Edited by G. Einsele and A. Seilacher
With 180 Figures
Springer-Verlag Berlin Heidelberg New York 1982
DM G(;.OO
Professor Dr. GERHARD ErNSELE Professor Dr. ADOLF SEILACHER Institut fiir Geologie und PaHiontologie Universitiit Tiibingen SigwartstraBe I 0 D-7400 Tiibingen
ISBN 3-540-11373-8 Springer-Verlag Berlin Heidelberg New York ISBN 0-387-11373-8 Springer-Verlag New York Heidelberg Berlin This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, spedfical!y those of translation, reprinting, re-use of illustrations, broadcasting, reproduction by photocopying machine or .similar means, and storage in data banks. Under §54 of the Gennan Copyright Law, where copies are made for other than private use, a fee is payable to "Verwertungsgesellschaft Wort", Munich, ©by Springer-Verlag Berlin Heidelberg 1982 Printed in Gennany The use of registered names, trademarks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. Offsetprinting and bookbinding: Briihlsche Universitii.tsdruckerei, Giessen 2132/3130-543210
v
Preface
The problem of bedding, a basic feature of most sedimenta ry rocks, is as old as the science of geology itself. We use bedding in structural geology, regional correlation and for estimating the time involved in the stratigraphic record. Nevertheless we still are far from fully under standing the processes involved. This is particularly true for carbonate rocks, where primary phenomena are sometimes difficult to separate from the secondary diagenetic over print. After new interest in the subject had arisen from the International Deep Sea Drilling Project and from pa leoecological studies in our own research group (Sonder forschungsbereich 53 '1Pal6kologie11 ) , a Rundgesprach (work shop) was held in TUbingen on April 25th - 27th 1980. The present volume, which resulted from this symposium, con tains a variety of contributions, including some by col leagues that were unable to attend the meeting itself. Papers whose authors did not submit an elaborated manu s�ript, are represented by abstracts in the form presented fo'r the me�ting. Our own interest in the problem envolved from studies of 11Fossil-Bonanzas11, such as the Solnhofen lithographic lime stones or the bituminous Posidonia shales, in which the unusual kind and preservation of fossils indicated extre me environmental conditions. During these studies (see sununary ;reports in Zbl. Geol. PaUiont. II, 1976 and N. Jb. Geol. Palaont., 157, 1978) we realized that even in these cases· one single-ell vironmental model is usually insuffi cient to explain the conflicting evidences. This lack in our approach became still more evident, when we left the extreme end members to study Fossil-Lagerstatten such as shell beds, in more "normal" types of facies. The title of the symposium focuses on the key questions: to what extent does bedding reflect the gradual cyclic and periodic changes of _our tellu'ric system or rather rare and unpredictable events that occur in almost any sedimen tary regime? Or, more pragmatically: in what environments have telluric cycles a chance to leave a sedimentary re cord that does not become wiped out by bioturbation or obscured by the effects of catastrophic events? Because this inherent dilemma commonly splits researchers into a .''cyclist11 and a 11 catastrophist " camp, we felt it necessary to have both views adequately represented in this volume. The con tribu·tions cover a broad spectrum of rock and facies types and paleoecological as well as sedimentological and diagenetic criteria. The well-knoW n turbidites, although being the most prominent group of the event deposits, are, however, dealt with only by a few .examples in relation to coexisiting cyclic or black shale phenomena, or in order to demonstrate the role of carbonate diagenesis. The au-
VI thors of this volume range from amateurs and undergradu ate students to established experts. In spite of such in herent heterogeneities we hope that this collection of papers in some way does justice to the scope of the prob lem and meets the needs of geoscientists trying to under stand the meaning of bedded sequences. Our work at Ttibingen including the workshop mentioned above was sponsored by the German Research Society (Deut sche Forschungsgemeinschaft) which is gratefully acknow ledged. Particular thanks are due to Mrs. L. Hagel, E. Himmel, H. Jurczyk, A. Lupke, R. Stephani for typewrit ing the manuscripts camera-ready, to Mr. H. Vollmer for help in drafting, and to Mr. ��. Wetzel for photographic work including the reduction of roost of the figures and tables to meet the page size set by the publishers. All this work had to be accomplished besides the daily tasks at our institute. We are grateful to Springer Verlag for accepting our home-made product despite of technical shortcomings, for which the editors alone bear the re sponsibility. We hope that in spite of these deficiencies, this volume may _convey some of the spirit that united the participants during the symposium and help to free stratigraphy from the blemish of being a dry and purely descriptive science.
TUbingen, March 1932
G. EINSELE A. SEILACHER
·i� . •·
I
I
; 1 .,, ..
. .'·�·. '. .1": .
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VII
Contents
Part I.
Limestone-Marl Rhythms and Climate-controlled
Facies Changes General Remarks About the Nature, Occurrence, and Recognition of cyclic Sequences (Periodites) G. Einsele (With 1 Figure) . . . . . . • . . . . . . . • • • ....... .. .
3
Limestone-Marl Cycles (Periodites) : ficance, Causes - a Review G. Einsele (With 14 Figures) . • • . • . .
. . ..
8
Observations on Well-bedded Upper Jurassic Lime stones W.M. Bausch, J. Fatschel, and D. Hofmann (With 8 Figures) .....................................
54
Origin of Marl-Limestone Alternation {Oxford 2) in Southwest Germany W. Rieken and c. Hemleben (With 3 Figures) . • • . • • • • •
•
.
63
Limestone-Shale Bedding and Perturbations of the Earth1s Orbit w. Schwarzacher and A.G. Fischer (With 7 Figures)
.
•
72
Diagnosis, .
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• •
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. • .
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. •
s·igni .
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•
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Rhythmic Sedimentation Documented in a Late Cretaceous Core (Abstract) L. Pr<:ttt.............................................
96
Ecology and Depositional Environments of Chalk-Marl and Limestone-Shale Rhythms in the Cretaceous of North America (Abstract) E.G. Kauffman........................................
97
Diagenetic Redistribution of Carbonate, a Process in Forming Limestone-Marl Alternations (Devonian and Carboniferous, Rheinisches Schiefergebirge, w. Germany) W., Eder (With 12 Figures) ............................
98
A Contribution to the Origin of Limestone-Shale Sequences 11. Walther {\Vith 2 Figures) • • . . . . • : . . . • • • . • • • . • .
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•
11 3
Deep-Sea Stratigraphy: Cenozoic Climate Steps and the Search for Chemo-Climatic Feedback W.H. Berger (With 2 Figures) ·····�················ . • . 121
VIII Part IIA. Event Stratification.. Calcareous and Quartz-sandy Tempestites General Remarks About Event Deposits A. Seilacher (With 2 Figures) • . . . • . . .
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Experiments on the Distinction of Wave and Current Influenced Shell Accumulations E. Futterer {With 2 Figures) . . . . . • . . . . . • . . . . . . . . . . .
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161 ':iJ '·
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•
175
Calcareous Tempestites: Storm-dominated Stratification in Upper Muschelkalk Limestones (Middle Trias, SW-Gerrnany) T. Aigne.r (With 10 Figures) . . • . . . . . • . . . . . . . . . . • • . . . . . 180 Allochthonous Coquinas in . the Upper Muschelkalk Caused by Storm Events? (Abstract) H. Hagdorn, and R. Mundlos . • • • . . . • • • • . • . . • • . . • . . . .
.
199
The role of Storm Processes in Generating Shell Beds in Paleozoic Shelf Environments R.D. Kreisa _and R.K. Bambach (With 2 Figures) . . . . . . . •
200
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•
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Rhythmic Bedding and Shell Bed Formation in the Upper Jurassic of East Greenland F.T. Flirsich (With 5 Figures) . . . . . . . . . • . . . . . . . . . • . . . . 208 Shell Beds in the Lower Lias of South Germany - Facies and Origin G. Bloos (With 7 Figures) . . . • . . . . • • . . . . • • . . . . • . • . • • . . 223 Storm Sedimentation in the Carboniferous Limestones Near Weston-Super-Mare (Dinantian, SW-England) D. Jef_fery and T. Aigner (With 1 Figure) . . . • . . . . • . . .
.
240
Event-Stratification in Nummulite Accumulations and in Shell Beds from the Eocene of Egypt T. Aigner (With 7 FigureS) . . . • • • . . . • • . . . . . • . . . . • . . . .
.
248
The !!Bank der kleinen Terebrateln u (Upper Muschelkalk, Triassic) Near Schwabisch Hall (SW-Germany} - a Tem pestite Condensation Horizon H. Hagdorn (With 13 Figures) . . . . . � • . . . • . • . . • . • . . . • . . . 263 Glauconitic Condensation Through High-Energy Events in the Albian Near Clars (Escragnolles, Var, SE-France) G. Gebhard (With 4 Figures) . • . . . • • . • . . • . . . . • • • . . • . . .
.
286
Muschelkalk/Keuper Bone-Beds (Middle Triassic, SW Germany) - Storm Condensation in a Regressive Cycle W.-E. Reif (With 11 Figures) . . . . . . . . . . . . . . . . . . . . . . . .
.
299
Condensed Griotte Facies and Cephalopod Accumulations in the Upper Devonian of the Eastern Anti-Atlas, Morocco J. Wendt and T. Aigner (With 2 Figures) . . . . • . . . . . . . • . 326 Distinctive Features of Sandy Tempestites A. Seilacher (With 7 Figures) . . . • • . . • • • . . .
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333
-�.
IX Multidirectional Paleocurrents as Indicators of Shelf Storm Beds D.I. Gr� y and M.J. Benton (With 2 Figures) . • . • . •
.
.
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.
.
350
Scour and Fill: The Significance of Event Separation R. Goldring and T. Aigner (With 2 Figures) . . • . . . • . . • • 354 Storm-surge Sandstones and the Deposition of Inter bedded Limestone: Late Precambrian, Southern Norway M. Tucker (With 5 Figures) . . . • . • . . . • • . . . . . • • • • • • . . . .
.
363
Flat Pebble Conglomerates, Storm Deposits, and the Cambrian Bottom Fauna J.J. Sepkoski, Jr. (With 4 Figures) . . . . . . . . • . . . • . • . . . 371
Part IIB. Event Stratification - Other Event Deposits Jurassic Bedded Cherts from the North Apennines, Italy: Dyscyclic Sedimentation in the Deep Pelagic Realm T.J. Barrett (With 5 Figures) • . . . . . . . . . . . • . • • • . • . . .
.
.
389
Quartz-sandy Allodapic Limestones as a Result of Lime Mud-Raising Clastic Turbidites u. Maier-Harth (With 8 Figures and 2 Plates) . . . . .
.
.
•
404
Belemnites as Current Indicators in Shallow Marine Turbidites of the Santonian Bavnodde Gr¢nsand, Bornholm (Denmark) R. Schmidt (With 2 Figures) . . . . . • • • • • • . . • . • . . . . . . . .
•
•
419
Habits of Zircon as a Tool for Precise Tephra stratigraphic Correlation J. Winter (With 1 Figure) . . . . . • • . • . • • . • • . . . • . . .
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423
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•
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431
Part III. Cyclicity and Event Stratification in Black Shales Cyclic and Dyscyclic Black Shale Formation A. Wetzel (With 5 Figures) . • • • • . • . . • • . • • . . .
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.
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Cyclicity and the Storage of Organic Matter in Middle Cretaceous Pelagic Sedime'nts P.L. deBoer (With 5 Figures) . . . . . . . . . . . . . . . . . • . • • • • . . 456 Types of Stratification in the Kupferschiefer J. Paul (With 2 Figures) . . . . .. . . . . • . . • • . . • . • . • .
.
.
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476
.
.
.
482
The Community Structure of "Shell Islands11 on Oxygen Depleted Substrates in Mesozoic Dark Shales and Laminated Carbonates (Abstract) E.G. Kauffman . . . . . . . . . . . . . . • • . . . . • . . . . . . . . . . . . . . • . . . •
502
.
.
.
.
Environmental Changes During Oil Shale Deposition as Deduced from Stable Isotope Ratios ·w. Klispert (With _5 Figures) . . . . . • . • . . . . • . . . • . . . . . .
X Ammonite shells as Habitats - Floats or Benthic Islands? (Abstract) A. Seilacher . • • . . . . . . . . . . . . . . • . . . . . . . • • . . . . • . • . .
.
•
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.
.
504
Palynology of Upper Liassic Bituminous Shales (Abstract) W. Wille . . . • . . . • . . . . . • . . . . . . . • • . . . . . . • . . . . . . . •
.
.
.
•
.
505
.
.
The Bituminous Lower Toarcian at the True de Balduc . ement de la Lozere, S-France) Near Mende (DElpart W. Riegraf (With 2 Figures) . . . . . . • . • • • . . . • • . . . . • • . ... 506 Bedding Types of the ToarCian Black Shales in NW-Gree.ce J.P. Walzebuck (With 6 Figures) . . . . . . . . . . . . . • • . ...... 512 Stratinomy of the Lower Kimmeridge Clay (Dorset, England) (Abstract) T. Aigner; . • . . . • • . . . . . . • • . . . . . . . • • . . . . • . . . . . . . • .
.
.
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•
.
526
The Formation of the Bituminous Layers of the Middle Triassic of Ticino (Switzerland) (Abstract) H. Rieber . . . • • . • . . . . . • • . . . . . . . • • . • . . . . • • • . . . • . . . . . . • . 527
*
s-ry Paleogeographic Significance of Tempestites and Periodites G. Einsele and A. Seilacher (With 2 Figures) • • . .•
• • . . .
531
XI
List of Contributors
1
*Aigner, T. 180, 240, 248 326' 354' 526 Bambach, R.K. 200 Barrett, T.J. 389 54 Bausch, W'.M. 350 Benton, M.J. Berger, W.H. 121 Blocs, G. 223 deBoer, P.L. 456 Eder, w. 98 *Einsele, G. 3, 8, 531, Fatschel, J. 54 Fischer, A.G. 72 Flirsich, F.T. 208 Futterer, E. 175 *Gebhard, G. 286 Goldring, R. 354 Gray, D. I. 350 Hagdorn, H. 199, 263 *Hemleben, C. 63 54 Hofmann, D. Jeffery, D. 240
Kauffman, E.G. 97, 502 Kreisa, R.D. 200 *Klispert, w. 482 *Maier-Harth, U. 404 Mundlos, R. 199 Paul, J. 476 Pratt, L. 96 *Reif, w.-E . 299 *Rieken, W. 63 Rieber, H. 527 *Riegraf, w. 506 *Schmidt, R. 419 Schwarzacher, w. 72 *Seilacher, A. 161, 333, 504' 531 Sepkoski, J.J. jr. 371 Tucker, M. 363 Walther, M. 113 *Walzebuck, J.P. 512 *Wendt, J. 326 *Wetzel, A. 431 *Wille, W. 505 Winter, J. 423
1A 11 authors marked with an' asterisk can be contacted under the following address: Geologisch-Palaontologisches Institut und Museum der Universitat SigwartstraEe 101 7400 TGbingen, FRG Please find the addresses of the remaining authors in the 11Address List11 (next pages)
XIII
Address List
Bambach,
R.K.,
Prof.
Virginia Polytechnic Institut.e and State University, Dept. of Geologi cal Sciences, 4044 Derring Hall, Blacksburg, Virginia 24061, USA
Barrett,
T.J.,
Dr.
University of TorontO, Dept. of Geology, Toronto, Ontario MSS 1A1, Canada
Bausch,
W.M.,
Prof.
Institut ftir Geologie u. Mineralo gie, Universitat Erlangen-Nlirnberg, SchloBgarten 5, 8520 Erlangen, FRG
Benton,
M.J.,
Dr.
Department of Geology University of Newcastle Upon Tyne, Newcastle, NE1 7RU, England
Berger,
W.H.,
Prof.
Scripps Institution of 09eanography University of California, San Diego La Jolla, California 92093, USA
�;
Blocs,
G.,
de Boer,
Eder,
W.,
Fatschel,
P.L.,
Dr.
State University of Utrecht, Insti tute of Earth Sciences, Budapest laan 4, P.O. Box 80.021, 3508 TA Utrecht, The Netherlands
Dr.
Geologisch-Palaontologisches Insti tut und Museum der Universitat, Goldschmidt-StraBe 3, 3400 GOttingen, FRG
J.
Institut ftir Geologie u. Mineralo gie, Universitat _Erlangen-Nlirnberg, SchloBgarten 5, 8520 Erlangen, FRG
Fischer,
A.G.,
Flirsich,
F.,
Futterer,
Staatliches Museum flir Naturkunde, Arsenalpla tz 3, 7140 Ludwigsburg, FRG
Dr.
E.,
Prof.
Dr.
Dr.
Princeton University, Dept. of Geological and Geophysical Sciences, Guyot Hall, Princeton, New Jersey 08544, USA Institut flir Palaontologie und Historische Geologie der Universitat, Richard-�agner-StraBe 10 II, 8000 Mlinchen 2, FRG Geologisch-Palaontologisches Institut der Universitat, Olshausen straBe 40/60, 2300 Kiel, FRG
XIV Goldring, R., Dr.
University of Reading, Dept. of Geology, Whiteknights, Reading Rg6 2AB1 England
Gray,
Department of Geology, University of Newcastle Upon Tyne, Newcastle, NE1 7RU, England
D.I.,
Dr.
Hagdorn, H. studienrat
Konsul-Uebele-StraBe 14, 7118 Ktinzelsau, FRG
Hofmann,
Institut flir Geologie u. Mineralo gie, Universitat Erlangen-Nlirnberg, SchloBgarten 5, 8520 Erlangen, FRG
D.
Jeffery, D.
Institute of Geological Sciences (Overseas Dept.) Keyworth, Nottingham, NG 12599, England
Kauffman,
Dept. of Geosciences, University of Colorado, Boulder, Colorado 80302, USA
E.G., Dr.
Kreisa, R.D.,
Dr.
Dept. of Geological Sciences, Ohio University, Athens, Ohio 45701, USA
Mundlos, R., Dr.
SchachtstraBe 6, richshall, FRG
Paul, J. , Dr.
Geologisch-Palaontologisches Insti tut urid Museum der Universitat, Goldschmidt-StraBe 3, 3400 GOttingen, FRG
Pratt, L.M.
Princeton University, Dept. of Geological and Geophysical Sciences, Guyot Hall, Princeton, New Jersey 08544, USA'
Rieber,
Palaontolog. Institut und Museum der ETH ZUrich, Klinstlergasse 16, 8006 ZUrich, Switzerland
H.1 Prof.
Schwarzacher, w., Prof.
Sepkoski, jr., Dr.
J. John,
Tucker, M.,
Dr.
J.,
University · of·Belfast1 Dept. Geology, Queen's University, Belfast - BT 71 NN1 Ireland
of
University of Chicago, Dept. of the Geophysical Sciences, 5734 s. Ellis Avenue, Chicago, Illinois 60637, USA Department of Geology, University of Newcastle Upon Tyne, Newcastle, NE1 7RU, England Geologisch-Palaontologisches Insti tut und Museum der Universitat, Goldschmidt-StraBe 3, 3400 GOttingen, FRG
Walther, M.
Winter,
7107 Bad Fried
Prof.
Geologisch-Palaontologisches Insti ·tut der Universi tat, Senckenberg Anlage 32-34, 6000 Frankfurt a.M., FRG
Part I. Limestone-Marl Rhythms and
Climate-controlled Facies Changes
I
I
I'
I
General Remarks About the Nature, Occurrence, and Recognition of Cyclic Sequences (Periodites) G.EINSELE
Abstract: Rhythmic sequences may be caused (1) by a succession of events or (2) by gradual periodic changes (minor cycles or per.iodites). Some characteristics of these principal me chanisms are demonstrated in sediment buildup-time curves for periodites, tempestites, turbidites, and black shales. The most prominent examples for periodites appear to be non turbiditic pelagic to hemipelagic limestone-marl rhythms. Under favorable conditions including diagenet�c enhancement of small primary alternations, they record a periodically changirig environment which is otherwise obscured or obliterated.
Cyclic or rhythmic sequences occur world-wide in presumably every stratigraphic system. Although such sequences have frequently _been dealt with in the past
1967; ELAM & CHUBER1
(e.g. MERRIAI1,
1972;
1964·; DUFF1
SCHWARZACHER,
1975) 1
HALLAM & WALTON1
many questions related
to this basic problem of stratigraphy are still unsolved.
Therefore
it is a challenge for earth scientists to do further research in this field in order to explain the striking phenomenon of cyclic sediments. Such an attempt seems particularly timely since new methods of inve. stigation have become available and our knowledge of Recent and Quater nary deposits in marine and lake environments has grown considerably. The term 11cyclic sediments" as used in the past comprises a wide varie ty of large-scale and small-scale phenomena of different nature and extremely diverse duration. In the p�esent context,
we cannot deal with
the whole spectrum of sedimentary seq�ences ranging from annual varves to lon�-period mega-cycles or cyclothems caused by lateral migration of a deltaic environment or by the rejuvenation of relief by endogenic
cyclic and Event Stratification (ed. by Einsele/Seilacher) ©Springer 1982
E..:.
4
.r ic se..... forces. The contributions to this volume focus on 1'1\0Xi.ne rhythr quences which DUFF et al
(1967 ) have described as "minor cycles". In a
first section we discuss cyclic sediments, which are controlled by factors not directly related to the mechanism of sediment transport and accumulation v,7ithin the sedimentary basin, outside of this area
but rather to processes
(allocyclic sediments).
1. Pure Type of Cyclic Sediments with a Certain Time Period
(Periodites) Some characteristics of these rhythms in comparison to event deposits are shown by Fiq. 1. The pure type of such rhythm.ic sequences consists of periodically alternating beds deposited in a pelaqic to hemipelag-ic environment below V.'ave bi:l:se. Sedimentation rate,
texture and fabri.c,
as well as composition of secUm.ents :rr.ay chang-e gJ:_"a.duai.ly within one period while features of omissio . n or erosion are atypical.
For that
reason, usually both alternating be·d types are affected by bioturbation anC no markeC change in the infauna can be observed. Base and top of sinqle beds show similar burrows;
breaks in the i.nfa.una occur only i:l:
oxygen depletion is involved near the sea floor.
Fig. 1 Simplified and idealized scheme of differences and interrelationships between periodites, tempestites, turbidites, and black·shale rhythms. Periodites are characterized by slow and periodic changes in sediment parame·ters, bioturbation, and sedimentation rates as well as by con tinuous vertical buildup with time (BT-curve). Change of primary com position includes texture and fabric; the curve for bioturbation re presents intensity of burrowing and also some changes in community. Tempestites and turbidites are the resul·t of erosional and dep_osi tional events, but sediment parameters, bioturbation, and sedimen tation rate of their background sediments may remain more or less unaffected. In the ideal case the material of tempestites is entirely autochthonous, whereas turbidites always contain sediment from distant sources (mixed with local material). In periodites as well as in the background sediments of tempestites and turbidites, the zone of bio turbation {B) is migrating solely upwards (Bupw), whereas the recolo nization of event deposits is starting at thelr surface and prograding . in a downward direction (B dw). Black shales may show both event strati fication (interval a) as well as periodic changes (interval b with periodites), but these are often superimposed by short-period changes generating thin !�ruinations as long as anoxic conditions are prevai ling. For further explanations see general remarks to part II (this vol.). Exp. = vertical exposure of sequence after diagenesis (compaction neg lected; 1, often lost by slow currentsi �, without lateral addition or loss of Sediment
5
TURBIDITES
PERIODITES
1 l
ALLOCHTH.
RATEOF SEDIM.
BIOTURBATtON
�
'
� 1
�
BACKGR. SED.
�
.
AN- l OXIC
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l
l
\ MEAN COMP
EROSl
ON
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TEMPESTITES
I.'l''' [ ��;:H I ��L�� �· COARSE
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CHANGE I OF PRIM I I I I " COMPOS. F INE "'tl '·' 'v'
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.
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EROSION
-
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bbft,k b fbik6�.
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6
As SCH1i17ARZACHER
(1975')
has pointed out,
there are cycles which carry
time information anC. others which do not. Many workers hold the opinion that rhythmic sequences as described above are induced or generated by some global mechanism with rr.ore or less constant time periods. If this is true,
such sequences can provide information about
stratigraphic a0e and accumulation rate as well as indications of �eneral qlobal processes. be termed
Therefore, sequences with this potential may
uPeriodites".
Mechanisms often quoted are the earth' s orbital cycles of precession, obliquity,
and eccentricity with·periods of approximately 21,000 y.,
41,000 y.,
and 100,000 years, respectively.
These periodic earth move
ments !l'lay affect the cliw.ate on the continents, the current systems of the Oceans, the elevation of the sea level and some physical and chemical prOperties of the sea water and thus al.so exert some in fluence on the sediments. The periodic change of on.e single sedimer..t parameter such as e.g. porosity rr.ay be sufficient to produce con spicuous rhythmic sequences,
particularly if it is enhanced by later
diagenesi-s. Marl-limestone rhythms are very sensitive to dia�enetic changes. Pro bably for that reason, these sequences have become the most prominent examples of periodites.
In view of the growing general interest in the
carbon cycle and the interchange of carbon dioxide between the atmos phere and hydrosphere, further studies on marl-limestone rhythms appear to be especially promising.
2.
Recognition and Modifications of Pure Periodites
Primary periodic alternations in sediments can be recognized only if they reflect substantial changes in sedQment parameters,
or if minor
variations are enhanced by secondary processes such as limestone dia genesis. Here,
the original symmetry of a sin�le bed can be lost due
to the uni-directional upward flow of pore water and its ability to dissolve and reprecipitate solid material. The recognition of perio dites may also be restricted to basins of a certain sedimentation rate. In areas of very slow deposition,
the sediments of alternating beds
can be completely mixed by bioturbation and thus the primary rhythm be obliterated. On the other hand,
high input of a sediment component not
implied in the periodic variations may dilute the alternating sediment components to such an extent that the · cycles become unrecognizeable. Periodic variations in a sedimentary sequence can also be obscured by aperiodic phenomena triggered by earth quakes,
storms and other events
Limestone-Marl Cycles (Periodites): Diagnosis, Significance, Causes - a Review G. EINSELE
Abstrac t : Pelagic to hemipelagic l imestone-marl rhythms are described as minor cycles with time information {periodite s ) They develop and can be recognized in the field only , if (a) the original sediment had a carbonate/clay ratio on the order of 4 and, under oxygenated ·conditions , if ( b ) the thicknes s of somewhat alternating beds was sufficient ( 5 to 10 em) to prevent complete mixing by bioturbation. Simple · models demonstrate the effect of fluctuating produc tivity and dissolution of (chiefly biogenic) carbonate as well as its dilution by 11clay11• Howeve r 1 small primary differences in carbonate content or texture - of alternating beds may be enhanced by diagenesis , including pressure solu tion, which strongly affect the final field aspect and the faunal content of the rhythmic sequence . Yet there are several criteria to prove the primary origin of such succession s . The exact correlation and timing of limestone-marl sequences is difficult in many case s , but it can hardly be doubted that the time periods of Quaternary and older carbonate cycles coincide in their order of magnitude ( 20 , 000 to 1 00 , 000 years) This i s also true for their sedimentation rate ( 0 . 5 to 3 cm/ 1 000 years) which is controlled by pelagic carbonate productio n . Limestone-marl rhythms reflect a depositional area below storm-wave base . In the case of continental margins or epicontinental seas , accumulation approximately balances subsidence , and alternating beds appear to be generated mainly by dilution. In the deep sea , cyclic carbonate de position is found on rises and plateaus , where it i s chiefly controlled b y dissolution i n conjunction wi·th a fluctuating CCD ·and/or changi ng bottom currents . _ Rhythms of this type occur wor�ide since Paleozoic time . They are probably caused by climatic variation s , but s imul taneous (generally small) sea level fluctuations may also play a part. During periods of widely extented and intensive plant grow·th on land, the .co2 content of surface waters may have been diminished for some time , whereas the river supply of dissolved old carbonate was increased. Both pro cesses tend to promote the production and preservation of carbonate in shallow and deeper water . •
•
Cyclic and Event Stratification (ed. by Einsele/Seilacher) © Springer 1982
9
1 . Introduction
Rhythmic sediments with carbonate as a principal phase occur in diffe rent environment s , the most important of which are { FISCHER, 1 9 8 1 ) : { 1 ) Pelagic to hemipelagic marine sedimentary conditions , { 2 ) marine carbonate platforms , ( 3 ) lacust,rine settings . This paper only deals with the first group of environments . The purpose is not to add a further case study Of a' limestone-marl sequence t0 the already existing extensive l i terature , but rathe·r to make an attempt to review the " state of the art" on this problem and, if possible, to add,.a few points which have not been discussed very much. It also appears to be high time to use the great amount of new information from the study of young and older marine sediments in the - oceans and to apply this knowledge to ancient rock sequences on the continents . work on this promising task has s tarted only a few years ago . Further more , sedimentation and dis solution of marine carbonate i s an important process of the world-wide hydrospheric and· endogenic carbonate cycles . Since these cycles are interlocked with the exogenic organic carbon { including carbon dioxide) cycle of the hydrosphere, atmosphere , and biosphere on land { e . g . GOLUBI C et a l . , 1 9 79 ) , cyclic deposition of carbonate may reflect certain fluctuations in the exogenic carbon cy qle . Therefore , this interrelationship is also briefly discussed in this paper. For a theoretical and statistical approach to the problem, the reader may start with SCHWARZACHER1s { 1 9 7 5 ) book . In this paper only a few very s imple models are used to quantify the effect of di fferent oszillating processe s .
2 . Definition and .F ield . Aspect o f Limestone-Marl Rhythms
As already pointed out in an introductory chapter to Section I of this volume (EINSELE , this vol . ) limestone-marl rhythms are a simple case of a cyclic sediment with a comparatively long time period ( FUCHTBAUER, 1 9 70) . DUFF et a l . ( 1 9 6 7 ) have described such an ABAB sequence a·s "minor cycle'' displaying regular alternations of shale or marl and fine-grained argillaceous limes tone of the order of a few decimeters per cycle . SCHWARZACHER ( 1 9 7 5 ) describes cycJ..ic sediments as an ordered
10
sequence o f lithologies which i s repeated i n a predictable pattern , although the orderliness of the sequence i s never perfect . According to him , limestone-marl rhythms are 11short cycles n which probably carry a certain time information (type cycles ) . This is the case , if the al ternating sequence is due to a mechanism with a statistically .con stant time period . Then the rhythms may be termed periodites (i:IHSELE, this volume) . Many l imestone-dolomite cycles as well as some evaporite cycles belong to this group . Limstone-marl type 1 cycles have to be distinguished from other car bonate-shale or marl sequences (after SCHWARZACHER "type 2" sequences) which reflect aperiodic events ( carbonate tempestites and .turbidite s , see EINSELE, this vol . ; AIGNER , this vol . ) . There are cases, where this distinction i s more diffic�lt than anticipated ( see MAIER, this vol . ) , especially when the carbonate phase - also i s fine-grained ( e . g . mud turbidites) and primary sedimentary structures have been overprin ted by strong diagenetic effects ( e . g . nodular limestones) . Of course there may be cases where both types of alternating sequences exist in the same stratigraphic section, but field examples have been hardly reported ( e . g . DEAN et al . , 1 9 7 7 ) . Both types of alternating sequences have beds which are laterally uniform in thickness and can be traced over long distances . In the Kimmeridgian of Southern Germany ZIEGLER ( 1 9 5 8 ) was able to establish a bed-by-bed stratigraphy over a distance on the order of 200 km wh·i ch , according to GWINNER ( 1 9 7 6 ) , however , is not_ clear everywhere . Another good example of bed correlation has been reported from Berriasian marl-limestone sequenCes of the southwestern Alps (BEAUDQIN et al . , 1 9 74 ) . Both examples probably represent type cycles with time information . Further examples are reported by DUFF et a l . ( 1 9 6 7 ) , L011BARD ( 1 9 72 ) , SCHWARZACHER ( 1 9 7 5 ) , FISCHER ( 1 9 8 1 ) , and KAUFFMAN ( this vol . ) . A further means to distinguish between type 1 and 2 sequences may be the study of bed thicknes s .(BEAUDOIN et al . , 1 9 74 ; SCHWARZACHER, 1 9 7 5 ) . True cyclic sequences of type 1 should exhibit equal or subequal beds , whereas event deposits show a large variety of bed thicknesse s . This criterion i s , however , no� always valid, because already a_ slight shift in the composition of limestone-marl sequences can considerably alter the ratio of bed thicknesses of the l imestone and marl layers ( see below) . For that reason ,. type 1 cycles also contain certain marker horizons like the type 2 successions .
11
3 . Limits for the Recognition o f Limestone-Marl Rhythms A sedimentary sequence consisting of the two principal phases "clay" and fine grained carbonate may vary in its composition without showing any distinct lithologic change or bedding in field exposures and dril ling core s . In ancient rocks , an observable transition from marlstones to limestones us �ally takes place when the carbonate content surpasses a certain minimum value Beyond this limit the material becomes hard rock arid more or less resistent to weathering. H�hce a rhythmic se que �ce can be easily recogn ized in the field only , if the alternating beds have a carbonate content fluctuating around this boundary ( Fi g . 1 ) . . osZ illations in the carbonate content below or above this limit do not significantly change the f ield aspect of the material as marlstone or limestone, respectively . However, as SCHNEIDER ( 1 9 6 4 ) has demonstrated for Early Cretaceous marine deposits of NW-Germany , alternat·i ons of claystones poor and somewhat richer in carbonate (average 8 % ) can be recognized in the field as rhythmic succes sions . As SEIBOLD ( 1 9 5 2 , 1 9 6 2 ) , HEILER { 1 9 5 7 ) , and OVERBECK ( 1 980) have pointed o u t , i n Jurassic marl-limestone sequences of Southern .Germany, the minimum carbonate content for l imestone beds or nodules of liDestone in marl (often des cribed as shale) is approximately 70% or even higher. In the Upper Cretaceous of �orthern Germany, at least 75-85% Caco are necessary to 3 fc/rm a hard limestone bed (ABU-MAARUF , 1 9 7 5 ) . Certainly this value varies between different occurrences of limestone-marl sequences (Fig . 2 ) , because it depends on several factors , the nature of the car bonate as well as the grain size distribution and mineralogical compo sition of the "cla)r" -phase , but for the models discussed below it is taken as a constant value. Consequently, the mean a·c cumulation rate of detrital silicates in the depositional area must be considerably lower than the biogenic carbonate accumulation (possibly minus dissolution) in order to get a limestone-marl sequence . Hence, this type of rhythm is strongly related to the production rate resp. sedimentati�n rate of fine-grained carbonate in the pelagic to hemipelagic environmen t . According to MILLIMAN ( 1 9 7 4 ) , i n the Holocene this rate (related to solid limestone) is on the order of 0 . 3 to 0 . 6 cm/1 000 y . on shelves and slope s , 0 . 4 to 0 . 7 5 cm/ 1 000 y . in 2000 to 3000 rn water depth of the large oceans ( in greater depth less ! ) , and 0 . 7 to 1 . 9 cm/1 000 y . in enclosed marine basins . •
.
If the pelagic carbonate production is diluted by a clay supply too high for the mean composition necessary for the transition from marl to limestone , the total sedimentation rate becomes higher than the values found for typical limestone-marl successions . This actually is
'
i ,f
12 TIME
Fig. 1 Sequence with an oszillating, but slowly increasing carbo nate-clay ratio (The ampli tude of the clay oszillation is 1 : 3 and remains constant in the course of time) Only oszillations crossing the transition line between lime stones and marls (here assumed to correspond to 70% carbonate, 30% clay) generate an alternating marl limestone sequence which can be easily recognized in the field •
MARL-LIMESTONE
the case for the example of Early Cretaceous marly claystones mentioned above , where over long periods the sedimentation rate was 5 to 9 em/ 1000 y . { SCHNEIDER, 1 9 6 4 ) . On the other hand, if carbonate anQ clay supply, although in the correct ratio, drop below a certain limit, the al ·te �nating beds run the risk to be destroyed and completely mixed by burrowing organisms . It is - difficult to fix a certain minimum bed thickness necessary for the _ preservation of primary bedding, but accor ding to field observa ·tions and special studies by SARNTHE"IN ( 19 7 2 ) , -
�
Fig. 2 . carbonate content in different limestone-marl sequences of the Jurassic in SW-Germany ( after HEILER, 195 7 ) . Marl layers are indicated by oblique hatchures , but the boundary between marls "and l imestones genera_lly is. transi tiona! and depends to some extend on the state of weathering. Inspite of this uncertainly it can be clarly seen that the "field aspect as limestones" starts at different car bonate content s . Maximum carbonate contents are found in the center of the l imestones . 'l'he exposures of Essingen and Al tenstadt show the same sequence 26 km apart
13
em
m
ESSINGEN
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OXFORDIAN IMALM fll LIMESTONES
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14
BERGER and HEATH ( 1 9 6 8 ) , BERGER et al . ( 1 9 7 7 ) , and A . WETZEL (pers . communication) this limiting thickness of one of the alternating beds may be on the order of 5 to 1 0 em. In Quaternary sediments from the deep slope off west- A·f rica ( 4 720 m water depth) , WETZEL { 1 9 8 1 ) has worked out the burrow-induced mixing of a 30 em thick layer rich in carbonate alternating with clay beds . Fig . 3 shows that a zone of several em at bottom and top of the bed contains strongly varying carbonate contents of burrow-fillings as well as a transition zone of medium carbonate content for the total sediment . In thi·s case , an alternating succession of 5 em thick beds certainly would have been completely obliterated. In oxygen-depleted environmentS , mixirig of alternating layers i s , of course, strongly reduced or absen·t . From this brief discussion it is evident that limestone-marl rhythms are restricted to environments with a comparatively narrow range of sedi ment accumulation. This rate- often varies between 0 . 5 and 2 cm/1 000 y . A further problem i s the distinction of true alternations and secon dary bedding planes . The number of observable beds in the same sequence may vary in relation to the degree of weathering ( SCHWARZACHER , 1 9 7 5 ) , if this distinction is not possible by using other criteria. This is a seiious problem for the determination of the time period of the limestone-marl cycles { Chapter 5 . 2 . ) . An oszillating supply of carbonate or clay in the right ratio which is discussed below as a principal cause of limestone-marl couplets , be completely masked by the substantial contribution o f a third pha se, e . g . silt or sand to the total sedimen t . For that and other reasons , limestone-marl-rhythms cannot develop in shallow water above wave base .
4 . Direct Mechanisms Producing Limestone-Marl Rhythms The different mechanisms leading to limestone-marls rhythms have been described, in a more or less complete way, by several authors {recent ly e . g . by DEAN et al . , 1 9 7 8 ; FISCHER, 1 9 8 1 ; VOLAT et al . , ·1 980) . The existing views can be summarized as follows: There are (1 ) primary or synsedimentary processes generating limestone-marl couplets and { 2 ) diagenetic processes producing or at least enhancing the genera tion of alternating beds . In the fol lowing discussion we assume that the basic requirements for the formation and recognition of a rhythmic sequence are complied with.
15
4.
L
Primary' Rhythms
4 . 1 . 1 . Periodic Fl uctuations of Carbonate Production (Productivity Cycles ) Primarily, marls and limestones are mixtures of a detrital phase of different silicate ·minerals , in general within the clay and silt size fraction ( usually described as "clay" ) and a biogenic phase consisting of the skeletons of plantonic and benthic organisms ,;<. (carbonates) . Two phase bedding cycles (FISCHER, 198 1 ) occur as soon as the contribution o:t at least one of these phases starts to fluctuate in the course of time ,(Fig . 4 ) . A pure productivity cycle is characterized by fluctua ting carbonate supply during steady contribution of clay (Fig. 4A) . This graph presents , however , only the alternating composition of the growing sediment colu_m n, but not the varying thickness of alternating beds . The bed sequence resulting from this mechanism is shown quantita tively for a still more simplified case in Fig . S , where a homogeneous marly sediment column ( 60% carbonate and 40% '1clay11 ) is transformed into a sequence of alternating marl-limestone beds . In this example , the periodically increased carbonate production must be higher by a factor of at least 2 in compqrisOn with the original production rate ( production fac tor 2 ) in order to get a hard and · resistent l imestone lay � .r contairiing_ at least 70% carbonate as mentioried above . With increasing production factor the periodically generated limestone beds become thicker (Fig . S) . For a production factor of. 5 , in our example the ratio of the bed thicknesses of limestones and marls L/M is 3 . 4 , for a factor of 1 0 , it is 6 . 4 . Carbonate-
t
'"''�--w;th Zoophy�os Plan
I
iles
carbonate: toyer
Planolites. with •
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high carbonate content low carbonate content
Fig . 3 I1ixing by bUrrowing at the upper and lower boundary of a layer riCh in carbonate alternating with clay bed s . Marine Quaternary a t the slope off west Africa (after NETZEL , 1 9 8 1 )
16
From this s imple test it can be seen that an already high original or 11background carbonate production" ( in our example contributing 60% of the total sediment) must be multiplied periodically by a factor of 1 0 to 20 to produce the common type of limestone-marl sequences with limestone beds alternating with thin marl layers . For that reason, e . g . SEIBOLD ( 1 952 , 1953 ) assumed a periodic inorganic precipitation of car bonate in order to explain Oxfordian limestone-marl sequences in southern Germany . Today we know ( HEMLEBEN, this volume) that these he mipelagic limestones as those of many other occurrences are composed mainly or entirely of biogenic materia+. The study of Quaternary marine sediments ( e . g . PRELL and HAYS, 1 9 7 6 ; RUDDI.MAN and t4ciNTYRE , 1976; PISIAS, 1 9 7 6 ) has reveaJ,ed that periodic changes in total biogenic carbonate production and in the faunal and mineralogical composition of the carbonate are indeed a wide-spread phenomenon for reasons some of which are discussed below. Whe·ther these fluctuations i.n carbonate production alone. are suff icient to produce the common type of rhythmic limestone-marl sequences as men tioned above i s , however , doubtful . It is difficult to assume that genic carbonate production is periodically multiplied by a factor of 1 0 or even 5 , if special envi�onmental conditions as upwelling are ruled out. For that reason cyclic productivity can be regarded only occasionally as the principal factor controlling limestone-marl rhythms . 4 . 1 . 2 . Periodic Increase of Detrital Sili cate Supply ( Dilution Cycles ) The pure type of this rhythm is generated when a steady production of biogenic carbonate is more or less diluted by fluctuating input of the detrital phase ('Fig . 4C) . If the cycles are symmetric , i . e . the half periods have the same length (duration) , the layers relat ively rich in "clay11 will become thicker than the alternating layers rich in carbo nate . The ratio L/M of bed thicknesses of l imestones and marls noW de pends on the mean composition of the sequence . If the mean carbonate/ clay ratio is just about at the transition from marls to l.imestones ( say 70% carbonate and 30% clay ) , the L/N-ratio will become < 1 . A higher mean carbonate content, however , enables the development of thicker l imestone beds altern·ating with thinner marl layers (Fig . 4C) A simplified quantitative study of these relationships is shown by Fig . 6 . Here a homogeneous sequence primarily consisting of 8 5 % carbo nate and 1 5 % clay ( s ame original composition as assumed by DEAN et al . , 1 97 8 ) is affected by periodic steplike increases of the clay input. •
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In this case doubling of the clay inPut (dilution factor 2 ) leads to a marly limestone with 2 6 % clay, and marl layers develop only when the dilution f_actor is greater than approximately 3. From this model and Fig . 4C one can clearly see that periodic high clay input into a car bonate-dominated environment probably is the most common mechanism to explain the occurrence of thin limestone layers alternating with thick
Ji�:L:._
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. F�g . 4 . Model ""t:o ·-exp'l·a:t:n....,..,..the combined result o f steady ( 8 1 , C 1 , DO, D 1 ) or fluctuating silicate supply ( S , nclay11 ) , carbonate sup ply ( C ) , and dissolution ( D ) in the course of time . The resulting total rate of deposition as well as the varying composition of the sediment is shown by (A) through '(E): for vari ations of S and c , but without dissolution, ( D ) and {E ) : for fluctuat ing C and s , either in pha se ( D ) or 180° out of pha se ( E ) . ( F ) and ( G ) : With fluctuating dissolution in combination with ( F ) s teady S and C , or ( G ) with inpha se fluctuating s and 18oo out of phase fluctuating c . Much more combinations are feasible, especially if the amplitudes 6f the fluctu ations (here causing vari ations of rates by a factor of 3 ) and the periods of the three different pro cesses are varied or the cycles become asymmetri c . The percentage o f marl and limestone in the re sulting sediments s trongly depends on the ratio of S 1/ C 1 (here 3/10 resp . 2 3 % clay and 7 7 % carbonate )
.
18 (->
Fig. 5. Simplified model for the transition of a homogeneous marly sediment column (A) i-nto a lime stone-marl succession ( C ) by peri odically increased carbonate pro duction ( B , productivity cycles ) . Note the relation between the thickness of limestone layers ( L ) and the carbonate production fac tor. ML, marly limestone
RfSUt T OF (B)
A
B
VOLUME OF PORE�FREE SEDIMENT TIME
c
RESULT OF (B)
A
8
VOLUME Of PORE-FREE SEDIMENT TIME
Fig. 6 . Simplified model for the transition of a homogeneous clay bearing carbonate Sediment (A) in to a limestone-marl success ion ( C ) b y periodically increased clay in put (B , dilution cycles ) . Note the growing thickness of the marl lay ers with increasing dilution fac tor, whereas the thickness of the limestones remains constan t . ML = marly limestone
19
marl beds. · In the example of Fig. 4 , a dilution factor of 20 enables a L/M-ratio of 0 . 2 6 . In previous work on alternating limestone-marl beds dilution as a controlling factor has not been stressed very much. However, for Pleistocene sediments of the deep sea1 it is generally assumed that during glaciations more fine and coarse grained material was delivered into the oceans than during interglacial s . Already a regression of sea by eustatic sea level fluctuations can induce this mechanisfu'.. Further more , in basins not far from land areas with a climate fluctuating between desert and more humid condi�ion s , the input of air-borne and river-borne fine-grained detrital material is fluctuating ( e . g . SARNTHEIN, 1 9 78 ) . In the eastern Ivlediterranean Sea , Quaternary periods of stagnation are characterized by several horizons of black shale which also reflect a reduction of detrital input from the neighboring land areas ( DOMINIK and STOFFERS , 1 9 7 9 ) . Hence in certain environments , par example on marine shelves or in shallow adjacent seas , where cyclic productivity and dissolution are less probable, strong variations in "clay11 input may be the most plausible cause for an alternating bed sequence . 4 . 1 . 3 . Periodic Dissolution of Carbonate - ( Dissolution Cycles ) Dissolution of carbonate can take place within the water column or in the uppermost sediment layer·s at the sea ·floor ( FLUGEL and PENNINGER, 1 9 6 6 ; EDER, this Vol . ) . In Fig . 4F a steady supply of clay and carbonate is superimposed by fluctuating dissolution of carbonate . With the re latively low original carbonate/clay .. ratio ( 7 7 % carbonate and 2 3 % cl ay ) of this example, the result o f fluctuating dissolution is a sequence of alternating thick marl and thin l imestone beds. On the basis of the ratio L/11 of bed thicknesses , this sequence can hardly be distinguished from the sequence of Fig . 4 C generated by cyclic dilution . Starting with a higher carbonate/clay ratio (the same as DEAN et al . , 1 9 77; have studied) Fig . ? demonstrates again in a simplified way by applying steplike changes , the effect of increasing dissolution on a primarily homogeneous clay-bearing carbonate mud (pore volume neglected) •
As already DEAN et a l . ( 1 9 7 7 ) have pOinted out, in this case a disso lution of 50% carbonate (dissolution factor 2 ) is not sufficient to produce a marl layer, but the thickness of thiS bed is already distinct ly reduced . InCreasing dissolution leads to thinning marl layers bet ween limestone beds of more or less constant thickness . If periodically 95% of the original carbonate is dis solved (dissolution factor 20) the
20 A VOLUME Of PORE- FREE SEDIMENT TIME 100%
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B
RESULT OF PERIODIC DISSOLUTION
PERIODIC DISSOLUTION OF ORIGINAL CARBONATE 100% 0 50
100% PORE.-fREE SfOIMENT
' '
'
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15
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5
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Fig . 7 . Transition o f a homoge .neous carbonate sequence ( A , taining 85% CaC0 3 and 1 5 % clay) into a limestone-marl succession ( C ) by periodic dissolution of primary carbonate (B) , dissolu tion cycles ) . Note the decreas ing thickness of individual marl beds with increasing dis solution
6
37
5
l
-=-�
0
l
L/M-ratio becomes 5 . 2 . A value in this order of magnitude appears to be characteristic for many limestone-marl sequences . Recently , periodic dissolution of carbonate .has been quoted by several authors as the main factor controlling fluctuating carbonate contents . In their review of Pleistocene sediments in the deep sea, VOLAT et al. ( 1 980) were able to show that during glacial periods in the Pacific much less calcareous skeletons were dissolved than during the warm periods (Fig . 8 ) . Therefore, the total carbonate content of the sediment is higher by 5 to 1 0 % than that of the alternating muds . �his relatively small fluctuation of carbonate content between 80 and 90% is, however , not suff icient to produce a marl-limestone sequence , if it is not enhanced later by diagenesi s . Stronger fluctuations in the carbonate content of Quaterpary marine sediments have been found else where in the Equatorial Pacific (BERGER, 1 9 7 9a) or in the wester� equatorial Atlantic (BEet al . , 1 9 7 6 ) . In contrast to the Pacific , here sections rich in carbonate ( 60 to 80%) correlate with warm whereas the glacial sediments contain only 4 5- 5 5 % biogenic carbonate (Fig . 9 ) . The main cause for this alternating carbonate content also appears to be dissolution rather than variations in species or of the belts of maximum production toward the equator ( see also et al . , 1 980) . The non-contemporaneous dissolution peaks in the appear to be associated with the changing circulation of Antarctic bottom waters. The same mechanism is quoted to explain several disso-
21
CORE DESCRIP TION
DEPTH !N em
S te o/POB
SEDIMEN TAT ION RATE
SOLUTION INDEX
TOTAL CARBONATE
30
COLOR
TEXTURE COLOR
40%
CJ1�2-3 IIllilll' ITIIIIJs 1::·:-:Js c::J7
Fig . 8 . Carbonate fluctuations in a Pleistocene deep-sea core from the Pacific by periodic climatically-controlled carbonate dissolution (redrawn and somewhat simplified after VOLAT et a l . , 1 9 80) . High values of solution index signify increasing dissoluti<;m intens.ity . Stippled areas correspond with glacial periods according to oxygen isotope studies
22
v25-59
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>0 20 30
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" 60 • • w > 70 • 0
• • w • w •
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• STADE 0 INTERSTADE Fig. 9 . Carbonate fluctuations in Pleistocene deep-sea se � iments of tbe western equatorial Atlantic (core V 25-5 9 , after BE et al . , 1 9 7 6 ) . carbonate peaks correlate with warm interstadial periods derived by MORNER { 1 9 7 2 ) for the region of the eastern Great Lakes, North America lution cycles found by KELLER ( 1 9 80) in Miocene deep sea sediments of (\ the North and Southeastern Pacific. A further well documented example of dissolution cycles has been re ported from the Eocene of the Sierra Leone Rise in the eastern Atlantid< ( DEAN et al . , 1 9 7 8 , 1 9 80) . Cores from the Deep Sea Drill ing Project, Site 3 6 6 , 2800 m water depth , have revealed an alternating sequence of clay or marl and chalk with carbonate contents f luctuating between lesS:; than 1 0 and 80 to 8 5 % (Fig . 1 0 ) . From careful examination of the bio genous components preserved it is evident that sections poor in car bonate reflect periods of strong and sel ected dissolution of nanno-
23 % CaC03 dl�$Oived
%clay added·dilution
I
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DISSOLUTION ONLY l
DILUTION ONLY A
Fig . 1 0 . carbonate fluctuations (A) in Eocene sediments from DSDP Site 3 6 6 , Sierra 'Leone Rise, 2800 m water depth ( DEAN et al . , 1 9 78 and 1 980) . Ass�ming an initial pelagic sediment with 8 5 % caco 3 and 1 5 % clay, the observed composition ( A ) results either from carbonate dissolution ( B ) or from dilution by clay ( C )
fossils and foraminifera rather than dilution by clay . Periodic disso lution here is explained by a fluctuating depth of the carbonate compen sation · depth ( CCD) and the lysocline { see e . g . BERGER, 1 9 7 9 ) . At pre sent the Sierra Leone Rise lies well above the CCD , but during Eocene time, the CCD had reached just about the depth where the cyclic sedi ments were deposited ( DEAN et al . , 1 9 80) . An example demonstrating Quaternary dis sOlution cycles by a fluctuating lysocline is reported by BERGER and MAYER ( 1 9 7 8 ) from the Ontong-Java Plateau. A somewhat different model using cyclic disso � ution and reprecipitation of basin floor ca-rbonate to explain shelf carbonates al ternating with shales, particularly black shales, is proposed by DEGENS and STOFFERS ( 1 9 76 ) . For a further discuss ion of bedding phenomena in black shales see 1VETZEL ( this vel . ) .
24
4 . 1 . 4 . Combinations of Fluctuating Carbonate Production , Dilution, and Dissol-ution The three synsedimentary processes described above which generate limestone-marl couplets can be combined in several way s . Some of these combinations which have a certain probability in nature are presented in Fig . 4 . Oszillating carbonate production in combination with steady clay supply and cOnstant dissolution (factors S 1 + C2 + D 1 ) does not cause �uch change in comparison to the pure productivity cycle (81 + C2 + Do, Fig . 4B ) apart from a general shift to a sequence more rich in clay with thicker marl layers. A combination of oszillating carbo nate productivity ( C 2 ) and oszillating clay supply ( S 2 , both in phase) leads to a pure limestone sequence, although the total sedimentation rate is strongly fluctuating (Fig . 4D) . I f , however, dilution and pro ductivity fluctuate 1 80� out of phase ( S 2 + C3 + Do, Fig . 4E) , again a limestone-marl sequence is deposited. Finally, Fig . 4G demonstrates the result of three f luctuating processes ( S 2 + C3 + D 2 ) , where clay and dis solution are in phase , but productivity is 1 80° out of phase . In this case , pure 11clay" beds can be generated alternating with marls and limestones ( 3 bed cycle) The result would be similar , if only C3 and D2 are combined and clay supply is kept constant . •
.
In the fol lowing chapters , these combinations are not further dis cussed, because the existing case studies usually do not give enough information for an interpretation in this way. In general , one of the three factors discussed i s , beside diagenesis , held to be chiefly responsible for the rhythmicity . 4 . 1 . 5 . Other Primary Factors Causing Minor Cycles in Carbonates In or or of
,. i;
general , there are many other processes leading to alternating beds cyclic sequences ( see e . g . DUFF et al . , 1 9 6 7 ) . Dealing with pelagic hemipelagic carbonates , the number of alternatives or modifications the mechanisms described above i s , however, sma l l .
Instead o f a gradually changing lithology a t the transition from marls to l imestones and viceversa, a sharp boundary may be visible . If tur bidites and other depositional events ar_e excluded , such sharp bounda ries can be explained as omission planes or hardgrounds , or as a com bination of both phenomena. Lithification of lime mud at or slightly below the present sea floor has been reported from many localities in shal·low and deep water { summarized e . g . by r.nLLH1AN, 1 9 74 ) .
25
can be accomplished within a period on the order of 1 0 , 000 years , and it can occur periodica�ly 1 _par example on the I4id-At1antic . Ridge { RN1SAY , 1 9 74 ) . Also Plant growth {possibly algae) at the floor of flat1 widely extended submarine banks has been quoted to form rhythmic succ essions of hardgrounds , e . g . in the Upper Cretaceous of Normandy {JUIGNET and KENNEDY, 1 9 7 4 ) . If kept free of sediments for ' some time, firm and hardgrounds often are identified by post-depositional burro wing or boring organisms (see e . g . HATTIN, 1 9 7 1 ; FURSICH , 1 9 7 7 ) , which indica te that there were long periods of non-deposition. There may be sequences with regular repetitions of suCh beds possibly due to periodic sea level fluctuations . There may also be composite cycles, where two or more independent cyclic processes are superimposed (SCHWARZACHER� 1 9 7 5 ) . In Upper Jurass.ic/Berriasian l imestone-marl successions, BEAUDOIN ( 1 9 7 7 ) has found 3 different time periods for cyclic processes controlling some minor features of these succession s . Favorable conditions for such a development can be expected in semi enclosed basins similar to the Red Sea, where sea level f luctuations were assoc iated with variations of (high) salinity (1>1-ILLIMAN et al . , 1 9 6 9 ) . Studying successions which contain penecontemporaneous , recolo nized firm and hardgrounds , we run, however , the risk to get an in cor�ect number of rhythms , because part of the sedimentary record may be missing. The chances for the preservation o f a total cyclic record again improve , when in a subsiding area a tidal flat and lagoonal en vironment is established . Then carbonate cycles of the Loferite-type may develop (FISCHER, 1 9 6 4 ) .
4 . 2 . Diagenetic ProcesseS Diagenetic processes have been quoted by several authors to be the only cause ( e . g . SOJKOWSKI , 1 9 5 8 : rhythmic unmixing) or at least the main factor producing a lternating limestone bedS ( diagenetic overprint) . In the last 1 0 to 20 years an enormous number of publications on the diagenesis of carbonates , m�ny of them summarized by BATHURST ( 1 9 7 1 ) and MILLH1AN { 1 9 7 4 ) has shown the str:,ong and early influence of dia genesis on the lithif ication of carbonates . There can be no doubt that diagenesis also must play a great part in the formation of limestone marl rhythms .
26
4 . 2 . 1 . Rhythmic Unmixing In the Hettangian of England, KENT ( 1 9.3 6 ) described limestones as se condary features , and in Oxfordian limestone-marl sequences of Poland SOJKOWSKI ( 1 9 5 8 ) observed no difference in the organic and mineral composition of both bed types . The limestones , however, were characte rized by a great percentage of calcitic cement and a better preserva tion of the macro- and microfauna than the marl layers , where smaller remains or more delicate shells had disappeared. SUJXOiVSKI concluded that primarily there was no difference in carbonate content and in the nature of the carbonate between the two bed type s , but he admitted that the diagenetically produced rhythmic changes might be due to very inconspicuous, chiefly textural primary alternations in the sequence ( see also HALLAH , 1 9 6 4 ) . According to our present knowledge on the diagenesis of carbonates , a rhythmi� unmixing of a completely homogeneous sediment column appears to be very unlike l y . In this case the result of diagenesis probably would be a random distribution of limestone nodules within a marly matrix or a nodular limestone with thin and irregular lenses of marl . Recent work on nodular limestones (NOBLE and HOWELLS, 1 9 7 4 i GARRISON and KENNEDY , 1 9 7 7 ) envisages such a pure diagenetic unmixing. However, as soon as limestone nodules are restricted to certain horizons wi·thin the sediment, a primary chemical or textural deviation from the mean composition appears ·to be necessary. For that reason , a purely d iage netic origin of the marl-limestone rhythms is rejected.
4 . 2 . 2 . ·Early Diagenetic overprint The most far-reaching effect of a diagenetic overprint is that mentio ned· above . Primarily the sedimentary column is homogeneous in respect to the carbonate/clay ratio, but inhomogeneous concerning the nature of the carbonate and/or the texture of the total sedimen t . A simple model for the last case starting with a carbonate/clay ratio of 4 (clay content 20% ) is represented in Fig. 1 1 . This example is based on three assumptions : ( 1 ) Gravitational compaction has reduced the origi nal pore volume to SO% , before diagene sis starts ( see e . g . SCHLANGER and DOUGLAS , 1 9 74 ) , ( 2 ) no further gravitational compaction takes place, i . e . the following reduction of porosity versus zero is solely due to carbonate dissolution and reprecipitation in the pore space, ( 3 ) carbonate is dissolved only from the subsequent marl layers ,
27
B
A
0
( '%VOLUME so
100
Fig . 1 1 . !1odel of lime stone-marl couplets purely produced by dia genes i s . A , primary fluc tuations of texture , pore size , etc . , but B , without alternation of . pore volume or composition ( carbonate/ clay ratio 4 : 1 ) . Early state of compaction (pore volume 50% ) i arrows show the succeeding migration of carbonate from marl (M) to limestone (L) layers . C , final composition and D1 field aspect of lime stone-marl couplets (pore volume 0) Limstones ( L ) consist o f 4 0 % primary carbonate cement and 1 0 % clay, but are not further compacted since B •
DEPTH
whereas carbonate precipitation is restricted to the texturally diffe rent subsequent l imestone beds . The result of this admittedly very simplified model is a limestone.:..rnar l sequence with a bed thickness ratio 2.5, whereas the original bed thickness ratio (after mechanical L/M compaction) was approximately 0.6. The carbonate content of the lime stone member has been diagenetically increased to 90% , but 50% of the total sediment volume is taken by carbonate migrated from the subse quent marls and �e�recipitated as cement. The marls have lost 75% of their original carbonate to the limestones , but due to "chemica-l com paction" they still contain 50% carbonate in the form of the original , mainly biogenic components . In reality limestone-marl diagenesis is much more complicated, but in this paper only a few additional points can be mentioned .( for further details see e . g . RICKEN and HEMLEBEN, this vol . , or WALTHER , this vol . ) : =
( 1 ) Already during mechanical compaction some dissolution and re precipitation of carbonate may take place . Then the dissolved carbo nate can be transported by the upstreaming pore-water into higher levels ( e . g . EINSELE , 1 97 7 ) and, und�r slightly changed physico-chemi cal condition s , start to cement such layers which primarily are rich in carbonate (containing suff icient nuclei of the stable carbonate phase) and/or have larger or different pores as well as a better per meability than the material below and above . Once cementation has star ted, compaction ceases in this horizon , but is proceeding in the neigh-
28 borhood . As a resu'l t, the carbonate content may reach a maximum in a certain level and decrease towards the base and the top of the growing limestone bed (Fig . 2 ) . Therefore , from the carbonate content alone it cannot be decided, whether the original carbonate phase was fluctuating or not. ( 2 ) As long as advective transport is involved, the maximum carbo nate content is not necessarily found in the center of a l imestone layer. However , if carbonate is migrating solely by molecular diffu sion, its rate and distance of transport becomes equal in an upward and downward direction (Fig . 1 1 ) . This certainly is the most important process for limestone diagenesis , although it takes a very long time and can transport dis solved species only over relatively short distan ces (em to m} concerning. the low concentration gradients involved ( e . g . BERNER, 1 9 7 1 ; FREEZE and CHERRY , 1 9 7 9 ) . The best examples for this process are limestone concretions within marls or shales (EINSELE and MOSEBACH , 1 9 5 5 ; SEIBOLD , 1 9 6 2 ; RAISWELL , 1 9 7 1 ; HUDSON, 1 9 78}. : (3) Tiny skeletons of aragonite and Mg-calcite usually are dissolved first, whether they occur in subsequent marls or limestones . Therefore ,: these constituents are generally miss ing in either bed type. At greater ·.·;: burial depth , also calcitic · foraminifera and nannofossils start being corroded or dissolved, especially if pressure solution becomes impor tant . Simultaneously calcitic cement is precipitat e d in the free pore space. In their study on pelagic carbonates from the Shatsky Rise, Northwest Pacific , MATTER et al . ( 1 9 7 5 ) have estimated, that in 1 00 m burial depth the percentage of cement in relation to the total carbonate con tent is already 6 4 % , and in 240 m 7 6 % . In a sequence with primarily alternating beds, the carbonate cement will be precipitated chiefly in layers, the grain framework of which is already stabilized by cementation against further gravitational compaction. In contrast, layers still affected by gravitational compaction will loose most of their dissolved · carbonate , because they provide only very small pores for cementation. Many authors indeed hold the view that carbonates be come lithified . at much Shallower depth than clayey sediments . This is a further reason , why a primary diff erence between the ·two alternating bed types appears to be mandatory. The better preservation of calca reous fossils in the limestones in comparison to the marls , however , cannot b e used a s evidence, neither for purely primary o r purely secon� dary origin of the limestone-marl couplets .
29
( 4 ) Magnesium released by the dissolution of Mg-calcite may be used form small quantities of authigenic dolomite (e . g . WALTHER, this to vol . ) . The occurrence of alternating l imestone-dolomite sequences, par example in the Upper Muschelkalk (Triassic) of Southern Germany, how
ever, cannot be explained by this mechanism alone. 4 . 2 . 3 . Pressure Solution and Stylolites
Pressure solution and the formation of stylolites can start durlng an
early burial stage and comes to an end when the pore space of the
surrounding sediment is completely filled by recrystallized carbonate { P�RK and SCHOT, 1 968) . In order to allow the migration of dissolved carbonate by diffusion, the pores must be water-filled. This inter stittal water is more or less stagnant, for as soon as meteoric water starts to circulate, the dissolved carbonate, or at least part of it,
is transported far away . The result is beginning karstification, which
is not dealt with in this pape r .
The· effects of pressure solution are evident in many limestone-marl sequences, par example in the Oxfordian sequences of Southern Germany
quoted above (Fig. 1 2 ) . The transition from a limestone to a marl bed as well as the marl beds themselves are often marked by a network of interconnecting irregular, but essentially bed-parallel solution seams ahd stylolites {WEILER, 1 957 ; RICKEN and HEMLEBEN, this vol ; GARRISON and KENNEDY, 1 97 7 ) .
By this mechanism, primary sedimentary structures are commonly distur
bed and difficult to recognize . The occurrence of these stylolites con firms the hypothesis that a great part of the limestone cement is deri ved from the accompanying marl layers .
During the main phase of stylolitization, not only the limestone beds,
but also the marls may have become firm and .gained a stabil ized grain framework . Therefore, stylolitization can be regarded chiefly as the last phase of the diagenetic overprint.
Studies on the carbon isotopes of limestone cement ( e . g . HUDSON, 1 975, 1 9 7 7 ) confirm the importance of cementation and pressure solution du
ring burial diagenesis. The source- of the cement is' often more or less identical with the marine carbonate sediment, because an influence of
isotopically light near-surface bicarbonate is subordinate or missing { see also CAMPOS and HALLAM, 1 979) . Therefore HUDSON assumed that chiefly shale's are the " donor beds11 for the cementation of the adjacent
limestones .
30
Further investigation s , e . g . the study of trace elements such as Sr, and Mn ( e . g . VEIZER, 1 97 7 ) , the fossil-matrix isotopic contrasts ( e.g. HUDSON , 1 97 7 ) as well as the influence of bacterial activity on the carbon isotope composition ( I RWIN et al., 1 97 7 ) have shown the 1 i ty to distinguish between different stages of diagenesis and tion . Essentially , all this work has underlined the great importance of diagenesis for the formation of limestones .
5 . Diagnosis and Interpretation of Limestone-Marl-Rhythms
5 . 1 . Criteria for Primary Origin Most of the criteria to be mentioned here are s imple and well-known to the experienced geologist. Much can be taken £'rom th.e preceding dis cussions and earlier publications ( e . g . HALLAM , 1 964 ; DEAN et al., 1 978) . Nevertheles s , a brief summary may be useful :
( 1 ) Composition and texture of the alternating beds . If the carbonate phase is· excluded , a primary alternating sequence may show some differences in the detrital sil icate phase , bo.th in texture and
composition. However, such differences must be used with caution, be
cause of the possibility of crystal growth and the formation of authi genic silicate minerals.
( 2 ) Fauna and bioturbation. The use of carbonate skeletons of rent faunal groups as a means to identify primarily
( SUJKOWSKI , 1 958) appears to be equivocal. As mentioned above, also
l imestones may have lost part of .their orig.inal fauna by dissolution, although the loss of biogenic carboD;ate from the marls is much greater.· On thE: other hand, extraction of fossils from. marls by standard mE't�hods is easy and may, therefore , pretend a very goad fossil preservation in these beds . Unfortunately , in this environment siliceous fossils are
very sensitive to diagenetic proces ses , too, and hence more or less useless for our purpose . More promising are all those relicts which are resistent to diagenetic dissolution , e . g. chitinous tests or parti culate organic matter . A better means for the distinction of primarily alternating beds is bioturbation , if the rhythmic sequence is not com
pletely destroyed by intensive burrowing (Chapt.2 ) . Burrows in one type
31
of the alternating beds may be filled by sediment from the other type (HALLAM , 1 9 64 ) . Sometimes a gradual change in the nature and intensity
of burrowing can be obserVed (Fig. 1 2) . However, it has to be taken into account that the marls have undergone stronger gravitationa l compaction · and chemical . dis �olution than the l imestones .
( 3 ) Inorganic - sedimentary structures should be also used as criteria
with caution. Small-scale lenticular or flaser bedding of marls may be entirely due to pressure solution (WEILER, 1 957 ; RICKEN and HEMLEBEN , this vol.) . There are, however, cases where a primary dif ference bet
ween the sedimenta.ry structures of both bed types is evident (Fig .1 2) . Thi:·s is probably one of the best evidences we have for the primary origin of the rythms . ( 4 ) The interpretation of oxygen and carbon isotope f l uctuations raises different problems mainly because of the strong influence of
diagenesis and interactions between the solid phase and interstitial Water {see e . g . HUDSON , 1 977; SCHOLLE and ARTHUR, 1 980) . These diffi culties may be particularly serious , if relatively smallscale varia-
/.
tions are considered. Recently , WEISSERT et al ( 1 979) and de BOER ( 1 980 and this vol . ) , however , have shown 0 1 3 c fluctuations between alternating limestones and black shales of Cretaceous age in the Alps.
WEISSERT et al. used bulk samples rich in calcareous nannofossils for
tfieir isotope study , whereas de BOER pointed out that the carbOn iso tope composition of organic matter is more stable and thus a better
indicator of primary environmental conditions . In both example s , the present limestope beds are isotopically lighter than the shale s , but
de BOER argues that in this case the original situation was opposite , because subsequent bacterial fermentation of organic ma'tter has altered 13 the 6 c value of the shale s . Obviously more work is necessary in this field to get a reliable tool for the identif ication of the primary
environment and its variations.
5 . 2 . Correlation and Timing of Limestone-Marl Rhythms 5.2 . 1 . General Constraints The timing of ancient l imestone-marl rhythms , i . e. the determination of the period of one cycle , is dealt with in more detail by FISCHER . ( 1 981 ) and SCHWARZ ACHER and FISCHER {this vol.) . Therefore , here only a few points have to be mentioned.
32
D
1cm
1----1
c
1cm
F
33
(1) The best method to aCcomplish this aim is to date bottom and top of a very thick sequence which contains a great number of cycle s . Thus the error for the timing of one cycle becomes re·l atively sma l l .
Neverthele ss ,,-the timing o f such sequences b y biostratigraphic methods remains rather uncertain. If , par example , the period of one cycle is 50,000 y. and the sequence between two datable levels contains 1 00
cycles , the difference in a �e of the two reference levels is only 5 m . y . Such a short time lriterval usually cannot be me.asured prec.is�ly by biostratigraphic methods . Hence the period of the cycles may easily vary between 40, 000 and 60 , 000 y .
Better results can be achieved with radiometric methods using volcanic
ash layers (KAUFF!1AN , 1977) or glauconite (ERNST et al . , 1979) . The
best timing of cyclic sediments has been accomplished for Quaternary marine sediments with the radiocarbon and thorium metho d , stable iso tope studies and magnetostratigraphy ( see e .. g . CLINE and HAYS , 197 6 ; SHACKLETON and OPDYKE , 197 3 ; IMBRIE and H1BRI E , 1979) .
( 2 ) The second problem is the determination of the number of cycles between two reference levels . As was pointed out earlier (chapters 3 and 4 . 2 ) , the recognition of primary alternations depends on several factors , the most important being the carbonate/clay ratio , biogenic
mixing in combination with the sedimentation rate , diagenetic overprint ahd weathering. It should be added that the common processes of con densation , offiission, and winnowing at the sea floor may, if they are
overlooked in a thick succession of similarly-looking beds , falsify the number of cyc les . Most of these factors tend to reduce the real number of cycles .
( 3 ) The diffi·c ulties mentioned above are obvious when exposures of the same succession have to be correlated over la·rge-distances . Thanks
�
Fig . 12 . A and B , pressure solution at the transition from limestone member (white) to marl (dark) Numbers in A indicate Caco 3 content (after WEILER, 195 7 ) . c and D , completely bioturpated limestone and marl beds with gradual transition from one bed type to the other (drill ing cores from the Lower Oxfordian , Heil3jura alpha, Hausen) . E, thick bioturbated limestone bed� alternating with thin evenly lami nated black marl layers . This relatively rare occurrence indicates the primary origin of the limestone-marl couplets . •
F , dark marl layer with sharp boundary to the underlying limestone and indications Of small-scale lenticular bedding due to pressure solu tion ( ? ) . (E and F represent drilling cores from Oxfordian l imestone marl succes sion , Weil3jura beta, Strassberg , Southern Germany)
34
to distinct master horizons (Chapt . 3 ) , usually a very good correlation
of certain sections of the total succession is possible { e . g . 1 96 6 ) . A bed-by-bed correlation i s , however, sometimes difficult .
Fig . 2 demonstrates �n example of well studied exposures in Oxfordian limestones of Southern Germany mentioned above (WEILER, 1 95 7 ) .
exposures lie 2 6 km apart and have been selected, because they contain
excellent mas�r beds with a typical fauna. The Essingen-section was , howeve r , 50 years exposed to weathering instead of only 1 0 years in the case of Altenstadt. This may be one of the reasons why on the
of the weathering profile and the very carefully determined carbonate content, the number of visible primary cycles appears t� be somewhat
different at both localitie s . Even for one exposure this number quite clear, because the thickness of. both bed types as well as that of the couplets varies very much . Such unc·e rtainties are, of course ,
multiplied, i f instead o f a 6 m sequence a much thicker �imestone marl succession is investigated. VJell documented examples for these difficulties (points 2 and 3 of this discussion) also have been repor ted from the Upper Cretaceous of NE Germany (SEIBERTZ , 1 979) .
5 . 2 . 2 . Discussion of Some Results For all these reasons , an exact timing of the limestone-marl periods
is difficult and often problematic . F ISCHER ( 1 981 ) prepared a list of well exposed and carefully investigated Cretaceous and Paleocene lime stone-marl successions in Europe and the United States and found
periods in the range of 1 2 , 000 to 1 00, 000 y . A great part of this
variation is caused by uncertainties in the absolute age of the stage boundaries , particularly for sequences Cenomanian to Coniacian in age.
Nevertheless, most values can be attached to two groups , one with values around 20, 000 y , another , b ut less distinct , with periods around 40, 000 y .
Some other values for the time· period of limestone-marl rhythms al).d
argillaceous rhythms in ancient rocks are listed in Table 1 . Values
for Quaternary carbonate cycles as observed in deep-sea sediments can
be taken from Table 2 .
Although this data collection is far from being comple.t e, it allows to draw some conclusions : ( 1 ) The scatter of data for the time period of limestone-marl
rhythms as well as redox cycles is considerable and covers the range me ll tioned above (FISCHER, 1 98 1 ) . This is also true of the Quaternary
marine cycles, which vary bet_ween 1 5 , 000 and 1 00 , 000 years, if one
example of a major cycle is neglected.
{ 2 ) The periods of the older rhythms have the same order of magni tude as the Quaternary cycle s . This is a strong argument that both
periodicities were contrOlled cl imatical l y .
( 3 ) Some authors , partly bY using modern statistical methods such as spectral analysis ( e . g . HAYS et al . , 1 97 6 ; de BOER, this vol . ;
SCHWARZACHER & FISCHER, this vol . ) have been able to put forward evi ,dence for the existence of certain discrete time periods .related to the orbital cycles of precession, obliquity, and eccentricity { periods of aroUnd 2 1 , 000 y . , 4 1 , 000 y . , and 1 00 , 000 y . , after MILANKOVITCH, 1 930) .
Personally , I am in doubt whether this is feasible with the information we have as yet , at least for sedimentary cycles older than the Quater nary period . As we have seen , the error in timing is still great and difficult to overcome .
( 4 ) For these reasons , it is not yet proven that limestone-marl
rhythms and other minor type 1 cycles really have a constant time period . The Quaternary marine carbonate cycles {Figs . 8 and 9) also re
present cycles of different length . Probably it is more realistic to say that the time period of a l l these rhythms varies around a certain me�n value . This mean time period is expected to be constant .
The time interval of event deposits such as tempestites and turbidites certainly is not constant { see e . g . AIGNER, this vol . ; SEILACHER, this vol . ) . From Late Quaternary marine sediments we know that turbidites
occur in time intervals between 500 and 1 0 , 000 years ( RUPKE and STANLEY , 1 97 4 ) . Their frequency depends very much on the sediment supply in shallow water and the configuration of the depositional area (EINSE.LE
and KELTS , 1 981 ) . I n basins with low input of sediment, their average
frequency may wel l be on the same order as that of the l imestone-marl periods {DEAN et al . , 1 97 7 ) . Therefore , similarities in the appearence
and sedimentation rate of cyclic limestone-marl sequences and fine grained al lodapic l imestone successions alternating with marls or clays are to be expected ( see MAIER-HARTH , this vol . ) .
( 5 ) Quaternary and older carbonate cycles coincide not only in their
range of time periods, but also in their rate of sediment accumulation . For wet Quaternary deep-sea sediments this rate is 1 -3 cm/ 1 000 y . , whereas older, more or less consol idated and lithified l imestone-marl successions , show rates between 0 . 6 and 2 . 8 cm/1 000 y . , if an extreme
case with the contributiori of chert is excluded. As already mentioned earlier, this rate is dictated by the limiting factor of carbonate
g;
Table 1 . a Average Time Periods of Pelagic to Hemipelagic Limestone-Marl Rhythms - older than Pleistocene ( for further data see FISCHER , 1981 ; WETZEL , this vol . ) Age
Oligocene to Miocene Eocene
Locality
Sierra Leone Rise (DSDP Site 3 6 6 ) "
I>iethod of timing biostrat. and radiometric "
Late Cretaceo us
NW-Germany
biostrat. and radiometric
Mid-Cretaceo us
Western Interior Basin, USA
radiometric
Mid-Cretaceous
Southern Europe
Barremian to Tithenian
C ape Verde Basin (DSDP Site 3 6 7 )
Kinuneridge
Oxford
Sedimenta- Time tionrate a per.i9d ( crn/1000 y) x1000 y
Remarks
Author { s )
0 . 9- 1 . 6
44
chalk-marl cycles
Dean· et al. ( 1980)
1. 4 5.2
19 7
altern. of green and cherty chalk
"
2.8
30-40
l imestone-marl cycles
Ernst ( 1979)
24
limestone-marl cycles
Kauffman (pers. conununic . )
limest. alternating with black shales
de Boer
altern. bioturbated l imestone and laminated dark marl s
Dean et al . ( 19 77)
biostrat. b
o.6
37
S Germany
biostrat. b
ca. 2
15-20
mean of c a . 110 couplets in 3 5 m pr
Ziegler d ( 1958)
S Germany-
biostrat. b
c a . 15
mean of 4 6 0 couplets in 120 m profile
Seibold ( 1952) d
c a . 3 . 4c
Il
-
�-
-�
- ;
-
- -
I
-
Table 1 . !?_ Average Time Periods of some Argillaceous Rhythms Eocene Late C�etaceous to Paleocene Hauterivian to Albian
biostrat.b
Cap Verde Bas.in "
NW-Germany
"
biostrat.
o.6
ca.SO
0.6-0.8
ca.50
1 .1-9
25-33
altern. greenishgray and dark carbonaceous claystones
altern. blue-green and reddish-brownish silty claystones altern. claystones and marly clayst., mean of 7 4 5 couplets
Dean et al. ( 1 97 7 ) "
Schneider ( 1 96 4 )
Table 1 . � Average Time Periods of some Peritidal or Lagoonal Limestone-Dolomite Cycles Early Cretaceous
I stria ( Jugoslavia)
biostrat.b
Triassic (Dachsteinkalk)
Austria
biostra t .
Triassic. ( Wettersteinkalk)
Austria
biostra t .
b b
ca.6
1 0-20
mean of 300-400 cycles
Flichtbauer and Tislja:r: ( 1 975)
ca. 1 0
20- 1 00
approx. 200 cycles in ca . 1.5 m . y .
Fischer ( 1 96 4 )
ca.20
ca.40
: · mean of
>
1 00 cycles
Bechstadt ( 1 97 3 ) -
a , dry , solid or lithified sediment
b , poorly defined base and top of succession, or poor determination due to other causes c , high value due to a relatively high percentage of marls d , authors did not calculate sedimentation rate or time period of cycles
'i
85
Table 2 . Average Time Periods and Sedimentation Rates o f Quaternary Carbonate Fluctuations in the Deep Sea Locality
Method of timing
Atlantic { Colombia Basin) Core v 1 2 - 1 22 Core V 1 8 - 357
stable isotops and radiometric
W equat. Atlantic Core v 25 - 59 NE-Atlantic Panama Basin Core V 28 - 2 3 8 Pacific (Solomon Rise, Core V 28 - 2 3 9 W-Pacific Ontong-Juva-Plat. Pacific Core 04 - KS06 Indian Ocean Core IC - 5 Indian Ocean E-Mediterranean Sea
"
"
"
"
(+biostratigr.and tephra:chronology) "
oxygen isot. and magnetostratigr. acoustic records in comb. with isot. and radiom.
stable isotops, radiom. , and foram. stratigr. stable isotopes , radiometric , and magnetostrat. stable isotopes and radiometric
Sedimenta- Time Period X 1 000 y tion rate (cm/1000 y) 2.3 ( wet) 2.0 ( wet)
Remarks
2.8 (wet)
60 50 20 - 30 15
mean of 2 cycles mean of 3 , or mean of S-7 cycles mean of 8.5 cycles of different duration
2.3 ( wet)
83 - 8 8
mean of 7 cycles
0 . 6 (dry, solid) 1 .o ( wet) ca.2 (wet) 1 .0 (wet)
23 1 00 25 90
2 2 5 - 350 75
1 . 7 ( wet) 3.0 (v.Jet)
aThere are also shorter periods due to turbidites
1 00 42 23 4 0 - 6 0a
, Author ( s )
Prell and Hays ( 1 97 6 ) B e et al. ( 1 97 6 ) Ruddiman and Mcintyre ( 1 97 6 )
for 6 1 8 o and carbon. Pisias ( 1 97 6 ) for opal accum. mean of minor cycles Shackleton and ( carbonate dissol.) Opdyke ( 1 97 6 ) mean of 5 major cycles mean of up to 7 cycles
Berger and 11ayer ( 1 97 8 )
mean of 5 different mean of 3 different
Volat e t al. ( 1 980)
t
cycles of duration cycles of duration
t
dominant in a record of minor cycles 4 50.000 y mean of 5 - 8 redox cycles
Hays et al. ( 1 97 6 ) Kidd et al. ( 1 97 8 Dominik and Stoffers ( 1 979)
�
39
production and not by rhythmic diagenetic processes { e . g . HALLAM, 1 9 6 4 ) . peritidal or lagoonal limestone rhythms , however , usually have been deposited with higher sedimentation rates ( 6-20 cm/1 000 y . , Table 1 c ) . ( 6 ) Therefore, also the thickness of a l imestone-marl couplet generally varies between narrow limits (commonly 1 0 to 50 em) With growing supply pf ,·clay" , the sedimentation rate as well as the thick ness of individual couplets inCrease s . In Upper Jurassic sediments of southern German y , the couplets of predominantly marly sequences (mean •
carbonate content 7 5 % ) reach an average thickness of 6 3 em, whereas in the l imestone succes.s ion (mean carbonate content 9 0 % ) they are only 36 em thick (calculated after data from SEIBOLD, 1 9 5 2 and 1 9 5 3 ) . For predominantly clayey deposits ( example in Table 1b after SCHNEIDER , 1 9 6 4 ) there is theoretically no limit for the thickness of single rhythms and their sedimentation rate . 5 . 3 . Conclusions for Tectonic Setting and Paleoceanography of the Depositional Area 5 . 3 . 1 . Principal Requirements for the Realization of Limestone-Marl Succession s From the preceding chapters one might draw the conclusion that the ge nesi,.,s of pure limestone-marl successions is restricted to rather special environmental conditions , the most important ones are ( see also chap� ter 3 ) : ( 1 ) Deposition . below strom wave base ; no eros ion or non-deposition by bottom currents ; no interruptions by turbidites or other events; ( 2 ) Maintenance of a certain carbonate/clay ratio on the order of 3 to 4 (after possible synsedimentary dissolution) . Since carbonate production is limited, this implies a sedimentation rate of about 0 . 5 to 3 cm/1 000 y . ( 3 ) More or less steady state conditions over a period on the order of 5 m . y . , if a rhythmic sequence of 1 00 m thickness is deposited. Con sequently, in a comparatively shallow basin (with a sea floor somewhat deeper than storm wave base) subsidence should at least balance sedi ment accumulation ( see ( 2 ) ) . In spite of these constraints , l imstone-marl successions have been rea lized rather frequently throughout geologic history ' s ince Paleozoic time. The reason is that the three requirements mentioned above often match the situation in present and ancient ocean basins and therefore not seldom also coincide .
40 5 . 3 . 2 . Consequences for Deep Shelves and Epicontine ntal Seas Divergent continentql margins show a certain hi story of subsidence (WATTS and STECKLER , 1 9 7 9 ; von RAD and EINSELE , 1 9 80) . During and shortly after rifting , subsidence is very fast (on the order of 1 0 to 50 cm/1 000 y . , 'but later slows down more or less exponent ial l y . One can assume that SO to 1 20 m . y . after rifting a rate of subsidence is reached which is in the correct order for the formation of limestone marl sequences . -At the margins of the Atlantic ocean , this situation was realized since Mid-Cretaceous time. In the course of the Tertiary, subsidence became too low to maintain the steady state conditions mentioned above . The shelves became too shallow, �nd reworking and winnowing of sediments started. Of course , this development was imposed by other important processes such as great climatic changes sea level fluctuations which cannot be dealt with in this contex t . Below epic9ntinental seas the rate o f subsidence usually i s low, but may reach the order of magnitude necessary for the formation of lime stone-marl rhythms , especially if subsidence coincides with a global sea level rise ( VAIL and HITCHUN., 1 9 7 9 ) . For that reason , in several regions of Europe limestone-marl rhythms have been deposited on top of continental crust (Variscan fold belt) during Jurassic and Creta ceous time s . However , their development was always in danger to be di s·turbed or modified by ( 1 ) clay input too high (transit ion to marl s ) ( 2 ) clay input too low (transition to pure limestones) , ( 3 ) shallowing sea ( reworking, omission , wedging out and channelling , interfingering and replacement by reef s , transition to carbonate tidal flats , or tion and replacement by sandy materia1) . Many successions in the Jura ssic of Southern Germany or in the Cretaceous of Northwestern Germany show these tYpes of facies change ( e . g . ZIEGLER, 1 9 7 7 ; SEIBERT Z , 1 9 7 9 ) . A s a result , in" .these regions the ideal type o f a more or less constant l imestone-marl success ion is verified only for relatively short time intervals on the order of 1 to a few mill ion years. For both settings , continental margins and epicontinental seas , it can be as sumed that carbonate dissolution does not play a great part for the genesis of the rhythms . Hence , periodic dilution and, to a minor degree and only in some case s , varying redox conditions and/or carbo nate productivity are held to be responsible for the primary of alternating beds which later are strongly enhanced by diagenes i s . I f dissolution can b e neglected, these successions tend to have ·thicker l imestone-marl couplets which are deposited under a higher rate than those of the deep sea ( compare Tables 1 and 2) •
41
5 . 3 . 3 . Consequences for Rhythmic Deep-Sea Carbonates Periodicity in deep-sea carbonates appears to be chiefly caused by dissolution due to a fluctuating carbonate compensation depth . This is often combined with a changing circulation system including inter actions between two ocean basins such as assumed for the Quaternary between the Antarctic water masses and the Atlantic and Pacific Ocean ( e . g . VOLAT et al . , 1 980) . During times of unstable stratification of the oceanic water masse s , caused by a smaller temperature gradient beto ween low- and high-latitude areas , fluctuations of the CCD or the ly � ocline probably' have been larger than at present time {BERGER, 1 9 7 9 ) and may , therefore, have affected a wide depth range . Thus, the occurr ence of dissolution cycles in the ancient record presumably is not re stricted to deep submar-ine rises or plateaus where they have been found preferentially in Quaternary s ediments. If carbonate dissolution at the sea floor is a major factor , the initial carbonate/clay r.atio must be higher than the final one . In other words, regions with l imestone-marl successions due to per�odic dissolution have a lower clay input and a lower mean sedimentation rate than dilu tion cycles, provided biogenic carbonate production was tDe same in both areas . As mentioned above , the values of Tables 1 and 2 obviously s�pport this assumption . Of course we also know areas of increased cai'bonate produc.tion in com bination with coastal uPwelling or ocean current divergence ( e . g . STOWE , 1 9,7 9 ) . The typical l imestone-marl succession i s , however , not associated with upwelling and an exceptionally . high production of phyto- and zooplankton reflected by high contents of organic matter and opaline silica (chert) in the corresponding sediments .
6 . Global Control of Limestone-Marl Rhythms ? 6 . 1 . General Remarks After having described the differen·t direct mechanisms controlling limestone-marl rhythms (chapter 4) , now an attempt is made to define these cycles in terms of a more or less global phenomenon. Such an approach appears to be justified, because these rhythms occur since Paleozoic time in many regions of the world. For a discussion of the
42
ultimate cause of theSe and other cycles with a corresponding of time period see e . g . HAYS et a l . ( 1 9 7 6 ) , IMBRIE and IMBRIE F ISCHER ( 1 9 8 1), and SCHWARZACHER & FISCHER .(this vol . ) These papers deal- with the astronomically controlled orbital cycles of precession, •
obliquity, and eccentriticy of the earth , based on the theory of Milankovitch . Obviously , these orbital cycles induce climatic with periods around 2 1 , 000 years , 4 1 ,000 years, and 1 00 , 000 years . Apparently there are many deviations from the orbital cycles, partly due to a time lag between the response of the carbonate system of the oceans to the change in insolation evident in the Quaternary marine dissolution cycles ( see e . g . LUZ and SHACKLETON , 1 9 7 5 ) or, still due to inaccurate timing of ancient sedimentary the relati-vely good agreement of Quaternary and well as the consistent sedimentation rates of carbonate as strong evidence that c limatic control also must have in older sedimentary basins . In their general conclusions et a l �
qp
the origin of cyclic sediments, DUFF
( 1 9 6 7.) have discussed changes of climate and eustatic sea level
separately and indicated that minor eustatic oscillations may provide a satisfying explanation for limestone and shale cycles in the transi tional marine-continental environment. Using our increased knowledge of the Pleistocene, we should consider the combined effect of both factors , because they are changing simultaneously and may supplement each other . Par example in a basin near the continent a globally-con trolled shift from a warm and humid climate to a colder and partly semi-arid c limate can have the following consequences : ( 1 ) Reduction of carbonate productivity , ( 2 ) Increase of terrigeneous input due to loss of vegetation cover and regressive sea . In this case , both factors work into the· same direction and intensify the dilution of carbonate . In. contrast, rising sea level combined with an extension of vegetation cover tends to reduce the input of "clay" . In non-glacial times of the earth history, relatively short-period eustatic sea level fluctuations were, if they existed, owing to the growing and waning of mountain glaciers, only smal l . They therefore chiefly affected very shallow water or peritidal . sediments (FISCHER, 1 9 6 4 ; BECHSTADT, 1 9 7 3 ) . Consequently it can be expected that marine l imestone-marl rhythms , which are restricted to environments below storm-wave base, are predominantly control led by c l imatic variations { see e . g . BRUCKNER, 1 9 53 ; WEILER, 1 9 5 7 ; ZIEGLER, 1 9 5 8 i DEAN et a l .
43
1 9 7 7 ; de BOER, 1 9 80) . These variations may exert influence on different processes such as ( 1 ) Water circulation controlling production and dissolution of carbonate as well as redox conditions at the sea floor . ( 2 ) Vegetation cover on land controlling input of terrigeneous material and the ratio of suspension load to dissolved river load entering the sea ( see e . g . GARRELS et a l . , 1 9 7 5 ) , ( 3 ) Variations in the exogenic carbon cycle chiefly controlling ihput and production of carbonate within relatively shallow wate r . I n ge'nera l , these processes are interlocked and sometimes their com bined effect accelerates a certain trend (also see BERGER, this vol . ) , but in the following especially the influence of the carbon cycle i s stressed. 6 . 2 . Plant Life as Main Factor Contro lling the Exogenic Carbon Cycle From a _number of recent publications on the global carbon cycle ( e . g . SIEGENTHALER and OESCHGER, 1 9 7 8 ; BOLIN et. al . , 1 9 7 9 ; BROECKER e t al . , 1 9 7 9 ; DEGENS and KEMP E , 1 9 7 9 ; WOODWELL , 1 9 7 9 ) the principal evidence relevant to our problem may be listed as follows : � 1 ) For cyclic variations with a time period of 2 0 , 000 to 1 00 , 000 years , only the exogenic carbon cycle (comprising the atmospheric car bon cycle and the carbonate cycle of the hydrosphere) are of interes t . The endogenic carbon cycle, which i s controlled by subsidence and up_ lift of the earth'1 s crust, approximately maintains steady-state con ditions for the exogenic cycle ( GOLUBI C et a l . , 1 9 79 ) , and induces , if any , only long-termed variations in the amount of carbon in the exogenic cyc l e . ( 2 ) The carbon content i n the present biosphere is nearly equal to that in the atmosphere, and about 9 8 % of the living biomass is concen trated in the plant cover on land. As far as dissolved and particulate organic matter is concerned, about one half is found on land ( soil hu mus ) , the other half in the ocean s . ( 3 ) The res idence times of carbon in the atmosphere and biosphere are very short in comparison to that in the hydrosphere . According to HOLLAND ( 1 9 7 8 ) , 1 0% of the atmospheric co 2 is drawn yearly into the organic carbon cycle and again mineralized, mostly by microbial pro cesses {Fig. 1 3 ) . Within the carbonate cycle of the hydrosphere, however, much less carbon is exchanged by precipitation and dis solution of
caco
3
.
44
Fig. 1 3 . Simplified s cheme of the carbon reservoirs o f the exogenic carbon cycles , their storage { S ) , annual exchange rates ( E ) , annual river transport ( T ) and deposition in the sea ( D ) . All values in 1 Q 1 5 g units C02 , rounded (here only orders of magnitude are importan t ) . From different sources , e . g . MILLIMAN ( 1 9 74 ) , BOLIN ( 1 9 7 7 ) , BOLIN et a l . ( 1 9 7 9 ) , BROECKER e t a l . ( 1 9 7 9 ) , GOLUBIC et al . ( 1 9 79 ) , WOODWELL ( 1 9 79 ) . CCD, carbonate compensation dep·th
ATMOSPHERE s � 700
A
ATM. C02 f-)?
ATM. COt { + ) ?
TERR {+)
Fig . 1 4 . Gain ( G ) or loss (L) in storage of carbon in the plant bio mass and soil humus (maximum value 1 x 1 o 1 5 g per year, after BOLIN et al . , 1 9 7 9 ) as well as increase ( + ) or decrease ( - ) of the C02 -con tent of the atmosphere ( ? ) and surface water s , the river input of dis solved bicarbonate ·and suspension load . A , during periods of increasing, and B , decreasing vegetation cover on land. Stage A is promoting production and preservation of marine car bonate ( lowering of the CCD) , whereas stage B may lead to reduction dilution of carbonate production and even to more dissolution ( rise CCD)
Therefore, the rates of incorporation and exchange within the organic carbon cycle are about 200 times higher than those of the carbonate cycle . From these points, even if the quantitative data are still uncertain, one can conclude that plant life on land is the most important factor
45
controlling the carbon cycle . Therefore, plant life also affects the hydrospheric carbonate cycle, because this cycle is interlocked with the atmosphere . 6 . 3 . Climatic Variations . and Their I nfluence on the Carbonate Cycle of the Hydrosphere From the Plei$tocene we know that, on a global scale , the vegetation . . cover dqring the glacial stages was considerably reduced in comparison
"to the interglac ial s . The released co 2 was taken up by the buffer system of the oceans and later turned back via the atmosphere to the biospher e , wheh vegetation and soils started again to advance over barren land surfaces (Fig , 1 4A) . Between .the land surface , the atmosphere , and also the uppermost zone of the oceanic water mas s e s , this exchange of co 2 was accompli�hed without great time delay. However, the exchange of co 2 bet·ween surface waters and the deep ocean took much longer time
(on the order of several thousands of years, see e . g . GARRELS et a l . , 1 9 7 5 ; LUZ and SHAKLETON , 1 9 7 5 ) .
As a result, the surface waters tended to become richer or poorer in
co 2 for some time , before the deep ocean water masses had buffered the
surplus or deficit of co 2 in the atmosphere and in surface waters . Whether these processes really led to a substantial variation in the co � content of the surface waters i s , however , not yet clear ..
Another mechanism, which is also associated with the growing and waning plant cover of the globe or of individual drainage basins, may be more important in oui context . A wide-spread vegetation cover promotes chemi cal weathering and thus increases the river input of dissolved carbonate into the oceans ( see e . g . AHNERT, 1 9 7 0 ; GARRELS and !1ACKENZ IE , 1 9 7 5 ; HOLLAND, 1 9 7 8 i B6GLI , 1 9 7 8 ) . As e . g . SHACKLETON ( 1 9 7 7 ) has mentioned , a higher input o f terrestrial Ca-bicarbonate into shallow seas enables an increase in the production and preservation of carbonate .. SCLATER et a l . ( 1 9 7 9 ) have calculated that an increase of the river input of 1 0% can generate a depression of the CCD of 3 50 m. 6 . 4 . · consequences for Ancient Limestone-Marl Rhythms Let us turn again to non-glaciated times of the earth history and accept that, owing to the orbital cycles, cl imatic variations resulting in an alternating increase and decrease of the global vegetation cover were possible · The half-periods with reduced vegetation cover may have • .
been either cooler or warmer (more arid) than the other halves of the cycles .
46
I f we further as sume that during certain times ocean circulation was much less developed than at present time (see e . g . ARTHUR and NATLAND 1 9 7 9 ; BERGER , 1 9 7 9 ) and thus the exChange of co 2 between surface and
deep waters was reduced , we have a relatively simple mechanism to ex
plain l imestone-marl rhythms (Fig. 1 4 ) . tation cover , both phenomena, decrease waters and higher input of terrestrial duction and preservation of carbonate .
During periods of growing of dissolved co 2 in surface bicarbonate , promote the pro Thus , layers rich in carbonate
can be forme d . In contras t , periods with globally reduced vegetaion cover resulting from wide-spread semi-arid and arid or colder cl imate create a tendency into the opposite direction (Fig. 1 4B ) . Carbon re leased by the reduction of plant biomass and degradation of soils is concentrated as co2 in the atmosphere and in surface waters. Input of dissolved bicarbonate by rivers into shallow seas is diminished, as the transport of detrital material into the oceans, at least nally, is increased. Thus , less production and more dissolution bonate is feasible, and in addition , also more sediments by terrigenous material may take place. In such a way , rhythmic changes in carbonate content and texture of sediments may be generated which are accentuated later by diagene s i s . Some detailed studies on the carbon i sotopes o f caco 3 i n alternating beds of l imestone and black shale mentioned in chapter 5 . 1 . possibly support this hypothesi s . The low (negative) Q 1 3c values of the lime stones may be caused by increased input of dissolved inorganic carbon from the neighboring continent ( 6 1 3 c - 6 . 5 ) , whereas " normal" l imestones have values not f a r f rom 1 . 5 { see e . g . SCHOLLE and ARTHUR, =
1 980) . WEISSERT et a l . ( 1 9 7 9 ) and de BOER ( 1 9 80 ) , however, have a different explanation for this phenomenon . 6 . 5 . Redox Cycles Of course, a globally controlled mechanism may be superimposed by other phenomena of regional or local nature . Of particular interest for rhythmic bedding are variations in redox conditions at the sea floor or above the sedim�nt-water interfac e . Moderate reducing condi tions with oxygen still present in small quantities at the sea floor promote the decomposition of organic matter and thus ·the dissolution of carbonate (WEFER, 1 9 7 6 ; BERGE R , 1 9 7 9 ) . As a result, the correspon ding beds (marls ) are somewhat depleted in Caco 3 and less or ly bioturbated. than the alternating l imestone beds . Some couplets of the Jurassic in southern Germany show a tendency into this direction {Fig. 1 2 ) , but most of them are intensively bioturbated.
47
In euxinic basins, carbonate produced in oxygeneted surface waters is better preserved at the sea floor, because under anaerobic conditions the decomposition of organic matter and, therefore , the redissolution of carbonate is hampered (BERGER, 1 9 7 9 ; GOLUBIC and SCHNEIDER, 1 9 7 9 ) . Hence, periods of oxygen deficiency may generate layers richer in car bonate than periods of better oxygenated bottom wate r s . In this case, however , the members rich in c-arbonate should be devoid of burrows produced by some infauna . For fUrther information on alternating bed sequences in black shales see WETZEL ( this vol . ) . This last model certainly does not describe the· typical limestone--marl successions , but probably it can be also explained by climatic variations which periodically induce an overturn and aeration of deep water masses ( DEAN et al . , 1 9 7 7 ) ; WEISSERT et al . , 1 9 7 9 ; de BOER, 1 98 0 and this vol . ) . 7 . Final Remarks , work in the Future This paper certainly has riot covered all the information already avai lable on l imestone-marl succession s , and a number of geol? gical , bio logica l , oceanographic , physical , and chemical processes which may also play a part in the genesis of these cycles have not been discussed. As some other workers have pointed out earlier, this problem is indeed ve �y complex . Therefore, it appears to be necessary that future studies in this field are conducted in a more interdisciplinary way than up to now. Some of the queStions not answered satisfactorily are : ( 1 ) Persistence of the l imestone-marl-type cycles or periods through the total Phanerozoic time? ( 2 ) Recognition of this cyclicity in sediments other than those rich in carbonate ,
( 3 ) Importance of and differences in diagenetic overprint , ( 4 ) Length and consistency of the time period of the rhythms , { 5 ) Special features of l imestone-marl sUccessions indicating diffe rent depositional environments , ( 6 ) Better evidence for a globally-controlled mechanism (by a combi nation of different methods including isotope studies) , ( 7 ) Use of rhythmic sediments (periodites) for a ref inement of the biostratigraphic and absolute geological time scale { e . g .._ ERNST et al . , 1 9 7 9 ) and continent-to-continent correlation s .
48 Acknowledgements W . Rieken, w . Schwarzacher , and A . Seilacher critically read the cript and made helpful suggestion s . Referen.ces ABU-MAARUF , M. ( 1 9 7 5 ) : Feingliederung und Korrelation der Mergel kalk-Fazies des Unter-Campan von Misburg, HOver und Woltorf im ostniedersachsischen Becken . - Ber . Naturhist. Ges . 112 : 1 27-204 , --Hannove r . BE, A . W . H . 1 DAMUTH , J .E. 1 LOTT, L . , FREE, R . ( 1 9 7 6 ) : Late Quaterna ry climatic record in western equatorial Atlantic sediment. I n : CLINE, R .M . , HAYS , J . D . ( eds . ) , Investigation of Late Quaternary paleoceanography and paleoclimatology . Geol . Soc . Amer . , Memoir H� : 1 6 s-2oo. . BEAUDOI N , B . ( 1 9 7 7 ) : Methodes d ' analyse sedimentaire et reconstruc tion du bassin : le Jurassique terminal-Berriasien des chaines subalpines meridional es . - These , Univ. Caen, Sciences de la terre et de � ' amenagement regional , 3 3 9 + 1 3 6 p . BEAUDOIN, B . , BIE, J . , CONARD, M . , GUY, B . , LE DEUFF, D . ( 1 9 7 4 ) : Essai d ' analyse des rythmes dans des formations marne-calcaires alternantes . � Bul l . Soc. geol . France ( 7 ) , Q¥J : 6 3 4 - 6 4 2 . BECHSTADT , T . ( 1 9 7 3 ) : Zyklotheme im h angenden Wettersteinkalk von Bleiberg-Kreuth (Karnten, �sterreich ) . Festschrift HEISSEL, Ver O f f . Univ. Innsbruck, Bd . �g : 25 -5 5 . BERGER, W . H . ( 1 9 79 a ) : Preservation o f foraminifera. I n : SEPM Short course No . 6 , Houston 1 9 7 9 (Foraminiferal Ecology and Paleocolo gy) , Soc. Econ . Paleontol . Mineralog . : 1 05 -1 5 5 . BERGER 1 W . H . ( 1 9 79 b ) : Impact of Deep-Sea Drilling on paleoceanogra phy. I n : TALWANI , M . , HAY , W . & RYAN , W . B . F . (Maurie Ewing Se ries 3 , Deep drilling results in the Atlantic Ocean: Continental , Am . Geophys . Union, margins and paleoenvironment , p . washington . BERGER 1 W . H . & HEATH , R . G . ( 1 9 6 8 ) : Vertical mixing in pelagic sedi ments . - J . Marine -Res . 1 �g: 1 3 4 - 1 4 3 . BERGER, W . H . , JOHNSON , R . F . & KILLINGLEY , J . S . ( 1 9 7 7 ) : "Unmixing 11 of the deep-sea record and the deglacial meltwater spike_.- Na ture , �g�, No . 5 6 3 0 : 6 6 1 -6 6 3 . BERGER , W . H . & MAYER , .C . A . ( 1 9 7 8) : Deep-sea carbonates : Acoustic reflectors and lysocline fluctuations . - Geology g : 1 1 - 1 5 . BERNER, R . A . ( 1 97 1 ) : Principles of Chemical Sedimentology . - McGraw Hill , New York, 240 p . BOER1 P . L . d e ( 1 9 80) : The paleo-environment of mid-Cretaceous black shale deposition as deduced from stable carbon isotopes . Compa rative Sedimentology Research Group, Institute of Earth Scien ces·, Utrecht, s e p . 4 2 , 1 9 p . , 8 Figs . ·
BOLIN, B . ( 1 9 7 7 ) : Modeling the oceans and ocean sediments and their response to fossil fuel carbon dioxide emissions . I n : ANDERSEN, N . R . , MALAKOFF , A . ( eds . ) The fate of fossil fuel co 2 in the oceans . - P lenum Press, New York London , p . 8 1 - 9 5 . BOLIN, B . , DEGEN S 1 E . T . , KEMPE , S . , KETNER , P . (eds . ) ( 1 9 79 ) : The global carbon cycle . - SCOPE 1 3 , Wiley, Chichester - New York Brisbane - Toronto , 4 9 1 s .
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50
FISCHER, A . G . ( 1 9 8 1 )' : The 2 1 . 000 year shale-limestone rhythm . - J . Geol . Soc . London (in press ) . FLtiGEL, H . , PENNINGER , A . ( 1 9 6 6 ) : Die Lithogenese der Oberalmer Schichten und der mikritischen Plassen-Kalke ( T'ithonium, NOrdli che Kalkalpen) . - N . Jb . Geol . PaUiont. Abh . l&J : 249-280. FREEZE, A . R . , CHERRY , J . A . { 1 9 7 9 ) : Groundwater . - Prentice-Hall, Englewood Cliffs N . J . , 604 p . FREYBERG, B . von ( 1 9 6 6 ) : Der Faziesverband irn Unteren Malm Franken s . Ergebnisse der Stromatometrie . Erlanger Geolog. Abh . , H . 6 2 : 1 - 9 2 . FtiCHTBAU.E R, H . , MULLER, G . ( 1 970) : Sedimente und Sedirrientgesteine . (Sediment-Petrolog i e , Teil I I ) . Schweizerbart, Stuttgart, 7 2 6 S . FtiCHTBAUER, H . 1 TISLAR1 J . ( 1 97 5 ) : Peritidal cycles in the lower Cre taceous of Istria (Jugoslavia) . - Sediment. Geology J� : 2 1 9 -2 3 3 . FURSICH, F . T . ( 1 9 7 7 ) : Corallian (Upper Jurassic) marine benthic asso� Ciations from England and Normandy . - Palaeontology &Q: 3 3 7-3 8 5 . GARREL S , R . M . , MACKENZ I E , F . T . & HUNT, C . ( 1 97 5 ) : Chemical cycles and . the global environment . - Kaufman n , Los Altos , 206 p . GARRISON, R . E . , KENNEDY , W . J . ( 1 9 7 7 ) : Origin of solution seams and flaser structure in Upper Cretaceous chalks of southern Eng land . - Sediment . Geology J� : 1 07 - 1 3 7 . GOLUBIC , S . 1 KRUMBEIN, W. 1 SCHNEIDER, J . ( 1 9 79 ) : The carbon cycle. I n : TRUDINGER, P . A . , SWAINE , D . J . (eds . ) Studies in Environmen tal Science 3 : Biogeochemical cycling of mineral-forming ele ments . - Elsevier , Amsterdam, .p. 29- 4 5 . GOLUBIC , S . , SCHNEIDER, J . ( 1 9 79 ) : Carbonate dissolution. I n : TRUDINGER, P . A . , SWAINE , D ' J . {eds . ) Biogeochemical cycling of mineral-forming elements . - Studies in Environmental Science 3 , Elsevier, Amsterdam, p . 1 07- 1 29 . GWINNER, M . P . ( 1 9 7 6 ) : Origin of the Upper Jurassic limes tones of the Swabian Alb (Southwestern Germany ) . - Contrib . - Sedimentology � : 1 -7 5 . HALLAM , A . { 1 9 6 4 ) : Origin of the limestone-shale rhythms in the Blue Lias of England : A compOsite theory . - J . Geology, Z§ : 1 57-1 6 8 . HATTIN, D . E . ( 1 9 7 1 ) : Widespread , synchronously deposited1 burrow mottled limestone beds in Greenhorn Limestone {.Upp . Cret . ) of Kansas and Southeastern Colorado . - Amer . Assoc. Petrol . Geol · . Bull . ��: 4 1 2- 4 3 1 . HAYS , Y . D . , IMBRIE, · J . & SHACKLETON , N . J . ( 1 9 7 6. ) : Variations of the easth ' s orb i t : Pacemaker of the ice ages . -- Science J.��: 1 1 2 1 -1 1 3 2 . HOLLAND , H . D . ( 1 97 8 ) : Chemistry of the Atmosphere and the .ocea�s . Wiley, New York, 3 5 1 p . HUDSON , J . D . ( 1 97 5 ) : Carbon isotopes and limestone cement . - Geology : 1 9 -2 2 . HUDSON, J . D . ( 1 9 7 7 ) : Stable isotopes and limestone lithification . Geol . Soc . London Quart. J . , v . l�� : 6 3 7 - 6 6 0 . HUDSON , J . D . ( 1 9 78 ) : Concretions , isotopes , and the diagenetic hi story of the Oxford Clay (Jurassic) of central England . - Sedi mentology &� : 3 39 - 3 7 0 . IMBRIE, J . & IMBRIE , K . P . ( 1 9 79 ) : Ice Ages , solving the mys,tery . MacMillan , Londo n , 2 2 4 p .
i l
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53
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Observations on Well-bedded Upper Jurassic Limestones W. M. BAUSCH, J. FATSCHEL, and D. HOFMANN
Abstract. Some studies of selected l imestone beds in the Oxfor dian of the northern and southern Franconian Alb again confirm the SEIBOLD-medel : A continuous de·p osition o f detrital clay is super imposed by a periodical changing calcium carbonate production . On this base1 fused (i . e . , doubled} l imestone beds are recognizeable . Fusion or splitting of beds depends on the clay . mineral associa tion: where kaolinite .is present , fusion occurs , and vice vers a . Ih the northern region investigated , thickness of sequences and carbonate(non-carbonate ratiO are positively correlated , in the southern area this correlation Ls negative . The lateral transition from a limestone/marl sequence into pure bedded limestones is a function of the thicknes s of individual limestone beds and their content of insoluble residue s . 1 . Introduction The Oxfordian of the swabian and Franconian Alb is developed as a regular intercalation of limestones' and marls (disregarding bio herms ) . In such a sequence , SEIBOLD ( 1 9 5 2 ) postulated that a con tinuous supply of "clay" was superimposed by a periodical precipi tation of carbonate . According to his mode l , each bed of limestone and each bed of marl represent approximate equal t�e spans . I f there i s appreciable precipitation o f carbonate , llmestone will be formed ; if ·not, the result is only mar l . I n the Swa.bian , and especially i n the Franconian Alb these sequen ces can be traced laterally over remarkably wide distances . V . FREY BERG and his co-workers established a most detailed system , accord ing to which sequences ·of beds and single beds may be identified over more than 100 km distance. Such a persistence of sedimentation rhythms is a strong argument for the concept of exact isochrony of these sequences . On the base of this " stromatometry" of v . FREYBERG (1 966 ) a group of 7 limestone and 9 marl beds from Malm B (planula�zone ) were
selected and studied in detail for S.edimentary parameters in 9 out crops in the northern and 6 quarries in the southern Franconian
Alb (Fig. 1 ) . Cyclic and Event Stratification (ed. by Einsele/Seilacher) © Springer 1982
55
Fig. 1 . Map of ?ampling locations within the Franconian Alb (dashed)
Isolation of non-carbonatic residues was done by monochloric acetic acid , followed by separation into fractions
>
and
<
2'p by the
ATTERBERG-method . Finally the mineralogical composition was deter mined quantitativel y (with simplifications} by X-ray diffraction .
2 . Fusion and Splitting o f Limestone Beds In the northern Franconian Alb we observed fusion as well as splitting of limestone beds by the disappearance of bedding planes
or by additional joints , respectively (Fig . 2 ) , such occurring in
a distance of about 40 km. These phenomena are not two independent effects 1 but o�ly the two sides of one coin. This is so because of the arbitrary counting scheme of the bed s .
56
According to Fig. 2 , fus£on and splitting of beds clearly is not
dependent on bed thicknes s . Fusion occurs e .. g. in beds No . 108[109 , in relatively thick profiles ( 6 , 7 ) as well as in thin ones (9 , 3 ) .
Splitting takes place e . g . in bed No . 110 , without any visible relation to thickness variations , occurring in a group of profiles of medium thicknes s . In fact , the fusion-splitting phenomenon is dependent on the quality of clay minerals in the insoluble residue s . Sections with maximum fusions ( 3 ) have the highest contents o f kao linite (Fig. 3 ) , whereas profiles with: separation of all beds (and additional splitting) exhibi,.t very little or no kaolin'ite . The six outcrops of the southern Franconian Alb show analogous relationships . The lnterpretation of the relationship of fusion/splitting with kaolinite contents may be possible according to WHITEHOUSE 1 JEFF REY & DERBRECHT (1960 ) . They described differing settling velo
cities of clay minerals 1 with kaolinite settling quicke s t , such being possible even the
when
water turbulence is high. In this case
tendency for formation of j o ints is relatively low� In con
trastr lack of kaolinite indicates relatively quiet sedimentation, leadlng to a maximum of horizontal joints� 3 . Lateral Variation of Non-Carbonates Grain s i z e fractionation of non-.carbonates into
>
and < 2
p
frac
tions separates the bulk of quartz and feldspars from the bulk of clay minerals , though not completely · so , with feldspars occurring only in the coarse , smectite in the fine fraction . In the coarse fraction feldspar and quartz + kaoliriite vary in versely. ·In the northern FrancOnian Alb, kaolinite is coming from NW1 in the southern area from W. Relative maxima of feldspar are found in the SE and E region s .
In the fine fraction , kaolinite varies inversely with illite
+
smectite� The regional d istribution of kaolinite corresponds to that of the coarse fraction� One sampling location (No. 1 ) , mostly protected by �urrounding quiet water bioherms, exhibits the relatively highest amounts of smectite. This may be explained by the low settling velocity of this mineral (WHITEHOUSE et al . , 1 9 60 ) .
57
s "'"
.o N
2m
111
1n 1100
110a 110
110
1m
109a
1090
109
109
108
108 107 106a 10 5 5
'
7
Fig.
2.
