico (Caracas) Publicion Especial 7 III, 2237–2270. Milani, E.J., Zalán, P.V. (1999) An outline of the geology and petroleum systems of the Paleozoic interior basins of South America. Episodes 22, 199–205. Milesi, J.P., Egal, E., Ledru, P., Vernhet, Y., Thiéblemont, D., Cocherie, A. et al. (1995) Les minéralisations du Nord de la Guyane française dans leur cadre géologique. Chronique de la Recherche Minière, France 518, 5–58. Montgomery, C.P.W., Hurley, P.M. (1978) Total rock U-Pb and Rb-Sr systematics in the Imataca Series, Guayana Shield, Venezuela. Earth Planetary Sci Lett 39, 281–290. Nogueira, S.A.A., Bettencourt, J.S., Tassinari, C.C.G. (2000) Geochronology of the Salamangone gold deposit host-rocks, Lourenço district, Amapá, Brazil. Revista Brasileira Geoc 30, 261–264. Nomade, S., Chen, Y., Pouclet, A., Féraud, G., Théveniaut, H., Daouda, B.Y. et al. (2003) The Guiana and the West African Shield Paleoproterozoic grouping: new paleomagnetic data for French Guiana and the Ivory Coast. Geophys J Int 154, 677–694. Norcross, C., Davis, D.W., Spooner, T.C., Rust, A. (2000) U-Pb and Pb-Pb age constraints on Paleoproterozoic magmatism, deformation and mineralisation in the Omai area, Guyana Shield. Precambrian Res 102, 69–82. Olszewski, W.J., Wirth, K.R., Gibbs, A.K., Gaudette, H.E. (1989) The age, origin and tectonics of the Grão Pará Group and associated rocks, Serra dos Carajás, Brazil: Archean continental volcanism and rifting. Precambrian Res 42, 229–254. Ordóñez-Carmona, O., Restrepo-Alvarez, J.J., Pimentel, P.M. (2006) Geochronological and isotopical review of pre-Devonian crustal basement of the Colombian Andes. J S Am Earth Sci 21, 372–382. Payolla, B.L., Bettencourt, J.S., Kozuch, M., Leite, W.B., Fetter, A.H., Van Schmus, W.R. (2002) Geological evolution of the basement rocks in the east-central part of the Rondônia Tin Province, SW Amazonian craton, Brazil: U-Pb and Sm-Nd isotopic constraints. Precambrian Res 119, 141–169. Pedreira, A.J., Bahia, R.B.C. (2000) Sedimentary basins of Rondônia State, Brazil: response to the geotectonic evolution of the Amazonic Craton. Revista Brasileira de Geociências 30, 477–480. Peulvast, J.P., de Claudino Sales, V. (2004) Stepped surfaces and paleolandforms in the northern Brazilian «Nordeste»: constraints on models of morphotectonic evolution. Geomorphology 62, 89–122. Peulvast, J.-P., Sales, V.C., Bétard, F., Gunnell, Y. (2008) Low PostCenomanian denudation depth across the Brazilian northeast: implcations for long-term landscape evolution at a transform continental margin. Global Planet Change 62, 39–60. Pidgeon, R.T., Macambira, M.J.M., Lafon, J.-M. (2000) The U-Pb isotopic systems and internal structures of complex zircons from an enderbite from the Pium Complex, Carajás province, Brazil: evidence for ages of granulite facies metamorphism and the protolith of the enderbite. Chem Geol 166, 159–171.

10/24/2009 10:41:06 Shobha

Geological evolution of the Amazonian Craton Pinson, W.H., Hurley, P.M., Mencher, E., Fairbairn, H.W. (1962) K-Ar and Rb-Sr ages of biotites from Colombia, South America. Geol Soc Am Bull 73, 907–910. Potter, P.E. (1994) Modern sands of South America: composition, provenance and global significance. Geologische Rundschau 83, 212–232. Potter, P.E. (1997) The Mesozoic and Cenozoic paleodrainage of South America: a natural history. J S Am Earth Sci 10, 331–344. Priem, H.N.A., Hebeda, E.H., Boelrijk, N., Verschure, R.H. (1968) Isotopic age determination on Surinam rocks, 3. Proterozoic and Permo-Triassic basalt magmatism in the Guiana Shield. Geologie en Mijnbouw 47, 17–20. Priem, H.N.A., Boelrijk, N.A.I.M., Hebeda, E.H., Verdurmen, E.A.T., Verschure, R.H. (1971) Isotopic ages of the Trans-Amazonian acidic magmatism and the Nickerie Episode in the Precambrian basement of Surinam, South America. Geol Soc Am Bull 82, 1667–1680. Priem, H.N.A., Boelrijk, N.A.I.M., Hebeda, E.H., Verdumen, E.A.T., Vershure, R.H. (1973) Age of the Precambrian Roraima Formation in northeastern South America: Evidence from isotopic dating of Roraima pyroclastic volcanic rocks in Suriname. Geol Soc Am Bull 84, 1677–1684. Priem, H.N.A., Boelrijk, N.A.I.M., Hebeda, E.H., Kroonenberg, S.B., Verdurmen, E.A.Th., Verschure, R.H. (1977) Isotopic ages of the high-grade metamorphic Coeroeni Group, SW Suriname. Geologie en Mijnbouw 56, 155–160. Priem, H.N.A., Boelrijk, N.A.I.M., Hebeda, E.H., Kuyper, R.P., Roever, E.W.F. de, Verdurmen, E.A.Th. et al. (1978) How old are the Supposedly Archean Charnockitic Granulites in the Guyana Shield Basement of Western Suriname (South America)? USGS, Open File report 78–701, pp. 341–343. Priem, H.N.A., Andriessen, P.A.M., Boelrijk, N.A.I.M., de Boorder, H., Hebeda, E.H., Huguett, A. et al. (1982) Geochronology of the Precambrian in the Amazonas Region of southeast Colombia (western Guiana Shield). Geologie en Mijnbouw 61, 229–242. Priem, H.N.A, Kroonenberg, S.B., Boelrijk, N.A.I.M., Hebeda, E.H. (1989) Rb-Sr evidence for the presence of 1.6 Ga basement underlying the 1.2 Ga Garzón-Santa Marta Granulite Belt in the Colombian Andes. Precambrian Res 42, 315–324. Reis, N.J., de Faria, M.S.G., Fraga, L.M., Haddad, R.C. (2000) Orosirian calc-alkaline volcanism and the Orocaima event in the Northern Amazônian Craton, Eastern Roraima State, Brazil. Rev Bras Geociênc 30, 38–383. Restrepo-Pace, P.A., Ruiz, J., Gehrels, G., Costa, M. (1997) Geochronology and Nd isotopic data of Grenville-age rocks in the Colombian Andes: new constraints for Late Proterozoic–Early Paleozoic paleocontinental reconstructions of the Americas. Earth Planet Sci Lett 150, 427–441. Rino, S., Motoki, A., Hirata, T., Maruyama, S. (2004) U-Pb Spot Dating by La/ICP-MS of single detrital zircon grains collected at the mouth of the Amazon River: Continental crustal growth history of the Amazon River Basin. Short Papers – IV South American Symposium on Isotope Geology, 108–110. Rivas, D. (1985) Geología de la subregion Atabapo, Territorio Federal Amazonas, Venezuela. Memoria I Simposium Amazónico, Puerto Ayacucho, Venezuela. Boletin de Geología Publicación Especial 10, 122–139. Rodríguez-Vargas, E., Koester, G., Mallmann, R.V., Conceição, K., Kawashita, M.B.I. (2005) Weber Mantle diversity beneath the Colombian Andes, Northern Volcanic Zone: constraints from Sr and Nd isotopes. Lithos 82, 471–484. de Roever, E.W.F. (1973) A provisional lithologic framework of the Falawatra Group. Boletino de Geología Publicación Especial

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7.2, 637–664; Geologisch Mijnbouwkundige Dienst Suriname, Mededelingen 23, 34–44. de Roever, E.W.F. (1975a) Provisional lithologic framework of the Falawatra Group, W. Suriname. Geologisch Mijnbouwkundige Dienst Suriname, Mededelingen 23, 34–44; also in Boletíno de Geologia Publicación Especial (1973) 7, 637–648. de Roever, E.W.F. (1975b) Geology of the Central part of the Bakhuis Mountains (W Suriname). Geologisch Mijnbouwkundige Dienst Suriname, Mededelingen 23, 65–101. de Roever, E.W.F., Lafon, J.M., Delor, C., Rossi, P., Cocherie, A., Guerrot, C., Potrel, A. (2003a) The Bakhuis ultra-high temperature granulite belt: I Petrological and geochronological evidence for a counterclockwise P-T path at 2.07–2.05 a. Géologie de la France 2003 (2–4), 175–205. de Roever, E.W.F., Kroonenberg, S.B., Delor, C., Phillips D. (2003b) The Käyser dolerite, a Mesoproterozoic alkaline dyke suite from Suriname. Géologie de la France 2003(2–4), 161–174. Ruiz, J., Tosdal, R.M., Restrepo, P.A., Murillo-Muñetón, G. (1999) Pb Isotope Evidence for Colombia-Southern México Connections in the Proterozoic. Geological Society of America Special Paper 336, 183–197. Sadowski, G.R. (2002) The fit between Amazonia, Baltica and Laurentia during the Mesoproterozoic Assemblage of the Supercontinent Rodinia. Gondwana Res 5, 101–107. Sadowski, G.R., Bettencourt, J.S. (1996) Mesoproterozoic tectonic correlations between eastern Laurentia and the western border of the Amazon Craton. Precambrian Res 76, 213–227. Santos, J.O.S. (1984) A parte setentrional do cráton Amazônico (Escudo das Guianas) e a Bacia Amazônica. In: Schobbenhaus, C., Campos, D.A., Derze, G.R., Asmus, H.E. (eds) Geologia do Brasil. Brasília: Departamento Nacional da Produção Mineral, Capítulo II, pp. 57–88. Santos, J.O.S. (2003) Geotectônica dos Escudos das Guianas e BrasilCentral. In: Bizzi, L.A., Schobbenhaus, C., Vidotti, R.M., Gonçalves, J.H. (eds) Geologia, Tectônica e Recursos Minerais do Brasil. Brasília: CPRM, pp. 169–195. Santos, J.O.S., Hartmann, L.A., Gaudette, H.E., Groves, D.I., Mcnaughton, N.J., Fletcher, I.R. (2000) A new understanding of the provinces of the Amazon Craton based on integration of field mapping and U-Pb and Sm-Nd geochronology. Gondwana Res 3, 453–488. Santos, J.O.S., Hartmann, L.A., McNaughton, N.J., Fletcher, I.R. (2002) Timing of mafic magmatism in the Tapajós Province (Brazil) and implications for the evolution of the Amazon Craton: evidence from baddeleyite and zircon U-Pb SHRIMP geochronology. J S Am Earth Sci 15, 409–429. Santos, J.O.S., Hartmann, L.A., Bossi, J., Campal, N., Achipilov, A., Piñeyro, D., McNaughton, N.J. (2003a) Duration of TransAmazonian Cycle and its correlation within South America based on U-Pb SHRIMP geochronology of the La Plata Craton, Uruguay. Int Geol Rev 45, 27–48. Santos, J.O.S., Potter, P.E., Reis, N.J., Hartmann, L.A., Fletcher, I.R., McNaughton, N.J. (2003b) Age, source and regional stratigraphy of the Roraima Supergroup and Roraima-like outliers in northern South America based on U-Pb geochronology. Geol Soc Am Bull 115, 331–348. Santos, J.O.S., van Breemen, O., Groves, D.I., Hartmann, L.A., Almeida, M.E., McNaughton, N.J., Fletcher, I.R. (2004a) Timing and evolution of multiple Paleoproterozoic magmatic arcs in the Tapajós domain, Amazon Craton: constraints from SHRIMP and TIMS zircon, baddeleyite and titanite U-Pb geochronology. Precambrian Res 131, 73–109. Santos, J.O.S., Reis, N.J., Chemale, F., Hartmann, L.A., Pinheiro, S.S., McNaughton, N.J. (2004b) Paleoproterozoic evolution of

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northwestern Roraima state – Absence of Archean crust, based on U-Pb and Sm-Nd isotopic evidence. Short papers IV South American Symposium on Isotope Geology, pp. 278–281. Sato, K., Siga, O. Jr. (2000) Evidence of the superproduction of the continental crust during Paleoproterozoic in South American platform. Implications regarding the interpretative value of the Sm-Nd model ages. Revista Brasileira de Geociências 30, 126–129. Schobbenhaus, C., Bellizzia, A. (2001) Geological map of South America 1:5,000,000. CPRM-DNPM Brasil (CD-ROM). Schobbenhaus, C., Hoppe, A., Lork, A., Baumann, A. (1994) Idade U/Pb do magmatismo Uatumã no norte do Cráton Amazônico, Escudo das Guianas, Brasil: primeiros resultados. In: 38th Congr Bras Geol, Balneário Camboriu, Extended Abstracts 2, 395–397. da Silva, A.J.P., da Cunha Lopes, R., Vasconcelos, A.M., Bahia, R.B.C. (2003) Bacias sedimentares Paleozóicas e Meso-Cenozóicas interiores [Paleozoic and Meso-Cenozoic sedimentary basins]. In: Bizzi, L.A., Schobbenhaus, C., Vidotti, R.M., Gonçalves J.H. (eds) Geologia, Tectônica e Recursos Minerais do Brasil. Brasília: CPRM, pp. 55–85. Soler, J.M., Lasaga A.C. (2000) The Los Pijiguaos bauxite deposit (Venezuela): A compilation of field data and implications for the bauxitization process. J S Am Earth Sci 13, 47–65. Tallarico, F.H.B., Figueiredo, B.R., Groves, D.I., Kositcin, N., McNaughton, N. J., Fletcher, I.R., Rego, J.L. (2005) Geology and SHRIMP U-Pb geochronology of the Igarapé Bahia Deposit, Carajás Copper-Gold Belt, Brazil: An Archean (2.57 Ga) example of iron-oxide Cu-Au-(U-REE) mineralization. Econ Geol 100, 7–28. Tassinari, C.C.G (1981) Evolução geotectônica da Província Rio Negro-Juruena na região amazônica. Dissertaçao de mestrado, Instituto de Geociências, Universidade de São Paulo, 99 pp. Tassinari, C.C.G., Macambira, M.J.B. (1999) Geochronological provinces of the Amazonian Craton. Episodes 22, 174–182. Tassinari, C.C.G., Macambira, M.J.B. (2004) A evolução tectônica do Cráton Amazônico. In: Neto, V.M. et al. (eds) Geologia do Continente Sul-Americano. São Paulo: BECA, pp. 471–486. Tassinari, C.C.G., Cordani, U.G., Nutman, A.P., van Schmus, W.R., Bettencourt, J.S., Taylor, P.N. (1996) Geochronological systematics on basement rocks from the Rio Negro-Juruena Province (Amazonian Craton), and tectonic implications. Int Geol Rev 38, 161–175. Tassinari, C.C.G., Bettencourt, J.S., Geraldes, M.C., Macambira, M.J.B., Lafon, J.M. (2000) The Amazonian craton. In: Cordani, U.G. et al. (eds) Tectonic Evolution of South America. Rio de Janeiro: 31st International Geological Congress, pp. 41–95. Tassinari, C.C.G., Munhá, J.M.U., Teixeira, W., Nutman, A., Palacios, T., Sosa, S.C., Calado, B.O. (2004a) Thermochronological history of the Imataca complex, NW Amazonian Craton. Short papers IV South American Symposium on Isotope Geology, pp. 121–123. Tassinari, C.C.G., Munhá, J.M.U., Teixeira, W., Palacios, T., Nutman, A., Sosa, S.C. et al. (2004b) The Imataca Complex, NW Amazonian Craton, Venezuela: crustal evolution and integration of geochronological and petrological cooling histories. Episodes 27, 3–12. Teixeira, W., Tassinari, C.C.G. Cordani, U.G., Kawashita, K. (1989) A review of the geochronology of the Amazonian Cration: tectonic impications. Precambrian Res 42, 213–227. Tohver, E., van der Pluijm, B.A., van der Voo, R., Rizzotto, G., Scandolara, J.E. (2002) Paleogeography of the Amazon craton at

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1.2 Ga: early Grenvillian collision with the Llano segment of Laurentia. Earth Planet Sci Lett 199, 185–200. Tohver, E., van der Pluijm, B., Mezger, K., Essene, E., Scandolara, J., Rizzotto, G. (2004a) Significance of the Nova Brasilândia metasedimentary belt in western Brazil: Redefining the Mesoproterozoic boundary of the Amazon craton. Tectonics, 23, Tc6004; doi: 10.1029/2003TC001563. Tohver, E., Bettencourt, J.S., Tosdal, R., Mezger, K., Leite, W.B., Payolla, B.L. (2004b) Terrane transfer during the Grenville orogeny: tracing the Amazonian ancestry of southern Appalachian basement through Pb and Nd isotopes. Earth Planet Sci Lett 228, 161–176. Tohver, E., van der Pluijm, B.A., Mezger, K., Scandolara, J.E., Essene, E.J. (2005) Two stage tectonic history of the SW Amazon craton in the late Mesoproterozoic: identifying a cryptic suture zone. Precambrian Res 137, 35–59. Tohver, E., Teixeira, W., van der Pluijm, B., Geraldes, M.C., Bettencourt, J.S., Rizzotto, G. (2006) Restored transect across the exhumed Grenville orogen of Laurentia and Amazonia, with implications for crustal architecture. Geology 34, 669–672. Trendall, A.F., Basei, M.A.S., de Laeter, J.R., Nelson, D.R. (1998) SHRIMP zircon U-Pb constraints on the age of the Carajás formation, Grão Pará Group, Amazon Craton. J S Am Earth Sci 11, 265–277. Vanderhaeghe, O., Ledru, P., Thiéblemont, D., Egal, E., Cocherie, A., Tegyey, M., Milési J.P. (1998) Contrasting mechanism of crustal growth. Geodynamic evolution of the Paleoproterozoic granitegreenstone belts of French Guiana. Precambrian Res 92, 165–193. Veenstra, E. (1978) Petrology and geochemistry of sheet Stonbroekoe, sheet 30, Suriname. Geologisch Mijnbouwkundige Dienst Suriname, Mededelingen 26, with addendum. Vigneresse, J.L. (2005) The specific case of the Mid-Proterozoic rapakivi granites and associated suite within the context of the Columbia supercontinent. Precambrian Res 37, 1–34. Vitte, A.C. (2001) Considerações sobre a teoria da etchplanação e sua aplicação nos estudos das formas de relevo nas regiões tropicais quentes e úmidas. Terra Livre (São Paulo) 16, 11–24. de Vletter, D.R. (1984) Contributions to the geology of Suriname, 8. Mededelingen Geologisch Mijnbouwkundige Dienst Suriname 27, 1–134. de Vletter, D.R., Aleva, G.J.J., Kroonenberg, S.B. (1998) Research into the Precambrian of Suriname. In: Wong, Th.E. et al. (eds) The History of Earth Sciences in Suriname. Amsterdam: Royal Netherlands Academy of Sciences, pp. 15–64. Voicu, G., Bardoux, M., Harnois, L., Crepeau, R. (1997) Lithological and geochemical features of igneous and sedimentary rocks at the Omai gold mine, Guyana,South America. Explor Min Geol 6, 153–170. Wanderley Filho, J.R., Gonçalves de Melo, J.H., da Fonseca, V.M.M., da Costa Machado, D.M. (2005) Bacias sedimentares brasileiras. Phoenix 7, 1–6. Zonneveld, J.I.S. (1969) Preliminary remarks on summit levels and the evolution of relief in Surinam. Proc 7th International Guiana Conf., 1966, Paramaribo. Verh Kon Nederlands Geologisch Mijnbouwk Gen 27, 53–59.

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THREE

The Paleozoic Solimões and Amazonas basins and the Acre foreland basin of Brazil Joaquim Ribeiro Wanderley-Filho1, Jaime Fernandes Eiras2, Paulo Roberto da Cruz Cunha3 and Paulus H. van der Ven3 1

Petrobras Exploration and Production, Manaus, Amazonas, Brazil PetroGeo – Serviços Geológicos S/S Ltd, Manaus, Amazonas, Brazil 3 Petrobras Exploration and Production, Rio de Janeiro, Brazil 2

Abstract The sedimentary basins of the Amazonian region are the fundamental elements that dictate the present configuration of what is generically known as Amazonia. The geological evolution of these basins established the landscape and the geomorphology of the rivers, lakes and marine systems that filled those depressions for the last 500 million years. The wide continental lowlands of Amazonia interacted dramatically with the uplift of the Andes during the last 23.5 million years (Neogene times), creating a special and unique region in terms of drainage, fauna and flora (‘the Amazon rainforest’). In this chapter we review the genesis and evolution of the intracratonic Solimões and Amazonas basins and the Acre foreland basin, which together formed the framework for the Neogene sedimentary record that is the subject of this book.

Introduction to the region’s geological evolution The Proterozoic evolution of the Amazonian region comprised two main tectonic events: the oldest was the assemblage of the granite-greenstone cratonic terranes and the high-grade metamorphic belts. These now constitute the basic elements of the Amazonian Craton, including parts of Brazil, French Guiana, Guyana, Suriname, Venezuela, Colombia and Bolivia. The Amazonian Craton underlies most of the northern South American platform and is divided into two shields: the Central Brazilian Shield to the south and the Guiana Shield to the north. These shields are surrounded by Early Proterozoic orogenic belts, which have been relatively stable since 1.0 Ga (see Chapter 2). The E–W-trending Paleozoic Amazonas, Solimões and Acre basins overlie and transect the Amazonian Craton (Fig. 3.1; see also Plate 8). Isotope studies and the definition of geochronological provinces provide a useful basis from which to understand crustal evolutionary processes and their tectonic implications at a continental scale. According to Tassinari et al. (2000), the Amazonian Craton can be subdivided into six major geochronological provinces based on age, structural trends, relative proportion of rock

Amazonia, Landscape and Species Evolution: A Look into the Past, 1st edition. Edited by C. Hoorn and F.P. Wesselingh. © 2010 Blackwell Publishing

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types, and some geophysical characteristics. The geochronological provinces of the craton comprise a stable Archean nucleus (Carajás-Iricoumé and Roraima blocks), which is included in the Central Amazonian Province, and several Paleoproterozoic and Middle Proterozoic provinces (see Fig. 3.1). During Middle Proterozoic times, a second series of roughly E–W-trending extensional events formed several NW–SE-trending basins, as evidenced, among others, by the Cachimbo Graben or Alto Tapajós Basin (Fig. 3.2). These basins were positively inverted during the Late Proterozoic (Wanderley Filho & Costa 1991). The structural grain established by these events exerts strong controls on the framework of the Amazonas Basin during the Paleozoic as well as on the structures formed later in Mesozoic and Cenozoic times. The inversion of the Cachimbo Graben, for example, resulted in the Purus Arch, the positive feature that separated the Amazonas from the Solimões Basin (see Fig. 3.2 and Plate 9). A NW–SE-trending extension of the Brazilian Shield, related to the early stages of opening and separation of the northern margins of Gondwana, resulted in an elongated E–W-trending intraplate trough in which the Solimões and Amazonas intracratonic sag basins were established. Greater subsidence was constrained by southern and northern hinge lines, separated from the outboard platform areas of lesser subsidence. The areal extent of these ‘sister’ basins was much larger than their modern configuration and during their geological history they occasionally communicated with the Parnaíba and Paraná

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ntic

> 2.3 Ga

Amazon Fan

Atla

1 56º 00m

68º

50º

Oc

ean

4º

GEOCHRONOLOGICAL AND GEOTECTONIC PROVINCES OF THE AMAZON CRATON Central Amazonian

2.2–1.95 Ga

Maroni-Itacaiúnas 1.95–1.8 Ga 1.8–1.55 Ga

> 2.3 Ga

nas azo Am asin B

Purus Manaus

Iquitos

es imó Sol sin Ba 1.5–1.3 Ga

Ventuari-Tapajós

0º Monte Alegre

> 2.3 Ga

Rio Negro-Juruena

Belem Gurupá

Rondonian-San Ignácio Sunsás

4º

Carajas Ridge

Goias Massif Neoproterozoic Belts

1.95–1.8 Ga

º

8

GEOLOGICAL UNITS

P. velho

Phanerozoic Covers Granitoids

1.8–1.55 Ga 0.65 Ga

1

. –2

Ga

12º

Felsic-Intermediate Vulcanic Covers

0

1.25–1.0 Ga

2.

Precambrian Sedimentary Covers Mafic Vulcanism

N

Greenstone Belts Granulitie Complex

240 km

Basement Structural High

Fig. 3.1 2000).

Major geochronological provinces and lithological association of the Amazonian Craton (modified from Tassinari et al.

05 00 00

00 00 00

– 05 00 00

– 10 00 00

– 70 00 00

– 65 00 00

– 60 00 00

– 55 00 00

– 50 00 00

– 45 00 00

– 40 00 00

Fig. 3.2 Main features of the Amazonian sedimentary basins and the Brazilian and Guyana Shields, showing the location of places and names referred to in the text. Depth to basement contours are in metres (see also Plate 9a).

Hoorn_ch03_Final.indd 30

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Solimões, Amazonas and Acre foreland basins North Platform (Manaus Platform)

B

B’

31

0m ⫺3000 ⫺6000

C

North ramp

South Platform (Mamuru Platform)

C’

0m

⫺3000

Legend Amazonas Basin

⫺6000

Cenozoic Upper Cretaceous

D

Urucuricaia High

D’

0m

Carboniferous-Permian ⫺3000

Devonian-Carboniferous Ordovician-Devonian Basement

⫺6000

Fig. 3.3 Cross-sections along the Amazonas Basin. Differential tilting along the basin during its geological evolution is evidenced by the thickness variations in important depositional sequences and the opposing north and south platform areas. Basins (see Fig. 3.2). Two Phanerozoic first-order sedimentary sequences comprise the sedimentary fill of the sister basins. The older and more significant Paleozoic section started in the Early Ordovician and was interrupted in the Permian by the onset of the collision between Gondwana and Laurasia, leading to the formation of a major unconformity and a long non-depositional hiatus throughout the Triassic, Jurassic and Early Cretaceous. The second and much younger first-order sedimentary sequence is the relatively thin Mesozoic and Cenozoic section.

The Amazonas Basin The Amazonas Basin is located in northern Brazil, covering an area of about 500,000 km2 (see Fig. 3.2 and Plate 9). The major axis of the basin trends east–west between the Guiana Shield in the north, and the Brazilian Shield in the south. In the west it is separated from the Solimões Basin by the Purus Arch. To the east the Gurupá Arch separates it from the Marajó Basin. Basin floor (basement) structural hinges roughly parallel and aligned to the northern and southern basin margins separate a central, deeper basin trough, with up to 5000 m of Paleozoic sediments, from relatively shallow ramps, herein designated as the North and South Platforms (Fig. 3.3). Despite its large dimensions, the Paleozoic sedimentary fill of the basin is rarely seen in outcrops because of the widespread overlying Mesozoic and Cenozoic sedimentary cover and the dense rainforest dominating the entire region. Most outcrops are concentrated in the basin margins, either along river drainage systems, which cut the stratigraphical succession along the basinal dip, or in limestone quarries. The only place where Paleozoic outcrops can be assessed and studied in more detail for greater distances is in the Monte Alegre Dome region (see Fig. 3.2). Such good quality outcrops are known

Hoorn_ch03_Final.indd 31

nowhere else, and therefore any regional study of the Amazonas Basin has to rely on subsurface well data and seismic data acquired for oil and gas exploration. Two main Phanerozoic first-order sedimentary sequences comprise the sedimentary fill of the Amazonas Basin (see Plate 10). The youngest is represented by the relatively thin Mesozoic and Cenozoic sections, and the older, much more significant and well-developed Paleozoic section consists of four second-order sequences (Cunha et al. 2007). From bottom to top these are: • the Trombetas Group (Upper Ordovician to lowermost Devonian); • the Urupadi and Curuá Groups (Lower Devonian to Lower Carboniferous); • the Faro Formation (Upper Visean); • the Tapajós Group (Upper Carboniferous to Upper Permian).

The Paleozoic section The first two Paleozoic sequences are essentially siliciclastic, with one glacial unit (Curiri Formation) representing the dominant cold climate of those times. From the Early Ordovician to Early Devonian the deposition of these rocks resulted from east–west transgressions onlapping the Purus Arch (see Fig. 3.2). At the eastern limit of the Amazonas Basin, the rising sea drowned the Gurupá Arch, connecting this basin with the northwestern African basins (Cunha et al. 2007). In the Early Devonian, following an episode of uplift and erosion coeval to the Caledonian Orogeny, a new transgressive-regressive cycle was established with marine and glacial sedimentation. A Permo-Carboniferous evaporitic sequence was deposited following a renewed subsidence event with a significant change in

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J.R. Wanderley-Filho et al.

climate to hot, dry conditions. A new marine transgression coming from the east and west drowned an extensive field of sand dunes across both the Amazonas and Solimões Basins, covering the Purus Arch and effectively connecting these two basins (see Plate 9). This process started with the deposition of fluvial, aeolian and shallow marine clastics, grading to carbonates capped by marine and lacustrine evaporites, subsequently followed by fluvio-lacustrine clastics and, less frequently, evaporites (Eiras & Matsuda 1998). In the lithostratigraphic nomenclature, this sequence is defined as the Tapajós Group, composed of the Monte Alegre, Itaituba, Nova Olinda and Andirá formations (see Plate 10). In Late Permian times the onset of the collision between Gondwana and Laurasia resulted in a long-lasting non-depositional and widespread erosional event that eroded much of the deposits of the Amazonas and Solimões Basins overlying the northern and southern platforms, leading to the formation of a major subcrop erosional surface and a prominent stratigraphic hiatus lasting throughout the Triassic, Jurassic and Early Cretaceous. It is still uncertain whether this hiatus is erosional or non-depositional. Considering that the upper sections of the Andirá Formation are only preserved in the central parts of the basin, at the time active erosion on the southern and northern platforms must have taken place. Igneous intrusions are widespread throughout the basin and occur in significant volumes. Previously these intrusions were attributed to a series of events during basin evolution. However, recent argon-argon (Ar-Ar) dating points to a uniform age of 204 Ma for the emplacement of these igneous rocks, suggesting that their origin is related to a single magmatic event, correlatable to the opening of the Central Atlantic Ocean. Maximum diabase thickness of 915 m is registered in the Tucumã number 1 well, located midway between the Tapajós and Xingu rivers (see Fig. 3.2). A total of 90,000 km3 of diabase in sills and dike emplacements in the sedimentary column of the basin has been calculated based on average diabase isopach maps (see also Chapter 2). The existence of transfer faults in this basin was initially recognized in the Nova Olinda region, where blocks tilting in opposite directions and separated by faults were observed. Gravity anomalies confirm the existence of structures trending at right angles to the general strike of the basin, and are also attributed to dextral transfer faulting. Isopach maps as well as geophysical maps of the region show that the Amazonas Basin is composed of a series of long segments with an ENE–WSW-trending orientation, which alternate with shorter E–W-trending segments. The configuration of this structural framework resembles a series of dog-legs. This pattern is a result of the interaction between normal NE–SW-trending faults from the Early Paleozoic trending rifts, and NW–SE-trending transfer faults. The main NW-SE trending faults in the Amazonas Basin were identified based on the interpretation of structural and isopach maps derived from geological and geophysical data. These faults are closely related to the six major geochronological provinces (see Fig. 3.1) and cut and displace the basin depocentres of almost all sedimentary sequences as well as positive gravimetric anomalies. Therefore, these faults have been active at least until the Neogene (Tertiary). The Purus and Gurupá arches and the border faults are certainly the main basin structural features of the Amazonas Basin (see

Hoorn_ch03_Final.indd 32

Fig. 3.2 and Plates 8 & 9). The Purus Arch is a NW–SE-trending structural element separating the Amazonas from the Solimões basins. It can be easily seen in gravimetric and isopach maps as well as in geological and seismic cross-sections. In the Bouguer map this arch corresponds to a Bouguer minimum, indicating the sedimentary nature of its composition, confirmed by a number of exploration wells that penetrated middle Proterozoic sedimentary rocks of the Prosperança Formation. The Gurupá Arch is the NW–SE-trending structural element that separates the Amazonas Basin from the Marajó Graben. This feature is very well defined from geophysical and geologic data.

The Cretaceous and Neogene section A long period of non-deposition and erosion dominated Amazonia from Triassic to Jurassic times, and subsidence was only resumed in both basins (Solimões and Amazonas) during the Late Cretaceous, as testified by the sedimentary record of lacustrine and fluvial systems with drainage towards the Pacific Ocean and Caribbean Sea. The resulting sand-prone sedimentary unit is the Alter do Chão Formation, which occurs in both the Amazonas and the Solimões basins (see Plate 2d). Acceleration of Andean exhumation ponded western Amazonia during the Neogene and inverted the continent-wide drainage system eastwards, creating the modern Solimões and Amazonas river systems. The Solimões Formation represents this new geographic configuration with widespread deposition of fining- and coarsening upward sequences. The Cretaceous and Neogene deposits are discussed in more detail in the chapters devoted to the fluvial systems of cratonic origin and megawetland (see Chapter 7).

The Solimões Basin The Solimões Basin consists of an east–west-oriented Paleozoic intracratonic depression of about 480,000 km2 (present-day limit of occurrence of Paleozoic rocks). This basin is bisected by the Solimões River and a network of hundreds of tributaries. Geographically it lies mostly within the Amazonas State in Brazil but also has northwards extensions into Colombia. Geologically it is bounded by the Guiana and Brazilian Shields to the north and south respectively. To the west it is separated from the Acre Basin by the Iquitos Arch, and from the Amazonas Basin, to the east, by the Purus Arch (see Fig. 3.2 and Plates 8 & 9). An intra-basin positive north–south-trending regional high denominated Carauari Arch controlled the distribution of sediments along the Solimões Basin, especially during pre-Mesozoic times. The Juruá sub-basin, to the southeast, is better known since 1978 due to decades of oil and gas exploration efforts by Petrobras. The Jandiatuba subbasin, corresponding to the western portion of the basin, is less known due to legal restrictions on its exploration imposed by the extensive indigenous reserves and national forest reserves (Eiras & Wanderley Filho 2006). Geologically, the structural and stratigraphic framework of the Solimões Basin is similar to other Brazilian intracratonic Paleozoic basins. The underlying Proterozoic basement upon which the Solimões Basin was installed is part of a number of mobile belts

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Solimões, Amazonas and Acre foreland basins accreted to an older nucleus denominated the Central Amazon Province (Cordani et al. 1984). This basement consists of metamorphic and igneous rocks under the Jandiatuba sub-basin (Rondoniense mobile belt). Under the Juruá sub-basin, the sedimentary rocks of the Purus Group were deposited in a succession of widespread Proterozoic rift systems, and are dominant over the cratonic metamorphic and igneous rocks of the basement. Intraplate tectonics generated folds and faults throughout the basin, but the largest and most important of these events is certainly what is now called the Mesozoic Juruá Transpressive Tectonic event. This event reactivated older structures and generated preferentially NE–SW-trending anticline folds and transfer faults. Other important structural elements include the Iquitos and Purus regional arches, the Carauari internal arch, some intrabasinal internal palaeo-highs, the northern and southern ramps, and the central depression (see Fig. 3.2). The sedimentary infill of the Solimões Basin and its igneous intercalations are the product of a combination of (i) tectonic subsidence, which created the necessary space for sediment deposition; (ii) sediment input, which controlled basin palaeobathymetry; and (iii) local climatic variations, which controlled the different types of sedimentary environments throughout the basin. Climate was an important factor in determining local environmental conditions with respect to temperature, winds and humidity. Hence, rocks representing a wide span of depositional environments, from glacial to desert/evaporitic conditions, can be found throughout the stratigraphic column. The Phanerozoic sedimentary infill can reach as much as 3800 m in thickness in the Juruá sub-basin, and 3100 m in the Jandiatuba sub-basin to the west. It can be divided into two firstorder sequences. The most important and significant one, the Paleozoic section, contains the seals, reservoir and source rocks of the proven petroleum system of this basin, and is intruded by extensive diabase sills and dikes (see Plate 11). This section is completely covered by Cretaceous and Neogene strata. Unlike the neighbouring Amazonas Basin, Paleozoic strata are not known to crop out anywhere in the Solimões Basin. The genesis of this basin is uncertain and subject to debate. The preferred working model admits progressive west-to-east regional subsidence in a flexural regime related to Ordovician rifting of the Gondwana continent. The Solimões Basin and other minor interior depressions and marginal basins would therefore have been formed by thermo-mechanical subsidence.

33

The Jutaí Formation is the stratigraphic representative of this section (see Plate 11). Further stratigraphic analyses of the three third-order sequences that compose this section confirm this hypothesis. During a third marine invasion, starting in the mid-Devonian, the Carauari Arch became evident. The climate was cold, as evidenced by glacial deposits capping this sequence. The Middle Devonian to Early Carboniferous Marimari Group occurs in the Juruá area and beyond the Carauari Arch. The sediments are quite muddy, particularly in the Jandiatuba area, and very rich in spicules of siliceous sponges. An important maximum flooding surface is present in the passage from the Upper Frasnian to the Lower Famennian. A condensed section of anoxic shales with the highest total carbon contents of the basin is found in this interval. The Tefé Group represents the fourth and last marine incursion in the basin from Late Carboniferous to the Early Permian. During this period the climate became hot and arid favouring the development of tidal bars and coastal aeolian dunes forming the best reservoir rocks of the basin. These were later followed by the development of the thick evaporitic sequence, which acts as an effective seal for the petroleum fields in the Solimões Basin. The control of the Carauari Arch over sedimentation can also be observed through the total isopach maps of this sequence, and halite isolith maps. Moreover, the erosional event separating the Paleozoic from the Mesozoic is more intense over the arch, as observed in the stratigraphic chart of the basin as well as in the regional geological cross-sections (see Plates 9 & 11). There is no evidence of any Triassic or Jurassic sedimentation in the Solimões Basin. During this period erosional processes were caused by Late Hercynian orogeny as well as uplifts generated by the extensive intrusion of diabase sills within the basin. At least three main bodies of diabase sills have been intruded within the Permo-Carboniferous section, following relatively uniform strata boundaries (see Plates 9 & 11). This major magmatic manifestation occurred at approximately 204 Ma during the Triassic Period (Ar-Ar dating in Wanderley Filho et al. 2005, 2007). This coincides with the Penatecaua magmatism, which is widely recognized along the Brazilian platform and is related to the opening of the Central Atlantic Ocean. Based on thermo-mechanical modeling and the analysis of the total thickness of diabase intrusions, it is estimated that at least 800 m of Permian section has been eroded during this period.

The Paleozoic section The Cretaceous and Neogene section During the Early Ordovician, the Juruá sub-basin area was a stable platform, separated from an eastern subsiding region (Jandiatuba) by a hinge line located approximately along the same lines where, in the mid-Devonian, the Carauari Arch would be established (see Fig. 3.2). The mid-Ordovician Benjamin Constant Formation represents the first marine incursion in the region, and is restricted to the Jandiatuba sub-basin. A transgression coming from the west is also well documented by proximal facies correlatable to those distal systems. The first manifestation of the Carauari Arch as a basin divide occurred during the Early Silurian. A marine incursion coming from the west covered the extreme west of the Juruá sub-basin.

Hoorn_ch03_Final.indd 33

After the Permian a long period of non-deposition and erosion dominated Amazonia. Subsidence resumed during the Late Cretaceous in both the Solimões and the Amazonas basins with clastic deposition from meandering to anastomosed fluvial systems, with aeolian reworking. This first cycle is followed by prograding deposits of fluvial, deltaic and lacustrine sediments intercalated with conglomerates. The sandstone beds are fine-to-medium-grained, red, variegated, kaolinitic, gillaceous, soft and generally cross-bedded with cutand-fill structures. Many massive limonitic bands occur throughout the section. The uppermost part sometimes forms plateaus capped by bauxite deposits. Waters from this fluvial system drained to

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J.R. Wanderley-Filho et al.

a palaeo-Pacific Ocean or Caribbean Sea. This sand-dominated unit occurs in both the Amazonas and the Solimões Basins and is named Alter do Chão Formation (see Chapter 7). The Cretaceous-Paleogene uplift of the Andean Cordillera started to isolate this fluvial basin. The overload of the Andean orogeny triggered lithospheric flexuring of the South American plate, shifting the Cenozoic sediment depocentre to the sub-Andean foreland basins (Acre-Ucayali-Marañón, also called Amazonian foreland basin). During the Miocene the fluvial systems gave way to large shallow freshwater lakes with low circulation fed by lowenergy meandering rivers, which deposited the predominantly muddy sections of the lower Solimões Formation (see Chapter 8). The high humidities of the early Cenozoic favoured the development of a rainforest, which was the precursor of today’s luxuriant Amazon rainforest. With the final emplacement of the Andes during the Neogene, the basin started to receive large amounts of sediments coming from the high mountains, starving the basin and reorganizing the fluvial system into a new one directed to the Atlantic Ocean: the modern Solimões and Amazonas transcontinental river system (Hoorn et al. 1995). During the Quaternary, as an isostatic compensation response to the Andean uplift, some rivers elevated their gradient, increasing their capability for carrying sedimentary loads. The Cenozoic sequence forms a sedimentary prism starting at the Purus Arch and finishing at the sub-Andean basins, where it can locally reach as much as 7000 m thick (Mosmann et al. 1986). The stratigraphical relationship between the Solimões Formation and the underlying Cretaceous strata is not well known due to the lack of outcrops and, in the case of wells drilled for oil and gas exploration, by the lack of interest in the shallow strata, which are not sampled or logged in oil and gas exploration wells. In the neighbouring Amazonas Basin a similar section is also termed the Solimões Formation. This formation is the lateral time-equivalent of the sedimentary sequence of the Amazon Fan, deposited in the offshore Foz do Amazonas basin in the northern Atlantic (see Plate 8).

the Uruaçuano Cycle (see Fig. 3.1), and has a NW–SE orientation as its main structural trend. The main structural feature of the Acre Basin is the large, north–south-trending Batã reverse fault to the east, and a system known as the Oeste de Batã Reverse Fault System associated with the Serra do Divisor Mountains along the Peruvian/Brazilian border. Another important feature is the east–west Paraguá-Jutaí Arch, which separates two internal depocentres: the Jaquirana depression to the north, and the Central depression. The Cruzeiro do Sul platform lies to the east (Fig. 3.5; see also Fig. 3.4). The Phanerozoic record of the Acre Basin is composed of up to 1500 m of Paleozoic sediments, which are covered by Mesozoic and Cenozoic sediments that can reach up to 4000 m and 2000 m in thickness, respectively (Feijó & Souza 1994). This succession comprises 11 second-order sequences, separated by important sequence boundaries or major regional unconformities, represented in the Stratigraphic Chart recently revised by Cunha (2007) (see Plate 12). The basis of the sedimentary sequence unconformably overlies the Proterozoic igneous and metamorphic basement. The Acre Basin is the only Brazilian basin that, during its structural evolution, was directly affected by Andean tectonics. It has a complex evolutionary history that started in the Paleozoic and was continually under the influence of the tectonic compressive regimes that acted along the western margin of the South American continent. Its evolution is related to two major depositional and tectonic cycles separated by orogenic episodes that affected the South American Plate: the Hercynian (‘pre-Andean’) and the Andean cycles (Barros & Carneiro 1991). The Hercynian, or pre-Andean, System is represented by the Silurian-Devonian (possibly occurring in the Acre basin), Devonian-Carboniferous and Permo-Carboniferous depositional cycles. The Andean System is represented by the Permo-Triassic, Juro-Triassic, Early Cretaceous/Paleocene and the Eocene-Pliocene cycles. These well-known cycles of the sub-Andean basins can be easily extended into the Acre Basin with minor adjustments.

The Paleozoic-Mesozoic foreland Acre Basin

The Hercynian (pre-Andean) System

The Acre Basin lies in westernmost Brazil, and constitutes the northern part of Acre State and the southwestern part of Amazonas State, near the border with Peru. This basin extends over an area of 150,000 km2 (Cunha 2006). Geologically it is limited to the east by the Iquitos Arch, separating it from the Solimões Basin, and bounded in the north and northwest by the Marañón Basin, and to the south and southwest by the Ucayali Basin. The Acre Basin is the eastern extension of the Amazonian foreland basins (or Neogene syncline) known in the neighbouring countries as the Marañón-Ucayali and Madre de Dios basins (see Chapter 5; see also Fig. 3.4 and Plates 8 & 9). The Acre Basin is a foredeep basin deposited over a rigid lithosphere associated with the genesis of a mega-compressional suture, the Andean fold belt (Bally & Snelson 1980, cited in Raja Gabaglia & Figueiredo 1990). The basin basement is composed of gneisses, migmatites and associated granites and forms an extension of the Rondoniano-San Ignácio mobile belt (Tassinari et al. 1996, 2000). This belt developed between 1.5 and 1.3 Ga, during

Silurian-Devonian(?) Cycle

Hoorn_ch03_Final.indd 34

In the Peruvian Marañón Basin this period is represented by the Contaya Group, and in the Brazilian Solimões Basin by the Benjamin Constant Group. In these basins this cycle is composed of siliciclastic sediments deposited in a neritic environment. Dalmayrac (1978) argues that central Peru was a positive feature during the Silurian and acted as the western border of the Early Paleozoic basin of this time. Barros & Carneiro (1991) and Ham & Herrera (1963) do not recognize the existence of this cycle in the Acre Basin, arguing that it was eroded during the Caledonian Orogeny. Seismic interpretation, however, supports the existence of sediments of this cycle in the Acre Basin. Devonian(?)-Early Carboniferous Cycle In the neighbouring basins of Peru a series of basal sandstones and conglomerates followed by a thick package of source rock quality

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Solimões, Amazonas and Acre foreland basins N

73ºW

74ºW

BRAZIL

International Boundary

06ºS

PERU Jaquirana Trough

Marañón Basin Acre Basin Oeste de Batã Fault System

Paraguá-Jutai Arch

Divis ierra

Cruzeir o do

or S

08ºS

Cruzeiro do Sul

sul Pla

Central Trough

tform

Bata Fault

Ucayali Basin

sediments such as red sandstones, mudstones and anhydrites of the Rio do Moura Formation (Silva 1983) overlie the previous sequence, and represent the re-establishment of continental environments. The Permo-Carboniferous sequence is ultimately truncated by erosion due to the late Hercynian Orogeny. Shale intervals within this sequence are probably chronostratigraphically equivalent to source quality Irati Formation shales of the Paraná Basin as well as the Pedra de Fogo Formation, of the Parnaíba Basin. Early to Middle Permian folding affected southern Peru whereas in central Peru this phase is marked by epeirogenetic regimes. An important erosional event marked the end of the Hercynian (‘pre-Andean’) System and the beginning of the subduction of the Nazca Plate and the break-up of Pangaea (Barros & Carneiro 1991).

The Andean System Cretaceous over Upper Paleozoic or Basement

Exploration Well

35

Permo(?)-Triassic Cycle

PERU 80 km

BRAZIL

Fig. 3.4 Main features of the Acre Basin and its sub-basins. Subcropping Paleozoic strata are covered by widespread Mesozoic-Cenozoic sediments of the foreland stage, developed due to the Andean Orogeny. shales of Devonian age have been recognized, but such a sequence has not yet been documented in the Acre Basin. Seismic data, however, show the existence of a basal Permo-Carboniferous unconformity overlying an older sedimentary sequence that pinches out to the east, towards the region where the Paleozoic platform lies directly over basement. This cycle underwent intensive erosion associated with the early Hercynian Orogeny (see Fig. 3.5). Permo-Carboniferous Cycle During the late Carboniferous and Permian periods, the Acre Basin was a cratonic marginal basin with flexural subsidence to the west, developing a hinge line associated with the Cruzeiro do Sul carbonatic platform. This sedimentary cycle formed during the late Hercynian orogeny and can be divided into three stages: 1

The first stage is represented by Carboniferous (late Mississippian) alluvial fan deposition following a distensive tectonic regime and an early Hercynian erosional event. The sedimentary record is represented by sandstones and conglomerates of the Apuí Formation (Feijó & Souza 1994). 2 The second stage, Pennsylvanian, comprises widespread transgressive marine sedimentation, extending from Ucayali to Acre. This phase is represented by the bioclastic carbonates, anhydrites and calcareous shales of the Cruzeiro do Sul Formation (Silva 1983) culminating with a maximum flooding surface. 3 A third and final stage is dominated by regressive marine to continental deposition. Permo-Carboniferous continental

Hoorn_ch03_Final.indd 35

In the Peruvian Ucayali and Marañón basins to the west this cycle is characterized by red beds and associated levels of evaporites (molasse deposits) of the Mitú Group. In the Acre Basin, this period is represented by the lower portion of the Juruá Mirim Formation (Triassic), which is formed under a distensive tectonic regime of normal synsedimentary synthetic faults. Triassic-Jurassic Cycle Significant subsidence marked the Late Triassic evolution of the intracratonic Acre Basin. Flexural subsidence gave room to the deposition of siliciclastic sandstones, carbonates and dark dolomites intercalated with shales, bituminous sandstones and evaporitic beds. An intrabasinal high developed in the position where later the Batã Fault would come into place. This cycle ended with Early Jurassic evaporite deposition. At the end of this cycle, a compressive tectonic regime developed low-amplitude and large-period folding, and associated halokinesis. This sedimentary regime changed gradually towards the top to a continental regressive sequence composed of ‘red beds’ intercalated by volcaniclastics and evaporites. Basic extrusive igneous rocks in the Acre Basin as well as in the Peruvian basins correspond to the intrusive diabases of the Solimões Basin. The term Juruá Mirim Formation is used to define this sedimentary association of fluvial, lacustrine, sabkha and restricted marine deposits grading upwards to aeolian environments. In Peru the base of this section corresponds to the Pucará Formation, whereas the upper section corresponds to the Sarayaquillo Formation. The most important compressive tectonic event affecting the Acre Basin occurred during the Late Jurassic to Early Cretaceous (the Juruá Diastrophism, defined by Campos & Teixeira 1988), when strong compression caused early displacements along the Batã Fault. West of this fault system major restructuring and faulting took place. This included the reactivation of older fault systems as well as halokinesis with the mobilization of salt layers to the axis of anticline features. A strong erosional event terminated this tectonic cycle.

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J.R. Wanderley-Filho et al.

,

A

Divisor Sierra 1000m OESTE DE BATÃ Fault System

BATÃ Fault

0 1.0 SECONDS

2.0 3.0 4.0

PE

RU

BRAZIL

,

A

Divisor

A

ACRE BASIN

AMA

Legend

ZON

Mount

ACR E

AS

0

50 km

Clay Sandstone Siltstone Grainstone Shale Basement

T -Cenozoic K -Cretaceous J-Tr -Jurassic-Triassic Up-Pz -Upper Paleozoic L-Pz -Lower Paleozoic

Fig. 3.5 Cross-section (NE–SW) through the Acre Basin. The basin was initially a marginal cratonic basin with flexural subsidence to the west until the Permo-Carboniferous. Later it was continuously affected by Hercynian and Andean orogenies, which transformed it into a typical foreland system. Early Cretaceous-Paleocene Cycle This is essentially a period of low tectonic activity. After a long period of erosion, four distinct sedimentary sequences were deposited. Sedimentation started with coarse clastic sediment input from the Guiana and Brazilian shields (Lower Moa Formation) deposited in fluvial-deltaic to neritic environments in a forelandtype basin adjacent to the Andean fold belt (Pereira 1995). This sequence is followed by transgressive marine sandstones and associated shales (Upper Moa Formation). A Cenomanian-Turonian shale sequence apparently represents the most significant event of maximum flooding since Triassic times (Pereira 1994). This is followed by fine marine clastics (grey to black shales) of the Rio Azul Formation, culminating with a maximum flooding surface in the Late Santonian. Basal fluvial sandstones of the Divisor Formation overlie the marine and transitional shales of the Rio Azul Formation. This cycle is concluded by the transgressive deposition of the Ramon Formation and a Paleocene maximum flooding surface. These units are grouped into the Jaquirana Group (Silva 1983), which is truncated at the top by the early manifestations of the Andean Orogeny. Early Eocene-Pliocene Cycle Following the first manifestations of the Andean Orogeny, a new sedimentary cycle was established. This cycle was characterized by the deposition of shallow marine and lacustrine shales intercalated by sandy layers and ‘red beds’ of the Pozo, Chambira, Pebas and Ipururu formations in western Peru, and by the Solimões Formation in the Acre Basin, where fluvial and lacustrine

Hoorn_ch03_Final.indd 36

sediments of this unit onlap its eastern border, as a result of the Andean tectonism. The Solimões Formation was especially affected by the Andean Orogeny from the Miocene to the Pliocene (the Quechua Phase in Oliveira 1994). The Batã Fault and the Oeste de Batã Fault System were reactivated uplifting the Serra do Divisor Mountains (see Figs 3.4 & 3.5) and reinstating halokinesis in the Jurassic section. This tectonic cycle and its associated depositional systems are still active, sourcing sediments to the Amazon, Magdalena and Orinoco drainage systems.

Conclusions The sedimentary record in the Paleozoic basins of Amazonia permits reconstruction of the Phanerozoic geological evolution of this region. Tectonic subsidence created the necessary space for sedimentation, and uplift of the regional arches controlled the infill and distribution of sedimentary environments in these basins. Basin uplift and subsequent intrusion of large volumes of Late Triassic diabases caused up to 1500 m of erosion of the sedimentary cover, ending the Paleozoic history of these basins. A long interval without sedimentation persisted from the Jurassic to the Early Cretaceous. Plate tectonics on both sides of the South American continent had major effects in post-Paleozoic evolution of the basins. The opening of the South Atlantic system during the Early Cretaceous was synchronous to uplift and erosion of the areas now represented by the northern and southern platforms of Solimões and Amazonas (the Juruá Diastrophism). Flexural subsidence towards

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Solimões, Amazonas and Acre foreland basins the west created renewed space for sedimentation. In the Solimões and Amazonas basins fluvial-deltaic sediments prograded into these basins coming from the Guiana and Brazilian shields (the Alter do Chão Formation). Marine incursions with neritic environments in Acre Basin eventually advanced as far as the Carauari Arch. Finally, Andean uplift in the Tertiary had a major and dramatic effect in Amazonia and inverted the continent-wide drainage system eastwards, away from the Pacific Ocean and the Caribbean Sea, creating the modern Solimões and Amazonas river systems. The Solimões Formation represents this new geographic configuration with widespread deposition of fining and coarsening upward sequences.

Acknowledgements We thank Petrobras for permission to publish the text on which this chapter is based, and our colleagues Pedro Victor Zalán and Edison J. Milani for their review and helpful comments. We also acknowledge the important contribution of Rodolfo Dino and Emilio A. Soares for sharing their understanding of the Neogene stratigraphy and sedimentology of the Alter do Chão and Solimões formations. The authors are extremely grateful to the draft technicians Aline Lima, Leila Pezzin and Rosalia Amaral (Petrobras) for their dedication and work on the illustrations. Finally, we thank Carina Hoorn for having invited us as contributors and her continuous support and patience throughout.

References Barros, M.C., Carneiro, E.P. (1991) The Triassic Juruá Orogeny and the tectono-sedimentary evolution of Peruvian Oriente Basin – exploratory implications. In: Simpósio Bolivariano 4, Bogotá, Colombia, 6, 1–44. Campos, J.N.P., Teixeira, L.B. (1988) Estilos tectonicos da Bacia do Amazonas. In: 35 Congresso Brasileiro de Geologia, Belém, Anais 5. Belém: SBG, pp. 2161–2172. Cordani, U.G., Neves, B.B.B., Fuck, H.A. et al. (1984) Estudo preliminar de integração do Pré-Cambriano com os eventos tectônicos das bacias sedimentares brasileiras. Ciência, Técnica, Petróleo 15. Rio de Janeiro: Petrobras. Cunha, P.R.C. (2006) Bacias sedimentaires brasileiras: Bacia do Acre. Aracaju: Phoenix 8, 86. Cunha, P.R.C. (2007) Bacia do Acre. Boletim de Geociências da Petrobras, Cartas Estratigráficas 2, 207–215 (http://vdpf.petrobras.com.br). Cunha, P.R.C., Melo, J.H.G., Silva, O.B. et al. (2007) Bacia do Amazonas. Boletim de Geociências da Petrobras, Cartas Estratigráficas 2, 227–251 (http://vdpf.petrobras.com.br). Dalmayrac, B. (1978) Géologie des Andes péruviennes. Géologie de la Cordillère orientale de la région de Huanuco: sa place dans une transversale des Andes du Pérou. ORSTOM, Travaux e Documents 93, 1–161. Eiras, J.F., Matsuda, N. (1998) Field Trip to the Tapajós River: Amazon Basin, Brazil. Rio de Janeiro: Associação Brasileira de Geólogos de Petróleo; Tulsa: American Association of Petroleum Geologists.

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Eiras, J.F., Wanderley Filho, J.R. (2006) Bacias sedimentares brasileiras: bacia do Solimões. Phoenix, Aracaju: Fundação Paleontológica Phoenix 8, 1–6. Feijó, F.J., Souza, R.G. (1994) Bacia do Acre. Boletim de Geociências da Petrobras 8, 9–16. Ham, C., Herrera, L. (1963) Role of sub-Andean fault system in tectonics of eastern Peru and Ecuador. American Association of Petroleum Geologists Memoir 2, 47–61. Hoorn, C., Guerrero, J., Sarmiento, G.A., Lorente, M.A. (1995) Andean tectonics as a cause for changing drainage patterns in Miocene northern South America. Geology 23, 237–240. Mosmann, R., Gonçalves, A., Nepomuceno Filho, F. (1986) Oil and gas potential of the Amazon Paleozoic Basins. In: Halbouty, M.T. (ed.) Future Petroleum Provinces of the World. AAPG Memoir 40, pp. 207–241. Oliveira, C.M.M. (1994) Estilos estruturais e evolução tectônica da Bacia do Acre. MSc thesis, Universidade Federal de Ouro Preto, Ouro Preto, 206 pp. Pereira, M.J. (1994) Seqüências deposicionais de 2ª./3ª. ordens (50 a 0,2 M.a.) e tectono-estratigrafia no Cretáceo de cinco bacias marginais brasileiras. Comparações com outras áreas do globo e implicações geodinâmicas. PhD thesis, Universidade Federal do Rio Grande do Sul, Departamento de Geociências, Porto Alegre, 430 pp. Pereira, M.J. (1995) A Bacia do Acre: estratigrafia de seqüências e perspectivas petrolíferas, em contexto com as bacias de Ucayali, Marañon (Peru) e Solimões (relatório de progresso). Relatório Interno. Rio de Janeiro: Petrobras. Raja Gabaglia, G.P., Figueiredo, A.M.F. (1990) Evolução dos conceitos acerca das classificações de bacias sedimentares. In: Raja Gabaglia, G.P., Milani, E.J. (eds) Origem e Evolução de Bacias Sedimentares. Rio de Janeiro: Petrobras, pp. 31–45. Silva, O.B. (1983) Relatório final do poço 1-RM-1-AC. Relatório Interno. Belém: Petrobras. Tassinari, C.C.G., Cordani, U.G., Nutman, A.P.,Van Schmus, W.R., Bettencourt, J.S., Taylor, P.N. (1996) Geochronological systematics on basement rocks from the Rio Negro-Juruena Province (Amazonian Craton) and tectonic implications. Intern Geol Rev 38, 1161–1175. Tassinari, C.C.G., Bettencourt, J.S., Geraldes, M.C., Macambira, M.J.B., Lafon, J.M. (2000) The Amazonian Craton. In: Cordani, U.G., Milani, E.J., Thomaz-Filho, A., Campos, D.A. (eds) Tectonic Evolution of South America. Academia Brasileira de Ciências, Special Publication of 31st International Geological Congress, pp. 41–95. Wanderley Filho, J.R., Costa, J.B.S. (1991) Contribuição à evolução estrutural da bacia do Amazonas e sua relação com o embasamento. In: Simpósio de Geologia da Amazônia, 3, Anais. Belém: Sociedade Brasileira de Geologia, pp. 244–259. Wanderley Filho, J.R., Melo, J.H.G., Fonseca, V.M.M., Machado, D.M.C. (2005) Bacias sedimentares brasileiras: Bacia do Amazonas. Phoenix, Aracaju: Fundação Paleontológica Phoenix 7, 1–6. Wanderley Filho, J.R et al. (2007) Bacia do Solimões. Boletim de Geociências da Petrobras, Cartas Estratigráficas 2, 217–225 (http:// vdpf.petrobras.com.br).

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FOUR

Tectonic history of the Andes and sub-Andean zones: implications for the development of the Amazon drainage basin Andres Mora1, Patrice Baby2, Martin Roddaz2, Mauricio Parra3, Stéphane Brusset2, Wilber Hermoza4 and Nicolas Espurt5 1

ECOPETROL, Instituto Colombiano del Petroleo, Santander, Colombia Université de Toulouse, France 3 The University of Austin, Texas, USA 4 REPSOL-YPF, Madrid, Spain 5 Université Paul Cézanne, Aix en Provence, France 2

Abstract The Andes and the Amazon River have been neighbouring geographical and geological features for at least the past 10 million years. However, the nature of the interactions between them remains unclear. The western margin of South America has been convergent since ~100 Ma, but only during the last 30 million years has there been an adjacent subduction orogen of the extent observed today. Instead, the configuration of the Amazon River evolved from 11 Ma and has remained largely unchanged at least for the past 6 million years. In this chapter we review the available data on the history of deformation, palaeoelevation and exhumation of the northern Central Andes, Northern Andes and adjacent sub-Andean basins in order to compare these data sets with the evolution of the Amazon drainage basin. The available data are far too scarce to propose definitive patterns, but do allow us to pose testable hypotheses on the interaction between the Andes, evolution of sub-Andean zones and the Amazon River. Deformation in the Andes began prior to the establishment of the modern Amazon drainage network and patterns. Although the modern Amazon is very young it appears to be closely related to the development of the Andes. This interrelated history of Amazon River and Andes is inferred from the acceleration in the denudation rates of the Eastern Cordillera, which coincides with the moment that Andean palaeoelevations became significant and began to constitute an orographic barrier and trap to moisture-bearing winds. However, this acceleration could also be related to the development of a denser drainage network in the Andean headwaters. All these factors, together with the presence of orogenperpendicular basement highs, may have prompted a greater and more focused water and sediment influx towards the Amazon lowlands, producing a river directed to its present-day delta plains in the Atlantic Ocean. As previously proposed, the synchronous development of intense deformation in the sub-Andean basins appears to be related to changing mechanical conditions in the foreland sedimentary wedge that prompted deformation to migrate to the lowlands.

Amazonia, Landscape and Species Evolution: A Look into the Past, 1st edition. Edited by C. Hoorn and F.P. Wesselingh. © 2010 Blackwell Publishing

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Tectonic history of the Andes

Introduction

locally understood, with numerous limitations in the resolution of the existing chronology. Data that evaluate the timing and rates of exhumation in the Andes (e.g. Fig. 4.2) are also scarce. In addition, the assessment of palaeoelevation of the Andes – and its implications – is difficult, in particular because palaeoelevation is reconstructed from data that are both scarce and variably interpreted. Therefore, rather than a final result, we here propose a working model of the relationships between the Andes and the

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The Amazon drainage basin occupies an area of about 7 million km², including portions of the territories of Colombia, Ecuador, Peru, Bolivia and Brazil. The Amazon drainage basin is the largest on Earth (Fig. 4.1) and is located adjacent to the Andean orogen, one of the world’s largest mountain chains. Most of the rivers feeding the Amazon are sourced in the Andes (see Fig. 4.1). It is therefore expected that the Andes and Amazon interact spatially and temporally in some way (see Fig. 4.1). The relationship between Andean orogenesis and development of the modern Amazon drainage system is poorly understood. Dobson et al. (2001) argue that the modern transcontinental Amazon drainage was not established until the Late Miocene. This raises the question why, in the light of the extended deformation history in the Andes, this major river system was established so recently. In reply, Hoorn (2006) proposed that the rise of the Andes mountains is the critical factor for the origin of the present-day Amazon River. Recently new data were generated on the orogenic processes in the Andean segments most closely related with the evolution of the Amazon sedimentary and hydrographic basin, the northern Central Andes (Oncken et al. 2006) and Northern Andes (Pindell et al. 1998; Cediel et al. 2003). However, there is no published comprehensive study on the relation between these data and the development of the Amazon drainage basin. Therefore, in this chapter we compare the timing of compressional deformation, surface uplift and exhumation in the Andean orogen with the timing of evolution of the Amazon drainage system. We find that the timing of the onset of compressional deformation in the Andes is only

39

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t en

ra l

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Fig. 4.1 Topographic map of northern South America, with the hydrographic network, including the Amazon (lighter coloured area) and Orinoco hydrographic basins. The Andean orogen is also present with two distinct geological segments – Northern Andes and Central Andes.

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Fig. 4.2 Shaded relief map of the Central and Northern Andes. Numbers in the figure denote different studies related to palaeoelevation: 1. Kennan et al. (1997); 2. Barke & Lamb (2006); 3. Gregory-Wodzicki et al. (1998); 4. Graham et al. (2001); 5. Garzione et al. (2006); 6. Burgos (2006); 7. Fields (1959); 8. Wijninga (1996). Studies relating to exhumation: 9. Benjamin et al. (1987); 10. Barnes et al. (2006); 11. Laubacher & Naeser (1994); 12. Garver et al. (2005); 13. Spikings et al. (2000); 14. Mora et al. (2008); 15. Kohn et al. (1984). Notice also the different names of the most important sub-Andean basins.

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Amazon River, one that must be verified with more data from the Amazon drainage basin and Amazon headwaters. This chapter also includes data from areas that were previously connected with the Amazon drainage basin, but are not part of that basin today (see Fig. 4.2). For instance, the Colombian Eastern Cordillera is partly adjacent to the Llanos foreland and partly adjacent to the North Amazonian foreland. However, the Llanos foreland was part of the palaeo-Amazon foreland until at least the Late Miocene (Hoorn et al. 1995; Hoorn 2006; Albert et al. 2006; see also Chapters 8 & 26) and therefore many of the processes that control the presence of a major river basin east of the Andes can be potentially understood in this area. In this compilation, first we review the available data on deformation, surface uplift and exhumation of the Central and Northern Andes (see Fig. 4.2). Then we explain in detail the location, role and evolution of a basement arch, the Vaupés Swell, which we consider as an instrumental feature that bounds the present-day Amazon drainage basin to the north and probably also controlled in part the Late Miocene deviation of the Amazon River towards the Atlantic. We then integrate and discuss the data sets to propose that intense precipitation in sediment source areas, focused by an extensive, high-elevation mountain range that intercepts moisture-laden air masses, is the reason why so many drainage basins coincide in a single river system. We consider that intense exhumation via erosion is the main factor leading to an overfilled Amazon drainage basin with most of the rivers flowing to the east. We also highlight the role of basement arches in bounding the Amazon drainage basin. The partial temporal correlation between these variables and the craton-directed growth of the sub-Andean ranges is also explored. As in the rest of this volume, this chapter is intended for all scientists with an interest in the development of Amazonia trough time. However, it is written by geologists using very specific geological terms. Thus, to make this manuscript understandable for a wider spectrum of readers, in Table 4.1 and Fig. 4.3 we define the most important geological concepts used here.

The history of the Amazon River in a regional geological context Hoorn et al. (1995) proposed that around 16 Ma there was no transcontinental Amazon River system with connection to the Atlantic Ocean. Instead, multiple lines of evidence suggest that during the Middle Miocene the present Amazon drainage basin constituted an extensive lacustrine basin, sporadically invaded by short-lived marine incursions. Within this wetland, there is a record of rivers flowing from the Andes since at least the establishment of this extensive lacustrine regime during the late Early to Middle Miocene (Hoorn et al. 1995; Hoorn 2006; see also Chapter 8). Evidence for this is gleaned from the lateral distribution of depositional systems (see Chapter 5), the presence of Andean schist fragments in the Middle Miocene sediments and pollen grains typical of mountainous areas (Hoorn et al. 1995), and the absence of Andean clastic sediments along the Atlantic margin (Dobson et al. 1997, 2001). Analysis of growth bands from mollusc shells indicates that a precipitation regime similar

Hoorn_ch04_Final.indd 40

to Present prevailed in the Amazon foreland basin (Kaandorp et al. 2005). Based on pollen analysis by Hoorn et al. (1995), it was noted that at that time biodiversity in the Amazon wetland was similar to, or even greater than today (Hooghiemstra & Van der Hammen 1998; Van der Hammen & Hooghiemstra 2000). Greater biodiversity may be an indicator of higher or similar temperatures and precipitation at that time. However, temporal and spatial resolution of palaeoprecipitation data is not yet enough to assess the presence or absence of orographic rain shadows in relation to the Andes. Damuth & Kumar (1975) hypothesize that the Late Miocene onset of deposition in the Amazon Fan is due to the origination of the transcontinental palaeo-Amazon River, although it was unclear whether at that time the river was similar in size and drainage network to the river we observe today. Data from offshore Brazil show that sediments unambiguously derived from the Andes began accumulating on the Brazilian coast by 10 Ma (Dobson et al. 1997, 2001). The transcontinental palaeo-Amazon river thus initiated in the Late Miocene, as suggested by Damuth & Kumar (1975), but the modern Amazonian network, namely an unambiguously documented similar distribution of depositional systems and flow patterns, is no older than the Pliocene (Roddaz et al. 2005; Espurt et al. 2007; Figueiredo et al. 2009).

The Andes and their relationship with the plate tectonic context The western margin of South America (see Fig. 4.2) has been characterized by subduction (i.e. the downward movement of the oceanic plate below the South American plate) for the past 100 Ma (Sdrolias & Müller 2006; Cobbold et al. 2007). The most notable change in the subduction regime during the Cenozoic was the break-up of the Farallon Plate into the Nazca and Cocos plates at ~23.4 Ma (Sdrolias & Müller 2006; Cobbold et al. 2007). Given this simple tectonic setting, an unresolved question is why significant orogenesis occurred during the late Cenozoic. It has been suggested that variations in the convergence rate between the Nazca oceanic plate and South American plates could be the main factor (see Fig. 4.2). Pindell et al. (1998) and more recently Oncken et al. (2006) and Babeyko et al. (2006) present convincing arguments about the major role played by the velocity of the westward drift of South America along the Northern (Pindell et al. 1998), Central and Southern Andes (Oncken et al. 2006) (see Fig. 4.1). In this context, Oncken et al. (2006) found a positive correlation in the Central Andes between the onset of shortening in the western margin of the upper plate (at ~48 Ma), and a positive difference between South American drift and the subducting slab rollback rate (recorded also since ~48 Ma). They define the onset of such a positive difference as the shortening threshold in the upper South American plate. A similar idea had been proposed by Silver et al. (1998), based on qualitative assumptions. Pindell et al. (1998) suggested that in the Northern Andes an additional factor is the arrival of the Panamá Arch to the western South American margin. However, it has been recognized that only after that onset of shortening, at ~30 Ma, can generalized mountain building with similar geographical extension as the present-day Andes

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Tectonic history of the Andes

41

Table 4.1 Definitions of some key geological terms. Term

Definition

Stresses

Applied forces that deform rock units constituting the Earth’s crust (uppermost portion of the Earth). They are in the simplest case horizontal and perpendicular to the anisotropies (earth discontinuities) in the rock units affected by stresses

Tensional stresses

Stresses that tend to expand particles or rock units

Compressional stresses

Stresses that tend to shorten the rock units (Marrett & Peacock 1999)

Transpressional stresses

Stresses oblique to the anisotropies in the rock units or earth discontinuities are termed transpressional when the main component is compressive (Sanderson & Marchini 1984)

Transtensional stresses

Stresses oblique to the anisotropies in the rock units or earth discontinuities are termed transtensional when the main component is tensile (Sanderson & Marchini 1984)

Contractional deformation or shortening

A permanent state of rock units prompted by the horizontal component of movement of those units and caused by compressional far-field or local stresses. For example, in Fig. 4.3 it can be seen that a rock mass with stratified units in an undeformed state (above) can have a horizontal component of movement (below), which caused the rear part of the rock units to be displaced horizontally to the left side of the cartoon. This component is dominant in those parts where deformation occurs along sub-horizontal faults

Thin-skinned tectonics

A style of deformation in contractional thrustbelts, and in some cases used in extensional settings, where rock units from the sedimentary cover are detached along a basal décollement from the underlying older and more rigid basement units

Thick-skinned tectonics

A style of deformation in contractional thrustbelts and in some cases used in extensional settings, where rock units from the older crystalline basement and the overlying sedimentary cover are involved

Rock uplift

Vertical movement of rocks with respect to the geoid (England & Molnar 1990)

Surface uplift

Vertical movement of the surface with respect to the geoid (England & Molnar 1990; see also Fig. 4.3)

Exhumation

Vertical movement of rocks with respect to the surface. Erosional denudation means, in most cases, exhumation (England & Molnar 1990), except in the case of tectonic denudation in normal faults

Orogenesis

All the aforementioned processes and forces can, under certain conditions, create mountains and therefore, if there is a widespread process of mountain building, the term orogenesis is used to define all the group of processes involved

Stream power

The energy that a river or water flow needs to be able to transport sediments; it depends on water discharge and surface slope (Whipple & Tucker 1999). The main factor controlling water discharge in tropical areas is rainfall

Orographic rainfall

In the Amazon basin most of the rainfall at its western side is caused by the orographic effect created by the Andes Mountains (Bookhagen & Strecker 2008). In this case, above certain elevations, the eastern side of the Andes focuses rainfall by retention of moisture-bearing winds on the eastern slopes of the mountain range (e.g. Mora et al. 2008)

be clearly documented, both in the Central Andes (see Fig. 4.1) (Allmendinger et al. 1997; Oncken et al. 2006) and in the Northern Andes (see Fig. 4.1; Gómez et al. 2001, 2005; Toro et al. 2004; Parra et al. 2005, 2009b; Bayona et al. 2008). The coincidence in the onset of deformation for the Central and Northern Andes is significant but is also expected. If a subduction margin is present west of the South American plate and this factor controls mountain building, a synchronous onset of deformation along the entire margin is likely. Allmendinger et al. (1997) proposed that the reorganization of Pacific plates at ~25 Ma may have caused generalized mountain building in the Central Andes. Babeyko et al. (2006) suggested that the velocity of the westward drift of South America was sufficiently high by ~25 Ma to trigger extensive deformation of the strong Andean lithosphere. For the Northern Andes, Pindell et al. (1998) even suggest that from the deduced flow lines of

Hoorn_ch04_Final.indd 41

convergence between the Caribbean and South American plates, the relative convergence rate between the two plates doubled by ~25 Ma. Based on this Pindell et al. (1998) predicted that, compared with previous periods, the Andean orogeny should have been particularly intense in the Northern Andes during the Late Oligocene. The previous discussion does not imply that structural styles do not need to change along strike. They are also due to other features such as subduction angle or different palaeogeographic and crustal features in the upper plate. For instance, there are two flat slab segments in the subducting Nazca Plate (see Fig. 4.2). One of them in the southern Central Andes (Jordan et al. 1997), and the other in the northern Central Andes, the latter aged at ~10 Ma (see Fig. 4.2; Gutscher et al. 1999; Hampel 2002). Both of them have been proposed to influence the deformation of the Andean

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A. Mora et al.

Average elevation (⌬) in shaded area X = Rock Uplift (RU)

Average elevation (⌬) in shaded area X = Surface Uplift (SU) Shaded area = X

SU = RU – D D = RU – SU D = ⌬X – ⌬X D=0 Shortening = S

Denudation (D) = ⌬X + Y – ⌬Y Average elevation (⌬) in shaded area Y = Surface Uplift

Shaded area = X

Average elevation (⌬) in shaded areas X + Y = Rock Uplift

SU = RU – D D = RU – SU D = ⌬X + Y – ⌬Y

Shaded area = Y

Shortening = S

Shaded area =X

Not all Rock Uplift generates Surface Uplift as some denudation counteracts Rock Uplift

Average elevation (⌬) in shaded area X = Rock Uplift

SU = RU – D SU = ⌬X – ⌬X SU = 0

Average elevation (⌬) in shaded area X = Denudation

No topography is generated as all the RU Is degraded by erosion Shortening = S

Fig. 4.3 Definition of the main geological concepts related to the deformation and erosion of the materials constituting the Earth’s crust.

foreland fold and thrust belts. However, as demonstrated by Kley et al. (1999) there is only a partial correlation between the segmentation of the Andean foreland fold and thrust belt and the geometry of the subducted Nazca Plate as initially proposed by Jordan et al. (1983). More convincing data on the coincidence in time and space between the subduction of the Nazca ridge and the Pliocene uplift of the Fitzcarrald Arch in the Peruvian subAndean zone (see Fig. 4.2) has been shown by Espurt et al. (2007). This broad low-elevation basement arch segments the Peruvian sub-Andean zone (see Fig. 4.2).

Cenozoic shortening history in the Central and Northern Andes Northern Bolivian and Southern Peruvian Andes Although compressional deformation events, starting in the Paleocene, have been documented elsewhere in the Andes, we focus on events that are synchronous with the Neogene origin of the Amazon River (Hoorn et al. 1995; Hoorn 2006). A recent synthesis of published deformation data suggests that in the Central Andes of Bolivia and southern Peru, deformation began at ~47 Ma in the pre-Cordillera (the Andean ranges closer to the Pacific coast; Fig. 4.4, Table 4.2). Deformation propagated eastwards, reaching the Eastern Cordillera first at ~40 Ma (Fig. 4.5;

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see also Figs 4.2 & 4.4) and virtually the entire Altiplano and Eastern Cordillera at ~30 Ma, with higher shortening rates in the Bolivian Eastern Cordillera (Oncken et al. 2006; see Figs 4.2, 4.4 & 4.5). Interestingly, deformation only reached the sub-Andean zones (see Fig. 4.2) at ~10 Ma (Kley 1996; Echavarria et al. 2003; Horton 2005). Baby et al. (1997) and Rochat et al. (1999) calculate total shortening of the Eastern Cordillera and the Bolivian subAndean zone from crustal balanced cross-sections (see Fig. 4.5). Total shortening varies from 180 to 230 km, and the Altiplano is interpreted as a piggyback basin carried on the crustal duplex of the Eastern Cordillera, linked with the underthrusting of the Brazilian Shield (see Fig. 4.5).

Northern Peruvian Andes and Ecuadorian Andes In the Peruvian Andes, pre-Eocene deformation events have been grouped under the term ‘Peruvian phase’. Although the spatial distribution of this deformation phase is unclear, this shortening event affected the Peruvian Eastern Cordillera but not the subAndean zone (Megard 1984; see Figs 4.2 & 4.4). An important exception is the Peruvian sub-Andean Santiago Basin (see Figs 4.2 & 4.4); seismic reflection sections show that this basin (Figs 4.6a & 4.7; see also Figs 4.2 & 4.4) was deformed by transpressive tectonic inversion of an Upper Triassic–Lower Jurassic rift, starting in the Late Cretaceous with low-amplitude/low-elevation structures rarely creating relief or substantial shortening (Navarro

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Fig. 4.4 (a–c) Summary of the different stages of deformation, exhumation and increased palaeoelevation in the Andes based on Oncken et al. (2006), the sources mentioned in the text and the sources shown in the maps. The black lines separate from west to east the Andean Western Cordilleras and pre-Cordilleras; the Altiplano-Puna Plateau domain and Andean Eastern Cordilleras, only separated in Colombia into the Central and Eastern Cordillera. The easternmost domain surrounded by black lines coincides with the sub-Andean zones. Notice that the deformation domains do not always coincide with the different divisions of the Andes. White dots are locations with documented deformation events. Black dots are areas with palaeoelevation data. Black dots with a white envelope are areas with documented exhumation rates from thermochronolgy. Each number is a different source. See a summary of data sources in Table 4.2 with numbers corresponding to the numbers used in this figure.

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A. Mora et al. (c)

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et al. 2005). This Late Cretaceous deformation is also well known in the Oriente Basin of Ecuador (see Figs 4.2 & 4.4), where it is associated with alkalic igneous activity (Barragan et al. 2005). The Early Eocene period in Peru is considered to be a time of tectonic quiescence and erosion of the Andes to the west of the subAndean zone (Hermoza et al. 2005; see Chapter 5). The Middle to Late Eocene Incaic phase has been regarded as the main Andean phase in Peru, at least in the Western Cordillera (see Figs 4.2 & 4.4) and adjacent Altiplano (Mégard 1984). In the Western Cordillera of Peru, pre-Late Eocene structures are eroded and unconformably overlain by ~40 million year-old clastic deposits (Noble et al. 1974, 1979), but structures of this age were not documented in the Eastern Cordillera (see Figs 4.2 & 4.4). Christophoul et al. (2002) describe Middle Eocene low-elevation/low-amplitude contractional structures (tectonic inversion) in the Oriente Basin (see Figs 4.2 & 4.4) of Ecuador. The Quechua phases would have affected the Peruvian Western Cordillera, Altiplano and Eastern Cordillera (see Figs 4.2 & 4.4) during the Late Oligocene and Miocene (Mégard 1984). However, it is unclear if these corresponded to a mostly continuous deformation, affecting different segments of the Peruvian Andes (like in the Northern Andes of Colombia, see below), or whether they can really be separated into discrete periods. Recent studies (Hermoza et al. 2005; Roddaz et al. 2005; Espurt et al. 2007) show

Hoorn_ch04_Final.indd 44

La Paz 17 22 21

Santa Cruz

a continuous Cenozoic eastward propagation of the Peruvian retro-foreland basin system from the Western Cordillera to the present sub-Andean zone (see Fig. 4.4). Laubacher & Naeser (1994) argue that the first signal of deformation, as reflected in AFT (apatite fission tracks) cooling ages from granitic rocks in the Peruvian Eastern Cordillera, occurred during the Early Miocene (see Fig. 4.4). However, this claim needs to be judged with caution as it is based on only three samples and no track length data. In contrast, a conformable deposition in the sub-Andean basins of Peru at that time (Early Miocene?) shows no evidence of Middle Eocene to Middle Miocene shortening affecting this structural province of the Peruvian Andes (Gil 2001; Hermoza et al. 2005). As will be explained below, deformation only reached the sub-Andean zone during the Late Miocene.

Northern Andes of Colombia Pre-Eocene deformation episodes in the Northern Andes of Colombia have been documented in the Central Cordillera (Fig. 4.8; Gómez et al. 2005), and even older deformation was recorded in the Western Cordillera (see Figs 4.4 & 4.8; Cediel et al. 2003). These events have also affected the area of the present-day Colombian Eastern Cordillera (see Figs 4.4 & 4.8; Bayona et al. 2006b), but

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Hoorn_ch04_Final.indd 45

Table 4.2 Summary of data sources used in Fig. 4.4; numbers in the left-hand column correspond to the numbers used in the figure. Number Reference

Location

Type of data

Interpreted process

Timing

Late Cretaceous-Early Tertiary 1

Gómez et al. (2005)

Central Cordillera (Colombia)

Thermochron/basin modelling

Deformation/exhumation

Late Cretaceous

2

Barragan et al. (2005)

Oriente Basin (Ecuador)

Documentation of contractional tectonics

Deformation

Late Cretaceous

3

Spikings et al. (2000)

Cordillera Real (Ecuador)

Low-temperature thermochron

Exhumation

Late Cretaceous/Paleocene

4

Navarro et al. (2005)

Santiago Basin (Peru)

Documentation of contractional tectonics

Deformation

Late Cretaceous

5

Megard (1984)

Eastern Cordillera (Peru)

Documentation of contractional tectonics

Deformation

Pre-Eocene

Late Eocene 6

Oncken et al. (2006)

Pre- and Eastern Cordilleras (Bolivia)

Documentation of contractional tectonics

Deformation

Eocene (starting at ~47 Ma)

7

Gómez et al. (2001)

Western flank of the Eastern Cordillera (Colombia)

Documentation of contractional tectonics

Deformation

Late Eocene

8

Bayona et al. (2008)

Western flank of the Eastern Cordillera (Colombia)

Basin modelling

Deformation

Late Eocene

9

Parra et al. (2009b)

Central part of the Eastern Cordillera (Colombia)

Basin modelling

Deformation/exhumation

Late Eocene

10

Noble et al. (1974, 1979)

Western Cordillera (Perú)

Documentation of contractional tectonics

Deformation

Late Eocene

11

Toro et al. (2004)

Eastern Cordillera (Colombia)

Documentation of contractional tectonics

Deformation

Oligocene

6

Oncken et al. (2006)

Central Andes

Documentation of contractional tectonics

Deformation

Oligocene

Oligocene

Parra et al. (2009b)

Eastern Cordillera (Colombia)

Thermochron/basin modelling

Deformation/exhumation

Late Oligocene

Megard (1984)

Western Cordillera, Altiplano and Eastern Cordillera (Peru)

Documentation of contractional tectonics

Deformation

Late Oligocene

Early Middle Miocene 7

Gómez et al. (2001)

Western flank of the Eastern Cordillera (Colombia)

Thermochron/basin modelling

Deformation/exhumation

Early Miocene

8

Bayona et al. (2008)

Eastern Cordillera (Colombia)

Basin modelling

Deformation

Early Miocene

Tectonic history of the Andes

12 5

(Continued)

45

10/24/2009 10:52:28 Shobha

46

Hoorn_ch04_Final.indd 46

A. Mora et al.

Table 4.2 Continued. Number Reference

Location

Type of data

Interpreted process

Timing

9

Parra et al. (2009b)

Eastern flank of the Eastern Cordillera (Colombia)

Thermochron/basin modelling

Deformation/exhumation

Early Miocene

13

Spikings et al. (2000)

Northern Cordillera Real (Ecuador)

Low-temperature thermochron

Exhumation

Middle Miocene

5

Megard (1984)

Western Cordillera, Altiplano and Eastern Cordillero (Peru)

Documentation of contractional tectonics

Deformation

Miocene

6

Oncken et al. (2006)

Central Andes

Documentation of contractional tectonics

Deformation

Miocene

Late Miocene 3

Spikings et al. (2000)

Cordillera Real (Ecuador)

Low-temperature thermochron

Deformation/exhumation

Late Miocene

14

Garver et al. (2005)

Peruvian Cordillera Blanca

Low-temperature thermochron

Deformation/exhumation

Late Miocene

15

Laubacher & Naeser (1994)

Peruvian Eastern Cordillera

Low-temperature thermochron

Deformation/exhumation

Late Miocene

16

Benjamin et al. (1987)

Bolivian Eastern Cordillera

Low-temperature thermochron

Deformation/exhumation

Late Miocene

17

Barnes et al. (2006)

Bolivian Eastern Cordillera

Low-temperature thermochron

Deformation/exhumation

Late Miocene

Late Miocene or Late Miocene/Pliocene 18

Wijninga (1996)

Colombian Eastern Cordillera

Palaeobotany

Increased palaeoelevation

Late Miocene/Pliocene

19

Fields (1959)

Colombian Eastern Cordillera

Palaeobotany

Increased paleoelevation

Late Miocene/Pliocene

20

Burgos (2006)

Cordillera Real (Ecuador)

Palaeosurfaces

Increased palaeoelevation

Pliocene

21

Garzione et al. (2006)

Bolivian Altiplano

Stable isotope palaeoaltimetry

Increased palaeoelevation

Late Miocene

22

Graham et al. (2001); GregoryWodzicki et al. (1998)

Bolivian Altiplano

Palaeobotany

Increased palaeoelevation

Late Miocene

23

Barke & Lamb (2006); Kennan et al. (1997)

Bolivian Altiplano

Palaeosurfaces

Increased palaeoelevation

Late Miocene

Late Miocene/Pliocene 24

Mora et al. (2008)

Colombian sub-Andes

Low-temperature thermochron

Deformation/exhumation

Late Miocene/Pliocene

25

Espurt et al. (2008)

Peruvian sub-Andes

Low-temperature thermochron

Deformation/exhumation

Late Miocene/Pliocene

26

Mora et al. (unpublished work)

Peruvian sub-Andes

Low-temperature thermochron

Deformation/exhumation

Late Miocene/Pliocene

27

Baby et al. (1997)

Bolivian sub-Andes

Documentation of contractional tectonics

Deformation

Late Miocene/Pliocene

28

Echavarria et al. (2003)

Bolivian sub-Andes

Documentation of contractional tectonics

Deformation

Late Miocene/Pliocene

10/24/2009 10:52:29 Shobha

Foreland Basin System Orogenic Wedge 16º – 18ºS

Total shortening = 177 km

Sub-Andean foothills Sajama

Altiplano

Cordillera Oriental

Brazilian Craton

Beni plain

Wedge Top

Foredeep

Forebulge

Backbulge

0

40

? 0

100 km

Subsidence

80

Cenozoic synorogenic deposits

Paleozoic - Mesozoic

Middle crust/Lower crust

Fig. 4.5 Regional cross-section at the latitude of the Central Andes south of the Bolivian orocline (~16–18°S), showing the main elements constituting the Andean orogen and adjacent foreland (modified after Baby et al. 1997). See its location in Fig. 4.2.

ault ca F

Boya

aga

Soap

Fa u

W

na

Floresta Massif Cusiana-4 Cusiana-2

Cu

Cormichoque-1

sia

Arcabuco Anticline

lt

Cordillera Oriental

E

Llanos Foredeep 0

Fault

(c)

km 30 Cordillera Oriental

LLANOS FORELAND BASIN

Quetame Massif Humadea-1

30 km

Llanos Foredeep

0

(b)

km 30

Santiago basin

North Amazonian Foreland Basin Marañón Foredeep Huitoyacu

Sungachi

Iquitos Forebulge Tunchiplaya

Bolognesi (projected)

0

50 km

(a)

km 30

Neogene

Cretaceous

Paleozoic

Paleogene

Jurassic

Guiana Shield basement

Fig. 4.6 Balanced cross-sections from the Northern Andes, Eastern Cordilleras and sub-Andean basins. (a) Geological crosssection along the Santiago Basin (see location in Fig. 4.2; after Gil et al. 2001). (b) Geological cross-section through the Central Eastern Cordillera of Colombia (see location in Fig. 4.2; modified after Mora et al. 2008). (c) Geological cross-section through the northern Eastern Cordillera of Colombia (see location in Fig. 4.2; after Toro et al. 2004).

Hoorn_ch04_Final.indd 47

10/24/2009 10:52:29 Shobha

5 2

Ca ue q tà Rive r

s Puru

Araracuara Range

Arch

rch iA

rch sA

ar

to ui

u ra Ca

Iq

Marañón Basin

Range

Bagre West-1 Putumayo-1

Santiago Basin

AMAZONIA

Huallaga Basin

South American Plate

Acre Ucayali Basin Basin Fitzcarrald Arch Madre de Dios 25 Basin

⫺10° Lima

? Cusco

26

27 Santa Cruz

Nazca Plate 3

⫺20° ⫺80°

-70°

28 Chaco Basin

Fig. 4.7 Shaded relief showing the sub-Andean zones (darker grey areas) and the data points supporting a Late Miocene and younger deformation in those areas.

deformation rates were so low that in most of the area sedimentation was faster than the vertical rock uplift (Mora et al. 2006). Deformation and associated erosional unconformities reached the present western foothills of the Colombian Eastern Cordillera and the Magdalena Valley (see Figs 4.4 & 4.8) before Early–Middle Eocene times (Restrepo-Pace et al. 2004), whereas the Central Cordillera was inactive at this time (Gómez et al. 2005). However, similar to the Central Andes, only by ~25 Ma was deformation ubiquitously present throughout the Eastern Cordillera. This was documented by growth strata in both the western (Gómez et al. 2001, 2005) and eastern foothills of the Eastern Cordillera (see Fig. 4.6; Parra et al. 2005, 2009b; Martínez 2006; Bayona et al. 2008) and by thermochronology (Toro et al. 2004; Parra et al. 2009b). Therefore, the Eastern Cordillera has been uplifted as an enormous Laramide-type basement block

Hoorn_ch04_Final.indd 48

0˚

100 km 73˚

Beni-Mamore Basin

La Paz

E

2˚

iquete

Oriente Basin

Ri v

Chirib

0°

sS we ll

4˚

DI AM IA AZ QU ONIA ORINO

e ang

pe

ORINOQUIA

viare River Gu a

R rena aca

Llanos Basin Rio Pescado-1

Va u

er

t Me

Candilejas-1

Payara-1

24

Medina-1

La M

Uribe-1

Cusiana-2A

Seismic Lines in Fig. 4.12

Vanguardia-1

SB

4 a

IL L

RD CO

1

3

VI D

10°

3

1 EA ST ER N

7 – 0 Ma

ER A

A. Mora et al. W ES CE TER N CO NT RDIL RA LER LC A OR DI LL MAG ER DALE A NA V ALLE Y

48

Fig. 4.8 Topographic map (see location in Fig. 4.2) including the main geographical features around the Vaupés Swell, which in fact acts as the drainage divide between the Orinoquía and the Amazonia. Notice the Chiribiquete, Araracuara and Macarena ranges and the position of the Orinoco-Amazon drainage divide. All these elements are defined by the Vaupés Swell low-elevation arch.The numbers indicate the location of different recent geological studies in the Eastern Cordillera: 1. Gómez et al. (2003, 2005); 2. Parra et al. (2009a, 2009b); 3. Bayona et al. (2008); 4. Toro et al. (2004); 5. Mora et al. (2008). Wells used in Fig. 4.11 are also shown (black squares) and seismic profiles used to make the composite seismic line in Fig. 4.12 (white boxes). SB, indicates the location of the high plain of Bogotá.

(e.g. Erslev 1986) since the Late Oligocene (see Figs 4.4, 4.6 & 4.8). Balanced cross-sections show that shortening rates increased dramatically in the Northern Andes by Late Miocene times (Mora et al. 2008). In contrast, the very narrow Colombian sub-Andean ranges (Fig. 4.7) underwent most of their shortening since Late Miocene times (Mora et al. 2008). Altogether, a strikingly evident synchronism along the Eastern Cordilleras of the Central and Northern Andes becomes apparent. For instance, the onset of deformation in the Eastern Cordilleras of both Colombia and Bolivia is dated to the Late Eocene. However, there are insufficient data to evaluate the timing of onset of deformation in the Peruvian Eastern Cordillera (see Fig. 4.4). The generalized Late Oligocene to Early Miocene deformation in the Andes, especially along the Andean Eastern Cordilleras, is also remarkable (see Fig. 4.4). Nevertheless, the lack of evidence of significant shortening in the sub-Andean zones prior to Late

10/24/2009 10:52:30 Shobha

Tectonic history of the Andes Miocene times is also clear (see Figs 4.4 & 4.7). It is worth noting that prior to the Late Miocene there is abundant evidence for deformation, but no evidence for high palaeoelevations along the Central and Northern Andes (see Fig. 4.4).

Surface uplift data in the Andes Palaeoelevation data from the Central and Northern Andes are essential to understand the evolution of the Amazon drainage system. This is because the amount and timing of surface uplift for a given region directly relate to the stream power on slope and topography and subsequently the palaeo-depositional settings and fluvial networks (e.g. Flemings & Jordan 1989; see Table 4.1). Equally, the presence of laterally extensive terrains with some degree of topographic relief may favour the presence of larger drainage areas, which in turn causes higher erosion rates. However, palaeoelevation data in the Andes are remarkably scarce (e.g. Gregory-Wodzicki 2000), and there is a need for additional palaeoelevation data from throughout the Andes. In the Central Andes, palaeoelevation was initially inferred based on analysis of palaeo-surfaces (Kennan et al. 1997; see also Figs 4.2 & 4.4). These authors proposed a topographic growth of ~2 km since the Late Miocene in the Bolivian Eastern Cordillera, where present-day elevations range from 2000 to 4000 m. More recently, Gregory-Wodzicki et al. (1998) and Graham et al. (2001) used palaeoflora and leaf physiognomy in the Bolivian Eastern Cordillera and Altiplano to suggest that from one-third to one-half of the surface uplift of the Eastern Cordillera and Altiplano had already occurred by the beginning of the Pliocene (see Fig. 4.2). In contrast, Garzione et al. (2006, 2008) used oxygen isotopic compositions of carbonates to suggest that the Altiplano reached its present-day elevation somewhere between ~10.3 and 6.8 ± 0.4 Ma (see Figs 4.2 & 4.4). Surface uplift was estimated at ~2.5– 3.5 km during that time, which is much faster than the surface uplift inferred from previous work. The authors conclude that such an enormous amount of surface uplift in such a short time can only occur if there is removal of dense eclogitic lower crust and mantle lithosphere. However, as noted by Oncken et al. (2006), the data of Garzione et al. come from a single section in the Corque syncline (see Fig. 4.2), which is located in the hanging wall of a very precise structure out of many that were involved in the uplift of the Altiplano. Nevertheless, the regional geological context (deformation rates) favours a progressive surface uplift during the Cenozoic deformation (e.g. Baby et al. 1997; Allmendinger et al. 1997; Horton 1998, 2005; Rochat et al. 1999; Victor et al. 2004; Oncken et al. 2006), rather than a very fast Late Miocene surface uplift, which would require either delamination of lithospheric mantle, or faster shortening rates. Indeed, there is no evidence for peak shortening rates during that specific period of time (e.g. Oncken et al. 2006). Barke & Lamb (2006) (see Figs 4.2 & 4.4) used the morphology of well-preserved regional palaeo-surfaces in the Bolivian Eastern Cordillera to propose that this range was uplifted ~2 km during the last 12 My, reaching the present-day average elevations of 3250 m.

Hoorn_ch04_Final.indd 49

49

Garver et al. (2005) (see Figs 4.2 & 4.4) review the palaeoelevations inferred from palaeo-surfaces northwards, in the Peruvian Eastern Cordillera, and state that it is likely that much of the final surface uplift of the Eastern Andes in northern Peru happened in the last 5–6 My. However, previous stages of surface uplift as old as 15 Ma are also documented. In Ecuador, Burgos (2006) deduced that much of the surface uplift of the Cordillera Real (see Figs 4.2 & 4.4) has occurred during the last 4 My, based on the comparison of the original and present-day position of the Lower Pliocene Pisiyambo surface. Northwards, in the Magdalena inter-Andean valley of Colombia (see Figs 4.2, 4.4 & 4.8), west of the Eastern Cordillera (see Fig. 4.8), the so called La Venta fossils (Fields 1959) record mangroves and other fauna typical of very humid climates in the ~15 My-old La Venta Formation. It is likely that this desert, now one of the few arid regions in the Northern Andes, is the result of an orographic rain shadow that prevented moisture-bearing winds from reaching the inter-Andean Magdalena Valley (see Fig. 4.8). This is a similar palaeo-environmental response as was reported by Kleinert & Strecker (2001) in the Argentinian Andes. In other areas in the Andes, elevations of at least 2000 m are required to have an effective orographic barrier limiting the propagation of moisture-bearing winds (Sobel & Strecker 2003; Blisniuk et al. 2005). Therefore, we can conclude that at ~15 Ma there was insufficient relief in the adjacent Eastern Cordillera to form such a barrier. Finally, in the Eastern Cordillera of Colombia, pollen data suggest that the Central High Plain of the Sabana de Bogotà (see Figs 4.2, 4.4 & 4.8) reached its present elevation progressively from ~15 to 3 Ma, with maximum surface uplift rates between 6 and 3 Ma (Wijninga 1996; Van der Hammen et al. 1973; Hooghiemstra et al. 2006). In general, the evidence shows that although deformation started as early as 30 Ma and earlier in the Andean Eastern Cordilleras, the available palaeoelevation data allow us to infer that present-day surface elevations over large areas were only reached during or after the Late Miocene.

Exhumation histories In the Central Andes, one of the first studies on the exhumation of the Bolivian Eastern Cordillera was done by Benjamin et al. (1987; see Figs 4.2 & 4.4). Masek et al. (1994) and Anders et al. (2002) pointed out that the data from Benjamin et al. (1987) show increased denudation rates around 10–15 Ma. More recently the AFT data from Barnes et al. (2006) confirm accelerated erosion since ~15 Ma, based on individual AFT ages and on modelling of track lengths (see Figs 4.2 & 4.4). For the Peruvian Eastern Cordillera, to the north, Laubacher & Naeser (1994; see Figs 4.2 & 4.4) deduced two main cooling episodes based on three AFT samples, but without length data. The first occurred during the Late Oligocene to the earliest Miocene, implying erosion of ~2 km of rock. The second cooling episode occurred from the Late Miocene to Present, with denudation of 3–4 km and rates of 0.33 mm/year. In the Peruvian Cordillera Blanca, Garver et al. (2005; see Fig. 4.2) produced ZFT (zircon fission tracks) and zircon uranium-thorium/helium (zircon [U-Th]/He;

10/24/2009 10:52:35 Shobha

A. Mora et al. Sediment Load/Total Subsidence (%) 70 60 50 0

Miocene

Pliocene

20

20

0

2

1

0

-1

-2

Oligocene

Onset of exhumation Oligocene

Lower Guayabo Fm.

Miocene M

km

3

L

4

Miocene

Upper Guayabo Fm.

0

Pliocene

5

Age (Ma)

50

C7 C8

60

40

10

20 ZFT (~0.27 mm/yr)

60

Palaeocene

C6

?

FT age (Ma)

C4

C5

Acceleration of exhumation

Palaeocene

Carbonera Fm.

Oligocene

C3

Age (Ma)

C2

E

0

Eocene

1

40 Eocene

C1

Eocene

León

2

Medina SMMVB

AFT (~1.45 mm/yr)

Mirador Fm.

2.5 0.5 ⫺1.5 ⫺3.5 Structural Elevation (km)

0

0.4

0.8 1.2 1.6 Depth (km)

2.0

Fig. 4.9 Comparison between exhumation data in a basement massif along the eastern foothills in the Colombian Eastern Cordillera (data after Mora et al. 2008; Parra et al. 2009b) – see location in Fig. 4.8 in the area number 2 and number 5 – and the subsidence and granulometric changes in the Neogene record of the adjacent foredeep along the eastern foothills (modified after Parra et al. 2009a). Exhumation is shown plotted by age versus structural position in order to derive exhumation rates based on zircon fission track data and apatite fission track data from the same basement massif. Notice that the ZFT data show a first period of lower exhumation rates, whereas the AFT data show a period with higher exhumation rates. The foredeep depozone is located in the surroundings of the Medina-1 well (see Medina-1 location in Fig. 4.8 and its regional correlation with other wells in Fig. 4.11). AFT, apatite fission tracks; Fm, Formation; FT, fission tracks; SMMVB, Southern Middle Magdalena Valley Basin; ZFT, zircon fission tracks.

or zircon uranium thorium-helium, ZHe) data suggesting that since ~6.2 Ma nearly 5 km of unroofing occurred along the Cordillera Blanca normal fault. For the Cordillera Real of the Ecuadorian Andes, Spikings et al. (2000; see Figs 4.2 & 4.4) presented ZFT, AFT individual ages and modelling data suggesting more rapid exhumation rates between 10 and 0 Ma compared with the previous 15 Ma. For the Colombian Eastern Cordillera, Parra et al. (2009b; see Figs 4.2, 4.4 & 4.8) demonstrated that denudation along the eastern foothills started at ~22 Ma. Calculated denudation rates during the interval 22 to ~10 Ma average 0.3 mm/year, with a maximum of 0.5 mm/year in a few locations (Parra et al. 2009b; Fig. 4.9). However, during the Late Miocene, the data from the Eastern Cordillera show a progressive acceleration until ~4 Ma, when exhumation rates reach values between 1.5 and 2 mm/ year (Mora et al. 2008) (see Fig. 4.9). Interestingly, Parra et al. (2009b) and Mora et al. (2008) found a positive correlation in the Colombian sub-Andes between increasing granulometry upsection, and more proximal depositional settings in the foredeep, synchronous with increasing denudation rates in the adjacent

Hoorn_ch04_Final.indd 50

Eastern Cordillera (see Fig. 4.9). In contrast, Parra et al. (2009b) found that faster subsidence rates, at ~30 Ma in the foredeep, correspond with the onset of deformation in the hinterland, whereas faster exhumation rates at ~6 Ma have no expression in faster subsidence rates (see Fig. 4.9). In the northernmost Venezuelan Andes, Kohn et al. (1984) obtained AFT ages of between 8 and 2 Ma; a few locations yielded Late Oligocene ages. The absence of profiles located in welldefined structures, and problems with inferred upwarping of isotherms make it difficult to estimate denudation rates, or the onset of denudation in this area. However, Pliocene AFT ages in the Mérida Andes partially coincide with Late Miocene-Pliocene acceleration of denudation rates in the Colombian Eastern Cordillera, as reported by Mora et al. (2008). In general, the data above show a Late Miocene acceleration of denudation rates in the eastern face of the Eastern Cordilleras of the Central and Northern Andes. However, it should be noted that there are significantly fewer data from the Peruvian Eastern Cordilleras than in other areas of the Northern Central and Northern Andes. Particularly interesting is that the timing of

10/24/2009 10:52:35 Shobha

Tectonic history of the Andes

51

(a) CHV-RB-96-204 MG-90-2070 1.7 ± 0.3 Ma 1.9 ± 0.8 Ma

4 Humadea-1 0

⫺4

⫺8 Cretaceous

Paleocene-Eocene

Paleozoic Units

Oligocene-Mid-Miocene

Basement

Neogene Growth-strata

ru

ri

bo

4

Ta m

Pu

MD 28 – 5.5 ± 1.5 Ma MD 22 – 11.0 ± 1.6 Ma

pa

ui nq

MD 29 – 4.5 ± 1.0 Ma

ta

Sy

Th

nc

MD 26 – 11.3 ± 1.5 Ma

st

lin

e

(b)

0

⫺4

⫺8

Fig. 4.10 Structural cross-sections from the sub-Andean zones of (a) the Llanos Basin (modified after Mora 2007) and (b) the Madre de Dios Basin. In the structural cross-section from the Llanos basin, AFT ages near the base of the Cretaceous sequence are shown; vitrinite reflectance data and the much younger AFT ages compared with the stratigraphic age of the units show that those ages are totally reset ages. Mora (2007) considers these ages as indicators of the age of initial thrusting. In (b), the section from the Madre de Dios area, the antiformal stack has totally reset ages of of 11 ± 1.6 Ma close to well-documented growth strata. This age is taken as a proxy for the onset of thrust-induced denudation and close to the age of the antiformal stack. See text for discussion and Fig. 4.2 for cross-section location.

higher denudation rates roughly coincides with coarser grained sedimentation in the sub-Andes (e.g. rivers with higher stream power) and faster sedimentation rates in the sub-Andean foredeeps, if these data are compared with those shown by Roddaz et al. in Chapter 5.

Structural styles and timing of the structures in the sub-Andean zones In Figs 4.5, 4.6 & 4.10 we present cross-sections along the thinskinned sub-Andean belts from the Benì area northwards through the Madre de Dios, Marañón and Santiago basins (after Baby et al. 1997, 1999; Gil et al. 2001; Toro et al. 2004; Hermoza et al. 2005; Roddaz et al. 2005; Mora et al. 2008) towards the Colombian

Hoorn_ch04_Final.indd 51

Llanos foothills to show the structural styles of all these subAndean basins (see Figs 4.2 & 4.4). Along strike structural changes are partially preconditioned by changes in the palaeogeography and thickness distribution of the deformed strata, as well as by the deformation regime and relative orientation of the stress vector with respect to the deformed belt, as noted by Kley et al. (1999), Gil et al. (2001) and Mora (2007). The highest reliefs of the Colombian Eastern Cordillera coincide with a wide Early Cretaceous rift zone, and the master faults of this rift system were reactivated during the Cenozoic deformation (see Figs 4.6 & 4.8). In turn, the adjacent narrow sub-Andean belt of northern Colombia is constituted by shortcuts splaying from the master faults and coinciding with an area where apparently much thinner Lower Cretaceous synrift units are present (see Figs 4.6 & 4.8; Mora 2007). The pinch-out of

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A. Mora et al.

such synrift units preconditions the foreland propagation of the thrust front and the southward extent of the sub-Andean ranges (see Fig. 4.6c). However, when compared with other sub-Andean foothill areas, it appears that the Colombian sub-Andean zone is much narrower and underwent less shortening. In this area, Mora (2007) used borehole breakout data and map patterns to document obliquity between applied stresses and the rift boundary. Based on analogue models, it appears that the obliquity between the inherited rift boundary and the applied stresses (e.g. Marshak et al. 1992; Macedo & Marshak, 1999) could be the factor causing such a narrow sub-Andean front. A close analogue to this situation is the Santiago Basin (see Figs 4.2 & 4.4 for location and Fig. 4.6 for cross-section) in northern Peru, where there is a narrow sub-Andean belt whose width coincides with the eastward pinch-out of evaporites of the Triassic-Jurassic Pucara Formation. In the Santiago Basin (Navarro et al. 2005), as in the Colombian Eastern Cordillera and sub-Andes and Oriente Basin of Ecuador (Barragan et al. 2005; see Figs 4.2 & 4.4), there is also a strong influence of inversion tectonics (Gil et al. 2001). Notably, southwards in the Huallaga Basin (for location see the central parts of Figs 4.2 & 4.4) the detachment level is still constituted by the evaporites of the Pucara Formation, but shortening in the Huallaga Basin reaches ~40% (Hermoza et al. 2005), whereas shortening in the Santiago Basin is only ~13.5%. The difference between the Huallaga (see Fig. 4.2) and Santiago Basins could be related to the width of the area where the Pucara Formation was deposited in both basins. However, Gil et al. (2001) proposed that the significant difference also could be due to the transpressional regime north of the Huancabamba deflexion (see Fig. 4.2), where the Santiago Basin is located. In this case, as in the Colombian subAndean zone, the obliquity between the inherited rift boundary and the applied stresses is the cause for such a narrow sub-Andean front. Therefore, as proposed by Gil et al. (2001), this is a typical feature of the Northern Andes. In comparison, there is more shortening in the Huallaga Basin (see Fig. 4.2) and southwards in the Madre de Dios (see Figs 4.2 & 4.10), where the Central Andes are characterized by their orientation almost perpendicular to the plate convergence vector. Gil et al. (2001) mention the virtual absence to the south of significant tectonic inheritance as an important factor that appears to precondition the degree of basement involvement (i.e. thinskinned vs thick-skinned deformation). Therefore, following Gil et al. (2001), the absence of a Mesozoic synrift sequence and associated normal faults south of the Abancay deflection (see Fig. 4.2), and the presence of a thick sequence of Paleozoic platform sediments, preconditions the Madre de Dios, Beni (location in Fig. 4.2 and cross-section through the Madre de Dios Basin in Fig. 4.10) and Chaco sub-Andean zones (see Fig. 4.2) to be characterized by purely thin-skinned deformation. In turn, the absence of such Paleozoic platform sediments conditions the transition from the Bolivian sub-Andes (see Chaco basin in Figs 4.2 and 4.4) to the thick-skinned deformation of the Pampean ranges (Allmendinger et al. 1997; Kley et al. 1999) (the Pampean ranges are located south of the Chaco basin; see location of the Chaco basin in Fig. 4.2). In spite of the marked changes in the structural style conditioned by the stress regime and palaeogeographic conditions, there is a unifying aspect that has been rarely noted in the literature. All the sub-Andean zones from Bolivia to Venezuela share a

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common feature, which is the youth of their main deformation phase (post-Late Miocene, see Figs 4.4 & 4.7). The main argument (Baby et al. 1995; Dunn et al. 1995; Casero et al. 1997; Gil 2001; Mora et al. 2007) that most of the deformation occurred after the Late Miocene, is that Late Miocene and younger rock units are involved in the deformation, whereas older growth stratal relations are scarce and only preserved in some places in Late Miocene or younger sequences. This hypothesis has been confirmed using thermochronology (AFT) in the Northern Andes of Colombia, where the frontal thrusts have ages for the onset of exhumation between 12 and 3 Ma (Mora et al. 2008, see one such locality in Fig. 4.10). In fact, Mora et al. (2008) were also able to demonstrate that the majority of the shortening in the Northern Andes of Colombia occurred after the Late Miocene. AFT data have also been used to show a Late Miocene minimum age for exhumation prompted by tectonic stacking in duplexes and thrust faults in the Santiago Basin (Baby et al. 2005), the Huallaga Sub-basin (AlvarezCalderon 1999) and the Madre de Dios Basin (Mora, A., Baby, P. & Roddaz, M., unpublished work; Fig. 4.2). For example, in the Madre de Dios Basin (Figs 4.2 & 4.10), Mora et al. (unpublished work) used AFT cooling ages in a triangular zone, located at the backlimb of the Puquiri syncline (see Fig. 4.10b) to propose a minimum age for the deformation in this triangular zone. Cooling ages from those samples passing chi-squared cluster around 11 Ma (samples MD 22 and MD 26) corresponding to ages measured on Tertiary, pre-Middle Miocene units. Sample MD 22 comes from a unit just below the base of documented growth strata (see Fig. 4.10b). However, there are younger populations in outcropping Permian and Cretaceous rock units. Based on these considerations, Mora et al. (unpublished work) say that folding in the eastern flank of the Punquiri syncline, and therefore shortening of the underlying duplexes, started around 11 Ma and has probably been active through Late Miocene and Pliocene times as deduced from younger populations in other samples. Late Miocene deformation has also been shown to the south in the sub-Andean ranges adjacent to the Chaco foreland (Moretti et al. 1996; Echavarria et al. 2003) and inferred in the Beni Basin (Strub 2006) (see Fig. 4.2). Following the aforementioned exhumation and palaeoelevation data, deformation in the sub-Andean thin-skinned ranges is normally synchronous with a period of faster denudation rates in the adjacent Eastern Cordillera. Even with the limited resolution of the data that we have, when comparing the aforementioned palaeoelevation data with the Late Miocene-Pliocene age of deformation in the sub-Andes, it appears that the sub-Andean zones began deforming close to the time when a certain elevation was reached in the hinterland Andean regions or at least synchronous with the main topographic growth in the Andean Eastern Cordilleras. The upheaval of the low-elevation subAndean ranges should have also influenced the river networks and the amount of material being transported from the Andes to the Amazon foreland.

The Vaupés Swell and the Fitzcarrald Arch Traditionally, the Mérida Andes (Northern Andes of Venezuela) was considered as the most important barrier in directing the

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Tectonic history of the Andes

Vanguardia-1

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Amazon River towards the Atlantic instead of towards the Caribbean (Hoorn et al. 1995). However, the real northern boundary for the Amazon hydrographic basin is the so-called Vaupés Swell (see Fig. 4.8), a western low-elevation promontory where the Guiana Shield is exposed or covered by a thinner sedimentary cover than in the Amazon lowlands to the south, or the Orinoquían savannas northwards (see Fig. 4.8). The Macarena, Chiribiquete and Araracuara low-elevation ranges represent the surface expression of the Vaupés Swell, where the Guiana Shield basement rocks crop out at elevations of up to 1000 m above sea level (see Fig. 4.8). The origin of this low-elevation Guiana Shield promontory is currently unknown. However, the timing of its deformation can be inferred from many lines of evidence. Regional stratigraphic correlations show (Fig. 4.11) that the Vaupés Swell has been a positive area since the Late Cenomanian, as deduced from the thinning of the units deposited in that region (see Fig. 4.11; Casero et al. 1997). Both Paleogene and Cretaceous units are thinner towards the Vaupés Swell, but the most dramatic thickness change is evident in the Miocene rock units (see Fig. 4.11). In addition, seismic lines (Fig. 4.12) demonstrate that the thinning observed in the Miocene units is related to deposition and subsequent erosion of that interval. This is deduced by the absence of

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Miocene or older growth stratal relationships associated with the dip of the Vaupés Swell in its northwestern face. In contrast, all the Cenozoic units appear to be bent by post-Middle Miocene or even post-Late Miocene movements (see Fig. 4.12). In addition, Wesselingh et al. (2006) mention that east of the Araracuara range, around the Apaporis River, there are some patches of Late Miocene sediments (Apaporis sand unit) resting on top of basement. All these observations confirm the Late Miocene-Pliocene origin of the Vaupés Swell. We therefore infer a Late MiocenePliocene uplift of the present-day relief of the Vaupés Swell and await thermochronological data from the Macarena, Araracuara and Chiribiquete ranges to confirm our inference. An analogue transverse feature in the Amazon drainage basin is the Fitzcarrald Arch, separating the Ucayali and Madre de Dios Basins in the Peruvian sub-Andes (see Fig. 4.2). The late Cenozoic origin of this basement arch has been documented by Espurt et al. (2007). In that case, a direct relationship has been proposed between the Fitzcarrald Arch and the kinematics of the Nazca ridge due to temporal coincidence between the formation of the former and the subduction of the latter, and the geophysically inferred spatial coincidence of both. In the case of the Vaupés Swell, such a relationship with subducted features has not yet been documented, but is possible.

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Fig. 4.12 Composite seismic line along the northwestern face of the Vaupés Swell (see location in Fig. 4.8). The frontal thrust of the eastern Colombian foothills (Guaicaramo Fault) is visible to the right. Further south (to the left) there is a progressive slow thinning of the Cretaceous to Miocene units. However, no evident growth stratal relationships can be identified. In contrast, the most significant feature is the post-Middle Miocene erosion that bevels the upper part of the Neogene sequence. This bevelling removes an amount of overburden equivalent to ~ 1 sec two-way travel time (TWT). Using velocities close to 4000 m/s this is ~2000 m of overburden removed. This is due to post-Middle Miocene northward tilting of the basin due to the uplift of the Vaupés Swell. A normal flexural response due to the load of the adjacent thrust would imply lower dip angles in the foreland plate (Bayona et al. 2008).

Discussion The Andes and their relationship to the Amazon foreland and Amazon River dynamics Based on a summary of the existing data on the history of the Northern and Central Andes and adjacent Amazon we here propose a hypothesis about the mechanisms that link Andean mountain building to reshaping of the continental drainage of northern South America. A key element in this hypothesis is the low-elevation basement arch that forms the present northern boundary of the Amazon drainage basin and separates the Caquetá hydrographic basin, with rivers flowing towards the Amazon, from the Guaviare hydrographic basin, with rivers flowing towards the Orinoco (see Figs 4.1 & 4.8).

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Hoorn et al. (1995), among other ideas, proposed that the palaeogeographic scenario prior to the Late Miocene included a palaeo-Orinoco, collecting most of the Colombian, Ecuadorian and Peruvian Amazon tributaries and flowing towards the Maracaibo Lake (see this area in Fig. 4.1; for details see fig. 1A in Hoorn et al. 1995). Oligocene initial uplift and exhumation of the Colombian Eastern Cordillera, as documented by Parra et al. (2005), Gómez et al. (2001) and Bayona et al. (2008), and Late Oligocene-Early Miocene uplift and exhumation (Kohn et al. 1984; Higgs 1993; Colleta et al. 1997; Pindell et al. 1998) of the Mérida Andes, discard a palaeo-Maracaibo outlet for most of the rivers flowing north at that time. However, sedimentological data from Ecuador to Colombia (e.g., Hoorn et al. 1995 and references therein; Christophoul et al. 2002; Parra et al. 2005) show that a northward flow of most of the sub-Andean fluvial systems collected

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Tectonic history of the Andes by a palaeo-Orinoco was likely. The Late Miocene or younger Vaupés Swell would have constituted a barrier from that time on, preventing the northern Amazon tributaries from flowing towards the Caribbean. We consider the Vaupés Swell to be a key element, but not the only one, causing the majority of the rivers to stop flowing towards the Caribbean, as previously documented by Hoorn et al. (1995). Our main argument for additional elements is a pattern in the rivers of the Amazon drainage basin, which is very evident only at some distance from the Vaupés Swell. These rivers almost always flow to the east rather than to the north, with the notable exception of the Ucayali River (see Fig. 4.1). This pattern is reminiscent of the drainage pattern described in overfilled foreland basins, where accommodation space created in the foredeep is not enough to store sediments coming from the adjacent range (Flemings & Jordan 1989), therefore the drainage in the basin is not parallel to the foredeep and the range, but almost perpendicular (Flemings & Jordan 1989). This behaviour occurs when the rate of relief creation in the forebulge is surpassed by the sedimentation rates. We hypothesize that during an underfilled stage, characterized by rivers dominantly flowing parallel to the Andes, the stream power of the rivers coming from the Andes should be capable of incising the Vaupés Swell. This is supported by the fact that rapidly uplifting ranges in the sub-Andes are always incised by rivers of different sizes, and by the fact that the Iquitos, Carauari and Purus arches (see Fig. 4.2 and Roddaz et al. 2005), which are basement arches similar to the Vaupés Swell, do not act as barriers for the eastward flow of the Amazon tributaries and Amazon River itself. In an overfilled stage of the Amazon basin, the influence of trasverse arches like the Vaupés Swell might be highlighted, as we will explain below. The transition from filled conditions in the Amazon foreland to overfilled conditions is therefore critical for the configuration of the Amazon flowing toward the Atlantic. Roddaz et al. (see Chapter 5) proposed that such a transition occurred in the Late Miocene all along the Amazonian and Llanos forelands. In relation to this the role of the Andes favouring or delaying such a transition should be assessed. Therefore the geological developments in the Andean hinterland that conditioned the transition from filled to overfilled stages in the Amazon foreland should be taken into account. In Chapter 5, Roddaz et al. explain in more detail the sedimentary signals of the transition from filled to overfilled. A synchronism exists between many processes along the northern Central and Northern Andes. First, an Early Oligocene onset of deformation in the northern Central Andes compared with the Eastern Cordillera in the Northern Andes. Second, generalized Late Oligocene, Early Miocene deformation in the Andean Eastern Cordilleras of Bolivia and Colombia at least, and possibly in Peru. Maximum surface elevations in the Andean Eastern Cordilleras were also progressively reached during the post-Late Miocene. There is also a pattern of accelerated Late Miocene to Pliocene/ Quaternary denudation in the eastern face of the Andean Eastern Cordilleras. Finally, after the Late Miocene, deformation migrated from the Eastern Cordilleras towards the sub-Andean zones. This partially synchronous chronology of geological processes and how these may affect the origin of the Amazon River is part of our hypothesis.

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Flemings & Jordan (1989) show that the amount of sediment transported from an uplifting range toward the foreland is proportional to the slope, and to a transport coefficient that varies according to precipitation and climate. In turn, a critical palaeoelevation in the hinterland should be reached for the moisturebearing winds to be trapped and generate focused precipitation (e.g. Bookhagen & Burbank 2006), as occurs today along the entire margin of the Andean Eastern Cordilleras (see precipitation maps in Chapter 14). Following Bookhagen’s map, most of the rainfall in Amazonia should be focused along the Andean eastern front. The critical elevation for topography to focus precipitation has been deduced in the Central Andes and other mountain belts (Sobel & Strecker 2003; Blisniuk et al. 2005; Bookhagen et al. 2005; Strecker et al. 2007; Bookhagen & Strecker 2008). In such cases a value of ~2 km has been proposed. Deformation was present since the Oligocene in the Andean Eastern Cordilleras but palaeoelevation data show that such critical elevations were not reached until the Late Miocene. Thus, the data summarized here show that it is likely that rainfall was focused along the Andean Eastern Cordilleras since the Late Miocene in response to the Late Miocene major surface uplift of the Central and Northern Andes Eastern Cordilleras. An increased slope may also have been associated with this configuration. In turn, the amount of rainfall in the hinterland should also be proportional to the denudation rates in the same areas (Willet 1999). Therefore, the late Cenozoic uplift of the Andean Eastern Cordilleras may also be responsible for increased denudation rates by increasing rainfall, as documented in detail by Mora et al. (2008) for the Colombian Eastern Cordillera. Noticeably increased granulometry in the Amazonian foredeep deposits (Christophoul et al. 2002; Roddaz et al. 2005; Parra et al. 2009b) is mostly synchronous with faster denudation rates (Mora et al. 2008; see Fig. 4.9). In addition, for the Ecuadorian foredeep Christophoul et al. (2002) document a late Cenozoic change in palaeocurrents from axial (parallel to the orogen) to transverse (perpendicular). Finally, in Chapter 5 Roddaz et al. show a shift in the Late Miocene record from tidal environments to fluvial all along the Central sub-Andean ranges. All these processes in the Amazon foreland reflect changing conditions in the Andean hinterland. In fact, based on a compilation of all the available sedimentological data in the sub-Andean basins, only partially mentioned here, Roddaz et al. (Chapter 5) propose a Late Miocene transition from filled Amazonian foreland to overfilled. This is in line with the processes happening in the Andean Eastern Cordilleras with increased surface uplift, inferred higher focused precipitation rates along the eastern side and faster denudation rates during the Late Miocene. It appears therefore that increased denudation rates in the Andean Eastern Cordillera have a causal relationship with the transition from filled to overfilled conditions in the adjacent foredeep, where higher sedimentation rates would have induced overfilled conditions. Such Late Miocene onset of overfilled conditions should have prompted the presence of an eastward-directed fluvial drainage pattern mostly typical of overfilled basins (Flemings & Jordan 1989). In this case, most of the sediment is transported outside the foredeep adjacent to the mountain front, as the foredeep is not capable of storing the sediment input.

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Therefore, transverse drainage – similar to present – dominates. In such a scenario, features like the Fitzcarrald Arch (Pliocene) and Vaupés Swell (Late Miocene-Pliocene) are less likely to be incised by rivers flowing parallel to the range, as in the case of the Iquitos Arch, and their role as basement arches confining the river flow patterns is highlighted. In turn, more rivers flowing east, in an overfilled basin, should find an easier way to flow towards the Atlantic. With this evidence, a two-phase evolution of the Amazon River can be outlined. First, during the Late Miocene the transcontinental drainage started under overfilled conditions and tributaries flowing perpendicular to the Andean deformation front. Second, unambiguously fluvial and coarser-grained sedimentary environments preserved in the record (see Chapter 5) document the presence of the modern Amazonian network since the Pliocene. We hypothesize that this final stage was prompted by Late Miocene-Pliocene uplift of the Vaupés Swell and Pliocene uplift of the Fitzcarrald basement arches, the fast deformation along the sub-Andean ranges, and an even more widespread drainage network in the Andean headwaters.

Migration of the sub-Andean ranges The previous discussion raises the question why deformation only migrated towards the sub-Andean ranges after more than 20 Ma of deformation events in the Andes and why deformation migration towards the Subandes is virtually coeval with the birth of the Amazon river. Many studies propose that there is a mechanical interaction between deformation migration towards the foreland and elevation in the hinterland, explained in terms of the critical taper wedge (Davis et al. 1983; Dahlen et al. 1984; Dahlen & Suppe 1988). Roeder & Chamberlain (1995) proposed that the Late Miocene and later migration of deformation from the Eastern Cordillera and Altiplano towards the sub-Andean ranges could be due to the Eastern Cordillera having reached a critical palaeoelevation along extensive areas in the Central Andes. This is in agreement with available palaeoelevation data in the Andes and has been documented in detail in the Colombian Eastern Cordillera by comparing the timing of the structures in the sub-Andes and the palaeoelevation data inferred from pollen analysis (e.g. Mora 2007). However, Roeder & Chamberlain (1995) say that simple wedge mechanics involving a simple shear mechanism of deformation for the entire Altiplano, Eastern Cordillera and sub-Andean ranges requires a complex succession of events, including a plateau collapse after a main phase of crustal thickening. Alternatively, Oncken et al. (2006) and Babeyko et al. (2006) suggest, based on modelling, that a change in the mechanical properties (cohesion and friction) of the subAndean sedimentary wedge is needed for deformation to migrate from the Eastern Cordillera to the sub-Andean foredeep. As already suggested by Babeyko et al. (2006), two possible scenarios can favour the change in mechanical conditions: increased pore pressure along the basal detachment due to hydrocarbon generation, and increased pore pressure in the wedge due to higher precipitation rates caused by the surface uplift of the Eastern Cordillera. The latter scenario has the problem that higher precipitation would make the entire sedimentary wedge

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weaker, not just the detachment. Wedge mechanics suggest that a less resistant wedge increases basal friction and decreases the effectiveness of the detachment horizon (Davis et al. 1983; Dahlen et al. 1984; Dahlen & Suppe 1988). In contrast, a documented late Cenozoic event of hydrocarbon generation from Colombia to Bolivia (Baby et al. 1995; Cazier et al. 1995) is more likely to facilitate the presence of a weaker detachment, given the fact that the hydrocarbon source rocks are also the basal detachments for the sub-Andean thrustbelt. Late Miocene hydrocarbon generation in the sub-Andean ranges from Colombia to Bolivia occurred at that time because only then was burial sufficient to heat the source rocks enough for them to enter the oil generation window. An increase in denudation rates in the Eastern Cordilleras producing a considerable amount of molasse deposits favours this scenario of rapid source rock burial and heating. Importantly, these molasse deposits are also coeval with the proposed onset of overfilled conditions.

Conclusions From the data presented above we suggest that the evolution of the Andes went through a stage of generalized rock uplift and – for the first time – deformation spread over similar geographical areas as today. This stage could be as old as 30 Ma, an age that is in line with the observations of Oncken et al. (2005) in the Central Andes. We propose that the origin of the Amazon River and its flow towards the Atlantic is not precisely related to this stage. Rather, we suggest that it is related to Late Miocene-Pliocene faster denudation rates in the Andes. Faster denudation rates probably started when the eastern margin of the Andes reached a critical elevation during the Late Miocene, in an already growing mountain range. We hypothesize that faster denudation rates coincide with threshold elevations in the Andean Eastern Cordilleras, because it was only at that time when orographic precipitation was fully developed as a consequence of the newly created topographic barrier. Probably then, drainage areas of high topography were also much more extended than in the previous times. In this scenario, it is likely that the Late Miocene transition from filled to overfilled conditions (see Chapter 5) in the Amazon drainage basin is the result of a sequence of processes in the Andean Orogen. Thus, a greater amount of sediments coming from the Andes may have caused the basin to evolve from filled to overfilled conditions during the Late Miocene. Our main hypothesis suggests that those conditions favoured the initial development of the transcontinental Amazonian drainage. An instrumental piece of evidence supporting this hypothesis, which links the development of the transcontinental network and the orogenic processes, is the fact that all the exposed Late Miocene timing of surface uplift and accelerated subsidence in the Andes and change in sedimentation patterns in the Amazon foredeep coincide with the initiation of the Amazon transcontinental network by 11 Ma, suggested by the sedimentary record from offshore Brazil (Dobson et al. 2001; Figueiredo et al. 2009). However, the modern Amazon network, namely the large drainage area, with similar distribution of depositional systems and flow patterns, sourced in the Andean Eastern Cordilleras and eastward drainage into the Atlantic Ocean, was only fully

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Tectonic history of the Andes established after the Pliocene (see Chapter 5 and Figueiredo et al. 2009). We consider that the Late Miocene to Pliocene uplift of the Vaupés Swell, the Pliocene and younger uplift of the Fitzcarrald Arch, the fast deformation along the sub-Andean ranges, and an even more widespread drainage network in the Andean headwaters may have prompted the final Pliocene development of the modern Amazon network. In addition we also propose that the following were the main factors that caused the palaeo-Amazon to stop flowing towards the Caribbean and be redirected to the Atlantic. We suggest that first the transition from filled to overfilled Amazonian foreland during the Late Miocene and second the Late Miocene-Pliocene uplift of the Vaupés Swell permanently separated the Orinoco from the Amazon drainage basins. Thus, in our view, surface uplift and deformation processes in the Andes not only controlled denudation rates, depositional systems, basin processes and thrust belt migration, but also the origin of the largest fluvial system in the world.

Acknowledgements The authors are indebted to the Instituto Colombiano del Petroleo (ICP), the Institut de Recherche pour le Développement (IRD) and PeruPetro for material and financial support. We also acknowledge Carina Hoorn for inviting us to contribute to this volume. The text greatly benefited by the careful reviews of Onno Oncken, Franck Audemard, Joel Saylor and an anonymous reviewer. Chris Moreno helped to improve English in the text. An earlier version of this manuscript was graciously revised by Eduardo Rosello, Ed Sobel and Manfred Strecker. Birgitt Fabian, Diego Ramos and Alejandro González helped with graphic work in some of the figures.

References Albert, J., Lovejoy, N., Crampton, W. (2006) Miocene tectonism and the separation of cis and trans–Andean river basins: evidence from Neotropical fishes. J S Am Earth Sci 21, 14–27. Allmendinger, R.W., Jordan, T.E., Kay, S.M., Isacks, B.L. (1997) The evolution of the Altiplano-Puna Plateau of the Central Andes. Annu Rev Earth Pl Sci 25, 139–174. Anders, M.H., Gregory-Wodzicki, K.M., Spiegelman, M. (2002) A critical evaluation of late Tertiary accelerated uplift rates for the Eastern Cordillera, central Andes of Bolivia. J Geol 110, 89–100. Babeyko, A., Sobolev, S., Vietor, T., Oncken, O., Trumbull, R. (2006) Numerical study of weakening processes in the central Andean backarc. In: Oncken, O. et al. (eds) The Andes – Active Subduction Orogeny. Berlin: Springer-Verlag. pp. 495–510. Baby, P., Moretti, I., Guillier,B., Limachi, R., Mendez, E., Oller, J., Specht, M. (1995) Petroleum system of the northern and central Bolivian sub-Andean zone. In: Tankard, A.J., Suarez R., Welsink, H.J. (eds) Petroleum Basins of South America. Tulsa: AAPG, pp. 445–458. Baby, P., Rochat, P., Mascle, G., Hérail, G. (1997) Neogene shortening contribution to crustal thickening in the back arc of the Central Andes. Geology 25, 883–886. Baby, P., Rivadeneira, M., Christophoul, F., Barragan, R. (1999) Style and timing of deformation in the Oriente Basin of Ecuador. In: 4th International Symposium on Andean Geodynamics (ISAG ´99), Göttingen, Germany, pp. 68–72.

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Baby, P. et al. (2005) Geodinámica mio-pliocénica de las cuencas subandinas peruanas: un mejor entendimiento de los sistemas petroleros. V International Seminar INGEPET, Lima, Perú. Barke, R., Lamb, S. (2006) Late Cenozoic uplift of the Eastern Cordillera, Bolivian Andes. Earth Planet Sci Lett 249, 350–367. Barnes, J.B., Ehlers, T.A., McQuarry, N., O Sullivan, P., Pelletier, J.D. (2006) Eocene to recent variations in erosion across the central Andean fold-thrust belt, northern Bolivia: Implications for Plateau evolution. Earth Planet Sci Lett 248, 118–133. Barragan, R., Baby, P., Duncan, R. (2005) Cretaceous alkaline intraplate magmatism in the Ecuadorian Oriente Basin: Geochemical, geochronological and tectonic evidence. Earth Planet Sci Lett 236, 670–690. Bayona, G., Cortes, M., Jaramillo, C., Ojeda, G., Aristizabal, J.J., ReyesHarker, A. (2006b) Distinguishing tectonic versus eustatic surfaces in the Llanos Basin of Colombia, and implications for stratigraphic correlations. Extended abstracts, IX Simposio Bolivariano de Exploración Petrolera en las Cuencas Subandinas, Cartagena de Indias, Colombia. Bayona, G., Cortés, M., Jaramillo., C., Ojeda, G., Aristizabal, J.J., Reyes-Harker, A. (2008) An integrated analyisis of an orogensedimentary basin pair: Latest Cretaceous-Cenozoic evolution of the linked Eastern Cordillera orogen and the Llanos foreland basin of Colombia. Bull Geol Soc Am 120. doi: 10.1130/B26187.1 Benjamin, M.T., Johnson, N.M., Naeser, C.W. (1987) Recent rapid uplift in the Bolivian Andes: Evidence from fission-track dating. Geology 15, 680–683. Blisniuk, P.M., Stern, L.A., Chamberlain, C.P., Idleman, B., Zeitler, P.K. (2005) Climatic and ecologic changes during Miocene surface uplift in the southern Patagonian Andes. Earth Planet Sci Lett 230, 125–142. Bookhagen, B., Burbank, D.W. (2006) Topography, relief, and TRMMderived rainfall variations along the Himalaya. Geophys Res Lett 33, L08405. doi: 10:1029/2006GL026037 Bookhagen, B., Strecker, M.R. (2008) Orographic barriers, highresolution TRMM rainfall, and relief variations along the eastern Andes. Geophys Res Lett 35, l06403. doi: 10.1029/2007gl032011 Bookhagen, B., Thiede, R.C., Strecker, M.R. (2005) Extreme monsoon events and their control on erosion and sediment flux in the high, arid NW Himalayas. Earth Planet Sci Lett 231, 131–146. Burgos, J.D.Z. (2006) Genese et progradation d’un cone alluvial au front d’une chaine active: example des Andes Equatoriennes au Neogene. PhD thesis, Université Paul Sabatier, Toulouse, 373 pp. Casero, P., Salel, J.F., Rossato, A. (1997) Multidisciplinary correlative evidences for polyphase geological evolution of the foot-hills of the Cordillera Oriental (Colombia). In: VI Simposio Bolivariano de Exploración Petrolera en las Cuencas Subandinas, Bogotá, Colombia, pp. 100–118. Cazier, E.C., Hayward, A.B., Espinosa, G., Velandia, J., Mugniot, J.F., Leel, W.G. (1995) Petroleum geology of the Cusiana Field, Llanos Basin foothills, Colombia. AAPG Bulletin 79, 1444–1463. Cediel, F., Shaw, R., Cáceres, C. (2003) Tectonic assembly of the northern Andean Block. In: Bartolini, C., Buffler, R.T., Blickwede, J. (eds) The Circum-Gulf of Mexico and the Caribbean: Hydrocarbon Habitats, Basin Formation and Plate Tectonics. American Association of Petroleum Geologists Memoir 79, 815–848. Christophoul, F., Baby, P., Dávila, C. (2002) Stratigraphic responses to a major tectonic event in a foreland basin: the Ecuadorian Oriente basin from Eocene to Oligocene times. Tectonophysics 345, 281–298. Cobbold, P.R., Rosello, E.A., Roperch, P., Arriagada, C., Gómez, L.A., Lima, C. (2007) Distribution, timing and causes of Andean deformation across South America. Geol Soc Lond 272, 321–343.

10/24/2009 10:52:37 Shobha

58

A. Mora et al.

Colletta, B., Roure, F., De Toni, B., Loureiro, D., Passalaqua, H., Gou, Z. (1997) Tectonic inheritance, crustal architecture, and contrasting structural styles in the Venezuelan Andes. Tectonics 16, 777–794. Dahlen, F.A., Suppe, J. (1988) Mechanics growth and erosion of mountain belts. In: Clark, S.P.J. (ed.) Processes in Continental Lithospheric Deformation. Geological Society of America Special Paper #218, pp. 161–178. Dahlen, F.A., Suppe, J., Davis, D. (1984) Mechanics of fold-and-thrust belts and accretionary wedges: Cohesive Coulomb theory. J Geophys Res 89(B12), 10087–10101. Damuth, J.E., Kumar, N. (1975) Amazon Cone: Morphology, Sediments, Age and Growth Pattern. Geol Soc Am Bull 86, 863–878. Davis, D., Suppe, J., Dahlen, F.A. (1983) Mechanics of fold-and-thrust belts and accretionary wedges. J Geophys Res 88(B2), 1153–1172. Dobson, D.M., Dickens, G.R., Rea, D.K. (1997) Terrigenous sedimentation at the Ceara Rise. In: Proceedings of ODP, Scientific Results 154. College Station, TX: Ocean Drilling Program, pp. 465–473. Dobson, D.M., Dickens, G.R., Rea, D.K. (2001) Terrigenous sediment on Ceara Rise: a Cenozoic record of South American orogeny and erosion. Palaeogeogr Palaeocl 165, 215–229. Dunn, J., Hartshorn, K., Hartshorn, P. (1995) Structural styles and hydrocarbon potential of the Sub Andean thrust belt of southern Bolivia. In: Tankard, A.J., Suarez, S.R., Welsink, H.J. (eds) Petroleum Basins of South America. Tulsa: American Association of Petroleum Geologists, pp. 523–543. Echavarria, L., Hernandez, R., Allmendinger, R., Reynolds, J. (2003) Subandean thrust and fold belt of northwestern Argentina: geometry and timing of the Andean evolution. AAPG Bulletin 87, 965–985. England, P., Molnar, P. (1990) Surface uplift, uplift of rocks, and exhumation of rocks. Geology 18, 1173–1177. Erslev, E.A. (1986) Basement balancing of Rocky Mountain foreland uplifts. Geology 14, 259–262. Espurt, N., Baby, P., Brusset, S., Roddaz, M., Hermoza, W., Regard, V. et al. (2007) How does the Nazca Ridge subduction influence the modern Amazonian foreland basin? Geology 35, 515–518. Espurt, N., Barbarand, J., Brusset, S., Baby, P., Roddaz, M., Hermoza, W. (2008) Kinematic evolution of the Camisea Subandean thrust belt from apatite fission-track thermochronology, Peru. Extended abstract in 7th International Symposium of Andean Geodynamics, pp. 199–202. Fields, R.W. (1959) Geology of the La Venta badlands, Colombia, South America.Univ Calif Publ Geol Sci 32, 405–444. Figueiredo, J., Hoorn, C., van der Ven, P., Soares, E. (2009) Late Miocene onset of the Amazon River and the Amazon deep-sea fan: evidence from the Foz do Amazonas Basin. Geology 37, 619–622. Flemings, P.B., Jordan, T. (1989) A synthetic stratigraphic model of foreland basin development. J Geophys Res 94, 3851–3866. Garver, J.I., Reiners, P.R., Walker, L.J., Ramage, J.M., Perry, S.E. (2005) Implications for timing of Andean uplift from thermal resetting of radiation-damaged zircon in the Cordillera Huayhuash, Northern Peru. Geology 113, 117–138. Garzione, C.N., Molnar, P., Libarkin, J.C., MacFadden, B.J. (2006) Rapid late Miocene rise of the Bolivian Altiplano: Evidence for removal of mantle lithosphere. Earth Planet Sci Lett 241, 543–556. Garzione, C.N., Hoke, G.D., Libarkin, J.C., Withers, S., MacFadden, B., Eiler, J. et al. (2008) Rise of the Andes. Science 320, 1304–1307. Gil, W. (2001) Evolution latérale de la déformation d’un front orogénique: Exemple des bassins subandins entre 0º et 16ºS. PhD thesis, l’Université Paul Sabatier, Toulouse. Gil, W., Baby, P., Ballard, J.F. (2001) Structure and palaeogeographic control of the Peruvian Subandean zone. Cr Acad Sci II A 333, 741–748.

Hoorn_ch04_Final.indd 58

Gómez, E., Jordan, T.E., Allmendinger, R.W., Hegarty, K., Kelley, S., Heizler, M. (2001) Controls on architecture of the Late Cretaceous to Cenozoic southern Middle Magdalena Valley Basin, Colombia. Geol Soc Am Bull 115, 131–147. Gómez, E., Jordan, T.E., Allmendinger, R.W., Cardozo, N. (2005) Development of the Colombian foreland-basin system as a consequence of the diachronous exhumation of the northern Andes. Geol Soc Am Bull 117, 1272–1292. Graham, A., Gregory-Wodzicki, K.M., Wright, K.L. (2001) Studies in neotropical paleobotany. XV. A Mio-Pliocene palynoflora from the Eastern Cordillera, Bolivia: Implications for the uplift history of the Central Andes. Am J Bot 88, 1545–1557. Gregory-Wodzicki, K.M. (2000) Uplift history of the Central and Northern Andes: A review. Bull Geol Soc Am 112, 1091–1105. Gregory-Wodzicki, K.M., McIntosh, W.C., Velasquez, K. (1998) Climatic and tectonic implications of the late Miocene Jakokkota Flora, Bolivian Altiplano. J S Am Earth Sci 11, 533–560. Gutscher, M., Olivet, J., Aslanian, D., Maury, R., Eissen, J. (1999) The lost Inca Plateau: cause of flat subduction beneath Peru? Earth Planet Sci Lett 171, 335–341. Hampel, A. (2002) The migration history of Nazca Ridge along the Peruvian active margin: A re-evaluation. Earth Planet Sci Lett 203, 665–679. Hermoza, W., Brusset, S., Baby, P., Gil, W., Roddaz, M., Guerrero, N., Bolaños, M. (2005) The Huallaga foreland evolution: thrust propagation in deltaic environment, in the northern Peruvian Andes. J S Am Earth Sci 19, 495–512. Higgs, R. (1993) Sedimentological evidence for early uplift (Oligocene) of the Venezuelan Andes. AAPG Bulletin 77, 325. Hooghiemstra, H., Van der Hammen, T. (1998) Neogene and Quaternary development of the neotropical rain forest: the forest refugia hypothesis, and a literature overview. Earth Sci Rev 44, 147–183. Hooghiemstra, H., Wijninga, V.M., Cleef, A.M. (2006) The Paleobotanical record of Colombia: Implications for biogeography and biodiversity. Ann MO Bot Gard 93, 297–325. Hoorn, C. (2006) The birth of the mighty Amazon. Sci Am 294, 52–59. Hoorn, C., Guerrero, J., Sarmiento, G.A., Lorente, M.A. (1995) Andean tectonics as a cause for changing drainage patterns in Miocene northern South America. Geology 23, 237–240. Horton, B. (1998) Sediment accumulation on top of the Andean orogenic wedge: Oligocene to late Miocene basins of the Eastern Cordillera, southern Bolivia. Geol Soc Am Bull 110, 1174–1192. Horton, B.K. (2005) Revised deformation history of the central Andes: Inferences from Cenozoic foredeep and intramontane basins of the Eastern Cordillera, Bolivia. Tectonics 24; doi:10.1029/ 2003TC001619. Jordan, T.E., Isacks, B.L., Allmendinger, R.W. et al. (1983) Andean tectonics related to geometry of subducted Nazca plate. Geol Soc Am Bull 94, 341–361. Jordan, T., Reynolds, J., Erikson, J. (1997) Variability in age of initial shortening and uplift in the Central Andes. 16–33.30ºS. In: Ruddiman, W.F. (ed.) Uplift and Climate Change. New York: Plenum Press, pp. 41–61. Kaandorp, R.J.G. et al. (2005) Seasonal Amazonian rainfall variation in the Miocene climate optimum. Palaeogeogr Palaeoclim 221, 1–6. Kennan, L., Lamb, S.H., Hoke, L. (1997) High altitude paleosurfaces in the Bolivian Andes: Evidence for late Cenozoic surface uplift. In: Paleosurfaces: Recognition, Reconstruction, and Interpretation. Geological Society of London, Special Publication, pp. 307–324. Kleinert, K., Strecker, M. (2001) Climate change in response to orographic barrier uplift: Paleosol and stable isotope evidence from

10/24/2009 10:52:37 Shobha

Tectonic history of the Andes the Late Neogene Santa Maria basin, northwestern Argentina. Geol Soc Am Bull 113, 728–746. Kley, J. (1996) Transition from basement-involved to thin-skinned thrusting in the Cordillera Oriental of southern Bolivia. Tectonics 15, 763–775. Kley, J., Monaldi, C.R., Salfity, J.A. (1999) Along-strike segmentation of the Andean foreland: causes and consequences. Tectonophysics 301, 75–94. Kohn, B.P., Shagam, R., Banks, P.O., Burkley, L.A. (1984) MesozoicPleistocene Fission Track ages on rocks of the Venezuelan Andes and their tectonic implications. Geol Soc Am Bull 162, 365–384. Laubacher, G., Naeser, C.W. (1994) Fission-track dating of granitic rocks from the Eastern Cordillera of Peru: evidence for late Jurassic and Cenozoic cooling. J Geol Soc Lond 151, 473–483. Macedo, J., Marshak, S. (1999) Controls on the geometry of foldthrust belt salients. Bull Geol Soc Am 111, 1808–1822. Marrett, R., Peacock, D.C.P. (1999) Strain and stress. J Struct Geol 21, 1057–1063. Marshak, S., Wilkerson, M.S., Hsui, A. (1992) Generation of curved fold-thrust belts: insight from simple physical and analytical models. In: McClay, K.R. (ed.) Thrust Tectonics and Petroleum Systems. London: Chapman and Hall, pp. 83–92. Martínez, J. (2006) Structural evolution of the Llanos foothills, Eastern Cordillera, Colombia: J S Am Earth Sci 21, 510–520. Masek, J.G., Isacks, B.L., Gubbels, T.L., Fielding, E.J. (1994) Erosion and tectonics at the margins of continental plateaus. J Geophys Res 99(B7), 941–956. Mégard, F. (1984) The Andean orogenic period and its major structures in central and northern Peru. J Geol Soc Lond 141, 893–900. Mora, A. (2007) Inversion tectonics and exhumation processes in the eastern cordillera of Colombia. PhD thesis, Potsdam University, Potsdam. Mora, A., Parra, M., Strecker, M.R. (2006) The role of penetrative deformation in orogenic processes. An example from the Eastern Cordillera of Colombia. 11. Abstracts of Symposium Tektonik, Struktur- und Kristallingeologie. Mora, A., Parra, M., Strecker, M.R., Sobel, E.R., Hooghiemstra, H., Torres, V., Vallejo-Jaramillo, J. (2008) Climatic forcing of asymmetric orogenic evolution in the Eastern Cordillera of Colombia. Bull Geol Soc Am 120, 930–949. doi: 10.1130/B26186.1 Moretti, I., Baby, P., Mendez, E., Zubieta, D. (1996) Hydrocarbon generation in relation to thrusting in the Subandean zone from 18º to 22ºS, South Bolivia. Petrol Geosci 2, 17–28. Navarro, L., Baby, P., Bolaños, R. (2005) Structural style and hydrocarbon potential of the Santiago basin. In: V International Seminar INGEPET, Lima, Peru, pp. 1–16. Noble, D., McKee, E., Farrar, E., Petersen, U. (1974) Episodic Cenozoic volcanism and tectonism in the Andes of Peru. Earth Planet Sci Lett 21, 213–220. Noble, D., Mégard, F., McKee, E. (1979) Early tertiary “incaic” tectonism, uplift and volcanic activity, Andes of Central Perú. Geol Soc Am Bull 90, 903–907. Oncken, O., Hindle, D., Kley, J. Elger, K., Victor, P., Schemmann, K. (2006) Deformation of the Central Andean Upper Plate System – facts, fiction, and constraints for plateau models. In: Oncken, O. et al. (eds) The Andes – Active Subduction Orogeny. Berlin: SpringerVerlag, pp. 3–27. Parra, M., Mora, A., Jaramillo, C., Veloza, G., Strecker, R. (2005) New stratigraphic data on the initiation of mountain building at the Eastern Front of the Colombian Eastern Cordillera. In: International Symposium on Andean Geodynamics, Barcelona.

Hoorn_ch04_Final.indd 59

59

Parra, M., Mora, A., Jaramillo, C., Strecker, M.R., Sobel, E.R., Quiroz, L.I. et al. (2009a) Orogenic wedge advance in the northern Andes: Evidence from the Oligocene-Miocene sedimentary record of the Medina Basin, Eastern Cordillera, Colombia: Geol Soc Am Bull 121, 780–800. Parra, M., Mora, A., Sobel, E.R., Strecker, M.R., González, R. (2009b) Episodic orogenic-front migration in the northern Andes: constraints from low-temperature thermochronology in the Eastern Cordillera, Colombia: Tectonics 28, TC4004. Pindell, J.L., Higgs, R., Dewey, J.F. (1998) Cenozoic palinspastic reconstruction, paleogeographic evolution, and hydrocarbon setting of the northern margin of South America. In: Pindell, J.L., Drake, C.L. (eds) Paleogeographic Evolution and Non-glacial Eustasy, Northern South America. Tulsa: Society for Sedimentary Geology, pp. 45–86. Restrepo-Pace, P.A., Colmenares, F., Higuera, C., Mayorga, M. (2004) A fold-and-thrust belt along the western flank of the Eastern Cordillera of Colombia. Style, kinematics, and timing constraints derived from seismic data and detailed surface mapping. In: McClay, K.R. (ed.) Thrust Tectonics and Hydrocarbon Systems. American Association of Petroleum Geologists Memoir 82, pp. 598–613. Rochat, P., Hérail, G., Baby, P., Mascle, G. (1999) Bilan crustale et contrôle de la dynamique sédimentaire sur les mécanismes de formation de l’Altiplano. CR Acad Sci 328, 189–195. Roddaz, M., Baby, P., Brusset, S., Hermoza, W., Darrozes, J.M. (2005) Forebulge dynamics and environmental control in Western Amazonia: The case study of the Arch of Iquitos (Peru). Tectonophysics 399, 87–108. Roeder, D., Chamberlain, C.P. (1995) Structural geology of subAndean fold and thrust belt in northwestern Bolivia. In: Tankard, A.J., Suarez, S.R., Welsink, H.J. (eds) Petroleum Basins of South America. Tulsa: American Association of Petroleum Geologists, pp. 459–479. Sanderson, D., Marchini, W.R.D. (1984) Transpression. J Struct Geol 6, 449–458. Sdrolias, M., Müller, R.D. (2006) Controls on back-arc basin formation. Geochem Geophys Geosyst 7, Q04016, doi: 10.1029/ 2005GC001090 Silver, P.G., Russo, R.M., Lithgow, B.C. (1998) Coupling of South American and African Plate motion and plate deformation. Science 279, 60–63. Sobel, E.R., Strecker, M.R. (2003) Uplift, exhumation and precipitation: Tectonic and climatic control of Late Cenozoic landscape evolution in the northern Sierras Pampeanas, Argentina. Basin Res 15, 431–451. Spikings, R.A., Seaward, D., Winkler, W., Ruiz, G. (2000) Lowtemperature thermochronology of the northern Cordillera Real, Ecuador: Tectonic insights from zircon and apatite fission track analysis. Tectonics 19, 649–668. Strecker, M.R., Alonso, R.N., Bookhagen, B., Carrapa, B., Hilley, G.E., Sobel, E., Trauth, M.H. (2007) Tectonics and climate of the southern Central Andes. Ann Rev Earth Planet Sci 35, 747–787. Strub, M. (2006) Evolution géomorphologique et tectonique au front des Andes centrales (Nord de la Bolivie). PhD thesis, Université Paul Sabatier, Toulouse, 191 pp. Toro, J., Roure, F., Bordas-Le Floch, N., Le Cornec-Lance, S., Sassi, W. (2004) Thermal and kinematic evolution of the Eastern Cordillera fold-and-thrust belt, Colombia. AAPG Hedberg Series 1, 79–115. http://www.aapg.org/pubs/aapgcopyright.cfm Van der Hammen, T., Hooghiemstra, H. (2000) Neogene and Quaternary history of vegetation, climate, and plant diversity in Amazonia. Quaternary Sci Rev 19, 725–742.

10/24/2009 10:52:37 Shobha

60

A. Mora et al.

Van der Hammen, T., Werner, J.H., Dommelen, V. (1973) Palynological record of the upheaval of the Northern Andes: a study of the Pliocene and Lower Quaternary of the Colombian Eastern Cordillera and the early evolution of its High-Andean biota. Rev Paleobot Palynol 16, 1–122. Victor, P., Oncken, O., Glodny, J. (2004) Uplift of the western Altiplano Plateau: evidence from the Precordillera between 20 degrees S and 21 degrees S: northern Chile. Tectonics 23, 4, TC4004. Wesselingh, F.P., Hoorn, M.C., Guerrero, J., Rasanen, M.E., RomeroPittman, L., Salo, J. (2006) The stratigraphy and regional structure

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of Miocene deposits in western Amazonia (Perú, Colombia and Brazil), with implications for late Neogene landscape evolution. Scripta Geologica 133, 291–322. Wijninga, V.M. (1996) Palynology and paleobotany of Neogene sediments from the high plain of Bogotá (Colombia): evolution of the Andean flora from an ecological perspective. PhD thesis, University of Amsterdam, 370 pp. Willet, S.D. (1999) Orogeny and orography: The effects of erosion on the structure of mountain belts. J Geophys Res 104, 28957–28981.

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FIVE

Cenozoic sedimentary evolution of the Amazonian foreland basin system Martin Roddaz1, Wilber Hermoza2, Andres Mora3, Patrice Baby1, Mauricio Parra4, Frédéric Christophoul1, Stéphane Brusset1 and Nicolas Espurt5 1

Université de Toulouse, France REPSOL-YPF, Madrid, Spain 3 ECOPETROL, Instituto Colombiano del Petroleo, Santander, Colombia 4 The University of Texas, Austin, USA 5 Université Paul Cézanne, Aix en Provence, France 2

Abstract In this chapter we present a synthesis of the Cenozoic evolution of the Amazonian foreland basin system, based on a review of the estimated ages, lithology and sedimentary structures, palaeontological content, and inferred depositional environments of sedimentary units in the basin. In addition, we have calculated maximum sedimentation rates for the Cenozoic formations of the northern Peruvian foreland basin and integrated these with existing data on sedimentation rates, subsidence analysis, migration of depocentre and depositional environments. Based on this information we propose a model for the Cenozoic evolution of the Amazonian foreland. The sedimentary architecture of this foreland basin indicates that Cenozoic evolution was marked by several periods, which were roughly synchronous and of similar effect, along the entire Amazonian foreland basin system. Tectonic loading of the Andes of Colombia, Ecuador, Peru and northern Bolivia, and development of the Amazonian foreland, was initiated during Late Cretaceous-Paleocene times and followed by an unloading stage during the Early-Middle Eocene period. The Middle-Late Eocene marine transgression and the increase in sedimentation rates, associated with westward migration of the depocentre, were all indicative of a renewed phase of tectonic loading of the Peruvian Western Cordillera and the Ecuadorian and Colombian Eastern Cordillera. Subsequent Oligo-Miocene increase in sedimentation rates and further migration of the depocentres towards the present-day sub-Andean zone, are all indicative for a thrust-induced uplift and loading of the Eastern Cordilleras of Peru, Bolivia and Colombia. This Oligo-Miocene loading stage maintained high subsidence rates that favoured the sedimentation of aggradational floodplain and coastal plain and tidally influenced deposits. Nevertheless, the processes that controlled the Early-Middle Miocene marine ingressions remain to be determined. Late Miocene ongoing thrust tectonic loading of the Eastern Cordillera, initial structuring of the sub-Andean zone and the onset of the main phase of Andean surface uplift induced flexural subsidence in the foredeep depozones of the entire Amazonian foreland basin. This process also drove the Late Miocene marine transgressions that characterized the filled stage of the Ecuadorian, Peruvian and Bolivian Amazonian foreland basin system. Valley incisions and full relief development in the hinterland during the Late Miocene-Pliocene provided increased sediment supply and overfilled the Amazonian foreland basin system. Finally, the flat-slab subduction of the Nazca ridge induced Pliocene (~4 Ma) uplift of the Fitzcarrald Arch and subdivided the Amazonian foreland basin into the northern and southern Amazonian foreland basins.

Introduction The sedimentary basins adjacent to the eastern side of the Central Andes form one of the best examples in the world of retroarc

Amazonia, Landscape and Species Evolution: A Look into the Past, 1st edition. Edited by C. Hoorn and F.P. Wesselingh. © 2010 Blackwell Publishing

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foreland basin systems (Fig. 5.1; Jordan et al. 1983; Jordan 1995; Horton & DeCelles 1997). Foreland basins are a favoured area for studying the interplay at different scales of tectonics, climate and sedimentation (see, e.g., Beaumont 1981; Burbank 1992; Jordan 1995; DeCelles & Giles 1996; Horton 1999; Catuneanu 2004 amongst many others) as they record the denudation of the adjacent mountain belt and hence the interaction between erosion and mountain growth. The stratigraphic record of foreland basins is generally very complete (Jordan 1995), and

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80ºW

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Fig. 5.1 Map delimiting the present-day location of the Amazonian foreland basin depozones. NAFB, North Amazonian foreland basin; SAFB, South Amazonian foreland basin. Modified from Roddaz et al. 2005b.

therefore they provide ample information on the geometry of thrust sheets, ages of movement on particular thrust sheets, rheology of the lithosphere, drainage history, denudation history, slab subduction rate and dip history, and climate history (Jordan et al. 2001). These processes all may have promoted isolated areas capable of fostering conditions for biodiversity development. The Andes are the second largest mountain belt in the world, spanning more than 50° of latitude with a maximum width of ~800 km and peak elevations exceeding 6.7 km. Regional

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shortening, mountain building and the initiation of the Andean foreland basin started in Late Cretaceous-Paleocene times (Balkwill 1995; DeCelles & Horton 2003; Barragan et al. 2005; Martin-Gombojav & Winkler 2008 and references therein). South of 22°S, although debated (see Jordan et al. 2007 and references therein), the Andean foreland basin initiated in the CretaceousEarly Tertiary (Arriagada et al. 2006) and migrated eastwards throughout the Paleogene (Carrapa & DeCelles 2008). In southern Bolivia, the Andean foreland basin defines a four-component foreland basin system (wedge-top, foredeep, forebulge and backbulge

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The Amazonian foreland basin system depozones, after DeCelles & Giles 1996) that propagated eastwards throughout the Tertiary since at least the Late Paleocene (DeCelles & Horton 2003). Two Miocene marine ingressions interrupted the continental foreland sedimentation. The first transgression occurred between 15 and 13 Ma and was tectonically and eustatically controlled; a second one, predominantly tectonically controlled, younger than 10 Ma, and was generated by tectonic loading of the Eastern Cordillera fold-and-thrust belt (Hernández et al. 2005). At least one other marine ingression occurred in the Western Amazon basin during the Eocene (Christophoul et al. 2002a; Hermoza et al. 2005b; Santos et al. 2008) but the extent and wider significance of this marine ingression have not been fully explored. In spite of the large size of Amazonian foreland basins, their Tertiary evolution received comparatively little attention. Some of the difficulties are the remote access, political instabilities, wide extension of sedimentary units into different countries (Bolivia, Peru, Ecuador and Colombia), difficulties in accessing subsurface data, poor stratigraphic control and numerous local formation names. Oil exploration started as early as the 1920s in the Amazonian regions of Peru and Ecuador, with a production boom arriving in the 1970s. Due to this renewed interest, many informal reports contain valuable regional data. Some recent studies have proposed regional basin analysis in Peru (Hermoza 2004; Hermoza et al. 2005a, 2005b), in Ecuador (Christophoul et al. 2002a) and in Colombia (Bayona et al. 2007; Parra et al. 2009) but lack stratigraphic homogenization. As a result, political frontiers have prevented an earlier synthesis of the Tertiary evolution of the Amazonian foreland basin. This chapter presents the first compilation of the Cenozoic evolution of Amazonian foreland basin that attempts to correlate the Paleocene to Recent development over the entire Colombian-toBolivian Amazonian foreland basin. The aims of this chapter are: (i) to present an overview of the stratigraphy and depositional environment of the Paleogene to Neogene infill of Bolivian to Colombian Amazonian foreland basin; (ii) to identify the main periods of foreland basin development and to discuss the controls on accommodation; and (iii) to emphasize how the sedimentary processes were controlled by the growth of the Andes.

63

An important difference between the pro- and retro-foreland basins is that retro-foreland basins are subjected to sublithospheric loads mainly caused by the drag force generated by viscous mantle corner flow beneath the retro-foreland (e.g. dynamic subsidence: Mitrovica et al. 1989; Gurnis 1992; Catuneanu et al. 1997; Pysklywec & Mitrovica 2000; Catuneanu 2004). Combined with orogenic supracrustal loading (load of the mountain belt exerted on the continental lithosphere), these are the primary subsidence mechanisms that control accommodation and sedimentation patterns in retro-foreland basin (DeCelles & Giles 1996; Pysklywec & Mitrovica 1999; Catuneanu 2004). Local-scale mechanisms such as reactivation of weak structures (Bayona & Thomas 2003), threedimensional (3D) configuration of the orogenic load (Whiting & Thomas 1994) or variable strengths of the lithosphere (Cardozo & Jordan 2001) can also play an important role in subsidence within a foreland basin. Orogenic loading leads to the partitioning of the foreland systems into four discrete depositional areas: the wedge-top, the foredeep, the forebulge and the backbulge depozones (DeCelles & Giles 1996) (Fig. 5.2). The wedge-top depozone is the mass of sediment that accumulates on top of the frontal part of the orogenic wedge, including ‘piggyback’ and ‘thrust top’ basins (DeCelles & Giles 1996). The foredeep depozone consists of the sediment deposited between the structural front of the thrust belt and the forebulge. The forebulge depozone is the broad region of potential flexural uplift between the foredeep and the backbulge depozones. The backbulge depozone is the mass of sediment that accumulates in the shallow but broad zone of potential flexural subsidence cratonward of the forebulge (DeCelles & Giles 1996). Renewed thrusting in the orogenic belt (orogenic loading) results in foredeep and backbulge subsidence and forebulge uplift (DeCelles & Giles 1996; Catuneanu 2004 and references therein), and the reverse occurs as orogenic load is removed by erosion or

Orogenic loading Foredeep

Forebulge

Load

Backbulge

Flexural uplift

+

Basic concepts Flexural subsidence

This section is intended for readers who are not familiar with foreland basin and basin analysis. Here we simply give basic definitions and concepts used in the chapter. For further details, the reader is referred to publications cited in this section. A foreland basin generally is defined as an elongate region of potential sediment accommodation that forms between a linear contractional orogenic belt and the stable craton, mainly in response to flexural subsidence that is driven by thrust-sheet loading in the orogen (Dickinson 1974; Beaumont 1981; Jordan 1995; DeCelles & Giles 1996). Foreland basins can develop on the subducting lithosphere in a forearc setting, when they are referred to as pro-foreland basins; or they can form on the overriding lithosphere (behind the orogenic belt), when they are termed retro-foreland basins (Catuneanu 2004 and references therein).

Hoorn_ch05_Final.indd 63

Orogenic unloading

Foreslop e

Foresag

Load

–

Flexural subsidence

Isostatic rebound

Fig. 5.2 Orogenic loading and unloading stages and the associated depozones in a retro-foreland basin. (+, −) refer to increases and decreases in orogenic load, respectively; see text for explanations. Modified from Catuneanu 2004.

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M. Roddaz et al.

extension (i.e. orogenic unloading, Catuneanu, 2004). The orogenic unloading stage causes the wedge-top and foredeep depozones to uplift and erode, referred to as the foreslope topographic zone, and the main zone of low sediment accumulation (foresag depozone) is located cratonward (Catuneanu et al. 1998) (see Fig. 5.2). The recognition of a forebulge depozone is thus of particular importance for reconstructing loading/unloading cycles. As forebulges are positive structures, their past existence has been inferred from progressively cratonward migration of distal unconformities and onlaps of the overlying sediments (Crampton & Allen, 1995; White et al. 2002). Although they are generally associated with non-deposition and/or erosion, forebulges can occasionally preserve thin and condensed sedimentary sequences (Crampton & Allen 1995; DeCelles & Horton 2003; Catuneanu 2004; Roddaz et al. 2005a; Dávila et al. 2007), which suggests that they can present some accommodation even during tectonic loading. Several causes can explain sediment deposition in the forebulge depozone including overfilling of the foreland basin (Crampton & Allen 1995; Dávila et al. 2007), temporal changes in the lithosphere rigidity (Garcia-Castellanos et al. 2002), overdensification of the lower crust (Leech, 2001) and large-scale dynamic subsidence (Catuneanu 2004). The forebulges can thus be the loci of interferences between long-wavelength dynamic subsidence and flexural uplift. It is difficult to discern the influence of these processes on accommodation, and this explains why the identification and exact localization of past forebulges remains a difficult task. Accommodation in a retro-foreland basin depends on the interplay of base-level changes and sediment supply and is mostly controlled by the interaction of tectonic and sublithospheric static and dynamic loading (e.g., Catuneanu et al. 1997; Catuneanu 2004). The degree to which the accommodation space is consumed by deposition is reflected in the depositional setting of the sedimentary record of the foreland basin system. Three stages are conventionally defined: underfilled, filled and overfilled, dominated by deep marine, shallow marine and fluvial environments, respectively (Sinclair & Allen 1992; Sinclair 1997). Foreland basins are generally viewed as evolving in a predictable way starting from an underfilled stage to finish with an overfilled configuration (Crampton & Allen, 1995). Jordan (1995) provided a different

Eastern Cordillera

Orogenic Loading

SAZ

Wedge-top dz

definition for continental foreland basins in which the underfilled basin is characterized by longitudinal drainage because of the subsidence in the foredeep due to tectonic loading and the overfilled basin is characterized by transverse drainage due to increasing erosion. Despite these differences in their definitions, both Jordan (1995) and Sinclair (1997) proposed an increase in tectonic shortening to explain the transition from underfilled to overfilled. In contrast, other authors (Blair & Bilodeau 1988; Heller et al. 1988; Burbank 1992; Heller & Paola 1992; Christophoul et al. 2003) suggest that this transition is mainly due to erosional unloading (isostasic rebound) succeeding tectonic loading.

Geological setting and stratigraphy of Amazonian foreland basin deposits The western Amazon drainage basin extends from southern Colombia to northern Bolivia. Since the Pliocene (Espurt et al. 2007; see also Chapter 6), the Amazonian foreland basin has been divided into two foreland basin systems (sensu DeCelles & Giles 1996): the North Amazonian foreland basin system and the South Amazonian foreland basin, separated by the Fitzcarrald Arch (see Fig. 5.1) (Roddaz et al. 2005b). The North Amazonian foreland basin system comprises the Colombian, Ecuadorian and Northern Peruvian foreland basins. The basins in the sub-Andean zone, including the Huallaga Basin, define the present-day wedge-top depozone and are separated from the foredeep depozone basins by the sub-Andean thrust front. The Oriente (Ecuador) and Marañón (Peru) delimit the foredeep depozone. We here include the Llanos foredeep (Colombia) as part of the Amazonian foreland until Late Miocene times (see Chapter 4); the Putumayo Basin (Colombia) and the Oriente basin (Ecuador) are considered as a single basin (Mora et al. 1998). Because the Oriente Basin is much bigger than the Putumayo Basin, we did not include the stratigraphy of the Putumayo Basin (Colombia); however, correlations were made where appropriate. To the east the foredeep depozones end with the high of the Iquitos forebulge. East of the Iquitos forebulge is the Pebas backbulge (Fig. 5.3; see also Fig. 5.1).

Llanos, Oriente, Marañón, Madre de Dios, Beni-Mamore basins

Tectonic subsidence Foredeep dz

Iquitos forebulge

Pebas backbulge

Beni forebulge-backbulge

Flexural uplift Forebulge dz

Backbulge dz

Fig. 5.3 Schematic cross-section depicting the concept of foreland basin system sensu DeCelles & Giles (1996). Depozone is labelled dz. SAZ, sub-Andean Zone.

Hoorn_ch05_Final.indd 64

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The Amazonian foreland basin system The South Amazonian foreland basin system comprises the Southern Peruvian and Northern Bolivian foreland basins. The sub-Andean zone basins define the present-day wedge-top depozone and are delimited from the foredeep depozone by the sub-Andean thrust front (see Figs 5.1 & 5.3). The Madre de Dios and Beni-Mamoré Basins define the foredeep depozone. As the topographic expression of the forebulge is minor, it is difficult to separate the forebulge and the backbulge depozones. These two depozones are grouped and termed Beni forebulge-backbulge depozones (see Figs 5.1 & 5.3).

Cenozoic sedimentary evolution of the Colombian foreland basin system Late Cretaceous to Early Paleocene depositional systems Following the widespread Cretaceous, rift-related shallow marine deposition in Colombia, sea withdrawal and establishment of nonmarine deposition up to ~1500 m thick is recorded in shallowing-upward Maastrichtian to Paleocene sedimentary units. This succession represents a doubly tapered geometry with a maximum thickness of ~1500 m along the western margin of the Eastern Cordillera, which disappears toward the Central Cordillera, to the west, and the Llanos Basin to the east (Cooper et al. 1995; Gómez et al. 2003, 2005). This doubly tapered geometry is locally irregular in the foredeep (Sarmiento 2002) and, although not properly documented, minor intra-foredeep structures cannot be discounted. Maastrichtian units To the west, in the Middle Magdalena Basin (see Fig. 5.4), ~80 m of calcareous mudstones and interlayered thin, cross-laminated sandstones comprise the Buscavida Formation (Gómez et al. 2003). This unit is overlain by up to ~80 m of multistoreyed cross-bedded conglomerates, fine-grained sandstones, minor mudstones, and foraminiferan- and mollusc-rich limestones that were interpreted as fan-delta deposits (Gómez & Pedraza 1994; Gómez et al. 2003). To the southwest, in the Upper Magdalena Valley Basin, lateral equivalent units (La Tabla and Monserrate Formations; Montes et al. 2005; Ramon & Rosero 2006) are continental (braided river deposits; Ramon & Rosero 2006). Palaeocurrent and provenance data for these units indicate a western source located in the present-day Central Cordillera (Gómez et al. 2003; Montes et al. 2005; Ramon & Rosero 2006). To the east, in the Eastern Cordillera and Llanos Basin, sediments consist of ~250 m-thick easterly derived, laterally continuous, fine-to-coarse-grained sandstones with lenticular and flaser bedding. At the top of this sequence the Labor and Tierna Formations are characterized by sandstones with large-scale crossstratification, which were interpreted as tidal flat deposits (Pérez & Salazar 1978). Paleocene units The Cretaceous units in the Eastern Cordillera are conformably overlain by the Guaduas Formation, which is constituted by up to 1400 m of variegated mudstones interlayered with

Hoorn_ch05_Final.indd 65

65

fine-to-medium-grained, cross-bedded sandstones and minor coal beds (Fig. 5.4). The Guaduas Formation progressively thins eastwards, beneath the first regional unconformity of the Llanos Basin. The formation is only ~50 m thick in the eastern foothills of the Eastern Cordillera and is absent further to the east (Santos et al. 2008). The Guaduas Formation and the laterally equivalent units in the Magdalena Valley Basins (Guaduala, Seca and Lisama Formations) register the onset of protracted non-marine deposition along fluvial plains. The Lower Hoyon Formation, which is restricted to the westernmost Middle Magdalena Valley Basin, consists of clast-supported, horizontally bedded conglomerates that are interlayered with lenticular-shaped cross-bedded sandstones, interpreted as alluvial fan deposits (Gómez et al. 2003). Gravel petrography suggests a sediment source situated in the west, the Central Cordillera, and the first appearance of plutonic lithic fragments.

Late Paleocene to Early Eocene Upper Paleocene-Lower Eocene units define an eastwardtapering wedge onlapping onto the Upper Cretaceous-Early Paleogene units toward the Llanos Basin. From west to east, this sedimentary wedge includes (see Fig. 5.4): 1

Eastward thinning, west-southwesterly derived, clast- and matrix-supported conglomerates and interbedded reddish mudstones and cross-bedded sandstone lenses that comprise the upper part of the Hoyón Formation in the Middle Magdalena Valley Basin (Gómez et al. 2003). These rocks coarsen upwards and represent alluvial fan sediments deposited in a proximal foredeep (Gómez et al. 2003). 2 An up to ~1200 m-thick fining-upward sequence evolving from sandstone-dominated braided plain deposits of the Cacho Formation to the variegated mudstones and sandstones of the Bogotá Formation, deposited in fluvial plains (Hoorn et al. 1987) within the foredeep depozone in the present axial Eastern Cordillera. 3 A lateral equivalent, ~700 m-thick fining upward sequence including the distal alluvial and coastal plain deposits of the sandstone-dominated Barco and mudstone-dominated Los Cuervos Formations (e.g. Santos et al. 2008), which are interpreted as the distal part of the Late Paleocene foredeep (Parra et al. 2009).

Eocene Lower Eocene units are absent in the Middle Magdalena Valley Basin (see Fig. 5.4). Middle to Upper Eocene rocks correspond to the Almacigos Member of the San Juan de Río Seco Formation (see Fig. 5.4). East of the Middle Magdalena Valley Basin, this unit is ~900 m thick, and onlaps westwards onto the substratum of the basin, defining a regional unconformity (Gómez et al. 2003). The Almacigos Member is made up of fining-upward, cross-bedded, conglomeratic sandstones with beds roughly metre scale in thickness (Gómez et al. 2003). To the east, in the axial Eastern Cordillera, Eocene units include the ~1000 m-thick Regadera and Usme Formations

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66

M. Roddaz et al. Series

Southern Middle Magdalena Valley MESA

Late

Foothills

GUAYABO

TILATÁ

Middle

SANTA TERESA

LEÓN

SAN JUAN DE RIOSECO

GU GU

Late

HIATUS

CARBONERA

Early

MIRADOR

EOCENE

USME Middle

REGADERA Early

BOGOTÁ Late Middle

HOYÓN

Early

MAASTRICHTIAN

PALEOCENE

PALEOGENE

Late

LATE CRETACEOUS

Llanos Foreland

HONDA

Early

OLIGOCENE

Eastern Cordillera SABANA

PLIOCENE

MIOCENE

NEOGENE

Pleistocene

CACHO GUADUAS

CIMARRONA

CUERVOS BARCO

TIERNA

BUSCAVIDAS

LEYENDA GU

Growth unconformity

Angular unconformity

late Cenozoic uplift

Facies change

Alluvial fan conglomerates

Non-marine sandstones Delta and coastal plain sandstones

Shallow marine sandstones Shallow marine mudstones

Non-marine mudstones

Delta and coastal plain mudstones

Volcaniclastic deposits

Lacustrine siltstone

Fig. 5.4 Stratigraphic overview (time–distance diagram, or Wheeler diagram) of the Paleogene to Neogene Colombian foreland basin. Modified from Parra et al. (2009).

(Hoorn et al. 1987). The Early to Middle Eocene Regadera Formation is 650–750 m thick and consists of tens of metrethick packages of interlayered (i) cross-bedded conglomeratic sandstones, clast-supported pebble conglomerates, and thin variegated mudstones, and (ii) variegated mudstones with minor, lens-shaped, fine-grained sandstones. The Regadera Formation was interpreted as deposits formed in a braided alluvial environment (Hoorn et al. 1987; Kammer 2003). The Late Eocene-Early Oligocene Usme Formation is ~300 m thick and is unconformably overlain by the Early Miocene Tilatá Formation. The lower 100 m of the Usme Formation consist of tens-of-metres-thick brownish and greyish mudstones; the upper 200 m are formed by medium-grained sandstones, variegated mudstones and minor coal interbeds. A deltaic depositional environment has been proposed for the Usme Formation (Hoorn et al. 1987). In the eastern part of the Eastern Cordillera, Eocene rocks are ~400 m thick and correspond to the Mirador Formation and the lower part of the Carbonera Formation (C8 Member; Parra et al. 2008; see next section). The Lower Mirador Formation consists of ~250 m of multistoreyed, metre-thick, cross-bedded

Hoorn_ch05_Final.indd 66

sandstones and sandy pebble conglomerates interbedded with minor variegated mudstone that was deposited in braided fluvial systems. To the east, the Eocene units progressively onlap onto the Llanos Basin substratum, resulting in a regional unconformity with an eastward increase in the chronostratigraphic gap (Jaramillo 2007). Thus, Eocene deposits in central Colombia display a double tapered shape along a transverse WNW–ESE section (see Fig. 5.3). Lower Eocene units are restricted to the central part of this wedge; they outcrop in the Eastern Cordillera but are absent in the Middle Magdalena Valley Basin and Llanos Basin. It is important to notice that the Late Eocene shaly units of the Upper Mirador Formation and Lower Carbonera C8 member indicate a change from fluvial conditions to marine-influenced conditions (Pulham et al. 1997).

Oligocene In the Eastern Cordillera, Oligocene units are restricted to the uppermost ~200 m of the Usme Formation (Hoorn et al. 1987;

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The Amazonian foreland basin system Parra et al. 2009), which are unconformably overlain by Early Miocene fluvial strata of the Tilatá Formation (see Fig. 5.4). Further to the east, Oligocene deposits of the Carbonera Formation show a progressively eastward thinning succession (Cooper et al. 1995; Gómez et al. 2005; Parra et al. 2009). In the eastern foothills of the Eastern Cordillera, this unit conformably overlies the Mirador Formation and has a maximum thickness of ~3400 m, which diminishes by a half ~100 km basinward (Bayona & Thomas 2003; Bayona et al. 2007). In the proximity of the Eastern Cordillera, the Carbonera Formation mainly consists of two interlayered facies associations (Parra et al. 2008) (see Fig. 5.4): 1

2

Up to 10 m-thick, thickening and coarsening-upwards intervals composed of tabular sandstone with minor dark-grey mudstone interbeds, common flaser and lenticular lamination, laminae rich in organic matter and dewatering structures; up to 100 m-thick intervals of dark-grey to greenish mudstone with occasional bioturbation and coal interbeds.

Freshwater molluscs are occasionally present in either of the facies associations. Punctuated marine influence is registered by the presence of discrete, thin levels with foraminiferal linings. According to the relative abundance of these lithofacies, the Carbonera Formation is divided into eight members (C8 to C1 members, from older to younger) of alternating sandstonedominated and mudstone-dominated deposits, that are interpreted as tidally influenced lacustrine and coastal plain deposits (Parra et al. 2009). Westward facies variations toward braided fluvial deposits in the lower part of the Carbonera Formation (C7–C6 members) have been documented in the westernmost reaches of the eastern foothills region ((Parra et al. 2009). Finally, a coarsening-upwards interval of fluvial deposits constitutes the ~uppermost 1 km of the unit (C1 member). Toward the east, the basal deposits of the Carbonera Formation become younger and overlie a progressively older substratum (Gómez et al. 2005; Santos et al. 2008).

Miocene to Present The Middle Miocene León Formation consists of a package of ~500 m of thin, laminated dark-grey mudstones and conformably overlies the Carbonera Formation (see Fig. 5.4). Subsurface and well data in the Llanos Basin indicate lateral continuity, albeit minor facies changes occurred across the Llanos Basin. Towards the north and west of the basin these changes include the presence of sandstone interbeds a few metres thick (Cooper et al. 1995). Scarce interlayered, thin fossiliferous horizons show relative high abundances of freshwater molluscs (Parra et al. 2006). Brackish-water palynological associations, dinoflagellate cysts and foraminiferal test linings (Bayona et al. 2007) suggest the punctuated influence of brackish waters within an otherwise continuous lacustrine environment. The León Formation is overlain by the Upper Miocene Lower Guayabo Formation, which is composed of a succession of very thick beds of channelized, texturally immature sandstones, conglomeratic sandstones and

Hoorn_ch05_Final.indd 67

67

conglomerates interbedded with very thick beds of sandy mudstones and mudstones where palaeosol development is common. The upward increase in grain size and the upward appearance of channelized conglomeratic levels in the top of the Lower Guayabo Formation suggest a change in depositional environment from meandering to braided rivers. The Upper Guayabo Formation consists of clast-supported cobble to pebble conglomerate that is organized in very thick beds and interbedded with isolated dark mudstones beds, of less than 2 m thick. The age of the lowermost dark mudstones level of the Upper Guayabo Formation is estimated as Late Miocene-Early Pliocene (Mora 2007). Thus, the upper parts of this unit are likely to have been deposited during the Pliocene (see Fig. 5.4).

Cenozoic sedimentary evolution of the Ecuadorian foreland basin system Paleocene The Tena Formation is poorly documented but known to consist of monotonous reddish shales interbedded with thin beds of fine sandstones that were deposited in a distal meandering floodplain (Fig. 5.5). The age is based on Paleocene charophytes that were found in the upper part of the formation (Fauchet & Savoyat 1973). The sediments of the Tena Formation derived from the Eastern Cordillera (Ruiz et al. 2004, 2007; Martin-Gombojav & Winkler 2008), and the formation is thought to be an analogue of the Rumiyaco Formation in the Colombian Putumayo Basin (Mora et al. 1998).

Eocene The Tiyuyacu Formation (Tschopp 1953; Baldock 1982) is divided into two members. The Lower Member has been dated by zircon fission track analysis at 51 ± 5 Ma (Ruiz et al. 2004). In the north of the sub-Andean zone, the lower part of the Upper Member contains a tuffaceous layer dated at 46 ± 0.4 Ma (argonargon [Ar/Ar] dating on biotite, Christophoul et al. 2002a). The Tiyuyacu Formation is fed by a higher grade of metamorphic rocks of the Eastern Cordillera than those of the Tena Formation (Martin-Gombojav & Winkler 2008). This unit can be correlated with the Pepino Formation in the Colombian Putumayo Basin (Mora et al. 1998). The Tiyuyacu Lower Member is variable in thickness (150–548 m) and the base is a regional erosional unconformity that can be identified in both outcrops and seismic sections (Christophoul et al. 2002a) (see Fig. 5.5). This member consists of conglomerates and reddish shales. The conglomerates are composed of 80–90% reddish chert clasts and 10–20% white quartz pebbles and metamorphic rock fragments (Marocco et al. 1996; Valdez Pardo 1997). These deposits range from conglomerates to mudstones and are organized in typical channel-filling, fining-upward, 10 m-thick sequences. Each base of a sequence corresponds to conglomerates and sandstones displaying trough cross- and planar stratifications and ripple crosslaminations deposited in gravel bars (Miall 1996). These deposits

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68

M. Roddaz et al. West

Series

Oriente Basin Alluvial fan

PLIOCENE

Chambira Fm

Erosion

?