107 106a 106
correlation of profiles of the northern sampling group. Distance from left ( N ) to right (S) : 40 km
proportional peak ratio
90
80
70
60
50
40
30
20
10
Fraction < 2p
0 texturized
r---�----�---L--�r�a�ti�o-n���-tiu---�--�
100
Legend : M+J
6
a
:--o
:--o
K : --a M + JIQ : --o M + J/K : --o
7
9
bed nr, 1 1 0 , horizontal correlation
5
2 0
Fig. 3 . Contents of smectite + illite ( !1 + J) , quartz ( Q ) and kaolinite ( K ) . Given values are not percentages . Note the disappearance of kaolinite in sample locations 1 , 4 , 5, 2 , 8 . This diagram is only valid for bed No J 1 0 ; however , all other beds show very similar results
58
4 . Absolute Non-Carbonate Amounts and SEIBOLD-Medel Relative amounts of insolubles in the limestone beds range in the
�orthern Alb between 2 and 6 % , in the southern Alb between 1 and 3% For mar l s , it is of little use to refer to percent values, as their carbonate content may be altered secondarily in the surface out crops . Howev�r, for marls the absolute values of non-carbonates , given in "em thickness 11 , provide reliable values for comparisons . The amounts of non-carbonates ( in em) , are rather constant in limestones as also in marls , as is shown by Fig. 4 for the northern
Alb , by Fig. 5 for the southern Alb. This would substantiate the
SEIBOLD-medel ( vd . Introduction) . It is only the different amount of carbonate ( in em thickne s s ) which causes the difference between limestones and marls . It also determines the thickness of a lime stone (valid for the section studied ; in higher or lower parts of the Oxfordian sequence there also occur gradational changes in the absolute non-carbonate conten� ) . The SEIBOLD-medel , developed in· the Swabian Alb , is thus now confirmed in the northern and southern Franconian Alb. There are two reasons for assuming that the separation into lime stone beds and intercalated marls is a primary feature , and not caused by diagenetic effects : ( 1 ) the constancy of absolute non-carbonate amounts {in em) , ( 2 ) the identity of bedding rhythms over distances of more than 100 km. The observations of HEMLEBEN & RICKEN ( this Vol . ) indicate indeed diagenetic effects which may have been very remarkable but they cannot explain the facts given above . With the aid of the absolute amount of non-carbonates {in em). it is also possible to detect the fused character of a double bed, as
may be seen by comparison of Figs . 2 and 4 . 5 . Disappearance of Marls
Whereas in the northern area marls are intercalated .rather regular ly between the limestone s , this is not seen in the southern area marls , which are present in the west and di sappear towards east . This could perhaps be explained thus :
]
59 3
Sample locat\on
nr.
5, vertic at correlation
�
22 0 c 0 "'
81 I
c 0 z
-
106
- - - " ,.,.
105a total
_,. .-. �
-
107
7" - - - - - - - - -
108
109
--
-
II
'
'
'
109a
'
'
,
,
'
'
'
'
'
'
''
'
'
'
'
'
'
'
110
2!J
> 2{J
Fig. 4 . Absolute contents of non-car}:)onates in em . Note that values for l imestone beds ( normal numbers) show the same level as those for marls ( numbers with a-suffix ) . Bed No . 1 1 0 exhibi·ts twice the "normal " amount, and is in fact a double bed ( proven by splitting, compare Fig . 2 )
em
00 2 40
Location 1 5 •
'
n o n - carbonate
o carbonate
30 0
10 Bed- Nc
Fig. 5 . Absolute contents of non-carbonates in em, southern Franconian Alb . As in the northern part (Fig. 4 ) , amounts in limestones and marls are nearly identical
Let us suppos e the SEIBOLD-medel is valid regarding l imestone beds of about 10 em th.ickness wi.th. only 1 % insolubles .. Then correspond ing marls would contain only 1 rom non-carbonate thickness .. Also if some carbonate content is added , the marl would reach a thick ness of not more than around 3 rom. Obiously at this stage , the
marl degenerize to bedding j o ints . With the aid o f the SEIBOLD-medel , the change o f a limestone-marl succession into a mere limeston·e sequence can be described by the following limiting conditions : Marls disappea r , when thickness of limestones x percentage of 1 mm . insolubles =
The data for this rule are given only by the remaining limestones of the sequence . Actually , proceeding from W to E within the
60
southern Franconian Alb, the thickness of limestone beds decreases to 10 em, and the contents of non-carbonate grade to the range of 1%. 6 . Comparison Between Non-Carbonates of Limestones and Marls Qualitatively , there are no differences of lLmestones and �arls in the non-carbonates. As regarded quantitative aspects , the follow ing differences are recognizable : Marls contain relatively more quartz in the coarse fractLon than limestones, but less in the fine fraction . An explanation may be seen in the different diagenetic history: it is reasonable to assume that the percolation of pore fluids is higher in marls than in pure limestones , which usually are cemented rather early. There fore, qua�tz i s enabled to grow by preference in marls . Then it forms part of the coarse fraction and becomes rarer in the fine fraction . Careful check of this hypothesis by e l ectron scanning microscopy will be done .
7 . Relationship Between Thickness and Carbonate/Non-Carbonate Ratios Fig. 6 shows a pos·itive correlation between total thickness of the profiles studied and the carbonate/non-carbonate ratio for the northern Alb, and a negative correlation for the southern Alb. The limestones and mar l s st�di.ed can be regarde<:'l. as a two-component system, built up by carbonate and non-carbonate. Both components are able to vary independently; they may vary both together and with �ifferent intensity. Quite understandable are the two possib le extremes : variation of only one component , constancy of the other .. Variation of t h.e carbonate component only creat.es a posJ..tive correlation . I f thickness variations base on varying non-carbonate contents, then a negative correlatio� is resulting. Fig. 6 shows the contrary behaviour of northern and southern Alb; in these identical seq·..J.enoes both cases are realized . 8 . Relations Between Carbonate/Non-Carbonate and the Grain S i z e of the Residues
There exists a negative correlation between purity of limes.tones and thetr coarse/fi.ne residue fractions quotient . This is shown for
61 70
15 •
14 •
12 •
:8 60 e
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13 •
0
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.0
8 I
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'-· • c
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20
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150
140
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2
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total
170
8 •
5
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200
thickness (em)
ratio Fig. 6 . Negative correla tion between ca·rbonate/non-carbonate and total thicknes s for the southern Francon ian Alb ( squares ) , and slightly positive correla tion for the northern _Francon ian alb ( c ircles)
0
35
�
e
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.0
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I
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�
'
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15
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Fig . 7 . Negative correlation between carbonate/non-carbonate ratio and < /> 2 }A- ratio for one single bed (No . 1 1 1 , northern Franconian Alb)
62
one single bed by Fig� 7 ; there i s , however , a remarY�ble disper sity. Th.is implies that relative pure limestones exhibit coarser non-carbonates . This effect can also be shown fo� single beds (Fig. · a ) . We would also note that there seems to be a relation between the slope of the correlation lines wLth the thickness of the beds (one exception) . At this stage in the studies , we will not attempt to explain this effect. . � 35 2 �
0 c
� 30 8
I c 0 c
' �
25
0 c 0
-e 20 8
,5 51,4 38
15 2.0
2.5
3.0
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4.0
4.5
5.0
5.5
6.0
<J, 2J.J
Fig . 8 . Correlation l.ines for the same relation as in Fig . ? , for single beds . Numbers : Average thickness of beds ( em) Acknowledgements
This study was supported by the German Research Foundation ( Deutsche Forschungsgemeinschaft) which is greatly acknowledged . References FATSCHEL , J . -11 . ( 19 79) : Variationen sedimentarer Parameter im t1alm der Feuerstein-Folge . Diplomarbeit , Universitat Erlangen .
B
FREYBERG, B . v . ( 1966) : Der Faziesverband im Unteren Malm Frankens . Ergebnisse der Stromatometrie . - Erlanger ge61 . Abh. &I, 112 p . HOFMANN, D . ( 198 1) : Laterale Variation sedimentarer Parameter im Oxford der slidlichen Frankenalb. Diplomarbeit, Univ. Erlangen ( in Bearbei tung) SCHMIDT-KALER , H . ( 196 2 ) : Zur Ammonitenfauna und Stratigraphie des Malm Al pha und Beta in der Slidlichen und Mittleren Frankenalb . Erlanger geo l . Abh. � ' 5 1 p . • .
SEIBOLD , E . ( 19 5 2 ) : Chemische Untersuchungen zur Bankung im Unteren Malm Schwabens . - N . Jb . Geol . Pa l . Abh. �' 3 3 7-370 . WHITEHOUSE , U . G . , JEFFREY, L . M . & DERBRECHT ,· J . D . ( 1960) : settling tendencies of clay minerals in saline waters . - Clays and Clay Min . l ·
n.-.m n
ofMarl-Limestone.Alternation (Oxford 2) in Southwest Germany W. RieKEN and C. HEMLEBEN
Abstract: A co'red Oxford 2 marl-limestone alternation shows diag.e netic overprint of primary marl-limestone rhythms . The clay con tent in the marl layers is enhanced due to pressure solution and the formation of marl seams . Well preserved and little compacted burrows throughout the limestone beds indicate early cementation. weathering emphasises the differences in prima_ry lithology .
1 . In traduction The Oxford 2 (20 m to 90 m thick) , exposed 200 krn along the bench land of· the Swab ian Alb {Fig. 1 ) , consists of two different facies :
( 1 ) Well bedded marl-limestorie rhythms interfingering with ( 2 ) the bioherm facies , which is composed of ( ? ) blue green algae and calci fied siliceous sponges and l-acks clear bedding 'in most cases . De tails of stratigraphy and facies distribution of the Swabian Alb have been summarized by GWINNER ( 1 9 7 6 ) and Z IEGLER ( 1 9 7 7 ) . rh the Oxford 2 only a few marl beds can be traced locally in both . facies , ind icating that the relief of the bioherm bod i es was low and flat. No real reef detritus has been· observed so far . Therefore,
all authors agree that the bioherms of the lOwer members of the Swa bian Upper Jurassic originated in relatively deep water, predominant ly below the storm wave base . Outcrop and facies distribution of the Oxford 2 are shown in Fig. 1 . Quarries and natural outcrops exhibit sharply alternating limestone beds { 1 0 to 30 em) and thin mar�y layers and marl j-oi nts (Fig. 2A) . The carbonate content in the limestones- ranges from 8 5 to 9 7 % while the intercalating marls contain 70 to 90 % cai'bonate (SEIBOLD1 1 9 5 2 ) . These data show that. a 5 % overlap exists between field determined marls and l imestones . This can be explained by weathering process e s . Furthermore , relict limestone particles within the marl beds cause an increase of the carbonate content . The noncarbonate part con 84 % clay {KNOBLAUCH, 1 9 6 3 ) . The distribution of carbonate Within an individUal limestone reaches its maximum in the middle of sists of
the bed {WEILER, 1 9 5 7 ) . Thick limestone beds are riche-r in carbonate Cyclic and Event Stratification ( e d . by Einsele/Seilacher) .0 Springer 1982
64
+
+ ,---, Crater C. ...)R ies \_...
+
+ site KB 6
• marl - l i mestone •
rhythm
algal-sponge facies
Fig. 1 . Outcrop and facies distribution of the Oxford 2 along the Swabian Alb after HILLER ( 1 96 4 ) . Thickness in m
than thin ones (.SEIBOLD , 1 9 5 3 ) . Quarries occasionally exhibit varia tions in thickness (Fig. 2B) . At the Eastern Alb WEILER ( 1 9 5 7 ) corre lated s.ingle be'ds over distances of more than 20 km by caco 3 analyses (EINSELE , Fig. 2 , this ·val . ) . However attempts of single bed corre lation have not been successful in all cases . Recently GWINNER ( 1 9 7 6 ) summarized correlations in the region of the Eastern Alb , which com prise a distance of ca. 70 km . He found that bed _by bed correlation seems to be problematic because of the .uniformity of the sequence and the lack of real marker beds . However, the increasing thickness of the Oxford 2 . from NE to SW (Fig. 1 ) suggests that the limestone marl rhythms started earlier or ended later towards SW; thus, not all lithological units can be traced over several tenth of km. In addition, the boundary between Oxford 1 and Oxford 2 is not well de fined by biostratigra�hy . Nearly all previous investigat'ions have been carried out on weathered material . Therefor e , we examined a drilling core, KB 6 , 7 em in dia meter from StraBberg { F i g . 1 ) . The initial core description { ETZOLD et al. , 1 9 7 5 ) shows the Oxford 2 to be 4 6 . 5 rn thick . Unfortunately the cored sequence is not entirely free of weathering. Particularly the lower 1 4 m above the underlying Oxford 1 marls show karst solution
65
and there.fore have not been used for any detailed studie s . Nevertheless , . prelimi riary results will be contributed herewith . 2 . observations and Interpretation The cored Oxford 2 (Fig. 3 ) exhibits alternations of marl and biomicri tic limestone containing mostly coccoliths . Some parts of the lime stone contain also debris of brachiopod s , echinoids , belemnites , ostracods and foraminifera (see also SEIBOLD & SEIBOLD, 1 9 5 3 ) . Some parts are rather r.ich ih Thoracosphaera, Pithonella and similar tests. 18 8 marl-limestone transitions were studied; 42 of which show only tranSitions in the rom-range . The relative amount of clay in Fig. 3 i s indicated by the descriptive weathering section. I n contrast t o out crops , the core shows intensive bioturbation . Planolites and Teichich nus structures occur quite often. Long burrows of ·2 to 4 em in diame ter and a maximum visible length of 4 5 . em ( ? athropod burrows) are re:Stricte4 to the middle and 'lower part of the cor e . The clay-rich upper part is predominantly bio"turbated by Chondrites - which are also common in the middle and the -lower part of the section. Here they are usually much smaller and arranged in · a nest-like pattern . In the limestones all burrows are well pres�rve d , because early cementation and negligible compaction took place . In contrast , burrows in the marl lay�rs are strongly compacted and thus badly preserved (Fig. 2 D) . Only 17% of the alternations show a rather distinct contact betwe�n marl and limestone (CD in Fig. 3 } as can be observed in outcrops (Fig. 2 F ) . In all other cases the transition between marl and lime stone is usually diffuse {CI in Fig. 3 ) and consists of bundles of single marl seams (MS in Fig . 3 } , causing a flaser-like structure (Fig. 2 c , D, E , G. H ) , which decreases towards the Kimmeridge 1 and disappears gradually. The bulk of the seams are fairly parallel to the stratification , although angles up to 4 5 ° occur producing nodular . features . In some cases marl seams are bound to small fissures (Fig. 2 E and at 6 3 . 7 m , 5 3 . 9 m , 4 1 . 8 m and 3 8 . 6 m in Fig . 3 ) . 22 % of the marls contain carbonate lenses {CL) often showing an internal flaser structure caused by marl seams (Fig. 2 H ; EINSELE Fig. 12 A , B , this vol . ) . As a rule , marl seams truncate t.he bioturbation structures indicating a diagenetic origin. Thu s , ' they cannot be interpreted as primary sedimen�ary structures . Thin sections and sometimes even rock specimens reveal the stylolytic nature of these marl seams . Marl seams in connection with fissures indicate that the marl seams were formed predominaritly after the initial cementation of the limestone beds at a late diagenetic stage . Competent components , for instance ,
67
belemnites or early diagenetic pyrite nodules , enable the reprecipi tation of carbonate , causing pressure shadow structures (PSS , Fig . 2 G , H ; SEILACHER et al , 197 6 ) . Marl seams caused :- by pressure solution are corrunon featur'e s in lime-· stones and even sometimeS in chalk {WEILER, 1 9 5 7 ; TRURNIT, 1 9 6 8 ; WANLESS, 1979 } . BARRET ( 1 9 6 4 ) described quite similar marl seams in oligocene _ calcarenites of New Zealand , which · are subparallel to the bedding. Diagenetic features , closely resembling the Oxford 2 marl seams , have recently been reported from the chalk of England ( GARRI SON & KENNEDY, 197.7 ) . These authors suggest that marl seams develop after the formation of early diagenetic nodules due to the overlying sediment of some 100 m . Evidence o f pressure solution can be observed not only within the marl sea�s but also in the marl beds where pressure shadow structures domi nate . Compared to adj acent limestone beds , marl beds seem to be · en riched in bioclastic detritus. Perhap s , this may be explained as the
Fig. 2 A, Oxford 2 limestones : Typical limestone-marl alternation. Scale : 1 m . Qudrry 3 km - southward o f Pfullingen , Swabian Alb. B, Upper part of OxfOrd 2 limestones : Branching of a limestone bed . Swelling and thinning of single bed is probably of concretionary origin. Sc'a le : 0 , 5 m. Road GOnningen-Genkingen , Swabian Alb. C , Bundles of single seams leading to nodular features . Scale: . lO em. Road G6nningen-Genkingen, Swabian Alb. D, Marl-limestone alternation exhibiting maximum carbonate content in the 'middle of the bed. Marl seams disseCt bioturbation structures . In th� marl layers , circular sections of burrows have been deformed to ellipses by compaction ( arrows ) . Scale: 1 em. KB 6 , 4 9 . 6 8 m - 4 9 . 8 1 m . E , Abundant marl seams i n the nodular part· of a limestone bed ( com pare 3 C ) . Marl seams developed at a small fissure ( arrow) , probably caused by different compaction around early lithified nodular struc ture . Scale: 1 em. KB 6 , 4 1 . 7 8 m - 4 1 . 9 3 m. F, Sharp marl-limestone boundaries . Upper cont-acts show stylolytic overprint. Marl seam is parallel to the bedding and continues into a joint, which is perpendicular to the bedding. Scale : 1 em. KB 6 , 53 . 7 5 m - 5 3 . 9 3 m . G, Marly limestone including a belemn ite shm.,ing "pressure shadow structurd '' . bcale: � em. KB 6 , 4 2 � 60 m - 4 2 . 7 3 m. H , Cross section of a thin limestone bed showincr compactional shearing and marl seam formation. All stages .of integration and formation of residual marl and limestone are developed . The originally limestones bed ( a ) -has been' converted into limestone lenses ( b , c ) . Belemnite shows small 11pressure shadow structures " . Scale: 1 em. KB 6 , 65 . 8 9 m - 6 6 . 0 1 m
68
co
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65
61
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66
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62
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67
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69
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48
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Fig . 3 . Marl-limestone alternation of core KB 6 , StraBberg , depth
51
69
boundary
oxford 21
CD HS
Cl Cl Cl
Cl
Cl Cl
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Cl Cl Cl Cl
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detritus ., 5%
abbreviations marl seams
MSS ·mart seam -stylolitic ,rmacroscapicl
(I
CO PSS CL
� ammonite
fossUs blue-grey white·
contact marl/ limestone indistinct contact marl / limestone distinct
pressure shadow structure carbonate lenses
0
( s!ie/1
/
belemnite echinoderm fragme'nt
bioturbgtion-sttucfurf!s mainly Planolites
o=
#
�
Teichichnus -..,. Chondrites
r}jg{},enefic structures ;;::_....,.,. . marl seams
�limestone lenses -<e><S>pressure shadow structures o o
pyrite crystals
70
result of compaction and leaching of matrix Carbonate (EDER, this GARRI SON & KENNEDY , 1 9 7 7 ) , Sometimes the contact to the overlying and underlying limestones very distinct and certainly altered by pressure solution. On the hand , a certain primary sedimentary origin of these marl beds cannot be excluded . 3 . conclusions Pr�liminary results obtained mainly from macroscopic observation sug-; gest that the alternations of limestones and marls are· of primary origin. Mixing of sediment by burrowing organisms , as can be observed in modern sediments ( EKDALE & .BERGER, 1 9 7 8 ) , destroys most of the limestone-marl boundaries : Further destruction of primary pattern depends on the clay content and i s due to pressure solution, resulting .in macroscopically rather scattered transitions between marls -·and limeStones . On the other hand , ear.LY diagenetic cementation of the limestone facilitates pressure solution at the marl boundary. Thus , sharp marl-limestone contacts may be explained probably as a late stage of pressure solution. The final 11product" of these proces-. ses are th� well layered lim�$�nes in outcrops of the Oxford 2 of the Swabian Alb, where weathering attacks mainly the marl layer s . Acknowledgments : The writers are indebted to the Sonderforschungsbe reich 11Pal5kologie" for financial support of this study. Prof. and Prof. Luterbacher reviewed the manuscript. Technical ass istance was provided by W. Ries and w. Wetze l . References
_
BARRETT , . P . J . ( 1 9 6 4 ) : Residual Seams and cementation in oligocene calcarenites , Te Kui ti Group . - J . Sedim. Petr . , vol . 1.1 , no . 3 : -524-5 3 1 . EKDALE , A . A . & BERGER, W . H . ( 1 9 7 8 ) : Deep-sea ichnofacies : Modern organism traces on and in· pelagic carbonates of the western equatorial Pacific . - Palaeogeog . , Palaeoclimato l . , Palaeoeco l . , �4 : 26 3-27 8 . ETZOLD, A . , HAHN, W . & KOERNER, U . ('1 9 7 5 ) · : Keuper , Jura .- und TertHir in Bohrungen dcr P lanungsgcmcinschaft BN - Stollen zwischen Bode,nse� und Ncckar . - Jh. geol . Landcsamt Badcn-WUrttemb_erg , J.,Z : 8 9 - 2 5 5 . GARRISON, R . E . & KENNEDY , N . J . ( 1 9 7 7 ) : Origin o f solution seams and flascr structure in Upper Cretaceous chalks of southern England . Sedim. Geology, l2 = 1 07- 1 3 7 . GNINNER , M . P . ( 1 97 6 ) : Origin of the Upper Jurassic limestones of the Swabian Alb ( Southwestern Germany ) . - Contr. Sedimentology , � : 1 -7 5 .
71
HILLER, K . ( 1 9 6 4 } : tiber die Bank- und Schwammfazies des WeiBen Jura der Schwabischcn Alb (Wlirttemberg) . - Arb . Geo l . Pal&ont. Inst . Technische Hochschule Stuttgart N . F . �Q: 1 -1"8 9 . KNOBLAUCH, G . ( 1 9 6 3 } : Scdimentpetrographischa und gcochcmischc Unter suchungen an WeiBj urakalkCn der gcschichteten Fazies im Gebict von urach und Neuffen . - Thesis Univcrsitat Tlibingen, 9 figs . , 1 06 p . SEIBOLD , · E . ( 1 9 5-Z ) : Chemische Untersuchungen zur Bankung im untcrcn Malm Schwabens . - N . Jb . Gcol . Palaont . , Abh . 2� ( 3 ) : 3 3 7 - 3 7 0 . SEIBOLD , E . & SEIBOLD, I . ( 1 9 5 3 ) : Foraminifcren und Kalkgehalt cines Profils im gebankten unteren Malm Schwabens . - N . Jb . Geol . Palaont. Abh . �!) ( 1 ) : 28-86 ., ' sEILACHER, A . ; ANDA:L I B , F . ; DIETEL, G . & GOCHT , H . ( 1 97 6 ) : Preser vational history of · comprEJssed JUrassic ammonites from Southern Germany . - N . Jb . Gcol . Palaont. Abh . l�� ( 3 ) : 307-3 56 . TRURNIT, P . ( 1 9 6 8 } : Analysis of pressure solution contacts and clasSi fication of pressure solution phenomena . - in: MULLER, G . ( cd . ) : Recent developments of carbonate sedimentology in Central Europe : 7 5-8 4 , Springer . WEILER, H . ( 1 9 5 7 ) : Untersuchungen zur Frage der Kalk-Mcrg£1-Scdimcnta tion im Jura Schwabcns . - Thesis Univcrsitat Tlibingen,. 57 p . ZIEGLER, B . ( 1 9 7 7 ) : The "Whithe" (Opper} Jurassic i n southern Gcrma:ny . Stuttgartcr · Beitr. Naturk . , Ser . B , �g : 1 -7 9 . •
Limestone-Shale Bedding and Perturbations of the Earth's Orbit W. SCHWARZACHER and A. G. FISCHER
Abstract: Stratigraphic sequences in carboniferous and Cretaceous limestones were measured with particular attention to strength and spacing of bedding planes . Spot sampling at regul·a r intervals transforms these data into numerically manageable form, replotted in smoothed Curves of sequential bed thickness . Beds are bundled into sets about 1 m thick. Power spectra of the Cretaceous data yield three frequency peaks , in the vicinity of 50 em, 1 m , and in the less-well defined low-frequency end. Depositional rates indicate that the bundles represent about 1 001 000 years , imply ing correlation with the Earth ' s 1.001000 year short cycle in eccentricity . That would match the 50 em peak with the orbital cycle in obliquitY and the low-frequency peak possibly with the long cycle in eccentricity . Sub-structure of the bundles , re"C vealed bY relative position of bedding planes , yields curves sho�ving three to four groupings of bedding planes per bundle. These are remarkably similar to insolation minima per eccentri city cycle, as calculated from BERGE R ' s algorithm, and suggest that the .main beds reflect the precessional cycle . It appears that limestone deposition, as well as Pleistocene ice volume , is a sensitive index to climate . It reflects the Earth ' s orbi tal perturbations at least through the latter half of Phanero zoic time. 1 . Introduction Although stratigraphy is basically defined as a science concerned with strata , most stratigraphers in fact pay scant attention to strata per s e , and to the bedding planes that bound .them. They are more concerned with packages of strata differentiated by compos i tion , sedimentary structures , or biotic character - the features that define the familiar formations and biostratigraphical zones . Most such units span episodes in the time range o f 1 05 to 1 07 m . y . This approach has sufficed for the normal applications o f strati graphy to problems of structural mapping, for recognition of epi sodes of Earth history, and for economic applications . Individual strata, however 1 may record events from this scale down to that of a year or less , and may offer clues to Earth behavior at much finer time scales . Classical along these lines are the stu dies of Holocene varve suites by de Geer and other workers . More recently, fine-scale studies of Pleistocene history have been made on Pleistocene deep sea cores, with or without reference to beds Cyclic and Event Stra·tification (ed. by Einsele/Seilacher) © Springer 1982
73
(e . g . HAYS et al . , 1 9 7 6 ) . These have confirmed the long-suspected tie of climates. to t�e Earth ' s orbital perturbations , and have 4 6 res olved patterns of ice behavior and climate in the 1 0 to 1 0 year range . We have been interested in sedimentary bedding and its origin during geologically earlier times (SCHWARZACHER, 1 9 5 4 , 1 9 5 8 , 1 9 6 4 , 1 9 7 5 , 1 9 8 1 ; FISCHER, 1 9 6 4 , 1 9 7 5 , 1 9 80 , 1 9 8 1 ) . Here we will describe techniques designed to convert field observa tions to. numerical data and methods of processing such data . Re-· suits are compared with astronomical data on orbital perturbations , and their effects on insolation (BERGER , 1 9 7 8 , 1 9 80) . Three sequences of limestones were treated in this manner. One is the Glencar Limestone , one of a series of Carboniferous shelf-edge sediments loca�ed in C . Sligo {NW Ireland ) , previously studied in some detail { S CHWARZACHER, 1 9 6 4 , 1 9 8 1 ) . The other two are in the pelagic Cretaceous section of central Italy {Umbria) , at Gubbio {ARTHUR and FISCHER, 1 9 7 7 ; ARTHUR, 1 9 7 7 1 1 9 7 9 ) . One is in the upper part of the Maiolica Limestone (Aptian) in the Bottaccione gorge at the town of Gubbio . The other deals with the Cenomanian part of the Scaglia Bianca, in the Valle de la Contessa, several ki lometers to the north of Gubbio . 2
·.-
Bedding in Limes tones
Bedding in these l imestones is defined by bedding planes , disconti nuities which are generally associated with a thin layer of shaly matter, largely o-f detrital origin. In the Glencar limes tone , some _ of these shaly or marly beds reach a considerable thickness . In the Maiolica and Scaglia Bianca limes tones , thin (rom-dimension) ra diolarian cherts or siliceous shales may define bedding planes . All locallities show a distinct grouping of beds into pakets or "bundles " , a feature which is readily seen in the field {Figs . 6 and 7 ) . A h ierarchy of alternatives may be considered for the origin of this bedding. Primary or Diagenetic?· The first of these alternatives centers on the question of wheth'er the bedding is original , or induced by a 11diagenetic unmixing " . SUJKOWSKI ( 1 9 5 8 ) and HALLAM ( 1 9 6 4 ) suggested that a rhythmic alternation of limestone with marly or shaly inter beds could be formed from a uniform primary ·sediment by diagenetic diffusion processes that removed carbonate from some zones and en riched it in oth-e rs . This suggestion appears to derive some
74
s trength from the following observations : ( 1 ) , lithified carbonate rocks such as the Maiolica generally show more sharply defined beds than do their poorly consolidated prototypes such as the chalk . ( 2 ) , many bedding planes show unmistakeable features o f dissolution . On the other hand, marl or shale beds in the examined sequences show features which can only be explained by primary sedimentation . For example, in the Scaglia limestone beds contain marl-filled burrows , and shale beds may contain carbonate-rich burrows , proving . that primary sediment varied from carbonate-rich to marly or shaly ( ARTHUR and FISCHER, 1 97 7 ) . Ripple cross bedding defines some bed ding planes of the Glencar limestone as primary . Furthermore , sedi mentary layering, whatever its origin, is inevitably parallel to success ive positions of the sedimentwater interface. If due to un mixing, such unmixing had to be directed .either by a preexisting layering, or by the - immediate proxirnni ty of . the seafloor . In either case it records depositional history. Diagenesis has profoundly modified these consolidated sediments . Many beds are now stylolytic . What are now sharp bedding planes or thin shale seams may not have been that initially. But we reject the view that well-developed and continuous bedding planes are unre lated to depositional history. We can imagine their loss through bioturbation (ARTHUR , 1 9 79b ) , but not their addition by diagenesi s . We therefore feel justified i n treating bedding planes as expres sions of historical events developed in stratigraphical sequence � At the same time , the field observer may find it difficult to draw the line between true bedding and a family of other discontinuities , which in their more distinctive expression are discontinuous and cross-cutting, but which may masquerade as "weak" beds . Some of these are biogenically induced ( for example , spreite· burrows o f the Zoophycos typ e . Others are stylolytic or non-stylolitic solution surfaces unrelated to bedding. Yet others , in folded terranes such as these, are tectonically induced fractures . Combinations of such features are likely . These inevitably constitute a kind of noise in the bedding signal, which will be . further discussed below. Which Phase has Fluctuated? Let us assume that the bedded limestone sequences record the alternation of two primary phases of sediment. The dominant one is rich in biogenic carbonate and contains only a small admixture of detrital quartz and clay . The subsidiary phase is richer in detrital components , and may or may not have been en-
75
. -hanced in character by diagenetic export of carbonate·. I t is gene rallY assumed that such alternation result from sedimentary settings in which one phase is supplied at a steady rate, and the other phase at a variable rate. Presumably there are limestone sequences in which the carbonate component is the more invariant one and the clay supply peaked at intervals ; and others in which the reverse occurred. There is no reason to exclude the third possibility , that the supply of both fractions fluctuated , Th� amount of detrital matter deposited per unit time is largely related to the rate at which terrigenous matter is supplied . The amount' of carbonate deposited per unit time fluctuates in response to two variables : the carbonate productivity of the contributing biota1 and the rate of carbonate dissolution from the sea floor ( ARTHUR and FISCHER, 1 9 7 7 ; DEAN et a l , 1 9 7 8 , 1 9 8 1 ; ARTHUR, 1 9 7 9 a , 1 9 79b) . The latter factor is presumably not significant in shoalwa ter settings , but becomes dominant as the carbonate compensation depth is approached . For the Sligo Carboniferous , SCHWARZACHER ( 1 9 6 4 ) has suggested the terrigeneous matter may be the variable phase; for the Italian pe lagic sequences , ARTHUR and FISCHER ( 1 9 7 7 ) have suggested a steady influx of the detrital phase and variations in the carbonate phase 1 due e ither to fluctuations in productivity or to changes in rate of seafloor solution . Any of these factors are highly sensitive to climatic control , and limestones may be amongs t the most sensitive barometers of climatic fluctuations , as suggested by' GILBERT ( 1 89 5 1 1 900) •
3 . Field Observations
A s tudy of bedding necessarily implies concentrating attention on the features that bound beds , namely bedding planes . Field notes were designed t? record two features of these surfaces : their stratigraphic distance above a common reference point- the base of the measured sequence (Fig. 1 column a ) ; and a semiquantitative ranking of the intensity of the bedding plane 1 plotted in column b . This intensity ranking was considered important because o f the uncertain boundary between true bedding and other surfaces of dis continuity, and because of di fferences in the expression of bedding depending on the nature of the exposure and the degree of weathering .
76 a
46 44
b c 1 /
1 0
/
0
22
2
0
16
3
0
1
2
42 40
0 0
32
2
36
8
5
1
0
3
4 3
0
Fig. 1 . Stratigraphic field measure ments , with accompanying codes . ( a ) , stratigraphic elevation of bedding planes above base of sequence, in em. (b) 1 ran king of bedding plane, from ( O ) , contact which is not a bedding plane, to { 4 ) , master bedding plane . ( c ) , code for type of contact : ( 0 ) , lime stone-limestone ; ( 1 ) , lime stone-shale; ( 2 ) , shale-l ime s ·tone; ( 3 ) , limestone-chert � ( 4 ) , chert-limestone
The ranking scheme used was as follows ! 0 - discontin�ity that is not a bedding plane, e . g . a boundary between l imestone and a chert nodule . 1 - a weak surface , planar or stylolytic , developed only locally ( i . e . , in dm domain) 2 a very weak bedding plane , discontinuous at outcrop scale 3 a strong bedding plane , continuous through outcrop, may show a visible thin film of clay 4 a master bedding plane, generally associated with a definite . shaly (or cherty) interbed , showing topbgrap�ic expression in the outcrop surfaC e . -
-
-
A third column ( F i g . 1 c ) records changes in rock type . It too may be coded numerically , for example : 0 - no lithic change - limestone to shale 2 shale to limes tone -
3 - limestone to chert
4 - chert to limestone
1
I
i
77
_ Additional. columns may be used to plot other features observed in the field or obtained from subsequent s tudy .
4 . Analysis of Data The columns of Fig. 1 contain all of the information at hand . The next steps will be ·'to make this information more suitable for pro ce;s sing by computer, or for vizualization. This invariably results in a ).oss of in'formation . Various alternative methods will be used, each of which sacrifices some kinds of information in order to illu minate others . one such method is to construct simple - frequency diagrams of some of the data , for exampl e , of bedding thickness (Fig. 2 } . In this , the
30 20 10 20 10
0
30
40
om
50
om
60
MAIOUCA
Fi9:. 2 . Frequency distributions of bedding thickne s s , Maiolica and Scaglia limestones_ . Vertical scale: observed frequency
time-series aspect provided by s tratigraphic superposition is dis carded, bUt useful information for comparing s tratigraphic sequences emerges . Multimodal thicknesses of limestone beds may suggest either non-random fluctuations in the rates of carbonate depos ition, or a non-random variation in timing of interrupting events .
78
We turn now to plots in which the stratigraphic integrity of the data is preserved . In the case of varved sequences , e ach bedding couplet represents one year . In the tradition of varve analysis , the vertical axis of a stratigraphic plot i s inverted to a true time axis by assigning equal intervals to each couplet. Plotting the thickness of varves on the horizontal axis then depicts the fluctua tions in sedi�entation rates , fiom year to year. In most sediments , including those here considered , the length of time represented by beds or bed-couplets is not known , nor is it necessarily constant: there is no assurance that thin beds bounded by bedding planes of grade 1 or 2 represent as much time as does a thick bed delimited by planes of grade 3 . Nor can we be sure that a thick bed is not the result o f amalgamation of several thin ones , by temporary suppression of events that cause bedding planes , or by subsequent bioturbation . Thus we cannot treat ordinary bedded sequences in the manner of varves , and are constrained to retain the initial field measurements as our closest approximation to time . This time axis i s true in di rection . For most formations its dimension - the rate of sedimenta tion - is probably fairly constant when sets of beds are averaged1 but deviates at the level of beds and inter-bed events . Most methods in time-series analyses require data obtained at equal interval s . Since t ime is not known , data are collected at constant vertical intervals . Choice of interval will vary from case to case: It makes no sense to choose intervals smaller than the thinnest beds encountered with some frequency . In the case of the Umbrian Cretaceou s , in which 1 0 em beds are commo n , a 10 em interval was used; in the case of the more coarsely structured Glencar limestone , a 20 em interval gave good results . Bed thicknesses are recorded only at such predetermined measuring points . In the case of limestone beds separated by simple bedding planes . or by shale partings of rnm dimensions , the thickness of the shales can be ignored , and a shale parting counted as a bedding plane. In the case of thicker interbeds of marl or shale , the observer is faced by a choice : Whether to ignore the shale , no matter what its thick ness, treating it in the manner of a bedding plane ; or whether to deal With the section as a sequence of shale-limestone couplets , adding the thickness o f the shale to that of the succeeding lime stone bed . The two methods yield di f ferent results , and we have
79
chosen the second alternative in our treatment of the Glencar lime stone (below) . Data obtained in this manner may be smoothed by means of a running average of 3 with the weight 1 , 2 , 1 , and plotted on the strati graphic base as sequential bedding variation curves (Fig . 3 ) . In spection of thes·e curves shows a fairly regular alternation between thicker and thinner beds - the "bundling" of strata that is apparent 1
2
3
'
5
r;
?
8
9
:�' :� O
I
9
1l
Kl
2
3
n
n
12
12
l. ,
5
13
14
a
6
15
14
1
16
!5
16
•
I
15
16
17
18
19
20 21
22
23
2J.
25
17
2&
V
8
9
18
r1
n
I
28 29
18
� 12
"
30
13
31
14
32
=� 20
25
�
�
Fig. 3 . Sequential variation in thickness of beds . Vertical scale: thickness of beds in centimeters; horizontal scale in meters . A - Scaglia Lime stone , B - Jilaiolica. Limestone . Numbered arrows are master bedding planes noted in field, and used to define cycles to th.e eye in the field or in photographs (Figs .. 6 , 7) . They may be tested for periodicities by means of power spectra of the unsmoothed data (Fig. 4 ) . The three cycles thus revealed - one of which pro duces the bundles - will be considered below.
80
Fig, 4 . Power spectra of strati graphic variation in bed thick ness , for Maiolica and Scaglia limestones . Interpretation : So em peak: 4 3 , 000 year obliquity cyclei 1 m peak : 1 00 , 000 year cycle in eccentricity. Peak at left ? 4 1 3 ,000 year cycle in eccentricity
5
-
• 0 " •
•
MAJOLICA
>
4 � 0
(
"'
Jl
SCAGL I A
cyc1.;2ocm
om
.1
200
BlANCA .2
100
.3
6M
.4
60
.5
40
While this approach reveals the periodicities of higher order, it does not by itself define the s tructure of the bundles , in as much as the beds which form the bundles serve as the units of measurement . In order to study the s tructure o f the bundles , it is necessary to do two things : first1 to find some reference point within the cycle a distinctive bed or bedding plane - which serves to break the chain of stratigraphic events into equivalent segment s ; and secondly1 to devise a method of depicting and comparing the structure of these segments , individually or in an integrated manner . For our reference or segmentation point we chose the Master bedding planes (bedding planes of grade 4 ) observed in the field . Fig. 3 shows that these are generally well coirelated with the curve of bedding variation1 although not inevitably so: In a few cases 1 two bundles seem to lack a dividing master plane; in others a bundle may have been split . We have not .attempted to "correct" these cases with the benefit of hindsigh t 1 but have retained the initial field iden tification for the analysi s . The s tructure o f bundles has been s tudied b y counting the number of beds in them (SCHWARZACHER, 1 9 5 4 ; 1 9 75 ; FISCHER1 1 9 80 ) . This places much importance on which planes to count in terms of their intensity and spacing. counts from which the weaker bedding planes (grade 1 ) have been omitted are shown in Table 1 .
81
Table 1 . Number of major beds per cycle No. of beds per cycle 1 2 3 4 5 6 7 mean
MAIOLICA 32 cycles 0 6 4 8 12 1 1 4 . 03
SCAGLIA 1 7 cycles 0 0 4 2 5 4 2 4 . 88
As an alternative-- -we have used the relative position of bedding planes : The thickness of a given bundle is normalized to 1 , and the relative stra-tigraphic position of each bedding plane within that bundle is recorded. A histogram of their distribution de-emphasizes the number of bedding planes , and expresses the divison of the cycle into segments of relatively uninterrupted carbonate deposition1 separated by portions in which interruptions are concen-trated. Rather than dealing with individual cycleS , we here present the average structure of_ bundles for a given stratigraphic sequence (Fig. 5 ) . These various methods provide for a given s tratigraphic sequence a series of numerical data. These document visible features ( e . g . the bundli�g shown in Figs . 6 and 7 ) , reveal additional structure ( i . e . cyclicity below and above the levels of the bundles observed) , and present the information data in form that permits comparison , with other stratigraphic sequences as well as with non-stratigraphic da ta (such as the insolation curve in Fig. 5 ) . 5 . Examples The methods discussed above were applied, in part or in whole , to three sedimentary sequences : . ( 1 ) , the Maiolica Limestone ( Lower Cre taceous ) of central Italy ( Umbria) ; ( 2 ) , the Scaglia Limestone , Up per Cretaceous , of the same · area; and ( 3 ) .the Glencar Limestone, Lower Carboni ferous , of Ireland. Maiolica Limestone The Maiolica (Fig. 6 ) is a whitish, fine-grained limestone , somewhat cherty , of Tithonian-Aptian age. It is a li'thified coccolith ooze containing an admixture of larger protists ( calpionellids , globige-
82 MAIOUCA
G LEN CAR
SCAG LIA
I N S O LATI O N M I N .
Fig . 5 . Structure of bedding cycles in Glencar, Maiolica and Scaglia l ime stone s , as defined by relative position of bedding planes within them . Master bedding planes at positions 0 and 1 . 0 ( top and base) . The relative frequency of bedding plane positions is plotted from left to right. Various position maxima are indicated by shading. The position of insol�tion minima is shown for comparison ( see text) rinids , radiolaria) as well as some pelagiq crinoids and ammonite aptychi . It is thus an example of" the ''Aptychus limestone11 facies widespread in the Tethyan realm during the Tithonian-Neocomian in terval (GARRISON and FISCHER, 1 9 6 9 ; HSU , 1 9 76 ) . It was deposited above calcite compensation depth , but below the zone of aragonite compensation. Part of .the Umbrian pelagic sequence , succeeding Ju rassic . radiolarites and preceding the radiolarian-rich shales (Scisti a Fucoidi) of the Aptian-Albian 1 it was probably deposited at depths on the order of 1 km. The measured sequence comprises the top 36 . 4 5 m of the formation on the northwest side of the Bottaccione Gorge at Gubbio , Italy ( ARTHUR and FISCHER, 1 9 7 7 ) , exclusive of a slump zone at the very top of the 'uni t . No evidence was found in this sequence for carbo-
83
nate resedimentation in form of turbidite beds , a common feature of these rocks elsewhere ( GARRISON, 1 9 6 7 ; GARRISON and FISCHER, 1 96 9 ) . We believe the measured section to lie entirely within the Aptian (PREMOLI S ILVA et al . , 1 9 7 7 ) . Bedding Character . In the ·sequence measured, beds show an average thickness of 1 5 em, but the distribution (Fig. 2 ) shows one mode at 6 em and another at 2 1 em. Most bedding planes are sharp and in some cases s lightly sutured, with little or no cl.ay visible . Much s �·ron'ger bedding planes are developed at interva l s of about one meter , and these generally contain 1 to 5 em of dark shaly matter. These" are the "master bedding planes 11 which produce step-like slopes where gently dipping beds reach the surface, and define a 11bundling" of strata evident in outcrop . Not all of these master bedding planes contain shale, and shale may occur in the minor bedding planes near a master plane, so that presence or absence o f shale i s not a reliable objective criterion for defining cycle boundaries . Detailed petrographic and/or chemical studies have not been made . Lower ·in the format�on (Fig. 6 ) bedding is more massive, and bundling is even more striking. Aside from the bedding,· the chief varial:?le noticed iri the field is the distributiOn of chert . Chert layers , at least some of which are i"ich in radiol � ria, generally occur dir�?tly in the bedding pla nes ( in place of shale ) , or within five centimeters above or below a bedding plane. With reference to the cyclical bundles , 5'5 % of the chert beds occur �n the lower third of the bundle, 25% immediately below the master bedding plane, and the remaining 20% are dispersed along bedding planes in the upper 2 / 3 of the cycle . Cherts are not universally present in these bundles , but the number of chert bands ( 6 7 ) is about double the number of cycles ( 3 3 ) . Thus cheits cannot be used as a definitive objective criterion for recognition of cyc les . Sequential Bedding Thickness . In s tudying bedding thickness , as outlined above , spot samples taken at 1 0 em intervals served to construct the variation curve shown in Fig. 3 . The same diagram al so shows the position of master bedding planes identified in the field and numbered in ascending order. The cyclic nature o f the se diments emerges clearly from the graph, which may be regarded as ob jective proof of the cyclic s tructure of this sequence . There are, however, a few inconsistencies . No maximum in bedding thickness was found between master bedding planes 3 and 4 , while two maxima occur
84
Fig. 6 , Maiol�ca Limestone , old quarry, Valle della Centes sa, This lies lower in forma ·tion than the sequence described in text , bUt shows similar cycle . Scale bar 2 . 5 m
. between planes 4 and 5 . No maximum is found between planes 1 0 and 1 1 , 20 and 2 1 , 2 2 �nd 2 3 . Master bedding plane 2 8 falls directly onto the only peak between planes 27 and 29 . We may be justified in re . trospect in concluding that master bedding planes 20 and 2 8 , which occur rather too closely spaced to their neighbors as wel l , are pro bably spurious , and that one has been omitted between planes 4 and 5 . Yet , we have not revised the original data, preferring to treat these cases as unresolved.
85
Fig. 7 . Scaglia Bianca Limestone , road cuts , Valle della Contessa, Umbri a . A , lower part of section described; B, middle part. scale bar 2.5 m
The mean thickness of cycles thus recognized is 1 1 0 . 3 em. 'l'he ave rage thickness of cycles 1 - 1 9 is 1 2 5 . 3 cm1 that o f cycles 1 9 - 3 3 is 9 0 . 0 em, but this difference disappears if, as suggested above , the former set counts one cycle . too few and the latter two cycles too many . Power Spectrum. An objective estimate o f cyclicity can be obtained by calculating the power spectra of the thickness indices . Indices were calculated with a spacing of 20 em to obtain the maximum amount of information in the low frequency bands . The spectrum ( Fig . 4 ) shows a well developed peak in the frequencies corresponding to a wave length of 1 26 to 1 3 3 cm1 slightly longer than the cycle calculated by use of the master bedding planes . A second prominent peak lies at wave length 52 em. This peak is not attributable to any cyclicity recognized in the field .
86
Higher frequencies ' have very little significance in the spectrum, ' inasmuch as they approach the dimensions of the units used in measu ring the thickness indices . Experiments with much more closely spa c_ed thickness indices lead to power spectra .which have no obvious terpretation. I t is not possible to establish further subdivisions the 1 m cycle by such means . More informa�ion is contained in the low frequency part of the spectrum, which shows a peak at a wavelength of approximately 8 m . A power spectrum calculated with a smaller band-width, using 3 0 em intervals , suggests that this peak lies more properly between 6 . 5 and 7 . 5 m , but the resolution of wavelength in these low frequencies . is not very good in a 35 m sequence . Th is cycle is not obvious in the field, but finds expression in the bed-thickness variation curve (Fig. 3 ) , in which four episodes of high amplitudes , each containing 4-6 cycles.' are separated by shorter episodeS of low amplitude. It thus appears that the bedding cycles in the 1 m range are grouped into sets ·of �bout seven . Relative Position of Bedding Planes . The high-frequency end of the spectrum in the Maiolica section is examined next by means of rela- · tive position dia grams , using the master bedding planes as cycle boundaries , in the manner discussed above . The �esults are shown on Fig. 5 . The pattern of bedding plane spacing is highly symmetrical . Maximum concentrations at 0 . 2 and 0 . 8 repre sent bedding planes which lie near the master bedding plane . There is a wide and less well defined zone of bedding plane frequency in the middle of the cycle, having three subsidiary peaks between 0 . 3 and 0 . 6 . The cycle is thus shown to have a general s tructure of four major beds , of which the middle ,ones may in turn show subdivision. Bed coun ts . Counts ( Table 1 ) confirm the analysis by relative posi tion of bedding planes : Th€y show a total of 1 2 9 major beds in 33 cycles, yielding a mean of 4 beds per cycle, and a mode of 5 . Scaglia Bianca The Scaglia Bianca is a light-colored , somewhat cherty limestone which represents a lithified coccolith-globigerinid ooze (ARTHUR and FISCHER, 1 9 7 7 ) . It thus resembles the Maiolica Limestone , from which it is separated by the Aptian-Albian Scisti a Fucoidi . The Scaglia Bianca represents Cenomanian and part of Turonian time (PRE MDLI SILVA et al. , 1 9 7 7 )
•
87
The section was measured in the cuts along the highway passing through the Valle della Contessa , some kilometers to the northwest of Gubbio. It comprises the 1 9 . 7 meters of Scaglia Bianca immedia tely below the distinCtive black cherts and shales of· the "Bonarelli bed" ( ARTHUR and ·FISCHER, 1 9 7 7 ) , and is strikingly rhythmic (Fig. 7 ) . As in the Maiolica, no evidence was found here of turbidite beds , although such beds are common in the Scaglia group in other parts of Italy (WEZEL, 1 9 79 ) . ,Bedding Characte r . Both the bedding and the lithologies are mOre com plicated than are those of the Maiolica. Continuous thin (rnm domain) layers of black and at least in part radiolarian chert 1 commonly associated with dark shale, occur commonly along the cycle boundaries . Light-colored chert i s more common within the limestone beds, and may occur either as nodules or in continuous layers . The 2 2 4 beds of Scaglia measured average 8 , 8 4 em thick, appreciably less than the beds in the Maio lica. A strong mode lies at 2 em, bey ond which there_ lies a second, diffuse multiple maximum ( F i g . 2 ) . The bed thickness distribution results from the following sequence, which occurs over and over again: (1 )
Strongly developed bedding plane , possibly shaly; (f) 2 em limestone; ( 3 ) weakly developed bedding plane; 1 0 to 1 5 em limestone ; (4) ( 5 ) weakly developed bedding plane; ( 6 ) 2 em l imeStone ; ( 7 ) s trongly developed bedding plane , possibly shaly etc. A complication factor is that the thinner beds may be s i liceous , and may turn into chert . The striking bundling of beds in this sequence (Fig. 7 ) is produced by the occurrence of more shale and associated or substituting black chert. Bundle boundaries were independently verified by SCHWARZACHER and FISCHER. 1 8 cycles were differentiated. Sequential bedding thickness . The cu.rve of sequential bed thicknes s·es (Fig. 3A) portrays these relationships graphically. As in the Maiolica , the pattern appears cyclic. Cycle boundaries as picked in the field are plottet on this curve , and as in the Maiolica, one could have second thoughts about the placement of a few of them : perhaps the cycle defined by planes 5 and 6 should have been spli t .
88 We were uncertain about the uppermost three cycles in the field , and this is reflected by the curve . Even so, we proceeded to pro cess the data without further revision. PoWer Spectrum. The average thickness of a cycle given by the field data and the bedding thickness plot is 1 07 em. This is well suppor ted by the spectral analysis of the unfiltered data (Fig. 4 ) . The high frequency peak at 5 1 em corresponds to one in the· Ma.lOlica spectrum, and its origin is revealed ln the bedding variation curve: various cycles - particularly those between planes 1 - 2 , 1 0- 1 1 , 1 1 1 2 and 1 3- 1 4 are split by a group of thin beds . Agai n , as in the Maiolica· , there is a peak in the low-frequency range, at a wave length of between 400 to 5 3 3 em. Structure of cycles. Relative position of bedding planes within the cycle is shown in Fig. 5 . The occurrence of numerous bedding planes in the middle portion of the cycle, responsibl� for the spectral peak at 0 . 5 1 m , is seen to be bimodal , falling on either side of a central bed. Two additional bedding plane maxima are seen in the lower part of the cycle , and near position 0 . 8 two peaks appear . Thus this Scaglia Bianca cycle contains more beds than the Maiolica cycle - an average of 4 . 9 major beds as shown on Table 1 - yet the overall structure is not too dissimilar . Glencar Limestone The Lower Carboniferous of County Sligo, Ireland, has been investi gated extensively by SCHWARZACHER ( 1 9 6 4 , 1 9 7 5 ) . Its basic sedimen tary cycle is probably seen best where a maximal compositional diffe rentiation occurs , in the Middle Glencar Limestone . A regular alter nation is found between 30 to 50 em layers of marly shales and 1 50-to 200 em units of bedded limestone . The individual limestone beds range in thickness from 5 to · about 30 em, with a mode at about 20 em, and are frequently ·separated by thin layers of marl. The bundles are thus approximately twice as thick as are those in the Maiolica and Scaglia limestones , and are distinguished by presence of a prominent basal shale bed . These cyclic packages of 2 to 2 . 5 m thickness are mappable units in the Middle Glencar Limestone , and their lateral variation can be studied. In the Sligo area the thickness of the marly shale beds decreases rapidly in SW direction, while the limestone gain thick ness in proportion. Thus the succession of. cycles in which the boundaries are defined by marly-shaly beds changes to one of lime-
89
stone in which the same boundaries are demarked by unusually well developed (master) bedding planes . From the paleogeographic set ting of the areas it is quite clear that this transition is connec ted with increasing distance from the land to the NE , that appears to have provided the clay· . Carbonate is largely of benthonic skele tal origin, with sponge spicules playing a large role . While some of it was produced in situ , some may have been excess supplied from carbonate banks and reef · knolls . The depth of the setting is estima t�d at about 1 40 m or less . Statistical Data. Bedding data on this sequence has · been provided elseWhere (SCHWARZACHER, 1 9 6 4 , 1 9 7 5 , 1 9 8 1 ) . While the actual num ber of limestone beds between marls or master bedding planes can be quite variable , the mean number arrived at by counting is 8 . 0 8 . Relative Position '9f Bedding Planes . For the present paper , the average structure of the cycles was computed by the relative po sition of bedding planes (see above ) . Fig . 5 shows essentially three main composite peaks in bedding plane position: at about 0 j , 0 . 4 and 0 . 8 . Thus the cycle is divided into four main segments , closely resembling �hat of the Maiolica. In bed-counts , this simi larity is obscured by the larger number of satellitic bedding pla nes in the Glencar Limestone . 6 . Discussion Communality of Pattern. In all three sequences investigated , l ime stone deposition "was interrupted by events which produced bedding planes and/or shale interbeds . This interruption occurred in a cyclic pattern. This is very similar for the Glencar and Maiolica lime stones , and slightly different for the Scaglia Bianca. Apparent similarities in timing will be discussed below . A s imple sedimen tation model which can explain these observations can be derived by superimposing two periodic functions having a frequency ratio of 1 : 5 . If this function is regarded as a probability measure for the formation of limestone beds , then profiles very similar to those observed can be generated (SCHWARZACHER, 1 9 8 1 ) . Sedimentology . At first glance, it may appear surprizing that such different sedimentary settings should respond to what appears to have been a similar cause . The Maiolica and Scaglia Bianca Limestones are pelagic deposits , probably formed near calcite compensation depth . In them, the
90
accumulation o f pelagic calcitic muds was interrupted periodically to allow the normally minor components of clay and biogenic silica to accumulate in pure form. Whether this signifies a drop in orga nic calcite production or an increase in rate of calcite dissolu tion on the sea floor remains uncertain, as does the question of whether the production of radiolarian biogenic silicy fluctuated . In the Glencar Limestone, periods dominated by calcareous matter, either of local biogenic origin or derived from near-by carbonate mounds and banks , alternated with episodes dominated by the terri genous component, possibly partly by an increase in supply of the latter. Thus bedding in each of these settings was controlled by local factors of different sorts . The communality of pattern suggests , however , that these local factors were driven in turn by some general rhythmic fluctuation in the state o f the outer Earth . Such fluctuations include eustatic oscillations and changes in climate . Change in sea leve l , possibly indirectly related to climate through fluctuations in the volume of ice , are according to FISCHER ( 1 9 6 4 ) probably the immediate cause o f some sedimentary cycle s . I t does not seem likely as a cause of cycles in pelagic depositS ·, such as the Maiolica and scaglia limestones . It might have played a role in the Glencar cycles, in which a drop in sea level would have increased the influx of terrigeneous debris while reducing the supply of carbonate from bank settings at times of emergenc e . Both the Italian Cretaceous and the Irish carboniferous settings might have responded to climatic factors , in other ways . Thus the supply of terrigeneous debris to the Glencar settings is likely to have been influenced by changes in the c limate of nearby/ lands, as well as by changes in wind-driven currents . Likewise , carbonate productivity of the carbonate banks presumably responded to changes in salinity temperature and wind-driven circulation·. Productivity of calcite-precipitating plankton in the Umbrian Cretaceous was presumably dependent upon supply of nutrients and thereby upon oceanic circulation , and rise and fall of the carbonate Compensa tion depth likewise must have mirrored changes in oceanic structure and circulation (FISCHER and ARTHUR, 1 9 7 7 ) ultimately responSive to climate. Thus climate appears to be the main conunon denominator o f the changes that induce interruptions o f carbonate depos ition . Indeed 1 limestone stratification may be one of the most sensitive recorders of climatic change , as suggested by GILBERT ( 1 89 5 ) . I
91
The Cenomanian sequence in Umbria is about 50 m thick (ARTHUR and FISCHER, 1 9 7 7 ) , and the duration of the stag� is calcula ted to lie between 2 . 5 and 8 m . y . ( OBRADOVICH and COBBAN , 1 9 7 5 ; VAN HINTE, 1 9 7 6 ) . Accordingly the mean depositional rate lies in the range of 6-20 m/m . y . ,- or 5 0 1 000 to 1 60 , 000 years per cyc le . Ess entially the same rate is indicated for the Maiolica . Similar considerations yield an estimate of 2 7 , 000 to 1 50 1 000 years for the cycles in the Glencar Limestone . These values are of the s ame order as those calculated for other bundled carbonate sequences (FISCHER, '!980) , and as the main climatic cycle recorded in Pleistocene ice flux, ( HAYS et al . , 1 9 76 ) . ==�
Link to Orbit. Regularity o f sedimentary cycles has led various authors ( among others CROLL , 1 8 7 5 ; BLYTT, 1 88 6 ; � ( GILBERT , 1 8 9 5 ; BRADLEY, 1 9 2 9 ; SANDER, 1 9 3 6 ; SCHWARZACHER, 1 9 5 4 ; VAN HOOTEN , 1 9 6 4 ; FISCHER, 1 9 80 ) to suggest that the sedimentary record is influenced by the perturbationS of the Earth ' s orbit . MILANKOVITCH was the first to calculate the variations in insolation caused by these perturba tions during the- :?leistocene . The recovery of continuous records of Pleistocene deposition in the deep sea , together with improved da ting , detailed paleontological studies and studies of oxygen _i sotope ratios have made it possible to compare the stratigraphic record of Pledstocene events - in particular the growth and shrinkage patterns of the ide sheets - with the orbital data and the insolation values calculated from them ( HAYS et al . , 1 9 7 6 ) . The patterns are remarkab ly similar, and leave little doubt that the orbital perturbations played a major role in the Pleistocene expansions and contractions of the cryosphere , and thereby on sedimentation . Such a precise comparison is not possible for the Carboniferous and Cretaceous sequences dealt with here because their timing is not well enough known . - Howeve r , the predominant cycle ·of sets of about five beds seems in each case to approximate 1 00 T OOO years � a factor of two. The spectral analyses of the marine record of Pleistocene glaciations also show their most prominent peak at this wavelength , which has been equated with the Earth 1 s short cycle in excentricity (HAYS et al . , 1 9 7 6 ) . If the sPectra are correlated on that basis , then the subdivisions of the bundle into approximately f. ive units ( the major beds) match the precessional cycle with frequency peaks at 1 9 1 000 and 2 3 , 000 years . The spectral peak at about 5 0 , 000 years then matches the ob-
92
liquity cycle, and the low-frequency peak would poS� ibly correspond to the long cycle in eccentricity noted by BLYTT ( 1 8 8 6 ) , calculated by BERGER at 4 1 3 1 000 years , and observed in the Pleistocene record by BRISKIN and BERGGREN ( 1 9 7 5 ) . A first-approach model may be formulated on the assumption that carbonate sedimentation is interrupted at times o f insolation extre ma, let us say minima . The following experiment was carried out to test this : Daily, insolation was calculated for the vernal equinoxes at the equator for the last 20 million years using the BERGER ( 1 9 7 8 ) algorithm. This shows a 2 0 , 000 year periodicity whose amplitude is modulated by a wave of approximately 1 00 , 000 years , which re sults in a series of 11minima of minima " . A program was written to find all o f the insolation miriima , and 11minima of minima11 which occur at roughly 1 00 , 000 year intervals . Assuming that the 11minima of minima'' find expression as master bedding planes, they were then used as points o f reference for establishing the relative positions of the minima (= normal bedding planes ) . The results , smoothed by a running average o f 3 with weight 1 , 2 , 1 , are shown in Fig. 5 . The pattern is that o f a five-fold cyc.le which may occasionally contain more beds , and is quite similar to the bedding plane position dia grams shown : especially to that of the Scaglia Bianca .
!i
We have already stated our hypothesis that cycles of the s ort ob served in the geological record are related in various ways to cli matic change . We do not wish to claim that insolation at the e'qua tor is directly responsible for turning carbonate deposition on and o f f , but view it as an index to general climatic · effects induced by orbital perturbations . Resemblance of the insolation curve to bed ding patterns is not proof of a causal relationship. It is consi stent, however , with the hypothesis that orbital perturbations in fluence climate , and climate in turn influences patterns of sedi mentation . Neither of these relations is likely to be simple and linear. 7 . Summary and Conclusions The Maiolica , Scaglia Bianca and Glencar limes tones show geome ·tri cally similar cycle s , seen in the field as a 11bundlii1g" of strata . These can be objectively described by converting the field data into diagrams of usequential variation in bed thickness " .
I·
Such di agrams do not resolve variations smaller than groups of beds . However, distinctive bedding planes or strata may be used to break
93
the depositional record into its cyclic segments , whose s tructure may then be compared . Bed counts offer the simplest approach , but statistical distribution of bedding planes , expressed in 11relative position11 diagrams , was found to be a better way of characterizing the structure of the cycles . Although the mechanism responsible for the cyclical interruptions of carbonate deposition are obscure and multiple, they are most like-· ly related to climate . This , in turn , is influenced by the Earth ' s orbital perturbations . Three and perhaps · four quasiperiodicities were found in the sedi mentary record, by means of spectral ·analysis of the sequential bedding curves and by the structure of the bundles . These have approximately the ratios to each other that characterize the quasi periodicities in the orbital perturbations . The actual period of the sedimentary cycles , while not accurately known, suggests 1 06., 000 years for the 11bundle " of beds , which would equate this with the short cycle of orbital eccentricity. This corre lates the main beds of the cyclic sequence with the Earth 1 s 1 9 1 0002 3 , 000 years precesSion. The power spectra further show what then appears as an expression of the 4 3 ,000 year cycle in obliquity, as wr ll as a low-frequency peak that may well correspond to the 4 1 3 , 000 year long Cycle of eccentricity . These results correspond to the patterns of Pleistocene climatic fluctuations a�d their effects on Pleistocene geochemistry arid faunas . We conclude that orbitally driven variations in climate were recorded in limestones arid perhaps in other sensitive sedimentary settings through the latter half of Phanerozoic time . As suggested by GILBERT { 1 900) the resulting cycles may indeed constitute a pulse which, once better understood, wi l l afford a basis for a re fined geochronology and for a better understanding of how the outer Earth functions . Acknowledgement The field s tudy of the Glencar limestone was made by SCHWARZACHER some years ago . The observations in Umbria were made jointly , with the additional help of Ms . Amanda PALMER , and under the sponsorship of National Science Foundation project EAR-77- 2 3 36 9 . Measurements and processing are by SCHWARZACHER, and for help in the field we are obliged to Pro f . Giovanni NAPOLEONE .
94
References ARTHUR, M . A . ( 1 9 7 7 ) : Sedimentology of the Gubbio sequence and its bearing on Paleomagnetism . Mem. Soc . Geol . Ital . J�, 9-20 . ( 1 9 79 a ) : NOrth Atlantic Cretaceous black shales : the record at Site 398 and a brief comparison with other occurrences . In RYAN , W. B. F . , SIBUEJ, J . C . , et a l . , Initial Reports of the Deep Sea Drilling Project 4z, p t. 2 . •
( 1 979b) : Sedimentologic and geochemical studies of Cretaceous and Paleogene pelagic sedimentary rOcks : The Gubbio sequence . Dissertation, Princeton Univers ity , Part I , 1 74 pp . ARTHUR, M . A . and FISCHER, A . G . ( 1 9 7 7 ) : Upper Cretaceous magnetic stratigraphy at Gubb i o , Italy: I , lithostratigraphy and sedimen tology . _Geol . Soc. A.rner . Bul l . �§: , p . 3 6 7-37 1 . BERGER , A . L . ( 1 9 7 8 ) : A simple algorithm to compute long-term ·varia tion of daily or monthly insolation. Universit8 Catolique de Louvain , Ins t . d ' Astronomie et de Geophysique George Lemaitre , Contrib . ;),§,, p . 1 - 1 7 . ( 1 9 86 ) : The Milankovitch astronomical theory o f paleoclimates : a modern r.e view . - Vistas in Astronomy , v . ,€4 , p . 1 03 - 1 2 2 . FISCHER, A . G . ( 1 9 6 4 ) : The Lofer cyclothems o f the Alpine Triass i c : i n MERRIAM, D . F . , ( ed . ) , Symposium on Cyclic SedimentatiOn , Kan sas State Geol . Surv. Bul l . Jg�, 1 , 1 0 7 - 1 5 0 . ( 1 9 7 5 ) : �idal deposits , Dachstein Limes.tone of the North-Alpine Trias s i c . In GINSBURG, R. N . , ( ed . ) ; Tidal Deposits : a casebook of Recent examples and fossil counterparts. Ch . �Z p . 2 35-2 4 1 . ·
( 1 9 80 ) : Gilbert - Bedding rhythms and geochronology . Geol . Soc. Amer . Spec. Paper j�� ' p . 9 3- 1 04 . ( 1 9 8 1 ) : Climatic oscillations in the biosphere. In NITECKI , M . H . ( ed . ) , Biotic Crises i n Ecological and Evolutionary Time . Acade mic Press , p . ·1 0 3- 1 3 1 . and ARTHUR, M . A . ( 1 9 7 7 ) : Secular variations in the pelagic realm. In COOK , H . E . , and ENOS, P . (eds . ) , Deep-water carbonate environ ments , Soc . Econ. Paleont. Mineral . Spec. Pub . g� , p . 1 9 -50. GARRISON-, R . E . ( 1 96 7 ) : Pelagic limestones of the Oberalm beds ( Upper Jurassic - -Lower Cretaceous ) , Austrian· Alps . Bul l . Canad . Petro l . Geol . v . ,1� , p . 2 1 -4 9 . and · FISCHER, A . G . ( 1 9.6 9 ) : Deep water limestones and radiolarites of the Alpine Juras s i c . Soc . Econ . Paleont. Mineral . Spec . Publ . :1,4 , p . 20-55 . GILBERT, G . K . ( 1 89 5 ) : Sedimentary measurement of geologic time . Jour. Geol . � � 1 2 1 - 1 2 7 . --- ( 1 900) : Rhythms and geologic time . Amer . Assoc. Adv . Sci . , Proc �� 1 - 1 9 .
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HALLAM , A . ( 1 9 64 ) : Origin o f the limestone-shale rhythms in the Blue Lias of England: A composite theory . Jour . Geol � J:€. ' 1 57 - 1 69 . HAYS , J . D . , IMBRIE, J . and SHACKLETON, N . J . ( 1 9 76 ) : Variations in the earth ' s orbit : Pacemaker of the ice ages . Science 12� , 1 121-1 132. HSU, K . J . ( 1 9 7 6 ) : Paleoceanog�aphy o f the Mesozoic Alpine Tethys . Geo l . Soc . America , Spec. Paper 1 701 4 4 pp .
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- MILANKOVITCH, M. ( 1 9 4 1 ) : ' Kanon der Erdbestrahlung und seine Anwendung _ auf das Eiszeitenprohlem . E d . Spe c . Acad . Roy . Serbe , Belgrade , v . JJJ, 6 3 3 pp . English transl . U . S . Dept. Commerce . OBRADOVITCH, J . D . and COBBAN , W . A. ( 1 97 5 ) : A time scale for the Late Cretaceous of· the Western Interior of North America. In CALDWELL , W . G . E . {ed. ) , The cretaceous System in the Western Interior of North America. Geol . Assoc. Canada Spec. Paper 1J_, -p . 3 1 -5 4 . PREMOLI SILVA, I . , PAGGI , L. , and MONECHI , S . ( 1 9 7 7 ) ; Cretaceous thro"ugh Paleocene biostratigraphy of the pelagic sequence at Gubbio , Italy . Mem. Soc . Geo l . Italy v . l� ' p . 2 1 -3 2 . SANDER , B . ( 1 9 36 ) ; Beitrage zur Kenntnis der Anlagerungsgeflige. Mi neral . Petrogr. Mitt. v . �� , p . 2 7 - 1 39 . --- ( 1,_9 5 1 ) : Contribution to the s tudy of depositional fabrics . Amer . Assoc. Petro l . Geol . , Tulsa, Oklahoma . SCHWARZACHER, W . ( 1 9 5 4 ) : Die GroBrhythmik des Dachsteinkalkes von Lofer . Mineral . Petrogr . Mit t . v . � , p . 44- 5 4 . ( 1 9 5 8 ) : The stratification o f the Great Scar Limestone i n the Settle District of Yorkshire . Liverpool and Manchester Ge6 1 . Jour . , v . £ 1 p . 1 24 - 1 4 2 .
( 1 9 64 ) : An application o f statistical time-series analysis to a liffie stone-shale sequence . Jour. Geol . , v . z�, p . 1 9 5- 2 1 3 .
( 1 975 ) : Sedimentation Models and Quantitative Stratigraphy . De velopments in .Btratigraphy 1 v . J,� , Elsevier , 3 8 2 pp . ( 1 9 8 1 ) : Quantitative correlation of a cyclic limestone-shale formation. Proc . Internat . Assoc . Mathema t . Geol . '· Symposium on Quantitative Con;elation, in press . SUJKGWSKI , Z . L . ( 1 9 5 8 ) : Diagenesis . Amer . Assoc. Petro l . Geo l . Bul l . , v . ii • p . 2 6 9 2 - 27 1 7 . VAN HINTE, J . E . ( 1 9 7 6 ) : A Cretaceous time scale . Amer . Assoc. Petrol . Geo l . Bull . , v . �Q , p . 4 9 8 -5 1 6 . VAN HOUTEN , F . B . L1 8 6 4 ) : Cyclic lacustrine sedimentation, Upper Triassic Lockatong Formation , New Jersey and adj acent Pennsylva n i a . Kansas Geo l . Surv. Bull . J,�� � p . 4 9 7-5 3 1 . WEZEL, F . C . { 1 9 7 9 ) : The Scaglia Rossa Formation of Central Italy : results and problems emerging from a regional study . Ateneo Parmens·e 1 Acta Nat . , v . J, � , p . 2 4 3 - 25 9 .
Rhythmic Sedimentation Documented in a Late Cretaceous Core (Abstract) L. PRAIT
Abstract : A continuous core taken through the Bridge Creek Limestone and Hartland Shale is the basis for a study of rhythmic in -l ate Cenomanian - early Turonian deposits in the Western Interior Basin of North America . The core has been cut in half , polished, and peeled, thus providing a detailed record of the large-scale bioturba tion rhythms , small-scale lamination rhythms , current events , and volcanic ash falls that occur in 30 meters of the section. Alterna ting beds of burrowed micritic limestone and laminated organic-rich marls characteristic of the Bridge Creek Limestone are well-suited to .the study of rhytmic stratal events and can be interpreted in terms of changing oceanographic conditions . The broad spectrum of sediment tYpes and sedimentation events documented in the peels indi cate that this Cretaceous sea oscillated between brackish and marine states, ·s tratified and well-mixed states, and between anaerobic and aerobic bottom conditions . The laminated marls consist of 8 major sediment types : 1 ) pelletal marl s , 2 ) pelletal and foraminiferal marls , 3 ) homogeneous ( bioturba ted) marls , 4 ) black b ituminous shale1 5 ) foraminiferal calcarenites, 6} calcarenites of foraminifera and inoceramid fragments , 7 } bone beds of fish debris , 8 ) volcanic ash bed s . Other features of sedimen- ,· tological importance i-n clude lumps of poorly sorted shell fragments and foraminifera that are probably coprolites produced by fishes and reptil e s , bedding-concordant and bedding-discordant inoceramid frag ment s , pyrite framboids , massive pyrite, and fish bone s .
..·,
�.. .
Diagenetic processes of 90mpaction, recrystallization, and neo formation have affected each microfacies differently. Foraminiferal chambers provide sites for many diagenetic reactions that can be studied with light and scanning electron microscopy , common fillings in the chambers being b locky calcite, dog-tooth calcite, clay crystals , and framboidal pyrite .
l. f I I
t�'
Cyclic and Event Stratification (ed. by Einsele/Seilacher) c Springer 1982
Ecology and Depositional Environments of Chalk-Marl and Limestone-Shale Rhythms in the Cretaceous of North America (Abstract) E. G. KAUFFMAN
Abstract : Chalk-marl and limestone-shale couplets less than a meter ' ·thick widely characterize pelagic Cretaceous sediments of the Western rnterior and Gulf Coastal Plain, United States and northern Mexico. These rhythmic depOsits are best developed during transgressive (eu static) peaks between Late Aptian and earliest Campanian time . In dividual couplets can be traced for hundreds of k ilometers , and from offshore epicontinental and marginal continental platforms into basi nal facie s , where they broadly grade into clay shales . Shallow and deep water couplets are similar in many ways . The�r cyclicity ranges between 30 and 50, 000 year s . measured radiometrically , suggesting climatic cycles . Most have graded lower contacts between shale and carbonate, but sharply defined boundaries on top of · carbonate units ; in some cases these are eroded and/or partially cemented surfaces (firmgrounds) . In each couplet, clay content diminishes upward as mainly biogenic pelagic carbonate increases . Many lines of evidence suggest diminishing sedimentation rate , and decreasing water content of the sediment , from the bottom to the top of each cycle. Marine varves consisting mainly of alternating clay enriched and coccolith enriched bands a nun or .less thick are variably preserved in the rhythms , and best studied in offshore deep basinal facies . Ecblogically restr i cted benthic faunas characterize these rhythms , with abundance and diversity decreasing offshore and beilthic inverte b iates disappearing completely in certain basinal settings . Environ mental constraints on benthic colonization are substrate texture and fluidity, turbidity of benthic water s , temperature as a reflection of depth , and especially sediment and benthic 1-1ater chemistry (o , H 2 S ) and carbonate dissolution potential . The principal macro 2 zoobenthos were burrowing polychaete worm and arthropods , big epi faunal bivalves of the families Inoceramidae and Ostreidae, · small epi- and endobionts on these shells (worms , sponges , bivalves , bryozoans , barnacles , etc . ) , and saltating epibenthic ammonite s . Ecological giadients can b e established both spatially (onshore offshore depletion and change in community st�ucture} and temporally for each cyclothem . Onshore - offshore gradients seem to be mainly oxygen and temperature related , and involve , fi i s t , loss of more specialized epibenthic and infaunal mollusks and- echinoderms , follo wed by loss of shell epibionts and specialized burrowing invertebra tes , followed by loss of inoceramids and all burrowing arthropods , and finally by reduction and loss of detritus-feeding polychaetes . Temporal changes involve upward increase in density and diversity of burrowers with decreasing turbidity . and water content of the sediment, followed by w idespread colonization of the substrate surface by ino ceramids and their epi- and endob ionts, and terminating at the top of some cycles by development of firmgrounds and local hardgrounds with their characteristic encrusting and boring biota. Alternating cool/wet and warm/dry cl imatic cycles best account for temporal changes within chalk-marl and limestone-shale rhythms . Cyclic and Event Stratification {ed. by Einsele/Seilacher) © Springer 1982
Diagenetic Redistribution of Carbonate, a Process in Forming Limestone-Marl Alternations (Devonian and Carboniferous, Rheinisches Schiefergebirge, W W. EDER
Abstract: Rhythmically developed l imestone-shale alternations in the Devonian and Carboniferous of the northern part of the Rheinisches Schiefergebirge reveal certain features which cannot be explaned in terms of sedimentary transport alone . Three different paleozoic environments are interpreted as pro duced (Letmathe sequence) or at least modified ( Helle sequence , Edelburg sequence) by early diagenetic carbonate migration. Re distribution of carbonat e , specific to the local environment , Could develop differences in an originally homogenous body of marly sediment. . This process starts with carbonate solution ( probably initiated by bacterial activity) within the uppermost 1 0 em of sediment, and is followed by subsequent reprecipitation of the same material immediately below sea floor . The newly cemented sediment-layer remains largely unaffected during a latter phase of carbonate-solution because of its coarser grain size .
1 . Introduction This paper presents the interpretation of some selected Palaeozoic limestone-shale cycle'S of the northern part of the Rheinisches Schiefergebirge ( F i g . 1 ) . I t is not intended to give a comprehensi ve accOunt of the world-wide phenomenon of wel l-bedded limestone shale rhythms , though it contains some general suggestions . Rhythmically or cyclically developed , regularly bedded l.imestones , the s O-called Plattenkalke ? f the Rheinisches Schiefergebirge, in general are s ituated between the sites of major production of car bonate - represented by reefs - and the "deeper" water areas where
This work forms part of the research program of the . Sonderfor s chungsbereich 48 " Entwicklung , Bestand und Eigenschaften der Erd kruste , insbesondere der Geosynklinalraurne 11 , University of G6t tingen (Federal Republic of Germany ) . Funds for this work were provided by the Deutsche Forschungsgemeinschaft (German Research Society) . Cyclic and Event Stratification (ed. by Einsele/Seilacher) © Springer 1982
�0
99
r
Fig. 1 . Occurrence of Devonian carbonate complexes in the Rhein ische s Schie ferge birg e with loca tion of the investi gated sections Let mathe , Helle , and Edelburg
Fig. 2. Palaeogeographical and environmental reconst1:-uction of the investigated sections : ( 1 ) " Schleddenhof Beds" near Letm.ithe ; subtidal inter-reef l imestone sequence of Upper Givetian age ( 2 ) "Garbecker Kalk" near Helle/Balve ; near-reef l imestone turbidites of Upper Givetian age ( 3 ) " Kulm-Plattenka lk" of Edelburg-quarry near Herner ; basinal l imestone turbidites of Lower Carboniferous age
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detrital carbonates ' accumulate. Often the s equences can be inter preted as limestone turbidites or allodapic limestones (MEISCHNER, 1 9 64 ) , but obviously various sedimentary and diagenetic �rocesses have played a part in their development .
Many of the coarse detrital calcareous turbidites show a fine-grained layer of microspar i te. This one to several centimeters thick layer is firmly attached to the overlying detrital b ed . MEISCHNER ( 1 9 6 4 ) called i·t "Nullphas e 11 , ''0-Phase 11 , or "Prephase " and suggested later (SEILACHER & MEISCHNER1 1 9 6 5 i MEISCHNER1 1 9 67 ) that diagenetic processes are responsible for the development of this layer . Studies of near-reef small-scale turbidites ( 11Garbecker Kalk11 , EDER, 1 9 70 , 1 9 7 1 ) confirm the view that the ''basal layeru ( equivalent to the "O-Phase" ) is a secondary product- of diagenetic redistribution of carbonate in solution. A common aspect of many shallow water sequences of Middle Devonian near-reef limestones i s that bedding is not alWays controlled by environmental changes . Conventionally , bedding planes are interpre ted as correlated to periods of non-depos ition or abrupt changes in the sedimentary delivery (CAMPBELL , 1 9 6 7 ) . But a cooperative project of the Sonderforschungsbereich 4 8 , comparing the sedimentological
and geochemical features ( S - , c - , and 0-isotope ratios , total car b onate- and Sr-content) of various limestone-marl rhythms , leads to . the suggestion that diagenetic processes play a part in forming re gularly bedded li�estone-marl successions .
2 . Geological Setting and Observations The area of study comprises the "Massenkalk" -region of the Remscheid Altena-Anticline ( F i g . 1 ) . Three examples of near-reef or off-reef depositional environments .( F i g . 2 ) , differing in their geological setting and age, have been investigated: "Sch leddenhof Beds " , "Garbecker Kalk " , and "Kulm-Plattenkalk u . 2 . 1 . 11S chleddenhof Beds11 of Letmathe ( Fig . 3 ) Geological setting: Northern and topmost part of the Hagen-Iserlohn Massenkalk (KREBS , 1 97 1 ) of Upper Givetian age; abandoned quarry east of Letmathe (Le , R 3 4 04 1 0 , H 5 6 9 3 20) , transition from reef-deposits to "deeper11 water environment.
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bed thickne5s
Fig , 3. Idealized example of the l imestone-marl sequence near Letmathe : - Alternation of dm-thick , bedded autochthonous shallow water limestones and calcareous shales Biogenic constituents , vagile and sessile benthos , are enriched in the " shales" ( light band) - Total carbonat_e cont�nt : 90 - 9 9 % in limestones , 80 - 90% in " shales'1 Insoluble residue : quartz , illite , pyrite - Frequency distribution of bed-thickness
· - ·.
Fig. 4 . s - , c- , and 0-istotope ratios from selected beds of the Letmathe section. No sig ni ficant di fferences between limestone and "shale 11
Observations : Alternation of dm-thick, autochthonous shallow water limestone beds and calcareous shales . Total thickness ca. 30 m . Gradual transitions from limestone bed to calcareous shale. No erosional features , but thinning out of individual beds . Total carbonate content : 90 - 99 % i'n limestones , 80 - 90 % in 11 shales 11 • Insolub,le res idue : Quar t z , illite, pyrite. M.i crofacies : Float- and boundstone (or biomicrosparite) in l ime stones and calcareous shale s , dissolution of b iofragments in the 11ShaleS 11 parallel to bedding.
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Biogenic constituents : Vagile and sessile benthos ( corals , stromato poroids , amphiporoids , brachiopods , gastropods , trilobites , sponges , bryozoa, algae, forams , and abundant crinoid ossicles ) , the fossils ar:e enriched in the "shale11 (up to 1 0 times ) . s-,
C-,
and 0-isotopes : no significant difference between lime
stone and 11Shale11 in bulk samples (Fig. 4 ) .
2 . 2 . ''Garbecker Kalk11 of Helle ( Fig. 5 ) Geological setting: SOuthern part of the Balve-Massenkalk (KREBS , 1 9 7 1 ) of Upper Givetian agei abandoned quarry .at. Helle/Balve ( H e , R 34 2 1 56 , H 56 89 7 2 ) , pelagic near-reef sedimentation influenced episodically by small-scale limestone turbidites (EDER, 1 9 7 0 , 1 9 7 1 ) . Observations : Alternation of dm-thick , richly fossiliferous detrital limestone beds and calcareous shales . Total thickness c a . 35 m . Switch from autochthonous t o allochthonouS sediment observable within single beds . 4 zones ( shale , basal layer , detritus , up per layer) distinguishab l e . Total carbonate content: 90 - 9 9 % in limestones , 7 0 - 83 % i n Sr/Ca-ratio x 1 000: 2 - 3 in the limestones , c a . 1 in 11shales" .
HELLE
GARBECKER K A L K , Givet
bed thickness
Fig . 5 . Idealized example of the limestone-marl sequence near Helle: - Alternation of dm-thick , bedded limstone turbidites with fine grained basal layer-beds and calcareous shales - Total carbonate content : 90 - 9 9 % in limestones , 70 - 8 3 % in - Sr/Ca-ratio x 1000: ca . 1 in "shales " , 2-3 iri limestones Frequency distribution of bed-thickness
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Mic rofacies : Wacke- or floatstone (or ·biomicrospcirite) in the fine-grained limestone zones and. the "shales " , wacke- or pack stone (or biomicrosparite) in the detrital zone. s- ,
c-,
and 0-isotopes : Allochthonous detrital zone : isotopically
"heavy " , autochthonoUs zones ( shale , basal layer and upper layer ) : isotoP i cally " l ight11 (Fig. 6 ) .
upper layer
detritus basal layer
� @, ��
shale
Fig. 6 . S - , c- , and 0-isotope ratios and mean Sr-contents of a se lected bed o f the Helle. section : Autochthonous shale , the basal layer and the upper layer are " isotopically" light, al lochthonous detritus is isotopically "heavy11
2 . 3 . 11Kulm-Plat::t enkalk11 of Edelburg ( F i g . 7 ) Geological setting: Outer part (maximal to distal ) o f the Upper Vis E: an "Kulm-Plattenkalk" (MEISCHNER , 1 9 6 4 ) ; abandoned quarry Edelburg east of Herner (Ed, R 34· 1 6 50 , H 56 9 7 00) , basinal limestone turbidites . Observations : Alternation of em- to drn-thick calcareous turbidites with pelagic shales1 sharp trans ition from shale to limestone. Total carbonate content: 8 5 - 9 8 % in l imestones , 30 - 5 5 % in sha les . Microfacies : Wackestone (or biomi�rosparite) in the shale and "0-Phase " , pack- and floatstone (biomicrosparite) in the alloch thonous detrital zone. S- , C- , and 0-isotopes : Allochthonous detritus : isotopically "heavy u , autochthonous shale and 110-Phas e " : 11 l ight11 (Fig . 8 ) .
1 04
Fig . 7, Idealized example of the limestone-shale sequence of - Alternation of em to dm-thick, bedded limestone turbidites with 110-phase" (basal layer) and pelagic shales - Total carbonate conten t : 85 - 9 8 % in the turbidites and the basal layer , 30 - 5 5 % in shales - Frequency distribution of bed-thickness
8%o 345
� shale
detritus 0- [lhase 0'
basal Ia er shale
6 %0 13( O%Xl18Q
�
� @ Gb
--
& @ @b � @@
F i g . 8 . s - , c- , and tope ratios of a bed o f Edelburg : Au.toch·' thonous shale and basal l ayer are characterized relatively ratios , the de-tritus is "heavy11
3 . Discussion The sedimentological , paleoecological, and geochemical s tudy of the three sections has shown that , except for its total carbonate con tent, the l imestones of Letmathe , the "basal" and "upper" layer of Hel l e , and the 110-Phase11 of Ede lburg , have a wide range of features
105
in common with the interbedded marls or shales . Especially the iso topic composition of the calcareous shales and the autochthonous limestone beds of all three s equences are more or less identical . This leads to the assumption that the lithological differences bet ween s'hales and fine-grained limestones in the investigated sequen ces are produced or at least intens if.ied by an early diagenetic car bonate migration or redistribution. The analysis of total carbonate content and bed-thickness of the L7·tmathe sequence shows no recognizable correlation between bed thickness and carbonate content (Fig. 9 and 1 0 ) . Similar observa tions' have been made by FLUGEL et a l . ( 1 97 1 ) , who studied Lower Permian beds of the Carnic Alps . This contrasts to investigations of limestone-marl cycles of the Upper Jurassic in southern Germany where SEIBOLD { 1 9 5 2 ) and BAUSCH ( 1 9 80) have shown that the carbonate content of a limestone bed is proportional to its thickness . It was inferred {SEIBOLD , 1 95 2 ) that the cycles were produced by periodic variations of calcium carbonate precipitation during a constant "background 11 of C lay deposition. Another result of FLUG�L et a l . ( 1 9 7 1 ) i s that not all bedding planes of their investigated section are due to primary variations in fossil content or partical s i Z e . Some bedding planes in the '
limestone rhythm could not be correlated to periods of abrupt changes in sediment supply or to periods of non-deposition. The same i s generally true for the Letmathe section, where no sedi mento logical or pa leoecological changes can be detected at the limestone/marl interface . Primary pulsat.ions of the carbonate- or clay-supply do not seem to provide a plausible explanation for the l imestone-marl rhythms . Investigators of P lattenkalk-sequences often noted that the density of calcareous biogenic constituents per unit volume of matrix de clines with increasing carbonate content (SEIBOLD & SEIBOLD, 1 9 5 3 ; HALLAM , 1 9 6 0 ; NEUMANN & SCHUMANN , 1 9 7 4 ; this paper F ig . 3 } . The conventional explanation, a relatively enrichment of fossils due to a slower rate of sedimentation, is not confirmed by the results of our paleoecological and isotopic analysis , which suggest that sedi mentation and diagenesis of the shales and of the fine-grained l i mestones took place under the same or similar conditions . An alternative explanation may be that solution of micritic matrix in the marl took place and lead to a relative enrichment of bioge-
106
Le9 llcm
le 29 9cm
le 60/1 .15cm
Le 60/2
17cm
le 60/3 18cm
Le 60/4 14,5cm
Le 68/1
13cm
Le 68/2 16cm
Le 8
llcm
Fig. 9. Distribution of total carbonate content ( light) and insoluble residue ( stippled) of nine selected beds of the Letmathe section. Range of samples is indicated by black columns
107 30%
LET MATHE
100%
104 bod1
� 20
z 0 u
< z
£ � u
90
"
Bf0-THICKNf55
Fig . 1 0 . Frequency distribution of bed-thickness of the Letmathe section and range of total carbonate content of 1 2 selected beds (black lines ) . No correlation of bed-thickness and carbonate content nic constituents . Additionally , the marls might have undergone stronger compaction than the limestones ( HALLAM , 1 9 60 ) . A similar diagenetic interpretation for the genesis of nodular limestones is based upon data from recent marine s ediments . So for example · the theory of GRUNDEL & R6SLER ( 1 9 6 3 ) explaining the forma tion o f calcareous nodules i s mainly based on ideas of ILLIES ( 1 9 4 9 ) , who proposed that differences in pH- and Eh-values in the oxidation zone and lower reduction-zone within the sediment are responsible for carbonate solution (in the reduction-zone) and concentration ( in the oxidation-zone ) . The importance of the diagenetic migration of calcium carbonate ( s e� gregation) in forming regular cyclic alternations of shale and ar gillaceous limestones is also described by HALLAM ( 1 9 6 4 ) . The formation of recent . to subrecent carbonate hardgrounds or hard layers in nearly all marine environment.s has been summari z ed by BATHURST ( 1 9 7 6 ) . He argues that submarine diagenesis must be regar ded as important as meteoric diagenesis in the lithification of limestone s . Therefore w e propose a diagenetic . origin o f the described limestone marl sequence of Letmath e , and a diagenetic overprint of the calca reous turbidite-shale alternations of Helle and Edelburg .
108 4 . Genetic Model · of Limestone-Marl Alternation
It is supposed that diagenetic redistribution-processes , recognized . by MEISCHNER ( 1 9 6 7 ) in the deeper turbidite environment of the nisches Schiefergehirge, may also have been effective in shallower environment . Measurements on recent carbonaceous sediments indicate that the lo
west pH-values occur in the uppermost 5 em of the s ediment (SEIBOLD·, 1 9 6 2 ; PAUL , 1 9 70) . I t is suggested that a sort of chemical bedding within the sediment is responsible for two levels : A lower one with carbonate solution1 · probably initiated by bacterial activity (CHILINGAR et al . 1 9 6 7 ; HARTMANN & NIELSEN, 1 9 69 ; GOLUBI C & SCHNEI DER, 1 9 79 ) , and a higher one with carbonate reprecipitation imme
diately below the sea floor (Fig. 1 1 ) . Pore-water dissolving Ca- � Mg-, and Sr-carbonate migrates upwards , until saturation is achieved and carbonate i s reprecipitated as a calcite cemen t.
Fig . 1 1 . Model of diagenetic differen tiation in a carbonate mud : subsurface carbo'nate solution ( initiated probably by bacte rial activity) and carbonate reprecipitation within the uppermost 1 0 to 1 5 em of ' sediment Fig. 1 2 displays an idealized dynamic model for the origin of dia genetic bedding. While solution and reprecipitation takes place in the shallower subsurface , sedimentation is continued . The zone of solution, depleted of carbonate , undergoes stronger compaction. Solu tion and compaction lead to a relative enrichment of fossils and in solubles . As sedimentation goes on1 the front of bacterial activity (possible sulfate reduction) and concomitant carbonate solution migrates upwards in the column affecting newly deposited carbonate matter . The dissolut ion/reprecipitation cycle is repeated . Carbonate precipitated during earlier cycles ( now buried) remains largely un affected because of its coarser grain-s ize .
109
�··
Fig. 1 2 . Dynamic model : continuous sedimentation of carbonate mud, subsequently overprint.ed by carbonate solution and reprecipitation. Homogenous mud acquires regular diagenetic bedding . Density of stippling is inversely proportional to carbonate content
5 . Interpretatiqn of the Investigated Sections Letmathe : The carbonate sediment, primarily more or less homogeneous , acqui�ed bedded character as a result of migration or s egregation of carbonate during early diagenesi s . This process possibly is controlled by bacterial activity ( sulfate reduction) in a partly "open system" of the uppermost layer of the sediment (NIELSEN, 1 9 79 ) . Helle: The deposition of au.tochthonous marl was interrupted by episo dic influx of calcareous turbidites . Early diagenetic redistribution of carbonate differentiated the autochthonous marl into a calcareous shale and a basal layer . The segregation of carbonate possibly was controlled by bacterial activity. Solution and reprecipitation of carboncite took place under partly 11open system11 conditions (NIELSEN , 1 9 7 9 ) within the uppermost 1 0 em of the sediment. The overlaying turbidite (detrital zone) remains isotopically " heavy 11 , because cementation took place under 11closed system" conditions .
1 10
Edelburg : The primary alternations ( allochthonous detritus 1 nous shale) were overprinted and to some extent obscured by ry , diagenetic car�onate migration possibly controlled by bacteria l ac.tivity as suggested for the Helle-sequence .
·
6 . Conclusions The hypothesis proposed here suggests that regularly bedded lime stone-marl alternations., apparently cyclic or rhythmic , in many cases (deep and shallow water carbonate environments) may be due to .e arly diagenetic redistribution of carbonate rather than to va riations in the primary delivery of materi a l . The supposed diagenetic redistribution process may a l s o have been effective in the Muschelkalk (see AIGNER, this vel . ) or in the Ju rassic P lattenkalk sections (KEUPP, 1 9 7 7 ; RICKEN & HEMLEBEN, this vol . ) or in the paleozoic sequences of Ireland (see WALTHE R , this vol . ) or the Alps ( FLUGEL et a l . , 1 9 7 1 ) . The controlling parameters of the proposed redistribution process are thOught to be local rather than global . Ac;knowtedgemen ts I feel very indepted to Prof . Dr . D . MEISCHNE R , who initiated and s t imulated the study of sedimentary and diagenetic processes in the Rheinisches Schiefergebirge. I would like to express my thanks to my colleages and friends Dr . w. ENGEL and Dr . W. FRANKE, who were incorporated in the field and brain work 1 and Prof . Dr .. J . HOEF S 1 Dr . H . NIELSEN, Dr . A . SCHNEIDER and Prof . Dr . H . WEDEPOHL for the'i r generous help with the isot�pe studies . H . GRIMME and B .· RAUFEISEN carried out the drawings . Last not least I want to thank the Deutsche Forschungsgemeinschaft (DFG1 "German Research Society) , who financed the investigations of our group in the "SFB 4 8 - Erdkruste " . References BATHURST1 R . G . C . ( 1 9 7 6 ) : Carbonate s ediments and their diagenesis , 2nd edn . , Elsevier1 Amsterdam Oxford New York . BAUSCH , W . M . ( 1 980) : Tonmineralprovinzen in Malmkalken . Erlanger Forsch . B , Naturwiss . u . Medizin � CAMPBELL, c . v . ( 1 9 6 7 ) : Lamina , Laminaset and Bedset. Sedimentology 1! = 7-2 6 .
111
G . V . ,. BISSELL, H . J . , WOLF , K . H . ( 1 9 67 ) : Diagenesis of carbonate rocks . I n : LARSEN, G . , CHILINGAR , G . V . ( eds . ) , Diage nesis in Sediments . - Developm. Sedimentol . � : 1 79 - 3 2 2 · Elsevier.
EDER1 w . ( 1 9 70 ) : Genese Riff-naher Detritus-Kalke bei Balve im Rheinischen Schiefergebirge (Garbecker Kalk) . Verh. Geol. B . -Anst . , J�ZQ= 5 5 1 -56 9 , Wien.
EDER , w. ( 1 9 7 1 ) : Riff-nahe detritische Kalke bei Balve im Rheini schen Schiefergebirge (Mittel-Devon , Garbecker Kalk) . - GOttin ger Arb . Geol . Pal&ont . , JQ · FLtJGEL, E . , HOMANN , W . , TIETZ , G . F . ( 1 97 1 ) : Litho- und Biofazies eines Detailprofils in den Oberen Pseudoschwagerinen-Schichten (Unter-Perm) der Karnischen Alpen . - Verh. Geo l . B . -Anst . 1�11: - --10-42 , Wien. GOLUBIC , S . , SCHNEIDER, J . ( 1 9 7 9 ) : Carbonate Dissolution. I n : TRUDINGER, P . A . , SWAINE , D . J . ( eds . ) , Biochemical cycling of mineral-forming elements. -Studies in Environffi. Science 1, Elsevier, Amsterdam Oxford New York : 1 07- 1 29 . GRUNDEL , J . , ROSLER, H . J . ( 1 9 6 3 ) : Zur Entstehung der oberdevonischen Kalkknollengesteine ThUringens. Geologie J� : 1 009 - 1 03 8 . HALLAM , A _. { 1 960} : A sedimentary and faunal study of the Blue Lias of Dorset and Glamorgan . - Phi l . Trans . Roy. Soc . London , Ser. B , 2 4 3 : 1 -4 4 . HALLAM , A . ( 1 9 6 4 ) .: Origin of the l imestone-shale rhythm in the Blue Lias of England: a composite theory . - J . Geo l . �� : 1 57 - 1 6 9 . ===
HARTMANN, M . , NIELSEN, H . ( 1 9 69 ) : Delta 3 4 S-Werte i n rezenten Meeres sedimenten und ihre Deutung am Beispiel einiger Sedimentprofile aus der westlichen Ostsee . - Geo l . Rundsc h . �� : 6 2 1 -6 5 5 . ILLIES , H . ( 1 9 49 ) : Die Lithogenese des Untereozans in Nordwestdeutsch i and._- Mitt.. Geo l . Staats ins t . Hamburg J,� : 7 - 4 6 . _ KEUPP , H . { 1 9 7 7 ) : Ultrafazies und Genese der Solnhofer P lattenkalke ( Oberer Malm, Slidliche Frankenalb) . - Abh . Naturhist. Ges . Nlirn berg, ��
KREBS , w. ( 1 9 7- 1 ) : ·Devonian Reef Limestones in the Eastern Rhenish Schiefergebirge . I n : MULLE R, G . ( ed . ) Sed imentology of parts of Central Europ e , VIII Int . Sed . Congr . 1 97 1 , Heidelberg, Guidebook to Excursions : 4 5 -8 1 . MEISCHNER, K . D . ( 1 9 6 4 ) : Allodapische Kalke , Turbidite in Riff-nahen Sedimentations-Becken . I n : BOUMA , A . H . , BROUWER , A. (eds . ) Turbi dite s . - Developm. Sedimentol . � ' Elsevier: 1 5 6- 1 9 1 . MEISCHNER, D . { 1 9 6 7 ) : PalOkologische Untersuchungen an gebankten Kalken - Ein Diskuss ions-Beitrag . - Geol . FOren. StOckholm FOrh. Jl� : 465-469 . NEUMANN , N . , SCHUMANN , D . ( 1 9 7 4 ) : Zur Foss·ilerhaltung1 besonders der Goniatiten in roten Knollenkalken vern "Arnmonitico Rosso11-Typ . · N . Jb . Geol . Palaont. Mh. J��� : 2 9 4-3 1 4 . NIELSEN, H . { 1 9 7 9 ) ! Sulfur I sotopes . I n : JAGER, E . , HUNZIKER, J . C . { eds . ) Lectures in Isotope Geology. Springer, Berlin Heidelberg: 2 8 3 -3 1 2 . PAUL , J . ( 1 970) : Sedimentgeologische Untersuchungen im Limskikanal und vor der istrischen Ktiste (nOrdliche Adria) . - GOttinger Arb- . Geol . Palaont. � SEIBOLD , E . ( 1 9 52 ) : Chemische Untersuchungen zur Bankung im unteren Malm Schwabens . N . Jb . Geol . Palaont. Abh . �g : 3 3 7 - 3 7 0 .
112
SEIBOLD , E . ( 1 96 2 ) : Untersuchungen zur Kalkfallung und KalklOsung am Westrand der Great Bahama Bank . - Sedimentol . J : 50- 7 4 . SEIBOLD , E . , SEIBOLD , I . ( 1 9 5 3 ) : Foraminiferenfauna und Kalkgehalt eines Profil im gebankten unteren Malm Schwabens . - N . Jb . Geol . Palaont. Abh . �� : 2 8 -8 6 . SEILACHER, A . , MEISCHNE R , D . ( 1 9 6 5 ) : Fazies-Analyse im Palaozoikum des Oslo-Gebietes . - Geol . Rdsch. �� ( ftir 1 9 6 4 ) : 596-6 1 9 .
A Contribution to the Origin of Limestone-Shale
Sequences
M. WALTI!ER
.
Abstract: Layered limes tones of the Sligo syncline in NW Ire land . indicate that the limestone-shale alternations are largely of diagenetic origin. The centre of limestone layers generally consists of largely pure calcium ca�bonate . This light-grey lime stone is often enveloped by chert. The oU ter parts of the layers are composed of dark-grey limestone with an increasing dolomite content towards the margins . Metastable components of modern lime muds are aragoni te , bioge nous silica, and Mg-calcite . In this order they are dissolved and precipitated in the pore space of the mud forming limestone layers with the internal structure described above .
. Introduction A 7PO-m section of Lower Carboniferous Limestone ( S 2 - D1 ) has been studied in coaStal ou-tcrops about 1 5 km
NW
of Sligo in NW Ireland .
The fossiliferous limestones were deposited in an open marine, shal low water environment . The sequence consists of more or less regu larly alternating nodular limestone layers (mostly micrite) separa ted by shale . A statistical time-serieS analYsis of parts of the section about 1 0 krn eastwards did not result in any definite me
chanism for the cha�ge between periods of limestone and shale depo sition ( S CHWARZACHER, 1 9 6 4 ) . Limestone-shale alternations are relatively common world-wide in any stratigraphic system. Most previous workers have related these
alternations to successive periods of limestone and shale depos � tion
~
which may have been controlled by various factors e . g . climatic cycles (BRUCKNE R , 1 9 5 1 , 1 9 5 3 ; BJ¢RLY�KE , 1 9 7 3 ) , permanent changes between shallow water and deep water environments (KREBS , 1 9 6 2 ) , turbidi�es (MEISCHNER, 1 9 6 4 ) , lateral facies migrations (B5GER,
1 9 6 6 ) , periodical C0 -increases ( FLUGEL & PENNINGER , 1 9 6 6 ) , or cycli 2 cal fluctuations of the microplancton productivity (GARRISON, 1 9 6' 7 ) . Cyclic and Event Stratification ( e d . by Einsele/Seilacher) © Springer 1982
1 14 Others pointed out the role of secondary carbonate migration tuating primary inhomogeneities ( HALLAM, 1 9 6 4 ; SEILACHER & 1 9 6 4 ; MEISCHNER, 1 9 6 7 ; OLDESHAW & SCOFF I N , 1 9 6 7 ; EDER, 1 9 7 1 ; & AMSTUTZ , 1 9 7 9 ) or · the possibility of an origin of l imestone ers without any primary pattern within the sediment (KENT, 1 9 3 6 ; EDER, 1 9 7 5 ) . The _ carbonate i s mobilized by solution o f aragonite and pressure solution ( OLDERSHAW & SCOFFIN, 1 9 6 7 ) , a decrease of pH-value (EDER, 1 9 7 1 , 1 9 7 5 ) , or pressure solution ( TRURNIT & ARM STUTZ , 1 9 79 ) .
2 . Field Observations · within mqst limestone l ayerS there are three different main compo nents easily distinguished i f the. lime S tone is intensively light-grey _limestone, chert and dark-grey limestone. In parts section the layers consist of light-grey and dark-grey limestone only. Light-grey l imestone is generally situated in the center of the layers . It occurs in straight layers or nodules with all transitions_ bet;ween both . AccumUlations ·o f· fossils , shell layers or coarser intercalations are generally s ituated in the centre of the layers of light-grey limestone . Such layers thicken around large fos � ils ( e . g . coral colonies) o r often terminate where lenses of fossils or shell debris disappear as wel l . Nodular thickening o f subse quent layers is generally complementary , i . e . : if the l ight-grey limestone of one layer is thickening, it is thinning in the subse quent layer . There is no compaction observed within the light-grey l imestone . Chert generally envelopes the nodules or layers of light-grey
l imestone . These envelopes _are between a few mm and about 5 em thick and consist of vitreous or granular chert ( ROBERTSON, 1 9 7 7 ) . The transition between l imestone and chert can be abrupt or grada tiona l . Dark-grey l imestone i s generally situated a t the outer parts o f the layers . If there is no chert between the two types of limes tone_ , the vertical and horizontal transition between both is gradual . The dark-grey l imestone reveals a certain amount of compaction increa sing towards the margins of the layers . Delicate fossils are gene rally crushed, stable ones only show deformation i f s ituated clo sely to the margin of the layers .
1 15
The· shale beds between the l imestone layers are up to a few em thick. Fossils within the shale beds are s trongly deformed by compaction. If shale layers pass through thickets of l ithostrotionide coral s , the coralites are s trohgly leached, sometimes only relicts o f the wall survived the diagenes i s . Neither the l imestone nor the shale l ayers are continuous i ri all cases . Quite commonly are thin l imestone layers surrounded- by shale only a few em wide . O ri e thick limestone layer splits up completely i�to two thinner layers within 25 m laterall y . The two thin layers enclose a shale bed . - Each of them has got the internal structure of limestone layers as- described above . Quite· often are shale layers which are developed only for a few em between successive nodules of light-grey limestone. Beyond the area of pressure emanating from the nodules these shale layers can be traced into dark-grey limesto ne . Gastropods or cephalopods have never been observed outside the light-grey limestone exept as moulds or PY rite replacements .
3 . Laboratory Investigations . The Mgo and CaO content of a l imestone layer from Serpent Rock (Lower Glencar Limestone ) consisting of light-grey limes tone in the centre and dark-grey l imestone in the marginal parts was analysed titrimetrically according to HERRMANN ( 1 9 7 5 ) . The results were plotted in F i g . 1 as Mg, Ca ( C0 ) 2 and caco 3 , the insoluble residue 3 wqs calculated from these data . x-ray diffraction analysis shows that the insoluble residue is largely made up of quartz . . Fig.
shows that the calcite (Caco 3 ) content is high in the middle of the laye r , and decreases towards the margins . Insoluble residue is low in the centre , but increases towards the margins with two
distinct peaks in the outer centres . The dolomite content ( C a , Mg (co ) ) is generally negligible but increases at the margins of 3 2 the layer, so that roughly a threefold zonation of the layer can be established: An inner zone with high calcite content is surroun ded by .intermediate zones with distinct quartz peaks and outer zo nes with- relatively high dolomite concentrations . The inner zone appeared as l ight-grey limestone , the rest of the layer as dark grey l imestone .
1 16
Fig. 1 . MgO and CaO (plotted as ' Mg , Ca ( C0 3 ) 2 and CaC0 3 ) content of a l imestone layer ( Serpent Rock , Lower Glencar Limestone ) . Insoluble Residue ( I R ) calculat ed . Left side : s chematic sketch of the weathered limestone layer cons1sting of l ight-grey and dark-grey limestone
80 90 wt 0/o
4 . Diagene tic Model Modern lime muds have porosities of between 40 and 90% (PRAY
&
CHOQUE-TTE , 1 9 6 6 i CHILINGAR et al . , 1 9 6 7 ) . When deposited above the lysocline , they generally consist of various amounts of aragonite, Mg-calcite , calcite , biogenous silica , and clay minerals , depending on the . environment in which they ar·e deposited. The porosity of ancient limestones , consisting mainly o f calcite , quartz and clay mineral s , is reduced to about 2 or 3 % (BATHURST, 1 9 70) . When the high porosity of lime muds is reduced to about zero with Only minimal or without compaction a source problem for the large amounts of carbonate needed for cementation emerges (BATHURS T , 1 9 70 , 1 9 7 6 ) .
The l imestones described above are assumed to be lit� ified by an internal diagenetic process , i . e . they derived their cements from ' indigenous sources by soluti on at one site and reprecipitation at other sites . The site of precipitation is assumed to be the original pore system of the lime mud where the l imestone layers are now , with areas of
117
solution mainly being the shale beds which are considered to be a compacted residuum of the original lime mud deplete-d of the meta stable carbonates ( aragonite and Mg-calcite) and biogenous silica . .A.s described above , the sequence generally cons ists o f four dif·fe rent types of rock : chert and two types of l imestone within the layers , and phale between the limestone layers ( Fig'. 2 ) . The .diffe rent rat·e s of - compaction of the various parts of the sequence show that each part was lithified at i ts individual time : the inner �arts of the layers ( light-grey limestone) do not reveal significant compaction , therefore i t i s concluded that they were lithified re latively soon after deposition prior to accumulation of overburden. The outer parts of the layers ( dark-grey limestone) and the shale beds show an increasing degree of· compaction, therefore must have been cemented later. The composition of the solut�on which cemented the pore space of a lime mud to form a limestone layer consisting of three different types of rock had to change three times . At the beginning it had to be oversaturated with respect to largely pure calcium carbonate to form the l ight-greY limestone in the �iddle of the laye·r . It is assumed that this oversaturation was achieved b y solution of the aragonitic components of the l ime mud (Fig. 2 ) . Later the solution be dame oversaturated with :r::e spect to Sio2 by dissolution of bioge
nous s i lica to cover the l ight-grey limestone with chert, and at the end it was oversaturated with respect to calcium carbonate and dolo mite to form. the dark-grey l imestone which is the result of so lution of the Mg-calcite . The shale beds remain as a compacted re siduum ( l ime mud depleted of aragoni te , b iogenous silica, and Mg calcite ) . According to the ' maturatio? . hypothesis ' ( ERNST & CALVERT , 1 96 9 ; WISE & WEAVER, 1 9 7 4 ) the first form of s ilica to be precipitated
is disordered cristobalite . In calcareous environments this silica
is transformed to quartz within an average rate of between 30 and 50 million years ( v . RAD, 1 9 79 ) . Metastable components of the same material undergo d ifferent types of ' stabi l ization ' (BATHURS T , 1 9 6 4 ; LAND , 1 9 6 7 ) . I t largely depends on the s i te where components are s i tuated whether they are dissol ved or Whether they recrystallize in situ . Metastable components s ituated at the site of precipitation or within alreadY lithified parts of the l ime mud recrystall· ize. in situ, outside those sites of recrystallization they are generally dissolved ( Fi g . 2 ) . This
118 S o l u t i on
of
Shale S o l u t i on Solution Dark-grey
of
of b i og,
+ dolomite
s i l ica
a r a g on i t e
Chert
L i ght-grey M g- c a l c i t e
l imestone
Chert
Solution Dark-grey
of
Solut ion
limestone
of a r a g on I t e
2
b i og.
Shale
Formation Sequence
of
even t s :
I ight-grey limestone
s i f ica
Formation of chert l imestone
Fig . 2 . Schematic model displaying the formation of a limestone layer consisting of l ight-grey limestone, chert and dark-grey limestone . Disordered cristobalite as an intermediate form during silica diagenesis ( see text) i s not shown
can be proven by gastropods or cephalopods which consisted largely of aragonite ( HALLAM & O ' HARA , 1 9 6 2 ; MAJEWSKE , 1 9 7 4 : MILLIMAN , 1 9 7 4 ) . They can not ·be observed recrystallized outside the ligh.t-grey limestone. A few exeptions in which they are preserved as moulds or
replaced by pyrite prove that the restriction of these fossils to light-grey limestone i s not primary . Factors other than the site such as Mg-content, grain size , crystal shape, and abrasion induced strain also determine the solubility of components ( CHAVE & SCHMALZ ,
1 9 6 6 ; LAND , 1 9 6 7 : NEUGEBAUER , 1 9 74 ) , therefore recrystallized fos sils are sometimes observed in areas of solution.
1 19
The s ite of precipitation seemed to be determined by primary inho mogeneit ies within the l ime mud such as shell layers , accumulations of fos s i ls , coarser intercalations , or even single l arge fossils in to which the carbonate ions migrated: If these inhomogenei·ties are absent, the l ight-grey l imestone occurs in nodules and the layers become more or less nodular in appearance . All transitions between limes tone nodules and l imestone layers can be observed , therefore the formation of both , nodules · and layers , is attributed to the same ( concretionary ) origin. Acknowledgements The author is indebted to Prof . D . MEISCHNER for consistent advice , constructive criticism and support . The manuscript has been impro ved by the critical reading of Prof . B . ERDTMANN.
References BATHURST , R . G . C . ( 1 9 6 4 ) : The replacement of aragonite by calcite in the molluscan shell wal l . I n : IMBRIE , J . & NEWELL, N . · (eds . ) : Approaches to paleoecology , Wiley & Sons 1 . New York : 3 5 7 -3 7 6 .
BATHURST, R . G . C . ( 1 9 70 ) : Problems of lithification in carbonate muds ·. -Proc. Geo l . Ass . 8 1 : 4 2 9 - 4 4 0 . BATHURST, R .- G . C . _ ( 1 9 7 6 ) : Carbonate sediments and their -diagenes is . � Develop� Sedimentol . 1 2 : 2nd edn . , 6 5 8 pp . BJ�RLYKKE , K . ( 1 9 7 3 ) : Origin of limestone nodules in the Lower Pa laeozoic of the Oslo Region. -Norsk Geo l . Tidsskr . 5 3 : 4 1 9-4 3 1 . BUGE R , H . ( 1 96 6 ) : PalaoOkologische Untersuchungen an gebankten Kal ken . -Geo l . FOren. Stockholm FOrh . 8 8 : 307-3 2 6 . BRUCKNER , w . ( 1 9 5 1 ) : Lithologische Studien und zyklische Sedimenta tion in der Helvetischen Zone der Schweizeralpen . -Geo l . Rdsch. 39 : 1 9 6 -2 1 2 . BRUCKNER , W . D . ( 1 9 5·3 ) : Cyclic calcareous sedimentation .as an index of cl imatic variations i n the pas t .· -J . Sed . Petro l . 2 3 : 2 3 5 -2 3 7 . CHAVE , K . E . & SCHMALZ , R . F . ( 1 9 66 ) : Carbonate - seawater interactions . Geochim. Cosmochim. Acta 3 0: 1 0 3 7 - 1 04 8 . CHILINGAR , G . V . , BI S SELL, H . J . & WOLF , K . H . ( 1 9 6 7 ) : Diagenesis of carbonate rocks . -Developm. Sedimentol . 8 : 1 7 9 - 32 2 . EDER, F . W . ( 1 9 7 1 ) : Riff-nahe detritische Kalke bei Balve im Rheini schen Schiefergebirge (Mittel-Devon , . Garbecker Kalk) . - GOttinger Arb . Geo l . Palaont. 1 0 : 66 pp. •.
EDER, F,. W . ( 1 9 7 5 ) : Riffe und Riff-detritogene Plattenkalke . - Ber. SFB 48 : Entwicklung , Bestand und Eigenschaften der Erdkrus te , insbesondere der Geosynklinalraume , Projektber. A : 1 1 7- 1 4 3 . ERNST, W . G . & CALVERT , S . E . ( 1 9 6 9 ) : An experimental s tudy o f the recrys talliza-tion of porcelanite and its bearing on the origin of some bedded cherts . - Am. J . Sc i . 2 6 7-A: 1 1 4 - 1 3 3 .