Fan delta Middle

Curaray Fm Arajuno Fm

Early OLIGOCENE

?

Growth strata

Late MIOCENE

NEOGENE

East

Mera Fm

PLEISTOCENE

Late

Fluvial

Distal floodplain deposits

Chalcana Fm

Early

Orteguaza Fm

Upper Tiyuyacu Mb. EOCENE

PALEOGENE

Late

Middle

Early

PALEOCENE

Lower Tiyuyacu Mb. Late

Tena Fm

Middle Early

Legend Marine deposits

Continental deposits

Clasts

Trough cross-stratification

Mud clasts

Palaeosol

Erosional surface

Mudstones

Lenticular/wavy bedding

Sandstones

Channel

Limestones

Fig. 5.5 Stratigraphic overview (Wheeler diagram) of the Paleogene-Neogene Ecuadorian foreland basin. Fm, formation; Mb, member.

grade upwards into coarse and then fine sandstones, indicating downstream accretion macroforms and lateral accretion macroforms (Miall 1996). The top of the sequence consists of fine siltstones and mudstones and palaeosols characterizing floodplain and overbank deposits. These elemental sequences are repeated, showing an overall fining upward trend. The Tiyuyacu Upper Member is variable in thickness and ranges between 150 and 548 m. This member outcrops continuously in the sub-Andean zone and its facies associations are quite similar to the Lower Tiyuyacu Member (see Fig. 5.5). The main difference is that 90% of the conglomerates are composed of well- to verywell-sorted clasts of white vein quartz in a rare blue clay matrix (Christophoul et al. 2002a; Ruiz 2002).

Hoorn_ch05_Final.indd 68

Early Oligocene The Early Oligocene Orteguaza Formation, formerly known as the Chalcana Formation (Tschopp 1953), conformably overlies the Upper Tiyuyacu Member (see Fig. 5.5) and has a variable thickness that ranges between 40 and 341 m. Palynological dating by Zambrano et al. (1999) suggest a Late Eocene to Early Oligocene age for the Orteguaza Formation (Table 5.1). The provenance of the Orteguaza Formation is similar to that of the Tiyuyacu Formation (Martin-Gombojav & Winkler 2008). In the sub-Andean zone, the Orteguaza Formation consists of fine fluvial deposits similar to the overlying Chalcana Formation. Eastwards within the basin, the Orteguaza Formation consists

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Hoorn_ch05_Final.indd 69

Table 5.1 Overview of main features of the different parts of the foreland basin system. Biostratigraphic and radiometric data are indicated separately. Foreland basin

Formations

Age

Barco

Early Paleocene

Los Cuervos

Middle-Late Paleocene

Mirador

Characteristic fossils

Radiometric dating

Depositional setting

Palaeocurrent directions

Representative outcrops

Colombia Fluvial braided rivers

Guadualera Creek

Bombacacidites annae, Foveotricolpites perforatus, Proxapertites operculatus, Tetracolporopollenites maculosus (Jaramillo et al. 2005)

Floodplain deposits

Piñalerita Creek

Eocene

Tetracolporites maculosus, Spinozonocolpites grandis (Jaramillo et al. 2009)

Coastal plain and estuarine deposits

Piñalerita Creek

Carbonera C8

Early Oligocene

Echitriporites trianguliformis orbicularis, Nothofagidites huertasii (Parra et al. 2009)

Coastal plain and estuarine deposits

Piñalerita Creek

Carbonera C7-C6

Early to Late Oligocene

Sandufouria seamrogiformis, Magnastriatites grandiousius, Mauritiidites franciscoi minutus and Verrucatosporites usmensis, Cicatricosisporites dorogensis (Parra et al. 2009)

Coastal plain and estuarine deposits

Carbonera C5-C1

Early Miocene

Coastal plain and estuarine deposits

Humea and Gazaunta Rivers

León

Middle Miocene

Lacustrine

Gazamumo River

Guayabo

Late MiocenePliocene

Braided river deposits and alluvial fans

Tena Fm

Paleocene

Tiyuyacu Lower Mb

Lower to Middle Eocene

Tiyuyacu Upper Mb

Middle to Late Eocene

NE–E (Parra et al. 2009)

E, NE–SW

Guadualera-Gacenera Creeks

Tontogüe Creek

Ecuador Charophytes (Faucher & Savoyat, 1973)

Floodplain deposits Fluvial deposits Tuff 46 ± 0.4 Ma age (Ar/Ar dating on biotite; Christophoul et al. 2002a)

Braided river deposits

(Continued)

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Hoorn_ch05_Final.indd 70

Table 5.1 Continued. Foreland basin

Formations

Age

Characteristic fossils

Radiometric dating

Palaeocurrent directions

Representative outcrops

Orteguaza Fm

Late Eocene-Early Oligocene

Palynology (Zambrano et al. 1999): Deltoidospora sp., Syncolpites sp., Cicatricosisporites sp., Cicatricosisporites dorogensis, Verrucatosporites sp., Striatricolpites catatumbus, Laevigatosporites sp., Retitricolpites sp., Magnaperiporites spinosus, Stephanoporites sp., Spinozonocolpites echinatus, Magnastriatites howardi, Verrucatosporites usmensis, Monocolpites sp.

Marine deposits

Chalcana Fm

Late Oligocene to Early Miocene

Miospore (Laevigatosporites sp.) and freshwater alga (Edwards, 1983) Sporomorphs (Cicatricosisporites criatus, Magnastriatites howardi and Verrucatosporites usmensis (Lorente 1986) Charophytes oogones (Tectochara cf. ucayaliensis)

Meandering or anastomosed river deposits

NW–SE, N–S (Christophoul et al. 2002b)

Napo and Aguarico Rivers

Arajuno Fm

Middle to Late Miocene

Foraminifera Bathysiphon sp., Psammosphaera sp., Trochammina sp. and Valvulina? (Bristow & Hoffstetter, 1977) Echiperiporites and rare Echitricolporites maristellae (Villano-2 well) Freshwater fern spore (Azolla)

Gravel-wandering river

WNW–ESE to WE and N–S (Christophoul et al. 2002b)

Arajuno and Napo Rivers

Chambira Fm

Middle to Late Miocene

Verrucatotriletes etayoi (Edwards, 1983) Crassoretitriletes vanraadshoovenii (Muller et al. 1987)

Gravel braided river

SW–NE, E–W

Pastaza Depression, Napo river

Curaray Fm

Early to early Late Miocene

Foraminifera: Ammobaculites (2 spp.), Sigmoilina sp., Polystomella sp., Rotalia sp. (Tiputini well) Cytheridea cf. kollmani, Cyprideis aff. howei (today Vetustocytheridea bristowi) in Tiputini well Retitricolporites guianensis, Zonocostites ramonae, Laevigatosporites sp. and foraminiferan Ammonia beccarii (Vinita well) Crocodilian and turtles

Tidal deposits

Mera Fm

Plio-Pleistocene

Apatite and zircon fission track dating (AFT and ZFT) ~ 22 Ma (Ruiz et al. 2004)

Depositional setting

Lahar deposits

Curaray River, Napo River

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Northern Peru Yahuarango Fm

Paleocene

Floodplain and lacustrine deposits

Ucayali Basin (Kummel, 1946)

Lower Pozo Mb (Pozo sands)

Middle EoceneEarly Oligocene

Ostracods, foraminifers, gastropods and palynomorphs (Sánchez & Herrera 1998; Seminario & Guizado 1976; Williams 1949)

Estuarine and shoreface deposits

Shapaja area (6.58555°S, 76.30250°W, Huallaga basin; Hermoza et al. 2005) Marañón foredeep (wells)

Upper Pozo Mb (Pozo shale)

Middle EoceneEarly Oligocene

Ostracods, foraminifers and pollen of Eocene-Oligocene age (Sánchez & Herrera 1998; Seminario & Guizado 1976; Williams 1949)

Shallow clastic shelf deposits

Juanjui-Tocache road (Peru, 7.23471°S, 76.74628°W; Hermoza et al. 2005 Marañón foredeep (wells)

Chambira Fm

Oligocene-Middle Miocene

Charophytes (Tectocara supraplana) in Santa Lucia 2X well (Hermoza 2004)

Tide-influenced fluvial deposits and distal floodplain deposits

Tarapoto–Bellavista road (6.70905°S, 76.28790°W) Bellavista area (7.07166°S, 76.57440°W) Marañón foredeep (wells) (Hermoza et al. 2005)

Ipururo Fm

Middle MiocenePliocene

Deltaic to continental deposits

SAZ only, Rio Sisa and Saposoa area Sacanche area (7.07235°S, 76.70231°W) Juanjui-Tocache road (7.53722°S, 76.68028°W) (Hermoza et al. 2005)

Pebas Fm

Early Mioceneearly Late Miocene

Juanjui Fm

Plio-Pleistocene

Fluvial to alluvial fan deposits

Only SAZ (Hermoza et al. 2005)

Marañón Fm

Late MiocenePliocene

Floodplain deposits

Marañón foredeep (wells) (Hermoza 2004; Wesselingh et al. 2006)

Corrientes Fm

Pleistocene

Floodplain deposits

Marañón foredeep (wells)

43.0 ± 9.9 Ma (Carmen 1508 well) and 35.1 ± 4.4 Ma (Corrientes 115) AFT dating on tuffs (Hermoza 2004)

See Chapter 18

(Continued)

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Table 5.1 Continued. Foreland basin

Formations

Age

Characteristic fossils

Radiometric dating

Depositional setting

Palaeocurrent directions

Representative outcrops

Southern Peru and northern Bolivia Hermoza (2004) Huayabamba Fm

Paleocene

Bala Fm

Oligocene-Miocene

Quendeque Fm

Late Miocene

Charqui Fm

Late Miocene

Ipururo/Madre de Dios Fm

Miocene

Masuko and Tutumo Fm

Plio-Pleistocene

Madre de Dios Fm

Pliocene

Fm, Formation; Mb, Member; SAZ, sub-Andean Zone.

Charophytes (Sphaerocharas and Tectochara supraplana) (Gutiérrez 1975) Charophytes (Sphaerocharas sp., Porochara gildemeisteri costata, P. gildemeisteri solensis, Sphaerocharas huaroensis) in Candamo 78–53-1X well (Carpenter & Berumen 1999) Charophytes (Nitellopsis supraplana) in Puqiri Syncline (Inambari River, Madre de Dios Basin; Hermoza 2004)

Floodplain and lacustrine deposits

Puquiri syncline, Inambari River (Madre de Dios Basin) (Hermoza 2004)

Charophytes (Tectochara ucayaliensis coronata) Foraminifera (Bathysiphon) in Candamo 78–53-1X well (Carpenter & Berumen 1999)

Fluvial deposits locally tideinfluenced

Southern Peruvian and northern Bolivian SAZ

8.7 ± 0.9 Ma and 7.96 ± 0.58 Ma (Ar dating on micas) in northern Bolivian SAZ (14.71°S, 67.58°W; Strub 2006; Strub et al. 2005)

Braided and meandering river deposits

Southern Peruvian and Northern Bolivian SAZ

9.01 ± 0.28 Ma (Ar dating on feldspars) (Campbell et al. 2006)

Tide-dominated deltas and estuaries

Madre de Dios foredeep

Fluvial to alluvial fan deposits

Southern Peruvian and northern Bolivian SAZ

Braided and meandering river deposits

Madre de Dios foredeep

Ambrosia sp., Multimarginites vanderhammeni, Kuylisporites waterbolki, Caryophyllaceae, Proteaceae, Corsinipollenites oculus noctis and Compositae/Polygonum (Mobil Oil Corp., 1998)

3.12 ± 0.02 (Ar dating on feldspars and biotites) (Campbell et al. 2006)

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The Amazonian foreland basin system of marine deposits composed of greenish shales and mediumto-coarse, locally glauconitic, sandstones (Christophoul et al. 2002a). Sedimentary structures such as flaser and wavy bedding, 2D ripple marks and trough cross-beddings indicate tideinfluenced deposition (Christophoul et al. 2002a) within a clastic marine depositional environment. As the Orteguaza Formation overlies continental deposits of the Tiyuyacu Upper Member, its base corresponds to a transgressive surface. Well log sequential analyses suggest that the Orteguaza Formation is composed of two transgression–regression cycles (T–R cycles) (Christophoul et al. 2002a). At the end of the second T–R cycle, the reddish shales of the Chalcana Formation rapidly prograded throughout the Oriente Basin.

Late Oligocene to Miocene The Late Oligocene-Miocene formations are formed of sediments issued from both the Eastern and Western Cordilleras (MartinGombojav & Winkler 2008, and references therein). Chalcana Formation The Chalcana Formation is Late Oligocene to Early Miocene in age (see Table 5.1) and mostly consists of reddish shales intercalated with rare fine-grained and thin sandstone beds displaying trough cross-bedded stratifications and horizontal laminations (see Fig. 5.5). These lithofacies represent floodplain fines and crevasse channels (Miall 1996) that possibly characterized the inter-distributaries of a sandy, low-sinuosity, meandering or anastomosed river system, similar to the present-day Amazonian plain. Palaeocurrent measurements show two main directions, NW–SE and N–S, which may be interpreted as transverse river systems perpendicular to the Andean proto-cordillera, debouching into river systems parallel to it. The thickness of the Chalcana Formation is variable (255–455 m). Arajuno Formation The contact between the Chalcana Formation and the overlying Arajuno Formation (Tschopp 1953) is conformable and gradual (Campbell 1970). Based on spores and fossil assemblages (see Table 5.1), its base is ascribed to late Early Miocene Zone 27 (Late Aquitanian-Burdigalian), with an age ranging from ~22 to 16.2 Ma (Muller et al. 1987; Rull 2002). Fission track dating on volcanic zircons and apatites from the base of the formation yields Early Miocene ages of ~22 Ma (Ruiz et al. 2004); the age of the upper part is less constrained. The Arajuno Formation is essentially composed of fine-to-coarse-grained sandstones with trough cross-bedded stratifications and horizontal laminations and conglomerates displaying horizontal bedding and trough cross-stratifications interbedded with minor siltstone beds that represent lateral accretion and downstream accretion deposits (Miall 1996). These associations of lithofacies can be interpreted as the distal part of gravel-wandering rivers. Palaeocurrent measurements indicate two drainage directions, WNW–ESE to W–E and N–S, as in the Chalcana Formation.

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73

Chambira Formation The Chambira Formation is of Middle to Late Miocene age, based on biostratigraphic markers (see Table 5.1) and stratigraphic relations with the underlying Arajuno Formation. Previous interpretations attributing a Pliocene age to this formation (Tschopp 1953) cannot be entirely ruled out but might only apply to the uppermost part of the formation. The Chambira Formation is composed of quartz pebble-bearing conglomerates included in a quartz-rich argillaceous matrix. The basal part contains trough cross-bedded and matrix-supported conglomerates. The upper part is composed of conglomerates with horizontal and trough cross-stratifications grading up to trough cross-bedded and ripple cross-laminated sandstones and massive siltstones (see Fig. 5.5). These assemblages characterize high-energy gravel-braided rivers with frequent mudflows (Miall 1996). Palaeocurrents indicate main flow directions, ranging from SW–NE to E–W, i.e. transverse drainage. Curaray Formation The Curaray Formation is of Early to early Late Miocene age based on the biostratigraphic markers and correlations with other, radiometrically dated formations. For instance, the Loyola Formation was dated at 13.9 ± 1.4 and 11.1 ± 1.0 Ma (Serravallian-Early Tortonian) and the Mangán Formation at 9.9 ± 1.2 to 9.5 ± 1.0 Ma (Tortonian: Hungerbuhler et al. 2002). The Middle to Late Miocene age confirms that the Curaray Formation is the easterly lateral equivalent of the Arajuno and Chambira Formations (see Fig. 5.5). This formation is made up of sandy to silty tidalites, containing crocodilians and marine turtles (Bristow & Hoffstetter 1977, but see Chapter 16 for the putative marine character of Neogene South American turtles), which indicate deposition in a tidal environment.

Pliocene to Present The Mera Formation unconformably overlies the Chambira and Arajuno Formations and is covered by volcaniclastic deposits (see Fig. 5.5). The age of the Mera Formation is poorly constrained and a Plio-Pleistocene age has been ascribed due to its stratigraphic position. The oldest dated deposits are 40,580 ± 1030 years BP in age (14C dating on charcoal; Bes de Berc et al. 2005). Typically, the lower part of the formation is composed of wellsorted rounded clasts (mainly volcaniclastics with minor metamorphic fragments from the Eastern Cordillera) that range from 1 cm to 20–50 cm, and are included in a volcanic sandy matrix. These sediments are arranged into at least three units, each 15 m thick, with poorly convex erosional bases, corresponding to wide and shallow channels. The middle part of the formation (20 m) is made up of unsorted angular andesitic clasts (60%), which are included in an ash-rich silty-to-sandy matrix. These beds are likely to represent lahars deposits. The upper surface of the lahars deposits is oxidized and hardened and corresponds to the Mera surface (Bes de Berc et al. 2005). The Mera Formation mainly crops out at the apex of the Pastaza Megafan (Räsänen et al. 1992) where it reaches its maximum thickness. The thickness decreases downstream along the Pastaza River.

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M. Roddaz et al. tuffaceous sandstones of the base of the Pozo Formation gave two stratigraphic ages at 43.0 ± 9.9 Ma and 35.1 ± 4.4 Ma (Hermoza 2004). The Pozo Formation is divided into two members, a lower sandy member (Lower Pozo Member or Pozo Sand) and an upper muddy member (Upper Pozo Member or Pozo Shale), both outcropping continuously in the sub-Andean zone and in the Marañón foredeep (see Fig. 5.6).

Cenozoic sedimentary evolution of the Peruvian and northern Bolivian foreland basin system Paleocene The Yahuarango Formation (northern Peru) and the Huayabamba Formation (southern Peru) are poorly dated and consist mainly of red siltstones and mudstones forming distal fluvial deposits (Figs 5.6 & 5.7) (Gil 2001; Hermoza 2004). Paleocene deposits are not encountered in Bolivia.

Lower Pozo Member In the sub-Andean zone the lower Pozo Member reaches up to 20 m in thickness. The base of the formation consists of unconsolidated conglomerates composed of Cretaceous sandstones, Paleozoic quartzites, and tuffaceous sandy clasts in a well-sorted sandy matrix, which are interpreted as lag pebble deposits (Hermoza et al. 2005b). Immediately following these lag pebble deposits is a succession of 30 cm-thick fining-upward sandy sequences composed of coarse-to-medium-grained sandstones with tidal bundles, sigmoid

Eocene Northern Peru: Pozo Formation Based on biostratigraphic markers, the Pozo Formation is Eocene-Oligocene in age (Williams 1949; Seminario & Guizado 1976; Sánchez & Herrera 1998). Apatite fission track dating on

Series

West (SAZ) Juanjui Fm

East (lquitos Fb.)

Corrientes Fm Marañón Fm

?

Growth strata

Pebas Fm

Middle Upper Chambira Fm

Fan delta

Early OLIGOCENE

Marañón foredeep

Alluvial fan

Late MIOCENE

NEOGENE

Q PLIOCENE

Lower Chambira Fm

Late

Distal floodplain deposits

Early Late

EOCENE

Middle

Lower Pozo (sand) Mb. Early

PALEOCENE

PALEOGENE

Upper Pozo (shale) Mb.

Late

Yahuarango Fm

Middle

?

Early

?

?

?

?

Legend Marine deposits

Continental deposits

Clasts

Trough cross-stratification

Mud clasts

Palaeosol

Erosional surface

Mudstones

Lenticular/wavy bedding

Sandstones

Channel

Limestones

Fig. 5.6 Stratigraphic overview (Wheeler diagram) of the Paleogene-Neogene northern Peruvian foreland basin. Fm, formation; Mb, member; SAZ, sub-Andean zone.

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The Amazonian foreland basin system Upper Pozo Member

laminations, planar foresets, and herringbone cross-stratifications (Hermoza et al. 2005b). These sandy sequences represent shoreface deposits (i.e. marine deposits; Hermoza et al. 2005b). In the Marañón foredeep, the thickness of the Lower Pozo Member varies from 20 to 56 m. The Lower Pozo Member consists here of well-sorted sandstones intercalated with silts and grey shales. An 18 m-long core of the Lower Pozo Member (Carmen1508 well; Hermoza 2004) showed that the upper part of the Lower Pozo Member is composed of well-sorted medium-to-coarse-grained sandstones (8 m thick) with abundant sigmoid, planar and trough cross-bedded laminations, followed by fine sandstones and grey to black siltstones and muds displaying flaser and lenticular bedding. Both sandstones and siltstones are strongly bioturbated. The Lower Pozo Member sediments were deposited within a tide-influenced deltaic and estuarine environment. Overall and in each part of the Marañón foredeep, the Lower Pozo Member defines a regressive system tract followed in the distal part by a transgressive system tract (Hermoza 2004).

Series

West (SAZ)

Madre de Dios/Beni foredeep

East

Alluvial fan Masuko Fm

Late

?

Growth strata

Middle

OLIGOCENE

Madre de Dios Fm ?

Quendeque Fm Tide-influenced

?

? Early

EOCENE

?

?

Bala Fm

?

Late

? Early

?

Late

Middle

?

?

Lower Pozo (sand) Mb.

Early

PALEOCENE

PALEOGENE

In the sub-Andean zone, the Upper Pozo Member consists of a succession of reddish/greenish shales associated with sandstones and shallow marine limestones containing marine ostracods and foraminiferans (see Table 5.1) that disappear westwards. These sediments were deposited in a shallow clastic shelf environment (Hermoza et al. 2005b). In the Marañón foredeep, the thickness of the Upper Pozo Member ranges from ~50 m to 156 m. It consists of a succession of intercalations of black-to-grey and green shales and siltstones with abundant glauconite and occasional pyrite. Limestone levels can also occur. These sediments are interpreted to represent shallow marine deposits. In the western part, the Upper Pozo Member is interpreted as regressive system tracts at its base followed by transgressive system tracts, whereas in the eastern part it consists entirely of transgressive system tracts (Hermoza 2004).

Charqui Fm

MIOCENE

NEOGENE

Q PLIOCENE

75

Late

Huayabamba Fm

?

Early

?

?

?

Middle

?

Legend Marine deposits

Continental deposits

Clasts

Trough cross-stratification

Mud clasts

Palaeoso

Erosional surface

Mudstones

Lenticular/wavy bedding

Sandstones

Channel

Limestones

Fig. 5.7 Stratigraphic overview (Wheeler diagram) of the Paleogene-Neogene southern Peruvian foreland basin. Fm, formation; Mb, member; SAZ, sub-Andean zone.

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76

M. Roddaz et al.

Southern Peru and northern Bolivia The Pozo Sand Member is preserved at some places in the frontal thrust of the southern Peruvian sub-Andean zone and conglomerates of this member were found in some wells, but in general Eocene deposits are absent (see Fig. 5.7). Eocene deposits are not encountered in Bolivia.

Late Oligocene to Miocene Northern Peru Chambira Formation The age of the Chambira Formation (Kummel 1946), which is a different unit from the Chambira Formation from Ecuador dealt with above, is poorly constrained. For Marocco (1993), it is Late Oligocene-Middle Miocene, whereas for Seminario & Guizado (1976) it is Miocene. Charophytes (Tectocara supraplana) found in Santa Lucia 2X well suggest an Oligocene to Middle Miocene age (Hermoza 2004; see Table 5.1). The Chambira Formation outcrops almost continuously in the sub-Andean zone and in the Marañón foredeep. The Chambira Formation has been divided into two members (Lower Chambira Member and Upper Chambira Member), both of them representing a similar depositional setting (see Fig. 5.6). In the sub-Andean zone, the Lower Member consists of a succession of sand bars with trough and planar cross-stratifications, mudstones and channels with sand-mud couplets. Several channels exhibit coarse-to-medium-grained sigmoid beds, sandstone and planar foreset stratifications. Mudstones and sandy bars with trough and planar cross-bedding indicate deposition within a meandering fluvial system. Sigmoid beds and sand-mud couplets suggest a tide-influenced system (Hermoza et al. 2005b). The Upper Member is thicker and is characterized by sequences of tidal sand bars, sigmoid bedded sandstones, and trough crossbedded sandstones, with intercalations of reddish to brownish argillites and silts. In comparison with the Lower Member, the silt/sand ratio is higher, but the Upper Member was deposited in a similar tide-influenced fluvial system (Hermoza et al. 2005b). In the Marañón foredeep, the thickness of the formation varies from 580 to 1500 m and is formed by an alternation of red siltstones and mudstones with intercalations of fine sandstones. The Lower and Upper Members are thought to represent distal aggrading floodplain deposits in a meandering fluvial system. Channel structures can be easily visualized on seismic lines and anhydrite occurrence is frequent (Hermoza 2004). Wesselingh et al. (2006) have added a small regressive basal subunit dividing the Chambira Formation into three subunits based on the study of three wells located in the Pastaza Megafan close to the Ecuadorian frontier. Ipururo Formation The Mio-Pliocene Ipururo Formation (Kummel 1946) is present only in the sub-Andean zone, is poorly dated and has an uncertain

Hoorn_ch05_Final.indd 76

thickness. Hermoza et al. (2005b) divided the Ipururo Formation into three members (see Fig. 5.6). The Lower Member consists of microconglomerates and mudstones followed upwards by medium-to-coarse-grained sandstones with oblique planar stratifications and low-angle cross-laminations. The Lower Member represents regressive system tracts composed of successive prograding deltaic lobes (Hermoza et al. 2005b). The Middle Member consists of marls and limestones associated with fine- and very fine-grained hummocky cross-stratified calcarenites. These deposits represent westward transgressive storm deposits (Hermoza et al. 2005b). The Upper Member is composed of conglomerates of wellrounded volcanic and quartzite clasts with trough and planar cross-bedding stratifications intercalated with siltstones and mudstones. This sequence is succeeded by trough and planar cross-bedded and horizontal-bedded sandstones. The Upper Member represents fluvial to braided river deposits (Hermoza et al. 2005b). Pebas Formation A detailed description of this formation can be found in Chapter 8. The basal part of this formation is not well dated. For Hermoza (2004) it is Middle-Late Miocene in age, whereas for Wesselingh et al. (2006) it is Early to early Late Miocene. The Pebas Formation is continuously present in wells of the Marañón foredeep (see Fig. 5.6). Based on the study of three wells, Wesselingh et al. (2006) determined the thickness of the Pebas Formation depending on the presence of coaly intervals; the first coaly interval encountered would be the base of the Pebas Formation whereas the last one would be the top. For these authors, the Pebas Formation is about 1000 m thick. Based on an extensive study of wells and seismic lines, Hermoza (2004) suggests that its thickness is fairly constant (400–500 m). For Hermoza (2004), the base of the Pebas Formation is marked by a lowering in sonic interval travel time, a lowering in gamma ray response and an increase in resistivity, which is interpreted as a transgressive surface. On seismic lines, the base of the Pebas Formation corresponds to a sharp reflector, and channel structures are absent. The basal part consists of glauconite-rich sandstones, and siltstones and mudstones with fish and ostracod remains. The upper part is made of blue mudstones typical of the Pebas Formation. Calcareous intervals are also present (Hermoza 2004). In the absence of biostratigraphic or radiometric dating, the exact thickness of the Pebas Formation remains unclear. However, the study of Hermoza (2004) is based on extensive studies of well and seismic lines covering the entire northern Peruvian foreland basin and delimitations are based on more criteria; for this reason we favour here a maximum thickness of ~500 m for the Pebas Formation. Southern Peru and northern Bolivia The Ipururo Group (Valdivia 1974) comprises Late Oligocene to Miocene deposits of southern Peru and northern Bolivia. The Ipururo Group is divided into three formations: the Bala Formation, the Quendeque Formation and the overlying Charqui Formation (see Fig. 5.7).

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The Amazonian foreland basin system Bala Formation The Bala Formation is considered as the age-equivalent of the Petaca Formation (Sempere et al. 1990; see also Chapter 7) and hence is Oligocene-Miocene in age. This formation unconformably overlies the Jurassic Beu Formation. Its basal part is estimated at ~27 Ma based on lithostratigraphic correlations (Baby et al. 1995). This formation has been poorly studied and no detailed sedimentological study exists. The formation is up to 200 m thick and is composed of fluvial sandstones and conglomerates intercalated with muddy palaeosol intervals. The clasts consist of cherts, quartzites and reworked sandstones of the Beu Formation and usually the matrix is sandy Fe-rich. Iron and siliceous nodules are frequent both in the sandstones and conglomerates. Lateritic palaeosols can occasionally occur at the basal part of the formation. The sediments of the Bala Formation were deposited by a fluvial system that developed on a very low topographic gradient. Abundant palaeosol horizons and low sediment thickness (< 200 m) indicate predominantly non-deposition. Lateritic palaeosols and Fe- and Si-rich nodules suggest intense meteorization compatible with a tropical climate (Strub 2006). Quendeque Formation Based on biostratigraphic markers (see Table 5.1), the base of the Quendeque Formation is Late Oligocene-Miocene in age. Stratigraphic correlation (Baby et al. 1995) further suggests a Late Miocene age for the basal part of the formation. In the Peruvian sub-Andean zone, the Quendeque Formation is up to 1500 m thick and consists of sequences of 6–8 m-thick red quartz and feldspar-rich sandstone bars separated by 10–15 m-thick siltstones and mudstones. The sandstones are characterized by trough and planar cross-stratifications and ripple crosslaminations. The siltstone and mudstone beds are massive. The Quendeque Formation deposits represent distal meandering and floodplain sediments (Hermoza 2004). In the Bolivian sub-Andean zone, the Quendeque Formation is about 2 km thick in the external part of the sub-Andean zone (Madidi syncline) (Strub 2006). The Quendeque Formation deposits consist mainly of aggrading anastomosed fluvial and floodplain deposits (Strub 2006). Tidally influenced point bar and estuarine/deltaic interdistributary bay facies coexisting with fluvial facies described above can occasionally occur (Hovikoski et al. 2007) Charqui Formation Argon-40/argon-39 (40Ar/39Ar) dating on a tuffaceous level of the upper part of the Charqui Formation in the northern Bolivian sub-Andean zone gave ages of 8.7 ± 0.9 Ma (Strub et al. 2005; Strub 2006), 7.96 ± 0.58 Ma (micas) and 7.79 ± 0.03 Ma (feldspars) (Hérail et al. 1994). In the Peruvian sub-Andean zone, the Charqui Formation is up to 1750 m thick and consists of conglomerates, quartz and feldspar-rich sandstones and rare massive mudstones. The conglomerates exhibit trough and planar cross-bedded stratifications and horizontal stratification whereas the sandstones have

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77

trough and planar cross-stratifications. The Charqui Formation sediments correspond to braided river deposits. Moreover, the formation also exhibits frequent syntectonic growth strata indicating that deposition is strongly controlled by thrust tectonics (Hermoza 2004). In the Bolivian sub-Andean zone, the Charqui Formation outcrops only in the Madidi syncline. Compared with its southern Peruvian sub-Andean zone equivalent, it exhibits higher proportions of sandstones and mudstones but similar syntectonic growth strata (Strub 2006). The Charqui Formation probably represents meandering and braided river deposits Madre de Dios foredeep: Ipururo and Madre de Dios Formations In the Madre de Dios foredeep, the Ipururo Group comprises the Ipururo Formation and Unit A and B of the Madre de Dios Formation sensu Campbell et al. (2001). Based on stratigraphic correlations, the Ipururo Formation is estimated to be Miocene in age (Hermoza 2004). The Madre de Dios Formation is Late Miocene (40Ar/39Ar dating on feldspars at 9.01 ± 0.28 Ma, Campbell et al. 2001). The Madre de Dios Formation can thus be considered as a lateral equivalent of the Charqui Formation. The Ipururo Formation is continuously present in well and seismic sections all along the Madre de Dios foredeep; the Madre de Dios Formation outcrops continuously across the Madre de Dios foredeep (Roddaz et al. 2004; see also Fig. 5.7). Subsurface data indicate that the thickness of the Ipururo Formation ranges from 1100 to 1300 m and that the thickness of the Madre de Dios Formation is fairly constant (~400 m in the three studied wells; Hermoza 2004). The upper part of the Ipururo Formation consists of subtidal channel sediments deposited in a tide-dominated delta (Roddaz 2004; Roddaz et al. 2004) and the Unit A and B of the Madre de Dios Formation were deposited in tide-dominated estuaries (Roddaz 2004; Roddaz et al. 2004; Hovikoski et al. 2005). For further details on tide-dominated estuaries see Chapter 9.

Neogene(?) to Present Northern Peru The sub-Andean zone (Juanjui Formation) The Plio-Pleistocene Juanjui Formation is about 100 m thick and can be found at various locations in the sub-Andean zone (Díaz et al. 1998; Sánchez & Herrera 1998). In areas close to Tocache, it is named the Tocache Formation (Díaz et al. 1998). The formation is composed of polygenic well-rounded conglomerates. The clasts are usually less than 15 cm long and consist of intrusive volcanic schist, gneisses, quartzite, limestones and sandstones. The conglomerates show frequent trough and planar cross-bedded stratifications. Clast-supported and inverse-grading facies are also present. The conglomerates usually coarsen upwards (see Fig. 5.6). The Juanjui Formation developed in fluvial to alluvial fan environments (Hermoza et al. 2005b).

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M. Roddaz et al. Marañón Formation

The Marañón Formation is poorly dated but considered to be of Pliocene age. Its thickness ranges from 220 to 600 m (Hermoza 2004). The base of the formation consists of well-sorted sandstones, some of which are glauconite-rich with intercalations of red siltstones and mudstones. The middle part of the formation is composed of massive sandstone beds (up to 30 m thick) intercalated with siltstones and mudstones. Pyrite, gypsum and anhydrite are occasionally found. Some limestone levels are also found. The upper part of the formation consists mainly of red siltstone and mudstone with thin sandstone intercalations (see Fig. 5.6) with occasional occurrences of anhydrite (Hermoza 2004). The Marañon Formation probably represents meandering and floodplain fluvial deposits. Corrientes Formation The Corrientes Formation represents Pleistocene deposits and ranges in thickness from 400 to 850 m. The formation is composed of massive sandstone beds (10–30 m thick) intercalated with red siltstones and mudstones (see Fig. 5.6). Coaly intervals are locally present. The Corrientes Formation represents aggrading channel infill and floodplain sediments (Hermoza 2004) deposited in a meandering fluvial system probably similar to present-day Amazonian rivers. Southern Peru and northern Bolivia The sub-Andean zone (Masuko and Tutumo Formations) The Masuko Formation is estimated to be Plio-Pleistocene in age. This formation outcrops in the Peruvian sub-Andean zone (see Fig. 5.7) and has a variable thickness. The formation consists of gold-bearing conglomerates of economic interest that are presently being mined. This formation is separated from the underlying Charqui Formation by an erosional unconformity (Hermoza 2004). The conglomerates are composed of intrusive volcanic, schist, quartzitic, gneissic and sandstone clasts, 15–30 cm in diameter. Trough and planar bedded crossstratifications are present. Massive clast-supported and inverse clast-supported facies also occur. This formation exhibits welldeveloped syntectonic growth strata indicating thrust-controlled deposition. These deposits correspond to prograding alluvial fan deposits controlled by the activity of sub-Andean zone thrusts (Hermoza 2004). The Plio-Pleistocene Tutumo Formation (Davila et al. 1965) comprises the Pliocene Bolivian sub-Andean zone deposits and Quaternary aggradational terraces of the Beni River and its tributaries. The Tutumo Formation is variable in thickness (20–700 m). Unfortunately, precise sedimentological studies of this formation are not yet available. It consists mainly of conglomerates of Andean origin and trough cross-stratified sands (Strub 2006). This formation is considered as the lateral equivalent of the Masuko Formation (Hermoza 2004) and hence probably represents braided river and alluvial fan deposits.

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Plio-Pleistocene sedimentation in the Madre de Dios and Beni foredeeps The upper part of Madre de Dios Formation (Campbell et al. 2006) is part of the Madre de Dios foredeep sedimentary record, and is of Pliocene age based on absolute dating of a tuffaceous level intercalated in these deposits (40Ar/39Ar dating at 3.12 ± 0.02 Ma; Campbell et al. 2001). These deposits have variable thicknesses (10–30 m) and exhibit several facies that are characteristic for braided river and meandering river systems (Roddaz 2004). The braided-river deposits consist of gravels in a sandy matrix and with clasts (1–5 cm in length) of quartzites, intrusive rocks or white weathered sandstones. Trough and planar cross-bedding as well as horizontal laminations are present. Meandering river deposits are characterized by muds, silts and sands. The sands exhibit trough cross-stratifications and have a channel-shaped base, defining channel infill deposits. They are associated with muds with faint planar laminations interpreted as oxbow-lake deposits. Red massive muds and silts are also frequent and represent floodplain deposits. Closely associated with these facies, welldeveloped palaeosols occur (Roddaz 2004). Macrofossil evidence of a pre-Holocene thorny bamboo similar to Guadua (Poaceae: Bambusoideae: Bambuseae: Guaduinae) has recently been found in these deposits (Olivier et al. 2009). Drainage systems of the Pliocene Madre de Dios deposits are similar to present-day drainage systems of the Madre de Dios Basin such as the braided Inambari River and the meandering Madre de Dios River. The modern sedimentation in the Beni foreland basin is dominated by episodic accumulation of floodplain deposits controlled by El Niño-Southern Oscillation (ENSO) cycles (Aalto et al. 2003; see also Chapter 14).

Sedimentation rates Colombian foreland basin Geohistory analysis in the eastern foothills area suggests limited subsidence during the Eocene and earliest Oligocene (Mirador Formation and C8 Member of the Carbonera Formation; Parra et al. 2009; see also Chapter 4). At ~31 Ma, subsidence rates increased and fluvial-dominated deposition was restricted to the proximal eastern foothills region (Parra et al. 2009).

Ecuadorian foreland basin Isopach maps and sedimentations rates for the Tiyuyacu, Orteguaza and Chalcana Formations are available in Christophoul et al. 2002a. The Tiyuyacu Lower Member is variable in thickness (150–548 m) and the depocentre is localized in the centre of the Oriente Basin. Calculated sedimentation rates range from 0.01 to 0.05 mm/year. The thicknesses of the Tiyuyacu Upper Member range from 59 to 319 m and its depocentre was located in the centre of the Oriente Basin. Calculated sedimentation rates are similar to those of the Lower Tiyuyacu Member and range from 0.01 to 0.05 mm/year. The Orteguaza Formation is variable in thickness (40 to 341 m) and its depocentre is localized in the

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in several papers (e.g. Ungerer et al. 1990). To calculate sedimentation rates, the program uses the backstripping method. The necessary input consists of stratigraphic time–depth information (thicknesses, ages, lithology and depositional water depths). The decompaction model is based on the algorithm described by Perrier and Quiblier (1974). To calculate sedimentation rates, we constructed fictitious wells where maximum thickness and lithology for each formation formed the input. Due to poor stratigraphic control regarding the Neogene formations, we proposed four different scenarios (Table 5.2). Scenario 1 ascribed a Late Oligocene age to the Chambira Formation and a Miocene age to the Pebas Formation. Scenario 2 proposed a Late Oligocene to Early Miocene age for the Chambira Formation and Middle to Late Miocene age for the Pebas Formation. Scenarios 3 and 4 are the same

centre of the Oriente Basin. Calculated sedimentation rates range from 0.009 to 0.07 mm/year. The thicknesses of the Chalcana Formation range from 255 to 455 m and its depocentre is located in the centre of the Oriente Basin. Calculated sedimentation rates (0.07–0.12 mm/year) increased when compared with underlying formations. Overall, sedimentation rates increased throughout the Cenozoic with the depocentre remaining at a constant place.

Northern Peruvian foreland basin For the reconstruction of the burial history and basin subsidence rates, we have used the Genex 1D basin modelling software (IFPBEICIP). The basic concepts of the Genex program can be found

Table 5.2 Calculated sedimentation rates for the northern Peruvian foreland basin. Maximum sedimentation rates calculated from GENEX 1D basin modelling software (BEICIP-IFP; see text for explanations). Maximum sedimentation rates are based on stratigraphic thickness, time as well as compaction. Formations

Period End

Start

Top

Bottom

Sedimentation rates m/Ma

mm/year

Scenario 1 Corrientes

0

1.8

0

850

725.3

0.7253

Marañón

1.8

5.3

850

1450

306.0

0.306

Pebas

5.3

23

1450

1950

83.1

0.0831

28.4

1950

3450

527.0

0.527

Chambira

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Depth (m)

23

Upper Pozo

28.4

37.2

3450

3606

38.3

0.0383

Lower Pozo

37.2

48.6

3606

3662

14.9

0.0149

Scenario 2 Corrientes

0

1.8

0

850

725.3

0.7253

Marañón

1.8

5.3

850

1450

306.0

0.306

Pebas

5.3

16

1450

1950

137.5

0.1357

Chambira

16

28.4

1950

3450

229.5

0.2295

Upper Pozo

28.4

37.2

3450

3606

38.3

0.0383

Lower Pozo

37.2

48.6

3606

3662

14.9

0.0149

Scenario 3 Marañón

0

5.3

0

1450

431.4

0.4314

Pebas

5.3

23

1450

1950

83.1

0.0831

Chambira

23

28.4

1950

3450

527.0

0.527

Upper Pozo

28.4

37.2

3450

3606

38.3

0.0383

Lower Pozo

37.2

48.6

3606

3662

14.9

0.0149

Scenario 4 Marañón

0

5.3

0

1450

431.4

0.4314

Pebas

5.3

16

1450

1950

137.5

0.1357

Chambira

16

28.4

1950

3450

229.5

0.2295

Upper Pozo

28.4

37.2

3450

3606

38.3

0.0383

Lower Pozo

37.2

48.6

3606

3662

14.9

0.0149

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as scenario 1 and 2 respectively with the exception that the Corrientes and Marañón have been grouped into one formation. Isopach maps for the Marañón Basin are available in Hermoza (2004) and can be provided upon request. The lowest sedimentation rate was found for the lower Pozo Member (~0.01 mm/year), which has its depocentre located in the distal part of the present-day Marañón foredeep. The depocentre of the Upper Pozo Member migrated westwards, close to the present-day orogenic front (Hermoza 2004), and its sedimentation rate increased at ~0.04 mm/year (see Table 5.2). The depocentre of the Chambira Formation migrated towards the present-day sub-Andean zone (Hermoza 2004; Hermoza et al. 2005a) and its sedimentation rate increased at 0.23 mm/year or 0.53 mm/ year, depending on the scenario chosen. The Pebas Formation has a constant thickness and its sedimentation rates decreases at 0.14 mm/year or 0.08 mm/year (see Table 5.2). The highest sedimentation rates are found, depending on the scenario chosen, for the Corrientes or Marañón Formations (~0.73 mm/year and ~0.43 mm/year), and the locus of the depocentre migrated in the present-day Marañón foredeep (Hermoza 2004).

Discussion Late Cretaceous-Paleocene: initial tectonic loading and partitioning of the foreland basin Although there is still debate, most recent studies suggest that the initiation of the Andean foreland basin started in Late Cretaceous-Paleocene times (Balkwill 1995; DeCelles & Horton 2003; Barragan et al. 2005; Martin-Gombojav & Winkler 2008 and references therein). These authors suggest that in southern Bolivia the Paleocene Santa Lucia Formation, outcropping in the Eastern Cordillera, was deposited in the Paleocene backbulge depozone of the Central Andean foreland basin (DeCelles & Horton 2003). In Colombia, the distribution of facies and thickness of the Late Cretaceous to Paleocene foreland deposits of the Magdalena

–

–

–

–

Valley, Eastern Cordillera and Llanos Basins is much more constrained and illustrate well the position of the tectonic load and the extent of the foreland depozones. In the Magdalena Valley Basin, several stratigraphic observations have been used to suggest Late Cretaceous initiation of uplift of the Central Cordillera (Gómez et al. 2003) and coeval associated foreland basin development. In addition, these authors calculated that about 7–13 km thickness of Central Cordilleran rocks were removed from the Campanian to the Eocene, due to kilometre-scale uplift of this range. Foredeep deposition occupied the Middle Magdalena Valley and the axial Eastern Cordillera. The accumulation of westerly derived coarse-grained deposits (Cimarrona and Hoyón Formations) occurred adjacent to the topographic front along the uplifting Central Cordillera. In contrast, the deposition of mudstone-dominated fluvial plain and estuarine deposits associated with high subsidence rates occurred in the distal part of the foredeep, along the axial part of the Eastern Cordillera. Further east, either erosion or limited deposition in the Llanos Basin indicated forebulge conditions. In Ecuador, the Tena Formation is the oldest formation to be derived from the Eastern Cordillera (Ruiz et al. 2004; MartinGombojav & Winkler 2008). Detrital zircon fission track analysis of the sediments of the Tena Formation suggests rapid exhumation and uplift of the Eastern Cordillera (Ruiz et al. 2004) consistent with the 65–55 Ma period of elevated cooling rates and exhumation rates of the Eastern Cordillera (Spikings et al. 2001). The fast exhumation and topographic growth of the Eastern Cordillera is related to the Late Cretaceous-Paleocene initial collision of the Caribbean with the South American Plate (Vallejo et al. 2006) and marked in the Ecuadorian foreland basin the onset of tectonic loading and related flexural subsidence. Unfortunately, no such studies exist for the northern Bolivian and Peruvian Eastern Cordillera. Additionally, no detailed sedimentological studies of the Eastern Cordillera of northern Bolivia, Peru and Ecuador have so far been undertaken, so that the existence of a Paleocene forebulge is difficult to establish. Hence, we suggest that the distal fluvial floodplain and continental deposits of the Huayabamba, Yahuarango and Tena Formations were deposited in backbulge or distal foredeep

–

–

–

–

–

Fig. 5.8 Paleogene palaeogeographic maps; black lines with black triangles indicate the positions of the Andean thrust front ; light grey indicates areas of marginal marine and lacustrine wetlands. ECC, Eastern Cordillera of Colombia.

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The Amazonian foreland basin system position, these depozones being formed as a response to initial tectonic loading of a proto-Andean Cordillera (Fig. 5.8).

Eocene: erosional unloading In the Colombian Llanos Basin, east of the Eastern Cordillera, the absence of Lower and Middle Eocene units (Santos et al. 2008) is ascribed to Eocene forebulge uplift (Parra et al. 2009). Thus the Colombian Eocene foredeep could have been much narrower than the Paleocene foredeep. At first glance, this evidence may suggest thrust loading and cratonward progradation of the orogenic front of the Central Cordillera (e.g. Gómez et al. 2005). However, there is no other direct and unambiguous evidence of renewed thrust loading and eastward progradation of the Central Cordillera during the Eocene. Most of the structures of the Magdalena Valley below a conspicuous Eocene unconformity are older than the Eocene (Suarez et al. 2000) and could correspond to a Late Paleocene deformation event. Moreover, the westward onlapping sequences of the Magdalena Valley most likely suggest erosional retreat of the Central Cordillera. In addition, to the east, along the eastern foothills of the Eastern Cordillera, the deposits of the Mirador Formation registered a 56–31 Ma slow sediment accumulation under estuarine and coastal plain conditions (Parra et al. 2009). Thus, if the absence of Lower and Middle Eocene deposits in the Llanos Basin is due to post-Middle Eocene erosion, then a much wider Early-Middle Eocene basin could have been possible. In such case, low Early-Middle Eocene sedimentation rates to the east and absence of coeval newly created accommodation space to the west, adjacent to the Central Cordillera, would coincide with a configuration typical of an erosional unloading stage (Catuneanu 2004). The exact significance of the Magdalena Valley unconformity remains unclear. It could be formed by Late Paleocene deformation in the valley followed by Early Eocene erosional unloading in the Central Cordillera or by Eocene advance of the orogenic front towards the valley. None of the two hypotheses can be ruled out with the available data, but in line with observations in Peru, Bolivia and Ecuador, we suggest that it is more likely that Early-Middle Eocene times corresponded to a stage of tectonic quiescence and erosional unloading in the Colombian Central Cordillera. In Ecuador, the Early-Middle Eocene period is marked by low sedimentation rates (max. ~0.05 mm/year), by the development of braided rivers fed by sediments from the Eastern Cordillera (Ruiz et al. 2004; Martin-Gombojav & Winkler 2008) and by lower exhumation rates of the Eastern Cordillera (Spikings et al. 2001). Therefore, the erosional base of the Lower Tiyuyacu Member and its associated coarse sedimentation is interpreted to mark the onset of tectonic unloading due to isostatic readjustment of the Eastern Cordillera. This tectonic quiescence stage lasted until the Middle Eocene with the deposition of the Upper Tiyuyacu Member. The Late Eocene-Early Oligocene transgressive marine deposits of the Orteguaza Formation are characterized by an increase in sedimentation rates (~0.07 mm/year) and by the appearance of high-grade metamorphic minerals, coming from the Eastern Cordillera (Martin-Gombojav & Winkler 2008). Associated with an increase in exhumation rates in the Eastern Cordillera (Spikings et al. 2001), these suggest the end of the isostatic readjustment and renewed tectonic loading of the Eastern Cordillera.

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81

In northern Peru, the Lower Eocene unconformity of the Pozo sand Member constitutes a regional subaerial unconformity that marked an important change in geodynamic conditions. Low sedimentation rates (max. ~0.04 mm/year), cratonward migration of the Pozo sand depocentre and reciprocal architecture of the depositional system tracts (regressive system tracts in the proximal part of the basin and transgressive system tracts in the distal part) characterized an orogenic unloading stage probably due to the erosion of the Western Cordillera. The Middle to Late Eocene Pozo shale Member defined transgressive system tracts occurring throughout the basin. These transgressive system tracts, as well as increased sedimentation rates and westward (toward the Andes) migration of the depocentre, characterized a change to a tectonic loading stage where thrust-related loading of the Western Cordillera provoked flexural subsidence and transgression in the foredeep depozone. Limited deposition of the Lower Pozo sand Member in southern Peru and the absence of deposition of the Pozo shale Member in southern Peru as well as the absence of Eocene deposition in northern Bolivia suggest low accommodation space compatible with an Eocene unloading stage (see Fig. 5.8). Based on this review, we suggest that the Colombian, Ecuadorian and northern Peruvian foreland basins were characterized by an Early-Middle Eocene unloading stage corresponding to the erosion of the Central Cordillera of Colombia, Eastern Cordillera of Ecuador and Western Cordillera of Peru. The configuration of the southern Peruvian and northern Bolivian foreland basin remains unclear. The Eocene erosional surface and low sediment accumulation could either mark an erosional unloading stage or be produced in a distal backbulge setting, as proposed by DeCelles & Horton (2003) for the southern Bolivian foreland basins. The Middle(?)-Late Eocene period marked the onset of tectonic loading of the Western Cordillera of Peru and renewed tectonic loading of the Eastern Cordillera of Ecuador.

Oligocene-Middle Miocene: generalized loading stage In Colombia, continued westward onlapping of fluvial OligoceneMiocene units in the Middle Magdalena Valley Basin reflects erosional retreat of the Central Cordillera (Gómez et al. 2003). This Oligocene-Miocene erosional retreat of the Central Cordillera was contemporaneous with generalized low shortening and uplift of the Eastern Cordillera (Gómez et al. 2003, 2005; Mora 2007; Parra et al. 2009). Generalized deformation of the Eastern Cordillera resulted in a Late Oligocene increase of tectonic subsidence in the eastern foothills of the Eastern Cordillera (Parra et al. 2009; see also Chapter 4) and Llanos Basin (Bayona & Thomas 2003). This episode thus reveals a stage of eastward migration of the foreland basin system. The observed greater subsidence patterns roughly coincide with the time of deposition of the coastal plain; tidally influenced deposits of the Carbonera Formation and are prolonged throughout the Miocene (see Chapter 4; Fig. 5.9). Consequently, because of the Oligocene uplift of the Eastern Cordillera, there is no record of Middle Oligocene to Middle Miocene deposits. The mountain-building and exhumation patterns recorded in the hinterland (Parra et al. 2009) were rather similar during the Late Oligocene-Middle Miocene. Thus, the

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Fig. 5.9 Oligocene and Middle Miocene palaeogeographic maps; black lines with black triangles indicate the positions of the Andean thrust front. ECC, Eastern Cordillera of Colombia. Middle Miocene Leòn tidal-influenced lacustrine transgressive deposits cannot be solely explained by the onset of Andean-scale mountain-building processes (Bayona et al. 2007). The Oligo-Miocene infill of the Ecuadorian Amazonian foreland basin comprises thick non-marine deposits (Chalcana and Arajuno Formations) passing eastward to shallow marine to lacustrine deposits (Curaray Formation). Increasing sedimentation rates contemporaneous with Oligocene exhumation of the Western and Eastern Cordillera (Spikings et al. 2001, 2005; Martin-Gombojav & Winkler 2008) are indicative of ongoing tectonic loading of the proto-Andes. Upward coarsening of the series, westward/upward passage from meandering to braided streams, and reduction of the floodplain/ channel infill ratio indicate an increase in slope and in erosion rate during Early(?)-Middle Miocene (Burgos 2006). The convergence of the palaeocurrent directions along with the channel instability shows that these deposits formed a distributary system with a fanlike arrangement. The sedimentary evolution thus records the evolution through time of a shallow-dipping alluvial fan grading into a large-scale fan delta to a piedmont fan prograding eastwards (see Fig. 5.9). Despite the absence of a visible transition, it should be postulated that a delta marked the transition from the piedmont deposits to the deposits of the Curaray Formation. Eastern onlaps of the Curaray Formation indicative of uplift of the basement and eastern progradation of the Arajuno and Chambira Formations suggest ongoing tectonic loading throughout Early-Middle Miocene times. In northern Peru, the depocentre of the Chambira Formation was located in the present-day sub-Andean zone and in this zone the Chambira Formation is tide-influenced suggesting marine ingression throughout the foredeep parallel to the Oligo-Miocene palaeo-thrust front (see Fig. 5.9). In the distal part, the OligoMiocene period is marked by increasing sedimentation rates (see Table 5.2) and aggrading floodplain deposits. Similar features are found in the southern Peruvian and northern Bolivian parts with locally tide-influenced sedimentation and significant thicknesses of the deposits of the Quendeque/Ipururo Formations in the sub-Andean zone and distal aggrading sedimentation (Ipururo Formation). Together with other tectonic evidence (see Chapter 4),

Hoorn_ch05_Final.indd 82

this sedimentological evidence indicates that sedimentation in the Peruvian and Bolivian parts of the Amazon foreland Basin was controlled by tectonic loading of the Eastern Cordillera. The Miocene Pebas Formation in the Marañón foredeep shows two transgressive-regressive cycles that could probably be correlated with tide-controlled sedimentation of the Chambira Formation of the sub-Andean zone and with the Ecuadorian Curaray and Colombian León formations. However, due to poor stratigraphic control it is difficult to unravel which processes controlled these marine ingressions (flexural subsidence, eustasy or a combination of both).

Late Miocene to present-day: loading and transition from filled to overfilled In the Colombian foredeep, there is a well-documented transition from Middle Miocene tidal-influenced lacustrine deposits of the León Formation, to fluvial environments of the Guayabo Formation. The transition from a meandering-to-braided river depositional environment of the Late Miocene Lower Guayabo Formation to alluvial fan deposits of the Late Miocene-Pliocene Upper Guayabo Formation documents an increase in grain size and a passage from a distal to a proximal fluvial depositional environment. Mora (2008), document that the main facial and granulometric change between the Lower and Upper Guayabo Formation roughly coincides with a dramatic Mio-Pliocene acceleration in denudation rates in the Eastern Cordillera, which reached a critical elevation. However, part of such an acceleration could be due to a progressively increasing size of the catchment areas in the hinterland because of widespread incision. Therefore it could be expected that the process of increasing exhumation is not a point in time but a time-range event. Provided that the calculated subsidence rates are roughly constant in the MiddleLate Miocene in the Colombian foredeep (see Chapter 4), then increasing denudation rates in the source areas may result in an overfilled foredeep.

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Fig. 5.10 Late Miocene, Pliocene and present-day maps. NAFB, North Amazonian foreland basin; SAFB, South Amazonian foreland basin; MA, Mérida Andes; ECC, Eastern Cordillera of Colombia; Iq Fb, Iquitos forebulge; PC fb, Puerto Cavinas forebulge; Ca, Contaya Arch; Fa, Fitzcarrald Arch; SAZ, sub-Andean Zone. Black lines with black triangles indicate the positions of the Andean thrust front.

Other parts of the Amazonian foreland sedimentation were widely controlled by deltaic and estuarine sedimentation during the Late Miocene (Fig. 5.10) including the wedge-top depozone (Hermoza et al. 2005b), the foredeep depozone (Hermoza 2004; Roddaz 2004; Roddaz et al. 2004; Hovikoski et al. 2005; Burgos 2006), the forebulge depozone (Roddaz et al. 2005a, 2006; Rebata-Hernani et al. 2006a, 2006b) and the backbulge depozone (Gingras et al. 2002; Roddaz et al. 2005a, 2006). The Late Miocene Amazonian foreland basin system may therefore be interpreted as a filled foreland basin system (Catuneanu 2004). The Latest Miocene to Pliocene sedimentation in the Amazonian foreland basin system is characterized by continental deposits (see Fig. 5.10), including prograding alluvial fan and braided river deposits in the wedge-top depozone, aggrading meandering rivers, and floodplain and lacustrine deposits in the foredeep and forebulge and backbulge depozones (Hermoza 2004; Roddaz 2004). The Amazonian foreland basin system may therefore be interpreted as overfilled (Catuneanu

Hoorn_ch05_Final.indd 83

2004). To reinforce this interpretation, it is worth noting that in the Latest Miocene, a shift in palaeoflow directions from parallel to the orogen to perpendicular to the orogen has been documented in Ecuador (Christophoul et al. 2002b; Burgos 2006). This change may also be symptomatic of a transition from filled to overfilled stages. The overfilled stage could also be reflected in the presentday sedimentation from Bolivia to Colombia as most of the subAndean rivers run perpendicular to the deformation front, like in overfilled systems (Jordan 1995). Transition from filled to overfilled is caused by a decrease in accommodation space, which in turn depends on the interplay between sediment supply and base level changes (see Catuneanu 2004 and references therein). In the case of the Amazonian foreland basin, this transition is marked by an increase in sedimentation rates in northern Peru (see Table 5.2) and by eastward cratonic migration of the depocentres in Peru and southern Bolivia. A global eustasy sea-level fall can be ruled out as the Late

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Miocene tidal sedimentation occurred when the sea level was lower or equal to its present level (Haq et al. 1987). Hence, Latest Miocene emersion of the Amazonian foreland basin and associated continental sedimentation cannot be due to eustatic sea level fall. Rather, the presence of growth strata in Late Miocene tidal wedge-top deposits associated with Late Miocene forebulge uplift and structuring and uplift of the Eastern Cordillera and the sub-Andean zone (see Chapter 4) indicate that the Late Miocene ingression was driven by flexural subsidence as a result of renewed thrust tectonic loading. The absence of Late Miocene tidal deposits in Colombia could be explained by the Late Miocene uplift of the Mérida Andes (Colletta et al. 1997; Audemard & Audemard 2002) that would have closed the connection with the Caribbean Sea. If correct, this suggests a southern connection for the Late Miocene Amazonian marine ingression. Increasing sediment supply from the Andean highland is the more plausible mechanism to explain the Neogene transition from filled to overfilled as suggested by the exhumation data from the Colombian Eastern Cordillera. We propose a two-step response of the Amazonian foreland basin system to the Neogene uplift and relief acquisition triggered by tectonic loading. A Late Miocene (~9 Ma) tidal transgression (filled stage) is roughly contemporaneous with the initiation of the inferred surface uplift and consequent increased load of the Andean wedge, as a result of increasing tectonic shortening of both the sub-Andean zone and the Eastern Cordillera (see Chapter 4 and Fig. 5.10). Later, widespread incision of the newly created high relief was probably due to the transition from topographic pre-steady state to steady state. As a consequence exhumation rates increased and more sediment was supplied to the Amazonian foreland basin, achieving overfilling of the Amazonian foreland basin at ~6 Ma. The Pliocene (~4 Ma) uplift of the Fitzcarrald Arch as a result of the flat-slab subduction of the Nazca Ridge (Espurt et al. 2007) is then responsible for the partitioning of the Amazonian foreland basin into the North Amazonian foreland basin system and the South Amazonian foreland basin system (see Fig. 5.10).

Conclusions During the Cenozoic, the development of the Amazonian foreland basin as recorded by its sedimentary architecture was strongly controlled by Andean tectonics and related subduction processes. Initial tectonic loading of the Andes of Ecuador, Peru and northern Bolivia occurred in the Late Cretaceous-Paleocene and favoured distal floodplain sedimentation in the Amazonian foreland. Similar processes occurred in the Colombian Andes with the onset of a Late Cretaceous-Paleocene foreland basin coupled with the Central Cordillera loading. Tectonic quiescence and an orogenic unloading stage prevailed during the Eocene. The Early-Middle Eocene period was marked by an unloading stage affecting most of the Amazon foreland basin. During the Middle(?)-Late Eocene, increasing sedimentation rates and migration of the depocentre westwards within the northern Peruvian Amazonian foreland basin indicate a tectonic loading stage, probably due to thrust-related uplift of the Western Cordillera. In Ecuador, Middle-Late Eocene renewed tectonic loading is also documented by high exhumation rates

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of the Eastern Cordillera (Spikings et al. 2001) and increases in sedimentation rates of the Orteguaza Formation. At the same time, in the Northern Andes significant tectonic loads were located in the Eastern Cordillera of Colombia. During approximately the same time, the southern Peruvian and northern Bolivian parts of the foreland basin were still in an erosional unloading stage. The Oligocene-Miocene period was marked by a general loading stage along the entire Amazonian foreland. Increasing sedimentation rates in Ecuador and development of a large-scale alluvial fan were largely controlled by thrust-induced uplift and loading of the Western and Eastern Cordillera. Increasing sedimentation rates and migration of the depocentres, which occupied the present-day sub-Andean zone, indicate a thrust-induced uplift and loading of the Eastern Cordilleras of Peru, Bolivia and Colombia. In Colombia, Oligo-Miocene loading of the Eastern Cordillera maintained high subsidence rates, reflected by coastal plain and tidally influenced deposits. Consequently, the Middle Miocene León tidal-influenced lacustrine transgressive deposits cannot be solely explained by the onset of Andean-scale mountain-building processes (Bayona et al. 2007). Similarly, the processes that controlled Early-Middle Miocene marine ingressions in other parts of the foreland remain to be determined Ongoing thrust-tectonic loading of the Eastern Cordillera and sub-Andean zone and the onset of the main phase of Andean surface uplift induced enhanced flexural subsidence in the foredeep depozones of the entire Amazonian foreland basin from Colombia to Bolivia and drove Late Miocene marine transgressions that characterized the filled stage of the Ecuadorian, Peruvian and Bolivian Amazonian foreland basin. Valley incisions and full relief development in the hinterland during the Late MiocenePliocene provided increased sediment supply and caused the Amazonian foreland basin to be overfilled. During that period, the Ecuadorian, Peruvian and Bolivian Amazonian foreland basin formed a unique Amazonian foreland basin system partitioned into the four discrete depozones. The flat-slab subduction of the Nazca ridge induced Pliocene (~4 Ma) uplift of the Fitzcarrald Arch (see Chapter 6) and divided the Amazonian foreland basin into the North and South Amazonian foreland basin systems. This first compilation of the foreland sedimentary and basin evolution from Colombia to Bolivia shows that, like subduction processes adjacent to the Andes, foreland basin processes are roughly synchronous and similar along the entire Amazonian foreland. This reinforces the point that subduction and foreland basin development have a close causal linkage to each other in the Andes.

Acknowledgments We thank the Instituto Colombiano del Petroleo (ICP), the Institut de Recherche pour le Développement (IRD) and PeruPetro for material and financial support. We also acknowledge financial grants from INSU-CNRS DyETI and ECLIPSE II programmes. This study has been supported by BQR grant « Mise en place d’équipements pour la thermochronologie basse température en Sciences de la Terre » from the Université de Toulouse. We apologize in advance to those whose work we may have unintentionally

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The Amazonian foreland basin system misrepresented or failed to quote. We thank Carina Hoorn and the editors for inviting us to contribute to this book. Comments by Frank Wesselingh were appreciated. This chapter benefits from constructive reviews by German Bayona, Victor Ramos and Wilfried Winkler.

References Aalto, R., Maurice-Bourgoin, L., Dunne, T., Montgomery, D.R., Nittrouer, C.A., Guyot, J.-L. (2003) Episodic sediment accumulation on Amazonian flood plains influenced by El Niño/Southern Oscillation. Nature 425, 493–497. Arriagada, C., Cobbold, P.R., Roperch, P. (2006) Salar de Atacama basin: A record of compressional tectonics in the central Andes since the mid-Cretaceous. Tectonics 25, TC1008. Audemard, F.E., Audemard, F.A. (2002) Structure of the Mérida Andes, Venezuela: relations with the South America-Caribbean geodynamic interaction. Tectonophysics 345, 299–327. Baby, P., Moretti, I., Guillier, B., Limachi, R., Mendez, E., Oller, J., Specht, M. (1995) Petroleum system of the northern and central Bolivian sub-Andean zone. In: Tankard, A.J., Suárez, R., Welsink, H.J. (eds) Petroleum Basins of South America. American Association of Petroleum Geologists, pp. 445–458. Baldock, J.W. (1982) Geología del Ecuador, Dirección General de Geología y Minería. Boletín de explicación del Mapa Geológico de la República del Ecuador, Esc. 1:1000000, 66 pp. Balkwill, H.R. (1995) Northern part of Oriente Basin, Ecuador; reflection seismic expression of structures. In: Rodrigue, G., Paredes, F.I., Almeida, J.P. (eds) AAPG Memoir. Tulsa, OK: American Association of Petroleum Geologists, 559 pp. Barragan, R., Baby, P., Duncan, R. (2005) Cretaceous alkaline intraplate magmatism in the Ecuadorian Oriente Basin: Geochemical, geochronological and tectonic evidence. Earth Planet Sci Lett 236, 670–690. Bayona, G., Thomas, W.A. (2003) Distinguishing fault reactivation from flexural deformation in the distal stratigraphy of the Peripheral Blountian Foreland Basin, southern Appalachians, USA. Basin Res 15, 503–526. Bayona, G., Jaramillo, C., Rueda, M., Reyes-Harker, A., Torres, V. (2007) Paleocene-Middle Miocene flexural-margin migration of the nonmarine Llanos foreland basin of Colombia. CTF Cienc Tecnol Futuro 3, 51–70. Beaumont, C. (1981) Foreland basins. Geophys J Roy Astr S 65, 291–329. Bes de Berc, S., Soula, J.C., Baby, P., Souris, M., Christophoul, F., Rosero, J. (2005) Geomorphic evidence of active deformation and uplift in a modern continental wedge-top-foredeep transition: Example of the eastern Ecuadorian Andes. Tectonophysics 399, 351–380. Blair, T.C., Bilodeau, W.L. (1988) Development of tectonic cyclothems in rift, pull-apart, and foreland basins: sedimentary response to episodic tectonism. Geology 16, 517–520. Bristow, C.R., Hoffstetter, R. (1977) Ecuador. Paris: Lexique Stratigraphique International, 5, 410 pp. Burbank, D.W. (1992) Causes of recent Himalayan uplift deduced from depositional patterns in the Ganges basin. Nature 357, 680–683. Burgos, J.D. (2006) Genese et progradation d’un cône alluvial au front d’une chaîne active: exemple des Andes équatoriennes au Néogène. PhD thesis, Université Paul Sabatier, Toulouse, 373 pp. Campbell, C.J. (1970) Guide to the Puerto Napo area, Eastern Ecuador with notes on the regional Geology of the Oriente Basin. Quito.

Hoorn_ch05_Final.indd 85

85

Campbell, J.K.E., Frailey, C.D., Romero-Pittman, L. (2006) The Pan-Amazonian Ucayali Peneplain, late Neogene sedimentation in Amazonia, and the birth of the modern Amazon River system. Palaeogeogr Palaeocl 239, 166–219. Campbell, K.E., Heizler, M., Frailey, C.D., Romero-Pittman, L., Prothero, D.R. (2001) Upper Cenozoic chronostratigraphy of the southwestern Amazon Basin. Geology 29. 595–598. Cardozo, N., Jordan, T. (2001) Causes of spatially variable tectonic subsidence in the Miocene Bermejo Foreland Basin, Argentina. Basin Res 13, 335–357. Carpenter, D., Berumen, M. (1999) Geological and geochemical modeling of the fold and thrust belt of southeastern Peru. In: INGEPET’99 – Exploration and Exploration of Petroleum and Gas, Lima. Carrapa, B., DeCelles, P.G. (2008) Eocene exhumation and basin development in the Puna of northwestern Argentina. Tectonics 27, TC1015. Catuneanu, O. (2004) Retroarc foreland systems – evolution through time. J Afr Earth Sci 38, 225–242. Catuneanu, O., Beaumont, C., Waschbusch, P. (1997) Interplay of static loads and subduction dynamics in foreland basins: Reciprocal stratigraphies and the “missing” peripheral bulge. Geology 25, 1087–1090. Catuneanu, O., Hancox, J., Rubidge, B.S. (1998) Reciprocal flexural behaviour and contrasting stratigraphies: a new basin development model for the Karoo retroarc foreland system, South Africa. Basin Res 10, 417–439. Christophoul, F., Baby, P., Davila, C. (2002a) Stratigraphic responses to a major tectonic event in a foreland basin: the Ecuadorian Oriente Basin from Eocene to Oligocene times. Tectonophysics 345, 281–298. Christophoul, F., Baby, P., Soula, J.-C., Rosero, M., Burgos, J. (2002b) Les ensembles fluviatiles neogenes du bassin subandin d’Equateur et implications dynamiques [The Neogene fluvial systems of the Ecuadorian foreland basin and dynamic inferences]. CR Geosci 334, 1029–1037. Christophoul, F., Elibana, B., Brusset, S., Soula, J.-C., Bessière, G., Roddaz, M. (2003) Deformational events and sequential balancing of alpine structures, on the eastern Pyrenees northern flank. In: McCann, T., Saintot, A. (eds) Tracing Tectonic Deformation Using the Sedimentary Record. Geological Society of London, Special Publications, pp. 229–252. Colletta, B., Roure, F., de Toni, B., Loureiro, D., Passalacqua, H., Gou, Y. (1997) Tectonic inheritance, crustal architecture, and contrasting structural styles in the Venezuela Andes. Tectonics 16, 777–794. Cooper, M.A., Addison, F.T., Alvarez, R., Coral, M., Graham, R.H., Hayward, A.B. et al. (1995) Basin development and tectonic history of the Llanos Basin, Eastern Cordillera, and middle Magdalena Valley, Colombia. AAPG Bulletin 79, 1421–1443. Crampton, S.L., Allen, P.A. (1995) Recognition of forebulge unconformities associated with early stage foreland basin development: Example from the North Alpine foreland basin. Am Assoc Petrol Geol Bull 79, 1495–1514. Dávila, F.M., Astini, R.A., Jordan, T.E., Gehrels, G.E., Ezpeleta, M. (2007) Miocene forebulge development previous to broken foreland partitioning in the southern Central Andes, west-central Argentina. Tectonics 26, TC5016. Davila, J., Vargas, C., Ponce de Leon, V. (1965) Informe sobre la geologia del bloque Andino noroccidental y la flaja subandina del norte. YPFB Bolivia internal report La Paz, Bolivia. DeCelles, P.G., Giles, K.A. (1996) Foreland basin systems. Basin Res 8, 105–123.

10/27/2009 12:59:47 Shobha

86

M. Roddaz et al.

DeCelles, P., Horton, B.K. (2003) Early to middle Tertiary basin development and the history of Andean crustal shortening in Bolivia. Geol Soc Am Bull 115, 58–77. Díaz, G., Milla, D., Huayhua, J., Montoya, A., Castro, W. (1998) Geología de los cuadrágulos de Tocache (17-j) y Uchiza (17-k). Lima: INGEMMET. Dickinson, W.R. (1974) Plate Tectonics and Sedimentation. Special Publication, SEPM (Society for Sedimentary Geology), pp. 1–27. Edwards P. (1993) Villano 2: Completion Report, Arco Oriente. PetroEcuador (unpublished report), pp. 4–13. Espurt, N., Baby, P., Brusset, S., Roddaz, M., Hermoza, W., Regard, V. et al. (2007) How does the Nazca ridge subduction influence the modern Amazonian foreland basin? Geology 35, 515–518. Fauchet, B., Savoyat, E. (1973) Esquisse géologique des Andes de l’Equateur. Revue de Géographie Physique et de Géologie Dynamique 15, 115–142. Finer, M., Jenkins, C.N., Pimm, S.L., Keane, B., Ross, C. (2008) Oil and gas projects in the western Amazon: threats to wilderness, biodiversity, and indigenous peoples. PLoS ONE 3, e2932. Garcia-Castellanos, D., Fernàndez, M., Torne, M. (2002) Modeling the evolution of the Guadalquivir foreland basin (southern Spain). Tectonics 21, 1018. Gil, W. (2001) Evolution latérale de la déformation d’un front orogénique: Exemple des bassins subandins entre 0° et 16°S. PhD thesis, Université Paul Sabatier, Toulouse, 150 pp. Gingras, M.K., Räsänen, M.E., Ranzi, A. (2002) The significance of bioturbated inclined heterolithic stratification in the southern part of the Miocene Solimões Formation, Rio Acre, Amazonia Brazil. Palaios 17, 591–601. Gómez, E., Pedraza, P. (1994) El Maastrichtiano de la región HondaGuaduas, límite norte del Valle Superior del Magdalena: Registro sedimentario de un delta dominado por ríos trenzados. In: EtayoSerna, F., Laverde, F. (eds) Estudios Geológicos del Valle Superior del Magdalena. Universidad Nacional de Colombia/Ecopetrol, Publicación Especial, pp. III1–III20. Gómez, E., Jordan, T.E., Allmendinger, R.W., Hegarty, K., Kelley, S., Heizler, M. (2003) Controls on architecture of the Late Cretaceous to Cenozoic southern Middle Magdalena Valley Basin, Colombia. Bull Geol Soc Am 115, 131–147. Gómez, E., Jordan, T.E., Allmendinger, R.W., Cardozo, N. (2005) Development of the Colombian foreland-basin system as a consequence of diachronous exhumation of the northern Andes. Bull Geol Soc Am 117, 1272–1292. Gurnis, M. (1992) Rapid continental subsidence following the initiation and evolution of subduction. Science 255, 1556–1558. Gutiérrez, M. (1975) Contribucion al conocimento micropaleontológico del Oriente Peruano. Boletín de la Sociedad Geológica del Peru 49, 25–52. Haq, B.U., Hardenbol, J., Vail, P.R. (1987) Chronology of fluctuating sea levels since the Triassic (250 million years ago to present). Science 235, 1156–1167. Heller, P.L., Paola, C. (1992) The large scale dynamics of grain-size variation in alluvial basins, 2: application to syntectonic conglomerates. Basin Res 4, 91–102. Heller, P.L., Angevine, C.L.T., Winslow, N.S., Paola, C. (1988) Twophase stratigraphic model of foreland-basin sequences. Geology 16, 501–504. Hérail, G., Sharp, W., Viscarra, G., Fornari, M. (1994) La edad de la Formacíon Cangalli: nuevos datos geocronologicos y su significado geologico. In: Memorias del XI Congreso Geologico de Bolivia, La Paz. Hermoza, W. (2004) Dynamique tectono-sédimentaire et restauration séquentielle du retro-bassin d’avant-pays des Andes Centrales. PhD thesis, Université Paul Sabatier, Toulouse.

Hoorn_ch05_Final.indd 86

Hermoza, W., Bolanos, R., Baby, P., Roddaz, M., Brusset, S. (2005a) El Cenozoico de la Cuenca Maranon: Dinamica sedimentaria e implicancias sobre los sistemas petroleros. Lima: INGEPET, 20 pp. Hermoza, W., Brusset, S., Baby, P., Gil, W., Roddaz, M., Guerrero, N. (2005b) The Huallaga foreland basin evolution: thrust propagation in deltaic environment, in the northern Peruvian Andes. J S Am Earth Sci 19, 21–34. Hernández, R.M., Jordan, T.E., Dalenz Farjat, A., Echavarría, L., Idleman, B.D., Reynolds, J.H. (2005) Age, distribution, tectonics, and eustatic controls of the Paranense and Caribbean marine transgressions in southern Bolivia and Argentina. J S Am Earth Sci 19, 495–512. Hoorn, C., Kaandorp, M.C.N., Roele, J. (1987) Tertiary Sediments of the Usme Valley, Colombia: a Palynological and Stratigraphic Approach. MSc thesis, University of Amsterdam, Amsterdam. Horton, B.K. (1999) Erosional control on the geometry and kinematics of thrust belt development in the central Andes. Tectonics 18, 1292–1304. Horton, B.K., DeCelles, P.G. (1997) The modern foreland basin system adjacent to the central Andes. Geology 25, 895–898. Hovikoski, J., Räsänen, M.E., Gingras, M.K., Roddaz, M., Brusset, S., Hermoza, W., Pittman, L.R. (2005) Miocene semi-diurnal tidal rhythmites in Madre de Dios, Peru. Geology 33, 177–180. Hovikoski, J., Rasanen, M., Gingras, M., Lopez, S., Romero, L., Ranzi, A., Melo, J. (2007) Palaeogeographical implications of the Miocene Quendeque Formation (Bolivia) and tidally influenced strata in southwestern Amazonia. Palaeogeogr Palaeocl 243, 23–41. Hungerbuhler, D., Steinmann, M., Winkler, W., Seward, D., Eguez, A., Peterson, D.E. et al. (2002) Neogene stratigraphy and Andean geodynamics of southern Ecuador. Earth Sci Rev 57, 75–124. Jaramillo, C. (2007) Quantitative biostratigraphy. In: IV Congreso Colombiano de Botanica. Medellin. Jaramillo, C., Muñoz, F., Cogollo, M., Parra, F. (2005) Quantitative biostratigraphy for the Paleocene of the Llanos foothills, Colombia: Improving palynological resolution for oil exploration. In: Powell A.J., Riding J. (eds) Recent Developments in Applied Biostratigraphy. Micropalaeontological Society Special Publication TMS001, pp. 145–159. Jaramillo, C., Rueda, M., Bayona, G., Santos, C., Florez, P., Parra, F. (2009) Biostratigraphy breaking paradigms: dating the Mirador Formation in the Llanos Basin of Columbia. In: Demchuk T., Waszczak R. (eds) Geologic Problem Solving with Microfossils. Society for Sedimentary Geology Special Publication (in press). Jordan, T.E. (1995) Retroarc foreland and related basins. In: Busby, C.J., Ingersoll, R.V. ( eds) Tectonics of Sedimentary Basins. Oxford: Blackwell Science, pp. 331–362. Jordan, T.E., Isacks, B.L., Allmendinger, R.W., Brewer, J.A., Ramos, V.A., Ando, C.J. (1983) Andean tectonics related to geometry of subducted Nazca plate. Geol Soc Am Bull 94, 341–361. Jordan, T.E., Schlunegger, F., Cardozo, N. (2001) Unsteady and spatially variable evolution of the Neogene Andean Bermejo foreland basin, Argentina. J S Am Earth Sci 14, 775–798. Jordan, T.E., Mpodozis, C., Muñoz, N., Blanco, N., Pananont, P., Gardeweg, M. (2007) Cenozoic subsurface stratigraphy and structure of the Salar de Atacama Basin, northern Chile. J S Am Earth Sci 23, 122–146. Kammer, A. (2003) La Formación Tilatá en los alrededores de Chocontá: Marco tectónico y ambientes deposicionales. Análisis Geográficos 26, 69–100. Kummel, B. (1946) Estratigrafía de la región de Santa Clara. Boletín de la Sociedad Geológica del Perú 14, 133–152. Leech, M.L. (2001) Arrested orogenic development: eclogitization, delamination, and tectonic collapse. Earth Planet Sci Lett 185, 149–159.

10/27/2009 12:59:47 Shobha

The Amazonian foreland basin system Lorente, M.A. (1986) Palynology and palynofacies of the Upper Tertiary in Venezuela. Dissertationes Botanicae 99, 1–222. Marocco (1993) Informe preliminar sobre el éstudio de la sedimentacion Neogena del Nororiente del Peru, en base al análisis de pozos. Lima: Petroperu. Marocco, R., Valdez Pardo, A.M., Rivadeneira, M. (1996). Sedimentología de las formaciones de edad eocena y oligocena de la Cuenca Orienta. IRD-Petroproducción. Martin-Gombojav, N., Winkler, W. (2008) Recycling of Proterozoic crust in the Andean Amazon foreland of Ecuador: implications for orogenic development of the Northern Andes. Terra Nova 20, 22–31. Miall, A.D. (1996) The Geology of Fluvial Deposits, Sedimentary Facies, Basin Analysis and Petroleum Geology. Berlin, Heidelberg and New York: Springer, 582 pp. Mitrovica, J.X., Beaumont, C., Jarvis, G.T. (1989) Tilting of continental interiors by the dynamical effects of subduction. Tectonics 8, 1079–1094. Mobil Oil Corporation (1998) Phase I. Technical Synthesis Tampobata Lote 78. Perupetro S.A, Lima, Peru. Montes, C., Hatcher Jr, R.D., Restrepo-Pace, P.A. (2005) Tectonic reconstruction of the northern Andean blocks: Oblique convergence and rotations derived from the kinematics of the PiedrasGirardot area, Colombia. Tectonophysics 399, 221–250. Mora, A. (2007) Inversion tectonics and exhumation processes in the Eastern Cordillera of Colombia. PhD thesis, Universität Potsdam, 133 pp. Mora, A., Venegas, D., Vergara, L. (1998) Estratigrafia del Cretacico Superior y Terciario Inferior en el Sector Norte de la Cuenca del Putumayo, Departamento del Caqueta, Colombia. Geologia Colombiana 23, 31–77. Muller, J., de Di Giacomo, E., Van Erve, A.W. (1987). A palynological zonation for the Cretaceous, Tertiary, and Quaternary of northern South America. American Association of Stratigraphic Palynologists Contributions 19, 7–76. Olivier, J., Otto, T., Roddaz, M., Antoine, P.-O., Londoño, X., Clark, L.G. (2009) First macrofossil evidence of a pre-Holocene thorny bamboo cf. Guadua (Poaceae: Bambusoideae: Bambuseae: Guaduinae) in south-western Amazonia (Madre de Dios – Peru). Rev Palaeobot Palynol 153, 1–7. Parra, M., Mora, A., Romero, O., Moron, S. (2006) Secciones estratigráficas detalladas de las Formaciones Carbonera (C1), León y el Grupo Guayabo en los Sinclinales de Nazareth y Medina, Piedemonte Llanero, Cordillera Oriental de Colombia. Internal Report, Smithsonian Tropical Research Institute. Parra, M., Mora, A., Jaramillo, C., Strecker, M.R., Sobel, E.R., Quiroz, L.I. et al. (2009) Orogenic-advance in the northern Andes: evidence from the Oligocene-Miocene sedimentary record of the Medina Basin, Eastern Cordillera, Colombia. Geol Soc Am Bull 121, 780–800. Pérez, G., Salazar, A. (1978) Estratigrafia y facies del Grupo Guadalupe. Geología Colombiana 10, 7–85. Perrier, R., Quiblier, S. (1974) Thickness changes in sedimentary layers during compaction history: methode for quantitative evaluation. Am Assoc Petrol Geol Bull 58, 507–520. Pulham, A.J., Mitchell, A., MacDonald, D., Daly, C. (1997) Sequence stratigraphic evaluation and production characteristics of the Eocene Mirador Formation, Cusiana Field, eastern Colombia. Papers presented at the Gulf Coast Section, Society of Economic Paleontologists and Mineralogists Foundation Annual Research Conference. Houston, TX: Society of Economic Paleontologists, Gulf Coast Section (GCSSEPM) Foundation, pp. 225–233 Pysklywec, R.N., Mitrovica, J.X. (1999) The role of subduction-induced subsidence in the evolution of the Karoo Basin. J Geol 107, 155.

Hoorn_ch05_Final.indd 87

87

Pysklywec, R.N., Mitrovica, J.X. (2000) Mantle flow mechanisms of epeirogeny and their possible role in the evolution of the Western Canada Sedimentary Basin. Can J Earth Sci/Rev Can. Sci. Terre 37, 1535–1548. Ramon, J.C., Rosero, A. (2006) Multiphase structural evolution of the western margin of the Girardot subbasin, Upper Magdalena Valley, Colombia. J S Am Earth Sci 21, 493–509. Rebata-Hernani, L.A., Gingras, M.K., Rasanen, M.E., Barberi, M. (2006a) Tidal-channel deposits on a delta plain from the Upper Miocene Nauta Formation, Marañón Foreland Sub-basin, Peru. Sedimentology 53, 971–1013. Rebata, H.L.A., Räsänen, M.E., Gingras, M.K., Vieira, J.V., Barberi, M., Irion, G. (2006b) Sedimentology and ichnology of tide-influenced Late Miocene successions in western Amazonia: The gradational transition between the Pebas and Nauta formations. J S Am Earth Sci 21, 96–119. Roddaz, M. (2004) Transition des stades alimentés à suralimentés dans les systèmes de rétro-bassin d’avant-pays: exemple du bassin amazonien. PhD thesis, Université de Toulouse, Toulouse, 332 pp. Roddaz, M., Hermoza, W., Hovikoski, J., Brusset, S., Antoine, P.-O., Baby, P. (2004) Late Miocene tidal sedimentation in the foredeep of the southwestern Amazonian foreland basin system (Madre de Dios basin, South Peru). In: Bartholdy, J., Pedersen, J.B.T (eds) Tidalites 2004. Copenhagen: Institute of Geography, University of Copenhagen, pp. 161–164. Roddaz, M., Baby, P., Brusset, S., Hermoza, W., Darrozes, J. (2005a) Forebulge dynamics and environmental control in western Amazonia: the case study of the arch of Iquitos (Peru). Tectonophysics 399, 87–108. Roddaz, M., Viers, J., Brusset, S., Baby, P., Herail, G. (2005b) Sediment provenances and drainage evolution of the Neogene Amazonian foreland basin. Earth Planet Sci Lett 239, 57–78. Roddaz, M., Brusset, S., Baby, P., Hérail, G. (2006) Miocene tidalinfluenced sedimentation to continental Pliocene sedimentation in the forebulge-backbulge depozones of the Beni-Mamore foreland basin (northern Bolivia). J S Am Earth Sci 20, 79–96. Ruiz, G.M. (2002) Exhumation of the Northern Sub-Andean Zone of Ecuador and its Source Region: a Combined Thermochronological and Heavy Mineral Approach. Zurich: ETH, 260 pp. Ruiz, G.M.H., Seward, D., Winkler, W. (2004) Detrital thermochronology – a new perspective on hinterland tectonics, an example from the Andean Amazon Basin, Ecuador. Basin Res 16, 413–430. Ruiz, G.M.H., Seward, D., Winkler, W. (2007) Evolution of the Amazon Basin in Ecuador with special reference to hinterland tectonics: data from zircon fission-track and heavy mineral analysis. Developments in Sedimentology 58, 907–934. Rull, V. (2002) High-impact palynology in petroleum geology: Applications from Venezuela (northern South America). Am Assoc Petrol Geol Bull 86, 279–300. Sánchez, A., Herrera, I. (eds) (1998) Geología de los cuadrágulos de Moyobamba (13-j), Saposoa (14-j) y Juanjui (15-j). Boletin, Serie A: Carta Geológica Nacional 122. Lima: INGEMMET, 268 pp. Santos, C., Jaramillo, C., Bayona, G., Rueda, M., Torres, V. (2008) Late Eocene marine incursion in north-western South America. Palaeogeogr Palaeocl 264, 140–146. Seminario, F., Guizado, J. (1976) Síntesis Bioestratigráfica de la Región de la Selva Del Perú. In: II Congreso Latino Americano de Geológia, Caracas, Venezuela, pp. 881–898. Sarmiento, L.F. (2002) Mesozoic rifting and Cenozoic basin inversion history of the Eastern Cordillera, Colombian Andes: Inferences from tectonic models. PhD dissertation, Vrije Universiteit, Amsterdam, 295 pp.

10/27/2009 12:59:47 Shobha

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M. Roddaz et al.

Sempere, T., Hérail, G., Oller, J., Bonhomme, M.G. (1990) Late Oligocene-early Miocene major tectonic crisis and related basins in Bolivia. Geology 18, 946–949. Sinclair, H.D. (1997) Tectonostratigraphic model for underfilled peripheral foreland basins: An Alpine perspective. Geol Soc Am Bull 109, 324–346. Sinclair, H.D., Allen, P.A. (1992) Vertical versus horizontal motions in the Alpine orogenic wedge: stratigraphic response in the foreland basin. Basin Res 4, 215–232. Spikings, R.A., Winkler, W., Seward, D., Handler, R. (2001) Alongstrike variations in the thermal and tectonic response of the continental Ecuadorian Andes to the collision with heterogeneous oceanic crust. Earth Planet Sci Lett 186, 57–73. Spikings, R.A., Winkler, W., Hughes, R.A., Handler, R. (2005) Thermochronology of allochthonous terranes in Ecuador: Unravelling the accretionary and post-accretionary history of the Northern Andes. Tectonophysics 399, 195–220. Strub, M. (2006) Evolution géomorphologique et tectonique au front des Andes Centrales (Nord de la Bolivie). PhD thesis, University of Toulouse, 182 pp. Strub, M., Hérail, G., Darrozes, J., García-Duarte, R., Astorga, G. (2005) Neogene to Present tectonic and orographic evolution of the Beni Subandean Zone. In: 6th International Symposium on Andean Geodynamics (ISAG 2005) Barcelona, pp. 709–713. Tschopp, H.J. (1953) Oil explorations in the Oriente of Ecuador. Am Assoc Petrol Geol Bull 37, 2303–2347. Ungerer, P., Burrus, J., Doligez, B., Bessis, F. (1990) Basin evaluation by integrated two-dimensional modeling of heat transfer, fluid flow,

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hydrocarbon generation and migration. Am Assoc Petrol Geol Bull 74, 309–335. Valdez Pardo, A.M. (1997) Reinterpretación sedimentológica, estratigráfica de la Formación Tiyuyacu y su relación con la tectónica del Terciario inferior. Universidad Central del Ecuador, 141 pp. Valdivia, H. (1974) Estratigrafia de la Faja Subandina de la region de Madre de Dios. Lima: Petroperu. Vallejo, C., Spikings, R.A., Luzieux, L., Winkler, W., Chew, D., Page, L. (2006) The early interaction between the Caribbean Plateau and the NW South American Plate. Terra Nova 18, 264–269. Wesselingh, F., Guerrero, J., Räsänen, M.E., Romero Pittman, L., Vonhof, H.B. (2006) Landscape evolution and depositional processes in the Miocene Amazonian Pebas lake/wetland system: evidence from exploratory boreholes in northeastern Peru. Scripta Geologica 133, 323–361. White, T., Furlong, K., Arthur, M. (2002) Forebulge migration in the Cretaceous Western Interior basin of the central United States. Basin Res 14, 43–54. Whiting, B.M., Thomas, W.A. (1994) Three-dimensional controls on subsidence of a foreland basin associated with a thrust-belt recess: Black Warrior basin, Alabama and Mississippi. Geology 22, 727–730. Williams, M.D. (1949) Depósitos terciarios continentales del Valle del Alto Amazonas. Boletín de la Sociedad Geológica del Perú, Vol. Jubilar parte 2. Zambrano, I., Ordoñez, M., Jiménez, N. (1999) Micropaleontologia de 63 muestras de afloramientos de la Cuenca Oriental Ecuatoriana. Petroproducción, distrito de Guayaquil.

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The Nazca Ridge and uplift of the Fitzcarrald Arch: implications for regional geology in northern South America Nicolas Espurt1, Patrice Baby2, Stéphane Brusset2, Martin Roddaz2, Wilber Hermoza3 and Jocelyn Barbarand4 1

Université Paul Cézanne, Aix en Provence, France Université de Toulouse, France 3 REPSOL-YPF, Madrid, Spain 4 Université Paris Sud, France 2

Abstract The Fitzcarrald Arch corresponds to a NE–SW-trending widespread dissected relief of about 400,000 km2 located to the east of the sub-Andean thrust front. Analyses of drainage maps demonstrate that this regional uplift controls the modern Amazonian drainage network. During the Miocene, the Fitzcarrald Arch did not exist and this area corresponded to the subsiding foredeep of the Amazonian foreland basin. Analyses of the Miocene deposits outcropping within the Fitzcarrald Arch area indicate that deposition was controlled by tidal current and consisted of tide-dominated deltas and estuaries. The tidal Miocene deposits were followed by fluvial conglomerate and sandstone sediments, probably Pliocene to Pleistocene in age. Modelling of the vitrinite reflectance data shows that the uplift of the Fitzcarrald Arch began in Pliocene times; this date is further confirmed by sedimentary evidence such as provenance and a change of palaeocurrent directions from the Pliocene and Pleistocene record. Geophysical data show that the Fitzcarrald Arch uplift superimposed on the eastern part of the Nazca Ridge flat slab segment. The flat subduction process linked to the buoyancy of the Nazca Ridge has been active since ~4 Ma. Thus, the coincidence of the initiation of flat slab subduction with the Pliocene uplift of the Fitzcarrald Arch implies that the flat subduction of the Nazca Ridge is the cause of uplift. The Nazca Ridge flat subduction is the process responsible for the modern configuration of the Amazon drainage basin and may be one of the decisive factors that triggered large-scale modification of the Amazonian landscape inducing drastic biota changes in the Amazonian basin during the last 4 million years.

Introduction The Fitzcarrald Arch is a major geomorphic feature of about 400,000 km2 in the landscape of the Amazon drainage basin. It divides the northern Amazonian foreland basin from the southern Amazonian foreland basin, both of which are driven by Andean orogenic loading (Roddaz et al. 2005). The Fitzcarrald Arch makes the Amazon foreland basin an atypical foreland basin, not like the fourcomponent model of DeCelles & Giles (1996) (see also Chapter 5). The causes of the appearance of Fitzcarrald Arch regional uplift are the subject of active discussion. Some authors have proposed Amazonia, Landscape and Species Evolution: A Look into the Past, 1st edition. Edited by C. Hoorn and F.P. Wesselingh. © 2010 Blackwell Publishing

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alternative hypotheses for the Fitzcarrald Arch uplift: surface expression of inherited Paleozoic heterogeneities (Jacques 2003; Carlotto et al. 2007), simple mega-fan geometry (Picard et al. 2008) or a relatively higher stress regime above a subduction zone (Dumont 1996). Based on geomorphic, geophysical and geodynamic evidence, Espurt et al. (2007) have demonstrated that the high elevation of the Fitzcarrald Arch was caused by the flat slab subduction of the aseismic Nazca Ridge below northern South America. These authors estimated the uplift of the Fitzcarrald Arch to be no older than Pliocene (Espurt et al. 2007). This chapter reviews the criteria that permitted Espurt et al. (2007) to link the subduction of the aseismic Nazca Ridge and the uplift of the Fitzcarrald Arch. We also constrained better spatial and time relationships between the Nazca Ridge subduction and the Fitzcarrald Arch uplift in the Amazonian foreland basin,

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through addition of new geomorphic, sedimentological and well log data. The consequences of the Nazca Ridge subduction and the related Fitzcarrald Arch uplift on the modern Amazonian basin evolution and its related biota are then discussed.

The Nazca Ridge feature The Nazca Ridge is one of the major oceanic ridges subducting below the South American plate (Fig. 6.1). This ridge has an average bathymetric relief of 1.5 km above the adjacent sea floor of the Nazca Plate, a maximum width of 200 km at its base, and an average crustal thickness of 18 ± 3 km (Woods & Okal 1994; Tassara et al. 2006). The Nazca Ridge formed from the activity of a hotspot located near the Pacific spreading ridge (Pilger 1981), and the Tuamotu mirror ridge is visible on the eastern Pacific Plate (Gutscher et al. 1999). The ridge migrates southwards below the South American Plate because the ridge segment is N45°E trending, oblique to the N78° present-day plate convergence (Gripp & Gordon 2002). The Nazca Ridge subduction beneath the Peruvian coast started 11.2 Ma at 11°S (Hampel 2002), and constitutes the southern edge of the Peruvian flat slab segment (Gutscher et al. 1999).

Andean Nazca Ridge subduction imprints The southward migration of the Nazca Ridge between 11°S and 17°S has been recorded in the geomorphology and tectonics of the Peruvian Andes (see Fig. 6.1). Along the forearc region, the ridge migration generated N–S variations of the rates of subsidence and erosion of the coastal margin (von Huene & Suess 1988; von Huene et al. 1996;

n ea nd

-A

b Su

–7000

Fig. 6.1 Geomorphic map of western South America. The dashed black line shows the boundary of the Amazon drainage basin (modified from Guyot et al. 2007). The Fitzcarrald Arch divides the western Amazonian foreland basin into two parts: the northern Amazonian foreland basin (NAFB) and the southern Amazonian foreland basin (SAFB). To the east, the arch is bound by the intracratonic Amazonian basins (EAB). The plate convergence vector is from Gripp & Gordon (2002). The projection of the Nazca Ridge beneath the South American plate is from Hampel (2002).

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Le Roux et al. 2000; Hampel 2002; Clift et al. 2003), and ~800 m maximum uplift of Pliocene-Pleistocene marine terraces (Hsu 1992; Macharé & Ortlieb 1992; Saillard 2008; Wipf et al. 2008). The Cordillera Blanca region (Central Andes), located northwards of the present-day Nazca Ridge subducted segment, shows recent extensional collapses in a prethickened continental crust, which are interpreted as the consequence of the underplating of the Nazca Ridge (McNulty & Farber 2002). In addition, the southward migration of the Nazca Ridge is recorded by Neogene counterclockwise rotations in the Eastern Cordillera (Rousse et al. 2003). The eastern continuation of the Nazca Ridge is marked by a seismic gap observed at the sub-Andes–Amazonian foreland boundary (Gutscher et al. 1999; Hampel 2002), which coincides spatially with the relief of the Fitzcarrald Arch (see Fig. 6.1) (Espurt et al. 2007).

Morphological records, drainage system and crustal structure of the Fitzcarrald area The structural and morphological Fitzcarrald Arch includes a large part of the western Amazon drainage basin (see Fig. 6.1). The Fitzcarrald Arch forms a WSW–ENE-trending upland in southeastern Peru and western Brazil that covers about 400 000 km2, i.e. ~15% of the total surface of the Amazon floodplain (Guyot et al. 2007). The Fitzcarrald Arch develops east of the sub-Andean thrust front and south of the Contaya-Moa thrusts, where no thrust deformation occurs (Fig. 6.2a). It develops between the northwestern Ucayali sub-basin to the northeast, and Madre de Dios sub-basin to the southeast, and is bounded to the northeast by the Acre sub-basin (Kronberg et al. 1998; see also Chapter 3). The Fitzcarrald Arch displays a higher mean elevation surface (~615 m asl [above sea level]) compared to its bounding Ucayali and Madre de Dios sub-basins (~120 m asl and ~150 m asl, respectively). The Fitzcarrald Arch forms a main drainage divide (Church 1901; Dumont 1996; Espurt et al. 2007; Regard et al. 2009) between the Ucayali sub-basin, the Acre sub-basin and the Madre de Dios sub-basin. The drainage network has been digitized from Shuttle Radar Topographic Mission (SRTM) digital elevation model 90 m using the TAS freeware (Terrain Analysis System; Lindsay 2005). Seventy-one sub-basins of seventh order, representing an average area of ~3660 km2, have been defined on the Fitzcarrald Arch (see Fig. 6.2a) according to Strahler’s method (1952) (see Regard et al. 2009 for more details). The mean orientation and elongation of each sub-basin have been measured. At a large scale, the Fitzcarrald Arch disturbs the present-day drainage network of the Amazon basin in generating an asymmetrical radial drainage. The main part of the drainage network displays a northeasterly direction (average value of N52°E; see Fig. 6.2b), toward the Acre sub-basin. The Fitzcarrald Arch divides the drainage basin of the Ucayali to the northwest, and that of the Madre de Dios to the southeast. The apex of the drainage divide is located east of the sub-Andean thrust front, at roughly 10.5°S, 72.5°W (see Fig. 6.2a). Most of the basins located west of the apex exhibit drainage directions towards the Andes (see Fig. 6.2a,b). At a local scale, seventhorder sub-basins develop on tilted Miocene and Pliocene surface bedding (Espurt et al. 2007; Regard et al. 2009). These sub-basins have elongated surfaces and are poorly organized with parallel and subparallel patterns (see Fig. 6.2a).

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The Nazca Ridge and Fitzcarrald Arch The Fitzcarrald Arch exposes mainly Miocene and Pliocene sedimentary deposits, whose bedding is parallel to the topographic surface (see Fig. 6.2a). Dip of the bedding has been measured along regional topographic cross-sections from the SRTM digital elevation model (see Fig. 6.2a). The eastern Brazilian flank of the arch shows low-magnitude bedding dipping ~0.02° to the ENE. On the northwestern Ucayali flank of the Arch, bedding dips ~0.3° northwestwards. The most prominent tilted bedding surface is found near the Ucayali River. This surface connects down with the Ucayali River and tops thick Pliocene conglomerate deposits (Dumont et al. 1991; Räsänen et al. 1995). The surface tilting seems to postdate the drainage over the surface of several tributaries of the Ucayali River (Dumont et al. 1991). The northern flank of the Fitzcarrald Arch is greatly incised and exhibits mainly Miocene strata. In contrast, the southeastern Madre de Dios flank of the arch presents more gently tilted bedding dipping ~0.1° to the southeast (see Fig. 6.2a). The NW–SE-trending regional seismic reflection profile provided by PeruPetro S.A. (see Fig. 6.2c) shows that the Fitzcarrald Arch forms a bulge of lithospheric-scale wavelength (340 km minimum).

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This bulge is underlain by ~2.5 km-thick Cretaceous and Cenozoic strata of nearly constant thickness. On this profile, the Paleozoic sedimentary basins, the Paleozoic thrusts (e.g. Paititi High) and other pre-Cretaceous structures (e.g. Manu Arch) are unconformably overlain by Cretaceous strata and present a local-scale wavelength of ~100 km. These older structures, eroded and sealed by Cretaceous strata, have been subsequently uplifted and involved in the lithospheric-scale structure of the Fitzcarrald Arch (Espurt et al. 2007). The Neogene sedimentary rocks are partially eroded and exposed in both flanks of the Fitzcarrald Arch. Thickness variations in the Neogene sediments, that could support a synsedimentary Neogene uplift of the Fitzcarrald Arch, are not found (see Fig. 6.2c).

Miocene to Pleistocene sedimentological record of the Fitzcarrald Arch area We describe here outcrops of the Neogene succession in three representative areas of the Fitzcarrald Arch (Fig. 6.3): the northwestern

(a)

(b)

Su b-

An

de an

(c)

Fig. 6.2 (a) Map of the seventh-order drainage sub-basins of the Fitzcarrald Arch area. The background corresponds to a digital elevation model from Shuttle Radar Topographic Mission (SRTM) 90 m data. Basins are bounded by white lines. The apex of the drainage sub-basins of the Fitzcarrald Arch is shown by the black star. The main structural features are shown. (b) Rose diagram of the modern drainage orientations of the drainage sub-basins of order 7. (c) Seismic reflection profile A–A’ of the Fitzcarrald Arch uplift showing the very-long-wavelength bending of the subsurface sediments of the Fitzcarrald Arch. The dashed black line shows the supposed top of the Miocene strata. SW, Sepa well; MW, Mashansha well; PW, Panguana well; PPW, Puerto Primo well; pjt, projected onto the seismic profile.

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Unconf.

Juanjui/Corrientes Fm (Pliocene ~3–4 Ma)

Well

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Madre de Dios Fm (Late Miocene ~9 Ma) Upper Ipururo Fm (>13 Ma)

Seismic line (see Fig. 2c) Outcrop discussed in the text

Lower Ipururo Fm (~13 Ma) Paleogene Cenozoic undifferentiated, Mesozoic and Palaeozoic rocks/structural highs

River City

Fig. 6.3 Geological map of the Fitzcarrald Arch. Numbers 1–14 refer to locations of outcrops discussed in the text: 1, Mapuya River (Espurt et al. 2006; Antoine et al. 2007); 2, Sepa River; 3, Urubamba River; 4, Purus River (Räsänen et al. 1995, Hovikoski et al. 2008); 5, (Cozzuol, 2006); 6, Patos Inca (Cozzuol 2006); 7, Alto Purus River (Campbell et al. 2001); 8, Manú River; 9, Madre de Dios River; 10, Inambari River (Roddaz 2004; Roddaz et al. 2006; Hovikoski et al. 2005); 11, Tambo River; 12, Chicosillo River (Espurt et al. 2006); 13, Las Piedras River (Campbell et al. 2001); 14, Lower Madre de Dios River (Roddaz 2004); 15, Ucayali River (Dumont et al. 1991). Locations of the Mashansha well (MW), Sepa well (SW), Panguana well (PW) and Puerto Primo well (PTW) are shown. The location of the seismic profile of Fig. 6.2c is also shown. Fm, formation.

flank of the arch (Ucayali sub-basin) where the oldest sediments outcrop; the northeastern flank of the arch (Acre sub-basin); and the southeastern flank of the arch (Madre de Dios sub-basin).

Miocene deposits The oldest sediments outcropping on the Fitzcarrald Arch area are Miocene in age and part of the Ipururo and Madre de Dios Formations in Peru (Kummel 1948), and of the Solimões Formation in Brazil (Oppenheim 1946; Silva 1988). The northern

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flank of the arch exhibits late Middle Miocene deposits, which are well exposed along the Sepa, Urubamba, Inuya and Mapuya Rivers, where the arch is the most deeply incised (see Fig. 6.3). The strata of the Ipururo Formation are observed along the Mapuya River (10.43086°S, 73.05145°W) (Fig. 6.4a; see also Fig. 6.3) (Espurt et al. 2006). The base of the section is composed of sandstones exhibiting cyclic lateral variation of foreset thicknesses underlined by mud clasts, interpreted as tidal sand bar and sandstones, with calcrete concretions interpreted as palaeosol deposits (see Fig. 6.4b). Palaeocurrents measured from foresets are mainly to the east. These sandy deposits are eroded by ~1.5 m-thick

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Juanjui Fm

Terraces Fluvial conglomerates

>2m

n = 35 (d)

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PLIOCENE

PLEIS.

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Tidal sand bars

1m E

(b) A Sf Sm Sg Cg

Legend Flaser bedding Planer bedding Herring-bone Climbing-ripple Carbonate concretion

Bioturbation Vertebrate fossil Tooth Wood

Fig. 6.4 (a) Representative section, from the Mapuya and Chicosillo Rivers on the northern flank of the Fitzcarrald Arch, of the Miocene deposits and unconformable Pliocene and Pleistocene syntectonic deposits (see detail in text). (b) Details of the basal conglomerates with wood fragments. (c) Detail of the tidal deposits showing mud-sand couplets defining tidal bundles, mud drapes and double mud drapes characteristic of tidal deposits. (d) Pliocene conglomerate deposits (Chicosillo River). (e) Aggradational Pleistocene terrace deposits eroding the Neogene strata (Chicosillo River). Fm, formation; Pleis, Pleistocene; A, clay; Sf, fine sand; Sm, medium sand; Sg, coarse sand; Cg, conglomerate; SU, unconformity surface.

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channel-shaped conglomerates (lithofacies Gt, sensu Miall 1996) (see Fig. 6.4b), which can be laterally replaced by planar cross-bedded sandstones (lithofacies Sp, sensu Miall 1996). The conglomerates contain sandstone and mudstone clasts, less than 5 cm in diameter, wood fragments and high concentrations of vertebrate fossils and teeth (Antoine et al. 2007; Salas-Gismondi et al. 2007), incorporated in a sandy matrix. The overlying interval is ~1.8 m thick and consists of alternating mud and sand layers exhibiting mud-sand couplets interpreted as tidal bundles (see Fig. 6.4c). Mud drapes and double mud drapes as well as cyclic variations of the bundles are frequent and characterize tide-controlled deposition (Nio & Yang 1991). In the lower part of this interval, the sands exhibit mud-sand couplets with opposite bidirectional stratifications (‘herring-bone’ cross-stratifications) and flaser to lenticular bedding. Reactivation surfaces as well as mud drapes and double mud drapes are common. The palaeocurrent directions are dominantly towards the northeast and southwest (see Fig. 6.4c). These deposits are commonly bioturbated. In the upper part of this interval, type 2 climbing ripples (Jopling & Walker 1986) with sigmoid stratification (Mutti et al. 1985) underlain by mud clasts are frequent and indicate tide-controlled deposition (Nio & Yang 1991). The sedimentary succession ends with mud and sand tidal rhythmites followed by palaeosol deposits in the top (see Fig. 6.4a). The diverse vertebrate Mapuya fauna (~30 taxa; Antoine et al. 2007; Salas-Gismondi et al. 2007) is strongly reminiscent of Late Middle Miocene Laventan faunas from Colombia, Venezuela and Brazil (13.5–11.8 Ma; Madden et al. 1997). In addition, a change in palaeomagnetic polarity data from normal to reverse provides a numerical age around 13 Ma, owing to Laventan biochronology and Global Magnetic Polarity Time Scale (GMPTS) (Madden et al. 1997; Antoine et al. 2007). On the eastern flank of the Fitzcarrald Arch, the outcrops of the Solimões Formation have been described by Räsänen et al. (1995) and Hovikoski et al. (2008) along the Jurua and Acre Rivers and the Purus River (8.80610°S, 68.80330°W) (see Fig. 6.3). These Miocene sediments commonly consist of various types of tidally influenced inclined heterolithic stratification (IHS)-bearing channel deposits that are intercalated with pedogenically altered horizons (Hovikoski et al. 2008). The palaeocurrents are variable between localities but are mainly toward the southeast and northeast (Räsänen et al. 1995; Hovikoski et al. 2007). The vertebrate faunal assemblage of the Neogene deposits of the Acre region indicates a Late Miocene age (Cozzuol 2006). The precise age of these deposits is estimated at 9.01 ± 0.28 Ma on the basis of argon-40/argon-39 (40Ar/39Ar) dating (mica and feldspar) of river level (Campbell et al. 2001; see Fig. 6.3) along the Purus River (10.41530 °S, 71.172799°W). The southern flank of the Fitzcarrald Arch displays Late Miocene deposits (Madre de Dios Formation) described by Roddaz (2004), Roddaz et al. (2004) and Hovikoski et al. (2005) along the Manú (11.90318°S, 71.39417°S), Madre de Dios (12.56787°S, 70.10265°W) and Inambari Rivers (12.92209°S, 70.34911°W) (see Fig. 6.3). The sedimentary series displays a 10–40 m thickness of several sharp based successions. The individual tidal point bars are ~3 m thick and consist of tide-dominated estuarine and delta deposits (Roddaz 2004; Roddaz et al. 2004; Hovikoski et al. 2005). The palaeocurrents are dominantly towarda the northeast, and towarda the east and west. A tuffaceous level dated within

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the tidal deposits – probably the same as dated by Campbell et al. (2001) in the Purus River – gives an estimated age at 9.31 ± 3.39 Ma (apatite fission track dating; Roddaz 2004; see Fig. 6.3) near Puerto Maldonado along the Madre de Dios River (12.5823°S, 69.21098°W). Late Middle-Late Miocene deposits show similar dominance of a tide-dominated depositional environment in the whole of the Fitzcarrald Arch area. The tides indicate sea-level setting as confirmed by pedogenically altered intervals (Hovikoski et al. 2008). Consequently, from the late Middle Miocene to the Late Miocene, the Fitzcarrald Arch area was completely flooded and its sediment deposition was controlled by tidal currents.

Pliocene deposits The Pliocene deposits consist of coarse-grained alluvial fan and fluvial deposits, ascribed to the Juanjui/Corrientes Formations in the Ucayali sub-basin, and to the Mazuko Formation in the Madre de Dios sub-basin. On the northern flank of the Fitzcarrald Arch, along the Tambo and Chicosillo Rivers (10.66257°S, 73.70055°W; see Fig. 6.3), the Pliocene deposits unconformably overlie the Miocene tidal deposits along an erosional surface (subaerial unconformity) interpreted as a foreslope surface (see Fig. 6.4a) (Catuneanu et al. 1999; see also Chapter 5). They are ~40 to 50 m thick and consist of unconsolidated conglomerates displaying well-rounded quartzite and volcanic pebbles and wood remains, included in a sandy matrix (see Fig. 6.4a,d). The pebbles are less than 15 cm in diameter and their volcanic and quartzite compositions indicate an Andean provenance. This unconsolidated conglomerate is topped by a tuffaceous level not yet dated, sandy deposits and palaeosol muddy deposits. Palaeocurrent data from imbricate gravels indicate a flow direction towards N355. On the southern flank of the arch, the Pliocene deposits consist of channel-shaped conglomerates (lithofacies Gt, sensu Miall 1996) with quartzite, volcanic and white sandstone pebbles and planar crossbedded sandstones (lithofacies Sp, sensu Miall 1996). The pebbles are from 1 to 5 cm in diameter. Conglomerates and sands are associated with red massive muds and silts and are locally bioturbated (Roddaz 2004). Along the Piedras River (12.05330°S, 69.90170°W), these Pliocene deposits are dated at 3.12 ± 0.02 Ma (40Ar/39Ar dating on micas from a tuffaceous level; Campbell et al. 2001; see Fig. 6.3). Like the Miocene sediments, the Pliocene deposits form a set of successive tilted and dissected morphostructural surfaces, like those evidenced by the digital elevation model of the Ucayali and Madre de Dios sub-basins (see Fig. 6.2a; Espurt et al. 2007). Field observations and image interpretation also indicate the unconformable disposition of the Pliocene on Miocene strata, particularly in the northern flank of the arch (see Figs 6.3 & 6.4a). On the northern flank of the arch, the age of the contact is not well constrained. However, the Pliocene age of these series is related to overlying Pleistocene terrace deposits made up of weathered conglomerates (Dumont et al. 1991). On the southern flank of the arch, the age of the contact between Miocene and Pliocene strata is much better and is bracketed between ~9 and ~3 Ma. The Pliocene deposits record a change in the geodynamic context of the Amazon drainage basin from a filled foreland basin

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The Nazca Ridge and Fitzcarrald Arch during the Miocene characterized by tidal deposits to an overfilled foreland basin characterized by continental deposits (Roddaz 2004). In the Fitzcarrald Arch area, the major unconformity developed at the base of the Pliocene deposits (see Figs 6.3 & 6.4a) is probably linked to the beginning of the arch uplift in Pliocene times (Espurt et al. 2006).

Pleistocene deposits The Pleistocene deposits of the Fitzcarrald Arch correspond to the Upper Solimões Formation (Silva 1988) equivalent to the Ucayali Formation in the Ucayali sub-basin. On the northern flank of the Arch, along the Tambo River and particularly along the Chicosillo River (10.66884°S, 73.69917°W; see Fig. 6.3), the Pleistocene deposits form ~5–8 m-thick aggradational terraces of pebbles made of quartzite and volcanic rocks, Miocene clasts of mudstones and sandstones, and continental Pliocene clasts of sandstones. The pebbles are included in a sandy and silty matrix (see Fig. 6.4a,e). The pebble imbrications indicate a main palaeocurrent direction of N250°. This direction is parallel to the present-day drainage direction of the Chicosillo River, which runs towards the west, i.e. towards the Andes. Dumont et al. (1991) mentioned a radiocarbon age of 8520 ± 440 years (10.59364°S, 73.91033°W and 10.73521°S, 73.60313°W) for stepped floodplain Pleistocene terraces along the Urubamba and Ucayali Rivers (see Fig. 6.3). Along the Mapuya River (10.42422°S, 73.05185°W), aggradational terraces located at about 50 m above the water level consist of fine sediments, quartzitic pebbles, carbon debris and palaeosol deposits (see Fig. 6.3). On the eastern flank of the arch, Kronberg et al. (1998) described Late Pleistocene(?) sediments that consist of lacustrine clays interlayered with thin carbonate beds rich in gypsum levels and vertebrate fossils (see Fig. 6.3). These sediments are interpreted to be deposited in a fluvial-lacustrine environment with variable hydrological conditions (Kronberg et al. 1998). These fluvial deposits suggest a depositional system similar to the present-day depositional system developed in the Fitzcarrald Arch area. These deposits are strongly controlled by recent vertical motions that are due to the uplift of the Fitzcarrald Arch.

Timing of the Fitzcarrald Arch uplift The subsidence curve and the evolution of the subsidence and sedimentation rates through time can be estimated by thermal modelling of vitrinite reflectance (Ro) – see Sweeney & Burnham (1990) and Allen & Allen (2005) for more details. In this study, we modelled Ro data using the one-dimensional basin modelling tool Genex 4.0.3 (IFP/Beicip). These results have been obtained from Airy back-stripping of the sedimentary succession, by calculating the decompacted thickness of each lithology and age, and considering appropriate palaeobathymetric and erosion values, as well as the isostatic readjustment of each sedimentary unit. We considered a Cenozoic geothermal gradient of 24°C/km, which is an average value in foreland basins and similar to the value estimated for other Andean foreland basins (Husson & Moretti 2002). We used data of the Mashansha well located on the northern flank of

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the arch (10.47202°S, 73.38603°W, and 326 m asl) and the Puerto Primo well located on the southern flank of the arch (12.10861°S, 69.72861°W, and 303 m asl) (Fig. 6.5; see also Fig. 6.3). The distance between these two wells is ~439 km. We also used additional data from the Sepa well (see Fig. 6.3) located ~60 km south of the Manshansha well. These data have been projected to the Mashansha well along the same stratigraphic levels. The interpretation of the well diagraphies (see Fig. 6.5a) completed with field data permit recognition of the different Cenozoic formations. The southern Puerto Primo well shows a 1.7 km-thick Cenozoic pile that consists of, in ascending order: a 475 m thickness of the Paleocene Huayabamba Formation; a 807 m thickness of the Miocene Ipururo Formation; and 405 m thickness of the Miocene-Pliocene Madre de Dios Formation. In contrast, the Mashansha well only displays a 1 km-thick Cenozoic pile, which consists of: 35 m thickness of the Paleocene Yahuarango Formation; 67 m thickness of the Eocene-Early Oligocene Pozo Formation; 410 m thickness of the Late Oligocene-Miocene Chambira Formation; 474 m thickness of the Miocene Ipururo Formation, and finally a 42 m thickness of Pleistocene alluvial deposits. When compared with the Puerto Primo well succession, the Late Miocene sediments of the Mashansha well are missing and have been eroded. Ro data have been used to identify the unconformities and estimate the amount of denudation according to the geothermal gradient (Allen & Allen 2005). Ro data come from wood in late Middle Miocene outcrops near the Mashansha well (Mapuya River) and from Paleozoic strata of the Mashansha well coupled with additional Ro data of the Sepa well (see Fig. 6.5b) (Espurt 2007). Thermal modelling of the Ro data shows a modelled curve with three jumps (see Fig. 6.5b), which can be correlated with three main unconformities and large stratigraphic gaps. The lower jump is correlated with the unconformity developed at the base of the Late Carboniferous series (see Espurt et al. 2008). The intermediate jump is correlated with the unconformity developed at the base of the Cretaceous series (see Jaimes & de Freitas 2006 and references therein; see also Fig. 6.2c). Finally, the upper jump is correlated with the unconformity developed at the base of the Pliocene continental series (see Figs 6.3 & 6.4). The model shows that ~700 m thickness of Miocene series have been removed below the Pliocene unconformity. Thermal data allow us to constrain the Cenozoic basin modelling of the Fitzcarrald Arch area (see Fig. 6.5b). During the Paleocene-Oligocene period, the Amazonian foreland basins was occupied by floodplain deposits (Wesselingh & Salo 2006). The sedimentation rates are about 10–25 m/million years and the subsidence rates are low in the Fitzcarrald area. The occurrence of tidal deposits since the Middle Miocene is marked by an increase in the subsidence rates from 40 to 90 m/million years at the base of the Ipururo Formation and the sedimentation rate increased from 50 to ~160 m/million years. The Ipururo Formation accumulated during a period of ongoing tectonic loading and recorded a strong increase of the sedimentation rate (see Chapter 5). This major change in the geodynamic context of the Amazonian foreland basins is related to the growth of the Andes and the eastern propagation of the sub-Andean thrust front (see Chapters 4 & 5). These results indicate that the Fitzcarrald Arch did not exist in Middle and Late Miocene times. Instead, the Fitzcarrald area was

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Well data Fitzcarrald Arch

Madre de Dios sub-basin

Mashansha well (326 masl)

Puerto Primo well (303 m asI)

400 MIOCENE

* 13 Ma(3)

m

200

100

DT (µs/m) 0

50

GR (°API)

200

400

POZO

?

1000

800

MIOCENE

NEOGENE

IPURURO

Estuarine domain with tide influence

Floodplain

CHAMBIRA OLIGOCENE

YAHUARANGO

MADRE DE DIOS

*

600

800

PALEOCENE

AGE PLIOCENE

600

EOCENE

FORMATION

Fluvial

9.31⫾3.39 Ma(2) 9.01⫾0.28 Ma(1)

Tidal domain

200

3.12⫾0.02 Ma(1) Fitzcarrald unconf ormity *

0

200

0

100

DT (µs/m)

Estuarine domain with tide influence

IPURURO

NEOGENE PALEOGENE

100

m 0

FORMATION

50

GR (°API) AGE

100

(a)

1000

m

9k

1600

? OLIGOCENE EOCENE PALEOCENE

Thermal history of the Mashansha well

2.0

CENOZOIC MESO. PALEOZOIC

1.6

Pliocene erosion (Fitzcarrald uplift)

IPUR.

⫺1.0

EOCENE

YAHUARANGO

OLIGOCENE POZO

MIOCENE

CHAMBIRA

⫺0.5 Cretaceous erosion

COPA.

AMBOBASEMENT

0.2

0.4

0.6

Sedimentation rate

0

Carboniferous erosion

I.F.P Ro EASY%Ro Ro Data

0.8

Vitrinite reflectance (%)

200

Foredeep depozone

CHAMB. PO.-YAH. VIVIAN CHONTA NIA

PLIO.

IPURURO

1.0

160

0.5

120 80 40 0

Subsidence rate

1.0 Total subsidence

⫺40

1.5

⫺80 2.0

⫺120

2.5

⫺160

Sedimentation and subsidence rates (m/Ma)

Depth (km)

PALEOC.

0.8 1.2

Basin modelling of the Mashansha well

Fitzcarrald uplift

0.0 0.4

(c)

Log

Subsidence (km)

(b)

PALEOGENE

1400

Floodplain

43

HUAYABAMBA

1200

⫺200

3.0 60

50

40

30

20

10

0

Time (Ma)

Fig. 6.5 Basin modelling of the Fitzcarrald Arch from well and vitrinite reflectance (Ro) data. (a) Well log records of the Mashansha and Puerto Primo wells and log data with stratigraphic correlations from gamma ray (GR) and sonic (DT) data (see well locations in Fig. 6.3). Field absolute age data (denoted by black stars) are projected to the well data: (1) Campbell et al. 2001; (2) Roddaz 2004; and (3) Antoine et al. 2007. (b) Fit of Ro data using the one-dimensional basin modelling tool Genex 4.0.3 (IFP/Beicip). Abbreviations of the sedimentary series are indicated. I.F.P. Ro and EASY%Ro correspond to model curve. Ro data come from Late Miocene (~13 Ma; Antoine et al. 2007) wood debris collected along the Mapuya River and from cutting of the Paleozoic rocks. Additional Ro data of the Sepa well have been projected to the Manshansha well. (c) Cenozoic subsidence curve and sedimentation/subsidence rates of the Mashansha well.

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The Nazca Ridge and Fitzcarrald Arch (a)

85°W

80°W

75°W

70°W

65°W

5°S

IA

Iquitos

u Per

15°S

ch

10°S

n Tre

Z

ile

SA

-Ch

AFB

Lima

e a dg M Ri 1.2 a 1 zc at Na ion sit o p

de Tid lta es do an m d ina es te tu d ar ie s

AN

DE

S

0

500 km

11.2 Ma: Ridge subduction (b)

85°W

80°W

75°W

IA

5°S

70°W

65°W

Iquitos

NAFB

ch

10°S

n Tre

Z SA

ile

-Ch

ru Pe

EAB

FA

NAZCA PLATE

SAFB

15°S

Lima

AN

dg

e

DE

La Paz

Na

zc

a

Ri

S 0

500 km

4 Ma: Flat subduction and uplift of the FA (c)

85°W

80°W

75°W

IA

5°S

70°W

65°W

Iquitos

NAFB

EAB

10°S

ch

FA Lima

15°S

SAFB m/a

78.5 m

zc

a

Ri d

ge

La Paz

Na

0

500 km

Present day Wedge-top depozone Forebulge depozone

Foredeep depozone Backbulge depozone

Orogenic wedge Uplift

Foreland basin system

Fig. 6.6 (a) The Nazca Ridge subduction history and its relation with the Amazonian foreland basin evolution. When the subduction of the Nazca Ridge started 11.2 Ma, the

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The reconstruction of the Nazca Ridge geometry beneath the South American lithosphere (Hampel 2002) indicates that the buoyancy of the Nazca Ridge supports a current ~700 km-long flat slab segment (Espurt et al. 2007). This flat slab segment reaches the Amazonian foreland basin below the Fitzcarrald Arch uplift. Hampel (2002) showed that the Nazca Ridge subduction below the South American Plate started 11.2 Ma (Fig. 6.6a). The beginning of the ridge subduction coincides with the onset of the eastward overthrusting of the Eastern Cordillera and sub-Andean zone, north of the Fitzcarrald Arch (Hermoza et al. 2005). During the Miocene, the Amazonian foreland basin constituted a single four-component foreland basin system (sensu DeCelles & Giles 1996; see Fig. 6.6a). The Fitzcarrald Arch did not exist and the area was subsiding and totally occupied by tide-dominated deltas and estuaries (see Fig. 6.6a). At the same time, the product of the erosion of the Andes filled the western Amazonian drainage basin (Roddaz 2004). The buoyancy of the Nazca Ridge produced a flat subducting slab beginning ~4 Ma, according to the cessation of arc volcanism (see Fig. 6.6b) (Soler & Bonhomme 1990; Rosenbaum et al. 2005). The coincidence of the initiation of flat slab subduction (Gutscher et al. 2000) with uplift of the Fitzcarrald Arch leads us to interpret the flat subduction of the Nazca Ridge as the cause of uplift (Espurt et al. 2007). Uplift of the Fitzcarrald Arch in Pliocene times is recorded by unconformities and by ~700 m of erosion within the arch area since ~5–4 Ma (see Fig. 6.6b). The Pliocene deposits

Z SA

n Tre ile Ch

ruPe

NAZCA PLATE

a subsiding foredeep depozone (in the sense of DeCelles & Giles 1996) and subject to Middle and Late Miocene marine incursions. The lack of Upper Miocene and Pliocene sediment on the Mashansha well is evidence of Pliocene erosion of about 700 m caused by the uplift of the Fitzcarrald Arch during Pliocene times (see Fig. 6.5). Accordingly, the Puerto Primo well shows that this erosion is restricted to the Fitzcarrald Arch, as evidenced by the thickness of Late Miocene Madre de Dios Formation sediments (~405 m) deposited in a subsiding area adjacent to the Fitzcarrald Arch uplift (e.g. in the Madre de Dios sub-basin).

Origin of the Fitzcarrald uplift

La Paz

NAZCA PLATE

97

Fitzcarrald Arch area of the Amazonian foreland basin (AFB) was a subsiding foredeep and dominated by tide-dominated estuaries and deltas. (b) When the flat subduction began at about 4 Ma, the eastern edge of the Nazca Ridge reached the Amazonian foreland basin and provoked uplift of the Fitzcarrald Arch (FA) dividing the Amazonian foreland basin into two subsiding basins: the northern Amazonian foreland basin (NAFB) and the southern Amazonian foreland basin (SAFB). The Fitzcarrald Arch is eroded by the Pliocene fluvial system. The drainage system of the Amazon Basin is reorganized inducing speciation of the freshwater fish (see Chapter 17). (c) Present-day architecture of the Amazonian drainage basin and the denudation of the Fitzcarrald Arch by the modern drainage system. Dashed arrows indicate drainage orientations. Reconstruction of the Nazca Ridge path is from Hampel (2002). EAB, eastern and central Amazonian intracratonic basins; SAZ, sub-Andean Zone; IA, Iquitos Arc.

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N. Espurt et al.

have been progressively uplifted and warped on the southeastern side of the arch by its continuing growth, which is probably still active today (see Fig. 6.6c). This recent uplift is also consistent with the change of the palaeocurrent directions observed between the Pliocene and Pleistocene deposits and the abnormal westward present-day drainage of some rivers towards the Andes (see Figs 6.2 & 6.6c).

Consequences of the Fitzcarrald Arch uplift on Amazonian basin evolution and its biota The Amazon drainage basin is a highly heterogeneous region in which, from the Miocene to the present day, geological history, landscape, climate, soils and biota have been closely related to large-scale subduction processes of the Nazca-South America convergence system (Roddaz 2004). Before 4 Ma, the role of the Nazca Ridge subduction seems to have been restricted to the Andes (e.g. Hampel 2002; McNulty & Farber 2002; Rousse et al. 2003) and the ridge does not influence the Amazonian foreland basin development (see Fig. 6.6a). In contrast, Pliocene-Pleistocene landscape evolution of the western Amazonian foreland basin coincides with the onset of the Nazca Ridge flat subduction (see Fig. 6.6b) (Espurt et al. 2007). Thus, the geomorphic Fitzcarrald Arch regional uplift may have induced large-scale modifications of the hydrological, erosional and sedimentological processes in the Amazon drainage basin since the Pliocene. The modern drainage network of the Fitzcarrald Arch eroded Neogene sediments away from an area greater than 400,000 km2 (see Fig. 6.2). These erosive processes induced new rivers in the Fitzcarrald Arch area, which are noticeably enriched in smectite, indicating strong recycling of Miocene sediments (Guyot et al. 2007). In any budget related to Andean erosion, the suspended sediment yields from the erosion of the Fitzcarrald Arch must be taken into account in order not to overestimate the erosion rates of the Andes. Tropical rainforests are well known for their high biodiversity (Groombridge 1992; Hooghiemstra & Van der Hammen 1998). The Fitzcarrald Arch uplift could have played a major role in the distribution patterns of the modern Amazon biota (e.g. Lougheed et al. 1999; Patton et al. 2000; Montufar & Pintaud 2006). Indeed, the uplift and dissection of sedimentary series of the Fitzcarrald Arch created a dynamic edaphic mosaic in the area that sustained high diversity. The Neogene geological formations outcropping on the Fitzcarrald Arch area are various and complex, and have distinctive lithological (e.g. Miocene Madre de Dios clays vs Pliocene-Pleistocene conglomerates), hydrological and geochemical characteristics (Guyot et al. 2007) with associated fertile soils. This edaphic control probably induces a distinct floral and faunal development, such as endemic species (e.g. Linna 2001; Klemola 2003; Ruokolainen et al. 2005), and probably controls the activities of autochthonous tribes (Hovikoski et al. 2007). At about 8 Ma, the modern eastward Amazon River system had become established (Hoorn et al. 1995; Dobson et al. 1997, 2001; Wesselingh & Salo 2006). The Fitzcarrald Arch uplift has created a formidable NE–SW-trending barrier within the Amazonian lowland forest since Pliocene times. This uplift also contributed to producing a drained area and may have permitted the development of isolated

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flora in drier soils (Haffer 1997; Daly & Mitchell 2000; Saatchi et al. 2008). Through its action as a drainage divide since the Pliocene, the Fitzcarrald Arch uplift has probably controlled the speciation of aquatic populations (see Fig. 6.6b), as the freshwater fishes on either side of the arch started to diverge ~4 Ma (Hubert et al. 2006). Thus we hypothesize that the Fitzcarrald Arch played a major role in increasing spatial Pliocene-Pleistocene biodiversity in the Amazon drainage basin.

Acknowledgements This research project was carried out thanks to the Institut de Recherche pour le Développement-PeruPetro SA research agreement, and supported by IRD (UR 154) and the French Institut National des Sciences de l’Univers – Centre National de la Recherche Scientifique programs DyETI (Dynamique et Evolution de la Terre Interne) and ECLIPSE II (Environnement et CLImat du PaSsé: histoire et Evolution). This study has been supported by BQR grant « Mise en place d’équipements pour la thermochronologie basse température en Sciences de la Terre » from the Université de Toulouse. We thank PeruPetro SA for providing seismic reflection profiles and for permission to publish their imagery. We also thank Carina Hoorn for inviting us to contribute to this book. We finally thank T.E. Jordan and J. Hovikoski for their interesting comments that helped to improve this chapter.

References Allen, P.A., Allen, J.R. (2005) Basin Analysis. Principles and Applications. Blackwell Publishing, 549 pp. Antoine, P.-O., Salas-Gismondi, R., Baby, P., Benammi, M., Brusset, de Franceschi, D., Espurt, N. et al. (2007). The middle Miocene (Laventan) Fitzcarrald fauna, Amazonian Peru. In: E. Díaz-Martínez, E., Rábano, I. (eds) 4th European Meeting on the Palaeontology and Stratigraphy of Latin America, Instituto Geológico y Minero de España, Madrid. Cuadernos del Museo Geominero 8, 19–24. Baby, P., Guyot, J.L., Deniaud, Y., Zubieta, D., Christophoul, F., Rivadeneira, M. (1999) The High Amazonian Basin: tectonic control and mass balance. In: International Symposium MANAUS 99, Hydrological and Geochemical Process in Large-scale Rivers Basins: Manaus (Brazil). Extended abstracts CD. Campbell, K.E. Jr., Heizler, M., Frailey, C.D., Romero-Pittman, L., Prothero D.R. (2001). Upper Cenozoic chronostratigraphy of the southwestern Amazon Basin. Geology 29, 595–598. doi: 10.1130/0091–7613(2001)029<0595:UCCOTS>2.0.CO;2. Campbell, K.E. Jr., Frailey, C.D., Romero-Pittman, L. (2006) The Pan-Amazonian Ucayali Peneplain, late Neogene sedimentation in Amazonia, and the birth of the modern Amazon River system. Palaeogeogr Palaeocl 239, 166–219. doi:10.1016/j. palaeo.2006.01.020 Carlotto, V., Cárdenas, J., Tintaya, D., Rodríguez, R., Valdivia, W. (2007) Altos estructurales en el control de la evolucion andina: la deflexion de Abancay y el Arco de Manú. In: XIII Congreso Peruano de Geología. Resúmenes Extendidos. Lima: Sociedad Geológica del Perú, pp. 248–251. Catuneanu, O., Sweet, A.R., Miall, A.D. (1999) Concept and styles of reciprocal stratigraphies: Western Canada foreland basin. Terra Nova 11, 1–8.

10/24/2009 11:03:48 Shobha

The Nazca Ridge and Fitzcarrald Arch Church, G.E. (1901) South America: an outline of its physical geography. Geographical Journal 17, 333–406. Clift, P.D., Pecher, I., Kukowski, N., Hampel, A. (2003) Tectonic erosion of the Peruvian forearc, Lima Basin, by subduction and Nazca Ridge collision. Tectonics 22, 1023. doi: 10.1029/2002TC001386. Cozzuol, M.A. (2006) The Acre vertebrate fauna: Age, diversity, and geography. J S Am Earth Sci 21, 185–203. Dalrymple, R.W., Zaitlin, B.A., Boyd, R. (1992) Estuarine facies models: Conceptual basis and stratigraphic implications. J Sediment Res 62, 1130–1146. Daly, D.C., Mitchell, J.D. (2000) Lowland vegetation of tropical South America – an overview. In: Lentz, D. (ed.) Imperfect Balance: Landscape Transformations in the pre-Columbian Americas. New York: Columbia University Press, pp. 391–454. DeCelles, P.G., Giles, K.A. (1996) Foreland basin systems. Basin Res 8, 105–123. Dobson, D.M., Dickens, G.R., Rea, D.K. (1997) Terrigenous sedimentation at Ceara Rise. Proc ODP, Sci Results 154, 465–473. Dobson, D.M., Dickens, G.R., Rea, D.K. (2001) Terrigenous sediment on Ceara Rise: a Cenozoic record of South American orogeny and erosion. Palaeogeogr Palaeocl 165, 215–229. Dumont, J.F. (1996) Neotectonics of the Subandes-Brazilian craton boundary using geomorphological data: the Marañon and Beni basins. Tectonophysics 259, 137–151. doi: 10.1016/0040–1951 (95)00200–6. Dumont, J.F., Deza, E., Garcia, F. (1991) Morphostructural provinces and neotectonics in the Amazonian lowland of Peru. J S Am Earth Sci 4, 373–381. Espurt, N. (2007) Influence de la subduction d’une ride océanique sur la dynamique de la plaque continentale chevauchante: exemple de la ride de Nazca et du bassin amazonien. Thèse de l’Université Toulouse III - Paul Sabatier, 328 pp. Espurt, N., Baby, P., Brusset, S., Hermoza, W., Antoine, P.-O., SalasGismondi, R. et al. (2006) Geomorphic and sedimentologic analyses on the Fitzcarrald Arch: evidence of a recent tectonic uplift. In: XIII Congreso Peruano de Geología. Resúmenes Extendidos. Lima: Sociedad Geológica del Perú, pp. 273–276. Espurt, N., Baby, P., Brusset, S., Roddaz, M., Hermoza, W., Regard, V. et al. (2007) How does the Nazca ridge subduction influence the modern Amazonian foreland basin? Geology 35, 515–518. Espurt, N., Brusset, S., Baby, P., Hermoza, W., Roddaz, M., Bolaños, R. et al. (2008) Paleozoic structural controls on shortening transfer in the Subandean foreland thrust system, Ene and southern Ucayali basins, Peru. Tectonics 27, TC3009. Gripp, A.E., Gordon, R.G. (2002). Young tracks of hotspots and current plate velocities. Geophys J Int 150, 321–361. Groombridge, B. (1992) Global Biodiversity: Status of the Earth’s Living Resources. World Conservation Monitoring Centre. New York: Chapman and Hall. 585p. Gutscher, M.-A., Olivet, J.L., Aslanian, D., Eissen, J.P., Maury, R. (1999). The “lost Inca Plateau”: Cause of flat subduction beneath Peru? Earth Planet Sci Lett 171, 335–341. Gutscher, M.-A., Spakman, W., Bijwaard, H., Engdalh, E.R. (2000) Geodynamics of flat subduction: Seismicity and tomographic constraints from the Andean margin. Tectonics 19, 814–833. Guyot, J.L., Jouanneau, J.M., Soares, L., Boaventura, G.R., Maillet, N., Lagane, C. (2007) Clay mineral composition of river sediments in the Amazon Basin. Catena 71, 340–356. Haffer, J. (1997) Alternative models of vertebrate speciation in Amazonia: an overview. Biodivers Conserv 6, 451–476. Hampel, A. (2002) The migration history of the Nazca Ridge along the Peruvian active margin: a re-evaluation. Earth Planet Sci Lett 203, 665–679.

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Hermoza, W., Brusset, S., Baby, P., Gil, W., Roddaz, M., Guerrero, N., Bolaños, R. (2005) The Huallaga foreland basin evolution: Thrust propagation in a deltaic environment, northern Peruvian Andes. J S Am Earth Sci 19, 21–34. Hooghiemstra H., Van der Hammen T. (1998) Neogene and Quaternary development of the neotropical rain forest: the forest refugia hypothesis, and a literature overview. Earth Sci Rev 44, 147–183. Hoorn, C., Guerrero, J., Sarmiento, G.A., Lorente, M.A. (1995) Andean tectonics as a cause for changing drainage patterns in Miocene northern South America. Geology 23, 237–240. Hoorn, C. (1993) Marine incursions and the influence of Andean tectonics on the Miocene depositional history of northwestern Amazonia: Results of a palynostratigraphic study. Palaeogeogr Palaeocl 105, 267–309. Hovikoski, J., Rasanen, M., Gingras, M., Roddaz, M., Brusset, S., Hermoza, W. et al. (2005) Miocene semi-diurnal tidal rhythmites in Madre de Dios, Peru. Geology 33, 177–180. Hovikoski, J., Gingras, M., Räsänen, M., Rebata, L.A., Guerrero, J., Ranzi, A. et al. (2007) The nature of Miocene Amazonian epicontinental embayment: High-frequency shifts of the low-gradient coasline. Geol Soc Am Bull 119, 1506–1520. Hovikoski, M., Räsänen, M., Gingras, M., Ranzi, A., Melo, J. (2008) Tidal and seasonal controls in the formation of Late Miocene inclined heterolithic stratification deposits, western Amazonian foreland basin. Sedimentology 55, 499–530. Hsu, J.T. (1992) Quaternary uplift of the Peruvian coast related to the subduction of the Nazca Ridge: 13.5 to 15.6 degrees south latitude. Quaternary Int 15/16, 87–97. Hubert, N., Duponchelle, F., Nuñez, J., Garcia-Da Vila, C., Paugy, D., Renno, J.F. (2007) Phylogeography of the piranha genera Serrasalmus and Pygocentrus: implications for the diversification of the Neotropical ichthyofauna. Mol Ecol doi: 10.1111/j.1365-294 X.2007.03267.x. Husson, L., Moretti, I. (2002) Thermal regime of fold and thrustbelts. An application to the Bolivian Sub Andean Zone. Tectonophysics 345, 253–280. Jacques, J. (2003) A tectonostratigraphic synthesis of the Sub-Andean basins: inferences on the position of South American intraplate accommodation zones and their control on South Atlantic opening. J Geol Soc Lond 160, 703–717. Jaimes, E., de Freitas, M. (2006) An albian-Cenomanian unconformity in the northern Andes: Evidence and tectonics significance. J S Am Earth Sci 21, 466–492. Jopling, A.B., Walker, R.G. (1968) Morphology and origin of rippledrift cross lamination, with examples from the Pleistocene of Massachusetts. J Sediment Petrol 38, 971–984. Klemola, L. (2003) Stratigraphy and properties of the Iquitos White Sand formation in Allpahuayo-Mishana Reserve, north-eastern Peru. MS thesis, University of Turku, 78 pp. Kronberg, B.I., Fralick, P.W., Benchimol, R.E. (1998) Late Quaternary sedimentation and palaeohydrology in the Acre foreland basin, SW Amazonia. Basin Res 10, 311–323. Kummel, B. (1948) Geological reconnaissance of the Contamana region. Peru. Bull Geol Soc Am 59, 1217–1266. Le Roux, J.P., Tavares Correa, C., Alayza, F. (2000) Sedimentology of the Rimac-Chillon alluvial fan at Lima, Peru, as related to PlioPleistocene sea level changes, glacial cycles and tectonics. Journal of South American Earth Sciences 13, 499–510. Lindsay, J.B. (2005) The Terrain Analysis System: a tool for hydrogeomorphic applications. Hydrological Processes 19, 1123–1130. Linna, A., 2001, Petrology of the Neogene-Quaternary fluvial sands of the Andean Foreland Basin in western Amazon in Peru (Licentiate thesis): University of Turku, 110 p.

10/24/2009 11:03:48 Shobha

100

N. Espurt et al.

Lougheed, S.C., Gascon, C., Jones, D.A., Bogart, J.P., Boag, P.T. (1999) Ridges and rivers: a test of competing hypotheses of Amazonian diversification using a dart-poison frog (Epipedobates femoralis). P R Soc B 266, 1829–1835. Macharé, J., Ortlieb, L. (1992) Plio-Quaternary vertical motions and the subduction of the Nazca Ridge, central coast of Peru. Tectonophysics 205, 97–108. Madden, R.H., Guerrero, J., Kay, R.F., Flynn, J.J., Swisher, C.C., Walton, A.H. (1997) The Laventan Stage and age. In: Kay, R.F., Madden, R.H., Cifelli, R.L., Flynn, J.J. (eds) Vertebrate Paleontology in the Neotropics. The Miocene Fauna of La Venta, Colombia. Washington DC: Smithsonian Institution Press, pp. 499–519. McNulty, B., Farber, D. (2002) Active detachment faulting above the Peruvian flat slab. Geology 30, 567–570. Miall, A.D. (1996) The Geology of Fluvial Deposits, Sedimentary Facies, Basin Analysis and Petroleum Geology. Berlin: Springer, 582 pp. Montufar, R., Pintaud, J.C. (2006) Variation in species composition, abundance and microhabitat preferences among western Amazonian terra firme palm communities. Bot J Linn Soc 151, 127–140. Mutti, E., Rosell, J., Allen, G., Fonnesu, F., Sgavetti, M. (1985) The Eocene Baronia tide dominated delta-shelf system in the Ager basin. In: Mila, M.D., Rosell, J. (eds), 6th IAS European Regional meeting, Lleida, pp. 577–600. Nio, S.D., Yang, C.S. (1991) Diagnostic attribute of clastic tidal deposits: a review. In: Smith, D.G., Reinson, G.E., Zaitlin, B.A., Rahmani, R.A. (eds) Clastic Tidal Sedimentology. Canadian Society of Petroleum Geologists, pp. 3–28. Oppenheim, V. (1946) Geological reconnaissance in southeastern Peru. Bull Am Assoc Petrol Geol 30, 254–264. Patton, J.L., da Silva, M.N.F., Malcolm, J.R. (2000) Mammals of the Rio Jurua and the evolutionary and ecological diversification of Amazonia. B Am Mus Nat Hist 244, 1–306. Picard, D., Sempere, T., Plantard, O. (2008) Direction and timing of uplift propagation in the Peruvian Andes deduced from molecular phylogenetics of highland biotaxa. Earth Planet Sci Lett 271, 326–336. Pilger, R.H. (1981) Plate reconstructions, aseismic ridges, and low-angle subduction beneath the Andes. Geol Soc Am Bull 92, 448–456. Räsänen, M., Neller, R., Salo, J., Jungner, H. (1992) Recent and ancient fluvial deposition systems in the Amazonian foreland basin, Peru. Geol Mag 129, 293–306. Räsänen, M.E., Linna, A.M., Santos, J.C.R., Negri, F.R. (1995) Late Miocene tidal deposits in the Amazonian Foreland Basin. Science 269, 386–390. doi: 10.1126/science.269.5222.386. Regard, V., Lagnous, R., Espurt, N., Darrozes, J., Baby, P., Roddaz, M. et al. (2009) Geomorphic evidence for recent uplift of the Fitzcarrald Arch (Peru): a response to the Nazca Ridge subduction. Geomorphology 107, 107–117. Roddaz, M. (2004). Transition des stades alimentés à suralimentés dans les systèmes de rétro-bassin d’avant-pays: exemple du bassin amazonien. Thèse de l’Université Toulouse 3, Toulouse, 332 pp. Roddaz, M., Hermoza, W., Hovikoski, J., Brusset, S., Antoine, P.-O., Baby, P. (2004) Late Miocene tidal sedimentation in the foredeep of the southwestern Amazonian foreland basin system (Madre de Dios basin, South Peru). In: Bartholdy, J., Pedersen, J.B.T. (eds) Tidalites 2004. Copenhagen: Institute of Geography, University of Copenhagen, pp. 161–164.

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Roddaz, M., Viers, J., Brusset, S., Baby, P., Hérail, G. (2005) Sediment provenances and drainage evolution of the Neogene Amazonian foreland basin. Earth Planet Sci Lett 239, 57–78. Roddaz, M., Brusset, S., Baby, P., Hérail, G. (2006) Miocene tidalinfluenced sedimentation to continental Pliocene sedimentation in the forebulge–backbulge depozones of the Beni-Mamore foreland Basin (northern Bolivia). J S Am Earth Sci 20, 351–368. Rosenbaum, G., Giles, D., Saxon, M., Betts, P.G., Weinberg, R.F., Duboz, C. (2005) Subduction of the Nazca Ridge and the Inca Plateau: Insights into the formation of ore deposits in Peru. Earth Planet Sci Lett 239, 18–32. Rousse, S., Gilder, S., Farber, D., McNulty, B., Patriat, P., Torres, V., Sempere, T. (2003) Paleomagnetic tracking of mountain building in the Peruvian Andes since 10 Ma. Tectonics 22, 1048. doi: 10.1029/2003TC001508. Ruokolainen, K. Tuomisto, H., Kalliola, R. (2005) Landscape heterogeneity and species diversity in Amazonia. In: Bermingham, E., Dick, C.W., Moritz, C. (eds) Tropical Rainforests: Past, Present & Future. Chicago: University of Chicago Press, pp. 251–270. Saatchi, S., Buermann, W., ter Steege, H., Mori, S., Smith, T.B. (2008) Modeling distribution of Amazonian tree species and diversity using remote sensing measurements. Remote Sens Environ 112, 2000–2017. Saillard, M. (2008) Dynamique du soulèvement côtier Pléistocène des Andes centrales: Etude de l’évolution géomorphologique et datations (10Be) de séquences de terrasses marines (Sud Pérou – Nord Chili). Thèse de doctorat de l’Université de Toulouse 3, Paul Sabatier, 312 pp. Salas-Gismondi, R., Antoine P.-O., Baby, P., Brusset, S., Benammi, M., Espurt, N. et al. (2007) Middle Miocene crocodiles from the Fitzcarrald Arch, Amazonian Peru. In: Díaz-Martínez, E., Rábano, I. (eds) Fourth European Meeting on the Palaeontology and Stratigraphy of Latin America, Madrid, pp. 355–360. Silva, L.L. (1988). A estratigrafia da Formação Solimões: uma análise crítica. Anais do XXXV Congresso Brasileiro de Geologia Belém. Pará 2, 725–737. Soler, P., Bonhomme, M.G. (1990) Relation of magmatic activity to plate dynamics in central Peru from Late Cretaceous to present. Geol Soc Am Special Papers 241, 173–192. Strahler, A.N. (1952) Dynamic basis of geomorphology. Geol Soc Am Bull 63, 923–938. Sweeney, J.J., Burnham, A.K. (1990) Evaluation of a simple model of vitrinite reflectance based on chemical kinetics. Am Assoc Petrol Geol B 74, 1559–1570. Tassara, A., Götze, H.-J., Schmidt, S., Hackney R. (2006) Threedimensional density model of the Nazca plate and the Andean continental margin. J Geophys Res 111, 09404, 10.1029/2005JB003976. von Huene, R., Suess, E. (1988) Ocean Drilling Program Leg 112, Peru continental margin, part 1, Tectonic history. Geology 16, 934–938. von Huene, R., Pecher, I.A., Gutscher M.-A. (1996) Development of the accretionary prism along Peru and material flux after subduction of Nazca Ridge. Tectonics 15, 19–33. Wesselingh, F.P., Salo, J.A. (2006) A Miocene perspective on the evolution of the Amazonian biota. Scripta Geologica 133, 339–458. Wipf, M., Zeilinger, G., Seward, D., Schlunegger, F. (2008) Focused subaerial erosion during ridge subduction: impact on the geomorphology in south-central Peru. Terra Nova 20, 1–10. Woods, M.T., Okal E.A. (1994) The structure of the Nazca Ridge and Sala y Gomez seamount chain from dispersion of Rayleigh waves. Geophys J Int 117, 205–222.

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PART II

Cenozoic depositional systems in Amazonia

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SEVEN

The Amazonian Craton and its influence on past fluvial systems (Mesozoic-Cenozoic, Amazonia) Carina Hoorn1, Martin Roddaz2, Rodolfo Dino3, Emilio Soares4, Cornelius Uba5, Diana Ochoa-Lozano6 and Russell Mapes7 1

University of Amsterdam, The Netherlands Université de Toulouse, France 3 Cidade Universitária, Rio de Janeiro, Brazil 4 Campus Universitário, Manaus, Brazil 5 Universität Potsdam, Germany 6 Smithsonian Tropical Research Institution, Balboa, Ancon, Panama 7 University of North Carolina, Chapel Hill, USA 2

Abstract The Amazonian Craton is an old geological feature of Archaean/Proterozoic age that has determined the character of fluvial systems in Amazonia throughout most of its past. This situation radically changed during the Cenozoic, when uplift of the Andes reshaped the relief and drainage patterns of northern South America. Here we review the sedimentary characteristics of Amazonian rivers and compare these with four fluvial depositional settings from the Meso-Cenozoic sedimentary record. These sedimentary units are the Alter do Chão Formation (Brazil, Late Cretaceous-Paleogene), the Petaca Formation (Bolivia, Late Oligocene to Middle Miocene), the Mariñame and Apaporis Sand Units (Colombia, Miocene), and the Iquitos White Sand Unit (Peru, Late Miocene-Pliocene). This review illustrates that the river systems born on the craton share features such as sediment texture and composition, depositional environments and transport directions. Evidence for the diminished role of cratonic fluvial systems and the onset of Neogene Andean uplift can be identified in the sedimentary record by changes in sediment provenance and transport directions. Although the Andean uplift and related processes discontinued the major Amazonian-born fluvial systems it also created new topographic features such as the Iquitos and Fitzcarrald Arches. These newly formed reliefs triggered a new generation of rivers, some of which are presently known as biodiversity hotspots.

Introduction The crystalline highlands that constitute the Amazonian Craton formed the main driver and source of sediment supply for fluvial systems during most of the geological history of South America. In spite of this, very little is known about the nature of these depositional systems and their implications for the palaeogeography. This is mainly due to the difficult access of the Amazonian terrain

Amazonia, Landscape and Species Evolution: A Look into the Past, 1st edition. Edited by C. Hoorn and F.P. Wesselingh. © 2010 Blackwell Publishing

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and the lack of stratigraphic control. An additional complication is that Amazonia is shared by many countries and the continuity of geological features between countries is not always clear. In this chapter we review geological evidence for past cratonic fluvial systems and we will discuss their significance in the wider South American palaeogeographic framework. Prior to discussing the fossil fluvial systems we summarize the most important sedimentary characteristics of the present rivers, such as geochemistry of the water, the composition of the sediment load and the proportions of bed load versus suspended load. These features are critical for identifying the provenance area and to estimate the dynamics of past river systems. Subsequently we will review four fluvial depositional systems from the sedimentary record

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Fig. 7.1 General map showing the Amazon drainage basin and the Amazonian Craton. The location of representative outcrops of the four fluvial units discussed in the text are indicated with symbols. The extent of the Alter do Chão Formation has grey shading; that of the Pebas/Solimões Formation is shown by the hatched area (see also Plate 6). that range in age from Cretaceous to Pliocene (Fig. 7.1). The stratigraphical, sedimentological, geochemical, palynological and palaeontological evidence from both outcrop data and exploration wells underscore the long-lasting role of the craton over a wide geographical region. These features also permit us to differentiate the cratonic systems from the younger Andean-dominated aquatic systems and allow a more complete palaeogeographic reconstruction of Mesozoic and Cenozoic history.

Evolution of the Amazonian Craton The rocks of the Amazonian Craton range in age between 1 and 2.3 Ga although at some sites ages of 3 Ga have been recorded, representing relics of the early history of the Earth. The craton is composed of a variety of metamorphic and magmatic rocks such as granites, migmatites, greenstone belt terranes and metasedimentary rocks, which are the result of a complex series of processes that included terrane amalgamation, orogenesis and rifting (Cordani & Sato 1999; Santos et al. 2000; Brito Neves 2002; see also Chapter 2, Fig. 7.1 and Plate 6). The Transbrasiliano Lineament separates a northerly continental mass, composed of the Amazonian and São Luis cratons, from a more southeasterly group of cratonic units. Until 1.7 Ga the crustal evolution of all these cratonic units was relatively similar and they possibly

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formed part of an Early Proterozoic supercontinent. However, during the Middle and Late Proterozoic the northerly continental mass remained relatively stable whereas the southeastern mass became fragmented (Cordani & Sato 1999; Brito Neves 2002). From Paleozoic until Early Cretaceous times the Amazonian, African and other cratonic units were connected and formed Gondwana, the southern part of the supercontinent Pangaea. During the Late Ordovician a failed E–W-directed rift created a low in the craton with the Guiana Shield to the north and the Brazilian Shield to the south. This rift zone gave origin to the Solimões and Amazonas Paleozoic sedimentary basins (Brito Neves et al. 1984; Brito-Neves 2002; Pedreira & Caliari-Bahia 2000; see also Chapter 3). Exploratory studies by the oil industry have revealed that from Late Ordovician until the end of the Permian, fluvio-deltaic and marine deposition took place in the Brazilian Paleozoic basins that form the subsurface of Amazonia (Wanderley Filho et al. 2005). The western margin of the Amazonian Craton is buried under a thick Phanerozoic sedimentary sequence. Seismic data indicate that structural trends in the basement define the shape of the sedimentary basins and that the basement continues below the Andean foreland (see Plate 9). Structural highs such as the Purus Arch have contributed considerable detritus to the Phanerozoic sedimentary systems prior to burial (e.g. Basu et al. 1990; Mapes et al. 2005; Chew et al. 2007).

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Large mantle plumes preceded the break-up of Pangaea and caused doming of the continental crust, which marked the beginning of the opening of the Atlantic Ocean, a process that was concluded during the Cretaceous (Cox 1989, 1998). A river system was born on these newly uplifted surfaces near the Atlantic margin and flowed towards the west along the low in the craton (Cox 1989; Potter 1997; Moore & Blenkinsop 2002). The Cretaceous Alter do Chão Formation, which we will discuss in this chapter, forms the sedimentary record of this fluvial system (Eiras et al. 1994; Cunha et al. 1994), but was discontinued during the early Paleogene. However, other cratonic fluvial systems, with similar transport directions, were born on the craton and continued to exist throughout the Paleogene and Neogene. This situation came to an end during the Neogene as a result of Andean tectonism. Contemporaneously with the E–W-orientated systems, smaller scale, W–E-directed drainage also existed, which flowed into the Atlantic (Dobson et al. 2001; Figueiredo et al. 2009). The increased uplift in the Eastern Andes – and the deformation of the South American and Caribbean Plates as a whole – brought to an end the principal role that the craton exercised on sedimentary systems. The new palaeogeography facilitated the development of a new and complex aquatic system, a mega-wetland of primarily Andean source. This system culminated during the Late Miocene in the establishment of the Amazon River. The sedimentary record of this system of lakes and swamps is represented in the Pebas and Solimões Formations and is further discussed in Chapter 8.

Modern cratonic rivers of Amazonia: classification and composition of the sediment load

Fig. 7.2 The meeting of a black-water, Amazon-born river rich in humic acid, and a white-water, Andean-born river with a heavy suspended load. Photo by Russell Mapes.

The Amazon River drainage basin is the largest hydrographic basin in the world, covering 6.1 × 106 km2 (Guyot et al. 2007). The rivers in this vast region can be differentiated based on sediment composition, texture and geochemistry, features that are all indicative for sediment origin and proximity to the source area. Sioli (1984) defined three types of Amazonian rivers based on the proportion of bed load versus suspended load and the acidity (pH) of the water. These are: white water rivers with high suspended load and Andean source (e.g. the Amazon River); and craton-born rivers, which can be subdivided into two types, clear-water rivers with little suspended load and high bed load (e.g. Tapajós River), and black-water rivers, which are rich in humic acids and have little suspended load (e.g. Negro River) (Fig. 7.2). Dunne et al. (1998) estimated that only around 1% of the total Amazon River sediment load travels as bed load, with the other 99% in suspension. The bed load of Andean rivers is characterized by immature, lithic arenitic sands rich in volcanic and metamorphic components and a variable clay mineral composition. Sands are further characterized by an unstable heavy mineral suite composed of epidote, garnet and chloritoid (Krook 1979; Hoorn 1993, 1994b; Ruiz et al. 2007). In its upstream reaches, Andean tributaries are characterized by a suspended load high in illite and chlorite, which originate from the erosion of a great variety of rocks. The illite and chlorite contents decrease downstream whereas smectite content increases due to lateral bank erosion of the Tertiary units in the Amazon lowlands (Irion 1984; Johnsson & Meade 1990; Guyot et al. 2007; Viers et al. 2008). Downstream

there is also an increasing sediment contribution from foreland tributaries that drain the Pastaza Megafan (Räsänen et al. 1992) and the Fitzcarrald Arch. There is little sediment deposition in the main channel and high suspended load is mostly deposited on the floodplains that harbour the highly fertile flooded forest called várzea. Cratonic rivers mainly carry bed load sediments composed of quartz sands and minor plagioclase and alkali feldspars (Franzinelli & Potter 1983; Potter & Franzinelli 1985; Johnsson et al. 1988; Potter 1994). Quartz is highly resistant to weathering whereas minerals such as plagioclase and alkali feldspar easily decay into clay minerals. For this reason, quartz forms the dominant component of sands in Amazonian rivers that are not of Andean descent. These sands are also characterized by their ultrastable and stable heavy mineral suites, which are composed of minerals such as tourmaline, zircon, small amounts of rutile, anatase, and the opaque minerals ilmenite and magnetite (Hoorn 1993, 1994a; Lopez-Africano 1998; Ruiz et al. 2007). The suspended load of cratonic rivers is characterized by a kaolinite-dominated clay mineral association (up to 100% in some rivers); however, the proportion of kaolinite to the main Amazon River suspended clay yield remains low (Klammer 1984; Guyot et al. 2007). Craton-derived rivers are characterized by channel-dominated processes where sand deposition takes place in the form of sandbars within river channels; floodplains are of limited extent and the flooded forests in these systems are called igapó.

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Trace element geochemistry and neodymium-samarium (Nd-Sm) isotope ratios are particularly useful in determining the sediment provenance (e.g. McLennan et al. 1993) and have been successfully applied to discriminate Andean from cratonic sediments (McDaniel et al. 1997; Roddaz et al. 2005b, 2006). In Fig. 7.3 the Nd-Sr isotopic composition of the Amazonian sediments (Roddaz et al., 2005b), Andean volcanic rocks (Kay et al. 1994; Rogers & Hawkesworth 1989; Barragan et al. 1998) and Andean sediments (Pinto 2003) define a hyperbolic relationship – one end member being the primitive Andean arc and the other the upper continental crust of the Brazilian Shield (Basu et al. 1990; Roddaz et al. 2005b). Sediments of cratonic origin plot closer to the Brazilian Shield end-member whereas sediments of Andean origin are related to the Andean magmatic end-member. Present-day cratonic rivers are clearly distinct from the Andean rivers. Cratonic rivers have lower ⑀Nd and higher 87Sr/86Sr isotope ratios than Andean rivers (Allègre et al. 1996). The Nd-Sm isotopic compositions of cratonic rivers clearly differ from those of sediments of the main Amazon River. Sediments are further characterized by high europium anomalies (Eu/Eu* < 0.65), low chromium/thorium ratios (Cr/Th < 6), high zirconium/scandium and thorium/scandium ratios (Zr/Sc > 15 and Th/Sc > 1) (Roddaz et al. 2005b, 2006). The Negro and Tapajós Rivers, large Amazon River tributaries, are typical examples of modern cratonic rivers. The Negro River is a black-water river that drains the Guiana Shield (Latrubesse &

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Fig. 7.3 Diagram showing ⑀Nd versus 87Sr/86Sr values. Black rectangle: Ecuadorian volcanic lavas (Barragan et al. 1998); AM, Amazon mouth (Parra & Pujos 1998). Mesozoic and Neogene volcanic rocks from Rogers & Hawkesworth (1989) and Kay et al. (1994). Data for Central Depression, Altiplano, Oriental Cordillera and sub-Andean Zone fields are available in Pinto (2003). Solimões, Amazon-20, Amazon-6, Madeira, Negro, Urucara, Trombetas and Tapajos isotopic compositions are taken from Allègre et al. (1996). NAFB, North Amazonian Foreland Basin sediments; SAFB, South Amazonian Foreland Basin sediments (both from Roddaz et al. 2005b). Fm, formation.

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Franzinelli 2005) and carries kaolinitic clay minerals (74–81%) and minor proportions of illite and chlorite (4–7% and 8–15% respectively) as suspended load (Guyot et al. 2007). The ⑀Nd(0) value of the Negro River (Allègre et al. 1996) is lower than that of Andean and Amazonian Craton-derived sediments, and its Nd-Sr isotopic composition plots outside of the Andean-cratonic mixing line that characterizes sediments issued from the Andes. The Tapajós River is a clear-water river that drains the central Brazilian Shield and carries a suspended load dominated by the clay mineral kaolinite (80%) and minor proportions of illite (7%) and chlorite (8%) (Guyot et al. 2007). However, the Tapajós River has lower ⑀Nd values and higher 87Sr/86Sr isotope ratios than, for instance, the Negro River. The Nd-Sm isotopic compositions plot outside of the Amazon River field and, together with the Trombetas River, are typical for the Nd-Sm isotopic composition of sediments derived from the craton (Allègre et al. 1996) (see Fig. 7.3). Other good examples of rivers of undiluted Amazonian Craton origin are the Marowijne and Corantijn Rivers in Surinam, which flow directly from the craton into the Atlantic.

Climate and weathering in the cratonic source area Long-term exposure of relatively stable continental crust together with intense tropical weathering provided a continuous supply of sediment to lowland Amazonian and cratonic rivers. The exposed bedrock was deeply weathered due to a warm climate and high precipitation, causing profound leaching and leaving a resistant residue composed mostly of quartz and secondary lateritic soil minerals. Over time this process resulted in the formation of thick lateritic soils that were enriched in quartz and iron and aluminium-rich minerals such as kaolinite, goethite, hematite and gibbsite (e.g. Aleva 1994; Lima-Costa 1997). The so-called ‘white-sands’ podzols are characterized by soil horizons, leaching and mottling features, and mineral alteration commonly associated with podzol and laterite formation in tropical climates. White-sand podzols typically occur on (i) granitic basement (Proterozoic rocks that form part of the Amazonian Craton); (ii) Cretaceous sandstone (Alter do Chão Formation); (iiic) low uplands or terra firme; (iv) levees in a floodplain or várzea; and (v) sand ridges or dunes of the coastal lowland (Whitmore & Prance 1987). Lateritization of both Proterozoic rocks (Théveniaut & Freyssinet 2002) and Mesozoic sedimentary sequences like the Alter do Chão Formation (Nogueira et al. 1999; Horbe et al. 2004) has been ongoing for many millions of years and has resulted in considerable rock volume loss from the Amazonian Craton throughout the Tertiary (Théveniaut & Freyssinet 2002). Nevertheless, tropical climate alone is ineffective to promote high erosion rates in tectonically inactive areas (von Blanckenburg et al. 2004). Théveniaut & Freysseinet (2002) suggest that 50–80 m of ‘landscape’ was eroded during the Oligocene, a figure that indicates an average denudation rate of about 5–8 mm/ka over the 10 million years that constituted the Oligocene. This rate is very low and within the range of denudation rates found in other tropical highlands such as that studied in Sri Lanka (von Blanckenburg et al. 2004). The palynological record from fluvial deposits in

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The Amazonian Craton and its fluvial systems the intracratonic basins further confirms that, at least since the Neogene, a tropical, humid climate characterized Amazonia (Hoorn 1994a).

Cratonic rivers, past and present: evidence from the sedimentary record in Amazonia In this section we review four different fluvial sediment units that range in age from Cretaceous to Late Neogene and all have a cratonic source area. These units are: the Alter do Chão Formation in Brazil (Cretaceous-Paleogene); the Petaca Formation in Bolivia (Oligocene-Miocene); the Colombian Mariñame and Apaporis Sand Units (Miocene); and the Iquitos White Sands (MiocenePliocene) (Fig. 7.4).

Cretaceous-Paleogene (Alter do Chão Formation, Brazil) Definition, stratigraphic relations, distribution and general characteristics The Alter do Chão Formation forms part of the MesozoicCenozoic Javarí Group that occurs in the Brazilian Amazonas and Solimões Basins (see Plates 2d & 3b). In the Solimões Basin the Alter do Chão Formation is only known from the subsurface. It has a discordant contact both with the underlying Paleozoic formations and the overlying Cenozoic Solimões Formation

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(Eiras et al. 1994; Eiras & Wanderley Filho 2006; Milani & Zalán 1999). In the Amazonas Basin, east of the Purus Arch, the formation can be observed at outcrops (Andrade & Cunha 1971; Cunha et al. 1994; Wanderley Filho et al. 2005). The best exposures are in the periphery of Manaus, where the Alter do Chão Formation has an unconformable upper contact with two thin siliciclastic units called Novo Remanso and Iranduba Formations (Rozo 2004, Rozo et al. 2005; Dino et al. 2006; Abinader et al. 2007; Soares 2007; R.W. Mapes, 2009). These sandy deposits are discontinuous and of limited extent and each of them are capped by a lateritic palaeosol. The Novo Remanso and Iranduba Formation will be further described by R.W. Mapes et al. (2009). The Alter do Chão Formation was first described by Derby (1879) based on geological information from Hartt (1870), who made reference to sequences of multicoloured mudstones and sandstones with worm tubes. At a later stage Kistler (1954) assigned the name Alter do Chão Formation to the reddish layers that cover the Paleozoic units in the Amazonas Basin. Subsurface data obtained from Petrobras’ oilwells indicate that formation thickness ranges from 50 to 600 m. The maximum of 600 m thickness is reached in the central part of the basin, but on average the thickness is around 300 m, and the formation is thinning westward in the direction of the Solimões Basin. The type section of the Alter do Chão Formation is the well 1-AC-1-PA, located on the eastern bank of the Tapajós River, near the village of Alter do Chão, Pará, Brazil (Fig. 7.5a; Caputo et al. 1971). At the surface a series of outcrops in the periphery of Manaus permit a broader view of the sedimentary characterisitics of this unit (see Fig. 7.5b).

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Fig. 7.4 Overview and stratigraphic relations of the four sedimentary units that are referred to in this chapter. E-Stm, clay unit; Fm, formation; A.S.U., Apaporis sand unit; N-Sc, sand unit (codes as used on geological map of Colombia). The arrow indicates transport directions.

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Fig. 7.5 (a) Map with location of the type section of the Alter do Chão Formation. (b) Principal outcrops of the formation in the Manaus region. The Alter do Chão Formation is characterized by red, yellow and white sandstones and mudstones with intercalated conglomerates, and rarely any fossils (Caputo 1984) (Fig. 7.6a). The sandstones are composed of reddish middle/coarse-grained quartz sand with a kaolinitic clay matrix, and display cross-bedding and cut-andfill structures (see Fig. 7.6b). Albuquerque (1922) applied the name ‘Manaus Sandstone’ to a series of predominantly sandy, discontinuous and silicified levels. This distinct horizon occurs, as the name suggests, in the Manaus area and consists of red to white, silty to clayey sandstones of variable thickness and bedding/lamination planes characterized by different grain sizes. The texture of the ‘Manaus Sandstone’ is fine/medium-coarse-grained, has low-medium grain roundness, is poorly-to-moderately sorted, and has variable grain rounding (Franzinelli & Rossi 1996, 1997). Sand grains are coated with siliceous and iron oxide cement and mainly composed of mono- and polycrystalline quartz. Alkali-feldspar grains and rock fragments (quartzite and siliceous sandstone) occur in minor proportions.

depth in well NO-1-AZ at Nova Olinda (Amazonas, Brazil). The age of the formation was narrowed down to EarlyCenomanian/Maastrichtian (Daemon & Contreiras 1971; see also Chapter 19) and Middle-Albian/Turonian (Daemon 1975) based on palynological data. The Cretaceous age of the Alter do Chão Formation is corroborated by data from the palaeosol at the top of the formation. This palaeosol was correlated with the SD1 surface (Horbe et al. 2001), which is considered to be of Paleogene age (Costa 1991). The Paleogene age is consistent with the palaeomagnetic data from lateritic crusts north of Manaus (Trinidade et al. 2006), which provided ages between 50 and 70 Ma (Late Cretaceous to Paleocene). Siliciclastic outcrops in the Itacoatiara and Manacapurú regions, east and west of Manaus respectively, further indicate that the Alter do Chão Formation predates the Middle Miocene. The two thin overlying units, informally known as the Iranduba (inferior) and Novo Remanso (superior) formations (Abinader et al. 2007; Rozo 2004; Rozo et al. 2005; Soares 2007; R.W. Mapes et al. 2009), are no younger than Middle to Late Miocene (Dino et al. 2006).

Age Lithofacies Price (1960) assigned a Cretaceous age to the Alter do Chão Formation based on a theropod dinosaur tooth. This tooth was collected in the upper part of the formation, at 193–196 m

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Sandstone outcrops north of Manaus (see Fig. 7.5b) are composed of quartz, opaque minerals, zircon and epidote, with

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Roads Ponta Pelada Airport

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(1) Souza (1974); (2) (1998), Soares (2007), Vieira & Nogueira (1998), Nogueira et al. (1999); (3) Horbe et al. (2006); (4) Nogueira et al. (2006); (5) Colônia Oliveira Machado; (6) Estrada do Turismo (Soares, 2007).

Continued.

a matrix of silt and clay (Souza & Medeiros 1972; Souza 1974). Locally, centimeter-scale manganese concretions with plant remains occur in sandy layers (Soares 2007). These concretions are of secondary origin and associated with siliceous or ferruginous cement. Vieira (1999) divided the Alter do Chão Formation into nine sedimentary facies. These were: 1 2 3 4 5 6 7 8 9

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sandstone with trough cross-bedding; sandstone with tabular bedding; sandstone with tabular cross-bedding; conglomerate with massive bedding; pelite with tabular cross-lamination; sandstone with massive bedding; sandstone with cross-lamination; sandstone with tabular lamination; sandstone with bioturbation and mud cracks.

Facies 1–4 were all indicative of a lower flow regime environment and bed load deposition related to migration of sandy bars and sheet sand in fluvial channels. Facies 5–9, instead, are indicative of suspension deposits and palaeosols related to floodplains and abandoned braided fluvial areas (Vieira & Nogueira 1998; Nogueira et al. 1999; Horbe et al. 2006). Along the Negro River and Highway BR-174 (Manaus– Caracaraí) the Alter do Chão Formation is composed of crossbedded kaolinitic sandstones with interbedded mudstones, consistent with deposition in an alluvial plain environment (Caputo & Sad 1974). Moreover, in the Presidente Figueiredo region, at the northern edge of the Amazonas Basin, the Alter do Chão Formation was deposited in isolated grabens and is unconformably overlying the Silurian rocks of the Nhamundá Formation (Nogueira et al. 1997). There the deposits represent alluvial fans and braided fluvial systems (Nogueira et al. 1999).

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Gamma-ray

Lithology and Cores depth (metres) 0

Lithological description Quartz sandstone, coarse to conglomeratic with quartzite fragments and ferruginous (limonitic) material; increasing clay content and colour variations with depth

1–6

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Conglomerates, medium rounded, well sorted and pale-brown clays

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Alter do Chão Fm. Andirá Fm. Very fine reddish sandstones, argillaceous, micaceous and slightly calciferous

The reddish coloration of the sediments and the terrestrial fossils in the Alter do Chão Formation are typical for alluvial plain and fan environments (Caputo 1984). In addition to this, the sedimentary sequences suggest a braided fluvial system, a conclusion that was based on the presence of straight and sinuous sand bars, wide and shallow channels (indicative of strong lateral migration), the unidirectional nature of the palaeocurrent indicators, and the low frequency of claystones. Nevertheless, using regional datasets, some authors believe fluvial-lacustrine depositional conditions existed (Caputo et al. 1972; Castro et al. 1988; Pereira 1988; Cunha et al. 1994). The trace fossils Scoyenia, Skolithos and Muensteria, which are characteristic of the Scoyenia ichnofacies, further confirm the continental nature of this formation (Nogueira et al. 1999). The traditional idea of a strictly continental palaeoenvironment for the Alter do Chão Formation was questioned by Rossetti & Neto (2006) who, based on sedimentological and ichnological analysis of outcrops near Careiro Island (east of Manaus), suggested a marine influence. Dino et al. (1999) subdivided the formation into two sequences: (i) an inferior Aptian succession, characterized by terrigenous influx associated with cycles of fluvial meandering to an anastomosing system; and (ii) a superior Cenomanian succession, deposited in fluvial, deltaic and lacustrine environments (see Chapter 19). According to these authors, the Alter do Chão Formation was deposited in a relatively dry climate but with some humidity, an assertion based on the abundance of spores and the absence of rimulate and polyplicate pollen grains. Furthermore, the presence of fungal spores inside amber fragments indicated a hot and wet climate during sediment deposition (Pereira et al. 2006). Palaeoclimate data throughout the formation are in agreement with the recurring palaeosols, abundant fossil root traces, and channel deposits that contain abundant silicified wood fragments. All these features are indicative of vegetated fluvial margins. Palaeogeography

Fig. 7.6 (a) Type section of the Alter do Chão Formation (modified after Caputo et al. 1971). (b) Outcrop of the Alter do Chão Formation in Manaus area (photo by Clauzinor da Silva).

Geochemical evidence The Nd-Sm isotopic ratios in sediment samples from the Alter do Chão Formation do not resemble values of the present Andean river sediments. Two samples have ⑀Nd values similar to that of the present Negro River, whereas the other samples have a similar Nd-Sm isotopic composition as the Trombetas and Tapajós Rivers (see Fig. 7.3). Therefore we conclude that the Amazonian Craton formed the source of sediment supply to the Alter do Chão fluvial system.

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Palaeocurrent directions, detrital zircon ages, isotopic signature, sedimentological characteristics and geographic distribution together suggest that the Alter do Chão Formation was formed by alluvial fans to braided fluvial systems fed from a cratonic source (Guiana Shield to the north and Brazilian Shield to the south) throughout the margins of the pre-existing Amazonas and Solimões basins. These braided fluvial systems, which were coming from opposite directions, merged in the central depression of the Amazonas and Solimões Basins and formed there a E-W-oriented trunk river. Contemporary with the earliest phase of Alter do Chão deposition, the Gurupá Arch (uplifted footwall of the Marajó Rift) also contributed with sediments eroded from uplifted Paleozoic strata. The latter also played an important role in driving the trunk river westwards, at least up to the middle Late Cretaceous (Santonian or Campanian) (Figueiredo et al. 2004). After that, the river split into two separate systems, possibly divided by the Monte Alegre high (Figueiredo 2004). One river continued to flow to the west, as a continuation of the Alter do Chão system, whereas the other flowed into the Foz do Amazonas

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12°

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Study sites of the Petaca Formation. Modified after Uba et al. (2005).

Basin (Limoeiro Formation) (Figueiredo et al. 2004; Pasley et al. 2005). The Alter do Chão River system was discontinued during the Paleogene.

Late Oligocene to Middle Miocene (Petaca Formation, Bolivia) Definition, stratigraphic relations, distribution and general characteristics The Petaca Formation occurs in the Chaco Foreland Basin and is situated south of the Bolivian orogenic belt (Fig. 7.7). This basin system comprises the central Andean-sub-Andean Zone and the Chaco plain morphotectonic units where sediment accumulation was initiated during the Late Oligocene (Gubbels et al. 1993; Uba et al. 2005, 2006). The lower boundary, between the Petaca Formation and the Cretaceous(?) Tacuru Group, is a sharp, low-angle unconformity. The distinct lithological change from the aeolian Tacuru sandstone to thick Petaca calcrete further marks this contact and can best be observed at the Angosto de Pilcomayo and Iguamiranti sections. The upper boundary, between the Petaca Formation and the Middle Miocene Yecua Formation, is a gradual transition from thin beds of mudstones and sandstones, which were deposited in a floodplain environment, into a mudstone-dominated lacustrine, marginal marine and fluvial environment. The transition from the Petaca to the Yecua Formation is accompanied by a

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66°

change in palaeo-drainage direction, which changes from E–W in the Petaca Formation to axially N–S in the Yecua Formation (Uba et al. 2005). In addition to this, petrographic analyses also show a change in mineralogical composition and provenance between these formations (Hulka 2005). The Petaca Formation can reach a thickness of up to 250 m and is well exposed along the sub-Andean foothills at Tatarenda, Iguamiranti and Angosto de Pilcomayo sections (Fig. 7.8; see also Fig. 7.7) in the southeast of Santa Cruz de la Sierra (Uba et al. 2005, 2006). Echavarria et al. (2003) and Uba et al. (2006) have shown that the Petaca and the overlying Yecua, Tariquia and Guandacay formations thin and young progressively to the east, and that the proportion of coarse-grained sediments increases to the west. This westward increase in coarse sediments is an indication of westward location of the basin centre and deformation front with respect to its present-day position. Uba et al. (2006) documented the progressive onlap terminations and thickeningupward trend of the Petaca strata and the younger strata on the Alto de Izozog to the east in seismic profiles, suggesting that the Alto de Izozog is a pre-Tertiary structure.

Age Recently, Uba et al. (2007) estimated the age of the top of the Petaca Formation at 12.4 Ma, based on volcanic ash at Angosto de Pilcomayo in the southern part of the Chaco Basin. No absolute age

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Fig. 7.8 Lithological columns of the Petaca Formation. Height in metres. Cg, conglomerate; Cl, clay; S, sand. Arrows indicate generally westward current directions. Modified after Uba et al. (2005).

data exist for the base of the Petaca Formation; however, reptilian and mammalian bone fragments from the Quebrada Saguayo (NW Chaco Basin) suggest a Late Oligocene age (~27 Ma) for the lower boundary (Marshall & Sempere 1991). Lithofacies The sedimentary characteristics of the Petaca Formation are described based on data from three sections (see Fig. 7.8). Tatarenda section The 60 m-thick Tatarenda section is exposed along the Grande River and contains ‘typical’ Petaca successions composed of conglomerates, sandstones and mudstones. In this section a polymictic conglomerate marks the base of the Petaca Formation whereas the characteristic basal calcrete found elsewhere is absent. The conglomerate consists mostly of poorly sorted intraformational (reworked) pedogenic clasts. The size of the conglomeratic clasts decreases upwards in the section, suggesting decreasing current

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energy. The sandstones and mudstones, which comprise about 75% of this section, form the top of the upper Petaca Formation. The medium-to-coarse-grained calcareous sandstone is characterized by trough cross-, planar and massive stratification. The mudstone consists of laminated and bioturbated beds and includes moderately to well-developed dark-red to red palaeosols. Iguamiranti section The Iguamiranti section is the easternmost section in the Chaco Basin. This section is situated along an ephemeral stream bed southeast of Santa Cruz de Sierra. In this section the Petaca Formation is up to 80 m thick and shows the best, and the thickest, exposed basal calcrete horizon of the entire Chaco Basin. The red calcrete bed is up to 12 m thick and marks the basal stratum of the Petaca Formation. This bed is dominated by purple to red, calcareous, clustered or coalesced, spherical to irregular nodules, occasionally containing intraformational conglomerates. Unlike the other sections, a reworked pedogenic conglomeratic body does not outcrop here; instead, sandstone and mudstone directly

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The basal calcrete horizon suggests extensive pedogenesis in response to low sedimentation rates in a semi-arid climate (Uba et al. 2005). This is substantiated by stable isotope analysis, which indicates that during this time the Chaco Basin had a dry climate (Strecker et al. 2006). Furthermore, Uba et al. (2005) postulated that the calcrete horizon may also indicate a long interval of basin stability. The distinct differences in grain size (conglomerate, sandstone and mudstone) within the Petaca Formation are indicative of variable, high-energy stream flows (Uba et al. 2006). The variation in stream energy coupled with other features such as channel structures, fining-upward sequences as a result of decreasing flood intensity and pedogenesis suggest that the Petaca strata were deposited in an ephemeral, braided stream depositional environment. The angular homogeneous reworked-pedogenic conglomerate clasts are thought to be a result of intrabasinal erosion and a product of uplift and exposure of the calcrete horizon. The absence of conglomeratic bodies to the east (Iguamiranti) is probably related to the distal position of the section relative to the palaeo-high axis to the west (Angosto and Tatarenda). Palaeogeography All exposures of the Petaca Formation display E–W-oriented palaeocurrent directions, which suggests that the braided fluvial system originated on the Brazilian Shield. Provenance analysis further supports this view and shows a predominance of cratonderived quartz sand and quartz-rich subarkose clasts in the Petaca strata (Hulka 2005).