120 FLUGEL , H . & PENNINGE R, A. ( 1 9 6 6 ) : Die Lithogenese der Oberalmer Schichten und der mikritischen Platten--Kalke (Tithonium , NOrd liche Kalkalpen) . - N . Jb . Geol . Palaont . , Abh . 1 23 : 2 4 9 - 2 8 0 . FOLK , R . L . ( 1 96 5 ) : Some aspects of recrystallization i n ancient l imestones . -Soc . Econ. Paleontol . Mineral . , Spec. Pub l . 1 3 : 1 4- 48 . GARRISON, R . E . ( 1 96 7 ) : Pelagic l imestones of the Oberalm Beds (Upper Jurassic - Lower Cretaceous ) , Austrian Alps . -Bul l . Cana . dian Petro l . Geol . 1 5 : 2 1 -4 9 . HALLAM , A . ( 1 9 6 4 } : Origin o f l imestone - shale rhythm i n the Blue Lias ·of England: a d.omposite theory . - J . Geo l . 7 2 : 1 5 7 - 1 6 9 . HALLAM , A . & O ' HARA, M . J . ( 1 96 2 ) : Aragonitic fossils in the Lower Carboni ferous of Scotland . - Nature 1 9 5 : 2 7 3 -2 7 4 . HERRMANN , · A . G . ( 1 9 7 5 ) : Praktikum der Gesteinsanalyse . - 204 pp . Springer; Berlin Heidelberg New York. KENT , P . E . (. 1 9 3 6 ) : The formation of the hydraulic l imestones in the Lower Lias . -Geo l . Mag. 7 3 : 4 76 - 4 7 8 . KREBS , w . ( 1 9 62 ) : Das Oberdevon der Prllmer Mulde/Eifel· unter Aus schluB der Dolomit-Fazies . - Notizbl . hess . L . -Amt Bodenforsch . 9 0 : 2 1 0- 2 3 2 . LAND, L . S . ( 1 9 6 7 ) : Diagenesis of skeletal carbOnates . - J . -Sed . Pe trol . 3 7 : 9 1 4 -9 30 . MAJEWSKE , O . P . ( 1 9 7 4 ) : Recognition of invertebrate fossil frag nents in rocks and thin sections . - 2nd edn . 1 0 1 p p . E . S . Bri l l ; Leiden. MEISCHNER, K . D . ( 1 9 6 4 ) : Allodapische Ka�ke , Turbidite in riff-na hen Sedimentations-Becken . - Developm. Sedimentol . 3 : 1 56 - 1 9 1 . MEISCHNE R, D . ( 1 9 6 7 ) : Pal&Okologische Untersuchungen an gebankten Kalken . Ein . Diskussions-Beitrag . -Geo l . FOren. Stockholm FOrh. 89 : 4 6 5 -4 6 9 . MILLIMAN , J . p . ( 1 9 74 ) : Marine carbonates . -3 7 5 pp . Springer; Berlin, Heidelberg, New York. NEUGEBAUER, J . ( 1' 9 74 ) : Some aspects of cementation in chalk ,.. Spec. Publs . int . Ass . Sediment . 1 : 1 49 - 1 7 6 . OLDERSHAW , A . E . & SCOFFI N , T . P . ( 1 9 6 7 ) : The source o f ferroan and non-ferroan calcite cements in the Halkin and Wenlock l imesto nes . - Geol . J . . 5 : 309-320. PRAY , L . C. & CHOQUETTE , P. W . ( 1 96 6 ) : Genesis of carbonate reservoir facies . - Am .' Assoc . Petroleum Geologists , Bul l . 5 0 : 6 3 2 . RAD , u . von ( 1 9 79 ) : SiO � -Diagenese in .Tiefseesedimenten . - Geol . Rdsch. 6 8 : 1 02 5- 1 036 . ROBERTSON , A . H . F . ( 1 9 7 7 ) : The origin and diagenesis of cherts from Cyprus . - Sedimentology 2 4 : 1 1 -3 0 . ·
SCHWARZACHER, w . ( 1 9 6 4 ) : An application of statistical time-series analysis of a l imestone-shale s equenc e . - J . Geo l . 7 2 : 1 9 5 -2 1 3 . SEILACHER , A . & MEISCHNER, D . ( 1 9 6 4 ) : Fazies-Analyse iffi Palaozoikum des Oslo-Gebietes . - Geol . Rdsch . 5 4 : 5 9 6 -6 1 9 . TRURNIT , P . & AMSTUTZ , G . L . ( 1 9 79 ) : Die Bedeutung des Rlickstandes von Druck-LOsungsvorgangen flir stratigraphische Abfolgen, Wech sellagerung und Lagerstattenbildung . - Geo l . Rdsch . 6 8 : 1 1 1 7- 1 1 24 WIS E , s . w . & WEAVER, F . M . ( 1 9 7 4 ) : Chertification o f oceanic sedi ments . - Spec. Publs . int . Ass . Sediment . 1 : 30 1 -3 2 6 .
.·
Deep-Sea Stratigraphy: Cenozoic Climate Steps and the Search for ChemocClimatic Feedback W. H. BERGER
Abstract : Results of paleoceanographic studies on cores recovered by Glomar Challenger suggest that the evolution of ocean circu1ati.on and climate is not gradual but is punctuated by periods of rapi.d change, or " steps " . These events may or may not be associated with the classical stratigraphic boundarie s , some apparently ar.e . There appear to be two extreme types of events : { 1 ) those which represent accelerations of a given trend, depending on positive feed back from hydrosphere and perhaps carbon sphere and ( 2 ) those which are independent of a trend and depend entirely on outsi.de forcing. Steps opposing the overall trend, but following on-trend steps , also exist. They are interpreted a s rebound events . The step-like transitions from one geochemical�climatologic setting to another are important as natural experiment s , · for the s tudy of response characteristics of the system, and for global high resolu tion s tratigraphy . Il nous restait ensuite a nous assurer' positivement si ces d:Lff€.rents terrains, ces diff6rents 6tage s , si tranch6s sur le sol de la Franc e , etaient l e r€.sultat des c:Lrconstances locale s , sp€.ci.ales a notre sol , ou s ' ils d€.pendaient de faits g€.n6raux qui se seraient produits sur tous les points du globe a la fois. , M . A . d ' Orbigny ( 1 8 5 1 )
1
•
In traduction
The question of whether or not there were times of rapid c limati.c change on a global scale can realistically be answered only by study ing continuous sequences in the deep · sea . We now have a reasonably complete (but by no means perfect) record of Tertiary ocean climate . Were there sudden changes? What can they teach us about the ocean atmosphere system in geologic time? The first question can now be answered in the affirmative; an updated list of events is given in Table 1 . The second question provides an opportunity for much speculation , because analysis of c l imate events in deep-sea stratigraphy is entirely in its infancy . The events usually considered include sudden large-scale increases in terrige nous component s , changes in sedimentation rates , hiatus development or cessation , drastic drops or rises of the carbonate compensation Cyclic and Event Stratification {ed. by Einsele/Seilacher) © Springer 1982
122 depth , and large changes in the proportion of organic matter (see .. reviews by Arthur, 1 9 7 9 and by Berger1 1 9 7 9 , 1 9 8 1 a ) .
, ·
Table 1 . Examples of fast c l imatic transitions in the Cenozoic ( for data sources see Arthur , 1 97 9 ;' Berger et a). . , 1 9 8 1 i Haq, 1981 ) ( 1 ) Cretaceous-Tertiary Boundary Event ( rv 6 5 m. y . ) . Large-scale extinction of oceanic plankton and othe:t:' organism groups . Large c:l'imatic fluctuations probable. { 2 ) Paleocene-Eocene boundary ewent. Short l ived peak warmi:ng· . Expansion of tropical faunas and ( 3 ) Earl,.y-to-mid-Eocene cooling steps . Shift of climate zones toward equator . Cooling of deep water. Chert formation . ( 4 ) Eocene Termination Event ( tV 3 8 m . y . ) . Cooling in · high and low latitudes , expansion of polar highs ._ Significant changes in deep-sea benthic fauri.a . Rapi.d drop ot the carbonate compensation deP th . ( 5 ) Mid-to-Late Oligocene Oxyg-en Shift ( ? ) ( rv 30 m . y . ) . F· irst occurrence of rather heavy oxygen isotope values in deep sea benthic foramini fera , presumably due to polar bottom water formation. ( 6 ) Mid-Miocene " Oxygen Shi ft" ( .rv 1 5 m . y . ) . Oxygen isotope ratios shift to heavier value s , presumably due to Antarctic ice buildup. ( 7 ) 6-mil l i on-year 11Carbon Shift " . Isotope ratios o f the ocean ' s carbon shift to li.ghter values 1 presumably due to organic carbon input from regressi.on and pco -change ( ? ) . 2 ( 8 ) Messinian "Salinity Cri.s i s " ( rv 5 . 5 m . y . ) . Isolation of Mediterranean through regression, strong cooling. ( 9 ) 3-mill ion-year Event. Onset of Pleistocene-type climatic fluctuations . ( 1 0) 1 -mill ion-year Event . Onset of large climati·c fluctuations after peri.od of quiescence. ( 1 1 ) 11Terminations'1 of the late Pleistocene ( last 0 . 8 m . y . ) . Rapid deglaciations , warming of mid-latitudes . pco change . 2
In the short time since the evidence for climatic-related steps has been e;x:tracted from Tertiary sequences in the deep sea · (Berggren, 1 9 7 2 ; Kaneps , 1 9 73 ; van Andel and Moore , 1 9 7 4 ; Benson , t 9 7 5 ; Savin et al . , 1 9 7 5 ; Shackleton and Kennett, 1 97 5 ; Kennett and Shackleton, 1 9 7 6 ) a tradition has alrea�y developed which leans heavily on pre sumed changes in ocean circulation produced by continental drift and
123
sea floor spreading to explain the timing of events and their sedi men tologic manifestation (Berggren and Hollister, 1 9 7 4 , 1 9 7 7 ; Berger and Roth , 1 9 7 5 ; Kennett , 1 9 7 7 ; McGowran, 1 9 7 8 i Thie�stein an� Be�ge�, 1 9 7 8 � Arthur and Nat!Land , 1 9 79 ; Talwan.i. et a l .
1
1 9 7..9) .
There are good reasons for ascribing basinwide sy:n.cl:u::onous facies changes t6 changes in ocean circulation. Surface ci.:r:culati.on. domi nates biogeography and productivity patterns , and deep cix:cul.a tion ' govern s · chemical and mechanical, erosion and .redeposition processe s . Thf.' echange o f water masses betw$en ocean basins i s a cruci.ai factor in de �ermining the ultimate resting place of chemical deposi'ts , that
is, carbonate and s ilica . Changing opportunities for basin�basin ex� change result from the re·arrangement of geography by plate tecton ic s . Thus , the opening and closing of the gatew.ays o f circulation must profoundly influence facies patterns . Even more to the point, in the context of climatic change, ocean currents are involved in largescale heat transport . Hence a change of geography allowing {or forcing) increased poleward transport of tropical water masses ( or increased sinking of polar waters) should have important effects on the planet ' s temperature and precipitation patterns . The disappearance of the
Tethyan seaway , the or � gin of the Gulf Stream, and the evolution of the C ircumpolar Current are recurrent themes in such discussions . The approach l_eads_ to a search for critical gateways and barriers affecting surficial and deep circulation. As a rule, this type of event analysis leads to a reasonably exact statement about the strati graphic position
6£
a climate-related event , an assertion about con
temporaneity with a poorly dated geophysical-geographic event, and speculative suggestions about how the one was a result of the other -. Here I shall take a di fferent approach to the treatment of events. I shall argue , in essence , that many of the observed steps are produced by internal feedback , notably albedo feedback , and that they do not in principle depend on minor rearrangements of geography. Also, I draw attention to the intimate involvement of the carbon cycle in climate c5 1 3c
steps as manifested in changes in carbonate deposition and in
composition of foraminiferal shells . It appears unavoidable to me that ' such involvement must result in change s in the atmospheric co content. 2 The present essay summar izes facts and concepts put forward earlier (Berger, 1 9 7 7 ; Thierstein and Berger , 1 97 8 ; Vincent et a l . , 1 98 0 ;
�erger and Vincent, 1 9 8 1 ; Berger e t a l . , 1 98 1 ) , and further develops some of these concepts especially with regard to carbon feedback and to the implications of the c5 1 3c signal in foram stratigraphy.
124
Just also albedo gOverns how much of the sun ' s radiation is by the earth ' s surface , the co 2 helps determine at vh�t te�pe��tu he the energy is to be reradiated . We have , then , an interpl.
put of an electric control system which pro� uces a change at the out put: the respons e . Such systems adjust their output by comparing it with the input signal and operating o n the difference ( = error ) .
Hence, they are characterized by " feedback " , that i s , information from the output signal helps control future Output. Feedback is "positive"
125
-2
'
CD 0 a.
0
"'
to
1\
DOUGLAS, SAVIN,
IIIll
1975, 1977
� =����� �� =��
PLANKTONIC FORAMS
- BENTHIC FORAMS SHACKLETON, KENNE TT, O
1975
AMS
R
0
20
40
60 m.y.b.p.
Fig. 1 . Oxygen isotope trends in Cenoz.oi.c foraminifera frox:n Pacifi.c deep-sea sediments . Data of Douglas and. Savin refer to cen tral Pacific (Douglas and Savin, 1 9 7 5 ; for Douglas and Savin , 1 9 7 7 see Douglas and Woodruff , 1 9 8 1 ) . Data of· Shackleton. and Kennett ( 1 9 7 5 ) refer to region south of E . Australi.a and New Zealand. Major steps are marked by arrows
when it i s in phase with the error, magnifying i t . I t is �'negative" if it is in opposing pha-s e , reducing the error. Positive feedback leads to instab i l ity , negative feedback stabilizes a system. The response to a step function describes the efficiency of a feed back system. A highly sensitive system responds very � apidly, thanks to lack of damping. However , such a system tends to go wel l beyond the new equil ibrium state and large overshoot is typical . As the overshoot builds up, the discrepancy between input signal and output signal rises and the feedback acts i n reverse . Again the system goes beyond the equilibrium point: i t oscillates. There are several ice age theories which make use of this property of systems with weak damping. Their s�ccess has been limited because they failed to account for the need of outside stimulation if the
126
oscillations are to C ontinue. This need has been fil led by the kovich mechanism {Hays et al . , 1 9 7 6 ; Pisias , 1 9 7 6 ; Imbrie and Imb �ie, 1 980) . As yet the typical time constant of the Pleis t'ocene climate system, its resonance peak, has not been found (or looked for in a systema·tic fashion) . Perhaps no such thing exists. With sufficient damping, overshoot -and hence osci llations- can be eliminated . A strongly damped system responds more slowly to a change in input, and reaches the new equilibrium after a much longer time . There seem to be , in the cOurse of the Cenozoic , times with strongly damped cl imate systems and · times with weak damping, characterized by large and frequent fluctuations . On the whole, instability appears to increase until it reaches its climax in the Pleistocene . The abundance of climate steps also increases. We have one step every 1 00 , 000 years or so for the last million years : the glacial-post glacial transitions known as " terminations" (Broecker and van Donk , 1 9 70) . I think it is unlikely that the terminations reflect strong damping, and I would , therefore, expect evidence for overshoot and oscillation. The fact that a very warm period (the AllerOd) was followed by a very cold one (Younger Dryas') near the end of the last termination supports the overshoot hypothesis (Berger, 1 98 1 b) . The resonance period, under this assumption, would be near 2000 years.
2 . 3 . Why S·tudy Climate Steps? A resonance period of 2000 years , if correct , would seem to hold little promise of being detected i_n older parts of the Cenozoic, un less extremely detailed studies are performed in various segments of the record . Neverthe l e s s , step analysis is very worthwhile because { 1 ) Times of rapid c limatic change are likely to be dominated by one or two causal factors whose influence may be obscured at other time s , when a multitude of .f actors has control . ( 2 ) If there is no clearly identifiable external cause forcing the change , one must then suspect the action of strong internal positive feedback , which needs to be identified; ( 3 ) the maximum rate of climat i c change . yields informa tion on damping, a most interesting property of the system; ( 4 ) a comparison of "before" and "after" should be very instructive the increase or decrease in stability ( " stabilizing11 versus "destabilizing" events ) . Last but not least , the record of
127 climate steps may produce an excellent means · f � r basin-shelf corre lation : it has done so a lready for basin-basin correlation (Haq et a l . , 1 980) . 2 . 4 . Albedo Feedback The c l imate machine is driven by the sun ' s energy . _ Of the radiation reaching the earth , a portion i s refl�cted into space before it can heat the surface: this i s the albedo. The planetary a lbedo is the
pJ:oportion of. radiat_ion refl � cted as measured from space. It is near 2 8 % overall and depends greatly on atmospheric conditions
regionally, especially on cloud cover. The surface albedo is the proportion of incident radiation reflected from ground or water sur face . It varies with the sun ' s angle. Surface albedo values for clear sky and high noon are listed in Table 2 . Table 2 .
Albedo-values o f ocean , and land surface s . (Reflectivity in percent o f incident light, during noon) ( Source: comp ilation of Humme l and Reck , 1 9 7 8 )
Annual global average :
Q£�gg
low latitudes : mid lat itudes : high latitudes:
Land
1 4 ( planetary albedo: 2 8 ) 4 to 7 4 to 1 9 6 to 50
min .
( surmne r ) :
6
max .
(winter) :
55
desert:
2 0 t o 30
grasslands, coni ferous & deciduous forests: wetlands :
15
10 tropical rain forests: 7 snow-covered land: 3 5 to 82
�rr��££��£-�2rr��rr�rr��1_f��-��E
pack ice in water:
as
40 to 5 5
The well-known features o f surface albedo distributions are readily seen : { 1 ) a dark ocean, especially in low latitudes where it is less ' fertile on the whole and , therefore , clear; ( 2 ) a great range of land
128
values , from dark foi'ests and wetlands to light deserts and bright snow fields; ( 3 ) the enormous contrast between ice and snow on the one hand and water surface on the other. Replacing water surface with snow fields , or vice v.e rsa , provides the greatest possible change in albedo. on the whole, the c limate history of the Cenozoic is marked by regression and ice buildup. Henc e , albedo has increased promoting cooling. The buildup of ice and snow was favored by the poleward movement of land masses ( Donn and Shaw, 1 97 7 ) and the thermal isolation o f Antarctica and the Arctic Ocean. Albedo changes contain positive feedback : cooling promotes snow cover , which increases albedo, and vice versa. There i s , however, also nega ·tive feedback: cooling decreases moisture in the air, hence cloud 9over, and hence planetary a lbedo decreases . Which of the effects will dominate presumably depends on the initial state: once snow and ice cover considerable areas , positive feedback must gain greatly in importance. The fol lowing feedback mechanisms apply:
( 1 } Warming
�
ice and snow cover reduction � a lbedo decreases
f'Urther warming ( 2 ) \Varming -7 ice mass reduction decreases � further warming
-7
sea level rises
-7
�
albedo
( 3 ) Warming � humidity rises � forest growth � albedo decrea ses � further warming ( 4 ) "(.�!arming --7 wirids decrease � upwelling decreases -7 albedo decreases � further warming Cooling engenders these steps in reverse . However, it must be kept in mind that when the sea level drop reaches the shelf edge there is much less feedback of type ( 2 ) . Cenozoic climate history is marked by an increase in the feedback mechanism ( 1 ) and ( 2 ) . The interaction of ice masses and sea level are crucial: the ice masses constitute large reservoirs of instability as it were. In this view, the onset of Pleistocene f luctuations is a matter of critical ice mass. 2 . 5 . Carbon Feedback The major climate steps are associated with changes in the carbon cycle. Both o 1 3 c signals and preservation-of-carbonate signals in dicate that this is so (Berger, 1 9 7 7 ; Shackleton, 1 9 7 7 ; Scholle and
129
Arthur, 1 98 0 ; Vincent et a l . ( 1_9 80) . Perhaps the most rema:r.-kab!Le phenomenon in this respect is the preservation spike wl1ich is. in variably associated with late Plei.stocene deglaciation steps (Berger and Vincent , 1 98 1 ) . we must now ask whether the role cf· the C?,;t:bop, cycle is entirely passive_ , or whethe;r i.t contributes to posi.ti.ve a:pd_ negative feedback surrounding climate steps. The idea that climatic variations are influenced by the a.tmospheri.c carbon dioxide content through the " greenhouse effect" is ove:r a
h,undred years old; it is associated with the writings of· J . Tynda;LJ..r
s . Arrhenius , and T . C . Chamberlin . Early in the century i.t exci.te<:;l considerable discussion ( see Dacque , 1 9 1 5 , p . 4 5 6 f f ) . Subsequep.tly
it fell into disfavor with _ many- textbook writers ( e . g . Wright, 1 93 7 ) but was then revived in connection with the present bui.!Ld.up o f co 2 in the atmosphere, from the burning of· fossil fuel ( P lass , 1 9 5 6 ) . Recent- calculations (see e . 9 . Hansen et aL , 1' 9 8 1' ) suggest that a doubling of the pco 2 will raise the average global surface tempera ° ture by about 2 C . Direct evidence for past changes in pco 2 now exi.st from the analysis of ice· cores (Berner et a l . , Delmas et al . 1 1' 980) . Such calculations , then , are indeed pertinent to the interpretation
of climate history. The deep-sea record shows that the intensity of volcanism cha.nged 1 through Cenozoic h i'story (Kennett and Thunell, 1 9 7 5 i Kari.g, 1: 9 7 5 ) a,nd so, presumably, did the supply of co 2 . Likewise, the intensity of weathering changed , as seen in deep ocean sedimentation rates (pavi.es et a l . , 1 9 7 7 ; Wotsley and Davi.e s , 1:979) 1 and thus the rate of· uptake
of co2 in the weathering process mus·t have changed. Fina1J.y 1 the accu mulation rate for organic carbon i.s not constant (Thi.erstei n, T 9 7 9 ) , implying that this maj or sink for co 2 varies .in eff'icien.cy·. F luctua.tions in the phosphorus cycle (Arthur and Jenkyns, 1 98 1' ) through i.ts in
fluence on the carbon cycle may be at least partly responsible (Broek ker, 1 9 7 3 ) . As concerns climate steps1 the possibility- of rapid additi.on of co 2 to the atmospher e , or extraction from i t , is of interest. The various carbon reservoirs which are in intimate contact with the atmosphere are listed in Table 3 . These reservoirs can change siz.e quickly enough to affect the pco 2 of the atmosphere on the time scale here considered. Several of these reservoirs are considerably greater than that ot: the atmosphere. Of central importance is the partitioning of the co 2 bet ween ocean and atmosphere . Obviously1 relatively small changes in this
130
5 Table 3 . Carbon reservoirs active on a 1 0 year scale {ACM pheric carbon mass ) . For data sources see Scholle and Arthur ( 1 980) and Vincent et al . ( 1 9 80) .
=
atmos
_Reservoir 6
Atmospheric co 2 Dissolved c in ocean (mainly HC03 l
350
58
0
1 00 5 50
1. 7 1 8
+ 1 ° /oo
·Accessible Organic Carbon Biosphere ( forests) Soil carbon ( 11humus " ) , peat, etc. Seafloor stirred layers
8 20 6
1 3 1
Stream flux, all carbon (per thousand years)
10
2
Accessible Carbonate Deep sea ( upper meter) Shelf (upper meter) Land ( 1 0 5 years ' leaching)
Volcanic co input (per thousand 2 years)
0. 5 - 2
- 25 °/oo
0. 2
partitioning and in the fluxes between the active carbon reservoirs can . Potentially have large effects on the atmospheric pco 2 . A cursory overview of potential feedback mechanisms in the carbon sphe re suggests a striking symmetry between carbonate and organi.c carbon (Table 4 ) . The processes which produce positive feedback in the carbo nate cycle, produce negati.ve feedback in the organic cycle, and vice versa. For example , when sea level rises , shelf carbonate i.s precipi _ tated which releases co 2 to the atmosphere , but also organi.c carbon accumulates - tying up co 2 . It is not clear to me which ef';E"ect wUJ., be stronger at any given time during a transgression. Presu�abl,y- the balance depends on the width of the tropical belt, the �orpho�ogy of the shelf, and the availabi-lity of· nutrients. Nutrient supp;Ly has opposing effects on carb
131
Table 4 .
Feedback mechanisms of the carbon sphere
Bas � c principle: pC?2 � n the atmosphere is linked to the . of reradJ..at1.on of energy to space, in the infra effJ..cJ..ency red spectrum. High pco 2 obstructs reradiation, hence atmos pheric temperature rises ( 11greenhouse " effect) .
Positive feedback Carbonate Sea level rise - shelf carbonate precipitation - CO release . warming - co 2 release - warming - further sea leve l rise ( i f ice present ) . Also: reduced co uptake by decreased land surface 2 in warm, humid climate) . { bu t : chemical weathering favored Sea level fal l : reverse of rise, but precipitation of deep-sea carbonates can proceed to balance shelf carbonate dissolution. Physical principle s : . lag of deep-sea carbonate dissolution behind shelf precipitation (not valid in reverse) , co 2 less soluble ln warm water than in cold, exposed land surface neutrallzes co 2 while submerged surface does not. Carbon
---
Sea level rise - decrease of erosion - fewer nutrients to sea reduction of fertility - less Cor produced - less uptake of co ? from atmosphere � warming _ �urther sea level rise ( i f ice present ) . Sea level fal l : reverse of rise . Physical principle: effect of ocean fertility on pCO ? ' through
photosynthetic carbon fixation.
Negative feedback Carbonate Sea level rise - decrease of erosion - fewer nutrients to sea decre-ase of carbonate precipitation - less co 2 released - cooling effect. Sea level fal l : reverse of rise, but effect may be more pronounced, since onset of new erosion can produce a nutrient spike. Physical principle: effect of ocean fertility on pco 2 , through rate of deep-sea carbonate accumulation. Carbon Sea level rise - drowned estuaries, marshes, lagoons collect orga nic-rich sediments - co 2 extracted from atmosphere - cooling. Also: increased precipitation on land (more sea surface, warming from albedo effect) builds up biosphere and associated " humus " , peat, etc . Sea level fal l : reverse of rise, but previously accumulated organic carbon is very accessible, hence onset of erosion can produce a carbon spike .
132
Table 4 {continued) Physical principle: effect of sea level change on forming or exposing traps of organic-rich sediments, and on precipitation patterns , influencing bio- and soil-carbon size . In addition to the overall tendency of carbOn and carbonate feedback mechansims to balance within _each cycle and between cycles, there also is a tendency for direct cancellation: release of co 2 , from organic carbon pools , is neutralized by dissolution of carbonate , and take-up of co 2 is similarly neutralized by carbonate precipita tion.
On the whole, the carbon sphere system lacks a simple and strong positive feedback mechanism of the albedo type. We should , there fore, see much less short-term high amplititude climatic variation in a world without ice. Nevertheles s , the buildup and subsequent
erosion of readily accessible reservoirs of organic carbon , and of nutrients, may provide opportunities for large-scale step changes
even in the Mesozoic .
2 . 6 . On Reading Proxy Signals In what follows we shall look more closely at oxygen istope and carbon isotope _records. The correct reading of these types of curves
is still a matter of research and discussion. The main points may be
briefly summarized as follows ( see also chapter on stable isotopes in
Lipps et a l . , 1 9 7 9 ) . Concerning oxygen isotopes, a change in the signal may indicate: ( 1 ) a rise or a drop in temperature, 4 to 5 ° c for 1 ° /oo chang e ; ( 2 ) a decrease or increase in the mass of conti 16 nental .ice (which enriches or depletes the ocean in o , the preferred
isotope in ice) ; · ( 3 ) a regional change in the composition of seawater,
due t'o changes in evaporation-precipitation pattern s. Lesser effects (vita l , seasonal , dissolution-related) also exist.
The choice of the effect or combination of effects to be credited with a change in oxygen is-otope values is generally qu�te arbitrary. Thus, the temperature scales given with or 1 80 records are not necessarily informative. I have left them off .
Concerning carbon isotopes , a change in the signal may indicate: ( 1 ) a rise or a fall in the degree to which 1 3 c is enriched in sur face waters with respect to deep waters , such enrichment being a function of both fertility and intensity of stratification; ( 2 ) a
133
change in the input or output ratios of carbonate a.n..d carb0n, to or from the . ocean-atmosphere system: such change wi_:u a.ffect the ratiO because of the low 0 1 3 c values of' organic carb0p_ (Table
regional change in the compos ition of
Hco;
1 3 c;1 2c 3) ; a
of deep waters , due to
changes in deep circulation.: old waters are enriched in 1 2c . Lesser effects {vital, seasonal, dissolution-related) also exi.s t, and. voi
canism also can inflUence
61 3c .
Related signals of the carbon cycle are carbonate preservation and
organic carbon preservation. A change in carbonate preservation. w.ay ' indicate one or more of these: ( 1 ) a change in the producti.vity of
the ocean ( increase in fertility decreases saturation} , ( 2 ) a change in the overall supply of calcium-carbonate to the deep ocean (bui.;I..d_
up or erosion of shelf carbonate , ridge crest volcanism) ,
(3)
regio
nal change from altered deep ci-rculation (within and between basin fractionation ) . Other effects also exist {Berger, t 9 'Z ? ) . The preser
vation of organic ca �bon in deep-sea sediments is a matter ot carbon and nutrient supply , of availability of calcium and magnesi.urn, and of oxygenation of the deep sea . The balance between eupelagi.c and hemi pelagic sedimentation is probably very important both for carbon. and carbonate signals in deep-sea sediments , as it is f.or silica depo
sition.
3.
Major Steps : the Evidence
3. 1 .
Abundance of Steps
As pointed out above , the abundance of c limate steps is a function
both of the variability of' the c limate and of the ( arbi.trary) stati.s
tical cut-off for "unusual " contrast between c limatic regimes follow ing one another. Traditionally , changes in fossil abundances , and changes in stable isOtope composition have been used to i.den.ti.fy periods of rapid change in the deep-sea record. On the whole, the abundance of events appears to increase throughout the Cenoz.oic
(Figure 1 ) . The probable reason has been mentioned : a general trend of increasing importance of albedo feedback and of ice-mass buildup in the Cenozoic . Only the major st.eps are considered here, obviating the n.eed for statistics to extract them from the available information.
134
3 . 2 . Cretaceous-Tertiary Boundary The most signi ficant information on the Cret a.ceous-Tertiary boundary event has been provided by the pelagic record . It concerns the whole sale extinction of oceanic plankton within a few thousand years combined with an absence of extinctions in the deep benthonic realm. Se.veral papers on this topic were published in Rosenkrantz and
Brotzen (editors , 1 960 ) , by J . P . Beckmann , H . M . Bolli and M . B . Cita , and W . A . Berggren . Subsequently Luterb.acher and Silva ( 1 96 4 ) , in the Gubbio section, established the Globigerina eugubina Zone, which became the criterion for comple teness of the record . The deep-sea record fully supports the earlier suggestions of planktonic ex
tinctions including pelagic coccolithophores (Bramlette , 1 96 5 ; see
Thierstein and Okada , 1 9 79 } . Also it allows an estimate for the time it took to replace Cretaceous faunas and floras with Tertiary ones, in the pelagi'c realm. It is a matter of a few thousand year s , at
most (Thierstein, 1 980) . The debate about the end-of-Cretaceous ex tinctions is in full swing (Christensen and Birkelund, 1 97 9 ; Rus sel l , 1 97 9 ) ; the arguments for a n extraterrestrial cause seem t o hold sway
at this time (Alvarez et a l . , 1 980; Emiliani , 1 980; HsU , 1 980; Smit and Hertogen, 1 9 80) � - The evidence . for the catastrophic impact of an extraterrestrial body lies almost exclusively with the discovery of
high iridium values within the K-T boundary . . The geochemistry of iridium, however, is poorly understood . Recently, sanid�ne spherules have been proposed as additional evidence (Smit and Klaver, 1 98 1 ) . There is now little doubt that the event was both catastrophic and
independent of a trend , so that gradualist explanations (Bramlette , 1 9 6 5 ) become rather unattractive. Whether the K-T boundary event teaches us anything about the climatic workings of the ocean-atmosphe re system is as yet a moot question . As things stand, it i.s a re
minder that Earth (and life on i t ) are subject to cosmic bombardment (Urey, 1 -97 3 ) , and it provides an outstandi.ng example for plankton radiation.
3 . 3 . Tertiary Trends The climate events within the Tertiary appear most cle�rly as steps in the oxygen isotope record , although other signals also show them .
Such signals are found in various sediment properties : carbonate con tent and related preservation of calcareous fossils, silica content, clay mineral composition, hiatus abundance , organic matter content,
and carbon i sotopes within calcareous shells (see Arthur , 1 97 9 ) .
135
The available isotope data, largely produced by Douglas and Sa�in ( 1 97 5 ) and by Shackleton and, Kenn.ett ( 1 9 7 5 ) , show the well-known. fact that the Cenoz.oic on the whole i.s marked by a cooling tre-nd
(Figure 1 ) . It is not always appreciated that this cooling trend is chiefly a high-latitude phenomenon. The tropi.cs are not a;f';fected
very much . Immediately, this discrepancy suggests a clue to the trend: It is something that happens in high latitudes only. The most likely cause whi.ch offers itself i.s that the land areas in hi.gh latitudes
cover themselves with snow and ice, reflecting much of the incoming sUnlight. An overall regression provides more land areas (which by
itsel� increases albedo ) 1 and continental drift leads to therma.l isolation of the Arctic Ocean. and the Antarctic Continent, favoring
buildup of snow (Donn and Shaw, l 9 77 ) .
The trend has steps : the threshold events . It appears that these steps essentially reflect the phase transitions from water to i.ce t on va_ rious parts of the globe .
The importance of sea level in modulating the temperature evolution on the planet - and hence the e f· fect of Cenoz:oi.c regression on the overal l cooling trend · - has been variously demonstrated in a compa rison of sea level changes and - the stratigraphy o f· stable is-otopes and CCD-level fluctuations ( see Berger , 1 9 77. ; Fi.scher a.nd A..r thur, 1 97 7 ; van Andel et al . , 1 97 7 ) . lilhen. one considers the anornali.es of
the shaded trend shown in Figure 1 , they clearly tend. to parallel
the sea level variation s , indicating that transgressions result in a warming of the ocean and an expansion of the tropical belt (Berger,
1 97 7 ) . The low stand of s.ea level in the Oligocene and the concurrent cooling of the planetary surface is especially noteworthy . I believe that this is largely an effect of the increased planetary albedo due to increased exposu"re of ( light) land at the expense of (dark) ocean. Cool tropics in the Oli.gocene also would have favored drought, · further
increasing albedo on land (and incidentally accounting for the low Oligocene sedimentation rate s : see Davies et al . , 1 97 7 ) . 3 . 4 . Early Eocene Eyents
The first distinct steps visible in Figure 1 are in the early Eocene . They are remarkably large , almost 1 ° /oo each. By the es tablLshed rules (Section 2 . 6 ) the steps describe drops in temperature or in-
136
creases in salinity 'in the region south of New Z:ealand, or else i.ce buildup on Antarctica. At the time when the site (DSDP Si.te 2 77J re corded the change it was some 1'5° furth,Qr south than now. I.t a.lso was shallower. The diagram shows that these steps mark the begi.nnin_g of a divergence of oxygen isotope values for high southern regions from those for central regions of the Paci.fic . I'n f·act , the overlap of the values i n the early Eocene i s surprising. A temperature gradi.ent must have existed from subtropics to subpolar z:ones . Thus, the hi.gh-iati.tucle values are 11too low" when interpreted in terms of temperature alone .. They must reflect excess precipitation of 1 6o-rich rain : The bi.ght between Tasmania and Campbell Plateau (Antarctica and Australia being close together) must have been, on the whole, an estuari..n e ba.sin wi.th salinity strati.fi.cation, situated in a rain belt� If the rain · belt hypothesis i.s accept.ed, we can. then unQ.ers.tand either step, or both , as an event establishing more nearly normal salinity. Since Australia separated f-rom Antarctica at about this time , an increase in the inf-lux of water from the I.ndi.an Ocean. could be invoked That such an ad hoc gateway �xplanation i.s not the whole story is suggested by the evidence for large climatic excursions near the Paleocene-Eocene boundary and in the middle Eocene , in the North Atlantic (Haq et al . , 1 977 ) . Haq ( 1 98 1 ) cites ev'idence that ·the latest Paleocene-early Eocene represents the warmest period of the entire Cenozoic in the mari,fle realm. Thus , the steps would seem to represent cooling events which lead from thi.s warm peak to more " nor mal 11 Cenozoic conditions . .•
The cooling and the rain belt mechanisms are not mutually exclusive. On the contrary: the high temperatures in subpolar regions require large-scale heat import . Transport of moisture from low to high lati tudes i.s the most efficient of the heat transfer mechanisms. Cooling and an equatorward movement of a hi_g h-latitude rain belt are compa tible with the data. The fact that there are steps, presumably, in� dicates positive feedback from albedo changes on Antarctica from . drought or snow. 'Large excursions in the 6 1 3c signals are associated with these steps (see Shackleton and Kennett , 1 975, Fig. 4 ) . J?os.t-step time (mid- to late Eocene) is characterized by increasing di£ferences 1 in 6 3c values of planktoni.c and benthic forams . A likely interpre tation is lowered fertility due to an increase in thermocli_pe develop ment: another clue to cooling as the step-producing factor.
137
The second of the two events , which may be called the " 50 m . y . step " , marks _ a change in high latitude land floras (Kemp, 1 97 8 i Wolfe, 1 9 7 8 ) and the end of a majOr episode of laterization of Gondwana land frag ments (according to McGowran, 1 9 7 8 ) . In the deep sea, chert deposits are common within this period of change . One possible explanation: laterization produced free silica {Leclaire, 1 9 74 ) , which was partly stored in shelf seas where nutrients were trapped . The development of a thermocline produced silica-poor surface water (j ust as it produced 13 e-rich surface waters) and the recently trapped s i l ica could thus be leached off the shelves and transported to the deep sea . Upwel ling areap were as yet poorly developed : thus there were no highly locali zed sinks for s i l ica, but the deposits were spread out. Alternative hypotheses are considered by Steinberg ( 1 9 8 1 ) . •
.
3 . 5 . End o f Eocene The next large step occurs near the Eocene-Oligocene boundary (Figure 1 ) . Tt · markedly .incr.e ases the d i fference in 61 8 o values between low and high latitudes - the planetary temperature gradient sharpens sud denly. The simplest explanation for this step is that the deep ocean started to fill up with cold water produced in high latitudes (Savin 1 975' ; Kennett and Shackleton , 1 9 7 6 ) . The supporting evidence � � _ that this is so comes from benthi.c ostracods (Benson, 1 9 7 5 ) and also from benthic foraminifera (Dougla s , 1 97 3 ; Keigwin, 1 9 80) . However, the change of benthic foraminifera across the boundary is rather more ° gradual than what one might expec.t if the temperature drop from 1 0 C ° to 5 C postulated by Kennett and · Shackleton ( 1 9 7 6 ) is real (see Cor l i s s , 1 97 9 ) . A change in fauna does take 'place, but it apparently takes millions of years rather than a hundred thousand. The change proceeds from abyssal depths upward (Douglas and Woodruf f , 1 98 1 ) , in dicating new and growing influx of heavy abyssal water. Of course, we need not accept the suggestion of a 5 °C drop in tempe •. ,
I
I
I
•
rature . It is only the first of several possibilities: ( 1 ) a strong cooling of Antarcti.c coastal waters resulted in_ density increase, sinking and production of cOld bottom waters ; ( 2 ) alternatively or in add ition, cold bottom water could have been produced in a newly acces sible Arctic Ocean· ('Thi.e rstein and Berger, 1 9 7 8 ) ; ( 3 ) salinity (and therefore 1 8 o content) of the deep water could have increased , by moving a rainbelt off the Antarctic coast toward the tropics ( see Kemp, 1 9 7 8 ) ; when this belt moves away from the Antarcti.c shelf , the
138
shelf waters will become heavier , and w.i ll have a better chance of sinking, even without substantial cooling; ( 4 ) buildup of a large amount of ice on Antarctica would have extracted 1 6 o from the t0us increasing the 0 1 8o values as observed ( e . g . Matthews 1 9 80) . - This latter possibility appears unattractive because , of independent evidence for such a buildup of ice . Which of these possibilities applies and in what combination? is little doubt "that a considerable temperature drop in mid- and high latitudes was associated with this event. Paleobotanic evidence for North America (Wol fe, 1 9 7 8 ) and the oxygen record of shallow water molluscs in the North Atlantic (Burchardt, l 9 7 8 ) attest to fact, among other clues. The low silica values in Oli.gocene pelagi.c sediments in the central Pacific (Leinen, 1 97. 9 ) and elsewhere that a newly born Antarctic Current (Kennett , 1 9 7.7.) rerroved silica world ocean on a large-scale. Such a current would favor thermal isolation of Antarctica, and hence promote cooling of this albedo increase there and i_n the polar high, and migration of the rain large ice buildup involved? If so, we should see an increase stability in climate . On the whole , this does not seem to be judging from facies variability (carbonate scatter) in the central Pacific (Berger, 1 9 7.7. ) . In any case, the suddenness of the Eocene/Oligocene event is stri king. Strong positive feedback appears to be involved, perhaps both from albedo changes and through extraction of co 2 from the by a cooling ocean. There i s a rapid deepening of the Pacific carbo""' nate cOmpensation depth at the Eocene/Oligocene bOundary (Berger and Roth, 1 9 7 5 i van Andel , 1 9 7 5 ; von Andel et �. , 1 97. 5 ) . Thus , there no question that the carbon cycl e is involv�d in this event. 3 . 6 . 1 5 Million Year Event
The Oligocene,with its low di.versity planktonic fauna., i.t s cool tro pic s , low sedimentation rates , and high carbonate-to-silica ratios somewhat of a mystery as far as paleoceanography is concerned. The phenomenon of Braarudosphaera blooms (see van Andel et · al . , l97. 7 ) is striking and has defied explanation. A weak thermocline and sporadi.C open-ocean mixing is a possibility (see Berger, 1. 9 7 9 ) . A late Oligo cene cooling step may exist (Figure 1 ) but it has received little attention.
I' ,I
139
may be related to an ex·t;reme ;Low stand of sea. level in the La.te (see Vail et a l . , 1' 9 7 7 ) . The beginning of the Miocene is an increase in the temperature gradi.ent between tropi.ca.l
areas , by an increase of cold bottom water production and' an
increase in the overall ferti.lity of the ocean . Basi.c all-y, the tropics are darkened as a result of the Miocene transgression., an.d th,e whi.ten.,.. 18 ing of high latitude continues . In the Mid-Miocene , the 6 o values
of calcareous plankton and o f benthic ;foramini;fera increases sharply (Figure . 1 ) . This is generally interpreted as reflecting the bui.idup o'f a large ice cap in eastern Antarctica {Savin, Dougl,as , Stehl i ,
1 975 ;,. $hackleton and Kennett , 1 97 5 ; Savin, 1. 9 77) . This bui.;I.dup pre 16 feren tially extracts H 2 o from the ocean, thus enri.ching i.t with 1 8 o. The rapidity of the glaciation again suggests positive teedback
within the causal factors . Presumably the growth of· ice coincidin.g with a warming of the tropics strengthens the temperature gradi.ent and favors wind transport of moisture from the tropics into white
and hence cold polar areas. I.'t i s noteworthy that the maximum buildup apparently occurs during maximum transgression and maximum wari,ni.ng, of the tropic s , which is favorable for supplying moisture to the air.
A detailed study by Woodruff et · al. .
( 1 98 1 ) , based on the isotopi.c
analysis of benthic foraminifera from DSDP Site 2 8 9 in the western equatorial Paci fic , puts the mid-Miocene transition event at between 16 and 1 4 mil l ion Years B . P . with the greatest change occurring in the
latter part .of the interval (see. Fig. 2 ) . The transition itself· is marked by rapid o � cillations , which are reminis-cent o f Pl.eistocene.. type cycles : an indication of instability in the system . The source
of this instabi l i ty, presumably, is the ease with · which snowfields and even large continental glaciers may be built up and destroyed , so that minor fluctuations in �he forcing functions ( e . g . Milankovi.tch
mechanism) can be greatly ampl ified through drastic changes in albedo. The carbon i sotope signal given by Woodruff et a l .
( 1 98 1 ) has some
very intriguing properties vis-a-vis the oxygen isotope curve. From inspection, one can see that the two stable isotope signals show
positive correlation throughout the time period investig_a ted ( 20 to 6 m . y . bp) , on a scale of one million years or less . Thus, "warming" . 1 2 c , isotopes within the shells of results in a· higher proportion o f
the transition, however, the positive correlation is opposed by a negative one, on a scale of several m. y . On this scale, and within the crucial interval ( 1 8 m . y . bp to 1 3 m . y . bp) , "warming" results 1 3 c and vice versa . in enrichment of shells with
140
-0.5 m 0 0..
0 � uo
0 +0.5
II I
LATE MIOCENE Nl6 NI
I iI
• 1 .0 ' 1 .5
m
� p
MIDDLE MIOCENE EARLY MIOCENE Nl5 Nl4 Nl3 Nl2 I I 10 9 8 N7 N6 N 5
lj/I I I I I I I J I I I 1 1 1i l li i l l l l l li i 'r1 , i l ill / 1 1 1 l i l l l l l l i lj 111 1 1 1 1rliJ, 1 IJJ II I 1 1 1 1 1 1 1 lu1
' 1 .0 + L5
(<) • 2 .0
11 1 1,1 1 111JI IIJJ1 1 1 J11l// lr / l l il l l lj,1 1 / J 11 1 1 II 11
jJI
WOODRUFF,$AVIN, DOUGLAS, 1981 •2.5 DSDP SITE 289 ( W.EQ.PACIFIC) . II CIBICIDES SPP. (BENTH.FORAM.) • 3 ·0 t:::;N 14;[1Ntl1ffi I7==;oN1J§: -z N�6I;�N�5d +3.0 3 N!II2�U [['f 16::J:�N[E15[[N[� 1 1:ffiiO[j}9�8[[]N[�]J • 2.5
6
8
10
12
14
16
18
20 m.y.b.p.
Fig. 2 . Oxygen and carbon isotope trends in Miocene benthi_c fora minifera from DSDP Si.te 2 8 9 , western equatorial Pacific. Data from Woodruff , Savin, an.d Douglas ( 1_ 9 8 1 )
It is dif;ficult to conceive of any one cause which could produce the positive and the nega ti.ve co�rela_ti.o n . Thus r we :wust sea+:ch, t·or least two. Also , the positive correlation must reflect more rapid chemical processes than the negative one. Various ways to produce in the carbon isotope composition o f foraminiferal shells have been .. · · · proposed (see Berger and Vincent, 1 98 1 ) . Briefly, they are ( 'I ) or release of organic carbon from temporary reservoirs (especially superficial sediments and soi l ) , ( 2 ) changes in forest mas s , ( 3 ) changes in ocean stratification and fertility, and ( 4') chang·es in circulation and · oxygenation (see Section 2 . 6 ) . In the present case, for example , positive correlation might be produced by factors ( 3 ) and ( 4 ) , since warming presumably leads to greater aging o f deep waters , hence loss of oxygen , hence gain of 1 2c - rich .co . The long 2 term negative correlation would be mainly based on factor ( 1 ) : warming leads to a buildup o f organic carbon reservoirs on land, through increased supply of moisture and on the shel.f (transgression) .
141
2 such buildup extracts 1 c preferentially from the sy·stem ( somewha.t in analogy to the 1 60 extraction by the bui.ldup of glaci.ers) . Upon cool ing - and regression - the temporari.ly stored organic carbon i_s freed and adds 1 2c to the system {Table 5 ) .
Table 5 . Effects of change in c limate on stable isotope rati.os in calcareous deep sea fossils {hypothetical) , to explain pos� tive and negative correlatioris between 61 8o and ot3c si.gnals recorded in· benthic foraminifera {Figure 2 ) . Change in Climate
Effect on 61 8o Positive correla tion with & 1 80
warming
•remperature: tS 1 8o decrease
lee-melting
1 6o release : 61 80 decrease
Cooling
Temperature : 61 8o increase
Ice buildup
l6o extraction :
61 8o increase
Negati:ve corre lation wi.th &1'8c
Aging of deep C reservoirs water, co ·lncrease, ( org 2 land and sea) 6 1 3c decrease 2 buildup. : 1 c extraction, b1 3 c increase .
Younging of deep water, co decrease, 2
0 1 3c increase
c
. org reservo1.rs (land and sea) diminished: cS1 3c decrease
We have , then , according to this hypothe s i s , lorig-term reservoir effects and short-term circulation effects , which produce opposing correlations between !>1 8o and d1 3c sign.a l s . Presumably, therefore , mutual cancellation and reinforcement of climate feedback from hydro sphere and carbonsphere likewise are a matter of time scale. Following the 1 5 m. y. cooli.;pg step, there seems to be a rebound in the oxygen isotope signal, centered on 1 2 m . y . This rebound , if such i.t be, i.s preceded by a change in the b 1 3c signal suggesting release
142
of 1 2 c to the system. If we fol low t� e generally accepted argument of ice buildup near 1 5 m . y . , we must admit global regression and heightened erosion of carbon-rich superficial deposits in coastal lowlands and on shelves . Such erosion would tend to descrease b1 3 c · in the ocean ' s carbon content . It would also , presumably, increase dissolution of deep-sea carbonates , by titration of carbonic acid with carbonate. There is indeed evidence for enhanced di.ssolution at the end of the middle Miocene . Furthermore , the release of carbon to the system should increase the atmosphere ' s co 2 content : hence the warming and hence the rebound .
3 . 7 . 6 Million Year Event An excellent period for study, in order to test the type of specu lation about carbon sphere involvement that - I just put forward, i.s the time near 6 m . y . bp . This i.s the t ime of the " (Magneti.c) Epoch Carbon shiftn ( Savin, 1 9 7 9 ; Vincent et al . , 1 97 9 ) . :tts course and signi ficance has been dis-cussed by Bender and Keigwin ( 1 9 7 9 ) i ( 1 97 9 ) ; Keigwin and Shackleton ( 1 980) ; Vincent � a l . ( 1 980) ; Berger and Vincent ( 1 98 1 ) ; and Vincent and Berger ( 1 98 1 ) . The best record of the 6 m . y . Event is that of DSDP Site 2 3 8 , in the Ind.ian Ocean, for which detailed mul tichann.el signals exist in the form of stable i sOtopes for several planktonic and benthic species (Vincent � a l . , 1 980) . Haq et al ( 1 980) showed that the event can be used globally as a chronologie marker . The Indian Ocean record i s intriguing in that it clearly shows a covariance between planktonic and benthic signa l s , demonstrating deep and shallow water chemistries were equally involved . Hence the case for release of organic carbon to the system, to change the entire ocean ' s 1 3c; 1 2c ratio, is very strong (Vincent � al . , 1 980) . Evidence for a substantial peak in carbonate dis solution in the eastern equatorial Pacific, during the time of the ME-6 Carbon Shift (Saito et al . , 1 9 7 5 ; Dunn et al . , 1 9 8 1 ) s�ggests ti.trati.on of newly introduced organic carbon against carbonate , in conjunction with fer-tility increase and changes in deep circulation patterns ( cooling would favor development of a North Atlantic carbonate trap) . I t is significant that the various stable isotope signals of the several ;foraminiferal species. show much l.ess cOherence before the
143
' e.vent than after i t . Before the shif-t, several unideDti.fi.ed factors, ' none strong and dominant , appear involved in producing the record. ourin g and after the transition a dominant signa�-guiding factor appea rs to emerge . This factor may be linked to the repea.ted desi.cca tion of the Mediterranean Sea at the time, discover-ed d.uring Leg 1'3 (Ryan , Hsu et a l .· 1 1 9 7 3 ) . Climatic variations paralielin.g the desic cation cycles from albedo changes i n and around the M'editerranean , presumably were amplified by i.ce mass variation in. polar regi.on_s , . ,which i n turn affected s e a levei a.nd hence s i 11 depth at the M.edi. terranean: a class � c case of posi.tive internal feedback . The pulsating production of deep saline waters whiCh must have accompanied the restriction of the Mediterranean also enters the pi.cture , th;rough i.ts effect on heat budget and ferti�i.ty var-i.a tion.
3 . 8 . 3 Million Year Event Glaciation of the northern hemisphere commenced about 3 mi.:!J,i.on. years ago . The time since has been. characteriz:ed by fluctuations in th,e s ize of northern_ ice caps; i t i s the geologic setting we live in and are familiar with. The buildup of ice and associated climatic change appear to hi:lve been very sudden (Berggren, 1 97 2 ; Shackl.et0n and Op dyke , 1 97 7 ) . As in the previously discussed 6 m . y . Event , instabil,i.ty increases during the 3 m . y . transi tion and a fter ( S hackleton and Op dyke 1 1 97 7 ; Keigwin and Thunell., 1'97 9 ) . Presumably instability arises for similar reasons as before: heightened albedo sensitivity, and
transient reservoirs in hydros Phere and carbon sphere. In the eastern equatorial Pacific the 3 m . y . Event i s marked by pronoun.ced carbonate dissolution (Dis solution Pulse GU3 ; Hays et a l . , 1 9 6 9 ; Saito et al . 1 1 97 5 ) . Again , the relationship between regression., cooling, in.stabi.li ty, and di ssolution is renri niscent of the 6 m . y . Event. L.ar:ge-scale introduction of newly eroded organic carbon , and increased fertiiLty from fresh nutrient supply, may be indicated. I f organic ca.rbon. was indeed released by transient reservoirs , we should see a d 1 3c -shift at this time toward light values . Kaneps ( 1 9 7 3 ) shows the GU3 dis ° solution pulse at 1 80 m in DSDP Site 1 5 7 . A shift of 0 . 5 /oo in the b1 3 c signal for the benthic foraminifera Uvigerina occurs at thi.s leve l , in �he expec ted direction (Keigwin, 1 9 79 ) . Orbulina 1 the
planktonic foram, does not show the shift, howeve r .
The most striking effect of the northern glaciations on the deep-sea record is the greatly increased supply of terrigenous material to the
144
deep sea , and the · increasingly large cli�atic fluctuations in carbonate cycles (Arrhenius , 1 9 5 2 ) and in isotope cycles 1" 9 5 5 , 1 9 6 6 ) . On a cooling Earth it was unavoidable that the �oving and rising continents would eventually collect snow and ice. Nevertheless , we may ask why di4 it happen so suddenly , and why at that particular time. Several possibilities can be envisaged. On land, the ·rise of ranges during the Pl iocene could have reached a critical level. covered and tree-barren highlands would have increased the albedo the point where cooling and snow buildup reinforced each other . In the ocean, the closure of the Panama Straits about 3 mill ion years ago would have obstructed the westward f low of Caribbean surface waters (Keigwin , 1 9 7 8 ) . These waters , then, would have been for northward transport in the Gulf Stream System. The have been a sharp intensification of the Icelandic low region and an increase in atmospheric moisture in high to evaporation from the warm ocean current . In would have been available for building up ice. This type of has turned up in several ice age theorie s . 3 . 9 . Pleistocene Terminations The last mill ion years have been· . characterized by an entire closely spaced steps , whose signi ficance is as yet obscure. and van Donk ( 1 9 7 0 ) drew attention to the peculiar " sawtooth " o f the i sotopic curves of Emiliani ( 1 9 5 5 , 1 9 6 6 ) . A maximum always ending at the same level of intensity - is followed b y a rapid deglaciation culminating in a peak warm period . There follows an overall increase in ice cover . The trend toward increasing glaciation on the gentle side of each sawtooth , the gradual buildup, is readily explained by positive do feedback , that i s , by increasing amounts of reradiation from whitening Earth. The rapid deglaciation is more difficult to Hwo does the deglaciat ion get started? What feedback prOcesses it? Why is it so rapid? These questions are being addressed ing box cores from deep-sediments, containing the record of the deglaCiation . Detailed stratigraphic analysis of box core material suggests that the sought-for positive feedback for deglaciation may be contained
145
in the - buildup of a low salinity layer on top of the ocean, once deglaciation s tarts. (Berger et a l . , 1 9 77 ) . Once such a layer is establi shed, i t would greatly facilitate the transport of heat from the ·tropices to high latitudes . This would induce additional melting and thus maintain the low salinity layer - a typical case of positive feedback . The like-lihood of a low salinity layer establishing itself increases during the course of deglaciation. It is not particularly great at the beginning, a lthough even then deglaciation apparently proceeded quite quickly. The answer to this riddle may lie in a low �alinity Arctic whi�h freshened during its isolation from the glacial world ocean, when sea level was lowered {Vigdorchik, 1 98 0 ) . Thus , after a substantial period of isolation , when sea level first rose , low salinity water would be available to initiate the meltwater lid effect. There i s little question that the carbon cycle would be affected considerably by a buildup of a low salinity layer {Worthington, 1 9 6 8 ) . The apparent rise Of pco in the a tmosphere in the early Holocene 2 (Berner et a l . , 1'9 8 0 ) and especially the ev i dence for a fluctuating pco 2 during the glacial-Holocene transition ( see data of Delmas et a l . , 1 9 8 0 ) , must be considered with the possible effects of a transient meltw�ter lid in mind. There is no doubt that the carbon cycle was profoundly affec'ted by deglaciation: a preservation spike in forami nifera and pteropods occu·rs during the maximum rate of change of sea leve l , over wide regions of the ocean floor {Berge r , 1 97 7 ; Shackleton,
1 977) . 4 . General Features o f Climate Steps
4 . 1 . Surprise Events and Threshold Events Regarding climate events , we wish to know whether or not it is ex ternally caused and how much internal feedback is involved . Also, we wbuld like to know whether an event, in principle , could have been predicted before it occurred , and how accurately . I suggest that among the events discussed there is only one . unpre dictable one: the Cretaceous termination. It is truly a " surprise event " , because nothing in the preceding record suggests that doom is near. The view that there are no prophecies, right up to the point when catastrophe strikes , is not generally accepted , of course. There are
1 4�
those who po.ir}t to- a drop in sea level , to cha:n.ges in, ca;r;bonate position pattern s , or to declining d:i versity in various groups organisms in order to advance the idea that the event was the nation of a trend. (For review see Russell , one suspends justif iable doubts about the quality o f the ev£dence offered , there remains the impression that the postulated tal changes are merely part o:::- the background t:'l. uctuati.ons the late Cretaceous . Indications are th�t the external interference ' whether cosmic or tectoni.c . A certain · of environmental preconditioning, say , by a drop in sea �eve�, have exacerbated the calamitous impact of the disturbance, but not by itself produce the event . On thi s , if anything, the record is clear (Thierstein and Okada , 1 97 9 ) . All other events discussed appear to be predlctable , The major climate steps in the Tertiary lie on a coo�ing trend : constitute acceleration of this trend . The most llkely mechanism acceleration is albedo feedback , and the most efficient duci� g phenomenon is the 'changeover from water to snow , (Table 2 ) . Each step, in this interpretation , reflects the of a portion of the earth ' s . surface , at the point where t<•mper2
.
i
..
4 . 2 . Rebound Events What about the events which oppose the general cooling trend? less spectacular than the major steps listed they are abundant out the Tertiary (Figures 1 and 2 ) and are especially Plei.stocene , as deglaciation events or " ter:t:nination s " . On the of analogy with the Pleistocene terminations ( albeit at the prejudging the issue ) , I suggest that the reverse steps are events " which owe their existence to three ingredients: ( 1' ) side force ' or a negative feedback overshOot temporarily cooling trend , ( 2 ) a sufficient buildup of ice all.owing level rise upon melting, and ( 3 ) positive albedo feedpack se of sea level , the disappearance of snow cover, and the vegetation . There is one corollary of the 11reboundu hypothe s i s : the reverse steps should get larger and perhaps more common. as ice build. up proceeds throughout the Cenozoic . Statistics on thi.s point are not
BIB/\ 108H KH l01 Qi\tA"Oillklll! lllll 110Y l�l1 KM MtHllKfi� mv""� s ava, wo"� ' but the assertion appears reasonable that rb��;;:::,;.;� ps increasingly large and abundant in the late Cenoz·oi c' and especial
_ _ _. _
Pleistocene proper . In fact, it would seem that the ing trend has run its course , so that any additional cooling steps only take place after a rebound step has provided the opportunity new ice buildup. In this view, the c limate of the last mill ion years cons ists of a compressed sequence of cooling steps and rebounds . Maximum instability has been reached because the areas affected by rospheric albedo change were never larger , and because ice masses .hy? peak glaciation were never more voluminous . Thus , the main sources of positive feedback are greater than ever before in the Cenozo ic .
4 . 3 . Forcing Events Unless we assume that c l imate is inherently unstable and flips over from one state into another more or less by chance, each cl imate chan must have an identifiable "ultimate cause " . Such a "cause" should outside the feedback system, and should be able to start the change . A plethora of possible causes for c limatic change has been listed in the literature . They may be grouped under the headings geologic settin-g , orbital variations , solar phys ics , and ·cosmic effects. Causes related to geologic setting have been invoked for the general Cenozoic cooling trend: the drift of northern continents .away from the equator , the isolation of Antarctica and of the Arctic , mountain building and global regression (Hamilton, 1 96 8 ; Donn and Shaw, 1 9 77 ) . Orbital variations (Croll , 1 97 5 ; Milankovich , 1 9 3 0 ) demonStrably dominate Pleistocene c l imatic f luctuations (Hay� et al . , 1 9 7 6 ) . Little is known about possible variations in solar output and about cosmic effects if any. Volcanic activity, changes in sea level from changes in seafloor spreadinq rate s , and changes in salinity due to buildup or release of salt deposits are other themes which turn up in dis cussions about climatic change . There is one class of potential forcing events which are of special interest because they might produce steps by themselves , without the aid of feedback . These are inundation and des iccation events which are produced by the rapid filling or emty.ing of semi-isolated ocean bas ins . They produce essentially instantaneous global regressions or transgressions (Berger and Winterer , 1 9 74 ) . Salt depos its in the �editerranean , in the Gulf of Mexico , in the South Atlantic , and in
148 the North Atlantic , suggest that large evaporite basins were at various times after the breakup of the Pangaea . It should be possible to find evidence for the associated periods o f instabil ' of sea level in the shelf sediments of the last 2 0 0 million years 4 . 4 . Gateway Events as Triggers As mentioned in the introduct ion, the idea that changes _in the opening and closin� of straits and passages especially ocean circulation and hence climate has gained considerable favor and currency. There are good reasons for this developing (Berggren and Hollister, 1 9 7 4 , 1 9 7 7 ; Savin et a l . , 1 9 7 5 ; Kennet, 1 9 7 7 ; McGowran, 1 97 8 ; Lance lot, 1 98 0 ; Berger, 1 98 1 a ; Haq , 1 9 8 1 ) . is certainly striking that major circulation adjustments can be associated with just about every one of the major Tertiary steps . Quite commonly there is an embarrassment of riches : one a choice of several aateway events which are close enough to a climate step to qualify as trigger events (Berggren , 1' 9 8 1 ; Haq, I n the ear·l y Eocene, there i s the separation of Austral,ia from arctica , development of a passage between North Atlantic and ·the incipient collision of India and Asia, closing a east-west passage . At the end of the Eocene , there is a deepening the passage between North Atlantic and Arcti c , and the incipient collision of India and Asia , closing a tropical east-west pas sag-e . the end of the Eocene, there is a deepening of the passage between Antarctica and Australi a , and of that from Atlantic to Arct i c , and ther e is the closure of the central Tethys . In the mid-Miocene themes again appear : opening of the Drake Passage in the Southern Ocean , closure of Tethys , and access from the Atlantic to the Nor wegian Sea and to the Arctic Ocean (Thiede , 1 98 0 ; Eldholm and 1 98 0 ; voget et a l . , 1 98 1 ) . For the 6 m . y . Event we have the isolation of the Mediterranean, and. for the 3 m . y . Event the closure of the Panama Straits . For the Plei� stocene terminations , fi_nal l y , we can invoke an isolated Arctic (Vigdorchick1 1 9 8 0 ) , if we so desire. The arguments for the importance of gateway trigger events range from plausible ( thermal isolation of the Antarctic by development of the Circum-�ntarctic Current; Savin et al . , 1 9 7 5 ) to �xtended ( importance of " in j ection even·ts " ; Thierstein and Berger , 1 9 7 8 ) . They are not
149
< .c:ornp'"" """" , because ( 1 ) the assignment of gateway events to climate commonly arbitrary ( the opening of Drake Passage , for examp 'le , has been invoked for c limate steps both of Eocene and of Miocene age) , and { 2 ) they suffer from a lack even of e lementary heat budget incidental l y , is true for the present essay) . summary , gateway triggers are a good ide a , but they may not be steps ,
4 . 5 . Stability and COherence as Criteria for Event Type In several instances we saw that a c l imate step may result in altered clima te stability and coherence between climate-·r elated signals. During the course of the Cenozoic , albedo feedback and ice masses increase in importance . I take this to be responsible for an apparent overall decrease in stability and an increase in coherence of signals since the Oligocene . By this argument, each step which results in decreased stability supports the presumption of ice mass buildup . A special type of instability source is the semi-isolated ocean basin such as the · Mediterranean and the Arctic . Geographic isolation im plies a potential for ' bU ildup of large reservoirs o f hypersaline or of br acki�h water. Beyond the intriguing possibility of total dis connection .and sudden reconnection o f such basins (Gartner and Keany,
1 9 7 8 ; Thierstein and Berget , 1 97 8 ) they constitute an important element of deep ci.rculation. The Mediterranean Sea with its deep
saline outflow is an actualistic example . Its outflow apparently is linked to the production of North Atlantic Deep Water (see Reid, 1 9 7 9 ) , and thus the Mediterranean c limate may be closely l inked to
the heat budget of the Norwegian Sea , itself a climatic amplifier of great power ( Ruddirnan and Mcintyre, 1 98 1 ) . Ice mas ses , and isolated basins with their "odd" water masses are hydrospheric examples of "transient reservoirs" which can be called upon to introduce geochemical diseqUilibrium to the ocean-atmosphere system. Others (carbon , phosphorus , salt} can b� envisaged {Berger et a l . , 1 9 8 1 ) . St i l l , for the Cenozoiq and especially the late Ceno zoic there is probably no transient reservoir effect which exceeds that of the ice masses and thei.r enormous leverage through sea level change .
150
5 . Conclusion and Outlook I have proposed that ciin,ate steps are r ea l , an.d that they r-eflect the action of feedback mechanisms mainly from hydrospheric (albedo effect) but also from changes in the carbon cycle effeft) . On the whol e , the greenhouse feedback appears less and the checks and balances within it lead one to suspect that feedback has a moderating rather than an arnpli.fying role . I.f so, intriguing possibility arises that we are dealing with a system fluctuates because of imbalances between opposing feedback rne:ct\ar1i :3(lU3 ;: The fast-response albedo feedback would provide for overshoot in direction of change , whi le the lagging greenhouse effect would be responsible for reversing directions . There is some indication the la� t great climate step, deglacia tion , displays properties such an oscillating system (Berger, 1 9 8 1b ) . Examples in vari.ous of the geologic record, and on various tilne sqales, must be to establish the reality of this postulated phenomenon . The importance of transient geochemical reservoirs other than ice ( water, carbon , phosphorus1 metal nutrients, sulfate and sulfide ; salt) in abetting climate instability i.s worth considering. The ,,,
up of Pangaea and the subsequent development of isolated basins and long pass ive margins should have greatly favored the development of such reservoirs ( see Hay, 1 98 1 ) . The crucial factor in their activa tion is sea level fluctuation, which rema ins as the central problem in global stratigraphy (Vail et al . , 1 9 7 7 ) . References
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Keigwin , L . D . , J r . ( 1 9 7 9 ) : Late Cenozoic stable isotope and plaeoceanography of DSDP Sites from the east equatorial north central Pacific Ocean. Earth & Planetary Science Letters 361-382. Keig.w in, L . D . , J r . ( 1 9 8 0 ) : Oxygen and carbon i sotope analyses from Eocene/Oligocene boundary at DSDP Site 2 7 7 . Nature 287 � 722-72 5 . Keigwin, L . D . , Jr . , Shackleton , N . J . ( 1 9 8 0 ) : Uppermost Miocene isotope stratigra Phy of a piston core in the equatorial Pacific . Nature 2 8 4 : 6 1 3-6 1 4 . Keigwin , L . D . , Thunell , R . C . ( 1 9 7 9 ) : Middle Pliocene climatic in the western Mediterranean from faunal and oxygen isotopic trends . Nature 2 8 2 : 294-29 6 . Kemp, E . M . ( 1 9 7 8 ) : Tertiary climatic evolution and vegetation in the Southeast Indian Ocean region. matology , Paleoecology 2 4 : 1 6 9-208 . Kennett , J . P . ( 1 9 7 7 ) : Cenozoic evolution of Antarctic glaciation , the C ircum-Antarctic Current and their impact on global pale oceanography . Journal of Geophysical Research 8 2 : 3 8 4 3 - 38 6 0 . Kennett, J . P . , Shackleton , N . J . ( 1 9 7 6 ) : oxygen isotopic evidence development of the psychrosphere 38 m . y . ago. Nature 2 6 0 : 5 1 3- 5 1 Kennett, J . P . , Thunel l , R . C . ( 1 9 7 5 ) : Global increase i n Quaternary explosive volcanism. Science 1 8 7 : 4 9 7 - 5 0 3 . Lance lot , Y . ( 1 9 8 0 ) : Environnements sedimentaires oceaniques : deve- . loppement de la pal€o-oc€anographie. Livre Jubilaire de la . soc. g€ol. de France, 1 8 3 0 - 1 9 8 0 . Mem. h . S€r . g9ol. de France, 1 98 0 , 1 0 , 3 5 1 -3 6 2 . Leclaire, L . ( 1 9 7 4 ) : Hypothese sur l ' origine des silifications dans les grands bassins oceaniques . Le rOle des climats hydrolisants. Bull. . Soc . Geol . France 1 6 : 2 1 4 -2 2 4 . Leinen, M . ( 1 9 7 9 ) : Biogenic silica accumulation in the Central Equa torial Pacific and its implications for Cenozoic paleoceanography Geological Society of America Bulletin 9 0 : 1 3 1 0- 1 3 1 6 . Lipps , J . H . , Berger, W . H . , Buzas , M . A . , Dougla s , R . G . , Ross, C . A . { 1 9 7 9 ) : Foraminiferal ecology and paleoecology . SEPM Short Course' No . 6 , Houston , Texas , 1 9 8 pp. Luterbacher, H . P . , Premoli-Silva, I . ( 1 9 6 4 ) : Biostratigrafia del limite Cretaceo-Terziario. Riv. I ta l . Paleont. 7 0 : 6 7- 1 2 8 . Matthews , R . K . , Poor e , R . Z . { 1 98 0 ) : Tertiary Q1 8o record and glacio� eustatic sea level fluctuations . Geology 8 : 5 0 1 - 5 0 4 . McGowran , B . ( 1 9 7 8 ) : Stratigraphic record o f Early Tertiary oceanic and continental events in the Indian Ocean region . Marine 26: 1-39. Milankovic h , M . ( 1 93 0 ) : Mathematische K limalehre und astronomische Theorie der Kl imaschwankungen. Handbuch der Kl�matologie, vol . 1 A , Berlin, Gebr . Borntraeger , 1 7 6 pp. Pisias , N . G . { 1 9 7 6 ) : Late Quaternary variations in sedimentation rate in the Panama Basin and the identification o·f orbital fre quencies in carbonate and opal deposition rates. Geological Socie ty qf America Memoir 1 4 5 : 3 7 5- 3 9 1 . Plass , G . N . ( 1 9 5 6 ) : The carbon dioxide theory of climatic change. Tellus 8 : 1 4 0 - 1 5 4 . Reid, J . L . ( 1 9 7 9 ) : On the contribution of the Mediterranean sea out-
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Ryan , W . B . F . , Hsli , K . J . , Cita, M . B . , Dumitrica, P . , Lort, J . M . , Mayne·, w . , Nesteroff , W . D . , Pautot , G . , Stradner, H . , Wez e l , F . C . ( 1 9 7 3 ) : I,flitial Reports of the Deep Sea Drilling Proj ect, VOl . 1 3 , parts 1 and 2 , U . s . GOvernment Printing Office , washington, D . C . , 1 44 7 pp.
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.Part ITA. Event Stratification
Calcareous and Quartz-Sandy Tempestites
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General Remarks About Event Deposits A. SEILACHER
Abstract: Rare, but geologica·lly common events such as severe storms , floods , turbidity currents , seismic shocks and volcanic eruptions leave behind bed units with distinctive sedimentological and ecological features and succ.e ssions . Event stratigraphy has pOtential applications in correlation , basin analysis and evolutionary research. The present generation of earth scientists has become aware that the history of the biosphere is not only one of gradual and stately changes but that it is accentuated by events of various kinds and degrees , most of which are so rar.e that they refute a uniformitarian approach . In the context of the present work shop we have not dealt with the dramatic "very rare events " that may be responsible for global faunal changes arid not even with the more common sea level changes due to glaciations and changes in the rates of sea floor spreading . We rather focuss e"d on more local catastrophies that are common enough to appear in any section, bu t in terms of human life spans and experience still deserve to be called "rare events " . 1 . Ki�ds of Rare, Event Deposits Volcanic ashfalls (and dust from meteoric impac�s) are examples in which geochemical and crista llographic " fingerprints" may be used to distinguish fndividual beds over long distances ( "tephra stratigraph y " ; WINTER, 1 9 7 7 and in press) ; but apart from their mineralogic composition , such beds are little distinctive with respect to sedimentological or ecological features . Similarly , seismic shocks may alter the structure of the gradationally compacted upper sediment layers in a dist'inctive way ( " seismites " , SEILACHER, 1 9 6 9 ) . But. their sedimentological and ecological effects are negligeabl� except through mass flows and turbidity currents that they may trigger. In this case , however , the resulting beds can no more be distinguished from olistostromes or turbidites released by other kinds of events . Cyclic and Even t Stratification (ed. by Einsele/Sei.lacher) © Springer 1982
162
The effects of turbulence events are more complex , because they imply sedimentological as well as ecological successions within the depositional unit. Storm deposits (tempestites } , turbidity current deposits ( turbidites } and f lood deposits ( inundites) have in common ; { 1 ) that they reflect the onset, culmination and waning of water turbulence during the event by distinctive erosional and depositional structures .
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( 2 ) that they re-distribute the organic and inorganic sediment material along a vertical {bottom to top) and horizontal (shallow to deep) gradient . ( 3 ) that they change the ecological situation for benthic organisms by altering the consistency and/or the food content of the bottom for a biologically relevant period after the event . In the following paragraphs only the effects of turbulence events � with special emphasis on tempestites , will be further discussed . 2.
Sedimentologic Conse'quences of Turbulence Events
In the ideal case we can assume a symmetric succession of erosional phases during the increase, and of depos itional phases during the decrease , of the episodic turbulence (Fig . l a ) . Erosional Phases of different magnitude can be distinguished in the record as far as they reflec·t a compactional gradient within an otherwise uniform substrate . Depending on the strength of the event , increasingly consistent layers will be reached by the erosion . The result is a succession of erosional phenomena , of which only the · last generation will be preserved as casts on the sole of t�e overlying event deposit . The gradation of the depositional phases during the waning of the epi�odic turbulence is two-fold . Given a uniform sediment , bed forms will change according to the decreasing modes of flow regimes and particle transport . In most cases , however , the transported sediment is not uniform, but contains particles of varying s i z e , shape and specific weight, which will be deposited in a graded succession according to their settling velocity . In the case of a polymodal particle content , sedimentation can
163
become discontinuous , the phases being separated by intervals of non-sedimentation . The symmetrY between erosional and depositional level in the ideal cycle is most Closely approximated in tempestites , because the turbulence is wave-domina·ted and more or less stationary , i . e . lateral sediment transport i s subOrdinate . In turbidites and inundites , however , the turbulence peak shifts later.ally during the even t , so that the levels of the erosional and the depositional phases in any given place will rarely correspond (Fig . l b ) . The . same - i s true in rip currents that compensate for coastal water build-ups during storm s . Another complication results from the fact that most substrates are not homogenous . In many cases the substrate already contains the sedimentologic memories of previous events that may change the erosion behaviour from layer to layer . This memory is less consequential in sandy sediments , where only shelly or congl.o meratic layers block the erosion. I t is more important in muddy sediments , in which previous erosion surfaces become further compacted by intermittent overload . The memory becomes most . enhanced in carbonate mud s , because early diagenesis tends to selectively cement erosional mud s·u rfaces buried under calcarenitic or S helly event deposits , so that these surfaces Qecome more and more resistant reference horizons during repeated re-burial and re-exposure by subsequent events . The processes involved are not yet well und�rstood , but field evidence suggests that they are laigely responsible for the formation of hardgrounds and their common association with tempestitic condensation horizons (FURSICH , 1 9 7 1 ) in shallow marine sequences , particularly during regress ive phases caused by lowering of the sea level or reduced subsidence . In principle , condensation phenomena may occur in all kinds o f turbulence events , but due to the higher rates of net sedimentation and lateral transport in turbiditic and flood regimes , they are less prominent in these than in storm-dominated sequences . In shallow seas , most background sediments become deposited only temporari ly , because the next storm is likely to rework them and carry them further down towards the center of the basin. What we eventually see in the sedimentary record is the final
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PROXIMALITY GRADIENTS SYM M E T R Y
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Fig. 1 . A) In the ideal case , levels of the erosional (a-c) and depositional regime {A-C) should cor respond at any point along the proximal-distal gradient of a turbulence event ( upper diagram) . In re ality ( lower diagram) , shoreward accumulation O f water masses (during a storm� , or gravity flows fol lowing the paleoslope , have commonly _ d1storted this syffimetry, so that lower levels of erosion are fol lowed by unproportionately high-level deposition. This effect is most pronounced in turbidites and flood deposi t s ; but i t may also occur in s torm deposits . As one consequence , firm ground representing high level erosion may be left without depositional cover in the proximal zone. B-H) These diagrams , with dramatically exaggerated vertical scale , visualize vertical and lateral modifications in a tempestite . Erosion is directly proportional to the energy level (decreasing from left to right) only in homogeneous substrate ( B ) . In gradationally compacted mud .(C-E ) it will decre ase at lower levels . In sediments strUctured by the sedimentary memories of previous events (F-H ) , erosion wil l follow ancient bedding planes and expose them over wide areas according to their ero sional resistivity . Deposition is continuous only if the available event sediment is uniform. Poly modal grain size distribution, usual in pre-sorted shallow marine sediments , introduces intervals o f non-sedimentation (black arrows ) , during which the exposed surfaces may b e modified by hydrodynamic (shell orientation; ripple reworking) or by biologic ( trace formation) processes
8l
166
result of countless erosional and depositiona! .episodes . Of these only the strongest can still be recognized , because this event tends to wipe out the traces of intermittent weaker events . The degree of cannibalism and event condensation is an important facies criterion , because it allows us to distinguish between proximal and distal zones in a lateral , and between times of faster or slower subsidence and deposition in a vertical, direction. 3 . Ecologic Consequences of Turbulence Events
From our human perspective we tend to overrate the immediate , destructive impacts of catastrophic events . In terms of geologic and evolutionary time , however , the less dramatic after-effects are the more important ones . Event erosion commonly disturbs and removes epibenthic and shallow infaunal populations over large areas , their dead remains becoming transported to other environments ( for instance to the beach) or incorporated in the overlying event deposits . Their fate will be discussed in the chapter on taphonomic consequences . Since only limited areas are affected by turbulence events , their. recolonization from unaffected regions in the neighbourhood is no problem . Neverthel�ss _the after-event community commonly differs in character and species content from the backgro�nd commuriity. This is readily understandable in proximal zones of asymmetrical event deposits , where the eroded surfaces have remained uncovered , or the sandy, shelly or gravelly sedimentation was not followed by a muddy tai l . The firmer or coarser substrates naturally attract a fauna different from the native background community (oysters , brachiopods , crinoids , stromatolites; firm ground burrowers of the Glossifungites association ) . Examples are firm- and hardground communities (FURS ICH , 1 9 7 1 ; see also contribution by AIGNER) and the attached communities or community successions of autochthonous coquinas (see contributio_n by HAGDORN & MUNDLOS ) . It should be
noted, howeve r , that event-condensation commonly produces "pseudo-successions" of communities separated by burial/reac � ivation intervals . Their regular sequence does not reflect biological interdependence , but the maturation of the substrate during subsequent events (Fig . 2 ) .
167
@ Soft ground continuously bioturbated by deposit feeders ( end of Maliere sedimentation)
Erosional firm ground burrowed by 1 s t . Tha l a s s inoides generation @ Reactivation of same horizon after burial & transfor mation into hard ground @ Below hard cavities are wa•sh.ed out along ancient burrows . . @ Encrus ters and borers set tle on exposed rock surfaces QD' cavities filled with soft mud ( Couche Verte) ® After reactivati on , compacted fills burrowed by 2nd
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generation
oncoids · & encrusta tion by stromatoli tes after repeated reactivation ( Congl .����� de Bayeux ) .
G) Multiple reburial by
oolites with size-graded mixture of fossils from different zones (Calc . a OolitheS Ferrugineuses ) .
Fig. 2 . The already complex history of a mid-Jurassic condensed section (FURSICH 1 9 7 1 ) can be better Uriderstood if we add a number of burial/reactivation events ( arrows) that have left no direct sedimentary record except in the oolithe ( i ) . In this modified view the observed faunistic sequences represent not true ecological successions , but independent communities that did respond to diffe rent substrate conditions in a maturing reference horizon and were separated by long burial intervals
168
But post-event communities also tend to differ from the background fauna if the top of the event deposit is muddy and granulometrica lly undistinguishable from the background mud . The difference is most pro?ounced in the turbiditic flysch facies , where the pr�-turbidite background burrows (preserved as casts on the sole faces) are consistently different from the burroWs that are dug to different levels by the one-phase post-turbidite community. This difference mainly reflects food supply : While the pre-turbiditic background fauna subsists on an impoverished but continuous rain · of degraded organic material from the photic zone , the post-turbidite fauna makes use of the fact that detrital food, derived from local reworking or . event-imported from shelf areas , is concentrated in the top layers of the graded event deposit . Accordingly the post-event communities are more opportunistic, i . e . their species diversity is lower. and differs from bed to bed in spite of high densities . There is also a difference in trophic s trategies : Direct sedi�ent - feeding prevails in the post-turbidite communities , while the less compact but highly organized patterns of pre-tur bidite burrows suggest food extraction .with the help of exo symbiotically 11farmed " microbionts {SEILACHER , 1 9 7 7 ) .
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Similar, t�ough less pronounced differences between pre- and post-event mud burrowers can be expected in f lood and storm beds . Distinction of the two would be useful to pin-point the top of the event-deposited muds either directly or by way of the penetration level of the stratified burrow community that is recorded on the interface between the coarser and the einer part of the event deposit. 4 . 'f<;phonomic Consequences of Turbulence Events
In the paleontological record , the immediate victims of catastrophic events play a - subordinate - role. Notable exceptions are autochthonous brachiopod communities that became buried in life position in the lower few centimeters of shallow marine sandstone beds (SEILACHER , 1 9 6 8 ) , obviously without previous erosion or transport. Event-imported arthropods preserved at the end of their last track at the sole of Solnhofen lithographic limestone beds (GOLDRING & SEILACaER, 1 9 7 1 ) may be cited as another example .
169
r
I
More mobile organisms may start to work their way up trough the smothering sediment , leaving escapement tracks behind. This reaction , however , depends on the thickness and the kind of the overlying sediment. In modern experiments , NICHOLS et a l . ( 1 9 7 8 ) found that escapement was not attempted i f the overload exceeded about 30 em. Escapement was more difficult o'r impos sible if the
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smothering sediment was finer grained. The common occurrence of completely preserved echinoderms at the top of the coarser part of event deposits ( ROSENKRANZ , 1 9 7 1 ) possibly ref lects the fcital c loggi-ng of their ambulacral system by the fine mud that was d�posited during the final phase of the event� Most body fossils found in event-deposits , however , are reworke d . In par-autochthonous coquinas (see contributions by T . AIGNER and HAGDORN & · MUNDLOS) they largely represent the local epi- and infauna of the background mud s . Their faunal spectrum may in fact , be less distorted in the coquinas than in their native muddy sediment , where the aragonitic shells tend to become selectively dissolved during early diagenesis . Repeated reworking , such as occurs in "cannibalistic" storm beds ,
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wi l l eventually change the faunal spectrum in the coquinas as wel l . Aragon i tic shells , diagenetically weakened during buria l , disaPpear first (although new ones may be added during each event) . Calcitic shells w i l l be next , probably followed by
echinoderm .remains 1 whose high-magnesium calcite becomes soon transformed into a rather resistant and eventually solid form of cal9ite. Most resistant are the apatitic skeletal remains , particularly teeth , because they also tend to gain, rather than loose , durability during burial in phosphorous-rich sediments . This is effiphasized by the fact that in the resulting bone beds
I
the originally soft coprolites , phospatized and brok �n during repeated burial and reworking 1 form a common element (see
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Thus all-shell coquinas , calcitic shell beds , crinoidal limestones and bone beds form a series of increasingly mature concentration
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contribution by
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deposits. Their. distribution in the stratigraphic co lumn reflects the degree of cannibalism and of regressi vi ty, wi t.h the bone
170
beds accentuating the final {or by renewed reworking the following initial) phase of a transgressive - regressive cycl e . A few deviations from this model should b e mentioned . 1.
Since new material becomes admixed during every eve:n,t , c annibalistic event deposits may always contain well preserved elements of low durability , though with a much lower percentage than in the original thanatocoenosi s .
2 . There are condensation horizons i n which 11ghost ?-nunonites" are
,. ·:.
as well preserved as the associated younger forms (FURSICH , 1 9 7 1 ; contribution by G . GEBHARD in this volume ) . Still their close association with manyphase hard grounds and i ncrustations below , plus the fact that the heterochronous ammonites are size-graded w�thin the condensed layer , strongly suggest repeated reworking. In these cases it is im�ortant to note that the host sediment largely consists of aragqnitic muds or oolites , by which the shells would be diagenetically protected , while the storm reworking .may have been too rare and too short to effectively abrade the fossils . 3 . It should also be remembered that the ecologic situation may
have changed considerably during the complex h istory of one bed . While the first tempestite cont�ined only reworked shells of · the soft-mud fauna � the basal surface may have subsequelltly. been colonized by firm- and eventually hard-bottom communities during later exposure s . Remains of all communities , plus current-imported allochthonous forms , will be incorporated i nto the final shell bed . 4 . Early diagenetic cementation is favoured not only below the
eroded mud surfaces . In addition, pressure shadow concretions may form in mudfilled shells , strongly increasing their duiability in subsequent cycles {contribution by GEBHARD ) These as well as ordinary concretions will increase the amount of reworkable diaclasts during success ive events . Multiple growth ( "hiatus concretions'' VOIGT1 1 9 6 8 and i-ncrustation or boring by sessile organisms {KENNEDY & GARRISON , 1 9 7 5 ) may reveal the complex histories of the individual clasts in such cases . · •
171
5 . Fields of Application
Because of the interrelationship of sedimentological and biologi cal processes , the concept of event-stratigraphy is relevant to dif ferent fields of research . 5 . 1 . Shelf Sedimentation apd Geomorphology
Taking a steady state ocean as a null-hypothes i s , we should expect the marginal sediment wedge to develope an equilibrium surface near the extreme s torm wave. base . Except in coastal �reas and aroUnd carbonate buildups , in which sediment pro duction and resistivity may far exceed rare storm erosion, extreme events would tend to winnow down excessive sediment accumulations to this equilibrium leve l , while the winnowed fines would gradually prograde the shelf edge basinwards . The sa·me basic model can be applied to epicontinental basins and lakes , but with a shallower shelf level corresponding to the reduced storm wave depth . . In this paradigm, negative changes in relative sea · level (caused . by tectoriic , e �s tatic or sedimentational processes ) will be
reflected by condensations of coarser and more durable sediments , 1 while a re·lative sea level rise Will favour more complete accu mulation of finer sediment s , on the shelf surface. In the case of a rapid rise i n sea leve l , however, the old shelf sbrface could become inactivated , forming a terrace below a new shelf prograding at a higher leve l . The imp lications o f this paradigm for the analysis o f lar ge-scale stratigraphic sequences and for the prospection of hydrocarbon and mineral deposits are obvious .
5 . 2 . Basin Analysis Just as turbidites , which have been used in the past to re _ · construct the geometry of flysch basins , storm and flood depo sits provide distality criteri� either individually or on a sta tistical basis (see contribution by AIGNER ) . Since s torm waves (and floods ) originate at sea level and reach deeper the stronger
172
they are , the number of tempestites between fingerprinted marker beds may be also used to contour the basin morphology (AIGNER, 1 9 79 ) , while their occurrence in general prov·ides . a paleobathymetric datum. 5 . 3 . High- resolution Stratigraphic Correlation Since the events here considered do last only for days , they potentially provide a measure stick for within-basin correlation that is many orders of magnitude more exact than the evolutionary change ·of index fossils . The usefulness of this measure stick in disparate outcrops depends on · our ability to identify individual beds by rhythmograms or 'by ecological or geochemical fingerprinting.
The success of seisrnostratigraphy (PAYTON 1 9 77 }/t�hich allows to trace physically distinct beds continuously , probably
rests to a large part on event stratification. 5 . 4 . Community Evolution Since turbulence events intermittently provide similar bottom conditions that allow the expansion of a specific post-event benthic fauna , . community structures · in subsequent colonisation episodes should provide an answer as to how random this structure actually is . Also there may be acci dental or evolutionary replacements if there is an upper limit to species diversity . 5 . 5 . Species Evolution In the discussion about a gradualist�c versus a punctuational mode of evolution , storm beds may also provide important evidence . I f our assumption is correct, population sizes of firm- and hardground dwellers became extremely expanded following erosional events and became reduced during the long intervals ,which in turn were the high time for the soft mud background fauna . Preliminary observations {Terebratula beds of the German Muschelkalk; Oyster and carolia beds in the Eocene of Egypt) suggest that there is a considerable change in form , size and behaviour from one autochthonous coquina to the next and that this change is not linear . I n comparison , the species of the background fauna seem to change con·siderably less ( SEILACHER , 1 9 8 1 ) .
173
In conclusion , we feel that the complexity of the processes involved and the variety of phenomena to be observed make· event deposits , in particular the tempestite s , an interesting field for integrated studies by sedimentologists , geochemists and paleontologists . References AIGNER, T . 1 9 7 9 : Schill-Tempestite im Oberen Muschelkalk , (Trias , SW-Deutschland) . - N . Jb . Geol.Palaont . ,Abh. ��z : 3 2 6 - 3 4 3 . F liRSICH, F . 1 9 71 : H�rtgrUnde und Kondensation im Dogger von Ca1vados . - N . Jb . Geol . PaUiont . , Abh.1 ,J.,4� : 3 1 3-342 . GOLDRING , R .
&
SE ILACHER , A . 1 9 7 1 : Limulid undertracks and their
sedimentological implications . - N . Jb . Geol .Palaont . , Abh .
,),;);( : 4 2 2- 4 4 2 . KENNEDY , W . J .
&
KLINGE R , H . C . 1 9 7 2 : Hiatus concretions and
&
GARRISON, R . E . 1 9 7 5 : Morphology and genes is of
hardground 'horizons in the Cretaceous of Zululand . Paleontology, J � : 5 3 9 - 5 4 9 . KENNEDY , W . J .
nodular phosphates in the Cenomanian Glauconitic Marl of South-east Eng��n d . - Lethaia , � : 3 3 9-360 . NICHOLS , J . A . ; ROWE , G . T . ; CLIFFORD , C . H . ; YOUNG, R . A . 1 9 7 8 : In situ experiments on the burial of marine invertebrates . J . Sed . Petr .1 �� : 4 1 9 - 4 2 5 . PAYTON , C . E . {ed .·) 1 9 7 7 : Seismic stratigraphy - Application to hydrocarbon exploration . - AAPG Mem . , �g . ROSENKRANZ , D . 1 9 7 1 : Zur Sedimentologie und 6kologie von Echinodermen-Lagerstatten . - N . Jb . Geol .Palaont . , Abh . 1 J,:j� : 2 2 1 -2 5 8 . SEILACHER, A . 1 9 6 8 : Origin and diagenesis of the Oriskany Sandstone ( Lower Devonian , Appalachians) as reflected in its shell fossi ls . - I n : Recent Developments in Carbonate Sedimentology in Central Europe , 1 7 5 - 1 8 5 , 7 Abb . , Heidel berg (Springer) 1 9 6 8 . SEILACHER, A . 1 9 6 9 : Fault-graded beds interpreted a s seismi tes . - Sedimentology , �� , 1 5 5 - 1 5 9 , Amsterdam.
174
SEILACHER, A. 1977 : P attern analysis of PaZeodietyon and related trace fossil s . - Trace fossils 2 (CRIMES & HARPER, Spec . Issue No . 9 : 2 8 9 - 33 4 , 1 5 figs ed. ) . Geol •
.
•
.
SEILACHER, A . 1 9 8 1 : Towards an evolutionary stratigraphy . In : Concept and Method in Paleontology ; Acta geo l . Hispan ica, lg: 39-44 . VOIGT , E . 1 9 6 8 : Uber Hiatus-Konkretionen (dargestellt an Bei spieleri aus dem Lias ) . - Geol . Rundsch .1 �� : 2 8 1 - 2 9 6 . WINTER, J . 1 977 : Stabile Spurenelemente als Leit-Indikatoren einer tephrostratigraphischen Korrelation (Grenzbereich Unte�-/Mitteldevon , Eifel-Belgian) . - News l . Stratigr . , g : 1 52 - 1 70 . WINTER, J . ( i n press ) : 11Exakte tephrostratigraphische Korre lation mit morphologisch differenzierten Z irkonpopulationen ( Grenzbereich Unter-/Mittel -Devon , Eifel-Ardenhen) . N . Jb . Geol . Palaont.
Experiments on the Distinction of Wave and Current ' Influenced Shell Accumulations E. FuTIERER
Abstrac t : Edgewise and stringer accumulations o f shells may b e formed by current as well as wave activity , but the results can be dis t�h guished by particular criteria. This distinction will be important for the recognition of wave-generated s torm coquinas . ExtenQing on earlier studies about current orientations of isolated shells , the occurrence of a 2nd stable position is referred to higher current velocities . Principles controlling the familiar orientation of biogenic par t:i cles ( stable positions use.d as current indicators) are valid mainly for isolated objects . This has been shown by many experi ments ( FUTTERER, N . Jb . Geol . Palaont . , Abh . l�g , 1 9 7 8 ) . In shell accumulations these stable positions are rare due to the reciprocal impediments exerted by neighbouring objects . Since the formation of shell accumulations is very complex, field observations must be supplement by experimental data , if we want to 11decode" the orig1..n and the hydrodynamic environment of fossil shell accumu lations .
1 . Edgewise Shell Accumulations
l
] ''
I
!
Edgewise coquinas are generally attributed to wave action (MI I , Res . Bull . , �g , 1 9 5 7 ; SCHAFER , Ecology and Palaeoecology o f Marine Envirortments , 1 9 7 2 ) in analog to flat pebbles placed vertically by wave action ( ROZANSKI , J . Geol . , �1 1 9 4 3 ; DIONNE , Canad . J . , Earth S c . , � � 1 9 7 1 ; SANDERSON & DONOVAN , J . Sediment. Petrol . , �� , 1 9 7 4 ) . Only few authors refer edgewise shell accumulations to --
current action (MULLER, lehrb . d . Palaozoologie ) . All edgewise shell accumulations have in common that they appear in shallow water and that the shells and pebbles involved are more or less flat . Flume studies with a sample of single valves of Mya arenaria demon
I
l l !.
strate that edgewis e shell accumulations form under wave as well as current conditions . Howeve r , the results can be distinguished in the two hydrodynamic regimes ( see Tab . 1 ) . Cyclic and Event Stratification (ed . by Einsele/Seilacher) © Springer 1982
176
Tab . 1 .
. Distinction of wave and current generated edgewise shell accumulations waves
currents
direction of long
di fferent
shell axes
orientations
:': uniform, at right angles to the current
inclination
various
constant , generally
directions
upcurrent
stacks in various directions
imbrication in one direction ( down-current)
accumulations
In shal low water , waves and currents commonly occur in combination, The dominant for ce , however, determines the type of the resulting edgewise shell accumulations . 2.
Linear Shell Accumulations
Longitudinal shell accumulations .nstringers·" with more than 20 specimens have been recorded · from fossil c:eposi ts by several au thors ( HAUF."F , Paleontographica, §,£, 1 9 2 1 ; KLii.HN , Jb . vaterl . Ver . Naturkd . Wlirtt . , �g , 1 9 2 9 ; MOORS , Sed . Geo l . , � � 1 9 70 ; BRENNER , N . Jb . Geol . Palao n t . Abh . , l�J , 1 9 7 6 ) . The observed accumulations are mainly composed of ammonites . Shells of pelecypods , ostracods and graptolites , however , can also form such linear accumulations . I t is d�fficult to decide whether their formation is due to wave or current action. Some criteria allowing this distinction have f.'.f,
been found by field observations . in the Wealden shales of s . Eng land ( together with T . AIGNER) and by supplementary flume experi 2 ) . I t is important to stress that longitudinal shell bands formed by currents run parallel to the current direc tion, whereas linear shell accumulations caused by wave action are
ments ( see tab
•
.
parallel to the wave crests . In shallow water environments wave motion is generally combined with currents , the direction of which is more or less transversal to the wave crests . Linear shell accumulations occur mostly in argillaceous deposits ( e . g . graptolite shales , Lias epsilon b ituminous shales , Wealden shales ) , deposited under quiet water conditions . As flume experi ments show, the occurrence of shell bands is an unequivocable in dication that water movement (waves or currents) has at least epi sodically influences
the
sediment.
177
Tab. 2 .
Distinction between linear shell accumulations caused by wave and current action waves
distance between
uniform, corres
shell bands
ponding to ripple
currents
variable
dis tance frequenc y of
always numerous
shell bands shell position wi th.im the bands deposition of very small biogenic par
imbrication in long axis of shells parallel to the bands one direction on both s i d e s only on the lee ward side of the of the bands bands
ticles direction of gutter casts caused by
often isolated 1 rarely numerous
transversal to the bands
parallel to the bands
temporary currents
3 . Appendix - Additional Studies with Isolated Biogenic Particles 3 . 1 . Single Pelecypod Valves I
A number of observations with single shell valves have shown that a second stable position occurs during their transport ( FUTTERE R , N . Jb . Geol. Palaont . , Abh . , l�g, 1 9 7 8 ) . Subsequently , left v�lves of abOut 20 different pelecypod species have been se-
I
J I I
1
I
I
I
systematic transport•stability studies -- depending lected for on the position of their umboes -- in an unidirectional current. The valves were placed in the current with the convex sides up in four different· umbo positions - each displaced by 90° . Fig . 1 shows the following results for 3 variously formed types of pele cypods : ( a ) Circular pelecypods ( Gtycimeri s ) are most stable when their umboes point down-stream; (b) Elongated tri angular pelecypods with terminal umbo ( My t i Z u s ) are also most stable when their umboes point down-stream• { c ) Elliptical pelecypods with the umbo at one of the long sides (Pe trico Z a ) are roo st stable when the long axis is parallel
to the current and the posterior end of the valve points up stream.
0
178 "'
§ 60 '
270°
+ o•
90"
180°
o::>
)I
...··
· ·· ...- · ·· · ·
.... .
40 .-
20
0
become •
• •
Glycimeris
Myfi/us
glycimeris
edu/is
Petr icofa
pholadiformis
270°
0°
Position of umbo
The principles hitherto believed to govern shell orientation (MULLER, Abh. Dt .. Akad . Wiss . Berlin, Kl . Math . u . All g . Natur wiss . , � � 1 9 50) are supplemented but not contradicted by these results . The occurrence of pelecypod shells with the s econd stable position in fossil ' deposits , however , suggests stronger currents (critical velocity of transport more than 50 cmjs ) . 3 . 2 . Gastropod Shells Evaluation ( together with T. AIGNER) of plano-spiral and troche spiral gastropod shells (Planorbina sp . and G alha s p . respecti vely) from Oligocene freshwater deposits ( Bembridge limestone) in S . England ( Isle of Wight) confirmed the stable orientation found in flume studies ( FUTTERER , N. Jb . Geo l . Palaont. Abh . , J�g, 1 9 7 8 ) . As the orientation roses show ( F i g . 2 } , Planorbina shells have their stable position with the aperture downstream and the margin of the aperture parallel to the current. The high spired Galba shells lie in a stable position with the long axis · parallel to the curren t , but apart from the predomina�t maximum with the apex-stream (most stable position ) , there is a smaller submaximum with the apex down-stream. Experiments in the flume have shown taht troche-spiral shells ( e . g . Lymnae a ) tend to get anchored with the aperture lying flat on the bottom, whereby the apex may turn either against or with the current .
179
Fig. 2 . Orientation roses for Planorbina ( 2 1 readings) and Galba ( 1 3 1 readings) from the Bembri'dge limestone ( O ligocene , Isle of Wight, s . England)
Planorbina
121I
Golba
1 1 31 I
15% '
N
t
//
/
'
/
' '
References FUTTERER , E . ( 1 9 7 8 ) : Untersuchungen tiber die Sink- und Transport geschwindigkeit biogener Hartteile . - N. Jb .. Geol . PaUion t . Abh . , d�� : 3 1 8- 3 5 9 . FUTTERER, E . ( 1 9 7 8 ) : Fossil-Lagerstatten Nr . 4 4 : Studien Uber die Einregelung , Anlagerung und E inbettung biogener Hartteile im StrOmungskanal . - N . Jb . Geol . Palaont . Abh . , J�g : 8 7 - 1 3 1 .
Calcareous Tempestites: Storm-dominated Stratification in Upper Muschelkalk Limestones (Middle Trias, SW-Germany) T. AIGNER Abstract : Storm-induced high energy events dominate stratifica tion phenomena in Upper Muschelkalk epicontinental carbonates . Within s torm beds ( tempestites) diagnostic sedimentological and paleoecological features show an orderly gradation from bottom to top and from proximal to dtstal facies . ThiS concept has implications for basin ana�ysis , stratigraphy, paleoecology and diagenesis .
1 . In traduction Stratification in carbonates is far from being fully understood . Apart from long-term allocyclic changes ( see section 1 of this vo lume ) , changes in 'energy level may be responsible for bedding and lamination in limestones { e . g . HARDIE
&
GINSBURG , 1 9 7 7 ) . In addition ,
diagenetic processes have commonly altered or enhanced the original picture. Studies in recent sediments demonstrate the importance of powerful storm event s , which may have significant effects in shelf morphology and sediment transport (reviews in KELLING & MULLIN 1 9 7 5 and in BRENCHLEY et ai . 1 9 79 ) . Although storm effects in ancient rocks cannot be proven with absolute certainty , the object of this paper is to demonstrate examples of event-stratification from the Upper Muschelkalk (Middle Trias , SW-Germany) , which are likely due to storms (AIGNER 1 9 77 , 1 9 7 9 ) and thus represent " tempestites 11 (AGER 1973) •
The Muschelkalk is a shallow marine epicontinental carbonate se quence , representing a marine ingression into the Germanic Basin separating_ the continental phases of the Lower and Upper Trias . Further SE , borehole data suggest a: belt of sandy, probably litto ral sediments along . the NE-SW trending shoreline (Fig. 1 ) . In an offshore direction , a zone of crinoidal or shelly , partly oolitic "shoals" is follOwed by the more open marine "Tonpla:tten " facies , whiqh comprises laterally persistent limestones interbedded with mar l s . It is in this facies that tempestites can be most clearly recognized (Fig. 2 ) . Cyclic and Event Stratification (ed. by Einsele/Seilacher) © Springer 1982
F 1 81
Fig. 1 .
Paleogeographical setting 1 i sopachs ·and overall facies cross-section of the Upper Muschelkalk basin (Modified after GEYER & GWINNER 1 9 6 8 , MUNDLOS 1 9 7 6 , and AIGNER & FUTTERER 1 9 7 8 )
2 . Descriptions 2 . 1 . Lithofacies Limest�nes vary between calcirudites , calcarenites and calcilutites . PackS tone depositional textures are most abundant , but mudstones ,
.:·. '
I
wackestones and even grainstones may also be found . Mol luscan , brachiopod (and occasiona l vertebrate ) bioclasts form the bulk of the components , while few smaller fossils can be recognised ·
in the calcilutites , except for Schizospha$re Z-la . Intraclasts (often traceable to subj acent layers) and extraclasts ( s i lt/fine sandstone and "Black Pebbles " ) are particularily common in thicker beds. Lithoclasts , usually subangular in shape, range in size from a few
mrn
up to 40 x 50 x. 5 ern ( in marginal areas )
•
Bed thickness ranges between a few ern and a few dm. Although the bedding is very regular , amalgamation of beds is conspicuous in many instances. A em-thick "1.!-nderbed" of microsparite firmly attached to the base of many tempestites , probably represents a diagenetic overprint (see 4 . 1 . ) .
182
Fig. 2 . Typical stratification sequences through , Muschelkalk tempestites : A } basal shell lag, followed by plane lamination and toppe d by wave-ripple lamination (Upper Muschel kalk , Kernmtihle ) . B ) parallel and low-angle lamination, followed by convolute bedding ( Upper Muschelkalk , Gottwolls hausen) C ) shell bed with platY interclasts showing imbrication. (Lower Muschelkalk , leg. R. RAUSCH) . •
D ) graded packstone (note mud-shelters below shells and imbricated f ibrics) , passing into slightly convoluted plane bed and into ripples (Upper Muschelkalk , Crailsheirn) 2 . 2 . succession Within Single Beds a) The base of an individual bed is always sharp and erosional . Sole marks include large scours (up to several m broad and 20 em deep ) , gutter casts and tool marks (Fig. 3d , e ) . Thes e , however , are commonly hidden by the encrusting underbed . b) Internally , limestone beds display three main types of succes sions :
(Fig. 2 )
( 1 } crudely graded bedding of litho- and bio
clast s , ( 2 ) couplet of a basal shell l�yer followed by laminated calcilutite and ( 3 ) laminated calcilutite alone . Bioclasts show all stages of disarticulation and shells may be imbricated,
183
edge-wise or convex-down , although orientations more or less pa rallel to the bedding plane predominate . Laminations include in this succession - plane lamination , low angle and hummocky cross-stratification , wave-ripple lamination . Rippledrift and convolute bedding also occur in some beds (Fig. 2 ) .
c ) Bed tops may be of four basic types : { 1 } gradational to the over lying marls , ( 2 ) with primary current lineation ( Fig . 3 c) , ( 3 ) as hardground ( 4 ) with wave-ripples (height ranging between 1 and 10 em, wavelenghths between 60-100 - .i n calcarenites/rudite s , Fig. 3 a , b - and 7 - 1 5 em - 'in calcilutite s ) . Wave ripples are bften slightly asymmetrical both in shape and internal structure , and their crests trend parallel or s lightly oblique to the suppo sed shoreline .
Fig. 3 . Some common top (a-c) and bottom features (d-e ) in calcareous tempestites . a) large-scale wave-ripples in cross-section with "spill over11 ( scale 2 5 em) . Troch i tenkalk, Kernmi.ihle . b ) wave-ripple field ( length of compass 1 1 em) . Upper Hauptmuschelkalk, Barenhaldenmlih l e . c ) pr1mary current lineation on bed top. d) scour with abundant rip up clasts . e ) tool marks at bottom surface of a distal tempe's tite
184
2 . 3 . Biofacies
a) The erosional bed bases may be burrowed by Glossifungites and Talassinoides Pebbles and larger reworked plates are commonly concentrated near the base and are often encrusted on both sides with several stages of colonisation (AIGNER 1 9 7 9 , Fig. 7 ) . b ) The shelly fauna within the coquinas may range between two types : ( 1 ) soft bottom association, dominated by shallow burrowing and epifauna·l bivalves , scaphopods and gastropods (Myophoria, Pseudoaorbula,
;·.::
Hoernesia,
Entolium, Enta l i s ,
Omphaloptyaha
etc. ) . In contrast ,, the deep-burrowing bivalve P l euromya is rare in the shell pave�ents while it is most abundant in the interstratified marls . ( 2 ) mixed faunas , containing epifaunal Terebratulids , Lima , Pecti nids ,. and epifaunal encrusters (Enantios treon_, Placunop s i s ) i n addition t o elements from ( 1 ) . c ) Three different biologic situations can be recognised at bed tops (Fi.g. 4 ) : ( 1 ) Topmost part o.f beds bioturbated and penetrated by spreite burrows (Rhizooora l lium_, Tei a h i a hnus ) , the degree of biOturbation significantly decreasing downward and commonly stopping at top of the lower coquinoidal part . ( 2 ) Epifaunal bivalves ( e . g . Lima_, Pectinids , partly in life attitude} and btachiopods colonising upper sur faces . { 3 ) Hardgrounds , perforated by borers and encrusted by Ena n t i o s treon and PZaaunopsi s .
Fig. 4 . Ecological variation in post-tempestite fauna. - softground, burrowed by Teiohiehnius (Upper Muschelkalk, Ummenhofen) - firmground, colonised by Lima and Pecten ( Lower Muschelkalk , leg . H . HAGDORN) hardground, encrusted by oysters (Upper Muschelkalk, Heldenmtihle) -
185 2 . 4 . Results
· ( 1 ) Stratification similar to the BOUMA-sequence suggests that many limestone beds formed under high-energy conditions in a waning flow regime . ( 2 ) Some of the beds , mainly the thinner and finer grained ones , were rapidly deposited as one-event beds . ( 3 ) Other beds , mainly the thicker and coarser ones , represent the accumulation of several erosive and depositional events , i . e . they are multiple-event beds . Mixed faunas and abundant extraclasts in these composite beds indicate a multiple pro venance or a complex series of truncation and colonisation episodes . IDEAL TEMPESTITE- SEQUENCE + HYDRODYNAMIC INTEPRETATION
1i•1@1#11;1·i1¥ I. ��== � � ��p��----L--=_O���� � = j I ! - � �� � - - - - t -
pelitic division w v
LAMINAR FLOW M
UPPER FLOW M -
graded bedding
redeposition of suspend ed detritus .
very low
--mOd;ra
-
w
high
I
I
1
-
-
-
-
-
very high
very low
Fig. 5 ,
The idealised tempestite sequence corresponds closely to the BOUMA-sequence , except for the wave-rippled top . Since plane lamination in most cases passes gradually into wave-ripple lamination , wave-ripples belong to the complete tempestite sequence and are not due to a subsequent phase of reworking
3 . Interpretation
3 . 1 . Processes a) Depositional Environment . The abundance of fine-grained shelf se diment suggests an environment below normal wave-base which was disturbed only episodically by storm waves . On the other h and , the f auna indicates a marine shelf platform.
186 Since many beds are laterally persistent for several kilometers , a relatively even topography with a low paleo-slope towards the basin center may be inferred . b) Frequency of Events . From Plaeunopsis growth rates , WAGNER ( 1 93 6 ) assumed 2 5 000 years for the deposition o f one meter of Muschelkalk sediment giving an average sedimentation rate of 4 cm/1000 y . Assu ming roughly 10 m . y . for the deposition of about 200 m sediment (re presenting the whole Muschelkalk) , sedimentation rate would be 2 cm/1000 y . Since 5 - 10 storm beds may be typically present in 1 m , a rough estimate suggests that each event took place every 2 500 . 5 000 years according to �AGNERs assumption and every 5000 - 10 000 years according to the second calculation . Comparable orders of magnitude (one event per 400 - 2 000 years) are given by GOLDRING & LANGENSTRASSEN ( 1 9 7 9 ) for Devonian sheet sandstones . In the Present , AGER ( 1 97 3 ) .reports a 9 5 % probability for a hurricane to occur once every '3 000 years at any one point of the Gulf of Mexico . For Ordo vician storm sandstones , BRENCHLEY et al . ( 1 9 7 9 ) assume a 10 000 1 5 000 year periodicity. Such frequencies are far too low compared to the 20 - 50 year fre quency of major storms in the North Sea ( REINECK et al . 1 9 68 ) . But it should be remembered that it is difficult to single out all the events condensed in composite beds or minor storms that have left little record of their passage . c ) Nature of Events Stratification similar to the BOUMA-sequence (Fig. 2 , 5 ) strongly suggests that many limestone beds were deposited by high-energy events . In shallow marine environments , storms are the most likely· initiating mechanism, but the variety of factors in .•
volved makes detailed int�rpretat!ons difficult. Two main categories of processes, occurring independently , or , more likely , in combina tion , have to be considered (KELLING & MULLIN 1 9 7 5 ) :
( 1 ) Waves : They are responsible for s'tirring-up and rewoiking bottom sediments in-situ and result in 11swell lags" ( BRENNER & DAVIES 19.7 3 ) . SPECHT & BRENNER ( 19 7 9 ) consider fabrics like geopetal voids and grain-sheltered patches as indicative for storm-wave winnowing. ( 2 ) Currents account for lateral transport and deposition of alloch thonous sediment . Various initiating factors (winddrift , back flow currents , storm surge tides) as well as various depositional mechanisms (suspension and bottom currents , see WALKER 1 9 7 9 , and rip currents) may be responsible.
187 Muschelkalk tempestites suggest that both categories were involved: Wave indicators (oscillation ripples , grain-shelters etc . ) are asso ciated with current featu"res (parting lineation , lateral shell transport etc. ) , but current-ripple lamination is conspicuously rare. HAMBLIN & WALKER ( 19 7 9 ) suggested that hummocky cross strati f i cation would be characteristic for density current deposition un der the influence of storm-wave surge below fair-weather wave base . Muschelkalk gutter casts are parallel to shore (AIGNER & FUTTERER 1 9 78 ) , while the oscillation ripples suggest on/Offshore winds and ' wave s ; internal tempestite features indicate offshore flow towards the NW, probably compensating for near-shore water build-up ( comp . HAYES 1 9 6 7 ) .
3 . 2 . Proximality a ) Concept. Storm effects decrease towards deeper, offshore bottoms . This obvious relationship has been demonstrated in modern environ ments (HAYES 1 9 6 7 , fig. 3 ; CURRAY 1 960 , fig. 6 ; POWERS & KINSMAN 1 9 5 3 ; REINECK & SINGH 1 9 7 5 , figs . 4 5 8 and 4 79 ) . The basinward de crease of both storm-waves and s torm-induced currents should be ref 1.ected in a lateral succession of bedforms; i . e . tempestite s , like turbidites , should range between proximal and distal end. types . Muschelkalk storm beds can be arranged within such a proximality framework: { 1 ) Proximal teffipestites are relatively thick-bedded , bioclast-dominated and coarse-grained calcirudite s , commonly for ming composite and amalgamated beds . ( 2 ) Distal equivalents are mud-dOminated and thinner one-event beds , mainly calcilutites (Fig. 6 ) . Detailed correlation of beds shows that massive coarse calCirudites actually do pass laterally into thin finerg�ained calcirudites , calcarenites and calcilutites (Fig. 7 ) . A tempestite-frequency map , computed from log-data of VOLLRATH ( 1 9 55 ; see AIGNER 1 9 7 9 , fig. 5 ) , shows a consistent relationship between the isopachs and 11iso-tempestites " , i . e . a gradual decrease in the number of h i s "SchalentrUmmerk�lke11 (roughly equivalent to proximal beds ) . from marginal to basinal localities in a time-equi valent part of the section. More detailed cross-sections (Fig. 8 ) , however, show that this relationship is probably too simpli fied , because one thick proximal bed may distally split up into several thinner ones (Fig. 7 ) . In any case , tempestite proximality de-
188
Fig . 6 . A ) Typical n proximal tempestite 11 • No'te overall grading, geopetal f i l l of voids , abundance of interclasts and extraclasts ( 11black pebbles11 and s i l tstone clasts . B , C). Typical 11distal tem pestite s 11 : dominance of fine-grained pelleted carbonate with fine bioclastic debri s . At base micro-scours and underbeds SSE
«p r o x i m a l»
pebbly calci rudite
« d i s t .a I»
NNW
3 km
rudite
F i g . 7 . Lateral change in tempestite s : a uproximal11 bed passing within about 3 km into several "distal11 beds . {Exposures Kernmi.ihle - Baren haldenmtihle near Crailsheim, right is basinwards) . This example emphasizes the composite nature of many proximal beds and indicates their potential function as source areas for distal beds
189
creases away from shore , as expressed by a decrease in bed thick ness , grain s i z e , bioclast content, and by a Change in sediment fabrics ( e . g . increase in abundance of mudstones , no grainstones
in basinal s-ection of Fig. 8 ) .
b) Control s . The distribution of proximal and distal tempestites i s likely . to be controlled by water depth and b y a variety of factors
other than depth. Thus , the general sedimentation model (Fig. 10)
of proximal (nearshore) and distal (offshore) beds will be modified
SE
marginal
b asinal
NW
fabrics
bed-0
bed-0
gram s1ze
gram s1ze
"L .,
" cu
. _
Fig. 8 .
.,;
_
� -
:::::
:
Change of tempestite facies in a section from marginal to basinal settings ( a Heldenmlihle , Unterohrn ) . Note the following trends : b =
=
- depositional fabrics : dominance of mudstones and no grainstones in basinal area. - bed-thickness : decrease towards basinal. - grain size: increasing abundance of calcilutites (black) in basinal , distal beds
190
by factors such as shelf morphology and by a number of factors in volving the nature of storms . Subsequent events may also be crucial in enhancing the 11proximal appearance" of a particular bed . Such secondary 11proximalisation11 will become most effective 1 ) in shallo wer near-shore zones , 2 ) in areas of s low rate of subsidence re lative to sedimentation, 3 ) in progradational/regressive s i tuations . Under each of these conditions , proximal tempestites should be more abundant and show evidence of repeated reworking or the amalgama tion of several events and ecologic communitie s .
3 . 3 . Results
·, .�
{ 1 ) Tempestites are deposited in shallow water bY sudden influx of sediment and rapid deposition caused by rare high-energy events , most likely storms and hurricanes. ( 2 ) The particular association of sedimentary str�ctures suggests the presence of both unidirectional and oscillatory currents . ( 3 ) Vertically , tempestite s equences are characteri sed by a) an upward gradient of sedimentary structures , and b) a downward countergradient it:t the degree of bioturbation. ( 4 ) Laterally , tempestites show a more or less pronounced proximality gradient, which may become exaggerated by subsequent ev.ents.