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Definition, stratigraphic relations, distribution and general characteristics The Middle Caquetá River area in southeast Colombia hosts a series of characteristic quartz-rich sand sequences that are informally named the Mariñame Sand Unit and are exposed adjacent and eastwards from the Araracuara table mountains (Fig. 7.9). The best sites are situated along the Agua Negra River (a tributary of the Yarí River) and near Isla de Mariñame and Tres Islas, along the margins of the Caquetá River (Hoorn 1993, 1994a; Ochoa-Lozano 2007). Other similar, but somewhat younger, sand unit deposits are exposed along the Apaporis and Pirá Paraná Rivers and are informally called the Apaporis Sand Unit (Hoorn 2006). The Miocene sand units in Colombian Amazonia are part of the northern extension of the Solimões intracratonic basin. Here these sand units were originally called ‘Terciario Superior Amazónico’, a name provided to distinguish them from the blue

80°0′0″W

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The Petaca Formation is exposed along the ephemeral Pilcomayo River near the town of Villamonte. At low river level approximately 15 m of calcrete, conglomerate and sandstone outcrop along the river bed. At the base there is a 1 m-thick calcrete bed that displays a blocky, massive or pseudo-prismatic form. This bed is multicoloured and characterized by polygonal desiccation cracks that are indicative of subaerial exposure. Overlying the calcrete horizon is a thin-bedded polymictic conglomeratic body. The conglomerate clast composition and size are similar to those at the Iguamiranti section. However, the high percentage of angular cobble-sized pedogenic clasts is unique to this section and indicates local synsedimentary interbasinal erosion. The sand bodies are coarser than at the other sections and occasionally contain pedogenic pebble strings; mudstone deposits, common in the other sections, are absent.

0°0′0″

Angosto de Pilcomayo

Tectonically, the Petaca strata were deposited when the deformation front was probably at the Eastern Cordillera and this part of the basin was the forebulge depocentre of the foreland system (Uba et al. 2006). The overlying Yecua unit is contemporaneous with the arrival of the deformation front in the present sub-Andean zone at ~12.4 Ma (Uba et al. 2007) and represents deposition in a distal foredeep depocentre (Uba et al. 2006). These events were marked by widespread well-developed palaeosols, low accommodation space and low sedimentation rates during the deposition of the Petaca Formation. In contrast to this, the widespread mudstone-dominated Yecua Formation was formed under conditions of high accommodation space and relatively low sedimentation rates (Uba et al. 2006, 2007).

5°0′0″S

overlie the calcrete horizon. The white to red, medium to very coarse-grained sandstone is characterized by crude horizontal, planar and massive stratification coupled with stacked channelized sand bodies. The sandstone is interbedded by red-to-darkbrown massive mudstone, with moderately developed palaeosols and bioturbation suggesting subaerial floodplain exposure. This section shows a general fining- and thickening-upward sequence of sandstone and mudstone.

113

Studied sites 1. Agua Negra 4. Santa Isabel

2. Mariñame 5. Apaporis

3. Tres Islas 6. Pirá Párana

Fig. 7.9 Map with locations of the main outcrops of the Colombian Miocene Sand Units.

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clays and lignites of the ‘Terciario Inferior Amazónico’ (Galvis et al. 1979; Proradam 1979). However, palynological analysis shows that the blue clays are actually younger than the sand units along the Caquetá River, but contemporaneous with (or perhaps even older than) the unit along the Apaporis River (Hoorn 2006; Wesselingh et al. 2006a). The contact between the sand unit and the deeply weathered Precambrian basement can only be observed at the Apaporis 5 section (Hoorn 2006) whereas the transition from the Mariñame Sand Unit into the blue clays of the Pebas/Solimões Formation is observed at Santa Isabel (Hoorn 1994b). The total thickness of the sand units could not be established but along the Caquetá and Agua Negra Rivers a minimal thickness of 60 m was measured. At the top the sand units are in unconformable contact with a conglomeratic series of Andean-derived sediments of Late Miocene age or younger.

composition the sediments at the Caquetá River and the Agua Negra sections belong to the Retitricolporites guianensis and the Crototricolpites-Psiladiporites zones. The absence of marker species such as Verrutricolporites rotundiporus, Crassoretitriletes vanraadshoovenii, and other older and younger biostratigraphic markers suggest an age of Early to early Middle Miocene (Hoorn 1993, 1994a; Ochoa-Lozano 2007). The Apaporis Sand Unit is estimated at a Middle to Late Miocene age based on the presence of the pollen taxon Grimsdalea magnaclavata and the absence of older and younger biostratigraphic marker taxa (Hoorn 1993, 2006). This implies that the ‘younger’ sand unit was contemporaneous with the Pebas/ Solimões Formation, and that the Amazonian mega-wetland (see Chapter 8) coexisted with the cratonic rivers that deposited these sediments. Lithofacies

Age The outcrops at Agua Negra, Mariñame and Tres Islas are rich in organic matter and contain abundant pollen. The sections were dated using palynological analysis and ages were defined by comparison with the Caribbean biostratigraphic markers and zonations (Germeraad et al. 1968; Lorente 1986). An adapted zonation was established for Amazonia. Based on the palynological

The outcrops along the Caquetá and the Agua Negra River are characterized by poorly sorted, subangular, medium to very coarse-grained sands with fine gravel in minor proportions (Fig. 7.10). These sands are composed of quartz and mica and alternate with light-coloured clays and dark-coloured organicrich clay and lignite beds in a fining-upwards sequence. The sands have a bimodal grain size distribution as fine to very fine sand

Palaeocurrent direction

km

km

km

Fig. 7.10 Overview of the Colombian Mariñame Sand Unit as observed along the Agua Negra and Caquetá Rivers (modified after Hoorn 1994a and Ochoa-Lozano 2007). Cg, conglomerate; Cl, clay; S, sand; f, fine; m, middle; c, coarse.

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The Amazonian Craton and its fluvial systems is usually missing from the succession. The sands also contain a heavy mineral suite composed of tourmaline, zircon, rutile, anatase and single grains of staurolite, andalusite, sillimanite and epidote. This assemblage is typical for Proterozoic source rock of the Amazonian Craton. However, blocks of the Proterozoic basement are also incorporated into the Andean Cordillera, therefore the assemblage is not exclusive to Amazonian cratonic areas. The best – and most characteristic – outcrops of the Mariñame and Apaporis Units are in southeastern Colombia, and detailed descriptions of these field sites can be found in Hoorn (1994a, 2006) and Ochoa-Lozano (2007). Here we summarize the main characteristics of these units: Mariñame section This is 60 m thick and characterized by tabular cross-stratified (1 m) sand beds with gravel concentrations along the foresets (see Fig. 7.10). These beds range from half a metre to several metres in thickness, represent large fluvial sand bars that were deposited in a channel environment, and indicate palaeocurrent directions toward the northwest. The massive sand bed sequence changes into a ~ 8 m-thick sequence of black clays that, at the top, grade into grey clays that contain foraminifera and mangrove pollen. The organic-rich clays in this sequence contain many pyrite nodules, which possibly are a secondary diagenetic feature that resulted from marine incursions into freshwater oxbow lakes and swamps. The clays and lignites represent a flood basin environment with brief marine influences. Tres Islas section In the 25 m-thick Tres Islas section, sand beds are less abundant and clay beds predominate. The latter often display reddish mottling, a feature typical of exposure and oxidation of the palaeosurface. The clay beds alternate with clay pebble beds, or coarse-grained sands alternate with thin clay beds. Together these features characterize an alternation of dynamic and slack periods, a succession typical of a crevasse splay environment. These deposits include plant as well as amber fragments. Also characteristic are the massive clay beds (4–7 m) with palaeosols at the top of each sequence. These beds are thought to represent mudflows in the flood basin that were exposed after each depositional cycle.

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sedimentary sequence was deposited in a fluvial environment with swampy conditions and some clastic influx. Apaporis section This section is situated along a mixed river, with clear- and blackwater tributaries, that has its origin in the Colombian Llanos and forms a tributary to the Caquetá River. Along this river, and its tributary the Pirá Paraná, there are a series of outcrops that resemble the ones along the Caquetá River (Hoorn 2006). The sedimentary successions are characterized by subangular, medium to coarse-grained sands, composed of quartz and mica that include many gravel beds and varicoloured clays and lignites. The succession is a fining-upward sequence that, like the sequences along the Caquetá River, presents a bimodal grain size distribution where fine sand is almost absent. In contrast with the Caquetá sections, gravel is more common in the Apaporis Sand Unit. These gravels frequently form stringers within the massive sand beds (5–7 m thick) and are thought to represent sheet flood deposits formed during high fluvial discharge and abundant supply of a coarse bed load. The sand beds rarely display tabular cross-stratification; however, in four different beds, north and westward transport directions were identified. Clays and lignites occur in equal proportion to sand and include fine-grained sand laminae and lenses. These fine-grained sands, which are otherwise rare, are possibly related to a marine inlet that affected the depositional system. The organic matter in the sandy clays is dominated by Zonocostites (a mangrove pollen), foraminifers and dinoflagellates, all of which are coastal indicators. This assemblage is ecologically similar to the one found at the top of the Mariñame 3 section, but the Apaporis section is younger and represents flood basin and coastal back swamp environments. Depositional environment and palaeogeography The sedimentary sequences exposed along the Apaporis, Pirá Paraná, Agua Negra and Caquetá Rivers represent sandbars, crevasse splay, and flood basin deposits that were formed in a low

Agua Negra section The Agua Negra section is 36 m thick (Fig. 7.11; see also Fig. 7.10 and Plate 2(b)) and situated adjacent to the Paleozoic table mountains of Araracuara. This section is characterized by a fining, thinning-upwards sequence of sands and clays similar to those observed at Mariñame and Tres Islas. The most important differentiating characteristic is that the sands and sandy clays are fine-grained, suggesting these sediments were deposited more distally from the source. The section is characterized by a large channel structure that is filled with lignite and organic-rich clay that alternates with sand. The clay beds show reddish colours and mottling, and at the base in some of the beds pyrite nodules are present. Sandstone beds exhibit planar lamination. Based on the lithological sequence and the palynological assemblage, we conclude that the Agua Negra

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Fig. 7.11 The Agua Negra section (Mariñame Sand Unit, Colombia). This section is stratigraphically close to the contact with the Paleozoic Araracuara Formation and the Precambrian basement. Photo by R. Corstanje.

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C. Hoorn et al. (Locations POM 37 and POM 40; see Fig. 7.13) (Roddaz et al. 2005a; Rebata et al. 2006a). West of Iquitos the same erosional surface separates the tidal deposits of the Nauta Formation (Nauta 1 Member) from the fluvial Nauta 2 Member (Roddaz et al. 2005a; Rebata et al. 2006b). The latter and the Iquitos White Sand Unit are presumed to be laterally equivalent. However, the Nauta 2 Member has Andean provenance and its sediments were deposited in a W–E direction towards Iquitos, whereas the Iquitos White Sand Unit has a cratonic provenance and its sediments were deposited in an NE direction towards Iquitos (Roddaz et al. 2005b). The upper contact of the White Sand Unit is an erosional surface that separates the White Sand Unit from the overlying informal Amazon formation (POM 38; see Fig. 7.13) and the Quaternary floodplain deposits (POM 37; see Fig. 7.13) (Roddaz et al. 2005a). The informal Amazon formation is composed of sediments deposited by a river drainage system with origins in the Ecuadorian Andes (Roddaz et al. 2005b). This formation could not be dated, but the maximum age is estimated as latest Miocene to Pliocene as it overlies the White Sand Unit and the Nauta 2 Member.

sinuosity to anastomosing river system. These fluvial systems had NW-oriented transport directions and originated on the craton. Modern analogues for the fossilized flood basin environments would be the present igapós – such as described by Sioli (1984). Modern analogues for the entire system would be the present tributaries of the Orinoco River and lowland Amazonian rivers. The main differences between the Apaporis Sand Unit and the Mariñame Sand Unit are the palynological composition and the granulometries. At the Agua Negra site, grain size varies between fine-grained sand and clays, whereas at the Apaporis sites, grain sizes vary from fine gravel/very coarse-grained sands to clays. These variations are probably due to the more distal position of the Agua Negra site to the source area.

Miocene-Pliocene (Iquitos White Sand Unit, Peru) Definition, stratigraphic relations, distribution and general characteristics The Iquitos White Sand Unit forms part of the Pastaza-Marañon Basin and was first described by Räsänen et al. (1998) based on outcrops along the Iquitos-Nauta road (km 0–33). Here we will discuss the main characteristics of these outcrops (Fig. 7.12). The Iquitos White Sand Unit is an informal name for conglomerates and white quartz sands with a clayey matrix that is mainly composed of kaolinite (Fig. 7.13). The basal contact of this unit is an erosional surface with a conglomeratic infill that separates the sands from the characteristic blue clays of the Pebas Formation

The age of the Iquitos White Sand Formation is uncertain as no absolute or relative dates are available. For this reason Räsänen et al. (1998) suggested that the age might range between 8 and 1 Ma. Based on field evidence, Roddaz et al. (2005a) proposed that the White Sand Unit is the eastern lateral equivalent of the Nauta 2 Member (the fluvial part of the Nauta Formation). Both

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Iqui tos-

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Studied sites of the Iquitos White Sand Unit (modified after Roddaz et al. 2005a).

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The Amazonian Craton and its fluvial systems the Nauta 2 Member and the White Sand Formation are overlain by the (informal) Amazon formation, which is no older than latest Miocene-Pliocene. Based on these stratigraphic relationships we tentatively suggest that the age of the White Sand Unit ranges between 8 and 5 Ma (Roddaz et al. 2005a). Lithofacies and depositional environment

Amazon Fm or Quaternary Floodplain deposits

Near Iquitos, the White Sand Unit is characterized by a 14 m-thick fining-upward sequence with conglomerates at the base of thesequence and shales at the top (Fig. 7.13). The basal conglomeratic layer is a 10–20 cm-thick trough cross-stratified horizon composed of quartz and gneiss pebbles (up to 10 cm long at outcrop POM 40) and clay clasts. These fine to medium/coarse-grained conglomerates gradually change into coarse (up to 2 m in thickness) and fine-grained sand (up to 8 m in thickness; Fig. 7.14).

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Generally, the sands are massive, but faint planar and trough crossstratification can also occur (POM 37 and POM 40). Locally, the sandy deposits change gradually into white shales (or mud) with slight horizontal laminations. Palaeocurrent directions measured on trough cross-stratifications and on channel axes indicate directions to the NE–E (Räsänen et al. 1998). Measurements on the channelized base of the White Sand Unit indicate transport directions to the SE–E. These lithofacies associations and the variation in palaeocurrent directions are typical for channel fill and point bar deposition, all features typical of a fluvial meandering system. The term ‘white sands’ is also used for Amazonian podzols (Whitmore & Prance 1987). Although fluvial and podzol sands at first glance can look similar – because of the light colours and quartz sand composition – there are also distinct differences. Fluvial white sands for instance present different granulometries between strata; the stratal sequence follows certain patterns such as fining upwards, coarsening upwards, or abrupt changes; and there are many sedimentary features, such as cross-bedding, climbing ripples, etc. that differentiate deposition from aquatic and/or aeolian environments. Based on the features observed in the White Sand Unit we agree with Räsänen et al. (1998) that these sediments represent a meandering fluvial system. Provenance

Iquitos White Sand Unit

Trace element concentrations and Nd-Sm isotopic compositions were measured in the quartz-rich sand of the Iquitos White

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Cl

fmc S Cg

Fig. 7.13 Lithological column representing the Iquitos White Sand Unit (Peru). c, ; Cg, conglomerate; Cl, clay; f, fine; Fl, ; m, middle; S, sand.

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Fig. 7.14 White Sand Unit at Iquitos. Photo by M. Roddaz.

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Sand Unit (IQ 2 and POM 001A) and the corresponding mud fraction (POM 001B and IQ 37A; Roddaz et al. 2005b, 2006). A comparison between Neogene Andean-derived sediments and the White Sand sediments shows that the latter has the highest europium anomalies (Eu/Eu* ~ 0.59 and 0.63), Zr/Sc ratio (174.6 and 44.7) and Th/Sc ratio (2.6 and 1.3) suggesting a felsic and/or recycled sedimentary source. In Fig. 7.3, the sand sample falls outside of the mixing trend that characterizes the sediments issued from Andean sources. Together, a low ⑀Nd(0) and the geochemical signature point to a cratonic origin for the Iquitos White Sand Unit (Roddaz et al. 2005b, 2006). The cratonic origin of this depositional system is further supported by the composition of the conglomerates at the base of the formation, which comprise gneiss and quartz pebbles, and are probably derived from a metamorphic and granitic basement. Finally, the pure quartz sand composition of the White Sands resembles the composition of present-day cratonic rivers (Franzinelli & Potter 1983; Johnsson et al. 1988; Potter 1994).

Palaeogeographic and biological implications The Iquitos White Sand outcrops are all situated on a topographic high, the Iquitos Arch or forebulge, which was formed in response to Late Miocene-Pliocene uplift of the Andes (Roddaz et al. 2005a, 2006, but see ‘discussion’ in Wesselingh et al. 2006a). The forebulge is a NW–SE-oriented arch that is a key location for the present Amazonian drainage systems. In this region, the Marañón River joins the Ucayali River to form the Amazon River, and 65 km eastwards the Napo River (the largest river draining the Ecuadorian Andes) joins the Amazon. The development of several Neogene formations that outcrop on the Iquitos forebulge was controlled by forebulge uplift (Roddaz et al. 2005a, 2005b, 2006), of which the Iquitos White Sand Unit is one example. The White Sand Unit records a flexural-induced uplift phase of the Iquitos forebulge, which acted as a drainage divide between transverse rivers issued from the Andes (Nauta 2 Member) and rivers issued from the eastern craton (White Sand Unit) in response to thrust-induced tectonic loading of the Eastern Cordillera and sub-Andean Zone. Uplift of the Iquitos forebulge subsequently created a high elevation area, and triggered development of cratonic river(s) that may have played an role in the development of Neogene Amazonian biodiversity. Regional uplift also exposed a variety of strata, increasing edaphic heterogeneity in the area (Räsänen et al. 1998); see also Chapter 22), and may ultimately have been responsible for the development of endemic species of birds in the white sand forest of Iquitos (Anderson 1981; Alonso & Whitney 2001, 2003; Whitney & Alonso 2005).

The shift from cratonic- to Andean-dominated fluvial systems The exhumation of the Andes can be identified by provenance change in the sedimentary record of the sub-Andean (foreland) basins and the intracratonic and pericratonic basins. In the Ecuadorian sub-Andean basins the onset of exhumation of the Cordillera Real is marked twice, once at the end of the Cretaceous

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and a second time during the Late Oligocene. Later uplift episodes are dated as ~15 and 9 Ma (Spikings et al. 2001; Ruiz et al. 2007; Martin-Gombojav & Winkler 2008). However, the related changes in sediment composition are not synchronous throughout the entire region and the changes in provenance differ in age from west to east. Provenance changes are directly related to the Andean uplift. As uplift proceeded during the Late Miocene due to thrustdominated processes a transition from underfilled to overfilled conditions occurred in the sub-Andean basins (see Chapters 4 & 5). Thus, the Amazonian drainage basin is a direct consequence of a sequence of processes in the Andean orogen (see Chapter 4). Pliocene intensification of the uplift is further marked by increased sedimentation in the sub-Andean basins (see Chapter 5). First evidence of compositional changes in the Amazonian sedimentary record is reported from the Early to Middle Miocene transition in the Solimões Basin. There, Early Miocene sediments evidence a distinctly cratonic source characterized by an ultrastable to stable heavy mineral suite. Between the Early and Middle Miocene the heavy mineral composition changed into a moderately stable assemblage typical for Andean parent rocks that were metamorphosed in the greenschist facies. Although the age of this change could not be pinpointed, due to the rare occurrence of sands in the sedimentary sequence, it is clear that between the Early and Middle Miocene there was a distinct change in sediment composition. This change is also evidenced in clay mineral composition and in palynological assemblages (Hoorn 1993, 1994a, 1994b; Hoorn et al. 1995). Besides a change in sediment supply, the change from craton- to Andean-driven fluvial regimes is also marked by a shift in depositional environments and transport directions. All the cratonic fluvial systems were channel-dominated systems with a general westward and northwestward transport direction. However, in both the sub-Andean foreland basins and the intracratonic basins the provenance change is accompanied by a change from fluvial to fluvio-lacustrine and lacustrine environments with an eastward transport direction. This environmental change is registered between the Late Oligocene and Early Miocene in the sub-Andean Marañón Basin (Wesselingh et al. 2006b), Early to Middle Miocene in the Solimões Basin (Maia et al. 1977; Hoorn 1993, 1994b) and during the Middle Miocene in the Chaco Basin (Hulka 2005).

Conclusions The fluvial history of Amazonia can be subdivided into two parts, an Amazonian Craton-dominated period (almost the entire Phanerozoic) and the Andean-dominated period (from Neogene onwards, ~16 Ma). In this chapter we have focused on understanding the craton-driven sedimentation history by looking at the characteristics of four different fluvial systems that range in age from Cretaceous to Neogene. These four sedimentary units are all of Amazonian cratonic origin and were characterized by: (i) sediments composed of mature quartz sands, modest kaolinitic clay fraction, and ultrastable to stable heavy mineral assemblages; (ii) isotope ratios characteristic for Proterozoic source rocks; (iii) bimodal distributions of grain size; (iv) predominant E–W transport directions; and (v) depositional environments that range from alluvial fan, to braided, low sinuosity to meandering fluvial systems.

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The Amazonian Craton and its fluvial systems The oldest system of this series, the Cretaceous-Paleogene Alter do Chão Formation (Brazil), represented alluvial fans to braided river systems that originated on the craton and crossed the continent as a trunk river in an opposite direction to the present Amazon River. Although the Alter do Chão fluvial system was discontinued during the Early Paleogene, fluvial drainage of E–W transport directions continued throughout the Paleogene and Early Miocene. Examples of this are the Late Oligocene to Middle Miocene Petaca Formation (Bolivia), which represented a craton-derived braided fluvial system that flowed towards the sub-Andean Zone. Another example of craton-derived fluvial sedimentation is represented in the Miocene Mariñame and Apaporis Sand Units of south-eastern Colombia. At the end of the Cretaceous and during the Paleogene, Andean uplift is imprinted upon the sedimentary record of the subAndean zone. Once Andean uplift accelerated, during the Neogene, the effect became established in the entire Amazonian sediment record. This transition from cratonic fluvial systems to Andean fluvio-lacustrine and lacustrine sedimentation, together with the reversal of drainage patterns, is preserved in the sedimentary sequences of the Solimões Formation (Hoorn 1993). Although the craton is still a source of sediment supply to relatively minor rivers up to the present, since the Miocene it no longer controlled the principal arteries of drainage in Amazonia. Instead, a new generation of secondary rivers was born on the new topographies – the forebulges – that resulted from Andean uplift in the sub-Andean Zone and in Amazonia. In summary, relatively recent geological processes created the present geography of Amazonia. The Andean river system and its nutrient-rich aquatic environments are not older than Miocene and replaced many of the craton-derived fluvial systems that dominated Amazonia during most of the geological history. This not only created a change in palaeogeography but also signified a biotic change. New environments were formed and barriers were created between areas that previously were joined, such as the Magdalena, the Caribbean coast and Amazonia. The Neogene shift in drainage and sediment source thus represents much more than a regional geological process and is relevant for understanding the entre northern South American landscape and hence species composition and distribution.

Acknowledgements We are very grateful for the extensive support that we have received from Paulus van der Ven and Joaquim Wanderley Filho (Petrobras) during the compilation of this chapter. We warmly thank Drew Coleman (University of North Carolina) and the drafts technicians Leila Pezzin, Aline Lima and Rosalia Amaral (Petrobras) for their work on some of the illustrations. Finally, we thank Jorge Figueiredo, Salomon Kroonenberg and an anonymous reviewer for their detailed comments that helped us improve earlier versions of this manuscript.

References Abinader, H.D., Nogueira, A.C.R., Mapes, R.W., Coleman, D.S. (2007) Estratigrafia do Cenozóico da porção centro-oeste da Bacia do

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119

Amazonas. In: XI Congresso da Abequa, Belém, Brazil, resumos in CD-ROM. SBG, Núcleo Norte. Albuquerque, O.R. (1922) Reconhecimentos geológicos no vale do Amazonas, Rio de Janeiro. DNPM, Boletim do Serviço Geológico e Mineralógico do Brasil 3, pp. 84. Aleva, G.J.J. (1994) Laterites – Concepts, Geology, Geomorphology and Chemistry. Wageningen: International Soil Reference and Information Centre, 169 pp. Allègre, C.J., Dupre, B., Negrel, P., Gaillardet, J. (1996) Sr-Nd-Pb isotope systematics in Amazon and Congo river systems: constraints about erosion processes. Chem Geol 131, 93–112. Alonso, J.A., Whitney, B.M. (2001) A new Zimmerius Tyrannulet (Aves: Tyrannidae) from white sand forests of Northern Amazonian Peru. Wilson Bull 113, 1–9. Alonso, J.A., Whitney, B.M. (2003) New distributional records of birds from white-sand forests of the northern Peruvian Amazon, with implications for biogeography of northern South America. Condor 105, 552–566. Anderson, A.B. (1981) White-sand vegetation of Brazilian Amazonia. Biotropica 13, 199–210. Andrade, C.A.C., Cunha, F.M.B. (1971) Revisão geológica da Bacia Paleozóica do Amazonas. In: Congresso Brasileiro de Geologia 25 (Anais), São Paulo, SBG, 3, 93–112. Barragan, R., Geist, D., Hall, M., Larson, P., Kurz, M. (1998) Subduction controls on the compositions of lavas from the Ecuadorian Andes. Earth Planet Sci Lett 154, 153–166. Basu, A.R., Sharma, M., DeCelles, P.G. (1990) Nd, Sr-isotopic provenance and trace element geochemistry of Amazonian foreland basin fluvial sands, Bolivia and Peru: implications for ensialic Andean orogeny. Earth Planet Sci Lett 100, 1–17. Brito Neves, B.B. (2002) Main stages of the development of the sedimentary basins of South America and their relationship with the tectonics of supercontinents. Gondwana Res 5, 175–196. Brito Neves, B.B., Fuck, R.A., Cordani, U.G., Thomaz, A.F. (1984) Influence of basement structures on the evolution of the major sedimentary basins of Brazil: A case of tectonic heritage. J Geodyn 1, 495–510. Caputo, M.V. (1984) Stratigraphy, tectonics, paleoclimatology and paleogeography of northern basins of Brazil. PhD thesis, University of California, Santa Barbara, 583 pp. Caputo, M.V., Sad, A.R.E. (1974) Geologia do Baixo Rio Negro e trecho da BR-174. Relatório Interno 675-A. Belém: Petrobras, 33 pp. Caputo, M.V., Rodrigues, R., Vasconcelos, D.N.N. (1971) Litoestratigrafia da Bacia do Amazonas. Relatório Interno Técnico, 641-A. Belém: Petrobras. Caputo, M.V., Rodrigues, R., Vasconcelos, D.N.N. (1972) Nomenclatura estratigráfica da Bacia do Rio Amazonas. In: Congresso Brasileiro de Geologia 26, Belém. Anais, SBG 3, 35–46. Castro, J.C., Eiras, J.F., Caputo, M.V. (1988) Paleozóico do rio Tapajós. In: Congresso Brasileiro de Geologia 35, Belém. Roteiro das Excursões, SBG, pp. 27–44. Chew, D.M., Schaltegger, U., Kosler, J., Whitehouse, M.J., Gutjahr, M., Spikings, R.A., Miskovic, A. (2007) U–Pb geochronologic evidence for the evolution of the Gondwanan margin of the north–central Andes: Geological Society of America Bulletin 119, 687–711. Cordani, U.G., Sato, K. (1999) Crustal evolution of the South American Platform, based on Nd isotopic systematics on granitoid rocks. Episodes 22, 167–173. Costa, M.L. (1991) Aspectos geológicos dos lateritos da Amazônia. Rev Bras Geociências 21, 146–160. Cox, K.G. (1989) The role of mantle plumes in the development of continental drainage patterns. Nature 342, 873–877.

10/24/2009 11:09:39 Shobha

120

C. Hoorn et al.

Cox, K.G. (1998) River flow patterns associated with Mesozoic and Cenozoic plumes; can this be expanded to Paleozoic and Proterozoic cases? Geol Soc Am Abstr Progr 30, 343–344. Cunha, P.R.C., Gonzaga, F.G., Coutinho, L.F.C., Feijó, F.J. (1994) Bacia do Amazonas. Boletim de Geociências da Petrobras 8, 47–55. Daemon, R.F. (1975) Contribuição à datação da Formação Alter do Chão, Bacia do Amazonas. Rev Bras Geociências 5, 58–84. Daemon, R.F., Contreiras, C.J.A. (1971) Zoneamento palinológico da Bacia do Amazonas. In: Congresso Brasileiro de Geologia 25, São Paulo. Anais, SBG, pp. 79–88. Derby, O.A. (1879) Contribuições para a geologia da Região do Baixo Amazonas. Archivos do Museu Nacional 3, 77–104. Dino, R., Silva, O.B., Abrahão, D. (1999) Caracterização palinológica e estratigráfica de estratos cretáceos da Formação Alter do Chão, Bacia do Amazonas. In: Simpósio sobre o Cretáceo do Brasil, 5, Rio Claro. Boletim de Resumos Expandido, SBG, pp. 557–565. Dino, R., Soares, E.A.A., Ricommini, C., Antonioli, L., Nogueira, A.C.R. (2006) Caracterização palinoestratigráfica de depósitos miocênicos da Bacia do Amazonas, região de Manacapuru, AM. In: Simpósio do Cretáceo do Brasil 7; Simpósio do Terciário do Brasil 1, Serra Negra. Boletim de resumos: IGCE/UNESP, p. 43. Dobson, D.M., Dickens, G.R., Rea, D.K. (2001) Terrigenous sediments on Ceará Rise: A Cenozoic record of South American orogenesis and erosion. Palaeogeogr Palaeocl 165, 215–229. Dunne, T., Mertes, L.A.K., Meade, R.H., Richey, J.E., Forsberg, B.R. (1998) Exchanges of sediment between the floodplain and channel of the Amazon River in Brazil. Geol Soc Am Bull 110, 450–467. Echavarria, L., Hernández, R., Allmendinger, R., Reynolds, J. (2003) Subandean thrust and fold belt of northwestern Argentina: Geometry and timing of the Andean evolution. Am Assoc Petr Geol B 87, 965–985. Eiras, J.F., Wanderley Filho, J.R. (2006) Bacias sedimentares brasileiras, Bacia do Solimões. Fundaçao Paleontologica Phoenix. Phoenix 8, 1–6. Eiras, J.F., Becker, C.R., Souza, E.M., Gonzaga, F.G., Silva, J.G.F., Daniel, L.M.F. et al. (1994) Bacia do Solimões. Boletim de Geociências da Petrobrás, 8, 17–45. Figueiredo, J.J.P.; Soares, E.F., Souza, J.M.P. (2004) Rio Amazonas – Uma história evolutiva do Eocretáceo ao Recente. In: Congresso Brasileiro de Geologia, 42, Araxá, Minas Gerais. CD-ROM. SBG. Figueiredo, J., Hoorn, C., van der Ven, P., Soares, E. (2009) Late Miocene onset of the Amazon River and the Amazon deep-sea fan: Evidence from the Foz do Amazonas Basin. Geology 37, 619–622. Franzinelli, E., Potter, P.E. (1983) Petrology, chemistry, and texture of modern river sands, Amazon River System. J Geol 91, 23–39. Franzinelli, E., Rossi, A. (1996) Contribuição ao estudo petrográfico e geoquímico do Arenito Manaus. In: V Simpósio de Geologia da Amazônia, Belém. Boletim Resumos Expandidos. Belém: SBG, pp. 209–211. Franzinelli, E., Rossi, A. (1997) Some aspects of the diageneses of the Manaus Sandstone (Amazonas Basin, Brazil). In: I Congreso Latino Americano de Sedimentología, 1997, Porlamar, Isla de Margarita. Memorias Tomo I: Sociedad Venezolana de Geólogos, pp. 269–274. Galvis, J.V., Huguett, A., Ruge, P. (1979) Geología de la Amazonia Colombiana. Boletin Geologico Ingeominas, Bogota 22, 1–86 Germeraad, J.H., Hopping, C.A., Muller, J. (1968) Palynology of Tertiary sediments from tropical areas. Rev Palaeobot Palynol 6, 189–348. Gubbels, T.L., Isacks, B.L., Farrar, E. (1993) High-level surfaces, plateau uplift, and foreland development, Bolivian Central Andes. Geology 21, 695–698.

Hoorn_ch07_Final.indd 120

Guyot, J.L., Jouanneau, J.M., Soares, L., Boaventura, G.R., Maillet, N., Lagane, C. (2007) Clay composition of river sediments in the Amazon Basin. Catena 71, 340–356. Hartt, C.F. (1870) Geology and Physical Geography of Brazil. Fields, Osgoods and Co., 620 pp. Hoorn, C. (1993) Marine incursions and the influence of Andean tectonics on the Miocene depositional history of northwestern Amazonia: Results of a palynostratigraphic study. Palaeogeogr Palaeocl 105, 267–309. Hoorn, C. (1994a) Fluvial palaeoenvironments in the intracratonic Amazonas Basin (Early Miocene-early Middle Miocene, Colombia). Palaeogeogr Palaeocl 109, 1–54. Hoorn, C. (1994b) An environmental reconstruction of the palaeoAmazon River sytem (Middle to Late Miocene, NW Amazonia). Palaeogeogr Palaeocl 112, 187–238. Hoorn, C. (2006) Mangrove forests and marine incursions in Neogene Amazonia (lower Apaporis River, Colombia). Palaios 21, 206–219. Hoorn, C., Guerrero, J., Sarmiento, G.A., Lorente, M.A. (1995) Andean tectonics as a cause for changing drainage patterns in Miocene northern South America. Geology 23, 234–240. Horbe, A.M.C., Nogueira, A.C.R., Horbe, M.A., Costa, M.L., Suguio, K. (2001) A lateritização na gênese das superfícies de aplainamento da região de Presidente Figueiredo-Balbina, nordeste do Amazonas. In: Reis, N.J., Monteiro, M.A.S. (eds) Contribuições a Geologia da Amazônia. Manaus: SBG, vol. 2, pp. 148–176. Horbe, A.M.C., Horbe, M.A., Suguio, K. (2004) Tropical spodosols in northeastern Amazonas State, Brazil. Geoderma 119, 55–68. Horbe, A.M.C., Vieira, L.C., Nogueira, A.C.R. (2006) Geoquímica de camadas vermelhas bioturbadas da Formação Alter do Chão, Cretáceo da Bacia do Amazonas. Rev Bras Geociências 36, 13–20. Hulka, C. (2005) Sedimentary and tectonic evolution of the Cenozoic Chaco foreland basin, southern Bolivia. PhD thesis, Freie Universität Berlin, Berlin, 100 pp. Irion, G. (1984) Clay minerals of Amazon soils. In: Sioli, H. (ed.) The Amazon – Limnology and Landscape Ecology of a mighty Tropical River and its Basin. The Hague: Dr W. Junk, pp. 537–579. Johnsson, M.J., Meade, R.H. (1990) Chemical weathering of fluvial sediments during alluvial storage: the Macuapanim Island point bar, Solimões River, Brazil. J Sediment Petrol 60, 827–842. Johnsson, M.J., Stallard, R.F., Meade, R.H. (1988) First-cycle quartz arenites in the Orinoco River basin, Venezuela and Colombia. J Geol 96, 263–277. Kay, S., Coira, B., Viramonte, J. (1994) Young mafic back arc volcanic rocks as indicators of continental lithospheric delamination beneath the Argentina Puna plateau, central Andes. J Geophys Res 99, 24323–24339. Kistler, P. (1954) Historical Resume of the Amazon Basin. Relatório Interno. Belém: Petrobras/Renor. Klammer, G. (1984) The relief of the extra-Andean basin. In: Sioli, H. (ed.) The Amazon. Dordrecht: Dr W. Junk Publishers, pp. 49–83. Krook, L. (1979) Sediment petrographical studies in northern Suriname. PhD thesis, Drukkerij Elinkwijk BV, Utrecht, 154 pp. Latrubesse, E.M., Franzinelli, E. (2005) The late Quaternary evolution of the Negro River, Amazon, Brazil: Implications for island and floodplain formation in large anabranching tropical systems. Geomorphology 70, 372–397. Lima-Costa, M. (1997) Lateritization as a major process of ore deposit formation in the Amazon region. Explor Min Geol 6, 79–104. López-Africano, P.E. (1998) Aspectos Geológicos, Mineros y Ambientales: Cuenca Media y Alta de los Ríos Guainía e Inírida. Contrato SENASecab nr. 0051–97. URL http://www.rds.org.co/aa/img_upload/44 fdd6beac515f4957a4a964392a2da8/mineria_guainia.pdf [accessed on 2 July 2007].

10/24/2009 11:09:40 Shobha

The Amazonian Craton and its fluvial systems Lorente, M.A. (1986) Palynology and palynofacies of the Upper Tertiary in Venezuela. Dissertionae Botanicae 99, 224 pp. Maia, R., Godoy, H., Yamaguti, H., de Moura, P., da Costa, F., de Holanda, M., Costa, J. (1977) Projecto de Carvao no Alto Solimões. CPRM-DNPM, 137 pp. Mapes, R.W., Coleman, D.S., Cox, R., Nogueira, A.C.R. (2005) Significant changes in the detrital zircon age signature along a transect of the modern Amazon River. Abstract, Salt Lake City Annual Meeting. Geol Soc Am Abstr Progr 37, 481. Mapes, R.W. (2009) Past and present provenance of the Amazon River. Ph.D. thesis, The University of North Carolina at Chapel Hill. 196 pp. Marshall, L.G., Sempere, T. (1991).The Eocene to Pleistocene vertebrates of Bolivia and their stratigraphic context: A review. In: Suarez-Soruco, R. (ed.) Fosiles y facies de Bolivia, Tomo 1: Vertebrados. Revista Técnica de Yacimientos Petrolíferos Fiscales Bolivianos pp. 631–652. Martin-Gombojav, N., Winkler, W. (2008) Recycling of Proterozoic crust in the Andean Amazon foreland of Ecuador: implications for orogenic development of the Northern Andes. Terra Nova 20, 22–31. McDaniel, D.K., McLennan, S.M., Hanson, G.N. (1997) Provenance of Amazon fan muds; constraints from Nd and Pb isotopes. Proceedings of the Ocean Drilling Program, Scientific Results, 155, 169–176. McLennan, S.M., Hemming, S., McDaniel, D.K., Hanson, G.N. (1993) Geochemical approaches to sedimentation, provenance and tectonics. In: Johnsson, M.J., Basu, A. (eds) Processes Controlling the Composition of Clastic Sediments. Geol Soc Am Special Paper 284, pp. 21–40. Milani, E.J., Zalán, P.V. (1999) An outline on the geology and petroleum systems of the Paleozoic interior basins of South America. Episodes 22, 199–205. Moore, A., Blenkinsop, T.G. (2002) The role of mantle plumes in the development of continental-scale drainage patterns: the southern African example revisited. S Afr J Geol 105, 353–360. Nogueira, A.C.R., Souza, V., Soares, E.A.A. (1997) Contribuição a tectônica Cenozóica da região de Presidente Figueiredo, Norte de Manaus – AM. In: V Simpòsio Nacional de Estudos Tectônicos, 6. Pirinopolis: SBG, pp. 123–125. Nogueira, A.C.R., Vieira, L.C., Suguio, K. (1999) Paleosolos da Formação Alter do Chão, Cretáceo-Terciário da Bacia do Amazonas, regiões de Presidente Figueiredo e Manaus. In: Simpòsio do Cretáceo do Brasil 5, Serra Negra. Boletim. Rio Claro: UNESP, pp. 261–266. Nogueira, A.C.R., Horbe, A.M.C., Paz, J.D.S., Motta, M.B., Rozo, J. (2006) Amazônia, 2006, Belém. Anais do Simpósio de Geologia da Amazônia, Resumos em CD-ROM. Belém: SBG/Norte. Ochoa-Lozano, D.P. (2007) Palinología de los depósitos fluviales de la sección Agua Negra e implicaciones en la geología del nw de la Cuenca Amazónica. Tesis de grado. Escuela de Geología, Universidad Industrial de Santander, Bucaramanga [see also URL http://www.cgares.org/index.php?id=15]. Parra, M., Pujos, M. (1998) Origin of late Holocene fine-grained sediments on the French Guiana shelf. Continental Shelf Res 18, 1613–1629. Pasley, M.A., Shepherd, D.B., Pocknall, D.T., Boyd, K.P., Vander, A., Figueiredo, J.J.P. (2005) Sequence stratigraphy and basin evolution of the Foz do Amazonas Basin, Brazil. Search and Discovery article #10082. URL: http://www.searchanddiscovery.net/. Pedreira, A.J., Caliari-Bahia, R.B. (2000) Sedimentary basins of Rondônia State, Brazil: Response to the geotectonic evolution of the Amazonic Craton. Rev Bras Geociências 30, 477–480. Pereira, E.R. (1988) Possibilidades metalogenéticas ná região do Domo de Monte Alegre-PA. In: Congresso Brasileiro de Geologia 35, Belém. Anais, SBG 1, pp. 286–295.

Hoorn_ch07_Final.indd 121

121

Pereira, R., Carvalho, M.A., Mendonça-Filho, J.G., Azevedo, D.A., Carvalho, I.S. (2006) Ocorrência de esporos de fungos preservados em âmbar cretácico da Bacia do Amazonas, Formação Alter do Chão. In: Simpòsio do Cretáceo do Brasil 7; Simpòsio do Terciário do Brasil, 1, Serra Negra. Boletim de Resumos. Serra Negra: IGCE/ UNESP, p. 101. Pinto, L. (2003) Traçage de l’érosion Cénozoïque des Andes Centrales à l’aide de la minéralogie et de la géochmie des sédiements (Nord du Chili et Nord-Ouest de la Bolivie). PhD thesis, University Paul Sabatier, Toulouse, 196 pp. Potter, P.E. (1994) Modern sands of South America: composition, provenance and global significance. Geol Rundsch 83, 212–232. Potter, P.E. (1997) The Mesozoic and Cenozoic paleodrainage of South America: a natural history. J S Am Earth Sci 10, 331–344. Potter, P.E., Franzinelli, E. (1985) Areias recentes dos Rios da Bacia Amazônica: Composição petrográfica, textural e química. Rev Bras Geociências 15, 213–220. Price, L.I. (1960) Dentes de Theropoda num testemunho de sonda no estado do Amazonas. An Acad Bras Ciênc 32, 79–84. Proradam (1979) La Amazonía Colombiana y sus recursos. Bogota: Instituto Geográfico ‘Agustin Codazzi’. Räsänen, M.E., Neller, R., Salo, J.S., Junger, H. (1992) Recent and ancient fluvial deposition systems in the Amazonian foreland basin. Geol Mag 129, 293–306. Räsänen, M.E., Linna, A.M., Irion, L., Rebata-Hernani, L., VargasHuamas, R., Wesselingh, F. (1998) Geología y geoformas de la zona de Iquitos. In: Kalliola, R., Flores Paitán, S. (eds) Geoecología y desarrollo Amazónico: estudio integrado en la zona de Iquitos, Perú. Ser A II. Turku: Annales Universitatis Turkuensis, pp. 59–137. Rebata, L.A., Räsänen, M.E., Gingras, M.K., Vanderlei Vieira, Jr, Barberi, M., Irion, G. (2006a) Sedimentology and ichnology of tide-influenced Late Miocene successions in western Amazonia: The gradational transition between the Pebas and Nauta formations. J S Am Earth Sci 21, 96–119. Rebata, L.A., Gingras, M.K., Räsänen, M.E., Barberi, M. (2006b) Tidalchannel deposits on a delta plain from the Upper Miocene Nauta Formation, Marañón Foreland Sub-basin, Peru. Sedimentology 53, 971–1013. Roddaz, M., Baby, P., Brusset, S., Hermoza, W., Darrozes, J.M. (2005a) Forebulge dynamics and environmental control in Western Amazonia: The case study of the Arch of Iquitos (Peru). Tectonophysics 399, 87–108. Roddaz, M., Viers, J., Brusset, S., Baby, P., Herail, G. (2005b) Sediment provenances and drainage evolution of the Neogene Amazonian foreland basin. Earth Planet Sci Lett 239, 57–78. Roddaz, M., Viers, J., Brusset, S., Baby, P., Boucayrand, C., Herail, G. (2006) Controls on weathering and provenance in the Amazonian foreland basin: Insights from major and trace element geochemistry of Neogene Amazonian sediments. Chem Geol 226, 31–65. Rodriguez, C.I., Warden, A.J. (1993) Overview of some Colombian gold deposits and their development potential. Miner Deposita 28, 47–57. Rogers, G., Hawkesworth, C.J. (1989) A geochemical traverse across the North Chilean Andes: evidence for crust generation from the mantle wedge. Earth Planet Sci Lett 91, 271–275. Rossetti, D.F., Netto, R.G. (2006) First evidence of marine influence in the Cretaceous of the Amazonas Basin, Brazil. Cretaceous Res 27, 513–528. Rozo, J.M.G. (2004) Evolução holocênica do Rio Amazonas entre a Ilha do Careiro e a foz do Rio Madeira. Dissertação (Mestrado), Instituto de Ciências Exatas, Universidade Federal do Amazonas, Manaus, 93 pp.

10/24/2009 11:09:40 Shobha

122

C. Hoorn et al.

Rozo, J.M.G., Nogueira, A.C.R., Horbe, A.M.C., Carvalho, A.S. (2005) Depósitos Neógenos da Bacia do Amazonas. In: Horbe, A.M.C., Souza, V.S. (eds) Contribuições a Geologia da Amazônia, Manaus: SBG, 4, pp. 201–207. Ruiz, G.M.H., Seward, D., Winkler, W. (2007) Evolution of the Amazon Basin in Ecuador with special reference to hinterland tectonics: Data from zircon fission-track and heavy mineral analysis. Dev Sedimentol 58, 907–934. Santos, J.O.S., Hartmann, L.A., Gaudette, H.E., Groves, D.I., McNaughton, N.J., Fletcher, I.R. (2000) A new understanding of the Provinces of the Amazon Craton based on integration of field mapping and U-Pb and Sm-Nd geochronology. Gondwana Res 3, 453–488. Sioli, H. (1984) The Amazon and its main affluents: Hydrography, morphology of the river courses, and river types. In: Sioli, H. (ed.) The Amazon: Limnology and Landscape Ecology. Monographiae biologicae 56. Dr W. Junk Publishers, pp. 127–165. Soares, E.A.A. (2007) Depósitos pleistocenos da região de confluência dos rios Negro e Solimões, porção oeste da Bacia do Amazonas. Tese Doutorado, Instituto de Geociências, Universidade de São Paulo, São Paulo, 205 pp. Souza, M.M. (1974) Perfil geológico da BR-174 (Manaus-Boa Vista) no trecho: Manaus-Serra do Abonari. In: Congresso Brasileiro de Geologia 28, Porto Alegre. Anais, SBG 2, 75–86. Souza, M.M., Medeiros, M.F. (1972) Contribuição ao estudo sedimentológico da região de Manaus. In: Congresso Brasileiro de Geologia 26, Belém. Anais, SBG 1, 129–140. Spikings, R.A., Winkler, W., Seward, D., Handler, R. (2001) Alongstrike variations in the thermal and tectonic response of the continental Ecuadorian Andes to the collision with heterogeneous oceanic crust. Earth Planet Sci Lett 186, 57–73. Strecker, M., Mulch, A., Uba, C.E., Schmitt, A.K., Chamberlain, R. (2006) Late Miocene onset of the South American Monsoon. AGU Fall Meeting, San Francisco. Abstract #T31E-06. Tassinari, C.C.G., Macambira, M.J.B. (1999) Geochronological provinces of the Amazonian Craton. Episodes 22, 174–182. Théveniaut, H., Freyssinet, Ph. (2002) Timing of lateritization on the Guiana Shield: synthesis of paleomagnetic results from French Guiana and Suriname. Palaeogeogr Palaeocl 178, 91–117. Trinidade, R.I.F., Horbe, A.M.C., Peixoto, S.F. (2006) Paleomagnetismo de crostas lateríticas da região amazônica: dados preliminares e implicações cronológicas. In: Congresso Brasileiro de Geologia 43, Aracaju. Anais, SBG, p. 136.

Hoorn_ch07_Final.indd 122

Uba, C.E., Heubeck, C., Hulka, C. (2005) Facies analysis and basin architecture of the Neogene Subandean synorogenic wedge, southern Bolivia: Sediment Geol 180, 91–123. Uba, C., Heubeck, C., Hulka, C. (2006) Evolution of the late Cenozoic Chaco foreland basin, Southern Bolivia. Basin Res 18, 145–170. Uba, C.E., Strecker, M., Schmitt, A.K. (2007), Increased sediment accumulation rates in the Central Andes during the late Miocene due to climate forcing. Geology 35, 979–982. Vieira, L.C. (1999) Depósitos fluviais da Formação Alter do Chão, Cretáceo-Terciário da Bacia do Amazonas, Ponta Negra, Manaus. Dissertação (Mestrado), Instituto de Ciências Exatas, Universidade Federal do Amazonas, Manaus, 53 pp. Vieira, L.C., Nogueira, A.C.R. (1998) Petrografia de arenitos da Formação Alter do Chão, Cretáceo-Terciário da Bacia do Amazonas, Praia da Ponta Negra, Manaus. In: Congresso de Iniciação Científica 3, São Carlos. Anais, Associação das Escolas Reunidas, p. 165. Viers, J., Roddaz, M., Filizola, N., Guyot, J.-L., Sondag, F., Brunet, P. et al. (2008) Seasonal and provenance controls on Nd-Sr isotopic compositions of Amazon rivers suspended sediments and implications for Nd and Sr fluxes exported to the Atlantic Ocean. Earth Planet Sci Lett 274, 511–523. von Blanckenburg, F., Hewawasam, T., Kubik, P.W. (2004) Cosmogenic nuclide evidence for low weathering and denudation in the wet, tropical highlands of Sri Lanka. J Geophys Res 109, F03008. Wanderley Filho, J.R., Melo, J.H.G, Fonseca, V.M.M., Machado, D.M.C. (2005) Bacias sedimentarias, Bacia do Amazonas. Fundaçao Paleontologica Phoenix. Phoenix 7(82), 1–6. Wesselingh, F.P., Hoorn, M.C., Guerrero, J., Räsänen, M., Romero Pittmann, L., Salo, J. (2006a) Stratigraphy of Miocene Amazonian deposits. Scripta Geologica 133, 291–315. Wesselingh, F.P., Guerrero, J., Räsänen, M.E., Romero Pitmann, L., Vonhof, H.B. (2006b) Landscape evolution and depositional processes in the Miocene Pebas lake/wetland system: evidence from exploratory boreholes in northeastern Peru. Scripta Geologica 133, 323–361. Whitmore, T.C., Prance G.T. (eds) (1987) Biogeography and Quaternary History in Tropical America. Oxford: Clarendon Press. Whitney, B.M., Alonso, J.A. (2005) A new species of gnatcatcher from white-sand forests of northern Amazonian Peru with revision of the Polioptila guianensis complex. Wilson Bull 117, 113–127.

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EIGHT

The development of the Amazonian mega-wetland (Miocene; Brazil, Colombia, Peru, Bolivia) Carina Hoorn1, Frank P. Wesselingh2, Jussi Hovikoski3 and Javier Guerrero4 1

University of Amsterdam, The Netherlands Naturalis, Leiden, The Netherlands, and University of Turku, Finland 3 Geological Survey of Denmark and Greenland (GEUS), Copenhagen, Denmark 4 Universidad Nacional, Bogotá, Colombia 2

Abstract The scenery of Western Amazonia once consisted of fluvial systems that originated on the Amazonian Craton and were directed towards the sub-Andean zone and the Caribbean. In the course of the Early Miocene these fluvial systems were largely replaced by lakes, swamps, tidal channels and marginal marine embayments, forming a mega-wetland. In this chapter we will review the characteristics of this mega-wetland and its different phases of development. These aquatic environments hosted a diverse fauna whereas the shores of these systems were fringed by palm swamps, and a diverse rainforest occurred in the peripheral dry lands. The genesis of this wetland was primarily driven by geological mechanisms such as the Andean uplift, and an increase in accommodation space in the sub-Andean and intracratonic basins. Additionally, high precipitation rates also played an important role in wetland formation. The earliest phase of wetland development is recorded in boreholes drilled in the sub-Andean foreland basins of Peru and Colombia, and in the intracratonic Solimões Basin of western Brazil. During the latest Oligocene to Early Miocene (~24 to 16 Ma) lacustrine conditions alternated with episodes of Andean and cratonic fluvial drainage as well as marginal marine influence. In Amazonia, marine incursions are intercalated as thin beds in the Middle to Upper Miocene fluvial strata and contain marine and coastal taxa (foraminifera, mangrove pollen). Lacustrine conditions expanded further during the Middle Miocene to early Late Miocene (~16 to 11.3 Ma; Pebas phase). During this period the lake-embayment and swamp systems – fringed by forested lowland – reached their maximum extension. This wetland was subject to marginal marine influence and sustained a large radiation of endemic aquatic invertebrate faunas. During its maximum extent the wetland covered an area of more than 1.5 × 106 km2 – comprising much of the Present western Amazonian lowlands. From the Late Miocene onwards uplift rates in the Eastern Cordillera, Cordillera Real and Cordillera de Merida substantially increased and the Andes became a continuous barrier. This barrier effectively separated lowland Amazonia from Orinoquia and the Magdalena Valley and closed off all lowland connections with the Pacific and the Caribbean. The wetland system became a complex environment where deltaic, estuarine and fluvial environments coexisted. This Late Miocene fluvial-tidal-dominated wetland (~11.3 to 7 Ma, Acre phase) hosted a species-rich vertebrate fauna, but (in contrast to the Pebas phase), the molluscan fauna was species poor and already strongly resembled the modern Amazonian fluvial fauna. This system represents the onset of the transcontinental Amazon River. From 11.3 Ma onwards, sediments of Andean origin reached the Atlantic continental shelf and initiated the build-up of the Amazon Fan.

Introduction The Neogene sediments in the sub-Andean and cratonic sedimentary basins contain crucial information about the history of Amazonia, Landscape and Species Evolution: A Look into the Past, 1st edition. Edited by C. Hoorn and F.P. Wesselingh. © 2010 Blackwell Publishing

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Amazonia. However, the degree of exposure of these strata is poor, and stratigraphical relations are difficult to establish. Furthermore, the paucity of radiometric ages and the different interpretations regarding the depositional settings have severely complicated the reconstruction of the Neogene history of lowland Amazonia. Outcrops along the Amazonian rivers can be an excellent source of sedimentological and palaeontological data, in particular during the dry seasons, when the river level drops by up to 10 m.

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However, outcrop data comprise relatively short time intervals and need to be used in combination with (large-timescale) subsurface data from oil companies and government agencies. Therefore data from the Companhia de Pesquisas e Recursos Minerais (CPRM), Petrobras, PetroPeru and Occidental Petrol de Peru S.A., obtained in the sub-Andean and the intracratonic basins, together with outcrop data have proved fundamental for the understanding of the dynamic Neogene history of Amazonia (Maia et al. 1977; Hoorn 1993a; Eiras et al. 1994; Vonhof et al. 2003; Hermoza et al. 2005; Roddaz et al. 2006a; Wesselingh et al. 2006b; Uba et al. 2007). In this chapter we will review the main features of the latest Oligocene to Miocene depositional environments by assessing information concerning the Pebas and Solimões Formations from exploration wells and outcrop data (Fig. 8.1). In this review we subdivide the history of the wetland into a fluvio-lacustrine precursor phase (~24 to 16 Ma), the mega-wetland or Pebas phase (~16 to 11.3 Ma), and the fluvio-tidal-dominated wetland or Acre phase (<11.3 to 7 Ma), which represented the incipient transcontinental Amazon River system. For an extensive discussion about the definitions of the sedimentary basins in Amazonia, the stratigraphy, the regional extent of the Neogene formations, and their relation with deposits in the sub-Andean zone we refer readers to Chapters 3 & 5. Miocene climate and its influence on the wetland is discussed in Chapter 12, whereas marine influence in the wetland and the regional character of the marine incursion will be discussed

in Chapter 9. Specific information about the wetland faunas is presented in Chapters 15–18, and the palaeoflora is reviewed in Chapter 19.

Definition of a wetland and mega-wetland According to the Convention on Wetlands (Ramsar 1971) wetlands are areas where water is the primary factor controlling the environment and the associated plant and animal life. They occur where the water table is at or near the surface of the land, or where the land is covered by shallow water. Ramsar further defines these environments as following: ‘Wetlands are areas of marsh, fen, peat land or water, whether natural or artificial, permanent or temporary, with water that is static or flowing, fresh, brackish or salt, including areas of marine water the depth of which at low tide does not exceed six metres.’ Five major modern wetland types are generally recognized: 1 marine (coastal wetlands including coastal lagoons, rocky shores and coral reefs); 2 estuarine (including deltas, tidal marshes and mangrove swamps); 3 lacustrine (wetlands associated with lakes); 4 riverine (wetlands along rivers and streams); 5 palustrine (marshes, swamps and bogs).

Apaporis Mariñame

Fig. 8.1 Map of the area with the main (documented) localities where the Solimões and Pebas crop out. The boreholes referred to in this study are indicated with black circles.

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Development of the Amazonian mega-wetland Further description of modern wetlands can be found in Mitsch & Gosselink (2000). In this chapter we use the term mega-wetland for the Miocene depositional system, composed of shallow lakes and swamps, which exceeded an area of one million square kilometres. The evolution of this Amazonian palaeowetland included four of the five wetlands types that are listed above.

Causes for wetland development during the Neogene in Amazonia The genesis of the Miocene wetland in the sub-Andean-Amazonian region was driven by geological processes in the Andes, which, in turn, were initiated by the collision of the Caribbean Oceanic Plateau with the Northern Andean margin (75–65 Ma) and break-up of the Farallon Plate during the Oligocene (27–25 Ma) (Martin-Gombojav & Winkler 2008). Although tectonic pulses in the Andes are documented from Mesozoic onwards (Horton & DeCelles 1997; Lundberg et al. 1998; Guerrero et al. 2000; Guerrero 2002a, 2002b; Bayona et al. 2007a; Mora 2007; Ruiz et al. 2007), it is mainly the Late Paleogene and Neogene uplift of the Eastern Cordillera and Cordillera de Merida (i.e. Guerrero, 1997; Villamil 1999; Mora et al. 2008; Uba et al. 2007; Garzione et al. 2008; Bermúdez-Cella et al. 2008; Parra et al. 2009), the Cordillera Real (Steinmann, et al. 1999; Ruiz et al. 2007), and Nazca Ridge subduction (Espurt et al. 2007) that directly affected the sedimentary history of Amazonia. The immediate consequence of the uplift of the Eastern Cordillera was subsidence in the sub-Andean foreland basins and sediment supply of Andean source into the sub-Andean foreland and intracratonic sedimentary basins in Amazonia (Cooper et al. 1995; Hoorn et al. 1995; Steinmann et al. 1999; Hermoza et al. 2005; Roddaz et al. 2005), a topic that is further discussed in Chapters 4 & 5. An Andean volcanic/metamorphic arc was present in the NW corner of South America since at least the Late Triassic and continued to be an important feature at the western flank of the backarc Cretaceous Colombian Basin (Guerrero et al. 2000, 2002a, 2002b). An increase in the rate of Andean uplift and the beginning of a foreland basin formation occurred during the Paleocene, as indicated by the presence of the relatively coarse-grained fluvial deposits of the Lower Socha Formation (Guerrero & Sarmiento 1996). This regressive facies belt, which includes several formations, overlies the Cretaceous marine strata in a distinct erosional unconformity. In Brazil evidence for Paleogene basin reactivation can be found in the Amazonian Solimões and Amazonas Basins and is represented by an unconformity between the CretaceousPaleogene Alter do Chaõ Formation and the Neogene Solimões Formation (Caputo et al. 1971, 1972; Maia et al. 1977; Eiras et al. 1994). Both subsurface records and outcrop data suggest that during the Cretaceous-Paleogene an E–W-directed fluvial system of cratonic origin existed that was followed by a period of exposure and non-deposition (see Chapter 7). From late Oligocene(?) to Early Miocene renewed basin subsidence occurred, and by Middle Miocene times a change from a cratonic to an Andean source was established. At the time, subsidence exceeded sediment input and resulted in the swampy and lacustrine environmental conditions

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that characterize most of the Solimões Formation (Maia et al. 1977; Hoorn 1994a; Wesselingh 2006). The Early Miocene incipient uplift of the Eastern Cordillera (Parra et al. 2009) thus formed a significant source of sediment supply that drove depositional regimes eastwards into Amazonia (Hoorn 1993a; Hoorn et al. 1995; Guerrero 1997; Steinmann et al. 1999; Villamil et al. 1999; Gregory-Wodzicki 2000; Hermoza et al. 2005; Burgos et al. 2005; Burgos 2006). In addition to this, isotope analysis of the mollusc shells (Kaandorp et al. 2005) and climate modelling (see Chapter 13) indicate that during the Middle Miocene Amazonia had a seasonal climatic regime with higher precipitation rates than at present. The combination of high precipitation rates, increased accommodation space and Andean sediment supply resulted in a mega-sized Amazonian wetland. The main uplift of the Eastern Cordillera, however, took place during the Late Miocene and Pliocene (Van der Hammen et al. 1973; Guerrero 1997; Villamil 1999; Mora et al. 2008; Uba et al. 2007; see also Chapter 4). This resulted in isolation of the NW part of the basin and the initiation of the north-directed drainage of the Magdalena River as represented by the Neiva Formation (Guerrero 1997). A large increase of sediment supply and sedimentation rates was noted in the sub-Andean foreland basins, but also as far away as the Amazon Fan on the Atlantic Margin (Dobson et al. 2001; Mora 2007; Uba et al. 2007; Figueiredo et al. 2009). The Late Miocene uplift coincided with a period of global climatic cooling (Harris & Mix 2002) and global sea level fall (Haq et al. 1987; Miller et al. 2005) resulting in a reduction in size of the Amazonian wetland and the fluvial environments – with tidal influence – that predominated in the Acre and Solimões Basins (see Chapter 9). At the same time the connection between Andes and Atlantic was established due to the breach of the Purus High (Figueiredo et al. 2009). During the Pliocene (~4 Ma), subduction of the Nazca Ridge (in the Pacific subduction zone) resulted in the uplift of the Fitzcarrald Arch and compartmentalization of the drainage systems in lowland western Amazonia (Espurt et al. 2007; see also Chapter 6). The sediment transport of the Amazon River increased during the Pliocene and the river acquired its present magnitude (Dobson et al. 2001; Figueiredo et al. 2009).

The Miocene Amazonian wetland: evidence from the sedimentary record The Pebas and Solimões Formations have a wide geographic distribution and their extent, together with the sedimentary, palaeontological, and geochemical characteristics, support the existence of the Miocene Amazonian mega-wetland. These two units, which are partially time-equivalent, occur over an area of about 1.5 × 106 km2 and, thanks to oil exploration wells in the Marañon Basin (Mathalone & Montoya 1995; Hermoza 2004; Wesselingh et al. 2006b) and the Solimões Basin (Maia et al. 1977; Eiras et al. 1994), we know that these units can reach a thickness of up to 1000 m. The name Pebas Beds or Pebas Formation has been applied since the end of the 19th century (e.g., Gabb 1869; Katzer 1903; Ruegg & Rozenzweig 1949; Pardo & Zuñiga 1975; Nutall 1990; Hoorn 1994b; Wesselingh et al. 2002; Räsänen et al. 1998; Wesselingh 2006). Boreholes in the Marañon Basin show the

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most complete record of the Pebas Formation; they registered the (transitional) contact with the underlying (Oligocene) Chambira Formation and the concordant, diachronical contact between the Pebas Formation and the overlying Marañon Formation. In the Jibaro well from Occidental Petrol de Peru, the contact with the underlying Chambira and the overlying Marañon Formations suggests that the Pebas Formation has a thickness of about 1000 m (Wesselingh et al. 2006b). The lower part of the Pebas Formation is typically represented by yellow sands and alternating layers of red and blue clays with some organic-rich beds and a low-diversity invertebrate fauna. The upper part of the formation presents the characteristic Pebas features described in existing literature. These features are: grey-blue smectite-rich mudstones and claystones, with minor interbeds of grey feldspar-rich sandstones, brown-black coals and organic-rich claystones. In the upper half of the Pebas Formation, sandstone and sandy shell beds are usually well expressed on the gamma ray and resistivity logs. Carbonate fossils are common throughout the Pebas Formation (Wesselingh et al. 2006a). The Solimões Formation has an estimated maximum thickness of 980 m and was first described by Moraes Rego (1930) and

Gamma-ray

Lithology and depth (metres)

validated as a formation by Caputo et al. (1971, 1972). The reference section is interval 5–765 m of Petrobras borehole 2-RJ-1 -AM (Fig. 8.2) and the additional reference section is 5–305 m in Petrobras borehole 1-BV-1-AM, both situated in the intracratonic Solimões Basin (Brazil) (Eiras et al. 1994). Ditch cutting samples of these wells are stored with Petrobras. Additional lithostratigraphic information, electrical log data and cores from this formation are property of CPRM and material is kept at the Manaus (Brazil) storage facilities. The lithological record from the CPRM well 1AS-4A-AM shows that at a 350 m depth, Early and Middle Miocene deposits (Psiladiporites-Crototricolpites pollen zone; Hoorn 1993a) represent a lithological transition from coarse subangular quartzrich cratonic sediments, to fine mixed sand of Andean origin. These sands can be differentiated based on heavy mineral composition (Hoorn 1993a) but also based on their grain size and angularity. A similar transition, albeit not dated, is observed in wells 4a, 27, 15 and 48. Maia et al. (1977) refer to the lower sandy unit as Ramon Formation (353 to 339 m depth; CPRM well 1AS-4A-AM, Fig. 8.3). However, this formation has not been formally described (J. Figueiredo, personal communication) and the

Lithological description

Resistivity (ohm m)

Clay, dark-grey

Solimões Fm.

250

Sandstone, fine, grey Clay, light-yellow

500

Clay, multicoloured

Sandstone, fine, white

Clay, dark-brown 750

765 m Alter do Chão Fm.

Fig. 8.2 The type location of the Solimões Formation in Brazil is Petrobras well 2-RJ-1-AM. Fm, Formation. Courtesy of Petrobras.

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1AS-4A-AM

1AS-4-AM

1AS-27-AM

1AS-51-AM

1AS-48-AM

Crassoretitriletes zone

Grimsdalea zone

Depth (m) Lithology 0

0

0

0

0

50

50

50

50

50

100

100

100

100

100

150

150

150

150

150

170

R3CP zone

Crototricolpites\P2p zone

180

200

200

200

Legend Sand 250

250

250

Silty clay Clay Lignite

Verrutricolporites zone

Igneous rocks Erosive contact

300

300

300

350

350

350

400

400

408

402

?

353

First report on fossils

Fig. 8.3 Lithological columns of the Solimões Formation based on Companhia de Pesquisa de Recursos Minerais (CPRM) well data. The shallowing, coarsening-upward cycles (sand, silty clay, lignite) mostly occur at ~ 10 m intervals. The lower section is dominated by sands and represents a fluvially dominated environment. The sediments from well 51 are estimated by Maia et al. (1977) as Miocene to Pliocene. The sediments of well 48 are undated, but resemble the fluvial sequences typical for Late Miocene sediments of the Acre region. The pollen zonation in the left bar applies to well 1AS-4A-AM.

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data suggest that most of the Solimões Formation (below 339 m depth) is of Paleogene age and was deposited in a fluvial environment. The Solimões Formation discordantly overlies the red clays and yellow sands of the Cretaceous-Paleogene Alter do Chaõ Formation (Eiras et al. 1994; see Chapter 7)

Fluvial environments and the incipient wetland (~24 to 16 Ma) (see Fig. 8.8a) Lithostratigraphy and distribution The lower Early Miocene sections of both the Pebas and Solimões Formations are poorly documented and, with the exception of the sub-Andean zone and the intramontane basins, which formerly were connected to Amazonia (e.g. Magdalena Basin, Colombia; Pilsbry & Olsson 1935; Pilsbry 1944), there are few outcrops representative of this time interval. The only evidence for a fluvial to lacustrine transition within the Early Miocene to Middle Miocene time interval can be observed along the Caquetá River (Santa Isabel; see Fig. 8.1). Other insights into the Early to Middle Miocene development of Amazonian environments derive from hydrocarbon exploration wells.

and lignites with shells and fine greyish sands of Andean origin continue until 25 m below the surface (Hoorn 1993a). Figure 8.3 represents five lithological columns based on CPRM borehole data of which 4, 4A and 27 illustrate well the transition from fluvial into wetland-dominated environments. The northward situated wells (51 and 48) are not dated but resemble the Late Miocene fluvial sequences that are known from the Acre region. Marañon Basin During the 1990s the Jibaro, Huayuri and Capahuari wells were drilled in the Marañon Basin by Occidental Petrol de Peru S.A., and these wells constitute our main data set for the lower Pebas Formation. In the Jibaro well the lower (Early Miocene) Pebas Formation occurs between 4700 and 2250 feet (~1400–700 m depth) and is predominantly composed of grey and occasionally varicoloured siltstones and claystones (Fig. 8.4). Lithic arenites,

Jibaro SP

1000

GR

RILD

Marañon Formation 1170

Colombian Amazonas Basin CZ4

2000 CZ3

Pronounced sequences

CZ2

Top compacted fossils Pebas Formation

3000 CZ1

4000

Crude mollusc zones

The Colombian Amazonas Basin is situated north of the Solimões Basin and no divisor seems to exist between them. This basin is relatively shallow judging by the frequency of basement outcrops in riverbeds. Along the Caquetá River, between Araracuara and La Pedrera (near the border with Brazil), sediments become stratigraphically younger and – from west to east – range from Early to Middle/Late Miocene in age (Hoorn 1993a, 1994a, 1994b; Wesselingh et al. 2006c). The Early Miocene sediments include fluvial deposits of cratonic source (Hoorn 1994a; see also Chapter 7) that gradually change into the typical blue clays of the Pebas Formation, i.e. the Santa Isabel outcrop (see Fig. 8.1). The sediments from the Santa Isabel outcrop were dated as Early to Middle Miocene based on pollen (Psiladiporites-Crototricolpites Zone; Hoorn 1994a). From Santa Isabel towards the east, sediments become younger and acquire the typical Pebas features (e.g. Puerto Caiman; see Fig. 8.1). The shallow depth of the sedimentary basin is illustrated on the Brazilian side of the border, where CPRM borehole 1AS-51-AM (along the Japurá/CaquetáRiver) registered the contact between the Solimões Formation and the Precambrian basement at 172.85 m (see Fig. 8.3b).

Poorly defined sequences

Solimões Basin 4696

The lower section of the Solimões Formation (340 to 170 m depth; Early Miocene) in well 1AS-4A-AM, is composed of multicoloured – mostly red and yellow – clays and subangular quartz sands with intercalations of blue-green clays and lignites, and rarely shell beds. The transition from a multicoloured lithology that is poor in shells into dark and blue-green clay beds and lignites with an abundance of shells occurs within the PsiladiporitesCrototricolpites Zone (from 250 m onwards) and is concluded at a 170 m depth (Crassoretitriletes Zone). The typical blue-green clays

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Chambira Formation 5000 depth (ft)

Fig. 8.4 The well log data – gamma-ray (GR), spontaneous potential (SP) and deep induction resistivity (RILD) – of borehole Jibaro in the Marañon Basin illustrate the physical properties of the rocks of the Pebas Formation in the subsurface, and distinguish sand and clay lithologies.

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Development of the Amazonian mega-wetland shell beds and lignite layers all occur in variable amounts, with rare occurrences of anhydrite crusts (Hermoza 2004; Wesselingh et al. 2006b). The mollusc fauna is relatively species poor in this part of the formation and is characterized by fluvial and lacustrine taxa. However, within the interval of 2250 to 1300 feet (~700 to 400 m depth) the molluscs are varied and abundant. The onset of these more diverse mollusc beds is mirrored by more densely spaced – and well-developed – sedimentary sequences that represent the boundary between the lower and the upper parts of the Pebas Formation. In the Huayuri and Capahuari a similar succession was observed (Wesselingh et al. 2006b). The contact between the Pebas and the overlying Marañon Formation is at 1150 feet (~350 m depth). Eastern Amazonia Rossetti & Góes (2004 and references therein) provide a detailed report on the calcareous lithologies of the Pirabas Formation and the clastic deposits of the Barreiras Formations. These formations represent the transition from littoral to fluvial deposition and occur in the Graben de Marajó and Plataforma de Pará, which neighbour the Foz do Amazonas Basin (Pasley et al. 2005). These sedimentary basins occur in the area where eastern Amazonia meets the Atlantic Ocean. The age of these formations was established through micropalaeontology and palynology as Oligocene to Early Miocene and Early to Middle Miocene, respectively. In addition the palynological and palaeobotanical data indicate the presence of tropical rainforest taxa. Other palaeontological studies further indicate the presence of a varied macrofauna. Other sub-Andean basins The stratigraphy of sub-Andean basins (also called Amazonian Foreland Basins) is reviewed in Chapter 5, but here we briefly discuss the sedimentary sequences that are relevant to the wetland development. During the Late Paleogene and Early Neogene the Colombian Llanos Basin was situated more westward relative to its current position and in direct connection with Amazonia and the Caribbean. In this area the late Oligocene-Early Miocene period is represented by the Carbonera Formation, which is formed by fine-grained sands and clays, and was deposited in a fluvio-deltaic system with drainage towards the NNE (Cooper et al. 1995; Bayona et al. 2007b). Although at some levels the palynological association contains marine taxa, the composition is mostly dominated by freshwater taxa. A distinct mollusc level near the top of the Carbonera Formation, called the Heusser Horizon, was dated as Early Miocene. This 10 m-thick level – which is overlain by a marine interval – characterizes the freshwater, lacustrine nature of this depositional system (Gomez et al. 2009). In the Putumayo Basin (Colombia) the Paleogene period is represented by clastic sequences that mostly are a product of the enhanced uplift of the Central Cordillera followed by lacustrine/ fluvial sediments of the Oligocene to Middle Miocene Orito-Belen Group (Gonçalves et al. 2002). In the Oriente Basin of Ecuador the lacustrine Curaray Formation is a contemporary equivalent of the Orito-Belen Group, which to the west grades into the predominantly sandy, fluvial Arajuno Formation (Burgos et al. 2005;

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Burgos 2006). The spatial and vertical extensions of the Curaray Formation are not well known yet; however, the formation is the lateral equivalent of the Peruvian Pebas Formation. The Curaray Formation consists of light-grey to blue-green or reddish clays alternating with fine-to-medium-grained sandstones and is rich in fossils (Burgos et al. 2005; Burgos 2006). The fossil assemblage contains freshwater taxa (Wesselingh 2006a), but also a variety of foraminifera indicative of marginal marine conditions and which are in agreement with the tidal sedimentary structures that have been reported on the Curaray Formation. Although the predominant depositional setting was fluvial, marine influence is evident due to the presence of marine fossils (Burgos et al. 2005; Burgos 2006). Age No radiometric ages are available for the lower Pebas and Solimões Formations; however, the base of the Pebas Formation was estimated by Wesselingh et al. (2006b) at 22.5–23.9 Ma (OligoceneMiocene boundary), based on a combination of palynology, mollusc studies, and a calculation of sedimentation rates from three wells in the Peruvian Marañon Basin. This date agrees with the base of the Curaray Formation in the adjacent Oriente Basin, which was estimated at 23 Ma (Burgos et al. 2005; Burgos 2006). In the Solimões Basin the lowest (accessible) part of the Solimões Formation is dated as Early to early Middle Miocene based on the presence of the Verrutricolporites, Retitricolporites and PsiladiporitesCrototricolpites Zones (274 to 181.8 m, well 1AS-4a-AM; Hoorn 1993a). The underlying Ramon Formation remains undated but is possibly of Paleogene age (Eiras & Wanderley 2006) and partially correlates with the Oligocene Chambira Formation of Peru. Depositional environment and palaeogeography During the latest Oligocene to Early Miocene the foreland basin axis formed a N–S-oriented depositional centre that received sediment input from the craton situated to the east and the Central Cordillera to the west. This N–S-oriented basin drained into the Caribbean and did not yet fully progress towards the east into Amazonia. At the time the fluvial overbanks repeatedly were exposed, as is suggested by anhydrite and red clay deposits. These mixed fluvial and lacustrine environments gradually changed into the humid, swampy and lacustrine wetland landscape. Life in the fluvial environments and incipient wetland Palynological analysis provided relative ages and an estimate of the palaeovegetation cover in Amazonia. The dark clays and lignites of the Early to early Middle Miocene (Verrutricolporites to Psiladiporites-Crototricolpites Zones) section of the Pebas Formation contain a highly diverse palynological assemblage with many species that have affinity with modern rainforest taxa. In addition, the abundance of Mauritia pollen suggests that palm swamps existed in proximity to the fluvial channels (Hoorn 1993a, 1994a; see also Chapter 19). The presence of mangrove pollen and foraminiferans in thin intervals within oxbow lake deposits further confirms that marine incursions reached the area at that time. Similar palynological features were identified further

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to the north in the latest Oligocene to Early Miocene sediments of the Carbonera Formation in the Llanos Basin (Bayona et al. 2007b). This suggests that the marine incursions, albeit shortlived, affected a wide region. Beside pollen and palaeobotanical remains such as amber, leaves and seeds, few other fossils were found. The only known mollusc fauna of this period was reported from La Cira in the Magdalena Valley. This species-poor assemblage was mostly composed of fluvial taxa and some endemic genera such as Pachydon and Dyris, which are indicative of lacustrine environments.

The mega-wetland (Pebas phase, ~16 to 11.3 Ma) (see Fig. 8.8b) Lithostratigraphy and distribution The central part of the mega-wetland was situated in Western Amazonia, and the Middle to early Late Miocene sediments of the upper Pebas and Solimões Formations are testimony to this. These sediments are characterized by blue-green, black and grey clays, lignites and intercalations of fine-grained grey-brown lithic arenites. One of the most distinct features of these sediments is the abundance, variety and extremely good preservation of a highly diverse aquatic fauna (Plate 3e). Typical exposures of this unit can be found along the Amazon River between Nauta (Peru) and Leticia (Colombia), along the Peruvian Napo River, and the Javari River in Brazil. Other exposures are known from the Putumayo and Caquetá Rivers in Colombia; a good example of the earliest known diverse mollusc faunas of the Pebas Formation can be found at La Tagua along the Caquetá River (Colombia) (Nuttall 1990; Hoorn 1994a; Wesselingh et al. 2006a). Isolated outcrops are reported from as far south as Shepahua (Ucayali, Peru) and as far north as the Serrania de la Macarena (Colombia). Colombian Amazonas Basin In southeastern Colombia, along the Amazon River and in the proximity of Leticia, some of the best exposures can be found at Puerto Nariño, Los Chorros and Santa Sofía (Hoorn 1993a, 1994b; Wesselingh et al. 2006a). The Colombian geological map (Ingeominas 2007) reports this geological unit as Terciario Inferior Amazonico (TIA) or ‘Paleogene’ (E-Stm code on map), but we favour the use of Pebas Formation instead, more so because the Terciario Superior Amazonico (N-Sc, or ‘Neogene’ on the map) is older than the TIA (Hoorn 1993a, 1994a, 1994b). Solimões Basin In the Solimões Basin the transition from lower (sandy) to upper (clayey) Solimões Formation is characterized by the disappearance of multicoloured reddish clays and coarse, subangular quartz sands of cratonic origin, in favour of dark-grey and blue-green silts and clays, lignites, and intercalations of Andean-derived fine, lithic sands. One of the most striking features is the appearance of a varied and abundant mollusc fauna. In well 1AS-4A-AM the transition occurs at 170 m depth (within the

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Psiladiporites-Crototricolpites Zone) and the clayey upper Solimões Formation continues until about 30 m depth, at the contact between the Solimões Formation and the overlying Iça Formation (Hoorn 1993a). Petri & Fulfaro (1983) attribute the lithologies of the Upper Solimões Group to the Pebas Formation and describe the extensive distribution of this unit in the subsurface of Brazilian Amazonia. Exploration wells in the southern and northernmost Brazilian Solimões Basin, and outcrops at the southern margins of the Acre Basin display an additional transition from bluish clays into reddish sand and silt sequences (Eiras et al. 1994; Cozzuol 2006). We attribute these younger sediments to the Late Miocene Acre phase (see below). Marañon Basin Middle and early Late Miocene deposits of the Pebas Formation are widespread in the subsurface of the Marañon Basin (Hermoza 2004; Hermoza et al. 2005; Wesselingh et al. 2006b) and were identified in the Jibaro, Huayuri and Capahuari wells. In the Jibaro well the transition occurs between 2250 and 1750 feet (~700 to 350 m depth) and is characterized by densely stacked 2–8 m sequences of blue smectitic clays, fine-grained lithic arenites, and lignite layers. This section is characterized by a rich and diverse assemblage of very well-preserved invertebrate and vertebrate fossils. This Middle to early Late Miocene part of the formation constitutes the ‘archetypical’ Pebas Formation, which we here call Pebas phase, and represents the mega-wetland at its maximum development. Eastern Amazonia In Eastern Amazonia the Middle Miocene period is represented by calcareous and clastic lithologies that were formed in a littoral to fluvial transition. These deposits are known as the Barreiras Formation and are further described in Rossetti & Góes (2004 and references therein). At the same time the Foz do Amazonas and the Ceara Rise registers clastic deposition from a fluvial system, and sediment analysis indicates a cratonic source (Dobson et al. 2001; Figueiredo et al. 2009). Other Andean and sub-Andean basins The Leon Formation in the Colombian Llanos Basin is contemporary with the upper Pebas Formation, and is composed of dark laminated mudstones and shales with some interbedded sands. The palynological assemblage is similar to that of the Pebas Formation and indicates a Middle Miocene age and a freshwater environment with some brief marine episodes (Bayona et al. 2007b). In the Oriente Basin of eastern Ecuador exposures of the partially time-equivalent Curaray Formation were recorded (Burgos et al. 2005; Burgos 2006). Outside of Amazonia other important coeval sediments are the fluvial deposits of the Honda Group in the Middle Magdalena Basin, Colombia (Guerrero 1997; Kay et al. 1997) and the Loyola Formation in the Cuenca Basin in Ecuador (Bristow & Parodiz 1982).

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Development of the Amazonian mega-wetland Age The first indication of a transition of fluvio-lacustrine environments into a species-diverse extensive wetland, with cyclical shallowing upward sequences, was registered within the Psiladiporites-Crototricolpites Zone, and was completed within the Crassoretitriletes Zone (Hoorn 1993a, 1994b; Wesselingh et al. 2006b). Therefore, based on the palynological marker species of the Psiladiporites-Crototricolpites, Crassoretitriletes vanraadshovenii, and Grimsdalea magnaclavata zones, a relative age of late Early to early Late Miocene was assigned to the upper part of the Pebas and Solimões Formations. The wetland, however, attained its maximum extent during the Middle to Late Miocene. Other sediments of similar age are part of the Curaray Formation in eastern Ecuador, and were dated through apatite fission track analysis. Here the Crassoretitriletes vanraadshoveni-bearing intervals were dated as 14.5 ± 2 Ma and 15 ± 5.1 Ma (Burgos et al. 2005; Burgos 2006). Additionally, the Loyola Formation was dated through zircon fission track analysis as ranging between 12.1 ± 1.2 and 13.9 ± 1.4 Ma (Steinmann et al. 1999). The Middle Miocene Honda Group from Colombia includes fluvial (mostly meandering) and lacustrine deposits proximal to the Central Cordillera in the west and connected at the time in the east with the Pebas Formation deposits. This unit was deposited between 13.5 and 11.5 Ma (Guerrero, 1997) and includes several fish species that at present occur in the Magdalena, Orinoco and Amazonas drainage basins, suggestive of the earlier connection between the areas in Middle Miocene times (i.e. Lundberg et al. 1998). During the late Early to early Late Miocene an endemic invertebrate radiation took place in the Pebas system and this permitted a further subdivision of the biozonation, based on mollusc and ostracod data (Muñoz-Torres et al. 2006; Wesselingh et al. 2006a). The disappearance of this endemic invertebrate fauna at the end of the Pebas phase most likely coincided with the establishment of the transcontinental Amazon River system (Lundberg et al. 1998), which is dated at 11.8–11.3 Ma (Figueiredo et al. 2009). The occurrence of mangrove pollen together with foraminiferan, marginal marine molluscan and ostracod taxa, and very rare barnacles is indicative of the marine incursions that reached Amazonia during both late Early and early Middle Miocene (Hoorn 1993a; Vonhof et al. 1998, 2003; Vermeij & Wesselingh 2002; see also Chapter 18). Additional indications from mollusc zone MZ9 (Wesselingh et al. 2006a; see also Chapter 9) in the uppermost Pebas Formation (late Middle to early Late Miocene) further extend the time range of marine influence in Amazonia. The regional character of the episodic marine influence during the Middle Miocene is further confirmed by similar findings in the Leon Formation (Llanos Basin) (Bayona et al. 2007b). Sedimentological features The upper Pebas and Solimões Formations are composed of bright-blue clays, greyish immature sands, and lignites. These sediments often are rich in organic matter and contain a wealth of carbonate fossils. The characteristic blue colour is derived from montmorillonite, a clay mineral from the smectite group, which also is responsible for the excellent preservation of organic and

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carbonate fossils (Räsänen et al. 1998). Towards the sub-Andean zone in the west, bluish clays laterally grade into reddish-grey immature sands and clay deposits, with some minor gravel components (Burgos et al. 2005; Burgos 2006; Wesselingh et al. 2006a). The upper Pebas strata are characterized by several facies including bioturbated mud, wave-ripple, current-ripple and combined-flow-ripple cross-stratified sand, various types of heterolithic bedding and lignite. Locally, sharp-based upward-fining successions occur, which can be interpreted as palaeochannels. Ripple structures and rare trough cross-stratification suggest low to moderate flow within the system. The rhythmic alternation of sands and clays indicates the tidal influence (Hoorn 1993b, 1994; Hovikoski et al. 2005) but is also partly of seasonal origin (Hovikoski et al. 2007; see also Chapter 9). The sedimentary sequences constitute recurring coarseningupward sequences of 2–8 m thickness, and represent transgressive-regressive cycles. Each sequence includes a succession of shallow, low-energy marine-influenced embayments that changes into tide-and wave-influenced bay-margins and swamps (Fig. 8.5) (e.g. Los Chorros – Vonhof et al. 2003; Santa Rosa de Pichana – Wesselingh et al. 2006d; Santa Teresa – see Chapter 9). At the top of these successions, the deposits grade into organic-rich mud/ muddy sand or lignite (swamps). In some places inclined heterolithic stratification (IHS, following Thomas et al. 1987) was recognized, which is typical for fluvio-tidal channel deposits. Heavy mineral analysis together with eastward transport directions, measured in the immature Pebas sands, indicates that the metamorphic rocks in the Ecuadorian Andes are the main source of sediment supply to Amazonia (Hoorn 1993a; Roddaz et al. 2006a). The Andes, however, is not the only source of sediment input into Amazonia. In the Colombian Apaporis region, sand outcrops with a distinct cratonic origin and palynologically dated as late Middle Miocene to Late Miocene suggest that the craton also continued actively to supply sediments into the intracratonic basins (Vonhof et al. 2003; Hoorn 2006). Depositional environment and palaeogeography During the Middle Miocene the wetland system reached its maximum development and consisted of a highly dynamic mosaic of lakes, embayments, swamps, rivers and fluvio-tidal environments. Along its periphery the system was bordered by alluvial plains of Andean rivers in the west, and lowland cratonic rivers to the east, whereas in the northeast, the wetland onlapped onto the cratonic rocks (Wesselingh et al. 2006a). The wetland lake and embayment system was shallow (possibly up to 10 metres in depth) and repeatedly went through cycles of drowning and shallowing upwards. Each of these stages had its own characteristic taphonomic association. The base of the transgressive part is typically burrowed with Glossifungites ichnofacies. In the deepest part of the successions (zone of maximum flooding) molluscs such as Pachydon and Dyris predominate. In the shallow (regressive) intervals, however, molluscs are virtually absent, and the tidally and wave-influenced deposits are burrowed with lowdiversity assemblages of mixed Cruziana-Skolithos ichnofacies (Wesselingh et al. 2006a; Hovikoski et al. 2007). These regressive episodes conclude with swamp development, which often is laterally continuous for hundreds of metres. Once the lakes were filled,

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Interpretation

Description NOTES

MF

IF

FACIES

SEQUENCES

19

IMF Massive mud with some shelly intervals

18

Sublittoral PO

TI Low-energetic littoral Shallow littoral

Bioturbated sandy mud Lignite, layered and bioturbated Bioturbated sand

17

G?

Wave-influenced upper shoreface

Interlayered rippled sand and mud, stratiform organic matter

15

Swamp Foreshore

Sand with abundant clay layers; deformed laminations

16

V

TSE

Ar PI GY

Laminated mud, starved ripples and organic debris

RPI

Wave-influenced lower shoreface

14

Massive mud with shells

13

IMF

Sublittoral SD

12

TI Bioturbated sandy mud Organic mud Organic mud with shell lenses Layered lignite Layered organic mud and sand strings Bioturbated sand with organic layers

11

10

Low-energetic littoral

TD TD TD

Shallow littoral P G

Backswamp Soil Wave influenced foreshore Wave influenced upper shoreface

Massive sand

9

IV

TSE

RPI

Bioturbated sand Massive sand, starved ripples and clay drapes (flasers)

8

G

Laminated/bedded mud, starved sand ripples

Wave-influenced lower shoreface

PO

7 Massive shell-rich mud

SD SD

Sublittoral

SD SD

6 Massive sandy silt Layered lignite-organic mud; wood; bone; Amazoniconcha Massive mud

5

3

Organic-rich mud with organic debris and leaves Massive mud with shells

2

Interlayered shelly mud and sand lenses

1

Unspecified mud

TI A

TE G

Low-energetic littoral (coastal) swamp

III

TSE

Backshore logoon Wave-influenced upper shoreface

Massive sands with some deformed layers Interlayered mud and rippled sand Deformation and bioturbation in sand

4

IMF

RPI

Wave-influenced lower shoreface PO

Low-energetic foreshore

PO SD SD SD

Sublittoral

TI

Low-energetic littoral

TSE

Backswamp

IMF

II

I

0 mud

H (m)

peb

sand

Fig. 8.5 The Middle to Late Miocene record at Santa Rosa de Pichana (Pebas Formation) shows the depositional cycles and shallowing phases that are characteristic for the wetland environment. IMF, interval of maximum flooding; PO, Pachydon obliquus assemblage; RPI, regressive/prograding interval; SD, Small Dyris assemblage; TD, Tall Dyris assemblage; TE, terrestrial molluscs; TI, transgressive interval; TSE, transgressive surface of erosion. Ichnofossils: A, Asterosoma; Ar, Arenicolites; G, Glossifungites; P, Psilichnus; Pl, Planolites.

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Development of the Amazonian mega-wetland vegetation development was initiated on these poorly drained soils and is mostly formed of palm associations. Locally, swamp channels accumulated a mixture of macrofossils; in particular, seeds, logs, and fish, turtle and crocodile remains are extremely abundant in both the lake margins and swamp deposits. At some places logs occur exclusively, which represent old forest floors (see Plate 3d & f). Geochemical evidence points to a predominance of Andean freshwater sources and limited influence of cratonic waters (Kronberg et al. 1989; Vonhof et al. 1998, 2003). Outcrop data from the Colombian Apaporis area further confirm that cratonic fluvial systems affected the eastern parts of the system. There is still some controversy about interpretations of salinity regimes of the Pebas Formation (e.g. Gingras et al. 2002a vs Vonhof et al. 2003). However, it is now believed that, although the repeated brackish water phases were temporally less significant than the freshwater phases, tidal processes played a significant role in the Neogene in Amazonia, indicating periods of prolonged connection between the sea and the depositional system (Hovikoski et al. 2005, 2007; Hoorn et al. 2006). Palaeogeographically the foreland basins, which are situated parallel to the Andes, received the bulk of the sediments through fluvial input from the emerging Eastern Cordillera. In Amazonia, however, fluvial channels are rare and, if present, of modest size and rather distant from the Andean source. The entire wetland system was exposed to marine influence from the Caribbean although an additional port of marine entrance might have been the Essequibo gap in the Guiana Shield (G.F.W. Herngreen, personal communication) and the Cuenca corridor in the west cannot be excluded either (Wesselingh & Salo 2006). Life in the mega-wetland Neogene outcrops of Amazonia form a true paradise for palaeontological and palynological research because well-preserved fossils and organic material are plentiful. Palynological data suggest that the vegetation was extremely diverse, and the arthropod fauna enclosed in Middle Miocene amber (Antoine et al. 2006) further confirms this. The main components of the terrestrial vegetation were rainforest taxa, whereas the swamps were mainly populated by palms such as Mauritia and an extinct taxon called Grimsdalea. In addition, grass pollen are relatively common and were probably related to (aquatic) grass meadows – a vegetation type currently known from the white-water rivers of Andean origin. Their presence in the Miocene wetland is probably linked to the Andean fluvial influxes into the wetland system as this group was absent prior to the Middle Miocene. Other Andean taxa such as Podocarpidiites (Podocarpus), Clavainaperturites (Hedyosmum) and pteridophytes (related to Andean tree-fern taxa) (Hoorn 1993a, 1994a) further confirm this. Although there are sediment layers with mangrove pollen, the abundance of freshwater algae and pteridophytes underscores the dominant freshwater character of the deposits. During the Pebas phase the molluscs and ostracods underwent extensive evolutionary radiations. Cochliopid snails dominate the mollusc fauna in species numbers, but in terms of abundance pachydontine bivalves prevail. The ostracod fauna is dominated by endemic species of the genus Cyprideis. To date, about 160 species of molluscs and about 45 species of ostracods have been described,

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and these numbers are only expected to rise with further work (Wesselingh 2006a; Muñoz-Torres et al. 2006; see also Chapter 18). The vertebrate fauna in the mega-wetland contained a diverse suite of fish species including piranhas, Amazonian stingrays (Potamotrygon) and sharks (Carcharinus) (Monsch 1998; see Chapter 17). In addition to this, a wide variety of turtles, crocodilians and mammal fossils was found in the swamp channels that are preserved in the upper Pebas Formation (Salas-Gismondi et al. 2007; see also Chapter 16). In general, Middle to Late Miocene wetland fauna was dominated by aquatic taxa of neotropical lowland character. In addition, the Middle Miocene terrestrial mammal assemblage – as reported in the proximities of the Fitzcarrald Arch (Peru) – was composed of taxa typical for an open environment (hypselodont notoungulates) and dense tropical forest habitats (astrapotheres, litopterns and xenarthrans) (Antoine et al. 2007; see also Chapter 15). The episodic marine character of the environment is illustrated by the presence of benthic foraminifera, barnacles and dinoflagellates, which are restricted to thin beds deposited during marine influxes (Hoorn 1993a; Vonhof et al. 1998; Vermeij & Wesselingh 2002). Additionally, the trace fossils described above further confirm the episodically brackish nature of the environment. Brackish water ichnofossils are most common in the lower part of sedimentary sequences, but occur also locally in the regressive part of the sequences (Gingras et al. 2002b; Hovikoski et al. 2007). The marine influence in Amazonia played a long-running role and traces of it can be recognized in Amazonia today. For instance the freshwater dolphin Inia together with fishes such as stingrays and needlefish, amongst others, are thought to have adapted from marine to fresh water in Miocene times. Although at present marine taxa can reach into the heart of the continents, these taxa are thought to have arrived in Amazonia through marine connections with the Caribbean (Lovejoy et al. 1998 , 2006; see also Chapter 25).

The fluvio-tidal wetland and the early Amazon River (Acre phase, ~11.3 to 7 Ma) (see Fig. 8.8c) Late Miocene sediments were documented in the Marañon Basin (northwest Peru), the Acre Basin (Brazil) and the Madre de Dios Basin (southeast Peru and adjacent Bolivia; see Fig. 8.1) and are here referred to as the Acre phase. These sediments are characterized by a series of fining-upward successions that are composed of sand and clay and are estimated to have a total thickness of 400 m (Hermoza 2004). They are known under a variety of names, depending on the sedimentary basin in which they occur. The strata are separated from the overlying conglomeratic units by a marked regional stratigraphic discontinuity (Campbell et al. 2001; see also Chapter 5), which is best observed in the Madre de Dios area (see Fig. 8.1). The main difference between the Acre phase and the Pebas phase is the absence of dark clay, lignite and carbonate fossil deposition during the Acre phase. Lithostratigraphy and distribution Upper Miocene strata that crop out in the Madre de Dios Basin (Peru) are informally referred to as Ipururo and Madre de Dios

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134

C. Hoorn et al.

formations – Kummel (1948) and Oppenheim (1946), respectively. Kummel (1948) applied the name Ipururo formation to strata outcropping in the Contamana area, ~600 km north from the Madre de Dios Basin. However, the original description lacks sufficient detail adequately to characterize the formation; therefore, it is proposed that the use of ‘Ipururo formation’ is modified or dropped in the Madre de Dios Basin (H. Nuñez et al. unpublished data). According to the original description, the overlying Madre de Dios Formation (Oppenheim 1946; Campbell et al. 2001, 2006) consists of a sequence of fining-upward sand and clay channel successions, and a conglomeratic unit that erosionally truncates the Miocene sediments (Fig. 8.6). This marks a major stratigraphic boundary in the area, which possibly is of Plio-Pleistocene age (M. Räsänen & L. Romero, unpublished 14C datings; see also Chapter 6). The conglomerates, which here are excluded from further discussion, have different formation names including Quimiri Formation, ‘Secuencias Superiores’ and Juanjui Formation (e.g. Hermoza 2004; Espurt et al. 2007) Solimões Basin In the Solimões Basin this interval is only known through well log data from wells drilled in the northern end of the basin (see CPRM wells 48 and 51 in Fig. 8.3). These data suggest the absence of carbonate fossils and lignites, and the presence of sand and clay sequences similar to those described in the Acre Basin. Marañon Basin In the Marañon Basin (Peru), the Acre phase is represented by the Marañon Formation (Hermoza 2004; Wesselingh et al. 2006b), which diachronically overlies the Pebas Formation and is only known from well data. The formation possibly correlates partially with the Nauta Formation in the Nauta area (Rebata et al. 2006), and Räsänen et al. (1998) refer to these deposits as Unit B and Unit C. The Marañon Formation is composed of reddish-grey immature sandstones and siltstones that were deposited in fluvial settings (Hermoza 2004; Wesselingh et al. 2006b). Lignites and carbonate fossils are absent in the formation.

In the Nauta area (Loreto, Peru) the outcrops of the Nauta Formation typically consist of 7–9 m-thick inclined heterolithic stratification (IHS)-bearing channel deposits similar to those of the uppermost Solimões Formation (see below). The base of the successions is commonly composed of mud-draped, trough cross-stratified sand that grades upwards into bioturbated IHS. The IHS is variable, containing interstratified sand and mud beds/ lamina of various orders. The strata from the Nauta Formation are thought to represent seasonally and tidally controlled estuarine or deltaic channel complexes (Rebata et al. 2006). Acre Basin In the Acre region outcrops are characterized by fining-upward sand-channel sequences that are attributed to the upper Solimões Formation and considered time-equivalent to the Ipururo and Madre de Dios Formations (Maia et al. 1977). These sediments represent megafan deposits (Latrubesse et al. 1997) and were dated using macrofossils that were collected from channel deposits (Cozzuol 2006). The Universidade Federal do Acre (at the Laboratório de Pesquisas Paleontológicas) stores an extensive collection of fossils that has been assembled during decades of research (see Chapters 15 & 16). Amongst others the collection holds examples of the giant caiman, Purussaurus brasiliensis. Eastern Amazonia The Late Miocene is characterized by a regional hiatus and lateritic palaeosols (Rossetti & Góes 2004, and references therein). However, the Foz do Amazonas Basin and the Ceara Rise (Dobson et al. 2001; Pasley et al. 2005; Figueiredo et al. 2009) register the arrival of the first Andean sediment influx. Around 6.8 Ma sedimentation rates increased and from 2.4 Ma onwards this river system resembled the modern Amazon River (Figueiredo et al. 2008). Other Andean/sub-Andean basins The Late Miocene alluvial fan deposition in the Andean/ sub-Andean basins was a direct result of intensified Andean uplift (Guerrero 1997; Mora 2007; Uba et al. 2007). In the Llanos region (Colombia) these Late Miocene deposits are known as the Guayabo Formation (Cooper et al. 1995); the conglomeratic Ospina Formation (Gonçalves et al. 2002) is their equivalent in the Putumayo Basin, and in the Oriente Basin it is the lower part of the Ecuadorian Chambira Formation (Hermoza 2004; see also Chapter 5). The latter is much younger than the Oligocene Chambira Formation of adjacent Peru and represents a more proximal fluvial setting. Age

Fig. 8.6 The contact between the Madre de Dios Formation and the overlying conglomerates. Location Inambari River (12.922098S, 70.349118W).

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Several radiometric ages are reported for the sediments deposited during the Acre phase. Campbell et al. (2001) presented two Late Miocene argon-40/argon-39 (40Ar/39Ar) ages, which are close to 9 Ma. Similarly, Roddaz (2004) dated the same tuffaceous level by apatite fission track dating at 9.31 ± 3.39 Ma (Puerto Maldonado; see Fig. 8.1). Continental vertebrate taxa from apparently coeval strata in the Acre region were assigned to the Huayaquerian

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Development of the Amazonian mega-wetland mammal age, which further confirms the Late Miocene (Tortonian) age (e.g. Frailey 1986: Cozzuol 2006: Latrubesse et al. 2007). Finally, pollen assemblages reported by Silva (in Latrubesse et al. 2007) and by Rebata et al. (2006) are attributed to the Late Miocene Asteraceae Zone. However, it is also possible that the latter relate to the Ipururo Formation, which is coeval with the upper part of the Pebas Formation (see Chapter 5). Sedimentological signature The Madre de Dios Formation and the uppermost Solimões Formation typically consist of various types of sharp-based, 2–15 m-thick, upward-fining successions, which are interpreted as channel deposits and are intercalated with pedogenic or rootbearing horizons (Fig. 8.7a,b). The lower part of the fining-upward successions generally consists of yellowish, massive or crossstratified fine-to-medium-grained sands. Upwards in the succession, the deposits grade into mud-dominated homolithic sediments, or into various types of IHS (following Thomas et al. 1987). The IHS consist of alternating beds of sand, mud and calcareous mud-bearing strata of variable thickness and lateral extent. At the top, the channel successions grade into massive, pedogenically altered clay horizons. Locally, rooted mudstone makes up for the majority of the sedimentary record, especially in the Acre region (Hovikoski et al. 2008). The IHS channel deposits are highly variable in terms of sedimentological features and fossilized burrowing traces of animals (ichnofossils; see Chapter 9 for methodology) and are thought to represent floodplain to tidally influenced estuarine conditions. Most commonly though, IHS units lack burrows or bear monospecific to low-diversity ichnofossil assemblages, including Gyrolithes, Skolithos and/or Cylindrichnus. Assemblages containing Asterosoma, Thalassinoides, Ophiomorpha and Laminites occur. Channels burrowed by near monospecific suites of Taenidium or Planolites, unnamed irregularly branching burrows, or suites containing elements of Mermia ichnofacies are locally present. The tops of most channel deposits and pedogenically altered horizons are commonly burrowed with terrestrial Scoyenia ichnofacies.

month cycles (e.g. Rebata et al. 2006; Hovikoski et al. 2005, 2007, 2008; Hovikoski 2006) and are further discussed in Chapter 9. The complexity of the depositional setting is well illustrated by the sedimentary sequence at the Acca biological station, studied by both Campbell et al. (2006) and Hovikoski et al. (2005). This sequence consists of three stacked channel belt successions. The lowermost succession overlies a palaeosol and is a typical fluvial point bar displaying nearly homolithic upward-fining succession. However, towards the top of the succession, the channel (a) Sharp contact Ripple foreset dip direction

Sigmoidal ripples

n=7 Sharp contact

Loading structures

Ripple foreset dip direction

Erosional contact (laterally discontinuous)

Interlaminated silt and clay

Heterolithic climbing ripples (wavy bedding)

Mud-draped, current ripple cross-stratification Cross-stratified sand with mud clasts Cross-stratified silty sand

Synaeresis cracks IHS (lenticular-flaser bedding)

n=8

Undichna Loading structures Pyrite Synaeresis cracks Erosional contact Diplichnites

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Heterolithic climbing ripples (lenticular bedding)

Interlaminated silt and clay

Depositional environments and palaeogeography The Late Miocene sediments in the Acre region were deposited in fluvial, deltaic and estuarine channel complexes that were interbedded with a variety of alluvial strata. The fossil record of these strata is dominated by continental vertebrates, which has led some researchers to consider the Acre phase as fully continental (Campbell et al. 2006; Cozzuol 2006; Latrubesse et al. 2007). In addition it was proposed that seasonal processes in a continental depositional system could explain the heterolithic patterns in these deposits (e.g. Westaway 2006). While the notion of prominent seasonal variation in energy levels is true, there is also strong evidence for semi-diurnal tidal processes in many IHS channels. These include local current reversals (measured from foreset of adjacent ripples), double mud-drapes, regular reactivation surfaces in heterolithic cross-strata, sigmoidal ripples and statistically verified cyclic rhythmites. The rhythmites show evidence of superposition of several tidal cycles including flood–ebb, diurnal inequality, neap–spring and anomalistic

135

Flaser-bedded very fine sand

Massive fine sand Interlaminated silt and clay

Pyrite Ripple foreset dip direction

Mud-draped, current ripple cross-stratification

Massive, sandy mud with mud clasts

n=2

Mud-draped, current ripple cross-stratification

Massive sandy silt

Fig. 8.7 (a,b) Examples of Late Miocene sedimentary successions from (a) Cocha Cashu (Madre de Dios, Peru) and (b) Tarauaca (Acre, Brazil). c, coarse; cl, clay; f, fine; m, medium; sa, sand; si, silt. Modified after Hovikoski et al. (2005) and (2008), respectively.

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(b)

– – –

n=3 Ta?

n=1

–

To, fu, Pa, Pl, La

n=8 Th, fu

n=3

Pl

n=3

-

Pl, Sk, Ta

Ps, Sk, (eq), Pl

n=1

Ta

n=2

Pl, Pk, Sk, Ps,

-

-

–

n=5

n=3 Pl, Pa, eq,

n=3 Pl

Pa

Gy, Pa

Fig. 8.7

Hoorn_ch08_Final.indd 136

Continued.

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Development of the Amazonian mega-wetland units display an increasing heterolithicity, including rhythmically occurring sand-clay couplets that form statistically verified semi-diurnal tidal rhythmites. Ichnological data suggest that the Acre phase tidal facies was accumulated in both brackish and freshwater settings in deltaic and middle-upper estuarine environments (Hovikoski et al. 2008; see also Chapter 9). Moreover, sediments with typical tidal features were described from various outcrops in the area (Madre de Dios – Hermoza 2004; Roddaz et al. 2006b; Hovikoski et al. 2005; Acre – Räsänen et al. 1995; Gingras et al. 2002b; Hovikoski et al. 2007, 2008 and see Chapter 9; Nauta – Rebata et al. 2006). The implications of these findings for the palaeogeographical interpretation of this region are still not entirely clear. At the time of the Acre phase the transcontinental Amazon River system had just connected with the Atlantic, and former pathways of marine influence into Amazonia along the northern Andean foreland basins were closed. Although this topic is subject to further study it would be plausible to expect a new route of marine influence, namely the Atlantic through the mouth of the newly formed Amazon River (see also Chapter 9). Life in the transition from wetland to Amazon River system Pollen records are rare for this interval; however, where available the records suggest the presence of rainforest combined with grasslands and/or floating meadows (Rebata et al. 2006; Latrubesse et al. 2007; see also Chapter 19). The vertebrate fauna of the Acre phase is extremely rich, both in abundance and in species numbers, and is typical for terrestrial and freshwater settings, although teeth of euryhaline sharks have also been reported (Räsänen et al. 1995; Monsch 1998). This fauna is further described in Chapters 15 & 16. In contrast, invertebrate faunas are relatively rare and poorly preserved. The mollusc faunas that were sampled from study sites in Acre are dominated by pearly freshwater mussel taxa that closely resemble present-day fluvial faunas of the region (Wesselingh et al. 2006e).

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the overlying sediments consist of a succession similar to that at the ACCA (Asociación para las Conservación de la Cuenca Amazónica) research station. The trace fossil fabric consists of reburrowed (by Planolites) Ophiomorpha, Thalassinoides and Gyrolithes. The Tarauaca outcrop (see Figs. 8.1 and 8.7b) is about 15 m high and can laterally be followed for ~100 m. The deposits form a 12 m-thick fining-upward succession. The lower contact is sharp and bears mud-clasts. The overlying strata consist of trough cross-stratified or low-angle, planar-stratified silty sand. The dunes are most commonly 10–30 cm high (highest 70 m), and as co-sets they form 1–3.5 m-high compound units. Where observed, the width of the larger troughs was typically more than 5 m. The cross-strata are commonly mud-draped, or bear minute mud-draped ripples. Double mud-drapes are present regularly. Upwards, these deposits grade into inclined heterolithic stratification (IHS) that consists of a coarse-to-fine (sand to mud) couplet. The members are cross-stratified or massive silty sand and inclined, laminated mud. The coarse member can be 0.5–2 m thick, and the fine member 20–60 cm thick. The contact from coarse-to-fine is sharp, although the grain size change is slightly gradational (normally graded). The contact from fine-to-coarse is sharp and often truncated. The fine-members form intraformational breccia in the down-dip direction. IHS can be followed laterally for up to 70 m. This interbedding is interpreted to reflect seasonal changes in energy levels (Hovikoski et al. 2008). Both the IS and IHS sets may bear poorly cyclic rhythmites (a cycle comprising 15–21 sand-clay lamina couplets, n = 5). These strata are overlain by rhythmically occurring, mud-draped current ripple-bearing sand and silt, and interlaminated mud and sand. The thickness of the coarse member ranges from 0.5 to 8 cm. The thicker layers form flaser-bedding as co-sets. The thickness of the fine member is generally 1–5 cm. Contacts between the members are most commonly bigradational (i.e. both contacts are gradational), typically marked by mud-draped climbing ripples. As couplets they form statistically verified cyclic tidal rhythmites. Asymmetric lamina couplets, countercurrent ripples, and scourand-fill structures are observable locally.

Case studies: the Cocha Cashu and Tarauaca outcrops The Cocha Cashu outcrop is situated along the Manu River, a few hundred metres down river from the Cocha Cashu biological station (see Figs. 8.1 and 8.7a). The outcrop is 10 m high and can laterally be followed for about 100 m. The facies consist of a bluish mud-dominated, inclined, heterolithic stratification, and form sharp-based fining-upward successions up to 3 m thick. The successions begin with massive or cross-stratified, fine-grained sands with mud clasts. Upwards in the section the deposits commonly turn into clay-draped, cross-stratified sand and silt. Statistical analysis shows that these strata are semi-diurnal tidal rhythmites and possibly also related to seasonalities. The top of the succession generally consists of interlaminated silt and clay, and softsediment deformation is common. Palaeocurrent directions are predominantly toward the east and west, whereas locally the foresets display bipolarity. Sedimentary accessories include synaeresis cracks and pyrite. Bioturbation is generally absent, but where present, the trace fossil genera observed include bedding-plane traces such as Diplichnites, Undichna and (?)Phycodes. Near the top of the outcrop, a low-density assemblage of Taenidium is observed. The contact with the overlying deposits is not seen in detail, but

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The Amazonian Miocene mega-wetland in its regional context The Miocene mega-wetland connected most of South America through both fluvial and marine pathways. Towards the north this wetland was linked with the Caribbean through the Llanos region, a connection that existed until the Late Miocene (see Fig. 8.8). This pathway permitted sporadic marine influences into the Amazon drainage basin. Proof of this are the similarities between Caribbean and Amazonian fish faunas (Lundberg et al. 1986; Lovejoy et al. 1998), the migration pathway of the mollusc Pachydon (Wesselingh & Mactosay 2006), the overall resemblance of the vertebrate faunas of La Venta, Urumaco and Amazonia (Cozzuol 2006) and the coeval Miocene marine-influenced deposits in the Llanos and Amazonia. No firm evidence exists regarding southern connections; however, possible fluvial pathways with the Paraná region are explored in Chapter 17. These data imply that prior to the Late Miocene the present Orinoco, Amazon and Magdalena drainage basins and their fauna and flora were all connected. During the Late Miocene

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Fig. 8.8 Palaeogeographical maps showing the evolution of the wetland through time. (a) The latest Oligocene to Early Miocene Andean and cratonic fluvial systems with an incipient wetland in the sub-Andean region. The palaeo-Magdalena flows eastwards and is connected with the sub-Andean and cratonic drainages, which all flow northwards to the Caribbean. Together these rivers form the sub-Andean River system. (b) The Middle Miocene wetland at its maximum but still disconnected from the Atlantic. (c) The Late Miocene fluvial system, with localized wetland development. The Purus Arch barrier is cut by the fluvial system, which now connects with the Atlantic.

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Development of the Amazonian mega-wetland and Pliocene, accelerated uplift of the Eastern Andes and uplift of the Cordillera de la Costa (Venezuela) led to the separation of these drainage basins (Hoorn et al. 1995; Diaz de Gamero 1996; Potter 1997; Lundberg et al. 1998; Albert et al. 2006). This provided a completely new geographical scenario for species evolution. At the same time, an additional new palaeographical setting was developed through the establishment of the Amazon River, a transcontinental fluvial route that connected the Andes and the Amazon (Hoorn et al. 1995; Figueiredo et al. 2009). This event subsequently led to another major palaeogeographical development, which was the start of deposition of the Amazon Fan (Pasley et al. 2005; Figueiredo et al. 2009). The onset of this giant new depositional system along Brazil’s equatorial Atlantic margin is dated by Figueiredo et al. (2009) as 11.8–11.3 Ma, a date that coincides with the onset of terrigenous sedimentation on the deep-sea Ceara Rise (Dobson et al. 2001). After the Andes–Atlantic connection was established the Amazonian mega-wetland became a fluvio-estuarine system with a distinct tidal signature (Hovikoski et al. 2005). Although this needs to be investigated further (i.e. by searching for Atlantic influence in these sediments) it is plausible that the tidal influence originated in the Atlantic. This is particularly likely if we consider that the Amazon drainage basin is the only basin in the world where, at present, tidal (wave) influence can reach as far as 1000 km inland (see Chapter 11).

Conclusions The Miocene Amazonian mega-wetland was one of the largest and longest-lived wetland environments in geological history. The evolution of this depositional system was controlled by a myriad of factors such as global sea level change, subsidence and a seasonally wet climate. The most important driving mechanism, however, was the uplift of the Andes. The immediate predecessor of the wetland was the Oligocene to Early Miocene fluvial system that originated in the Central Andes and the craton and flowed towards the Caribbean. The transition from a fluvially dominated environment towards lacustrine-dominated conditions occurred during the Early to early Middle Miocene (Psiladiporites-Crototricolpites pollen zone), and by the Middle Miocene (Crassoretitriletes pollen zone) the megawetland reached its maximum extent. This predominantly freshwater lacustrine system had sediment input from the Andes and marginal marine influence from the Caribbean. At the same time the Amazonian Craton remained an additional source of sediment supply, and minor fluvial systems contributed to the wetland system. One of the most outstanding features of this depositional setting was the highly diverse aquatic vertebrate and invertebrate fauna. Although the vertebrate fauna remained diverse throughout the Late Miocene, the endemic invertebrates virtually disappeared and have remained comparatively poor ever since. The termination of this fauna during the early Late Miocene correlates with the establishment of the modern Amazon River. Andean uplift dramatically increased during the Late Miocene and the sedimentary wedge progressed to the east, beyond the sub-Andean and Amazonian sedimentary basins. As a result the

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wetland disappeared and, after connecting with fluvial systems of eastern Amazonia, the Andean fluvial systems merged into the Amazon transcontinental river system. Around 6 Ma further uplift in the Andes caused relocation of the depositional centre towards the east and abandonment of the continental basins. From then on the bulk of sediment deposition took place in the Foz do Amazonas Basin (Equatorial Atlantic Margin). The Miocene Amazonian wetland system was unique not only because of its extent and species diversity but also because it was concurrent with major changes in northern South American geography. After the disappearance of this system Amazonia suffered a decline in species diversity as result of the geographical changes and climatic cooling, the latter the possible cause for a decrease in diversity of the reptile fauna. The nature of this complex inland environment, where freshwater, marine and tidal conditions coexisted under the influence of a warm, humid climate, as well as its transition to the present, still poses many questions that remain unanswered. Ultimately, a deeper understanding of this system and its evolution up to the present might help us to understand the evolution of Amazonian biodiversity.

Acknowledgements We kindly thank Mathias Harzhauser, John Lundberg and Hubert Vonhof for their comments, which helped us to improve the manuscript. Jan van Arkel (University of Amsterdam) is thanked for the production of the palaeogeographic maps.

References Albert, J.S., Lovejoy, N.R., Crampton, W.G.R. (2006). Miocene tectonism and the separation of cis- and trans-Andean river basins: Evidence from Neotropical fishes. J S Am Earth Sci 21, 14–27. Antoine, P.-O., De Franceschi, D., Flynn, J.J., Nel, A., Baby, P., Benammi, M., Calderón, Y. et al. (2006) Amber from western Amazonia reveals Neotropical diversity during the middle Miocene. PNAS 103, 13595–13600. Antoine, P.-O., Salas-Gismondi, R., Baby, P., Benammi, M., Brusset, S., de Franceschi, D. et al. (2007) The Middle Miocene (Laventan) Fitzcarrald fauna, Amazonian Peru. In: Díaz-Martínez, E., Rábano, I. (eds) Fourth European Meeting on the Palaeontology and Stratigraphy of Latin America, Instituto Geológico y Minero de España, Madrid. Cuadernos del Museo Geominero 8, pp. 19–24. Bayona, G., Cortés, M., Jaramillo, C., Ojeda, G., Aristizabal, J., Reyes, A. (2007a) La Deformación Pre-neógena de la Cordillera Oriental definida por depósitos de antepaís. Boletín de Geología 29, 51–59. Bayona, G., Jaramillo, C., Rueda, M., Reyes-Harker, A., Torres, V. (2007b) Paleocene-Middle Miocene flexural-margin migration of the nonmarine Llanos foreland basin of Colombia. Ciencia, Tecnología y Futuro 3, 141–160. Bermúdez-Cella, M., Van der Beek, P., Bernet, M. (2008) Spatial and temporal patterns of exhumation across the Venezuelan Andes from in-situ and detrital apatite fission-track analysis: Implications for Cenozoic tectonics. In: Garver, J.I., Montario, M.J. (eds) Proceedings from the 11th International Conference on Thermochronometry, Anchorage, Alaska, pp. 25–27.

10/24/2009 11:16:16 Shobha

140

C. Hoorn et al.

Bristow, C.R., Parodiz, J.J. (1982) The stratigraphical paleontology of the Tertiary non-marine sediments of Ecuador. Bull Carnegie Mus Nat Hist 19, 1–53. Burgos, J.D.Z. (2006) Genese et progradation d’un cone alluvial au front d’une chaine active: example des Andes Equatoriennes au Neogene. PhD thesis, Université Paul Sabatier, Toulouse, 373 pp. Burgos, J.D.Z., Christophoul, F., Baby, P., Antoine, P.-O., Soula, J.-C., Good, D., Rivadeneira, M. (2005) Dynamic evolution of Oligocene–Neogene sedimentary series in a retro-foreland basin setting: Oriente Basin, Ecuador. In: 6th International Symposium on Andean Geodynamics (ISAG 2005, Barcelona), extended abstracts, pp. 127–130. Campbell, K.E., Heizler, M., Frailey, C.D., Romero-Pittman, L., Prothero, D.R. (2001) Upper Cenozoic chronostratigraphy of the southwestern Amazon Basin. Geology 29, 595–98. Campbell, K.E., Frailey, C.D., Romero-Pittman, L. (2006) The PanAmazonian Ucayali Peneplain, late Neogene sedimentation in Amazonia, and the birth of the modern Amazon River system. Palaeogeogr Palaeocl 239, 166–219. Caputo, M.V., Rodrigues, R., Vasconcelos, D.N.N. (1971) Litoestratigrafia da Bacia do Amazonas. Relatorio Interno. Belem: Petrobras, 92 pp. Caputo, M.V., Rodrigues, R.,Vasconcelos, D.N.N. (1972) Nomenclatura estratigrafica da Bacia do Rio Amazonas. In: Congresso Brasileiro de Geologia 26, Belem. Anais 3, 35–46. Cooper, M.A., Addison, F.T., Alvarez, R., Coral, M., Graham, R.H., Hayward, A.B. et al. (1995) Basin development and tectonic history of the Llanos Basin, Eastern Cordillera, and Middle Magdalena Valley, Colombia. Am Assn Petrol Geol Bull 79, 1421–1443. Cozzuol, M.A. (2006) The Acre vertebrate fauna: Age, diversity, and geography. J S Am Earth Sci 21, 185–203. Diaz de Gamero, M.I. (1996) The changing course of the Orinoco River during the Neogene: a review. Palaeogeogr Palaeocl 123, 385–402. Dobson D.M., Dickens G.R., Rea D.K. (2001) Terrigenous sediment on Ceara Rise: a Cenozoic record of South American orogeny and erosion. Palaeogeogr Palaeocl 165, 215–229. Eiras, J.F., Wanderley, J.R. (2006) Bacias sedimentares brasileiras, Bacia do Solimões. Fundaçao Paleontologica Phoenix: Phoenix 8, 1–6. Eiras, J.F., Becker, C.R., Souza, E.M., Gonzaga, F.G., da Silva, J.G.F., Daniel, L.M.F. et al. (1994) Bacia do Solimões. Boletim Geociencias Petrobras, Rio de Janeiro 8, 17–45. Espurt, N., Baby, P., Brusset, S., Roddaz, M., Hermoza, W., Regard, V. et al. (2007) How does the Nazca Ridge subduction influence the modern Amazonian foreland basin? Geology 35, 515–518. Figueiredo, J., Hoorn, C., van der Ven, P., Soares, E. (2009) Late Miocene onset of the Amazon River and the Amazon deep-sea fan: Evidence from the Foz do Amazonas Basin. Geology 37, 619–622. Frailey, C.D. (1986) Late Miocene and Holocene mammals, exclusive of the Notoungulata, of the Rio Acre Region, Western Amazonia. Contributions in Science, Natural History Museum of Los Angeles County 374, 1–46. Gabb, W.M. (1869) Descriptions of fossils from the clay deposits of the Upper Amazon. Am J Conchology 4, 197–200. Garzione, C.N., Hoke, G.D., Libarkin, J.C., Withers, S., MacFadden, B., Eiler, J. et al. (2008) Rise of the Andes. Science 320, 1304–1307. Gingras, M.K., Räsänen, M.E., Pemberton, S.G., Romero, L.P. (2002a) Ichnology and sedimentology reveal depositional characteristics of bay margin parasequences in the Miocene Amazonian foreland basin. J Sediment Res 72, 871–883. Gingras, M.K., Räsänen, M.E., Ranzi, A. (2002b) The significance of bioturbated inclined heterolithic stratification in the southern part

Hoorn_ch08_Final.indd 140

of the Miocene Solimões Formation, Rio Acre: Amazonia Brazil. Palaios 17, 591–601. Gomez, A., Jaramillo, C.A., Parra, M., Mora, A. (2009) Huesser horizon: a lake and a marine incursion in northwestern South America during the Early Miocene. Palaios 24, 199–210. Gonçalves, F.T.T., Mora, C.A., Cordoba, F., Kairuz, E.C., Giraldo, B.N. (2002) Petroleum generation and migration in the Putumayo Basin, Colombia: insights from an organic geochemistry and basin modeling study in the foothills, 2002. Mar Petrol Geol 19, 711–725. Gregory-Wodzicki, K.M. (2000) Uplift history of the Central and Northern Andes: A review. GSA Bulletin 112, 1091–1105. Guerrero, J. (1997) Stratigraphy, sedimentary environments and the Miocene uplift of the Colombian Andes. In: Kay, R.F., Madden, R.H., Cifelli, R.L., Flynn, J.J. (eds) Vertebrate Paleontology in the Neotropics: The Miocene Fauna of La Venta, Colombia. Washington DC: Smithsonian Institution Press, pp. 15–43. Guerrero, J. (2002a) A proposal on the classification of systems tracts: application to the allostratigraphy and sequence stratigraphy of the Cretaceous Colombian Basin. Part 1: Berriasian to Hauterivian. Geología Colombiana 27, 3–25. Guerrero, J. (2002b) A proposal on the classification of systems tracts: application to the allostratigraphy and sequence stratigraphy of the Cretaceous Colombian Basin. Part 2: Barremian to Maastrichtian. Geología Colombiana 27, 27–49. Guerrero, J., Sarmiento, G. (1996) Estratigrafía física, palinológica, sedimentológica y secuencial del Cretácico Superior y Paleoceno del Piedemonte Llanero. Implicaciones en exploración petrolera. Geología Colombiana 20, 3–66. Guerrero, J., Sarmiento, G., Navarrete, R. (2000) The stratigraphy of the W side of the Cretaceous Colombian Basin in the Upper Magdalena Valley. Reevaluation of selected areas and type localities including Aipe, Guaduas, Ortega, and Piedras. Geología Colombiana 25, 45–110. Haq, B.H., Hardenbol, J., Vail, P.R. (1987) Chronology of fluctuating sea-levels since the Triassic. Science 235, 1156–1166. Harris, S.E., Mix, A.C. (2002) Climate and tectonic influences on continental erosion of tropical South America, 0–13 Ma. Geology 30, 447–450. Hermoza, W. (2004) Dynamique tectono-sédimentaire et restauration séquentielle du retro-bassin d’avant-pays des Andes centrals. PhD thesis, University of Paul Sabatier de Toulouse III, 296 pp. Hermoza, W., Brusset, S., Baby, P., Gil, W., Roddaz, M., Guerrero, N., Bolaños, R. (2005) The Huallaga foreland basin evolution: thrust propagation in deltaic environment, in the northern Peruvian Andes. J S Am Earth Sci 19, 21–34. Hoorn, C. (1993a) Marine incursions and the influence of Andean tectonics on the Miocene depositional history of northwestern Amazonia: Results of a palynostratigraphic study: Palaeogeogr Palaeocl 105, 267–309. Hoorn, C. (1993b) Geología del Nororiente de la Amazonia Peruana: la Formacion Pebas. In: Kalliola, R., Puhakka, M., Danjoy, W. (eds) Amazonia Peruana – Vegetación Húmeda Tropical en el Llano Subandino. Jyväskylä, Finland: Proyecto Amazonia Universidad de Turku (PAUT) and Oficina Nacional de Evaluación de Recursos Naturales (ONERN) pp. 69–85. Hoorn, C. (1994a) Fluvial palaeoenvironments in the intracratonic Amazonas Basin (Early Miocene-early Middle Miocene, Colombia). Palaeogeogr Palaeocl 109, 1–54. Hoorn, C. (1994b) An environmental reconstruction of the palaeoAmazon River sytem (Middle to Late Miocene, NW Amazonia). Palaeogeogr Palaeocl 112, 187–238. Hoorn, C. (2006) Mangrove forests and marine incursions in Neogene Amazonia (Lower Apaporis River, Colombia). Palaios 21, 197–209.

10/24/2009 11:16:16 Shobha

Development of the Amazonian mega-wetland Hoorn, C., Guerrero, J., Sarmiento, G.A., Lorente, M.A. (1995) Andean tectonics as a cause for changing drainage patterns in Miocene northern South America. Geology 23, 237–240. Hoorn, C., Aalto, R., Kaandorp, R.J.G., Lovejoy, N.R. (2006) Miocene semidiurnal tidal rhythmites in Madre de Dios, Peru: Comment. GSA, Forum. Geology; doi: 10.1130/G22115.1. Horton, B.K., DeCelles, D.G. (1997) The modern foreland basin system adjacent to the Central Andes. Geology 25, 895–898. Hovikoski, J. (2006) Miocene Western Amazonia in the light of sedimentological and ichnological data. PhD thesis, Annales Universitatis Turkuensis, Ser. AII, University of Turku, 138 pp. Hovikoski, J., Räsänen, M.E., Gingras, M., Roddaz, M.,Brusset, S., Hermoza, W. et al. (2005) Miocene semidiurnal tidal rhythmites in Madre de Dios, Peru. Geology 33, 177–180. Hovikoski, J., Gingras, M., Räsänen, M., Rebata, L., Guerrero, J., Ranzi, A. et al. (2007) The nature of Miocene Amazonian epicontinental embayment: High-frequency shifts of the low-gradient coastline. GSA Bulletin 119, 1506–1520. Hovikoski, J., Räsänen, M.E., Gingras, M., Ranzi, A., Melo, J. (2008) Tidal and seasonal controls in the formation of late Miocene IHS deposits, western Amazonian Foreland Basin. Sedimentology 55, 499–530. Ingeominas (2007) Atlas Geológico de Colombia. URL: http://www. ingeominas.gov.co/ Katzer, F. (1903) Grundzuge der Geologie des unteren Amazonasgebietes (des Staates Para in Brasilien). Leipzig: Max Weg, 298 pp. Kay, R.F., Madden, R.H., Cifelli, R.L., Flynn, J.J. (eds) (1997) Vertebrate Paleontology in the Neotropics: The Miocene Fauna of La Venta, Colombia. Smithsonian Institution Press, 592 pp. Kaandorp, R.J.G., Vonhof, H.B., Wesselingh, F.P., Romero-Pittman, L., Kroon, D., van Hinte, J.E. (2005) Seasonal Amazonian rainfall variation in the Miocene Climate Optimum. Palaeogeogr Palaeocl 221, 1–6. Kronberg, B.I., Franco, J.R., Benchimol, R.E., Hazenberg, G., Doherty, W., VanderVoet, A. (1989) Geochemical variations in Solimões Formation sediments (Acre Basin, western Amazonia). Acta Amazonica 19, 319–333. Kummel, B. (1948) Geological reconnaissance of the Contamana Region, Peru. Geol Soc Am Bull 69, 1217–1266. Latrubesse, E.M., Bocquentin, J., Santos, J.C.R., Ramonell, C.G. (1997) Paleoenvironmental model for the late Cenozoic of Southwestern Amazonia: paleontology and geology. Acta Amazonia 27, 103–118. Latrubesse, E., da Silva, S., Cozzuol, M., Absy, M. (2007) Late Miocene continental sedimentation in southwestern Amazonia and its regional significance: Biotic and geological evidence. J S Am Earth Sci 23, 61–80. Lovejoy, N.R., Bermingham, E., Martin, A.P. (1998) Marine incursion into South America. Nature 396, 421–422. Lovejoy, N.R., Albert, J.S., Crampton, W.G.R. (2006) Miocene marine incursions and marine/freshwater transitions: evidence from Neotropical fishes. J S Am Earth Sci 21, 5–13. Lundberg, J., Machado-Allison, A., Kay, R.F. (1986) Miocene characid fishes from Colombia: Evolutionary stasis and extirpations. Science 234, 208–209. Lundberg, J.G., Marshall, J.G., Guerrero, J., Horton, B., Malabarba, M.C.S.L., Wesselingh, F. (1998) The stage for neotropical fish diversification: a history of tropical South American rivers. In: Malabarba, L.R., Reis, R.E., Vari, R.P., Lucena, Z.M.S., Lucena, C.A.S. (eds) Phylogeny and Classification of Neotropical Fishes. Porto Alegre, Brazil: EDIPUCRS, pp. 13–48. Maia, R., Godoy, H., Yamaguti, H., de Moura, P., da Costa, F., de Holanda, M., Costa, J. (1977) Projecto de Carvao no Alto Solimões. CPRM-DNPM, 137 pp.

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Martin-Gombojav, N., Winkler, W. (2008) Multiple recycling of Proterozoic crust in the Andean Amazon foreland of Ecuador: Implications for orogenic development of the Northern Andes. Terra Nova 20, 22–31. Mathalone, J.M.P., Montoya, M. (1995) Petroleum geology of the subandean basins of Peru. In: Tankard, A.J., Suárez-Soruco, R., Welsink, H.J. (eds) Petroleum Basins of South America. Memoir 62, American Association of Petroleum Geologists, pp. 423–444. Miller, K.G., Kominz, M.A., Browning, J.V., Wright, J.D., Mountain, G.S., Katz, M.E. et al. (2005) The Phanerozoic record of global sealevel change. Science 310, 1293–1298. Mitsch, W.J., Gosselink, J.G. (2000) Wetlands, 4th edn. New York: John Wiley & Sons, 600 pp. Monsch, K.A. (1998) Miocene fish faunas from the northwestern Amazonia basin (Colombia, Peru, Brazil) with evidence of marine incursions. Paleogeogr Palaeocl 143, 31–50. Mora, A. (2007) Inversion tectonics and exhumation processes in the Eastern Cordillera of Colombia. Dissertation. Institut für Geowissenschaften, Universität Potsdam, Germany, 177 pp. Mora, A., Parra, M., Strecker, M.R., Kammer, A., Dimaté, C., Rodríguez, F. (2006) Cenozoic contractional reactivation of Mesozoic extensional structures in the Eastern Cordillera of Colombia. Tectonics 25, TC2010. doi: 10.1029/2005TC001854. Mora, A., Parra, M., Strecker, M.R., Sobel, E.R., Hooghiemstra, H., Torres, V., Vallejo-Jaramillo, J. (2008) Climatic forcing of asymmetric orogenic evolution in the Eastern Cordillera of Colombia. Bull Geol Soc Am 120, 930–949. Moraes Rego, L.F. (1930) Notas sobre a Geología do territorio de Acre e da bacía do Javarí. Manaus: Cezar, Cavalcante and Cia, 45 pp. Muñoz-Torres, F.A., Whatley, R.C., Van Harten, D. (2006) Miocene ostracod (Crustacea) biostratigraphy of the Upper Amazon Basin and the Cyprideis genus evolution. J S Am Earth Sci 21, 75–86. Nuttall, C.P. (1990) A review of the Tertiary non-marine molluscan faunas of the Pebasian and other inland basins of north-western South America. Bull Brit Mus Nat Hist (Geol) 45, 165–371. Oppenheim, V. (1946) Geological reconnaissance in southeastern Peru. Geol Soc Am Bull 30, 254–264. Pardo, A.A., Zuñiga, F. (1976) Estratigrafía y evolución tectónica de la región de la selva del Perú. (l) Paleozoico y (ll) Mesozoico y Cenozoico. Memorias Segundo Congreso Latinoamericano de Geología. Bol Geol., Caracas, Publ. Esp. 7(2), 588–608. Parra, M., Mora, A., Jaramillo, C., Strecker, M.R., Sobel, E.R., Quiroz, L. et al. (2009) Orogenic wedge advance in the northern Andes: Evidence from the Oligocene-Miocene sedimentary record of the Medina Basin, Eastern Cordillera, Colombia. GSA Bulletin 121, 780–800. Pasley, M.A., Shepherd, D.B., Pocknall, D.T., Boyd, K.P., Vander Andrade, Figueiredo, J.P. (2005) Sequence stratigraphy and basin evolution of the Foz do Amazonas Basin, Brazil. Search and Discovery Article #10082. URL: http://www.searchanddiscovery. net/documents/2005/pasley/images/pasley.pdf. Petri, S., Fulfaro, V.J. (1983) Geologia do Brasil. São Paulo: Editora da Universidade de São Paulo, 631 pp. Pilsbry, H.A. (1944) Molluscan fossils from the Rio Pachitea and vicinity in eastern Peru. Proc Acad Nat Sci, Philadelphia, 96, 137–153. Pilsbry, H.A., Olsson, A.A. (1935) Tertiary fresh-water mollusks of the Magdalena embayment, Colombia. Proc Acad Nat Sci, Philadelphia, 87, 7–39 Potter, P.E. (1997) The Mesozoic and Cenozoic paleodrainage of South America: A natural history. J S Am Earth Sci 10, 331–344. Ramsar, Iran (1971) The Ramsar Convention Manual. A Guide to the Convention on Wetlands, 4th edn, 114 pp. URL: http://www.ramsar. org/lib/lib_manual2006e.pdf.

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Räsänen, M.E., Linna, A.M., Santos, J.C.R., Negri, F.R. (1995) Late Miocene tidal deposits in the Amazonian foreland basin. Science 269, 386–390. Räsänen, M., Linna, A., Irion, G., Rebata Hernani, L., Vargas Huaman, R., Wesselingh, F. (1998) Geología y geoformas de la zona de Iquitos. In: Geoecología y desarollo Amazónico: estudio integrado en la zona de Iquito, Peru. Annales Turkuensis (AII) 114, 59–137. Rebata, L., Gingras, M., Räsänen, M., Barberi, M. (2006) Tidalchannel deposits on a delta plain from the Upper Miocene Nauta formation, Marañon Foreland Sub-basin, Peru. Sedimentology 53, 971–1013. Roddaz, M. (2004) Transition des stades alimentés à suralimentés dans les systèmes de rétro-bassin d’avant-pays: exemple du bassin amazonien. PhD thesis, Université de Toulouse, Toulouse, 332 pp. Roddaz, M., Baby, P., Brusset, S., Hermoza, W., Darrozes, J.M. (2005) Forebulge dynamics and environmental control in Western Amazonia: The case study of the Arch of Iquitos (Peru). Tectonophysics 399, 87–108. Roddaz, M., Viers, J., Brusset, S., Baby, P., Boucayrand, C., Herail, G. (2006a) Controls on weathering and provenance in the Amazonian foreland basin: Insights from major and trace element geochemistry of Neogene Amazonian sediments. Chem Geol 226, 31–65. Roddaz M., Brusset, S., Baby, P., Herail, G. (2006b) Miocene tidalinfluenced sedimentation to continental Pliocene sedimentation in the forebulge-backbulge depozones of the Beni-Mamore foreland Basin (northern Bolivia). J S Am Earth Sci 20, 351–368. Rossetti, D.F., Góes, A.M. (2004) O Neógeno da Amazônia Oriental. Belém, Pará: Museu Paraense Emílio Goeldi, Coleção Friedrich Katzer, 222 pp. Ruegg, W., Rosenzweig, A. (1949) Contribución a la geología de las formaciones modernas de Iquitos y la Amazonia superior. Volumen Jubilar de la Sociedad Geologica del Perú (II)3, 1–24. Ruiz, G.M.H., Seward, D., Winkler, W. (2007) Evolution of the Amazon Basin in Ecuador with special reference to hinterland tectonics: Data from zircon fission-track and heavy mineral analysis. Dev Sedimentol 58, 907–934. Salas-Gismondi, R., Antoine, P.-O., Baby, P., Brusset, S., Benammi, M., Espurt, N. et al. (2007) Middle Miocene crocodiles from the Fitzcarrald Arch, Amazonian Peru. In: Díaz-Martínez, E., Rábano, E.I. (eds) 4th European Meeting on the Palaeontology and Stratigraphy of Latin America, Cuadernos del Museo Geominero 8. Madrid: Instituto Geológico y Minero de España, pp. 355–360. Steinmann, M., Hungerbühler, D., Seward, D., Winkler, W. (1999) Neogene tectonic evolution and exhumation of the southern Ecuadorian Andes: a combined stratigraphy and fission-track approach: Tectonophysics, 307, 255–279. Thomas, R.G., Smith, D.G., Wood, J.M., Visser, J., Calverly-Range, E.A., Koster, E.H. (1987) Inclined heterolithic stratification; terminology, description, interpretation and significance. Sediment Geol 53, 123–179. Uba, C.E., Strecker, M.R., Schmitt, A.K. (2007) Increased sediment accumulation rates and climatic forcing in the central Andes during the late Miocene. Geology 35, 979–982. Van der Hammen, T., Werner, J. H., van Dommelen, H. (1973) Palynological record of the upheaval of the Northern Andes: A study of the Pliocene and lower Quaternary of the Colombian

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Eastern Cordillera and the early evolution of its high-Andean biota. Rev Palaeobot Palynol 16, 1–42. Vermeij, G.J., Wesselingh, F.P. (2002) Neogastropod molluscs from the Miocene of Western Amazonia, with comments on marine to freshwater transitions in molluscs. J Paleontol 76, 265–270. Villamil, T. (1999) Campanian-Miocene tectonostratigraphy, depocenter evolution and basin development of Colombia and western Venezuela. Palaeogeogr Palaeocl 153, 239–275. Vonhof, H.B., Wesselingh, F.P., Ganssen, G.M. (1998) Reconstruction of the Miocene western Amazonian aquatic system using molluscan isotopic signatures. Palaeogeogr Palaeocl 141, 85–93. Vonhof, H.B., Wesselingh, F.P., Kaandorp R.J.G., Davies, G.R., van Hinte, J.E., Guerrero, J. et al. (2003) Paleogeography of Miocene Western Amazonia: isotopic composition of molluscan shells constrains the influence of marine incursions. Geol Soc Am Bull 115, 983–993. Wanderley Filho, J.R., Gonçalves de Melo, J.H., Medina da Fonseca, V.M., da Costa Machado, D.M. (2005) Bacias sedimentarias, Bacia do Amazonas. Fundaçao Paleontologica Phoenix. Phoenix 7, 1–6. Wesselingh, F.P. (2006) Miocene long-lived lake Pebas as a stage of mollusc radiations, with implications for landscape evolution in western Amazonia. Scripta Geologica 133, 458 pp. Wesselingh, F.P., Macsotay, O. (2006) Pachydon hettneri (Anderson, 1928) as indicator for Caribbean-Amazonian lowland connections during the Early-Middle Miocene. J S Am Earth Sci 21, 49–53. Wesselingh, F.P., Salo, J.A. (2006) A Miocene perspective on the evolution of the Amazonian biota. Scripta Geologica 133, 439–458. Wesselingh, F.P., Räsänen, M.E., Irion, G., Vonhof, H.B., Kaandorp, R., Renema, W. et al. (2002) Lake Pebas: a palaeoecological reconstruction of a Miocene, long-lived lake complex in western Amazonia. Cainozoic Res 1, 35–81. Wesselingh, F.P., Guerrero, J., Räsänen, M., Romero Pitmann, L., Vonhof, H. (2006a) Landscape evolution and depositional processes in the Miocene Amazonian Pebas lake/wetland system: evidence from exploratory boreholes in northeastern Peru. Scripta Geologica 133, 323–361. Wesselingh, F.P., Kaandorp, R.J.G., Vonhof, H.B., Räsänen, M.E., Renema, W., Gingras, M. (2006b) The nature of aquatic landscapes in the Miocene of western Amazonia: an integrated palaeontological and geochemical approach. Scripta Geologica 133, 363–393. Wesselingh, F.P., Hoorn, M.C., Guerrero, J., Räsänen, M.E., Romero Pittmann L., Salo, J. (2006c) The stratigraphy and regional structure of Miocene deposits in western Amazonia (Peru, Colombia and Brazil), with implications for Late Neogene landscape evolution. Scripta Geologica 133, 291–322. Wesselingh, F.P., Kaandorp, R.J.G., Vonhof, H.B., Räsänen, M.E., Renema, W. (2006d) The nature of aquatic landscapes in the Miocene of western Amazonia: an integrated palaeontological and geochemical approach. Scripta Geologica 133, 363–393. Wesselingh, F.P., Ranzi, A., Räsänen, M.E. (2006e) Miocene freshwater Mollusca from western Brazilian Amazonia. Scripta Geologica 133, 419–437. Westaway, R. (2006) Late Cenozoic sedimentary sequences in Acre state, southwestern Amazonia: Fluvial or tidal? Deductions from the IGCP 449 fieldtrip. J S Am Earth Sci 21, 120–134.

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Marine influence in Amazonia: evidence from the geological record Jussi Hovikoski1, Frank P. Wesselingh2, Matti Räsänen3, Murray Gingras4 and Hubert B. Vonhof 5 1

Geological Survey of Denmark and Greenland (GEUS), Copenhagen, Denmark Naturalis, Leiden, The Netherlands 3 University of Turku, Finland 4 University of Alberta, Edmonton, Canada 5 Vrije Universiteit Amsterdam, The Netherlands 2

Abstract Marine influence in Amazonia during the Miocene is a controversial issue, one about which nearly opposite theories (continental vs marine) have been proposed. Increasing multidisciplinary palaeoenvironmental data sets from various Miocene stratigraphic levels and parts of Amazonia have revealed depositional complexities triggered by repeated, high-frequency base-level changes and a very low depositional gradient. As a result, Early-Middle Miocene strata (Pebas phase) are organized into recurring 3–10 m-thick transgressive-regressive bay-margin successions. An array of evidence indicates some sort of marine influence in these deposits. This evidence includes mangrove pollen, foraminifers, rare brackish-water mollusc species, barnacles, common brackish-water trace fossil assemblages and tidal sediments. The ongoing debate now focuses on the degree of saltwater influence in the Pebas phase strata. The Late Miocene (Acre phase) stratigraphic record comprises mainly channelized deposits, and fossil faunas are dominated by terrestrial and freshwater vertebrates. The evidence for marginal marine influence in these strata is mainly confined to inclined heterolithic stratification (IHS)-bearing channels and includes tidal sediments, restricted brackish-water ichnofossil assemblages, and locally teeth of euryhaline sharks and mangrove pollen (Nauta and Madre de Dios Formations). In particular, the tidal signature in these deposits is well developed, pointing to local development of deltaic/estuarine settings. There are also suggestions of marine influence on other stratigraphic intervals throughout Amazonia, particularly in Pliocene and Quaternary strata. The allegedly marine conditions have been proposed based on purported global sea-level highstands, the low altitude of the Amazon region and supposed coastal geomorphological features such as deltas. We conclude that these suggestions are misconceptions and that geological data to support such marine settings during the Late Neogene are lacking.

Introduction Marine settings have been postulated to occur in western Amazonia in several stratigraphic intervals and in many localities (Fig. 9.1). For some periods the evidence is scant (e.g. herringbone structures in the Oligocene Chambira Formation of Peru: Hermoza et al. 2005) or ambiguous (Pliocene and Quaternary terrace deposits, alleged Pliocene deltas: Klammer 1984; Nores 2004; Campbell et al. 2006). Even in Miocene strata, where clear Amazonia, Landscape and Species Evolution: A Look into the Past, 1st edition. Edited by C. Hoorn and F.P. Wesselingh. © 2010 Blackwell Publishing

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evidence for marine influence is present, the nature and extent of the ingressions is still debated. In the past nearly opposite theories (freshwater versus marine) have been proposed regarding the aquatic ecosystems of Miocene Amazonia (Räsänen et al. 1995 and Webb 1995; vs Hoorn 1996; Marshall & Lundberg 1996; Paxton & Crampton 1996). Although controversies still exist, several of the more recently proposed depositional models are sharing more common ground, and some extreme scenarios can be ruled out. For instance, the Miocene depositional system was not a deep, long-lived seaway with fully marine salinities, nor a pure Pantanal-like freshwater lake-swamp complex. Increasing multidisciplinary data sets from various Miocene stratigraphic intervals in different parts of Amazonia have revealed depositional

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Fig. 9.1 A map of study locations. 1–5: Indiana outcrop group (Loreto, Peru); 6: Santa Teresa (Loreto, Peru); 7: Tamshiyacu (Loreto, Peru); 8: San Rafael; 9: Porvenir (Loreto, Peru); 10: Ave Maria (Loreto, Peru); 11: Santa Julia (Loreto, Peru); 12: Santa Rosa de Pichana (Loreto, Peru); 13,14: Rio Yavarí (Amazonas, Brazil); 15: Buenos Aires (Amazonas, Colombia); 16: Los Chorros (Amazonas, Colombia); 17: Purus (Amazonas, Brazil); 18: Seringal Amapa (Acre, Brazil); 19: Community Katukina (Acre, Brazil); 20: Tarauacá (Acre, Brazil); 21: Volta Grande (Acre, Brazil); 22: Rio BrancoBolivia Hwy (Acre, Brazil); 23: Boca de San Pedro (Acre, Brazil); 24,25: Cocha Cashu (Madre de Dios, Peru); 26: Pastora (Madre de Dios, Peru); 27: Rio de las Piedras (Madre de Dios, Peru). C do Sul, Cruzeiro do Sul.

complexities such as repeatedly generated shallow but widespread accommodation space, and seasonal and tidal sedimentation. Typically, the marine and non-marine palaeoenvironmental indicators are closely intercalated or occur as mixed, reworked assemblages in the sedimentary record (Gingras et al. 2002a; Wesselingh et al. 2006; Hovikoski et al. 2007a). Understanding the temporal and spatial nature of marine ingressions is not a trivial issue, since they have potentially been important for the development of modern Amazonian biodiversity. For instance, the present-day Amazonian aquatic taxa are exceptionally rich in groups that are closely related to near-shore marine species (e.g. Lovejoy & Collette 2001; Lovejoy et al. 2006; see also Chapter 25). The aim of this chapter is to summarize the main palaeoenvironmental characteristics of the Miocene incursion levels, discuss their temporal and spatial occurrence, and how they can be distinguished from the interbedded freshwater deposits.

Methods Below is a brief review of different methods that have been applied to distinguish marine from non-marine signals in the Amazonian geological record.

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Sedimentological data consist of textural and primary sedimentary structures of sedimentary rock. Interpretation of those properties permits reverse modelling of a sedimentary deposit – i.e. the dominant hydraulic conditions at the time of sediment accumulation can be deduced. In addition, numerical analysis (e.g. fast Fourier transform) can be applied to the thickness data of heterolithic deposits to extract the possible cycles (tidal, seasonal) in depositional processes (Hovikoski et al. 2005, 2007b, 2008). In this context, the distinction of ocean-driven tidal from seasonal or random fluvial processes is one of the key issues, thus their signature in the geological record is briefly addressed below. A tide is a cyclic fluctuation in water level generated by geophysical forces, primarily by the tractive gravitational forces of the Moon and to a lesser extent of the Sun (Dalrymple 1992). The most important tidal cycles observable in the rock record are flood–ebb (current period of 6–12 h), semi-diurnal (12.42 h), diurnal (24.8 h), tropical half-month (13.6 days), syzygy (14.76 days) and anomalistic month (27.5 days) (e.g. Visser 1980; Tessier & Gigot 1989; De Boer et al. 1989; Nio & Yang 1991; Choi & Park 2000). Such cycles can be preserved in the sedimentary record either as cyclic thickness variation in heterolithic (e.g. mud-sand couplets) cross-strata or in vertically accreting, small-scale heterolithic sediment series (‘tidal rhythmite’) (Fig. 9.2). In addition to cyclic variation in flow-power, tides are also characterized by local current reversals. Finally, the unsteady flow characteristic of tides tends to generate sigmoidal ripples, various types of heterolithic bedding, inclined heterolithic stratification (IHS of Thomas et al. 1987) and regularly occurring reactivation surfaces in cross-stratified media; these features are typical though not diagnostic for tidal facies. Therefore, decisive recognition of tides typically requires the presence of several of these sedimentological attributes. Tides can occur in any large aquatic basins including lakes. Internal tides in large lakes (e.g. the Great Lakes), however, have a minimal tidal range (up to centimetre scale), and are typically masked by other factors causing water level fluctuations such as winds and barometric pressure changes. Moreover, an equatorial lake would most likely bear diurnal tides. Therefore, the widespread tidal facies and semi-diurnal tidal rhythmites in Miocene western Amazonia have been interpreted to indicate a hydrographic connection to an oceanic basin (e.g. Hovikoski 2006).

Ichnological indications for marine influence Ichnofossils (or trace fossils) are biogenic sedimentary structures that include fossilized burrows, tracks and trails, mostly produced by invertebrates. Trace fossils are palaeontological features that were emplaced in the sediment as a biological response to the prevailing physico-chemical conditions (e.g. salinity, oxygenation and sedimentation rates) during and after deposition. Based on the morphology of the trace fossils, they can be divided into ichnogenera or ichnospecies. Ichnofossils commonly occur as specific ethologically related groupings that are called ichnofacies

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A month Apogean Spring tide

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Fig. 9.2 Ideal superimposition of tidal cycles as they appear in cross-strata in semi-diurnal, coastal depositional settings. Scenarios for two-element (no deposition during reversing current) and four-element (deposition also during reversing current) daily deposition are shown. White illustrates sand, black-lines on foresets mud.

(e.g. Ekdale et al. 1984). Ichnofacies commonly correlate to particular sedimentary facies and can thus be a powerful tool in reconstructing depositional environments. In this chapter, the focus of the ichnological approach is to recognize and define the nature of the ingression levels. Brackish-water environments, particularly those that undergo tidal influence, are ecologically challenging, imposing multiple stress factors on invertebrates. These stresses may include fluctuating salinity, locally rapid depositional rates, shifting substrates, increased turbidity stress, low substrate consistency (fluid mud), and/or episodic subaerial exposure. Few organisms can tolerate such settings, and consequently brackish-water environments are typified by low-diverse to monospecific assemblages of opportunistic, infaunal marine organisms that are capable of rapid reproduction and colonization (Fig. 9.3). Bioturbation intensity may be rapidly fluctuating due to high mortality rates and subsequent rapid recolonization as suitable depositional conditions return in

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brief colonization windows. Trace fossils in such environments tend to be small due to the dominance of young individuals and osmotic stress. Ethologically, the burrowing organisms are commonly generalists capable of various feeding styles, and may display behavioural adaptations that help to protect them from environmental stresses. Common ichnogenera of such settings include Gyrolithes, Cylindrichnus, Arenicolites, Teichichnus, Thalassinoides, Ophiomorpha, Chondrites, Skolithos and Planolites (e.g. Pemberton & Wightman 1992; MacEachern et al. 2007). Brackish-water trace fossil assemblages are impoverished marine assemblages comprising elements of mixed Skolithos and Cruziana ichnofacies. Finally, in brackish and marine realms, consolidated substrates such as those produced by tidal or wave ravinement (e.g. transgressive surface of erosion), are commonly colonized by a firm ground suite called Glossifungites ichnofacies. These typically comprise monospecific suites of, for example, Thalassinoides or Diplocraterion.

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Mermia/Scoyena ichnofacies Brackish-water species Monospecific to low diversity assemblages comprising mixed Skolithos and Cruziana ichnofacies

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Fig. 9.3 Generalized relationship of species diversity and salinity levels. Expected ichnological signal is also shown. Modified from Pemberton & Wightman 1992; Buatois et al. 1997.

Invertebrate palaeosalinity indicators Palaeontological data have been widely used to recognize marine influences in Pebas phase strata. These data include occurrences of coastal fish taxa, mangrove pollen, foraminifers, barnacles, ostracods and molluscs (Hoorn 1993, 2006; Monsch 1998; Wesselingh et al. 2002; Muñoz-Torres et al. 2006; see also Chapter 18). Mollusc and ostracod species of a few endemic genera dominate the Pebas fauna of western Amazonia. The sheer dominance of endemic species indicates that the Pebas system could not have been an ordinary marginal marine system. The latter systems are as a rule inhabited by non-endemic, widespread species (Wesselingh et al. 2006; see also Chapter 18). Molluscs and ostracods are widely used in palaeoecological reconstruction, including palaeosalinity estimates, through an actualistic approach. This approach is based on the assumption that ecological tolerances of living taxa can be used as a proxy for fossils of the same or similar taxa. This assumption needs testing (Wesselingh 2007). The endemicity rates of the Miocene invertebrates of western Amazonia are so high that a standard actualistic approach does not work throughout (Wesselingh et al. 2002, 2006; Wesselingh 2007), with the exception of so-called incursion faunas in the Pebas system and fluvial faunas of the Pebas and Acre systems (see below).

Isotope geochemistry A range of isotope geochemical methods is available in order to assess palaeosalinities. In the Miocene deposits of Amazonia, much attention has been given to the application of Sr (strontium) isotope geochemistry (Vonhof et al. 1998, 2003), with additional information from stable oxygen and carbon isotopes (Kaandorp et al. 2006; Wesselingh et al. 2006). Strontium isotope signatures of molluscan shells, ostracods and fish teeth can be used to infer relatively precise palaeosalinities in brackish-water environments of low salinities. Strontium isotope geochemical

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techniques have been applied to the abundant and well-preserved bivalves of the Pebas Formation (Vonhof et al. 1998, 2003). The technique is based on the clear isotopic contrast between strontium dissolved in (Miocene) marine waters and that in different Amazonian freshwater sources (Vonhof et al. 2003). Unlike other environmental isotope techniques, 87Sr/86Sr composition as measured in the aragonitic shells is not sensitive to climate, biological or environmental processes but directly records the composition of the water in which the shells lived. As a result 87Sr/86Sr analyses in shells from the Pebas Formation can easily be converted to palaeosalinities, provided the Sr-isotope composition of the freshand marine water end members, and the age of the sediments are known (see Vonhof et al. 2003, for details). In Miocene Western Amazonia, fresh water dominantly derived from Andean catchments, resulting in comparatively low 87Sr/86Sr composition of freshwater shells from the Pebas Formation. Just a minor influx of seawater would drive up shell 87Sr/86Sr values in this system considerably, because seawater has both higher 87Sr/86Sr ratios and significantly higher Sr concentrations than fresh water. Seawater Sr-isotope systematics are very well known for the Miocene, so the Miocene marine end member can be determined rather precisely. Although not all the details of the Sr-isotope systematics of Miocene Andean fresh water are known to us, it is believed that the general modern Sr-isotope systematics can safely be applied to the Middle and Late Miocene situation, because the Andes mountain range was largely in place already in this time interval. By application of a simple binary mixing calculation between these end members, 87Sr/86Sr values of fossil bivalves can be transferred into palaeosalinities. These data are particularly precise for the low salinity range of brackish-water mixtures. From approximately 10–15 practical salinity units (PSU) onwards, the 87Sr/86Sr composition of brackish water can no longer be distinguished from the marine end member.

Other alleged indications of marine influence: eustasy and geomorphology Marine influence in the Amazonian geological past has been suggested through the application of alleged global sea-level curves, such as the Haq curve (Haq et al. 1987; Nores 2004; Campbell et al. 2006). These curves were found to be non-reproducible, the underlying data have not been made available, and the assumptions underlying the whole concept were found to cause random pseudo-correlations that easily fail statistical testing – see Miall (1997) for a serious review of criticism on this matter. Furthermore, specific stratigraphic studies with well-constrained temporal resolution and plausible tectonic assumptions delivered completely different global sea-level curves with, for the Neogene, much smaller magnitudes of sea-level change then implied in the Haq curve (see e.g. Miller et al. 2005). Unfortunately, the Haq curve is still widely and uncritically applied in Amazonian biogeographical and geological literature (Nores 2004; Campbell et al. 2006). Although eustatic curves may predict sea-level highstands in a given time interval, this in itself is no proof of the presence or absence of marine conditions in any area at that time. Marine settings derived from sea-level curves need ‘hard’ geological (sedimentological, geochemical or palaeontological) corroboration.

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Marine influence in Amazonia Other alleged indicators of marine influence in Amazonia that have been proposed in literature are terraces and deltaic morphologies (Klammer 1984; Campbell et al. 2006). Terrace deposits can result from several processes, and in themselves are no proof of a specific depositional environment: additional study of their architecture, sedimentology and palaeontology is required in order to confirm the palaeoenvironment. In case of the Amazon region, numerous terraces in the eastern part (between Manaus and the Amazon River mouth) have been attributed to past high sea levels (Klammer 1984). However, the terraces are made up of deep tropical palaeosols developed within fluvial sediments (Irion 1984), and are therefore not marine in origin. We are very suspicious about the deltaic morphologies reported by Campbell et al. (2006). In their illustrations, the deltaic features seem to correspond to valleys actually draining seasonally flooded terrains. In summary, a number of methods have been used to demonstrate marine influence in the geological history of lowland Amazonian. Sedimentology has demonstrated the presence of semi-diurnal tidal processes and a locally elevated tidal range, and thus a hydrographic connection to a sea. Ichnology, floral and faunal evidence as well as isotope geochemistry have provided more insights into what kind of salinities actually occurred in lowland Amazonia. In contrast, neither the application of eustatic sea-level curves nor geomorphology can be taken as evidence for marine influence in Amazonia.

Geological indications of marine influence in Miocene Amazonia The extent of marine influence in Amazonia during the late Early to early Late Miocene Pebas phase (Pebas Formation in Peru and Colombia and lower part of the Solimões Formation in Brazil) and the Late Miocene Acre phase (upper part of the Solimões Formation) is reviewed below. The stratigraphy and regional extent of these formations are discussed in more detail in Chapters 5 & 8.

Late Early to early Late Miocene (Pebas phase) Deposits of the Early to early Late Miocene (Pebas/Solimões Formation) outcrop widely in erosive riverbanks in northeast Peru and adjacent southeast Colombia and Brazil (see Fig. 9.1). A characteristic lithological succession is presented in Fig. 9.4. Common aspects of Pebas sequences are summarized below and in Fig. 9.5. Depositional architecture of the Pebas Formation Pebas Formation deposits are commonly composed of stacked, 3–8 m-thick, sharp-based, upward-coarsening successions (Räsänen et al. 1998; Gingras et al. 2002a; Wesselingh et al. 2002; Hovikoski et al. 2007a; see Fig. 9.5). The lower contact is laterally extensive and typically possesses Thalassinoides or Psilonichnus. Thalassinoides burrows are smooth-walled, regularly branching, and bear bulbous enlargements at burrow intersections. Such burrow morphologies have been observed only in marginal-marine

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147

to marine deposits, and are attributed primarily to the work of thalassinid shrimps (Rice & Chapman 1971; Frey & Howard 1975; Dworschak 1982, 2000; Gingras et al. 2000; Nesbitt & Campbell 2002). In most examples, the trace fossils are passively filled, descend into organic-rich mud, cross-cut rhizoliths and define a Glossifungites ichnofacies-demarcated discontinuity (Gingras et al. 2002a; Fig. 9.6a,b). This surface is interpreted to represent a transgressive surface of erosion (TSE) (Gingras et al. 2002a; Hovikoski et al. 2007a). The deposits overlying the contact typically consist of 0.5–1 m-thick, upward-fining, massive appearing, blue-coloured sandy mud. Unburrowed, shell-hash-rich intervals also occur. This interval bears a low-to-moderate-diverse, mixed-ethology suite of trace fossils. Observed ichnogenera include Thalassinoides, Ophiomorpha, Arenicolites, Chondrites, Asterosoma, Planolites and Teichichnus. Such assemblages contain elements of a mixed Cruziana-Skolithos ichnofacies that is characteristic of brackishwater settings (e.g. Pemberton & Wightman 1992). Typically, the shell-hash-bearing intervals contain partially reworked mollusc faunas that are dominated by species attributed to the Tryonia and tall Dyris assemblages of Wesselingh et al. (2002). The former is interpreted to represent marginal lacustrine settings, the latter lacustrine shoreface and foreshore settings. Also, (reworked) fluvial taxa (Sheppardiconcha and Diplodon species) can be found in this lower interval. By far the majority of the mollusc fauna in this interval comprises endemic taxa, but strict freshwater taxa are not uncommon. In one section, juvenile spat of Corbula cotuhensis, an alleged brackish-water inhabitant, occurred in low numbers in this interval (Wesselingh et al. 2006). Upwards, the sandy mud facies grades into Planolites-Chondritesbearing, massive to laminated, clayey mud. Bioturbation intensity is evenly distributed in this facies and is typically high (60–100%). Molluscs in this interval are well preserved, and often (partially) in life position. The fauna changes from the highly diverse endemic small Dyris association into the endemic, but less diverse, Pachydon obliquus assemblage (Wesselingh et al. 2006). The former association is interpreted to represent lacustrine bottoms between fair-weather and storm-wave base, the latter dysoxic lake bottoms below storm-wave base. Strict freshwater taxa are very rare in this interval (some in situ Diplodon pairs have been found). In addition, indicators of elevated salinities are lacking. The stable oxygen/ carbon signatures reach their most negative values in these, indicating substantial input of terrestrial dissolved inorganic carbon (DIC) in the system (Kaandorp et al. 2006) and pointing to the freshwater nature of the host water. In contrast, Chondrites is a common trace fossil of brackish (?upper oligohaline) to marine environments, and has not been described with certainty from freshwater settings. The co-occurrence of Planolites and Chondrites is characteristic for dysoxic settings (cf. Martin 2004). The clayey mud gradually coarsens upwards into interbedded sand and mud that is lightly burrowed by a low-diversity suite comprising lined and unlined Skolithos, Arenicolites, Teichichnus, Ophiomorpha, Gyrolithes, Cylindrichnus, Thalassinoides, Planolites and/or fugichnia (see Fig. 9.6c). Large, rhythmically infilled, horizontally oriented Psilonichnus are present in places. Equilibrium structures (Teichichnus) and dense monospecific suites of Arenicolites and lined Skolithos are locally common. Upward-coarsening sediments, signs of wave-reworking (wave

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Interpretation:

Description: FACIES

ICHNOLOGY A

Ar, Gy, Te

10.5 Cy

9.5

Monospecific Ch

Massive mud

Interlayered gradual contact Bioturbated contact Drapes of organic matter

9.0

8.5

Drapes of organic matter

Lignite Roots fu, Sk, Th, Pl, Rh, Cy, Di Flaser bedding

PlanolitesChondrites bearing mud

High (50–75%)/Moderate

FS

Abbreviations: Arenicolites (Ar) Chondrites (Ch) Cylindrichnus (Cy) Diplocraterion (Di)

Ch, Pl

MFS

Gyrolithes (Gy) Ophiomorpha (Op) Palaeophycus (Pa) Planolites (Pl), Psilonichnus (Ps), Rhizocorallium (Rh) Skolithos (Sk) Teichichnus (Te)

Section 6.5 moved 80 m Mollusk Axis direction

Complete (75–100%)/Large

Moderate (25–50%)/Small

Sk

7.5

7.0

Bioturbation degree/ Size of the trace fossils:

Ch

Convolutions

8.0

C = Size of the biggest traces observed

Sparse(5–25%)/Diminutive

MFS

Glossifungites surface

6.0

Clay pebbles Drapes of organic matter

5.5 n = 30

Thalassinoides (Th)

Sk, Ar, Ch, Pl Organic-rich bioturbated mud

Th Tr, Pl, Pa

Trichichnus (Tr)

TSE

fugichnia (fu)

Swamp/ lagoon

10.0

Sequence III

Ripple crossstratified sand

B = Bioturbation degree

Sequence II

Rare Gy

A = Observed genera/ethology

Swamp

11.0 n =10

Ch, Pl, Pa, Ar, Sk, Tr

ICHNOLOGY

shoreface

Ripple foreset 11.5 dip directions

Legend

Bay-margin

Cosets of upwardfining or coarsening sand and organic rich mud

C

Dysoxic bay-offshore

Lignite 12.0

B

ENVIRONMENT and PROCESS

Swamp/ lagoon

12.5

MOLLUSCS

Dysoxic bay-offshore

NOTES

MOLLUSCS

Low-angle crossstratified sand

>20 spp.

Ripple foreset dip directions 4.0

Flaser bedding Soft sedimentary deformation

Pl, Sk, Te, Ch Gy, Ar

3.5 n = 18

n = 11

Pl, Pa, Te

Ca-rich interlayers

3.0

2.5

n = 40

2.0

PlanolitesChondritesbearing laminated mud

1.5

1.0

Small Dyris assemblage Tryonia assemblage

Ch, Pl, Pa

MFS

ENVIRONMENT AND PROCESS Abbreviations: Transgressive surface of erosion (TSE) Maximum flooding surface (MFS) Flooding surface (FS)

Relative Sea Level Change

Th? Progradation Retrogradation

Ch Ar

0.5

0

Pachydon obliquus assemblage

Sk, Ar, Pl

Laminations 3–5 mm thick

n = 25

0–9 spp.

Dysoxic bay-offshore

4.5

Pl, Sk, Ar, fu

Sequence I

Gradual contacts

Seasonally and tidally influenced shoreface

10–19 spp. 5.0

f m c

Ch,Pl, Pa, Cy

cl si sa

Fig. 9.4 Example of a Pebas Formation sedimentary succession, Santa Teresa (Loreto, Peru). c, coarse; cl, clay; f, fine; m, medium; sa, sand; si, silt.

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Marine influence in Amazonia

149 18

Sequence III

Root-bearing mud/sand

3 – 10 m

Sequence II

Sporadic Taenidium, and irregular passively filled burrows Rare recognizable trace fossils

Heterolithic bedding (wave, combined-flow and current ripples) Bioturbated to laminated mud

Sequence I

Bioturbated or shell-lag

Low-diversity to monospecific assemblages containing Planolites, Arenicolites, Skolithos, Gyrolites, Teichichnus, rare Ophiomorpha, and/or Thalassinoides

Planolites-Chondrites suite

Zone of MF

Lignite

TSE

Progradation

Glossifungites ichnofacies-demarcated surface

Mixed Cruziana-Skolithos ichnofacies

Glossifungites ichnofacies-demarcated surface

TSE

18

␦ O/␦ C Tryonia assemblage and pulmonate snails –4.0/ –5.2 Reworking and mixing common

Molluscs lacking; very rarely pavements of juvenile bivalve spat

Pachydon obliquus assemblage In situ faunas Small-Dyris assemblage Mostly in situ faunas

–7.6/ –8.3

Tall-Dyris and Tryonia assemblage Reworking and mixing common

–4.0/ –5.2

–6.1/ –7.4

Fig. 9.5 A generalized synthesis of common aspects of transgressive-regressive sequences of the Pebas Formation. MF, maximum flooding; TSE, transgressive surface of erosion. ripples, low-angle laminae truncation) (Fig. 9.7a) and mixedethology, ichnofossil assemblages indicate progradation and increased oxygen availability. A number of features point to tidal influence in this interval: (i) various types of heterolithic bedding; (ii) double mud-drapes preserved, for example in tubular burrow infills; (iii) millimetre-scale rhythmites; (iv) locally bipolar palaeocurrents; and (v) local upward-fining, bay-margin/shoreface profiles (see, e.g., Gingras et al. 2002a; Hovikoski et al. 2007a for further discussion). The asymmetric lamina couplets point to a semi-diurnal or mixed regime. Towards the top of the heterolithic units, ichnological diversity further decreases and bioturbation becomes sporadic: only rare Skolithos, Ophiomorpha, Arenicolites or indistinct, diminutive burrow mottling are present in some sequences. Contemporaneously, synaeresis cracks become common in the mud interbeds. Molluscs are in general lacking in this interval. An exception is the presence of strings of juvenile Pachydon species in butterfly orientation, found in one of the Indiana sections. These strings suggest a short window of colonization and very rapid mass mortality of these bivalves, implying the general hostile character of the environment during the formation of the upward-coarsening interval within sequences. Given the common preservation of sedimentological structures as well as the presence of an alleged dysoxictolerant Pachydon obliquus assemblage in the underlying interval, Wesselingh et al. (2006) interpreted oxygen stress as the likely limiting factor for life in this interval. Hovikoski et al. (2007a) attributed the stress to fluctuating low salinity (oligohaline-freshwater) based on very stressed ichnofossil assemblages, alternating burrowed and unburrowed intervals, and synaeresis cracks. Synaeresis cracks are known to form, albeit not exclusively, in swelling clays in response to fluctuating salinity and are exceedingly common, for instance in deltaic coastal settings (Plummer & Gostin 1981). Similarly, the corkscrew shape of Gyrolithes has been interpreted to reflect behavioural adaptation of the burrowing organism to low/fluctuating salinity stress. Near the top of the succession, the heterolithic bedding grades into a root-bearing mud and finally into a 20–150 cm-thick organic-rich mud or lignitic interval, that contains common freshwater fossils such as turtles, caimans and bones of continental

Hoorn_ch09_Final.indd 149

vertebrates such as giant sloths (L. Romero, personal communication, 1999). In these lignites also terrestrial snails have been found, and freshwater molluscs may occur as well (Wesselingh & Gittenberger 1999; Wesselingh et al. 2006). Apart from this standard type of sequence, sharp-based, laterally limited, upward-fining successions that have been interpreted as channels have been reported from the Pebas Formation deposits. Their lower contact is erosional, trough-shaped and laterally discontinuous. These channels are filled with mud-draped cross-stratification or vertically stacked deposits. Inclined heterolithic stratification (IHS) is a descriptive term for lithologically heterogeneous deposits that exhibit primary dip (previously also known as epsilon cross-stratification). Bipolar dune foresets are common in the Pebas Formation strata. The IHS comprises millimetre-scale mud, sand and organic debris alternations, but decimetre-scale sand–mud alternation is also observed. Soft sediment deformation is common. Hovikoski et al. (2007a) interpreted these as fluvio-tidal channels. Different interpretations of salinity regimes in Pebas phase deposits Molluscs and ostracods The Pebas invertebrate fauna is characterized by diverse assemblages of endemic aquatic species (Wesselingh 2006; Muñoz-Torres et al. 2006; see also Chapter 18). Because most of the Pebas taxa are endemic and extinct, their salinity tolerances cannot be assessed straightforwardly (Wesselingh et al. 2002; Wesselingh 2006). The endemic nature of the ostracod and mollusc fauna indicates that the Pebas Formation was not an ordinary marginal marine system: worldwide, marginal marine environments are occupied by lowdiverse faunas composed of widespread species often belonging to genera with a wide distribution (Wesselingh et al. 2002, 2006). The ostracod faunas have been interpreted in different ways: Whatley et al. (1998) and Muñoz-Torres et al. (2006) suggested that the Pebasian endemic ostracods lived in a thalassic or athalassic inland (brackish) sea, because the dominant genus Cyprideis is well known and abundant in brackish low-energy settings. Their interpretation was also based on the rarity of cosmopolitan

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(a)

Bay-offshore

Bay-shoreface

Marsh/lagoon/ Floodplain

Erosion of coastal peat/ lagoonal deposits, formation of a firm ground surface

Redeposition of freshwater molluscs and organic debris

Advancing Tidal creek complex

Flood plain/ Forest

Transgression

Colonization of a Glossifungites– demarcated surface Peat-organic rich mud TSE2 TSE1

(b)

(c)

5 cm

(e) (d)

5 mm

(f)

(g)

5 mm

Fig. 9.6 (a) Schematic diagram illustrating interpreted processes involved in the formation of Glossifungites ichnofaciesdemarcated surface that is commonly present at the base of Pebas sequences: Transgression exposes terrestrial environments (lagoons, marsh, ponds and floodplains) to tidal and wave erosion. This leads to formation of a firmground surface, and winnowing and redeposition of organic debris and freshwater molluscs into the transgressed embayment. Prior to the burial, the firm ground surface is colonized by Thalassinoides-producing organisms, which generate a Glossifungites ichnofacies-demarcated transgressive surface of erosion (TSE). In theory, both the tidal and wave processes may cause separate ravinement surfaces (tidal ‘TSE1’ and wave-generated ‘TSE2’). However, the Pebas sequences bear most commonly only one TSE surface, and double TSEs are rarely developed/preserved. (b–e) Examples of brackish-water trace fossils. (b) An example of a Glossifungites ichnofaciesdemarcated surface (Locality 9, Upper Pebas Formation [Fm]). Thalassinoides burrows descend into pedogenically altered mud. The burrows are passively filled with shell hash and organic debris. (c) Ophiomorpha burrowing muddy sand (Locality 13, Upper Pebas Fm). (d) Close-up of Asterosoma (undefined Miocene stratigraphic level, Madre de Dios). Asterosoma is a starshaped trace fossil in which concentrically laminated, bulbous arms horizontally radiate from a central burrow. It is a specialized feeding burrow, thought to be generated by various polychaetes. (e) Monospecific suite comprising diminutive Gyrolithes (Locality 20, Upper Solimões Fm). Gyrolithes is a corkscrew-shaped, vertically oriented trace fossil. It is only common in very low/ fluctuating salinity settings. (f,g) Examples of continental/freshwater trace fossil suites. (f) Scoyenia from pedogenically altered horizon (Late Miocene, Madre de Dios). (g) Arthropod-generated bedding plane trackways called Diplichnites (Locality 26, Madre de Dios). It is a common element of Mermia or mixed Mermia-Scoyenia ichnofacies. Pen shown for scale is ~0.6 cm wide.

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(a)

(b)

150 mm (d)

(e)

2 cm (c)

5 cm (f)

(g)

5 cm

Fig. 9.7 Facies interpreted to be tidally influenced from Miocene Amazonia. (a) Heterolithic combined-flow ripples. Locality 7, Middle Pebas Formation (Fm) (b) Regularly occurring reactivation surfaces in mud-draped dune cross-strata (Locality 27, Ipururo/ Madre de Dios Fm). (c) Heterolithic climbing ripples forming lenticular bedding. Black arrow indicates the climbing angle (Locality 24, Ipururo/Madre de Dios Fm) (d) Inclined heterolithic stratification (IHS) consisting of mud-draped current-rippled sand and massive mud. Interbedding is interpreted as seasonal (Talisma, Upper Solimões Fm). (e) Low-angle IHS consisting of statistically verified cyclic rhythmites (cf. Hovikoski et al. 2008). Black arrows indicate bipolarity (Locality 24, Ipururo/Madre de Dios Fm). (f) Heterolithic bedding and reactivation surfaces (Locality 26, Madre de Dios). (g) Heterolithic lamination that grades into highly sigmoidal cross-stratification (e.g. dashed lines). White arrows point to abundant soft sedimentary deformation. Black arrows indicate examples of double mud-drapes (Locality 24, Ipururo/Madre de Dios Fm).

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J. Hovikoski et al.

0.7090

0.7088

35 psu seawater 5.0 psu salinity line

Nuevo Horizonte Porvenir and Los Chorros

0.7080

Fresh water

0.7084

87

Sr / 86Sr

0.7086

0.7082

0.5

Pebas and Indiana Sta Rosa

0.7078

0.0

Wesselingh et al. (2002, 2006) interpreted the Pebasian endemic molluscs as lacustrine. Their interpretation was based on stable oxygen and carbon isotope signatures of the endemic Pebasian taxa, corroborated by Sr isotope analyses that pointed to the freshwater nature of the host waters of these molluscs (Vonhof et al. 1998, 2003; Kaandorp et al. 2006; Wesselingh et al. 2006; Fig. 9.8). Furthermore, these authors noted the complete lack of common coastal taxa, such as oysters, mussels and mangrove littorinine and cerithoidean snails. Together with the endemic Pebasian mollusc taxa, low numbers of in situ freshwater taxa, such as pearly freshwater mussels, were found. In the entire Pebas fauna, 31 mollusc species (20% of the Pebas fauna) were restricted to fresh waters, whereas only eight species (5% of the fauna) were considered to be indicative of elevated salinities. Hovikoski et al. (2007a) suggested that both the Pebasian endemic molluscs and ostracods were probably euryhaline, and that the ability to tolerate salinity fluctuations (fresh to occasionally slightly brackish) was probably one of their key ecological strengths. This interpretation was based on the observation that the endemic invertebrates co-occur locally with mangrove pollen, euryhaline foraminifers (Hoorn 1993; Muñoz-Torres et al. 2006), brackish-water neogastropods (e.g. Melongena woodwardi and Nassarius reductus: Vermeij & Wesselingh 2002; Chapter 18), commonly with tidal sediments, and stressed brackish-water trace fossil assemblages. Isotope data indicate that one endemic Dyris species records oligohaline

Oligohaline water

freshwater ostracod species, local co-occurrences of euryhaline foraminifers (e.g., Elphidium), and ostracods more typical of elevated salinities (e.g. Macrocypris). Endemic Cyprideis flocks also occur in some modern long-lived lakes, which led Ramos (2006), Ramos et al. (2007), Wesselingh et al. (2006) and Wesselingh & Ramos (see Chapter 18) to suggest that the Pebasian endemic ostracods are probably lacustrine. Very low ␦18O ratios in pilot stable isotope analyses of some Cyprideis specimens performed by H. Vonhof (unpublished data) also point to freshwater settings. Athalassic, evaporation-driven settings can be ruled out: such systems (e.g. Caspian Sea, Lake Eyre in Australia) are characterized by strong evaporation that depletes light oxygen isotopes. Instead, the oxygen isotope signature of the Pebasian fossils is commonly light (Vonhof et al. 1998, 2003; Kaandorp et al. 2006; Wesselingh et al. 2006). The endemic mollusc fauna is dominated by two groups, the pachydontine bivalves and cochliopid gastropods. The former group is part of the widespread marine and brackish water family of the Corbulidae, which led Nuttall (1990) to infer a common fluctuating brackish palaeoenvironment for the Pebas system. The Corbulidae contains a single surviving pachydontine representative, which lives in Amazonian fluvial and upper estuarine environments (Wesselingh 2006). Cochliopid taxa occupy any marine and freshwater habitat (Wesselingh et al. 2002, 2006) and are thus uninformative as to the extent of palaeosalinities in the Pebas phase deposits.

5.0

10.0

15.0 Age (Ma)

psu

sal

init

20.0

25.0

y li

ne

30.0

Fig. 9.8 Examples of molluscan 87Sr/86Sr ranges of Pebas Formation outcrops from a predominantly Andean catchment. Few samples (Indiana) that are interpreted to bear cratonic influence are excluded. Superimposed are the palaeosalinity fields for this part of the Neogene calculated with the Sr-isotope-based binary mixing model from Vonhof et al. (2003). The black line marked ‘35 psu seawater’ is the marine 87Sr/86Sr reference curve for this time interval. This plot shows that most Pebas Formation molluscan 87Sr/86Sr data plot in the freshwater field of the model (<0.5 psu salinity), including the alleged brackish-water molluscs Melongena woodwardi and Odostomia sp. from Porvenir. The excluded Buenos Aires outcrop data are interpreted to indicate an oligo-mesohaline interval. Sta Rosa, Santa Rosa; the stratigraphic ages of the Nuevo Horizonte, Porvenir and Los Chorros localities have recently been re-estimated to around 11–12 Ma.. Figure after Vonhof et al. (2003).

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Marine influence in Amazonia values (Vonhof et al. 1998, 2003). Fluctuating salinity stress would also explain the rarity of cosmopolitan freshwater invertebrate fossils and the absence of their stenohaline counterparts in the Pebas phase strata. However, stable isotope profiles through growth bands of Pebasian molluscs have not shown indications, for example, of seasonal-scale salinity stress (Kaandorp et al. 2003, 2005, 2006). Pollen and fish fossils Mangrove pollen is present in various stratigraphic levels in the Pebas Formation strata (Hoorn 1993, 1994, 2006), particularly in the lower and upper parts of the Pebas phase. Based on the presence of coastal elements such as mangroves (Zonocostites), benthic foraminifers and dinoflagellates, Hoorn (1993, 1994) proposed that during the Pebas phase the area was influenced by marginal marine conditions caused by several episodic marine incursions. Furthermore, the same author suggested that a fluctuating relative sea level caused alternating stages of coastal plain and alluvial plain conditions. Monsch (1998) reported that freshwater, brackish-water and marine taxa occur together as reworked assemblages more or less throughout the Pebas Formation, causing him to suggest episodic marine incursions in the area. The ecological interpretations of Monsch have been reviewed by Lundberg et al. (see Chapter 17). These authors concluded instead that the Pebasian fish faunas are dominated by strict freshwater taxa and that the euryhaline taxa also might represent freshwater conditions. Stable isotopes Vonhof et al. (1998, 2003) used 87Sr/86Sr analysis from mollusc shells to reconstruct palaeosalinities throughout the Pebas Formation. Their main conclusion was that the vast majority of bivalves from the Pebas Formation had lived in fresh water. Only in the Buenos Aires outcrop (Colombia) were increased salinities (oligo-mesohaline) recorded in the Sr-isotopes (Fig. 9.8). This observation was corroborated by the composition of bivalve fauna, and by mangrove pollen found in this outcrop (Vonhof et al. 2003). Although most of the analysed Pachydon shells record freshwater chemistries, there are a few Pachydon samples that show somewhat elevated 87Sr/86Sr, ␦13C and ␦18O values (cf. Vonhof et al. 2003): these can be interpreted as due to minor brackish-water or cratonic influences in these samples. Vonhof et al. (2003) preferred the latter explanation. A large number of ␦18O and ␦13C analyses were performed on Pebas Formation molluscs (Vonhof et al. 1998; Kaandorp et al. 2005, 2006; Wesselingh et al. 2006). Comparatively low ␦13C and ␦18O values of bivalves are indeed in good agreement with values found in the modern freshwater system in Amazonia, but not with typical brackish-water stable isotope values. Furthermore, stable isotope analyses were performed at subseasonal resolution through the growth increments of the bivalves. The resulting seasonal cyclicity in bivalve ␦18O values closely resembles wet to dry season cyclicity in modern Amazonian freshwater environments.