4 . Implications 4 . 1 . Diagenesis Many detrital tempestites show a em-thick layer of microsparite firmly attached to the erosive bas� of the shelly bed. Similar 11underbeds" have been described by MEISCHNER ( 1 9 6 7 ) and EDER ( 1 9 70 , 1 9 7 1 , 1 9 7 5 , t h i s volume) from calcareous turbidite s . EDER interprets this layer as a secondary e f fect of diageneti c carbonate redistribu tion. The following observations suggest an early diagenetic origin also for the Muschelkalk underbeds : ( 1 ) uncompressed preservation of Rhizocorallid spreite burrows in the underbeds indicat�s early ce mentation ; ( 2 ) gutter casts are commonly rimmed by a cementation aureole cutting across internal laminations ( Fi g . 9 a , b ) ; ( 3 ) intra clasts may be traced to subjacent underbeds ;
( 4 ) where penecontempo
raneous erosion has removed again the whole shell bed , the exhumed underbed could already act as a firm- or hardground for specific
191
burrowers , borers and encrusters , or even get broken up into large plates (AIGNER 1979 , Fig. 2 , 7 ) . Thus , " underbed s " may be regarded as a type of concretion controlled by the sudqen porosity change at the base of the tempestite .
Fig. 9 . Underbeds as diagenetic aureoles below event surface s . A ) Concretion-like pillow below small gutter cast (Upper Muschelkalk , Kernmlihle} B} Cementation aureole around a gutter cast below shell bed preserves even the lamination of a previous smaller-scale event ( Upper Muschelkalk , Garnberg) . C ) Underbed below crinoidal limestone preserves the strongly burrowed firmground s tage ( Lower Haupt muschelkalk, Neckarwestheim, leg. 0 . LINCK) . D ) Composite bed: underbed of first event ( lower part) probably remained covered with sediment , whereas underbed of second event ( upper part) became exposed and burrowed as a f irmground prior to final burial ( Upper Muschelkalk , Tiefenbach )
192
Since reworking plays a'n important role in shallow-marine sedimenta tion , the underbed mechanism may be important 1 } in enhancing the resistance of the underlying muds against sub sequent erosion and in hardground formation (LINDSTRtiM 1 9 79 ) . 2 ) as an indicator for composite beds : burrowed or bored and en
crusted firm- or hardgrounds record previous phases of event-de position and reexposure of the underbed (Fig. 9 c , d ) . 3 ) In acting as a local base ( "reference horizon" ) on which several subsequent events are only partly recorded due to 11cannibalistic" reworking and partial condensation .
4 . 2 . Paleoecology
a) General o As we have seen, erosional or depositional storm events alter substrate conditions on the sea floor and p�rticularly so in carbonate muds . The biologic response to such events will therefore be mainly· controlled by a change in substrate consistency . Principally , the establishment o f a post-tempestite fauna i s simi lar to that known from turbidites (SEILACHER I960) ' but it may be complicated ( a ) by subsequent phases of event reworking and ( b ) by interaction with early diagenetic processes ( in carbonates ) . As demonstrated above , recolonized surfaces on tempestite tops or ba ses may be softground s , firrngrounds or hardgrounds (Fig. 4 ) . Commu nity succession may involve all phases from soft- to firm and - in case of gradual lithification - to hardgrounds , or be unpredictably modified through subsequent events . b ) Distortion of Faunal Spectrum. KREISA & BAMBACH (this vel . ) show, that faunas o f Paleozoic storm-packstones closely correspond to those in the unreworked i X:t.e rstratified sediments . In the Upper Muschelkalk , 11rich" faunas in the tempestites do not in general correspond to the 11poor" faunas of the interlayered mud s . In the latter , shells were mainly aragonitic and are preseryed as inter nal molds , with the deep-burrowing bivalve Pleuromya predominating (mostly in life attitude ) . In contrast, Pteuromya is extremely rare in the shelly tempestite s , while epifauna and shallow burrowers are abundant . Faunistic reconstructions based on storm-reworked shell material should therefore consider possible distortions of the faunal spectrum caused by several factors :
193
1 . distortion by level of erosion: shallow levels of a stratified benthic community are more likely to be eroded , while deeper burrowers remain undisturbe d .
2 . Diagenetic distortion : non-reworked aragonitic shells may have eventually been dissolved· in the muds , whereas the. same shells . survived when reworked into the different diagenetic regime of a coquin a . Storm reworking thus increased the preservation poten tial of these s he l l s . 3 . 9omposite b e d distortion: due t o repeated phases of burial and re-exposure , shell beds may have served as substrates for epi and infaunal colonisation by different organisms , that led to an increase ip eventual diversity . 4 . transport distortion : a "miXed fauna11 including e lements from different habi tats , may also result from lateral and commonly selective , transport of shell material . REINECK et a l .
( 19 6 8 ) for
instance , have recorded living and dead Hydro b i a shells displaced from tidal flats into offshore storm layers.
4 . 3 . Basin Analysis a) Lateral Sequences . Although proximal or distal tempestites may dominate in di fferent parts o f a sequence , the relative abundance of the two types appears to show a significant variation across the basin ( see Fig . 8 ) . Proximal tempestites are more corrunon in a '1near -shore facies " , wh ich can be distinguished from an "open shelf fa cies " , dominated by distal tempestites ( Fi g . 10) . Further work should treat tempestites in a more quantitative way of analysis , an approach which has been successfully applied to deep turbidite basins ( WALKER 1 9 6 7 , 19 70; LOVELL 1970) . Eventual l y , tempes'tite features may b e transformed into contour-type map s , re vealing the geometry and facies-pattern of shallow storm-dominated basins . b) Vertical Sequences . Apart from two major shallowing cycles in the Upper Muschelkalk (Fig. 1 ) , smaller-scale cycles , a few meters thick, may also be present . For instance , distal type tempestites may pass upwards into proximal ones and eventually into massive shell accumulations (up to 2 m thick e . g . Spiriferina-Bank in Crailsheim area) , which are commonly cross-bedded . This may be in-
L
194
FACIES - MODEL offshore , •• ...
TEMPESTITES
«proximal»
facies
bioclast-dom.
paleocurrents base
J. to
shore
channelled
':N•••.
, ....�-�--
« distal .. mud-dominated
II to shore impacts
grain size intra-extracl. amalgamation
Fig. 10.
Tentative facies model accounting for the lateral change of tempestite facies in terms of proximality as shown in th·e Muschelkalk exampl'e (W wave base, storm wave base) . Paleocurrents in distal SW areas are mainly deduced from gutter casts =
=
terpreted as a shallowing sequence , probably due to the prograda tion of the source area . Simil ar cycles have been described from terrigenous clastic facies ( e . g . GOLDRING & BRIDGES 1 9 7 3 and HOBDAY & READING 1 9 7 2 ) .
4 . 4 . Stratigraphy Many Muschelkalk tempesti t_e s , in particular thicker shell beds 1 can
be traced for a few kilometers (see AIGNER 1 9 7 9 , fig . 4 ) , less fre quently for some lO ' s of kilometers. A few distinctive shell beds
with tempestite features (Spiriferina-beds of the Lower and Upper Muschelkalk 1 see SCHWARZ 1 9 7 5 , 1 9 7 7 and SCHKFER 1 9 7 3 ) can be traced nearly throughout the basin 1 except the very basin c enter. , Since storms form instantaneous and isochronic events , tempestites are potentially ideal for correlation on a regional and in certain cases probably even on a basin-wide scale ( BALL 1 9 7 1 ) . 11Event-stra tigraphy11 based on such horizons might refine biostratigraphic re solution by several orders of magni tude .
195
Basin-wide marker beds are rare because they require a particular combination of sedimentary factor s . Low shelf bottom relief and low rates of subsidence· and sedimentation (which favour the quantum like accumulation of many events over a long period and even the establishment of a new fauna) may be important . No. matter how many events are condensed in one bed, it will be the final large event that ultimately coins the overall aspect of the stratification. ,· 5 . Conclusions
Stol::m-induced high energy events are important processes in genera ting stratification within the Upper Muschelkalk : 1 . Storms control the development of various types of bedform se quences in tempestite s , which may be differentiated, as in turbi dite s , · between proximal and distal end members . 2 . These proximality gradientS help to understand vertical and la teral facies sequences in storm-dominated shelf environment s . 3 . Deposition o f calcareous tempestites (or o f calcareous turbidites) may cause early diagenetic cementation of underlying muds ( under beds ) , thus enhancing the formation of intraclast s , as well as firm- and hardground s , during subsequent events .
4 . Paleoecologically , storms i'nfluence the establishment and succes sion of particular communities mainly by changing the consistency of the substrate s . The overall faunal spectrum in an individual shelly tempe s ti t e , however , may be distorted by several factors . 5 . Since storms are instantaneous and i sochronic event s , storm beds and their ecologic impact are useful tools for high-resolution stratigraphy , at least on a regional scale . 6 . Tempestites With particular associations of sedimentary structures ( e . g . presence or absence of wave ripples) may also prove to be valuable as paleodepth indicators . Acknowledgements I am grateful to ·the following individuals and institutions who helped me in many ways to carry out these studies whil e I was an undergraduate student: Pro f . Dr. J . R . L . ALLEN , Prof . Dr. P � . ALLEN , Dr. R. GOLDRING (Sedi mentology Research Lab . , Univ. of Reading/Englan d ) , H. HAGDORN (Klinzelsau ) , Pro f . Dr . H . -E . REINECK ( Institute of Marine Geology and Biology , Wilhelmshaven) and Pro f . Dr. A . SEILACHER (Dept . of Geology and Paleontology , Ttibingen Univ . ) . He , Prof. Dr . G. EINSELE , Dr. R. GOLDRING and H . HAGDORN reviewed the manuscript. Mr: RIES and Mr. WETZEL (Ttibingen ) provided technical assistance . Specimens are deposited in the Institut� and Museum for Geology and Paleontology , Univ. of Tlibingen (No . 1 5 74 ) .
196
References AGER, D . V . ( 1 974 ) : Storm . deposits in· the Jurassic of the Maroccan High Atlas . - Pa.leogeogr . , Paleoclimat . , Paleoeco l . , J:� : 83-9 3 . AIGNER, T . ( 1 9 77 ) : Schalenpflaster i m Unteren Hauptmuschelkalk bei Crailsheirn (Wlirtt . , Trias , mol} - Stratinomie , Bkologie , Sedi mentologie . - N . Jb . Geol . Pal . , Abh . !��: 19 3-2 1 7 . - ( 1 9 7 9 ) .: Schill-Tempestite im Oberen Muschelkalk ( Trias , SW-Deutsch- · land ) . - N . Jb . Geol . Pal . , Abh . J�J: 3 2 6 - 34 3 . AIGNER, T .
&
FUTTERER, E . ( 1 9 78 ) : Kolk-T6pfe und -Rinnen (pot and
gutter casts) im Muschelkalk - Anzeiger fUr Wattenmeer ? - N . Jb . Geol .Pal . , Abh .
J�� : 2 8 5-304 .
BALL; S . M . ( 1 9 7 1 ) : The Westphalia Limestone of the northern Midcon tinent: a possible ancient storm deposit . - J . sedim. Petr . ,
�J : 2 1 7-2 3 2 .
BRENCHLEY , P . J . i NEWALL , G .
&
SANISTREE.T , J . G . ( 1 9 79 } : A storm s-urge
origin for sandstone beds in an epicontinental platform sequen c e , Ordovician, Norway . - Sedim. Geol . , �� : 1 8 5 -2 1 7 . BRENNER,- R . L . & DAVIES, D . K . ( 1 9 73 ) : Storm-generated coquinoid sandston e : genesis of h i gh-energy marine sediments from the Upper Jurassic of ·Wyoming and Montana . - Bul l . Geol . Soc . 1685-1697 . CURRAY , J . R.
Am.
, 84 : ==
( 1960) : Sediments and history of Holocene transgres
sion , northwestern Gulf of Mexico . - In : F . P . SHEPARD , F . B . PHLEGER & Tj . van ANDEL (Eds . ) , Recent Sediments , Northwest Gulf of Mexico . - Am . Ass . Petro l . Geol . : 22 1-26 6 . EDER, W .
( 19 70) : Genese riff-nahe'r Detritus-Kalke bei Balve im
Rheinischen Schiefergebirge ( Garbecker Kalk ) . - Verb. Geo l . B . -A . ,
!�Z�: 5 5 1 - 56 9 .
- ( 1 9 71 ) : Riff-nahe detritische Kalke bei Balve irn Rheinischen Schiefergebirge (Mittel-Devon , Garbecker Kalk) . - G6ttinger Arb . Geol . P a l . EDER, W.
�� , 66 pp.
( 19 75 ) : Riffe und Riff-detritogene Platte'n kalke . - Bericht
Sonderforschungsbereichs 4 8 , Projektber . A, GBttingen 1 9 7 5 , 1 17 - 1 4 3 . EDER, W . ( 19 8 2 ) : On the genesis o f limestone-marl alternation in the Rheinisches Schiefergebirge {this volume ) .
197 &
GEYER, O . F .
GWINNER, M . P . ( 19 6 8 ) : Einflihrung in die Geologie von
Baden-Wlirttemberg . - 2 . Aufl . , Schweizerbart . KELLIN G, G . & MULLIN , P . R . ( 1 9 7 5 ) : Graded limestones and limestone quarzite couplets: possible ancient storm deposits from the Mo roccan . carboniferous . - .Sediment . Geol . J,J : 1 6 1 - 1 90 . KREISA, R .
&
BAMBACH , R . ( 1982) : The role o f storm processes in gene
rating shell beds in Paleozoic shelf environments . - (this volume ) . GOLDRING, R . & BRIDGE s , P . · ( 1 9 7 3 ) : Sublittoral sheet sandstones . J . Sedim. Petr . , 4� : 7 36-7 4 7 . GOLDRING , R . & LANGENSTRASSEN , F . ( 19 79 ) : Open shelf and near-shore clastic facies in the Devonian . - Spec . Pap. in Palaeont . g� � 8 1 -9 7 . &
HAMBLIN , A . P .
WALKER, R . G . ( 1 9 7 9 ) : Storm-dominated shallow marihe
deposits : the Fernie-Kootenay (Jurassic) transition , southern Rocky Mountains . - Can . J . Earth Sci . �g: 1 6 7 3- 1690 .
HARDIE , L . A .
&
GINSBURG , · R . N .
( 19 77 ) : Layering : the origin and en
vironmental significance of lamination and thin bedding . - In : �IE , L . A . (Ed . ) : Sedimentation on the Modern Carbonate Tidal Flats of Northwest Andros Islan d , Bahamas . - Johns HopkiUs Univ.
Studies in Geology , No . ��· 1 HAYES", M. O . ( 1 9 6 7 ) : Hurricanes as geological agents , south Texas coast . - Bul l . Am. Assoc. Petro l . Geol . 5 1 : 9 37-9 4 2 . ==
HOBDAY , O . K .
&
READING , H . G .
( 1 9 7 2 } : Fair weather versus storm pro
cesses in shallow marine sand bar sequences in the Late Pre cambrian of Finnmark , North Norway . - J. sedim. Petrol . ��, 3 18-324 . LINDSTROM , M.
( 1 9 7 9 ) : Diagenesis of Lower Ordovician hardgrounds in
Sweden . - Geologica et Palaeont . ��: 9-30 . MEISCHNER, D . ·( 1 96 4 ) : Allodapische Kalke, Turbidite in riff-nahen Sedimentationsbecken . - In: Turbidite s , Developments in Sedi mentology , J: 1 56 - 1 9 1 .
MUNDLOS , R . ( 1 9 7 6 ) : Wunderwelt im Stein . Fossilfunde - Zeugen der Urzeit . - Bertelsmann Lexikon-Verlag . POWERS , M . C . & KINSMAN , B . ( 1 9 5 3 ) : Shell accumulations in underwater sediments and their relation to the thickness of the traction zone . - J . sediment . Petr . , �� : 229-2 34 .
198 REINECK , H . -E . ; D5RJE S, J . ; GADOW, S . & HERTWECK , G . ( 1 9 68 ) : Sedi mentologie , Faunenzonierung und Faziesabfolge vor der Ostktiste der inneren Deutschen Bucht . - Senckenb . Leth . , i�: 2 61-309 .
REINECK , H . -E .
&
SINGH , I . B . ( 19 75) : Depositional Sedimentary En
vironments . - Springer , Berlin-Heidelberg-New York . SCHKFER 1 K . A . ( 1 9 73 ) : Zur Fazies und Palaogeographie der Spiriferi na-Bank (Hauptmuschelkalk) im n6rdlichen Baden-Wtirttemberg . N . Jb . Geol . P a1 . , Abh . J4�: 5 6 - 1 1 0 . SCHWARZ , H . -u. ( 1 9 75 ) : Sedimentary structures and facies analysis of shallow marine carbonates (Lower Muschelkalk , Middle Triassic Southwestern Germany ) . - Contrib. Sediment.
�: 1-100 .
- ( 19 7 7 } : Sedirnentationszyklen und stratigraphisch-fazielle Probleme der Randfazies des Unteren Muschelkalks (Kernbohrung Mersch/Lu xemburg) - Geol . Rdsch . , .•
SEILACHER, A .
��: 34-6 1 .
( 1 9 6 2 ) : Paleontological studies on turbidite sedi
�entation and erosion . - J. Geol . , JQ: 22 7-2 34 .
SPECHT , R . W .
&
BRENNER, R . L .
( 1 9 79 ) : Storm-wave genesis of bio
cla� tic carbona�e s i n Upper Jurassic epicontinental mudstones ,
East-central Wyoming . - J . sedim. Petrol . , 4� : 1 30 7 - 1 32 2 .
VOLLRATH , A . ( 1 9 55 ) : Stratigraphie des Oberen Hauptmuschelkalks (Schichten zwischen Cycloides-Bank und Spiriferina-Bank) in
Baden-Wlirttember g . - J h . geol . L . -A . Bad. -Wtirtt . , J : 190-21 6 . - ( 19 5 5 ) : Zur Stratigraphie des Hauptmuschelkalks in Wlirttemberg . Jh . geol . L . -A . Bad. -Wlirtt . , J : 7 9 -1 6 8 ( 1 9 5 5b ) .
WAGNER , G .
( 1 9 3 6 ) : Riffbildung als MaBstab geologischer Zeitraume .
Aus der Heimat ,
��: 1 5 7 - 1 6 0 .
WALKER,, R . G . ( 1 9 6 7 ) : Turbidite sedimentary structures and their re lationship to proximal and distal depositional environments . J ·. sediment . Petrol . , �z : 2 5 -4 3 . WALKER, R . G . ( 1 970) : Review on the geometry and facies organisation of turbidites and turbidite-bearing basins . - I n : ( L . LAJOIE , ( Ed . ) Flysch-Sedimentology in North America - Spec . Pap . geol.
Ass . canada, z : 2 1 9 - 2 5 1 .
•.
- ( 1 9 79 ) : Facies Models . - Geoscience Canada , Reprint Series 1 .
Allochthonous Coquinas in the Upper Muschelkalk Caused by Storm Events? (Abstract) H. HAGDORN and R MUNDLOS
I
! !'
1 Abstract : In the shallow-marine Upper Muschelkalk , epifaunal colo ri f es comprise ( 1 ) cemented species · ( P taounopsis� Enan t i o s t r e o n ; Enarinu s ] as W,ell as ( 2 ) flexibly at·tached forms ( Coen'o thyris" Spiriferina, Tetrao t i ne l la ; Plagiostoma, Pleuron e o t i t e s ,
Mya l ina,
Myt i lu s ) .
Their large-scale pioneer settlement requires firm substrates , such as : 1 . storm-swept surfaces , on which suspended mud did not re-settle in situ but became initially or subsequently washed into dee per parts of the basin . Such surfaces _may b e : a ) tops o f shelly tempestites b ) erosive tempestite bottOms , re-exposed during a subsequent storm. c ) shell talus derived from adj acent bioherms. 2 . hardgrounds on top of oolithic shoals ; paleogeographically restricted to submarine swells . Once established , a large-scale corrununity will be able to maintain the �ecessary substrate for a longer period, because patches killed off by intermittent mud accumulation can become re-populated from unaffected areas . The final kill-off by a major event of mud-deposition is maiked by the preservation of articulated specimens (brachiopods , Enerinus ) on the top of the- autochthonous debris of previous generations . In detail , beds formed b y cemented and flexibly attached species , differ in ecologic structure and geometry: a) biohermal bodies, consisting of frame-buildung cemented spe cies (Ptacunops i s , terquemiids and Encrinus ) plus loosely attached brachiopods and pelecypods . b ) b iostromal bed s , mainly brachiopods . In Muschelkalk stratigraphy, autochthonous shell beds of the bio stromal type can be used as regional marker horizons a) because the · particular combination of event and ecologic conse sequences was relatively rare b) because the dominating species were dif ferent in most instances (Spir:Lfera-Bank, Cycl'Oides-Bank , Hauptterebratelbank) 1
(A detailed version is in print in N . Jb . Geol . Palaont . )
Cyclic and Event Stratification (ed. by Einsele/Seilacher) <0 Springer 1982
The Role of Storm Processes in Generating Shell Beds in Paleozoic Shelf Environments R. D. KREISA and R. K BAMBACH
Abstract: Storm related processes were a principle agent of stratification in Paleozoic shelf environments . During peak storm conditions the shallow sea floor was eroded , coarse materials including shells formed winnowed lags , while fine sand, silt and clays were put into suspension . As storms wane d , the fine sand and silt settled out o f suspension rapidl y . Some infiltrated the upper portions of _ the coarse grained winnowed lag deposits and the bulk formed laminate d , weakly graded beds with distinctive wave generated structure$ . Shell beds formed by these proces ses contain reworked· but untransported fos s i l s . Biases of preservation resulting from such activity include the destruction of preserved comrnu"nity succession , under representation of originally aragonitic fossils and an enhanced preservation of whole , uncorroded fossils . 1 . Introduction Storms were the principle agent of stratification for Paleozoic age rocks in eastern North America deposited in open-marine shelf environments . -Storm-generated beds have been recognized and their mechanism of deposition worked out during detailed study of the Upper Ordovician in Virginia ( KREISA , 1 9 8 0 ; KREISA , 1 9 8 1 ) Similar storm-generated features have been noted in shelf facies of 4
Cambrian through Carboniferous age throughout the folded Appa lachians (BAMBACH , 1 9 6 9 : BOWEN et a l . 1 9 7 4 ) . The recognition of storm beds is based on sedimentary structures and fabrics . which reflect scouring and hydraulic s�rting during the high-energy phase of storms and extremely rapid deposition during the waning energy phase. Predominantly wave-generated sedimentary structures serve to distinguish storm beds from· superficially similar depo sits such as turbidite s . The influence of storms is not limited t o stratification and se� dimentary structure s . Storms also modify the foss i l · record through bias in preservation o f body fossils , fossil associations (paleo communitie s ) and trace fauna. Recognition of storm reworking of the fossils in most shell beds is .e ssential for paleoecologists . Cyclic and Event Stratification (ed. by Einsele/Seilacher) <0 Springer 1982
201 2 . Recognition of Storm Deposits A model that defines the interrelationships among sedimentary struc tures and textures found in storm beds is presented as Figure 1 . The model is useful both for interpreting the origin o f individual sedimentary features and for recognizing storm-influenced shell beds and strati fication in the geologic record. It will be discu� sed in terms of two phases of storm activity: the high-energy storm peak and i:: he waning-ener"gy late-storm/post-storm phase .
MUOROCK
LAMINA TEO BEO
Plane l am i n a t i o n to hummocky l am i n a t i o n to c l imbing r i pp l e l am i n a t i o n Escape burrows Weakly graded rrow repop u l a t i o n
��������B:u
SHELL BEO
Prefe�red shel l o r i e n t a t i o n I n f i l tration fabrics ( I nsert) lntracl as ts Mudcoated s he l l s
BASE
MUOROCK
l-:
Sharp/Eros i o n a l Gutter casts Tool marks
. 1 . Storm-generated features
2 . 1 . Storm Peak During peaR storm conditions , the depth of water in which surface waves will disturb the sea floor (wave base) is greatly increased compared to fair weather . This is due to the increased wavelength of the surface waves and to storm-wave qenerated turbulence . Evidence o f this episodic increase in energy at the sea floor is found at the base of storm layers in the form of smooth to irre gular erosional surface s . These surfaces commonly bear tool marks
202
and elongated scour�and-fill structures termed gutter casts (WHITAKER, 1 9 7 3 ) . AIGNER & FUTTERER ( 1 9 7 8 ) have suggested that gutter casts are useful in basin analysis because their a ligned orientations seem to reflect consistent current systems within the basin of deposition. Similar alignment of gutter casts has been observed also· in preliminary investigations of the Cincin natian Series (Ordovician) in Ohio. The sediments which overlie the erosional bases may be graded due to differerttial suspen·sion during the storm, and deposftion of gradually finersized sediment as the storm wanes ( e . g . KELLING
',•:: ; ·.;:
& MU�LIN, 1 9 7 5 ) . However, . in many cases storm energy results in a distinctly segregated upwar4-fi�ing sequence of lithologies , each member of which is abruptly gradational to the next (Fig. 1 ) . Where the appropriate grain size is available , the basal coarse deposit develops as . a winnowed lag. Intraclasts similar to underlying l ithologies , exhumed mud-cOated fossils, and preferred shell or clast orientation are evidence that these beds consist of mechani cally reworked material. In Ordovician rocks in Virginia, analysis of shell orientation in storm beds revealed shells dominantly oriented parallel to bedding { 7 5 % ) and· convex-Up ( 2 ! 1 ) . In some cases large flat shells or intraclasts ( such as in " flat pebble conglomerates " ) are oriented vertically or are imbricated . such shell pOsitions are not predicted by studies of the life habits _ of the organisms involved or by shell orientation on modern shel ves (EMERY , 1 9 6 8 ) . The portion of a storm bed formed during the peak o f the storm includes the sharply marked erosional base of the bed and the bulk _ of the overlying shell bed ( i f present) . The shell bed represents reworked , disturbed material winnowed in place ra·ther than having been transported long distances .
2 . 2 . Waning Phase Finer grained sedimen t , especially very fine sand and mud , is transported during the high-energy phase of stor�s ( H�YES , 1 9 6 7 ; REINECK & SINGH , 1 9 72 ) . Although details of the storm processes by which this occurs remain unclear, it is evident that terrigenous sand can move away from the shore zone during storms·, and that sand and mud on the open shelf are transported by storm-generated cur rents with net unidirectional flow (SMITH & HOPKIN S , 1 9 7 2 ; BUTMAN
203
et a l . , 197 9 ) . Sand and mud sized carbonate sediment is also sub ject to transport by storms {AIGNER, 1 9 7 9 ; K.REISA, 1 9 80 ; KRE;ISA ,
1981) . As the storm passes , the finer sediment drops rapidly from suspen sion . This results in abu'ndant infiltration fabrics such as shel ter porosity , micrograded sediment perc�ed on individual shells , and sediment screening in the shell (and intraclast) beds (Fig. 2 ) . Sediment screening occurs where fine sediment is partially blocked from infi ltrating the grain-supported fabric of the lag deposit by the large shells . and results in a decrease downward in inter stitial fine sediment (Fig. 2 ) . Although local shelter porosity and micrograded sediment could be due to processes other than in filtration , these processes are also associated with storms . For · example , only partial winnowing of the shell lag can produce micro graded sediment , and shelter porosity is observed where a single shell has come to rest concave downward on the substrate and is rapidly buried by deposition of sand or silt. Infiltration fabrics found in relatively thin shell beds interbedded with rnudrock ap pear to be excellent evidence o f episodic high-energy events in a normally low-energy environment .
Fig. 2 . Shell bed showing in filtration fabrics . Note · shel ter porosity beneath shells ( now filled With bladed calcite ce ment) ; coraser peloidal sediment and skeletal debris perched on large shells and grading upward tO micrite under sheltered voids ; increase in before-cement pore� sity caused by screening action of large shells at bottom of section. Width of field of view is approximately 1 ern . Sketched from photomicrograph
204 The fine sand and silt in laminated storm-beds and laminated upper portions of shell bed-laminated couplets were deposited rapidly during single waning-energy events . They contain escape burrows and· are weakly graded (KREIS'A, 1 9 81 ) . In addition, the lamination itself suggests deposition from a suspension cloud rather than movement of sediment as bed load. Sediment of this size would be expected to move by foreset accretion in the proces s producing cross-lamination if transported by traction currents . The lamina tion in storm beds is typically paralle l , either planar or undula tory. Where cross-lamination occurs it is of a type characteristic of rapid fallout from suspension . This is true both for cross lamination which formed in unidirectional flow ( ripple drift lamination , JOPLING & WALKER , 1 9 6 8 ) , and in wave-generated oscil latory flow {climbing wave-ripple lamination , BLOOS , 1 9 7 6 ) . Laminated storm beds commonly display ordered sequences of lamina tion ( e . g . DE RAAF et a l . , 1 9 7 7 ) . The transition upward from plane or hummocky stratification ( HARMS et a l . , 1 9 7 5 ) to climbing wave ripple lamination (Fig. 1 ) suggests waning velocity of oscillatory currents according to the relationships defined by INMAN ( 1 9 5 7 ) , ALLEN ( 1 970) and KOMAR
&
MILLER ( 1 9 7 5 )
(KREISA, 1 9 8 1 ) . REINECK
&
SINGH { 1 9 7 2 ) suggested that parallel lamination can also form when storm-suspended clouds of sediment are deposited in slowly moving water ( 20 em/sec) as the storm abate s . Deposition o f laminated beds o f sand and silt i s followed b y depo sition of suspended very fine grained sediment after the storm is over and the sea floor i s then repopulated by epi- and endobionts . Repopulation by burrowing infauna is observed as intense bioturba tion in the fine grained mudstones above the laminated port�ons of storm beds and a decrease in the amount of bioturbation down ward from the top of storm layers . When burrowing is both abundant and deep, bioturbation can destroy primary bedding competely (MOORE
&
SCRUTON 1 9 57 ) . Most Paleozoic age shelf sediments show in
tense downward penetrating bioturbation through only 3 -8 em. It may be that the depth of active burrowing in open shelf environ ments has increased through the Phanerozoic (BAMBACH 1979) .
� SEPKOSKI ,
Laminated sand or silt beds with wave associated structures and some portion of the overlying mudrock represent the portion of a storm bed deposited from suspension as storm energy wanes. Bio turbation usually obscures the contact relations between the
205
Il
j, '
I
1
I
finer grain�d storm peposited material and the overlying sediments that gradually accumulated under calmer conditions .
3 . Influence o f Storms on ·Foss i l Preservation The fossils contained in storm reworked shell beds are little transported although they are reworked . The sediment adhering to and filling shells is lithologically identical to the underlying beds from which they are eroded. The species in the shell beds and ' interstratif,ied mudrocks are the same and they may even occur in the same relative abundances (Table 1 ; PLANTS , 1 9 77 ; BOWEN et · a l , 1974) . Some features are obscured or biased by storm disturbance, however.
i
I I L l l I
storm reworked shel l beds can not preserve the , ty succession such a s those reported b y WALKER Aragonitic skeletons are often dissolved early ginally aragonitic fossils may be preserved as
i
& ALBERSTADT ( 1 9 7 5 ) . in diagenesis . Ori molds in unreworked
sediments but molds are destroyed during erosion and resuspension Table 1.
Comparison of · relative abundance of calcite skeletons from reworked beds interbedded with unreworked beds (Pack stones and shales, respectively) . Data from Martinsburg Formation (Upper Ordovician) Catawba Mountain , Virginia . Sowerbye l la-Zygospira community interval ( 8 1 m thick ) . 10 collections from shale
i
I
details of communi
7 collections from packs tones .
Sowerbye l la " z ygospil"a
27%
30%
28%
22%
Dalmane l l a
12%
17%
i
Ramose Bryozoa
14%
14%
i
Rafinesquina
5%
5%
I
I
j
.I
{After PLANTS , 1 9 7 7 ) o f enclosing sediments by storms . Storm generated shell beds commonly contain a much smaller proportion of originally aragoni tic fossils compared to calcitic forms than unreworked inter stratified mudrocks . In col lections from Silurian rocks at Arisaig, Nova Scotia, over 80% of the assemblages fiom unreworked mudrocks have a complete representation of molds of aragonitic as wel l as
206
calcite fossils whereas only 3 7 % of the storm beds contain origi-_ nally aragonitic forms in proportions equal to those in unreworked beds {BAMBACH, 1 9 6 9 ) . Paleontologists must take great care to re cognize storm disturbed beds to prevent misinterpretation of the fauna collected from them. Despite these negative taphonomic effects there is one positive factor associated with fossils in storm bed s . The. rapid burial of . shells by storms prevents the continued destruction and loss that would have occ �rred to exposed s�ells (DRISCOLL, 1 970) . Storm deposition thus contributes to preferential preservation of whole, uncorroded shell s .
References AIGNER , T . { 1 9 7 9 ) : Schill-Tempestite im Oberen Muschelkalk (Trias, SW-Deutschland) . - N . Jb . Geol . Palaont . Abh . , !�Z : 326-34 3 . AIGNER, T . & FUTTERER, E . ( 1 9 7 8 ) : Kolk-TOpfe und -Rinnen {pot and gutter casts) im Muschelkalk - Anzeiger flir Wattenmeer? . N . Jb . Geo l . Palaont. Abh . , !�g : 2 8 5-304 . ALLEN , J . R . L . ( 1 970) : Physical Processes o f sedimentation . New York, American Elsevier Pub. Co . , 2 4 8 pp. BAMBACH , R . K . ( 1 9 6 9 ) : Bivalvia of the Siluro-Devonian Arisaig Group , Nova Scoti a . - Unpub . Ph . D . dissertation, Yale Universi ty ' 376 pp. BAMBACH, R . K . & SEPKOSKI , J . J . , Jr . ( 1 9 7 9 ) : The increasing influen ce of biologic activity on sedimentary stratification through the Phanerozoic . - G . S . A . Abstracts with programs , ! ! : 3 8 3 •
. BLOCS , V . G . ( 1 9 7 6 ) : Untersuchungen Uber Bau und Entstehung der feihkOrnigen Sandsteine des Schwarzen Jura (Hettangium u . tiefstes Sinemurium) im Schwabischen Sedimentationsbereich . Arb. Institut Geol . Palaont. Vniv. Stuttgart , 2 7 7 p . BOWEN , Z . P . , RHOADS , D . C . & MCALESTER, A . L . ( 1 9 7 4 ) : Marine benthic communities in the Upper Devonian of New York . - Lethaia z : 9 3- 1 2 0 . BUTMAN , B . , NOBLE , M . , FOLGER , D . W . ( 1 9 7 9 } : Long-term observations of bottom current and bottom sediment movement on the Mid-At lantic Continental Shelf . - J. Geophys . Res . �� {C3 ) : 1 1 82-1205 . DE RAAF , J . F . M . , BOERSMA, J . R . & VAN GELDER, A . ( 1 977 ) : Wave generated structures and sequences from a shal low marine suc cession, Lo.,.1er Carboni ferous , County Cork , Irelanq . - Sedimento logy �� : 4 5 1 � 4 8 3 . DRISCOLL, E . G . ( 1 970 ) : Selective Bivalve She l l Destruction i n Ma rine Environments , A Field Study . - Sed . Petro l . �Q: 898-905 . EMERY , K . O . ( 1 9 6 8 ) : Positions of Empty Pelecypod Valves on the con tinental Shel f . - J . Sed. Petrol . � �: 1 2 6 4 - 1 2 6 9 .
207
'>Ill'""'" ' '
J . C . , SOUTHARD, J . B . , SPEARING, D . R. & WALKER, R . G . ( 1 97 5 ) : Depositional environments as interpreted from primary sedimen tary structures and strati fication sequence s . - s . E . P . M . Short Course � , 1 6 1 pp.
, M . O . ( 1 96 7 ) : Hurricanes as geologic agents , South Texas coas t . - Ame r . Assoc . Petrol . Geologists Bul l . �� : 9 3 7 - 9 5 6 . , D . L . ( 1 9 57 ) : Wave-generated ripples i n nearshore sands . Dept. Army Corp . of Engr . , Beach Erosion Board Tech . Mern. ),QQ . 6 5 pp. LING, A . V . & WALKER, R . G , ( 1 9 6 8 ) : Morphology and origin o f JOP ripple-drift cross-lamination, with examples from the Pleisto cene of Massachusetts . - Petrology �� : 9 7 1 - 9 84 . KELLIN G , G . & MULLIN , , P . R . ( 1 97 5 ) : Graded limestones and limestone quartzite couplets : posible storm-deposits from the Moroccan Carboniferous . - Sedimentary Geol . �� : 1 6 1- 1 9 0 . KOMAR, P . D . & MILLER, M . C . ( 1 9 7 5 ) : The initiation of oscillatory ripple marks and the development of plane-bed at high shear str.esses under waves . - Jour. Sed . Petrology �g: 6 97-703 . KREISA, R .-D . ( 1980) : The Martinsburg Formation {Middle and Upper Ordovician) and related facie s , southwestern Virgini a . - Ph . D . Dissertation, Virginia Poytechnic Institute and State Universi ty , Blacksburg, VA. KREISA, R . D . ( 19 8 1 ) : Storm-Generated Sedimentary Structures in Sub tidal. Marine Facies with Examples from the Middle and Upper Ordovician of Southwestern Virginia . - Jour. Sed . Petrology 21 -( in press ) . MOORE , D . G . & SCRUTON , P . G . ( 1 9 57 ) : Minor Internal Structures of Some Recent Unconsolidated Sediments . - Bull . Amer . Assoc . Petrol . Geol . ik • 2 7 2 3 -2 7 5 1 . PLANTS , H . F . ( 1 977 : Paleoecology of the Martinsburg Formation at Catawba Mountain , Virginia . - Unpub . M . S . T hesis , Virginia Poly technic Institute and State University , Blacksburg, 2 1 7 pp. REINECK ·, H . E . & SINGH, I . B . ( 1 97 2 ) : Gene sis· of laminated sand and graded rhythmites i n storm sand layers of shel f mud . - Sedimen tology },� : 1 2 3 - 1 2 8 . SMITH, J . D . & HOPKINS , T .. S . ( 1 97 2 ) : Sediment transport on the continental shelf o f f of Washington and Oregon in light of recent current measurements . - I n : SWIFT , D . J . P . , DUANE , ·o . B . & PILKEY 1 O . H . (eds .. ) 1 Shelf sediment transpor t : Process and pattern . - Stroudsburg , Pa . , Dowden., Hutchinson & Ross , Inc . : 1 4 3- 1 8 0 , WALKER, K . R . & ALBERSTAD T , L . P . ( 1 97 5 } : Ecological Succession As Pa leobiology An Aspect of Structure in Fossil Communities . }, : 238-257 . WHITAKER, J . H . McD. ( 1 97 3 } : ' Gutter Casts ' s a new name for scour and-fill structures : With examples from the Llandoverian of Ringerike and MalmOya , southern Norway . - Norsk Geol . Tidsskr . i:i;J, : 403- 4 1 7 .
Rhythmic Bedding and Shell Bed Formation in the Upper Jurassic of East Greenland F. T. FDRSICH
Abstract: In the Middle Volgian of Milne Land, shelf sediments deposited well below wave base eXhibit rhythmic alternations of sh�lly sandstones and largely unfossiliferous , uncemented sandy shales . They reflect a primary distribution pattern of the fauna which was only s lightly distorted by diagenetic pro cesses . This primary distribution pattern is due largely to differen ces in the sedimentation rate, shell beds forming during pe riods of reduced sedimentation or omission . Shell beds are either autochthonous accumulations or else parautochthonous . The latter . type were built up by repeated in-situ reworking and winnowin g , caused by events such as storms which periodi cally disturbed the sea floor . 1 . Introduction The Middle Volgian Pernaryggen Member of Milne Land , East Green land, consists predominantly of micaceous , silty , fine-grained sands and shelly sandstones , commonly with an admixture of glau conite. They form part of a regress ive , offshore sequence and are overlain by the highly micaceous , very fine-grai fled sands of the Astartedal Member which in turn are capped erosively bY shal low-water sands and sandstones of the basal Hartz F j eld Formation (also �id-Volgian in age)
(BIRKELUNG , CALLOMON
&
FURSICH, 1 9 7 8 ) . In
any section (most of the information is drawn from the Pernaryggen section east of Kronen , but the �eatures described below are found in all other sections as wel l ) largely unfossili ferous fine-sands alternate with thin, often concretionary layers of fossiliferous sandstone (Fig. 1 ) . The restriction of shell beds largely to thi.n cemented horizons poses the problem, whether this rhythmic accumu lation o f shells is largely a primary feature which influenced the diagenetic history, or else whether there was originally a fairly uni form faunal distribution out of which certain levels were fa voured by diagenetic processes and remained preserved , whilst the majority of the fauna was dissolved . We thus have two models to choose from (Fig. 2 ) , a ' diagenetic extinction ' model and a ' dia genetic reprint ' model . As such bedding phenomena are widespread Cyclic and Event Stratification (ed. by Einsele/Seilacher) © Springer 1982
209
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Ill c ::>
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c 1 Fig . 1 . Section through the Pernarygsren Member ( M . Volgian) at Pernarygge n , East of Kronen (Milne Land) . Black : cemented layers/concretions . Grain size : f = fine sand; m = medium sand; c = coarse sand . Glauconite content : l = low ; m = medium; h = high . Faunal density : r = . rare; o .= occurs ; c = common ; a = abundant . 1 - 8 : benthic assemblages , dominated by either Isognomon ( 1 ) , Plagios toma ( 2 ) , Entolium ( 3 ) , Rou i l leria ( 4 ) , Gramma t odon (Cosmetodon) { 5 ) , Thraaie ( 6 ) , Grammatodon ( Gramma todon) ( 7 ) and Isoayprina ( 8 )
210
in the sedimentary record it i s worth attempting to explain their origin, particularly as for both, a palaeoeco_logical analysis of the fauna and an interpretation of the ancient environment, the faunal distribution pattern is of major importance .
'diagenetic extinction mod e l '
8
'diagenetic reprint mode l '
Fig . 2 . The two models explaining the relationship of cemented layers to shell beds in the Pernaryggen Member
2 . Diagenetic History
First of all we must Concentrate pn the role of diagenesis in shaping the present distribution pattern .
2 . 1 . Evidence Supporting the ' Diagenetic Extinction ' Model a) In some very rare cases, shell beds are preserved in unconsoli
da-ted sands . They usually consist, however·;-··only o � fairly thick , calcitic shells ( e . g . Isognomon� Camp tonectes (BoPeionee tes ) , belemnites ) . Sporadic small phosphatic concretions within
these shell beds , however, contain small , thin-shelled aragoni tic faunal elements , indiCating that solution of aragonite
211
took place during diagenes is . Similar bias through diagenetic processes occurs in· several beds where the fauna is of low density and consists largely of calcitic elements , but where a highly diverse aragonitic fauna is again preserved in some small calcareous concretions . b ) In parts of the section , small round to oval concretions occur containing monotypic associations of the bivalve Gramma todon. From biostratinomic data it is obvious that they do not repre sent clusters of the bivalves on the sea floor, but are re lics of shell beds preserved in laterally discontinuous con cre tionS , the remaining shells having been dissolved during diagenesis . Thus there is evidence that selective as wel l as complete shell dissolution took place during diagenes is . 2 . 2 . Evidence Supporting the ' Diagenetic Reprint' Model a ) In the upper part o f the section, faunas commonly occur in both cemented and uncemented layers , although in the latter at much lower dens itie s . As the faunal ·compos"iti·on hardly varies between the two , it seems unlikely that diagenesis affected the unce mented lay.ers in a different way from the cemented ones . Thus «ifferences in faunal densities would appear to be largely pri mary . b ) Although extremely rare, isolated shells or very small shell clusters can be found in several of the uncemented sands in the neighbouring sections of equivalent age . c ) There are several scattered Concretions or concretion levels which contain little or no shelly faun a . As these concretions formed in the same way as the highly fossiliferouS ones , this feature appears to be primar y . 2 . 3 . Discussion Looking at the available evidence , it seems that both models are appropriate , but that the importance of the processes shaping the present-day fauna l distribution varied . Support for the ' diagene tic extinction ' model is sparse except for the evidence of shell beds restricted to laterally discontinuous concretions . The latter seem to represent cases where insufficient carbonate was
212
available for the concretions to join together laterally and form one continuous cemented layer. An additional factor must not be neglected: the influence of late diagenetic processe s . Most of the shells found in loose sands are very poorly preserved and show clear signs of present-day weath ering. This may locally have completely dissolved the shells and thus removed evidence of their former presence in the uncemented layers . There are also cases where late diagenetic processes in fluenced already cemented layers by dissolving shells and leaving only moulds . These cases are , however , very rare . In contrast, evidence Supporting the ' diagenetic reprin t ' model is much more abundant and it seems likely that the present-day faunal distribution is largely a primary pattern which was modi fied , to some extent , by selective early diagenesis and by tela genetic processes . Fig . 3 i llustrates the various diagenetic processes which influ enced the distribution pattern of the fauna in the Pernaryggen Member . The time of diagenetic stage I can be determined by the temporal relationship between cementation and compaction. In many cases there is evidence that compaction preceded cementation : the s teinkerns of thin-shelled burrowing bivalves such as P Z euromya are frequently distorted by compaction , and some of the brachio pods (particularly terebratulids) show compactional fracturing of the shell . On the other hand there are also plenty of cases where fossils are not distorted by compaction. This does not necessa�ily imply that cementation took place before compactional forces were active , but may simply be due to an early l i thification of the in fil lings .of the shells (steinkern formation) . It thus seems like ly that cementation was generally preceded by compaction and did not take place very early .in diagenesis . Diagenetic stage I I is though.t to represent comparatively late stage diagenetic processes , that is largely during exposure of the sedimentary sequence to meteoric waters . Thes e processes seem to be. largely confined to dissolution of shell material in unconsoli dated sediments and rarely occur in sandstone layers . · I n case ( a ) and ( f ) in Fig. 3 faunal density is relatively low· pro bably related to ·this there is not enough carbonate present to ce ment the sediment . Consequently the s ediment remains loose and con-
213
Di agenetic original faunal distrib ution
p roc e s s e s diagenetic
diagenetic
..
1
2
a
..
b
c
''''''''' ''"""
d
.. ..
.. .. k:'''';\{l> 2
1
e
�
f
C> 3
Fig . 3 . Diagenetic processes modifying the primary distribution pattern of the fauna . 1= common; 2= occurs ; 3= rare . Further explanation in the text .
I
tains a sparse benthic fauna { f ) or is unfossiliferous ( a ) , the fauna being removed by Recent weathering processes . Cas·e ( a ) is reckO ned to be rare, ( f) to be common . In cases ( b ) and ( d ) , an origi nally high shell density facilitates cementation of the sediment during diagenetic stage I . Whilst in (b ) no further diagenetic pro cesses affect the shell distribution, later diagenetic solution pro cesses remove the shell material in (d ) 1 leaving voids (upper half) , or even lead to dissolution of the cement ( lower half) so that the sediment is again loose , but unfos s i liferous . Case ( d ) is very rare , case ( b) widespread . In case ( c) an originally high density shell bed is only locally ce mented ( in the form of nodules ) . As the nodules grow , the shells between the incipient nodules dissolve and supply carbonate to the cementation process . On the other hand, these shells could dissol ve during diagenetic s tage II (not i l lustrated) . Case ( c ) is wide spread. In case ( e ) a sparse benthic fauna dissolves during dia genetic stage I , supplying carbonate to cementation processes el sewhere.
214
There are , of course, further variations on this theme . Thus , solution can be confined to aragonitic shells , whilst calcitic elements remain unaffected. Again this process is relatively rare. In summary , it thus seems that diagenetic processes largely en hanced , but only in few cases drastically altered1 primary dis con tinuities in the distribution pattern of the fauna . Drastic alte rations are mainly restricted to beds with concretionary nodules where horizontal rather than vertical dissolution seems to have prevailed and where the primary vertical shell distribution was thus not altered . The rhythmic bedding is therefore mainly a pri
' �.
mary phenomenon and we hav-e to look for depositional or biological phenomena in order to unravel its origin.
3 . Depositional and Biological His tory 3 . 1 . The Substrate The sediment is fairly uniform and consists for the most part of micaceous, silty , fine- to very fine-grained sands �nd sandstones with varying amounts of glauconite (Fig. 1. ) . Grain size analysis shows that in most samples mica flakes and predominantly angular ,•·•
to subangular quartz grains smaller than 1 00 microns form over 50 percent of the sediment . The glauconite pellets in the remainder exhibit sharp boundaries and range from fine- to medium-grained. The two distinct peaks in the grain size distribution (one in the silt/very fine sand range, the other in the fine to medium sand range ) indicate that the glauconite was not deposited together with the remaining particles , but probably formed in situ on the sea · floor. Evidence for the autoChthonous nature of the glauconite pellets is afforded by the observation that in thin-sections most glauconite pellets exhibit extensive shrinkage cracks caused by dewatering processes of the original hydroxid gel . It seems highly unlikely that such grains could have survived transport without breaking up into small particles . Early diagenetic formation of the glauconite within the sediment seems to have been rare . Glauco nite pellets formed in this way commonly do not have sharp bounda ries and lack shrinkage cracks ( e . g . WILDBERG , 1 9 80 ) .
215
distribution of glauconite grains in the sediment may caused by the intensive bioturbation of the sediment below) . OnlY a t the base of the Pernaryggen Mem9er was some coarse-grained material deposited, and this is the only level at which sedimentary structures , ranging from .large-scale trough to planar crossbedding, have been observed . It is only there that g lauconite grains are arranged in seams and are possibly allochthonous . Judging from the grai� size distribution, a substantial part of the sediment was deposited from suspens :ton , whi.lst another part (the glauconite) formed syngenetically on the sea floor. Bed load trans port s eems to have played a comparatively insignificant role except in the formation of some beds. Glauconite is a mineral which typi cally forms on the sea floor during periods of reduced sedimentation or omission. The widespread occurrence of autochthonous glauconite in the Pernaryggen Member thus points to low rates of deposition. 3 . 2 . The Nature of the Shell Accumulations More than 90 percent of the fauna consists of benthic · elements ; bi . valves dominate, followed by brachiopods , gastropods and some ser pulids . Common nektic e lements consist of ammonites and belemnites . The bivalves were partly deep and shallow burrowing members of the infauna, partly by?sally attached or reclining members of the epi fauna . Gastropods and brachiopods were epifaunal . The shells are usually well preserved, s i gns of boring and encrustation being rare. Of the infaunal elements , the deep burrowing forms are fre quently preserved in life position, and epifaunal elements commonly occur in clusters , indicating little post-mortem disturbance. The degree of fragmentation varies considerably; it i s , however, rarely very high and sometimes absent. It need not have been caused by physical processes, but might have been due, at least partially , to the activity of scavenging or predating organisms . Faunal density varies from loosely packed shell beds to s cattered shells . Among the shell bed s , three types can be reCognised: a ) She l l Beds Exhibiting Signs o f Current Influence
I
i
I
I
i
t
Current influenced shell beds have been found only at two horizons . In one case , large disarticulated valves of the bivalve Plagiosloma form a shell pavement in which the position of the shells is con vex-up . This occurs at the base of the member , where the medium- to
216
coarse-grained sediment and relic sedimentary structures have al ready independently provided evidence for currents . Higher up in the section a shell bed occurs , at the base of which burrowing bi v�lves in life position are found followed by largely articulated epifaunal shells . These grade up into a shell layer where most valves are disarticulated and in a convex-up position ( 50 out of a sample o f 6 1 shells ) . b ) Shell Beds Exhibiting Signs of In-Situ Reworking Beds with a random orientation of the shells and a varying degree of fragmented shells probably indicate in-situ reworking. Whilst burrowing forms are still predominantly articulate d , epifaunal elements are usually disarticulated. Lateral transport of the sheills can be excluded as the number of right and left valves is roughly the same; • no size sorting has taken place ; * •
•
no signs of abrasion and wear have been found ; and shells do not show any preferred orientation .
This type of shell bed i s quite frequent. c) She l l Beds Formed Without Any Signs of Physical Disturbance Equally common as · type ( b ) are shell beds , in which a large percen tage of the fauna is still in-situ: This is particularly true of the infauna; epifaunal elements , such as terebratulids or the bi valve Isognomon , are however also preserved in clusters in life position . Most shells are articulated, even forms such as pectinids , and the degree of shell breakage is usually very low . Such shell beds must reflect l ack of any disturbance and a relatively rapid burial . Otherwise the epi faunal species could not have been pre served in life pos ition . The nature of the shell beds indicates that large-scale transport of the fauna can be excluded, and that in a large number of cases even in-situ reworking did not play any role. Biostratinomic data thus point to a low energy environment which was only rarely di sturbed by an increase in the energy level . The only faunal mixing which occurred was due to reduced sedimen tation or omission which led to the accumulation of various seral stages of a community sequence in one horizon ( condensation ) . Furthermore , the faunal mixing was enhanced by subsequent biotur bation (see also FURSICH, 1 9 7 8 ) .
217
faunal composition (nearly exclusively suspension-feeders ) and moderate faunal diversity suggest a shelf environment of moderate depth, although probably still less than 100 m .
3 . 3 . Bioturbation Apart from very rare levels where sedimentary structures can be ob served1 the sediment has - been bioturbated intensively. Most of the bioturbation was the work of a small deposit-feeder which left thin1 sinuous , refilled burrows (Maearoniehnus ) , orientated ran ' q;mly . In addition, the trace fossils Curvo li t h u s , Thalassinoides.,
·viploeraterion and Chondr i t e s occur . In many cases the burrowing
fauna represented by the traces may have re-oriented shells and thus effected an in-situ mixing.
3. 4 . Discuss·i on The high degree of bioturbation and the presenc� of autochthonous glauconite point to a low rate of deposition , .as do the shell beds formed in-situ. The co�paratively restricted reworking of the shelly fauna indicates that deposition took place below wave base, out 1of reach of fair weather disturbance by waves or tidal pro?es ses . On the other hand, relic s edimentary structures , trace fos sils such as Diploerater:io n , and preservation of clusters of epi faunal organisms in life position point respectively to the occa sional presence of more turbulent conditions and a relatively high rate of depos ition. We seem therefore to be dealing with disconti nuous , relatively short-lived sedimentation events (by bed load transport) which were interrupted by long periods of reduced or non-sedi�entation, recorded as condensed shell beds . Background sedimentation was provided by sediment settling out of . suspension. Considering that the number of shell beds which are present is about 20 , there is no need for condensation of ammonites of diffe rent subzones or even zones into one bed . A calculation of depo si tional rate s , based on the average duration of a Jurassic ammo nite zone yields a figure of about 1 . . 6 cm/1 000 years (assuming continuous deposition ) . This relatively low figure suggests that the environment was rather offshore and that it was only affected infrequently by clastic input . In shallow-water clastic environments such as the North Sea , sedi mentary deposits represent only a small proportion of the passage
218
of time and much of the history of the basin is unrecorded ( e . g. REINECK , 1 9 6 0 ) . It is tempting to interpret the sequence of the Pernaryggen Member along similar lines , but for the following reasons this temptation must be forgon e : Whilst in shallow-water basins such as the North Sea reworking processes dominate , there is no sign of substantial reworking in the Pernaryggen Member . Furthermore, the postulation of many undetectable hiatuses meets with the difficulty of having to explain what happened to the ben th.ic fauna at these times . Whereas breaks in sedimentation can be explained relatively easily { shift of current patterns , reduction in the rate of supply from the source area , etc . ) , it is difficult to understand why there should be a similar reduction in faunal density . Faunal density should increase with decreasing sedimenta tion rate and indeed we have evidence of this in the shell beds of the Pernaryggen Member . Unrecognisable hiatuses imply either rne �hanical destruction or dissolution of the shel-ly fauna (of which there are no s i gns ) , or else, an environment inhospitable to ben thic faunas . There are also no data in support of the last possi
bility . We therefore have to come to the conclusion, that the rate of colonisation of the sea floor was fairly constant throughout the time of deposition of the Pernaryggen Member . In order to ex plain the scarcity of shelly fauna inbetween the shell beds , the organic productivity (and thus the rate of organic shell must surely have been fairly low .
explanation for this may be the depth of the environment,which could well have been at the An
lower end of the photic zone, i f not deeper . Combini�g the avai lable evidence , the following model of the depositional and biolo gical processes is proposed (Fig. 4 ) : In the lower subtidal zone of a sedimentary basin bordered to the west by Palaeozoic basement rocks , deposition occurred by sporadic influx of sand-sized quartz particles as well as by settling out of silt- and clay-sized particles and mica flakes from suspension. At times when sedim�ntation rates were comparatively high, there was insufficient_ time for a benthic fauna to develop and cover large areas of the sea fioor. However, extensive reworking by soft-bodied infaunal organisms was possible . The r�sult was the production of a largely unfossiliferous ,- bioturbated silty fine sand (A 2 ) . Frequently , hOweve r , no or only very little sediment was deposited and during these omission periods there was suffi cient time for an abundant benthic fauna to establish itself {B 2 , c
2 ) . The faunal density eventually reached such high levels that
219
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shell beds forme d . Often , they contain many elements preserved in their positions of growth (B 3 ) . This was caused most likely by an increase in the rate of sedimentation which led to burial of the shell beds and protected them from further d'isturbanc e .
220
Quite commonl y , howeve r , sea floor colonisation was interrupted by short-term in-situ reworking of the shells which caused winno wing, exhumation of some shal low burrowing species , of valves of epifaunal species and possibly some breakage After such disruptions, renewed colonisation could take place (C 4 ) , again fol lowed by in-situ reworking . By repetition of these processes autoChthonous shell beds built up which differ from the undi'sturbed shell beds in various ways (see above ) , most important ..: ly in representing a much longer span of time . Reworking was also caused by bioturbation; there are several cases where shells are concentrated in large crustacean burrows ( :i'ha lassinoide s ) . Simi larly, the breakage of shells is probably partly due to biological rather than physical processes . At some stage after the formation of a given shell bed1 sedimentation resumed and the shell bed was buried ( C 5 ) . During phases of reduced sedimentation or omission , ·g-lauconite pellets formed on the sea floor , transferring part of the finer sediment fraction to a larger grain size. Their original distri bution was subsequently somewhat altered by the burrowing infauna# Finally (Fig. 4 ) , it has now also become clear, why shell beds were preferrentially cemented during diagenesis . DUring periods of reduc'ed sedimentation or omission the proportion of biogenic car bonate is higher1 due to condensation . It is logical to suggest that beds with a higher amount of carbonate have a higher cementa-· tion potential than beds with only a very low content . It thus seems that cementation processes were largely influenced by the original shell distribu-tion .
5 . In-situ Reworked Shell Beds : Interplay of Omission and Storm Events It has been demonstrated above that currents were not responsible for the reworking of the fauna . Bioturbation can only have played a minor rol e , so storms were probably the major agents . It is suggested that, on rare occasions , the effects of particularly heavy storms extended down to regions of the sea floor usually well below wave base . This is compatible with biostratinomic data and not incompatible with the good ·preservation of the fauna. These in�situ reworked shell beds may thus be shell accumulations re flecting very slow or halted sedimentation and reworking by storms
221
to exceptional depths . The genesis of these shell beds be compared to that of the famous. condensed Fe-ooli tic limestooverlying a prominent hardground in the Bajocian of the Normandy coas t (FURSICH , 1 9 7 1 ; see also contribution by SEILACHER) and to that of a glauconitic condensation horizon at Escragnolles , south eas tern Frances (LoWer-Upper Aptian; contribution by GEBHARD) . The Greenland examples differ in being far less Spectacular , the de gree of omission and of disturbance_ by storms being lower and thus less easily recognised. Such shell beds could be quite widespread in the lower parts o� the ·subtidal zone. , It is highly likely that s torms also affected the sea floor during times of higher sedimentation rates� and that traces in the sedi ments were subsequently destroyed by bioturbation (due to lack of hard-shelled faunas no biostratinomic data are available) . The storms most probably were also responsible for deposition of the coarser fraction of the sediment by bed-load transpor t . Thus , one can differentiate between storms which cau-sed short-term reworking of the sea floor without supplying substantial amounts of sediment and s torms which introduced sediment into the environment . The positional sequence .of the Pernaryggen Member can therefore be envisaged to consist of numerous small-scale , irregular oscilla
tion� in the rate of sedimentation superimposed on which we-re (in
t i m e
rates of deposition
Fig. 5 . Relationship be tween shell bed formation, rates of deposition, time , and frequency of storm events (the latter indi cated by stars) . Letters ( B , C ) have the same con notation as in Fig. 4
222
geological terms ) frequent storm events of varying effect. The terplay of the two mechanisms led to the formation of shell beds. · In conclusion, the greatest spans of time are usually represented by the shell beds, whilst the largely unfossiliferous units in between represent � except when heavily glauconitic - only short intervals of the depositional history ( F i g . 5 ) . The shell beds
can be classified as condensed shell bed s . In this respect they differ from the storm-generated shell beds typical of shallower soft bottom environments ( e . g . AIGNER, 1 9 7 9 ; see also contribUti on by AIGNER) , but show close similarities to sheli beds associated with hardgrounds equally from the shallower range of the ' bathyme tric scale ( e . g . FURSICH , 1 9 7 1 ) .
Acknowledgements I would like ·to thank R . BATHURST , Liverpool , T . BIRKELUND, Copenha gen, J . H . CALLOMON , London , A . JOHNSON , Munich, and A . SEILACHER, TUbingen, for useful O.iscussions and critical remarks . The work was· carried out at the Institut for Historisk Geologi og Palaeontologi of the UniVersity of �openhagen during the tenure of a Heisenberg Fellowship,
which is gratefully acknowledged . The paper is
published with the permi�sion of the Director of the Geological Survey of Greenland . References AIGNER1 T . ( 1 9 7 9 ) : Schill-Tempestite im Oberen Muschelkalk (Trias , SW-Deutschland) . - N . Jb . Geol . Palaont . Abh . J�J : 3 2 6 - 34 3 . BIRKELUND1 T . , CALLOMON, J . H . & EURSICH , F . T . ( 1 9 7 8 ) : The Jurassic. o·f Milne Land, central East Greenland . - Rapp . Gr¢nl . GeOl. Unders:. !),Q : 9 9 - 1 06 . FURSICH , F . T . ( 1 9 7 1 ) : Hartgrlinde und Kondensation im Dogger von Cal-. . vados . - N . Jb . Geol . Palaont. Abh . lJ� : 3 1 3- 3 4 2 . FURSICH, F . T . ( 1 9 7 8 ) : The influence o f faunal condensation and ffiixing on the preservation of fossil benthic communities . Lethaia : 2 4 3 -2 5 0 . REINECK , H . E . ( 1 9 6 0 ) : Uber Zeitllicken in rezenten Geol . Rundsch. j,� : 1 49 - 1 .6 1 . WILDBERG , H . G . H . ( 1 9 80 ) : Glaukonitgenese und Lithofazies im Cenoman von Dortmund (Westfalen) . - N . Jb . Geol . Palaont . Mh . J��Q: 52-6 4 .
...,..,,�
Beds in the Lower Lias of South Germany t
Abs tract: The described shell beds reveal a complex history . They reflect strong rework.ing as well as calm periods and differ in litho- and biofacies from the background sedi ments . Most probably the shell beds were accumulated during times of reduced sedimentation and modi fied by storms . Abundant indications of storm activity in the background sediments and their significance for the shell bed formation are discussed .
1
.
intrOduction
Shell beds -- sheet-like thin and wide-spread -- are a characteri stic constituent of many sha llow marine sediment co�plexes of fine-clastic type . They commonly form excellent marker beds , for which the lower Lias (mainly Hettangian) of Southern Germany pro vides good examples . This sequence of shales and fine-grained sand'
stones is preserved right to the ancient shoreline ( see figs . 1 + 2 ) . This is a valuable advantage , as it prevents the introduction
of hypotfe tical eroded facies belts . Of course· the results cannot be generalized 1 but they might be taken into consideration in similar cases . For more details see SCHLOZ 1 9 72 and BLOOS 1 9 7 6 . 2 . Facies Types and Their Interrelationships The shell beds under ·c onsideration get up to several decimeters thick and occur isolated or in groups . They suggest a strong increase of water agitation compared to the fine-grained sediments below and abov e . Therefore most authors. considered them as indicators of shallowing and regression. Cons idering, however, not only isolated profiles but the whole extent o f the · sequence and all its facies relatiorlships , the model of changing sea level is unsatis facory (see below) .
Cyclic and Event Stratification ( ed . by Einsele/Seilacher) © Springer 1982
-------:----·--·
224
Fig . 1 . Paleogeography of the Hettangian and early Sinemurian in Middle Europe . 1 : Shale with limestone beds ; unknown areas . 2 : Shelf sand be lt . 3 : Estuarine and fluviatile . 4 : Highlands
Shell beds may also be formed by extraordinary heavy storms in shallow seas . We shall discuss whether or not this model is suffi cient to explain all aspects in the present cas e .
A third model for the shell bed formation is that o f continuous current activity, preventing __ sedimentation (see e . g . SCHLOZ 1 9 72 } . The fourth model discussed is the interruption of sediment supply · by change in current directions , combined with changes of other factors . The shell ·beds considered are not uniform in facies , but show an ob vious lateral change in facies from west to eas t , i . e . · normal to the ancient coast line . a ) The beds reaching farthest west are fine , marly , bioturbated limestones that as a rule contain no sand, no reworked nodules , and no primary sedimentary structures . The scattered macrofossils are embedded without preferred orientatio n . Chondr i t e s is the
225
predominant type o f burrows . The faunal diversity is high; lower and upper boundaries of the beds are transitional . The background sediments in this deepest part of the basin ( 11belt of bit. shales 11 in fig . 2 ) are marls and b ituminous (black} sha les . Indications of s torm activity are rare .
Towards the east follows a transitional, rather- broad belt, in
which the s i lt content in the shell beds increases and more pri mary sedimentary struutures have escape·d bioturbation. In this 'belt new shell beds develop from biodetritic marl horizons . The low�r boundaries of the beds are usually eros�ona l . Iron oolites and reworked nodules occur. The diversity of the fauna is still
high and the shells show many types of borings . Thalassinoides
burrows are common at the base o f the shell beds ; in fact, they reach their optimum in this facies . The background sediments in this belt are non-bituminou s , s ilty
shales (see fig. 2 ) with thin siltstone layers (a few millime tres thick) and some carbonate content. Shell pavements of
mud-dwelling peleCypods (mostly Modiolus and Cardinia ) different from those in the regarded shell beds are common . Bioturbation
occurs only sparcely in these shales .
c) still farther eas t , _the sandy limestone beds become calcareous , foss iliferous , fine-grained sandstones or coars-e shell hashes , but their thickness remains low . Argillaceous arid mud-sized calcareous matr.,ix disappears . Reworked infra- and intrastratal components ( nodules , fossils , oxydized pyrite , sediment partic les ) are common. Oolites are still common , but become rarer to wards the coast . Lower and upper boundary of the beds are ero sional;· erosional surfaces occur also within the beds . Primary sedimentary structures prevail (even or oblique bedding of fossi l fragments, convex-up shell orientation; less frequent are wave ripples and graded bedding) Most horizons wedge out already in considerable distance from the shore· . •
Among the - scattered burrows s imple and U-shaped ones are most com mon in this marginal belt ; Thci l a s s inoides at the base disappears . The diversity of the fauna becomes reduced , some groups being rare or absent ( e . g . brachiopods , corals , Gryphaea ) , whereas others be
come very frequent ( e . g . gastropods ) . An example o f decrease of
frequency and size is given in fig. 3 . Borings in the shells are abundant with a wide spectrum of forms . In contras t , the diversity
Belt
of
O f f s h o re b i t. s h a l e s
s h a l es
Shelf
Shales, storm silt toyers, bioturbatE'Cl
homogenous limestones
limestone beds with shE'll conc(>ntraUons
w
]
Belt of shelf sand bodies Sea of nearshore shales
Be l t o f s i l-ty s h a l e s
Shales, bit. shales. marls,
sands
] Estuarine Fluviatile
Shales, irregular sand toyers.
Sheet sands, shales, shell be-ds
! with sand matrix 1
Li t t o ral
channel-shaped scours -
E
km
1 00
Vertical scales: water depth 1--1 :-20-25m sediment thickness .,_... = "' 5-7m
Oecrt"asing bitumen contents. Increasing silt contents and frequency of
Increasing groin size and sortin,g,
Deereosing portion of sandstones
Decreasing marine
Almost no sand contents and
incrli'
and diversity of fauna.
influence. Shales
neation and tool marks. Decrea
Optimum of ichnofauna,predomi and fine-to coarse-
sing carbonate contents of shales.
nonce of spreite borrows.
grained sondstor"le'S Poor ichnofouna,
sing f�ency of iron oolites.
rootlet beds.
reworking. No bitumencontents.no sand
no primary sedimentary struc- stone bodi�.o!most no primary sedimenture-s in limffstones. Fossils
tory structures in limestont>s. Sholescalca-
scatt(>!'ed, high diversity.
rt>ous. Chondritespredominant borrow. In-
No iron oolilffs in limestont"S.
Crfflsing frequency of iron oolitesinlimesl
I
lnmasing frequ�cy Of primary S(>
dimentary structur� in shell beds.
I
No sondfree limestone-s. DeClll'O
I
I
Fig. 2 . S chematical cross section through Hettangian sediments in Southern Germany with inferred water depths . Minor variations in thickness and lateral shifts of shelf sand bodies are disregard�d
lll
227 Plagiostoma giganteum:
E
rnmm very frequ. lllllJD frequ. IDJl not frequ.
Fig. 3 . Faunal change within a shell bed to wards the ancient coast: decrease in frequency and size of PLagio s toma giganteum ( Modified from SCHLOZ 1 9 7 2 , Fig . 1 0 )
of the incrusting sessile epifauna remains surprisingly low (mainly LiO s tr e a and serpulids)
•
The background sediments of this facies belt are -- besides silty shales -- shelf sand bodies ( 1' 5-70 km wid€. , 40-
200 km long, up to
4 m thick 11sheet sands'') parallel to the basin axis ( see figs . 1 + ' 2) . The grain s i ze increases from west to east 'from silt to very
fine sand { fig. 4 ; note the strong asymmetry of grain size distri bution ) ; at the same time the degree of sorting increases . Both trends indicate wi�nowing of the finer fractions in the eas t . The range of grain sizes ends abruptly at about 100 microns , probably
reflecting the upper limit of transport in suspension at a given turbulence leve l . Towards the coas t , these shelf sand bodies
grade into an intercalation of shale (poor in carbonate) and thin,
irregular layers of siltstone (see fig. 2 : "belt of nearshore sha les11 ) . Channel-like , sandfilled scours ( "gutter casts " ) are the most characteristic feature of these intercalations . Their maximum
depths of erosion increases from west to east ( 1 m at most) . In the eastern part o f the shelf sand belt current lineation can be ob served frequently, associated with concentrations of heavy mineral s . The fauna of the shelf sands also shows lateral zonation: a t their
western margin · it is very poor, in th'e middle it reaches the highest diversity (s ometimes nearly as high as in shell beds of this belt) and near the coast it decreases strongly (almost monotypic Liostrea� Cardinia , and gastropod associations ) . Shell accumulations
228
a " 0
rs::::SJ siltY sand+ sand
Fig. 4 . I ndicators of some oceanographic factorS .
Left: Grain size distribution in one of the shelf sand bo dies , reflecting shallowing from west to .east. The small interval of grain sizes segregated over a large distance indicates a very low depth gradient . Cross bedding shows s'ediment transport to soutq.
Right : Directional indicators oriented SE
-
NW probably in
dicating episodic storms from SE . The indicators are best developed in the eastern part of the shelf sand belt
in the shelf sands are always . more or less lenticular . Iron oolites are lacking; reworked calcareous , mostly spheroidal sandstone concretions become more and more frequent towards east. Borings in shells are rare .
The lateral facies transition in both, shell beds and background se
diments , though di f ferent in detai l , reflects an increase in inten sity and frequency of sediment reworking by' storms from west to easL This indicates shallowing in this direction . In the east , the fine
detritus of the shell beds became winnowed to get deposited farther west. There , in deeper waters, the fossilisation potential of pri mary sedimentary structures was low, because storm ev�nts were sepa- · rated by lbng times of bioturbation . In the shallower water to the east , the chance -of complete bioturbati0n in the shell beds was re _ duced because storm reworking occurred more frequently ahd because the coarser sediment was less attractive for sediment feeders .
229
lateral dif ferentiation of litho- and biofacies and its obvious to water depth i'n dicates a simultaneous origin of in shell beds over a considerable depth range . This excludes
origin by a shifting narrow ( e . g . littoral) facies belt as
during a regression . Additional arguments against such a
are: diachrony o f shell beds has never been documented by the coast line did not shift considerably during the
truely coastal sediments (sands) are quite di f ferent
from those of the shell beds . Moreove r , regression would have cau sed merely a shift of . the e�isting facies belts , e . g . of the shelf sand belt, rather than producing such a striking lateral change of facie s . Therefore it can be inferred that change of sea level -
eustatic or tectonic -- was no dominant factor in the origin of the shell beds .
According to the second mode l , an increase of storm effects must not
be due to shallowing. It can be also produced by climatic changes , i . e . by storms o f increased energy able to affect the sea floor to
depths way beyond the reach of normal storms . Therefore the indica
tion of storm action should be regarded in more detail .
3 . Storms and Their Effects in the Basin 3 . 1 . Storm Layers UbiquitOus Traces of strong, · short-lasting turbulence are ubiquitous in all sediments of the basin , except for those of the bituminous belt .
There was actUally no sediment movement, of neither mud nor sand,
without participation of storm-induced turbulence in this basi n : the sedim-entation was discontinuous throughout the sequence . Almost every sediment layer -- even in deeper parts of the basin (belt of silty shales) -- rests on an erosional base indicating the
climax of a storm. Features of upward grading in the layers -
reflecting the waning of storm action -- are quite common . . Two types of grading can be dis tinguished : in shaly sediments a decrease of the silt and an increase of the argil'·laceous component upwards ; in sands , however , a structural grading from even lamination at the
base into different· types and scales of wave-formed oblique bedding ( e . g . micro-scale "millimeter" ripples , small-scale "normal " wave
ripples , large scale "hummocky " cross bedding) . No grain s i ze gra
ding can be seen in the fine sands because they were too well sorted .
230
\'
All primary sedimentary structures seem to be wave-formed or at least wave-superimPosed . There are no structures indicating ous strong currents independent from storms ( e . g . current ripple s or · dunes as a dominant structure of sandstone beds ) . The of storm layers varies between some millimeters and some maximum thicknesses occur in the shelf sand bodies . 3 . 2 . Directional Indicators There are many indicators of directions or at least orientations currents and waves. ( see fig . 4 ) . Remarkably, the distribution of directions is not as random as may be expected in a shelf area. main orientations can be observed : southeast-northwest ( that means oblique to the coast and the basin axis) and almost north-south (parallel to the basin axis ) . The orientation southeast-northwest
documented by current lineation , tool marks , gutter casts , and aligned elongate particles ( e . g . echinoid spines , wood fragments ) . Prod casts on erosional surfaces ( e . g . at the base of sandstone or on the sole and the walls of gutter casts) show always two site maxima , indicating a bipolar change of current direction
·( northwest -- southeast, see f i g . 5 ) . In contras t , the mechanism
of infill of the gutter casts was unidirectional : the low angle oblique bedding of the sand fill always dips to the southeast.
Also unidirectional is the maximum of the dip azimuths of wave ripple lee-side laminae : northwest (main orientation o f ripple
crests southwest-northeas t ) . All these features indicate that the storms came predominantly from southeas t .
The second important direction of water movement w a s -- as j ust
mentioned
parallel to the basin axis , almost from north to
south. It i s mainly expressed in an asymmetry of- the large-scale
low angle cross bedding of the shelf sands , the so-called 11hununocky cross bedding" ( HARMS et al . 1 9 7 5 : 87-88 i detailed description in
BLOCS 1 9 7 6 : 1 3 8- 1 54 ) . It i s also reflected in a minor maximum of ripple crest orientation (west-east with lee side la�inae dipping to the south ) . Howeve r , these are the only structures indicating this direction . No current lineation, no tool mark s , �o gutter casts , no long objects are oriented north-south .
The most important indication of water movement from north to south is the origin of the fine shelf sands . This sand was not supplied from the adjoining land mass in the east ( Bohemian massive) , be-
231
Fig . 5 . Tool marks on a gut ter cast from the nearshore shales . Prod casts point in opposite direction, indicat ing bipolar changing currents
· cause there are s i gnificant differences in. mineraloglcal composi tion between the shelf sands and local delta sands along the
coast . There is also a gap of grain sizes between both types of
sands . The main source of the fine sediments in the eastern part of the �iddle European sea seems to have been the mouth of the large drainage pattern of Eastern Europe which was situated south of the
Baltic .
The differences between the two groups of currents are striking . One cause may be the different orientation to the coast. Storms oblique to the coast (SE-NW) caused a - tide effect strongest in very
shallow parts of the basin. The effect of northerly storms was ob viously stronger in the adjoining deeper area (belt of shelf sand bodie s ) . Both types of storms probably acted antagonistic.ally , in , fluencing the main current directions and the direction of net se diment transport . Shifts of their relation with regard to energy and frequency can be imagined; as a consequence a directional change o f the prevailing currents and of the net sediment transport seems
poss ible .
4 . Storms - Main Factor in Shell Bed Formation? As shown in the previous chapter, storms did not only form the depo sitional and erosional structures in most sediments of the basin ;
232
they also controlled the supply of terrigenous sediments cing the currents . It is quite obvious that storms also
shell beds ( erosion , reworked material , orientation of shell s , _ ting) . The question . remains , however , whether all features of beds are, direc·tly or indirectly 1 storm-related. 4 . 1 . Intrastratal Reworked Components The sand content of the shell beds depends conspicuously on the stence of sandstones below . Calcareous sandstone concretions in shell beds ( sOmetimes as ammonite casts) suggest that the sand mainly reworked . Shell beds poor in sand rest mainly on shales their whole extent .
In spite of the extreme frequency of storm erosion in the basin,
calcareous nodules within the shales were never reworked shell beds . This indicates an unusually deep
and that may mean
strong -- act of erosion. However , this must not be the result of a single .event . In times of non-deposition, the effects of many minor acts of erosion might have accumulated.
Not reworked from other sedimen·ts are the iron coli tes , because they_: occur only in the shell beds' . Since they enclose typica l compone·nts
of these beds , they must have been formed during the formation of the shell beds themselves . They cannot either have formed at the coast
and transported offshore , because sediments are different therei
moreover iron oolites are lacking in the coastal sediments .
Similar observations apply to the fauna . It is different from that
o f the background sediments as well as from that of the coastal zone. 4 . 2 . Evidence of In trastratal Rewor�ing
Fossi+s are the. most common intraclasts . They can be recognized by different sediment fill or - more commonly - by oxydized pyrite.
Early-diagenetic pyrite grains in the shells have become oxydized ,
causing the yellow colour of the fragments . After each subsequent burial, a new generation of pyrite was formed . Only the pyrite of
the last burial remained fresh . It is impossible to �ist·inguish and count the generations of oxydized pyrite in a shell fragment; but it can be assumed that shells did not survive many acts of rewor
king.
4.3.
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Factors Causing the Faunal Change
233
As shown before, the sediments of the basin reflect mainly storm events , that means short and exceptional . conditiqns which were comroonly letal for parts of the benthic fauna . The fauna was mainly in-
fluenced by the conditions in the i�tervening periods , in which no majc;>r. sediment movement occured; i t is these backgrbund conditions that the fossils reflect. Three types of differences can be r.ecognized between the fauna of
the shell beds and that o f the shales ( see fig. 6 } : 1 . there are speci es restricted either to t�e shell beds or to the shales ; 2 . the
frequency and 3 . the size of species occurring in both types of se diment are different. There are als·o minor differences , e . g . in the manner of growth ( fig . 7 ) . Some spec�es ( e . g . Lios tre a } show no re , cognizable differences .
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Fig. 7 . Different growth of Cardinia in shales .and shelf sands ( left) and in shell beds (right) . Growth line intervals decrease regularly during ontogeny in the accompanying se d iments (left) whereas they vary irregularly in the shell beds (right) ( From SCHLOZ 1 9 7 2 ,. Fig. 1 2 )
In general , the faunal change shows a shift to the calc-i tic shells of suspension feeders that lived near or at the sediment surf_ace i many of them are fixosessile ( see SCHLOZ 1 9 72 ) . Strikingly there is
no increase of incrusting sessile organisms . The proportion of arago nitic pelecypods dwelling deeper in the sediment is reduced accor dingly .
From the fauna one may conclude what environmental factors were
mainly responsible for this change . Substrate
The most conspicuous environmental change from shale to shell bed
seems to be that of the substrate 1 which partly consisted _of accu
mulated shells and shell fragments , partly of silt- to fine sand sized particles (matrix} . Partly it was hard (shells , •reworked no
dules : secondary hard ground) . A comparison with the background se
d iments shows that such substrates were also commonly available in the shales as silt layers and shell pavements ; in the shelf sands
the substrate can be almost identical to the shell beds . Never theless the typical fauna of the shell beds did not develop in these
235
�reas . This shows that the cha� ge of substrate was not the only an'd not even the most important cause for the faunal change . sea water Under this heading many factors are comprised, the respective in
fluences of which are difficult to distinguish. These include phy� sical properties such as turbulence, current velocity and direction, temperature , contents of suspended matter , chemical properties such as ,salinity and carbonate contents , ·and biological ones such as
blooms or larvae from. other regi·ons .
Life conditions of suspension feeders became" improved . This may in dicate an increase of the avera·ge turbulence. Important in this re gard seem to be the boring pelecypods in reworked nodules . They occur only in some cases and are always s·cattered 'on the nodules , never crowded like in littoral environments . This suggests that
the average tUrbulence remained at a minimum and often j ust below the level required by borihg pelecypods . The suggested increase of turbulence may be due to increased avera ge wind action as well as to increased current velocities . It is difficult to imagine sudden changes of average win·d energy , there fore, an increase of current velocities seems more likely .
There is little evidence of changes in chemical properties . The men tioned decrease of diversity and size of the fauna towards t.he coast probably reflects stronger variations of salinity adjacent to river mouths .
Variations of an unknown fa.ctor are reflected by· the ·trr·egul·ar· growth of Cardinia in the· shell beds . In contrast, shell growth is regular in the background sediments ( shales and shelf sands ) . Here , the di stances of growth interruptions decrease regularly ( see. fig . 7 ) .
Another unknown factor seems to have influenced the aromo·n ite fauna .
The diversity of species is considerably higher in the shell beds th.a n in the background sediments , though the few forms of the back· ground community sometimes occur more frequently . Iron oolites indicate other unknown factors . Their origin is still
uncertain. If turbulence would be the main factor for their forma tion, they should be found in nearshore areas throughout the se
quence . Similarly, glauconite . is found only in shell beds , which also indicates changed conditions .
·· n ·I!
236
Another striking difference between shell beds and background ments is the strong to complete bioturbation of the shell horiz ons in large parts of the basin , in contrast to the scarcity of this phenomenon in the background sediments . The interpretation is dif·
ficult . Grain size di fferences are too small to be responsible above ) . The strong bioturbation could reflect increased . of endobiontic organ isms . But on the other hand scanty could, during long time periods of non-reworking, also plete bioturbation . The only indication of changed conditions may
be the size of Tha l a s sinoides burrows at the base of the shell which are up to six times as wide as in the background sediments . There are indications that two different realms existed in
of Western Europe at that time: prevailing carbonate rocks in the
south, terrigenous sediments in the north (see HALLAM 1 9 6 9 ) . Sou thern Germany belongs to the northern realm. But the shell beds may reflect times of increased influence from s Outhwes t , as indica ted by a comparable increase of faunal diversity in this direction. Rate of Sedimentation The fauna of the shell beds , dwelling as it was near or at the se
diment surface, was probably less tolerant to rapid deposition of
fine, especially argillaceous sediment than that of the background
sediments . This may be particularly true for corals and boring or
ganisms . One reason why shell pavements and silt la"yers in shales did not become settled may be the fact that they were too soon co vered by mud . There are indications of reduced sediment s upply during the forma
tion of the shell beds. The large , s ize of calcareous nodules in the shales below many shell beds s uggests a longer period of precipita tion. Intrastratal reworking ( e . g . oxydized pyrite in ' shells) also
indica·tes a more or less constant sediment level . Sometimes a con densation of ammonite subzones can be shown . Therefore the material of the· shell beds was not supplied by one single event , but con sists o f the remains of m·any generations of organisms . By rewor king from time to time1 earlier shell generations became destroyed
so that the beds grew very slowly .
The areas of reduced sediment supply were not extended over the
whole basin . Differences existed particularly in !J1er·idional di
rection. Ammonite subzone that are represented in Sou ·th Germany only by one shell bed1 may reach many meters of shale and sand-
237
in Northwest Germany and vice vers a . This is another indi cati on that the formation of the shell beds was not a question of water depth .
�
In contrast to the background sediments , a considerable . portion of th e shell beds -- . the hard parts of organisms -- were supplied du rin g the times between storm events . Therefore storm e f fects seem to be rather an accessory phenomenon in the shell beds (see also
be low ) .
5 . t-1odel of Shell Bed Generation The first two models mentioned above (regression and one-event ge neration) will not be discussed again in this chapter , since the third model ( i ncreased current velocity) seems more probable (chap ter 4 ) . But it seems questionable whether currents were strong
enough to prevent at least the sedimentation of fine sand. It should be remembered that f ine-grained material reworked by storms did settle after the storm had waned over large parts of the basin and became winnowed only in its eastern part . That means if such
sediment would have been supplied , it would have been also deposi
ted in spite of the increased current velocity . Therefore the third model is not satis factory either.
The most adequate �odel to explain the phenomena of the regarded shell beds seems to be realized on modern shelves ( interruption of sediment supply , no . 4 in chapter 2 ) , where sediment is not supp
lied uniformly from all parts of the adjoining coas t . In most cases
one large river mouth dominates the s edimentation on a shelf over
hundreds of kilometers a long the coast, whereas smaller rivers con tribute only small or neglectible portions ( see e . g . CURRAY 1 9 60) . . Sediment supply thus depends on the cu�rents that distribute sedi ment in suspension from the respective large river mouth . If the
direction of sediment transport changes , new areas receive the se . diment, leaving the former depositional areas with conditions of non-sedimentation (accumulation of har'a parts of organi sms, genera tion of glauconite, etc . ) . An example of recent shell beds formed in this way is described by COLEMAN
the Mississippi delta.
&
GAGLIANO ( 1 9 6 5 ) from the area of
238
As mentioned above ( 3 . 2 . ) , the situation in Middle Europe was
rable in the beginning of the Lias . The dominant sed.iment south of the Baltic supplied material as far as South Germany .
the net transport from the north was reduced or a change of the prevailing currents , conditions began . Currents from the southwest brought no sediments ,
there was no comparable source of fine sediment . On the other hand ; those currents may have brought sea water with dif �erent physical and chemical pr.operties as mentioned above ( 4 . 3 . ) . The model is not yet confirmed in all details . But it seems to ex
plain ' more aspects of the 'shell beds than any other. I t shows that
the effect of storms , though dominating the appearance o f the beds , were of minor importance in regard of their origin. Only rectly they might have been the main cause : if the change of sedi
ment supply was due to a change of the prevailing winds as mentioned above ( 3 . 2 . ) . 6 . Other Traces of Rare Events ? As shown , shell concentrations in shallow marine sediments may be
formed in d ifferent ways . The regarded shell beds obviously are not the product of rare events in spite of the common occurrence of
storm event structures . It seems that a single storm, as rare in
strength as it might be, cannot cause an interruption of the sedi
mentation for thousands of years . And it cannot explain why sedimen tation went on as before after such a long period. O f course a
strong storm is able to provide a secondary hard ground; but this can only be colonized if the conditions are d i fferent from those
before the storm. Otherwise the shell concentration remains without settlement like many others in the shales . The regarded sequence represents a long period and is made up al
most totally of storm-induced layers . It ·seems that these are good
conditions to find traces of rare events . It is difficult, however, to recognize extraordinary horizons , which could be referred to a single rare event with certainty . Perhaps horizons of exceptionally large gutter casts may be the result of such events . But even such
horizons can be traced only over restricted areas ; they are by far no means of basin-wide correlation. The only markers suitable over larger distances are the regarded shell beds, and they are no re sult of rare events .
·
239
AI GNER, T . ( 1 9 80 ) : Storm dep6sits as a tool in facies analys is . I . Calcareous Tempestites . - Internat . Assoc. Sedimentologists 1 st Europ . Meeting , Bochum 1 9 8 0 , Abstracts : 4 4 -4 6 .
BLOCS , G . ( 1 9 7 6 ) : Untersuchungen liber Bau und· Entstehung der feink6rnigen Sandsteine des Schwarzen Jur·a (Hettangium und tiefstes Sinemurium) irn schwabischen Sedimentationsbereich . Arb . Inst. Geol .
Palaont. Univ. Stuttgart , N . F . �l = 1 - 2 6 9 .
( 1880) : Primary sedimentary structures in fine-grained shelf
sands - a fossil example . - Internat . Assoc. Sedimentologists 1 s t Europ. Meeting, Bochum 1 9 80 , Abstracts : 3 5 - 3 8 .
COLEMAN , J M. & GAGLIANO , S . M . ( 1 9 6 5 ) : Sedimentary structures : Mis .•
s issippi River deltaic plain . - In : G . V . MIDDLETON ( e d . ) : Pri
mary sedimentary struc tures and their hydrodynamic interpre tation . - Soc .
,l §, : 1 3 3- 1 4 8 .
econ . Pa.leontologists Mineralogists Spec. Pub l .
CURRAY, J . R . ( 1 960) : Sediments and history of Holocene transgres sion , continental she l f , northwest Gulf of Mexico . - I n : F . P .
SHEPARD , F . B . PHLEGER 5 T . H . VAN ANDEL (ed. ) : Recent sediments , '
northwest Gulf of Mexico: 2 2 1 - 2 6 6 .
HALLAM , A . ( 1 9 69 ) : Faunal realms and facies in the Jurassic . - Palae ontology ,
,l� : 1 - 1 8 .
HARMS , J . C . , SOUTHARD , J . B . , SPEARING , D . R . & WALKER, R . G . ( 1 9 7 5 ) : .
Depositional environments as interpretated from primary sedi
mentary structure and stratification sequences . - Lecture notes for Short Course No. 2 , Dallas/Texas ( S . E . P . M . , Tulsa) .
NEWTON, R. S .
( 1 9 6 8 ) ·: Internal structure of wave-formed ripple marks
in the nearshore zone . - Sedimentology , lJ : 2 7 5-29 2 .
REINECK , H . -E . & SINGH, I . B . ( 1 9 7 2 ) : Genesis of laminated sand and graded rhythmites in stotm-sand layers of shelf mud . - Sedimen tology , ,1� : 1 2 3- 1 2 8 . SCHLOZ ,
w.
( 1 9 7 2 ) : Zur BildungsgeschiChte der Oolithenbank ( Hettan
gium) in Baden-Wlirttemberg . - Arb . Inst. Geo l . Palaont. Univ.
stuttgar t , N . F . gz: 1 0 1 -2 1 2 .
SCHMIDT 1 H . ( 1 9 3 9 ) : Bionomische Probleme des deutschen Lias-Meeres . Geol . der Meere und Binnengewasser , � : 2 3 8-2 5 6 .
Storm Sedimentation in the Carboniferous Lrrne�;torles Near Weston-Super-Mare (Dinar1tiar1, SW-Englar1d) D. JEFFERY and T. AIGNER
Abstract ·:
Dinantian carbonate rocks exposed in the cliffs at Middle Hope Weston-super-Mare, are composed of diffe rent lithofacies units accumulated on a tide-dominated shelf durin'g the upper half of one of RAMSBOTTOM ' s ( 1 9 7 3 ) regressive cycles . Interbedded within the carbo nate facies are basaltic tuffs , lavas and thin limesto ne units . Grading, bioturbation and ripple marks within tuffs and thin carbonate units , as well as biofabrics in shell beds , indicate that the beds cons ist of alter nating storm- and £air-weather layers , which form cha l!acteristic couplets . Storms are inferred to have been· an important factor in Dinantian sedimentation , and to have generated a characteristic lithofacies which would not have been otherwise developed i f simple eustasy or vertical tectonics (or both} were the sole regional controls on the carbonate sedimentation .
1 . Graded Tuff and Limestone Sheets 1 . 1 . Descriptions 1 . 1 . 1 . Limestone-Dominated Subunits The beds are carbonate s iltstones and mudstones , with variable
amounts of fine-grained volcanic � etritus . The s iltstones are terised by symmetrical ripple s , often wi.th complex and partly bio turbated internal cross-lamination . The mudstones separate the silt stones and fill the ripple - troughs : the base of each siltstone is therefore sharp and flat. Red mudstones of horizon 10 bear distinc""". , tive wrtnkle-marks. Wavy bedding, similar to that of the oolites in both scale and direction , is also developed . Loadcasts, flame
structures and very small-scale recumbent cross-lamination and a " suite of trace fossils (which include Rhizoaora l lium� MonooraterionJ Diplooraterion J Sko lithos and Teiahiahnus) record a complex series
of rapid depositional and scouring events , followed by quieter con-
Cyclic and Event Stratification (ed. by Einsele Seilacher) © Springer 1982
241
allowing the development of a strongly bioturbated layer shell lags .
1 . 1 . 2 . Tuff-Dominated Subunits rhythmites composed of sets of fining-upward bands of
thickness , within each of which are sets of normally gra
( sometimes with graded cross- lamination) of centimetre and millimetre thickne s s . The tops of the thicker bands are marked by very thin seams or flat lenses of black micrite or chertified car bo'nat e . 1 . 2 . Interpretations 1 . 2 . 1 . Carbonate-Dominated Subunits These are interpreted to have been deposited from suspension and to
have been reworked by waves and burrowing animals . The beds show features of storm deposits ( e . g . HAYES 1 9 6 7 , KUMAR and SANDERS , 1 97 6 ; REINECK et a l . 1 9 6 7 , 1 9 6 8 ; SCHAFER , 1 9 5 6 ) . The process-response mo del of KELLING and MULLIN { 1 9 7 5 ) for· storm-generated sequences en visages normal sedimentation, storm stirring, the introduction of a coarse increment , then a return to normal · sedimentat'ion. In such
deposits , the bedding is a function of £air-weather and storm' weather conditions . During fair weather, organisms can colonise and
grow on essent ially muddy substrates ; storms stir up bottom sed iment and smaller shells (which go into suspension) , and concentrate the larger ones as shell lags . After the passage of the storm swe l l ,
the suspended matter settles out into graded beds with shell laYer s ,
and fair weather conditions al low bioturbation o f rippled sediments.
Intensive bioturbation itself may accentuate or produce grading of shell beds ( c f . RHOADS and STANLEY , 1 9 6 5 ) . These graded laminated
limestones are thus the probable analogues of the sheet sandstones described by GOLDRING and BRIDGES ( 1 9 73 ) . 1 . 2 . 2 . Graded Tuffs ( I ncluding Lapillistones ) Accepting a storm origin for. . the above beds we can interpret the majority of the graded tuffs as the result of the fall-out from aqueous suspension (during fair weather and following storms ) of detritus derived by current and wave activity from the prograding front of a lava - hyaloclastite complex some distance to the west and southwest . The thin micrite seams and lenticles at the top
242
of each graded bed represent the final settling of the less dense carbonate mud stirred up at the same time as the volcanic The thicker beds of tuff represent more rapid deposition due to increased amounts of detritus being available as a result of sified volcanic or storm activity or both .
2 . Carbonate Blankets : .Crinoid Gr-ains tones Though Michelinia can be found in growth position, more usually the fossil debris has been badly abraded and sorted , to produce an crinoidal grainstone with two modes - a major one at 0 . 5 mm median grain diameter, and a weaker one at 2
mm .
Complex internal cross-stratification of the kind f igured by KLEIN
( 1 970) .is visible. Trough cross-bedding was observed in one .locali ty , where it appears to merge with complex cross-stratification. The tabular cross-bedding occurs on two scales , and the upper parts of some of the units have been thoroughly bioturbated ; apparent dips · are a lways bipolar and bimodal . The cross-bedding in these horizons indicates that the original sediment was moulded into dunes and sandwaves (terminology of HARMS , 1 9 7 5 ) . As shown by the bioturbated tops of most of the units , the . cross-bedding was produced by inter mittent reversing currents . The bioturbation i s often s o extensive and complete that time must have elapsed between deposition of one dune layer and the next. The base and top of composite cross-bedded biosparite units are both sharp and they were origirially overlain by biomicrites (now largely dolomitised ) . In s imple eustatic models we would need to invoke a gradual fall of sea leve l , followed by a sudden rise, with a return to a sudden f al l . Moreover ,the observed lateral gradation of cross-bedded biosparites into biomlcrites would also be inexplicable . A model of pulsed tectonism to account for depth differences also leads to the same difficulties. An alternative explanation is therefore offered. The limestones might represent carbonate analogues of "blanket sand stones" described by ANDERTON '( 1 9 7 6 ) . He has reconstructed a series of events based on known processes operati� in modern siliciclastic seas to account for the tabular sandstone bodies of the late Pre-
243
cambrian Jura QUartz ite of Western Scotland. The sedimentary struc ,tures of his sand bodies closely resemble the structures observed in the crinoid biosparites . Complex dunes are considered by KLEIN ( 1 9 7 0 ) to be c l imbing dunes : at Middle Hope , they are associated with pebbly beds and weak channel structures just as they are in �NDERTON ' s model . The post-storm conditions mark the return of
mud-sedimentation over and adj acent to the area occupied by the gravelly sand (pebbly biosparite in carboDate analogue ) . The thick
b lanket sands are built up of successive storm units: previously deposited post-storm mud is winnowed away before deposition of the next , layer ( amalgamation effect ) .
3 . Coquinas Within Ooid Grains tones (F'ig. 1 ) The oolites are clearly high-energy depos its : bottoms were constant ly and rapidly shifting;-, and frequent agitation is also implied by
the concentric architecture of the ooids (DAVIES et al . , 1 9 7 8 ) . Oscillation ripples observed at seveal exposures indicate wave aCt-ivity, but most of the energy was provided by reversing currents ,
which are usually interpreted to be tidal (SELLEY, 1 9 6 8 ; KLEIN , 1 9 7 0 ) . The scale and style of the cross-bedding indicate that the currents worked the ooid sand into small dunes and sandwaves ( terminology of HA�S , 1 9 7 5 ) . All these features are consistent with those observed from some modern ooid accumulations ( e . g . LOREAU , 1 9 7 3 ) . - No evidence of subaerial emergence was found , nor any sedimentary structures in dicating intertidal or beach environments (but this may be due to
poor exposure in the upper parts of the oolites ) . It is inferred that the bulk of the oolites were rarely emergent broad bars or sheets of ooid sand accumulating on an open shelf . Coquinas , mainly consisting of Produatus and Megachonetes shells
with sorn·e gastropods are frequently intercalated with these oolites . These shell beds always show an erosive, often irregularly scoured , base; the shells are commonly imbricated . According to fabrics and the orientation of the shells, two types of c Oquinas may be diStinguished: Type A: wackestones in carbonate mud/sand matrix. Lmbrication dominantly convex-down or shells standing in 11end-on" positions . Type B : packstones-grainstones, matrix largely washed away . Imbrication domin�y convex-up.
244
COQUINAS IN LW. CARBONIF. OOLITES
SHELL IMBRICATION
oolite shoals
STORM·
TYPE A
i;t::\
TYPE B
Fig. 1 .
Two types of storm-generated coquinas with characteristic imbrication features. For further explanation see text
The orientation of concave-convex particles has been used as an indicator for depositional processes by several authors (MIDDLETON',
1 9 6 7 , EMERY 1 9 6 8 , SANDERSON & DONOVAN 1 9 7 4 , review in FUTTERER 1 MIDDLETON ( 1 96 7 ) demonstrated that 2 3 - 6 9 % of concave-convex deposited from experimental turbiOity currents were found in down or' in a standing-on-end position. On the other hand , RICHTER ( 1 9 4 2 ) showed that convex-u� orientation { 11Einkippung'' } is typical for unidirectional currents ( see also FUTTERER 1 97 8 ) . The observed fabrics in the two types of coquinas may therefore be interpreted as follows : (Fig . 1 Type A : periodic influx of shell-rich sediment on ooid bars/shoals and rapid sedimentation from suspension as indicated by the dominant convex-down position of she l l s . No subsequent reworking due to immediate burial .
245•
Type B : in contrast to Type A , shells became subsequently reworked by current activity . Consequently , the fines were winnowed away ( packstones ) and shells were preferentially imbricated in a convex-up position. In conclusion, periodic influx o f shells onto ooid shoals is inter preted to be_ due to storm events . However , storm-generated struc tures (Type A coquinas ) have often been overprinted by tidal re working of the sediment during £air-weather periods ( Type B coqui � s ) .
4 . Conclusions The recognition of storm events
casts some doubt on a s imple eusta
tic model for the minor cycles of the Dinantian. GEORGE (pubs . to 1 9 7 8 ) has long been a critic of eUstasy , invoking instead tectonic controls to both major and minor cycles. However, severe storms can superimpose unlike facie s , even over wide areas - recent hurri canes may have diameters of 650 krn ( BARRY and CHORLEY , 1 9 7 1 ) and affect up to 500 km of coastline. Because they rarely follow exact ly the same path s , storms are significant agents of resed imentation over.r regions greater than their individual diameters ( PERKINS & ENOS , 1 9 6 8 ) � Adopting a uniformitarian view o f { i ) the distribution of chlorozoan carbonate associations and ( i i ) the behaviour of the earth ' s atmosphere , leads inevitably to the conclusion that much of the Lower CarbOniferous sedimentation must have been grossly modified by deep tropical cyclones , events which , though catastro phic on our modern human scal e , may very well be geologically normal. An acceptance of catastrophe as normal does not necessarily invali date the concept of the major cycles in the Dinantian but it does point up the emphasis that has been placed on exogenic non-sed imen tological controls to the development of minor cycles . The mode l of ANDERTON ( 1 97 6 ) demonstrates the generation o f sequences of alter nating sand and mud units , metres thi�k, in a stormy tide-dominated sea without recourse to minor oscillations of sea level (eustasy) or of sea floors (tectonic s ) . It is suggested that regional interpretation o f major portions of the c arb . Lst . sequence in terms of sil iciclastic mod els would be a
246
fruitful contribution to basin analysis of the Dinantian rocks of. Britain and NW Europe . Acknowledgements This note is based on a MSc-Thesis submitted by DJ to the of Reading/England and grew out from a j oint field session with TA in 1 9 7 9 . We thank our colleagues and the members of staff of the Geology Department at Reading for discussion and Prof. Dr . A. lacher for reviewing the manuscript.
References ANDERTON , R. ( 1 97 6 ) : Tidal shelf sedimentation: an. example Scottish Dalradian . - Sedimentology , �� : 4 2 9 -4 5 8 . BARRY , R . G . & CHORLEY , R . J . ( 1 9 7 1 ) : Atmosphere, Weather and Methuen and Co . , London , 3 7 9 p . DAVIES , P . J . , BUBELA , B .
&
FERGUSON , J . ( 1 9 7 8 ) : The formation of
ooid s . - Sedimentology , �g : 703-7 2 7 . EMERY , K . O . ( 19 68 ) : Positions o f empty pelecypod valves on the con tinental shel f . - J . sed. Petr . , ��: 1 2 6.4- 1 2 6 9 . FUTTERER , E.
( 1 97 8 ) : Studien tiber die Einregelung, Anlagerung und Einbettung biogener Harteile im StrOmungskanal . - N . Jb. Geol . Palaont. Abh . , 1gg : 87-131 .
GEORGE , T . N . ( 1 9 7 8 ) : Eustasy and tectonics : sedimentary rhythms and stratigraphical units in British Dinantian correlation . - Proc. Yorks . geol. Soc . , 4� : 229-2 6 2 . GOLDRING, R. J . sed •
&
BRIDGES , P . ( 1 97 3 ) : Sublittoral sheet sandstone s . Petr . , �;j : 736-747 .
HARMS , J . C . ( 1 9 7 5 } : Stratification produced by migrating bedforms . Soc . Econ. Paleont. Minera l . Short Course, � : 4 5-6 2 . HAYES , M . O . ( 1 9 6 7 ) : Hurricanes as geological agent s : Case studies of Hurricanes Carla , 1 9 61 and Cindy , 1 9 6 3 . - B� r . · Econ. Geol. Univ. Texas rept . Invest. 6 1 . KELLING, G . & MULLIN , P . R . ( 1 9 7 5 : Graded l imestones and limestone quartzite couplets : possible storm deposits from the Moroccan Carboniferous . - Sediment . Geol. �� : 1 6 1 - 1 9 0 .
247
KLEIN , G . de V .
( 1 9 7 0 ) : Depositional and dispersal dynamics of inter-
tidal sand bars . - J. sed.
Petr. ,
�Q: 1095-1 1 2 7 .
KUMAR , N . & SANDERS , J . E . ( 1 9 7 6 ) : Modern and ancient storm deposits . J . sed. Petr . , iJi : 1 4 5- 1 6 3 . LOREAU , J . -P . , ( 1 9 7 3 ) : Nouve lles observations sur la genese et l a signification des oolithe s . - Sciences Terre, ��: 2 1 3 - 2 4 4 . MIDDLETON , G . V . ( 1 9 67 ) : The orientation of concave-convex particle� deposited from experimental turbidity currents . - J . sed . Petr . ,
;a� : 2 2 9 - 2 3 2 . &
PERKINS, R .. D .
ENOS , P . ( 1 9 6 8)' : Hurricane Betsy in the Florida
Bahama area - geologic effects and comparison with Hurricane Donna . - J . Geo l . , �§ : 7 10-7 1 7 .
RAMSBOTTOM , W . H . C . ( 1 9 7 3 ) : Transgressions and regressions in the Dinantian: a new synthesis of British Dinantian stratigraphy . Proc . Yorks . geol . Soc . , 42: 567-607 . REINECK, H . -E . , GUTMANN , W . F .
&
HERTWECK , G .
( 1 9 6 7 ) : Das Schlick
gebiet sUdlich Helgoland als Beispiel rezenter Schelfablagerungen . Senckenbergiana , ��: 2 1 9 - 2 7 5 •
.:... - . - - , D5RJES ,
J . , GADOW, S .
&
HERTwE C K , G . ( 1 9 6 8 ) : Sedimentologie,
Faunenzonierung und Faziesabfolge vor der OstkUste der inneren Deutschen Bucht. - Senckenbergiana , �2: 2 6 1 - 3 0 3 RICHTER, R . ( 1 9 4 2 ) : Die Einkippungsregel . - Senckenbergiana, �� : 1 81-20 6 .
RHOADS , D . C . J . sed·
&
STANLEY , D . J . ( 1 9 6 5 ) : Biogenic graded bedding . Petr . , 3 5 : 9 56 - 9 6 3 . ==
SANDERSON , D . J . & DONOVAN , R . N . ( 1 97 4 ) : Packing of shells and stones on some recent beache s . - J. sed. Petr . , �� : 680-6 8 8 . SCHKFER, w . ( 1 9 5 6 ) : Gesteinsbi ldung im Flachenseebecken, am Beispiel der Jade . - Geol. Rdsch . , i� : 71-8 3 . SELLEY, R . C . ( 1 9 68 ) : A classification of paleocurrent mode l s . J . Geol . , 7 6 : 9 9- 1 1 0 . ==
Event-Stratification in Nummulite Accumulations Shell Beds from the Eocene ofEgypt T.AIGNER
Abstract : The concept o f event-strati fication may successfully be applied to two type.s of bioclastic deposits in the Eocen·� of Egypt : 1 . Middle Eocene Gizehensis-Bed: physical processes ( 11winnowing events " ) are significantly involved in structuring and in-situ accumulating nummulitic sediment bodies . 2 . Upper Eocene shell beds : reveal a complex series of erosion and colonisation events over a certain time interva l , each 11physical process '' being followed by a "biological response" . 1 . Introduction The Eocene of Egypt comprises shallow-marine and marginal sediments that were deposited in narrow and elongated tectonic basins forming embayments of the Tethys . Ear-ly and Middle Eocene were times of car bonate sedimentation , but due to regression, sediments became essen tially terrigenous from the Late Eocene onwards . Compilations on stratigraphy and sedimentation patterns are provided by SAID ( 1 9 6 2 ) and SALEM ( 1 9 7 6 ) . The object of this paper is to apply the concept of " event-stra tification'' to two rather different kinds of shell deposits :
1 . Accumulations of nummulites as a very special case of hie clastics. 2 . Several prominent and laterally persistent shell beds , that are being used for refined stratigraphic correlation (STROUGO 1 9 7 7 ) . 2 . Nummulite Accumulations Structure'd by Physical Events 2 . 1 . Previous Work The sedimentological aspects of such bioclastic deposits that con tain nummulites in rock-forming quantities have so fa r received little attention . Generally, nummulites are believed to have formed autochthonous "banks11 or 11bioherms11 or they are even considered as reef-builders (ARNI 1 9 6 5 , ARNI LANTERNO 1 9 7 9 ) .
&
LANTERNO 1 9 7 2 , 1 9 7 6 1 DECROUEZ
&
Cyclic and Event Stratification (ed. by Einsele/Seilacher) © Springer 1982
MOKA T TA M
249
sands congtomer<>les
E
Ain
Musa
sands shell
{dolomite)
STRANOLINE ?
wilh beds
L I T T O R AL
z
D
.,Taf!a .. (gypsilerous)
RESTRICTED LAGOON
0
C
Bryozoa - l imestone
PROT E C T E D PLATFORM
Ul
UJ
8
Upper Building
B A C K - BANK Stone
OPEN
SHELF
a:
UJ
A
G i z e h e n s i s - Bed
NUMMULITE - BANK
Lower Building
FOREBANK
Slone
a:
?
Fig. 1 . Stratigraphic summary log of Middle and Upper Eocene in the Mokattam-Hills ( E of Cairo) and tentative environmental interpretation . The Gizehensis-Bed (Mokattarn-Formation) and several shell beds (Maadi-Formation) are discussed here
However , nummulites have also been recognised ·to occur wave-affected (ARNI & LANTERNO 1 9 7 2 ) , reworked in shoals and fore-reef channels (SANTISTEBAN & TABERNER 1 9 8 0 ) , in cross-bedded littoral series
250
( RONIEWICZ 1 9 6 9 ) and in turbidites ( ENGEL 1970 ) . FOURNIE ( 1 97 5 ) compared certain nummulite accumulations with ooids lated them to an analogous hydrodynamic regime . 2 . 2 . Strati fication Types in the Gizehensis-Bed The Middle Eocene Nummu lites gizehensis- Bed was studied in classi cal exposures at the Giza Pyramids Plateau (W of Cairo) Mokattarn Hills (E of Cai.r o, Fig . 1) , where limestones appear ge nerally massive· or relatively thick-bedded ( 0 . 5 - L S m) with 1 50 em marly intercalations . They consist largely o f nummuli•e,-r>> C> micrites showing wackestone/packstone depositional textures but grainstones may also occur. The following features bear on origin (Fig. 2 ) : ( l' ) Firmground Horizons .
The bases of many nummulite beds are sharp and erosional , showing large-scale scours and small-sCale pockets (pot holes ) , as well as burrows , both typical for firm� ground conditions (GZossifungi·teB .-facies , SEILACHER 1 9 67 ) . Maj·�·r: hydrodynamic events ( hurricanes? ) eroding the seafloor down td already compacted levels are inferred to have exposed these laterally fairly persistant firrnground surfaces.
( 2 ) Erosive Pockets and Pot Holes . Pot holes and pockets occur in association with firmgrounds as well as within nummulite bed S (Fig. 3 ) . They commonly show a densely packed grainstone fill edgewise imbrication and fan position of nummulites and are most common in the shallowest parts towards the top of the Gizehensis-7· · · Bed ( probably a "Nurrunulite Bank" ) , where they indicate the com posite and amalgamated nature of the beds (Fig . 3 ) . These pockets resemble the pot holes described by DORR & KAUFFMAN ( 1 963} and AIGNER & FUTTERER ( 1 9 7 8 } , which are referred to strong vor tex currents during storm events . ( 3 ) Scour-and- Fill Structures (Fig. 2 ) . Within massive, apparently unstratified bed s , scour-and-fill structures occur in various orders of magnitude. They are much broader (up to several dm) and filled with nurnmulite-packstones . Their lower boundary is commonly shallow channel-like or sinusoidal but normally non erosive. ( 4 ) Planar Lags . {Fig. 2 ) . Thin planar layers of nummulite packsto ne are also common and may a lmost exclusively consist of the larger B-forms of .Nummu l'ites gizehensis, which are often imbri-
251
Events in Nummulite-Banks
;::r:...��,, -
�tJfitr
�
Fig . 2 . Biofabrics in Nummulite-accumulations ( 11Gizehensis-Bed n ) reflect physical processes as structuring and stratifying agents . Note: - extensively bUrrowed firmgrounds with erosive relief , erosive pockets filled with biosparite, imbrication , - stratification reminiscent of cross-bedding, - Nummulite-concentrations on planar scours , - small-scale scour and fill structures
252
Composite Bed em
wackestone
Fig . 3 . Levels with erosive grainstone-filled pockets within a massive Nummulite-wackestone bed indicate at least three discrete episodes of penecontemporaneous erosion and a com posite, amalgamated nature of the whole bed ( about 20 me tres lateral outcrop are projected into this diagram) . This is a bed from the top of a "nummulite bank11 • (Gizehensis-Bed, quarry along road to Gebel Mokattam)
cated . These laterally persistent packstone- " sheets" seem to
have formed by winnowing of wackest?nes ( c f . SPECHT & BRENNER 1 9 7 9. ) and removal of smaller particles including the A-forms of the nummulites.
( 5 ) Erosive Ripples . . Bedding planes may show more or less cal undulations reminiscent of large-scale ripples (wave length 60-100 em , height 10-25 ern) , but because of the absence of inter nal structures typical for ripples and due to the highly variable . shape , these undulations more likely represent "erosive ripples" ( c f . REINECK & SINGH 1 97 5 , Fig. 8 ; GOLDRING 1 97 1 , Fig . 1 5 ) or 11scour ripples" ( BAILEY 1 9 6 6 , Fig . 1 ) rather than normal ripples . ( 6 ) Imbrication . Both 11 contact" and " i solate11 imbricatiOn ( LAMING 1 9 6 6 ) is common , especially in local concentrations of the larger and f latter B-forms of N. gizehenai.s (Fig : 2 ) , while edge wise imbrication was only occasionally observed . Current action thus seems to have been more important than pure wave action. ( 7 ) Sorting and Fragmentation . Sorting i s normally poor , expressed by the association of small A-forms and larger B-forms of N ·
253
gizehensis . Thin-sections reveal the worn and fragmented appea rance of many nummulite tests as well as the abundance o f "nummu litic hash " . Abraded edges of tests are most conspicuous , while abrasion on equatorial surfaces seem to be less common. 2 . 4 . Nummulites as Sedimentary Particles considering that nummulite tests were extremely porou s , they should
be very susceptible to reworking , so that their se9imentological Pe J;laviour may have resembled that of crinoid .remains originally (SEILACHER 1 9 73 ) , but may have been drasticall y altered by prefossi lizcltion.
In order to get an idea about the sedimentological behaviour of nummulites, a few simple exper�ments and calculations have been carried out:
( 1 } Porosity. SEM·�studies , experimental and mathematical determina tions show that a considerable poros ity is still preserved in many nummulite tests . Depending o n the surrounding rock type
and on the s i ze o f the nummulite , porosity was found to range between 1 up to 5 4 % . In recent Amp hisorus tests from the Phi r
lippines ( specimens sampled by A . �,:�i:ILACHER) , porosity reached
up to 7 2 % . A similarly high original porosity can be inferred
for nummulites ; the impli.cations for their potential as hydro carbon reservoirs are evident.
( 2 ) Bulk Density. Like the porosity , the bulk density values are highly variable and may be as low as 1 . 2 8 g cm- 3 . Due to inter nal cementation within the nummulite tes t , however , bulk density
may , in some case s , approach values typical for pure calcite . ( 2 . 7 1 g em 3 ) . For recent Amph�so�us , only 0 . 3 05 g em- 3 has been found .
Settling Velocit�.
Determination of the settling velocity allows
to deduce the diameter o f hydraulically equivalent quartz grains ( e . g . FUTTERER 1 9 7 7 , Fig . 1 0) . In this way, nummulites with a dia meter of 7 - 24 mm were found to be equivalent to 1 . 0 - 1 . 8 5 mm sand grains ( ''very coarse sand " ) .
Critical Transp.o rt Velocity.
Firstly, hydraulic equivalents de
rived from settling experiments were used in the HJULSTROM-dia
gram: here , values for the critical transport velocity are most -1 ly around 30 - 40 em s . Secondly , the critical transport ve-
254
GIZEHENSIS - B ED AlB - R AT I O
L I THOLOGY
IMBRICATION
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.