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Sedimentology and ichnology Sedimentological and ichnological data indicate that the Pebas Formation was influenced by repeated changes in accommodation space that organized the deposits into recurring successions of environments (Räsänen et al. 1998; Gingras et al. 2002a; Hovikoski et al. 2007a). As a result, the Pebas Formation consists of several tens of 3–8 m-thick sequences that have been interpreted by these authors as brackish to freshwater bay-margin successions. Ichnological data suggest that the lower (transgressive) part of these sequences was commonly accumulated in brackish water (oligo-mesohaline), whereas the regressive part is more variable in terms of water chemistry. However, in many sequences, very stressed brackish-water ichnofossil assemblages are locally observed together with tidal sediments and synaeresis cracks in the regressive part of the successions. The top of a sequence accumulated in fresh water in all data sets. The freshwater- and marine-influenced strata are closely intercalated in the sedimentary record. Accordingly, Hovikoski et al. (2007a) suggested that the Pebas Formation was a low-gradient depositional system, which was influenced by recurring, shallow and restricted marine ingressions. The ingressions were interspersed with rapid progradations that allowed advancement of continental environments.

Scenarios explaining the conflicting data sets In summary, the different lines of evidence clearly show that the Pebas system was located at sea level, was open to the sea and experienced some marine influence. Stable isotope data have shown a freshwater system with only a single meso-oligohaline ingression level in these strata so far, whereas some palaeontological data (e.g. mangrove and microfossils), and particularly ichnofossil data, indicate more common saltwater influence in the Pebas phase deposits. Wesselingh et al. (2006) proposed two possible explanations to solve the apparently conflicting indications of the extent of marine influence. The first possibility was that the brackish-water ichnofossils were deposited in brief time windows and that the endemic invertebrate species with their freshwater geochemical signals were deposited in other time windows. In the Peruvian outcrops Porvenir and Nuevo Horizonte, beds were found that comprised an admixture of marginal marine molluscs and foraminifers, endemic molluscs as well as fluvial molluscs, pointing to the complex background of some of the fossiliferous intervals. The second possibility proposed by these authors was that the tidal signatures were freshwater tidal signatures and that the ichnotaxa evolved, like several of the invertebrate taxa, beyond their usual ecological tolerances (supralimital evolution: Wesselingh 2007), in this case into fresh water. In a similar vein, at least in part, Gingras et al. (2002a) and Hovikoski et al. (2007a) suggested that much of the controversy could be explained by the complex depositional setting, characterized by the before-mentioned low-gradient coastline and repeatedly generated shallow accommodation space. Such depositional circumstances allowed rapid changes in shoreline

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positions, lateral environmental variability, and amalgamation of the sedimentary record. Thus, vertically closely spaced and temporally separated palaeoenvironmental indicators can represent very different environmental conditions. The observation that an array of coastal elements such as mangrove pollen, foraminifers, tidal sediments, coastal fish fossils and brackish-water trace fossils co-occur locally in several Pebas phase sequences, led these authors to propose that there are probably resolution gaps in the stable isotopic data sets. Hovikoski et al. (2007a) argued that the hypothesis that Pebasian trace fossil assemblages can be ascribed to adaptation of marginal-marine burrowing invertebrates to a long-lived freshwater lake environment is problematic: trace fossils represent behavioural adaptations that demonstrate successful survival strategies for a set of depositional parameters. Although organisms may physiologically adapt to fresh water, transporting behaviours – especially entire ichnofacies – from one environment to another is not simple. No bioturbation similar to brackish-water bioturbation has been described from long-lived freshwater lakes. Finally, as the Pebasian trace fossil assemblages occur frequently with other marginal marine indicators, as summarized above, these authors believed that the unique crossing of the salinity barrier is not the easiest interpretation. Currently it seems that the conflicting evidence for different salinity regimes could be best explained by temporally separated events (Wesselingh et al. 2006). Resolving the methodological controversy and the salinity regimes of the Pebas Formation requires more work in the future, particularly bed-by-bed comparison of ichnological, in situ palaeontological and stable isotopic data.

Upper Miocene (Acre phase) deposits outcrop in riverbanks and road cuts in Acre (Brazil), Madre de Dios and Loreto (Peru) (see Fig. 9.1). They comprise a variety of strata, including palaeosols, root-bearing mud, and organic-rich, homolithic, upward-fining successions interpreted as fluvial point bars. Inclined heterolithic stratification successions similar to those that occur locally in the Pebas phase strata are very common. Indicators of marine influence are largely confined to these channels.

Summary of Acre phase IHS channels The Late Miocene IHS deposits are diverse, displaying major variation in sedimentological and ichnological properties (Hovikoski et al. 2007b; see Fig. 9.9). Most commonly they comprise sharpbased, 3–16 m-thick, upward-fining successions that are gradationally capped by root-bearing mud or pedogenically altered horizons. The basal contacts are commonly erosional, troughshaped and contain mud clasts. The lower part of the successions commonly consists of massive or cross-stratified silty sand. The cross-strata are typically mud-draped, or display minute, superimposed, mud-draped ripples. Double mud-drapes, sigmoidal ripples, and regular reactivation surfaces occur frequently (see Fig. 9.7b–g). Upwards, the sandy deposits grade into IHS, which commonly comprises sedimentary rhythmites or laterally extensive, decimetre-scale sand and mud alternations (see Fig. 9.7d,e). Bipolar sediment transport indicators (opposing current ripple foresets) and soft sedimentary deformation are common.

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IHS-channel sediments in the Madre de Dios (Oliveira et al. 2007; see also Fig. 9.1). Otherwise the pollen record of the Acre phase is continental (see Chapter 19 for details).

Synaeresis cracks are locally present (Rebata et al. 2006; Hovikoski et al. 2007b, 2008). Although the above-mentioned deposits are usually unburrowed, they locally contain Skolithos, Cylindrichnus, Planolites, Palaeophycus, Laminites and/or Gyrolithes (see Fig. 9.6e). Smoothwalled, regularly branching Thalassinoides, Ophiomorpha and Asterosoma rarely occur (see Fig. 9.6d). The IHS successions grade upwards into pedogenically altered sediment, burrowed with continental Scoyenia ichnofacies (see Fig. 9.6f). This facies is locally present in all the studied outcrops.

Different interpretations of the extent of marine influence during the Late Miocene The extent of marine influence during the Late Miocene also has been the subject of debate. Based on fossils, some palaeontological field parties have argued that the Acre phase strata represent entirely freshwater and terrestrial settings (Campbell et al. 2006; Cozzuol 2006; Westaway 2006; Latrubesse et al. 2007). These authors also proposed that seasonal processes in a continental depositional system may explain the heterolithic patterns in these deposits (e.g. Westaway 2006). Although the notion of prominent seasonal variation in water levels is true (see below), there is also very strong evidence for tidal processes in many IHS channels. These include: statistically verified tidal rhythmites that point to semi-diurnal tidal cycles; superimposition of cycles attributable to flood–ebb, diurnal inequality, neap–spring and anomalistic month cycles; current reversals measured from current ripple foresets; double mud-drapes; regular reactivation surfaces, sigmoidal ripples; and a variety of IHS successions (e.g. Nio & Yang 1991; Hermoza 2004; Roddaz et al. 2004; Hovikoski et al. 2005, 2007a, 2008; Rebata et al. 2006; Fig. 9.10; see also Fig. 9.7).

Palaeontology The invertebrate fauna from Acre phase deposits is poorly preserved and so far only mollusc assemblages strongly reminiscent of modern Amazonian fluvial faunas have been found in rootbearing horizons (see Chapter 18). The vertebrate fauna of the Acre phase deposits, however, is extremely rich in species and locally well preserved, and has shed much light on the depositional system. The fauna is dominated by terrestrial as well as freshwater taxa (Cozzuol 2006; Latrubesse et al. 2007; see also Chapters 15 & 16). Rarely, remains of presumed euryhaline sharks and sawfishes are found (see Chapter 17 for an ecological reappraisal of these taxa). Finally, Rebata et al. (2006) reported mangrove pollen from IHS-channels from the Nauta Formation. Mangrove pollen were also found sporadically in preliminary analysis of Cocha Cashu

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Fig. 9.10 (a) Predicted (i.e. ideal) modern synodic high- and low-water tidal station data from Recife, Brazil (NOAA 2001). Both ebb and flood currents are included. The vertical axis indicates the height of tides; the horizontal axis denotes the event number. A, apogean spring tide; P, perigean spring tide. (b) Fourier transformation of the same data. Flood–ebb (2.4 to 2), semi-diurnal (4.3–3.7) and neap–spring (~60) cycles develop. (c) Rhythmite thickness data from Madre de Dios. The vertical axis shows the thickness of sand-mud lamina couplets, the horizontal axis the couplet number. A, interpreted occurrence of apogean spring tide; P, interpreted occurrence of perigean spring tide. (d) Fourier transformation of the same data. White noise test indicates a non-random origin for the series (p < 0.0001). In the present example, peaks interpreted as flood–ebb (2.1) and neap–spring develop (~59) develop. Modified from Hovikoski et al. (2007b).

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Fig. 9.11 A diagram illustrating elements of a riverine estuary. Inclined heterolithic stratification (IHS) forms in laterally accreting fluvio-tidal channels in deltaic and estuarine environments. Modified from Dalrymple et al. 1992.

Based on sedimentological and ichnological data, the IHS deposits are interpreted mainly as deltaic and/or middle-upper estuarine (cf. Dalrymple 2006, i.e. estuary is transgressive) channel complexes that are interbedded with a variety of alluvial strata (Gingras et al. 2002b; Hovikoski et al. 2005, 2007a, 2008; Rebata et al. 2006). Generally, tidally influenced IHS is believed to form as a result of migration of turbidity maxima, which is caused by estuarine circulation and intermingling of tidal and fluvial currents (e.g., Dalrymple & Choi 2007). This transitional zone constitutes an overall energy minimum along the riversea transect, and is typically characterized by laterally accreting channel deposits (Fig. 9.11). Its location is controlled by coastal slope and relative strength of tidal and fluvial currents, but most commonly it occurs in coastal zones where fluvio-tidal channels are fringed by alluvial plains rather than tidal flats (Dalrymple & Choi 2007). Due to flocculation of clays and water turbidity, the turbidity maxima result in elevated fine-grained sediment accumulation. The proportional strength of tidal and riverine processes varies through tidal and seasonal cycles causing lateral shifts of the turbidity maxima and, consequently, alternating coarser- and finer-grained bedding on the point bars (i.e. IHS). In many modern estuaries and deltas notable seasonal variability in depositional processes has been recognized despite the tidal environment (e.g. Van den Berg 1981; Lettley et al. 2007). Similarly, the major IHS generating cycle is interpreted to be seasonal within a tidally influenced system in the Acre phase deposits. This interpretation is based on the thickness of the IHS couplets (up to 2 m), locally high bioturbation intensity in decimetre-scale IHS, and the number of tidal cycles superimposed on the lower frequency cycles (Gingras et al. 2002a; Rebata et al. 2006; Hovikoski et al. 2008). Statistically verified neap–springgenerated IHS occurred only in low-energy, combined-traction and suspension-driven locals, such as in tidal creeks and deltaic channels that retain an estuarine character after becoming separated from the fluvial system. In many seasonally dominated IHS

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deposits, tidal cycles appear as subordinate and are superimposed on the annual cycles. The salinity regime of the IHS in Acre phase deposits is interpreted to vary from rare presumably mesohaline (moderate diversity assemblages containing Thalassinoides, Ophiomorpha) to freshwater (unburrowed, or near monospecific Planolites, Taenidum, or unnamed irregular burrows dominated suites, or assemblages containing elements of Mermia ichnofacies: see Fig. 9.6g). Very low diversity suites containing Cylindrichnus, lined Skolithos and Gyrolithes are common. These are best interpreted as very restricted (fluctuating salinity and/or oligohaline) brackish-water suites (see Fig. 9.6e). The palaeoenvironmental significance of Laminites is in need of further study. Inclined heterolithic strata are closely intercalated with a variety of non-marine strata such as alluvial plains, palaeosols and fluvial channels. These strata are typically burrowed with monospecific to low-diversity suites of meniscate trace fossils (Taenidium, Beaconites or Scoyenia) and/or root bioturbation, defining continental Scoyenia ichnofacies (see Fig. 9.6f).

Discussion Modern and ancient analogues of Miocene Amazonia Early-Middle Miocene (‘Pebas phase’) There is no single modern analogue that adequately resembles the Pebas system: we live now in an interglacial period and large sea-level epicontinental depositional systems are rare. The Pebas system was a long-lived basin with brackish to freshwater salinities. No long-lived lake/embayment with a marine connection exists today, whereas they did so in the past (e.g. the Miocene Maeotian Sea of Eastern Europe). Long-lived brackish lakes do exist (the Caspian Sea) and did so in the past as well (e.g. a

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Marine influence in Amazonia variety of Neogene Paratethyan long-lived lakes in Central and East Europe: see references in Wesselingh 2007). However, these lakes all have in common their separation from the oceans and maintenance of salinity by evaporation. Given the common preservation of organic carbon and the very negative oxygen isotope signatures of the carbonate fossils, the Pebas system had a very strong precipitation/run-off surplus instead. Vonhof et al. (2003) suggested that the Pebas was predominantly a freshwater system that maintained a limited connection to the sea, rather like the early 20th century Lake Maracaibo (northern Venezuela). Prior to the excavation of shipping lanes, which allowed marine waters to enter the lake, Lake Maracaibo was mostly a freshwater lake. The lake only developed during the latest Pleistocene-Holocene, and is therefore not a long-lived basin. Lake Maracaibo nowadays harbours an impoverished coastal fauna containing widespread brackish taxa, such as Nerita, Rangia and Polymesoda species. Its waters range from mesohaline in its northern portion to oligohaline in its southern portion. A similar young embayment is the Baltic Sea, which has experienced alternating brackish and freshwater phases in its history. The Pebas Formation has similarities with any shallow, tidally influenced bay-margin depositional systems, where accommodation is repeatedly generated. Such ancient examples include, for example, the Pennsylvanian Brazil Formation in the USA (Kvale & Mastalerz 1998), and the Upper Cenomanian Kaskapau Formation in Canada (Plint & Kreitner 2007). The latter was deposited under general greater salinity than the Pebas Formation. Both transgressive/regressive bay-margin successions of variable salinity and IHS-channels have also been described from the Cretaceous Mannville Group in Alberta, Canada (e.g. Hayes et al. 1994). Another potential analogue combining very low gradients resulting in strongly shifting (restricted) marine and freshwater facies is the Paleocene Tongue River Formation of Wyoming, USA (Belt et al. 2005; Anderson et al. 2006). Late Miocene (‘Acre phase’)

hundreds of kilometres above the river mouth (Kvale & Archer 2007). The modern Amazon River hosts exceptionally strong tidal currents that penetrate up to 800 km inland (Archer 2005). Although the preservation potential of the tidal signature is low due to strong fluvial input in the Amazon trunk river, the tributaries are of low energy and characterized by less fluvial input. Such rivers (particularly the Xingu River) may host mesotidal currents in a freshwater environment, and due to a limited fluvial component the potential for preserving the tidal signal is higher (Kvale & Archer 2007). Such settings may be analogous to local occurrences of freshwater tidal deposits in the Acre phase (Madre de Dios, Acre; Hovikoski et al. 2007b). The closest ancient analogies to the Late Miocene IHS are the similarly interpreted IHS deposits in the Cretaceous DinosaurProvincial Park, and estuarine Horseshoe Canyon and McMurray Formations, Alberta, Canada (e.g., Smith 1988; Lettley et al. 2007).

Pathways of marine incursions In the literature, five different incursion pathways have been proposed for the Pebas system (Fig. 9.12; Nuttall 1990; Wesselingh et al. 2002). So far, biogeographic and geological evidence has

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Palaeontological studies have suggested that the Acre phase depositional setting is similar to the modern Pantanal, dominated by lake-swamp complexes and fluvial channels (Cozzuol 2006; Latrubesse et al. 2007). Recent sedimentological work suggested a laterally complex depositional setting, akin to modern estuarine and deltaic systems. Like the Pebas Formation, the Upper Miocene depositional system was also characterized by extremely low relief, evoking rapid autocyclically and allocyclically driven shifts in the shoreline. In modern settings, such as the Fly River delta in Papua New Guinea, such conditions promote the juxtaposition of continental delta-plain facies/forest, fluvially dominated, active deltaic channels, and tidally dominated channels that maintain an estuarine character following abandonment of the fluvial system. The closest modern analogies for the Acre phase IHS are mesotidal reaches of fluvio-tidal rivers and tidal creeks such as those in the Guayaquil Bay (Babahovo estuary, Ecuador) and Willapa Bay estuary (Washington, USA) (Smith 1988; Gingras et al. 1999). Freshwater tidal rhythmites/IHS may (theoretically) develop also in low-energy tributaries of the Amazon River, up to some

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Fig. 9.12 Potential pathways of marine influence in the Early to early Late Miocene Pebas phase. For details see text. Likely pathways: (1) Connections through Ecuadorean basins; (2) Llanos-Maracaibo connection; and (3) Llanos-East Venezuela Basin pathway. Unlikely pathways: (4) Essequibo-Rio Branco corridor; (5) Amazon corridor; and (6) Paraná-Plate River corridor. Ruled area indicates a potential marine pathway running through the lower western margin of the Guiana Shield during the late Middle to early Late Miocene (possibly also late Early Miocene). The Late Miocene marine pathways are uncertain; possible sources for marine influence are through corridors 5 and 6.

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shown that marine influence reached western Amazonia at least from the north (the Llanos Basin of Colombia: Hoorn et al. 1995; Villamil 1999; Wesselingh & Macsotay 2006; Bayona et al. 2007). Bayona et al. (2007) demonstrated that the marine palynomorphs are mainly concentrated on three major Early and Middle Miocene flooding surfaces in the Llanos Basin, whereas elsewhere in the Miocene series they occur sporadically in the basal part of minor upward-coarsening successions. Analysis of foraminiferan content of these assemblages suggested rapid brackish-water and freshwater alternations. Their data also showed that mangrove pollen intervals occur similarly in Oligocene and Eocene sediments. During the Early-Middle Miocene, marine influence may have reached the Llanos Basin from the proto-Caribbean Sea through the Maturin and Barinas Basins (Guzman & Fisher 2006). The presence of lowland aquatic connections between the Barinas-Apure area and the northern Venezuelan coast is still the subject of research (e.g. see Chapter 26). A lowland connection with the East Venezuela Basin is also supported by the presence of Pachydon hettneri in the Early Miocene Chaguaramas Formation (Wesselingh & Macsotay 2006). That occurrence is a very clear sign of biogeographic closeness between the Amazon region, Llanos region and the East Venezuela Basin region. A direct marine connection through the Colombian Magdalena Basin as envisaged by Nuttall (1990) seems implausible (Wesselingh & Macsotay 2006). Marine incursion pathways between the Llanos and western Amazonia episodically may have extended over the lower western margin of the Guiana Shield in the vicinity of Vaupes region (Hoorn 2006). Although there has been speculation about direct pathways through the emerging Andes towards the Pacific (see references in Nuttall 1990), only Steinmann et al. (1999) have shown the possibility of restricted connections through the present-day Ecuadorean Andes. The presence, size and duration of these possible Pacific connections need further investigation. Given the Miocene fluvial strata present in central and eastern Amazonia, it is unlikely that marine incursions entered western Amazonia from there. Finally, no geological and biogeographic data support a southern (Paraná-Plate River Basin) origin of marine influence during the Pebas phase. The origin of marine influence in western Amazonia during the Late Miocene (Acre phase) is uncertain. In the Late Miocene, uplift of the Vaupes region in Central Colombia (see Chapter 4) closed the previous northern pathways (Hoorn et al. 1995). The Ecuadorean Pacific lowland corridor was also closed by 9.5 Ma (Steinmann et al. 1999). Possible pathways for marine influence during the Late Miocene are through the modern Amazon valley and Paraná Sea (Argentina). The modern Amazon River was established around 11 Ma (Figueiredo et al. 2008, 2009): the hypothetical ingressions through this route would have been linked to eustatic transgressive episodes. Distal (Chaco)-proximal (Madre de Dios/Acre) relationships of apparently contemporaneous strata, and some palaeocurrent data collected from the Late Miocene fluvio-tidal facies in western Amazonia could be easily explained by the Paranán connection (Hovikoski et al. 2007b). However, field studies in the Chaco Basin have suggested that the marine influence in southern Bolivia could be better explained by northern pathways (e.g. Hulka et al. 2006). Resolving the Late

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Miocene palaeogeographic setting requires more work in the future.

Conclusions Marine settings throughout the Neogene and Quaternary have been proposed in the literature. We can only find certain evidence for a marine influence in Miocene western and western-central Amazonia. Miocene Amazonian strata consist of high-frequency intercalations of marine-influenced and continental strata. The Early-Middle Miocene Pebas/Solimões Formation was a sea-level depositional system in which accommodation space was repeatedly generated. The recurring base-level changes arranged the deposits into repeating 3–8 m-thick, transgressive to regressive bay-margin sequences. Each sequence grades through Glossifungites and/or mixed Cruziana-Skolithos ichnofacies into pedogenically altered, continental fossil-bearing, organic-rich sediments. The invertebrate faunas in these deposits are species rich and highly endemic. Stable isotope data have documented only one meso-oligohaline ingression level in these strata, whereas some palaeontological data (e.g. mangrove pollen and microfossils), and particularly ichnofossil data, have suggested a more common brackish-water influence in these deposits. Ichnological data suggest that brackish water influence (up to mesohaline) was strongest in the transgressive part of the sequences, whereas the regressive part was laid down under freshwater/fluctuating salinity conditions. Resolving this methodological controversy requires more work in the future, particularly bed-by-bed comparison of ichnological, palaeontological and stable isotopic data. The Late Miocene strata comprise a variety of IHS-channels that are intercalated with diverse alluvial facies. Palaeontological studies have suggested that these deposits were accumulated in an environment similar to the modern Pantanal, dominated by lakeswamp complexes and fluvial channels (Cozzuol 2006; Latrubesse et al. 2007). These authors also proposed that seasonal processes in a continental depositional system may explain the heterolithic patterns in these deposits (e.g. Westaway 2006). Recent detailed sedimentological studies have also documented strong evidence for semi-diurnal tidal processes in the IHS-channels. The tidal signature locally co-occurs with restricted, brackish-water trace fossil assemblages, and is commonly superimposed on prominent seasonal variability (e.g. Rebata et al. 2006; Hovikoski et al. 2008). Based on these data, these authors proposed that Late Miocene western Amazonia hosted a laterally complex depositional setting, similar to modern estuarine and deltaic complexes. The accelerated tectonic subsidence of the Andean foreland during the MiddleLate Miocene (Hermoza 2004) caused extremely low relief and evoked rapid auto- and allocyclically driven shifts in depositional settings. In modern environments such as the Fly River delta in Papua New Guinea, such conditions promote the juxtaposition of continental delta-plain facies/forest, fluvially dominated, active deltaic channels, and tidally dominated channels that maintain an estuarine character following abandonment of the fluvial system. The continental environments were most probably temporally and/or spatially more significant than the co-occurring, restricted marginal-marine strata.

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Acknowledgements We would like to thank reviewers Jose I. Guzman and George Postma, and editor Carina Hoorn for their constructive comments that improved the manuscript. The authors would also like to thank INGEMMET, UFAC, SERGEOMIN and IRD for their kind cooperation.

References Anderson, L.C., Hartman, J.H., Wesselingh, F.P. (2006) Close evolutionary affinities between freshwater corbulid bivalves from the Neogene of western Amazonia and Paleogene of the northern Great Plains, USA. J S Am Earth Sci 21, 28–48. Archer, A.W. (2005) Review of Amazonian depositional systems. In: Blum, M., Marriott, S., Leclair, S. (eds) Fluvial Sedimentology VII. Special Publication of the International Association of Sedimentologists, vol. 35, pp. 17–39. Bayona, G., Jaramillo, C., Rueda, M., Reyes-Harker, A. (2007) Paleocene-middle Miocene flexural-margin migration of the nonmarine Llanos foreland basin of Colombia. Ciencia, Tecnología y Futuro 3, 141–160. Belt, E.S., Tibert, N.E., Curran, H.A., Diemer, J.A., Hartman, J.H., Kroeger, T.A., Harwood, D.M. (2005) Evidence for marine influence on a low-gradient coastal plain: ichnology and invertebrate paleontology of the lower Tongue River Member (Fort Union Formation, middle Paleocene), western Williston Basin, U.S.A. Rocky Mt Geol 40, 1–24. Buatois, L.A., Mangano, M.G., Maples, C.G., Lanier, W.P. (1997) The paradox of nonmarine ichnofaunas in tidal rhythmites: Integrating sedimentological and ichnological data from Late Carboniferous of eastern Kansas, USA. Palaios 12, 467–481. Campbell, K.E., Frailey, C.D., Romero-Pittman, L. (2006) The PanAmazonian Ucayali Peneplain, late Neogene sedimentation in Amazonia, and the birth of the modern Amazon River system. Palaeogeogr Palaeocl 239, 166–219. Choi, K., Park, Y. (2000) Late Pleistocene silty tidal rhythmites in the macrotidal flat between Youngjong and Yongyou Islands, west coast of Korea. Mar Geol 167, 231–241. Cozzuol, M. (2006) The Acre vertebrate fauna: Age, diversity, and geography. J S Am Earth Sciences 21,185–203. Dalrymple, R. (1992) Tidal depositional systems. In: Walker, R.G.E. (ed.) Facies Models: Response to Sea Level Change. Toronto: Geological Association of Canada Publications, pp. 195–219. Dalrymple, R. (2006) Incised valleys in time and space: an introduction to the volume and examination of the controls on valley formation and filling. SEPM Special Publication 85, 5–12. Dalrymple, R., Choi, K. (2007) Morphologic and facies trends through the fluvial-marine transition in tide-dominated depositional systems: A schematic framework for environmental and sequencestratigraphic interpretation. Earth Sci Rev 81, 135–174. Dalrymple, R.W., Zaitlin, B.A., Boyd, R. (1992) Estuarine facies models: conceptual basis and stratigraphic implications. J Sediment Petrol 62, 1130–1146. De Boer, P.L., Oost, A.P., Visser, M.J. (1989) The diurnal inequality of the tide as a parameter for recognizing tidal influences. J Sediment Petrol 59, 912–921. Dworschak, P.C. (1982) The biology of Upogebia pusilla (Petagna) (Decopoda, Thalassinidae). Mar Ecol 4, 19–43. Dworschak, P.C. (2000) Global diversity in the Thalassinidea (Decapoda). J Crustacean Biol 20 (Special 2), 238–245.

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Ekdale, A.A., Bromley, R.G., Pemberton, S.G. (1984) Ichnology: Trace Fossils in Sedimentology and Stratigraphy. Society of Economic Paleontologists and Mineralogists Short Course 15, pp. 1–317. Figuereido, J., Hoorn, C., van de Ven, P., Soares, E. (2008) Birth of the Amazon River and stages of development: New evidence from the Amazon Fan (Foz do Amazonas, Late Miocene to Present). EGU 2008, Geophys Res Abstr 2 pp. Figuereido, J., Hoorn, C., van de Ven, P., Soares, E. (2009) Late Miocene onset of the Amazon River and the Amazon deep-sea fan: Evidence from the Foz do Amazonas Basin. Geology 37, 619–622. Frey, R.W., Howard, J.D. (1975) Endobenthic adaption of juvenile thalassinidean shrimp. Bull Geol Soc Denmark 24, 283–297. Gingras, M.K., Pemberton S.G., Saunders, T., Clifton, H.E. (1999) Ichnology of modern and Pleistocene brackish-water deposits at Willapa Bay, Washington: variability in estuarine settings. Palaios 14, 352–374. Gingras, M.K., Hubbard, S.M., Pemberton, S.G., Saunders, T. (2000) The significance of Pleistocene Psilonichnus at Willapa Bay, Washington. Palaios 15, 142–151 Gingras, M.K., Räsänen, M.E., Pemberton, S.G., Romero, L.P. (2002a) Ichnology and sedimentology reveal depositional characteristics of bay margin parasequences in the Miocene Amazonian foreland basin. J Sediment Res 72, 871–883. Gingras, M.K., Räsänen, M.E., Ranzi, A. (2002b) The significance of bioturbated inclined heterolithic stratification in the southern part of the Miocene Solimões Formation, Rio Acre: Amazonia Brazil. Palaios 17, 591–601. Guzman, J., Fisher, W. (2006) Early and Middle Miocene depositional history of the Maracaibo Basin, western Venezuela. AAPG Bulletin 90, 625–655. Haq, B.U., Hardenbol, J., Vail, P.R. (1987) Chronology of fluctuating sea levels since the Triassic. Science 235, 1156–1167. Hayes, B.J.R., Christopher, J.E., Rosenthal, L., Los, G., McKercher, B., Minken, D. et al. (1994) Cretaceous Mannville Group of the Western Canada Sedimentary Basin. In: Mossop, G.D., Shetson. I. (Comp.) Geological Atlas of the Western Canada Sedimentary Basin. Calgary: Canadian Society of Petroleum Geologists and Alberta Research Council, pp. 317–334. Hermoza, W. (2004) Dynamique tectono-sédimentaire et restauration séquentielle du retro-bassin d’avant-pays des Andes centrals. PhD thesis, Université de Paul Sabatier de Toulouse III, 296 pp. Hermoza, W., Brusset, S., Baby, P., Gil, W., Roddaz, M., Guerrero, N., Bolañoz, M. (2005) The Huallaga foreland basin evolution: thrust propagation in a deltaic environment, northern Peruvian Andes. J S Am Earth Sci 19, 21–24. Hoorn, C. (1993) Marine incursions and the influence of Andean tectonics on the Miocene depositional history of northwestern Amazonia: Results of a palynostratigraphic study: Palaeogeogr Palaeocl 105, 267–309. Hoorn, C. (1994) Fluvial palaeoenvironments in the intracratonic Amazonas Basin (Early Miocene-early Middle Miocene, Colombia). Palaeogeogr Palaeocl 112, 187–238. Hoorn, C. (1996) Late Miocene tidal deposits in the Amazonian foreland basin: Comment. Science 273, 122. Hoorn, C. (2006) Mangrove forests and marine incursions in Neogene Amazonia (Lower Apaporis River, Colombia). Palaios 21, 197–209. Hoorn, C., Guerrero, J., Sarmiento, G.A., Lorente, M.A. (1995) Andean tectonics as a cause for changing drainage patterns in Miocene northern South America. Geology 23, 237–240. Hovikoski, J. (2006) Miocene Western Amazonia in the light of sedimentological and ichnological data. PhD thesis, Annales Universitatis Turkuensis, AII, pp. 1–138.

10/24/2009 11:30:05 Shobha

160

J. Hovikoski et al.

Hovikoski, J., Räsänen, M.E., Gingras, M., Roddaz, M.,Brusset, S., Hermoza, W. et al. (2005) Miocene semidiurnal tidal rhythmites in Madre de Dios, Peru. Geology 33, 177–180. Hovikoski, J., Gingras, M., Räsänen, M., Rebata, L., Guerrero, J., Ranzi, A. et al. (2007a) The nature of Miocene Amazonian epicontinental embayment: High-frequency shifts of the low-gradient coastline. GSA Bulletin 119, 1506–1520. Hovikoski, J., Räsänen, M., Gingras, M., Lopez, S., Ranzi, A., Melo, J. (2007b) Palaeogeographical implications of the Miocene Quendeque Formation (Bolivia) and tidally-influenced strata in the southwestern Amazonia. Palaeogeogr Palaeocl 243, 23–41. Hovikoski, J., Räsänen, M.E., Gingras, M., Ranzi, A., Melo, J. (2008) Tidal and seasonal controls in the formation of late Miocene IHS deposits, western Amazonian Foreland Basin. Sedimentology 55, 499–530. Hulka, C., Gräfe, K.-U., Sames, B., Heubeck, C., Uba, C. (2006) Depositional setting of the middle to late Miocene Yecua Formation of the central Chaco foreland basin, Bolivia. In: Hoorn, C., Vonhof, H. (eds) New contributions on Neogene geography and depositional environments in Amazonia. J S Am Earth Sci 21, 135–150. Irion, G. (1984) Clay minerals of Amazonian soils. In: Sioli, H. (ed.) The Amazon – Limnology and Landscape Ecology of a Mighty Tropical River and its Basin. Monographiae Biologicae. Dordrecht: Junk, pp. 537–579. Kaandorp, R.J.G., Vonhof, H.B., Busto, C. del, Wesselingh, F,P., Ganssen, G.M., Marmol, A.E. et al. (2003) Seasonal stable isotope variations of the modern Amazonian freshwater bivalve Anodontites trapesialis. Palaeogeogr Palaeocl 194, 339–354. Kaandorp, R.J.G., Vonhof, H.B., Wesselingh, F.P., Romero Pittman, L., Kroon, D., Hinte, J.E. van. (2005) Seasonal Amazonian rainfall variation in the Miocene climate optimum. Palaeogeogr Palaeocl 221, 1–6. Kaandorp, R.J.G., Wesselingh, F.P., Vonhof, H.B. (2006) Ecological implications from stable isotope records of Miocene western Amazonian bivalves. J S Am Earth Sci 21, 54–74. Klammer, G. (1984) The relief of the extra-Andean Amazon basin. In: Sioli, H. (ed.) The Amazon – Limnology and Landscape Ecology of a Mighty Tropical River and its Basin. Monographiae Biologicae. Dordrecht: Junk, pp. 47–83. Kvale, E., Archer, A. (2007) Paleovalley fills: Trunk vs. tributary. AAPG Bulletin 91, 809–821. Kvale, E.P., Mastalerz, M. (1998) Evidence of Ancient freshwater tidal deposits. SEPM Special Publications 61, 95–107. Latrubesse, E., da Silva, S., Cozzuol, M., Absy, M. (2007) Late Miocene continental sedimentation in southwestern Amazonia and its regional significance: Biotic and geological evidence. J S Am Earth Sci 23, 61–80. Lettley, C., Pemberton, G., Gingras, M., Ranger, M., Blakney, B. (2007) Integrating sedimentology and ichnology to shed light on the system dynamics and paleogeography of an ancient riverine estuary. In: MacEachern, J.A., Bann, K.L., Gingras, M.K., Pemberton, S.G. (eds) Applied Ichnology. SEPM (Society for Sedimentary Geology) Short Course Notes 52, 144–162. Lovejoy, N.R., Collette, B.B. (2001) Phylogenetic relationships of New World needlefishes (Teleostei: Belonidae) and the biogeography of transition between marine and freshwater habitats. Copeia 2, 324–338. Lovejoy, N.R., Albert, J.S., Crampton, W.G.R. (2006) Miocene marine incursions and marine/freshwater transitions: Evidence from neotropical fishes. J S Am Earth Sci 21, 5–13. MacEachern, J.A., Pemberton, S.G., Bann, K.L., Gingras, M.K. (2007) Departures from the archetypal ichnofacies: Effective recognition of

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physico-chemical stresses in the rock record. In: MacEachern, J.A., Bann, K.L., Gingras, M.K., Pemberton, S.G. (eds) Applied Ichnology. SEPM (Society for Sedimentary Geology) Short Course Notes 52, 65–94. Marshall, L.G., Lundberg, J.G. (1996) Miocene deposits in the Amazonian Foreland Basin. Science 273, 123–124. Martin, K.D. (2004) A re-evaluation of the relationship between trace fossils and dysoxia. In: The Application of Ichnology to Palaeoenvironmental and Stratigraphic Analysis. Geological Society, London, Special Publication 228, pp. 141–156. Miall, A.D. (1997) The Geology of Stratigraphic Sequences. Berlin: Springer-Verlag, 433 pp. Miller, K.G., Kominz, M.A., Browning, J.V., Wright, J.D., Mountain, G.S., Katz, M.E. et al. (2005) The Phanerozoic record of global sea-level change. Science 310, 1293–1298. Monsch, K. (1998) Miocene fish faunas from the northwestern Amazonia Basin (Colombia, Peru, Brazil) with evidence of marine incursions. Palaeogeogr Palaeocl 143, 31–50. Muñoz-Torres, F.A., Whatley, R.C., Van Harten, D. (2006) Miocene ostracod (Crustacea) biostratigraphy of the Upper Amazon Basin and the Cyprideis genus evolution. J S Am Earth Sci 21, 75–86. Nesbitt, E.A., Campbell, K.A. (2002) A new Psilonichnus ichnospecies attributed to mud-shrimp upogebiain estuarine settings. J Paleontol 76, 892–901. Nio, S.D., Yang, C.S. (1991) Diagnostic attributes of clastic tidal deposits: A review. Canadian Society of Petroleum Geologists Memoir 16, 3–28. NOAA (National Oceanic and Atmospheric Administration) (2001) Tide tables 2001: east coast of North and South America including Greenland. International Marine, 376 pp. Nores, M. (2004) The implications of Tertiary and Quaternary sea level rise events for avian distribution patterns in the lowlands of northern South America. Global Ecol Biogeogr 13, 149–161. Nuttall, C.P. (1990) A review of the Tertiary non-marine molluscan faunas of the Pebasian and other inland basins of north-western South America. Bull Brit Mus Nat Hist Geol 45, 165–371. Oliveira, F. A., de Capuzzo, J.P., Barberi, M., Hovikoski, J., Räsänen, M. (2007) Caracterização da vegetação do distrito de Madre de Dios (Amazônia Peruana) durante o Mioceno Superior com base em análise palinológica e paleoecológica. In: Anais do VIII Congresso de Ecologia do Brasil, Caxambu, MG, Brazil. Paxton, C.G.M., Crampton, W.G.R. (1996) Miocene deposits in the Amazonian Foreland Basin. Science 273, 123. Pemberton, S.G., Wightman, D.M. (1992) Ichnological characteristics of brackish water deposits. In: Applications of Ichnology to Petroleum Exploration. Calgary: SEPM Core Workshop 17, pp. 141–168. Plint, G., Kreitner, M. (2007) Extensive thin sequences spanning Cretaceous foredeep suggest high-frequency eustatic control: Late Cenomanian, Western Canada foreland basin. Geology 35, 735–738. Plummer, P.S., Gostin, V.A. (1981) Shrinkage cracks; desiccation or synaeresis? J Sediment Res 51, 1147–1156. Ramos, M.I.F. (2006) Ostracods from the Neogene Solimões Formation (Amazonas, Brazil). J S Am Earth Sci 21, 87–95. Ramos, M.I.F., Celestino, E.A., Paz, J.D.S. (2007) Nova ocorrência de ostracodes Miocênicos da Formação Solimões. In: Anais, XX Congresso Brasileiro de Paleontologia, Búzios, Rio de Janeiro, 2007, p. 186. Räsänen, M., Linna, A., Irion, G., Rebata Hernani, L., Vargas Huaman, R., Wesselingh, F. (1998) Geología y geoformas de la zona de Iquitos. In: Geoecología y desarollo Amazónico: estudio integrado en la zona de Iquito, Peru. Annales Turkuensis (AII)114, 59–137.

10/24/2009 11:30:05 Shobha

Marine influence in Amazonia Räsänen, M.E., Linna, A.M., Santos, J.C.R., Negri, F.R. (1995) Late Miocene tidal deposits in the Amazonian foreland basin. Science 269, 386–390. Rebata, L., Gingras, M., Räsänen, M., Barberi, M. (2006) Tidalchannel deposits on a delta plain from the Upper Miocene Nauta formation, Marañon Foreland Sub-basin, Peru. Sedimentology 53, 971–1013. Rice, A., Chapman, C. (1971) Observations on the burrows and burrowing behaviour of two mud-dwelling decapod crustaceans, Nephrops norvegicus and Goneplax rhomboids. Mar Biol 10, 330–342. Roddaz, M., Hermoza, W., Hovikoski, J., Brusset, S., Antoine P.-O., Baby, P. (2004) Late Miocene tidal sedimentation in the foredeep of the southwestern Amazonian foreland basin system (Madre de Dios basin, South Peru). In: Tidalites 2004, Conference Abstract Volume, Copenhagen, pp. 161–164. Smith, D.G. (1988) Modern point bar deposits analogous to the Athabasca oil sands, Alberta, Canada. In: de Boer, P., van Gelder, A., Nio, S.D. (eds) Tide-Influenced Sedimentary Environments and Facies. Dordrecht: Reidel Publishing Company, pp. 417–432. Steinmann, M., Hungerbühler, D., Seward, D., Winkler, W. (1999) Neogene tectonic evolution and exhumation of the southern Andes: A combined stratigraphy and fission-track approach. Tectonophysics 307, 255–276. Tessier, B., Gigot, P. (1989) A vertical record of different tidal cyclicities: an example from the Miocene marine molasses of Digne (Haute Provenance France). Sedimentology 36, 767–776. Thomas, R.G., Smith, D.G., Wood, J.M., Visser, J., CalverlyRange, E.A., Koster, E.H. (1987) Inclined heterolithic stratification; terminology, description, interpretation and significance. Sediment Geol 53, 123–179. Van den Berg, J.H. (1981) Rhythmic seasonal layering in mesotidal channel fill sequences, Oosterschelde Mouth, the Netherlands. In: Holocene Marine Sedimentation in the North Sea Basin. International Association of Sedimentology Special Publication 5, pp. 147–159. Vermeij, G., Wesselingh, F. (2002) Neogastropod molluscs from the Miocene of Western Amazonia, with comments on marine to freshwater transitions in molluscs. J Paleontol 76, 265–270. Villamil, T. (1999) Campanian-Miocene tectonostratigraphy, depocentre evolution and basin development of Colombia and western Venezuela. Palaeogeogr Palaeocl 153, 239–275.

Hoorn_ch09_Final.indd 161

161

Visser, R. (1980) Neap-spring cycles reflected in Holocene subtidal large-scale bedform deposits: a preliminary note. Geology 8, 543–546. Vonhof, H.B., Wesselingh, F.P., Ganssen, G.M. (1998) Reconstruction of the Miocene western Amazonian aquatic system using molluscan isotopic signatures. Palaeogeogr Palaeocl 141, 85–93. Vonhof, H.B., Wesselingh, F.P., Kaandorp, R., Davies, G., van Hinte, J., Guerrero, J. et al. (2003) Paleogeography of Miocene western Amazonia: Isotopic composition of molluscan shells constrains the influence of marine incursions: Geol Soc Am Bull 115, 983–993. Webb, S.D. (1995) Biological implications of the Middle Miocene Amazon Seaway. Science 269, 261–262. Wesselingh, F.P. (2006) Molluscs from the Miocene Pebas Formation of Peruvian and Colombian Amazonia. Scripta Geologica 133, 19–290. Wesselingh, F.P. (2007) Long-lived lake molluscs as island faunas: a bivalve perspective. In: Renema, W. (ed.) Biogeography, Time and Place: Distributions, Barriers and Islands. Dordrecht: Springer, pp. 275–314. Wesselingh, F.P., Gittenberger, E. (1999) The giant Amazonian snail (Pulmonata: Acavidae) beats them all. The Veliger 42, 67–71. Wesselingh, F.P., Macsotay, O. (2006) Pachydon hettneri (Anderson, 1928) as indicator for Caribbean-Amazonian lowland connections during the Early-Middle Miocene. J S Am Earth Sci 21, 49–53. Wesselingh, F.P., Räsänen, M.E., Irion, G., Vonhof, H.B., Kaandorp, R., Renema, W. et al. (2002) Lake Pebas: A palaeoecological reconstruction of a Miocene, long-lived lake complex in western Amazonia. Cainozoic Res 1, 35–81. Wesselingh, F.P., Kaandorp, R.J.G., Vonhof, H.B., Räsänen M.E., Renema, W. (2006) The nature of aquatic landscapes in the Miocene of western Amazonia: an integrated palaeontological and geochemical approach. Scripta Geologica 133, 363–393. Westaway, R. (2006) Late Cenozoic sedimentary sequences in Acre state, southwestern Amazonia: Fluvial or tidal? Deductions from the IGCP 449 fieldtrip. J S Am Earth Sci 21, 120–134. Whatley, R.C., Muñoz-Torres, F., van Harten, D. (1998) The ostracoda of an isolated Neogene saline lake in western Amazon basin. In: What about Ostracoda? Bulletin des Centres de Recherches Exploration-Production Elf-Aquitaine 20, 231–245.

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TEN

Megafan environments in northern South America and their impact on Amazon Neogene aquatic ecosystems M. Justin Wilkinson1, Larry G. Marshall2, John G. Lundberg3 and Mikhail H. Kreslavsky4 1

NASA-Johnson Space Center, Houston, Texas, USA Arizona Museum of Natural History, Mesa, Arizona, USA 3 The Academy of Natural Sciences, Philadelphia, USA 4 University of California-Santa Cruz, USA 2

A good theory points to possible factors in the real world that would otherwise remain hidden and thus stimulates new forms of empirical research. Even a first, crude theory can have these virtues. MacArthur & Wilson (1967)

Abstract A megafan is a low-angle, partial cone of fluvial sediments that can reach hundreds of kilometres in length. Its size and depositional processes differentiate the megafan from alluvial fans and deltas. Megafans are a mesoscale, entirely continental depositional feature. This landform has received little attention in either modern or palaeogeographic reconstructions of northern South America. We present a model for landscape development in Neogene Amazonia examining the role of the ‘inland delta’ or megafan. This environmental setting should be taken into account in mesoscale palaeogeographic reconstructions because it is so widespread in modern foreland landscapes in South America and elsewhere. We describe both megafan and multimegafan patterns, the latter introduced by means of a roughness map of northern South America, which also assists in identifying landscapes of different types. We distinguish seven landscape phases; in each we evaluate a specific set of physical environmental processes in megafan settings that have biological significance. The processes are based primarily on stream avulsion, but also on stream incision and drainage-net breakdown. We conclude that fluvial sedimentation of the Andean foreland depression probably was accomplished primarily by megafan sedimentation, with declining estuarine and lacustrine sedimentation as the Neogene progressed. Because megafans are often juxtaposed, hydrological connections between neighbouring megafan rivers were probably repeatedly established – with all the associated biological ramifications of range expansion, population mixing, introgression, reinforced genetic isolation and competition. Regional connections between suites of nested megafans, and between major drainage basins, probably existed at times via critically situated megafans. Some modern fish distributions coincide with the mountain-front zone of megafan development, suggesting that megafan-mediated biological processes could have contributed to speciation. Accordingly, our attempt to reconstruct Miocene and younger landscapes in greater detail, with the benefit of the modern megafan analogue, seems justified.

Introduction We examine one element of landscape development in northern South America that feasibly accompanied the Late Miocene Amazonia, Landscape and Species Evolution: A Look into the Past, 1st edition. Edited by C. Hoorn and F.P. Wesselingh. © 2010 Blackwell Publishing

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tectonic uplift of the Andes Mountain chain – the little-known yet unquestionably prevalent geomorphic feature, the megafan (Figs 10.1 & 10.2). Then we explore possible implications for biological diversification of such landscape evolution. The geographical ranges of species and higher clades result from, and can reciprocally reflect, the old landscapes that promoted their dispersal or vicariance. The great time depth for biological events that have generated modern aquatic species and

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Megafan environments and their impact (a)

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Fig. 10.1 Megafan geometries – internal and external relationships to trunk rivers and basin margins. (a) Megafan sediment cones generally have a triangular fan shape (shaded). The apex is located where the river exits upland (patterned area, left margin). Fans (dashed outlines) are usually nested; trunk river (right margin of the panel) is at distal basin margin. (b) Geometries of megafan river and basin trunk river. T/L, transverse/longitudinal (left, and (a) above); T/T, transverse/ transverse (right) (sensu Miall 1996). (c) T/L drainage pattern between two margins formed by Himalaya Mountains and Deccan uplands: the triangular Ghaghara, Burhi Gandak and Kosi megafans in the confined Ganges valley contrasts with

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Fig. 10.2 The Aripuanã palaeo-megafan. Fine, sinuous lines (centre) indicate sandy channels of a relict megafan (outlined), on the northwest margin of the Brazilian Shield. The arrows show previous courses of the Aripuanã River radiating from the apex (image centre 6.5S, 61W; NASA image STS094–745-25).

their associations (Lundberg 1998; Lundberg et al. 1998; Lima & Ribeiro, unpublished) has driven attempts to reconstruct landscapes since Miocene times, despite the small quantity of evidence available. The fluvial megafan is a global landform that provides a new component in erecting more realistic palaeogeographies. Megafans are barely described in the geological literature, even though a few have been examined in detail in the geomorphic literature of South America, Africa and India (see references below). Many megafans are difficult to discern in the landscape, which may explain why they are less recognized than seems warranted from their global distribution. Although our palaeogeographic reconstructions for Amazonia are speculative, they are based on simple geomorphic patterns that pertain worldwide. Our biological arguments are directed towards aquatic organisms, and are based on a set of megafan river behaviour models and their associated biological dynamics (Wilkinson et al. 2006). These models are only one cluster in a wider group of evolutionary models developed in the last few decades (see reviews in Salo et al. 1986, and Marroig & Cerqueira 1997). We note that fluvial dynamics at a variety of scales have been invoked in the post-refuge studies of the floral richness of Amazonia (see especially Salo 1986; Salo et al. 1986; Räsänen et al. 1987, 1990;

the T/T pattern of the diamond-shaped Tista megafan, which occupies the single-margin unconfined basin east of Deccan. Curved lines show convexities in trunk river (Ganges) course induced by megafan cones.

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Salo & Räsänen 1989; Tuomisto & Poulsen 1996; Tuomisto et al. 2003; and collections of essays in Kalliola 1992; Kalliola et al. 1993; Kalliola & Paitán 1998). Many fluvial features and patterns mentioned in these works notably address characteristics and scales of operation that typify megafans.

Contour maps and cross-sections We constructed several contour maps with different contour intervals (not illustrated here), as well as 37 topographic cross-sections, 28 regional (750–1850 km long) and 9 local (400–600 km long), (a)

Data and methodology Remotely sensed images Analysis of modern stream courses was based on remotely sensed images from two sources in particular, namely true and false colour imagery from the handheld camera image archive at the Johnson Space Center (Earth Sciences and Image Analysis, 2008; URL: http://eol.jsc.nasa.gov), and from true and false colour Landsat imagery of both the 1990 series and 2000 series, both of which include an infrared component.

Gui

Topographic roughness Roughness map To illustrate the geographical variability of modern Amazon topography we employ a roughness map of South America constructed from an algorithm first applied to Mars by Kreslavsky & Head (1999, 2002). Topographic roughness is a measure of slope variability along baselines of different length. Kreslavsky & Head (1999, 2000) generated a global map of topographic roughness of Mars at three baselines of 0.6, 2.4 and 9.6 km, using highprecision topographic profiles obtained by the Mars Orbiter Laser Altimeter (MOLA) from the Mars Global Surveyor mission. These maps can be seen at http://www.planetary.brown.edu/planetary/ rough/. A similar algorithm was applied to the SRTM (Shuttle Radar Topography Mission) digital topographic Earth data (available at ftp://e0srp01u.ecs.nasa.gov/) for a similar array of baselines (0.6, 2.2 and 8.7 km). The details of the data processing algorithm will be published elsewhere. The roughness map for South America is shown in colour (see Plate 1) and combines the roughness measures at all three baselines in a single colour map as a redgreen-blue (RGB) composite, with these channels coding the longest, intermediate and shortest baselines, respectively. In each channel, lighter shades denote greater roughness, i.e. mountainous/hilly landscapes such as the Andes Mountain chain. By contrast, the darkest tones indicate flat and smooth plains landscapes. Shades of blue indicate that the subkilometrescale roughness is relatively more prominent than roughness at longer scales; shades of green indicate the presence of a characteristic kilometre-scale horizontal length of topographic variability, etc. We also render the colour map in grayscale, in which the darkest (smoothest) and intermediate greys are the areas of interest in this research (Fig. 10.3a). The map shows the major geomorphic distinction between eroding landscapes (light-toned surfaces) and depositional landscapes, such as widespread smooth megafan surfaces and a few delta surfaces (also dark surfaces).

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Modern megafans

Fig. 10.3 (see also Plate 1) (a) Topographic roughness map of South America rendered in grey tones (see text for detail concerning construction). The map shows four broad landscape categories and major foreland trunk rivers (see text). Lightest tones indicate the roughest and steepest landscapes. Darkest tones represent the smoothest, lowest-sloped landscapes, dominated by megafan plains – Chaco (Pc), Beni (Pb), Pastaza-Marañón (Pp), Llanos plains of the Orinoco drainage basin (Pl) and Pantanal plains (Pt). Some larger constituent megafans are outlined, those in the Chaco (Pc) taken from Iriondo (1984, 1987, 1993). 1, Napo River; 2, Pastaza River; 3, Corrientes River; 4, Madeira River; 5, Parapetí megafan; 6, Pilcomayo megafan. Intermediate tones indicate landscapes of intermediate roughness: constituent rougher landscapes (lighter grey) of western Amazonia (zone Aw) and smoother (darker grey) in central Amazonia (zone Ac) separated by white dotted line. Trunk rivers of foreland shown as bold white lines (Orinoco, MamoréGuaporé, Paraguay-Paraná). Late Miocene axis of the foreland basin – grey dashed line (after fig. 19 in Lundberg et al. 1998). AA’, BB’, CC’ are upper, middle and lower section lines in (b).

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Megafan environments and their impact (b)

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C’

Fig. 10.3 (b) Topographic cross-sections showing roughness in different zones (see text). Upper panel: Chaco plains megafan landscapes: section AA’ from southwestern Brazil (Serranía de Santiago) to central Argentina (Sierra de Norte) (20ºS–30ºS) showing flatness (short baseline roughness dominance) and great extent of transverse slopes (four neighbouring megafans); regional topography varies by only 70 m over a distance of >1100 km (double-headed arrow). Middle panel: western Amazonia–lower Río Negro cross-section BB’ aligned ~2°S: modern megafan plains east of the Morona River (zone Pp) with maximum local altitudinal range of <8 m. In zone Aw (western Amazonia) relief varies by >50 m, compared with zone Ac (central Amazonia), where local relief (excluding major valleys) varies by <10 m. Lower panel: western Amazonia–Mérida Andes (Caribbean) cross-section CC’ (south to north): smooth plains of modern megafans of Pastaza plains (zone Pp; megafans constructed by Morona, lower Pastaza and lower Corrientes Rivers), with maximum local relief generally <10 m; these give way northwards to relatively rough landscapes of incised sedimentary bodies (inferred Pastaza, Caquetá-Putumayo, Guaviare and Meta megafans – see detail in Fig. 10.7) of zone Aw, with maximum local relief (excluding hilly shield terrain) of >50 m. In the Llanos Basin north of the Meta River, smooth megafan-dominated plains of the modern Apure system (zone Pl) have local relief <10 m. Vertical exaggeration ~1100; note bend in sections.

variously aligned. The maps and sections were also based on SRTM data manipulated with the Microdem freeware mapping program, which displays digital elevation models, map images, GIS databases, etc. (http://www.usna.edu/Users/oceano/pguth/website/ microdem.htm ). The vertical accuracy of the SRTM data is everywhere better than 5 m; horizontal spacing of SRTM data points varies around 90 m (Rodriguez et al. 2005; Gesch et al. 2006). The latter authors mention inaccuracies induced by dense vegetation. Generally, vegetation only influences topographic roughness in

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a minor way, because we use horizontal scales of roughness that are significantly larger than the characteristic horizontal scale of roughness contribution by vegetation. Detailed comparison of the roughness map with LANDSAT images in the Llanos region shows that the boundary between smoother and rougher terrains does not coincide with the boundary between savannas and rainforest, but angles across the savanna. The rougher appearance of the Amazon drainage basin generally reflects real topography rather than vegetation.

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Biotic distribution data The biological dynamics of our megafan models are oriented to aquatic organisms. In order to assess the reality of these models good data are needed on the geographic ranges of species and higher clades. Distribution maps of aquatic taxa are generally imprecise with large blank areas due to the lack of biological collecting. In addition, poor taxonomic resolution or specimen identification adds inaccuracy to range maps. Aware of these issues of quality control, we draw our examples from recent taxonomic studies based on large, vouchered museum samples and modern principles of systematics.

~200,000 km2. Nested megafans in the Bolivia-Paraguay-Argentina sector of the Andean foreland cover the immense area of ~750,000 km2 (see Fig. 10.3a) (Wilkinson et al. 2006) (megafan surfaces in South America total 1.2 million km2; M.J. Wilkinson, unpublished observations, 1996). Sets of nested megafans are known from several parts of the world, as described in India (Geddes 1960), the southern Sudan (Wilkinson et al. 2008a) and the Kalahari Desert (Wilkinson et al. 2008b). Wilkinson (2001, and M.J. Wilkinson, unpublished observations, 1996) has demonstrated that these features occur in all climates.

Megafans and palaeogeographical reconstruction

Megafan characteristics

The following additional perspectives inform our palaeogeographical reconstructions.

Modern megafans

Megafans versus floodplains, alluvial fans and deltas

Megafans are partial cones of river-laid sediment of wide extent (radii up to 700 km) and low slope (Gohain & Parkash 1990; Sinha & Friend 1994; Wilkinson 1999, 2001, 2003, 2005; M.J. Wilkinson, unpublished observations, 1996, among others). Megafans comprise mainly numerous alluvial tracts sensu lato (composite features, primarily channel-levee systems and overbank environments) but include windblown, lake and marsh facies and very widespread pedogenic units. General descriptions appear in Geddes (1960), Chorley et al. (1984), Miall (1996), Horton & DeCelles (2001), Leier et al. (2005) and Wilkinson et al. (2006). The partial cone morphology induces the megafan river to switch from one position to another, with prior channels commonly visible radiating from the fan apex (see Fig. 10.2). The global study of larger megafans (radii defined as >100 km in order to focus on the mesoscale) shows that apexes are located at the margins of mountain ranges, cratonic uplands and positive topographic features of all kinds, such as Amazon Trough arches, forebulges and resistant bedrock structures (M.J. Wilkinson, unpublished observations, 1996). Two processes provoke a river to switch courses, avulsion (Slingerland & Smith 2004) and stream piracy. Avulsion occurs when the formative river builds up its levees as much as several metres above the surrounding floodplain surface. This unstable situation leads to the potential for sudden shifts of the river course. The Kosi River (India), one of the best documented fan systems, is known to have shifted progressively westwards across the fan surface in a period of 213 years (Mookerjea 1961; Holmes 1965; Gole & Chitale 1966; Singh & Singh 1971; Wells & Dorr 1987), avulsing on average every ~20 years to a new position on the megafan surface. Stream piracy is a process more related to incisional phases of drainage evolution. We examine this topic no further here, as it depends on drainage analysis in the kind of detail that is as yet almost non-existent for Amazonia. Megafan radius is generally determined by the width of the foreland (or foreland-backbulge) depression (see Fig. 10.1b, left). Megafans can thus reach lengths of hundreds of kilometres between the apex and the trunk river floodplain. Accordingly the trunk river (see Fig. 10.1b,c) occupies the most distal location in the topographic depression. Megafan areas vary from ~7000 to

Wilkinson (2001, M.J. Wilkinson, unpublished observations, 1996) has argued that it is useful to distinguish clearly between these four fluvial landscapes and their unique sedimentary bodies. Megafans ought to be distinguished from floodplains for several reasons, one of the most important being that floodplains are confined within valley sides, whereas megafan rivers are unconfined. Among other things, this topographic control gives rise to various biological dynamics (see below) and significantly alters internal architectural geometries (M.J. Wilkinson, unpublished observations, 1996). Megafans differ from alluvial fans mainly due to the different combination of processes that act upon them as a result of their differences in scale (Group 9 vs 8 in Miall’s 1996 hierarchical schema – Table 10.1) (Wilkinson 2001, and M.J. Wilkinson, unpublished observations, 1996). Larger deltas appear to be classed as Group 9 architectural units in Miall’s (1996) schema – i.e. as mesoscale features. We suggest that megafans probably also occupy this level (see Table 10.1) (M.J. Wilkinson, unpublished observations, 1996). Sets of nested megafans would occupy the next level. Deltas are a commonly invoked fluvial component of Neogene Pebas landscapes. However, we suggest that megafans were probably far more common, no less so than on the margins of shallow, epicontinental waterbodies (M.J. Wilkinson, unpublished observations, 2004). Several features distinguish megafans from deltas, especially controlling process – proximal avulsion for megafans versus stream flow stagnation for deltas. The delta model may be less appropriate therefore for such waterbodies. Although little recognized, examples of megafans entering continental waterbodies are common; for example the Dulce River megafan of Argentina forms the northern shoreline of lake Mar Chiquita. Other examples are the Ili River megafan, which comprises 300 km of the 500 km southern margin of Lake Balkhash (Kazakhstan); and megafans comprising the northern, western and southern coastlines of Lake Chad in the southern Sahara Desert. During the lengthy phases of shallow wetland and lake landscapes in Amazonia (~23 to 7 Ma – Wesselingh & Salo 2006; see also Chapter 8) when regional continental aggradation was taking place, we envisage a palaeogeography dominated by nested

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Table 10.1 Miall’s (1996) hierarchical schema of fluvial sedimentary bodies (architectural elements). The third column shows rivers, alluvial fans and deltas from Miall (1996) and DeCelles et al. (1991). The fourth column contains suggested equivalent position of megafans and sets of megafans (from Wilkinson, unpublished observations, 1996). Group Timescale (years)

Fluvial sedimentary bodies (architectural elements) – abbreviated hierarchy Rivers and alluvial fans

Megafan distributary systems (radii >100 km)

6

102–103

Shallow channel, large streambed macroform

Shallow channel, large streambed macroform

7

103–104

8

Channel, fan trench backfill

Channel

4

5

Channel belt, alluvial fan

Channel belt

5

6

10 –10

9

10 –10

Floodplain, alluvial fan tract (bajada), delta, major depositional system axis (Gulf of Mexico coast depositional axes)

Megafan

10

106–107

Smaller basin-fill complexes (Tertiary formations, Gulf of Mexico coast)

Set of nested megafans

11

7

10 –10

8

Larger basin-fill complexes (Triassic Molteno Formation, Karoo basin)

megafans, some possibly of great length, sloping into waterbodies along hundreds of kilometres, or more, of coastline (Fig. 10.4). Modern megafan landscapes in South America as analogues for the past Zones of active sedimentation appear on the roughness map as dark areas, i.e. areas of smoothest and flattest topography (see Plate 1 & Fig. 10.3). Such areas are commonly thought of as floodplains of major rivers. However, the distribution of modern megafans in South America (Iriondo 1984, 1987, 1993; Wilkinson et al. 2006) coincides strikingly with almost all the smoothest and flattest zones. These vast plains occur primarily within the foreland depression (see Fig. 10.3a): • Chaco plains of central and northern Argentina, and western Paraguay (zone Pc) – largest area of contiguous megafans; various megafans described by Cordini (1947), Iriondo (1984, 1987, 1993), Malagnino (1989), Clapperton (1993), Comisión Nacional del Río Pilcomayo (1994) and Meyer (1996); • Beni plains (zone Pb) (Llanos de Mojos) of Bolivia – megafans described by Allenby (1988), Dumont & Fournier (1994); • Llanos, Orinoco Basin (zone Pl) – small, active Ariari-Güejar megafan documented by Wilkinson et al. (2006). Smaller areas of localized aggradation are the Pastaza plains (zone Pp) in eastern Peru (modern megafans described in detail by Neller et al. 1992, and Räsänen et al. 1990, 1992) and the Pantanal plains, southwestern Brazil (zone Pt), mainly the Taquari River megafan (Assine 2005). Comparatively insignificant areas of smooth, flat topography are the Orinoco delta, the Huallaga lowlands, floodplains of the Marañón and lower Amazon rivers, the Araguaia plains of northeastern Mato Grosso, Brazil, and a small interfan zone in Venezuela. Topographic cross-sections of the Chaco plains (see Fig. 10.3b, upper panel) show the smoothness and large transverse distances occupied by nested megafans. The middle and lower panels (see

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Fig. 10.3b) contrast modern accumulational (megafan) plains with rougher Amazonian landscapes – zones Aw and Ac in Fig. 10.3a – that are central to this study. To apply these patterns in Amazonia we have examined the potential palaeogeographies that arise if the known palaeogeographic criteria are matrixed: (i) wide regional fluvial sedimentation (region-wide overfilled foreland basin like the Gran Chaco) versus localized situations (shifting depocentre model of Räsänen et al. 1987, 1998); (ii) presence versus absence of major waterbodies; (iii) alternation of minor aggradation and erosion phases (many such phases are identified by Hovikoski et al. 2007, among others) (Table 10.2). Four of seven resulting potential landscapes typify conditions in western Amazonia under conditions of longterm regional sedimentation, as seems appropriate for the Late Miocene (regional depository; see Table 10.2 & Fig. 10.4). Regional drainage geometries and asymmetric sediment supply Miall (1996) has identified simple, recurring rectilinear drainage geometries based on river orientation with respect to the mountain front: the transverse/longitudinal (T/L) pattern is the classic pattern for most foreland basins where transverse tributary (megafan) rivers give way to a longitudinal trunk river (parallel with mountain front). In South America the Paraguay-Paraná drainage pattern is the classic T/L pattern of the two-margin foreland (see Figs 10.1a,b & 10.4). The existence of two basin margins (proximal mountain front and distal cratonic upland) gives roughly triangular megafan shapes. By contrast, the transverse/transverse (T/T) pattern arises in basins with only one margin (mountain front), with tributary rivers (megafan river) and the trunk river oriented broadly transverse to the mountain front (much of Amazonia, where the foreland basin intersects the Amazon Trough; see Figs 10.1b right & 10.3a). Diamond-shaped megafans are more typical (M.J. Wilkinson, unpublished observations, 2004). The clearest modern landscape relationships appear on the roughness map (see Fig. 10.3a) in T/L sectors in which trunk drainages (Orinoco and Mamoré-Guaporé and Paraná sectors – bold

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M.J. Wilkinson et al. Megafan aquatic environments Major megafan river Minor megafan river Lakes, swamps

River habitats Upland Alluvial fan Main megafan Minor megafan Delta distributary Trunk river

Wetland habitats Trunk river floodplain Delta interdistributary Coastal Open Megafan lakes, swamps (b)

Fig. 10.4 Putative regional lowland landscapes during major phases of fluvial aggradation, showing aquatic environments. (a) Regional sedimentation: fluvial zone (dominated by nested megafans up to hundreds of kilometres long, with minor alluvial fans) gives way downstream to a wetland/lake system during lake-estuary phases (left – Pebas and post-Pebas times), or to a trunk river during non-lake/estuary phases (right). (b) Minor incision: landscape pattern as in (a), but with minor incision of megafan rivers. Small deltas or incipient megafans form where megafan rivers enter waterbodies (left).

white lines) have been displaced eastwards by megafan sediment cones to positions against the cratonic uplands (Guiana and Brazilian Shields). The controlling factor – the asymmetry of sediment supply – is documented in the rivers of the Orinoco drainage basin with similar water discharge yield profoundly different quantities of sediment to the trunk river, depending

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on whether they rise in the Andes orogen or on the Amazonian Craton (Warne et al. 2002) (Fig. 10.5a,b). The Ganges foreland plain also provides a persuasive analogue for northern South America: the left side of Fig. 10.1c shows the T/L river pattern, with the Ghaghara, Burhi Gandak and Kosi megafans as neighbouring triangular features where the Ganges is

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Table 10.2 Matrix of potential Neogene landscape dynamics in Amazonia. Fluvial sediment bodies accord with Groups 10 or 11 of Miall (1996). Potential regional landscapes in major sectors of Andean foreland basin Regional depository (fluvial compenent is suites of nested megafans; this study)

Localized depositories: major floodplains, localized megafan landscapes, with major intervening erosional landscapes (shifting depocentre model) Sedimentation Mainly fluvial: modern sectors of Pastaza megafan, parts of Marañón and Llanos basins

Erosion Much of modern western Amazonia, parts of Orinoco drainage basin

Sedimentation Fully continental Mainly fluvial: modern (regional waterbodies absent) Gran Chaco of Bolivia, Paraguay, Argentina

Erosion Western Amazonia, Orinoquia, Mamoré-Guaporé drainage basins

Regional Sedimentation waterbodies present Fluvial and lacustrine/ estuarine (Pebas and similar environments)

Sedimentation Erosion Erosion Fluvial and lacustrine/estuarine On local uplifts (arches?) Episodically during lowered base levels (shrinking/ disappearing Pebas wetlands)

oriented east–west – a geomorphic parallel for the classic Andean foreland. Immediately to the east, the Ganges trunk turns southwards. The right side of Fig. 10.1c shows the resulting T/T pattern in which the trunk Ganges, tributary Tista River and the diamondshaped Tista megafan provide a geomorphic parallel for our reconstruction of the Amazon Trough sector. Prior indications of megafans in the literature Theories of evolution of the western Amazonia sector of the Andean foreland include the shifting depocentre theory of Räsänen et al. (1987, 1998). Interestingly, this theory is replete with facts and research conclusions that corresponded with many megafan characteristics. For example, some investigators have proposed that shifting rivers have dominated sedimentation of the Neogene Andean foreland. Furthermore, they have proposed that these remarkably widespread fluvial units have since been raised tectonically to form the Serra de Divisor and Contamana Arch of Acre and central Amazonian Peru (see particularly Salo et al. 1986; Salo & Räsänen 1989). Without invoking megafans per se, Räsänen et al. (1987, pp. 1398–1400) note features and processes strongly suggestive of megafan-scale and multi-megafan features, mentioning ‘channel migration’, ‘shifting floodplain networks’ and ‘channel diversions of the aggrading rivers’. Processes reminiscent of megafan dynamics are also invoked to explain the more recent sedimentary units of Amazonia. Wesselingh & Salo (2006, p. 442) suggest that Late Miocene and Pliocene Amazon ‘fluvial systems may have been poorly delimited and shifted drastically over lowland Amazonia’. Räsänen et al. (1987, pp. 1398– 1400) consider that ‘the uppermost terra firme areas resemble those being currently deposited by the meandering and braided rivers … [in the modern] Andean forelands’, and they speak of ‘laterally migrating Pleisto-Holocene rivers, similar to present ones’. Present actively aggrading rivers include megafan rivers in South America. These authors also invoke the Himalayan foreland as their analogue – one of the few areas from which megafans are well known in the geological and geographical literature. Similarly, Räsänen et al. (1992) specifically relate widespread remnants of fluvial beds on the Fitzcarrald Arch to sedimentary environments such as those of the modern Himalayan and Mesopotamian foreland basins.

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To our knowledge, only Latrubesse and co-workers (Latrubesse 1992; Latrubesse et al. 1997) and Bès de Berc et al. (2005) explicitly refer to palaeo-megafans as a depositional environment in filling the trough sector of the foreland. Further south in Bolivia the megafan model has been invoked by Horton & DeCelles (2001) and Uba et al. (2006) in their sedimentary models of Andean Tertiary rock successions. Some workers have questioned the megafan model of Andean foreland sedimentation (Campbell et al. 2006), but we see the modern ubiquity of the megafan feature as a persuasive analogue, for past landscapes. Unconfined river sediments constitute the major architecture in this scenario (~Groups 8–9 elements, Table 10.1). An alternative style that can account for regional fluvial aggradation is that of stacked confined valley fills, an architecture termed the PV mega-architectural element (palaeo-valley fills, Group 8? – see Miall 1996). This scenario implies numerous phases of aggradation and incision, which we regard as improbable in a situation of rapid regional alluviation that characterized Late Miocene Amazonia.

Inferring palaeo-megafans All the above indications and perspectives are employed in our palaeogeographic reconstructions of western Amazonia. Global studies have provided a sufficiently large sample of modern megafans for, at minimum, an understanding of locational controls – to the point that the existence of a megafan without obvious features in remotely sensed imagery was successfully predicted in southern Angola (Wilkinson et al. 2008b). Several criteria inform our interpretations of the few subrecent and inferred palaeo-megafans in this study: • • • • • •

roughness signatures of aggradational surfaces; diverging channel patterns; a feasible apex point; planiform convexity of the trunk stream away from an impinging megafan; convexity of transverse topographic (vertical) sections; river courses located in feasible inter-megafan depressions (a clear post-aggradation pattern).

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(a)

Water discharge Orinoco basin OrinocoRiver drainagebasin

Atlantic Ocean Orinoco R Caroni R

Megafans and biodiversity

Caura R

Ventuari R

N 120 Mean water discharge (1010 m3/yr) 0

upper Orinoco R 200 km

(b)

ES

Asymmetric sediment discharge Orinoco basin Atlantic Ocean

G

MEGAFAN --

D

EA

N

R

A

N

Orinoco R

upland margin GUIANA N SHIELD 150 Mean suspended sediment discharge 0 (106 ton/yr)

A

N

D O M I N AT E D

PLAINS 200 km

(c)

á ur

Suspended-sediments discharge (106 tons per year)

ro eg

p Ja

á

Iç

N

Gu 600

á ru Ju ira e ad M

ure

AMAZON

Ap

av iar Me e ta

1200 150

ORINOCO 0

0

á

í ron Ca

ura Ca

I Upnírid Or per a ino co

0

Tap ajó s Xin gu

P Ma urús de ira

Ju Ju taí r uá

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ORINOCO

0

km

Distance above delta apex 0 1200

km

Wilkinson et al. (2006) illustrated several ways in which stream dynamics on megafans may have promoted speciation in aquatic fauna and mixing of their populations (Table 10.3 & Fig. 10.6). Their models were based on three major stream behaviours (two not unique to, but widespread on, megafans). Megafan stream dynamics not only play a part in present-day population dynamics, but also operated in this way in the past. The models describe specific stream dynamics: (i) stream course switching; (ii) drainage net dislocation/break-up, generally caused by climatic change and tectonic factors; and (iii) stream incision into megafan surfaces. Real-world combinations of megafan river dynamics provide further models that need to be considered as part of the evolutionary biology toolkit. One such combination, Model 4, was attempted by Wilkinson et al. (2006). Examples of the specific stream dynamics known in South America are given, although these have not been investigated empirically with fish populations or species. Stream incision as a dynamic on megafans introduced here as an additional model (see Table 10.3). Incision here implies sufficient downcutting that the megafan river is unable to avulse across the megafan surface, so that subsequently incised streams produce disjunct habitats. Those species unable to negotiate the trunk river consequently experience some, or all, of such processes as range fragmentation, population divergence, speciation and extirpation (Model 5-v, Table 10.3). Stream incision can be a long-term condition (millions of years). Such timespans extend the period of vicariance to the point that genetic drift can occur, a likely state for populations dwelling in incised megafan streams. We thus denote this model as both an intrafan and interfan diversity model. A simple test of this model is to compare lists of aquatic species in megafan rivers (for species that cannot negotiate the trunk river): megafan streams incised in the more distant past ought to reveal greater evidence of vicariance than megafans incised more recently. Geomorphic analyses can often identify differences in relative age.

1

Gu

Iç

av Vic iare ha Me da ta Ap ure

Ja p

ur á Ne gr o Tr om be

ta

s

Water discharged (1012 m3 per year) 6

AMAZON

3000

Singly, none of these criteria is sufficient indication of past megafans, but in concert they add confidence to our interpretations. A test for the existence of the inferred megafans would be the presence of fluvial sediments with a homogeneous mineralogical signature across the entire area of the proposed feature.

0

Fig. 10.5 Asymmetry of sediment supply from craton-sourced rivers versus Andean rivers in the Orinoco and Amazon drainage basins. (a,b) Orinoco River water and sediment discharge, showing asymmetric supply of sediment to the Orinoco River by rivers from the Andes Mountains. (c) Quantitative measures of water and suspended sediment in the Orinoco and Amazon drainage basins. Compare especially the Negro and Madeira affluents of the Amazon River, and insignificant sediment contributions of the Guiana Shield versus Andean tributaries in the Orinoco drainage basin (after Warne et al. 2002).

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Table 10.3 Megafan river behaviour: models of vicariance and habitat connection or reconnection (from Wilkinson et al. 2006). Models based on river behaviours

Potential biological effects

Stream reorientation: Relict lakes, relict streams; connection/disconnection between megafan streams and major basins Models 1.1 and 1.2: Stream reorientation on individual megafans (intrafan diversity) 1.1-v Lake formation – disconnection of water body from Range fragmentation, population divergence, speciation, drainage net extirpation 1.1-c

Lake reintegration into drainage net

Range expansion, population mixing, introgression, reinforced genetic isolation, competition

1.2-v

Abandonment of course (river beheading)

Range fragmentation, population divergence, speciation, extirpation

1.2-c

Reoccupation of course

Range expansion, population mixing, introgression, reinforced genetic isolation, competition

Model 1.3: Connection and separation of rivers on neighbouring megafans (interfan diversity) 1.3-v Disconnection of megafan streams Range fragmentation, population divergence, speciation, extirpation 1.3-c

Connection between megafan streams

Range expansion, population and faunal mixing

Model 1.4: Connection between rivers of major basins (interbasin diversity) 1.4 Switching behaviour connects major basins Range expansion, faunal mixing Drainage net disintegration and reintegration on megafans (intrafan and interfan diversity) Model 2: River discharge fluctuations due to climate change 2-v Dry trend: river endpoint retreat Range fragmentation, extirpation, population divergence, speciation 2-c

Wet trend: river endpoint advance and drainage reconnection

Range expansion, population mixing, introgression, reinforced genetic isolation, competition

Model 3: River blockage due to channel sedimentation (with associated river endpoint retreat) 3-v Fan river–trunk river separation Range fragmentation, population divergence, speciation, extirpation 3-c

Fan river–trunk river reconnection

Range expansion, population and faunal mixing, introgression, reinforced genetic isolation, competition

Combinations of river behaviours and effects Model 4: Connected megafans separated from the trunk river (interfan diversity) 4-v Fans connected; fan and trunk rivers disconnected Range fragmentation, population divergence, disconnected speciation, extirpation 4-c

Fans connected; fan and trunk rivers connected

Range expansion, population and faunal mixing, introgression,reinforced genetic isolation, competition

Megafan stream dynamics cease due to stream incision Model 5: Fan-wide incision prevents stream avulsion and thus connection between megafan streams (intrafan, interfan diversity) 5-v Fan streams prevented in longer term from connection by Range fragmentation, population divergence, speciation, avulsion extirpation 5-c

Fan streams reconnect after regional alluviation again allows stream avulsion on megafans

Range expansion, population and faunal mixing, introgression, reinforced genetic isolation, competition

-v, habitat disconnection; -c, habitat (re)connection.

Landscape evolution and possible biodiversity implications We examine landscape evolution under four broad constraints, and in seven timespans. The constraints are: 1

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the apparently critical Late Miocene Andean uplift event at the beginning of the period of interest (see Chapter 4);

2 3 4

the final drainage orientation of the Amazon Basin in an easterly direction; a related regional cover of continental sediments; the likelihood of megafans as the medium of fluvial continental deposition.

Several workers have presented palaeogeographies of Amazonia, especially of Neogene times (among others Hoorn 1993, 1994a, b;

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2

2

1

2

1 1

3

Model 1.1

1

2

1

2

Model 1.3

3

Model 2 and 3

Zone A Model 4

A 2 1

1

2

3

Model 1.2

Model 1.4 B

Model 5

Fig. 10.6 Summary of speciation and dispersal models. One model is summarized in each panel. The main river in each diagram is the trunk river (arrowed), into which megafan rivers flow. Model 1.1: Megafan river with original fish population (symbol – –) switches to a new position (stage 1), leaving relict populations in lakes along the original course. Relict populations speciate (symbols +, ×, o; stage 2). The megafan river switches back to the original position, intersecting with the lakes (stage 3) and introducing new species to the river network. Model 1.2: The megafan river shifts course, carrying the original population (symbol +) (stage 1). The relict population in the beheaded river sector speciates (symbol o; stage 2). The trunk river (sinuous) is assumed to be inhospitable to the original species and acts as a barrier to migration. The megafan river switches back to its original course, introducing new species into the main megafan river (stage 3). Model 1.3: Rivers on neighbouring megafans simultaneously avulse together, mixing two species (symbols + and o) below the confluence (stage 1). When switching later occurs (stage 2), mixed populations occupy both rivers. Model 1.4: The megafan occupies a divide between river basins A and B. The megafan river alternately delivers water into each basin. The population in A (symbol o) is thus introduced into basin B (stage 1). At the next switching event, any basin B fish (symbol +) that invade the megafan sector of the drainage are delivered into basin A (stage 2). Models 2 and 3: River reach with active flow becomes detached from the trunk river (stage 1); see text. The original fish population (symbol +) is divided. The megafan population speciates (symbol o; stage 2). When the megafan river reconnects with the trunk (stage 3), new species are introduced into the trunk. Model 4: This is a combination of Models 1.3 and 2 (or 4), one of many plausible combinations. This multi-fan model shows disconnection of megafan rivers (during arid climatic phases) from the trunk river (stage 1, short arrow). Splitting of the original fish population (symbol +) in the trunk river allows speciation in one megafan river (population with symbol o, circled, stage 2). The new population is subsequently fed down the shaded piedmont zone A from river to river whenever rivers on neighbouring fans avulse together (curved arrows; stage 3). Model 5: Incision by megafan streams (hatching symbols along stream courses) prevents stream switching and hence connection between megafan streams. Every stream thus becomes a vicariant habitat, with potentially distinct aquatic populations (symbols –, +, ×, o). Adapted from Wilkinson et al. 2006.

Hoorn et al. 1995; Potter 1997; Lundberg et al. 1998; Räsänen et al. 1998; Gingras et al. 2002; Hovikoski et al. 2005, 2007; Roddaz et al. 2006; Rosetti & Goés 2005; Campbell et al. 2006; Wesselingh 2006a; Wesselingh & Salo 2006; Wesselingh et al. 2006a, 2006b, 2006c). In the sequence of events we portray, precise timing is relatively unimportant. We follow the sequence outlined by Hoorn et al. (1995), Lundberg et al. (1998) and Wesselingh & Salo (2006), and their extensive supporting citations. Alternative interpretations and timings for major events have been argued for recently by Rosetti & Goés (2005) and Campbell et al. (2006). Landscape developments in each timespan are then evaluated against the megafan-related models of vicariance and habitat connection presented above (see Table 10.3 & Fig. 10.6). We have attempted to limit the observations below to those that are testable. A great variety of distributional patterns is exhibited by species and higher clades of Amazonian aquatic organisms. Some

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distributions are singular with no discernible correlation to those of other taxa or to geography. Some distributions stand out with respect to the megafan models outlined above. Species or groups of species (which may be clades) with current distributions in the zone between the low-altitude streams with trunk rivers on one hand, and higher altitude Andean streams on the other, may be the best candidates for recent and current groups that could have been influenced by megafan dynamics.

Late Oligocene-early Middle Miocene (Fig. 10.7a) Landscape evolution Various kinds of data reveal that palaeo-drainage directions emanating from the Guiana Shield dominated northwest Amazonia before the Late Miocene uplift of the Andes (Räsänen et al. 1992;

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Fig. 10.7 Schematic stages of landscape evolution in western and central Amazonia and the Orinoco Basin showing hypothesized craton- and Andes-sourced megafans. Bold curved arrows suggest possible hydrological and biological connections between neighbouring megafans; bold, long, straight arrows represent directions of hypothesized transverse megafan development (see text). (a) Late Oligocene to early Middle Miocene. Craton-sourced megafans dominate a mesoscale fluvial depositional environment. (b) Late Middle Miocene. Andes-sourced rivers are major suppliers of sediment to the foreland basin, producing megafans. The Pebas System separates diminishing Guiana megafan-dominated landscapes to the northeast from expanding Andes-dominated megafans and wetland landscapes to the west. Documented megafans, dark shading; inferred megafan, light shading; hypothesized megafans, no shading. Dashed line suggests east-flowing trunk river (palaeo-Amazon). (c) Late Miocene to Pleistocene. Continued Andes-sourced megafan growth (and new megafans). Trunk rivers (Amazon, Japurá) occupy depressions between megafans, directed broadly eastwards. Both the Orinoco and Negro trunk rivers are situated against the craton margin. Megafans shown are based partly on documented examples (dark shading); megafans inferred from probable relicts in the landscape are in light shading. Bolivian sector megafans are hypothesized (not shaded; adapted from Latrubesse et al. 1997). Documented and inferred megafans are named after formative rivers: Apure Ap; Aripuanã Ar, Caquetá-Putumayo CP, Demini De, Guanare Ga, Guayabero-Caquetá GC, Guaviare Gu, East Japurá Je, Jiparaná Ji, West Japurá Jw, Juruá Ju, Meta Me, early Pastaza Pa1 (Pastaza adapted from Neller et al. 1992; Räsänen et al. 1992; Bès de Berc et al. 2005; Aripuanã and Jiparaná from Latrubesse 2002). Maps adapted from Wesselingh et al. (Chapter 26 and Plates 14–16).

Hoorn 1993, 1994a, 1994b; Hoorn et al. 1995; Räsänen et al. 1992; see also Chapter 7), as suggested by the small arrows in Fig. 10.7a. During phases of widespread sedimentation/aggradation, regional tectonic events suggest that drainage from the Guiana and Brazilian Shields provided most of the sediment to the depression. We hypothesize that these drainages generated megafandominated landscapes (open megafan symbols in Fig. 10.7a) that perhaps varied from near-continuous, in the style of the Chaco plains (zone Pc in Fig. 10.3), to discontinuous. Rivers flowing into

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the depression from the low Oligocene-Miocene Andes chain are documented (Hermoza et al. 2005), but they apparently supplied relatively little sediment compared with craton-sourced rivers. All landscape reconstructions include a great inland estuarylake system that stretched from the Maracaibo area in the north at least as far south as Acre state, Brazil (see references above), which persisted for millions of years (Wesselingh & Salo 2006; see also Chapter 8). We envisage megafan sediment wedges prograding into these waterbodies (diagrammed in Fig. 10.4) during the

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apparently repeated marine incursions that flooded parts of the foreland depression (Hovikoski et al. 2007). The highly fluctuating nature of waterbodies in the Pebas estuary-wetland regions is mentioned frequently (e.g., Hovikoski et al. 2007, and references therein) and implies that dry land existed at times over large tracts between the Andes and cratonic uplands, at least in southwest Amazonia. During such fully continental conditions, megafans can be expected to have extended distally to swamps and trunk rivers in axes of low topography (see Fig. 10.4a,b, right). Alternating phases of regression and transgression, as proposed by many researchers in the Pebas lake-estuary system, probably induced incision and aggradation phases, so that the dynamics of aggradation-related Models 1–4 alternated with those of incision-related Model 5. The landscape evolution model we apply suggests that a westflowing trunk river occupied the depression developed at the distal limits of sediment cones emanating from northern and southern (craton) margins of the trough (see Fig. 10.7a). Possible biological dynamics We expect that all the biological processes described in Models 1–5 (see Table 10.3 & Fig. 10.6) probably operated during this long phase along the foreland-facing and Amazon Trough margins of the Guiana and Brazilian Shields. The vicariant mode of the models (see Table 10.3) describes such effects as range fragmentation, population divergence, speciation and extirpation. The habitat-connection mode describes such processes as range expansion, population and faunal mixing, introgression, reinforced genetic isolation, and competition. One of the most salient results of our analyses concerns the connection mode and implies regional species homogenization along at least sections of the individual shield margins (curved arrows, Model 1.3-c) in Fig. 10.7a. It is also possible that shield margins on both sides of the Amazon Trough may have been connected at times, acting as a single biological zone for some animals. Examples of fish distributions that may have involved megafan dynamics are apparent from distribution maps. Brazilian scientists have recently discovered undescribed catfishes of the genera Gladioglanis and Scoloplax that seem to be endemic in the Aripuanã megafan river (Rocha et al. 2008a, 2008b). However, congeneric relatives occur in the Tapajós, Negro and Xingu drainages, and some species even span the upper Amazon tributaries extending into the upper Paraná drainage basin. Range expansion or migration ‘laterally’ along craton margins (lower curved arrow in Fig. 10.7a) is thus a hypothesis to be tested. The ‘leaky’ divide between southern tributaries of the Amazon and the Paraná drainage basins appears to operate today in the Goiás region. Hydrological connections are also known to operate on the Parapetí megafan, which occupies this divide (Iriondo 1987). This megafan probably mediated water, sediment and biological exchanges between the Mamoré-Guaporé and Paraguay-Paraná Basins (Model 1.4-c). Species adapted to megafan-sector rivers may thus have been transported regularly between the basins, although, as Wesselingh & Salo (2006) note, this connection does not appear to have been strong. Many examples exist worldwide of megafans situated on drainage divides between major basins: Zambezi-Congo and Zambezi-Okavango divides in southern

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Africa; the Wadi Saoura-Taoudenni basin divide (southwestern Algeria); the Muglad-Salamat basin divide (southwestern Sudan); the Ganges-Indus divide in South Asia; and the Huang He-Chang Jiang (Yellow-Yangtze) divide in East Asia (M.J. Wilkinson, unpublished observations, 2001). Therefore, we are confident that ancient fluvial connections probably existed between Amazonia and the Paraguay-Paraná Basin – quite apart from any marine or lacustrine connections that may have existed.