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20
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~ 100
A
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255
locity was directly determined by flume experiments carried out
by nr . E. FUTTERER (Kiel ) . Here, velocities range between 1 8 -1 7 7 em s , which is in the same order of magnitude as the values
derived from settling experiments . Dr . FUTTERER also observed , that nummulites are transported b y sliding, saltation and rolling . 2 . 5 . Conclusions ( 1 ) Stratification types and biofabrics in nummulite accumulation
show that physical sedimentary events were responsible for their structure . The , smaller A-forms of N.Jri.zehensis largely dominate
' the assemblages (Fig . 4 ) , as BLONDEAU ( 1 9 7 2 ) has postulated for their original communities . Therefore nummulites seem to have
mainly accumulated in situ through winnowing events and thus have formed extensive sediment bodies (Fig . 5 } .
There is no evidence for reef-like buildup in the "Nummulite Banks" of the " Gizehen.si8-Bed " . Due to the mechanisms involved , this type of buildup may be called "nummulite tells" .
( 2 } Simple experiments indicate that nummulites must have been very light and susceptible to reworking at current velocities as low as 20 -1 - 8 0 em s . According to LOGAN et al . ( 1 96 9 , Fig. 3 } , storminduced wave-current velocities of · -this order of magnitude occur in waters as much as 1 00 m deep.
( 3 } Physical processes involved in the formation of "Nummulite Banks" may be comparable to the role of physical s torm-related sedimen tation in mOlding the character of present-day carbonate mud banks around Florida (WANLESS 1 9 79 } .
� Fig. 4 . Detailed log showing variations of microfacies , depositional 'J fabrics , grain s i ze and nummulite A/B-ratio within the "Nummulites gizehensis-Bed" ( northern escarpment of Giza Pyramids Plateau, along Fayum road ) . A-forms dominate throughout most of the sequence , but layers with relative enrichment of B-forms often correspond to pack stone textures , indicating winnowing and removal of the fine parti cles including the A-forms . Note also abundance of imbricated fa brics (each measurements based on about 1 00 nummulites)
256 BUILD UP
N U M M ULITE ACCUMULATIONS : Result of in-situ winnowing events
nummulitic sediment body
nummulite community
TIME
Fig. 5 . The time-buildup diagram suggests that episodic winnowing ( probably storm-generated) is responsible for structure , depositional fabrics and growth of nummulite accumulations . Repeated winnowing events o f varying intensity have washed away Substantial amounts of lime-mud that probably represen ted the original substrate of nummulites (according to BLONDEAU 1 9 7 2 ) . Thus , the resulting bioclastic accumulation is s lowly growing upwards like an archaeological "tell struCture11 ( "nummulite tell ��") and consists to a large extent of event-generated in-situ lag deposits , but does not repre.:.· sent the actual habitat conditions o f the nummulites . Due to gradual shallowing, winnowed fabrics become more abundant towards the top of nummlllitic sediment bodies
3 . Shell Beds : Interplay Between Physical Processes and Biological Responses
3 . 1 . General Problems Shell beds have been widely used for community reconstructions ( e . g . McKERROW 1 9 7 8 ) . I t has also been debated whether detailed and even quantitative reconstructions concerning mortality rates , diversity , trophic structure etc . can be derived from such accumu � ations of biologic hard particles . After all , they may have suffered consi derable taphonomic distortion. Apart from postmortal transport , fau nal changes due to minor environmental fluctuations may have obli terated the original faunal spectrum of a " community" ( cf . WILSON 1 96 7 , FURSICH 1 9 7 8 , BOUCOT 1 9 7� .
257
In general , shell beds should not be viewed indiscriminately as paleoecological portraits of paleocommunities , but should rather be analysed in their taphonomic details in order to appreciate their full sedimentary history. 3 . 2 . Taphonomy and Stratification in Upper Eocene Shell Beds Upper Eocene shell beds , usually 0 . 5 tp 1 . 5 m thick , have been s tu died in the Mokattam Hills ( c f . Fig . 1 ) , the Giza Pyramids area , and in , Quasr.-el-Sagha (Fayu m Oasis } . Generalised they show the following attributes (Fig. 6 ) :
( 1 ) Shell beds develop on scoured surfaces which are commonly burrowed by SpongeLiomorpha , indicating firm substrate condi tions ( 11 firmgrounds " ) . ( 2 ) In most cases , these firmground horizons were colonised by Caro lia as a pioneer. Similar pioneer colonisations by this epibyssate bivalve usually take ' place repeatedly on internal erosion surfaces within shell beds . ( 3 ) Shell bed development is characterised by (often repeated) changes in faunal compos·i tion and sediment . fabrics . Thus , Caro lia colonies become frequently replaced by ostreids or by Pli c dt u la , the former being in many cases ·c·emented onto Carolia shells . The oysters in turn may later be encrusted by corals , eventually leading to small-scale coral banks . ( 4 ) Subsequent erosion may have led to partial or complete reworking
of original epibenthic assemblages and to the production of shell debris : This ·she'lly subs-trate in turn became inhabited by specific organisms ( e . g . Op hiomorpha with pellet-ligned walls for stabilisation ) . ( 5 ) Intercalations o f endobenthic organisms (Spatangids , Turrite l la )
indicate phases of intermittent mud-sedimentation and softground conditions . Since the fine sediment itself has later became re worked and winnowed away , these shell lags represerit the only "memoryn ( SEILACHER , this volume) of softground intervals within mostly epifaunal shell beds . 3 . 3 . Conclusions ( 1 ) Shell bed ±nitiation in the Upper Eocene requires strong erosion (major storm event?) to create a firmground substrate, which could be used by Caro lia as a pos t-event pioneer colonizer .
258
SHEL L BED S : a
events , . success1ons, pseudo - successions
b
EVENT RESPONSE
K
shell
EVENT RESPONSE
hash
Turritella
corals, algae shell hash
scours Spatangoids scours
shell
hash
I �
oysters
Carolia erosion she ) l
hash
with
Ophiomorpha
'P
I I I
'P
�
reworking
reworking
Plicatula corals
Carolia pebbles Carolia firmg·round. pot casts
Spongeliomorpha
I
'P
I 0
oysters Carolia oysters Carolia lirmground Sponge!iomorpha
Fig. 6 . Shell beds revealing a complex series of erosional/depositio nal events , followed by specific biological responses . Some appear to be true ecological successions , other are merely n pseudo-succes- . sions " . (Black bar = epifaunal response , clear bar = endofaunal re spons e . a = "Plicatula-Bed " , b = 110strea-Bed " , Mokattam-Casino section, Maadi-Formation)
259
( 2 } Shell bed development: shell beds accumulated on such firmgrounds are essentially autochthonous , but they record a series o f � dimentary events (erosion/deposition ) , each of which may · be follo wed by a specific biological response (epi/endobenthic coloni sation of soft , shelly .or firm substrate, see Fig . 7 ) . Although some of the resulting sequences resemble " community successions 11 , faunal changes within shell beds seem to be primarily controlled by environmental fluctuations ( 11Communi ty replacement " , BOUCOT 1 9 7 5 , HOFFMAN & NARKIEWICZ 1 9 7 7 ) , rather than by biologically
i nduced changes . Thus they represent only "pseudo-successions " .
( 3 ) Bfostratinomy : The mechanism responsible .for shell bed develop ment can be described as a quantum-like accumulation of a larger number of 11process-response couplets 11 (physical event biqlogical response ) over a certain time interval . Fiimground/ shell bed complez:es have acted as "·r·e ference horizons " , recor ding the interplay between ecological and sedimentary events during longer time periods . Due to their large geographic distribution and their isochronous character major shell beds are useful for
( 4 ) Stratigraphy :
high-resolution stratigraphy on a regional scale .
SEDIMENTATION
ll!l!lll
SOFTGROUND
REWORKING
PHYSICAL PROCESS
-
SUBSTRATE-CHANGE
-
BIOLOGICAL RESPONSE
Fig . 7 . Different sedimentary events or "physical processes " ( sedi mentation/reworking/eros ion) alter substrate conditions on the sea floor and are followed by specific "biological re sponses " on the newly created soft-, shell- or firmground s . Repetition and accumulation o f such "process-response cou plets'' represent the basic mechanism responsible for the development of the complex shell beds described here
260
( 5 ) Evolution : Since many similar shell beds with analogous but morphologically d ifferent faunas occur at different levels , they might provide the opportunity to s tudy the tempo and m ode of evolutionary changes in the 11pos-t-event fauna" as compar ed to the 11background fauna" (SEILACHER 1 9 8 1 ) . Acknowledgements This paper represents prel-iminary results of work towards my Diplorn-Thesis , supervised by Prof . Dr . A . SEILACHER. I thank him for discussion and reviewing the manuscript. Earlier drafts have also been critisized by Prof . Dr . G. EINSELE and by Dr . A . HOFFMAN· . During my stay in Egypt I benefited in various ways from support by Prof . Dr . H . ABBAS S , Drs . EISSA, I SSAWI , HAMZA, STROUGO and A . M. ZIKO. Furthermore, thanks are due to Dr . E . FUTTERER ( Kiel} for carrying out the flume experiments , and to Dr . A . WETZEL for sugge stions concerning porosity and bulk density determination . Technical assistance was provided by W. WETZEL { fotos ) , W . RIES { thin sections) and J. GHIOLD { SEM ) Field work was supported by the SFB 53 , which is gratefully acknowledged . •
References AIGNER, T .
&
FUTTERER , E . ( 1 9 7 8 ) : Kolk-TOpfe und -Rinnen {pot and
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261
DECROUE Z , D . & LANTERNOv E . ( 1 9 79 } : Les "Banks A Nummulite s " de l ' Eoc6ne mesog9en et leurs implications . - Arch. Sc . Geneve
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&
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&
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SEILACHER , A . ( 1 9 7 3 ) : Biostratinomy : The Sedimentology of Biologi cally Standarized Particles . - I n : R . N . GINSBURG ( Ed . ) : Evolving Concepts in Sedimentology . Johns Hopkins Univ. Press . ·sEILACHER, A . ( 1 9 8 1 ) : Towards an evolu.tionary stratigraphy . - I n : Concept and Method i n Paleontology ; Acta geol . Hispanica, Jg: 3 9 -4 4 . SPECHT , R . W .
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( 1 9 6 7 ) : Palaeoecological studies on shell-beds and
associated sediments in the Solway Firth. - Scott. J . Geol . J : 3 2 9 - 37 1 . [:,
The "Bank der kleinen Terebrateln" (Upper Muschelkalk, Triassic) Near Schwiibisch Hall (SW-Germany) a Tempestite Condensation Horizon H. HAGDORN Abstract: A facies model is proposed for the slope o f a submarine swell in the shallow marine epicontinental Muschelkalk sea . A belt of hardgrounds encrusted by PZaaunopsis is recognized between the center of the swell (oolithic " Kornstein "-facies , above normal wave base) and the "Tonplatte n " -facies in the basin center . These hard groun�s acted as reference horizons , on which the background fauna became mixed and condense d , together with the post-event fauna , by storm events . 1 . Introduction High-energy events are most Clea.rly recognizeable in tempestites within cephalopod-bearing, thin-bedded marl/limestone alternations ( "Tonplatten11 ) , a uniform basinal facies type throughout the Upper Muschelkalk (aompre s s u s - to nodo s u s - zone) of SW-Germany (AIGNER, 1 97 7 , 1 9 79 , and this volume ) . Due to regress ion , howeve_r , NE-Wtirttemberg became part of the mar ginal realm from the upper nodosus -zone onwards , and typical tempe stites were no longer forming . At the ·same time , vertical and la teral facies patterns are more differentiated on a local scale and form a complex facies mosai c . Paleogeographic structures ( 11SUd deutsche Haupts chwelle " , comp . VOLLRATH , 1 95 5 ) can be traced by massive, cross-bedded· oolithic biosparrudites ( "Kornsteine " ) . At times when these ,swells were not active, the muddy " Tonplatten fa cies 11 extended much further to the East and towards the coastline . According to its marginal and shal.low-water setting, this area should be expected to show significant traces of storm-induced high energy events . Sedimentologically, these events are less con spicuous because of lacking shelly tempestite s . This is due to cannibalistic condensation ( SEILACHER, this volume) of many such events , in addition to increased winnowing and lateral transport of the fines into deeper and more quiet parts of the bas in . The purpose of this paper is to illustrate the ecological and tapho nomic consequences of high-energy events by means of analyzing various fossil assemblages in the transition between the nodo s u s Cyclic and Event Stratification (ed. by Einsele/Seilacher) © Springer 1982
264
and discoceratite-zone of the Kocher valley near Schwi9.bisch Hall ( Tonhorizont
�
-
Bank der kleinen Terebrateln)
•
2 . Stratigraphic Frame ( Fi g . 1 ) 2 . 1 . Lithostratigraphy WAGNER ( 1 9 1 3 ) defined the 11Bank der kleinen Terebrateln 11 as a fossi · l iferous , marly nodular limestone ( "Brockelfels 11 ) , below a thin horizon of dolomitic marls ( "Dolomi t.ische Mergel 0( M I I I of WAGNER ; see lithostratigraphic nomenclature of GWINNER , 1 9 6 8 ) . =
The 11Bank der kleinen Terebrateln" rests on an up to 3 m thick ooli thic shelly limestone , which WAGNER called 11Kornstein I 11 or 11Mu schelquader I 11 and which was later termed 110bere Schalentrtirrrrner bank" by VOLLRATH ( 1 9 5 5 } . This unit is underlain by the " Tonhori zont � 11 , which is developed in Tonplattenfacies near Schwabisch
Hal l , and the nuntere Schalentrtirruuerbank " , which shows ''Kornstein11facies farther towards the East. 2 . 2 . Biostrat'igraphy Biostratigraphically , "Tonhorizont � " and "Obere Schalentrlimmer bank" belong to the nodo s u s -zone (Ladinian, Langobard) . Many spectacular specimens of Ceva t i t e s nodosus have been found in the 11Tonhorizont
� 11 near Schwabisch Hal l , but only few cerati tes
( still belonging to C. nodosus ) , strongly sculptured with extreme ly broad ventral side , are recorded from the "Obere Schalentrtirruner bank 11 . · Ceratites become more frequent in marly parts of the "Bank der kleinen Terebrateln11 , but they are mostly small binodose juve niles , which are difficult to identify . . Although C. nodosus still occ.urs , the discoceratite-zone with Disao oera t i t e s intermedius and D . Z e v a l 'l o i s i starts with the "Bank der kleinen Terebrateln 11 • 3 . 110bere Schalentrlimmerbank
u
3 . 1 . Description 3 . 1 . 1 . Facies Near Gottwollshausen, the most important and beststudied locality, the "Obere Schalentrtimmerbank" consists of 80 ern o f low-angle cross-
265
Tonhori zont t:;
Fig. 1 . Stratigraphy developed in ( a ) displays arenites and teln 11 (b) · as
(Gottwollshausen quarry ) . · "Tonhorizont "7 · " " Tonplattenfacies " . "Obere Schalentrtimmerbank" parallel to low-angle cross-laminated fine oolithic rudites . "Bank der kl'i= inen Terebra clay-rich nodular l imestone
stratified , finely areniti c , often graded, biopelsparites ( Fig . 4 , unit a ) , which laterally interfinger with ruditic bio-oosparites (unit b ) . Towards the E and NE , the 110bere Scha:lentrlimmerbank" gra des into mature , cross-bedded bio-oosparites which are dominated by rapidly burrowing , deposit feeding bivalves (aragonite-shelled Myophoria�
Tr1:gonodu s ) , byssate bakevelliids and pectinids
(Ento
Z iu m ) . Towards the w ( i . e . towards the center of the basin ) 1 the
" Upper Schalentrtimmerbank" rapidly decreases in thickness and gets replaced by '"l'onplatte n " -facies . The isopachs of the oolithic fa cies are shown in Fig. 2 . In the Schwabisch Hall area1 unit b i s followed by up to 30 em of laminated 1 unfossili ferous mudstones (unit c) in irregular, patchy di stribution. Within no more than 2 5 m distance 1 unit c wedges out
266
o Unt.O
Bank der kleinen Terebrat eln PALAEOGEOGRAPHY & FACIES -100'- lsopachs of Obere Schalen trDmmerbank {oolithic) in em
Ob.Schatentrb. oolithic
�
Placunopsis-Biostromes
Bank derk/einen Terebrateln wedged out
Fig. 2 . Paleogeography and facies . Overview map shows the distribu tion of ooli thic facies wi.thin the "Obere Schalentriimmer bank " , contouring a submarine swell in front of the 11Vinde lizisches Land" ( after VOLLRATH 1 9 5 5 ) . According to the de · t·ailed isopach map, PZaaunopsis biostromes mainly occur in a broad belt along the 1 00 m - isopach line . Towards the E , the 11 Bank der kleinen Terebrateln" gets replaced by the 110bere Schalentrtimmerbank ( "Kornstein 11 -facies) . (Thickness values of Obere Schalentrlinunerbank 11 after WAGNER 1 9 1 3 , VOLLRATH 1 9 5 5 and . personal observations i list o f - localities : IGPT) u
and gets replaced by unit b i but it may re-appear after several 1 0 ' s of meters . Occasionally , larger resediment-blocks and intra clasts ( 0 . 1 - 1 0 em) of unit c are · found within unit b ( Fi g . 3 ) .
267
Fig. 3 . Erosive remnant of unit c with debris talus
3 . 1 . 2 . Plaounops1:s-Bios tromes In Freas where unit c is eroded and unit b was exposed on the sea floor, its surface became partly encrusted by the Terquemiid pelecy pod Plaaunopsis os tracina . Both bioherffial and biostromal aggregates of Placunopsis have been described by WAGNER ( 1 9 1 3 , 1 9 3 6 ) , HULDER ( 1 9 6 1 ) , KRUMBEIN . ( 1 9 6 3 ) , and have more recently been 'Studied by BACHMANN { 1 9 7 9 )
{ compare also HAGDORN % MUNDLOS , in this volume ) .
Biostromes (unit d 1 ) consist of single , densely arranged pillars ( 1 0-20 em i n diameter) 1 which grew steeply up from the substrate .
In the section, individual biostromes can be traced 2 -5 m laterallY ;
they thus would cover about 3- 1 5 m2 ( i n case o f 2:: circular distri bution} . In many outcrops around SchwabisCh Hal l , biostromes may be found every 30-50 m; hence a significant proportion of the sea floor was covered with Plaounopsis -biostromes . For their regional distribution see Fig. 2 .
As BACHMANN has already pointed out, these biostromes superficially resemble algal stromatolite s , although no algae have contributed to their formation. Plaeunopsis b iostromes may reach 4 0 em in high, but at no time did they rise more than 2-3 em above the surface of the contemporary shell bed surface (unit d2 ) .
268 2
m
Obere SchalentrUmmerbank Gottwollshausen
Fig.- 4 . Facies mosaic of " Obere Schalentrlirnmerbank 11 in Gottwolls hausen quarry . Presumably , unit b cuts in a channel-like fashion through unit a down to 11Tonhorizont" ( intraclasts ) . Only where unit c was removed , Ptaaunopsi's biostromes were able· to develop on a hardground-crust of unit b Many Plaaunopsis colonies do not develop directly on the surface · of unit b, but on top of 3-40 em thick slab-like intraclasts (Fig. 5 , 6 , 7 ; compare also BACHMANN 1 97 9 , Fig . 9 b ) . These clasts are not bored , but may shov; thin l irnoni tic linings i th�y are mostly found imbricated or oriented parallel to bedding, resting directly on unit b . Three main types of encrustation may be distinguished ( Fig . 6 ) : · 1 . mm-thin , discontinuous crusts , mostly on. smaller clasts ; 2 . Up to 5 ern thick , forming a crescent-shaped cape on upper the surface, but only a thin crust on the lower surface of the cla.s t . Encrustation on upper surface commonly cuts discordant ly across encrustation of lower surface;
3 . On larger plate-like clasts , crusts of type 2 , may in course of their further growth get se�arated into several pillars . Secondary re-joining of pillars has not been observed, but all of them are oriented upwards . Discordant and two-phase overgrowths are also common { Fig . 7 , 8 ) . 3 . 1 . 3 . Bioerosion on PZaounopsis Bioherms Borings in the PZaounopsis bioherms described here show an uneven distribution , in contrast to those from other stratigraphic levels. BACHMANN ( 1 9 7 9 ) found 3 types of borings , two of which also occur in our cas e :
269
:' i
!I
":l
Fig. 5 . Oolithic · clasts encrusted by PZaaunopsi s . a ) Type 1 , only weakly encrusted. Type 2 forms thicker pillows on pebble . Type 3 develops individual pillars . Abundant borings of right pillar is caused by Catoiroda . Schwabisch Hall, quarry at Heimbacher Steige ( IGPT ) . b ) Two imbricated oolithic pebbles overgrown by PZaounops i s . Reworking i s indicated by re-adjustment Of growth direction in the right clas t . Gottwollshausen
1 . tunnels of Talpina gruberi ( circular cros s-section, 1 50-200 u wide ) ; according to VOIGT ( 1 9 7 5 ) produced by a phoronoid . 2 . 1 000 u wide tunnels of Ca lcir>oda kraiohgov1: a e , whose taxono mic position ( Phoronoidea? ) is not certain (VOIGT, 1 9 75 ) . In contrast to Talpina> Calairoda penetrates several Plaaunopsis generations up to 1 em deep.
270
Fig . 6 . Two-phase encrustation of oolithic clast as indicated by discordant contact between first and second phase of encrustation and the pebble itself . Close-up from Fig . 5a (peel)
Fig . 7 . PZacunopsis -biostromes a ) Although PLaeunopsis -overgrowths are attached to isolated pebbles , the bios.trome appears to form a rigid frame work , similar tO LLI-I-type and SS-type s tromatolites . Note vertebra of Nothosaurus in - bonebed veneer ( arrow) . a ' ) cross-section. Schw&bisch Hall, quarry at Heimbacher Steige ( I GPT ) . b ) PZaaunopsis-pillars developing on low-angle cross-lami nated hardground (unit b ) . Growth forms are probably con trolled by constant current direction . (Go�twollshausen SMNS 2 6 2 7 9 ) . c ) vertically embedded GermanonautiZus , heavily encrusted by PZaounopsis on inside and outside surface s . Gottwolls hausen ( SMNS 2 6 2 7 8 )
271
272
F ig . 8 . Top surface o f reierence horizon (unit d ) . Left hand side iritensively bored top of PZaaunop s i s -pillar , right hand side shell pavement . Detailed view: Calairoda- ( c ) and TaZpina-borings ( t ) on Placunops i e -pillar. ( CHK 1 0 5 3/ 1 )
Talpina i s present i n the entire biostrome , but em-thick levels may be more extensively bored . Ca leiroda� on the other hand, is striated to the top o f the biostromes. These surfaces are heavily affected by the bioerosion so that, the originally convex contours become flattened ; on such surfaces , individual valves can no more be distinguished and the whole surface shows a crater-like · relief
( Fi g . 9 ) . In spite of. the intensive bioerosion, the original biostromal fabr�c was preserved . Borers mainly affected the aragonitic Plaounopsis hypostracum, while th� outer, calcitic shell layers were preserved, their margins sticking out as minute ridges on the fla�ks of the pillars (Fig. 9 ) . The crevices between individual Plaounops i s p illars are filled with poorly sorted and bioturbated biomicrosparrudites (wackestones ) with local concentrations of silt and fine sand , in addition to abundant
273
Borings and the i r relationship to sediment leve l . Abundant Calciroda-borings (thicker- tunnels ) on top of pillar , formed above sediment leve l . Short-term discordant encrustation of pillar flank by PZaaunopsis and spirorbis during event-gene rated lower sediment l.e ve l . Renewed ·sedimentation prevented intensive boring. Increasing abundance of Talpina (thinner tunnel s) marks stil lstand phase during growth
small vertebrate remains . From Gottwollshausen towards the S and SE , unit d decreases in thickne s s , and within biostromes the pro portion · of overgrown pebbles increases . Nevertheles s 1 biostromes appear to be attached to the Surface of unit b (Fig. 7 a , a ' , i ; BACHMANN , 1 97 9 : F i g . 9 a , b ) . PZQ:eunoPsis-biostromes are lacking in areas in which the "Obere Schalent.rlirnmerbank 11 is entirely ooli thi c . Lat·erally, the biostromes pass into bioturbated biomicrosparrudites (packstones) containing well-preserved and poorly . sorted shells and a shell-paved top ( Fig . 8 ) . Microsparitic , slightly limonite-impregnated intraclasts may be bored by Trypani te s weisei� and the whole surface of unit d is covered by a rom-thin silty bonebed (with well preserved vertebrate remains ) , which also fills interstices between the biostrome s . As far as identification from polished slabs is possibl e , small gastropods and sediment-feeding bivalves (Nuculidae , Myophoridae) dominate the fauna . Isolated valves of epibenthic suspension feeders (Plaaunopsis� PZ.euron e a t i t e s � P.Z.agiostoma) are also very abundant. Convex-up Pleurone a ti tes -valves are commonly encrusted by em-thick Placunopsis -colonies (Fig. 1 1 a , b ) . Cephalopod tests are heavily encrusteq from all sides , in the body chamber even on the inner sur faces (Fig. S di PZ.aaunopsis "rolling-re·e f " , H5LDER, 1 9 6 2 ) . Towards the SE , Sphaeroaodium kokeni_, a Girvane l la- oncoid, becomes more abundant.
274
3 . 2 . Genetic Interpretation 3 . 2 . 1 . Facies Development . Units a and b are considered as facies of a paleogeographic swell in front of the 11Vindelizi,,,,,o,,,, }
Land11 (VOLLRATH , 1 '9 55 ) . ·Erosional features ( intraclasts ) and like replacement of unit a by unit b indicate an allochthonous ture o f the oolites , which may be derived from higher swell
The contours of this swell-structure are well illustrated by isopach pattern of the 110bere Schalentrtinunerbank " (Fig . 2 ) . Unit (mudstones) was extensive_ly deposited during a period of reduced water turbulence . Later on , it became eroded and was transported into deeper parts of the . basin; only a few lens shaped fragments remained in situ, but many of them disintegrated into smaller which may finally have become incorporated into graded tempestites The thus d i fferentiated facies pattern 6£ mudstone remnants and
.•
scoured ool. ithic su� faces has further developed in two different ways : 1 . into a biogenic hardground (on top of
.
P Z aaunopsis biostromes )
2 . into shellgrciund (in be.tween of biostrome s )
(Fig. 8 )
3 . 2 . 2 . Ecologic and Taphonomic Inferences from PZacunopsis BioStromes In spite o f the lack of borings and of mineralisation, the surface of unit b appears to have been cemente d , except if we assume that PZacunopsis was able to encrust even loose shell debris .
The intergranular voids o·f the coarse-grained sediment of unit b are likely to have been cemented while this unit was covered by unit c , analogous to Persian Gulf hardgrounds (SHINN , 1 9 6 9 ) . It was not possible for PZacunopsis to settle on the mudstone remnants of unit c. Within softground assembla·g es o f the basin interior, PZaaunops·i s does occur, but it is restricted to encrustations of larger hard particles which acted as "shell is lands 11 (LINCK , 1 9 5 6 i BACHMANN ,
1 9 7 9 ) . Only after extensive and deep erosion, which created hard
surfaces , the "opportunist" Plaaunopsis could suddenly become eco. logically extremely successful . A prerequisite for long-term colonisation of PZacunopsis is ommission; in every case , mud had to be removed and transported away towards the basin center. This interpretation is supported by the following two observations :
:·/
275
thin bonebed-veneers on top o f unit d . Quartz sand and phosphatic vertebrate remains , having a high preservation potential , are likely to become condensed through repeated reworking caused by storm-events, while aragonitic and Calcitic particles may be lost diagenetically ( SEILACHER , this volume) . Fine-grained material has been winnoWed away into the basin interior . In a way this rom-thin bonebed is genetically equivalent to the much more spectacular mo/ku-Grenzbonebeds ( RE I F, 1 9 6 8 and this volume ) . ? . Single PZ.aaunopsis shells were able to settle on surfac·e s that
experienced very · short-time exposures . Since these valves are ollly negligeably affected by boring s , they must soon have been re-buried . Zones with increased abundance of Calciroda borings thus mark penecontemporaneous sediment surface levels . Based on calculations of WAGNER ( 1 9 1 3 : 1 7 4 ; 1 9 3 6 ) , who attributed 4 years as average individual age of PZacunops is , and cOnsidering that as an average 20 generations are present in 1 em of biostrome (BACHMANN , 1 9 7 9 ) , about 3200 years are represented in 40 em of the Placunopsis -biostrome at Gottwollshausen. According to inferences made along s imil�r lines by , AIGNER (this . volume ) , several major storm events are likely to have occurred during a time interval of this order · of magnitude . A few such events are sufficient for re peated turn-over of encrusted clasts to allow allround overgrowth . From a certain critical pebble size onwards , hydrodynamic energy was not high enough to turn them over; from then on Placunopsis preferentially grew upwards until if became finally buried . As shown in Fig. Sb , individual colonies may have changed their growth direction in adjustment to reworking. The process of pebble- formation remains to be debated (Fig. 1 0) . Dis.co·rdances within Placunapsis -overgrowths may indicate, that in many cases only the cemented surface of unit · b has been encrusted (phase 1 ) . During major storms , these encrusted patches became ero ded and possibly turned over . This interpretation is more plausible if we assume that unit b was cemented only superficially . Once this crust got broken up during a storm, undercutting facilitated disintegration. The remaining hardground fragments could become further encrusted (phase 2 ) , thus showing discordant contacts to overgrowth-phase 1 .
276
A 2-PHASE OVERGROWTH
F ig. 1 0 . Two-phase overgroWth of oolithic pebbles . A) Phase I : Plaounopsis colonies start growth on cemented crust of unit b . B ) Due to storm erosion , crust o f unit b gets broken up into oolithic clasts , the Plaounops i s - colonies protec t;ing them from further disinteg,ration . C ) Reworked clasts are encr.usted in a second phase (Phci.:Se I I ) by PZaounops i s , showing discordant contacts to overgrowths of phase I This pro�ess seems to have tak�n place near Schwabisch Hall and Westhei�, where unit d is only 1 5-20 ern thiCk , and Plaounopsis pillarS are J.'"fl.ain)..y developed on pebbles {Fig. 7 } . Near Gottwollshau sen, pillars are firmly cemented onto unit b . Qther clasts do not appear to have been encrusted before hardground-erosion, and hardground fragments were. scattered on the seafloor
• .
The abundance of borings on top o� the Plaaunop s i s pillars further indicates , that biostromes stopped active growth prior to final bu-· rial . This may have happened during a previous phase of burial that caused the extinction of Placunops,i s . After re-exposure of the dead biostrome surface s , phoronoids occupied this ecological niche and prevented :new settlement of Flacunop s i s . Other epi faunal suspension� feeders later attached themselves to such biostrome surface s .
i
It is not clear, why · crin�ids with encrusting roots (Encrinus l i l ii . (Enan tio s t:t>.eon� Newaag i a ) are not pre
formi s ) and large Terquemiids
sent at. all in this st tuation , whil e they are main frame builders iri , genetically and paleoecologically comparable pelecypodjcrinoid-bio herms lower in the Upper Muschelkalk (HAGDORN , 1 9 7 8 ; HAGDORN & MUND LOS , this volume ) .
Shells of dead primary or secondary colonizers of biostromes accumu lated in areas between them and provided potential substrates for attachment of other epibionts.
277 Sphaerooodium does not necessarily indicate shallow water, but
rather quiet conditions combined wit� low sedimentation rates and low subsidence , which is in agreement·-- with our present picture (P ERYT 1 9 80 ) . 4 . "Bank der - kleinen Terebrateln
u
4 . 1 . Description 4 . 1 . 1 Facies . The 11Bank der kleinen Terebrate�n 1 following unit d , ' coffip rises 1 , 2 - 1 , 4 m of irregularily bedded 1 nodular microsparitic biorudites with mud- and silt-streaks . In the lowest - and uppermost •
n
part of the unit, mudstone lenses reach several em in thicknes s , . thus causing preferential weathering (recesses in outcrops ) . These parts are characterised by strongly bioturbated , poorly sorted wackest.ones , whereas the middle part of the unit is dominated by packstones which show several · leve ls with varying particle sizes and packing densities and are generally poor in mud .
ToWards the W and · Nw ( deeper parts _of the basin ) , the " Bank der kleinen Terebrateln" grades into "Tonplattenfacies 11 ; towards the E and NE it gets replaced by the "Obe're SchalentrUmmerbank11 (Fig. 2 ) . 4 . 1 . 2 . F&una. Framboidal pyri& ic moulds ·of minute gastropods and
Nuculids weather out in great abundance from clay-rich parts of the "Bank der kleinen Terebrateln " . Representatives of the infauna are dominating: deposit-feeders like Pa�aeonuauLa> PseudoaorhuLa> Myopho ria:, gastropods (A·a taeonina> 11Neritari a ", Loxonema ) . Larger suspen sion feeders are also very abundant , such as the endobyssate bivalve with shell torsion and a thick umbo used as an anchor
HoeVnesia
(McGHEE, 1 9 7 8 ) , as well as the deep-burrowing sinupalliate P�euro Myophoriids , P leuromyids and Hoernesia are occasionally pre served in life position, which fs documented by compactional stria tion. Suspension-fee � ing epibionts include PLeuro n e e t i t e s , P�agio
mya .
and Coenothyris . Many larger shells , as well as internal molds , are encrusted by PLa·c unopsis ,. including stoma> PLaeuno p s i s >
Spirorbis >
internal molds of the deepburrowing P t euromya . Only r�rely , intra
vital encrustation , or encrustation i_!l life .position occurs (Fig. 1 1 ) . Towards N and w, the fauna decreases in diversity . Epibenthos is restricted to isolated "is lands " provided by larger cephalopod shells . Towards the E (wedging-out of "Bank der kleinen Terebrateln " ) , a fauna typical for oolithic substrates (Ento�_L ium> Trigonodus, Myo phoriids) becomes increasingly dominant.
279
4 . 2 . Genetic Interpretation 4 . 2 . 1 . Mixed Fossil Assemblage . Facies and fauna of the "Bank der kleinen Terebrateln" · indicates deposition below wave b as e . However , this seems to be in contrast to the· abundant and diverse epifauna1 which is not restricted to " shell islands " . Abundance and excellent preservation o f double-valved pelecypods and brachiopods exclude allochthonous import from higher swell areas . Mixing of two different faunal assemblage s , however, may also be explained by condensation1 altho �gh mineralisation and subsolution as Well as mixing of index-fossils have not been recorded (WENDT , 1 9 70) . Internal molds of the bur�owing PZeuromya� that have been encrusted by Placunops i s , provide the key for this interpretation. 4 . 2 . 2 . Encrustation of P l e uromya as a Taphonomic Key. For encrusta tion, Pleuromya has to be exposed on the sediment-water interface. None of the about 300 specimens studied show preferential overgrowth at their rear ehd, as observed on recent Mya arenar i a , that was only partly eroded but remained in life . POSition (HERTWECK 1 9 79 , REINECK 1 9 80) . In this case, encrustation Of the s he l l interior shoUld also occur, which has never been recognised on Pleuromya molds . Consequently 1 encrustation took place after Pleu :r·omya -shel i. s were
filled with sediment and after this fill became differentially ce-
Fig. 1 1 . Pelecypods with Placunopsis-encrustations from the "Bank der kleinen Terebrate ln " , Gottwollshausen ( CHK 1 05 3 / 2 1 0 5 3 / 1 ) . All scale bars 1 em. A) P�euroneatites laevigatus . Left valve of double-valved specimen with one generation of Plaaunop s i s , arranged in polygonal mos a i c . B ) Cross-section through left valve of Pleurone a t i t e s > heavily encrusted b y Plaaunop s i s in several generations. C) Hoer•n e s i a socia l i s,right valve , encrusted in l i fe posi tion. D) Hoernesia sooia l1; s , left valve , encrusted after reworking . E) Plaounopsis o s traoina , double-valved specimens F) Myophoria intermedia , intravital encrustation G) Myophoria inte:rmedia , calcitic Plaaunopsis shows xeno morph sculpture of now dissolved aragonitic Myopho:ria shel l . H ) Internal molds of deep-burrowing Pleuromya m u s a u loides a s evidence for. deep erosion due to storms
'
280
'
E
! DEPOSITION f EROSION Fig
.
'
1 2 . Bank der kleinen Terebrateln : Genetic sequence . A) Plaaunopsis growing on re-exposed hardground crust B ) Event reworking of crust and Plaaunop s i s . Renewed overgrowth including overturned clasts . C) Plaeunopsis grow into pillars , competing with net mentation. Shel·ls of byssus-attached epibionts accumu lating. D ) Burial and reactivation . Borings on re-exposed Plaeu nop s i s tops . Bonebed veneer indicating condensation above referenqe horizon . E ) Soft grOund accumulates 1 thick enough for inf�·unal shells to form pressure shadow concretions . F ) Event re-exposes surface D ; remnant pelecypod concre tions (black) still with shell , overgrown by solitary Plaaunop s i s .
After that return to E
281
mented. Caused by bioturbation and the loose packing of fecal pel lets , their fill s ediment was also more porous than the surrounding sediment, which further enhanced -.�he formation of such "pressure shadow concretions " (SEILACHER et �1. , 1 9 7 6 ) .
Inteinal molds are never· cOrroded or bored, hence the or�ginal shell was still present when encrustation took plac e . On the other hand, reworking of internal molds may in some instances also have occurred after dissolution .of the aragonitic shell . This is demonstrated by · b9red and limonite-impregnated - internal molds of ceratites in thin bonebeds . _l\dditional. evidence is pr'e�ented with an intern.a l mold of Myop_hbria int ermedia (Fig. l1 G) , on which encrusting P Z aaunops i s has preserved the xenomorph sculpture o f the host after dissolution of the aragonitic shell substrate . Precondition for encrustation of endobionts i s intense reworking of softgrounds and removal of the fine sediment . After the surface of the 110bere Schalentrlimmerbank" (unit d ) was covered by sediment, it became further consolidated and finally cemented into a hardground , which must have provided considerable resistance a·gairist reworking and thus acted as a reference level for. subsequent event erosion.
4 . 2 . 3 . Reference-Horizon . Due to reworking and erosion, bioclasts and t.concretions be'came concentrated on the preexisting reference horizon of the "Obere Schalentri.immerbank 11 (SEILACHER , this volume ) . After erosive re-exposure :?f this surface, it became available for shorttime colonis ation by epifaunal suspension-feeders (post-eVent _ . community) • . The PZaeunops i s -biostrome s , howeve-r , did not bec_ome re-activate d , probably because omission did not last long enough . Background-sedimentation soon s tarted again and smothere·d the epi bionts , which are mostly preserved double-valved . When soft bottom conditions started to predominate again, the background fauna was able to re-immigrate from deeper areas which were not affected by the storm event . It is likely, that erosional and depos itional events were repeated several times on top o f the reference horizon, finally resulting in the mixed fauna observed in the "Bank der kleinen Terebrateln11 • The few pelecypods that have been found in· life position can be attri buted to the last softground phase that was no more affected by reworking. Apart from pressure shadow concretions , pyritic molds of small gastropods and Nuculids were condensed on the reference horizon.
282
-
FACIES MODEL
-
OOLITE COMMUNITY
KORNSTEIN
ft\i!)@cJ\%1 TEMPESTITIC CONDENS. HORIZON � �
TONPLATTEN
mud transport during events
Fig. 1 3 . Facies mode l . A · tempestitic condensation horizon with pat chily distributed Plaeunopsis biostromes developed on the gentle slope of a submarine swell between wave base and storm waVe bas e . - During storm events , mud was winnowed away into deeper parts of the bas in . Within the oolithic "Kornstein" -facies in the center of the swe l l , rapidly burrowing deposit-feeders (Myophoria� T.rigonodu s ) and byssate suspension--feeders ( Bakeve t Z i a , En to Z ium) predo minated , whereas On storm-swept hardgrounds and shell grounds epifaunal suspension feeders Plaeunop s i s , Pleu roneo t i t e s , PZagios toma, Coenothyri s ) were more abundant. Softgrounds represented by "Tonplatten-facies 11 were inha bited by a background-fauna including deposit- feeders Myophoria� Paleonueula� Ent a l i s ) and suspension feeders (Boern e s i a � Pleuromya J . compare SEILACHER, 1 9 81 , Fig . 1
According to HUDSON
&
PALFRAMAN ( 1 9 6 5 ) and HUDSON ( 1 9 7 8 ) pyritic
molds originate in organic-rich sediments , but un�er oxidising con ditions . Under shal low burial , anaerobic bacteria are able to reduce
so42-
from seawater within shell spaces .
283
to mixing of two different faunal assemblages , the "Bank der klei Terebrateln" should not be analyzed according to the " community concept " , although the bulk of the fauna is preserved doublevalved . Thus , the factors listed by FUR�ICH ( 1 9 7 8 ) that cause faunal mixing and condensation may be extended by the present example representing tempestitic condensation . 5 . Results
During a regressional phase , P�aounop s i s biostromes developed on the gentl'e slope of a submarine swell in a zone between normal and storm wave base . These biostromes and adjacent shell grounds subsequently acted as an effective reference horizon ( SEILACHER, this volume ) . During phases of its burial , the newly created so ftgrounds were colonised by a diverse background fauna . Internal molds of burrowing bivalves { for med as early diagenetic pressure shadow concretions) became subse quently reworked and condensed on top of the reference horizon du ring major storms , while the fine sediment was winnowed away into deeper parts of the basin. The internal molds were then available for encrustation. Boriebeds , which are abundant throughout the mo3 , together with re
worked concretions, are also indicative for storm condensation combined with lateral transport of the fines .
Such tempestitie condens.ation horizons are typical for regressio nal phases and thus are most abundant in the mo 3 . They are most pronounced on gentle slopes of submarine swells , where a certain relief guaranteed basin-ward removal of fine sediment . These ideas may be tested by s imilar studies in comparable facies types of the Trochitenkalk or of the "Quaderkalk" in the WUrzburg area. It would be particularly interesting to s tudy the distribu tion of P�acunaps1: s -biostromes and bioherms in the "Hauptterebra telbank" and in the transition of 11Toriplattenfacies " to "Kornstein facies 11 in the Trochitenkalk of the J-agst valley near Crailsheim. Acknowledgements This paper would not have been possible without the stimulating discussion with Prof . Dr. A. SEILACHER and T . AIGNER ( both Dept. o f Geology and Palaeont . , Univ. o f TUbingen ) . Furthermore , I thank
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284 Prof . Dr . G . H . BACHMANN (Hannover) for discussion. Thanks are due to Mr. w . WETZEL ( TUbingen ) for photographic ass istance. remain in my private �ollection ( No . CHK 1 1 5 3 ) , unless deposited
in the Dept. and Museum of Geology and Palaeonto logy , Tlibingen ( IGPT) or in the Staatliches Museum fUr Naturkunde Stuttgart ( SMNS ) . T . AIGNER and Pro f . Dr. A. SEILACHER reviewed and translated the manuscript into English . References AIGNER, T. . ( 1 9 7 7 ) : Schalenpflaster im Unteren Hauptrnuschelkalk bei Crailshe im ( WUrtt . , Trias, moll ) - Stratinomie , Okologie , Sedi mentologi e . - N . Jb . Geol . Palaont. Abh . , 2�� : 1 9 3-2 1 7 . AIGNER, T . ( 1 9 7 7 ) : Schill-Tempestite im Oberen Muschelkalk {Trias, SW-Deutschland) . N . Jb . Geol . Palaont. Abh. l�Z : 326- 3 4 3 . BACHMANN , G . H . ( 1 9 79 ) : Bioherme der Muschel Ptae ;._n o p s i s o s t raeina v. SCHLOTHEIM und ihre Diagenese . - N . Jb . Geo l . Palaont. Abh . , Jo,:j)l : 3 8 1 -407 . FURSICH, F . T . ( 1 9 7 8 ) : The influence of faunal condensation and mi xing on the preservation of fossil benthic communities . Lethai a , ;)J, : 2 4 3-250 . FURSICH , F . T . { 1 9 78 ) : Genesi s , environments , and ecology of Ju rassic hardgrounds . - · N . Jb . Geo l . Palaont . , Abh . , l�� : 1 �6 3 . GWINNER, M . P . ( 1 9 7 0) : Revision der lithostratigraphischen .Nomenkla tur im Oberen Hauptmuschelkalk des nOrdlichen Baden-Wllrttemberg. - N. Jb .. Geol . PaUi.ont . , Mh . : 77-87 . HAGDORN , H . ( 1 9 7 8 ) : Muschel/Krinoiden-Bioherme im Oberen Muschel kalk ( mol , Anis) von Crailsheim und Schw.3.bisch Hal.l { Stidwest deut.schland) . - N . Jb . Geol . PaL3.ont. , Abh. , J,gg : 3 1 -86 .
HERTWECK , G . ( 1 9 7 9 ) : Aufwuchs von Seepocken auf Hartteilen anderer Tiere . - Natur und Museum, J,g� : 305- 3 1 1 . HOLDER, H . ( 1 9 6 1 ) : Das Geflige eineS Plaeunopsis -Ri££s aus dem Hauptmuschelkalk . - Jber . u . Mitt. oberrhein . geol . Ver . , N . F . , �,:) : 4 1 -4 8 .
HOLDER, H . ( 1 9 6 1 ) : Muschelriffe im Muschelkalk . - Natur und Museum, l1� : 2 4 3 -2 5 2 . HUDSON , J . D � { 1 9 7 8 ) : Pyrite in Ammonite Shells and in Shales . N . Jb . Geol . PaUiont . , Abh . , J,�J : 1 90- 1 9 3 . HUDSON , J . D . & PALFRAMAN , D . F . B . ( 1 9 6 9 ) : The ecology and preserva tion of the Oxford Clay fauna at Woodham, Buckingharnshire . Quart . Journ . o f the Geo l . Soc . o f Londo n , l�i = 387-4 1 8 . KRUMBEIN, w . ( 1 9 6 3 ) : tiber Riffbildung von Plaaunopsis o s t racina im Muschelkalk von Tiefenstockheim bei Marktbreit in Unterfranken . - Abh . Naturwiss. Ver. WU.rzburg , � : 1 - 1 5 . LINCK , 0 . ( 1 9 5 6 ) : Echte undunechte Besiedler (EpOken) des deutschen Muschelkalk-Meeres . - Aus der Heimat, 2i= 1 6 1 - 1 69 .
285
Mc,GEiEE , G . ( 1 9 7 8 ) : Analysis of the shell torsion phenomenon in the Bivalvia . - Lethaia, JJ : 3 1 5 - 3 2 9 . , T . M . ( 1 980) : Structure of "Sphaerocodium kokeni WAGNER" , a Girvanella-oncoid from the Upper Muschelkalk (Middle Triassic) of Wlirttemberg ( SW Germany )' . - N . Jb . Geo l . Palaont . , Mh . : 2 9 3302 . EI F , W . -E . ( 1 9 7 1 ) : Zur Genese des Muschelkalk-Keuper Grenzbonebeds R i n Slidwestdeutschland . - N . Jb . Geol. Palaont . , Abh . , 1�� : 3 6 9 -404 . REINECK , H . -E . ( 1 9 80 ) : Steinkerne in der Entstehung . - Natur und Museum, lJQ: 44-47 . SCHMIDT, M. ( 1 9 2 8 ) : Die Lebewelt unserer Trias . - Hohenlohische Buch handlung F . Rau , . Ohringen . SEILACHER, A . &· al . ( 1 9 7 6 ) : Preservational h istory of compressed Jurassic ammonites from Southern Germany . - N . Jb . Geol . Pa laont . , Abh . , l�� : 307- 3 5 6 . SHINN , E . A. ( 1 9 69 ) : Submarine Lithification of Holocene Carbonate Sediments in the Persian Gulf . - Sedimentology , J� : 1 0 9 - 1 4 4 .
VOIGT, E . ( 1 9 7 5 ) : Tunnelbaue rezenter und fossiler Phoronoidea . Palaonto l . z . , 4� : 1 3 5- 1 6 7 .
VOLLRATH , A . ( 1 9 5 5 ) : Zur Stratigraphie des Hauptmuschelkalks in Wlirttemberg . - Jh. geol. Landesamt Baden-Wlirttemberg , l : 7 9 - 1 6 8 . WAGNER, G . ( 1 9 1 3 ) : Beitrage zur Stratigraphie und Bildungsgeschichte des OberEm Hauptmusch_e lkalks und der Unteren Lettenkohle in Franken . - Geol . -palaont. Abh . , N . F . , J� ( 3 ) : 1 - 1 80 .
WAGNER , G . ( 1 9 3 6 ) : Riffbildung als MaBstab geologischer Zeitraume . Aus der Heimat, ��: 1 57 - 1 6 0 . WENDT, J . ( 1 9 70 ) : Stratigraphische Kondensation i n triadischen und j urassischen Cephalopodenkalken der Tethys . - N . Jb . Geo l . Pa li:iont . , Mh . : 4 3 3-4 4 8 . Geological Maps Geologische Karte von Baden-WUrttemberg 1 : 2 5 000 : Blatt 6 9 2 4 Gaildorf (ErL3.uterungen E . EISENHUT ) , Stuttgart 1 9 7 4 Blatt 6 8 2 4 Schwi:ibisch Hall ( Erli:iuterungen A . VOLLRATH ) Stutt gart 1 9 7 7 Geologische Uber_sichtskarte von Baden-WUrttemberg 1 : 200 000 , Blatt 2 .
Glauconitic Condensation Tirrough High-Energy in the Albian Near Clars (Escragnolles, Var, SE-France) G. GEBHARD
Abstract: Condensation in this famous 11Fossil-Lagerstatte " is by repeated reWorking through rare events , probably storms between normal and storm wave base. The condensed sequence lenseshaped rock-body in a syntectonic depression.
1::
Fig . 1 . Location map of the exposures studied (black points ) .· Hatched line in ferred coast-line
Cyclic and Event Stratification (ed. by Einsele/Seilacher) © Springer 1982
287 1.
,
Introduction (Fig.
1)
The Albian glauconitic condensation from Clars (Escragnolle s , Var, sE-France ) 1 famous for the abundance and excellent preservation of its fossil content , was described for the last time by PARONA & BONARELLI ( 1 89 7 } , who only gave � general description of its fauna . Later on , the cond'ensatiOn of Clars was mentioned several times in the literature (RITZEL, 1902 ; SPATH , 192 5 ; SPATH , 1 9 2 3 -4 3 ; BREI STROFFER, 1 9 4 7 ; COLLIGNON , 1 9 4 9 ; COTILLON , 1 '9 7 1 a . o . ) , but a de tailed study concerning the biostratigraphy , facies and genesis of these deposits is still lacking. ' The Purpose' of this paper is to evaluate a) the stratigraphic range , b ) the genesis , and c ) the paleogeographic situation of the condensed sequence . ( Fig. 1 )
2 . Stratigraphy SPATH ( 19 2 3- 4 3 ) ; BREISTROFFER ( 19 4 7) ; DESTOMBEB & DESTOMBES ( 1 9 6 5 ) and OWEN { 19 7 1 ) have proposed biostratigraphic zonations for the European Albian , which are 'summari Z ed in Table 1 . Table 1 .
Biostratigraphic zonation of the Albian
Upper Albian (part)
Zone
SubzOne
DipZoaeras O:t'is tatum
DipZoaeras aristatum
Euho p Z i t e s lautus
Middle Albian
Euho p Z i t e s lorioa tus
Hop Z i t e s dentatus
Lower Albian (part )
Douv i l Z e ioe:t'as mamm i Z Zatum
Anahop Z i t e s daviesi Euhop l i t e s n i tidus Euho p Z i t e s meand:t'inus Mojsisoviasia subdeZaruei Dimo:t'phop Z i t e s niobe Anaho'p Z i t e s intermedius Hop Z i tes spathi Lye Z Ziaeras Z y e Z Zi Hop Z i t e s eodentatus
288
cristatum - zone (pars) den tat us - zone (pars) to crista tum - z o ne ( p ars)
J
� w
" ·E
' 0 0 0
ro � c
� "' w ro X 0 0
E .:"
mammiltatum- zone to dentatus- .zone {pars)
@
�':'.<<:."·./
�m � .2' c
mamm-illatum - z o n e
Barremian
-� X
_
gtauco�\te sand
limestone
1:.:\
phosphorite nodules
!go�)
l i mestone pebbles
�
Cl 0 ' o <( o -
40cm 20cm O cm
Fig. 2 . Profile and biostratigraphical zoning of the condensed Albian of Clars
289 Within this framework the stratigraphy of the condensed sequence can be summarized as follows (Fig . 2 ) : The base of the condensed succession i s formed by Barremian limesto ne with a hardground containing stromatolite s , borings and limonitic crusts at its top . Aptian and the lower part of the Lower Albian are missing in the biostratigraphic record . For some time during this interval the stromatolites were growing on the hardground. Detailed fossil collection at different levels enabled the author to subdivide the Albian of Clars biostratigraphically into three parts . The lower part belongs to the mammi llatum zone , the middle part shows a mixed fauna of mammi llatum
and lower dentatus zone , to the
and the upper part contains a mixed fauna from the dentatus cristatum zcme . (Fig. 2 ) 2 . 1 . The Lower Part
is about 80 em thick . Douvi Z Z e ioeras mamm i Z Z a t u m ( SCHLOTHEIM) , D. inaequinodum (QUENSTEDT) and Tegooeras gladiator ( BAYLE) show that this part of the sequence belongs to the rnammillaturn · zone . 2 . 2 . The Middle part is 1 10- 120 em thick . Ammonite faunas of both the rnammi llatum- zone a� d the lower deUtatus zone were found associated in this horizon, i . e . they indicate condensation (mixed fauna I ) . ' The mammi llatum
zone is represented by Dou v i Z Z e ioeras mammi Z Zatum, D . inaequinodum, CZeonioeras que�oifo lium
( D 1 0RBIGNY) and Tegooeras spp . , the lower
dentatus zone by Hop l i t e s (Hop l i t e s ) eodentatus CASEY , H . ( H . ) b a y Z e i SPATH , Lye Z Z i oeras sioosta tum
spp. and Branoooeras (Eubranoooera s ) ver
(MICHELIN ) .
2 . 3 . The Upper Part is formed by a 2 5 - 30 em thick layer with a · very diverse ammonite fauna consisting of representatives from the dentatus , loricatus and lautus zones of the Middle Albian. The Upper Albian is represen ted by the cristatum-zone. Typical for the dentatus zone are Hop t i t e s (Hop L i t e s ) b a y l e i� H. ( H . ) dentatus
( SOWERBY )
1
H . (!{ . ) paronai SPATH , H . ( H . ) rudis PARONA &
290
& BONARELLI , Lye Z liaeras spp . , a . o . , whereas the loricatus zone is represented by Hop li te s ( H . ) dorsetensis SPATH , H. {Anahop l i · inter>medius SPAT H , H . (A . ) praecox SPATH , H . (Dimorphop l i t es ) D. (M. ) a l t eY.natus (WOODWARD ) and DipoZoeeras (Mojs i sovicsi a ) spp . af cornu tum (PICTET) and D . (M. ) ·semioornutum (SPATH) may belong to the lautus-zone . The dentatus zone shows all subzones except those of the loricatus and lautus zone which are only partly represented . The cristatum zone is only partly condenSed into the mixed fauna I I , but conti nues into an uncondensed sequence . Characteristic are D i p o l oceras (Dipoloaera s ) eristatum
( DELUC) and Hysteroaeras simp Z i e i e o s t a
SPATH .
3 . Genesis
3 . 1 • General Remarks
Since A . HEIM ( 19 3 4 ) introduced the term 11stratigraphic condensa & SEIT Z , 1 9 3 4 ; SCHAUB , 1 9 4 8 ; MENSINK , 1960; DIETL, 1 9 6 9 ; WENDT, 1 9 7 0 ; GEYER & HINKELBEIN , 1 9 7 1 , a . o . ) . Most of the authors have proposed their ow� definitions , the most useful being that of WENDT ( 19 70) . He favours a strictly descriptive definition :
tion11 , several authors have dealt with this term (HEIM
11 • •
• ein Kondensationslager ist ein geringmachtiger Ge
steinskOrper, in dem verschieden alte Faunenelemente
lagerungsmaBig nicht mehr trennbar nebeneinander liegen. 11
According to this definition the condensed Albian of Clars consists of two condensation layers {mixed fauna I and I I ) .
3 . 2 . Mechanism of Formation The condensation of Clars i s caused by repeated reworking and re sedimentation (Fig . 3 ) . The hardgroUnd at the base of the conden sation was formed during the Upper Barremian and continued as a reference horizon into the Lower Albian . Subsequently, tectonic movements caused partial erosion of the hardground as indicated by reworked pebbles . These pebbles, possibly after some transport, were deposited together with faunas of the Lower Albiau on top of
291
·1 .
Fig. 3 .
Genesis of condensation (explanation see text)
292
the hardground {Fig. 3 A , ammonite shell in an unfilled state ) . The final site of deposition was a tectonic depression ( see below) . At the same time allochthonous glauconite sand was
deposited,
which covered the pebbles and biogenetic hard particles (Fig. 3B) . In the buried stage , early-diagenetlcally phosphoritlzed moulds and concretions were formed (Fig. 3B, ammonite shell shows the formation of mould and concretion ; the original shell is still preserved) •
After 'this firs ·t phase of sedimentation and lithification, rewor king and resedimentatlon started again with the result that finer sediment was partly removed through winnowing (Fig . 3 C ) . Reworking is indicated by :
1 } Fragmentation of prefossilized skeleton s . 2 ) Mechanical abrasion o f parts o f the original shell not protected by concretionary moulds (Fig. 3 C , affimonite shell) . 3 ) se·s sile foraminifera (Coscinophragminae ) are encrusting the moulds directly (F i g . 3C , ammonite shell ) , not by diagenetic compaction. 4 ) Early diagenetic concretions 1 representing an intermittent finer sediment 1 contain more planctonic foraminifera (Hedberge l la spp . ) than the surrounding nonlithified sediment , from which they were later winnowed away .
Storms may be responsible for the reworking and resedimentation . The erosive activity of storms has rec ently been recognized by several authors (FtlRSICH, 1 9 7 1 ; KELLING & MULLIN , 1 9 7 5 i AIGNER , 1977 ; BRENCHLEY et al . , 1 9 7 9 ; LINDSTROM, 1 9 7 9 a . o . ) . (Fig. 3 ) In our case sedimentary structures that would be indicative for
storm events are obl i terated _by intensive bioturbation . But there is other evidence for the occurrence of short high-energy events . Firstly , the excellent preservation o f fossils allows only short phases of reworking (FtiRSICH, 1 9 7 1 ) . Secondl y , pebbles , concre tions and moulds are encrusted rarely and then only by incipient pioneer faunas such as sessile foraminifera (Coscinophragminae) , small serpulid worms and juvenile oysters .
293
Thus these hard particles were available for settlement as secondary hardgrounds only for a short time and became sediment-covered be fore the oysters rea·ched the adult state. Under the influence of permanent bottom currents , more intensive settlement on the secondary hardgrounds would have been possible , because the currents would have prevented burial for a longer time . According to these observations , the condensed sequence probably was deposited at a depth between the bases of normal and s .torm waves . Above normal wave base more intensive settlement , but also st�onger abrasion and poorer. fossil preservation would be expected . On the other hand , stromatolites at the base of the condensation indicate that t.he whole sequence was deposi ted in shallow water . Phase A to C (Fig� 3 ) took part during the rnarnmillatum zone. Already during this time 1 glauconite sedimentation started and continued un til the lower dentatus-zone (Fig. 3 D ) . At the same time phosphori tic moulds and concretions were fOrmed. Subsequently, the relatively quiet phase was followed by •another storm event, which again caused reworking, resedimentation and winnowing, and which resulted in a mixing of mamrnillatum- and lower dentatus-zone faunas (Fig. 3 E ) . Neither the basal hardground nor1• the lower part of the first condensation was affected by this event , because the first . condensation contains only a mammillatum zone fauna ( compare_. stratigraphy and Fig . 2 ) . This second phase of reworking was _ fol lowed by another cycle of sedimentation, which continued into the cristatum-zone . A third phase of reworking during the crist-aturo-zone caused mixing of faunas containing ammonites of the dentatus- to the cristatum-zone in the same level (mixed fauna II , Fig. 2 ) . In reality, additional minor reworking events alternating with minor phases of sedimentation were probably also involved, but their record has become obliterated by subsequent stronger ev·ents .. The condensation of Clars thus results from many phases ·of repeated storm-induced reworking and resedimentation. In the absence o f any reworking , sedimentation would have been sufficient t o create
294
a thin but biostratigraphically well zoned sediment without cc>nclen,�0 sational mixing ( GEBHARD , 197 9 ) . In this respect, the present �c•n-, , ; ,cc densation is similar in genesis and facies to the one described by �URSICH ( 1 9 7 1 ) in the Dogger of Calvado s . 3 . 3 . Paleogeographic Setting (Fig. 4 ) �he condensation horizons of Clars were deposited as a lens shaped sediment body in a shallow marine environment , filling a syntectonic depression. Fig . 4 shows a scheme of the paleogeographic development from the Upper Barremian onwards . o·uring Barremian times we deal with a monotonous blanket o f carbonates parallel to the ancient coastf1ine ( strike NNW-SSE , Fig . 1 ) . The top of these Barremian l imestones marked by a hardground that can be traced for more than hundred kilometers along the strike. By the end of Barremian and during Aptian times , the formerly flat relief became more differentiated and small depressions , controlled ' by flexure-like deformations , acted as sediment-traps . At the same time the hardground· was eroded (Fig. 4 B ) , but only in the mOre elevated parts of the relief (Fig. 4 ) . Glauconite sand was then transported over the hardground by weak bottom currents ( COTILLON , 1 9 7 1 ) , as indicated by glauconite grains entrapped by stromatolites and borings . The bulk of the glauconite sand was eventually preserved in the above mentioned depress ions . SE of Escragonolles such a depression has formed during the Aptian (Fig. 4 B ) . The fill of this depression contains abraded fossils� from the Upper Barremian , but also a well preserved fauna of the Upper Aptian (Aaantohop l ites spp . , Cheloniaeras ·spp. a . o . ) . Because tectonic movements ceased at that time, the filling was sealed by a new hardground . This hardground prevented younger faunas to be come mixed with preyiously deposited sediments . The hardground environment continued to the Uppe:- Albian.
295
NW
A
l
Col deVa!f rrlr�re h
a
Ctars r
Barremian SE
Escragnoltes
I
d
g
I
o
u
n
d
������
0
h
a
d
g
Lower Albian u
d
n
Middle Albian o
u
n
d
Upper Albian
Fig . 4 .
Facies development from Barremian to Albian t imes in the area of Escragnolles (vertical scale strorigly exaggerated , faults are inferred to be flexure-like}
i i' ' l
296
Near Clars another depression started to form in the upper Lower Albian (Fig. 4 C ) . This small depression became filled with re worked pebbles and a fauna of the mammillatum-zone . As local sub s idence continued, marginal areas , as the Col de Valferriere section (Fig. 4 D) were included into the depression. In this section the oldest fossils are Middle Albian in age. The storms first caused reworking and condensation near Clars , while the hard ground environment still continued to persist outside the
'
It was only during higher Upper Albian time s , that the whole region became buried under glauconite sand (Fig. 4 E ) . This could be due to changes in the current regime , resulting in increased rates of glauconite sedirnen�ation. One reason for this might have been the uplift of the " Isthme Durancien" , a land -bridge to the west, another the increased production of glauconite sand. The deposition of these thi·c k glauconite sands prevented the formation of further condensations in the whole area. These paleogeographical considerations of the region of Escragnolles are · in accordance with those proposed by COTILLON ( 1 9 7 1 ) for the "arc subalpine'' and some new observations of my own . Acknowledgements This paper represents parts of my Diplorn-Thesis , supervised by Prof . D r . J . Wiedmann . I thank him for his support, particularily for his help in identifying the ammonite fauna. Prof. Dr . A . Seilacher , Dipl . -Geo l . J . Reitner and T . Aigner criticized the manuscript . For stimulating discuss ions thanks are due to my wife and roy colleagues . Field work during summer 1 9 7 8 was supported by SFB 5 3 .
297
References AIGNER, T. ( 1 9 77 ) : Schalenpflaster im Unteren Hauptmuschelkalk bei Crailsheim (Wlirtt . , Trias , mol) - Stratinomie , 5kologie , Sedimentologie . - N . Jb . Geol . Palaont.1 Abh . , �g� : 1 9 3 -2 1 7 . --- ( 19 79 ) : Schill-Tempestite im Oberen Muschelkalk (Trias , SW Deutschland) . - N . Jb . Geol . Palaont. 1 Abh . , l��: 3 2 6-34 3 .
BREISTROFFER ( 1 94 7 ) : Sur les zones d ' Ammonites dans ! · ' Albien en France et d ' Angleterre . Trav .Lab . Geol . Grenoble , �g : 1-88 . BRENCHLEY, P . J . , NEWALL , G . · STANISTREET , I . G . ( 1 9 79 ) : A storm surge Origin for sandstone beds in an epicontinental Platform sequence , Ordovician, Norway . -Sediment . Geol . , g£ : 1 85-2 1 7 . COLLIGNON , M. ( 1 9 49 ) : Recherches sur les faunes albiennes de Mada gascar. 1 . L ' Albien d ' Amba�imaninga. - Ann . geol . Serv. Mines , l�· 1 - 1 2 8 . COTILLON , P . ( 1 9 7 1 ) : Le Cretace inferieur de l ' arc subalpine de Castellane entre l ' Asse et le Var. Stratigraphie et sedimentologie . Mem. Bur. Rech. geol . min . , g� : XVII a . 3 1 3 p . DESTOMBES , P . & DESTOMBES , J . P . ( 1 9 6 5 ) : Distribution zonale des ammonites dans l ' Albien du Bassin de Paris. Colloque Cretace Inferieur, Lyon 1 9 6 3 . - Mem Bur. Rech. geo l . min . , l� : 2 55-270. DIETL, G. ( 1 9 6 9 ) : Biostratigraphische und biostratinomische Unter suchungen im Dogger Keltiberiens . Unpubl . Dip l . -Thesis Tlibingen 1 9 6 9 , 82 p . FtiRSICH, F . ( 1 9 7 1 ) : Hartgrlinde und Kondensation irn Dogger von Calva dos . - N . Jb . Geo l . Palaont. 1 Abh . , !4� : 3 1 3 -3 4 2 . GEBHARD , G . ( 1 97 9 } : Glaukonitische Kondensation im Alb der sub� alpinen K� tten (Clars , Escragnolles , SE-Frankreich ) , deren Ammonitenfauna und Kartierung in der· Umgebung von Escragnolles . Unpub l . Dipl. -Thes i s , Tlibingen 1 9 7 9 , 1 5 2 p .
GEYER, 0 . F . & HINKELBEIN 1 K . ( 1 9 7 1 ) : Eisenoolithische Kondensations horizonte im Lias der Sierra de Espuna (Prov. Murcia, Spanien) . N . Jb . Geo l . PalB.ont . 1 Mh . , ,!,2,ZJ: : 3 9 8 -4 1 4 .
298 HEIM, A . { 1 9 34 ) : Stratigraphische Kondensation . - Eclogae geol . Helvet . , gz : 372-383 ·.
HEIM, A . & SEITZ , o . ( 1 9 3 4 ) : Die rnittlere Kreide in� den helvetischen Alpen von Rheintal und Vorarlberg. und das Problem der Denkschr. schweiz . naturf . Ges . , g2 : I-XI a . l 85-310 . HITZEL , M . E . ( 1 9 02 ) : Sur les fossiles de l ' etage Albien recueillis_ par M . A . Gu€bhard dans la r€gion d ' Escragnolles (A . -M . ) . Bul l . Soc . g€ol . France, ( IV) ,€-: 874-87 9 . KELLING , G . & MULLIN , P . R . ( 1 9 75 ) : Graded limestones and limestone guarz ite couplets : possible storm- deposit from the Moroccan Carboniferous . - Sedimen t . Geol . , !J : 1 6 1 - 1 9 0 . LINDSTRlSM , M·. ( 1 9 79 ) : Diagenesis of Lower Ordovician hardgrounds Sweden . - Geologica Palaeontologica, �4 : 9-30 . MENSINK, H . ( 1 960) : Beispiel fUr die stratigraphische Kondensation Schichtlilckenund den Leitwert von Ammoniten aus dem Jura Spaniens im Vergleich zu NW-Europa . - Geol . Rdsch . , �2 : 70-8 2 . OWEN , H . G . ( 1 9 7 1 ) : Middle Albian stratigraphy in the Anglo-Paris Basin . - Bul l . Brit . Mus.. (nat. Hist . ) , Geology , Suppl . � : 164 p . PARONA , C . F . & BONARELLI , G . ( 1 89 7 ) : Fos s i 1 i Albiani d ' Escragnolles, del Nizzardo e della Liguaria occidentale . - -Palaeontographia ital . , � ( 1 89 6 ) : 53- 1 1 2 . SPATH , L . F . ( 1 9 2 5 ) : Sur quelques ammonites du Gault , nommee par P . REYNBS . - Ann . Mus . H i s t . nat . Marseille , �Q : 96- 105 . --- ( 1 9 23-4 3 ) : A monograph 'of the Ammonoidea o f the Gault . Part 1- 1 6 . - .Palaeontogr . Soc . , ,Z�;22 = XIV a . 787 p . WENDT, J . { 1 970) : Stratigraphische Kondensation i n triadischen und j urass ischen Cephalopodenkalken der T_ethys . - N . Jb . Geo l . Palaont. Mh . , �2ZQ: 4 3 3 -4 4 8 .
Muschelkalk/Keuper Bone-Beds (Middle Triassic, SW-Germany) Storm Condensation in a Regressive Cycle
W.-E. REIF
Abstract: Iri the southwestern part of the Germanic Basin 1 8 bonebeds occur below and above the Muschelkalk/Keuper boundary . They have small lateral extension except for the "Grenzbonebed11 , which can be traced from Hildesheim to the German/Swiss ·border . The Tempestite Model turns out to be a valuable instrument for · the sedimentological interpretation of these · bone-beds . They were formed below normal wave base by an alternation of long quiet periods , during which the vertebrate remains became fossilized ( " prefos s i l i zed" ) and of short erosive storin inter missions . Various characters of sediment structure and compo sition support this mode l . Slab joints on top of the Grenzbone bed can be explained in a similar way as the more familiar in tertidal slab joints .
1 . Introduction In the English l iterature the term " bone-bed" is applied to any layer containing boneS or bone-fragments ( e . g . PARRISH , 1 97 8 ) . However , se dimentological studies over more than 1 00 years of particular bone concentrations occurring in England , U . S . A. and Germany , led to a very specific definition: "A bone-bed is a sediment which is enriched in highly fractured and abraded vertebrate bone s . Very often the bone fraction i s well-sorted with grain sizes of fine to coarse gravel. Bone-beds appear abruptly in sections which are otherwise poor in vertebrate remains ; as a rule they have high lateral persistence ( up . to 50 000 km2 ) and a thickness of several mm up to 20 em ; often they
form series of 2 to 20 layers within one section'� ( REIF , 1 97 6 , p . 2 5 2 ) . ANTIA ( 1 97 9 ) made further terminological suggestions and reviewed the literature on American and European bone-beds .
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cyclic and Event Stratification {ed . by Einsele/Seilacher) © Springer 1982
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Discussions of the ffiode of bone-bed formation ( REIF , 1 97 6 ; ANTIA, 1 97 9 ) showed that the geochemical problem, namely a high concentra tion of phosphate in sections which are otherwise poor in phosphate , is still largely unsolved ( see also COOK & MCELHINNY , 1 979 ) . The· question is why the phosphorous which was supplied by rivers was not diluted in the sea , but rather was stabilized as sedimentary phospha te (bones , teeth, scales , coprolites, inarticulate brachiopods and probably inorganic. phosphorite nodules } . Many bone-beds . were formed in shallow water, either in the early part of · a transgr'ession cycle (English Rhaeti c ; SYKES 1 97 7 ; Austrian Tertiary ; SCHULZ , 1 9 7 2 ) or close to the end of a regressive cycle ( Ludlow/Downton , England ; . Muschelkalk/Keuper , South Germany and Po land) . It is very likely , though not clearly demonstrated , that all these bone-beds were formed in a similar manner . For the Muschelkalk/ Keuper bone�be d s , REIF ( 1 96 9 , 1 9 7 1 ) suggested an alternation of quiet phases of sedimentation on an open. shelf , below the normal wave base and phases of rapid reworking by strong currents and turbulence. In a shallOw open shelf area the required energy for the reworking can only come from storms . AGER ( 1 9 73 ) , KELLING & MULLIN ( 1 9 7 5 ) , BRENCHLEY et al . ( 1 9 7 9 ) and AIGNER ( 1 97 9 ) drew attention to the fact that storms can have an eminent effect on shelf morphology , sediment transport and sediment composition in shelf areas below the normal wave base. The storm layers were called tempestites by AGER { 1 9 7 3 } . The purpose of .the present paper is to test the " Tempestite Model 11 for all bone-beds occuring near the Muschelkalk/Keuper-boundary, and to show that this model explains many features of s ediment structure and composition. Some features have never been explai ned, others were never previously recognized. 2 . Lithostratigraphy and Facies Analysis In the South German pa:i:t of the 11gerrnanotypic11
basin ( fig . 1 ;
SCHWARZ , 1 9 7 5 ) the Upper Muschelkalk (Ladinian) consists of cal careni t·e s , calcirudi tes 1 calciluti tes , marls , claystones .and in the area south of Stuttgart - dolomites ( WAGNER, 1 9 1 3 ; GEYER & GWINNER, 1 9 6 8 ; AUST , 1 9 6 9 ; BACHMANN , 1 9 7 3 ; BACHMANN• & GWINNER,
1 86 1 ; AIGNER, 1 9 7 7 , 1 9 79 , 1 9 8 1 ) . A lithostratigraphic system based on ''Leithorizonte11 (marker-beds} was first developed by WAGNER ( 1 9 1 3 ; see also VOLLRATH, 1 9 5 5 ; SCHAFER, 1 9 7 3 ; AIGNER,
301
J
"'
i ...,
Fr<mklvrt
I
I
\
IOOkm
\
Fig. 1 . Distribution of the Grenzbonebed in the southern part of the 11germanotypic 11 basin ( diagonal hatching ) . Horizontal hatching: Outcrop areas of the Muschelkalk were the Grenzbonebed has not been found. Horizontal line with two arrows : Transsect of Fi9. 2 . Dotted outline NE of S tuttgart : Sand lens which is explained in the text ( after REI F , 1 9 7 1 , modi fied) ·
1 9 8 2 ) . Biostratigraphic . s ·tudies of cephalopods ( WENGER , 1 9 5 7 ; URLICHS & MUNDLOS , 1 9 80 ) and conodonts have since shown that the 11Leithorizonte11 are in fact i sochronous (KOZUR 1 9 7 4 a , b ; 1 9 7 5 ) . The Cy�lo ides -Bed is i sochronous between Poland and France {KOZUR 1 9 7 4 a ; J . TRAMMER, 1 9 7 5 ) . Other guide-beds can be reliably used for stratigraphic inference at least within smallei areas of the whole 11 germanotypi c " basin . 1 8 sections up to 5 m in thickness of the Muschelkalk/Keuper-boun dary were studied em. by em. in the province Hohenlohe (Eastern Baden-Wlirttemberg; NE of Stuttgart) in a marginal facies ( REIF , 1 96 9 ) . Additionally the Grenzbonebed itself (the bone-bed di rectly at the Muschelkalk/Keuper-boundary) was studied in 60
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' E
()
� ss
"'
outcrops between Waldshut (upper Rhine valley, w. of Lake of Constance) and Schweinfurt (NE of Wtirzburg)
•
Based on these stu
dies and on data given by WAGNER ( 1 9 1 3 ) and BRUNNER ( 19 7 3 ) , a schematic transsect from the marginal facies in the east (Gammes feld) to the basinal facies in the west ( Sinsheim) is given in fig . 2 (black line in fig. 1 } .
303
Fig. 3 . section in the eastern , marginal part of the transsect (Crailsheim-Gammesfeld area) . The horizontal bars indicate where bone-beds have been found . Lengths of the bars des cribe the lateral persistence of the bone-beds { not to scale ! ) . The change in mode of sedimentation at the Mu schelkalk/Keuper - boundary is thought to be responsible foi the bone-bed formation
In the western part of the cross-section the uppe rmost Muschelkalk (consisting of Obere Terebratelbank , Bairdien-Ton and Glaukonitkalk) is complete , with a thickness of about 6 . 7 m. It pinches out to the east and is absent in Gammesfeld. The obere Terebratelbank is
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least affected by this thinning out so that it can still be iden tified in a section 9 km SW of Gammesfeld. Hence along the trans sect the Grenzbonebed overlies unconformably the uppermost Muschel kalk and the mittlere Terebratelschichten . Though the thickness of the bone-bed may vary from outcrop to outcrop, it is systematically thicker in marginal than in basinal areas ( 1 5 to 30 em compared to.
1 em or less) . The Muschelkalk/Keuper-boundary marks a significant change in the ' mode of sedimentation . The Lower Keuper is characterized by clay stone s , dolomites and dolomitic marls ( and very rare l imestones ) . Deltaic sandstone bar f in,gers are furrowed deeply into the lower Keuper from a formation which lies above the Estherienschichten.
All , or at least most terrig_enous material of the Lower KE;:uper must ··
have been derived from the north (PATZELT, 1 96 4 ; BRUNNER, 1 9 7 3 , 1 980; ESSIGMANN , .1 9 7 9 ) . In contra s t , the small amount of sand which is found in the Upper Muschelkalk was derived from the east {Vindelici an land and Bohemian land) . The sandy nearshore facies of the Muschel kalk in eastern F�anconia was described by $CHROEDER { 1 9� 3 ) . It has its source in the Bohemian land, but will not be discussed in this pape r . The whole section under consideration undoubtedly was for�ed in normal marine to slight-ly brackish water below normal wave base (REIF, 1 9 7 1 ) . The differences in water depth between basinal areas (NW of Stuttgart) and marginal areas (NE of Stuttgart) probably were not very large . Yet, the lithologic differences ·are very conspicuous . The Upper Muschelkalk is more than 1 00 m thick in the western part of the transsect ( Fig . 1 i GEYER & GWINNER, 1 9 68 ) ; but only 60 m at the eastern end . Much of the eastern section is characteriz.ed by
crinoid and mollusc coquina s , which tend to be very th �n in basinal areas (contribution by _ AIGNER) . Both dif ferences are partly
due to a higher subsidence in the basin than at the margin . A much more imp-ortant factor 1 however , may be a low net sedimen-
tation rate which left enough time for the sediments to d ifferen-. tiate , through repeated reworking of marginal sedime�ts by . storms .
The sedimentation rate of the terrigenous material that came from the north in the lower Keuper was pro"Qabl y sign.i.ficantly faster and hence left no time for a differentiation between
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originally basinal and originally marginal areas . As the transport was predominantly from the north , it is not surprising that the thickness of the Lower Keuper increases significant�y from south to north ( PATZELT , 1 9 6 4 ; GEYER & GWINNER , 1 96 8 ; AUST 1 1 96 9 ; BRUNNER, 1 9 7 3 ) . In the northern part of the whole "germanotypic" basin the Keuper sedimenta.tion started much earlier than in south Germany . In Poland the Keuper sedimentation began directly above the cya Z o i de s -bed . This means that the upper 2 / 5 of the South German .Muschelkalk are contemporaneous with lower Keuper in Poland (KOZUR, 1 9 74a , b; TRJI_MMER , 1 97 5 ) . Presently it is unknown how many bone-beds occur in the basinal part of the transsect below and above the Muschelkalk/Keuper boundary in addition to the Grenzbonebed, itself . In the marginal part, bone-beds have been found in 1 8 different stratigraphic posi tions between Obere Terebratelbank and Estherienschichten (Fig . 3 ; REI F , 1 9 7 4 ) ." Their sedimentology ·and genesis are more or less iden tical . Except for the Grenzboneb e d , these bone-beds are very thin , little persistent laterally and tend to be stratigraphically crowded close to the boundary, while they decrease in number further below and above (Fig . 3 ) . Some of the sand bars of the delta contain a bone-bed which is of placer type ( e . g . locality Bibersfeld , s of sehwab isch Hall) and carries a different fauna. Unless otherwise spec ified, the following lithologic data are based on observations of the Grenzbonebed in the eastern part of the cross-section. 3 . Sole of the Bone-Bed In all cases studied the bone-bed sole is an erosive d iscontinuity , al though , due to the pronounced facies differentiation of the upper most Muschelkalk , the l i thology of the underlying bed may vary sig nificantly. The sole i s usually flat , rarely with a relief of up to 20 em within the area of a small outcrop . In most cases the eroded sediment must have been soft rather than lithified , although it does no � show any particular features (Fig . 6 a , b , d , 9 , 1 1 ) . In a few cases partly eroded Rh.i zoaor>a.t Zium- burrows and steep tube shaped burrows have been found which were f illed with bone-bed material ( Fig . 6c ) . G Z o s s i fungi t e s occurs at the base of the bone-bed in the Estherienschichten ( locality Steinbach ) .
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Fig. 4a. Groove-casts at the sole of the Grenzbonebed . Locality Barenhalde/Sattelweiler near Crailsheim/Jagst . 4 b . Gutter-cast_ in cross section , filled with bone-bed mate rial and covered with a thick mud-cover . Compaction of the mud cover led to the formation of slab-joints which form a right angle to the gutter cast ( and hence are not seen in t_he picture) and later to the crack which is in dicated by the arrows . Barenhalde ( Sattelweiler · near Crailsheim/Jags t ) . Grenzbonebed
307 Gutter-casts (WHITAKER , 1 9 7 4 ; AIGNER et al . , 1 9 7 8 ) with widths of 7 to 9 em and lengths of more than 25 em occur in those outcrops . of the marginal fac ies where .the underlying bed is a thin layer of pure claystone or clayey marl (Fig. 4b) . In the locality Barenhalde near Satteldorf/Crailsheim long groove � occur on a slab which was found on a heap of rocks ( Fig . 4 a ) .
Their widths are up to 6 em , but can be traced .only for 2 5 em ·because of the slab size . The groove casts have very sharp ridges·
and furrows and probably were formed by dragging of lithified angular pebble s . These pebbles were transported by a strong. current in
a.
s.edirnent suspension of high density ( A . SEILACHE R , pe �s . c omm . ) . Though most groove casts on the- spec imen are parallel to each other , some diverge from the main direction . ·This i's partly due to an overprinting by pebbles , which ha? a differen:t direction1 and partly due to a collisi·on of individu-al pebbles . A small angular mic-r itic l imeStone pebble was found at the end of one groove cast . It is unknown over what distance the pebble had been trans ported to this ·site of fLnal deposition , where the bone-bed over lies soft clayey marls . Shortly after the . high energy event , the energy waned rapidly , because the groove casts show sharp ridges and furrows , without any_ overprint by secondary erosion caused by samdloaded eddies . The bone-bed of this slab is structurally a one-event depos i t , with graded bedding1 a large _ number of clay pebbles , a mud-cover ( see below) and a thickness of 6 em. Indications of a l ithified sole (hard-ground) are rare. Only in a few localities very thin iron-crusts and phosphorite crusts were foun d . 3. 4 . Sedime�tary Structures The Upper Muschelkalk and the lower part of the Lower Keuper in South-We st Germany was formed be low normal wave base ( REIF , 1 9 6 9 ; AIGNER , 1 9 79 , 1 9 8 2 ) . In the light of the Tempestite Model it is assumed that the bone-beds are "condensation" deposits , where a slow sedimentation of lime and clay muds and skeletal remains was interrupted by episodic storms which winnowed out the mud s , formed pebble s , transported sand and abraded the skeletal remains . As the net sedimentation was very slow, espec ially in the marginal facies at the end of the Muschelkalk , there is no succession of storm
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layers ; rather , a high number of storms mu S: have reworked one and the same sediment at the top of the Muschelkalk and most of the mud was transported into deeper parts of the basin ·. If the last storm event led to a complete reworking of the concen trated unlithified clastic sediment ( including big pebble s , see be low) , only this last event is documented in the sediment structure . This Will be called a "one event bed" (sensu AIGNER , 1 97 9 ) . From the compos ition of the sediment it ·i s obviou S , however 1 that this storm was not the only reworking even t , but rather the last one in a long series of reworking events (see be1ow) Thus the sediment does retain a "memory" of earlier events {contribution by SEILACHER) •
A "multiple event bed" resulted if the last two or more storms decreased in energy relative to each other . Every new storm eroded on�y the top layer of the sediment deposited by the preceding storm. Thus we get an amalgamated succession of incomplete b ed s , covered by a thin bUt complete bed of the last storm . The structure and the composition of the Grenzbonebed varies con siderably from outcrop to outcrop . A major cause for this variabi lity is the superposition of a high number of storms with diffe rent energies and different local centers relative to the basin ge'?graphy. 4 . 1 . One Event Beds The minimum ene �gy necessary for a one-event bed to form was at least sufficient enough to stir up the pelitic and psammitic components of the soft sediment on the sea floor. Only a part of the mud was actually suspended , the remaining sediment was stir red up at the sea floor . This led to a mixed sedim�nt with a stochas�,ic .d istribution of · pebbles , vertebrate rema in s , quartz and mud .
There is almost no graded bedding , and the mud from · the
Fig. S a . Grenzbonebed with big lithified pebbles , a bone (white asterisk) and a thin mud-cover (black arrows ) . Ummenhofen, Blihlertal . Sb . Part of Grenzbonebed , R of Rothenburg/Tauber . Lower pebble with small bore holes . The pebble in the middle is broken either at the end of the transport or by compaction. In the left part this pebble contains bone-bed material which was already lithified when the pebble was transported . 5c . Pebbles in the Grenzbonebed , Tiefenbach/Jagstta l . Black arrows point at the limestone sole of the bone-bed
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suspension forms a thin mud cover {see below) . This lithotype is very common i n the exposures o f the Grenzbonebed (Fig. 4 b , Sa) . Lithified and soft pebbles can be found '� floating" in the middle of the bone-be d . In one locality (.Erkenbrechtshausen) a flat lithified pebble of 8 x 5 X o . 6. em was found "floating"- in the bed with an angle of about 6'5 ° to the horizontal dire�tion. The higher the energy , the more rna�erial ( clay and sand) is placed in suspension and the better the graded bedding will become . This high energy leads to . a separat-ion of pebb_l es , sand and clay . The pebbles lie on the sole and the clay forms a mud cover ( see below} . Small vertebrate fragments are t;ransported and deposl ted together ·with hy draulically equivalent smaller quartz grai�s ( o . 2 5 mm) . Quartz has a higher density than the fossilized vertebrate particles (REIF , 1 97 1 ) .. A combination of wave action and current action occurs i'n _any storm . ( see contribution by AIGNER) . In localities whe:r:e the .bone-bed is very rich i n ' s.and (more than 50� quartz grai-n s ) 1 parallel. lamination, hummocky cross stratification and low angle lamination can be obser
ved. -The storm energy probably waned rapidly 1 because this sandy bone bed litho type is cover'ed by symmetr�c ripples of low wavelength ( 7 to 1 0 em) and low amplitude (0 . 5 to 1 . 5 em) , which- rarely occur as interference ripples ( F i g . 7 a , b , c ) . The weak water agitation con tinued, while the mud particles already _ began to settle. This lead to 11spill-over ripples�' (Fig. 7 a ; ,SEILACHER , this volume ) with sand spilling over the crests. · Similarities between structures in tempestites and turbidites have been observed by AGER ( 1 9 7 4 ) and AIGNER ( 1 979 and this volume) . In the case of the Gr�nzbonebed, division A (grading ) , B ( parallel lamina tion) and -.c ( cross-lamination) of the BOUMA-sequence (ALLEN / ' 1 9 70 , Fig. 6 , 7 ) are not well separated , but all three features can be observed .
Fig. 6 a . Grenzbonebed from SchOl lmann-quarry , Sattelweiler N of heim/Jagst. The black arrow points at the erosive sol e , which is very rich in pyrite ( small black dots ) . In the bone-bed , layers enriched in mollusc shells alternate with layers en riched in vertebrate sand (black dots) . 6b . Finely laminated Grenzbonebed from Pleidelsheim/Neckar . 6 c . Grenzbonebed from Gelbingen/Kochertal . Erosive sole with a burrow which is partly filled with bone-bed sed'iment. 6 d . Divided Grenzbonebed from Erkenbrechtshausen/Jagsttal . Upper and lower member are incompletely separated by a mud layer which led to convolute bedding
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,,
Fig. 7 a o Sandy facies of the Grenzbonebed , Kirchberg/Jagst ; low angle lamination at the base of · the bed; symmetric ripples with spillover structures on top . The bone-bed is covered by a mud-cover (black color) which is only partly preserved in the specimen. 7b . Ludlow bone-bed from the type locality Ludlow . Erosive sole on a s·ilty' bed (horizontal lines ) ; low angle lamination / symmetric ripples and mud cover (back arrows ) . The counter part to this specimen is kept in the Town Museum of Ludlow . 7 c . Interference ripples on top of the Grenzbonebed , Burgbern heim, NE of Rothenburg/Tauber . The mud cover is weathered out. Burrows at the interface between bone-bed proper and mud cover are filled with bone-bed material
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The Upper Muschelkalk is very po?r in silt. Division D of the turbi dite sequence (mud and silt laminae ) , can consequently not be e�pec ted to occur. Because of this wide gap in part-icle size between quartz sand and mud ( see below) , a sharp discontinuity is usually developed on top of the psammitic/psephitic-part of the bone� bed,
especially so in localities where the bone-bed i s very rich in sand. The overlying mud cover corresponds to divis ion _ E of the. BOUMA-se
quence. It is usually 1 to 2 . 5 em thick and consists of a c layey micrite. It is separated by another discontinuity from the claystone of the Vi triolschie.fer above . This mud cover is thinly bedded and
forms the end of the cycle ( F i g . 4 b , 7b : arrows ; 9 : me) . It is over printed in three different way s : a ) Bioturbation bY sediment feeders which produced thin , irregular , more or less horizontal burrows . They were dug from above the mud cover and can reach down to the interface between psarnmi-tic se quence and mud dover. Tubes on this level are filled with bone-bed sand (Fig. ? c ) . b) Slab joints ( ,; Sigmoidalklliftung " ) are very common in the mtid cover (Fig. 8a) . They are v�ry steep (but not vertical) and are spaced 2 to 5
mm
apart . Their direction varies from outcrop to outrcrop, but remains constant over. a distance of many meters and probably was induced by very shallow local s lopes of the sea bottom. The perfect correlation of slab joint direc-t ion and s lope
is shown by slab joints above ripple marks which are always parallel
to the ripple crest direction (Fig . 8b) . The model .given by SCHWARZ ( 1 9 70, 1 97 1 , 1 9 7 5 ) 'for · intertidal slab joint formation in the Lower Muschelkalk can easily be adapted to our cas e : The mud cover was deposited very rapidly and hence had a very high water content . .The mud cover was then covered · by clay of the Vi triolschiefer. _This sealed off the mud cover and created an excess pore pressure , which reduced the shearing strength of the mud cover sediment and led to the formation of t·he s- lab joints . c ) Dewatering vents - (water escape structures ) . The· high pore pres sure resulting from the sealing off of the mud cover by the Vetriol schiefer led also to the formation of dewatering vents ( Fig . 8c) . They have a width of a few millimeters and a length of several cen timeters . The pore water pressure led to an injection of the vent by bone bed sand .
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Fi9. Sa. Grenzbonebe d , Talheim/Vellberg, Bi.ihlertal . The black arrow points at sigmoidal slab-joints in the mud-cove r . 8b . Ripple o n top of the Grenzbonebed, Kirchberg/Jags t , with a mud cove r . The slab joints in the mud cover run parallel to the ripple crest. B e . Top o f Grenzbonebed with mud cover , Tiefenbach/Jagst. The arrow points at a dewatering vent with soft sediment in jection from below �
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4 . 2 . M,u ltiple Event Beds . All transitions can be observed from a bone-bed lithotype whiCh is finely laminated (each lamina being the result of a different erosiOnal and depositional event (Fig . 6b ) to a lithotype in which several complete cycles follow i n succes sion . This second extreme is by far the more interesting one, be cause it required long intermissions between each (preserved) cyc le �nd an energy regime strong enough for lateral transport of the bone-bed material , but too weak for erosion o f the previous cycle . One example is the Grenzbonebed in Neudeck/Brettachtal with 4 comPlete cycles (Fig . 9 ) . They are graded and have mud �covers with slab j o ints . In the other extreme ( the laminated lithotype ) individual cycles can no longer be separated . ( There is probably a strong diagene-
Fig. 9 . Grenzbonebed with four cycles consisting o f bone-bed proper (bb ) and mud-cover (me) . Neudeck/Brettachtal, NE of Heilbronn
316
tic overprint ) . In the SchOllmann quarry {Sattelweiler, Crailsheim) , for example, the bone-bed is up to 1 5 em thick and consists of numerous laminae in which concentrations of bivalve debris and of vertebrate debris alternate ( Fig . 6 � ) . Where the mud cover of the lower bone-bed was spottyly eroded , lower and upper bone-bed will not be separated completely. Com paction and dewatering will hence be different in areas where the lower and the upper bone-bed grade into each other and in areas where they are sealed off from each other by a fairly unpermeable clayey mud layer. This leads to convolute bedding (Fig. 6d) . 3 ) Outside Hohenlohe ( a province NE o f Stuttgart) the Grenzbone bed is yery thin and does not show the whole spectrum of sediment structures described above . The same is true fOr all other bone beds of Muschelkalk and Keuper in Hohenlohe and outside this pro vince . Locally, however , Small bone-beds may increase in thickness : ( a ) -obere Terebratelbank , Sattelweiler (Creilshe im) i 1 . 5 em thick1
with lithifi�d and soft pebble s , graded bedding and a mud cover (REIF 1 1 9 6 9 ·;_··:-:Fig . 2 ) ; (b ) Vi triolschiefer 1 Garrunesfeld, 5 ern thick (including 2 em mud cover) , with graded bedding and a re
markable concentration of coprolites (REIF , 1 9 6 9 ) ; ( c ) Esterien schichten, Steinbach ( Schwabisch Hall.) ; up to 20 em thick, very immature bone-bed with a high clay content and la·rge pieces o f wood (REI F , 1 9 80 ) . 5 . Sediment Composition The Grenzbonebed and the other bone-beds under consideration vary considerably in composition from outcrop to ?utcrop . Their sedi ment components are : Micrite ( as sumed to have been aragonite mud originally; BACHMANN 1 1 _9 7 3 } ; clay ;· weli sorted fine-grained quartz sand (REIF1 1 9 7 1 ) ; pellets ; pebbles (Fig. 5 ; soft and lithified; intraclasts and extraclasts 1 with a maximum diameter of up to 10 em ( some lithified pebbles are incrusted by PZaaunops i s ) ; mollusc shells and debris ( Fig. 6 a ) ; ostracods ; authigenic glauconite and pyrite . Vertebrate remains fall into two categories : ( a ) Well-pre served skeletal parts . Articulated bones have not bee U found 1 though there are local concentrations o f vertebrae within a few 2 m , which probably belonged together . In two cases parts of tooth sets of colobodontid fishes were found. The individual teeth were originally held together by soft connective tissue , which is now
317
Fig. 1 0 . Vertebrate sand > 1 mm grain s i ze . The bone-bed was dissolved in acetic acid, washed and sieved with a 1 mesh-size sieve ; Tiefenbach/Jagsttal
Fig. 1 1 .
�
.,_,
mm
Erosive sole of the Grenzbonebed (b lacJ(- and white line ) . Leonberg W of Stuttgart
318
replaced by spary calcite cement. ( b ) Vertebrate sand ( s ensu REIF1 1 9 6 9 ) , which consists of highly abraded and sorted frag ments of bone s , teeth, scales and coprolites (Fig. 1 0 ) . This ver tebrate sand must have been formed by a repeated alternation of two different phases , ( a ) quiet sedimentation, buria l . and fossili zation ( i . e . phosphatization) of the vertebrate remains ( inclu ding faece s ) in mud and ( b ) reworking, abrasion , fracturing and transport of the vertebrate remains (REIF, 1 9 7 1 , 1 9 7 6 } . It is in full a�cordance with the Tempestite Model that the well-preserved vertebrate remains , which must have entered the sediment very late, are no.t concentrated on the top o f the bed; . rather they can be found anywhere in the bed . The bearers of the tooth-sets must have died during the storm or shortly prior to i t . This means that the sediment was completely mixed during the last storm in cer.t ain areas . Locally the rnicri tic sediment, in w}1ich the fresh vertebrate remains were first deposited a�d fossilized in order to form vertebrate sand later on , was not completely reworked . This led to small local beds of marl below the bone-bed, which contain well preserved vertebrate fossi ls .
6 . Discussion The above description showed that all sedimentary features of the bone-beds in the vicinity of the Muschelkalk/Keuper-boundary o� Southwest Germany are in good accordance with the predictions made by the Tempestite Model . Thus the major factors causing this bone-bed . formation were ( a ) water depth between normal and storm wave bas e , and ( b ) repeated alternation of phases of quiet sedimentation and of rapid reworking (which formed vertebrate sand and pebbles and transported quartz sand from shallow areas to the open shelf)
•
It is impossible to determine the number of reworking events which must have affected the sediments of the Grenzbonebe d , especially in the marginal facies. In genera l , the maturity of . the bone-beC.s is much higher than that of the shellY tempestites described by AIGNER ( 1 9 7 9 and this volume ) . As fossilized vertebrate remains are much more resistant to abrasion and fragmentation ,� one can predict a gradation from tempestites formed by single storm events a·na hence must be rich in well-preserved mollusc shells - to tempe stites formed by a very high number of storms (bone-beds ) . Only a few of the last storms or the single last storm are
319
documented in the struCture of the bon·e-beds . A "memory" of earlier events i s contained in 't.h8 fragmentation and abrasion of verte brate remains and of lithified pebbles . Several indicators 0£ increasing maturity can be singled out: particles ; shells;
{ a ) winnowing out of clay ( c ) loss of mollusc
(b) roundne ss of lithified pebble s ;
{d) fragmentation,. wear and sorting of fossil iz.ed verte
brate remains . If the maturity increases beyond a certain point , the "memory" will eventually be destroyed : the vertebrate partic: les will be ground into a fine powder, which will eventually be dissolved. The maturest l ithotype of the Grenzbonebed occurs in . the area aroun d Kirchberg/Jagst in the middle of the sand lens ( see belov.•) . There, the bone-bed consists of over 6 0 % well- sorted quartz sand (average grain size approximately 0 . 2 rnm) and of more than 3 0 % well sorted vertebrate sand (average grain size approxi mately 0 . 4 mm) . The rest of the rock consists mainly of pyrite and of very small quantities of clay. Identifiable vertebrate particles are rare . It is not possible to tell how much of the vertebrate sand has already been ground down to fine power and has either been dis solved or winnowed out . If this process would continue by repeated storm action , the end product would be a pure sandstone . The high variability of bone-bed thickness and the patchy dis tribution of all bone-beds except for the Grenzbonebed can be explained by current action of the storm s . I f the stratig�aphic interpretation of F i g . 2 is correct , the Grenzbonebed in marginal areas ( = proximal areas , sensu AIGNER, this volume) took much 'longer to form than the bone-bed in the ba sinal ( i . e . distal) areas . For a long time , muds were winnowed out in marginal areas and transported basinward where limestone and c laystone sedimentation continued . Thickness. of the Grenzbonebed can hence , by and large, be regarded as a proximality criterion . The composition of the vertebrate sand however, does not differ between proximal and distal areas and can not be used as proxima lity criterion. Another proximalita criterion i s provided by pebbles . Their occur rence can be traced along a narrow belt ranging from Rothenburg/ T auber, to Tiefenbach/N Crailsheim, to Eschenau and Urnmenhofen/ Btihlertal (which area seems to form the center with the highest number of large pebble s ) , to Gelbingen and Westheim/Kochertal , to Marbach/Neckar, to Rutesheim and Weil der Stadt/W of Stuttgart and
320
finally to the Wutach valley . This belt has roughly a NE-SW di rection and is thus parallel to the coast of the Vindelician Land in the SE (Fig. 1 ) . It must have been formed in the shallowest part of the accessible bone-bed area. Southwest of this belt the Upper Muschelkalk is largely covered by younger sediments . A third indicator of shallow depth is an eliptic sand lens in the Grenzbonebed 1 which lies between Wlirzburg and Schweinfurt in the NE and Stuttgart in the SW (Fig. 1 ) . Its longitudinal axis is 1 20 km long , running parallel to the coast . Its short axis is at least 40 km long. In the center o f this lens the Grenzbonebed contains more than 5 em sand , calculated. as pure quartz sand; no sand has been found in the Grenzbonebed outside the lens . The center of the sand lens lies only a few kilometers basinward of the pebble belt . Fossil and Recent examples for sand-sheets like the one described are well known . The sand is transported ontO the open shelf by storm-tides (REINECK , 1 9 70) . If one plots one-event beds and multiple-event beds on a map1 no regular pattern can be observed . This is not surprising, because it is merely a statistical phenomenon independent of water depth, whether or not the last storm was a very strong one. AEPLER & REIF { 1 9 7 1 ) distinguished three different types of bone-beds : 1 ) Placer bone-beds : Concentration of vertebrate remains ( including coprolites ) , due to their high density, in ri�er channels and mean dering streams . 2 ) Transgression bone-beds : Concentration of heavy vertebrate re mains in a transgression conglomerate .
3 ) Condensation bone-beds : Concentration of vertebrate remains by drastically reduced net sedimentat.ion rate on a shallow sea floor . All bone-beds discussed in the present paper fall into the third ca tegory , as do most of the fully marine bone-beds studies so far in Europe and America. In several of the cases not specifically men tioned above it i s very l.ikely that they can also be interpreted as tempestites like the Muschelkalk/Keuper bone-beds . This is true for tlle Upper Ludlowian/Dowtonian bone-beds of the We�sh borderland which were formed during a regressive cycle and lie a few meters below a deltaic sandstone (see ANTIA, 1 9 7 9 , for references ) . The bone-bed of the Ludlow type - locality at the road intersection south of Ludlow is usually thin. There i s , however, a very rare specimen in the town museum of Ludlow , where the bone-bed is up to
321
5 , 5 em thick (excluding tPe mud cover) and covered by symmetric ripple marks . Another pr.obably example of tempestitic bone-beds are the Middle Devonian bone-beds of the Central U . S . A . ( WELLS , 1 9 4 4 ; CONKIN e t al . , 1 9 7 3 ; CONKIN & CONKIN, 1 9 7 5 ; ANTIA , 1 9 79 ) . .There is
no doubt that the Tempestite Model will prove a major tool in �he sedimentological analysis of condensation bone-beds . Acknowledgements
,To restudy field data , rock samples and thin sections after 1 0 years in the light · o f the new model for the symposium was a great chal lenge . This seems all the more important as several classical exposures , that I could visit then and that were already described by WAGNER ( 1 9 1 3 ) , are no longer accessible today . I thank A . SEILACHER for the stimulus and T . AIGNER, J . GHIOLD , H . HAGDORN and J . TRAMMER for substantial advice and suggestions . The photographic work was carried out by w. WETZEL.
References AE(PLER, R. & REI F , w. -E . ( 1 9 7 1 ) : Origin of bone-beds . - Abstr . , VIII . Internat. Sedimentol . Congr. Heidelberg 1 9 7 1 , p . 1 . AGER, D . V . ( 1 9 7 4 ) : storm deposits in the Jurassic of the Moroccan High Atlas . - Paleogeogr . etc , l�: 83-9 3 . A�GNER, T . ( 1 9 7 7 ) : Schalenpflaster im Unteren Hauptmuschelkalk bei
Crailsheim ( Wlirtt . , Trias , mo 1 ) . - Stratinomie· , 15kologie, Sedi mentologie . - N. Jb . Geo l . Palaont . , Abh . , l��: 1 9 3 -2 1 7 .
AIGNER, T . ( 1 9 7 9 ) : Schill-Tempestite im Oberen Muschelkalk (Trias, SW-Deutschland) . - N. Jb . Geol . Palaont . , Abh . , J�Z, : 3 2 6 - 3 4 3 . AIGNER, T . : Calcareous Tempestite s : s torm-dominated stratification in Upper Muschel�alk limestones (Middle Triassic , SW-Germany) . This volume . AIGNER, T . , HAGDORN, H . & MUNDLOS , R.
( 1 978)
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Biohermal , biostromal
and storm-generated coquinas in' the Upper Muschelkalk. N . Jb . Geol . PaHiont·. Abh . , l�Z : 4 2 - 5 2 . i -
ALLEN, J . R . L .
( 1 9 7 0) : Physical Processes of Sedimentation . - 2 4 8 pp . ,
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322 ANTIA, D . D . J . ( 1 9 79 ) : Bone-beds : A review of their classification, occurrence, genesi s , diagenesi s , geochemistry, palaeoecology , weathering and microbiotas . - Mercian Geologis t ,
Z ( 2 ) : 93-1 7 4 .
AUS T, H . ( 1 9 6 9 ) : Lithologie , Geochernie und Palaontologie des Grenz bereiches Muschelkalk-Keuper in Franken . - Abh . Naturwis s . Ver. Wlirzburg ,
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BACHMANN , G . H .
( 1 9 7 3 } : Die karbonatischen Bestandt�ile des Oberen
Muschelkalkes (Mittlere Trias) in Slidwest-Deutschland und ihre Diagenese . - Arb . Ins t . Geol . PaL3.ont. Univ. Stuttgart, N . F . ,
!\!! = 1 -9 9 . BACHMANN , G . H . & GWINNER , M . P .
( 1 9 7 1 ) : Nordwtirttemberg . - Samml . Geol .
FUhrer . �4 , 1 6 8 pp . , Berlin, Stuttgart ( Gebrlider Borntraeger) . BRENCHLEY, P . J " � NEWALL , G . · & SANISTREET , J . G . ( 1 9 79 ) : A storm
surge origin for sandstone beds in an epicontinental platform
sequence , Ordovician, Norway . - Sedim. Geol . , ·,§,§ : 1 85 -2 1 7 . BRUNNER, H . ( 1 9 7 3 ) : Stratigraphische und sedimentpetrographische Untersuchu�gen am Unteren Keuper ( Lettenkeuper , Trias) im nOrd lichen Baden-Wtirttemberg . - Arb . Inst. Geo l . Palaont. Univ. Stuttgar t , N . F � , BRUNNER, H .
Z2 : 1 -8 5 .
( 1 980) : Zur Stratigraphie des Unteren Keupers (Letten
keuper, Trias) im nordwestlichen Baden-Wtirttemberg . - Jber. Mitt. Oberrhei n . geol . Ver . , N . F . , g,§ : 207-2 1 6 . CONKIN, J . E . & CONKIN , B .M . ( 1 9 7 5 ) : Middle Devonian bone beds and the Columbus-Delaware (Onondagan-Hamiltonian) contact in Cen tral Ohio . - Bul l . Am. Pal . , gz: 9 9 - 1 2 2 . CONKIN , J . E . , CONKIN , B . M . & LIPCHINSKY , Z . L . ( 1 9 73 ) : Middle Devo nian (Hamiltonian) stratigraphy and bone beds on the east · side of the Cincinnati Arch in Kentucky : Part 1 . Clark, Madison and Casey counties . - Univ . Louis'vi lle Studies in Palaeontology and Stratigraphy , § , 44 pp . COOK , P . J . & McELHINNY , M . W .
( 1 9 79 ) : A reevaluation of the spatial
and temporal distribution of sedimentary phosphate deposits in the light of plate tectonics . - Econ. Geol . ,
: 3 1 5 - 3 30 .
ESSIGMANN , J . H . ( 1 9 7 9 .) : · Stratigraphische und sedimentpetrographi sche Untersuchungen am Unter-Keuper im stidlichen Baden-Wtirt temberg . - Arb . Inst. Geol . . Palaont . Univ. Stuttgart , N . F . ,
Zi = 7 1 - 1 3 9 .
323
GEYER, O . F . & GWINNER, M . P . ( 1 9 6 8 ) : Einflihrung in die Geologie von Baden-Wtirttemberg . - 2nd ed . , Schweizerbarti Stuttgart. HAGDORN , H . ( 1 9 7 8 ) : Muschel/Krinoidenbioherme im Oberen Muschel kalk {rna 1 . 1 Anis} von Crailsheim und Schwabisch Hall (Slid westdeutschland) . - N. Jb . Geol . Palaont . , Abh . , l�g : 3 1 -8 6 . KELLING , G . & MULLIN, P . R . ( 1 9 7 5 ) : Graded limestones and lime stonequarzite couplets : possible ancient storm deposits form the Moroccan ·CarboniferoUs . - Sedim. Geol . , 1 6 1 - 1 90 . I
MULLER, A . H . ( 1 9 7 3 ) : Uber Ammonoidea · (Cephalopoda) aus der Grenz dolomitregion des germanischen Unterkeupers . - z . geo�. Wiss . , J : 9 35-9 4 5 .
PARRISH, W . C . (. 1 9 78 ) : Paleoenvironmental analysis of a Lower Per mian Bonebed and adjacent sediments , Wichita County , Texas . Palaeoge·ogr . etc . , �� : 209 - 2 3 7 . PATZELT , W . J . ( 1 9 6 4 ) : Lithologische und palaogeographische Unter suchungen i � unteren Keuper Stidwestdeutschlands . - Erlanger Geol . Abh . , ��� 30 S . REIF, w . -E . ( 1 9 6 9 ) : Bonebeds an der Muschelkalk-Keuper-Grenze in Ostwlirttemberg . - Dip l . Thesis (unpubl . ) 1 2 3- pp . , University of Tlibingen . REIF , W . - E . ( 1 9 7 1 ) : Zur Genese des Muschelkalk-Keupe-r-Grenzbone beds in Stidwestdeutschland . - N . Jb . Geo l . Palaont . , Abh . , J4�: 369 -404 . I
REIF , W . - E . ( 1 9 7 4 ) : Profile des Muschelkalk-Keuper-Grenzbereichs im Jagsttal (Trias ; Baden-Wtirttemberg) . - Oberrhein. geol . Abh . , il,;l : 4 3 -54 . REIF , W . - E . ( 1 9 7 6 ) : Sedimentologie und_ Genese von Bonebeds . - Zbl . Geol . Palaont. Teil II , 1 9 7 6 ( 5 /6 ) : 2 5 2 - 2 5 5 .
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REIF , w . - E . ( 1 9 80) : Tooth enameloid as a taxonomic criterion: 3 . A new primitive shark family from the lower Keuper . - N . Jb . Geol. PalMont . , Abh . , lgQ: 6 1 -7 2 . REINECK , H . -E . ( 1 9 70) : Marine SandkOrper, rezent und fossil . Geol . Rdsch . , gQ : 302-320. SCHltrER, K . A . ( 1 9 7 3 ) : Zur Fazies und PaUiogeographie der Spirife rina-Bank (HauptmUschelkalk) im nOrdlichen Baden-WUrttemberg . N . Jb . Geol . PaHiont � , Abh . , 1 4 3 : 5 6 - 1 1 0 . SCHRODER, B . ( 1 9 6 3 ) : Gliederung und Lagerungsverhaltnisse in der Randfazies der Trias bei Weiden - Parkstein (Opf . ) . - Geol . Bl . NO-Bayern , lJ: 9 8 - 1 4 1 . SCHULZ , 0 . ( 1 9 7 2 ) : Eine Fi�chzahn-Brekzie aus dem Ottnangien (Miozan) OberOsterreichs . - Ann . Naturhist. Mus . Wien, Jg : 4 8 5-490 . SCHWARZ , H . -u . ( 1 970) : Zur Sedimentologie und Fazies des Unteren Muschelkalkes in Stidwestdeutschland und angrenzenden Gebieten . Ph . D . Thesis (unpubl . ) , 2 9 7 pp . , University o f Tlib ingen. SCHWARZ , H . -U . ( 1 97 1 ) : Facies analysis of shallow marine carbonates (Lower Muschelkalk , Middle Triassic) . - I n : German Mtiller (ed . ) , Sedimentology of parts of Central Europe , Kramer , Frankfurt/ Main, p . 1 2 5-1 3 2 . SCHWARZ , H . -u . ( 1 9 7 5 ) : Sedimentary structures and facies analysis of shallow marine carbonates (Lower Muschelkalk , Middle Trias sic , Southwestern Germany ) . - Contrib. Sediment . , � : 1 - 1 00 . SEILACHER , A . : General remarks about event deposits . - This volume. SKUPIN, K . ( 1 970) : Feinstratigraphische und mikrofazielle Unter suchungen im Unteren Hauptmuschelkalk (Trochitenkalk) des Neckar-Jagst-Kocher-Gebietes . - Arb . Inst . Geo l . Palaont. Univ. Stuttgart, N . F . , g�, 1 73 pp. SYKES , J . H . ( 1 9 7 7 ) : British Rhaetic bone-beds . � Mercian Geologis t , g : 1 9 7 - 23 9 . TRAMMER , J . ( 1 9 7 5 ) : Stratigraphy and facies development of the Muschelkalk in the southwestern Holy Cross Mountains . - Acta Geol . Pol . , �� : 1 79-2 1 6 . URLICHS, M . & MUNDLOS , R . ( 1 9 80) : Revision der Ceratiten aus der atavus -Zone (Oberer Muschelkalk, Oberanis) von SW-Deutschland . Stuttgarter Beitr . Naturk . , B , i� : 4 2 S .
325
VOLLRATH , A. ( 1 9 5 5 ) : Zur Stratigraphie des Hauptmuschelkalks in Wlirttemberg . - Jh . geo l . L . -A . Bad. -Wlirtt . , J : 7 9 - 1 6 8 . WAGNER, G . ( 1 9 1 3 ) : Beitrage zur Stratigraphie und Bildungsgeschich te des Oberen Hauptmuschelkalks und der Unteren Lettenkohle in Franken . - 1 80 s . , Fischer-Verlag , Jena. WELLS , J . W . ( 1 9 4 4 ) : Middle Devonian bonebeds of Ohio . - Bull . Geo l . Soc . Amer . , ? 7 3 -302 . WENGER, R. ( 1 9 5 7 ) : Die germanischen Ceratiten . - Palaeontographica , A , J,Q�: 5 7- 1 2 9 . WHI'TAKER , J . H . McD. ( 1 9 74 ) : "Gutter casts " , a new name for scour and fill-structures with examples from the Llandoverian of Rj ngerike and Malm6ya, southern Norway . - Norsk. Geo l . Tidsskr . , :?,� : 403-4 1 7 .
Condensed Griotte Facies and Cephalopod Accumulations in the Upper Devonian of the Eastern Anti-Atlas, Morocco J. WENDT and T. AIGNER
Abstract: The Upper Devonian Griotte in the Tafilalt includes several types of thin variegated cephalopod limestones , ranging from a condensation facies to crinoidal limestones and nodular limestones . Excellent outcrops and the high fossil content faci litates reconstruction of depositional history and palaeogeogra phy . Stratification patterns and facies indicate that condensed Griotte and crinoidal limeStones were deposited in shallow water, temporarily under turbulent condition s , while nodular limestones may have been formed in a slightly deeper neritic environmen·t . While the Upper Devonian of the Tafilalt represents a particu larly shallow shelf , palaeobathymetric values of the same order of magnitude · may be applied to other Palaeozoic Griotte facies and to equivalent Mesozoic facies types : Condensation facies and crinoidal limestones are neritic sediment s , deposited on swells or on large (mostly carbonate) platforms in water depths of a few up to about 200 m. The more marly nodular limestones can ·partly be attributed to the same setting, but may also · occur at greater depths { e . g . reef or platform slopes ) , as indicated by s lumpings , resedimented breccias, turbiditic intercalations and by vertical transitions into bathyal sequences {radiolarites , flysch ) . Variegated cephalopod limestones are a characteristic member of a geodynamic facies succession documenting s low sUbsidence of shallow neritic areas ( generally carbonate platforms 1 rarely sandy shelf deposits or volcanic swells} into the bathyal . In significant lateral c� ange of facies and an extension over se veral thousands of km indicate that the environmental conditions were uniform over large areas . 1 . Introduction
Relatively thin sequences of variegated cephalopod limestone repre sent a characteristic fac·ies 'in many "miogeosyncline11 settings . In the Mesozoic , these l imestones are known from nearly all stages from the Lower Triassic to the Lower Cretaceous 1 although with varying abundance ( e . g . Hallstatt Limestone in the Triass ic , Ammonitico Rosso in the Jurassic ) . In the Palaeozoic, however, su�h facies ar� most widespread from the Upper Devonian to the Lower Carboniferous . � The term "Marbre Griotte" has been used more commonly for these de posits than "Cephalopodenkalk " . The best known examples can be found in France (Montagne Noire , Pyrenees ) , Spain (Pyrenees , Cantabrian Mountains) and Morocco (Anti-Atlas , High Atlas ) . This facies geneCyclic and Event Stratification (ed. by Einsele/Seila.cher) © Springer 1982
327 rally is cons idered to represent a characteristic stage within geo s yncline successions , called 11Periode de vacuitf§" by AUBOUIN ( 1 9 6 7 ) and "Nivellierungsstadium" by KOLLMANN & SCHCJNENBERG ( 1 9 7 5 } . How ever, the exact palaeogeographical position of this facies type is still unclear . 2 . Facies Characteristics o f Variegated Cephalopod Limestones Three main types of Griotte cephalopod limestones can be recognized. They interfinger with each other both laterally and vertically and are characterized by. a large regional extension (often several thou 2 sand km } and by minor or absent terrigenous clastic influx. 2 . 1 . Crinoidal Limestones This · facies does not belong to the variegated cephalopod lime . stones in the stricteSt sense since cephalopods are rare and the typical nodular appearance is not conspicuous . However, it should be discussed within the overall palaeogeographic context as it is closely related to the other two facies . Main characte ristics of this facies are rapid lateral changes in thickness , good sorting, abundant cross-bedding and a low�divers ified fauna (almost exclusively crinoid remains ) . Autochthonous cri noid bioherms are rare and are restricted to small-scale sub marine swells . 2 . 2 . Condensation Facies This facies consists of several em to a few meters of massive variegated , mostly reddish limestones which are often very fos siliferous . Cephalopod shells are still preserved . Omission surfaces, hardgrounds and submarine solution of sediment-filled shells indicate reduced sedimentation often related to reworking events . This facies type is well known from Mesozoic red l ime stones but is less common in the Palaeozoi c . In the Lower Carbo niferous of the Cantabrian Mountains for instance, it seems to be present only as reworked components within the nodular limestone facies . 2 . 3 . Nodular Limestones A relatively thick sequence (up to a few tens of meters ) of + marly , thin bedded , nodular or flaser-bedded limestones {Griotte facies or Ammonitico Rosso in a restricted sense } , interbedded
328
with red pelites and locally with cherts . Cephalopods show strong diagenetic overprin·t and are preserved only as internal moulds (NEUMANN & SCHUMANN 1 9 7 4 ) . 3.
Fauna
At a first glance the faunal spectrum with cephalopods and plancto nic organisms (calcispheres, radiolaria, styliolinids) predominating has a typical "pelagic" aspect. An abundance of benthonic organisms , however , is revealed through microfacies analysis and consists o f : thin-shelled pelecypods , corals (mostly solitary and small ) , Tabula� ta (.small} , brachiopods .(rare) , crinoids , trilobites (mostly ments ) . ' ostracods , conodonts , foraminifera and trace fossils g i t e s � Chondri t e s � etc . ) Nearly all these faunal elements are thonous or parautochthonous and therefore not helpful for logic and · palaeogeographic reconstructions . Hardgrounds within the condensation facies often show benthic organiSms (microscopic bo rings of algae or fungi , sessile foraminifera, crinoids, solitary corals ) . These faunas represent a particular habitat of pelagic hEtril��;;;� grounds (WENDT 1 9 70) . 4 . Griotte-Facies of the Tafilalt (Eastern Anti-Atlas ) 4 . 1 . Basin Evolution The Morrocan Devonian is part of an epicontinental facies belt north of the Sahara craton . Although detailed maps do not exist, the following basiq facies pattern can be re cognized : In the western part of the basin ( Tindouf Basin, Dra area) a sandy-shaly facies extends for about 5000 meters in thickness , while in the eastern part (Tafilalt) one observes a 2 50 m thick sequence of carbonates . After a period · of tectonic stability during the Middle Devonian, local facies differentia tions are recognized from the Upper Givetian to the Upper Devo nian. Early hercynian movements caused differential subsidence together with a swell and basin topography , especially in the eastern part of the area (MASSA et al • . 1 9 6 5 ) . G:i:iotte limestones. deposited in shallow waters are replaced by cri"l1oidal limestones :. and finally by flysch towards the basin. By the end of the De vonian a general transition to clays and deltaic sandstones is observed (MICHARD 1 9 7 6 ) .
329
UPPER
DEVONIAN , TA F I L A LT
Fig . 1 . Stratigraphic summary log (schematic) of the Upper Devonian in the Tafilalt, eastern Anti-Atlas , Morocco 4' . 2 . Stratigraphy of the Upper Devonian (Fig. 1 )
Givetian argillaceous and dolomiti c , often nodular limestones and marls are overlain by grey to reddish marls and limestones of the Lower Fra'snian, locally marked by an unconformity . After another hiatus , which is locally developed as an angular unconformity , Upper Frasnian black limestones follow with a rich fauna of Goniatites (Mant·ieooer>as and others) and orthoceres , the latter frequently being current..:.transported to form so-called 11battle fields " . After a period .of emersion, a typical Griotte is wide spread in the Lower Famennian, beginning with the condensation facies . It is characterized by massive, thick-bedded red limesto nes with abundant well preserved cepha lo'pod shells ( Ch e i loeer>as and orthoceres predominating) It is this facies that is quarried locally and known as the "Pier;re d 1 Erfoud " . The limestones con tinuously decrease in thickness towards the top while marl con tent and nodularity increase (Griotte facies in a restricted sense) and eventually grade into the clay and sandstone sequence ·of the uppermost Devonian/Lower Carboniferous . In the South o f the Tafilalt, nodular Griotte limestones become progressively re•
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.· I • ·
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II;.
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placed by crinoidal limestones which only occur as thin in the North . 4.3.
Depositional Environment The following criteria indicate a shallow water origin for the Upper Devonian sediments in the Tafilalt : 1 . Rapid changes in thickness and facies , in some levels wtth erosion and hiatusses . The disconformity at the base the Famennian II beta is most prominent and may locally cut down to the Upper Givetian (HOLLARD 1 9 6 7 , 1 9 7 4 ) . 2 . Birdseye- and teepee-structures in the Upper Frasnian. 3.
Carstification of the Upper Frasnian, following emersion; lution cavities are filled with red calci lUtites of the over lyin�? Griotte.
4.
Orientation of orthoceres indicates strong current events ring the Lower Famennian. In the South and East of the the current uniformly flows in a southeastern direction the opposite direction predominates in the North and West (Fig . 2 ) . Although a marked current orientation of orthoceres is also observed in the Upper Frasnian black limestones , in sufficient data make it difficult to predicate the exact current direction.
5 . Goniatites show reworking, imbrication and accumulation pat
terns caused by current events ( storms?) within the condensed levels of the Upper Frasnian and Lower Famennian. Reworked early diagenetic cements occur in the Upper Frasnian black limestones . 6 . Local occurrence of s·tromatoli tes in the Upper Famennian
Griotte. 7 . Large-scale trough cross-bedding in Famennian crinoidal lime stones . In conclusion, the lower part of the Upper Devonian o.f the Tafi lalt represents a shallow shelf environment whic � was temporarily and locally emergent . Synsedimentary tectonics has created a gentle swell-and-basin topography . Crinoids florished on swells , and their remains have been accumulated to form several metre thick crinoidal limestones , particularly in the southern area.
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In the · upper part o'f the Upper Devonian, red nodular limes tones and marls indicate more stable and uniform conditions . Renewed uplift caused these deposits to grade into the deltaic sequence of the uppermost Devonian/Lower Carboniferous . Acknowledgements We would like to thank Dr. M . BENSAID, Dr . A . HILALI , Dr . G . SUTER (all Geological St;Lrvey of Morocco, Rabat) for generous support of our field work and helpful discussions . Financial support of the field work by the SFB 53 is gratefully acknowledged . References AUBOUIN, J . ( 1 9 67 ) : Quelques probl8mes de sedimentation g8osyncli nale dans les chaines de la M8diterranee moyenne . - Geo l . Rdsch . , :ill = 1 9 �68 . HOLLARD , H . ( 1 9 67 ) : Le D8vonien du Maroc et du Sahara nord-occiden tal . - Int . Symp . Devonian System, � : 203- 2 4 4 . ( 1 9 7 4 ) : Recherches sur la stratigraphie" des formations du D8v0nien moyen , de l ' Emsien sup8rieur au Frasnien, dans le Sud du Tafilalt et dans le Ma ' der (Anti-Atlas oriental) . . - Notes M8m. Serv. g8o l . Maroc, �g4: 7-68 . KULLMANN , J . & SCH6NENBERG , R . ( 1 9 75 ) : Geodynamische und paUi.oOkolo gische Entwicklung im Kantabrischen Variszikum (Nordspanien) Ein interdisziplinares Arbeitskonzept . - N . Jb . Geol . Palaont .Mh . , HZ:i = 1 s 1 - 1 6 6 . MASSA, D . , COMBAZ , A . & MANDERSCHEID, G . ( 1 9 6 5 ) : Observations sur les s8ries siluro-devoniennes des confins alg8ro-marocains du Sud ,a , 1 87 S . , · g ( 1 9 5 4 - 1 9 5 5 ) . - Notes M8m. Comp. Franc . P8troles , Taf . , Pari s . MICHARD , A . ( 1 9 7 6 ) : Elements de g8ologie marocaine . - Notes M8m. Serv. g8ol . Maroc , .£�� , 408 S . , 6 Taf . , Rabat. •
NEUMANN , N . & SCHUMANN , D . ( 1 9 74') : Zur Fcissilerhaltung, besonders der Gonlatiten , in roten Knollenkalken vern "Ammonitico Rosso" -Typ . N . Jb . Geol . Palaont . Mh . , J�Z4: 2 9 4- 3 1 4 . WENDT, J . ( 1 9 70) : Stratigraphische Kondensation in triadischen und j_urassischen Cephalopodenkalken der Tethy s . - N . Jb . Geol . Pali:lont. Mh . , l2;bg: 4 3 3 - 4 4 8 .
Distinctive Features of Sandy Tempestites A. SEILACHER
Abstract: Sedimentological and biological features allow to distinguish wave-generated storm sands from current generated sandy turbidites and flood deposits . The origin of some of these features remains still uncertain . In many respects , . storm sands ( GOLDRING & BRIDGES 1 9 7 3 , BRENCHLEY et · al . , 1 9 7 9 ) form a link between shelly storm deposits and the sandy deposits caused by current events such as floods ( TUNBRIDGE · 1981) and turbidity currents . The characteristics of shelly storm beds in the carbonate facies have been discussed in the preceding contribution by T . AIGNER. They also apply to the sandy facies , but here the shell remains have commonly been wiped out by early diagenetic solution . On the other hand, sandy event deposits have a higher fossilisation potential for a variety of physical and biogenic structures . Therefore the distinctive features of storm sands will here be discussed in comparison · to the well known cha racteristics of sandy turbidites and flood deposits (Fig. 1 ) . 1 . Physica l Sedimentary Struc.tures
The grading of grain sizes and of erosional and depositional sedi mentary struct�res ( BOUMA 1 9 6 2 ) within one unit is commonly consi dered as a distinctive feature of turbidites . In principle , however it should be expected in all event deposits . The same is true for ·structures related to the dewatering of quickly deposited sands . Differences are subordinate and related to a) the nature of the turbulence , in that storm deposits are wave dominated , while · flood deposits and turbidites are current dominated . b) the grain size distribution of the available materi al , which depends largely on the processes acting during the background pe-riods . These effect a pre-sorting of the sediment in fluvial shallow marine environments (mud versus sand grade ) . Turbidity currents, in contrast, carry material , in which pre-sorting inherited from shallower depositories has become largely wiped out during the long transport and pick-up between the source cyclic and Event Stratification ( e d . by Einsele/Seilacher) © Springer 1982
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SANDY RARE-EVENT DEPOSITS storms
INUNDITES
TEMPESTITES
\_
ICHNOFACIES:
Fig. 1 .
SCOYENIA
rip channel deposits
CRUZIANA
TURBIDITES
NEREITES
Although being similar in their graded character (sand/ clay couplets ) sandy · flood, storm and turbidity current deposits may be distinguished by sedimentological and · biological. features r
and the depositional area· , in which the sediment is l�ttle affected during the long illtervals between the events except by bioturbation. On this background we shall now discuss. the potentially distinc tive sedimentary structures . 1 . 1 . Sole Marks 1 . 1 . 1 . Washed-out mud burrows are very common on turbidite sole
faces . The small and d8licate tunnel systems of ' Pa teodiotyon and other " graphoglyptids" (SEILACHER 1 9 7 7 ) occur only in this preservation. Nevertheless their contours are usually very sharp and hardly fluted . This reflects the ''shock ero sion" in front of a fastly approaching density current , which was immediately followed by sand sedi�entation in distal areas .
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In shalloW waters , event erosion tends to be less sudden and more tractional from the beginning. Therefore washed-out burrows are less perfectly preserved on the soles of storm and flood beds. Only the large crustacean burrows (ThaZassi noides� Fig. 2a) , which represent a deeper bioturbation level in more compacted mud , can still be recognized as secondary casts. 1 . 1 . 2 . Tool ma'rks are found in all sandy event deposits , but they alrrer .ln directionality . While they are well aligned and unidirection�l in the current-dominated turbidites and flood sands, their vector commonly varies or points in opposed directionS in storm deposits (see contributions by GRAY &
,'
BENTON and by BLOOS ) . 1 . 1 . 3, Flute casts , common on the soles of turbidites and flood sands , are lacking in storm sands . These, in turn , are cha racterized by pot and gutter casts (AIGNER & FUTTERER, 1 9 7 8 ) No such markings have been observed on modern mud bottoms , which suggests that they -� similar to most tool marks - form under a sand-saturated flow and become immediately cast by coarser sediment. 'rt is worth mentioning in this context , that the impact marks on· the walls of gutter casts have been found to point in opposite direCtions (BLOOS 1 9 7 6 and own observations ) .
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1 . 1 . 4 . Frondescent cast� represent the scars of larger flakes (Fig . 2b ) that were- ripped off from well consolidated mud s . Pre-existing burrows facilicate this process and therefore commonly form the axes of the leaf-like structures . In turbidites , frondes cent casts are usually we l l aligned. In storm sands , however , they point in different directions (Fig . 2 c ) , reflecting wave rather than current activity. In general , the four groups of structures represent, in this se quence , increasing levels of turbulence and- erosion during the erosional phase of the event . 1 . 2 . Internal Structures The vertical transition from higher to lower flow regimes ( i . e . from even lamination into ripple lamination , as described by the "BOUMA sequence11) also reflects the difference between current and wave-dominated event deposits . Although there may be some
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. lateral component in the ripple-laminated top · part-o·f-Sto�·�· ·� ��d� , 1
we rarely find the climbing current ripples , nor the rip-and furrow structures that are so common in turbidites and flood de posits (BOUMA � 9 6 2 , WURSTER 1 9 6 4 ) . Hummocky cross stratification (H�LIN et al, 1 9 79.; BLOOS 1 9 7 6 and contribution in this volume.) , on the other hand , is common in storm sands and seems to be an other expression of w �ve action at a higher level of turbulence . Convolute lamination ( see Muschelkalk contribution by AIGNEE, Fig. 2 ) , probably related to thixotropic dewatering of the sa turated· sand during sedimentation, does also occur in storm sands , . though less commonly than in turbidites and less continuous la terally . 1 . 3 . Top Surface Features The tops of storm sands , being usually very sharp , present the most distinctive phenomena. 1 . 3 . 1 . Osci l lation ripples are very common in the form of parallel crests or ot reticulate interference ripples ( " tadpole nests " ) . Corresponding ripple laminae may be underlying themi but com monly the crests still contain unreworked even laminae from a previous , higher flow regime (Fig . l ) . This indicates {a) that the ripples did fdrm after sand sedimentation had stopped and (b) that they were not active long enough to completely rework the uppermost sand layer as should be expected in a product of fair weather reworking (KUMAR & SANDERS , 1978 ) . Even more telling are the spill-over ripple crests of linear ripples . They · resemble the flat-topped ripples that form in the tidal zone under a thin film of receding water ; but the
�ottom features of sandy tempestites a) Washed-out ThaZassinoide s burrows ; L . Lias , Helmstedt. Note rounded contours with e·rosional bedding plane . b) Frondescent casts experimentally produced by flushing holes pierced into mud surface . c ) Fronde_scent casts; L . Lias , Helmstedt . Flake-off started from burrows that now form the axes of leaf-l.ike struc tures . Divergent directions disagree with origin by unidirectional current . d) Concentric injections . L . Lias , Hlittlingen. Circular sand dykes into underlying clay slope in tepee fashion towards the center ( see also Fig. 4d)
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tempestite examples commonly show a distinc�ive apron on one or both flanks , indicating that the same wave action went on after the trough's had already been� . filled by ·the subsequent
mud sedimentation (Fig . 3 )
•
1 . 3 . 2 . uKinneya11 • These strange , small scale sedimentary structures have been variously interpre-ted as rain drop imprints ( 11Re
gen.tropfenplatte11 QUENSTEDT 1 8 5 8 ) or as minute ripple marks . Both interpretations are in conflict with the almost vertical slopes , that are commonly observed at the flanks of the small pits , too steep to persist at - the sand/water interface. Kinneya structures may occur either on laminae within the sand or on top surfaces . In the latter case they are commonly asso ciated with oscillation ripples . They are then usually restric ted to the flattened ripple tops and contour the · margins of the crests (Fig . 4a ) . This fact and the oversteepened profiles suggest that Kinneya has formed later than the ripples and after the surface had become mud-covered. Another strange feature has been observed by BLOCS ( 1 97 6 ) . He found an undeformed lamina of x-ray resistant magnetite at the base of Kinneya structures . The origin of this structure i s stili problematic , but the observations suggest an intrasedimentary o�igin , posstbly re lated to differential de-watering and settling during the
SPILL-OVER
Fig . 3 . Assumed origin of spi 11-over osci llation ripples . Crests became flattened by continuing wave activity after mud had started to settle in ripple troughs . Spill-over aprons were later tilted down by mud compaction. For examples see Figs . 4 b , Sb , Sd
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event-sedimentation or immediately afterwards . This would associate Kinneya with load casts and convolute lamination rather than with ripple marks . But it remains to be explained why this structure has never been found in turbidites.
1 3 . 3 . 11Aris tophycus" . .These delicate dendroid structures are always .
found as positive relieves on sandstone tops . Like ftondescent casts , they commonly radiate from burrows of various affilia tion (Fig. 4b ) , but they may also radiate from ripple crests (Fig. 4 c } . If adequately preserved, they show a faint marginal depres sion ( i l lustrations in HliNTZSCHEL & REINECK 1 9 6 8 ) . � A biological origin (Th. FUCHS , 1 89 5 ; A . H . MULLE R , 1 9 5 5 ) i s ruled out by the variable shape , the long time range { Cambrian to Tertiary) and. the. low facies specifity (marine and non-marine) of this structure . The sharp and steep contoUrs clearly indi cate that Aristophycus is formed not at the surface, but wit hin the sediment . Nor did these structures form immediately after the deposition, because burrows from which they radiate or which they deform (Fig. 4c ) belong to the post-event gene ration . Without adequate experiments we can only speculate about the . true nature of Aristophycus . Most probably it originated when pore water was pressed out of the sand, preferably through burr·ow cavities , and on ripple crests . This water may carve minute distributary channels into the overlying ·lJIUd cover as it seeps out along the interface . The marginal depression · would then correspond to the mud washed-out from the groove lets .
1 . 3 . 4 . Concentric Injections . Another enigmatic structure is commonly found on lower and upper surfaces of Jurassic flagstones. It consists of concentric ridges of sand , filling corresponding cracks in the under- or overlying mud layer. Sometimes , but not in all cases , there is a vertical burrow in the center. If not broken away , the minute sand dykes are inclined away from the center . There is again no doubt that we deal with a phy sical structure that formed within the sediment and be low the zone o f Gyroahorte , which it Cuts across . It probably has to do with shrinkage of the mud by syneresis , in which burrows may have acted as centers �Figs . 2d and 4d) .
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None of the structures described in this paragraph has to my knowledge been found on the tops of sandy turbidite or - f lood de posits . This can be partly explained by the more uniform grain size distribution in turbidite flows, which makes sedimentation go on without interruption during the waning of the event. In contrast, most storm sediments are strongly pre-sorted, so that an interval of non-sedimentation separates _the deposition of storm sands and storm muds . This interval i s reflected by a well . defined sand/mud interface , which seems to be required for the ' described structures to form. It is also reflected in· some of the.. . epichnial trace fossils described in the next paragraph . 2 . Biogenic Sedimentary Structures Trace fossils found on sole and top surfaces of sandstone beds are most unlikely to have formed at the sediment/water interface , but not for the same reason as the inorganic structures . At the soles pre-existing surface trails will be destroyed by the erosion . of unconsolidated surface mud preceding the deposition of most sand layers . A sandy top surface , on the other hand , is too mobile surface trails for a longer time and with the deto preserve tails necessary for determination .. In addition , turbulence events do not last long enough �or truely infaunal burrowers to perform their ·norma l , trace forming activities during the event sedimen tation . Therefore it can be assumed, that the bulk of the trace fossils found on the soles, as well as on the tops , of sandy event deposits represents burrowing after the event was over . Combining these assumptions With the frict that each species in a community of endobenthic animals penetrates only to a certain
Fig. 4 . Inorganic sedimentary structures on tops of tempestites. a ) Kinneya on f lattened tops of interference ripples . Beduh Shales (Lower Triassic ) , Sinat ( N . Iraq ) . In this example Kinneya structures contour ripples and are thus younger . b) Aristophycus structure ( 11 figure de viscosite 11 , T h . FUCHS 1 8 9 5 , pl . 9 , Fig . 4 ; Dogger) starting from burrow and following ripple crest. Note marginal depression on right corner . c ) Aristophycus following ripple crest and crossing intra sedimentary burrow (Gyroahor t e ) . L . Lias , Htittlingen. d) Concentric injections arround vertical burrow cross Gyroohorte in lower part of picture . L . Lias , Helmstedt
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depth below the sediment surface , one can possibly use the trace fossil inventory as a gauge for the depth to which that particular surface had become buried by the event sedimentation. In - sequences of sandy turbidites it has been shown that post-tur biditic burrows reach the soles only of thinner beds and that dif ferent species disappear in thicken beds in the order of their pe netration depths (SEILACHER 1 9 6 2 ) Such studies have not yet been made in tempestites . Nor do we know the relative penetration le vels of dif�erent ichnospecies in shallow marine sands ; but in general it may be assumed that larger animals can burrow deeper than smaller ones . For instance the large crustacean burrows (Tha lassinoides� Fig . Sa) represent a deeper bioturbation level than the burrows of worms and irregular echinoids (Sao l i a i a ) . •
In a similar way , the burrows preserved on the top of a sandy event deposit ref·lect the thickness _of the mud layer that was subsE;1quently deposited by the same event . Limulid undertracks , for instance ( GOLo- · RING & SEILACHER, 1 9 7 1 , Fig . 9 ) are too sharp to have formed on a free sand surface and were under a mud blanket no more than a few millimeters thick . This thickness was exceeded in the ripple troughs . Simi larly, the pelecypod burrows in Fig . Sc are largely restricted to the ripple crests, indicating a slightly thicker mud cover . In contrast , the crustacean burrow in Fig . Sa suggests se veral decimeters of top mud . A s teri a a i t e s
More telling with respect to the sedimentational history of the event itself are the resting burrows of starfishes (Fig . Sb} .
Fig. 5 . �urrows on tops of tempestite s a) Thalassinoides and other burrows c.utting from overlying mud into smooth. ripple crest , on the right-hand s lopes of which earlier ripple laminae are visible. L . Lias , Helmstedt. b) Epichnial· resting burrows of ophiurans ( A s teriaai tes Zumbriaa l i s ) are concentrated in ripple trough s . Their st·eep s lopes indicate that the animals left- only after , mud sedimentation had started . L . Lias , Helmstedt. c ) P�lecypod burrows { Pe l eaypodiahnus) indicate mud cover of only a few centimeters , through which animals reached mainly the higher crests of the rippled sand surface . U . Jurassic , Boulogne s . M . d . Unnamed trace o f infaunal sediment feeder that followed sand mud interface along ripple troughs and crests . U . Jurassic , Boulogne s . M .
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Their stretched arms show that they represent shallow burrowing - enough for hiding the animal in the sand . SuCh resting tracks are found as postdepositional casts on the soles of finger-thick sand layers , but they alSo occur as epichnial grooves on the os�•••a<>C>n� rippled tops of thicker beds . In the latter c�se the star-shaped bUr,... row is lined by a ridge of dug-out sand , whose outer s lope i s much toq steep to have survived on a sand/water interface . The same is true for the imprints of the ambulacral feet that lead away from the burrow (SEILACHER, 1 9 5 3 , P l . 10, Fig . 2 ) . The asteropectinid and ophiuran starfishes responsible for these burrows are mainly sand dwellers and might have been brought-in from adj acent sand flats by the storm itself . The bodies of dead animals became buried more or less intact at the base of the storm· ·. sand . Live individuals ,that managed to remain at the surface until sand sedimentation had stopped, came to rest in the troughs of the now . forming oscillation ripples and dug themselVes in. When , after·. an intermission of several hours , the water had calmed down enough for the clay fractiOn to settle , the animals left their· hiding pla·:... ces . By this time a thin mud cover already protected the resting trace and allowed the preservation of sharp undertracks in some cases . In other cases mud sedimentation happened so rapidly, that the echinoderms , having a mud-sensitive arnbulacral system, became smothered . Their carcasses are then found as a conservation depo sit at the sand/mud interface { ROSENKRANZ , 1 9 7 l ) . Thus the preser vation of starfishes and their traces agree with a tempestite origin. Cruziana
The trilobite burrows (Cruziana ) , which commonly occur in sand/day _ sequences of Lower Paleozoic age, do nqt agree as well with the tempestite model . Trilobi.tes lived largely on sandy and silty bot toms , from which they gained their food by digging the sediment up' . with their endopodites. The self-made suspension was then strained by the feather-like epipodites within the filter. chamber provided by the dorsal Shield. This activity i s indicated by the orientation of the terrace lines on the ventral doublures ( SCHMALFUSS 1 9 7 8 ) and b y the fact that the legs dug towards the median Iine rather than moving the sediment laterally away from under the body (SEILA CHER 1970) . While the trilobite origin o f most Cruziana species i s beyond doubt , we agree with WHITTINGTON ( 1 980) that the soft trilobite legs would
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Fig. 6 . a) Cast of large trilobite burrow ( Cruziana pedroana ) . San Pedro Sandstone ( U . S il . ) ; Bonar ( N . Spain ) ; leg . W . FUhrer; Tlibingen collection . b) Details of s cratch marks are perfectly preserved, while impact casts on adj acent bedding plane record strong erosion preceding sand sedimentation . c ) Nevertheless the ripple marks on · top of the slab, which is only 3-4 em thi ck, are undisturbed above the burrow. This indicates that this sheet sand was not deposited as the lower part of a tempes tite couplet
have had difficulties to dig in semi-consolidated mud (GOLDRING, this volume ) , in which also the feeding from a self-made suspension might have been impossible or at least too costly. Tri lobite burrows are almost exclusively found as hypichnial casts and always with well preserved scratches . Therefore I assume , in
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contrast to GOLDRING (this volume) , that they were produced inter nally under a sand cover, rather than at the mud/water interface. On the other hand , trilobites almost certainly dug close to the sediment surface , so that we would expect to find their burrow casts only on the so-les of thinner tempestite sands . We would e�pect the interface between the tempestite sand and its mud cover to be disrupted above the hypichnial cast. Neither is case . Even above very large trilobite burrows , whose width far exceeds the thickness of the sand bed, the rippled surface remains completely undisturbed (Fig. 6 c) . To account for this discrepancy , we must assume that the trilobite'S dug only into sandy surfaces and that the beds in which we find them were not typical , · mud-topped storm sands . This example may also warn us not to apply the tempestite ·model exclusively -to all sand/clay alternations . 3 . Rip Channel Deposits So far , w-e have considered storm sands as sheet-like sediment bodieS dominated by wave action. In well exposed sequences , - these sonsi stent and relatively, thin bedS are commonly associated with channel: fills a few meters in thickness and several tens of meters wide in cross sect-ton s . Their size and the lack of characteristic longitudi'":"': nal cross bedding (REINECK & SINGH, 1 9 BO , Fig . 1 6 7 ) sets them apar.t from tidal channel s , which would also be incompatible with the subti dal environment indicated by the storm sands on their bank s . I n an Upper Ordovician sequence o f Sou_thern Jordan , cross bedding and other current features show that these channels drained in a northward direction, i . e . from the Arabian shield intO the basin (Fig. 7 ) � Several types of Cruziana found in these channels are consistently oriented against this current . Since Cruziana is most likely the product of trilobites , it can be concluded that the cur rent carried not fresh but normal marine water . To account for these observations , I conclude that we deal with rip channels ( COOK 1 9 70) , in which the undertow , compensating for wind-generated coastal water bui ld-up , flowed back to the sea . Such channels may extend well into the subtidal zone, where they could be active most of the time . But their initial erosion and final fill-up would likely be the work of the rare energy peaks during severe storms . This question as well as the relationships of
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Fig. ? .
Channel - sands in the Sabellarifex Sandstone { U . Ordovician) of the Qaa Disa area ( S . Jordan) diained off the AraPian shield in northerly direction s . Nevertheless they are not fluvial in origin, because they laterally grade into tempe stites and contain marine trace fossils . They are therefore interpreted as rip channels . Such channels may also control the present topographic relief (map in background from BENDER 1 9 6 3 )
the larger channels to the smaller scours discussed b y GOLDRING & AIGNER in the following contribution would deserve to be studied in more detail . References AIGNER, T . & FUTTERER , E . 1 9 7 8 : Kol�-TOpfe und -Rinnen (pot and gutter casts) im Muschelkalk - Anzeiger fUr Wattenmeer? N . Jb . Geol . Palaont . , Abh . , 1 5 6 : 285-304 . ===
BENDER , F . 1 9 6 3 : Stratigraphie der "Nubischen Sandsteine" in SUd j ordanien. - Geo l . Jb . , �� : 2 3 7 .
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BLOOS, G . 1 9 7 6 : Untersuchungen tiber Bau und Entstehung der feinkOr nigen Sandsteine des Schwarzen Jura alpha (Hettangium und tief stes Sinernurium} im schwabischen Sedimentationsbereich . Arb . Inst . Geol . Pal . Univ . Stuttgart, N . F . Z� · BOUMA , A . H . 1 9 6 2 : Sedimentology of some flysch deposits : A graphic approach to facies interpretation . - Elsevier (Amsterdam) . BRENCHLEY, P . J . ; NEWALL , G . & SANTSTREET , J . G . 1 9 7 9 : A storm surge origin for sandstone beds in an epicontinental platform sequen c e , Ordovician , Norway . - Sedim.Geol . 1 �g : 1 85 - 2 1 7 . COOK , D . O . 1 9 7 0 : The occurrence and geologic work of rip currents off Southern California. - Mar . Geol ., � : 1 7 3 - 1 8 6 . FUCHS, Th . 1 8 9 5 : Studien tiber Fucoiden und Hieroglyphen . Denkschr . math .nat .Kl . Akad . Wiss . , Wien, g� : 3 6 9 -4 4 8 . GOLDRING, R . & SEILACHER , A . 1 9 7 1 : Limulid undertracks and their sedimentological implications . - N . Jb . Geol . Palaont . , Abh .1 ,!,�£ : 422-44 2 . GOLDRING, R . & BRIDGES , P . 1 9 7 3 : Sublittoral sheet sandstones . J . sed .Petr .1 ,:!,� : 7 36 - 7 4 7 . HAMBLI N , A . P . , DUKE , W . L . & WALKER , R . G . 1 9 7 9 : Hummocky cross stra tification - indicator of storm-dominated shallow marine environments . - Am. As s . Petrol . Geol . , Bul l . �J : 460-4 6 1 . HKNTZSCHEL, W. & REINECK , H . E . 1 9 6 8 : Faziesuntersuchungen im Hettan 'gium von Helmstedt (Niedersachsen ) . - Mitt . geol . Staatsinst. Hamburg1 �Z : 5- 3 9 . KUMAR, N . & SANDERS , J . E . 1 9 7 8 : Storm deposits . - In : FAIRBRIDGE , R . W . & BOURGEOIS ( ed . ) : The encyclopedia of sedimentology . Encyclop. Ea:r:.th Sci . Ser . , g : 767-770. MULLER , A . H . 1 9 5 5 : tiber die Lebensspur Isopodiahnus aus dem Oberen Buntsandstein (Unt . R6t) von GOschwitz bei Je,na und Abdrlicke ihres rnutmaBlichen Erzeugers. - Geologie . � : 4 8 1-489 . REINECK , H . E . & SINGH , I . B . 1 9 8 0 : Depositional sedimentary environ ments ( 2nd . ed . ) . - Springer Ver l . Berlin, Heide �berg, New York . ROSENKRAN Z , D . 1 9 7 1 : Zur Sedimentologie und �kologie von Echinoder men-Lagerst�tten . - N . Jb . Geol . Palaont. , Abh . , l�� : 56 - 1 00 . SCHMALFUSS, H . 1 9 7 8 : Constructional morphology o f cuticular terraces in trilobite s , with conclusions on synecological evolution . : 1 64- 1 6 8 . N . Jb . Geol . Palaont . , Abh . ,
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SEILACHER, A. 1 9 5 3 : Studien zur Palichnologie . I I . Die fossilen Ruhespuren (Cubichnia) . - N . Jb . Geol . Pal&ont . , Abh . , �� : 87-1 2 4 . SEILACHER, A . 1 9 6 2 : Paleontological studies on turbidite sedimenta tion and erosion . � J . Geol . , ZQ : 2 2 7 - 2 3 4 . SEILACHER, A . 1 9 70 : Cruziana stratigraphy of 11non-fossiliferous" Palaeozoic sandstones . - I n : CRIMES & HARPER (eds . ) Trace fossils ; Geo l . J . Spec . Issue No . 3 : 4 47-4 7 6 . SE�LACHER, A . 1 9 7 7 : Pattern analysis o f Paleodictyon and related trace fossils . - I n : CRIMES & HARPER (eds . } Trace fossils 2 . Geol Spec . Issue N o . 9 : 2 8 9 - 3 3 4 . TUNBRIDGE, I . P . 1 9 8 1 : Sandy high-energy flood sedimentation - some criteria for recognition , with an example from the Devonian 79 -9 5 . of S . W. England . - Sedim. Geol . , �� WHITTINGTON , H . 1 9 8 0 : Exoskeleton , moult stage , appendage morpholo gy , and habits of the Middle Cambrian trilobite O�enoides serratus . - Pa1aeontology, �4 : 171 - 204 . WURSTER , P . 1 9 6 4' : Geologie des Schilfsandsteins . - Mitt. geol . Staatsinst . Hamburg , �� ·
Multidirectional Palaeocurrents as Indicators of Shelf Storm Beds D. I. GRAY and M.J. BENTON
Abstract Distal parts of shelf storm sequences , below the zone of cross stratification, may differ little from turbidites . nal paleocurrent indicators are described from the Lower y ar e�:r�;;���;: Hughley Shales o f the Welsh Borders of England, and as an important criterion for the recognition of storm in���e currents .
�
1 . Introduction Storm events in proximal shelf environments have been well in recent l. iterature. They have been recognised primarily by cross · s tratification in sediments interpreted as deposited or modi fied from suspension fallout in lowered ( storm) wave base conditions. Storm events on the distal she l f , however , where the depth of storm wave base approaches water depth , are less readily recognised. It is in this zone· that bottom currents induced by storm surge ebb events prevail , modifying suspension fall-out, and ultimately continue as density currents into most distal shelf environments . In this paper, we present evidence for storm events on the distal she l f . We studied the Lower Si lurian Hughley Shales (Upper Llandovery, c 5 Substage , g�iestoniensis Zone) which outcrop in the Welsh Borders region of England (Fig . 1 a ) . We examined various localities , and collected extensively and logged sections (Fig . 1 b ) at localities A (Devil ' s Dingle temporary dam site) and B (Hughley stream) . The sequence consists of mudstones with thin interbedded sandst'one units that present many typical turbidite features . However, cer tain features of the sandstones sugges·t storm effects . 2 . Sedimentology
Maroon to grey-green uniform or finely �aminated mudstones form more than 7 5 % of.. the measured section (Fig. 1 b ) . They contain a diverse benthic fauna , dominated by brachiopods and corals . The interbedded sandstones vary from 1 to 20 em in thickness , but are laterally persistent sheets that pinch and swell gently . Each sheet has the basal surface tool-marked by indigenous , unabraded Cyclic and Event Strati fication (ed. by Einsele/Seilacher) © Springer 1982
351
KEY
0 coral colony (!». solitary Cdml '-'&' brochiopods o;a' gdStltlp<.>d , � Plonolites
g�1i) �
b
I" Sheinton .
Hughley Shales
a
c
� · �-.+��·,_._.. ;,;,;.::...,:;,;,:·� . ··· . . . ·;'t'"•"· ����1�. The Hughley Shales , Upper Llandovery , Welsh Borders .
a . Map o f outcrop showing sample localities A and B ; b . sedimentary log showing position of sandstone units , fossils and .trace foss ils ; c . cross section through single sandstone unit showing coq�ina, trace fossils , lamination , and ripple lamination
352
fossils , and a bioturbated top (.Pa laeophyaus� Chondr i t e s ., Diplocra terion., trilobite trace s ) . These sheets may contain laterally 1m persistent basal or centrally plc-.ced coquinas and coarse sand lay ers·, but the remainder of the bed is 20- 1 00 urn laminated sands and silts . Internal tool-marked surfaces may also occur. The sand sheets are dominated by millimeter-scale parallel and ripple lamination . Ripple lamination is most common in the middle and basal parts of the beds , as poorly developed types 1 , 2 , and in-phase climbing ripples . Many beds contain s cour and fill cross laminated sets, and flaser silt drapes may accentuate this cross lamination. There are no unequivocally wave-produced sedimentary structures . 3 . Palaeocurrents Two oriented slabs , each about 1 m square , were collected from loca lities A and B (Fig. 1 a ) . Palaeocurrent measurements were made from unidirectional biogenic tool marks (prod �nd groove marks ) on each slab, and plotted in 20° segments on a rose diagram . The deepest part of the tool mark was assumed to point downstream (Fig. 2 ) . The results are rather different from what would be expected in a typical palaeoslope turbidite model . Slab A , containing a confused mass of cross-cutting tool marks , has no dominant current direction . Slab B has a less confused array of tool marks (Fig. 2 ) , and shows two major directions - one directed SSW ( from 020° ) , and the other running WSW ( from 070° ) . Neither direction can be said to have occurred first over the whole slab. 4 . Environment of Deposition Current paleogeographic models place the upper�ost Llandovery coastline up to 1 50 km to the eas t , and the shelf margin roughly 40 krn west of the study area. Although the sandstone units resemble turbidite s , evidence for deposition induced by shelf storm events includes : 1 . The shelf setting with locally derived benthic organisms . 2 . The wide directional spread of the tool marks . 3 . The p_resence of internal coquinas and coarse layers , both above a�d on the bed bas e , that attest to storm surge currents . 4 . Well sorted nature of the sands and silts , reflecting suspension load in the overlying �ater column . 5 . Abrupt transition to background mud deposition on bed tops .
353
A
n = 75
B
n = 120
Fig. 2 . Photograph of part of tool-marked base of sandstone unit from sample locality B ; rose diagrams for palaeocurrents based on unidirectional tool marks for localities A and B
Scour and Fill : The Significance of Event Separation R. GOLDRING and T. AIGNER
Abstract: Scour structures where cut and fill events were separated by a significant time interval .are described. Palaeontologically this interval is important: (a) in allowing faunal colonisation of scoured surfaces ( communities) and (b) in enhancing preservation potential ( fossil traps ) Both sand filled scours , and the lesser known mud-filled ones cut into sands , are particularly typical of shelf facies . •
1.
Introduction
Scour structures with a fill coarser than the host sediment are of many types a.nd may be found in many fac � es . It is generally assumed , and may be demonstrated , that little or no time ·elapsed between the cutting and filling of the structures . But scour structures , often similar in form to sand-filled scours , may have a muddy or banded mud and sand fi l l . Some mud-fills may have formed immediately after the cutting event; but where colonisation of the scoured surface took place prior to the fill event , an interval of time clearly se parated the two events . A banded ( sand/mud) fill is likely to have been deposited over a period of time . Where separation o f the two events can be recognized, it is logical and desirable that the form of the scoured surface and its mechanism of formation should be considered separately from the nature and sedimentary history of the fill . The purpose of this paper is to give an indication of the di stribUtion and the sedimentological and palaeontological significan ce of scour and fill structures wh�re the two events were separated .
ii: i I
r
'I
II, I
'
i,
The time-separation of erosive and depositional events in these areally restricted sedimentary structures is analogous to the com posite nature of many extensive and p1anar hardgrounds (FURSICH 1 9 7 9 ) 1 as well as to certain storm beds (see contributions o:E SEILACHER , HAGDORN and AIGNER in this volume) , in which repeated r_aisings and lowerings (ups and downs ) _of the sedimentary surface and a complex series of truncation and colonisation events are recog�ized. The erosional structures discussed he.re are mostly in terrigenous sediment of mud , silt and sand and have appreciable relie f . Planar surfaces do not appear to exceed a few metres in lateral extent . Cyclic and Event Stratification (ed . by Einsele/Seil acher) © Springer 1982
355
imm e d ia l e
f
typical gutter a cost
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partial sand fill (starved gutt e r )
t9 bonded fi!l
a
/
b
1
e
r
c
fossil trap
0 ?
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bioturbat ion I co lonisation
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Fig. 1 .
Five main types of gutter and the related physical and biological response for each
2 . Sedimentological Aspects Mud-filled· scours are· common ( SHROCK 1 9 4 8 ) but have received rela tively little attention . Indeed , in some shallow marine facies they appear to be as common as sand-filled scOur s , though they are gene rally much less conspicuous beCause they lack the relative enhance ment of the sand-filled scours by lithification and weathering·. The typical sandy gutter cast (Fig. la) , is the fill of a gutter cut into muddy sediment , whereas the typically observed mud-filled scour (Figs . ld, e ) is a gutter or runnel exhumed from, a fine-grained sand stone . Observations in the Lower and Upper Devonian of Devon and the Upper S ilurian of South Wales (below) suggest , that the range of form of the mud-filled scours is similar to those with a sandy
356 fill. A gutter-form ist the most common type and shows form described by WHITAKER ( 1 97 3 ) , occurring singly or groups . Parallel mud-filled gutters , cut into fine grained sandstone tend as to weather out exhumed sand ridge s . Such structures have been preted by BALDWIN & JOHNSON ( 1 9 7 7 ) as positive indication of emer� gence ; but GOLDRING & LANGENSTRASSEN ( 1 9 7 9 ) concluded that in the Devohian o f the Baggy Beds (GOLDRING, 1 9 7 1 ) and in the Psam mites du Condroz . (MACAR & EK 1 9 6 5 ) were cut and fi lled subaqueously�' BLOOS ( 1 9 7 6 ) also regarded similar structures in the German Lias as of subaqueous origin. The muddy fill to some scour structures may be only partial and follow an incomplete sand fill (Fig � lb) or may comprise mainly pelleted sediment . Larger scours of less regular outline with muddy fills are also common with the muddy fills weathering out to leave pan scours (GOLDRING 1 9 7 1 , pl . 2b; BLOOS 1 9 7 6 , p . 1 7 6 , fig . 5 6 ) . The margins may be- fluted (BLOOS 1 9 7 6 , fig. 5 6 ) or highly incised and gullied with considerable relief ( GOLDRING 1 9 7 1 1 pl . 2b) . With · further erosion 1 only erosional remnants of a· sand bed may remain below the muddy fill ( GOLDRING 1 9 7 1 , fig . 12a) . There thus appears to be a range of form from gutters to groups of paral lel gutters to amalgamated gutters , to pan scours and finally erosional remnant s . This does not imply any genetic gradation. Quite slight truncation of a graded sand sheet with removal of the muddy crust is common and the sandy substrate is often the site of colonization (below) •
In many instances the sediment exposed by penecontemporaneous ero sion clearly had substantial cohesion , sufficient to allow overhangs (Fig. 2 D ) in coarse silt/very fine sand grade sediment , and sharp ,, claw marks to be formed (below) . Substantial cohesion is also indicated by G Z o s sifungi t e s burrows at the top of Lower Kimmeridgian sheet sandstones (Gres de la Creche) of Portel 1 Normandy . Gyroohorte, formed under a mud co.ver of the sand/mud interface (SEILACHER 1 9 5 5 ) may later be cut by GZos sifung i t e s (Fig . 2A) 1 followin� erosion of the mud cove� down to cohesive levels . In banded sand/mud sediment the mud was eithe� preferentially eroded (Fig . 2C ; GOLDRING 1 9 7 1 , pl . 4c) , or at most , was scarcely more resistant than the sand . This contrasts with the effects of penecontemporaneous erosion on modern fluvial and intertidal sediments, where clayey layers tend to erode
357 G l o ssifu ngi tes
A
c
Diptocro t e r io n
·
Som
�
0--IO cm
coarse siltstone
0
10cm
Fig. 2 . A, B. Penecontemporaneous erosion and colonisation of sheet sandstones . A. GZossifung i te s , typical for relatively firm substrates , at top of Lower Kimmerid gian {Gres de la creche) sheet sandstones , Porte l , Normandy . One DipZooraterion cuts through Gyrochorte previously formed in still unconsolidated sediment . B. DipZocraterion on scoured top of sheet sandstone (Tertiary, Paracas , Peru) . c , D . Relative cohesiveness of host sedimen.t . C . Si lt stone-filled gutters incised into banded s iltstone and shale . Gutter margins are .incised more deeply into the shale , though s l ight flowage of silt may have taken place. Noi-th wall of south Cwar Glas quarry · (SN 7 2 7 2 4 8 ) . Upper Silurian Black Cook Beds. D. Shale-filled �nd undercut scour structure in parallel- laminated fine grained sheet sandstone with sole marks (Meatfoot Bed s , Lower Devonian, below Kilmorie, Torquay , Devon S X 9 3 63 ) . (A similar structure is present in unit below Corning Shale, West Falls Group , u . s . route 20 near Towanda, Pennsylvania , Middle Frasnian) proud to form potential mud pebble s . The reason for such substantial cohesiveness apparently close to the sediment-Water interface is not entirely clear , but is probably due to t.he muddiness of the sediment, the s low rate of overall sedimentation and repeated degradational 1 Cannibalistic ' events emphasising diagenetic affects . However , sand filled gutters , some with strongly undercut or stepped margins ( GREENSMITH et a l . 1 9 80) are related to synchronous fill . We have frequently observed a vertical repetition of scour structures (scou� in-scouri e . g . GOLDRING 1 1 97 1 , pl . 7b) . This is to be expected,
358
because a hollow may be only partly filled and act as a site of vor- tex initiation. Also, each fill is less compacted than the sediment adjacent to it . where scour and fill were separate events , care must be taken in interpretation of the direction of the eroding current from evidence in the fill sediment as KUDRASS ( 1 9 6 7 ) has noted. AIGNER & FUTTERER ( 1 9 7 8 ) noted that imbrication and climbing ripples indicate opposite directions of fill in adjacent gutter casts from the German Muschel kalk· WHITAKER ( 1 9 7 3 ) noted that transport directions indicated by sandy gutter casts and by ripple fores·ets at the top of the casting bed varied from 0 - 90° in the Lower Silurian of Norway . If discrete events led to cut and fill , then currents of quite different direc tion and origin may be involved . Neverthe.l ess in many case s , (for example GOLDRING 1 9 7 1 ) , there is a high deg�ee . of preferred orien tation of mud-filled scours of different types , as of those with a coarser fill , through a considerable thickness· of strat a . Erosional struc-tures with variable f i l l and those i n which the cut and fill events were separated by a significant period of time seem to be typical of fossi.l shelf sediment s , especially the shoreface ' to offshore facies . The closest modern analogue are furrows ( FLOOD ·, 1981 ) cut into cohesive estuarine sediment in Southampton Water , England , .some of which have remained open for several years . These furrows are attributed to helical secondary circulation . The grooves described from Lake Vattern (NORRMAN 1 9-6 4 ) and from the Baltic ( SEIBOLD 1 9 6' 3 ) appear to be similar. Mud-filled scours seem to be uncommon in turbidite facies . For in stance in the turbiditic Lower and Middle Cambrian of. North Wale s , the Hell ' s Mouth Grits ( BASSETT & WALTON 1 9 60) show evenly graded bed s , occasionally slumped but without erosion at bed tops . In con trast the upper part of the following Cilan Grits (CRIMES 1 9 70} are associated with common mud-filled scours , slumping, an apparent decrease in late:tal persistence , ·and at one level with Rusophyaus Dipl oaraterion and PhycOdes . It therefore remai.ns to pe debated, whether the Cilan Grits are proximal turbidites or storm deposits. :o
GOLDRING & LANGENSTRASSEN ( 1 9 7 9 ) suggested that the outer mud-line of continental shelves (STANLEY & WEAR 1 9 7 8 ) might correspond with the seaward limit of mud-filled scours . The suggestion was made because the mud-line represents the outer limit of ef fective ero-
359
sion on the shelf . Further seawards , turbidity currents are usually the principal erosing agent ; but deep water currents are today responsible at least for keeping furrows unfilled in depths up to 6 km ( e . g . FLOOD & HOLLISTER 1 9 80) . 3 . Palaeontological Considerations The palaeontological signif icance of separation of scour and fill events is two-fold. Firstly, (Fig. le) the scoured surface became immediately �vailable for epifaunal colonisation and for organisms ' moving on, into and through the substrate . Where graded beds are bio,turbated , minor erosion of the top, preceding the colonisation, may often be evident (Fig. 2b; GOLDRING & BRIDGES 1 9 7 3 , fig. ldi GOLDRING & LANGENSTRASSEN 1 9 7 9 , fig . 5) . Secondly (Fig. 1d) , the depressions became sites of enhanced . preservation potential, be cause they are relatively protected against further erosion and re latively more likely to suffer obrution. The scoured surfaces , parti cularly local hollows , represent potential local Fossil-Lagerstatten for the surrounding fauna and for elements washed into the hollows . ROSENKRANTZ ( 1 9 7 1 ) has described an echinoderm Fossil-Lagerstatte from such an erosional hollow in the Lower Jurassic of southern-Ger many and BLOCS ( 1 97 3 ) describes echinoids in a Liassic mud-filled sc:our . 4 . An Example of 11Community11 Preservation in the Upper S ilurian of South Wales In the Upper Siluri �n Black Cock Beds (POTTER & PRICE 1 9 6 5 ; CALEF & HANCOCK 1 9 7 4 ) of South Wales , the more southerly of the Cwar Glas quarries (SN 7 2 7 2 4 8 ) displays a sequence of coarse fossiliferous sheet siltstones and of shales interbedded with thin siltstones . The thicker siltstones are strongly lenticular due to penecontemporane ous erosion. Ar{ ovoid and partly mud-filled, east-west trending scour hol low ,. ( 3 . 0 x 1 . 2 x 0 . 08 m) , cut into a calcareous coarse silt stone1 displayed (April - June 1 9 7 9 ) the remains o f three articulated Craniops s p . (dorsal surface downwards ) , a nearly complete crinoid with a stern 30 em long, numerous paired valves of Pteroni t e l la sp . (also convex down) , several orthocones and numerous disarticulated but entire valves of Fergan e l la nuau la (mostly convex up and near to the south rim of the hollow) Also present were Sphaerirhynahia �i lsoni, Leptaena sp . , Protoahonetes ludloviensis, Pa laeonei l o sp . , . Po lenmita sp . , compressed membranous annelid tube's and very abundant •
360
carbonaceous {?arthropod} fragments . Otherwise fossils at this zon in the Silurian are found strongly fragmented as local tions within the sheet siltstones . It is from such beds that CALEF & HANCOCK ( 1 9 7 4 ) deter�ined their SaZopina community. A reconstruc tion of this community by COCKS ( in MCKERROW 1 9 7 8 ) is broadly con sistent with the fauna of the muddy fill , which would seem to re present a better indication of the 11community" present than that from the siltstones . 5 . Conclusions
Scour structures 1 where cut and fill events were separated by an interval of time are common and typical of many shelf facie s . They indicate rare to uncommon erosional events , with erosion down to levels already altered by compaction and diagenesis . Such penecon temporeaneo�s erosion substantially modified body and trace fossil preservatiOn, emphasising unusual modes of preservation and occa sionally allowing for exceptional · preservation . Acknowledgements We are grateful to Dr . R . D . FLOOD (Lamont-Doherty Geological Obser� vatory) for discussion and unpublished information , to Prof . Dr . A . SEILACHER (Tlibingen University) for discussion , and to Dr . D . WOODROW {Hobart and William Smith Colleges , Geneva) for demonstra ting sections in New York and Pennsylvania . Financial. assistance was provided by the Royal Society (RG) and the Adenauer Foundation (TA) . References AIGNER, T . & FUTTERER, E . ( 1 9 7 8 ) : Kolk-TOpfe und -Rinnen (pot and gutter casts) im Muschelkalk - An�eiger flir Wattenmeer? N . Jb . Geo1. Pa1aont . , Abh . , l�g ( 3 ) ; 2 8 5 - 304 . BALDWIN, C . T . & JOHNSON , H . D . ( 19 7 7 ) : Sandstone mounds and associa ted facies sequences in some late Precambrian and Cambro-Ordo viciah inshore tidal flat lagoonal deposits . - Sedimentology , 24 : 801 - 8 1 8 . =
BASSET, D . A . & WALTON , E . K . ( 1 960) : The Hell ' s Mouth . Grits : Cambrian greywackes in the St. Tudwal ' s peninsula, North Wales . Quart. J . geol . Soc. Land . , llg : 8 5 - 1 1 0 .
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BLOOS, G. { 1 97 3 ) : Ein Fund von Seeigeln der Gattung Diademopsis aus dem Hetangium Wlirttembergs und ihr Lebensraum . - Stuttgarter Beitr . Naturk . , B, � : 1 - 2 5 . BLOO S , G . ( 1 976 ) : Untersuchungen Uber Bau und Entstehung der fein k6rnigen Sandsteine des Schwarzen Jura (Hettangium) und , tief stes Sinemurium im Schwabischen Sedimentationsbereich. - Arb . Inst . Geo l . Palaeont. Univ .. Stuttgart , N . F . Z,�: 1 - 2 6 9 . CALEF , C . E . & HANCOCK, ·N . J . ( 19 7 4 ) : Wenlock and· Ludlow marine commu nities in Wales and the Welsh Borderland. - Palaeontology , �Z \,7 9 - 8 1 0 . CRIMES , T . P . ( 19 70 ) : A facies analysis of the Cambrian of Wales . Palaeogeogr . etc . , z : 1 1 3-170. FLOOD, R . D . ( ) : Distribution, morphology , and origin of 1981 sedimentary furrows in cohesive sediments , Southampton Wate r . Sedimentology , 28 : 5 1 1 - 529 . FLOOD , R . D . & HOLLISTER, C . D . ( 198o) : Submersible studies of deep-sea furrows and transverse ripples in cohesive sediments . - Mar . Geol . , gg: M 1 - M 9 . FURSICH, F , ( 19 7 9 } : Genesis , environments and ecology of Jurassic ' hardgrounds . - N . Jb . Geol . Palaont . , Abh . , !g� : 1-6 3 . GOLRING, R . ( 19 7 1 ) : Shallow-water sedimentation as i llustrated in the Upper Devonian Baggy Beds. - Mem. geo l . Soc . Lend . , � 1-80 . GOLDRING , R , & BRIDGES , P . H . ( 19 7 3 ) : Sublitoral sheet sandstone s . J . sediment . · Petrol . , £� 736 - 7 4 7 . GOLDRING, R . & LANGENSTRASSEN, F . { 1 9 79 ) : Open-shelf and near-shore clastic facies in the Devonian . - Spec . Papers Palaeontology , 23 : 81-97. GREENSMITH , J . T . , RAWSON , P . F . & SHALABY , S . E . ( 19 80 ) : An associa tion of minor fining-upward cycles and aligned gutter marks in the Middle Lias ( Lower Jurassic) of the Yorkshire coast. Proc . Yorkshire Geol . Soc . , �� : 525 - 5 3 8 . ==
KUDRASS ,,• H . -R. { 19 6 7 ) : StrOmungsmarken im unteren Muschelkalk des SE-Schwarzwalde s . - Ber . naturf . Ges . Freiburg i . Br . , gz : 203-206 . MACAR, P . & EK, c . ( 19 6 5 ) : Un curieux phenomene d ' erosion famenniene : les 'pains de gr€s ' de Chambrelles (Ardenne Belge ) . - Sedimento logy, i : 5 3 - 6 4 .
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MCKERROW , W . S . ( 19 7 8") : The ecology of fossils . - 3 8 4 pp . Duckworth , London . NORDMAN , J . O . ( 1 9 6 4 ) : Lake Vattern . Investigations on shore and bottom morphology . - Meddn . Upp . Univ. geogr . Instn . , ��i : 1-238 POTTER, J . F . & PRICE , J . H . ( 1 9 6 5 ) : comparative sections of Ludlovian - Downtonian age in the Llandovery and districts . - Proc . geol . Assoc . , b� : 3 7 9 - 40 2 . ROSENKRANTZ , D . ( 1 9 7 1 ) : ·zur Sedimeintologie und tikologie von me:n-Lagerstatten . - N . Jb . Geol . P-aUiont .Abh . , ��� : 2 2 1 - 2 5 8 . SEIBOL D , E . ( 1 96 3 } : Geological investigations of near-shore sand transport , 3-70. In SEARS , M . : ' Progress in Oceanography ' , k ' Pergamon , London . SEILACHER, A . ( 19 5 5 ) : SpUren und Fazies im Unterkambriurn. In SCHINDE� WOLF , o . & SEILACHE R , A . Beitrage zum Kenntnis des Kambriums in der Salt Range (Pakistan ) . - Abh . Akad . Wissen . Lit . Mainz . 1 ��; 3 7 3 -3 9 9 , SllROCK, R . R. ( 1 9 4 8 ) : Sequence in layered rocks . - 507 pp . McGraw New York. STANLEY , D . J . & . WEAR , c . M . ( 1 9 7 8 ) : The 'mud-line ' : an erosion-depo sition boundary in the upper continental slope . - Mar. geo l . , �� : 1 9 -2 9 . WHITAKER, J . H .McD. ( 1 97 3 ) -: ' Gutter casts ' , a new name for scour-and fill structures : with eXamples from the Llandoverian of Ringerike .. and Malrnoya, southern Norway . - Norsk Geol � Tidskrift, ��: 403-417-.�
Storm-surge Sandstones and the Deposition of Interbedded Limestone: Late Precambrian, Southern Norway :M. TuCKER
Abstract: In the . Late Precambrian Biri Formation of southern Nor way, graded sandstones are intercalated with thin-bedded rnicrites and flakestones . The · sandstones , interpreted as storm-surge de positS , commonly show parallel lamination passing up into cross lamination and theY have offshore-directed current ripples on their upper surfaces . The thin-bedded micri tes 1 initially lookin_g like tidal flat or lagoonal deposits , are locally disrupted into intraclast horizon s , and intraclasts occur within sandstone bedS. The occurrence of -the· storm sandstones indicates a subtidal, be low wa·ve-base shelf depositional environment for the micrite s . The intraclasts are thought to form by storm disruption o f sur ficial micrite layers which were lithified· on the seafloor. Thick flakestone units in the sequence are probably storm-channel fills .
1 . Introduction
The origin and depositional environment of ancient carbonate muds ' are still the subject of much discussion. Tidal flats , lagoon s , · quieter , deeper-water open shelves and platforms , slopes and ba sins are all possible sediment-ation site s . With Phanerozoic micritic limestones , depositional environments are · usually deduced from contained macro- , micro- , and/or ichnofossils. Back in the Precambrian, quite sophisticated facies analysis can now be at tempted if stromatolites are pi:esent. However ; there are many Precambrian limstones {or dolomites ) where a cryptalgal origin cannot be demonstrated so that inerpretations of depositional en vironment and mode of precipitation beco11te more speculative Many Precambrian micrites are attributed to precipitation on tidal flats , especially if there occur horizons of iritraclasts { forming flakestone s ) , supposedly generated by desiccation . •.
This short paper {details in prepaxation) considers the deposition of some Precambrian limestones of this type ,- from southern Norway , in the light o f interbedded sandstones . The sandstones are inter preted as storm deposits , formed by offshore-directed storm surge currents , and demonstrate subtidal , below normal wave base shelf sedimentation for the micritic limestones and flakestones . Cyclic and Event Stratification ( ed . by Einsele/Seilacher) © Springer 1982
364 2.
Background Stratigraphy
During the late Precambrian in southern Norway , several km of se diment were deposited within the Central Sparagmite Basin, an aUlacogen located on the northWestern margin of the Scandinavian European plate , on the south-eastern side of the opening lapetus Ocean (Fig . 1 ) •
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Fig. 1 . Location map of Late Precambrian Sparagmite deposits of Southern Norway
The carbonate-siliciclastic sequence described here occurs in the Biri Formation (Fig . 2 ) This is underlain by the Brottum Forma tion, consisting of basinal greywacke turbidites , and overlain by the arkosic f luviatile Ring Formation before the classic glacial deposit , the Moelv Tillite of Vendian age ( for a review of Sparag mite Basin sedimentation , see BJ¢RLYKKE et · al . 1 9 7 6 ) . In the mar ginal areas of .the basin, the Biri Formation: consists of laminated·, organic-rich limestones ; oolitic and catagraphic limestones ( TUCKER, in preparation) , in addition to the micrite-flakestone-sandstone association discussed here . In the center of the basin , laminated mudrocks predominate, with some resedimented units . �n the lower part o f the Biri Formation, there occur several basin-margin •
wedges of con glOmerate and sands tone (the Biskopas Cong lomerate ) . These are f an-delta and submarine fan deposits , which could reflect" a much earlier upland glaciation of surrounding continental areas (BJ¢RLYKKE et al . , 1 97 4 ) •
365
Vangsas Formation { fluviatile ) E k r e Shale M oelv Tillite
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R i n g Formation I fl u v i a t i l e
Biri Formation with
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3 . '·sedimentology In the highway exposures· of the Biri Formation at Kremmerodden near Biri on Lake Mj¢sa ( 2 5 km south of Li llehammer ) , much of the sequence is comPosed of thin-bedded micritic limestones , with horizons of intraclasts . The latter occur in thin beds .and lenses on the one hand ,_ and form prominent f lakestcine unit_s up to 2 me ters thick on the other. Intercalated with the micrites are sand stone beds (see Figs . 3 , 4 and 5 ) . The sandstone beds are mostly of fine to very fine · sand {chiefly quartz ) , grading to very fine sand coarse silt in their upper parts . Intraclasts of the micritic limestone are common in the basal parts of many sandstone beds and rounded, micritic peloids of sand-size are common throughout . Some sandy beds contain many intraclasts , often imbricate d . The sandS tones vary _ from 0 . 5 to 1 0 em in thick nes s , with many being in the range of 1 to 5 em . Bases are sharp , often with small-scale scours and less common ly load structures , i f overlying mudrock . All sandstone beds are normally graded. The two common internal structures are flat-bedding {parallel lamina tion) and cross lamination ( s ingle sets mostly ) . Some wavy , undu-
366
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micritic limestones and. graded sandstones,
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lating lamination occurs . Many sandstone beds show parallel l�ina tions , capped by cross lamination (BC sequences in BOUMA turbidite terms ; Fig·. 4 ) . Many s andstones show ripples on their upper sur faces (Fig . 4 ) and although bedding plane views are not common , most ripples are asymmetric with s traight to sinuous crests and wavelengths of 5 to 1 0 em. Most sets of cross lamination are con cordant with the rippled tops; some ripples are discordant with the internal lamination s . The features of the ripples indicate a current and wave-current origin. Sediment transport as shown by the ripples and cross laminations was dominantly nort�ward , that i s , offshore. Deformed and contorted laminations arising from se diment dewatering also occur . The features of the sandstones show periodic sand transportation into the area, and deposition from decelerating currents . The con text of the sandstone s , indicating a shelf regime, and the off shore directed nature of the currents suggest that ebb currents
367
Fig. 4 - 5 . · Field photographs ; Fig. 4 showing thin graded sands toneS with flat bedding and rippled tops 1 with micrite in troughs . More continuous micrite beds towards top possess vertical cracks ; Fig. 5 showing graded sandstones interbedded with thin bedded micrite. In lower part, micrite bed is disrupted locally to give flakestone lenses generated by s torm surges were responsible . Modern storm-surge sands have been documented by HAYES ( 1 9 6 7 ) , following a hurricane in the Gulf of Mexico, and by GADOW i REINECK ( 1 96 8 ) from the North Sea. Storm-deposited sandstones are now well-known from the
368
geological record; two' closely related example s , where storm sand stones are interbedded with carbonates , occur in the ._Ordovician of southern Norway ( BRENCHLEY et al . , 1 9 7 9 ) and the late Pre cambrian of Scotland {FAIRCHILD, · 1 980) . Major storms al9ng a coast line inducing offshore sand transport are likely to have been in frequent events . It is envisaged that between such events depo sition of the micrite - (and occasionaliy mudrock) took place . The paucity of wave reworking of the tops of the sandstones , the rela tive thinness of the sandstones , and lack of hummocky cross s trati fication ( WALKER, 1 9 79 ) suggest deposition below normal wave base and towards the depth of storm wave base . A siliciclastic shore line with significant wave activity is also indicated. The micritic limestones are generally thin-bedded (0 . 5 to 3 em) and parallel-sided, with mm thick mudrock partings between beds . In fact , in many cases mudrock grades upwards into micrite. Some micrite beds show vague horizontal laminations through a quartz silt rich , quartz-silt poor layering and single · spaced form sets of cross lamination are not uncommon . O f particular interest is the occurrence of micrite infilling the troughs of ripples on the tops. of sandstone beds (Fig . 4 .) . These features of the micrite beds just indicate "qlli ¢ t -water sedimentation " . Intraclasts of micrite are common in the sequence ( e . g . F ig . 5 ) and form thin b6ds and lenses within the bedded micrite ; 'they also form the thick flakestone units described below and they occur within the sandstone beds _ as noted above . The presence of micrite intraclasts in shelf carbonate sequences is frequently taken to signify subaerial exposure and desiccation on a tidal flat. The intercal ation with sandstones deposited below normal wave-base suggests that this was not the case . A Conspicuous feature of the intraclasts i s their shape : they are invariably rectangular in two-dimensional section and thicker ones often have 90° re-entrants at their ends , reflecting fracture across internal -laminations . Vertica l , parallel-sided crackS, reminiscent of desiccation cracks , cross some ·micri�e beds although , unfortu nately , there are no bedding plane - exposures at these horizons to see if there is a cracking· pattern·. Vertical cracks "are parti cularly well developed immediately beneath the sandstone beds apd they are infi l led by sand. The cracks are similar to pull apart structures or pseudo-mudcracks , which are often ascribed to se diment creep on the sea floor (see PFEIL
&
READ , 1 9 80 , for example ) .
r:-"
369
Where a micrite bed is overlain by a storm sandstone , it may be brecciated laterally (Fig. 6 ) and then totally removed. In several instances where a micrite bed terminates , s-and of the bed above also partly underlies the micrite , showing that s cour o f sediment
beneath the limestone had taken place .
The interbeddin9 of s ·torm sandstones and micrites s hows that
micrite deposition took place in the subtida l , near, and below and
beyond normal waVe base . The formation of the intraclasts i s ascri
bed to storm di'sruption of surficial micrite beds . This , together , with intraclast shape _and the scouring beneath micrite beds show
that the micrites were at least partially lithified and formed sur face crusts and hardgrounds through seafloor cementation .
The thick flakestone horizons occurring within the sequence {Fig . 3 ) consist of micrite intraclasts in a matrix of sand, mudrock and
micrite. Although the flakestones often appear to be a chaotic assemblage of intr&clas ts , some are clearly composite, consisting
of several unitS of differing average intraclast size . Some flake stones have markedly irregular bases _while others are lenticular.
A channel origin for these flakes�ones is most likely (as s ug
gested by BJ¢RLYKKE et a l . , 1 9 7 6 ) and in view of their sedimen
tolo��cal context, possibilities are the subtidal portion of major tidal channe ls , storm surge channels (c . f . BRENNER & DAVIES, 1 9 7 3 ;
JAMES , 1 9 80 ) or rip channe ls . The se diment itself may ·have been
emplaced by local debris flows , also generated by storm activity . 4 . ConclusiOn
The purpose of this note has be.en to illustrate how storm-deposited sandstones can be used to infer the depositional environment of
associated fine-grained limestones . It is s uggested that intra
clasts of bedded micrite are supplied by storm disruptiori of sur
face micrite layers and that the features of the intraclasts and the micrite beds-sandstone rela� ionships indicate seafloor lithification of the micrite . Acknowledgements
Fieldwork in southern Norway was s upported by the Natural Environ ment Research Counci l . This paper was written while in receipt of a· Lindemann Trust Fel lowship, which is grat.efully acknowledged .
------- · ' ''
370
References BJ�RLYKKE, K . , �LVSBORG , H . i H�Y, T . { 1 9 7 6 ) : Late Precambrian se dimentation"";"in the central sparagmite basin of s_outh Norway . Nor. Geol. Ti'd . , :i,g : 2 3 3-290 . BRENCHLEY, P . J . , NEWALL , G . & STANISTREET , I . G . ( 1 979 ) : A storm sur ge origin for sandstone beds in an epicontinental platform sequence , Ordovician, Norway . - Sedim . Geol . , �� : 1 85-2 1 7 . BRENNER, R . L . & DAVIE S , O . K . ( 1 9 7 4 ) : Storm-generated coquinoid sandstones : genesis of high energy marine sediments form the Upper Jurassic o f Wyoming and Montana . - Bul l . Geol. Soc . Am . , !l4= 1 6 85- 1 6 9 8 . FAIRCHILD, I . J . ( 1 9 80 } : Sedimentation and origin o f a late Pre cambrian ' Dolomi te' from Scotland . - J . Sedim. Petrol . , ��: 4 2 3-4 4 6 . GADOW , S . & REINECK , H . E . ( 1 9 6 9 ) : Ablandiger Sandtransport bei S turmfluten . - Senckenbergiana Marit . , J,: 6 3-78 . HAYES , M . O . ( 1 9 6 7 : Hurricanes as geological agents : case studies of hurricanes Carla, 1 96 1 , and Cindy, 1 9 6 3 . - Rep . InVest. No . g.J, , Bur. Econ Geol . , Uni v . o f Texas, 54 pp . JAMES , w . c . ( 1 9 80 ) : Limestone channel storm complex ( Lower Creta ceous ) , Elkhorn Mountain s , Montana . - J . Sedirn. Petrol . , �Q: 4 4 7-455 . PFEIL, R . W . & READ , J . F . ( 1 980) : Cambrian carbonate platform margin facie s , Shady Dolomite, Southwestern Virgini a , USA . - J . Sedim. Petrol . , :i,Q : 9 1 - 1 1 5 . WALKER, R . G . ( 1 9 7 9 ) : Shallow marine sands . - I n : Facies Models ( ed. , R . G . WALKER) , Geoscience Canada. •
·
.
·
Flat-Pebble Conglomerates, Storm Deposits, and the Cambrian Bottom Fauna J. J. SEPKOSKl, Jr.
Abstract : Flat-pebble conglomerates , which are very common in Cambrian strata, are formed from thin limestone beds that have been ripped up and redeposited, mostly during storms . Condi tions.. for genesis include episodic - deposition of thin beds , rapid lithification, and subsequent erosion and redeposition. Expansion of the infauna during the Ordovician eliminated the widespread potential · for rapid submarine cementation of thin carbonate layers , thereby reducing. the frequency of deposition of flat-pebble conglomerates . ·
Flat-pebble conglomerates are intraformational limestone conglome rates (calcirudites ) composed of rounded , tabular intraclasts . The se conglomerates are comm�n in shallow-water carbonate facies throughout the geol� gic column but are especially abundant in early Paleozoic strata. Extensive flat-pebble cong-lomerates are present in Cambrian and Lower Ordovician strata in many parts of North ' America ( e . g . RODGERS 1 9 5 6 : LOCHMAN-BALK 1 97 1 ) and in northern · china and Korea (KOBAYASHI 1 9 5 6 , 1 9 6 6 ) . Younger carbonate facies on all continents contain much fewer flat-pebble conglomerates , giving the impression of a verY uneven distribution of this _ rock type in geologic time . In this paper , I shall briefly discuss the nature , genesis , and temporal distribution o f flat-pebble conglomerates with two pur poses in mind : 1 . To document one partiCular kind of tempestite common in shallow wat-er deposits of Cambrian agei 2 . To provide an example of the impact of an evolving benthos on sedimentation and on the preservation of sedimentary structures . In the discussion below , I briefly re �iew the stratigraphic setting of flat-pebble conglomerates in western North America, then describe the nature of these conglomerates arid associated lithologies , and finally conclude with some considerations pertaining to their ge nesis. Cyclic and Event Stratification ( e d . by Einsele/Seilacher) © Springer 1982
372
1 . Geologic Settlng The distribution of early Paleozoic flat-pebble conglomerates is controlled largely by facies distributions . In western North Ame rica , Cambrian lithofacies on the non-orogenic craton and miogeo cline are organized into three magnafacies belts (Fig. 1 ) , as first recognized by PALMER ( 1 9 6 0 ) : 1 . Inner detrital belt, consisting principally of nearshore sands and shelf muds on t-he craton ; 2 . ·Middle carbonate belt, consisting of shallow subtidal to supra tidal carbonates deposited in a . bank environment straddling the outer craton and inner miogeocline ; 3 . Outer detrital belt, consisting of dark-colored terrigenous and calcareous muds on the outer shelf and. s lope portions of the miogeoclin� and also including deeper water siliceous se diments· traditionally cla$si fied as eugeosynclinal .. Intraformational conglomerates are present in many lithofacies within the middle ·carbonate belt but are most abundant in shaly facies of the· inner detrital belt (McKEE 1 9 4 5 ; LOCHMAN 1 9 5 7 ; AITKEN 1 9 6 6 ) Flat-pebble conglomerates are rare ·i:n arenaceous shoreline facies and occur only in calcareous turbidites in the outer detrital belt .. . •
My studie� of flat-pebble conglomerates and associatE7d lithologies (SEPKOSKI 1 9 7 7 ) have concentrated on Dresbachian ( lower Upper Cambrian) strata in portions of Montana, Wyoming , and South Da kota (Fig. 1 ) As shown by the stratigraphic cross-section in Fig .. 2A, Dresbachian rocks in this region encompass facies in the inner detrital belt and eastern portion of the middle carbonate belt .. Reconstructed depositional environments .for these facies are illu ..
strated in Fig 2B The sandstones in. the east were deposited in a complex mosaic of coastal environments overlying an i rregular Precambrian basement. Represented environments include beach , bay, bar , :tidal platform, and shoreface .. In -the west , the carbonate bank comprised a para� le l , although less complicated se� of peri tidal environments ; these included a subtidal platform of pelletal sands ( " ribboned limestone " facies) , channeled stromatolitic tidal flats ( stromatolitic dolomite facies) , and supratidal algal flats (algalaminate facies ) . At one stage in development , a long, sinuous thrombolitic stromatolite biostrome , reminiscent of a barrier reef, ..
..
373
'� '
Fig. 1 . Index map of western North America showing Cambrian facies belts and regions , from which flat-pebble conglomerates have been de9cribed . The brick pattern de limits the .average position of the middle carbonate belt. The area between this belt and the stipp ling to the east encompasses the inner detrital bel t . ( The· heavy line adjacent to the stippling demarcates the eastern erosional li mits of Cambrian strata in western North America . ) · The horizontal ruling west of the middle carbonate belt indicates areas in which sediments of the· outer detrital belt are still preserved . Cambrian flat-pebble· conglomerates have been described fr<;>m a number of re gions in western North America; particularly noteworthy studies have been published by AITKEN {region A) , LOCHMAN {region B-B ' ) , and McKEE ( region f) ---
built · up just offshore from the peritidal bank . All of the bank facies were deposited over an early Dresbachian oolite wedge and were covered in turn by a second oolite wedge formed by wave and tidal currents traversing the drowned bank . The local erosional unconformitY in the far western portion of the study region was produced by late Dresbachian tensional uplift along the hingeline between the stable craton and downwarping geosync line . Between the eastern coastal complex and western peritidal bank was a broad "shelf lagoon" with a width of greater than 500 krn in place s . As documented in Fig. 2 C , flat-pebble conglomerates are
374
A. Stratigraphic
crass-section.
PILGRIM FORMATION
upper HASMARK FORMATION
middle
lower
"GROS VENTRE" FORMATION
DEADWOOD FORMATION
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B. Depositional
environments.
PERITIDAL BANK Supra� Inter- lnfro-
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Percent
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INTERTIDAL BIOSTROME
flat-pebble
SUBTIDAl.
LAGOON
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Sondflots, boys, etc.
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Fig. 2 . Stratigraphic setting, deposi·tional environments , and abun-. dance of Dresbachian flat-pebble conglomerates along transect B-B ' (Fig. 1 ) . A . Stratigraphic cross-section showing the distribution of formations-and facies in -the inner detrital belt and eas ·tern middle carbonate belt. Numbered units within the Pi lgrim and Hasmark For mations are 1 -thrombolitic stromatolite facie s , 2 - "ribboned lime stone'' facies . , 3-stromatoliti� dolomite facies , and 4-algalaminate facies . �· Reconstructed deposition.al environments · 'fo:i= the strati graphic interval just bel·ow the upper oolite ( approximate time pla ne indicated by marginal arrows in A) . C . Proportion of flat-pebble conglomerate in the lithofacies represellted in Bi arrowheads indi cate control points for data . (After SEPKOSKI 1977)
375
most common in the lagoonal facies , attaining their greatest -abundance just east of the margin of the carbonate bank . conglome rates in this area constitute more than 2 5 % of the thickness of the lagoonal faq ies and more than 50% of the thickness of some 10-m intervals . 'The discussion below will concentrate on these flat-pebble conglomerates. 2 . Flat-Pebble Conglomerates and Associated Lithologies S�diments of the Dresbachian lagoonal facies consist of alterna ting mm- to crn-thick . beds of shale and s iltstone or limestone punctUated by em- to dm-thick beds of flat-pebble conglomerate . The shale is an olive�green , illitic clay shale with variable ad mixtures of mica but little terrigenous silt or sand ; it is ge nerally devoid of calcareous pellets and bioclasts , probably as a result of diagenetic diss.olution (SEPKOSKI 197 8 ; see also MARTINSSON 1 9 6 5,) . On outcrop , the shale breaks up into small flat chips , and not. papery lamellae, i,ndicating thorough bioturbation {BYERS 1 9 7 4 ) . Although some of the shale may represent the '1muddy tai ls" of storm deposits , much seems to have been slowly deposi ted under quiet-w2tter conditi�ns . The associated thin-bedded coarse siltstones and pelletal lime' stones , on the other hand , have characteristics indicative of rapid deposition under turbulent conditions ., presumably induced by storms . Most of these beds are flat laminated , with quartz silt and· fine bioclasts at the bottoms of individual laminae , variable amounts of sand-sized micritic pellets in the middles , and some clay or micrite at the tops. Some of the thicker beds are strongly graded , as illustrated in Fig. 3A. This bed con sists of an unlaminated basal portion o f Coarse silt with some tri lob ite bioclasts , whiCh ·grades upward into coarse laminae ;
the laminae f'ine toward the top of the bed, becoming intercala ted with very thin shale partings . Beds that do not grade upward continuously, often shift to undulatory lamination or cross-la mination laid down by oscillation or tran9verse current ripples on the tops . Very thin ( 5 mm) bed s , which are frequently mi critic, tend to pinch and swell latetally, often changing into layers of narrow pods ( GRANT 1 9 65 ) . Undersides of the thin beds commonly record the erosive event that preceded deposition of the silt or pellets (Fig. 3B) . Groove casts , prod marks , and rare gu-tter casts (AIGNER 197 9 ) , all indi-
376
377
-eating scouring of the muddy lagoonal bottom, are present on the lower surfaces . Also present are nondescript hypichnial casts of partially washed-out intrastratal burrow s , exhumed from under lying muds ; casts of epistratal ( surface} . traces are quite rare, however . The intraclasts in the flat-pebble conglomerates are clearly de rived from the associated thin-bedded storm deposits . �s illu strated by Fig . 3C , the intraclasts are thin (mostly less than 5 mm .thick) with diameters generally in the range of 0 . 5 to 5 em ( although occasionally reaching 30 em) . Shapes vary from ellipti cal (mOst commonly) to polygonal or irregular, all with rounded edges . Nearly all intraclasts are composed of flat-laminated pelletal to more homogeneous ·micritic limestone identical in com position and fabric to the associated thin limestone bed s � some intraclasts even have hypichnia on one surface . Thus , the intra clasts must have formed when partially lithified beds were broken up and r-edeposited as j umbled masses to form the conglomerates . Some conglomerates can be seen to cut through as much as 2 5 em of underlying shale and thin storm layers and , rarely , to pass laterally -- through a zone of partially disrupted bedding - into undisturbed , flat-lying 0beds,_ Significantly , however , none of th'€� intraclasts shows any evidence .of plastic deformation i thus , they must have been deposi t"ed as rigid discs and plates rather than partially consolidated "mud lumps" .. • .
The flat-pebble conglomerates only very rarely grade upward into laminated siltstone. or limestone .. Instead, they are usually co vered directly by ·shale , pe�haps suggesting fairly lengthy ex posure on the lagoonal bottom . This suggestion is corroborated
� Fig.
3 . Storm deposits from the Dresbachian shaly facies . �- Et ched section through a comparatively thick s iltstone layer showing graded beddingi trilobite bioclasts are concentra ted in the band that intersects the lower margin of the scale . B . Underside of a .thin siltstone bed exhibiting prod marks , grooVe casts , and hypichnial burrows . c . Etched sec tion through a flat-pebble conglomerate with edgewise intra clasts ; the intraclasts are composed of laminated, s i lty , pelletal limestone (note scatt8red endichnial burrows) . Q . Upper surface of a flat-pebble conglomerat� showing variab le orientations of edgewise intraclasts ; the scale along the right margin is marked in em · and is 26 em long. (A and B from Bighorn Mountains , Wyoming ; c from Cody , Wyoming ; Q from Beartooth Mountains , Montana)
378
by glauconite rinds · an micritic intraclasts near the tops of some conglomerates ; these rinds probably formed s lowly at the interface between seawater and the internally reduced intraclasts Skeletal fossils of encrusting organisms are absent · from the conglomerates , however, probably because an encrusting fauna did not exist in the Dresbachian shelf lagoon 6 The matrices of the flat-pebble conglomerates vary from mixtures of silt and granule-sized intraclasts to, much more frequently, clean bioclastic sands , often containing detrital glauconite . The bioclasts , which consist largely of trilobite and eocrinoid debris , are thicker ( i . e . _ heavier) but more fragmented than those in the graded , flat-laminated beds . A few bioclasts contain clay within intraskeletal void s , indicating exhumation from the now unfossili ferous shales . Beds of flat-pebble conglomerate vary from about 3 ern to nearly 1 m in thickness . Intraclasts within the thinner beds tend to be small and flat lying, whereas in thicker beds intraclasts are larger and often lie at high angles to bedding. The intraclasts in such "edgewise conglomerates" are rarely imbricated , however ; rather , they exhibit jumbled · ·or swirled patterns without any con sistent orientation (Fig. 3D) . Similar fabrics have been observed on beaches where breaking waves re-orient and· pack discoidal pebbles ( e . g . SANDERSON & DONOVAN 197 4 ) ; similar re-orientation presumably could be effected by strong oscillatory currents pro duced during storms (see contribution by FUTTERER; also , KREISA 1981 ) . Individual beds of edgewise conglomerate tend to be variable in thickness . Near the margin with t�e carbonate bank , some edge wise conglomerates are lenticular and appear to have been depo sited as low bars or megaripples with wavelengths on the order of 10 m or more. Most of t·he flat-pebble conglomerates , however , are fairly continuous and can be traced for tens to hundreds of meters along an outcrop . The maximum lateral continuity of a conglomerate in unknown ; however , some 5-m thick intervals with 50%) can be traced particularly abundant conglomerate ( i . e . for more than 100 km across depositional strike {McKEE 1 9 4 5 i SEPKOSKI 1 9 7 7 ) , providing considerable resolution i n stratigraphic correlation .
379
To summarize , flat-pebble conglomerates are one of t·wo distinct, but interrelated, types of tempestites in the Dresbachian shaly lagoonal facies of western North America: 1 . Thin, flat�laminated beds , deposited on leve l , scoured bottoms , possibly from storm-indUced suspension clouds ( REINECK & SINGH 1 97 2 ) that moved s ilt away from the shoreline and/or pellets away from the carbonate bank ; 2 . Intraformational flat-pebble conglomerates formed from partially lithified thin limestone beds torn up during particularly inten s � storms and/or ln areas of greatest storm impact ( the conglo merates are most frequent near the margins of the shelf lagoon where water was· presumably shallowest; c f . AIGNER 1 9 7 9 ; KREISA 1 9 8 14 . storm erosion and redeposition together represent only one o f se veral critical factors in the genesis of flat-pebble conglomerates . The dual problems of lithification and temporal distribution are considered next. 3 . Genesis and Temporal Distribution of Flat-Pebble Conglomerates Flat-pebble conglomerates have been used by some workers ( e . g . LOC HMAN-BALK 1 9 70) as enviroTiment� l indicators o f intertidal de position. Modern intraformational congloillerates have been observed forming on supratidal flats where case-hardened, mudcracked sedi ments have been r.eworked bY spring or storm tides (e . g . ROEHL 1 9 6 7 ) . Analogous conglomerates are present in supratidal facies of the Dresbachian carbonate bank ; these consist of small , light-colored, angular chips of algally-laminated dolomite. But such intraforma tional breccias are very different from the f lat-Pebble conglome rates of the shaly facies , suggesting a different genesis . Other considerations that argue against intertidal formation of the flat-pebble conglomerates include the following: 1 . The shaly facies o f the inner detrital belt is too wide to re
present frequently f looded tidal flats . 2 . The conglomerates are not associated with any other intertidal
features , such as stromatolites , flaser bedding , and channel structures (e . g . imbricated pebbles ) .
380 3.
The undisturbed · Dresbachian shales and thin s torm layers lack any features indicative of subaerial exposure, such as dessica tion cracks , fenestrae , etc .
4'. There are no shale intraclasts in the conglomerates and nonpelle tal siltstone intraclasts are rare, contrary to what would be expected i f case-hardened , mudcracked sediments were the source of the intraclasts . This last point is particularly noteworthy . Even in parts of the shaly facies where siltstone composes most of the thin storm lay ers , nearly all flat-pebble conglomerates are composed of lime stone intraclasts. This suggests , then , that the key to intra clast . formation in this case is not subaerial dessication but rather early submarine cementation of limestone beds . Submarine cementation is currently taking place over extensive areas of the Persian Gulf and Bahaman Platform. BATHURST ( 1 9 7 5 ) , in reviewing these modern hardgrounds , lists three factors belie ved to be important in promoting their cementation: 1 . Presence of a suitable substrate, specifically porous and _per
meable carbonate grainstones (carbonate cements are conspicuous ly absent from quartz sand in the Persian Gulf) ; 2 . Slow rates of sedimentation and general lack of movement of bottom sediments ·, as might result from bottom traction or, pre
sumably , infaunal reworking;
3.
Presence of a sufficient reservoir of seawater supersaturated with respect to Caco (which may be enhanced by bacterial pro 3 duction of bicarbonate below the oxidizing zone in the sedi ments ; ALLER 1 9 8 1 ) .
_ Deposition of thin storm layers of porous pelletal sand in the normally quiet Dresbachian she1f lagoon seems to have· provided appropriate substrates for early submarine cementation. But equal ly important must have been the protection of these layers not only from immediate current reworking but also from continuous bioturbation . There was certainly an abundant infauna in the Dres bachian shelf lagoon 1 as evidenced by the character �f the shale and the abundant hypichnia beneath the storm layers . However, most of this infauna evidently was small and capable of burrowing only to very shallow depths . Some of the storm layers (especially the very thin beds of micritic pelletal limestone ) do show some bio-
381
600 (/) Q)
·-
·-
'0 400 1.1-
E
-
0
Q), ..0 200 �
E
='
z
0
Paleozoic -
v
600
£
500
Geologic
400
Ti m e
300
Fig . 4 . Diversity of marine animal families through the Paleozoic Era (after SEPKOSKI 1 9 79 ) . The inset diagrams schematically illustrate bottom sediments with storm . layers in shallow s he l f environments (black represen.ts mud or shale; white represents calcareous and/or terrigenous grainstones ) . In the Cambrian Period, when diversity was low , shallow sub tidal _sediments tended not to be extensively bioturbated , and thin storm layers were frequently preserved , permitting formation of flat-pebble conglomerates . Following the ini tial phases of the Ordovician radiations , bottom sediments were more extensively bioturbated as a result of a larger and more diverse infauna , and thin storm layers were no longer regularly preserved
turbation ; however, intense bioturbational churning rarely exten ded deeper than about 5 mm into these beds . Thus , the small size of the Dresbachian infauna must have had two imPortant consequen ces : 1 . Thicker layers of pelletal sand, as well as thin layers covered
by 11muddy tails11 , escaped reworking and destructive mixing with adjacent muds ; 2 . Restricted burrow ventilation resulting from the absence of
deep burrowers must have sustained only a very shal low oxidi-
382
zing layer, permitting production of bicarbonate near the sedi ment surface ( c f . ALLER 1 9 8 1 ) . Both of these factors would have promoted rapid cementation of the pelletal sands . The Cambrian Period seems to have had a uniquely small and depau perate infauna in shallow, storm-swept shelf environments . This is probably the critical factor in the uneven distribution of flat pebble conglomerates in geologic time . Following the Cambrian, the marine fauna underwent a tremendous evolutionary expans ion, with familial diversity more than tripling over the course · of the Ordovic'ian (Fig . 4 ) . Although this radiation primarily affected epifaunal- suspension feeding organisms (SEPKOSKI 1 9 79 ) , a number of new infaunal groups expanded , including several kinds of bi valves and echinoderms ; some predominantly soft-bodied infaunal groups evidently also diversified ( e . g . polychae�e worms ; see SEPKOSKI 1 9 8 1 ) . This new infauna did not appreciably change the preserved diversity of trace fossils (SEILACHER 1 9 7 7 ) . However , it did increase the depth and inten�ity o f bioturbation in shelf se diments . Thus , by the Late Ordovician, the �ery thin storm layers widely distributed in Cambrian sediments were no longer preserved in shallow shelf situations · ( c f . KREISA 1 9 8 1 ) , greatly reducing (although not ent�rely eliminating) the potential for rapid cemen tation of thin, platy limestone beds . Concomitantly, deposition of flat-pebble conglomerates appears to have become restricted to areas of intense bottom erosion ( i . e . to sediment depths greater than occurred in the Dresbachian lagoon) and to environments with restricted infaunas ( e . g . peritidal environments} . This admittedly speculative explanation solves only half of the problem of the temporal distriPution of flat-pebble conglomerates ; the problem of their infrequency i n the Precambrian has not been broached. But thi s , too , may involve evolution of the infauna. Prior to the Vendian ( i . e . latest Precambrian ) , algal stromatoli . tes were abundant and diverse in a variety of shal low subtidal carbonate environments ; subaqueous erosion of these stromatolites did produce intraformational conglomerates but of a very different nature than the Cambrian flat-pebble conglomerates . The onset of the Vendo-Cambrian metazoan radiations greatly reduced the di versity and environmental range of stromatolites1 evidentlly as a result of grazing by newly evolved bottom-feeding animals (GARRET 1 9 7 0 ; AWRAMIK 1 9 7 1 ) . Thus 1 the initial radiation of marine animals
383
,not only freed subtidal sediments from the strong binding action of blue-green algae but also resulted in par-autochthonous pro duction of well�sorted carbonate grains in the form of fecal pellets . These pellets , then , could be redeposited as thin , per meable beds subject to rapid cementation. Therefore , the abundant flat-pebble conglomerates of the Cambrian and Early Ordovician may , in effect, represent a 1 00-myr "window" in which the co-evolution of animals and sediments resulted in unique conditions for the widespxead formation of intraformational conglomerates . 4 . Conclusions Flat-pebble conglomerates , consisting of rounded intraclasts . of limestone , are uniquely abundant in Cambrian and Lower Ordovician shallow subtidal carbonate sediments . Three conditions appear .to have been necessary for the formati.o n o f these conglomerates : 1 . Episodic deposition of thin, permeable calcareous beds separa ted by shale partings or thin beds; 2 . Preservation of these beds near the sediment-water interface where they could become rapidly cemented; 3 . Erosion and redeposition of the partially lithified beds by s'torms or other erosional events .
The first condition may have been realized only after the initial Vendian radiations of marine animals , when bottom grazing restric ted stromatolite distribution �nd led to production of abundant sand-sized carbonate fecal pellets . The second condition, on the other band, may have been no longer realized after the Ordovician radiation s , when the expansion of the infauna led to greater bioturbation of bottom sediments and increased destruction of thin pelletal beds prior to cementation . As a result, deposition o f flat-pebble conglomerates became confined to environments where the infauna was rest-ricted and/or bottom sediments were ero ded to considerable depths . Acknowledgements The ideas presented here benefited greatly from discussions with R . C . ALLER and especially R . K . BAMBACH .
384
References AIGNER, T. ( 1 97 9 } : Schill-Tempestite im Oberen Muschelkalk (Trias , , SW-Deutschland . - N . Jb . Geol . Palaont . Abh . l�Z: 326 -34 3 . . AITKEN , J . D . ( 1 9 66 ) : Middle Cambrian to Middle Ordovician cyclic sedimentation, southern Rocky Mountains of Alberta . - Can. Petro l . Geol . Bul l . l� : 405-4 4 1 . ALLER, R . C . ( 1 9 8 1 ) : Carbonate dissolution in nearshore terri gene ous muds : The role of physical and biological reworking . - J . Geol. � 2 , i n press . AWRAMIK, S . M . ( 1 9 7 1 ) : Precambrian columnar stromatolite diversity : Reflection of metazoan appearance . - Science JZ� : 8 2 5 -8 2 7 . BATHURST , R . G . C . ( 1 9 7 5 ) : Carbonate sediments and Their Diagene sis, 2nd ed . - Elsevier; Amsterdam. 6 5 8 p . BYERS 1 c . w . ( 1 9 7 4 ) : Shale fissility: Relation to bioturbation . Sedimentol. �J: 4 79-484 . GARRET 1 P . ( 1 9 70) : Phanerozoic stromatolites : Noncompetitive eco logic restriction by grazing and burrowing animals . - Science lg�: 1 7 1 - 1 7 3 . GRANT , R . E . ( 1 9 6 5 ) : Faunas and stratigraphy o f the Snowy Range Formation (Upper Cambrian) in southwestern Montana and north western Wyoming . - Geol. Soc . Amer. Mem. 9 6 . 1 7 1 p . KOBAYASHI1 T . ( 1 9 56 ) : The Cambrian of Korea and its relation to the other· Cambrian territories . - I n : J . RODGERS ( ed. ) El Sistema Cambrico . 20th Int . Geol . Congr . , Mexico: 3 4 3- 3 6 . KOBAYASHI , T . ( 1 9 6 6 ) : The Cambro-Ordovician formations and faunas of South Korea . Part X . Stratigraphy of the Chosen Group in Korea and south Manchuria. Section B . The Chosen Gro�p of North Korea and northeast China . - J. Fac. Sci . , Univ. Tokyo , Sec . I I . Jg : 209-3 1 1 . KREISA, R . D . ( 1 9 8 1 ) : Storm-generated sedimentary structures in sub tidal marine facies with examples from the Middle and Upper Ordovician of southwestern Virginia . - J . Sed. Pet . �J , in press . LOCHMAN , c . ( 1 9 5 7 ) : Paleoecology of the Cambrian in Montana and Wyoming . - Geol . Soc . Amer. Mem . gz ( 2 ) : 1 1 7- 1 6 2 . LOCHMAN-BALK1 C . ( 1 970) : Upper Cambrian faunal patterns on the craton . - Geol . Soc . Amer. Bull . �J : 3 1 9 7- 3 2 2 4 . LOCHMAN-BALK, C . ( 1 97 1 ) : The. Cambrian of the craton o f the United States . Pp . 7 9- 1 6 8 . - In : C . H . HOLLAND (ed . ) , Cambrian of the New World . Wiley-Interscience; New York . MARTINSSON, A. ( 1 96 5 ) : Aspects of a Middle Cambrian thanatotope on Oland . - Geol . FOren . Stockholm FOrh . �z : 1 8 1 - 2 3 0 . McKEE , E . D . ( 1 9 4 5 ) : Stratigraphy and ecology of the Grand Canyon Cambrian, Part I . In ·: E . D . McKEE & C . E . RESSER, Cambrian Hi story of the Grand Canyon Region . Carnegie Inst . Wash . Publ . ;ig,}: 5 - 1 6 8 . PALMER, A . R . ( 1 960) : Some aspects o f the Upper Cambrian stratigra phy o f White Pine County , Nevada , and vicinity . - Intermountain Assoc . Pet. Geol . Guidebook : 5 3 - 5 8 .
385 REINECK , H . -E . & I . B . SINGH ( 1 9 7 2 ) : Genesis o f laminated sand and graded rhythmites in storm-sand layers of shelf mud . - Sedi mentol . l�: 1 2 3-1 28 . ROEHL , P . W . ( 1 96 7 ) : Stony Mountain (Ordovician) and Interlake (Silurian) facies analogs of Recent low-energy marine and subaerial carbonates , Bahamas . - Amer. Assoc. Pet . Geol . Bul l . :?,J : 1 9 79-2032 . RODGERS , J . { 1 9 5 6 ) : The known Cambrian deposits of the southern and central Appalachian Mountains . - I n : J. RODGERS {ed . ) , El Sistema cambrico . 20th Int. Geol . Congr . , Mexico : 3 5 3-38 4 . SANDERSON , D .J . & R . N . DONOVAN { 1 9 7 4 ) : The vertical packing of shells and stones on some recent beaches . - J. Sed. Pet. -44: 680-6 88. SEIL�CHER, A. ( 1 9 7 7 ) : Evolution o f trace fossil communities . - In : A . HALLAM ( ed . ) , Patterns o f Evolution . Elsevier; Amsterdam. SEPKOSKI , J . J . , Jr . ( 1 9 7 7 ) : Dresbachian ( Upper Cambrian} Strati graphy in Montana, Wyoming , and South Dakota . - Unpub l . Ph. D . dissert. Harvard Univ . ; Cambridge, Mas s . SEPKOSKI , J . J . , Jr . ( 1 9 7 8 ) : Taphonomic factors influencing the lithologic occurrence of fossils in Dresbachian ( Upper Cambrian} shaly facies . - Geo l . Soc . Arner . Abstr. Program l�· 490. SEPKOSKI , J . J . , .Jr . ( 1 9 79 ) : A kinetic model of Phanerozoic taxo nomic divers ity . I I . Early Phanerozoic families and multiple eqUilibria . - Paleobiology g : 2 2 2 - 2 5 1 . SEPKOSK I , J . J . , Jr. ( 1 9 8 1 ) : A �actor analytic description of the marine fossil record . - Paleobiology £ : 36-5 3 .
Part llB. Event Stratification - Other Event Deposits .
Jurassic Bedded Cherts from the North Apennines, Italy: Dyscyclic Sedimentation in the Deep Pelagic Realm T. J. BARRETT
Abstract: Bedded cherts overlying ophiolites in the North Apennines consist essentially of rhythmic alternations of. radio larite and siliceous mudstone · beds . Sedimento logical ev� dence suggests the radiolarite beds represent radiolarian rich turbidites introduced into local basins where ambient radiolari�n-poor sediment accumulated s lowly . Hence , the bedding in the cherts owes its origin to dyscyclic rede positional processes . There is no evidence that telluric climatic controls have been ins trumental in producing the radiolarite/siliceous mudstone alternations . However , locally occurring shalier rhythms , which are separated by intervals consisting of normal radiolarite/siliceous mudstone alternations , could conceivably reflect such controls . 1 . Introduction In the North Apennines of Italy , Upper Jurassic bedded chert stra tigraphically overlies ophiolitic rock s , and . is stratig+aphically overlain by Catpion e L Z a limestone (Neocomian pelagic l imestone ) , PaZombini shale (neocomian to Albian shale and pelagic limestone) , and later shales and arenaceous' flysch (ABBATE & SAGRI , 1 9 70; ABBATE et al . , 1 9 80) . This sequence , which constitutes the Vara Complex, represents oceanic crust and overlying sediments which have been emplaced onto the continental platform as allochthonous thr.ust sheets during ocean closure in the Lower Tertiary (REUTTER & GROSCURTH , 1 9 7 8 ; ABBATE et al 1 9 80) (Fig. 1 ) . • i, .
The purpose of the present paper is to present a brief description of the bedded che�t, and an interpretation of �he bedding based on sedimentological evidence , and hydrodynamic considerations . The cherts were apparently deposited in an ocean basin of at least 1 . 5 km depth , as indicated by the non-vesicular ( 2 % vesicles) and com monly variolitic nature of the under lying pillow lavas ( c f . MOORE, 1 96 5 ; FURNES , 1 9 ? 3 ) . Detailed aspects of the stratigraphy , sedi mentology and geochemistry of the cherts are reported elsewhere ( BARRETT, 1 9 8 1 a , b) . For details on the s tratigraphy and paleotec tonic environment of the ophiolitic rocks underlying the cherts, see ABBATE et a l . ( 1 9 80) , and references therein . Cyclic and Event Stratification ( e d . by Einsele Seilacher) © Springer 1982
390
§ §
OCEAN FLOOR
WJra [omplex(Jur-Paleoc.)
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CONTINENTAL MARGIN
IIIliill Tuscan Sequence (0/igo.-Hio)
0
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Vara [amp/ex IJur.-Pa/eoc.)
Elba Flysch IU [ret)
CONTINENTAL MARGIN
ITIIlilJ Tuscan Sequence([arb.-Jur )
0
2.5
Skm
Fig . 1 . Index map and general geology of parts of East Liguria ( top left) and East Elba (right side ) . Ocean floor sequer:tces are represented by the Vara Complex, which contains ophio lites , and the Trebb ia and Canetelo sequences , which consist of flysch sequences and _ were probably formed in a continental rise-slope environment (REUTTER & GROSCURTH , 1 9 78 ) . Continental margin sequences are known or inferred to have had a continen · tal basement. These sequences were deposited in both relatively shallow and relatively deep environments , as the result of foundering o f fault-bounded marginal blocks to varying depths ( c f . BERNOULLI et al . , 1 9 79 ) . Letters indicate localities as follows . East Liguria: NC = Nascio-Cassagna ; MC = Monte Capra ; MT = Monte Tregi n ; RV = Rocchetta d i Vara. East Elba: PR = Pietre Rossi ; MC = Monte Capannello ; ML = Monte Castello; MA = Monte Arco
391 P.ROSSI
M.STREGA
M.CAPAN NELLO
VOLTER RAIO
M.CAS� TELLO
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0
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Serpentinite
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Chert wit,h limestone interbeds
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Fig . 2 . Stratigraphic columns , East Elbar measured relative to base o f Calpionella limestone . rn East Elba, the stratigraphic _ succession is consistently pil low lava/chert/Calpionella limestone (Palombini shales are in tectonic contact ) . This sequence can be traced along strike more th�n 6 km . In East Liguria, the stratigraphic succes sion is more variable . Chert overl-ies serpentinite, sedimentary ophiolitic breccias , or pi llow lava , and is generally overlain by a cher�Calpionella limes tone/Palombini shale sequence� Locally, however , , limestone is absent and chert passes up into shale, or chert is replaced laterally by shale . In East Liguri a , lateral facies changes and changes in basement composition can occur over distances of only a few km
392
393
General chert, best exposed in East Liguria and on Elba , is m thick, but occasionally reaches 1 50-200 m in (Fig. 2 ) . It consists of two main alternating �itholo siliceous mudstone (SM) ?tnd radiolarite {R} . In many areas , sections show characteristic stratigraphic changes . The basal few metres are dominated by thick reddish-brown ferruginous SM beds . The lower cherts display a striking rhythmic alternation of R and ferruginous SM beds . In the middle cherts , SM beds are much less ferruginous . In some sections , shalier rhythms , which are separa 'ted by normal intervals of· R/SM alternations , are als ? present. In the upper cherts , R beds are less frequent · and SM beds are essen tially non-ferruginous .
Representative Photographs o f the bedded chert from different in tervals within the chert sequence on Elba are shown in Fig. 3 , and bed thickness histograms in Fig. 4 . It is worth noting that a unique graded s andstone bed, consisting of phosphatic SM clasts and . radiolaria , occurs near the base of the Elban chert at three loca lities spaced over a distance of two kilometres . Thick SM beds
Fig. 3 : (A)
(B)
(C)
(D)
Monte Capannello chert section, Elba. View from base towards .top of section . Basalt's occur a few metres below cherts at base o f _ photo; l imestone begins on other side of ridge crest. Monte Capannello lower chert ; outcrop forms basal scarp in (A) . Note regular alternation of R (light} and SM (dark) beds . SM beds in lower cherts are reddish-brown , and occasionally reach 30 em in thickness . Harroner length = 30 em. Thick R bed containing elongate SM clasts overlying an SM bed which consis.ts of two layers . The lower Of the two layers is darker and more homogeneous than the upper one , and may represent turbiditic mud associated with the under lying R bed (not sampled) . Sample etched in HF; height = 1 1 em; E-6 4 , Pietre Ros s i . Monte strega middle cher t ; regularly spaced shalier inter vals impart a second rhythmicity to the sequence . _The chert intervals contain typical R and SM beds (two such R beds are inarked by arrows } . Within the shalier intervals , , thin ( 0 . 5cm) R beds may be present. Hammer length = 2 5 em.
394
BED THICKNESS HIS TOGRA M S IA ! CAP-1
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2
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Fig. 4 . Bed thickness histograms for radiolarite and s i l iceoUs mudstone beds in different parts of the chert section (CAP-1 to CAP-3 ) at Monte Capanello, Elba. This is the thickest section examined, with a thickness of about 200 m. The interval in metres foi each histogram re presents the distance of the measured section above basalt basement. {A) is from lower cherts , (B) and ( C ) are from middle cherts . Relative to lower cherts , R beds in middle cherts ·tend to be less frequent ( in terms of number per metre) and somewhat thicker. In Upper cherts (not shown) , R beds are notably less frequent. This trend is interpreted as the result of gradual reduction o f seafloor topo graphy during continuous sedimentation, leading to less frequent generation of radiolarian turbidites
and groups of R/SM alternations in the 1 . 5 m of sedime-nt overlying this marker bed closely correlate over a distance of at least 7-00m , indicating that bedding can have considerable lateral continuity . Normally, individual beds cannot be traced laterally for more than about 50 m, due to s imilarity in appearance of beds , non-continu ous exposure, and small-scale tectonic ef.fects.
395
3 . Radiolarite Beds R beds cons ist of densely packed radiolaria . in a matrix which con stitutes less than 10-20% of the bed. Mineralogically , R beds con sist of 80-90% quartz , 5--1 5 % clays and generally 1 % hematite . Radiolaria within R beds are filled with chalcedonic silica and/or microcrystalline quartz , and generally surrounded by a much finer grained matrix consisting of microcrystalline quartz , radiolarian debris r and clay minerals .
sOme R beds , particu �arly in lower cherts 1 show size grading of radioaaria , concentration of clastic detritus near their base, in crease in the amount of matrix near their top , or some combination of these (Fig. 5 ) . Such R beds , as well as many non-graded R beds , _ are also parallel-laminated. Similar features have been described for bedded cherts from various areas in' the North Apennines by GARRISON ( 197 4 ) , FOLK & McBRIDE ( 1 9 7 8 ) , McBRIDE & FOLK ( 1 9 7 9 ) , and KALIN et al. ( 1 979 ) , and from Greece and Japan by NISBET & PRICE ( 1 9 7 4 ) and IMOTO & FUKUTOMI ( 1 9 7 5 ) , respectively . Continuous and smooth size grading of radiolaria within R beds has not been observed . Grading commonly takes the form of a con cen �ration of larger radiolaria, often with ophiolitic and SM clasts , near the base of the bed, a central interval comprising most of the bed where grading is poorly or not developed, and a concentration of smaller radiolaria at . the top o f the b�d. Sor ting is poor throughout most R beds . In rare beds , a series of layers occur , each of which contains .slightly finer-sized radio laria than the preceding lower one .
On ·the basis of the sedimelltological evidenc e , R beds are therefore interpreted as turbidites ( c f . NISBET & PRICE , 1 9 7 4 ) . The gross features of R beds are in fact similar to those of distal clastic turbidites (cf . PIPER, 1 9 7 2 , 1 9 7 3 ) , namel y , bed thicknesses of a few centimetres or less , silt to fine sand grain size, and common parallel lamination. It should be. noted that structureless R beds , w�ich lack both grading and parallel le�ination, are relatively common in the bedded cherts . However·, since their megascopic appearance and mode of occurrence are the same as the graded and/ or lamina.ted beds , they are also interpreted as turbidites. This interpretation receiVes some j ustification from calculated radio larian settling velocities , as discussed in a later section.
397
(A) : Graded R bed. Matrix of bed increases and size o f radiolaria decreases i n top half of bed. I n this and following negative prints , radio laria (and silica) are black , SM beds and matrix material white. Scale bar = 5 rom . Sample 2 - 2 1 4 , Nascio . (B) : Well-graded R bed with concentrations o f SM clasts · (white} near base . Upper part of bed parallel-lamina ted. Negative print; scale bar = 8 mm. Sample 2 - 1 20 , Monte Capannello. ( C ) : Upper compound R bed and lower ungraded R bed, together with two SM beds . R bed ungraded . The upper layer of the upper R bed is parallel- laminated and displays a low-angle erosional contact with lower one ; such features indicate deposition from currents. SM beds contain a sprinkling of radiolaria. Negative print. Scale bar = 5 mm. Sample 2- 1 1 4 , Monte Capannello. (D) : Detail of SM bed. Lenticular clay-silica aggregates are common , and radiolaria rare. Note finegrained matrix around aggregates ; some aggregates merge into matrix artd -are poorly defined. Sample E- 2 2 4 , 1 krn west of Cima del Monte . Photomicrograph ; scale bar = 1 00 fl
4 . Siliceous Mudstone Beds SM beds are very fine-grained , almost devoid of whole radLolaria, and display a dense vitreous to slightly granular texture. They consist of 50-70% microcrystalline quartz , l 5 - 3 5 % chlorLte and muscovite , 0 - 1 5 % hematite, and up to a few percent corroded ra-· diolarian fragments . Microscopic claysilica aggregates , though rather vague in oUtline , can commonly be discerned. The aggrega tes occasionally have roughly spherical shapes of the same size range as the radiolaria. More commonly, they are moderately to highly lenticular , bedding paralle l , and closely pressed together . The near-absence of radiolaria in SM beds must be partly the re sult of near-surface dissolution . If dissolution had only occurred during progressively deeper burial of the siliceous sediments , then the radiolaria of R beds also should have been affected by a simi larly high degree Of corrosion. The roughly spherical c lay-silica aggregates are s imilar to those· occurring in near-surface sed iments in parts of the Pac·i fic ( SAYLES & BISCHOFF , 1 97 3 ; JOHNSON , 1 9 7 6 } . These authors found that siliceous microfossils were largely re moved by dissolution in the uppermost few centimetres or decime tres , whereas smectitic aggregates were formed , occasionally as pseudomorphs of radiolaria. RIEDEL & FUNNELL { 19 6 4 } have also
, ,,.,. '; ,:, '
:)ii'Ji!l i ): r·11
398
recorded near-surface dissolution of radiolaria in parts of the Pacific. SM beds are therefore interpreted as having formed at least partly in an analogous manner , i . e . ambient pelagic sediment accumulated slowly and experienced s ignificant near-surface disso lution of radiolaria together with the formation of clay-silica aggregates . Although most SM beds are uniform throughout , some are compound with twO distinct layers {Fig. lc) . The upper one resembles · ty pical SM, while the lower one is slightly darker and finer-grained -, and texturally more homogeneous . The lower layer contains somewhat more hematite and clays ; ·radiolaria, though scarce , are not strong ly corroded. The characteiistics of the lower layer are very simi lar to those of the mud (or mudstone) uppermost division of fine grained clastic or pelagic turbidites ( c f . RUPKE & STANLEY, 1 9 7 5 ; HESSE , 1 9 7 5 ·; 0 1 BRIEN et al. , 1 9 80) . Hence the lower SM layer is interpreted as representing the top of the underlying radiolarian turbidite . Turbiditic mud could also be present in SM beds which appear texturally uniform (cf. FOLK & McBRIDE , 1 9 7 8 ) , but its extent is not readily demonstrable . 5 . Comparison with Oceanic Siliceous Sediments
Until recently , no analogues to the North Apennine bedded cherts were known from modern oceans. Chert re_covered by the Deep Sea Drilling Project occurs eitl,ler as nodules and layers associ.ated with siliceous semi-lithified sediments , or as nodules within calcareous or procellaneous sediment (c. f . LARSON , MOBERLY et al . , 1 9 7 5 ) . Rhythmically bedded chert ,had not been reported , although chert had been recovered with a crudely bedded appearance resul ting from alternation of dense chert with porcellaneous sediment or chalk {KEENE , 1 9 7 5 ; RIECH & von RAD , 1 9 7 9 , and references the rein ) . Recently, however , drilling _in the Western Atlantic , n.ear · the Bermuda Rise, has intersected Lower Albian-Cenomanian nearly lithified sediment which in places displays lithologi � al and bed ding properties very similar to the North Apennine cherts . In the lower part of Hole 3 8 6 , Leg 4 3 , sharply-def'ined alternations o f radiolarite and mudstone are present , particularly i n cores 6 4 and 4 4 {TUCHOLKE , VOGT , et al . , 1 9 79 ) . The R beds are parallel-
399
laminated and/or graded , and some contain elongate mudstone clasts . structureless R beds are also present. Occasional mudstone beds contain two layers, the lower of which is darker and may represent turbiditic mud associated with underlying R bed. 6 . Calculated Radio"larian Settling Velocities R beds are commonly ungraded despite containing radiolaria which vary in size by a facto.r of up to several times . In an attempt to determine whether or not such a fea�ure is consistent with a tur bidity current origiri for. these R beds , model calculations were made to determine the settling velocities of radiolaria under va rious physical conditions (calculations were performed by J . LOR SONG , at Cambridge University , England} . The results indicate that for radiolaria between 50 and 250 v in thickness , having shell thickness of up to 1 5 % o f their diameter ( i . e . typical radiolaria) , and transported in a fluid of up to 20% clay content , settling velocities range from 0 .02-1 cm(sec. These settling velocities , which are maxima because non-spherical radio laria would settle more slowly , -are . lm., and of restricted range compared to the settling ve.locities of ab()Ut � .:..J Q em/sec for cla stic particles of the same size range lRUBEY, 1 9 3 3 ) . Relative to clastic particles , therefore , radiolaria should have less of a tendency to grade hydraulically. Developmen-t. of grading may be favoured by ( 1 ) -turbidity currents which carry radiolaria of large size range and de"crease in energy very slowly .and evenly with time , or ( 2 ) the presence within the currents of sediment- or cement filled radiolaria, which would act as hydraulically heavl.er clasts { such radiolaria are commonly found together with lithic clasts at the base of graded. R b�ds ) • ._.