Late Middle Miocene (Fig. 10.7b) Landscape evolution Mimicking drainage reversals of the Late Cretaceous in the Neuquén Basin in southern South America (Potter 1997), similar reversals start to develop in the foreland basin in the Miocene. Uplift events in the northern Andes at this time (Marshall & Sempere 1987; Jordan & Gardeweg 1989; Kroonenberg et al. 1990; Hoorn 1993, 1994a; Hoorn et al. 1995; Lundberg et al. 1998; Garzione et al. 2008) were probably accompanied by megafan sediment wedges prograding significantly into the foreland depression, and impacting waterbodies during marine incursions or when lakes existed. The distribution of megafans may have been discontinuous along the mountain front, or continuous. We show the latter (Chaco plains) scenario (see Fig. 10.3a & Table 10.2) because the amount of sediment implied by post-Pebas units is so large and distribution is so widespread that we conclude that Andes-sourced megafans were an integral component of the landscape – as has been suggested by Latrubesse et al. (1997) for southern Peru (see Fig. 10.7b). We include areas now characterized by regional erosion, but where young fluvial sediments are probably ubiquitous (Räsänen et al. 1987; Campbell et al. 2006). By contrast with the T/L drainage pattern of the Orinoco and Mamoré-Guaporé sectors, the T/T pattern of the Amazon Trough was probably characterized by transverse Andean rivers giving way downstream to transverse trunk drainage, as in the abovementioned Ganges example. Some megafans for which at least partial evidence exists, are rendered in Fig. 10.7b to show the early hypothesized phase in this development. The Pastaza megafan, in particular, appears in our topographic cross-sections as one of the largest Neogene sedimentary edifices on the eastern flank of the Andes (see Figs 10.3b & 10.7b). The geomorphic results in the Amazon Trough therefore appear to be emplacement of aggradational surfaces regionally, in the form of many megafan surfaces, some of which prograded into waterbodies occupying the axis of the depression. The great bend formed by the Andes in Ecuador further helps to explain the alignment of the evolving Amazon drainage: since megafans typically develop transverse to geological structure we suggest megafans prograded from the NE- and SE-trending arms of the Andes – with a resultant depression oriented roughly eastwards along the present generalized course of the Amazon River (see Fig. 10.7b). Possible biological dynamics Biological processes described in Models 1–4 (see Table 10.3) probably operated, but were located in this phase on the Andean

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Megafan environments and their impact margin of the foreland. Vicariant and habitat-connection modes of megafan rivers imply population divergence and speciation, etc., among aquatic populations below the Andean foothills. Older relict megafans whose remains are more securely interpreted in the landscape are few. However, it seems clear that the extensive surface of an aggrading Pastaza megafan (Pa1 in Figs 10.7b & 10.8) would have allowed connections between aquatic populations in the Pastaza and Napo Rivers (1, 2 in Fig. 10.3a), and further downstream between the Pastaza and Corrientes Rivers (3 in Fig. 10.3a), a past connection detailed by Neller et al. (1992) and Räsänen et al. (1992). Taken together, the individual connections above imply at least the possibility of regional population homogenization, along the margins of both cratons facing the Amazon Trough, as well as along some or even much of the Andean side of the foreland basin (curved arrow in Fig. 10.7b) (because megafan systems were probably continuous along the mountain front). In this phase, a newer connection between southwest Amazonia (Mamoré-Guaporé Basin) and the Paraguay-Paraná drainage basin is even more likely, since the divide is occupied by the modern Parapetí River megafan (5 in Fig. 10.3a), which leads water into both basins today. For the same reason, therefore, species adapted to this Andes-based megafan (or one nearby situated on the divide) may also have transported aquatic animals regularly between the basins. We hypothesize that a test for timing of basin-connecting events should be Late Miocene or later dates. This geomorphic reconstruction reveals the geographical correspondence between the Andes-margin high fish diversity zone and megafans. Megafan landscapes almost everywhere in South America lie below 200 m (except for some apex zones of small area). This is the altitudinal zone within which it is generally accepted that the greatest fish biodiversity of Amazonia presently exists. We suggest that this altitudinal coincidence may point to the operation of the suite of megafan river dynamics that we have described (Wilkinson et al. 2006), in this and later stages of landscape evolution. However, although aquatic connections between the Amazon and Paraguay-Paraná drainage basins are very likely, the altitudinal control may help explain why the connection is comparatively weak (Wesselingh & Salo 2006), because the responsible Parapetí megafan lies well above the 200 m contour line. A northward riverine connection between Amazonia and the Caribbean Sea is inferred (e.g. Hoorn 1994a, 1994b; Lundberg 2005; Lovejoy et al. 2006; see also fig. 19 in Lundberg et al. 1998) between 10 and 8 Ma. Lundberg et al. (1998) quote the evidence of the existence of several freshwater animals (large serrasalmid, pimelodid and arapaimatid fishes, mata-mata turtle, iniid dolphin), closely related to those of Amazonia and the Orinoco today, in the upper Urumaco Formation delta sediments in the Maracaibo region. Further south, Hoorn (2006) has observed that aquatic connections between Amazonia and the Caribbean may have persisted in a more easterly position along the exposed Guiana Shield margin (presumably close to the modern Orinoco course), even after local uplifts in early Late Miocene time (Vaupes Arch, V–V, in Fig. 10.8). We distinguish Andean from craton megafan habitats because they were apparently geographically isolated from one another, at least during the life of the Pebas and later estuarine-lacustrinewetland habitats. Wesselingh & Salo (2006) have drawn attention

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Fig. 10.8 Topographic roughness map of northern South America: Lightest tones, high roughness zones of mountains and cratonic uplands. Intermediate tones, two roughness zones of central and western Amazonia, Ac and Aw (intermediate greys) separated by a grey dotted line. Darkest tones, modern megafan surfaces: Llanos plains, Pl, comprise three modern megafans (named after formative rivers: Apure, Ap; Guanare, Ga; and Ariari-Güejar, AG). Pastaza plains, Pp, comprise three modern component fans (short dashed lines: Morona, Mo; younger Pastaza, Pa2; and Corrientes, Co), which occupy a modern basin within the large, inferred Pastaza megafan (Pa1). Beni plains, Pb, comprise mainly Beni megafan (Be) and small neighbours (unmarked). The Demini (De) is a modern feature, one of several (unmarked) on the southern margin of the Guiana Shield. Inferred palaeo-megafans (this study, unless otherwise noted): Caquetá-Putumayo (CP); Guayabero-Caquetá (GC ); Guaviare (Gu); East Japurá (Je); West Japurá (Jw); Juruá (Ju); Meta (Me) and Pastaza (Pa1: Bès de Berc et al. 2005). Aripuanã (Ar) and Jiparaná (Ji) megafans (Latrubesse et al. 2002); V–V, Vaupes Arch; I–I, Iquitos Arch (Roddaz et al. 2006); F–F, Fitzcarrald Arch; Río Negro, black dashed line; EC, Eastern Cordillera. Adapted partly from Allenby (1988), Dumont (1996), Latrubesse (2002), Neller et al. (1992), Räsänen et al. (1990, 1992) and Wilkinson et al. (2006).

to this separation. Species associations in rivers on the craton margins might thus be expected to differ measurably from those along the eastern flank of the Andes Mountains. Reinforcing the physical isolation due to distance is the strong physico-chemical contrast between rivers of the Andes-based megafans and rivers of the craton-based megafans (also noted by Wesselingh & Salo 2006).

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We suggest further that the craton and Andean megafan zones should reveal diachronically distinct phases of speciation, with cratonmargin habitats being older than those of Andes-based megafans.

Late Miocene-Pliocene times (Fig. 10.7c) Landscape evolution As the Eastern Cordillera of the northern Andes range was rapidly uplifted, a high sediment supply from the Andes probably promoted continued progressive transverse megafan growth and new megafans. By the Pliocene the modern river pattern was probably fully established. The closure of the Panama Isthmus occurred during this period and affected the regional climate. The implications of Pliocene climatic change for sediment influx Amazonia are further discussed in Chapter 14. Two major landscape trends are completed in this period. Fluvial landscapes become completely dominant; and lake and estuarine environments as well as the older craton-based megafans are fully eclipsed by the sloping sediment cones advancing from the Andes. These trends are plausibly related to longer wavelength tectonics (Hermoza et al. 2005) extending well east of the Andes, with foredeep and backbulge basins providing accommodation for sedimentary fills. Andean megafan growth reaches its fullest extent This stage sees the development of the present sequence of megafans for much of the 3000+ km along the eastern slopes of the Andes (from central Argentina to the llanos of Venezuela) – nested continuously in places for perhaps hundreds of kilometres along the mountain front, as in the modern Bolivia-central Argentina sector. Megafan slopes conceivably occupy the entire width of the foreland basin in the Orinoco and Mamoré-Guaporé sectors (see Figs 10.3a and Orinoco sector in Figs 10.7c & 10.8). By contrast, we infer that megafan sediment bodies extended far east into the Amazon Trough in the process of final reorientation of trunk rivers towards the Atlantic seaboard. This T/T pattern is strongly suggested by a sequence of inferred diamond-shaped megafans: Caquetá-Putumayo → West Japurá → East Japurá (CP, Jw, Je in Figs. 10.7c & 10.8). These features existed in their present form, or nearly so, during this phase. Other such sequences are likely. The strong relationship between discharge increase and channel slope decrease downstream in wet fluvial basins implies that downstream megafans ought to be of gentler slope, a relationship suggested in the cross-section constructed through these inferred megafans (middle panel, Fig. 10.3b). Some control by recurring activity of the Iquitos Arch seems likely. Roddaz et al. (2005; see also Chapter 5) show the likely role of the Iquitos Arch as a forebulge feature, and show its control on the orientation of smaller drainage basins. Based on a geochemical study of terminal Miocene fluvial sediments, Roddaz et al. (2006, p. 57) suggest that the Iquitos forebulge ~8–6 Ma acted as a subaerially exposed ‘divide between Andean and cratonic drainage systems’. Positive relief along the line of the arch (as mapped by Roddaz et al. 2006 – shown in Fig. 10.8), would have acted as the distal margin of megafans sourced in the Andes (e.g. inferred Pastaza megafan, Pa1 in Fig. 10.8), and also acted as an anchor for the apex

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of the next-downstream megafan (the inferred Caquetá-Putumayo megafan, CP in Fig. 10.8). These patterns accord precisely with the model of megafan evolution and controls of megafan apex location in cratonic lowlands. More locally, the possible effects of major (macroscale) geological structures such as the Iquitos Arch and Iquitos-Araracuara anteclise of Wesselingh et al. (2006c) are compatible with the mesoscale influences of megafan sediment cones on the evolution of the drainage pattern. We see broad tectonic control of the position and extent of the Pastaza megafan (between the mountain front and some expression of the Iquitos Arch) and mesoscale fluvial control of the conical megafan-surface and radial drainage. The Napo River follows the distal margin of the megafan and also roughly parallels one reconstruction of the arch. Eclipse of estuaries, major lakes, major wetlands and many craton-based megafans The advance of fluvial sediment wedges precipitates the complete eclipse of larger waterbodies. Fluvial and young lacustrine sediments finally bury the Pebas and Solimões sedimentary units (mainly in the central and western Amazonian basins where these sediments have been mapped – Hoorn 1994a, 1994b; Roddaz et al. 2006; Campbell et al. 2006; Wesselingh et al. 2006b). The megafans that prograded from the craton are also progressively buried by opposing megafan sediment wedges in the Orinoco and Mamoré-Guaporé sectors from such Andean rivers as the Apure, Guaviare, Beni and Grande (and many others). Final emplacement of the Orinoco and Mamoré Rivers along the exposed margins of the Guiana and Brazilian Shields (see Figs 10.3a & 10.7c) resulted in partial or entire erosional removal of some craton-based sediment cones. This important hydrological and sedimentological turning point resulted in water and sediment being directed entirely offshore (except where active basin subsidence occurs) at the Orinoco delta and the Amazon submarine fan (see below) – with relatively little sediment retained onshore, compared with Pebas/ Acre wetland times. In the Amazon Trough sector trunk rivers appear to have been similarly (but less obviously) displaced away from the Andes sediment source. Thus the Negro River, like the modern Orinoco, arises as a trunk river in a depression formed where Andessourced sediment cones finally reached the craton. The extensive Japurá megafan sedimentary bodies in central Amazonia (Jw, Je in Figs 10.7c & 10.8) are feasible causes of the broad arc, convex to the northeast, described by the main course of the Negro River. A contributary cause is the negligible mineral sediment supply from the northern, cratonic tributaries of the river (see Fig. 10.5c). Similarly, the Madeira River lies on or near the Brazilian Shield. Surface expressions of sedimentary bodies that may have influenced its course are difficult to discern in remotely sensed data, however. In this stage (the latest Miocene) the Amazon drainage became fully reoriented into its eastward course, debouching on the equatorial Atlantic coast. Three processes conceivably contributed to this major realignment. An almost inevitable result of eastward extension of sediment cones in the Amazon Trough is the elongation of the Amazon River progressively eastwards, partly along the depressions between fans, a typical control of trunk river location. Regional megafan and Pebas/post-Pebas lacustrine aggradation may have helped overtop a low point in the Amazon

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Megafan environments and their impact drainage basin rim somewhere in the Purus Arch-Barreiras region. Dating of the Amazon reorientation event – from siliciclastic sedimentation on the Ceara Rise offshore in the Atlantic – is variously claimed as ~8 Ma or later (e.g. Lundberg et al. 1998 and references therein; Figueiredo et al. 2009) and more recently at ~2.5 Ma as argued for by Campbell et al. (2006). The offshore data can be variously interpreted (Campbell et al. 2006, but see Chapter 26). However, recently published data from the Amazon Fan provides conclusive evidence that onset of the Amazon River was between 11.8 and 11.3 Ma and that high sedimentation rates in the Amazona Fan (similar to Present) start at 2.4 Ma (Figueiredo et al. 2009). Finally, the east-northeast extension of the Fitzcarrald Arch, dated as active in the Amazon Foreland Basin from ~5 Ma (Baby et al. 2005) – a region of more intense erosion within zone Aw (F–F in Fig. 10.8) – probably encouraged the new drainage orientation. Poorly defined episodes of incision punctuated the general infilling of the western and central Amazonian sedimentary basins. At least nearer the Andes chain, progressive uplift undoubtedly caused episodic stream incision during this and prior stages. As yet, however, the effects are difficult to evaluate, and can only be seen in terms of the well-known unconformities between the younger and older Neogene formations in Amazonia, and between the various units comprising the younger formations. A clear picture of degrees of incision and the distribution of incision is really only available in the most recent landscapes as revealed, for example, on the roughness map (see Figs 10.3a & 10.8). Progressive uplift implies that incision-related biological dynamics probably occurred at ill-defined times before the present landscape had developed as it now appears. We note the dating by Campbell et al. (2006) of their widespread Ucayali Unconformity between ~15 and ~9.5–9.0 Ma, when incision into megafan surfaces probably occurred in places. Possible biological dynamics Today’s rivers crossing the megafans of the western Amazon, Orinoco and Paraná drainage basins harbour hundreds of species of fish and other organisms. The local distributions of all these species must be controlled by megafan stream dynamics. Many of these species range beyond the megafans into lowland trunk rivers or upland mountain streams, or even onto the shield areas. Some species and species groups, however, have distributions that more closely coincide with the Andean zone of megafans between the eastern front of the mountains and the lowland trunk rivers. Five examples are illustrated in Fig. 10.9: characins of the genus Creagrutus (Vari & Harold 1998, 2001), armoured catfishes of the genera Otocinclus (Schaefer 1997) and Panaque (Schaefer & Stewart 1993), and characins of the genera Xenurobrycon (Weitzman 1987) and Brachychalcinus (Reis 1989). Other fishes occurring for the most part on megafans are species belonging to the characin genus Steindachnerina (see figs 14, 41 & 60, pp. 34, 65 & 82, in Vari 1991), the armoured catfish genus Farlowella (Retzer & Page 1997), and the cichlid genera Bujurquina, Crenicichla, Aequidens and Tahuantinsuyoa (see fig. 3, p. 29, in Kullander 1986). For taxa exhibiting such fidelity we have suggested (see Table 10.3 & Fig. 10.6) that megafan stream dynamics may have played a significant historical role in moulding their geographical ranges and in driving their diversification.

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Of special interest is the possible role that megafan drainage evolution has played in facilitating range expansions between major drainage basins. Some species, for example Otocinclus vittata (see fig. 35, p. 52, in Schaefer 1997) and Steindachnerina guentheri (see fig. 60, p. 82, in Vari 1991), are widespread across much of the Eastern Andes margin. These fishes, occurring in the uplands of sub-basins of the Amazon and Orinoco, suggest that lateral dispersal within and among megafans is common. Biotic crossovers between basins could involve simple flooding of low, swampy headwater divides, as for example in the Pantanal drainage basin between headwater tributaries of the Amazon and Paraguay-Paraná drainage basins. Crossovers with drainage rearrangements caused by megafan avulsions and stream piracy could lead to repeated dispersals and, in some cases, subsequent divergence and diversification. Sister species or multispecies clades distributed in two or more basins imply vicariant divergence following dispersion. To date, however, firm cases of this are elusive. Relationships among the many species within Creagrutus and Otocinclus are insufficiently resolved to detect vicariant pairs centred in the megafan zone. We have hypothesized elsewhere (Wilkinson et al. 2006) that the Parapetí megafan (5 in Fig. 10.3a), or one similarly situated on the Amazon-Paraná divide, may have transported aquatic fauna regularly between southwestern Amazonia and the ParaguayParaná Basin. The allopatric and nearly adjacent distributions (see Fig. 10.9d) of Xenurobrycon polyancistrus and X. macropus (Weitzman 1987), and Brachychalcinus copei and B. retrospina (Reis 1989), are suggestive of Amazon-Paraná transfer and speciation. However, in these cases too, species relationships are not fully resolved. An undescribed species of small endemic banjo catfish (Aspredinidae) has been identified in the Pilcomayo River (6 in Fig. 10.3a) in lowland Bolivia (J. Friel, personal communication). Its nearest relative is found in the Madeira River (4 in Fig. 10.3a), with others further north in the Pastaza and Napo drainages (1, 3 in Fig. 10.3a). Scenarios to explain the distribution of these small fishes may relate to the activity of the Parapetí River (5 in Fig. 10.3a) (Wilkinson et al. 2006). The full development of the Fitzcarrald Arch in this period, as a zone of erosion separating the Juruá and Purus drainage basins, has probably separated these basins biologically for at least a few million years.

Pliocene-Holocene times Landscape evolution The geomorphic roughness map (see Fig. 10.8) provides data on some of the most recent events in western and central Amazonia, with phases that probably overlap the last part of the phase described immediately above. The roughness map suggests four sequential phases of geomorphic development in western Amazonia. The precise relationship between the stages outlined below, and existing regional geologies, has yet to be defined. (a) Incision (western Amazonia) Landscapes of western Amazonia all appear rougher (lighter zone Aw between the Andes Mts and the dotted line in Figs 10.3a & 10.8) than modern megafan surfaces or those in the central Amazon

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(b)

(c)

(d) X. heterodon X. polyancistrus X. macropus

B. nummus B. copei B. retrospina

Fig. 10.9 Examples of fish distributions that overlap known and inferred megafan sedimentary systems. (a) The species-rich characiform genus Creagrutus (Characidae) occurs on both sides of the Andes (Harold & Vari 1994; Vari & Harold 2001). Mapped here are cis-Andean collection records for 56 species of the genus from Vari & Harold (2001: fig. 18, p. 44). The collection localities (larger dots) for some 34 species are collectively distributed along the eastern Andean margin from Venezuela to Bolivia. Locality records for species distributed in distant lowland trunk rivers and shield areas are plotted as smaller dots. (b) The armoured catfish genus Otocinclus (Loricariidae, Hypoptopomatinae) distributed especially in northern Llanos, but all along the Andean margin and south into Paraná drainage basin (from Schaefer 1997: fig. 8, p. 21, records from distant lowland trunk rivers and southeastern Brazil not shown). (c) The armoured wood-eating catfish Panaque dentex clade (Loricariidae, Ancistrinae) distributed mainly along the Andean margins in western Amazonia (from Schaefer & Stewart 1993: fig. 7, p. 317). (d) Some species of the characin genera Xenurobrycon (Characidae, Glandulocaudinae) and Brachychalcinus (Characidae, Stethaprioninae) have allopatric distributions in western Amazonia and upper Paraguay-Paraná drainage basins (spot maps respectively from Weitzman 1987, fig. 4, p. 119; and Reis 1989, fig. 29, p. 59).

drainage basin (darker zone Ac in Figs 10.3 & 10.8). Zone Aw is the classic region of widespread incision, even by small streams, with the resultant hummocky topography of small hills in a dense pattern, generally with a few tens of metres of relief (terra firme landscapes – see e.g. Klammer 1984; Bigarella & Ferreira 1985;

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Räsänen et al. 1987; Campbell et al. 2006). This regional incision was presumably mainly the result of Andes- or arch-related uplift and sea level fluctuations. The rivers incise into Late Miocene fluvial deposits and earlier deposits. This was not the first such phase of regional incision or non-deposition, as discontinuities

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Megafan environments and their impact in Neogene sediments attest (e.g. Campbell et al. 2006). The combination of stream rejuvenation, due to the breakthrough of the Amazon River to the Atlantic Ocean, with later repeated lowered glacio-eustatic sea levels, were undoubtedly important (Irion 1984; Miller et al. 2005; see also Chapter 11). (b) Aggradation (central Amazonia) Sediment deposition culminates in an aggradational surface, which is evident as the complex dark area of interfluves in zone Ac (see Fig. 10.8). From its regional extent, smooth low-slope interfluvial surfaces, diverging surface streams with points of divergence fixed at the topographic boundary between zones Aw and Ac, we interpret this to be the almost pristine depositional surface of continental megafans. Stream patterns here suggest – reasonably – that the Japurá River sometimes combined with the Solimões as a single trunk river, providing sediment for megafan development (in the form of inferred megafan Je in Figs 10.7 & 10.8). The topographic cross-section through the inferred West and East Japurá megafans shows a summit smoothness typical of megafan surfaces (compare Morona and Apure megafan surfaces, see Fig. 10.3b, middle and lower panels). We repeat that the inferred West and East Japurá megafans in turn probably influenced both the location and arc-like course of the Río Negro. This phase of sedimentation may be the depositional equivalent of the incisional phase described above. But this phase does seem to constitute the final stage of megafan sedimentation (Late Miocene-Pliocene(?) – Fig. 10.7c). Intriguingly, the major topographic division between the west and central Amazonian (Aw, Ac) landscapes coincides with the zone where the Carauari Arch is mapped (see, e.g., Räsänen et al. 1987), despite the fact that this is a deeply buried structure (Caputo 1991; Wesselingh & Salo 2006). Furthermore, both the West Japurá and Juruá inferred megafans lie immediately downstream of the Carauari/Jutaí Arch, suggesting the influence of a positive topographic manifestation as an anchor for fan apexes. At least three megafans remain unburied above the level of regional sedimentation in central Amazonia on the craton margins (Demini, Aripuanã and Jiparaná megafans; see Figs 10.7b,c & 10.8). (c) Incision (central Amazonia) Zone Ac is modified by widespread linear incision (lighter toned, barbed sinuous lines in Fig. 10.8), which indicates downcutting by all major streams. The incision is incomplete so that much of the above-mentioned original flat surface – see ‘(b) Aggradation (central Amazonia)’ above – remains in the landscape. This phase of incision may have extended upstream into zone Aw, thereby increasing its roughness signature compared with that of zone Ac. The Carauari-Jutaí Arch may have localized at least the northern half of the prominent north–south line that separates zones Aw and Ac, with which it coincides (dotted line in Fig. 10.8). The pattern of erosion suggests that the arch acted perhaps as a hinge line with greater uplift to the west. The Vaupes Arch, Iquitos Arch (as mapped by Roddaz et al. 2006) and Fitzcarrald Arch also coincide fairly closely with lines of distinctly lighter tone within the Aw zone (see V–V, I–I and F–F in Fig. 10.8) indicating a greater degree of incision.

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(d) Aggradation Modern megafans, especially in the Colombian Llanos (e.g. the small Ariari-Güejar megafan, Wilkinson et al. 2006) and the Pastaza and Beni plains (zones Pl, Pp and Pb in Fig. 10.8) with their immediate neighbours, evolve smooth currently forming surfaces. All the larger examples consequently show the strong signature of modern aggradation (dark surfaces in Fig. 10.8). They are largely unincised and probably younger than those of zone Ac in central Amazonia. These youngest, active megafans in South America are almost all located near the Andean mountain front. They are indicative of renewed sedimentation, and presumably result at least partly from recent enhanced local subsidence (Räsänen et al. 1987) within the widespread incisional Aw zone. An erosional zone such as zone Aw lying closer to the actively uplifting Andes follows one theory of foreland basin evolution, in which erosion and subsidence operate alternately on proximal and distal sides of a foreland basin (the reciprocal stratigraphy of Catuneanu et al. 1997). Major floodplains, wide enough to be detected in the roughness algorithm, also appear as narrow dark zones paralleling parts of larger rivers, most notably the Solimões-Amazonas (see Figs 10.3a & 10.8). Possible biological dynamics During the aggradational phases (b) and (d) described above, processes as described in Models 1–4 occurred and are probably still occurring not only in Amazonia (zones Pl, Pp and Pb – Fig. 10.8a), but also on the Chaco and Pantanal plains (zones Pc and Pt – Fig. 10.3a). We are almost as confident of our interpretation of inferred megafans of the incision phase described above (East and West Japurá and the Juruá – Je, Jw and Ju in Fig. 10.8a), and hence of associated biological dynamics. The modern megafan surfaces in western Amazonia and the Orinoco drainage basins (darkest surfaces in Fig. 10.8) indicate a series of potentially important local river connections at present or within the geologically recent past. Aquatic populations in the following rivers very likely have experienced the connection of their habitats on biologically recent timespans (see Fig. 10.8): between the Apure and Guanare Rivers via the Apure megafan; between the Meta and Guaviare Rivers via the small Ariari-Güejar megafan (Wilkinson et al. 2006); between the Pastaza and Morona Rivers, and the Pastaza and the Corrientes Rivers (rivers of active megafans on the south slopes of the great Pastaza megafan, zone Pp in Fig. 10.8); between the Beni and Mamoré Rivers via the Beni megafan; and between the Demini and the Xeriuini Rivers via the Demini megafan (Branco River region, northern Brazil). These connections have presumably been important at least for those organisms that cannot negotiate environments of the local trunk rivers (or river sectors) (Models 1.2 and 1.3 in Wilkinson et al. 2006). In the aggradational phase (b) (central Amazonia), trunk rivers may have been similarly affected. For example, the abovementioned East and West Japurá and the Juruá megafans (Je, Jw and Ju in Fig. 10.8) would have facilitated connections between the Japurá, Solimões and Negro rivers, hundreds of kilometres upstream of the present confluences between these rivers. The stream connections proposed above, and many others, must have been broken during incisional phases. The incisional phase (a) (western Amazonia), well developed against the

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Andes mountain front in zone Aw, is probably Pliocene or early Pleistocene in age. Each river that now flows off the original Pastaza megafan (Pa1 in Fig. 10.8) is incised, with significant interfluvial relief between them (see Fig. 10.3b, lower panel) (Morona, Pastaza, Tigre, the local trunk river, the Napo and others). We suggest that each incised river may have acted as a vicariant environment for some species. The argument for vicariance by incision can be applied to all the features we class here as inferred megafans in zone Aw. Because incision probably extended sequentially far from the mountain front, we hypothesize further that vicariant biological events of Model 5 may be younger with increasing distance from the mountain front (larger straight arrows in Fig. 10.7b,c), at least measurably between zones Aw and Ac. Despite regional incision, interbasin connections probably remained, since the Parapetí and Ariari-Güejar megafans are presently active on the major basin divides. As yet we are unaware of fish distributions that might have some relationship to the four phases of landscape evolution described in this section.

Discussion and conclusions We have aimed first to present a sequence of palaeogeographical events, evaluating in particular the role of the inland delta or megafan. Secondly, we have evaluated a specific set of physical environmental processes that have biological significance – the dynamics outlined in Table 10.3 and Fig. 10.6 after Wilkinson et al. (2006) – in each of the palaeogeographic stages. The biological processes proposed depend entirely on ‘other factors being equal’. Here we do not attempt to evaluate such factors, which may have countered or overwhelmed any signal of megafan-related processes.

Palaeogeographical reconstruction in western Amazonia Probing the Neogene of Amazonia for causes of diversification in aquatic species is severely hampered by lack of data. Our attempt to reconstruct Miocene and younger landscapes with the benefit of a new landscape analogue, the megafan, therefore seems justified. The megafan appears to be critical to mesoscale palaeogeographic reconstructions because it is so widespread in the modern landscapes of South America, but also in other parts of the world such as the Himalayan foreland. Megafans are mesoscale non-coastal fluvial sedimentary bodies distinguishable from smaller alluvial fans and from coastal deltas. We introduced a number of characteristics and geographical patterns of megafans, whose significance is generally unappreciated in the geological and biological communities, before attempting the palaeogeographical history. We showed numerous but oblique indications in the literature on the South American Neogene that point to megafan depositional environments. Occasional mention of individual megafans has been made. We have built on this work. The megafan feature provides a feasible synthesis of various apparently disparate pieces of data. To assist the landscape analysis we introduced a map of topographic roughness characteristics of northern South America,

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based on pioneering work on the roughness geography of Mars (Kreslavsky & Head 1999, 2000), to identify landscapes of different types, and to provide new perspectives on the sequence and distribution of some more recent events. A major conclusion of this analysis is that megafans probably filled the Andean foreland depression more or less continuously, especially during the rapid and widespread phases of Late MiocenePliocene continental deposition, as the dominant mesoscale fluvial form. This was no less the case during phases when shallow waterbodies occupied the depression, when true deltas (sensu stricto) often fail to form. Because the evidence for shallow and fluctuating waterbodies in the depression is so widespread we believe that megafans were the dominant fluvial mesoscale landform. This conclusion is less surprising considering that recent continental depositional styles of most of the world’s forelands are dominated by megafans. Argentina’s Chaco plains are a prime example of a foreland depression dominated by megafans, with a trunk river displaced by megafans to the most distal parts of the depression. These modern plains landscapes may provide our most secure model for the river-dominated palaeogeographies of the aggrading Orinoco drainage basin and Mamoré-Guaporé sectors of the Neogene Andean foreland. Although less obvious, the Amazon Trough sector of the foreland was also probably filled by sediment from transversely oriented megafans and transverse trunk rivers. Such patterns would explain large areas of flat landscape, the modern course of the Negro River, and perhaps shed light on the operation of the Iquitos and Carauari arches. We have aimed to do no more than introduce the topic of megafans as a likely central player in the Neogene sedimentation of northern South America, and to apply known patterns at different scales in a generalized sequence of events. The megafan perspective has sometimes been applied to local palaeogeographical reconstructions in the western Amazon drainage basin (Pastaza, Jiparaná and Aripuanã discussed above), but our global perspective suggests that it can be thought of as a fluvial norm in regionally aggrading continental environments. A geomorphic continuum? Theories of evolution of the Andean foreland have been summarized (Räsänen et al. 1987, 1998; Campbell et al. 2006) as the shifting depocentre theory (mentioned earlier), the megafan theory (Latrubesse et al. 1997) and the Lago Amazonas theory. As a partial synthesis, we suggest that the shifting depocentre and megafan theories occupy opposite ends of a geomorphic continuum, with localized fluvial deposition at one end and regional fluvial deposition at the other (see Table 10.2). The former is exemplified by the modern environment of western Amazonia, with zones of localized fluvial deposition alternating with zones of erosional topography. The Pastaza-Ucayali depocentre is a classic example of a deep but localized, modern fluvial depository with surrounding erosional landscapes. Local tectonic control has apparently caused confined floodplains to migrate laterally. The latter, regional fluvial sedimentation, is represented by the overfilled, multi-megafan depositional environment of the Chaco plains, in which along-strike dimensions of Pleistocene/ modern fluvial units are typically hundreds of kilometres (Iriondo 1984, 1987, 1993) – distances that match the along-strike

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Megafan environments and their impact extent of Neogene fluvial units described by Uba et al. (2005, 2006; especially fig. 15 in Uba et al. 2006). The distinction between regional Sevier and local Laramide depositional styles in the Cretaceous-Paleogene sequences of the North American foreland is reminiscent. The interplay of sediment supply, sea level and foreland basining style, all dependent on tectonics, appears to shift between the end members identified, with resulting effects on local geology and geomorphology. We consider that widespread fluvial deposition of regional extent characterized western Amazonia for a time, and concomitantly induced the reorientation of the Amazon drainage. Megafans typify fluvial depositional environments at the mesoscale everywhere on the continuum. Biologically, this implies that megafan-style dynamics could well have impacted aquatic animals throughout the foreland.

Possible biological dynamics Megafans as lowland environments with implications for biotic distribution The physical processes on megafans with possible biologically significant effects are encapsulated as Models 1–5 (Wilkinson et al. 2006; see Table 10.3 & Fig. 10.6). We expect that the processes in each model operated to some degree (except perhaps those of Model 2, megafan stream segmentation from the trunk river due to aridity, the common application of Model 2). The vicariant mode of the models (see Table 10.3) describes such effects as range fragmentation, population divergence, speciation and extirpation. The habitat-connection mode describes such processes as range expansion, population and faunal mixing, introgression, reinforced genetic isolation and competition. Megafan landscapes in South America are almost everywhere situated below 200 m (except for some apex zones of small area). A broadly accepted generalization in South American ichthyogeography is that fish diversity declines precipitously above the 200 m contour line. We suggested that this altitudinal coincidence may point to the operation of megafan river dynamics that we have described (Wilkinson et al. 2006). Furthermore, many fish groups are located in the sub-Andean zone, where megafans probably developed extensively at the earliest date in the evolution of the Amazon drainage basin. We presented examples of several fish groups distributed in the Amazon lowlands at the foot of the Andes. It is at least possible that vicariance-related river dynamics on megafans contributed to the diversity of forms. But more generally, examples of aquatic species and groups that may relate to the dynamics discussed above are not obvious. Various constraints operate that limit the investigation of fish distributions with respect to modern and past megafan distributions. Incomplete distribution maps and collection remains a major problem in discussing distributions. However, for those species and monophyletic groups with ‘good’ range data we see that most are widespread and not limited to modern megafan landscapes. Some of these taxa may have originated on megafans or achieved current distribution via megafan stream dynamics leading to dispersion beyond megafans. Detecting these as groups

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possibly related to megafans is not easy because resolved phylogenetic trees are needed in addition to the distribution of the terminals – that is, widespread species or groups with true origins on megafans might escape detection. Furthermore, extirpation alters native distributions. There are at least a few fish groups that have current distributions above the lowland Amazon and other large river valleys but below the higher altitude Andean streams – that is, within a generalized megafan altitudinal zone. These seem to be the best candidates, of which we presented examples, for groups that may have been influenced by megafan stream dynamics. Population connections by river switching on megafans – local to interbasin scales The maps generated in the analysis suggest that megafans could well have been widespread in western and central Amazonia, formerly from the direction of the cratons, where relict and modern megafans still exist, and later from the Andean side of the foreland basin, where megafans are best developed today. Connections between the megafan rivers – as rendered in Models 1.3, 1.4 and 4 with the biological ramifications of range expansion, population mixing, introgression, reinforced genetic isolation and competition – probably existed, in light of evidence from the modern geomorphology of South America. Connections could well have existed when the megafan-mediated habitats may have operated regionally, as probably between the Guiana and Brazilian Shield margin megafans; between the basins of western Amazonia and the Paraguay-Paraná Basin; between western Amazonia and the Orinoco drainage basin; and even perhaps between central Amazonia and drainage basins on the Atlantic coast before reorientation of the Amazon drainage. We have been impressed by the fact that two Andes-sourced megafans operate today on the Amazon-Orinoco and Amazon–Paraguay-Paraná watersheds, despite the fact that widespread erosional landscapes operate in most of Amazonia. These connection points, with others, probably explain the interbasin allopatry of sister species pairs, which can be attributed to vicariant divergence after the basin barrier formed. Habitat chronosequences There must be a marked chronological gap between the development of earlier major craton-based sedimentation (and potential speciation events on associated megafans) and Andes-based sedimentation and potential speciation there. We suggest therefore that these highland margins should show significant diachronism in megafan-related biological events. Later in the Neogene, biological effects related to megafan dynamics would have started earlier against the Andean chain, and later on central Amazonian fans. Topographic zones defined by roughness zones in western and central Amazonia (zones Aw and Ac) are particularly interesting because they appear to be prone to processes described in most of the biological megafan models. Thus, vicariance processes on single megafans reinforce one another, both during the active stages (Models 1.1–1.3, 2 and 3) and relict stages of megafan development (Model 5). These are also the most recent likely processes, so that a division between aquatic populations in smaller rivers in the wide areas west and east of the Carauari Arch (zones Aw and Ac in Fig. 10.8) might well appear in the field.

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Less secure, in our opinion, is a possible diachronic relationship within zones Aw and Ac, whereby sedimentation successively distant from the Andean mountain front translates into successively later megafan development. Although this sequencing of megafans in a distal direction is visible in various parts of the world, it may have happened too rapidly in Amazonia to have promoted significant biological events.

Acknowledgements MJW’s colleagues and management have provided encouragement over the years at the NASA-Johnson Space Center. We acknowledge the skill and dedication of astronaut crews in imaging continental depositional systems. MJW thanks Martín Iriondo in particular, and Carlos Ramonell (both of Santa Fé, Argentina), and the late Luis Meyer Jou (Asunción, Paraguay) for generously introducing him to the landscapes and literature of the Chaco. MJW also thanks Martín Iriondo for discussions on the Pastaza megafan. We thank Peter DeCelles (Tucson, USA) for fruitful discussion, G. Marroig and R. Cerqueira (1997) for directing our attention to the epigraph, and especially our reviewers, known and anonymous.

References Allenby, R.J. (1988) Origin of rectangular and aligned lakes in the Beni basin of Bolivia. Tectonophysics 145, 1–20. Assine, M.L. (2005) River avulsions on the Taquari megafan, Pantanal wetland, Brazil. Geomorphology 70, 357–371. Baby, P., Espurt, N., Brusset, S., Hermoza, W., Antoine, P-O., Roddaz, M. et al. (2005) Influence of the Nazca ridge subduction on the Amazonian foreland basin deformation: preliminary analyses. In: 6th International Symposium on Andean Geodynamics, Extended Abstracts, pp. 83–85. Bès de Berc, S., Soula, J.C., Baby, P., Souris, M., Christophoul, F., Rosero, J. (2005) Geomorphic evidence of active deformation and uplift in a modern continental wedge-top–foredeep transition: example of the eastern Ecuadorian Andes. Tectonophysics 399, 351–380. Bigarella, J.J., Ferreira, A.M.M. (1985) Amazonian geology and the Pleistocene and Cenozoic environments and palaeoclimates. In: Prance, G.H., Lovejoy, T.E. (ed.) Amazonia – Key Environments. Oxford: Pergamon, 49–71. Campbell, K.E., Frailey, C.D., Romero-Pittman, L. (2006) The PanAmazonian Ucayali Peneplain, late Neogene sedimentation in Amazonia, and the birth of the modern Amazon River system. Palaeogeogr Palaeocl 239, 166–219. Caputo, M.V. (1991) Solimões megashear: intraplate tectonics in northwestern Brazil. Geology 19, 246–249. Catuneanu, O., Beaumont, C., Waschbusch, P. (1997) Interplay of static loads and subduction dynamics in foreland basins: reciprocal stratigraphies and the “missing” peripheral bulge. Geology 25, 1087–1090. Chorley, R.J., Schumm, S.A., Sugden, D. E. (1984) Geomorphology. London: Methuen, 605 pp. Clapperton, C. (1993) Quaternary Geology and Geomorphology of South America. Amsterdam: Elsevier, 779 pp. Comisión Nacional del Río Pilcomayo (1994) El Río Pilcomayo: Su Problemática y Sus Desafíos. Asunción, Paraguay: Ministerio de Obras Públicas y Communicaciones, 20 pp.

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Cordini, R. (1947) Los Ríos Pilcomayo en la Región del Patiño. Anales I, Dirección de Minas y Geología (Buenos Aires), 82 pp. DeCelles, P.G., Gray, M.B., Ridgway, K.D., Cole, R.B., Pivnik, D.A., Pequera, N., Srivastava, P. (1991) Controls on synorogenic alluvialfan architecture, Beartooth Conglomerate (Paleocene), Wyoming and Montana. Sedimentology 38, 569–590. Dumont, J.F. (1996) Neotectonics of the Subandes-Brazilian craton boundary using geomorphological data: the Marañón and Beni basins, Tectonophysics 257, 137–151. Dumont, J.F., Fournier, M. (1994) Geodynamic environment of Quaternary morphostructures of the subandean foreland basins of Peru and Bolivia: characteristics and study methods. Quaternary Int 21, 129–142. Earth Sciences and Image Analysis, NASA-Johnson Space Center (2008) The Gateway to Astronaut Photography of Earth. URL: http://eol.jsc.nasa.gov/; accessed 3 July 2008. Figueiredo, J., Hoorn, C., van der Ven, P., Soares, E. (2009) Late Miocene onset of the Amazon River and the Amazon deep-sea fan: Evidence from the Foz do Amazonas Basin. Geology 37, 619–622. Garzione, C.N., Hoke, G.D., Libarkin, J.C., Withers, S., MacFadden, B., Eiler, J. et al. (2008) Rise of the Andes. Science 320, 1304–1307. Geddes, A. (1960) The alluvial morphology of the Indo-Gangetic plains. Trans Inst Brit Geographers 28, 253–276. Gesch, B., Muller, J.-P., Farr, T.G. (2006) The Shuttle Radar Topography Mission – Data validation and applications. Photogramm Eng Rem S 72, 233–235. Gingras, M.K., Räsänen, M., Pemberton, S.G, Romero, L.P. (2002) Ichnology and sedimentology reveal depositional characteristics of bay-margin parasequences in the Miocene Amazonian foreland basin. J Sedimetary Res 72, 871–883. Gohain, K., Parkash, B. (1990) Morphology of the Kosi megafan. In: Rachocki, A.H., Church, M. (eds) Alluvial Fans. London: Wiley, pp. 151–178. Gole, C.V., Chitale, S.V. (1966) Large fan building activity of Kosi River. American Society of Civil Engineers, Proceedings Paper 4722, v. HY2: 111–126. Harold, A.S., Vari, R.P. (1994) Systematics of the trans-Andean species of Creagrutus (Ostariophysi: Characiformes: Characidae). Smithsonian Contributions to Zoology 551, 1–31. Hermoza, W., Brusset, S., Baby, P., Gil, W., Roddaz, M., Guerrero, N., Bolaños, M. (2005) The Huallaga foreland basin evolution: thrust propagation in a deltaic environment, northern Peruvian Andes. J S Am Earth Sci 19, 21–34. Holmes, A. (1965) Principles of Physical Geology. New York: Ronald Press, 1288 pp. Hoorn, C. (1993) Marine incursions and the influence of Andean tectonics on the Miocene depositional history of northwestern Amazonia: results of a palynostratigraphic study. Palaeogeogr Palaeocl 105, 267–309. Hoorn, C. (1994a) An environmental reconstruction of the palaeoAmazon River system (Middle–Late Miocene, NW Amazonia). Palaeogeogr Palaeocl 112, 187–238. Hoorn, C. (1994b) Fluvial palaeoenvironments in the intracratonic Amazonas Basin (Early Miocene–early Middle Miocene, Colombia). Palaeogeogr Palaeocl 109, 1–54. Hoorn, C. (2006) Mangrove forests and marine incursions in Neogene Amazonia (Lower Apoporis River, Colombia). Palaios 21, 197–209. Hoorn, C., Guerrero, J., Sarmiento, G.A., Lorente, M.A. (1995) Andean tectonics as a cause for changing drainage patterns in Miocene northern South America. Geology 23, 237–240. Horton, B.K., DeCelles, P.G. (2001) Modern and ancient fluvial megafans in the foreland basin system of the central Andes, southern

10/24/2009 12:17:21 Shobha

Megafan environments and their impact Bolivia: implications for drainage network evolution in fold-thrust belts. Basin Res 13, 43–63. Hovikoski, J., Räsänen, M., Gingras, M., Roddaz, M., Brusset, S., Hermoza, W. et al. (2005) Miocene semidiurnal rhythmites in Madre de Dios, Peru. Geology 33, 177–180. Hovikoski, J., Gingras, M., Räsänen, M., Rebata, L. A., Guerrero, J., Ranzi, A. et al. (2007) The nature of Miocene Amazonian epicontinental embayment: high-frequency shifts of the low-gradient coastline. Bull Geol Soc Am 119, 1506–1520. Iriondo, M.H. (1984) The Quaternary of northeastern Argentina. Quaternary of South America 2, 51–78. Iriondo, M.H. (1987) Geomorfolgía y Cuaternario de la Provincia Santa Fé (Argentina). D’Orbygnyana. (Corrientes, Argentina) 4, 54 pp. Iriondo, M.H. (1993) Geomorphology and late Quaternary of the Chaco (South America). Geomorphology 7, 289–303. Jordan, T.E., Gardeweg, P.M. (1989) Tectonic evolution of the Late Cenozoic central Andes. In: Ben-Avraham, Z. (ed.) The Evolution of the Pacific Ocean Margins. New York: Oxford University Press, pp. 193–207. Kalliola, R. (1992) Abiotic control of the vegetation in Peruvian Amazon floodplains: environment change and pioneer species. Reports, Dept. Biology, University of Turku (Turku, Finland) 30. Kalliola, R., Paitán, S.F. (1998) Geoecología y Desarrollo Amazónico. Turku, Finland: Turun Yliopisto, 544 pp. Kalliola, R., Puhakka, M., Danjoy, W. (eds) (1993) Amazonia Peruana. Vegetation húmeda tropical en el llano subandino. Annales Universitatis Turkuensis (Turku, Finland), Series AII (114), 265 pp. Klammer, G. (1984) The relief of the extra-Andean Amazon basin. In: Sioli, H. (ed.) The Amazon. Dordrecht: W Junk, pp. 47–84. Kreslavsky, M.A., Head, J.W. (1999) Kilometer-scale slopes on Mars and their correlations with geologic units: Initial results from Mars Orbiter Laser Altimeter (MOLA) data. J Geophys Res 104, 21911–21924. Kreslavsky, M.A., Head, J.W. (2002) Kilometer-scale roughness of Mars’ surface: Results from MOLA data analysis. J Geophys Res 105, 26695–26712. Kroonenberg, S.B., Bakker, J.G.M., van der Wiel, A.M. (1990) Late Cenozoic uplift and paleogeography of the Colombian Andes: constraints on the development of high Andean biotas. Geologie en Mijnbouw, Special Issue 69, 279–290. Kullander, S.O. (1986) Cichlid Fishes of the Amazon River Drainage of Peru. Stockholm: Swedish Museum of Natural History, 431 pp. Latrubesse, E.M. (1992) The Cenozoic of the Purus basin in the Acre State, Brazil. Doctoral thesis, Universidad Nacional, San Luis, Argentina, 219 pp. Latrubesse, E.M. (2002) Evidence of Quaternary palaeohydrological changes in middle Amazonia: the Aripuanã-Roosevelt and Jiparaná ‘fans’. In: Mossa, J., Latrubesse, E., Gupta, A. (eds) South and Central American Rivers. Zeitschrift für Geomorphologie, Supplementband 129, 61–72. Latrubesse, E.M., Bocquentin, J., Santos, J.C.R., Ramonell, C.G. (1997) Paleoenvironmental model for the late Cenozoic of southwestern Amazonia: paleontology and geology. Acta Amazonica 27, 103–118. Leier, A. L., DeCelles, P. G. & Pelletier, J. D. (2005) Mountains, monsoons, and megafans. Geology 33 (4), 289–292. Lovejoy, N.R., Albert, J.S., Crampton, W.G.R. (2006) Miocene marine incursions and marine/freshwater transitions: evidence from Neotropical fishes. J S Am Earth Sci 21, 5–13. Lundberg, J.G. (1998) The temporal context for diversification of Neotropical Fishes. In: Malabarba, L.R., Reis, R.E.,Vari, R.P., Lucena, Z.M., Lucena, C.A.S. (eds) Phylogeny and Classification of Neotropical Fishes. Porto Alegre, Brazil: Edipucrs, pp. 49–68.

Hoorn_ch10_Final.indd 183

183

Lundberg, J.G. (2005) Brachyplatystoma promagdalena n. sp., a fossil goliath catfish (Siluriformes: Pimelodidae) from the Miocene of Colombia, South America. Neotrop Ichthyol 3, 597–605. Lundberg, J.G., Marshall, L.G., Guerrero, J., Horton, B., Malabarba, M.C.S.L., Wesselingh, F. (1998) The stage for neotropical fish diversification: a history of tropical South American rivers. In: Malabarba, L.R., Reis, R.E.,Vari, R.P., Lucena. Z.M., Lucena, C.A.S. (eds) Phylogeny and Classification of Neotropical Fishes. Porto Alegre, Brazil: Edipucrs, pp. 13–48. MacArthur, R.H., Wilson, E.O. (1967) The Theory of Island Biogeography. Princeton: Princeton University Press, 218 pp. Malagnino, E.C. (1989) Paleoformas de origen eolico y sus relaciones con modelos de inundacion de la Provincia de Buenos Aires. Simposio Latinoamericano de Percepción Remota, and IX Reunión Plenaria SELPER vol. II, Bariloche, Argentina, pp. 611–620. Marroig, G., Cerqueira, R. (1997) Plio-Pleistocene South American history and the Amazon Lagoon hypothesis: a piece in the puzzle of Amazonian diversification. J Comp Biol 2, 103–119. Marshall, L.G., Sempere, T. (1993) Evolution of the Neotropical Cenozoic land mammal fauna in its geochronologic, stratigraphic, and tectonic context, In: Goldblatt, P. (ed.) Biological Relationships Between Africa and South America. New Haven: Yale University Press, pp. 329–392. Meyer, L.A. (1996) El Río Pilcomayo: un caso de estudio. In: Atti del Corso: Sviluppo e Gestione dei Bacini Idrografici (Course text: Watershed development and management). Cuaderno ILLA, Serie Cooperación No. 6 (Istituto Italo-Latino Americano), Rome, pp. 303–314. Miall, A.D. (1996) The Geology of Fluvial Deposits. New York: Springer, 582 pp. Miller, K.G., Kominz, M.A., Browning, J.V., Wright, J.D., Mountain, G.S., Katz, M.E. et al. (2005) The Phanerozoic record of global sea-level change. Science 310, 1293–1298. Mookerjea, D. (1961) The Kosi – a challenge in river control. J Inst Eng (India) 42, 117–142. Neller, R.J., Salo J.S., Räsänen, M.E. (1992) On the formation of blocked valley lakes by channel avulsion in Upper Amazon foreland basins. Zeitschrift für Geomorphologie NF 36, 401–411. Potter, P.E. (1997) The Mesozoic and Cenozoic paleodrainage of South America: a natural history. J S Am Earth Sci 10, 331–344. Räsänen, M., Salo, J.S., Kalliola, R.J. (1987) Fluvial perturbance in the western Amazon basin: regulation by long-term sub-Andean tectonics. Science 238, 1398–1401. Räsänen, M., Salo, J.S., Jungner, H., Romero-Pittman, L. (1990) Evolution of the western Amazon lowland relief: impact of Andean foreland dynamics. Terra Nova 2, 320–332. Räsänen, M., Neller, R., Salo, J.S., Jungner, H. (1992) Recent and ancient fluvial deposition systems in the Andean foreland basin, Peru. Geol Mag 129, 293–306. Räsänen, M., Linna, A., Irion, G., Rebata Hernani, L., Vargas Huaman, R., Wesselingh, F. (1998) Geología y geoformas de la zona de Iquitos. In: Kalliola, R., Flores Paitán, S. (eds) Geoecología y Desarollo Amazónico: Estudio Integrado en la Zona de Iquitos, Peru. Annales Universitatis Turkuensis (Turku, Finland), Series AII (114), 59–137. Reis, R.E. (1989) Systematic revision of the neotropical characid subfamily Stethaprioninae (Pisces, Characiformes). Comunicações do Museu de Ciências de PUCRS 2, 3–86. Retzer, M.E., Page, L.M. (1997) Systematics of the stick catfishes, Farlowella Eigenmann & Eigenmann (Pisces, Loricariidae). P Acad Nat Sci Phila 147, 33–88. Rocha, M.S., Olivera, R.R., Rapp Py-Daniel, L. (2008a) A new species of Gladioglanis Ferraris & Mago-Leccia from Rio Aripuanã,

10/24/2009 12:17:21 Shobha

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Amazonas, Brazil (Siluriformes: Heptapteridae). Neotrop Ichthyol 6, 433–438. Rocha, M.S., Olivera, R.R., Rapp Py-Daniel, L. (2008b) Scoloplax baskini: a new spiny dwarf catfish from Rio Aripuanã, Amazonas, Brazil (Loricarioidei: Scoloplacidae). Neotrop Ichthyol 6, 323–328. Roddaz, M., Baby, P., Brusset, S., Hermoza, W., Darrozes, J. (2005a) Forebulge dyamics and environmental control in western Amazonia. Tectonophysics 399, 87–108. Roddaz, M., Viers, J., Brusset, S., Baby, P., Hérail, G. (2005b) Sediment provenances and drainage evolution of the Neogene Amazonian foreland basin. Earth Planet Sci Lett 239, 57–78. Rodriguez, E., Morris C.S., Belz, J.E., Chapin, E.C., Martin, J.M., Daffer, W., Hensley, S. (2005) An Assessment of the SRTM Topographic Products. Technical Report JPL D-31639. Pasadena, CA: Jet Propulsion Laboratory. Rossetti, D.F., Góes, A.M. (2004) O Neógeno da Amazônia Oriental. Belém, PA: Museu Paraense Emílio Goeldi, Coleção Friedrich Katzer, 222 pp. Salo, J. (1986) Rainforest diversification in the western Amazon Basin: the role of river dynamics. Reports, Dept. Biology, University of Turku (Turku, Finland) 16, 52 pp. Salo, J., Räsänen, M. (1989) Hierarchy of landscape patterns in western Amazon. In: Holm-Nielsen, L.B., Nielsen, I.C., Balslev, H. (eds) Tropical Forests. Botanical Dynamics, Speciation and Diversity. Oxford: Academic Press, pp. 35–45. Salo, J., Kalliola, R., Häkkinen, I., Mäkinen, Y., Niemelä, P., Puhakka, M., Coley, P.D. (1986) River dynamics and the diversity of Amazon lowland forest. Nature 322, 254–258. Schaefer, S.A. (1997) The neotropical cascudinhos: systematics and biogeography of the Otocinclus catfishes (Siluriformes: Loricariidae). P Acad Nat Sci Phila 148, 1–120. Schaefer, S.A., Stewart, D.J. (1993) Systematics of the Panaque dentex species group (Siluriformes: Loricariidae), wood-eating armored catfishes from tropical South America. Ichthyol Explor Fres 4, 309–342. Singh, R.L., Singh, K.N. (1971) Middle Ganga plain. In: Singh, R.L. (ed.) India – A Regional Geography. Varanasi: National Geographic Society of India, 992 pp. Sinha, R., Friend, P.F. (1994) River systems and their sediment flux, Indo-Gangetic plains, Northern Bihar, India. Sedimentology 41, 825–845. Slingerland, R., Smith, N.D. (2004) River avulsions and their deposits. Ann Rev Earth Planet Sci 32, 257–285. Tuomisto, H., Poulsen, A.D. (1996) Influence of edaphic specialization on pteridophyte distribution in neotropical rain forests. J Biogeogr 23, 283–293. Tuomisto, H., Ruokolainen, K., Yli-Halla, M. (2003) Dispersal, environment, and floristic variation of western Amazonian forests. Science 299, 241–244. Uba, C.E., Heubeck, C., Hulka, C. (2005) Facies analysis and basin architecture of the Neogene Subandean synorogenic wedge, southern Bolivia. Sediment Geol 180, 91–123. Uba, C.E., Heubeck, C., Hulka, C. (2006) Evolution of the late Cenozoic Chaco foreland basin, Southern Bolivia. Basin Res 18, 145–170. Vari, R.P. (1991) Systematics of the neotropical characiform genus Steindachnerina Fowler (Pisces, Ostariophysi). Smithsonian Contributions to Zoology 507, 1–118. Vari, R.P., Harold, A.S. (1998) The genus Creagrutus (Teleostei: Characiformes: Characidae): monophyly, relationships, and undetected diversity. In: Malabarba, L.R., Reis, R.E.,Vari, R.P., Lucena. Z.M., Lucena, C.A.S. (eds) Phylogeny and Classification of Neotropical Fishes. Porto Alegre, Brazil: Edipucrs, pp. 245–260. Vari, R.P., Harold, A.S. (2001) Phylogenetic study of the neotropical fish genera Creagrutus Günther and Piabina Reinhardt (Teleostei:

Hoorn_ch10_Final.indd 184

Ostariophysi: Characiformes), with revision of the Cis-Andean species. Smithsonian Contributions to Zoology 613, 1–239. Warne, A.G., Meade, R.H., White, W.A., Guevara, E.H., Gibeaut, J., Smyth, R. C. et al. (2002) Regional controls on geomorphology, hydrology, and ecosystem integrity in the Orinoco Delta, Venezuela. Geomorphology 44, 273–307. Weitzman, S.H. (1987) A new species of Xenurobrycon (Teleostei: Characidae) from the Río Mamoré basin of Bolivia. P Biol Soc Wash 100, 112–120. Wells, N.A., Dorr, J.A. (1987) Shifting of the Kosi River, northern India. Geology 15, 204–207. Wesselingh, F.P. (2006a) Miocene long-lived lake Pebas as a stage of mollusc radiations, with implications for landscape evolution in western Amazonia. Scripta Geologica 133, 1–17. Wesselingh, F.P. (2006b) Molluscs from the Miocene Pebas Formation of Peruvian and Colombian Amazonia. Scripta Geologica 133, 19–290. Wesselingh, F.P. Salo, J.A. (2006) Miocene perspective on the evolution of the Amazonian biota. Scripta Geologica 133, 439–458. Wesselingh, F.P., Guerrero, J., Räsänen, M., Romero-Pittman, L., Vonhof, H. (2006a) Landscape evolution and depositional processes in the Miocene Amazonian Pebas lake/wetland system: evidence from exploratory boreholes in northeastern Peru. Scripta Geologica 133, 323–361. Wesselingh, F.P., Hoorn, M.C., Guerrero, J., Räsänen, M., RomeroPittman, L. & Salo, J. (2006b) The stratigraphy and regional structure of Miocene deposits in western Amazonia (Peru, Colombia and Brazil), with implications for Late Neogene landscape evolution. Scripta Geologica 133, 291–322. Wesselingh, F.P., Kaandorp, R.J.G., Vonhof, H.B., Räsänen, M.E., Renema, W., Gingras, M. (2006c) The nature of aquatic landscapes in the Miocene of western Amazonia: an integrated palaeontological and geochemical approach. Scripta Geologica 133, 363–393. Wilkinson, M.J. (1999) Fluvial dynamics in the Andean foreland: megafans and mechanisms for biodiversity in aquatic biota. Geol Soc America, Fall Meeting Abstracts, Denver, Colorado. Wilkinson, M.J. (2001) Where large fans form: interim report of a global survey. In: 7th International Conference on Fluvial Sedimentology, Program and Abstracts, University of Nebraska-Lincoln, Lincoln, Nebraska (USA), 6–10 August 2001, p. 282. Wilkinson, M.J. (2003) A geomorphic classification of sedimentary subbasin types in the south American foreland – tectonic and drainage pattern controls. In: Alluvial Fans 2003, Conference Abstracts, Sorbas (Almería, Spain), 8–13 June, 2003. URL: http://alluvialfans. net/Abstracts.htm; accessed 29 November 2003. Wilkinson, M.J. (2005) Large Fluvial Fans and Exploration for Hydrocarbons. NASA Tech Briefs 29(1), 64. URL: http://www. nasatech.com/Briefs/ps.html; accessed 3 July 2008. Wilkinson, M.J., Cameron, N.R., Burke, K. (2002) Global geomorphic survey of large modern subaerial fans. Houston Geol Soc Bull 44, 11–13. Wilkinson, M.J., Marshall, L.G., Lundberg, J.G. (2006) River behaviour on megafans and potential influences on diversification and distribution of aquatic organisms. J S Am Earth Sci 21, 151–172. Wilkinson, M.J., Allen, C.C., Oehler, D.Z. (2008a) A new fluvial analog for the ridge-forming unit, northern Sinus Meridiani/ southwest Arabia Terra, Mars. In: 39th Lunar and Planetary Science Conference, Abstract #1392. Wilkinson, M.J., Kreslavsky, M.H., Miller, R.McG. (2008b) Megafans of the Northern Kalahari Basin. In: Third Southern Deserts Conference – Kalahari 2008 (Oxford University, School of Geography), Molopo Lodge, Northern Cape, South Africa, 16–19 September, 2008.

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ELEVEN

Long-term landscape development processes in Amazonia Georg Irion1 and Risto Kalliola2 1

Research Institute Senckenberg, Wilhelmshaven, Germany University of Turku, Finland

2

Abstract The Amazon River system developed into its present form in a comparatively short period over approximately the last 10 million years. The western part of the drainage basin was mainly controlled by exhumation of the Andes. The Cenozoic Andean foreland consisted of a series of subsiding aggradational basins, interrupted by segments of basement (arches), which were particularly active during the Neogene. In this geodynamic landscape a system of highly instable rivers developed. During the Quaternary, landscape evolution was controlled by glacio-eustatic changes. The central to eastern part of the basin (i.e. the lower reaches of the AmazonSolimões River) was most affected by glacio-eustacy and the effect extended upstream beyond the halfway point of the continent. The present várzeas and the numerous associated ria lakes are an indication of incomplete infilling of the palaeovalleys that were incised during the low-sea-level periods. In the Andean forelands, the soils in terra firme are highly variable due to their different sedimentation histories, parent materials and drainage conditions. The heterogeneity of the edaphic site properties influences the distribution patterns of the Amazonian biota, with highly specialized habitats occurring in the most extreme conditions. Denudation with deep alteration of the former aggradational plains in western Amazonia modifies the forest ground with time; also in the Precambrian shields in northern and southern parts of Amazonia, forest soils have been altered by deep weathering under the hot and moist tropical climatic conditions of the region. In this chapter we describe in particular the evolution of the fluvial landscapes in the Amazon drainage basin from the Miocene onwards, with particular emphasis on the Quaternary.

Introduction The Amazon drainage basin constitutes the world’s largest fluvial region, occupying about 5.5 million square kilometres. The Amazonian landscape can be described as a vast, forest-covered, lowland drainage basin that is shaped and characterized by the abundance of rivers (Sioli 1984; Toivonen et al. 2007). Most of this area is flat, and with the exception of the Andean foothills and some scattered mountain peaks in the lowlands, relative elevation differences are within a few tens of metres. The largest rivers that form the Amazon-Solimões stream are the Ucayali, Marañón, Napo, Putumayo, Japurá, Negro-Branco, Juruá, Purus, Madeira, Tapajós and Xingú Rivers. The Amazon River enters the Atlantic Ocean as a single large stream, which delivers about 6000 km3 of

Amazonia, Landscape and Species Evolution: A Look into the Past, 1st edition. Edited by C. Hoorn and F.P. Wesselingh. © 2010 Blackwell Publishing

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water and one billion tons of continental sediments annually into the Atlantic (Meade 1994). The present chapter portrays some long-term landscape development processes in the Amazon drainage basin from the Miocene onwards. These processes are determined by the co-occurrence of autochthonous and allochthonous factors. The physical geographical framework of the Amazon region consists of the Andean foothills, the Guiana and Brazilian Shield areas, and a vast funnel-shaped sedimentary basin consisting of the sub-Andean zone and the central Amazon Trough (Fig. 11.1). The Andean foreland is subject to continuing tectonic activity and is subdivided into structural basins and arches, all of which are mostly buried under a cover of Cretaceous and Cenozoic deposits (Dumont et al. 1991; Rossetti et al. 2005; see also Chapters 3–5). Near the border areas between Peru and Brazil, the Serra do Moa anticlinal structure peaks through the sediment beds and exposes the older geological strata. The Amazon terminates at the Atlantic, where it forms a large river delta that extends beyond the continental slope, merging into the Amazon submarine fan. Since the Late Miocene, a total of some 700,000 km3 of continental

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Fig. 11.1 Map of Amazonia with names and the major geological structures and locations mentioned in the text. Numbers on white discs refer to other figures.

sediments have been deposited in the subaquatic delta of the Amazon River (Damuth & Flood 1984). Recent biostratigraphic analyses revealed that the first sediments with Andean signature were deposited in the Amazon Fan between 11.8 and 11.3 Ma (Figueiredo et al. 2009). The Amazon region is characterized by its low elevation and perceptible uniformity of the physical landscapes. Most of the Amazon drainage basin has a wet or humid tropical climate, with the northern and southern margins being highly seasonal. However, areas of high and low precipitation coexist in the basin, and precipitation rates differ per area throughout the year (Hoffman 1975; see Chapter 14). Therefore drainage areas of the Amazonian streams are located in different climatic areas, and the annual precipitation regimes of the joining rivers may be different (Meade et al. 1991). Along some of the largest rivers, particularly the Amazonas-Solimões River, local water level fluctuations may not coincide with the annual precipitation patterns of the corresponding area. The inundation patterns in the major floodplains are still poorly known because of the vast size and remoteness of many areas and the difficulty of using optical remote-sensing technologies in flood studies (Hamilton et al. 2002).

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The vegetative landscape in Amazonia is densely forested and remarkably homogeneous in broad terms, and so it has been throughout the Tertiary and Quaternary (Hoorn 1993; Van der Hammen & Hooghiemstra 2000). However, other special types of vegetation also occur, including palm swamps, other swamps, savannas, liana forests and white sand areas with reduced biomass (Sioli 1984). Modern biogeographical research in Amazonia emphasizes regional differences (see Chapter 21) in the vegetation cover as a response to variations in the edaphic site conditions (Salo et al. 1986; Tuomisto et al. 1995). These edaphic variations, in turn, are a consequence of the region’s dynamic geological, geomorphological, climatic and hydrological conditions (Tuomisto 2007). As shown in other chapters of this book, the puzzle of the changing palaeoenvironments and the complex environmental dynamism within Amazonia is becoming better understood, and many previous assumptions can now be assessed using solid data. This situation contrasts with many earlier publications regarding the Amazon environment and its development, as some influential notions have been phenomenally speculative and based on vague evidence, such as those concerning the Pleistocene refugia (Haffer 1969),

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Long-term landscape development Lago Amazonas (Tuomisto et al. 1992) or the Middle Miocene Amazon seaway (Webb 1995).

Andean tectonics and the development of the Amazon River system The Amazon River system as we know it today was initiated during the Late Miocene after a phase of extreme uplift in the Eastern Cordillera of the Andes (Hoorn et al. 1995; Dobson et al. 2001; Figueiredo et al. 2009; see also Chapters 4 & 6), while east of the Andes a large wetland was formed (Hoorn 1994; Wesselingh et al. 2002; see also Chapter 8). These lacustrine and brackish-water wetland areas were episodically influenced by marine waters due to an open connection to the Caribbean Sea (Hoorn et al. 1995), and a late Neogene embayment system probably even reached Rio de la Plata and the southern Atlantic (Räsänen et al. 1995). The sediments deposited in the foreland basin areas were largely homogeneous and fine-grained with high contents of beidellite, which is a smectite group clay mineral, indicating humid tropical conditions in the surroundings of the wetland during the initial stages of Andean exhumation (Wesselingh et al. 2002).

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After the rise of the Venezuelan Cordilleras, the outflow from the Amazon to the Caribbean closed and around 10 Ma the presentday fluvial system started to develop. As the Andes reached their modern configuration the prevailing flow direction of the Andean watershed turned entirely towards the Atlantic Ocean. This transitional period involved large-scale palaeoenvironmental changes over a continental scale (Hoorn et al. 1995; Figueiredo et al. 2009; see also Chapter 8). Andean orogeny still maintains active foreland dynamism in the western parts of the Amazon Basin. The compression in the sub-Andean fold and thrust belt (Dumont et al. 1991; DeCelles & Horton 2003) has induced foreland shortening, and the differentiation between two distinctive geologically determined landscape categories in the region: retro-arch intraforeland sub-basins and emerging arches (structural heights) (Räsänen et al. 1990). Lacustrine and fluvial sediments from the Andes have continually filled the subsiding basins, and emerging arches have typically experienced a change from depositional to denudational (see Chapter 5). The Serra do Moa Arch is the major anticline formation in the region, comprising a belt of mountain peaks that are distinctly visible in remote-sensing imagery (Fig. 11.2a). This, roughly north–south-directed, emerging axis has forced the

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(c)

(d)

Fig. 11.2 Landsat TM imagery from the western Amazonia lowlands reveals a dynamic landscape evolution (scale bar on photos is 10 km). (a) Uplifting Serra do Moa mountains at the border between Peru and Brazil. (b) Ucayali River near Contamana with alluvial fans and sheetflood deposition in the West. (c) Junction of the Pastaza River with the Marañón River, with features of the Pastaza fan formation, blocked valley lakes (Laguna Rimachi) and a remnant island of dissected terra firme surrounded by swampy vegetation. (d) Swamp formations in the upper Tapiche River in the eastern margin of the Pastaza-Marañón sub-basin, with the Ucayali River running in the northwest.

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Ucayali River to run towards the north just as the Huallaga River and the upper Marañón River do in their respective sub-Andean valleys The penetration of the Nazca Plate’s Nazca ridge under the South American continent has been of particular importance in the entrenchment of the present river system. This process has uplifted parts of the lowland belt (Fitzcarrald Arch) creating a water divide that forces the rivers to run either northwards through the Ucayali-Solimões-Amazon river system, or southwards through the Madre de Dios-Madeira drainage system. Consequently, various river channel relocations occurred, and the rivers in the central parts of the Amazon region, particularly in the Acre region, presumably got to receive much less run-off than they had in the Late Miocene (Räsänen et al. 1990; see Chapter 6).

Fluvial landscapes in the Andean foreland basin The Andean foreland basins comprise the western portion of the Amazon drainage basin. Tectonic compression and tilting modified these areas and induced complex fluvial dynamism in the region. The subsiding foreland basins are widely inundated each year, and they act as major sinks for the sediments coming from the Andean source areas. Details of sediment deposition in the Amazonian forelands are still debated (Campbell et al. 2006), but neotectonics has an undeniable influence in shaping the presentday aggradational river systems in the Upper Amazon (Dumont et al. 1991). From the Late Miocene onwards, the sedimentary basins developed into an overspilling state (see Chapters 4 & 5), with the Andean flanks eroding to fill subsiding basins along the Andean foreland, followed by secondary erosion of sediments and their further transport by rivers to the Atlantic Ocean (Colinvaux & Oliveira 2001). The large rivers that originate in the Andes are rich in suspended sediments and provide the main pathway for the erosion products coming from the mountains into the Amazonian lowlands and further to the sea. The headwaters originating in the Andean Cordillera enter the Amazonian lowlands through narrow canyons, and they often have braided channels, steep gradients and gravel in their bedload. In Peruvian Amazonia close to the Andes many of the upstream rivers also exhibit a great channel sinuosity, high rates of bank erosion and continuous channel migration (Salo et al. 1986; Puhakka et al. 1992). Floodplain surfaces are dominated by alternating meander scrolls and swales, and, in places, lakes and islands are also abundant albeit small and short-lived. In the non-inundated areas, the sub-Andean forelands are often poorly dissected and characterized by aggradational features such as sheetflood deposition in alluvial fans (see also Chapter 10). Scars indicating river translocations are distinct, and in many places non-active floodplains and remnant palaeochannels can be found. Stream valleys dissect the older areas of alluvium in terra firme, and progressing denudation may gradually remove the entire top layer over vast extensions. Quaternary aggradational plains are still well preserved near the Brazilian, Peruvian and Bolivian borders (Räsänen et al. 1992). Soils texture varies from from clay to sand and often shows abrupt changes associated with remnant fluvial features (Hamilton et al. 2006). Also

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gravel conglomerates are commonly found in incised river valleys, and in places they direct and shape the river courses and obstruct navigation. South from the Fitzcarrald Arch, the Madre de Dios trunk river system collects the water run-off from the southern parts of Peru, from Bolivia and from the state of Rôndonia in Brazil. The Peruvian part of this region is composed of Neogene to Quaternary terraces that are in their early stages of denudation (Räsänen et al. 1987). Some of the Pleistocene terraces are asymmetric suggesting tectonic tilting (Räsänen et al. 1990). Poorly drained soils occur on the present flood basins and on depressions above the reach of present-day river floodplains in, for example, ancient river courses. Fluvial aggradation is active in the Beni region of Bolivia, where some rivers flow on levee-bordered channels with lower-lying basins between. These highly seasonal areas are climatically at the margin of the rainforest vegetation (e.g. Hess et al. 2003), due to which the interfluvial lands are often covered by a palm or savanna vegetation instead of broadleaved forest. North of the Fitzcarrald Arch, the Urubamba-Ucayali drainage collects the sediments and water run-off from both the Andean rivers and the tributaries from the eastern lowlands as far as the Serra do Moa Arch. The Ucayali has a highly dynamic meandering channel and a meander belt tens of kilometres wide; on the eastern side there are Quaternary fluvial terraces that are probably of climatic and tectonic origin (Dumont et al. 1991). In the central parts of the Ucayali sub-basin, major river avulsion occurred after the 1500s in an area of relative subsidence southeast of the city of Pucallpa in Peru (Kalliola et al. 1992; Pärssinen et al. 1996). This avulsion shifted the course of the Ucayali River closer to the Andes, leaving behind a large meander plain, and resulting in the submersion of one of its lowland tributary valleys, the currently blocked-valley lake Laguna Imiria. Further to the north, the Ucayali sub-basin is bordered by the Contaya Mountains, which are just a few tens of kilometres away from the Andean foothills (see Fig. 11.2b), making the entire Ucayali sub-basin susceptible to the consequences of a possible further uplift event in the Contamana region. The Pastaza-Marañón sub-basin forms a vast area of modern fluvial aggradation (60,000–70,000 km2; Räsänen et al. 1995) with extensive subsiding depressions and a widely flooded relief (see Fig. 11.2c). Fluvial interconnections between the Marañón and Ucayali Rivers exist up to 200 km upstream of the main confluence (Dumont et al. 1991). Sediment deposition into this fluviolacustrine environment is a consequence of the relative uplift of the Iquitos Arch, which is interpreted as a forebulge of the northwestern Amazonian foreland basin (Roddaz et al. 2005), and the subsidence of the Pastaza-Marañón sub-basin. The Marañón River enters the lowlands through a narrow Andean gorge, after which it has a meandering to anastomosed channel morphology. The Huallaga River is its only major tributary from the south, and there are vast inundated lands with abundant scars of previous river courses between its lower course and the Ucayali River, also known as the Ucamara region (Villarejo 1979; Salo et al. 1986). This region is constantly being reshaped by fluvial and vegetative processes and it has a complex mosaic of habitats that are defined by combinations of hydrological, physical, chemical and biological site conditions (Kalliola et al. 1991a; Kvist & Nebel 2001).

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Long-term landscape development Channel avulsions of the large rivers have left behind scars that are in places opportunistically occupied by the small tributaries of the Samiria and Pacaya Rivers (Neller et al. 1992). Palm swamps and open swamps are widespread, indicating the high water stress, and some wetlands are reportedly being created by sudden tectonic events, for example in the headwaters of the Tapiche River (fig. 3b in Dumont & García 1991). The northern major tributaries of the Marañón River are the Santiago River, Pastaza River and Tigre River, all with a southward course and with their uppermost drainage in Ecuador. The Pastaza River and its predecessors have deposited a vast (approximately 60,000 m2) continental fan with alluvium originated in the Ecuadorian Andes, rich in pyroclastic debris, hence giving rise to ‘black soils’ (Räsänen et al. 1990). This formation comes out in the satellite imagery in a very distinctive pattern, since the canopy physiognomy of the alternating palm and other swamps, and diverse types of forests make the appearance of this region mosaic-like. At the southwestern margin of this formation recent sediments top parts of the subsiding terra firme. A large blocked valley, Laguna Rimachi, and a couple of other similar formations are present in the remaining dissected landscapes, which are surrounded by the recent alluvium of the Pastaza fan (see Fig. 11.2d). The junction of the Ucayali and Marañón Rivers gives rise to the Amazonas River, which transports the waters and sediments over the Iquitos Arch into the várzeas of the Solimões-Amazon River. It has a large meandering to anastomosed channel and highly dynamic floodplain characteristics. The lateral channel erosion rate of the Amazonas River in places reaches up to tens of metres annually (Kalliola et al. 1992). Also the seasonal water level amplitude is distinctly high (10–20 m). The islands of the Amazonas River are unstable even on a short timescale, yet relatively stable river straights also occur (Tuukki et al. 1996; Kalliola et al. 1999). The major trunk rivers joining the Amazonas River include the Napo River, with a relatively straight braided to anastomosed course and its main drainage in the Ecuadorian Amazon. The Putumayo River has a meandering course and much of its catchment is in the former foreland basins that are now uplifted and subject to denudation processes. Within the catchment of both of these rivers, Paleozoic sandstone massifs are present, particularly in Colombian Amazonia. All in all, the rivers north from the equator possess relatively more stable and restricted floodplains than those in the south. No major Pleistocene or Holocene aggradational terraces are usually found beyond the close vicinity of the present rivers, which usually run in incised valleys in dissected terrain

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Manaus (1400 km inland) and 60 m above msl at the Brazilian/ Colombian border, some 3000 km inland. The resulting average slope increases from 0.7 cm/km over the first 700 km to 7 cm/km at the Brazilian/Colombian border. The daily tide influences on the Amazon River reach beyond the city of Obidós, i.e. some 1000 km inland. The várzea of the Amazon River was shaped by Quaternary sea level oscillations, which in this region extended exceptionally far inland. In general the influences of eustatic sea level changes are restricted to coastal areas and estuaries, but in the várzea the influences of the Quaternary sea level oscillations extend beyond half of the continent. During the Pleistocene, sea level changes probably experienced many high and low stages. During Early Pleistocene times sea level drop was within a few tens of metres, which is low compared to that of the Middle to Late Pleistocene (from about 700,000 years ago), when the sea level dropped to more than 100 m below msl (Berger & Wefer 1992). According to Irion et al. (2009) a sea level of ~40 m below the msl would lower the mean river level at Manaus by a maximum of 13 m. This is only a few metres below the average yearly minimum level of the Amazon at this place. In contrast, during the Last Glacial Maximum (LGM) the mean river level at Manaus was lowered by more than 40 m. The undeniable importance of the Quaternary sea level changes, as established during the last decades, gives the sedimentological development of the Amazon River valley a special importance. The present (Holocene) high sea level corresponds with the high water levels of the Amazon River, thus decreasing the gradient of the river. This appears as a backwater effect that also reaches the tributaries by forming ria lakes in their lower reaches. In rivers rich in suspended load the ria lakes have already been filled with sediments, but in the remaining cases the ria lakes still exist (Gourou 1950; Fig. 11.3). Ria lakes are present land inwards up to the mouth of the Jutai River, at a distance of some 2500 km from the sea. This is the upper limit of the backwater effect of the present sea level height. There are thousands of ria lakes in the lower section of the tributary of the Amazon River and its várzea. The length of these lakes ranges from a few hundred metres to more than 150 km (Tapajós). terra firme

terra firme

Várzeas in the lower reach of the Amazon River várzea

The Solimões-Amazon River forms an extended floodplain in its lower reach (i.e. the last 2500 km), generally known by the Portuguese word várzea. The slope of the Solimões-Amazon River is here extremely gentle and the overall elevation of central and eastern Amazonia is very low, with the 100 m contour enclosing an area of about 1,300,000 km2. The river level is 5 m above mean sea level (msl) at Santarém (700 km inland), 23 m above msl near

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km

terra firme

Fig. 11.3 Ria lakes west of Manaus. NASA radar imaging.

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Santarèm

Atlantic

Manacapuru

Metres 20 above and below 0 MSL*

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Amazon River slope

today 2.7 cm/km

10,000–BP 11,000– BP

–20 –40 –60

LGM

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–40 –60

–66 m

–80

0

7.6 cm

/km

12,000–BP

–80 –100

–100

*Present day mean Sea-level

–120

Fig. 11.4 Slope of the Amazon River since the Last Glacial Maximum (LGM). Based on studies carried out in central Amazonia near Manaus and in the Tapajós River. Detailed studies of ria lakes in Central Amazonia and the eastern Amazon River (Irion et al. 2006) have allowed estimates of the variance in slope over the last 20,000 years. According these studies in the section between Manaus and Santarém the slope has decreased since the Last Glacial Maximum (LGM) from about 7.6 to about 2.7 cm/km, whereas between Santarém and the river mouth there has been a decrease from 7.6 to 0.75 cm/km. Modified after Irion et al. (2009). The sea-level-dependent variation of the slope of the Amazon River may be regarded as an outstanding phenomenon in Quaternary history. This change significantly altered the current velocity and the sedimentation processes in the main valley. Detailed studies of ria lakes in the central (Irion et al. 1999; Keim et al. 1999) and eastern Amazon River (Irion et al. 2006) have made it possible to estimate the slope variance for the last 20,000 years. In the reach between Manaus and Santarém the slope decreased in the time since the LGM from ~7.6 to ~2.7 cm/km, whereas the variation between Santarém and the river mouth decreased from 7.6 to 0.75 cm/km (Fig. 11.4; Irion et al. 2009). During the LGM the Amazon River was probably a braided river with a large, very unstable floodplain and reduced turnover time of sediments deposited in the floodplain. At present, the extensive and highly complex várzea wetlands along the Solimões-Amazon River commence only after the junction of the Jutaí River (Irion 1976a), which is probably due to the backwater effect of the sea level heights that reach to this point. Prominent features of the central Amazon várzea include a distinct ridge and swale topography modified by the deposition of fine sediments, and large dish-shaped floodplain lakes and varied sized ria lakes (Hess et al. 2003). Compared to Upper Amazonia, the middle and lower Amazon River has a rather stable river bed and floodplain. Most major hydromorphological structures have an age of several thousand years, such as Marchantaria Island upstream of the confluence of the Amazon and Negro Rivers (Irion et al. 1983; Keim et al. 1999). The várzea corridor was formed as incised river valleys during the Pleistocene glaciations, and these have been repeatedly drowned due to the fluctuating sea level (Irion 1979). It may be assumed that the formation of the sea-level-dependent várzea started with the initial Pleistocene sea level lowering, but there is no unambiguous geological evidence for this. Anyhow, the foremost development in these areas during the Quaternary has been the filling of the drowned valley by Andean-derived sediments transported by the Solimões-Amazon and its white-water tributaries. This is an ongoing process, but sedimentation in the várzea

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has captured only a small proportion of the sediments transported through this corridor. This filling process is less advanced towards the east as indicated by an increasing proportion of open lake bodies in this direction (Sioli 1957). The palaeo-várzea can be recognized by the sequences of ridges and swales that result from the rhythmic depositional processes predominantly at the slipoff slope of the river course. Those ridges and swales create numerous lakes across the várzea zone (Fig. 11.5). The palaeo-várzea exhibits the same structure as the recent one, but its features are significantly weakened. Additionally, in many areas of the palaeo-várzea previous ria lakes are found, proving that they must have formed earlier than the LGM, during a time when the sea level was higher than at present. The total area of palaeo-várzeas can be estimated as more than 50,000 km², with the largest extensions north of Tefé in the area of Lago Amaná, and south of the Solimões River, between Lago

Paleo-várzea

Lago Aiapuá

5 km

Fig. 11.5 Radar map of the region of Lago Aiapuá on the Purus River – Sangamonian (or Eemian, i.e. the last warm period some 115,000 year ago) várzea with ridges and swales. After Irion (1976a).

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Long-term landscape development Coari and downstream of the Ilha do Careiro and at the rim of the várzea in the Juruti-Obidós-Santarém area (Irion et al. 2009). The palaeo-várzea is generally characterized by a widespread distribution of ria lakes formed predominantly during the LGM. One of the largest of those lakes formed in older várzea planes is Lago Coari, with a length of 100 km. Most ria lakes show a high degree of sediment infilling, and in many lakes the bottom is exposed during low water levels, and creeks may form for some months as a local drainage system. The water depth in ria lakes scarcely exceeds 10 m during seasonal high water, and equilibrium has presumably developed between sedimentation and erosion during the high- and low-water stages. The large extension and the comparatively low sediment load of the Negro and Tapajós Rivers, however, resulted in greater water depths in the corresponding ria lakes (respectively 100 m and 50 m). The ria lakes of the Negro and Tapajós Rivers have been studied in more detail (Irion 1976b; Irion et al. 2009). Some 140 km downstream of the junction of the Negro and Branco Rivers, the Negro River leaves the area of the Devonian shale and enters the Cretaceous sandstones, which are, with some exceptions, relatively vulnerable to erosion (Fig. 11.6; Projeto Radambrasil 1978). Further downstream the deeply incised river bed, or trough, has formed, reaching a maximum depth of 100 m. When this valley was incised and formed some 700,000 years ago, all relicts of a smaller pre-existing ‘ria valley’ might have been destroyed. The formation of this valley presumably took place during the Pleistocene low-water stage, when a similar low sea level was reached, for example 120 or 130 m below msl, as during the LGM. This resulted in a lowering by at least 40 m of the water level at the mouth of the Negro River. A 3.5 kHz profiling in the Negro River valley has shown that during the Holocene high sea level stages a 10 m-thick sediment layer was deposited in large parts of the valley (Irion et al. 1999). The volume of this Holocene deposit is estimated to be three to five times the total sediment mass deposited during the time of existence of the deep incised trough of the Negro River valley. When considering that similar processes occurred during each sea-level rise of the Late Quaternary, the filling of the trough is

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comparatively young, i.e. not older than a few glacial cycles. This fits very well in the general trend of sea level fluctuations during the Quaternary, as can be obtained from the δ18O variations in deep-sea sediment cores. There is evidence of a change to more extended glaciations between approximately 600,000 and 900,000 years ago, the so-called Mid-Pleistocene revolution (Berger & Wefer 1992).

Fluvial landscapes in other parts of the Amazon Basin The major rivers joining the Solimões River from the south are highly meandering white-water rivers with suspended sediment loads – ranging from tens of milligrams per litre to over 100 mg/l. The most important rivers are the Javari, Jutaí, Juruá and Purús Rivers. They drain predominantly fine-grained Tertiary sediments in the southwestern lowlands. The Amazon River is joined from the south by the Madeira River, whose headwaters reach into the Andean Cordillera. The sediment load of the Madeira River equals that of the Amazon River at the junction of the two rivers. Further to the east the Tapajós and Xingú Rivers have a major part of their catchments on the Brazilian Shield, where the proportion of material vulnerable to erosion is small and hence the rivers are characterized as clear-water rivers. North of the Solimões River there are three major rivers joining the main river. These are the Putumayo-Iça and Caquetá-Japurá Rivers, both carrying white water, and the Negro River, which is extremely low in suspended load. However, in terms of volumes of water delivery the Negro equals the Madeira River. Generally speaking the rivers north of the Amazon River are smaller. Their catchments are predominantly on the Guiana Shield and hence they carry clear to black water. These are the Urubu, Preto da Eva, Uatumá and Trombetas Rivers.

Landscape evolution in terra firme Remote-sensing data, particularly SLAR (Side-Looking Airborne Radar) and satellite imagery, have revealed the main geomorphology

NW

SE Arquipelago das Anavilhanas

Manaus

Lake sand Sand

Narrow of Tatu

Mud Mud with some sand

Max. incision during low sea-level stages

Mud

Amazon River sediment 20 m

Cretaceous sandstone 200 km

Fig. 11.6 Longitudinal cross-section through the ria lake of the Negro River. The archipelago of the Anavilhanas formed during various sea level heights during the last 600,000 years. The sediments consist chiefly of fine-grained material that was deposited as a result of decreased river flow in the upper 100 km. At the narrow of Tatu, where the width of the river decreases to merely 1.6 km, the deposition of mud is interrupted, but it continues when the ria lake expands again directly upstream of Manaus. Maximum incisions of the Negro River are reached during sea level lowstands.

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types in the non-flooded parts of the Amazonian lowlands, or terra firme. Both planar surfaces with scattered incised river valleys and diverse types of convexo-concave geomorphologies are present. All the different landscape types represent developmental stages of the incision of drainage systems into the loose surface soils. As most of the lowlands are sedimentary in origin, the denudation process actually deforms the original depositional surfaces: any planar relief is bound to become deformed with time once deposition turns into erosion. The speeds and patterns of the surface deformation processes vary according to local factors such as the parent material, vegetation and climate, which also determine the vulnerability of these soils for further surface erosion. The same factors also influence the local drainage conditions, such as surface run-off and ground-water flow. In response to the region’s high precipitation rates, the drainage system, formed by small upstream rivers, becomes increasingly detailed and subsequently terra firme gradually intermingles with the Amazon’s fluvial system through the repetitive valley-creek structure. In western Amazonia, the average distance from any point of land to the nearest river is only 12 km even when the smallest creeks are not taken into consideration, a total of 1.1% of the surface is covered by the open-water bodies, and riverine corridors including active channels and floodplain lakes occupy 17% of the land area (Toivonen et al. 2007). When exposed to soil-forming processes the topmost sediments alter depending on a combination of local geological, geomorphological, hydrological and climatic factors. The soil-forming processes are important for the Amazonian biota as they promote the edaphic site conditions. A general trend is a directional impoverishment of the soils as, over time, parent material diminishes. In western Amazonia the soils are in many places more varied than generally mentioned in standard textbooks; this is because the signal of the parent rock still prevails over large areas. In western Amazonia local-scale soil variability is well documented between the cities of Iquitos and Nauta (Peru) where, over a distance of only 100 km, the soils range from white quartz sands through diverse clays of the poorly altered Pebas Formation, to the highly altered soils on sediments of Tertiary tidal-channel deposits on a delta plain with a degree of similarity to the below mentioned ‘Belterra Clay’ (Kauffman et al. 1998; Linna et al. 1998; Rebata et al. 2006). Soil alteration also serves as a key to interpret the past environmental changes. For example, sediment cores taken from the palaeo-várzea in Brazil show significant alterations in mineral composition due to weathering processes at and near the surface; however, with depth (several metres) the mineral composition is exactly the same as that of the nearby recent river deposits (Irion 1976a). The alterations in these areas consist of increased amounts of kaolinite, gibbsite and iron oxides at and near the surfaces of the palaeo-várzea (Irion 1984a), sometimes accompanied by a decrease in smectite. The palaeo-várzea is mostly of Sangamonian age (or Eemian, the last warm period some 115,000 years ago). In accordance with the advanced mineral alteration there are also regions with palaeo-várzeas of the older Pleistocene stages. These show a higher degree of alteration reaching to a greater depth than that of the Sangamonian várzea (Irion 1976a, 1976b). According to the NASA georeferenced maps the level of the Sangamonian and older várzea, for instance near Lago Aiapuá (Purus River),

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is 40 m and 60 m above msl. This is some 10 m too high to be explained by backwater effects of a higher sea level during that time. The easiest explanation would be a general rise of the continent west of 60°W longitude. The Precambrian shields, the Paleozoic formations and the Alter do Chão Formation, form the predominant drainage areas of the tributaries east of 60°W longitude. On these formations the soil surface has been exposed to long-term alterations under the humid tropical climate, leading to thick weathering profiles. The poorly soluble quartz together with some heavy minerals such as zircon and anatase resist weathering and become enriched, whereas the less resistant and soluble minerals alter and easily get carried away. The results are thick lateritic soils that are enriched with quartz, newly formed kaolinite and minerals composed of iron and aluminium such as goethite, haematite and gibbsite (i.e. Aleva 1984; Irion 1984a). The laterization of both Proterozoic (Théveniaut & Freyssinet, 2002) and Paleozoic rocks, and the Mesozoic sequences such as the Alter do Chão has been ongoing for many millions of years and has stripped the Amazonian Craton of a considerable volume of rock throughout most of the Tertiary (Irion 1978; Théveniaut & Freyssinet, 2002). Nevertheless, von Blanckenburg et al. (2004) suggest that the tropical climate alone is too ineffective to promote high rates of erosion in tectonically inactive areas. However, as Théveniaut & Freyssinet (2002) suggest that 50–80 m of the soil surface has been eroded during the Oligocene alone, it only implies an average denudation rate of about 5–8 mm/ky over 10 million years. This is very low and is within the range of denudation rates found in tropical highlands of Sri Lanka (von Blanckenburg et al. 2004) and is in accordance with the observation of Sioli (1957) and the results of Irion (1978). One of the most important soil-related enigmas in the Amazon literature has been the case of the so-called ‘Belterra Clays’ (Sombroek 1966). These ochre clays extend over more than one million square kilometres and range in thickness from a few decimetres to 10 m. They cover large parts of the Guiana and Brazilian Shield areas and occur on top of both, the Paleozoic and Cretaceous sediments of eastern Amazonia. Sombroek (1966) interpreted these clays as lake deposits, and this lake allegedly existed between Late Tertiary and/or Early Pleistocene times. More detailed studies by Irion (1978, 1984a) and Chauvel et al. (1987) revealed that the clays represent the uppermost part of a deep weathering profile. There is a distinct difference of weathering sequences on different sediments and rocks. Profiles on Paleozoic shales are shallower due to poor drainage, and start at the bottom with minerals inherited from the shale. However, in the top few decimetres the geochemical and mineralogical differences with the typical Belterra Clay of the Cretaceous sandstones (Alter do Chão Formation) are small. A typical ‘Belterra’ profile on the Alter do Chão Formation starts at a depth of some 20 m with sediments or rocks with wellpreserved original structures but with mineral compounds that are, with the exception of quartz, predominantly transformed into kaolinite that is well ordered along its b-axis. A secondary transformation takes place in the surface horizon of some 1–10 m thickness where due to seasonal variation the phreatic water, or moisture levels, are oscillating and oxidation-reduction conditions are changing. In this layer iron oxides, mainly haematite,

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Long-term landscape development are precipitated forming a pellet structure; quartz and primary kaolinite are dissolved instead, and secondary kaolinite, which is poorly ordered to the crystallographic b-axis, is formed. The resulting uppermost layer is ochre-coloured and mainly composed of kaolinite and some relict quartz. Based on the differences in quartz content in the lower and uppermost layers and on the silica content of the waters leaving the formation, the minimum age of the soil profile is calculated as 10 million years (Irion 1978). The real age, though, is most probably in the range of 20–50 million years. This means that during the past at least 20 million years the surfaces on the ochre-coloured clays, particularly when they are several metres thick, were exposed to very limited erosion or none at all. The key inference we can draw from the presence of these ochre-coloured clays is that for many millions of years there was a continuous cover of dense vegetation that preserved the surface from erosion. This is further confirmed by present forest clearance in hilly areas, for example around Manaus, where erosion immediately starts and in matter of years deep gullies are formed. The composition of the sediments and their geochemistry tells us about the origin of the rivers and Amazonian landscape evolution. The fingerprint of the river’s source areas is indicative of both the original parent material and the alteration history that the different sediment grains have experienced. The geochemical signature is particularly notable in the upper tributaries (Kalliola et al. 1993), whilst in the trunk rivers that collect waters and sediments from different sources, geochemical signatures become less distinctive (Irion 1976b). In the Amazonas-Solimões River, there is, due to the low sediment contribution of its tributaries, a general constancy in the composition of the susp