7 . Discussion
Bedded chert is a relatively common sedimentarY facies in Mesozoic orogenic belts {see GRUNAU , 1 9 65 ) . The chert was originally depo sited either on ocean crust or in foundered basins of the conti nental margin (BERNOULLI et al . , 1 9 7 9 ; WINTEHER. & BOSELLINI , 1 9 81 ) . There appears to be a number of ways in which bedd5.ng rhythmicities can be developed. In North Apennine cherts overlying ophiolites , the b�dd j ng results primarily from the redeposition of siliceous sediment by turbidity currents , which produces radiolarian-rich
400
beds separated by radiolarian-poor siliceous mudstone of similar or greater thickness (cf . Fig. 3b) . Other bedded cherts display a different style of bedding. For example , in parts of Greece . (NISBET & PRICE, 1 9 7 4 ) and Tuscany, Italy (KKLIN et a l . , 1 9 7 9 ) 1 chert beds alternate with thin shale intercalations which, becaus e they weather recessively, are responsible for the bedded aspect visible in the outcrop . A given chert bed is , however , · generallY composite , and consists of up to four " sets " , ·e ach .set comprising a radio.larian-rich interval overlain by a mud-ri.ch interva l . Sets are interpreted by the cited authors as representing individua� turbidites , whereas the shaly intercalations are considered to be primarily inter-turbidite sediments . In theory , telluric-c:limatic fluctuatiOns could produce long-term cycli� variations within ambient pelagic sediments , with more fre-' quent dyscyclic sediment redeposition by turbidity currents super imposed upon this. Such sequences have in fact been recovered from modern oceans by the Deep Sea Drilling Project ( c f . MALDONAD0 1� 979 1 p . 3 8 5 ) . It is conceivable that the shalier intercalations occur ring in the middle portion of some chert sections could owe their origin to · telluri..c-cl imatic flucttlation s . These shalier interca lations 1 which are corrmonly 5-15 em thick, phase in and out gradu ally and are separated by up to several R/SM alternations (cf . Fig� 3d) . This indicates that the intervals between periods of increased clay deposition were notably longer than the time between deposition of radiolarian turbidites . Generation of radiolarian turbidites would be favoured in areas of strong topographic relie f , such as slow-spreading ridges or foundered continental margin basins . Differential relief is clearly indicated in EasL Ligurian by the underlying large masses of fault-scarp derived breccias , and is implied on Elba by signi ficant lateral variations in chert thickness which reflect sedi ment pending. Another possible environment would be young ocean basins , where redeposited material could be derived from adj acent continental margins . The occurrence of radiolarian turbidites within carbonate-free chert sequences requires that both source and depositional areas lay below the carbonate compensation depth (CCD) . Prior to the Cretaceous , it is likely that. the CCD occurred at shallower depths than today (BOSELLINI & WINTERER, 1 9 7 5 ) .. Henc'e , s i liceous sediments
401
could accumulate in environments which , in modern oceans , are characterised by calcareous sedimentation ( e . g . spreading ridges ) . Chert sequences which overlie ophiolites but lack radiolarian turbidites conceivably could have formed in environments such as fast-spreading .ridges , where topography is subdued and sediment redeposition limited, .or on basement highs above the range of in fluence of turbidity currents . Acknowledgements '
I am particularly grateful . to Dr . J . LORSONG for computer calculations �of radiolarian · settling velocities and analysis of the data (carried out at Cambridge University, England) . Dr . J . R . REIN (U . S . Geological Survey) critically reviewed an earlier version of the manuscript from \vhi.ch this paper is dra�m� This research was largely supported by the Natural Sciences and Engineering Research council of Canada .
References ABBATE , E � & SAGRI , M . n 1970 : The eugeosynclinal sequences . I n : Development of the Northern Apennines Geosyncline ( G , SESTINI , ed. ) , Sediment. Geol. i : 2 5 1 - 3 4 0 . ABBATl!! , E . , BORTOLOTTI . V . & PASSERINI , P . , 1 9 8 0": AP,ennine ophiolites: a peculiar oceanic crust. - I n : Tethyan Ophiolites V . 1 ( G . ROCCI , ed. ) , Special Issue , Ofioliti ( Bo l l . Gruppo Lavoro Qfiol Med . ) : 59-9 6 . •.
·
BARRETT, T . J � , 1 98 1a : Geochemistry and mineralogy of Jurassic bedded chert Overlying ophiolites in the North Apennines . Ital. Chern. Geo l . { in press ) . BARRETT , T . J . , 1 9 8 1 b : Stratigraphy and sedimentology of Jurassic bedded chert overlying ophiolites in the North Apennines , Italy. · - Sedimentology { in pres s ) . BERNOULLI , D Q , KXLIN , O . & PA�ACCA , E . , 1 97 9 : A sunken contin ental margin of the Mesozoic Tethy s : the . Northern Apennines . - Assoc. Sedim. franc. Spec. Pub l . � : 1 9 7 - 2 1 0 . BOSELLINI , A. & WINTERER, E . L . ·, 1 9 7 5 : Pelagic · limestone and radiolarite of the Tethyan Mes0zoic: a genetic . model. Geology ,4 : 279-2 8 2 . FOLK , R . L . & McBRIDE , E . F . , 197 8 : Radiolarites and their rela tion to subjacent "oceanic crust'' in Liguria, Italy. J . Sediment , Pet. �� : 1069-1 102 . FURNES , H . , 1 9 7 3 : variolitic structure in Ordovician pillow lava and its possible significance as an environmental indicator. - Geology ! = 27-30.
402
GARRISON , R . E . , 1 9 7 4 : · Radiolarian cherts, pelagic limestones and igneous rocks in eugeosynclinal assemblages . - In: Pelagic Sediments : On Land and Under the Sea ( K . H . HSU & H . C . JENKYNS, eds. ) , Spec. Pub. Int . Assoc. Sediment. ! : 3 6 7- 3 9 9 . GRUNAU , H . R . , 1 9 6 5 : Radiolarian cherts and aSsociated roCks in space and time. - Eclog. Geo l . Helv. �� : 1 57-208 . HESSE, R . , 1 9 7 5 : Turbiditic and non-turbidic mudstone of Cre taceous flysch sections of the East Alps and other turbidite basins. - Sedimentology �� : 3 87- 4 1 6 . IMOTO , N . , & FUKUTOHI , M. , 1 9 7 5 : Genesis o f bedded chert·s i n the Tamba Belt, Southwestern Japan. - Geol . Collaboration Soc. of Japan , Mon . �� : 3 5- 4 2 . JOHNSON , T . C . , 1 9 7 6 : Biogenic opal preservation in pelagic sedi ments of a small area in the eastern tropic Pacific. Bull. Geol. soc. Am . �z : 1 27 3 - 1 2 8 2 .
KXLIN , 0 . , PATTACA , E . & RENZ , O . , 1 9 7 9 : Jurassic pelagic deposits from Southeastern Tuscany : aspects of sedimentation and new biostratigraphic data. - Eclog. Geo l . Helv. · z� : 7 1 5-76 2 . KEENE , J . B . , 1 9 7 5 : Cherts apd porcellanites from the North Pacific, DSDP Leg 3 2 . - In: Initial Reports of the Deep Sea Drilling Proj ect , Vol. XXXII ( R . L . LARSON , R. MOBERLY , et al. ) . u . s . Govt. Printing Office , Washington , o . c . , 4 2 9-507 .
LARSON , R . L . , MOBERLY , R. , et al . , 1 97 5 : Initial Reports of the Deep Sea Drilling Project, Vol . XXXI I . u . s . Govt . Printing Office , Washington D . C . , 980 pp.
MALDONADO , A. , 1 9 7 9 : Upper Cretaceous and Cenozoic depositional processes and facies in the distal North Atlantic continental margin of Portugal , DSDP Site 3 9 8 . I n : Initial Reports of the Deep Sea Drilling Project, Vol . XLVII , Part. 2 ( J . -C . SIBUET , W . B . F . RYAN , et al. ) , u . s . Govt . Printinq Office , Washington D . C . ; 3 7 3 -406 . McBRIDE , E . F . & FOLK , R. L . , 1 97 9 : Features and origin of Italian Jurassic radiolarites deposited on continental crust. J . Sedimen t . Pet . ��: 8 3 7 - 8 6 8 .
MOORE, J . G . ; 1 9 6 5 : Petrology o f deep-sea basalt near Hawaii. Am . J . Sci . &gg: 40- 5 2 .
NISBETH, E . G . & PRICE, I . , 1 9 7 4 : Siliceous turbidites: bedded cherts as redeposited ocean ridge-derived turbidites. - I n : Pelagic Sediments : On. Land and Under the Sea ( K . J . HSU and H . C . JENKYNS., eds . ) ; Spec. Pub l . Int. Assoc. Sedirn�;nt. 1 : 3 5 1- 3 6 6 .
0 1 BRIEN, N . R. , NAKAZAWA , K . & TOKUHASHI , S . , 1 9 8 0 : Use of clay fabric to distinguish turbiditic �nd hemipelagic siltstones and silts. - Sedimentology �z : 47-6 1 . PIPER, D . J . W . , 1 9 7 2 : Turbidite origin of some laminated mudsto nes . - Geol. Mag. �Q�: 1 1 5 - 1 2 6 .
403
PIPER, D . J . W . , 1 9 7 3 : The sedimentology of silt turbidites from the Gulf of Alaska. - I n : Initial Reports of the Deep Sea Drilling Project, Vol. XVIII { L . D . KULM, R. von HUEN E , et al . ) . U . S . Govt . Printing Office , Washington D.. C . : 8 47--8 6 7 . RIECH, V . & von RAD , U . , 1 97 9 : Silica diagenesis i n the Atlantic Ocean: diagenetic potential and transformation s . - I n : w . HAY and W . B . f . RYAN ( ed s . ·) , Deep Sea Drilling Results in the Atlantic Ocean: continental Margins and Paleoenvironment , M . EWING Series d , American Geophysical Union (Washington, D . C ) : 3 25-340. RIEDEL, W . R. & FUNNEL L , B . M . , 1 9 6 4 : Tertiary sediment cores and microfossils from the Pacific · ocean floor. - Geol. , Soc. London Quart. Journ . �&Q: 305-3 6 8 . REUTTER , K . -J . & GROSCURTH , J . 1 1 9 7 8 : The pile of nappes i n the Northern Apennine s , its unravelment and emplacement. In : Alps , Apennines and Hellenides . Inter-Union Comm . on Geodynamics Scient . Rep . No . J�; 2 34-24 3 . RUBEY , W . M . , 1 9 3 3 : Settling velocities of gravel , sand and silt. - Am. Jour n . Sci . �g : 325-33 8 . RUPKE , N . A. & STANLEY , D . J . , 1 9 7 5 : Distinctive properties of turbidite and hemipelagic mud layers in the Algero-Balearic Basin , Western Mediterranean Sea . - Smithson . Contrib . Earth S C i . No . lJ r 40 pp . SAYLES , F . I . & BISCHOFF , J . L . , 1 9 7 3 : Ferromanganoan sediments in the equatorial East Facific . - Earth Planet. Sci . Lett. l� = 3 30-3 3 6 . '
TUCHOLKE , B . , VOGT , P . R . , et al . , 1 979 : Initial Reports of the Deep Sea Drilling Project, Vol . XLI I I . u . s . Govt . Printing Office , Washington D . C . , 1 1 1 5 pp . . WINTERER, E . L . & BOSELLINI , A . , 1 9 8 1 : Subsidence and s edimenta tion on JurasSic passive continental margin , Southern Alps , Italy. - Am . Ass . Pet. Geol . Bul l . g�: 394-4 2 1 .
Quartz-sandy Allodapic Limestones as a Result of Lime Mud-Raising Clastic Turbidites U. MAIER-HARTH
Abstract: The aim of this study was the interpretation of a Maastrichtian rhythmic limestone-marl succession in the southern Pyrenees , Spain. Bedding features including flute mark s , small scale sedimentary structures , s lumping and faunal content of the limestone layers as well a:s the facies distribution within the basin indicate turbidity current . origin. Sedimentation patterns were controlled by carbonate · producing organisms , synsedimentary tilting of a fault block with the up-lift of the Montsech anti cl�ne and finally an advancing delta . At the front of this- mi grating delta clastic turbidites were initiated by earthquakes or oversteepEming of the delta slope. On thei.r way toward the basin floor these turbidity currents raised and absorbed shelf and prodeltaic c_occolithic mud . The resulting allodapic limesto nes were deposited in the Ar9n area in a water depth of about 300 m .
1 . Introduction The present study iS apreliminary report of a thesis and deals with the filling of a narrow, synsedimentary subsiding basin of Maas trichtian age ( Upper Cretaceous ) in the Spanish Pyrenees around
Aren 70 km north of Lerida ( Fi g .
1).
• MADRID
5 PA IN
Fig . 1 . Location of the study area in Spain Cyclic and Event Stratifi cation (ed . by Einsele/Seil acher) © Springer 1982
405
In good exposures parallel to the basin axis basin plain mudstones
of Campanian age as well as interbedded marl and limestocies , deep water mudstones and a nearshore sandstone sequence of Maas,t rich tian age can be examined (Fig. 2 ) .
Garumnian red beds Orrit-member : sandstone lscles-member : siltstone, marl Sapeira-member ' sandstone
�
Upper Sobrecasfell- member: limesto e with mad Lower Sobrecastell-member: mar! and lliicoe,ton•e
witi;l limestone
I
2km
��-�
0
Section Village
,Fig/ 2 . Geological sketch map of the study area
The aim of this s tudy was the .interpretation of a Maastrichtian
rhythmic liffiestone-marl success ion in the southern Pyrenees , Spain. Because the Maastrichtian sediments are eroded north o.f the study area and are overlain by younger sediments south of ArBn, only a
partiaL analysis of the basin was poss'ible .
Previous research in the study are� and its surroundings was published by GHIBAUDO , MUTTI and ROSELL ( 1 9 7 4 ) , LIEBAU ( 1 9 7 ) ) , NAGTEGAAL ( 1 9 7 2 ) and VAN HOORN ( 1 9 70 ) among others . 2 . Stratigraphic Sequence o f the Ar&n-Basin The sedimentary sequence of the study· area begins with bluish-grey marls , here called Sola Member , which belong to the Vallca�ga For
mation (Fig. 3 ) . They are interpreted as offshore sediments w Some channels with well preserved foraminifera, alg,ae and corals can be also seen in this member . After GHIBAUDO, MUTTI and ROSELL ( 1 9 6 7 )
the boundary between Campanian and Maastrichtian should be in this
�
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[
,l 1
6
7
8
''" '' ,, , :..:.��;;}·:�-,!� �;o:!�c:�:,;;,·;�;-:O;-i� �t\.i tbid\tes re' :·,-�-�&�· �ft"S ___
offshoro
-�
nfkl
offshore
" ,not measured I flute marks, md . ... not meosuredl
a) Stratigraphic cross section of the Aren area: ( 1 ) Garumnian red beds , ( 2 ) nearshore sandston e , ( 3 ) offshore siltstone and marl, ( 4 1 5 1 6 ) sandstone- { 7 ) limestone with mar l , ( 8 ) marl and limestone , ( 9 ) marl with limestone and ( 1 0) offshore marl .
b ) Facies interpretation and directions· of current and s lumping
407
Plate I ( 1 ) Interbedded marl and limestones of the Barreda Member ( 2 ) S lump scar in the Upper Sobrecasteil Member . ( 3 ) Channel filling consisting of intraclasts , bioclasts (bryozoans ) and sandstone in the upper part of the layer. ( Scale is 1 em) ( 4 ) Horizontal laminated limestone showing alterna,ting b iomicrite and quartz-rich microsparite . {Scale is 1 em)
408
' Vallcarga Formation but was not found at the present time by the author. The overlying Barreda Member consists of marls with some· calcareous intercalations ( Plate I/ .1 ) . In the lower Maastrichtian
unit, the Sobrecastell Member , interbedded. marls and limestones dominate and become more and more calcareous in its upper part ( Plate I / 2 ) . In the eastern part, the percentage of quartz and shallow water fossils ( coquinas , arenaceous foraminifers)
towards �he overlying deltaic sediments (Sapeira Member ) . Both Sobrecastell and Sapeir a Member show intensive penecontemporaneous slumping related to tilting movements (Plate I/2 ) . The overlying
mudstones ( Iscles Member) grade upward into nearshore sandstones (Orrit Member = Aren sandstone after MUTTI , ROSELL and GHIBAUDO, · 1 9 75) .
. 3 . The Marl-Limestone Sequence of the Sobrecastell and Barreda Member 3 . 1 . Field Observations The beds have been simply divided into calcareous ones and marl .
Some of the calcareous beds are rich in quartz sand. In weathered condition all lower bedding contacts are sharp and chiefly graded bedding can be seen in coarse-grained and quartz-rich beds . In fresh exposures without weathering a d ifference between marl and limestone is hardly perceptible . Some of the marl beds and some calcareous layers seem to be homogenized by bioturbation.
The thickness of calcareous beds varies between 3 em and 36 em
but only a small portion ( mostly channel fillings) is more than 1 5 em thick (Fig. 4 ) . %
10
median
bed
dista l - fan fan
thickness :
fringe
fringe - outer ton
channel
7 10 16
2 2 >23
em cm em
em
F i g . 4. Thickness variation of calcareous layers o f the Barreda-Member , Lower and Upper Sobrecastell-Member, represented as frequency curve
409
Sediments filling flat channels often show bed thicknesses changing rapidly { 3 5 em to 0 ern within a horizontal distance of 5 m) and are commonly coarser grained (Plate I /4 } . The thickness of marl beds varies from 0 em ( erosion by overlying calcareous bed) up to s everal meters in the Barreda Member and from 0 em to 28. em in the Sobreca stell Member .
Apart from composition , grain size , grading etc . , a difference in
bed thickness can be discerned between more distal tUrbidites and more proximal turbidites or channel fillings . The average bed thick ness of distal turbidites amounts to about 7 em, whereas channel fillings have a mean bed thickness of about 1 6 em.
A sequence analysis after RICCI LUCCHI ( 1 9 7 4 ) shows thinning-up cyc
les , es�ecially in the western area, reflecting lateral migration or abandonment of a channel or regrading of a depositional lobe. In
contrast the thickening upward cycles are a result of prograding de
pos.itional lobes pr-evailing ih the east (Fig. 5 ) .
Only a few erosional flute and groove casts can be found . · Most o f them a s well a s channels indicate currents coming more o r less from
the s outh (SE-SW} . The channels which are common in the Sola Member and in the western Sobrecastell Member can be divided into two groups : { 1 ) rWide channels with gently dipping flanks , more than 1 0 m Wide . and up to -3 5 . em deep . ( 2 ) Deeply eroded channels with steep flank s ,
some dozens o f meters wide and u p to 2 . 4 m deep. Both types are fi l led with coarse grained material (grains of quartz and micro breccia, bioclasts like bryozoans , bathysiphons, corals and larger foraminifera , Plate I / 3 ) .
Slumping has left behind s lump scars and s lump deposits (Plate I / 2 ) .
The latter are preserved as rotational s lumps or, if suspension was
reached , as turbidites . Measurements of s lump axes indicate an orientation which changes from SE-NW in the Barreda and Lower Sabre castell Member to SW-NE in the Upper S6brecastell Member . This shows that slumps moved firstly in a northeastern direction and then due to subsidence · in the western part of the basin in a northwestern direction {Figs . 3 and 5 ) .
3 . 2 . Internal Sedimentary Structures In some calca�eous layers , mainly in sandy and not totally biotur
bated beds , the whole Bouma sequence can be observed. Graded bed ding occurs preponderantly in the a-divis ions , but can also be seen
@ Section :
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111
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I �'-
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.
w
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Fig. 5 . Vertical sequence analY.sis (a) from the Barreda Member to the Upper Sobrecastell-Member showing regrading cycles ( deepening) in the west and prograding cycles ( becoming shallower) in the east. se·ction ( a ) and section along the long axLs of the basin (b J'd ) as well a9 the _.paleio ge9graphical maps. ( c , _e) are- not to scale! · =
0
\ •AREN
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itl
l -<1 ��
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=
_
411 in ea'ch laminae of the b-division . Ripple-drift and convolute larrli nation in the c-division indicate current directi Ons according to the flute marks (Fig. 3 ) . Load casts and micro-faults caused by different compaction are fai.rly common in this interval . Most of
the calcareous layers sta-rt with this c-division or the overlying
d-division which has well developed parallel laminae passing gra ·dually into the interturbidite e-division. Some layers show rhythmic grading probably caused by ' subevents ' or spill-overs of submarine channels ..
Some channel fillings , e . g . at the transition from the calcareous
Sobr'e castell Member to the deltaic Sapeira Member , show typical grain flow texture (MIDDLETON and HAMPTON , 1 9 7 6 ) with inverse gra
ding, imbrication struc.ture , intraclasts , bryozoans and large bored shells . 3 . 3 . Petrography In the calcareous beds the carbonate content varies between 4 7 %
in the Barreda Member and 9 1 % in the uppermost part of · the Sabre castell Member . Biomicrite dominates here and consists mainly of re crystallized coccolith-skeletons and smaller quantities of bryo zoans , particularly in the western part of the basin. Biosparite
occUrs only in coarse grained channel fillings . Microsparite is
the result o f a recryStallization of micrite-calcite after Mg++ � ion migration to clay minerals has occured (FOLK, 1 9 7 4 ) . This process is commo� in poorly sorted clay containing micrite such as in debris f lows . In this study microsparite was found as dark la minae in the horizontal laminated d-divisions (Plate I /4 ) .
Quartz which can be found in all beds amounts up to 40% and attains
grain sizes between 0 . 04 mm and 0 . 1 6 mm. Sphericity varies between 0 . 5 and 0 . 9 and the aVerage roundness is 0 . 3 . This points to a sand neither of windblown nor of beach and nearshore origin (KRUMBEIN and SLOWW, 1 9 5 1 ) . Feldspar, pyrite , limonite , ruti le , and in coar .1"
'
ser beds intraclasts are less frequent. Content
The faunal content of channel fillings and proximal turbidites
not only differs in grain size but also in mal turbidites mainly consist of spiculae, and fragments of coccoliths and planctonic the channels contain typical shallow water
composition. The proxi
calcisphere s , pelloids foraminifera, whereas organisms such as red
412
grainsize
PIPER-
;
m"d
m"d
m"d m"d mud-silt
structures
division
sediment, f!lOStly bioturbated
., ., ., d
silt-
ripple and convolute lamination I
""d
,.,.
,.,.
b
to early diagenetic
lamination with graded laminae
imbrication sole marked surface
Fig . 6 . Ideal Bouma sequence. of sedimentary structures in a turbidite bed ( a fte� PIPER, 1 9 7 8 } algae, corals , bryozoans , benthonic and arenaceous foraminifera de
rived from small reefs of submarine sw.el],s or the surrounding coast line. 3. 5 . Biot.urbation Four types of trace fossils can be· distinguished:
( 1 ) The grazing
trace of a meandering phyaosiphon which is typical for flysch depo
sits (Plate II/ 3 ) . This is a good facies indicator 1 but it gives no evidence concerning water depth (SE ILACHER, oral communication) . ( 2 ) The feeding trace of ahondrites which descends through the
upper part of the layer and spreads out in a certain level (Plate I I/ 1 ) . It is conspicUous that 'large forms are restricte·d to sandy and silty calcareous layers (Plate II/4 ) . Chondrites gives neither information on bathymetry nor facies but clearly make visible the
partial erosion of turbidite layers by following events (Plate II/1 ) . ( 3 ) The backfill structure of sao l i a ia parallel to the bedding pla
ne and ( 4 ) unidentified, unbranched tubes o f about 0 . 5 em to 1 em in diameter passing oblique to the bedding plane .
413
Plate II ( 1 ) Two events at· the top ( 2 ) As terosoma ( 3 ) Meandering ( 4 ) Chond'!'i tes
in one calcareous layer . Recolonisation by Chondr i t e s . (Scale is 2 em). s p. Phyeosiphon sp.
sp.
The only trace indicating a reliable water depth i s the radial form of a s terosoma (Plate I I / 2 ) which occurs just below wave base ( CHAMBERLAIN , 1 9 7 8 ) . Here it is restricted to the top surface of the Sapeira delta formation and shows that the final deposition of allo dapic limestone took place in a water depth of about 1 50 m .
414
4. Discussion 4 . 1 . The Turbidite Origin of the Sobrecastell and Barreda Member The sandy and calcareous beds of the Sobrecastell and Barreda Member are here interpreted as depositions of turbidity cm;rents for:· the following reasons :
( 1 ) They show ful l , top-cut or base-missing Bouma sequences with graded bedding, horizontal lamination etc . . ( 2 ) They often truncate underlying beds , and therefore qn autochtho
nous microfauna is rare . ( 3 ) Load casts and above all ripple-drift cross-lamination are symptoms of rapid sedimentation which amounts to 1 5-20 em in 1 000 years i . e . , for one limestone-marl-couplet. Accordingly one event occured ,nearly every 1 000 years .
{ 4 ) Beds wedge out and sometimes show a rapid change of thickness
due to channeling. ( 5 ) The ichnofauna shows typi�al post-:-turb.idite grazing traces and f�eding traces , repopulating the turbidite layers from top to
bottom.
4 . 2 . Facies Model There are some reasons to believe in a transition from basin plain sedimentation to outer fan sedimentation in the Aren-Orrit area ( sections K, L, M) during lower Maastrichtian time :
a ) The Sola-Member and parts of the Barreda Mamber -- very flne la minated calcareous layers of clay- and silt-size (maximum bed
thickness 1 0 em) alternating with thick marl layers (Bouma d -and e ) -- represent a very distal sedimentation of turbidity currents
in a basin plain environment . .Grazing and feeding traces are fre
quent and some ripples and current marks indicate currents flo
wing more or less towards the north . Only ·small-scale s lUmp de posits occure in these members . The Lower Sobrecastell Member shows calcareous layers of a maxi mum bed thickness of 1. 5 em interbedded with marl . Horizontal la mination with grading ( Bouma c and d) Or Bouma bed ·sequences with ripples , load casts and fine lamination can be found often in coarser beds and point to a fan fringe environment .
Ungraded massive calcareous layers of a grain size between clay
and sand prevail in the Lower and mainly in the Upper Sobreca stell Member . The maximum bed thickness amounts to 20 em whereas
415
middle fan: distributary channel
ungraded mataive bOO
outer fan : ,..., Bouma a
maximum bed thickness 20 em Sllnd·, silt- and day-size
origin: rapid deceloration of
turbidity current at the end of distributary channel stump scars frequent
fan fringe : Bouma bed or cd horl�onta!ly !aminated bed �lth grading
rrmximum bed thicknMS 15 em fine·SIInd, silt and clayey silt
origin: slow deceleration of turbidity current ""
break-up of clayey floes and
gradual deposition of coarse mllterial slump deposits frequent basin plain: Bouma d very fine laminated
bed thickness below 10 em
silt end clayey silt
origin: distal sedimentation of turbidity current only sma!!�le slump deposits
Fig . 7 . Fan model with horizontal facies distribution (partly after PIPER, 1 9 7 8 and VAN VLIET, 1 9 7 8 )
t
• -
-
the marls are only a few em thick. Feeding traces are more frequent than grazing traces . These facts ·as well as the presence
of slump scars and channels point to an outer fan ¢nvironrnent. After PIPER ( 1 9 78 ) ungraded layers trace to a rapid deceleration of a turbidity Current at the end of a ffiiddle fan dis tributary
channel (Fig. 6 ) . b ) A sequence analysis from the Barreda Member to the Upper Sobre cast;ell Member shows regrading c Ycles in the east and prograding cycles in the west {Fig. 5 ) .
Fig . 7 shows a fan mOdel with a horizontal facies distribution obtai ned by transposing the vertical section in Fig. 3 in a horizontal sequence . 4 . 3 . Mixed Type Fan Deposits The most probable explanation for the occurence of turbidites is the emergence of the Montsech anticline in the south and subsidence
of the Aren area in the north , as well as the rapid advancing of
the Palaeo- Ribagorzana delta from the south which shed cl-astic turbidi.tes into shore region {Figs . Sa and b ) . Passing over pelagic coccolithic mud the turbidity current took up this calcareous mud,
416
and the mixture floWed towards the basin through submarine channels ( see also : SCHOLLE , 1 9 7 1 ) . However , in times with several short
interval events , not enough lime mud was available (because cocco lithic lime production is s lower than clastic accumulation) and
some clastic turbidity currents reached the area of deposition
out absorbing any lime mud . These quartz-sandy beds as well as zons containing plenty of s lump-scars may reflect periods of inten
se tilting, triggering turbidites by earthquakes and oversteepening of the· delta s lope by rapid accumUlation of terrigeneous material in the Palaeo-Ribagorzana delta. 5 . Palaeogeography and Concluding Remarks During the Maastrichtian the Tremp basin in its western part was . subdivided in.to the Puimolar submarine swell a�d the Ar&n-Sapei ra basin (see a:1so VAN HOORN , 1 9 70 ) At the same time uplift of the .
Montsech anticline caused a steepening relief and an accumulation of terrigenous sediments at the advancing Palaeo-Ribagorzana delta front. From time to time d�lta front turbidites were formed , rai sing and absorbing the surrounding coccolithic mud and flowing
through submarine Channels towards the basin in the north (Fig. Sa) .
As a consequence of subsidence in the western part of the basin (which is probably due to salt movements in the underlying - Triassic sediments ) and perhaps due to Coriolis effects ( READING , 1 9 7 8 ) channel migration from east to west occured . This is indicated by slumps and current directions in the Aren area ( Fig . 8b) . �he predominance of relativly thin Bouma ( c , d , e ) turbidites1 the
absence of great lateral facies c�ange , and the thickening upward
sequences lead to the conclusion that most of the calcareous lay ers were deposited in the Ar&n area in an outer ·tan .environment.
Only on the submarine swell west o f Aren sedimentation of alloda pic limestone started in a middle fan environment , indicated by
channelin g , coarse sediments and thinning-up sequences (see also VAN VLIET , 1 9 7 8 ; MUTTI and RICCI-LUCCH I , 1 9 7 2 ) . The early deposi
tion of sandy allodapic l imestones may have taken pla� e at a water depth of 3 00-500 m and ended at a depth of about 1 50 m . Tilting mo vements of the A!&n block caused rapid prograding o f the Palaeo
Ribagorzana. delta and regrading in the wes t . In the Sapeira area the depositional rate was greater than the rate of subsidence allo
wing the accumulation of nearshore sandstone (MUTTI , ROSELL,
417
a
0
n
t
'
.
'
h
b
' 7
z. N
Fig . 8 . Model of the western Tremp basin during (a') early and (b) midd_le Maastrichtian time'
GHIBAUDO and OBRADOR, 1 9 7 5 ) . In contraSt, in the subsiding western
part offshore mudstone was deposited . The filling of the entire
basin ended with the deposition of the nearshore ArEm sands·tone .and the overlying continental sediments .
Acknowledgements I wish to · thank Professor EINSELE for his guidance and continued support throughout the course of this work . I also thank Professor LUTERBACHER, Professor SEILACHER and Dr . LIEBAU for their informa tive discussions and helpful suggestions . My thanks also go to my
colleagues for their constructive criticism and �riendly discussions . This is a preliminary report of my thesis and was financially supported by the SFB 5 3 which is gratefully acknowledged .
418
References CHAMBERLAIN , K . C . ( 1 9 7 8 ) : Recognition of trace fossils in cores . I n : SEPM Shourt course No . 5 : ·Trace fossil concepts , Oklahoma \ City. FOLK , · R . L . ( 1 9 74 ) : The natural history of crystalline calcium car bonate : Effect of magnesium content and salinity . -J . Sed . Petr·. 44 / 1 , 40-5 3 . GHIBAUDO, G . , MUTTI , E . & ROSELL , J . ( 1 96 7 ) : Las aren�scas de Aren ( Cretacio sUperior en su · localidad tipo) . I n : "ROSELL , J . : VII Congreso del grupo espanol de sedimentologia. GHIBAUDO, G. , MUTTI , E . & ROSELL , J. ( 1 9 7 4 ) : Le spiagge fossili d.e lle arenarie di Ar6n (Cretacio superiore) nella valle Noguera Ribagorzana (P irenei centromeiidionali , province .di L&rida e Huesca, Spagna) . I n : Mernorie della Societa Geologica Italiana, vol . �' 4 9 7 - 5 3 7 . KRUMBEIN , W . C . & SLOSS, L . L . { 1 9 5 1 ) : Stratigraphy and Sedimentation . Freeman and Company, San Francisco.
LIEBAU , A. ( 1 9 7 1 ) : Die Ableitung der palOkologischen Systematik einer oberkretaZiSchen Lagune . Bull . Centre Rech . . Pau SNPA, �� 577-59 9 . MIDDLETON , G . V . & HAMPTON , M . A . ( 1 9 7 6 ) : Subaqueous sediment trans port and deposition by sediment gravity f lows . I n : STANLEY, D . J . & SWIFT, D . J . P . (ed . ) _: Marine sediment transport and enyiron m�ntal management , 1 9 7-2 1 8 , John Wiley, New York . MUTTI , E . & RICCI-LUCCHI , F . ( 1 9 7 2 ) : Le torb±diti del l ' Appennino settentrionale : indroduzione all ' analisi di facies . - Mern. Soc. geol . Ital . , 1.1_, 1 6 1 - 1 9 9 . MUTTI , E . , ROSELL, J, GHIBAUDO , G. & OBRADOR, A . ( 1 9 7 5 ) : The upper Cretaceous Ar€m sandstone in its type-area. I n ; The sedime-ntary evolution of the paleogene south pyrenaen basin . - I . A . S . 9 t� International Congre s s , Nice. NAGTEGAAL , P . J . C . ( 1 9 72 ) : Depositional history and clay minerals of the upper Cretaceous basin in the southcentral Pyrenees , Spain . Leidse Geologische Mededelingen , 4 7/2 , 2 5 1 - 2 7 5 . PONS , J . M . ( 1 9 7 7 ) : Estudio estratigraphico y paleontologico de los yacimientos de rudistidos del cretacico s up . del Prepirineo de la prov. de L6rida . - Universidad Autonoma de Barcelona , Publi caciones de Geologia No 3 . READING, H . G. (ed . ) ( 1 9 7 8 ) : Sedimentary environments and facies . Blackwell Scientific Publications , Oxford London Edinburgh Melbourne . RICCI-LUCCH I , F . ( 1 9 7 5 ) : Depositional cyCles in two turbidite for mations of northern Apennines (Italy) . -J . sedim. Petrol . , 4 5 , 3- 4 3 . SCHOLLE , P . A . ( 1 9 7 1 ) : Sedimentology of fine-grained deep-water carbonate turbidites, Monte Antol a Flysch ( Upper Cretaceou s) , Northern Apennines , Italy . - Bull . geol . soc. Amer . ,, vel. -82 , 6 2 9 -6 5 8 . VAN HOORN , B . ( 1 9 70 ) : Sedimentology and paleogeography· of an upper Cretaceous turbidite basin in the south-central Pyrenees , Spain . - Leidse Geologische Mededelingen , val . �� 7 3 - 1 5 4 .
VAN VLIET, A . ( 1 9 7 8 ) : Early Tertiary deepwater fans o f Guipuzcoa, Northern Spain . In : STANLEY, D . J . and KELLING, G . : Sedimentation in submarine canyons 1 fans and trenches , 1 90-209 ; Dowden, Hutchin son and Ros s , I nc . , Stroudsburg, Pennsylvania .
Belemnites as Current Indicators in Shallow Marine Turbidites of the Santonian Bavnodde Gr0nsand, Bornholm (Denmark) R. SCHM!DT
Abstract: Belemnite orientations suggest turbidite origin triggered by local faults . The Bavnodde Gr¢nsand is a sequence of glauconitic sandy marls , 1 80 m in thickness , which are rhytmically interbedded with distinct layers of sands ·tone and nodular sandstone · {Fig. 1 ) . These sand stone beds are typically graded in the following sequence ( from bottom to top) :
a ) Fine gravel and coarse sand containing remairis of· sponges , pelecypods and belemnites . The disarticulated pelecypod valves
{preserved as hollow casts) are commonly embedded · in a vertical position; rostra are variously inclined to the bedding plane, but show a unimodal azimuth orientation {F�g . 2 ) indicative for alignment by currents ( FUTTERER , 1 9 7 8 ) . Characteristically,
this coarse unit has become silicified during diagenesis . b ) Medium tO fine sand with planar lamination in the lower and ripple cross lamination in the upper part. c) Medium to fine sand similar to ( b ) , but bioturbated from the
top, which is covered by a layer of limonitic clay . In thinner
sandstones this bioturbation may reach down to the coarse grained
bottom unit ( a ) . Since the nodular sandstones are always thin ner, they are bioturbated throughout. The observed features clearly indicate rare event deposition . Considering the "marginal pelagic habitat" to which these sedi
ments are ass igned ( DOUGLAS & RANKIN, 1 9 6 9 ) , one might expect
storm events to be the most likely cause for the graded sandstone
r.
deposition. Nevertheless , a turbidite origin is assumed for the following reasons :
1 . The' unimodal belemnite orientation suggests a current rather
than a wave event (which should result in a bimodal orientation ) . 2 . The high concentration of belemnites ( about 30 specimens in 4 dm 3 of sediment) in unit ( a ) indicates accumulat i on by lateral Cyclic and Event Stratification (ed. by Einsele/Seil acher)
420 Profil
Profil 1
2
I e
d
c b a
f----- 1 m --------j
/Jlilli[]] 8 �
-
fi!iillill
non-cemented glauconitic sandy marl (clay , silt , fine. sand)
less cemented sandstone nodule layer ( fine gravel to medium sand) non-·cemented medium to fine sand with hardened lirnoititic clay on top ( uni·t c )
bioturb
lamellar and cross bedded medium to fine sand, non cemented (unit b ) fine gravel t o coarse sand with orientated bel �mnites a t base, partially biotubated (unit a ) , commonly silicified
Fig. 1 . Studied section of the Bavnodde Gr¢nsand. Diagrams show grain size of washed-residue >0 . 1 mm in weight percent for samples a-k. Classi fication of grain size follows DIN 4 1 8 8 ( 5 7 )
421
Cretaceous Tr i a s s i c - J u r a s s i c Pa l e o z o i c Pre - C a m b ri a n
Fa u lts
�{�[\-;�
�
�
CD '<
N •
•
•
•
• • • • • •
5 km
I
40%
• •
•
• • • • •
•
•
•
Fig. 2 . Geological map of Bornholm (modifled after GRY , 1 9 60 ) with sma l l arrow lndicating area of s tudy . Rose-diagram to the right shows the current direction indicated by belemnite orientation (Statistical error ± s o ) transport rather than by ,autochthonous winnowing, which would
require reworking of about 50 m of marl for each bed .
3 . The current direction in all measured beds is perpendicular to the strike of � local fault system ( F� g . 2 ) . Since these faults exceed 1 krn of vertical displacement , they may well have been
active already by Mesozoic times {GRY , 1 9 6 0 ) . They thus provided the paleos lope necessary to create turbidity currents following disturbances by storms or earth quakes in shal lower areas .
E
422
Acknowledgements Thanks are due to the Bundesanstalt flir Geowissenschaften und Roh stoffe , Hannover and to T . AIGNE R ( Tlibingen) , D r . E . FUTTERER (Kiel) Pro f . Dr . A . SEILACHER (Tlibingen) for valuable advices .
r
References DOUGLAS, R . G . & RANKIN, c . ( 1 9 6 9 ) : Cretaceous Planctonic Foraminife ra from Bornholm and their zoogeographic significance . - Lethaia, � · 1 85 -2 1 7 .
FUTTERER, E . ( 1 9 7 8 ) : Untersuchungen tiber die Sink- und Transportge s chwindigkeit biogener Hartteile . - N . Jb. Geol . Palaont . Abh . 1 J,�� · 3 1 8- 3 5 9 . FUTTERER, E . ( 1 9 7 8 ) : Fossil-Lagerstatten Nr . 4 4 : Studien tiber die Einregelung , Anlagerung und Einbettung biogener Hartteile im StrOmung_skanal . - N. Jb . Geol PaHion t . Abh . , J�g : 8 7 - 1 3 1 . GRY, H . ( 1 9 6 0 ) : Geology of Bornholm (Guide to excursions NOS A40 and C 4 5 ) : - IGC Copenhagen (Th . SORGENFRE I ) . •
I
'
.
· Habits of Zircon as a Tool for Precise Tepbrostratigraphic Correlation J. WINTER
Abstract : Airborn volcanic ash layers can be fingerprinted not only by mineral composition , trace e lements etc . , but also by the crystalographic habits of heavy minerals , which are preserved even in highly altered bentonite s . The value of suCh "crystalographic index fossils" for high-resolution correlation is shown for circons in Devonian bentonites of the . Eifel-Ardennes region. In many sedimentary sequences series of volcanic ash layers are . abunditn t , especially in orogenic belts with volcanic-arc and ocean. island volc�nism. Some series in Europe are the bentonitic ash layers in the Ordovician of Wales and Sweden, highiy altered ash layers in the Devonian and Carboniferous of the Variscan, and
triassic tuff layers in the. Alpine fold belt.
Volcanid air-fall ash layers are used as key beds in stratigraphy because o f their wide-spread and rapid deposition. T�e discrete subaerial or subaquatic air-fall ash layer is produced by a vol
canic event . This event may be a very shoitlived sin� le eruption or a series of eruptions lasting for some months or even years . Air-fall ash layers extending over vast areas are produced by
highly explosive volcanism as it is demonstrated by the latest eruptions of Mount St. Helens . Ash particles are ej ected to a , high altitude , where the ash is transported downwind by tropo spheric winds of high velocity.
Each air-fall ash layer is a geologically . ins tantenous deposition.
But use of these potential timemarks for precise tephrostratigraphic correlation is practicable only under the condition that individual ash layers can be identified1 since most series .contain several layers .
There is no difficulty in identifying. layers of Quaternary age by mineral composition , refractive index of volcanic glas s , or
·trace-element geochemistr y ( I ZETT et al . 1 9 70 , BORCHARDT et al . 1 9 7 1 , RICHARDSON
&
NINKOVICH 1 9 7 6 ) . But mineral and chemical com
positions o f geologically older ash layers are in most cases
Cyclic and Event Stratification ( e d . by Einsele/Seilacher) © Springer 1982
424
highly altered . Volcanic glass becomes hydrated and devitrified . The new principal mineral constituents of altered ash layers are
montmoril lonite, montmorillonite-illite mixed layers , illite, chlorite, quartz , authigenic feldspar , and calcite. The result of the alteration are commonly montmorilloni te-dominated bentonites or - more characteristic for the palaeozoic sequences - K-bento nites or meta-bentonites dominated by montmorillonite-illite
mixed layers . As the mineralogical and chemical make-up of altered ash layers is governed by secondary1 induced uniformity and homoge
neity, only two principal possibilities are left to i dentify indi vidual ash layers by their . Primary properties : stable trace ele ments or stable magmatogenic mineral constituents . In Lower/Middle Devonian gions a series of highly ded in index sections of The series of bentonites
boundary beds of the Eifel-Ardennes re altered bentonitic ash layers was recor the Eifel area (WERNER & WINTER , 1 9 7 5 ) . was found to be exactly corresponding
in three index sections , situated within a small area of the
southern part of the "Eifeler Kalkmuldenzone" . A rapid and pre cise tephrostratigraphic correlation within this small area of
s imilar sedimentary facies waS thus possible .
An
attempt to corre
late another series of altered ash layers of more or less the same age at the southern limb of the Dinant syncline initially
failed because o f sedimentary facies changes .
An attempt to use stable element characterization ( contents in Z r , Th, Ti, Nb , and Y ) of the bentonitic layers seemed to produce a possible tool in tephrostratigraphic correlation after testing
contents of corresponding layers in the neighboured index sections of the Eifel Area (WINTER , 1 9 7 7 ) . In comparing layers extending oyer vast areas, howeve r , sorting of air-fall ash proved to in fluence the contents in heayy minerals to a higher extent. The
downwind decreasing content of minor element bearing minerals 1 like
zircon 1 modifies tephra-spezific contents o f stable trace elements like Zr , Th, and Y . There �ore , stable trace element correlation of altered ash layers proved to be a practicable method within smaller areas, but not for far-distance correlation.
Stable magmatogenic mineral constituents of the altered ash layers are heavy minerals such as apatite , zircon, biotite, magnetite, and augite . Isotropic volcanic glass shards , glass spheres 1 and idiomorphic magmatogenic quartz were found in some localities
only, while idiomorphic zircons were found in the heavy mineral
425
Ard e n n es
Eifel St. Joseph
Eau Noire
232,8
ash layer
r---;
We t t e ldorfer
PSJ5
Richtschnitt
pre l i m i n a r y biostratigraphic correlation see WEDDIGE et al.1979
lynx
0
CD
G)
80
[. . libra
libra
V
IV
Ill
g ro u p
190
II
56
50 .
Horologium g r o u p
32 . 29
EAUN 7 7
I � r. 17
•
...
EAUN1
•
170,6
�e r c u l es 156 1 53 !50
Fig . 1
(
P S J 4a
�
�
-;: "'
I g rou
PSJ4
P SJ 3
Explanation in the text )
.
Hercules II
I .
:. e 0 =
�
426
fractions of all asn layers . Their maximum grain size in most cases is below 0 . 1 6
mm,
but some layers of the Ardennes region
contain zircons up to 0 . 4 2 mm . Corresponding eruption centers are not known , neither -i n the Eifel nor in t9e Ardennes regions . I n some of the subma:r;ine air-fall ash layers 1 however , an origin in
the W to SW is indicated by a grain size decrease of the idio morphic z i� cons . Even though the exact source of the ashes is not yet known , volcanic centers of the Variscan may now be covered by younger depos its . Thickness of the alt_ered ash layers ranges from some millimeters to 30 centimeters within the whole region, but conservation and thickness of the ash layers are influenced by changing depositional environments from Eifel area .
NW
to SE within the
Serial investigations of zircons from the air-fall ash layers under the SEM have shown , that the zircon populations of diffe rent ash layers can be distinguished from one another on the ba sis of certain significant features of tracht and habit (WINTER, in press ) . These investigations , now including 2500 circons , have pro duced evidence of wide ranging variations in the morphology of magmatogenic zircons . Unif·orm populations ·containing zircons of only one certain habit are distinguishable from complex popula tions comprising two or more morphological types of zircon. Further differentiations are possible . becaus e , besides normal type zircon , · malacon type meta�ict z ircon affected by radioactive isotopes does also occur. The causes for the morphological variations in the magmatogenic zircon populations are to be sought in the variable conditions affecting the parent magmas . Physico-chemical parameters within the magma chamber would control the velocity _o f advance of. developing crystallographic planes in t.h'e zircon crystals and would therefore be responsible for the dominance or suppresion of particular crystallographic surfaces . It is already known , _ for instance, that chemical parameters affect the development of prisms ( 1 00) , which
is appareritly favoured bY high Y content.. It also has .a hearing on the development of ( 1 1 0 ) prisms , which seems to be positively correlated with the cerium earths content ( BROTZEN , 1 9 5 2 ) . But the
morphological variation of successive zircon populations within a series o f ashfall layers is not only attributed to different magma chambers but also to an 11evolution11 of zircon morphology within a given magma under the changing physico-chemical conditions
427
through time . For exampl e , heterogeneity in physico-chemical con ditions of the magma chamber may have been induced by volcanic eruptions or assimilation processes . Of peculiar interest is a comparison between the results obtained
by bioStratigraphic and thephrostratigraphic correlation (Fig. 1 ) . An attempt was made by bios tratigraphers to correlate the LowerMiddle Devonian bound_ary beds within the classic Eifel-Ardennes
areas (WEDDIGE, WERNER & ZIEGLER, 1 9 7 9 ) . In equivalent parts of the section of both areas ( 1 - 3 , Fig. 1 ) , highly altered ash layers
could be identified by zircon morphology (WINTER, in press ) . I n alr sections occur ash layers o f the Hercules I group , which are characterized by zircons with pencil-like terminations dominated by { 3 1 1 ) terminal plane s . In the overlying beds of all sections we find layers of. the Horologium group , which show malacon type meta mict zircons with ( 1 1 1 ) terminal planes . The Horologium I ash lay er , characterized by an additional small { 3 3 1 ) terminal plane was found in all sections . Further up in the sections , layers of the
Libra group were identified in similar ways . . Precise comparison o f the tephrostratigraphic results with bio stratigraphic correlation (based particularly on icriodid conodont faunas) indicates that the position of the Lower-Middle Devonian '
boundary , as defined by preliminary biostratigraphic studies , is
too low in section 1 (Eau Noire Section) and too high in section 2 ( S t . Joseph ) O f the Ardennes region . According to KLAPPER & JOHNSON , 1 9 80 , most endemic species, especially o f Ieriodus , appear to be
confined to near-shore facies , so that Conodont distribution may be generally influenced by the temporal and spatial shift in biofacie s . Although there are no direct palaeontological criteria for recog nizing the proposed Partitus -boundary in the Ardennes region , the position of this boundary can be determined, because ash layers of the Horologium group occur in both regions and the partitus -boun dary was found to be immediately above the Horologium group in the Eifel type section s .
References BORCHARDT, G . A . , HOWARD , M . E . & R . A . SCHMITT ( 1 97 1 ) : Correlation of volcanic ash deposits by activation analysis of glass s epara tes . - Quat. Res . l : 2 4 7 - 2 6 0 , New York - London.
428
BROTZEN , 0 . ( 1 9 5 2 ) : Die zonaren Z irkone des Ramberggranites . - Geol . FOren . FOrb . , ,Z� : 1 73 -1 8 4 , Stockholm.
G . A . DEN ? BOROUGH ( 1 970) : The Bishop ash bed, a Pleistocene marker bed in the western ·
IZETT, G .A . , WILCOX , R . E . , POWERS , H .A .
&
·
United States . - Qua t . Res . l = 1 2 1 - 1 3 2 , New York - London. KLAPPER, G .
&
J . G . JOHNSON ( 1 9 80 ) : Endemism and Dispersal of De
vonian conodonts .. - J . Paleont . , ��- : 400- 4 5 5 , Lawrence (Kansas ) . D . NINKOVICH ( 1 9 7 6 ) : Use o f K 2 0 , Rb , Z r , and Y versus Si0 2 in volcanic ash layers of the eastern Mediterranean to trace ·their source·. - Geo l . Soc . Ame r . Bul l . , ,§,2 : 1 1 0- 1 1 6 ,
RICHARDSON, D .
&
Boulde r .
WEDDIGE , K . , WERNER, R.
&
Z IEGLER, W .
( 1 9 79 ) : The Emsian-Eifelian
Boundary . An Attempt at Correlation between the Eifel and Ardennes Reg:i ons . - News l . Stratigr . , � : 1 5 9. -1 6 9 , Berlin Stuttgart . WERNER, R . & WINTER, J . ( 1 9 7 5 ) : Bentonit-Horizonte im Grenzbereich Unterdevon/Mitteldevon in den Eifeler Richtschnitten . - Senck. le�h . , gg : 3 3 5 - 3 6 4 , Frankfurt am Main . WINTER , J . ( 1 9 7 7 ) : 11Stabile Spurenelemente als Leit-Indikatoren einer tephrostratigraphischen Korrelation (Grenzbereich Unter /Mitteldevon , E i fel-Belgien) . - Newsl . Stratigr . , g : 1 52 - 1 70 , Berlin - Stuttgart . WINTER , J . : Exakte tephrostratigraphische Korrelation ffiit morpho logisch differenzierten Z irkonpopulationen (Grenzbereich Unter-/MitteldeVon , Eifel - Ardennen ) ·. - N . Jb . Geol . PaU:iont . Mh .
(in press ) .
Part III. Cyclicity and Event Stratification in Black Shales
Cyclic and Dyscyclic Black Shale Formation A. WETZEL
Abstract: Cyclic and dyscyclic_ sedimentary processes forming black shales are discussed. Based ·on cycle length and its lithologic character four main types of variations can be distinguished: ( 1 ) Mega-scale cycles correspond to periods of the earth ' s history witp frequently ocCurring black shale s . ( 2 ) l1acro-scale variations form lithologic unit s . They result from long-term .changes in bottom water circulation, supply of organic matter, and sea level fluctuations . ( 3 ) Heso-scale variations are documented in black shale strata. Cyclic fluctuations of sea level and oxygen concentration and organic matter supply predominate , whereas dyscyclic processes are rare, i . e . rapid input of sediment or organic matter . ( 4 ) t1icro-scale variations are documented in laminae or layers within black shale strata . The preservation of cyclic sedimentary processes (annual varves) is mostly restricted to rapidly accumulated depos its , whereas dyscyclic events predominantly form layers and laminae . Furthermore , the composition of the fossil communities is often influenced by dyscycliC proces s e S . .
1
•
-
Introduction
The term black shales stands for argillaceous , well fissible, and dark-colored sedimentary rocks showing a distinct lamina tion . Black shales can be formed in various environments; in lake s , swamps , or in the ocean from shallow wate·r down to the deep-sea. Accordingly, the type of black shales and the condi tions under which these sediments have been deposited may vary in a wide range. Therefore , the black sediments show ·a varying lithologic composition; black limestones , marls , mudstones , and claystones with a varying amount of other constituents , e . g . phosphatic minerals or siliceous compounds , may be accumulated. In the following text, the term black shales is used for sedi mentary rocks showing mainly two typi�al lithologic features : Dark color is due to finely dispersed iron sulfides and/or a high content of organic matter .
Cyclic and Event Stratification (ed. by Einsele/Seilacher) 4:1 Springer 1982
432
Laminae consist of alternating diverse organic compounds and/ or argillaceous material . Lamination is normally very well pre served in black shales . This phenomenon is due to the reduced activity of benthic organisms indicated by the scarcity or ab sence of trace fossils and a reduced or lacking benthic fauna (POTTER et al . , 1 9 80 ) . Normally , below 0 . 5 ml o 2 ; 1 H 2 o biotur bation is missing ( CALVER� , l9 6 4 ) . Accordingly , black shales are normally deposited in an oxygen-deple.ted-.·environment . Due to the absence of bioturbation an excellent record o f the depositional history of the sediment may be preserved including cycles or rhythms , and events . The timing of these processes will be discussed. For this purpose literature on black shales has been reviewed and the data available were grouped into a newly proposed timing scheme . This scheme is based on bedding �n4 lamination phenomena; each lamina or bed represents a se dlmentary process with- a certain duration . The recurrence of these processes is evident in the repet� tion of laminae or beds and can be directly observed in both outcrops or Cores . There fore , cyclic or dyscyclic processes ( for definition of these terms see EINSELE , this Vol . ; SEILACHER, this VOL } are reflected by l amination phenomena within the biack shales as well as by the recu�rence of black shale facies . The sedimentary processes involved - will be described in accordance to lithologic features and scale: ( i ) lamination · { l owest category ) , { i i ) strata, ( i i i ) lithologic - units , and ( iv) longer periods of earth history · (the highest category to complete the scheme ) . Obviousl y , laminae or strata in different black shale deposits may represent di fferent time intervals . Thus , the categories described can ' t be sharply differentiated in terms of their individual time duratio n . However, the vertical recurrences of .typical lithologic features are well recognizable and clearly define these categories . · The sedimentary variations documenting cyclic or dyscyclic pro cesses are defined- and labeled from the highest to the lowest category as follows (Fig. 1 ) : ( 1 ) Mega-scale variations correspond to periods in earth history with increased occurrence of black shales , e . g . , the Mid-Paleozoic and Middle to Upper Mesozoic times. These megacycles last in the order of hundred mil l ion year s .
'
'
' ,<'
· mega-scale
macro-scale
meso-scale
micro-scale
Fig. 1 . Different scales of variations documented in black shales . Hega-scale variations correspond to long periods in the eax:th 1 s hi story with recurring black shales . Macro-scale variations from l ithologic units as cited in Deep Sea Drilling Sites . Meso-scale variations .are represented in strata. Hicro-scale variations generate laminae or thin layers within black shal� strata ( 2 ) Macro-scale variations lead to the deposition of lithologic units consisting of successions of black shale deposits possibly
interbedded with �ther sedimentary rocks , e . g . , the Upper Lias in different parts of Europe ( RIEGRAF , this Vol . ) , the Kimmerid gian London Clay, or li'thologic units as defined for DSDP Drilling
Sites . One unit may represent hundred thousand to about a million year s . ( 3 ) Meso-scale variations form ( :r;epeatedly) black. shale strata, e . g . ,
the redox cycles observed in the Upper cretaceous sediments in the Atlantic Ocean . The deposition . of one stratum spans some
hundreds to a million of years . ( 4 ) Micro-scale variations form the �ypical b lack. shale laminatio n .
These fluctuations 'represent periods o f one year to hundreds of years.
In the present context macro-, meso-, and micro-scale variations
will be discussed. Mega-scale Cycles are described and interpreted by BERRY & WILDE ( 1 97 8 ) , DEMAISON & MOORE ( 1 9 80 ) , JENKYNS ( 1 9 80 ) ,
SCHLANGER
&
JENKYNS ( 1 9 80 ) , THIERSTEIN
&
BERGER ( 1 97 9 ) .
' " ' .
::\:i
434 2.
Macro-scale Variations
Black shale units form over long time spans . The duration of oxygen depletion is controlled by· various processes. Four ma jor environmental situations can be disting-u ished: 2.1.
Thermohaline stratification
Water of higher density (due to higher salt content or lower temperature) forms a more or less stable lower water layer in an oceari basin ·. For most of che black shales formed under conditions of thermohaline stratification , deposition in more or less silled basins or depressions with restricted circula
tions is likely ( DEMAISON & MOORE, 1 980) . Anoxic conditions on the sea bottom may result from a certain supply of organic matter and restricted circulation. For black shales formed under comparable conditions, RYAN & CITA ( 1 9 7 7 ) assume a low to medium surface productivity , e . g . for the Cretaceous b laCk shales deposited in the North Atlantic Ocean. An anoxic period is commonly terminat�d by increas ing water circu lation or tectonic movements changing the configuration of the basin and thus increasing water circulation. The deposition of these units takes 3 to 3 0 million years ( 7 data) . 2.2.
High Supply of Organic Matter (Upwel ling)
Black shale units resulting from increased organic matter supply · are commonly found in upwelling region s . They are mainly s ituated at the E margins of the oceans , e . g . , the Permian Phosphoria Formation (McKELVEY et al . , 1 9 5 9 ) , the chiefly Tu ronian black marls of the Morrocan coastal basins (EINSELE
&
WIEDMANN , 1 9 81 ; WIEDMANN et a l . , 1 9 7 8 ) or parts o f the present day slope sediments off NW and SW Africa , S America, and other regions ( e . g . , DIESTER-HAAS S , 1 9 7 8 ) .
Upwelling
provides a hfgh surface productivity; an O?Cygen
minimum may develop in the underlying water (WYRTKI , 1 9 6 2 ) . This can cause the formation of black shales. Howeve i , upwelling does not necessarily generate anoxic conditions because deep oxygen supply from the deep-sea may compensate even for the very strdn;r oxygen consumption by oxydation of organic matter ( DEMAISON
&
MOORE , 1 9 80 ) .
435
This kind of black shales is often associated with phosphatic minerals and sometimes with .c.Qert (NOTHOLT , 1 9 80) . Changes in water circulation, relative changes of sea leve l , and/or in the configuration o f the ocean basin may terminate an upwel l ing-induced anoxic period . Units formed under these conditions correspond to a period of about 5 to 15 million years ( 5 data ) .
2 . 3 . Periods of High Sea Level A hi�h sea level favors increasing surface productivity in coastal waters and, hence , anoxic conditions in the underlying water and on the sea floor . Therefore , black shales are often formed during tr·ansgressive maxima . The superposition on com paratively rapid tectonic movements may lead to repeated for mation of black shales, e . g . , in marginal-marine environments (coal cyclothems ) , or in restricted marine basins (Black Sea ) . These units characteristically show a lower proportion o f black shales in comparison to the ones described above . and commonly maintain the same position within a cycle . Therefore , .t hese cycles are comprised of units. The formation of such a unit spans bet ween 2 and 2 0 million years ( 4 data) .
2 . 4 . Sediment-starved Lakes The most famous b lack shales deposited in a continental en vironment are those from playa lake complexes in semiarid re gions, e . g . , the Eocene Green River Shale {EUGSTER 1978 ) .
&.
HARDIE ,
These black shales are deposited in a sli.ghtly subsiding are a . The formation o f black shales depends on ( 1 ) climatic and geographic conditions providing shallow water conditions necessa ry for algal growt h , ( 2 ) low input of terrigeneous materia l , ( 3 ) a very low input of sulfate ( otherwise the organic matter will not be preserved ) , and ( 4 ) a salinity not exceeding the limit tolerated by algae . The formation of these units is mostly terminated by basin fill or vertical tectonic movements . A macro-scale unit has been deposited in about 1 to 2 million years ( 3 data ) .
436
2 . 5 . Timing of Macro-Scale Variations Most of the data are available from w,e l l dated Mesozoic and Cenozoic units , whereas relatively few data are known from Paleozoic rock sequences .
( 1 ) Black shale units formed in a tectonically stable marine en vironment represent , on the average , periods of about 10 million years ( 4 2 data between ,4 and 27 million years , 27 data between 8 and 1 3 million years ) . Sometimes a recurrence can be found in large depositional areas , e . g . , the Atlantic Ocean. A re currence time of 4 to 30 million years has been determined ( 1 2 data ) . ( 2 ) Black shale units formed in tectonically active regions , e . g . , rapidly subsiding shelve s , represent various depositional periods between 2 and 40 million years . A macro-scale recurrence can be rarely observed. Presently, an interpretation of these time-range data is rather
speculative . A period of about 10 million years remarkably re sembles the second order cycl�s of relative sea level changes defined by VAIL et al . ( 1 97 7 ) . Conditions leading to macro scale cycles containing black shale units are possibly con trolled by the same driving forces , but regional tectonic movements may also influence the realization of black shales as well as their cycle length . Therefore , macro-scale 'lglobal11 phenomena are only partly reflected in black shale formation.
3 . Meso-Scale Variations Meso-scale variations lead to the deposition of strata sequences consisting of black shales and other sedimentary rocks . They represent fluctuations in environmental conditions which ' are chiefly controlled by oxygen content in the pore or bottom water as well as by the sUpply of sedi �ent and organic matter . Meso-scale processes appear t o be often cyclic, whereas dyscyclic processes occur of a lower order of frequency. Fig. 2 shows some typical sections with black shale strata formed in various environments (Chapter 2 ) . Dyscyclic and cyclic sedi ments have been separated in this scheme , though both types of sediments may be found together in the same profi le .
437
playa-lake complex
shelf
ridge (island)
ocean
basin
3
§. :::> rt>
li\il black shale
�
mudstone
�.
�h��Wat
IZiitl
so.ndstone
lo ol tong\omerate
lo ol salt
coal
Fig. 2 . Schematic sequences with blae-k shales formed in- various en vironments . The cyclically formed sections are deposited as (a) chemi cal cycle; (b) detrital cycle; ( c ) coal cyclothems ; (d) coastal up well ing cycle; (e+f) productivity or oxygenation cycle {e above and f , below the CCD) ; (g) upwelling cycle close to oceanic islands/ridges; (h+i) deep-basin cycle ( h , Mediterranean type and i, Black Sea type) ; ( k ) shallow-basin type cycle. -- Possible lateral correlation. -- P9ssible range of dyscyclic units intercalated in cyclic desposits
It should be noted, that strata succe ssions, e . g . , limestone layers may also De formed by diagenetic changes . They can be distinguished from primary strata by their geometry and by the preservational features of their fossil content ( SEILACHER et al . ., 1976) . Principally, there are two characteristics for the lithologic se quences formed in different environments (Fig. 2 ) : ( 1 ) pelagic sequences commonly consist of only two alternating members � and ( 2 ) with decreasing distance from the coast , the sections tend to contain an increasing number of other lithologic members due to changes o f sea level which inf luence the supply of terrigeneous material. Nevertheles s , in a stable shallow water environment uniform sequences may also be found. The various types of sequences containing black shales will now be discussed in relation to the depositional processe s . Some o f these processes are closely inter-related, e . g . , oxygenation
438
and supply of organic matter . Nevertheles s , these processes are here grouped under different categories .
( 1 ) Relative changes of sea level are the predominant factor in shallow water environments because they affect the supply of sediment and organic matter . ( 2 ) Changes of oxygenation due to changing circulation patterns , while the input of organic matter remains � constant . ( 3 ) Continuous supply of organic matter combined with + constant Circulation . ( 4 } Discontinuous input of organic matter ( due to "event" sedimen tation) may form bed sequences consisting of black shales interbedded with other sediments . Obviously , these categories are not equivalent to each otheri ( ' 1 ) is important in shallow water environments , whereas ( 2 ) - ( 4 ) are preferentially documented in deeper water deposits . These four types o f sequences are demonstrated by schematic time-buildup diagrams (Fig. 3 } . 3 . 1 . Relative Changes of Sea Level Eustatic fluctuations and/or local or global tectonic movements are causing relative sea leve·l change s . They may influence terri geneous input and circulation ( see section 2 . 3 ) . Furthermore , rising sea level may favor the influx of higher salinity waters into an adj acent basin. 3 . 1 . 1 . Deltaic to Marine Environment Typical sediments deposited in this environment are coal cyclo thems consisting of various l�thologic members . These cycles also contain sediments formed in a non-marine environment ( in contrast to 3 . 1 . 2 ) . Within each cycle black shales can be formed in two situation s : ( 1 ) Black shales are accumulated previous to the first marine transgression in a swamp environment with stratified water con ditions . I t has been proposed , that black shale organic matter in this case is the product Of floating algal mats that reduce oxygen supply to the bottom water layer ( ZANGERL & RICHARDSON , 1 9 63 ) . The duration of these cycles may vary between ten thousand and one million year s , but periodicity has rarely been demonstra ted.
439
( 2 ) Black shales are formed in marine environments during trans gression maxima {Fig. 2 c ) . The oxygen minimum zone - is caused by high surface productivity in the neritic water body , whiCh may be accentuated by upwelling (HECKEL , 1977 ) . These black shales are accumulated at depths between 30 and 200 m. On the average , the formation o f one cycle represents ·more than a million years. Each cycle is about 5 m thick and the black shale strata vary iri thickness. . , 1 . 2 . Littoral and Sublittoral Environment In shallow water 1 black shales are usually deposited during a transgression maximum, often close to the upper boundary of the oxygen depleted zone . They occur in numerous regions and consist completely of marine deposits ( in contrast to 3 . 1 . 1 ) . In respect to the lithologic features, "normal" shelf regions (without upwelling) can be distinguished from those strongly in fluenced by upwelling . (l)
Shelf Without Upwel ling. A cycle starts with rapid deepening
and deposition of calcareous mudstones , which are overlain by black shales formed during the transgression maximUm. Lateran , dur�ng slight regression , marls and limestones were deposited in a coarsening upwards sequenc e . The lithologic boundaries are usually transitional and bioturbation is common ( SELLWOOD , 1 9 70 ) . According to the regional setting , the calcareous sediments may be replaced by silty or sandy material (PAYTON
&
THOMAS , 1 9 59 ) .
The thickness o f the black shales varies .considerab ly , depending on the duration of the high sea level situation . ( 2 ) Coastal Upwelling. Black shales alternate with other sedimen tary rocks ( l imestone , mudstone , sandstone ) . Furthermore , diage netically formed layers or nodules of phosphatic minerals or chert may indicate minor cyclic variations in environmental con ditions (Fig . 2d , 3 d , 4 ) . Sediments rich in phosphates indicate s low sedimentation close to the upper or lower boundary of the o'2 minimum ( F i g . 3 d , 1978) .
4;
READING,
Therefore , minor fluctuations in the circulation pattern may pro duce cyclic layers rich in phosphates within the black shales due to an episodic increase in surface productivity and a lowering of
440
d y s c
y
c
l
� � li' �
�� L L B o
(a)
L
;L L �L organiC matter turbidites
I:.:: T T
o
(b)
fll ] S cm
�=L L
(c )
L L L
S cm
L
0 -
B
I c
t
t l t ll
r
! S cm 'r '
T r T r TT
t
sediment turbidites L hemipelagic sediment
T turbidites
· Other g raphic symbols as in figure 2
t
pulsing input of organiC matter
'
t
::�� f Js� o���=t l � � � ld. ���-� A [ gtl ��� :� g; �= =�=, c
t
y c
l
I c
B- ;.;.{::; (d) upwelling
t
on
subsiding shelves
iBl
(e)
:��
.
deep basm
th ! S cm
(g) g�l i th(f)ological boundaries,�sharp,
t
�
=-=-
redox cyctes in open ocean or basin deposits
B
bioturbation;
0
(h)
t transitional,
-
- -- - -- -- �
t
I bioturbated
lacking bioturbation, t time; th thickness
F i g . 3 . Time-buildup diagrams for some meso-scale black shale sequences ( drawn after description of the cited authors ) . Dyscyclic deposits : ( a ) GRACIANSKY et a l . & VON RAD ( 1 9 7 2 ) ; ( c ) DEAN et al . ( 1 9 7 8 ) .
( 1 979 ) ;
( b ) BERGER
Cyclic depos its : ( d ) McKELVEY et al . ( 1 9 59 ) ; ( e ) ROSS & DEGENS ( 1 9 74 ) ; ( f ) McCAVE ( 1 9 7 9 ) ; ( g ) WEISSERT et a l . ( 1 9 79 ) ; DEAN et al . ( 1 9 7 8 ) ; ( h ) McCAVE ( 1 9 79 ) . For more details see text
the sedimentation rate on the sea floor ( NOTHOLT , 1 9 80) . Inter calations of chert or limestone may have formed similarl y , but more commonly they are of diagenetic origin or accentuated by diagenesi s .
441
upwelling black shale
' , ' , ' , ' , ' i 1 c arbonate 1 1 1 1 1 , ' , ' , - - - - chert
� = � � � � � = � � ��
phosphatic minerals
(a) I 1m
I'��J ;
B bioturbation
0 lacking bioturbation
lithological boundaries: - sharp I
transitional
Fig . 4 . Meso-scale black shale formation in coastal upwell ing region s , followirig descriptions of McKELVEY ( 1 9 5 9 ) , HECKEL ( 1 9 7 7 ) and DIESTER HAASS ( 1 9 7 8 ) . Sediment sections redrawn from ( a) HcKELVEY et a l . ( 1 9 5,9 ) and (b) DIESTER-HAASS ( 1 9 7 8 )
Major fluctuations , i . e . larger variations o f sea level may lead to fluctuations oi terriqeneous input and thus to interbedding with mudstones (deepening) or limestone/sandstone ( shal lowing) . The cycles are in general formed non-periodically in ten thousand to one million year s . They are between 1 and 10 m in thickness .
3 . 1 . 3 . Basinal Environment Sea level changes and/or tectonic movements may favor the influx of higher salinity water into marginal basins , thus establishing stable water stratification . Sediment sequences containing black shales are well known from the Black Sea ( DEGENS & STOFFERS , 1980; ROSS & DEGENS , 1 9 7 4 ) .
Two types of depositional processes can be observed: ( 1 ) A chemical system with the two end members seekreide and sapropel is usually developed in shallow water (Fig. 2k) . In this system carbonate precipitation proceeds in the oxic (upper) water
442
layer, whereas the blaCk shale f.acies develops in the anoxic (lower) water layer. The vertical fluctuation of pycnocline is responsible for alternation beds of shale and limestone ( DEGENS et al . , 1980) . Long-term cycles ( 0 . 5 to 1 my} as well as short-time cycles ( some ky � "megavarves " ) can be distinguished. ( 2 ) A detrital system with terrigeneous muds , sapropel , and coccolith limestone is usually developed deeper in the basin (Fig . 3 1 ) . Terrigenous muds are accumulated during the oxic stage s , while influx of higher salinity water establishes an anoxic water l ayer on the sea floor. Further inflow of saline water makes the H 2 s;o2 interface rise . Simultaneously , sapropelic muds are depo sited. After some time the H 2 s;o2 interface will be stabilized at a certain depth, with diffusion processes taking place and coccolith limestone/terrigeneous mud sequences being deposited. In the Black S1=a this situation corresponds to the Quaternary·
interglacial periods with a resulting rise of sea leve l . The se dimentary cycles are a few meters thick with a black shale unit of about 5 decimeters . Individual cycles span between 10 and 120 thousand years (Fig . 3 e ) . Similar sequences have been found in the Kirnmeridg.ian London Clay (TYSON et al . , 1 9 7 9 ) . Observed cycles lasted about 15 thousand years . For these deposits , how�ver , the Black Sea model is not appropriate ; paleontological data suggest lateral tran sitions into an oxic environment which i s imcompatible with a stable water stratification {AIGNER , 1 9 80 ) . A very complex formation of a black shale section due to re lative changes of sea level has been described by MALDONADO & STANLEY { 1 9 7 6 ) from the Eastern Mediterranean Sea (Fig. 2g) Here , black shales were formed during Quaternary warm periods
• .
that provided { 1 ) a strong fresh-water influx from the Black Sea, { 2 ) a stable thermohaline stratification, ( 3 ) frequently occurring turbidity currents (caused by a rising sea leve l ; a comparable mechanism has been described by SARNTHEIN & DIESTER-HAASS , 1 9 7 7 ) , and ( 4 ) a higher supply of organic matter transported from the Nile Cone into the deep-sea . When climatic conditions stabilized, anoxic conditions were terminated with the development of the typical Mediterranean circulation pattern . The observed black shales are synchronous with those known from the Black Sea.
443
3 . 2 . Oxygen Supply Meso-scale redox cycles are caused by variations in the intensity of circulation, assuming a + constant supply of organic matter (Chapter 2 ) .
Such variations in the oxygen content of the bottom water are typically reflected in cyclic sediments , whereas dyscyclic event oxygenation provides only thin layers of oxic sediments (section 4.2) . ' 3 . 2 . 1 . ,Discontinuous c irculation Due to Salinity Changes Caused by Fluctuating Evaporation For an early stage of the Atlantic Ocean ARTHUR & NATLAND ( 1 9 7 9 ) suggested that higher salinity water may b e produced on the shelves due to increased rates of evaporation. This water will eventually sink to the sea bottom. Therefore , they assume a climatically in duced circulation pattern which corresponds to the intensity of · evaporation. According to the depth of the CCD, grey carbonate/black marl or green/black shale sequences might be deposited. The lithologic boundaries of the black sediments are sharp to bioturbated (Fig. ' 3 f , g , h } , the individual cycles lasting about 10 to 50 thousand years with thickness of several decimeter s . 3 . 2 . 2 . Discontinuous Circulation Due to Temperature Changes This type of stratification reflects rapid changes from a warm, dry climate to a cooler humid one (WEISSERT et al . , 197 9 ) . Barre mian black shales which are intercalated in the carbonates depo sited . in the Tethyan Ocean might have been produced during the cooling periods . One can observe 6 to 16 black•shale episodes ( 0 . 2-0 . 5 m thick) in about 6 million years ( 6 - 1 2 m carbonate ) . Lower and upper boundaries are usually bioturbated (Fig. 3g) . 3 . 3 . Input of Organic Matter An
increasing input of organic matter may also lead to black shale
formation . ( 1 ) Cyclic processes (periodically increasing and decreasing surface productivity) and ( 2 ) dyscyclic proc·e sses ( input by turbidity currents ) can be distinguished.
444
3 . 3 . 1 . Variations in Surface Productivity Two different types o f productivity cycles may be distinguished : ( 1 } Black shale formation due to an increasing ( from low to ·medium) surface productivity has been described by McCAVE ( 1 9 7 9 ) from the Cretaceous Atlantic Ocean . It: has been called " long bloom cycle" , because the black shale strata frequently contain layers rich in radiolarian skeletons . Depending on the depth of the CCD, black/green shale or black marl/grey limestone sequences have formed . The lithologic bounda ries are gradual at the boti;.qm · ( " slow blackening" ) and sharp at the top ("!'rapid returnu ; Fig . 3 f , h ) . A sedimentary cycle has an average thickness of about 2 5 em and represents about 20 thousand year s . Because of the recurrence time of these cycle s , MCCAVE ( 1 9 7 9 ) assumed that the described changes might have been climatically col).trolled . ( 2 ) Upwe l l ing ( see also Chapter 2 and section 3 . 1 . 4 ) . There exist some observations of upwelling conditions close to oceanic islands due to diverging currents . The resulting high surface productivity causes an o minimum. Therefore, black shale could be episodicallY 2 deposited in hemipelagic qrey to green colored sequences .on the slopes of the i slands (or ridges)
•
Usually, this type of black shales i s not very thick {Fig . 2 g) ( some decimeters) and not widespread . Association with high amounts o f volcano-clastic material and/or chert as well as sharp litho logic boundaries are characteristic . These cycles do not show definite recurrence times within them, the deposition of black shales spans only several thousand years ( THIEDE et a l . ) . 3 . 3 . 2 . Input of Organic Matter by Turbidity Currents In an anoxic to oxic environment with benthic organisms ( green to grey, bioturbated sediments , E 0 level a few centimeters h below the sediment water interface) organic matter is brought in =
by turbidity currents . Due to the suddenly increased oxyg�nation of organic matter, this leads to an upward movement of the E h 0 level and to a sharp contact between black shales and under =
lying sediments before biOturbation could obscure it (Fig. 3 a) .
445
Grey limestone or green mudstone/black shale alternations may also occur in this s ituatio n , the carbonate content of the host sediment being related to . the depth of the CCD. GRACIANSKY et a l .
( 1 97 9 ) described repeatedly occurring organic
matter turbidites and calculated an average recurrence time for
these dyscyclic events of about 20 and 40 thousand years for two different units . Because of these ·values (without periodicity in a strict sense ! ) they ass �me climatic reasons for the release of
turbidites , e . g . , input of organic matter by rivers during wet \ periods. A single sequence is between 20 and 50 em thick, sometimes �with intercalations of detrital turbidites ( section 3 . 4 } .
Lithologically similar sequences have been investigated by DEAN
et a l . ( 1 97 8 ) . In contrast to GRACIANSKY et a l . ( 19 79 ) , they inter pret these phenomena by ''pulsing input " . The black shales contain plant fragments and show sharp lithologic boundaries at tops and bottoms ; erosional phenomena have not been described (Fig. 3 c ) . DEAN et aL { 1 9 7 8 } have also calculated aver·age recurrence times
of about 40 to 56 thousand years . Because of this duration they assume "cyclically" occurring "pulsing events'! induced by climatic ,
e . g . , wet-dry , changes on the neighbourin� co�tinen t .
3 . 4 . Input o f Other Sediment Meso-scale black shale sequences resulting from this process
were usually formed dyscyclically, with other sediment particles being imported into a black shale environment by turbidity currents . The lower contact of each turbidite is sharp , while the upper is sharp or graded. Normally , bioturbation is lacking (Fig. 3 b ) .
These dyscyclic events can superimpose other meso-scale processe s , such a s the cyclic or dyscyclic input o f organic matter. Comparable processes have been described by BERGER
&
VON RAD ( 19 7 2 )
or DEAN et a l . ( 1 9 7 8 ) . I n these cases the intercalated turbidite layers are some centimeters to decimeters in thicknes s .
3 . 5 . Climatic Control o n the Continent The most important black shales formed on the continent are known from semiarid regions ( see section 2 . 4 ) .
! ';., r·:
446
According to the ratio between rain and evaporation in the drainage area cycles more terrigeneous (wet; Fig. 2 b ) or more chemical (dry i Fig. 2a) in natura will be fOrmed ( VAN HOUTEN , 1964) . In this case , the formation of a meso-scale sequence took about 21 thousand years , the black shale part { stratum) representing several thousand years . 3 . 6 . Timing of Meso-Scale Variations In general , cyclic proce.s ses predominate in the formation of black shale strata, but moSt of these cycles are not periodic in a strict sense. Relative changes of sea level or changes in the oceanic current system may result from different factors such as global climatic changes (e . g . , ice ages ) , global tectonics ( e . g , sea floor sprea •.
ding} , and regional tectonic movements . If several of these variations are superimposed no regular recurrence time can be expected .
Climatic fluctuations of short duration ( < 100 thousand years) are commonly reflected in periodites ( see EINSELE , this Vol . ) . Arid/humid cycles are particularly well documented, because both the flora {protecting soils against erosion) on the neigh bouring continent and the hydrog+aphy are directly influenced by factors such as temperature , rain/evaporation ratio, and wind . Cyc les formed in semi-arid regions on the continents are very sensi tive to ··these change s . I n comparison, cool/warm climatic cycles i n higher latitudes appear to have been formed less regularily -- at least in the earth ' s earlier history. In summary , the duration of meso-scale cyclic fluctuations varies between 10 thousand and about one million years. The majority of them have periods in the order of � 1 0 1 '\120 1 "" 40 to IV 5 0 , and about 100 thousand years, mostly corresponding to periqdiC clima tic change s . The periodicity is closely related to variations of the orbital elements of the earth ( see FISCHER, thi� Vol . ) . 4 . Micro-Scale Variations Micro-scale variations are documented within black shales as thin layers or laminae (Fig. 5 ) .
447
Sediment supply ( inorganic' particles and organic matter) , fluctua tions of the H 2 s;o 2 interface , and bottom currents are the most important processes involve d . 4 . 1 . Sediment and Organic Matter Supply Variations in the supply of sediment and organic matter normally lead to the typical lamination of black shales ( see section 4 . 2 ) . Laminae can be produced during various time spans . Varves form Cyclically due to annual fluctuations as well as dyscyclically duri�g longer time spans . In order to be recognized , annual varves must be thicker than 0 . 1 -0 . 4 rom , i . e . , they will be found only in rapidly accumulated sediments ( > 10 cm/1000 year s } . In more slowly deposited sedi ments ( < 10 cm/1000 years} laminae mostly represent dyscyclic
events ( see section 4 . 2 } . Lamination is due to a change in the supply of at least one of the various constituents which contribute to the sediment. · 4 . 1 . 1 . Cyclic Processes The following sedimentary processes are known to form annual varve s : '
( 1 ) Continuous supply of detrital material plus seasonal input of organic matter occasionally mixed with other biogenic con stituents ( e . g . , CALVERT, 1 9 64 ) ; ( 2 ) Continuous supply of organic matter superimposed by seasonal input of detrital material ( e . g . , EUGSTER & HARDIE , 1 9 7 8 ) i { 3 ) Seasonally alternating input o f both organic matter and other particles ( e . g . , STACKELBERG, 1 9 7 2 ) . 4 . 1 . 2 . Dyscyclic Processes Dyscyclic processes cause the following sedimentary features : { 1 ) lamination , ( 2 ) turbid layer sediments, ( 3 ) " contourite s " , and { 4 ) thin veneers of volcanic materia l . Lamination can also be caused by dyscyclic input of organic matter and sediment , such as plankton blooms or rapid input of terrigeneous material , super imposed on a more or less continuous background sedimentation. However , these laminae are difficult to distinguish from those formed by cyclic processe s . Perhaps , the thickness of the laminae is less regular. Furthermore , their number in relation to the age o f the whole section will be a criterion .
448
current direction (
(d) (c)
(b) (a)
Fig. 5 . Micio-scale variations documented in black shale strata. ( a ) current events forming 11contourites" ( b ) layer of fossils due to rising H 2 S/02 interface ( c) current event combined with short-term oxygena·tion on the sea floor and afterwards mass mortality of benthic organisms ( d ) current events leading to band accumulations of she l l s 1 orien·ted fossi l s , pot and gutter casts Turbid layer sediments consist of fine-grained sediments which have been transported by low density, low velocity flows called turbid layers (MOORE , 1 9 69 ) . This sediment type often shows la mination as well as graded bedding. The l aminae are more longish than those of " contourites " . The lower face of t-he turbid layer sediments is commonly sharp , but erosional structures can be rarely found (AIGNER, 1 9 80 ; HtiLSEMANN & EMERY , 1 9 6 1 ) . 11Countourites'1 are composed of uneven laminae and lenses of coarser material , sqmetimes associated with scour marks resulting from bottom currents which winnow out fine grained particles; The lenticular type of bedding can be well observed in black shales (AIGNE R , 1 9 8 0 ) . Layers of volcanic ash are caused by dys cyclic eruptions of volcanoes . Howeve r , cyclic pumice layers have also been observed . In this case the pumice was transpOrted by rivers into the depositional area (ANDERSON , 1 9 6 4 ) . 4 . 2 . Position of the H2 s;o 2 Interface Fluctuations of this interface may result from cyclic changes in the ratio of organic matter input to o2 supply. Sometime s , there
449
are short-time fluctuations of the position of the H s;o inter 2 2 face caused by dyscyclic bottom currents (see section 4 . 3 ) . Cyclic fluctuations o f the o2 content can be observed in various types of layers : ( 1 ) Annual varves may result from S easonal fluctuations of the H 2 s;o 2 interface ( ZANGERL & RICHARDSON , 1 9 6 3 ) , while organic matter and other sediment particles are supplied continuousl y . This type
of black shales is known from subtropical swamp areas . During �ainy periods the w�ter level is higher. favoring less anoxic con ditions ; whereas during dry periods more anoxic sediments are depOsited.
( 2 ) Layers of fossils may result from changes in the depth po sition of the H s;o interface relative to the sediment surface . 2 2 Layers with .predominantly planktonic organisms document a rising H 2 s;o 2 interface within the water column. When this interface reaches the euphotic zone , this leads to a mass mortality of organisms (BERRY & WILDE , 1 9 7 8 ) . Layers preferably containing benthic organisms result from fluctuations of the H 2 s;o interface close to the sea bottom. As 2 is indicated by benthic organisms and/or by bioturbation horizons (AIGNER , 1 9 80 ; KAUFFMAN , l 9 7 8 ) , black sediments may also be formed ' under moderately oxic condition s . Howeve r , layers o f macrofossils can also be the result o f bottom currents (see section 4 . 3 ) , which case can be proven by the orien tation of the foss ils . Reconstructions of black shale environments are usually based on fossil communitie s , which are normally controlled by dyscyclic oxygenation eventS and fluctuations of the H s;o interface 2 2 -- as described above . I n relation to the total duration of . black shale accumulation, periods forming layers of fossils .may repre sent only short time intervals . However , the large amouTit of fossils preserved often gives the impression of continuously good living conditions and/or long-term oxygenation periods on the sea bottom. Therefore , the commonly short-term character of the ob served iayers of fossils should always be considered in order to avoid incorrect paleoecological interpretations . ( 3 ) Bioturbation horizons may indicate short phases of moderately . oxic conditions on the sea floor. During these periods a short-term colonization of the sea bottom by infaunal and epifaunal organisms
450
is possible. One can commonly find a single-phase bioturbation· horizon ·with distinct burrows or (hardly recogni zable) homoge nized sediment . This is overlain by some well preserved epi faunal remains reflecting mass mortality .due to the following rise of the H 2 s;o2 interface .
4 . 3 . Bottom Currents Bottom currents affecting black shale sedimentation are typical dyscyclic phenomena , unpredictable and likely- to occur in any part of the depositional area . They form characteristic fabrics from which the current direction may be reconstructed ( BRENNER, 1 9 7 6 ; FUTTERER, 1 9 7 8 ) . Natural l y , the current direction can be most reliably det·ermined in outcrops parallel to the bedding plane . Oriented fossils , shell stringer s , band accumulations of shells , shell pavements , pot and gutter casts , ripples of coarse grained particles1 small channels and lenticular bedding ( see section 4 . 1 . 2 ) are typical current indicators . These fabrics have been well described by AIGNER ( 1 9 80 ) , BRENNER & SEILACHER ( 1 9 7 8 ) , and HAUFF ( 1 92 1 ) . In this way , bottom current events were found to have occurred more frequently ·than was previously assumed (TWENHOFEL, 1 9 39 ) . 5 . Conclusions ( 1 ) Cyclic and dyscyclic sedimentary processes leading to black shale formation are grouped in four categories corresponding to the vert�cal recurrence of black shale units or certain bedding phenomena within black shales : ( a ) Mega-scale variations represent long periods ( 50-100 million years} in the eartl1 's history with increased frequency of black shale formation;
(b) Macro-scale variations form lithologic units ; ( c ) Meso-scale variations are represented in the order of beds; ( d ) Micro-scale variations· are responsible for the laminae and thin layers within black shale sequences . ( 2 ) Macro-scale variations span between 1 and 30 million years . They are caused . by long-term oxygen deficiency in certain environ ments . Recurrence in the same region has been rarely observed. ( 3 ) Meso-scale variations have durations of thousands to about 1 million years and are mainly controlled by cyclic processe s ,
451
as sea level changes and variations in supply of oxygen and organic matte r . Secondarily, dyscyclic input of organic matter can form black shale strata. Probably, most of the meso-scale are caused by c limatic change s . { 4 ) Micro-scale variations represent one t o hundreds o f years o f the depositional history. Cyclic processes o f this order ( annual varves ) can only be recorded in rapidly accumulating sediments with sedimentation rates greater than 10 cm/ 1 . 000 years. For this re?-son dyscyclic processes predominate in the control of observed laminae and layers . They may result from fluctuating supply of organic matter and terrigeneous sediment , the depth of the H s;o 2 interface , and bottom currents. Bottom currents are more 2 common in black shale environments than was previously assumed . Acknowledgements The author would like to thank Prof. G . EiNSELE for suggesting this study and for stimulating discussions , and Pro f . A . SEILACHER for reviewing the manuscript and making helpful comments .
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:i
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.3 i',:RJ ;,,;,i.\ft\\1{;:(��
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ments of Santa Barbara Basin as controlled by organisms and water character . Jour . Geol . , 6 9 , 2 79-290 . JENKYNS , H . C . ( 1 9 8 0 ) : Cretaceous anoxic events. : from continents to ocean s . J . Geol . Soc . London , 1 37 , 1 7 1 - 1 8 8 . KAUFFMAN , E . G . ( 1 9 7 8 ) : Benthic environments and paleoecology of the Posidonienschiefer (Toarcian ) . N . Jb . Geol . PaUiont . Abh·. , 1 57 , 1 8- 3 6 . MALDON�DO , A . , STANLEY , D . J . ( 1 9 7 6 } : The Nile Cone : submarine fan development by cyclic sedimentation . Mar . Geol . , 20 , 2 7 - 4 0 . McCAVE, I .N . { 1 9 7 9 ) : Depositional features of organic rich b lack and green mudstones at DSDP Sites 386 and 3 8 7 , western North Atlanti c . I n : TUCKOLKE , B.', VOGT, P . et a l . Initial Reports of the Deep _Sea Drilling Project, !.2., 4 1 1 -4 1 6 . · MCKELVEY , V . E . , WILLIAMS , J . S . , SHELDON , R . P . , CRESSMAN, E . R . , CHENEY , T . M. , SWANSON , R . W . ( 1 9 5 9 ) : The Phosphoria and Shedhorn Formations in the western phosphate field . U . S . Geol. Survey Pro f . Paper 3 1 3-A, 47 pp . MOORE , D . G .
( 1 9 6 9 ) : Reflection profiling studies of the California continental borderland: structure · and Quaternary turbidite
basins . Geo l . Soc. Am. Spec . Pap . , 1 07 , 1 4 2 pp . NOTHOLT, A . J . G . ( 1 980) : Economic phosphatic sediments : mode of occurrence and stratigraphical distribution . J . Geo l . Soc. London , 1 37 , 7 9 3-805 . PAYTON , C . E . , THOMAS , L . A . ( 1 9 5 9 ) : The petrology of some Pennsyl vanian black '1shales 11 • J. Sed. Petrol . , � ' 1 7 2 - 1 7 7 .
POTTER, P . E . , MAYNARD , J . B . , PRYOR, W . A . ( 1 9 8 0 ) : Sedimentology o f shales . 306 pp . , Springer, New York-Heidelberg-Berlin.
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DIETL 1 G . , GOCHT 1 H . ( 1 9 7 6 ) : Preserva tional history of compressed Jurassic ammonites from Southern
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VAN HOOTEN , F . B . { 1 9 64 ) : cyclic lacustrine sedimentation , Upper triassic Lockatong Formation, central New Jersey and adjacent Pennsylvania. GeC?1 · Surv. Kansas Bul l . , 1 6 9 , 4 9 7 - 5 3 1 . WEISSERT, H . , MCKENZIE , J . , HOCHULI , P . ( 1 97 9 ) : Cyclic anoxic events in the early Cretaceous Tethys Ocean. Geology, z, 1 4 7 - 1 5 1 . WIEDMANN , J . , BUTT, A . , EINSELE , G . ( 1 97 8 ) : Vergleich von marokka nischen Kreide-KUstenaufschllissen und Tiefseebohrungen (DSDP) : �
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Cyclicity and the Storage of Organic Matter in Middle Cretaceous Pelagic Sediments P. L. DEBOER
Abstract: Pelagic sedime'nts of Middle Cretaceous age in Umbria · ( I taly ) exhibit a rhythmic pattern of lithological alternations . The most conspicuous featUre of these a lternations is a regular fluctuation of the carbonate content . Sediment intervals which were deposited during periods with l_imi ted renewal of oxygen in deep wate r , are characterized by relatively high amounts of orga nic carbon , signs of anoxity during deposition, and a low carbo 4 nate content . This pattern is valid for shorter ( 1 0 - 1 0 5 yr) as well as for longer ( 1 06 - 1 0 7 yr) time periods . Stable oxygen isotope fluctuations suggeSt that the carbonate of the carbonate-rich intervals was formed in wate� which was cooler than that of the carbonate-poor intervals . Stable carbon isotope values of carbonate in organic carbon-rich intervals (black shales) . show relatively high values as compared to adjacent carbonate-rich layers with less or hardly any organic carbon . It is suggested that this di fference is the result of early diagenetic proces ses , in this case addition of heavy Co 2 set free by bacterial fermentation . A reversed pattern is shown by 0 1 3 ·c of the organic matter and, in a less pronounced way 1 by 1 3 c of carbonate from series which lack signs of oxygen deple 6 tion during deposition . There, 6 1 3c values are positively corre lated with the carbonate content of the relevant samples . A model is proposed which takes into account climate dependent fractionation e f fects between ocean and the atmosphere . This mo del may partly explain the observed 0 1 3 c f luctuations .
1 . Introduction Pelagic s ediments in the North Atlantic and Tethyan domain of Middle Cretaceous age are known for the frequent occurrence of black shales (Fig. 1 ) . These are clayey and marly pelagic sedi ments with abundant organic carbon , deposited under low-qxygen or anoxic condi tions . I_ncrea:sing attention is being given _ to this kind of sediments ( SCHLANGER & JENKYNS , 1 9 76 ; RYAN & CITA1 1 9 7 7 ; THIEDE &
VAN ANDEL , 1 9 7 7 ; ARTHUR1 1 9 79 ; JENKYNS , 1 9 80 ) . 60% of 'the total
proven oil reserves of the world has been derived from Middle Creta ceous ( 1 1 0 - 80 million years) source rocks ( I RVING, NORTH & COUILLARD , 1 9 7 4 ) . Cyclic and Event Stratification (ed . by Einsele/Seilacher) © Springer 1982
(
457
13 The increase of 6 c values of marine carbonates is also indicati ve of an extraction of much organic carbon from the ocean during this period ( SCHOLLE
&
ARTHUR, 1 9 80; VEI ZER, HOLSER
&
WILGERS , 1 9 80 ) .
Changing environmental conditions within the oceans have led to the extinction of a considerable number of foraminiferal species ( WONDERS , 1 9 80 ) . In general , hoWever1 epochs with a high sealevel , such as the Middle Cretaceous , have been suggested to be polytaxic (FISCHER & ARTHUR , 1 9 7 7 ) . The remarkable increase of species diversity of dine• fl,age llate cysts ( BUJAK -& WILLIAMS , 1 9 7 9 ) during the Middle Cretace ous , is in accordance with this idea. '
The basic cause of the particular phenomena that characterize Middle Cretaceous pelagic sedimentation seems to be the great global rise of sealeve l . The combination of sealevel rise , warm climates , and black shale deposition has also been recognized in other stratigra phic intervals ( c f . BITTERLI , 1 9 6 3 ; HALLAM & BRADSHAW , 1 9 7 9 ) . 2 . Storage of Orgariic Carbon in Middle Cretaceous Pelagic Sediments It has been suggested that the storage of large amounts of organic matt:,er in Middle cretaceous pelagic sediments was the result of a catastrophic transfer of terrestrial organic matter to the ocean, . by the flooding of densely Vegetated lowlands . However , the amount of organic matter that can be transferred from the terrestrial into the oceanic realm by the destructive action o f an unique transgression is relatively sma l l . Estimates of the amount of organic matter, which was stored within Middle Cretaceous pelagic sediments , are of the order of 1 0 20 to 1 0 2 1 gr c . compared to this figure the total terre strial biomass { l iving 6 - 9 x 1 0 1 � dead 1 0 - 30 x 1 0 1 7 gr C , BOLIN et a l . , 1 9 7 9 ) is too small for a significant contribution to the organic matter in Middle Cretaceous pelagic sediments in case of a catastrophic transfer . The Holocene sealevel ris e , caused by the melting of icecaps , pro bably occurred at a rate some orders o f magnitude greater than the Cretaceous sealevel rise . Yet , anoxity in pelagic settings remained restricted to local , isolated basins and, no recent analogues exist for the extensive oxygen exhaustion of deep ocean waters that must have occurred during the Middle Cretaceous .
458
There are no indications that, despite its latge extent, the Creta ceous transgression did h ave a catastrophic characteri ipso facto it did not result in a huge wash of terrestrial organic matter into the ocean . BERGER ( 1 9 7 9 ) explains that, under present-day conditions , anaero . bism and storage of organic matter in anoxic pelagic sediments cannot be wide-spread, but can only be a local or regional phenome non . . involving nutrient influx from outside the relevant system. The Middle CretaceouS anoxic settings were relatively wide-spread, and the anoxic events do not seem to fit BERGER ' s mo.del . However , du ring the Cretaceous some factors , which are considered to be con stant in present-day-ocean modelling, changed . The s lowing of cir culation velocity of Cretaceous ocean wa·ters must have contributed to the storage of organic matter . Under normal conditions the amount of organic life at the ocean surface is limited by a lack of nutrients . Supply of nutrients occurs by recycling due to chemical and biological oxidation of organic matter during sinking through the water column and at the OCean floor, and by supply from the land . Poor renewal of oxygen to deep water may occur, because of a restricted water circuiatiori, either or not favoured by spillage of saline waters from shallow · evaporitic settings (ARTHUR & NATLAND , 1 9 79 ) , an irregular topography of the ocean bottom, and/or decrea sed solubility of oxygen in warmer seawater . Under such conditions , a part of the sinking dead organic matter may nOt be oxidized and is thus removed from circulation and stored into the fossil sediment reservoir. Despite the relatively smaller surface area ( less than 1 /2 that of the oceans ) , the present-day bulk production of organic car�on on
land is more or less equal to that of the seas and oceans , (terre strial photosynthesis 0 . 5 - 0 . 8 x 1 0 1 7 gr C/yr ; marine photosynthe sis approx . o . s x 1 0 1 7 gr C/yr , BOLIN et al . , 1 9 7 9 ) . On average the
organic production per unit� area is therefore higher on land than in the sea . AJTAY , KETNER & DUVIGNEAUD ( 1 9 7 9 ) present data on pri mary production in terrestrial and marine ecosystems . Algal beds and reefs , and estuaries appear to be the only marine environments which approach the high productivity of terrestrial ecosystems . In terms of total surface area, however, highly productive marine eco system fade into insignificance when compared to land areas with high production .
459
The rise of sealevel caused the flooding of large parts of the conti nents, as a result of which about 80% of the earth ' s surface was cove red by seas and oceans . As compared ·to the present situation, an area equal to 1 /3 of the present land surface was flooded, but an even higher proportion of the lower lying land, on which formation and fossilisa·tion of soils is possible , was lost . The volume of terrestrial soils and the amount of nutrients stored within them , therefore , would have gradually been reduced to less than 2/3 of the original value (assuming a pre-sealevel- rise land surface more or less comparable to t the present situation) . Considering the lower production capacity of . aquati€ environments , it follows that the amount of nutrients that , in the absence of a transgression, would have been used by and stored within the terrestrial cycle on the now flooded parts of the continents , surpassed the amount necessary for the normal marine organic cycle . Together with the slow replenishment of oxygen in deep water during this period, deposition of black shales was clearly the most logical way to remove the excess nutrients. If, e . g . , the phoSphorus cycle is considered, reduction of the land surface by about one third must lead to an extra phosphorus influx tO the ocean sufficient for the synthesis of very large quantities of organic matte r . �he amounts of organic matter calculated by this metho'd agree well with amounts deduced from other data . This subject will be discussed in more detail elsewhere . It is noticeable that STUMM ( 1 9 7 3 ) states that small increases of the phosphorus concentration of the oceans will increase the proportion of the present day ocean floor covered by anoxic wate r s . 3 . Results from the Middle Cretaceous o f Umbria 3 . 1 . Relationship Between Productivity and Preservation of Organic Carbon The sedimentary sequence studied near the cemetery of Moria (Umbrian Apennine s , Italy (Fig . 1 ) , covers the upper part of the Albian and the entire · cenomanian (from the middle part of the Ticinella breg giensis zone (Pseudothalmanninella subticinensis zone of WONDERS ( 1 98 0) ) to the top of the Rotalipora �ushmani zone ) , and it has a thickness of 7 5 m (DE BOER & WONDERS , 1 9 8 1 ) . According to the time scale of VAN HINTE ( 1 9 7 6 ) this interval covers a time-span of about eleven million year s . This gives a mean sedimentation rate of about 0 . 7 cm/1 000 years . In generaL the percentage of carbonate in this s equence varies between 60 and 80% . Microscopic inspection shows
460
that dissolution has not significantly redueed the amount of carbonate. Thus a mean carbonate production of less than 0 . 002 gr/cm2 .yr is calculated. This places the carbonate providing surface waters · of this time in the category of present day areas with very low pro ductivity { c f . LISITZIN, 1 9 7 2 ) . If the organic matter within the black shale intervals is considered a similar picture develops . A very small primary productivity can produce the organic matter which is present in Cretaceous black shales, such as · found in the Umbrian Apennines . The "Livello Bona relli " ( Cenomanian/Turonian boundary ) , present at the top of the stu died sequence, and Well-known for its high organic carbon content (up to 20%) , is a good i l lustration. Near Moria it has a thickness of slightly more t�an one meter. Assuming a constant rate of sedimen tation of insoluble matter during the whole Cenomanian, the time for accumulation is of the order of 500. 000 ye'ar s . Settling and burial 2 at a rate of about 0 . 1 gr org C/cm • 1 000 yr woUld have been suffi cient to create this particular interval . In recent anoxic environ ments it has been measured that up to 20% of the primarily produced organic carbon eventually reaches the sediment surface ( DEUSER,
1 9 7 1 ) . Thus, even in periods of maximum black shale deposition, a 2 surface productivity of less than 0 . 001 gr C/cm . year would have sufficed. This again is comparabl e to recent oceanic areas With an extremely low produCtivity. 3 . 2 . Stable Isotope Composition The rather regular fluctuation of the carbonate content which is pre sent in pelagic series such as shown in Fig. 2 , points to cyclic changes of surface productivity . Calcimetric analyses of the sequen ce show that the amount of non-cclrbonate matter in carbonate-marl couplets is practically constant (DE BOER & WONDERS , 1 9 8 1 ) . Whereas microscopic inspection reveals that the carbonate-marl alternations are origina l , the variation of thickness of the successive carbona te beds must be the result of cyclic changes of the amount of car bonate produced in the surface layer. In order to learn if the li-thological alternations are related to cl imatic variations , some analyses of the stable isotOpe composition of intervals adjacent to the above sequence have been made (Figs .
3 and 4 ) 6 1 3 o values of carbonate-rich parts of the sequence are higher than those of adjacent carbonate-poor intervals . During Middle Cretaceous •
(
461 L
s
TURONIAN
·CENOMANIAN
ALBIAN
APTIAN
BARREMIAN
100%
0%
carbonate content organic matter
1 - 20%
Fig. 1 . Schematic representation of the character of pelagic sediments in the North Atlantic and Tethyan realm during the Middle Cretaceous . L , Long term trend; S , Short term ( 1 0 4 - 1 o 5 yr) trend
time s , the climate was warm and equable and no· indications of the presence of icecaps have been found. Therefore , fluctuations of 6 1 3 o cannot be ascribed to variations of the volume of icecaps , as
it is during the Quaternary . They must be due for the greater part to smaller variations in climate and thus to temperature dependant fractionation effects during the formation of. carbonate . The varia tion of oxygen isotope composition thus indicates relatively low
water temperatures for the periods in which the carbonate of the car-
Fig. 2. Lower part of the sedimentary sequence near Horia cemetery . Note the regular fluctuation of thickness of the carbonate-rich beds, and the marly intervals with organic matter bonate rich layers was forme d . The marly intervals were deposited during periods with relatively warm surface waters . Differences of water temperature between "warm11 and 11cold" periods may have been not more than some 2 or 3 degrees . This suggests variations of clima·te as a possible origin of the observed rhythmicity . The interpretation of the carbon isotope data is more complicated-. In successions with black shale ( low carbonate) intercalations , the heavy carbon isotopes are present in higher concentrat.ions in the caco
fraction of the carbonate-poor black shale inteFvals (Fig. 3 ) . 3 Similar high stable carbon isotope values in carbonate within orga nic e-rich intervals have been reported by DEUSER, DEGENS & STOFFERS ( 1 9 78 ) 1 vffiiSSERT, MCKENZIE & HOCHULI ( 1 9 79 ) , and CAMPOS & HALLAM ( 1 9 79 ) . It is suggested that the heavy carbon isotope composition of caco in the black shale intervals is the result of the activity of 3
463 0
E5::!l
••
carbonate
black shale
carbonate
-3.00
•
-2.60 o/oo
2.2
carbonate
0
2.4 2.6 o/oo
13
C organic
matter
�.27 " 26 "25 "24 %o
em.
30
20 10t 0
- 3.20
· 2.so
- 2.40 %o 2.2 2.4
2.6 %o
27 "26 " 25
·
24 %o
Fig . 3 . I sotope data of a pelagic sequence with black shale intervals , Upper Barremian , west of Acqualagna , Umbria 0
0
{25 C) {24 C) -----�----- ----�-----
I
I'
il :i
I
I
{. II
i
I
'
:':1
:.1 '
'I
20
','j
15 10 5 em. 0
40
50
carbonate
60 %
�2.80 2. 90 •
•
3.00
o 1 3 c (vs PDB )
00 -3. 10
- 3.00 -2.90 -2.80 %o
i) 18 o (vs PDB)
Fig . 4 . Isotope data of a sequence without black shale intervals , Fucoid i>1a rl s , Moria
464
methane prod�cing baCteria. This mechanism was suggested by a . o . I RWIN , CURTIS & COLEMAN ( 1 9 7 7 } . Methane producing bacteria become active during burial . After oxygen and sulphate have been consumed, ° the bacteria produce methane with a very low 6 1 3 c (-70 /oo} and 13 co 2 with high & c values . Indeed, C�ROTHERS & KHARAKA ( 1 980) re 13 port 6 c values of Hco 3 in oil field pore waters with a low phate content, as h igh as + 2 5 'J oo . I n anoxic settings, the light thane is inactive and has to migrate to higher levels, where is present, in order to 'be oxidized. The heavy co 2 , however , can
precipitate if calcium ions are present, adding heavy carbon i-soto- · pes to the carbonate fraction within the sediment . Such an exchange 13 is here suggested to be the cause of the high 6 cCaC0 values in 3 the black shale intervals . The carbonate-poor intervals of the section of Fig . 4 do not show excessive amounts of organic c , nor signs of anoxity during deposi 6 1 3 c values of the carbonate in these intervals tend to be 13 slightly lower than in the adjacent carbonate-rich beds . b c va
tion;
lues of organic matter are also lower in carbonate-poor intervals (Fig . 3 ) .
Vari.ous mechal,"lisms have been proposed to explain f luctuations of 13 6 1 3 c in marine sediments . I n general , low 0 c values of organic carbon are ascribed to increased influxes of terrestrial organic
matter . However , although a part of the organic matter may have a terrestrial origin, the location of the area at the time of deposi tion suggests that the bulk of the organic matter from the section of Figure 3 must be of marine origin. The apparent absence of pollen in the samples (pers. comm. A . W . VAN ERVE} supports this idea. 3 . 3 . Cyclicity 3 . 3 . 1 . Description of Couplets and Time Period I n the above section near the cemetery of Moria, 4 6 9 bedding rhythms, ( carbonate-marl couplets ) , are present. As this sequence covers a time-span of about eleven million years , this -yields a mean cycle duration of 2 3 . 400 years . The carbonate-marl couplets _hav·e a very constant amount of insoluble res idue (DE_ BOER & WONDERS , 1 9 8 1 ) . The marly intervals show no signs of dissolution . Beds with mass-flow phenomena were found only occasionally and are distinguished by ano malous absolute amounts of insoluble matter . These data suggest that this sedimentary sequence , with alternating carbonate-rich and marly
465
intervals , is the result of a continuous and regular sedimentation of non-carbonate sedimen t , with a regularly f luctuating supply of carbonate superimposed . The origin of this feature must be a fluctua tion of biogenic carbonate production in the surface waters during the time of deposition . As already indicated by the stable oxygen isotope data, variations of climate are likely to have played a ro le . Indeed, climate is · the main driving force behind water circulation and thus behind the n�trient supply . Thi s , and the average periodicity of the cyc'les of 23,.000 years , suggest influence S of the precessional cycle upon clima�e as the principle driving cause of the above mentioned cycli city in carbonate production. 3 o 3 . 2 . Influence of Orbital Parameters on Climate Variations of the earth 1 s orbital elements as a cause of climatic changes, often referred to as Milankovitch parameters , have been suggested before ( c f . FISCHER, 1 9 80) . It has been shown that a relationship exists between the theoretical ly calculated influence of astronomical parameters upon climate and the volume of polar icecap S during the last few 1 00, 000 years ( e . g . . HAYS , IMBRIE & SHACKLETON, 1 9 7 6 ) . However , only 30% of the variarice '
of the ice v�lume record can be ascribed to variations of astronomical parameters ( KOMINZ et a l . , 1 9 7 9 ) . This might be explained by the suggestion of OERLE!.ffi.N ( 1 980) that ice-sheet/bedrock dynamics have an 1 00 , 000 year cycle of their own , and thus reduce · variations of orbital parameters to a trigger mechanism for the start of growth of ice-caps . The large amplitude of climatic changes during subrecent geological
history coul � thus be mainly the result of the very process of gro
wing and melting of icecaps , eclipsing the influence of varying or bital parameters . In geological history , polar icecaps have probably not been a common phenomenon , and they seem to have been absent du ring the Cretaceous period. In the absence of polar icecaps , fluctua tions of c l imate are likely to be significantly smaller than during the last few 1 001 000 years, and to be dictated more than at present by the influence of astronomical parameters .
466
In order to check whet'h er or not the carbonate-marl alternations found in the field are related to· astronomi cal parameters, thickness values of the lowermost 1 50 carbonate-rich beds from the Moria sec tion ·have been analysed by means of spectral analysis (JENKINS & WATTS , 1 96 8 ) . The result (Fig . 5 a ) clearly reveals a dominant peak at a frequency of 0 . 2 3 4 , indicating a period of about 4 . 3 eRg (car bonate-rich beds) within the sequence of bedding thicknesses . This value fits well the interference between orbital parameters , eccen tricity ( 1 001 000 years) and the most important term of the pre cession ( 2 3 1 000 years ) . As a check on this result, a model curve representing the influence of astronomical parameters upon climate was generated, using the most important amplitudes and periods of the astronomical parameters as given by BERGER ( 1 9 7 8b ) . From the model curve , the successive peak heights were analysed by means of the same method as app_l ied to the Moria data. This curve (Fig . Sb) , and the one based on the field data show a striking resemblance. This leads to the conclusion that the rhythmic carbonate-marl alter nations from the Albian pelagic sequence in the Apennines - and pre sumably also from comparable sequences in other places - do reflect the influence of astronomical .parameters upon pelagic sedimentatio:a. 3 . 3 . 3 . Cenomanian - Turonian Boundary Carbonate is almost absent in the Livello Bonarelli , the organic C-rich interval at the Cenomanian - Turonian boundary in Umbria. However , lithologica l - alternations of clayey and silica-rich inter vals can be recognized. The multicoloured claystones in the middle part of the Bonarelli level do not al low the number of lithologic alternations in this part to be counted exactly . The total number of couplets wi.th different lithology and/or colour has been estimated to be in the range of 2 3 - 3 3 ( 2 8 ± 5 ) . I f lithologic alternations also reflect as ·tronomical ( h . c . precession cycle) influences in this ca s e , then the Bonarelli level would represent a time-span in the range of 5301 000 to 7 6 0 , 000 years . Calculations on basis of the as sumption that the sedimentation rate of non-·carbonate matter has been constant, also indicate a timespan of the order of soo , ooo years ( see above ) . Strikingly, FISCHER ( 1 980) reports the presence of 30 bedding rhythms in the Bridge Creek Member of the Greenhorn Fm. in Colorado ( U . S . A . ) which also spans the Cenomanian - Turonian boundary and
,):
467 7.0
0
6.5 6.0
5.5 5.0
4.5 4.0
3.5 3. 0
2.5
'
2.0 1.5 1 .0
.5 20
13
10
7
6
3
5
200
400
100
75
60
3
50x10
b
12
11
y.
10
9 B
7
6
5
4
2 .oo
. 05
.10
. 15
.20
.25
.45
.so
Fig . 5 . � Spectral density function for thickness of 1 50 . successive carbonate beds of Upper Albian/Lower Cenomanian age (Moria , Apennines , Italy ) . b spectral density function for the amplitudes of success ive peaks of a "climate curve" generated on basis of data of BERGER ( 1 9 7 8b ) . The values on the horizontal scale are the reciprocals of those in Fig . Sa ; vertical .axi s : spectral density
468
probably is the equiValent of the Bonarelli Level in Italy. FISCHER estimates the duration of this interval in Colorado to be Boo, ooo years . This resemblance may support the idea of a world-wide spe cial cyclic phenomenon which characterizes the Cenomanian-Turonian boundary . 4 . Discuss ion and Conclusions Anoxity of deep ocean waters and formation of black shales has occu rred during the Cretaceous on a scale unparallelled in recent ana logues . It has been demonstrated above that there was a comb ination of low surface production and excess storage of organic matter within pelagic .s ediments during the Cretaceous . An analogue is found in the Recent Black Sea, where the sediments with the highest amount of organic carbon are found below the zones with the lowest primary productivity_ (SHIMKUS & TRIMONIS , 1 9 7 4 ) . Thus a decrease rather than an increase of marine organic production characterizes those parts of the Cretaceous oceans where black shale deposition has occurred. Slow circulation velocity as a result of the small climatic con trasts between different latitudes (FRAKES , 1 9 79 } , and because of the absence of direct connect·ions between the North Atlantic and Tethys Ocean, and polar areas (c f paleogeographic maps of SMITH & BRIDEN, 1 9 7 7 } , caused a diminishing of oxygen renewal in deep water . As mentioned above , spillage of saline waters may have amplified this effect. Reconstructions by FRAKES ( 1 9 7 9 } suggest that the temperature of the seawater during the Cretaceous was higher than in any other part of the Phanerozoic. Solubility of oxygen in seawater i s dependant on temperature : a 5 degree rise in temperature causes a 1 0% decrease of solubility of oxygen ( RILEY & CHESTER, 1 97 5 } . Thu.s any rise of temperature will result in a diminished transfer of oxygen from the surface to the deep per unit volume of seawater . Even in the pre sent-day s ituation with a good circulation, the original content of oxygen in polar bottom waters is reduced to about 1 0 % when arriving at low latitude s . " I f there were only 2 rnl per liter l<;!ss oxygen in deep waters below 1 km, much of the present Pacific would be anoxic down to 2000 rn depth " (BERGER, 1 9 7 9 } . Thus , se,;eral mechanisms
may have attributed to the wide-spread anaerobisrn during parts of
the Middle Cretaceous . Storage of so much organic matter in pelagic sediments has been made possible by reduction of the land surface
(
469
by the large transgression, and the consequent extra transfer of nutrients into the oceanic realm. Oxygen depletion of bottom waters enabled nutrient removal as a part of organic e-rich sediments . 4 . 1 . Fluctuations o f Content of Carbonate and Organic Matter Both supply of nutrients to the surface water s , with resulting pro duction and sinking of organic matter , and replenishment of oxygen in deep water are a function of water circulation velocity. The association of high carbonate content and low organic C content with in a bed or vice versa is not contradictory . A high carbonate con tent reflects moderate surface productivity , and a related low or ganic carbon content reflects a re latively good refreshment of deep water , supplying enough oxygen for the oxidizing of the sinking organic matter . The lower carbonate content of the black shale and marly intervals reflects a very low surface production . The relati vely still more strongly reduced supply of oxygen to the bottom water, however, a llowed anoxity and burial of a part of the small amoUnt of organic matter ·that was produced at the surface . 4 . 2 . variations o f Climate and Fractionation of Carbon I so topes Oxygen isotope data indicate that the carbonate-rich layers we·re for me.¢! at a temperature a few degrees lower than the carbonate-poorer intervals . It is attractive to attribute these temperature f luctua tions to variatio'ns of climate , either on a regional or on a g lobal scale .
b 1 3 c in the ocean can be influenced negatively by a diminishing of
the terrestrial b iomass , such as during glacial periods ( c f . SHACKLE TON , 1 9 7 7 ) . However , even if the climatic variations, which· caused the formation of the rhythmic sequence in Umbria, were of a wider than local scale , they were probably still too small to have caused signi . ficant fluctuations in the volume of the terrestrial biomass . In the short term, a net removal of organic matter from the photic zone into pelagic sediments can have only a very small influence upon the mean carbon isotope ratio of the oceanic reservoir . tt was shown above that even during periods. of maximum black shale deposi2 tion at most 0 . 1 gr org C/cm . 1 000 year was extracted from the ocean.
This is less than 0 . 1 % of the amount of carbon present within the
photic zone in a column with basal area of one square centimeter . At present, residence time of carbon in the oceanic surface layer is
470
estimated to be of the · order of 5 to 1 0 years (KESTER & PYTKOVITCH , 1 9 7 9 ) . The surficial exchange with the atmosphere is the most im portant source of replenishment . A value of 200 gr c (as co 2 ) per 2 em . y·r was measured in the Pacific by Keeling ( RILEY & CHESTER, 1 9 7 1 ) � · Therefore, residence time of carbon in the surface water will not be significantly influericed by a s lowing of ocean circulation. In contrast to deep ocean water , replenishment of carbon in surface waters must have been fast enough to cOmpensate for the small change of 0 1 3 c caused by the extraction of low amounts of organic matter . Increased diffusion rates at higher temperatures may even favour the exchange of co 2 between the ocean apd the atmosphere. In spite of the wide spread deposition of b l ack shales during the Middle Creta ceous , anoxity of bottom waters was confined to parts of the oceans . Thus , the atmospheric realm has not been the only buffer of the effects of extraction of organic matter at these sites . Instead, those parts of the oceans , where deposition of black shales was an unimportant phenomenon , e . g . the Pacific, formed part of the buffe ring mechanism, by exchange of co 2 via the atmosphere . In contrast to the long-term effects shown by SCHOLLE ( 1 980) and VEIZER, HOLSER
&
&
ARTHUR
WILGERS ( 1 980) , short-term storage of
organic matter in pelagic settings thus can hardly have influenced 0 1 3 c of inorganic carbon in the oceanic surface laye r .
13 I n equilibrium conditions, b c o f atmospheric co 2 i s about 7 to 1 0 ° /oo l.e ss than that of Hco - dissolved in seawater ( EMRICH , 3
EHHALT & VOGEL, 1 97 0 ; MOOK , BOMMERSON & STAVERMAN , 1 9 7 4 ) . This dif ference is dependant on temperature ; for each degree rise of tempe rature must therefore necessarily lead to a net transfer of 1 3 c from the surface . layer of the ocean into. the atmosphere , and to a slight but consistent lowering of 0 1 3c in surface water s . A s with marine organic matter , fractionation during photosynthe�is causes the land plant organic matter also to become depleted in 13 c , relative to its inorganic so�rce . The rate of depletion is
dependant. on temperature: 13 /':, 6 c = 0 . 4 °joo /°C ( FRASER, FRANCEY & PEARMAN , 1 9 7 9 ) . At
higher temperatures relatively more C- 1 3 is incorporated into the organic tissue . Indirectly , this may also contribute to a lowering of 6 1 3 c in surface water in the event of a general climatic warming.
II
471
13
It is · suggested that the low 6 c values of carbonate and organic matter in marly intervals (Fig . 3 org. C � Fig . 4 caco ) is at least 3 partly the result of these temperature dependant fractionation ef
� j
l
fects � In case of extensive activity of methane producing bacteria, however , the original 6 1 3 c values of the carbonate in black shale intervals may be strongly modified (cf . Fig . 3 ) .
Fluctuation of the position of the caloric equator and the tropical
upwelling zone ( s ee below) is the result of the changing distribut�on of insolation over the g lobe . Thus , Of the above mechanisms, espe c i ally the temperatu�e dependance of fractionation over the air/wa ter i�nterface seems important for 6 1 3c iri the surface water at a
certain place. The continents are, in general , not equally dis tribu
ted over both hemispheres . Therefore , the position of the caloric equator , north or south of the geographic equator in relation to the
distribution of terrestrial organic production may indirectly also influence 6 1 3c of surface waters . The s low circulation of the Middle Cretaceous oceans must have caused the above 6 1 3c effects in the photic zones to be damped less than during other periods by exchange
With deep ocean water.
4 . 3 . Cretaceous-Tertiary Boundary Event Thet temperature. dependance of carbon isotope fractionation between
water, air , and the terrestrial biomass also offers an explanation for the strong negative shift of 6 1 3 c over the Cretaceous-Tertiary boundary , observed in many places (cf . ROMEIN , 1 9 8 1 ) . Although opi nions about the exact nature of this event differ, it is agreed
that it happened within a short time interval and resulted in a glo bal warming ( e . g . CHRISTENSEN & BIRKELUND , 1 9 80) . Besides possible e-ffects such as a diminishing of the terrestrial biomass , such a warming i s likely to have caused a transfer of 1 3 c from the surface layer of the ocean into the atmospheric realm and to have qontribu ted to che C-1 3 depletion of surface water and marine carbonates of that age. 4 . 4 . Depositional Model several authors have calculated the periodicity of rhythmic litho
logic alternations in sediments from pelagic and other settings . It is striking that periodicities of about 20F OOO years are often found
in sediments which have been deposited at paleolatitudes of some 2 5 to 3 5 degrees (cf . SCHWARZACHER, 1 9 54 ; VAN HOOTEN, 1 9 6 2 ; ARTHUR & FISCHER, 1 9 7 7 ; McCAVE , 1 9 7 9 ; FISCHER, 1 9 80 ) .
I
b
I
I
I
472
Between about 30
°
° N . L . and 30 S . L . upwelling causes the organic - pro
duction to be higher than north and south of this "high productivity equatorial zone11 (SCHOPF , 1 9 80 ) . The position of the caloric equator , which lies roughly in· the middle of the high productivity zone, has been shown to be related to astronomical variables {mainly precession ° and eccentricity} and to deviate up to about 1 0 from the geograph ic equator ( BERGER, 1 9 80a) . Thus , areas located at latitudes of some 2 5 to 3 5 degrees North o r South will alternately fall inside and outside the sphere of influence of this high productivity zone. If deposition 1
occurs above the carbonate compensation depth, then a fluctuating pro
duction of carbonate at such sites will be recognizable after fossi lisation ( c f . Fig . 2 ) . Acknowledgements I thank Drs . S . O . SCHLANGER, P . MARKS , A . A . H . WO�DERS and G. VAN GRAAS for discussions and cooperation during fieldwork . I am very pleased to acknowledge the critical reviews of Drs . P . MARKS , E . NICKEL , G . EINSELE , and T . DE MOWBRAY .
Fieldwork was financially supported by the Netherlands
Organisation for the Advancement of Pure Research ( Z . W . O . ) .
References AJTAY , G·. L . , KETNER, P . & DUVIGNEAUD , P . ( 1 979 ) : Terrestrial primary production and phytomas s . In : BOLIN , B . et a l . (eds . ) : The glo bals carbon cycle, 1 2 9 -1 8 1 . ARTHUR, M . A . ( 1 9 7 9 ) : North Atlantic black shales : the record at site 3 9 8 and a brief comparison with other occurrences . I n : RYAN , W . B . F . , SIBUET , J . C . et a.l . Init . Rep . D . S . D . P . 47/2: 7 1 9 -73 8 . ARTHUR, M . A . & FISCHER, A � C . { 1 9 77 ) : Upper Cretaceous - Paleocene magnetic stratigraphy at Gubbio , Italy. - I . LithostratigraphY and sedimentology . Bull. Geo l . Soc . Am. 88: 3 6 7 - 37 1 .
ARTHUR , M . A . & NATLAND , J . H . ( 1 979 ) : Carbonaceous s ediments in the North and South Atlantic: the role of salinity in sJ::.a ble stratifi cation of early Cretaceous basins . In: TALWANI , M . , HAY , W. & RYAN , W . B . F . (eds . )" , Deep Drilling Results in the ]\tlantic Ocean : Continental Margins and Paleoenvironment . - Am . Geoph . Union , Maurice Ewing Series , l= 3 7 5 -40 1 .
BERGER , A . L . ( 1 9 78a) : Long-term variations of caloric insolation re sulting from the earth ' s �rbital elements . Quat. Res . �: 1 3 9 - 1 6 7 .
BERGER , A . L . ( 1 97 8b ) ; Long-term variations o f daily insolation and Quarternary climatic changes . J . Atm. Sci. 3 5 : 2 3 6 2 - 2 36 7 .
473
BERGER, W . H . ( 1 9 79 ) : Impact of deep-sea drilling on paleoceanography.. I n: TALWANI , M . , HAY, W . & RYAN , W . B . F . (eds . ) , Deep drilling results in the Atlantic Ocean: continental margins and paleoen vironment. -Maurice Ewing Series �, Am. Geophys . Union, 2 9 7 -3 1 4 . BITTERLI , . P . ( 1 96 3 ) : Aspects of the genesis of bituminous rock se quences . -Geo l . Mijnbouw 42: 1 8 3-201 . DE BOER, P . L . & WONDERS1 A . A . H . { 1 9 8 1 ) : Milankovitch parameters and bedding rhythms in Umbrian Middle Cretaceous pelagic sediment s . I . A . S . 2nd Eur. Meeting, Bologna Abstr . , 1 0- 1 3 . BOLIN, B . , DEGENS , E . T . , DUVIGNEAUD , P . & KEMP E , S . ( 1 9 79 ) : The glo bal biogeochemical carbon cycle. In : BOLIN, B . et al. (eds . ) , The global carbon cycle , 1 -5 3 . BUJAK , J . P . & WILLIAMS , G . L . ( 1 9 79 ) : Dinoflagellate diversity through �ime . - Mar . Micropaleontol . i= 1 - 1 2 . CAMPOS, H . S . & HALLAM, A . ( 1 9 7 7 ) : Diagenesis of English Lower Ju rassic limestones as inferred from oxygen and carbon i sotope ana lysi s . - Earth Plan. S c i . Lett. 45: 2 3 -3 1 . CAROTHERS , W . W . & KHARAKA, Y . K . ( 1 9 80) : Stable carbon isotopes of Hco - in oil-field waters - implication for the origin of co . 3 2 Geochim. Cosmochim. Acta 44: 3 2 3 - 3 3 2 . CHRISTIANSEN , W . K . & BIRKELUND , T . (eds . ) ( 1 9 80) : Cretaceous .- Ter tiary boundary events . - Symposium,vol � proceedings . DEUSER, W . G . ( 1 9 7 1 ) : Organic-carbon budget of the Black Sea . - Deep Sea Res . 1 8: 9 9 5- 1 004 . DEUSER, W . G . , DEGENS, E . T . & STOFFERS , P . ( 1 9 7 8 ) : 0- 1 8 and C- 1 3 contents of carbonates from Deep Sea Drilling Sites in the Black Sea . - Init. Rep. D . S . D . P . � 6 1 7- 1 07 7 . EMRICH , K . , EHHALT , D . H . & VOGEL, J . C . ( 1 9 70 ) : Carbon isotope frac tionation during the precipitation of calcium carbonate . - Earth Plan. Sci . Lett. �: 3 6 3 -3 7 1 . FISCHER, A . G . ( 1 9 80) : Gilbert - Bedding rhythms and geochronology . G . S . A . Spec. _Pap. 1 83 : 9 3 - 1 04 .
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Types of Stratification in the Kupferschiefer J. PAUL
Abstract: The Kupferschiefer of the Central European Zechstein basin is a typical black shale. Three types of stratification are dis cernible : - Alternating varve-like light and dark laminae , most likely seasonal lay�rs . - Cycles of carbonate-rich and more shaly layers superpose the lami nation. They are the result of long-term fluctuations in plaricton productivity . - Episodic intercalations of shell-beds in black shale s , which are restricted to sites adjoining to "schwellen'' . These schwellen projected above the level of the chemocline. The shell-beds are interpreted as tempestites . 1
. In traduction
Black shales are suitable for studies of various bedding phenomena. As bioturbation is lacking such shales assume various types of la
mination , fluctuations and event-depo.s its of the sedimentary histqry .
Individual cycles can be correlated over large distance s . The water body has a chemocline (redox-discontinuity-layer ) and its influence . on the character of the bedding is an object o f special i�terest . Long- and short-term variations in the position of the chemocline should be reflected Ln the accumulated sediment . 2 . General Setting of the Kupferschiefer The Kupferschiefer is a
thin
bLtuminous marly layer which extends
over the entire area of the Central European Zechstein basin. Fac�es and thickness are uniform from England to Poland throughout the
basin. Facies changes are restricted to margins and schwellen.
Because of its economic s ignificance the Kupferschiefer is one of
the best studied cases in geology. Howeve.r , concentrations of heavy metal s , from which its name is derived , reach economically exploi table levels (BANAS , .1 9 80 ; WEDEPOHL, 1 9 80) only in few,. place.s of 2 this immense area (600. 000 km ) �
There is evidence that the deposition of the Kupferschiefer was isochronous throughout the basin. FUCHTBAUER ( 1 9 6 8 ) and 5LSNER ( 1 9 5 9 )
estimated the duration o f the Kupferschiefer deposition b y comparison with recent black shales and by counting the laminae respectively. Cyclic and Event Stratification (ed . by Einsele/Seilacher) «:! Springer 1982
477
Both authors obtained correspond�ng values in the magnitude of 10 4 5 to 10 years . The Kupferschiefer depositional .environment �1as a restricted $ea in which a euxinic sapropelitic facies prevailed (Fig. 1 ) . Comparison of thickness and facies of the over lying sediments ( Z echstein l imestone
and Werra anhydrite) on top of a schwellen within the basin provides good indications for the paleowater-depth . It is estimated to have
reached some hundred meters. The schwellen facies is an excellent indicator of processes in-· volving the basin as a whole such as the progress of the Zechstein tr�nsgression and the buildup and breakdown of the chemocline . 3 . Basin Facies
The Kupferschiefer of the basin facies is a 0 , 20 - 0 , 50 em thick , laminated bituminous marly shal�. The carbonate content , mainly cal
cite, reaches 3 0% (WEDEPOHL , 1 9 6 4 ) . The amount of organic matter averages 6 % (JUNG & KNITZSCHKE , 1 97 6 ) . Iron and other heavy metals occur as sulfides as in other black shales . Benthic communi�ies are totally absent. Only nec�ic and planctic faunas are present, tor
instance the famous well preserved fish Pataeonisaum fre i e s t e b e n i .
Lamina tio n : The megascopically visible lamination is due to an alter nation of Iight carbonate-rich and dark layers , with a high content
of clay and organic matter. Thickness of laminae is less than 100
micron. Microscopic examination reveals that the lamination conSists
of a fine flasery bedding, perhaps the result of diagenetic processes. The lamination is caused by periodic (probably seasonal ) variations in plancton production. By its metabolism and decay the plancton
strongly influences the chemistry of the water and the sediments , for instance th.e precipitation of carbonate and the amount of organic matter . Cyc L e s : Small scale variations of clay and organic carbon content can
be traced and correlated over distances of some hundred km ( RENTSCH , 1 9 6 5 ) . In the Thuringian basin, Which is best investigated due to
mining activity, three cycles are dis ·t inguishable . (GERLACH & KNITZSCH KE , 1 9 7 8 ) . Each cycle starts with a clay-rich basal portion , while high amounts of carbonate occur in the middle (Fig. 2 ) . Towards the end of a cycle the clay content of epeirogenetic movements inducing
transgressions and regressions of the sea . A possible solution of this problem is provided by the interpretation of the schwellen facies .
�
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Fig. 1 . Schematic model of the depositional area and of the Kupferschiefer bed . A , basinal laminated black shale facies ; B , s lope facies consisting of laminated black shales with intercalations of tempestites ; C , foSsiliferous schwellen facies above the chemocline , un bedded ; D , schwellen facies in small-depressions with carbonates at base ( "MutterflOz " ) . Above : intercalations of tempestites in laminated black shales; E , schwellen facies : alternations of laminated carbonates and shaly beds below the chemocline .1 , pre-Zechstein beds ; 2 , laminated black shales ; 3, caybonates with autochthonous fauna ; 4 , Shell beds explained as-tempestites ; 5 , alternations of carbonates and shales; 6 , Zechstein Limestone
479
3
Fig. 2 . Carbonate content of the Kupferschiefer in the Thuringian basin ( after GERLACH & KNITZSCHKE , 1 9 7 8 ) . Three cycles arB distinguish able
2
0
40
80%
CARBONATE·
4 . Schwellen Facies of the Kupferschiefer The schwellen facies is very variable in co�trast to the more uni form basin facies (see Fig . 1 ) . Facies and thickness depend mainly
upon the relief. of the pre-Zechstein surface and the position of the cheroocline. The carbonate content, mainly dolomite , is generally high ( 60-8 5 % ) and the amount of organic matter less than in the basin.
Thy tops of the schwellen often projected above the level of the chemocline into the oxygenated surface-waters ( F ig . lC) . These
sites provide� markers for the position of the chemocline . Here fossils are frequent . The numerous brachiopods , bryozoans , molluscs ,
crinoids , and corals of the Zechstein sea have survived the un
favourable anoxic conditions of the deeper parts of the basin, and from here they spread again towards the bas·i n after the Kupferschie fer deposition had ceased. On the schwellen, the Kupferschiefer is locally red-coloured , due to syngenetic formation ·of iron oxide in
dicating, like the fossils , oxygenated sea water .
On s lope or somewhat deep·er shoals , the lower part of the Kupfer schiefer succession is represented -by fossiliferous carbonates {Fig.
l ) , the so-called 11Mutterfl0z 11 • Due to benthic activity bedding is onlY poorly develope d . This carbonate -horizon is time-equivalent with
the black shale facies of the basin. This means that the schwellen
were above the level of the chemocline at the beginning of the
deposition of the Kupferschiefer . In the course of further trans gression , the euxinic Kupferschiefer facies spread over the Mutter-
480
flOz horizon. The chemocline rose to wave base (Fig. � ) . Well ex
pos·ed sections in a schwellen region (PAUL , in prep . ) show that the sea transgressed continuously during the deposition of the Kupfer schiefer .
Lamination: As in ' the basin fac�es the brownish-black sediment
(Fig. lA) displays a varve-like alternation of light carbonate-rich and darker , more bituminous layers . In contrast .to the basin facies the laminae reach a thickness of some millimeters , and especially the carbonate-rich layers increase in thickness thus causin.g the high carbonate content of the schwellen facie s .
Cyc l e s : The above-mentioned cycles of the basin can be traced to the
slopes of the schwellen. On top of a schwellen-region or in small
depressions within the schwellen, however , the Kupferschiefer deve lopment differs considerably. There are frequent alternations of carbonate-rich and more shaly layers with a thickness of about 5 to
em (Fig. l E ) . These changes in carbonate content are interpreted as the result of long-term fluctuations in . plancton productivity.
lO
I n basin areas the large and uniform water-body reacts with con siderable time�lag and reduced amplitude to changing environmental conditions . Therefore· , all seCtions show the same sequence over large distances . On the contrary, in schwellen areas and in smaller
depressions sedimentation i s · strongly influenced by .local conditions Ill general , closed systems of smaller dimensions react with greater
.•
amplitude to changes of hydrological parameters .
In some places shell beds occur in variable thickness on s lopes within the euxinic black shales (Fig. l B , D ) . They consist mainly of productids and other brachiopods . These beds are interpreted as tempestite s . Intermittently storms, transported the benthic animals , living on shoals above the chemocline, to adjoining deeper areas
with stagnant conditions . 5 . Conclusions
Bedding in the basinal sediments of the Zechstein devel?ped under the influence of several factors :
The major cycles in the Kupferschiefer and the Zechstein Limestone were controlled · by fluctuating environmental changes such as primary productivity and the position of the chemocline . These f luctuations effected the whole Zechstein basin. Episodic intercalations of shell
481
material in black shales were caused by high energy event s . They are of local occurrence only� The lamination is aSsumed to be caused by periodic (seasonal ) variations in plancton production. Acknowledgement I would like to thank Prof. Dr. W. Riegel 1 G0ttingen 1 for critical comments and vetting the English manuscript . Ref�rences BANAS , M. { 1 9 80} : Zechstein copper ,d eposits in Poland . - In : Euro pean Copper Deposits . Proc . Belgrade Symp . 1 9 8 0 , 1 36-14� FUCHTBAUER , H . ( 1 9 68 ) : Carbonate Sedimentation and subsidence in the Zechstein basin (Northern Germany ) . - In : MULLER , G . & FRIEDMAN , G . (eds . ) : Recent Developments in Carbonate Sedimentology i n Central Europe , Springer , Berlin ffeidelberg New York , 1 9 6 -204 GERLACH , R. & KNITZSCHKE, G. ( 1 9 7 8 ) : Sedimentationszyklen an der Zechsteinbasis (Z 1 ) im slid5stlichen Harzvo'rland und ihre Bezie hungen zu einigen bergtechnischen Problemen. - z . angew. Geol . , 112 : 462-467 JUNG 1 W. & KNITZSCHKE , G . (�9 7 6 ) ": Kupferschiefer in the German Democratic Republic (GDR) with special reference to the Kupfer schiefer deposit in the southeast Harz Foreland . - I n : ' WOLF , K . H . (ed. ) , Handbook of Stratabound and Stratiform Ore Deposits , vol� VI , Elsevier, Amsterdam, 3 5 3 -406 OLSNER, o . ( 1 9 59 ) : Bemerkungen zur Herkunft der Metalle im Kupfer schiefer . - Freiberger Forschungs h . , C 5 8 : 106-1 1 3 RENTSCH, J . (196 5 ) : Die feinstratigraphisch-lithologische FlOz lagen parallelisierung im Kupferschiefer am SUdrand des norddeutschen Beckens . - z . angew. Geol . , !!: 1 1- 1 4
WEDEPOHL,- K . H . ( 1 9 6 4 ) : Untersuchungen am Kupferschlefer in Nordwest deutschland ; ein Beitrag zur Deutung der Genese bituminOse'r Gesteine . Geochim. Cosmochim. Acta , 2 8 : 305- 3 6 4 WEDEPOHL , K . H . (1980) : The geOchemistry o f the Kuferschiefer bed in Central Europe . In : European Copper Deposits . Proc. Belgrade Symp . 1 9 8 0 , 129-135
Environmental Changes During Oil Shale Deposition as Deduced from Stable Isotope Ratios W. KDsPERT
Abstract; In sections throuqh Lower ToarcLan oil shale s , parallel variations in the 1 3-C/ 1 2-C ratios of carbonate and organic matter point to pronounced changes in the oxygen level of the water column� Fauna and flora, fossil preservation , paleogeogra phy, and certain sedimentological features support this inter pretat·ion and justify the distinction of three litholoaic isotopic facies types . carbon isotopes might be generally useful in tracing the extent of 11stagnation11 during deposition of bitu minous sediments . Although much lower in 0 1 3 C than comparable Recent marine plankton , the bulk of the organic matter (as well as the oil shale carbonate) in the Posidonia Shales - i s ultimately derived from phytoplankton which inhabited the oxygenated surface layer of the ocean . Diagenetic processes such as oxidation , cementation , dolomitisaticin , isotope exchange , impregnation and migration have led to partial redistribution of carbon and oxygen i sotopes with in the sediment-pore water syst�m . 1 . Introduction The orig.in of the bi.tuminous Lower Toarcian Posidonia Shales of Germany (Lias epsilon) and their equivalents from other parts of Central and western Europe (Jet Rock Series , n schistes cartons" etc . ) has been a matter of debate among geologists ever since Pompeckj (1901) compared their depositional environment to that of modern Black Sea sediments laid down in a barred , stratified , anaerobic basin. Hallam ( 1 9 67 ) , however , regarded the sole triggering mechanism to be a slight eustatic deepening of a very shallow epicontinental sea lacking an adequate circulation system to maintain sufficient oxygen supply to the near-bottom water . Among others., FisCher ( 1 9 6 1 ) quest:Loned the pr-evalence o f a · permanently anaerobic water column, and favoured repeated alternation between "true11 s a p r o p e 1 and g y t t j a conditions . A quite different model was proposed by Jordan ( 1 9 7 4 ) , who visualized brine formation by submarine salt dome leaching, perhaps in connexion with extensive oil and 9}'iS seepage , to be _ responsible fOr extinction of benthic organisms and, consequently, excellerit preservation of organic matter , primary bedd ing , and other vd se disintegrated fossils such as vertebrates and echinoderiP S . Bot torr current orientated fos s il s , observed by Brenner ( i n : Brenner & Seilacher, 1 9 7 8 ) , Cl.o not corroborate the hypothesis Of a severely Cyclic and Event Stratification ( e d . by Einsele/Seilacher) © Springe-r 1982
483 isola ted, circula tionless deep "'·ater body . In an entirely nev,r paleo ecolog-ical approach, Kauf fman ( 1 9 7 8 ) postulateC. a very sharp o2 ; H s 2 boundary fluctuating- between the sediment and the water irr.rr.eC.iately above i t . This configuration wa� considered the result of an algal fungal mat which periodically covered the sediment surface , and would have allowed limited epibenthic life (mainly a low diversity bivalve fauna ) to persist for more than 90% of the depositional history of the Posidonia Shales . To gain more insight into actual sedimentation processes , a stable risotope study was carried out at Tlihingen University as part of an interdisciplinary Oil shale research program. In the course of this < study , the Toarcian of Southern Germany and stratigraphically adj acent strata (Upper PlienSbachium to Lower Aalenium) were in vestigated for the carbon and oxygen isotopic composition of c_ar bonates (sedimentary carbonate , belemnites , diagenetic calcites) and the carbon isotopic composition of organic matter {kerogen , bitumen, jet) . Most of the 500 samples analysed consisted o f core material taken .from 5 shallow bore holes drilled along the foreland of the Schwabische Alb (Wlirttemberg) by BEB , Hannover . Remaining samples came from 13 outcrops in Wlirttemberg, Bavaria , Northern German y , Englarid , France , and Switzerland. Carbonat e , bitumen , and organic carbon content were measured for many of the sediment samp' les. The organic ·matter content of selected oil shale samples was analysed for elemental composition and deuterium content, and studi.ed using infra-red spectroscopy , light microscopy , column chromatography, -and capillary gas-liquid chromatography. I n addi tion, a series of sediment samples were analysed for major and minor elements and pyri.te content . In the following, only a brief summary of the results is given . For complete data presentation and detailed discussion the reader i.s referred to Klispert (1 9 7 7 , 1 9 8 1 ) . Basic informations on the Swabian Posidonia Shales may be drawn from the pionier studies of Quen stedt (1858 ) , Hauff (1921 ) , and Einsele & Mosebach - ( 1 9 55 ) •· Br6ckarnp ( 1 9 4 4 ) and v . Gaertner et a l . ( 1 9 6 8 ) comprehensively surveyed the Lias epsilon of Northern Germany. In England, Howarth ( 1 9 62 ) , Hallam ( 1 9 67 ) , and Gad et al . ( 1 9 6 9 ) worked on the Upper Lias of _ Yorkshire. Tissot et al . (1 9 7 1 ) , Durand et al . (197 2 ) , Espitalie et al. (1 97 3 ) , Alpern & Cheymol ( 1 9 7 8 } , and others thoroughly investigated the organic matter from Lower Toarcian shales of the Pari.s Basin.
484
(
Fiq . 1. Cor.centration and carbon isotopic composition of carbonate and orga�ic matter in the Upper Lias and Lower Dogger alpha of Southwest Germany (Vllirttemberq) . Bore holes 1 002 Zimmern near Bechingen and 1 005 t'Grtinqen . �:-:ate approximate vertical extension of facies A , B , and C . Bituminous limestones were only sawpled at site 1002 . Cros s-hatchinq : bituminous shales ; black : limestones ; horizon tal hatching : non-bituroinous shales ; ....L : grey marl s ; D : dolomitic mar l s . Vertical scale: each division 1 m =
485
2 . Results and Discussion
In the oil shale facies , the
d 1 3 c of the total
o r g a n i c c a r b o n varies systematically between -27 and - 3 3 %"o l ) . All sections show nearly identical carbon isotope distribution pat tern s . In WUrttemberg, the 1 3 c content reaches a minimum in the Unterer . Schiefer and a maximum around the Oberer Stein (Fig. 1 ) . Intermediate values characterize the Upper Lias epsilon (Bifrons zone ) . Particularly prominent is a layer of unusually low J 1 3 c , cdnfined essentially to the basal part of the F a 1 c i f e r u m� zone (Elegantulum and Exaratum subzones} , which can be traced from Southern Germany to Northeast England and Southern France (Fig. 2 ) . It is predictable that this layer should also exist in the Paris Basin. Surprisingly, oil shale c a r b o n a t e ( d' 1 3c : mainly +2 to -4%o ) almost perfectly parallels the isotopic fluctuations of the organic carbon, with an average fractionation factor of 1 . 0306 ( approximately corresponding to a 2 9 . 6%o difference} between carbonate and kerogen (Fig. 1 ) . 2 . 1 . Carbon Isotopes o f Carbonate
The , swabian Posidonia Shales generally contain 1 5 - 4 5 % carbonate , principally derived from calcareous· phytoplankton . In addition to coccoliths , s c h i z o s p h a e r e 1 1 a p u n c t a t a , a genus of uncertain affinity, · occurs as a rock.-forming nanno fossi.l. Hemleben ·et al . (1 9 80) s tudied the calcareous nannoplankton in detail, but, on a macroscopic scale , did not find any fundamental changes in nannoplankton[total carbonate ratio , relative contribution of S c h i z o s p h a e r e 1 1 a , or diversity and species composition of the coccolith association throughout the sequence . Hence , variations in the origin of the oil shale carbonate appear not to be responsible for the observed J 1 3 c variations. Varve-type laminae are common in the low J 1 3 c layer as well as . where the 1 3 c content is high. If cement should significantly contribute to oil shale carbonate , it does not noticeably affect the 6 1 3 c of the latter , and r therefore , must have formed rather late during diagene1)
d e 1 t a values are given in the typical notation: R lsample) - R ( standard) 1 3 c ;1 2 c , 1 B o; l 6 o , D/H x lOOO%, ; R R ( standard) , 1 8 3 J 1 c and J o values are reported relative to PDB , J o values d
=
relative to SMOW.
=
486
Whitby Yorkshire
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o13C org. C -32 �30 -28
613c org. c SUdfrankreich 32 -30 -:zs Mende
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� � c
f--Q ;��i}"i
!.ii$�$�)7?-J
�
Ovatum Band
�
�
-32 -30 -28
-32 -30 -28
Fig . 2, CarbQn isotopic composition of total orqanic carbon in the Toarcian of ·Northeast Enc;:rland (Yorkshire) and Southern France (Dept . . Lozere ) . Subzone s : Sem, S e m i c e -1 a t u m ; Em, E 1 e g a n t u 1 u m i Ex , E x a r a t u m i Es , F 1 e g a n s ; Cr, C r a s s u m . Cross-hatchinq: b ituminous shale s i black: lime stones ; horizontal hatching : non-bi'tuminous shales ; vertical hat ching: siderite . vertical scale : each division 1 m =
sis from dissolution of pre-existing carbonate in the irmnediate vi cinity. While oil shale carbonate has largely retained its primary carbon isotopic composition, interbedded bituminous 1 i m e s t o n e s
(
487
( o.t 1 3 C : + 2 to -17%o ) and calcareous concretions are depleted in l 3c by variable degrees compared with adj acent oil shale (Fig. ) . Non l bituminous concretionary limestones from the Upper Lias delta (Costa tenkalke ) and some of the nodular Lias zeta limes·tones· are also im poverished in the heavy isotope td1 3 c : +l to -6%o ) . In both cases, 1 3 c depletion results from early cementation in the sulfate reduc tion zone. I sotopically 11light" disso lved inorganic carbon , which. usually accumulates in pore water of marine sediments as consequence of bacterial oxidation of organic matter, has been incorporated intd liffiestones and concretions via sulfate reduction stimulated early cementation . Up to 45'% of total carbonate carbon in bituminous lime stones may be such 110xidized", formerly organic carbon. No extremely l 3 c-enriched carbonates , which are present in certain oil shales and most probably precip itate in the fermentation zone as a result of other bacterial activities , could be detected in the Posidonia Shales . In the bituminous Lias epsilon of Southern Germany , d o 1 o m i t e typically occurs a s a subordinate mineral . Only in the Altdorf swell region southeast of Nlirnberg does it occasionally outweigh calcite as the dominant carbonate . Grey marls from the Upper Lias delta (Blaugraue Mergel) and Middle & Upper Lias zeta, however , often wer¢ found to contain· high quantities (up · to two-thirds of the total carbonate) of an apparently iron-rich, disordered Ca-dolomite . Approaching the bituminous facies , the dolomite content always ra pidly decreases . Dolomitisation lowered the d l 3 c of the total car bonate only by about l%o and should have taken place fairly late in diagenesis . This is confirmed by the fact that dolomite i.s· never observed in interbedded early diagenetic limestone s . Non-dolomitic grey marls of the Lower Lias epsilon {Aschgraue Mergel ) and Lower Lias zeta ( Variabilismergel) have J 2 3 c values close to +l%o , which i s also the mea:n for bituminous intercalations within these marls . 2 . 2 . Oxygen Isotopes of Carbonate According to their oxygen i sotopic coropo�ition , the investigated carbonates may be grouped into three cateqories : a)
8 {d1 o : mainly 0 to - 3 %o , B e 1 e ro n i t e r o s t r a d r u e a r l y d i a g e n e t i c Fig . 3 ) and rare s y c a 1 c i t e tc5 1 8o : - 1 . 5 to -2. S%11 ) formed in or
488
close to equil{brium with more or less normal seawater at moderate temperatures; they have largely preserved their primary i.sotope ratios .
-8
BOHRUNG
1001
-7
-6
-5
-4
-2
-3
0
-1
BELEMNITEtv
KARBONATFRAKTlON t-0-i 1 2
Opalinuston Tonmerge! im Op.-Ton
1---0-1 1 1
K)-1 1 0
Jurensismerge\
l
. ..
•
Kalke d. Lias zeta bituminOse Kalke
. -
Fazies C
-
Fazies 8 Fazies A
04
bitum. Einschaltungen
1--0---1
Aschgr. Merge\
.
. · -·
3
.. . ..
t----0-1 2
Blaugr. Merge\
t----0----i 1
Costatenkalke
1----0----i
Merge\., Profit St. Nicolas, COte Vendee
-8
-7
-6
-5
-4
13
-3
l )
Opalinum Levesque! Thouarsense Variabilis
Bifrans u Fa\cifer
Tenuicostatum und Spinatus
· -
-2
-1
0
Fig . 3, Oxygen isotopic composition of sedimentary carbonate (means and ran9es for various lithological uni t s , total of 74 sarrples) and belemnite . rostra from the Upper Lias and Lower Dogger alpha of South west Germany (bore hole 1 00 1 Dotternhausen ; belemnites from various localities ) and from the T e n u i c o s t a t u m zone of S t . Nicolas , COte Vendee , France. Li tholoc;rical units : ( 1 ) concretionary lirr:es.tones and ( 2 ) grey marl of the Upper Lias delta ; ( 3 ) grey marl of the Lower Lias epsilon; ( 4 ) bituminous intercalations within grey re.arl s ; ( 5 , 6 , 7 ) oil shale of facies A , B , and c and (8 ) bituminous limestones of the Middle & Upper Lias epsilon; ( 9 ) limestones and ( 1 0 ) grey marl of the Lias zeta; { 1 1 ) interbedded Il'.a rls and ( 1 2 ) non-bituminous clay o f the Lower Dogger alpha; ( 1 3 ) grey rr.arl of St. Nicolas
489
d i a g e n e t i c c a 1 c i t e s (c.one-..1nL a t e cone and drusy calcite s , and calcitic fault--f illings ) , with d' 18 o values- ranging from -9 to -ll%o ; these precipitated from 1 8 o-depleted local groundwaters without participation of "oxi dized" organic carbon cJ 13 c : +1 . 5 to -1 . 5%o ) . c) S e d i m e n t a r y c a r b o n a t e ; this has values lying between groups ( a ) and ( b ) , . with oil shale carbonate depleted in the heavy isotope ( J 1 8 o : -6 . 5%o at site 1001 Dottern hausen) , and limestones (mainly -2 to - 5. 0/00 ) and non-bituminous clay from the Lower Dogger alpha (Opalinuston, -5%,,) enriched (Fig . 3 ) . Sedimentary carbonate also originated in the_ marine environment , but obviously has undergone large scale post-de positional oxygen isotope exchange with 11light11 meteoric for mation waters during late stage diagenesi s . b)
Variations i n the permeability of the different rock types to cir culating w a t e r s appear to control their f o r m a t i o n 18 final o content to a great extent . Samples froffi the vicinity of a small fault at site 1003 MOssi.ngen had l-2%o lower J 1 8 o values than comparable samples from other sites , indicating that significant oxy gen isotope exchange between groundwater and already consolidated calcitic country rock can take place even at rather low temperatures (and probably within geologically relative short time spans ) . The exact mechanism of the isotope exchange i s at present not known with certainty . Nevertheles s , 6 1 8 o data could be useful in studying long term water flow through highly impermeable carbonate-bearing sedi ments (e . g . , dense shales 1 suitable for the underground distribution of man-made substances harmful to human health) . B e 1 e m n i t e r o s t r a clearly tend to become 11l ighter" wi.th diminishing J 1 8o of the host rock. Thi.s trend can be easily seen by comparing belemnites · from the. Posidonia Shales with specimens from grey marls of the T e n u i c o s t a t u m zone of S t . Nicolas , COte Vendee , a region where the Toarci.an i.s not developed i.n oil shale facies (Fi.g. 3 ) Thu s , 6 1 8o values o f belemni.tes from the Swabi.an Upper Lias suggest either a slight increase in water tempera ture and/or decrease of salinity at the beginning of the Toarcian, or , more probably � post-depositional shif t ing of c5 1 8 o values by prefe rential recrystallization and/or cementation of rostra from the Posi donia Shales in contact with 1 8 o-depleted formation waters . Regard less of the di.ageneti.c overprint, oxygen as well as hydrogen isotopes (see below) , together with nannofossil studies by Hernleben et al . •
;:; ,
II
i ·'·· ' · ·. I•· I' /· . ·.. r'.
l
I: I
I..
I
!
I
I
490
t e m p e r a t u r e ( 1 9 80) , point to relati.ve constancy of during oil shale deposition. s a 1 i n i t y
a,nd
2 . 3 . · Organic Matter I ,I i•
li
I
,,
li I
In the Posidonia Shales of Southern Germany , the o r g a n i c c a r b o n c o. n t e n t of the oil shale i.t self ranges from 2 to 1 8 % (average 9 % ) , wh.i.le bituminous limestone values scatter around 2 % . " D ilution11 of organic ·matter by . carbonate seems to be the most important factor governing the organic carbon content . It may satis factori l y explain why organic carbon values reach a maximum in the low J 1 3c layer , where carborlate content rarely exceeds 2 5 % , i.n con trast to 30-45% for the rest of the sequence . Organic carbon content, therefore , is probably not a very valuable parameter in geochemical facies - analysis of bituminous sediments. Non-bituminous clay (Lower Opalinuston) and grey marls contain 1 % and 0 . 5 % o_rganic carbon, res pectively. The o r g a n i c m a t t e r is made up of roughly 10% bitumen {ex
tractable by benzene�ethanol mixture ) and 90% insoluble kerogen . Neither bi tumen ·nor kerogen exhibit any relati.onship between their chemic al or opti.cal properties and their 6 1 3 c values , as far as the main components of both are concerned. Some of the accessory con stituents such as palynomorphs , terrigeneous detritus , and isopre noids , however, show unexpected affinities in amount or composition to the 6 1 3c of the organic matter. The .oi.l shale k e r o g e n belongs to type II (as defined by Tissot et_ al . , 1 9 7 4 ) on the basis of its elemental composition (H/C: L 30 , O/C : 0 . 09 , N/C : O .Ol9 at site 1002 ; H/C : 1 . 2 8 , O /C : 0 . 06 , N/C : 0 . 021 at site 1005 ) . B i t u m e n contains more hydrogen (H/C : l . 50 at site 1002 ) , but less heteroelements ( 1 0 . 8 % by weight) than the kerogen (17 . 2 % heteroelernent s ) . Their nearly identical d 1 3 c values emphasize the 'genetic relationship between bitumen and kerogen (Fig . 1 ) . Minor c5 1 3 c d i fferences between them suggest scm� upward m i g r a t i o n of " light11 b itumen from the low 8 1 3 c layer into overlying str�ta . A m o r p h o u s m a t e r i always amounts to --at least 80% of the kerogen and dictates the value of the total organic matter . Predorninance . of amorphous kerogen in Lower Toarcian oil shales has previously been established by Teichmliller & Ottenjann ( 1 9 7 7 ) and Alpern & Cheymol (197 8 ) . P a 1 y n o m o r p h s ( algae and pollen) d e t r i t u s (vitrinites and continent-derived o r g a n i c
491
and inertinites) are present as minor components . Only the low 6 1 3 c layer is nearly devoid of coaly detritus (Wille, 1 9 80 ) , and, therefore, may De recogni.zed by i.ts somewhat more Drownish. color .
i. m m a t "\1 r e c.R' 0 . 5 % , positive · alteration of f luorescence of. liptinitic m substances) . Bitumeh yields (ClS+ ) and elemental analys·i.s of kerogen indicate that organic matter from site 1 005 Nlirtingen (Extr . /org . C : The organi_c matter can be classifi.ed as still more or less
1 2 . 7 % ) , located wi.thin the urach-Kirchheim geothermal area, has mere ly achLeved a slightly higher · degree of maturity than at site 1 002 z imm:ern near Heching�n (Extr . /org. C : l0 . 8 % ) . At site 1 005 , the organic
matte\!'" i.s at the earli.est stage of oil generati-on 1 which corresponds to a paleodepth near 1 500 m in the Pari.s Basin (Ti.ssot et al . 1 1 97 1 ) .
Evaluation of i n f r a r e d spectra of bitumen and kerogen indicate that both 1 3 c-rich and 1 3 c-poor organi.c materi.al are identi cal in terms of the relative contribution of aliphatic vs . naphthenic -·
vs. aromatic structural uni.ts to infra-red absorption. The
h y d r o
g e n i s o t o p i. c c o m p o s i t i. o n of the kerogen CO D : close to -1 1 1 %o ) agrees well �th published data for marine organic matter . Bi.tumen i.s more depleted in deuterium (cfD: close to -·1 4 1 %�:� ) ; however 1 some isotope exchanse during sample extractl:on cannot be ruled out. Nei.ther in bi_tumen nor kerogen do the 6 D values change with 1 3 c -'content . At site 1 005 , the porti.on of hydrocarbons - in th.e non-volatile extract lies somewhere between 30 and 50% . Distribution of c 5+ n - a 1 k a 1 n e s shows a maximum at n..,c 7 , no odd-even-p�edomi'nance , and only 1 minor amounts of long-chain paraffins . This is quite typical for many
marine sediments receiving little or no continental run-o f f . The organic matter of bituminous: liroes:tones was found to be indistingui sh able from that of surrounding oil shale in terms of 0 1 3 c , cS' D , Extr . /org . c . , and el emental composition .
Che!ni.cal and petrographi.c examinations suggest that aquatic microor gani..s:ms acted as the principal source ' for most of the organic material. From the stri.ct paralleli.sm i n 6J. 3c between oi_l shale carbonate and kerogen and from. th.e interpretation of the
6 1 3c
variations given below,
it is concluded that _ the bulk of the .organic JUatter also must have ultimately been generated by p h y t o p l a n k t o n which in
habited the oxygenated surface layer of the ocean (but probably not by calcareous nannoplankton) Anaerobic photosynthetic ·bacteria or benthic algae did not signif-icantly participate in formation of the •
I
I I
( i'
492
organic maf:eria·l . colnpared with Recent plankton from tropical and 3 temperate seas ( or 1 C : close to -20%o ) , organic carbon in the Posido-. 3 nia Shales ( - 2 7 to - 3 3 °/o"' ) is strongly depleted in 1 c and rather re- 1 sernbles that of terrestrial plants . Considering its incontestably aqua tic origin, this clearly demonstrates that carbon i sotope ratios alone cannot be used to reliably differentiate . marine from terrigeneous orga nic matter in ancient sediments . The organic carbon (but not the car bonate) of non-bituminous rocks ( J 1 3 c : -2 6 to -2 8%o ) is always en riched in 1 3 c by about 2 %0 relative to adj acent -bituminous shales ,
but probably also ori ginated largely in the marine environment .
A striking correlation · exists between the 6 1 3 c of the bitumen and the concentration o f the isoprenoid hydrocarbons p r i s t a n .and p h y t a n , as measured in relation to total n-alkanes (Fig. 4 ) . The . changing isoprenoid concentration apparently reflects variations in the chlorophyll content of the phytoplankton , since the phytyl-.side chain of chlorophyll must have acted as the source for most of the acyclic i soprenoids. The reason for the J 1 3 c-coupled variations in chlorophyll content is s o far not fully understood ( see below) . The pristan/phytan ratio , on the other hand, does not depend on the 6 1 3c of the bitumen , but clusters about a value of 1 . 4 5 (Fig. 3 ) .
D r i f t w o o d" , which is commonly found in the Posidonia Shales , has been mostly converted to jet. The elemental composition (H/C : l . l2 ,
O;c : 0 . 09 , N/C : 0 .0 1 5 ) and deuterium content (6 D : - 1 1 5 %o ) of this mate rial , together with petrographic observations , indicate that 70-80% of
it consists of now-polymerized compounds , taken up from the host rock during " j etification " . The impregnating compounds most likely existed
in the form of lipid-rich humic substances representing low molecular weight precursors of the amorphous kerogen. In terms of J 1 3 c values,
jet ( - 2 5 to - 3 2 %o ) , therefore , correlates with the surrounding sedi ment. However , it is not suitable for tracing secular 6 1 3 c variations in atrnosph.e ric co . 2 P y r i t e content of the oil shale fluctuates around 5 % , regard less of J 1 3 c and organic ·carbon content Only . two-thirds of the total iron is bound i'n pyrite. On the averag'9 , one-third of �.the Fe in pyri ...
te must have been supplied by an extraneous source, presumably sea water; the rest was derived from Fe-bearing clastic minerals . Availa
bility of iron was apparently the. limiting factor in pyrite formation .
493
�
Pr/Ph
R , 0,84
n-Alkane
0.70
1002 •
taos •
0.60
'"
O,LO
•
0.20
'"
a
Pr .. Ph �
·32
_
,
-30
-��
0,3
0,1
• •
•
•
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•
0,00
'" 0,20
6 1:C Extrakt
Pr/ Ph
1,6 1,4
-·•
- 30
----�. -.
•
1,2
•
_,
R= 0,22
1,8
•
•
•
-29
•
-18
•
\3
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-26
& C Extrakt
• •
•
1,0
0,8 0,6
•
0,4 0,2
.6
b
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•
____ ___. c
·'n
1002 . 1005 .
•
02
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-29
0,6
0,4
•
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• •
0,80
R:: 0,1B
0,5
•
1,00
•
0,30
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1,20
• •
R , 0,28
1,lO
•
•
1,80
"
"
"
"lo org.
C
d
"
"
"
�1. org. C
Fig . 4 . Oil shale bitumen from the Swabian Posidonia Shales, bore holes 1 002 and 1' 005 . ( a ) F.elative concentration of pristan aP..d phy tan v s . 0 1 3 c of the biturr.en and ( c ) vs . orgard.c carbon conteDt. ( b ) Pristan/phytan ratio vs. 0 1 3c of the bitumen and (d) vs . organic carbon content. I soprenoid concentration is measured relative to total n-alkanes 1 00 % . R: c6rrelation coefficient �
2 . 4 . Origin of o 1 3 c Variations Despite the undoubted synsedimentary nature of the parallel b 1 5 c variations in oil shale carbonate and organic carbon (Fig . 1 ) , the isotopic fluctuations obviously cannot be explained by changing chemi cal or petrographic composition of the carbOnate and the organic
494
3 matter (accessories db not noticeably affect the 6 1 c values } . Fur
thermore , the parallelism i s most likely not caused by -mixing of carbonate and organic matter from two different sources , an isotopi cally " light 11 and an isotopically "heavy" one . Throughout the oil
shale sequence , the same types of planktonic organisms always seem to have supplied the bulk of the material . Therefore , only three possible reasons can be ·invoked for the parallel 6 1 3 c variations : a)
" a g e e f f e c t " , related to the global inorganic carbon reservoir of atmosphere (C0 ) and hydrosphere (DIC: dis 2 solved inorganic carbon ) . e f f e c t 11 , related solely to b ) An " e n v i r o n m e n t a 1 An
DIC in local seawate r . c) A " b i o 1 o g i c a 1 e f f e c t 11 , related exclusively to the cytoplasmatic DIC pool of the individual phytoplankton cells,
from which carbon is withdrawn for assimilation and carbonate pre
cipitation.
age effect can be dismissed mainly in view of the magnitude and rapidity of the 6 1 3 c variation s . There is also some reason to consider the biological effect as unimportant ( e . g . , one would have to assume
An
that the calcareous nannoplankton provided nearly all of the organic
matter ) . Further implications of a biological effect are discussed
elsewhere (Klispert , 1 9 8 1 } . I f , however , the environmental effect is the predominant contro·l of the· 6 1 3 c variations , then the variations should reflect the extent of 11 s t a g n a t i o n " of the water column , as follows. 3 Organic matter is depleted in J.1 c relative to the inorganic carbon available. to the green plant, since assimi.lati_on pronouncedly di.s
criminates against the heavy isotope (negligible fractionation effects are involved i.n carbonate precipitation ) . In the surface waters of the sea , planktonic algae synthesize organic matter, which becomes
oxi.dized at depth . In an anaerobi.c basin, oxidation proceeds by sul fate reducing bacteri a , while removal of DIC introduced to the deep w�ter is severely restricted . Therefore , more and more 1 3 c-depleted , 11oxi.dized11 carbon will acCumulate in near-bottom water of :Stagnant basins (as in the Black Sea ; Deuser , · 1 9 7 0 ) . This may lo�er the 0 1 3 c value of the total deep water DIC by several permil .
During long lasting periods of very strong stagnation , isotopically " light 11 deep water DIC will escape to the photic zo'ne by convection and diffusion and in turn be taken up by the plankton for calcifi-
495
cation and photosynthesis . Since DIC in oceanic surface waters usual ly has a 6 1 3 c value very close to +2%o due to equilibration with at-
mospheric co , only an extraordinarily intensive stagnation-induced 2 r e c y c 1 i n g c a r b o n could lower its 0 1 3 c by the ·necessary 3 -4%o •
On the basis o f the d 13 c variations in combination with paleontologic and lithologic evidence , three new f a c i e s t y p e s (A, B , and C ) can be distinguished in the Posidonia Shales (Fig. l ) . F a· c i e s A refers to the low 8 13 c' layer ( carbonate < -0 . 5 %o , kerogen
k - 30%o ) , apparently deposited under highly anaerobic condition s , with the o 2 ;H s interface not far beneath the oxygenated photic zone and 2 vigorous carbon· recycling. Maximum S 13 c values in f a c i e s B (carbonate > +l%o , kerogen > -28 . 5%0 ) indicate optimum oxy.genation o f
near-surface water (assimilatory activity enriches the photic zone i n 1 3 c and o ! ) and return to a conventional mode of DIC replenishment . 2 Bioturbation horizons and occasional benthic microfossils prove that oxygen was temporarily supplied even to the sediment . In one respect , however , facies B is a transitional stage between A and c , in that
palynomorphs as well as belemnites , after a phase of restriction during A , only gradually recolonized a previouslY obstructed biotop e .
F a c i e s C ranges in carbon isotopic composition between A and B and shows only minor 6 1 3 c fluctuations. It represents an essentially nstabilized " , intermediate situation, but with DIC still more or less
!
belemnites and epibenthic ( ? ) bivalve s , and a diverse microflora.
I
approaching isotopic equilibrium with atmospheric co , and near-sur 2 face water condi�ions perhaps similar to those during grey marl sedi mentation. Facies c i s characteri.zed by abundant- macrofauna , including Facies A and B comprise the Middle Lias epsilon, and facies C the Up per- Lias epsilon. Although a clear cS" 1 3 c maximum near then oberer Steinn is to be recognized everywhere in the Swabian Posidonia Shale s , a
distinction between facies B and C is probably not possible through out the whole basin (e . g . , in Yorkshire , Fig . 2 ) .
A number of arguments supports the environmental effect and the pro posed facies types:
a ) J l 3c values of oil shale carbonate and organic matter from facies
B and c correspond to those of non-bituminous rocks and bituminous intercalations; carbonate ( moreover, fal·l S close to the average of +0 . 5% o for Jurassic marine carbonates (Veizer et al . r 1 9 80) . In con trast r facies A is significantly depleted in 1 3 c (only carbonate from the Opalinuston (-1 . 5 %o ) is also somewhat lower in J 1 3 c than that from facies B and C )
•
I
496
b) Bands of large calcareous concretions and thick concretionary
limestones (e . g . , Whale Stones , Unterer Stein) are very common in 1 facies A (Fig. 1 , 2 ) , and are always more depleted in 3 C than any other li.mestones in the succession. This poi.nts to an exception-al accumulation of "oxidized carbon in pore water as a consequence of n
anaerobism in the overlying seawater.
c) Facies
A
coincides with the wldest regional occurrence of bituminous
rocks in Euro�e (Hallam, 1967) and , hence , marks the period of strong
est anaerobism. 1 d) Belerrmites from facies C have the same range of c5 3 c values as belemnites from grey marls , while those from facies B are consistently enriched in 1 3c (Fig. 5 ) . Furthermore, in WUrttemberg , belemnites al
most completely disappear at the exact beginning of facies A (imme diately above the Fleins) and bec·ome moderately abundant again only in facies B (above the Steinplatte) . This observation has been fully con
firmed by W . Riegraf (personal communication, 19 8 1 ) . High diversity and abundance of belemnites are typical for facies c. Unlike ammonites , belemnites seem to have lived somehow dependeni on at least partly oxygena�ed near-bottom water .
• •
"'
0 .,
•
• Fo�oes 8 Fo�ies C F!eins TofeiHeins
·2
•
0
e
1
13
2
3
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•
• •
•
.,
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·3
. ..
•
•
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•
•
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• • •• • •
·2 s
& C Karbonatfraktion d. Begteitgesteins
.,
·3
·2
·I
0
Fig . 5. Isotopic composition of belemnite rostra from the Upper r,ias of Southwest Germany . Left: Q1 3 c of belerr.rli tes from bi:·tuminous shales ( facies B and c , Flein s , TafelfleinS) and non-bituwinous rr_arls (Upper Lias delta , Lower Lias epsilon, Lias zeta) v s . b1 3 c of host rock carbonate . Host rock data partly inferred from neigh
bouring section s . Hypothetical field for belemnites from facies 01 3 C - 61 8 o dia�rare for the same belemnite s . R: corre A . Bight: lation coefficient
I
6lao Belemnit
497
e) coqUinas ( 11Posi.doni.a beds " ) and dense bivalve pavements (wb.e ther P o s i d o n i a , I n o c e r a m u s , or P s e u -
dominated · by
d o m o n o t i s ) , which might be interpreted as autochthonous ep1benthos rather than scattered s he ll s , frequently occur in facies C as well as at the base of the bituminous sequence {Fleins) , but never within facies A (perhaps with one exception) . f ) Wi lle ( 1 9 8 0) �bserved a distinct relationship between 0 1 3 c values and acid-resistant microfossils (dinoflagellates , Prasinophyceae ,
acritarchs , and pollen ) . Entering facies A , diversity and abundance mf algae ( including Prasinophyceae , otherwise very common to black
shales) and diversitY o.f pollen drastically decrease , leaving a single ' pollen form, I n a p e r t u r o p o 1 1 e n i t e s o r b i c u 1 a t u s ,· as the dominant palynomorph. A diverse flora recurs only after 0 1 3 c values have increased to normal values in facies B. Facies
C displays a very uniform species composition. The. environmental sens itivity of the palynomorphs , as far as the planktonic algae are concerned , could possibly be the result of a benthic phase during their life cycle. g} Roscher ( 1 9 8 1 ) investigated Upper Liassic calcareous microfossils and discovered a moderately diverse , most probably autochthonous fauna in facies B , including benthic foraminifera and ostracOds. Microfossils from the same stratigraphic level have also been des' cribed by F i scher (1961 ) . No comparable fauna , however, has been hitherto found in facies A. h ) Several previously overlooked bioturbation horizons , barely visible in the outcrop (but easily identified in vertically cut cores ) , occur near· the 0 1 3 c maximum below and above the Oberer Stein (facies B) r but never in facies A. This kind of bi.oturbati.on is basically manifested as indistinct "perturbation 11 of the bedding (including- rare fucoid s ) , and should not be confused with the well known bioturbation horizons of thE!.. 11 Seegrasschlefer'' type , always present where bituminous_ shales grade into grey mar l s . Bioturbation wlt�n facies B has also already been recognized by Hauff ( 1 9 2 1 ) and Einsele & Mosebach ( 1 9 5 5 ) . i ) Ichthyosaur carcasses seem to be found preferentially in facies A . Moreover , the great majority o f specimens showing " skin-preservation'' was collected from this facies (Hauff, 1 9 2 1 ; Brenne r , 1976) . j ) In WUrttemberg , the transition from facies A to B is probably linked to a reversal of prevailing bottom current directions , as established by Brenner (in : Brenner & Seilacher, 1 9 7 8 ) for the Rolzmadffil area. Repeated hlgh energy events r causing erosion and condensation happened during phase C , but not during A. 1
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k ) Poverty of terresttial organic detritus and .monotony in the pollen assemblage indigeneolls to facies A (Wille, 1 9 80) may be attributed either to transport phenomena or to temporary changes in the vegeta tion cover of the nearby land mas ses . Correlation between isoprenoids
and d13c perhaps results from variations in nutrient supply to the
photic zone by admixing of deep water , or from changes in mean growth. ·depth of the phytoplankton.
3 . Conclusions Synsedimentary as well as post-depositi.onal factors determine the '
11
distribution of carbon and o�ygen . i sotopes in carbonates and organic matter. Among the diagenetic processeS , oxidation of organic matter , followed by carbonate precipitation, greatly influenced the 61'3c
values of coil.cret:Lons and early lLthi.fied limestones Dolomitisati,on . of grey rnarls only slightly affected their isotopic composition. Oxygen isotope exchange with meteoric formation waters certainly played a ·major role in lower ing the 61 8o values of sedimentary car bonate . Most diagenetic calcites formed in equ ilibrium with. such 1 80-depleted meteoric waters . No systematic changes of temperature or salinity during oil shale deposition could be detected . 013c and 0 1 8o values of belemnites are related to the isotopic. composition of the host rock. .•
Organic matter from the Posidonia Shales is �uch more impoverished in 1 3c than would be expected for organic material derived from mari ne organisms. Nevertheless , 110rdi.nary11 phytoplankton provided the bulk of the organic matter as well as most of the oil shale carbonate. Diagenetic transformation of the organic substances , however , i s probably not the main reason for the low 613c value s . Carbon isotope ratios can be used for tracing the primary migration of bitumen with in the oil s hale seam . Driftwood has been heavily impregnated with lipid-rich humic substances from the surrounding sediment . The un expected correlation between b13c of the bitumen and isoprenoid concentratLon is not fully understood. . The parallel 0 1 3c variations in oil shale carbonate and organic car bon are not caused bY changing compO sition of the mat�rial but rather seem to be environmentally controlled . Stagnation-induced carbon recycling i.s the proposed mechanism for 61 3c depletion. Carbon i.sotopes reveal that a. uni.form sequen"Ce of bituminous sedi ments was apparently deposi.ted under very di.fferent conditions with
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respec·t to the oxygen level of the water column. Current oi1 shale
models insufficiently- describe the sedimentary processes whi.ch opera ted during formation of the Posidonia Shales , and need revision in light of the new data.
The interpretation given for the
0 13 c variati.ons does not agree with
the hypothesis of - a thin high-salinity li_d' or an algal-fungal mat covering the sediment surface. The facies-type conception requires an anaerobic water column during deposition of facies A, precluding
the existence even of opportunistic .epibenthic (0 -respiring) orga 2 nisms. On the other hand , during deposition of facies B and c improved aer�tion of the deep water evidently tolerated at least occasional sea floor colonisations . In the future , sedimentary features such as varvin g , bioturbation , limestone deposition , shell bed formation ,
erosion etc. should be discussed bearing in mind the general background of long-term 613c variations . Carbon isotope ratios might be generally
helpful in tracing the extent of ''stagnation" in reducing environ ments.
At present, it is not clear whether an inadequate circulation system or a somewhat increased stratification of the water column (or both) was the driving ;force favoring oxygen depletion during the Toarci.an . HoWever , the geographical position of the epicontinental basin under coqsideratio n , open to the tropical Thethys as well as to the Arctic Ocean, might have predestined it in particular for the development of temporary stable density stratification , leading to wide-spread
oxygen deficiency, and , in consequence, oil shale· depositio n .
I'
Acknowledgements I wish to express m y thanks to my doctoral superVi.sor ,
H..
Friedri.ch
sen (Tiibingen) , for his appreciative parti.ci.patipn and continuous
int€.rest in this study. To B . Roscher (now at Karlsruhe) I am espe cially grateful for numerous discussions of the subject· and much valuable information. I am also greatly indebted to Ch. Hemleben , W. Riegraf , and W. Wille CTlibingen) for their friendly cooperation
and for providing samples . U . Rommel and St . Hoernes (now at Bonn) have been helpful in laboratory work; To lL w. Hagemann (Aachen) I am
very obliged for coal petrographic analyses . M . Schoell provided the opportuni.ty to visi.t the laboratori.e s of the BGR, Hannover . I thank T . I . Barrett (Tlibin�en) for useful comments on the manuscript and im
proving the English. Financial support has been granted by BEB , Hannover .
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References ALPERN , B . , CHEYMOL D . ( 1 9 7 8 } : Reflectance et fluorescence des organo· clastes du Toarcien du Bassin de Paris en fonction de la profondeur et _de la temperature . ReV. Inst. Franc;: . du Petrole 3 3 : 515-535 BRENNER , K. (197 6 } : Biostratinomische Unterschungen im PosidonienschieM fer (Lias epsilon, Unteres Toarcium) von Holzmaden (Wlirttemberg, Sliddeutschlan4 ) . Zbl . Geol . Palaont . , 1 97 6 , Teil I I : 223-226 BRENNER , K . 1 SEIL'A CHER, A . ( 1 9 7 8 ) : New aspects about the origin of the Toarcian Posidonia Shales . N . Jb . Geo l . Palaont . , Abh . 157 : 1 1 -18 BROCKAMP , B . ( 1 9 44 ) : Zur PaHiogeo_graphie und Bitumenflihrung des Posi donienschiefers im deutschen Lias . Arch. Lagerstatt·e nforsch. 7 7 . DEUSE R , W . G . (1970) : Carbon-13 i n Black Sea waters and implications for the origin of hydrogen sulfide . Science 1 6 8 : 1575-1577 DURAN D , B . , ESPITALit , J . , NICAISE , G . , COMBAZ , A . { 197 2 ) : gtudes de la matiere organique insoluble (k9rog8ne) des argiles d u Toarcien du Bassin de Paris . Premiere partie : ttude par les precedes optique , analyse 916mentaire , etude en microscopie et d i f fraction electro niqUe s . Rev. Inst. Fran9. du Petrole 2 7 : 865-·8 8 4 EINSELE , G . , MOSEBAC H , R. (195 5 } : Zur Petrographie , Fossi lerhaltung und Entstehung der Gesteine des Posidonienschiefers im Schwabischen Jura. N . Jb . Geol . Palaont . , Abh. 101 : 3 1 9 - 4 3 0 ESPITALI� , J . , DURAND , B . 1 ROUSSEL, J . -C . , SOURON , C . ( 1 9 7 3 ) : :E:tude de l'a matiere ·organique insoluble (kerog8ne) des argiles du Toarcien du Bassin de Paris. Deuxi8me partie : �tudes en spectroscopie infra rouge , en analyse thermique differentielle et en analyse thermogra vim9trique . Rev . Inst. Fran9 � du P9trole 2 8 : 37-6 6 FISCHER, W. ( 1 9 61 ) : tiber die Bildungsbedingungen der Posidonienschie fer in Sliddeutschland . N . Jb . Geol . Palaont . , Abh . 1 1 1 : 326-340 GAD , M . A . , CATT , J . A . , LE RICHE , H . H . ( 1 9 69 ) : Geochemistry of the Whitbian (Upper Lias) sediments of the Yorkshire coast . Proc . York shire Geol . Soc. 37 : 105-139 GAERTNER , H . R. v . , KROEPELIN , H . , SCHMITZ , H . -H . , FESSER, H . , �DLER, K. , HOFFMANN , H . J . & K . ( 1 9 6 8 ) : Zur Kenntnis des nordwestdeutschen Posidonienschiefers . Beih. geo l . Jh. 5 8 HALLAM, A . ( 1 9. 6 7 ) � An environmental study of the Upper Domerian and Lower Toarcian in Great Britain . ' Phi'l . Trans . Roy. Soc . London , B 252 : JgJ-4 4 5 HAUFF , B . Sr. ( 1 9 21 ) : Untersuchung der Fossilfundst&tten von Holzmaden im Posidonienschi.efer des Obe.ren Lias Wlirttem.bergs . Palaeontogr . 6 4 : 1-42 HEMLEBEN, Ch.. , ROSCHER. B . , GRXTZEL , E . , JAKOB , li. 1 MfJHLEN , D . , ROSENAU , E . ( 1 9 80 ) : Stratigraphische , paUi.ontologische und sedimen tologische Ergebnisse . In : Untersuchungen zur Genese des Posidonien schiefers (Unter-Toarciu:tn) in Siidwest-Deutschland. Unpublished report , Ins t . f . Geol . u . Pal&ont . , TUbing en HOWARTH, M . K. { 1 9 6 2 ) : The Jet Rock Series artd the Alum $hale Series of the Yorkshire coast. Proc . Yorkshire Geo l . Soc . 3 3 : 381-422 JORDAN , R . ( 1 9 7 4 ) : Salz- und Erd61/Erdgas-Austri tt als Fazies bestim mende Faktoren im Mesozoikum Nordwest-Deutschlands . Geo l . Jb. A 1 3 KAUFFMAN , E . G . (197 8 ) : Benthic envtronments and paleoecology o f the Posidonienschiefer (Toarcian) .. N . Jb . Geo l . Palaont . , Abh . 1 5 7 : 1 8-36 ·'
501
KtlSPERT , W. (19 7 7 ) : Sauerstoff- und Kohlenstoff- lsotopenverhaltnisse von Karbonaten aus Lias delta bis Dogger alpha SUdwestdeutschlands. Unpublished Diplomarbei t , Inst . f. Geol . u. Palaont . , Tlibingen , KUSPERT , W. ( 1 9 8 1 ) : Faziestypen des Posiddnienschiefers (Toarcium , Sliddeutschland) . Eine i sotopengeologische , organisch-chemische und petrographische Studie . D i s sertation, Tlibingen POMPECKJ, J . F . 0.901 ) : Die Jura-Ablagerungen zwischen Regensburg mid Hegenstau f . Geognost . Jb . 1 4 : 1 3 9-220 QUENSTEDT , F .A . (1858 ) : Der Jura. H. Laup p , Tlibingen ROSCHER, B . ( 1 9 81 ) t Dissertation , Tlibingen (in preparation) TEICHMULLER, M. 1 OTTENJANN , K . ('1 9 7 7 ) : Liptinite und lipoide Steffe in � einem ErdOlmuttergestein. ErdOl u. Kahle 30 : 387-3 9 8 TIS�OT, B . , CALIFET-DEBYSER, Y . , DEROO , G . , OUDIN, J . L . ( 19 7 1 ) : Origin and evolution of hydrocarbons in Early Toarcien shales . Amer. Assoc. Petrol . Geol . Bul l . 5 5 : 2 1 77-2 1 9 3 TIS SOT,. B . , ·DURAND , B . , ESPITALil� J. 1 , COMBAZ , A. ( 1 9 7 4 ) : Influence of nature and diagenesis of organic matter - in formation of petro leum. Amer . Assoc. Petrol . Geo l . Bull . 5 8 : 4 9 9 -506 1 3 12 c; c VEIZER, J . , HOLSER, W. T . , WILGUS , Ch . K . ( 1 9 80 ) : Correlation of and 34-S/32-S secular variations . Geochim. Cosmochim. Acta 4 4 : 579-587
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WILLE , w. { 1 9 80 ) : Zur Palynologie des Lias eoSilon� In : Untersuchungen zur Genese des Posidoniensch,iefers ( Unter-Toarc ian) in Siidwest Deutschland . Unpublished report , Inst. f Geol . u . PaHiont . , Tlibin-• gen . •
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The Community Structure of"Shell Islands" on Oxygen Depleted Substrates in Mesozoic Dark Shales and Laminated Carbonates (Abstract) E. G. KAUFFMAN
Abstract: Dark clay shales and laminated carbonates which are finely and evenly bed decl, and - sp:rrsely to non-bioturbated, haye been intf3XPreted as inhospitable substra tes for Mesozoic zoabenthos. 'lhese facies are cornrronly cited as deposits of oxygen starved basins with anax:ic , H2s enriched benthic sediments. Yet a paucispecific to moderately diverse suite of normally benthic invertebrates is oommonly found with such facies, sometimes in great abundance., fanning benthic shell pavements. Attempts to explain these occurrences as debris from rra.ss flO'Jl, or fran "pseudoplanktonic rain11 are unsatiSfactory in that they do not fit the great bulk of the evidenCe. Both shelly turbidites and examples of nonnally benthic invertebrates attached to floating lCXJS and living ammonites are kno.-m from Mesozoic dq.rk shale and laminated carbonate facies. But rrost occurrences of zoobenthos in these facies are not as sociated with mass flON" fabrics, nor with pseudoplanktonic hosts in life association. 'lhe latter accounts for less . than 5 percent of the macrozoobenthos associated with these facies. Another interpretation is needed to explain their occurence in sup posedly. toxic environments. Detailed field sbJ.dies reveal an. abundance of densely inhabitOO shell islands - main ly large dead amrocmite shells and inoverarnid or ostreid bivalves which apparently colonized inhospitable substrates - in these Mesozoic facies. The communities of these shell surfaces are lON" to moderate diversity asserrblages dcminated by cemented and byssate epibionts (bivalves , brachiop::xls , bryozoans , barnacles , S};Onges, worms) and small boring invertebrates (wonns , bam.acles, bryozoa, sp:mges mainly) . '!hey are camnanly preserved in life association on the shell islands. The CCJITliX>sition of these communities closely parallels the composition of t�e dead shell assemblages of benthic invertebrates which are corrrn rro ly associated with dark shales and laminated carbonates, strongly suggesting a benthic rather than a pseudoplanktonic origin for these taxa 'Ihis :in tum contradicts the stagnant basin mcx:lel for these facies, and instead suggests inhospitable chemical environments restricted to the benthic sedi ment, the sedimentwater :Interface, and a thin zone of benthic water column directly overlying the interface ; evidence suggests that the anoxic and/or H2s p::>ison boun dary migrated frequently back and. forth across the interface . 'Ihe rnmimum size of the colonized shell islands, and the distribution patterns of epibenthos on larger shells allaH rreasurement of tl)e p:!sition of the anoxic botmdary relative to the in terface at any one time (stratigraf:hic level) . Epibenthic encrustation patterns suggest that the oxygen and/or H S gradient v.as narra;...r , as taxa tolerant of low oxygen are. succeeded up.vard on trl:e same shell by less tolerant taxa within a few em. Further, the number of generations of epibiont species, and the coilplexity of the shell island ccmnunity, reflect the duration of habitable benthic environments in these facies. .
•
Cyclic and Event Stratification (ed. by Einsele/Seilacher) © Springe·r 1982
503
Depending up:m benthic oxygen levels and the longevity of habitable environments, one to five stages of "island" canm.mity succession are preserved on encrusted bi valve or dead amnonite shells. To generalize, the five successive colonization sta ges are: ( 1 ) Direct colonization of the substrate by eurytopic inooeramid or ostreid bivalves; (2) initial colonization of shell islands by boring worms , barnacles and/ or sponges, and encrusting serpulid wo:rms ; (3) overgrowth by one or more generations of s:rrall Ostreidae and subsequent generations of stage 2 organisms; (4 ) addition of diverse byssate bivalves , brachiopcds, encrusting and boring Bryozoa, boring polychaetes, and vagrant epilienthic invertebrates; ( 5) overgrowth by sp::>nges and/ or pedunculate barnacles and sUccessive generations of �sters and stage 4 epi bionts. Carnnunity structure evolves in complexity from Jurassic through Cretac.eoos time; in pnt this is related to changes in the size and morphol<Jg'ical comple:x:ity of shell islands themselves. (For� a more detailed version see GP'P:i , BOUcar & BERPY (eCs. ) "Corrmunities of the Past" , Hatchinson Foss Publ. , 1981 : 311-380) .
Ammonite Shells as Habitats - Floats or Benthic Islands? (Abstract) A. SE!LACHER
Abstrac t : In Erle KAUFFMAN ' s view, which has brought s o much new air into the stagnant Posidonia Shale discussion, "shell i s lands" acted as outposts for benthic life on an otherwise uninhabitable mud bottom. Of the potential is lands , vertebrate bones and belemni te rostra became available only after soft part disintegration . They never show epibionts . T.he ·remaining substrates - driftwood and ammonite shells - could become colonized also while they were still afloat. The following observations regard this distinction: 1 . Uneven colonization : In monospecific ammonite clusters , over growth cont�asts in degree and kind from one shell to the other . 2 . Substrate selectivity: None of the epibionts occurs with similar frequency on all substrates . Inoce�amus� Seiroqrinus and goose neck barnacles occur almost exclus ively on drift wood , Orbicu loi dea� oysters (2 kinds) and byssus-attached bivalves other than Inoceramus (Pectinids , Limids , Bakevelliids , Monotids} , only on ammonite shells . Even more specific, Serpula has been found only on Harpoceras and Naut i lu s , which also carries bryozoa. This selectivity holds true even if wood and ammonite shells are now closely associated On the same bedding plane . 3 . Equal encrustation on both flanks : Right and left side of the examined ammonite,s never differ significantly witP, respect to kinds, sizes , diqt�ibution and numbers of encrusters (Ostrea� Exogyra� Serpu la ) , suggesting s imultaneous rather than subse quent colonization. 4 . Unequal distribution around ambitus : In contrast to the encru sters , byssusattached pelecypods (Gerv i l l i a � P l a g i o s toma� Pecti nids ) on Lythoceras are significantly accumulated on the side that faced down in the swimming or floating cephalopod shell. This is also true for one Harpoceras specimen with Inoceramus. 5 . S lope orientation: While oysters , and serpulids fail to show preferred orientations , 1 77 Orbiculoidea on the flank of a large Lythoceras ar.e statistically aligned conforming with the swimming positi'on of the host shell . Conclusion i s that all 2 5 shells analyzed ( in a much larger number overgrowth was absent or insufficient for statistical evaluation) were colonized before they sank to the bottom. It should be empha sized, however , that large specimens were selected for this study and that almost all came fiom the lower fa lciferum Zone (=facies A of KUSPERT , this vel . ) , i . e . the most 11 stagnant " part ··o f the Posidonia Shales . (For a more detailed version see N . Jb . Geo l . Palaont . , in print)
Cyclic and Event Stratification (ed . by Einsele/Seilacher) © Springer 1982
Palynology of Upper Liassic Bituminous Shales (Abstract) W. WILLE
Abstract: Among Liassic acid resistant microfossils three groups are most important: terres'trial coniferous pollen grains , marine. dinofla gellate cysts , and equally marine Prasinophycean alga (Tasmanites and related genera) . Of these, pollen grains and Prasinophyceae have little or no stratigraphic value, while dinoflagellates look rather promising. The vertical transition from Upper Pliensbachian argillaceous limesto nes to Lower Toarcian b ituminous shales and back again to Upper Toar cian marls and limestones allows us to test the facies control of marine algae. Among the dinoflagellate species persisting from Lias delta to Lias zeta, the tolerance to euxinic conditions varies in the following order : Manoodinium semitabu Z a tum is most sensitive , followed by Comparodinium punotatum and Nannooeratopsis defZandrei , while only "Arg e n t ie l Z.a c f . p a t t e i 11 and lfApiodinium glabrum " endure the most euxinic conditions , indicated by the maximum bitumen values in the e Z. egantuZ.um and exaratum subzones of the lower faZ.oiferum zone. It is well known from Silurian to Tertiary sediments that Pras inophy cea commonly abound in marine "black shales 11 • In the Swabian Toarcian their diversity and quantitiy increase rapidly with the bitumen con tents in the upper fa Zeiferum and bifrons zones, but they are again verr rare in the highly bituminous "e.ZegantuZ.um shales" of the lower fal0iferum zone. Here the environment seems to have been too poor in oxygen even for Tasmanit e s and its allies . Surprisingly , the land-derived pollen assemblage also decreases in diversity in the " e ZegantuZ.um shales" until one species (Inaperturo p o Z. Z en i t e s orb i o u Z a t u s ) is absolutely dominant to the near exclusion of other pollen forms . Palynology and isotopic geochemistry studies ( KUSPERT) show that the controls for these facies relationships must have been effective from Yorkshire to Swabia and southern France. The Toarcian bituminous shale basin has been adverse npt only f,or benthic organisms , but also for those planctonic algae that include a benthic phase in their life cycle . In dinoflagellates this is the encystment stage : The cysts form in the photic zone and sink down to the sea floor for a resting phas e . When the Protoplast tried to leave the cyst again by way of specially designed lids , it was killed by the anoxic bottom waters . Lias epsilon cysts still show the somewhat loosened opercula in situ , while Lias delta or Lias zeta specimens indicate successful hatching by the absence of lids . Since the life cycle of the recent marine Prasinophycea is not adequately known , we can not yet explain their preference of moderate and medium euxinic environment.
Cyclic and Event Stratification (ed . by Einsele/Seilacher) © Springer 1982
The Bitunlinous Lower Toarcian at the True de Balduc Near Mende (Departement de 1a Lozere, S-France) W. R:IEGRAF
Abstract: The bituffiinous shales ( Lower Toarcian} of the True de Balduc represent the deepest facies of a symmetrical cycle of marine sediments . There are close faunistic and facies connec tions between Southern France an4 Southwest Germany . A few lime stone layers in the bituminous facies can be correlated over very long distances in Europe . Some micropalaeontological re sults concerning the forami � ifera and ostracods are presented . 1 . Stratigraphy and Basin Developme nt In the area of the True de Balduc marine sedimentation started with a Rhaetian transgres sion over crystalline basement (Fig . 1 ) . During the Lower Jurassic the deposition of shallow marine depos its : cla stic rocks , dolomites and oolithes continue . Later , limestones with shell hashes appear . In ·the Middle Liassic monotonous clay series were deposited on top of s ilica-rich limestone/marl alternations . The basin reached its maximum depth. in the Lower Toarcian which is . the peak of Lower Jurassic transgression (VAIL & MITCHUM, 1 9 79 } . Shales , marls and limestones of a b ituminous facies are widely distributed not only in the True de Balduc bas'in, but in large parts of Europe ( 110il Shale Event11 ) . In the Upper Toarcian the b ituminous facies gra des into a thick series of marls and shales which grade into ooli thes and unstratified dolomites indicative of shallow water . Thus , the bi·tuminous shales of the Lower Toarcian in the True de Balduc are an example of a nearly complete symmetrical cycle { cyclothem) in the sense of HALLAM ( 1 9 6 1 ) 1 HALLAM & BRADSHAW ( in press) and KAUFFMAN ( 1 978) . 2 . Bios tratigraphy In the area of the True de Balduc the zone of Dae t y L ioeeras tenui e o s ·t atum is clearly reduced { 0 . 1 -0 . 9 rn, Fig. 2 ) , as it is in Franco nia ( Southern Germany ) . The zone of Rarpoceras fale ifeP ( 4 . 0 - 8 . 0 m) is completely represented . The zone of Hildoeeras bifr �ns is unusual ly thick ( 20-30 m ) , comparable to the situation in Whitby (York shire ) . For subzones and biostratigraphic comparison with other are as see DEAN , DONOVAN & HOWARTZ ( 1 9 6 1 ) , HOWARTH ( 1 9 6 2 ; 1 9 7 3 ; 1 9 7 8 ) , WEITSCHAT ( 1 9 7 3 ) , RIEGRAF ( 1 9 80) and RIEGRAF , WERNER & LORCHER ( in preparation) .
Cyclic and Event Stratification (ed . by Einsele/Seilacher) © Springer 1982
11
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Fig. 1 . Position of the bituminous sqales of the Lower Toarcian in a nearly symmetrical cycle ( cyclothem) . 1 Dolomite , 2 Limestone with shell-hashes, 3 oolithe, 4 silica-nodules , 5 limestone , 6 marly limestone , 7 arenitic limestone, 8 shaleS, marly Shales , marls , 9 b ituminous shales, 1 0 liffiestone concretions , 1 1 sandy facies� 1 2 basal conglomerate, 1 3 metamorphic series, 14 syenogranite, T5 ammonites , riautilids�1 6 belemnites , 17 bivalves , 1 8 gastropods , 1 9 solitary corals , 20 vertebra te remains , 2 1 trace fossils;-22 plants (driftwood)
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Fig. 2. B iostratigraphy of the two types of Lower Toarcian sequen ces of the True de Balduc area in comparison with the incom plete sequence at Dotternhausen cement quarry . The aggluti nating foraminifera mostly used quartz grains and in one case glauconite. The benthic (burrowing) bivalves do not in c lude Steinmannia (= Posidonia pars) and B o s i t r a . BOth ge nera are believed to be epibenthic or pseudoplaDctoni c . The gastropods are forms like Coelodiscus minutus q;cHtiBLER) and c f . AmberZeya� c f . Oo l i thica , etc . The ostracods are mostly ogmoconchs . Sculptured species occur only in the higher sub zone of Peronoceras fibulatum to the Upper Toarcian
509
3 . Facies Development and Faunal Contents Facies and fauna of the Lower Toarcian of Southern France show close relationships to Southwest Germany (Fig. 2) and also to the Whitbian area (RIEGRAF:�· 1 9 80 ) . A limestone layer called 11Untere_ Bank" in the True de Balduc occurs at the same stratigraphic level and in the same facies in Alsace-Lorraine (SCHIRARDIN , 1 9 1 4 : 3 4 5 ) and in South west Germany ( "Unterer Stein, E I I 5 " , HAUFF , 1 9 2 1 ) . It has an equi valent of same stratigraphic age in WHITBY ( "Whalestone " of HOWARTH , 1 9 6 2 : 387 ) . Also the "Oberer Stein, S II a '' (HAUFF, 1 9 2 1 ) in the middle part of the subzone -of Harpoceras e �egans has been observed at' the True de Balduc. Based on their facie s , two types of bitu minou� series are distinguished in the area of the True de Balduc; Type 1 with normal thickness and 2-3 limestone layers , but rare concretions . It is deposited under shallower water conditions ( "Swabian Type " ) ; Type 2 with 1 2 or more limestone layers , abundant and commonly very large concretions and a few layers of single concretions en closing large drift-woods ( "Jet " ; "Jet-Rock'' in the Whitby area, (HOWARTH 1 9 6 2 : 3 8 5 ) ; the thickness of this sequence is clearly increased indicating deposition in a special basin ( "Whitby Type " ) . The two types of facies also differ geochemically . The bituminous facies (subzone of Protogrammoaeras p a � tum to subzone of Peronooeras fibu Z a t u m ) sta�ts with a sharp boundary of nonbituminous marls/bi tuminous shales . In their lowermost part the bituminous shales con tain many very small foraminifera, echinoderma.t a, gastropods and b ivalves . Later, oxygen-rich currents ( BRENNER, 1 9 7 6 ) often impro ved the living-conditions on the seafloor for a short time . These oxygenating events are reflected in bioturbation and benthic faunas during the fa�aifer-zone and by decreasing amounts of bituminous ma teria l . Lamination is rare in the bituminous shales . 4 . Micropalaeontology In the Upper Liassic the ostracods are good indicators for different oxygen levels . They disappear suddenly before the onset of the bi tuminous facies (same as in Southwest Germany) and are miss ing in all highl¥ bituminous horizons - with the exception of bioturbation-ho rizons in the subzone of Harpooeras e Zegans . At the beginning of the b ituminous facies one can observe relatively species-rich and unusu ally small foraminiferan assemblages . They are probably dwarfed forms ,
I
510
partly of autochthonouS and partly of allochthonous (drifted) origin. At the same time , agglutinating foraminifera (Hap laphragmo ides� Rheo phax� and other genera) become relatively common , but special prepa ration methods are necessary to obtain them. The same foraminifera occur in Southwest Germany ( for example in Dotternhausen and Gomarin gen) in the non-bituminous marls of the tenuiaos.tatum-zone . Further investigations must clarify whether these agglutinating foraminifera occur so commonly only at the bases of bituminous sequences . Biotur bation horizons of the sub'zone of Harpoceras e 'l egans commonly contain allochthonous ( Southwest Germany) ' or autochthonous (True de Balduc) foraminifera faunas . of low diversity. In the subzone of Daatytioaeras aommune foraminifera are again found in larger specimens at the True de Balduc · as well as in Southwest Germany (Dotternhausen) . 5 . Conclusions At the True de Balduc the boundary between anoxic/oxic conditions was at or somewhat above the sea-floor, but became transmitted epi sodically into the substrate by infrequent oxygen-rich currents . Such events resulted in bioturbation layers with benthic faunas . Characteristics of the bituminous facies are :
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special Foraminifera and coccolites; these orga nisms seem to have been specially adapted to oxygen-depleted conditiop s ; some of them are enriched by s trongly reduced hemi pelagic sedimentation. P s eudomon o t i s �
At the onset of the bituminous facies, or somewhat before, micro fossils disappear in the following sequence : ( 1 ) Sculptured and smooth ostracods , holothurian fragments ( 2 ) Calcitic fora�inifera (Miliolids , Marginu Z ina etc . ) 1 starfish ossicles , small benthic bivalves , gastropod s , brachiopods ( 3 ) Foraminifera of the Lentiau lina and Dentalina types . ( 4 ) Agglutinating foraminifera, echinoids . When the b ituminous facies gradually changes back into marls this succession is inversed .
51 1
Referen�es BRENNER, K . ( 1 9 7 6 ) : Ammoniten-Gehause als Anzeiger voh Palaeo-StrO mungen . - N . Jb . Geol . Palaont. Abh . JgJ: 1 01 - 1 1 8 , Stuttgart. DEAN, W . T . , DONOVAN , D . T . & HOWARTH , M . K . ( 1 9 6 1 ) : The Liassic ammo nite zones and subzones of the North-West European Province . Bul l .brit . Mus . nat. Hist. {geol . ) � : 4 3 7 -505 , London . HALLAM , A . ( 1 9 6 1 ) : Cyc lothems 1 transgressions and faunal change in the Lias of North-West Europe . - Trans . Edinburgh geol. Soc. J� : 1 2 4- 1 7 4 , London . HALLAM , A . & BRADSHAW, M . J . { in press ) : Bituminous shales and ooli thic ironstones as indicators of transgress· ions and regressions . J . geol. Soc � , London. HAUFF , B . s r . ( 1 9 2 1 ) : Untersuchung der Fossilfundstatten von Holzrna defl im Posidonienschiefer des Oberen Lias Wlirttembergs . - Palaeon tographica (A) g � : 1 - 4 2 , Stuttgart. HOWARTH /- M . K . ( 1 9 6 2 ) : The Jet Rock Series and the Alum Shale Series of the Yorkshire Coas t . - Proc . Yorkshire geol . Soc . gJ : 3 8 1 - 4 22 , Hul l . { 1 97 3 ) : The stratigraphy and ammonite fauna o f the Upper Liassic Grey Shales of the Yorkshire Coast . - Bul l . brit . Mus . nat . Hist. (geol . ) �� : 2 3 5-277 , London. ( 1 9 7 8 ) : The stratigraphy and ammonite fauna of the Upper Lias of Northamptonshire . - Bul l . brit . Mus . nat . Hist . ( geol . ) �� : 2 3 5-28 8 , London . KAUFFMAN , E . G . ( 1 9 7 8 ) : Benthic .environments and paleoecology of the Posidonienschiefer (Toarcian ) . - N . Jb . Geol .Palaont .Abh. Jg1 : 1 8- 3 6 , Stuttgart. I RIEGRAF , w. ( 1 9 80 ) : · Kartierung im I
Bedding Types of the Toarcian Black Shales in NW-Greece J. P. WALZEBUCK
Abstract: The background sedimentation of the Toarcian b lack shales in NW-Greece was frequently interrupted by h igh-energy ievents, i . e . "dyscyclic rhythms 11 • Even some lamination phenomena can be interpreted as event stratification. The study of the bedding types provides new insights on the pa leobathymetry and paleoenvironment of these b lack shales . In the Lower · Toarcian there exiSted small , shallow-marine, stagnant basins within the Ionian trough which were influenced by upwel ling in the Upper Toarcian. 1 . Geological Frame The Ionian zone of NW-Greece constitutes part of 'the most external zones of the Hellenides ( Zante .zone, Paxos zone, Ionian zone, Gavrovo zone, see Fig. 1A) . In the . lowermost Jurassic NW-Greece was covered by a huge carbonate platform which, in the Early Jurass i c , was intensively block-faulted. This process lead to the formation of platform and trough areas. Whereas the production of platform-car bonates persisted through the whole Jurassic in the Zante , Paxos and Gavrovo zone s , the Ionian zone became an area of stronger subsi dence and faulting showing vertical and lateral facies differentia tions ( F i g . 1 , B and C ) . For detailed information on the regional geology of NW-Greece the reader is referred to RENZ ( 1 9 55 ) , AUBOUIN ( 1 9 5 8 , 1 9 59 ) , INSTITUT DE GEOLOGIE ET RECHERCHES DU SOUS-SOL, ATHENES , ET INSTITUT FRANCAIS DU PETROLE , MISSION GRECE ( 1 9 6 6 ) , BERNOULLI ( 1 9 6 7 , 1 96 9 ) , BERNOULLI & RENZ . ( 1 9 70) , and references the rein •
. 1 . Lower Jurassic The 11 Pantokrator Limestone " represents the carbonate-platform facies . It consis-ts of massively bedded , white packstone with onko:i,ds , and ' also of pelletal and intraclast' packstone , grainstone and pelletal lime mudstone (after BERNOULLI & RENZ , 1 9 7 0 , p . 57 7 ) . The "Siniais Limestone " shows white or greyish, medium to thin bedded mudstone and wackestone with bioclasts ; the limestone contains chert layers and chert nodules . Cyclic and Event Stratification ( e d . by Einsele/Seilacher) © Springer 1982
513
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The Toarcian of the 'Ionian zone shows a characteristic facies difte rentiation. 5 types of facies can be distinguished: ( 1 ) Posidonia black shale, ( 2 ) Ammonitico Rosso marl and nodular limestone , ( 3 ) interfingering o f Posidonia black. shale and limestone with fi laments (Farmakovouni section in Fig . 2 ) , ( 4 ) marly argillaceous shale which vertically grades into Ammo nitico Rosso limestone and vice versa ; ( 5 ) swell areas are characterised b y stratigraphical gaps , some ranging from Lower to Upper Jurassic . The thickness of the Toarcian deposits varies between 0 - 1 00 meters .
514 1 .2.
Middle/Upper JurasSic
The Middle Jurassic is represented by the " Limestone with Filaments 11 (Calcaires en Plaquettes , Calcaire a Filaments , Pelagic Lamel libranch Limes tone , according to former authors ) , i . e . white mudstone and wackestone with pelagic lamellibranchs ( f ilaments ) . This sequence shows numerous intercalations of several meters thick intraformatio nal breccias . In Korfu a several meters thick layer of oolithic lime stone has been observed . The 11Upper Pos idonia Beds" ( U . Tithonian - L . Senonian) consist of very thin bedded s iliceous argillite and abundant chert, with radio larians and Posidonia . The 11Vigla - Limestone " resembies the alpine Majolica, with typically well-bedded ·white mudstone and chert . Its stratigraphical range is defined by the occurence of tintinnids (BERNOULLI & RENZ , 1 9 70 , p . 588) . A more detailed description of the Jurassic carbonate facies of the Ionian zone is given by BERNOULLI & RENZ ( 1 970) . 2 . Bedding Types 2 . 1 . Introduction This report presents part of a Ph . D . thesis entitled " Sedimentology of the Toarcian Black Shales of NW-Greece 11 • Here, observations on the type · of bedding within the black shale sequence are stressed . In con trast to West- and Mi4-European equivalents , this sequence shows nu merous clastic intercalations . The normal background sedimentation was interrupted by high-energy sedimentary events , i . e . "dyscyclic rhythms " (compare SEILACHER, this Vol . ) . 2 . 2 . Differentiation of Small Black Shale Basins Sections of the Toarcian black shales of the Ionian zone show varia tions in facies and thickness (Fig. 2 ) . Due to vertical and la·teral facies differentia'tions it is not possible to present on �. type sec tion for the study area nor to establish a detailed regional litho s tratigraphic correlation . The 6 sections schematically represented in Fig. 2 are characteri stic of the major Toarcian black shale basins within the Ionian trough: ( 1 ) Korfu basin , ( 2 } Igoumenitsa basin, ( 3 ) Farmakovouni basin, ( 2 ) Chionistra basin , ( 5 ) Kalokhori basin ( compare Fig. 1 B ) .
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F�g. 2 . Schemat�c sections o f the Toarcian black shales in NW-Greece . The litho logies of the shales can be classified in the ternary system carbonate quartz - clay : the major rock types are marly black shale and calcareous/ sil�ceous black shale; clayey black shale (white part of triangle ) is very rare . Note : clastic layers are not in scale . Further explanations see part 2 . 2 . For previous descriptions of sections of the same area see INSTITUT DE GEOLOGIE ET RECHERCHES DU SOUS-SOL , ATHENES ET INSTITUT FRANCAIS DE PETROLE ( 1 9 6 6 ) and references therein
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is a marked facies change from marly/calcareous facies to siliceous facies, due to a rapid increase of radiolarian content , ( 3 ) Posido nia occurs only in the upper part of the sections . The sections especially differ in their intercalations of clastic layers . Although some clastic layers reaCh a thickness of more than 5 meter s , their lateral extent is limited. The discontinuity of clastic layers is interpreted as a result of bottom relief (BERNOULLI, 1 Q 6 9 ) which is also indicated by frequent synsedimentary s lumping, folding .and fracturing. An important factor influencing the facies of the Toarcian black shales throughout the Ionian zone is the relative increase of radio larian content of the sediment in the middl� and upper part of the sections . In some of the samples radiolarians make up 50% of the rock . The radiolarian layers are well stratified and build up the upper black shale sequence as one of the main rockforming components (see part 2 . 3 ) . The relative increase of radiolarian deposition most pro bably reflects the influence of high fertility of ocean waters , as it is known from recent and ancient . upwelling areas (DIESTER-HAASS et al . , 1 97 3 ; EINSELE & WIEDMANN , 1 9 7 5 ; D IESTER-HAAS S , 1 9 7 8 ; and re ferences therein) . 2 . 3 . Bedding Types The following bedding types have been observed : ( 1 ) even lamination, ( 2 ) wavy lamination, ( 3 ) graded lamination, ( 4 ) stratified radiola rian-rich sequences, ( 5 ) bedding in connection with bioturbation, ( 6 ) clastic layers ( see part 2 . 4 } . Even Lamination ( terminology ac. - : - _,rding to CAMPBELL, 1 96 7 ) : Dark and light laminae. have a thickness between 1 - 1 0 millimeters (Fig. 5 ) A systematic thinni'ng-up or thickening-up of the sets of laminae has not been observed . Very often the lower boundary of an individual lamina is well defined in contrast to the top boundary . This probably is caused by .grading within some of the lamin�e . •
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The primary cause of lamination is not yet known . Inorganic and organic geochemical analyses are in progress . Wavy Lamination : Fig. 3 shows lamination which is build up of periO dic alternations of thin Posidonia layerS and minor. sets of laminae free of shells. The Posidonia shell layers have a characteristic biostratonomic pattern : the shells are accumulated convex-up stacks
517
directly upon _each other . This so-called "Ineinanderschachtelung" is generally attributed to wave action ( HAENTZSCHEL, 1 9 3 6 ; SEILACHER, 1 9 7 3 ; FUTTERER, 1 9 78 ) . The top of such a s�ell layer shows grading of Pos idonia shell debri s . I n the same rock .f:�pe a:bout 5 nun thick layers o f matrix-free edge wise coquina (consisting of small Posidonia shells) have been ob served . These coquina layers form single laminae . The sets of laminae free of bioclasts are deformed by the imprint of the overlying s·hell layers . Minor, very thin laminations actually are alternations of quartz-rich and quartz �poor laminae ; the quartz fills cavities and obViously · is a product of dissolution of opaline silica . This tYpe of lamination is interpreted to be the result of periodical high energy events which interrupted the normal black shale sedimentation. Each shell layer probably represents a storm event . The wave aCtion washed out the matrix and produced the characteristic biostratonomic pattern observed in the coquina . The wavy laminites build up the uppermost few meters of the Siniais section {Korfu) , - of the Mavron Oros section ( near Igoumenitsa) and of the Chionistra sections . Hence this facies can be correlated over cons iderable distances in the Ionian trough . Ip fact, the thin coquina layers can hardly be classified as "back ground sedimentation " . But their frequency and periodicity contrasts with the 11rare events" described in part 2 . 4 . Graded Lamination : In the middle part of the Toarcian black shale of Korfu an intercalation of a 6 - 1 0 em thick graded laminite bed was found . Its bedding features are (Fig . 4 ) : ( 1 ) erosive bas e , ( 2 ) al ternation ( lamination) _of Pos idonia layers and thin silt layers , ( 3 ) each lamina is graded ( grading of Posidonia and silt respective ly) , ( 4 ) decrease of the maximum grain size from lower to upper la minae (graded lamination) , ( 5 ) bioturbation in the uppermost part of the bed. (The maximum grain-size of silt layers has been determined in thin section in accordance with SCHEIDEGGER & POTTER, 1 9 6 5 ) . As indicated by the granulometric differences between the laminae and by the repeated grading ( " graded � rhytlunite " ) , each lamina is suggested to be a single depositiorial unit which represents a short depositional event. The impulse of the short events wanes upwards and thus the maximum grain size decreases from lower tu upper lami nae. This type of lamination can be explained as the result of wa ning impulses during one single storm event . This interpretation is
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WAVY LAMINITE THIN SECTION NO. 29623
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Fig . 3 . Wavy laminite : alternation of Posidonia layers and laminae free of shell s . Note: the shells are accumulated convex-up stacks directly upon each other. Redrawn from thin section, uppermost Toarcian black shale , section north of Siniais , N:-.Korfu
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Fig . 4 . Graded rhythmite : alternation of Posidonia layers and silt layer s . Left side : each lamina either of silt or of Posidonia-bioclasts is graded. Right side: the maximum grain-size and the length of bio clasts decrease from lower to upper laminae. Note : erosive base of the clastic layer and burrows in the top parti silt laminae in the lower part smoothen the bottom relief . (Total number of measured grains is 220 and total number of measured bioclasts is 2 6 8 ) . · J.1iddle Toarcian black shale , Viglatzuri section, N-Korfu
Fig . 6 . Debris flow breccia as clastic intercalation within the Toarcian black shale near Igoumenitsa , directly at the boundary between Toarcian black shale and Mid-Jurassic n limestone with filaments " . Note : erosive base and 11freezing" of components in a muddy matrix without any gra ding; rare grain-to-grain contacts of this type of breccia consist of white or reddish angular clasts of marly l imestone and/or of limestone with filaments . Mavroudhi section
520
in accordance with REINECK & SINGH ( 1 9 7 2 } who described similar gra ded rhythmites in storm layers ; however there are no shell layers in cluded in their description. Graded lamination is it is problematic to nae in the course of are hydrodynamically
also known from turb idites (PIPER, 1 9 7 2 ) . But explain the alternation of shell and s ilt lami a turbidity current , unless the Posidonia shells equivalent to , . the silt-sized grains .
Stratified Radiolarian-rich Sequences : In the middle and/or upper part of the Toarcian b lack shale sections of the Ionian trough ra diolarians are the major fa�nal component . Alternating layers with high or low content of radiolarians generate lamination. The radio larian-rich layers are stronger siliCif,ied by diagenesis than the other layers . The enrichment of radiolarians in single laminae can be caused by the action of bottom currents or turbidity currents. (WEISSERT, 1 9 7 9 , p . 1 1 2 ) . This interpretation i s i n agreement with other sedimentary structures which requ.i re the activity of bottom currents in the ra diolarian laminite sequence : grading of radio'larians, tractional transport of bioclasts forming small " fossiliferous peloids 11 • Bedding in Connection with B ioturbation : Normally the Toarcian black shales of the Ionian zone have a good fissility , but there are in tercalations of homogeneous beds of centimeter or decimeter thick nes s . Some of these homogeneous beds show burrows , e . g . Chondrites . These burrows often are small-scale ( 1 mm in diameter ) at the top of the beds but thicker in the lower part. They are interpreted as 11one-phase benthic horizons '' ( BRENNER & SEILACHER, 1 9 7 8 ) . Although a change of lithology is not evident, the bioturbated sediment forms well defined beds . Limestone Beds and Laminae : Alternations of shale and marly limesto ne beds show a rapid non-gradual change· of carbonate content from 40-75% (shale) to 7_5-85% (marly limestone) , combined with a change in rock-colour . In addition, fissili ty is reduced o.r missing in most of the limestone beds . Bedding surface's are well def..ined and even; beds have lateral continuity in thickness ( 1 em to more than 1 meter) and facies . A thinning-up or thickening-up or a � regular alternation of limestone and shale beds has not been observed . In at least some of the limestone beds coccoliths and calcispheres and thei � debris build up a considerable part of the rock. The amount of s ilica varies ; chert nodules are frequent.
521
According to the terminology proposed by WETZEL {this Vol . ) , even lamination, stratification of radiolarians and bedding in connection wi.th short-termed b ioturbation ( 110ne-phase 11 ) are the response of "micro-scale" variations in the depositional conditions. The inter calation of limestone beds, if they are of autochthoneous character , should be attributed to "meso-scale'' variations. 2 . 4 . Turbidites , Breccias , and Olis tostromes � Clastic layers within th� '-Toarcian black shales of the Ionian zone have a thickness ranging from less than 1 em to more than 1 5 meters . Th6 character of the clastic intercalations varies in the individual black shale basins . Turbidites : Intercalations of graded clastic layers occur in almost every Ionian b lack shale section and in Arnmonitico Rosso sections as well (BERNOULLI , 1 9 67 , 1 96 9 ) . The clastic graded layers within the black shale sequences show the divisions A, B and E in terms of the BOUMA-sequence_; division c has not been obserVe d . Erosional structu res at the base of these turbidites are not distinct . usually the redeposited material consists of sand/silt-sized limestone grains , Posidonia shells and minor shale fragments ( BERNOULLI , 1 9 6 7 , 1 96 9 ) . T9- e graded rhythmite described in part 2 . 3 . 3 should be listed among the graded clastic layers as wel l . However i t is . interpreted to be a storm deposit rather than a turbidite . Debris Flow sediments ( Fig. 6 ) : Debris f low sediments are exposed in the Igoumenitsa area. The components of this type of breccia reach several em in diameter and consist of white or reddish lime stone or marly limestone . Both, angular clasts and synsedimentary deformed pebbles and lumps have been observed . The components of the breccia res emble the nodular marly limestone of Ammonitico Rosso de posits which probably cover a.d j acent swell areas . However no rede posited fossils have been found . The fabric of the breccia corresponds to the description of debris flow sediments by MIDDLETON & HAMPTON { 1 97 3 ) , including groove casts at the base, the 11freezing" of com ponents in a muddy matrix, and the poor sorting. Nodular Breccias : Nodular breccias are exposed at Viglatzuri (NE Korfu) and reach a thickness of 6 meters . Synsedimentary deformed pebbles and boulders of more than 1 meter in diameter are lying in a clayey-silty matrix . In ·contrast to the debris flow sediments (part 2 . 4 . 2 ) the density of packing in the nodular breccias is re-
522
latively �igh and the clasts a�e in contact with each other . Stra tif ication within the breccia is not c lear . The pebbles and boul ders �onsist of limestone and marly limestone some of them contain small Posidonia. The redeposited sediment is of the Anunonitico Rosso type ; together with the clasts many fractured - ammonites have been found (a faunal list was published by RENZ , 1 9 5 5 ) . The nodular breccias are laterally not continuous : in outcrops only a few hundred meters apart they cannot be correlated. The base of the breccia is erosive. Olistostromes : SE of Igoumenitsa the Toarcian black shale wedges out towards a swell area within a few hundreds of meters . .The tran�ition between the basin and the swell is characterised by a penecontempora neous fault. Near the fault redeposited sediments reach a thickness of more · than 1 5 meters and contain large sheets of contorted shale beds and boulders of eroded older sediments , i . e . Lower Jurassic Pantokrator limestone . Towards the more distal areas the thickness of this "olistostrome" decreases rapidly to less than 1 meter ; within a few hundredS of meters the olistostrome laterally gr�des into a debris f low ·sediment. 2 . 5 . Interaction Between Clastic Layers and Black Shale In Fig. 4 the uppermost part of the event deposit shows pellet filled burrows . Obvi·o usl_y anoxic conditions indicated by the lamination and fissility of the black shale were interrupted by the sedimentary . event. Oxic conditions were only short termed as the subsequent sha le sedimentation continued w.ith anoxic conditions . In the Igoumenitsa sections several thin debris f low breccias have been fourid ( 1 0-20 em thick) Which have an irregular top and are covered by an about 1 meter thick homogeneous limestone bed. Also sedimentary breccias are preferably found at the top of the Toarcian black shale sequence, underlying the Mid-Jurassic limestone sequence ( 11 limestone with fi laments11 ; see Fig. 6 ) . These breccias a_pparently ar·e linked to lithological changes . This observatiOn can be explained as follows : some of the facies changes in the TDarcian black shale basins were set off by tectonic events which changed the basin morphology and depositional condition s , e . g . the opening of small stagnant basins to better water circulation. The tectonic events are recorded by the sedimentary breccias .
523
3 . Remarks on Paleobathymetry and Paleoenvironment Former authors present contradictory interpretations on the paleo bathymetry. of the Ionian trough during the Toarcian. RENZ ( 1 9 55 ) and AUBOUIN ( 1 9 5 9 ) have assigned the Toarc �an sequence to a deep marine, pelagic environment. Colleagues from the INSTITUT DE GEOLOGIE ET RE CHERCHES DU SOUS-SOL, ATHENES , ET INSTITUT FRANCAIS DU PETROLE , MISSION GRECE ( 1 9 6 6 ) clearly distinguished basinal areas with a com plete succession and areas with remarkable stratigraphic gaps (Fig . 1 ) . Tf. e stratigraphic gaps \.�fe thought to represent regions of local emergence . After BERNOULLI & RENZ ( 1 9 70 ) however the Toarcian Posi doni� beds have been deposited in at least some hundreds of meters of water depth . The same authors attribute the stratigraphic gaps to submarine highs which did not reach the photic zone (proposed water depth for the submarine highs is 200 - 300 meters ; BERNOULLI & RENZ , 1 9 70 ) . The study of bedding provides additional data for the interpretation of the paleobathymetry of the Toarcian black shale in NW-Greece : - In the upper part of the Korfu, Igoumenitsa and Chionistra sections the biostratonomic pattern of coquina indicates periodical wave action (part 2 . 3 . 2) . Therefore it is assumed that the Toarcian b lack shales of pw-Greece have been dep?sit�d at least temporarily above the storm wave base. In the Lower Toarcian the Ionian trough was subdivided into small shallow-marine basins , due to penecontemporaneous blockfaulting. Some of these baSins were isolated from open ocean water circulation and became areas of black shale deposition. However the mqdel of . stagnant basins is not valid for the upper part of the Toarcian black shale sequence . Here, the sedimentary structures indicate wave and current action (see part ·2 . 3 . 2 and 2 . 3 . 4 ) . In addition the black shale basins were open to upwelli �g ocean waters of high fertility . Two factors might be responsible for the lacking of b ioturbation in the Upper Toarcian black shales : ( 1 ) As known from upwelling regions , the oxygen-content in the sea water near the sea floor as well as in the unconsolidated sediment was reduced by the oxidation of settling abundant organic matter , ( 2 ) a sediment of alternating shell and mud laminae (see wavy lamination part 2 . 3 ) probably was a unfavourable substrate for burrowing organisms . A more detailed reconstruction of the paleoenvironment is expected when additional data on organic geochemistry are available.
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Acknowledgements I thank Prof . J . WENDT for discuss ions and for the introduction to the fie'ld study. Prof . G . EINSELE gave helpful recommendations and kindly reviewed this report. I thank all ·members of the " Sonderfor schungsberei.ch 5 3 11 of the University of Tlibingen for their helpful support and discussions . The Institute o f Geological . and Mineral Exploration Athens ( IGME Athens) kindly provided the work permit for the field trips . I would like to thank Dr . BORNOVAS , Dr. GAITANAKIS, Dr. ANGELOPOULOS , and Dr . TSAILA-MONOPOLIS for their interest and support . I �hank the field party of IGME in Ioannina for . the guidance in the field . This study was supported by the "Sonderforschungsbereich 5 3 " of the University of Tlib ingen. References AUBOUIN, J . ( 1 9 58 ) : Essai sur l 1 evolution paleogeographique et le d&ve loppement tecto-orogenique d 1 un systBrne geosynclinal: le secteur grec des Dinarides (Hellen-id_e s) . - Bul l . So<;: . geo l . France ( 6 ) ;! : 7 3 1 -7 4 9 . ( 1 9 59 ) : Contribution a l ' etude geologique de l a GrBce s eptentrio nale : les confins de l 1 Epire et de la Thessali e . - Ann. g&ol . Pays hellen. ( 1 ) J,Q : 1 -403 . BERNOULLI , D . ( 1 9 6 7 } : Probleme der Sedimentation im Jura Westgrie chenlands und des zentralen Apennin . - Verhand l . Natur f . Ges . Ba sel, J�LJ, : 3 5 -5 4 . ( 1 969 ) : Redeposited pelagic sediments in the Jurassic of the central Mediterranean area . - Ann . Ins t . Geol . Pub l . Hung . , LIV/2 : 73-90. --- , RENZ , 0 . ( 1 9 70 ) : Jurassic carbonate facies and new ammonite fau nas from Western Greece . - Eclogae geo l . Helv . , gJL§ : 57 3-607. BRENNER, K . , SEILACHER, A. ( 1 9 7 8 ) : New aspects about the origin of the Toarcian Posidonia shales . - N. Jb . Geol . Palaeont. Abh . , l�z, No . 1 / 2 : 1 1 - 1 8 . CAMPBELL, C . B . ( 1 96 7 ) : Lamina , laminaset , bed and bedset . - Sedimen tology , � : 7-2 6 . DIESTER-HAAS S , L . ( 1 9 7 8 } : Sediments as indicators of upwelling . - In: BOJE, R . , TOMCZAK, M . ( ed . ) Upwelling ecosystems , Springer-Verlag , Berlin Heidelberg New York: 2 6 1 - 28 1 . � - -- , PFLAULMANN , U . , RUEHL , N . , SCHRADER , H . -J . , THIEDE , J . ( 1 9 7 3 ) : AuftriebseinfluB in Sedirnenten . - Geowis s . Tag . 1 9 7 3 Frankfurt/M . : 3-4 . EINSELE, G . , WIEDMANN, J . ( 1 9 7 5 ) : Faunal and sedimentological evi dence for upwe lling in the Upper Cretaceous coastal basin of Tarfaya, Morocco . - IX th International Congress of Sedimentolo gy , Nice , Theme 1 : 6 7 - 7 2 .
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FUTTERER, E . ( 1 97 8 ) : Studien Ube·r die Einregelung , Anlagerung und Einbettung b iogener Hartteile im StrOrnungskanal . - N . Jb . Geol. Palaeont. Abb . , l�gbl' 87-1 3 1 . HAENTZSCHEL, W. ( 1 9 3 6 ) : Die Schichtungs-Formen rezenter Flachmeer Ablagerungen im Jade Gebiet . - Senckenbergiana leth . , l�: 3 1 6-35 6 . INSTITUT DE GEOLOGIE ET RECHERCHES DU SOUS-SOL ATHENES ET INSTITUT FRANCAIS DE PETROLE , MISSION GRECE ( 1 9 6 6 ) : Etude geologique de l ' Epire (Grece nord-occidentale) . - �III + 306 p . , 9 plates, 1 0 1 ---figs . ,. Paris (Technip) . MIDDLETON , G.V. , HAMPTON , M . A . ( 1 9 76 ) : Subaqueous sediment transport and deposition by sediment gravity f lows . - In: STANLEY, D . J . , SWIFT , D . J . P . (eds . ) Marine sediment transport and environmental management . John _ Wiley & . Sons, New York London Sydney Toronto: · c_ 1 9 7 -2 1 8 . . PIPER, D J . w . ( 1 9 7 2 ) : Turbidite origin of some · laminated mudstone:s . Geol . Mag . l£12 ( 2 ) : 1 1 5- 1 2 6 . REINECK, H . E . , SINGH, J . B . ( 1 9 7 2 ) : Genesis of laminated sand and gra ded rhythmites in storm-sand layers of shelf mud . - Sedimentology, l� ' 1 23 - 1 2 8 . RENZ , c . ( 1 95 5 ) : Die vorneogene S tratigraphie der normal-sedimenta ren Formationen Griechenlands . - Institute for Geology and Subsur face Research Athens, 6 3 6 p . , 4 text-figs . 1 1 1 figs . , 6 maps . SCHEIDEGGER, AoE. 1 POTTER1 P . E . ( 1 9 6 5 ) : Textural studies of graded bedding . Observation and Theory . - Sedimentology, � : 289-30 1 . SEILACHER, A . ( 1 9 7 3 ) : B iostratinomy : the sedimentology of bi9logical ly $tandarized particles . - In: GINSBURG , R . N . (eds . ) Evolving concepts in sedimentology . Johns Hopkins University Studies in Geology No. 2 1 . The Johns Hopkins Univ. Press Baltimore & Lon don: 1 59 - 1 7 7 . WEISSERT, H . J . ( 1 9 79 ) : Die Palaeoozeanographie der slidwestlichen Tethys in der Unterkreide . - Dissertatiori ETH ZUrich, 1 7 4 p . • .
Stratinomy of the Lower Kimmeridge Clay (Dorset, England) (Abstract) · T. AIGNER
Abstrac t : The Lower Kimmeridge Clay combines two alternating ecologi cal situation s : ( 1 ) homOgeneous mudstones and ( 2 ) finelY laminated bituminous sha1e s . The fossils in th� two lithologies are essentially the same , but differ in mode of occurrence . 1 . In the mudstone s , the scattered occurrence of mainly convexdown s ingle-valved shells iS compared to modern continental shelf muds and referred to the activity .o f carnivores turning the shells and to burrowers reworking all primary structures . The biValve fauna, being abundant but of low diversity, is dominated by infaunal luci nids , 'characteristic of low-oxygen environmE?nt s , but cOntains no nucul ids . 2 . In the bituminous shale s , sedimentary structures suggest fluctu ation between "hemipelagic" deposition and current and/or gravity flow events . Fissility , preservation of fine laminae and absence of an autochthonous infauna suggest that reducing conditions reached above the sediment-Water interface during shale deposition . Laminae of graded carbonate-rich material qs well as shell pavements call for hydrodynamic events . The allochthonous bivalves appear to be largely derived from the mudstOne fac·ies , which· would agree with the inferred lateral transition of bituminous shaleS into mudstones . Sub sequent to oXygenating current events , opportunistic bivalves (oysters) were able to colonise the shell pavements for short period s . . The · occurrence of vertically embedded ammonites supports previous palaeobathyreetric estimates for relatively shallow water durin� Kimmeridge Clay sedimentation . ·
AIGNER, T . ( 1 980) : Biofabrios and stratinomy of the Lower Kimmeridge Clay ( U . Jurassic, Dorset, England) - N . Jb . Geol . Pal .Abb . , 1 59 : 3 2 4 -3 3 8 .
cyclic and Event Stratification (ed . by Ein·sele Seilacher) © Springer 1982
The Formation of the Bituminous Layers of the Middle Triassic ofTicino (Switzerland) (Abstract) H. RIEBER
Abstract: The black shales of the Middle Triassic of the L imestone Alps of Ticino are famous for their perfectly preserved marine verte brates , reptiles. and fishes . They are also welh·known for their high content of organic matter (up to about 50% ) . The so-called Grenz bitumenzone is the most famous bituminous member within the Middle TEiassic of the Ticino. It is s ituated at the boundary Anisian/ Ladinian and well exposed near Serpiano at the Monte San Giorgio and at Besano ( Italy ) . It consists of finely laminated , highly bituminous claystones , more or less laminated bituminous dolomites and some thin layers of argillaceous tuffib�� · Completely preserved skeletons and isolated bones of vertebrates were found in the bituminous shales as well as in the dolomitic layers . Recently apparatuses of Conodonts have .been discovered in the bi tuminous shales. Other invertebrate remains , however , can be recogni zed only within the dolomites . Among these , remains of auchthoch tonous benthic organisms are missing. According to the succeSsion of speci�s of the pelecypod Daon e l Z a and of the Ammonoidea it can be concluded that sedimentation rate was very low during the formation of the bituminous member s . The lack of benthos , the high content of organic matter and the completely preserved skeletons indicate that there was no oxygen at the bottom during sedimentation of the Grenz b itumenzone. It is supposed that the sediments of the Grenzbitumen ,zone were deposited in a reef-bordered basin 8 - 1 0 km in diameter , with stagnant conditions prevailing in the lower part of the strati fied' water column .
cyclic and Event Stratification (ed . by Einsele/Seilacher} © Springer 1982
