Stromatolites

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DEVELOPMENTS IN SEDIMENTOLOGY 20

STROMATOLITES

FURTHER TITLES IN THIS SERIES

1. L. M. J. U. V A N S T R A A T E N , Editor DELTAIC AND SHALLOW MARINE DEPOSITS 2. G.C. AMSTUTZ, Editor SEDIMENTOLOGY AND ORE GENESIS

3. A.H. BOUMA and A . BROUWER, Editors TURBIDITES 4. F.G. TICKELL THE TECHNIQUES OF SEDIMENTARY MINERALOGY 5. J. C. INGLE Jr. THE MOVEMENT OF BEACH SAND .6. L. V A N D E R PLAS THE IDENTIFICATION OF DETRITAL FELDSPARS

I. S. DZULYN S K I and E.K. WA L T O N SEDIMENTARY FEATURES OF FLYSCH AND GREYWACKES 8. G. L A R S E N and G. V. CHILINGAR, Editors DIAGENESIS I N SEDIMENTS 9. G. V. CHILINGAR, H.J. BISSELL and R. W. FAIRBRIDGE, Editors CARBONATE ROCKS 10. P. McL. D. DUFF, A. H A L L A M and E.K. WALTO N CYCLIC SEDIMENTATION 11. C.C.R E E V E S Jr. INTRODUCTION TO PALEOLIMNOLOGY 12. R.G.C. B A TH U R S T CARBONATE SEDIMENTS AND THEIR DIAGENESIS 13 A.A. M A N TEN SILURIAN REEFS OF GOTLAND 14. K.W. GLENNIE DESERT SEDIMENTARY ENVIRONMENTS 15. C.E. W E A V E R and L.D. P O L L A R D THE CHEMISTRY OF CLAY MINERALS 16. H.H. RIEKE III and G.V. CHILINGARIAN COMPACTION OF ARGILLACEOUS SEDIMENTS 11.M.D. PICARD and L.R. HIGH Jr. SEDIMENTARY STRUCTURES OF EPHEMERAL STREAMS 18. G.V. CHILINGARIAN and K.H. WOLF COMPACTION OF COARSE-GRAINED SEDIMENTS 19. W. SCHWARZACHER SEDIMENTATION MODELS AND QUANTITATIVE STRATIGRAPHY

DEVELOPMENTS IN SEDIMENTOLOGY 20

STROMATOLITES EDITED BY

M.R. WALTER Bureau of Mineral Resources, Geology and Geophysics, Canberra, A.C.T. (Australia)

ELSEVIER SCIENTIFIC PUBLISHING COMPANY 1976 Amsterdam -.I Oxford -- New York

ELSEVIER SCIENTIFIC PUBLISHING COMPANY 335 Jan van Galenstraat P.O.Box 211, Amsterdam, The Netherlands

Distributors for the United States and Canada: ELSEVIER/NORTH-HOLLAND INC. 52 Vanderbilt Avenue New York, N.Y. 10017

Library of Congress Cataloging in Publication Data

Main e n t r y under t i t l e : Strornatolites

.

(Developments i n sedirnentology ; 20) Bibliogrqhy: p. Includes index. 1. Stromatolites. I. Walter, M. R.

QE955.S82 561l.93 ISBN 0-444-41376-6

11. S e r i e s .

76-17546

Copyright 0 1976 by Elsevier Scientific Publishing Company, Amsterdam All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of the publisher, Elsevier Scientific Publishing Company, Jan van Galenstraat 335, Amsterdam Printed in The Netherlands

LIST OF CONTRIBUTORS

S.M. AWRAMIK Department of Geological Sciences University of California Santa Barbara, Calif. 93106 (U.S.A.)

J.A. DONALDSON Department of Geology Carleton University Ottawa 1, Ont. (Canada)

E.S. BARGHOORN Biological Laboratories Harvard University Cambridge, Mass. 02138 (U.S.A.)

E.C. DRUCE Bureau of Mineral Resources, Geology and Geophysics Canberra City, A.C.T. 2601 (Australia)

J. BAULD Department of Microbiology University of Melbourne Parkville, Vic. 3052 (Australia)

J.R. EGGLESTON Geologic and Economic Survey Morgantown, W. Va. 26505 (U.S.A.)

J. BERTRAND-SARFATI

Facultd des Sciences C.N.R.S., Centre Gkophysique et Gkologique de Montpellier 34060 Montpellier (France) T.D. BROCK Department of Bacteriology University of Wisconsin Madison, Wis. 53706 (U.S.A.) A. BUTTON Economic Geology Research Unit University of the Witwatersrand Johannesburg (South Africa) A.E. COCKBAIN Geological Survey of Western Australia Perth, W.A. 6000 (Australia) W.E. DEAN Syracuse University Syracuse, N.Y. 13120 (U.S.A.)

K.A. ERIKSSON Department of Geology University of the Witwatersrand Johannesburg (South Africa) C.D. GEBELEIN Department of Geological Sciences University of California Santa Barbara, Calif. 93106 (U.S.A.)

S.GOLUBIC Department of Biology Boston University Boston, Mass. 02215 (U.S.A.)

R.B. HALLEY U S . Geological Survey University of Miami Fisher Island Station, School ofMarine Science Miami, Fla. 33149 (U.S.A.)

VI

LIST O F CONTRIBUTORS

L.A.HARDIE Department of Earth and Planetary Sciences The Johns Hopkins University Baltimore, Md. 21218 (U.S.A.)

G. PANNELLA Department of Geology University of Puerto Rico Mayaguez 00708 (Puerto Rico)

P.G. HASLETT Department of Geology and Mineralogy University of Adelaide Adelaide, S.A. 5000 (Australia)

R.K. PARK Phillips Petroleum Company Denver, Colo. 80202 (U.S.A.)

P.F. HOFFMAN Geological Survey of Canada Ottawa, Ont. K1A OE8 (Canada) H.J. HOFMANN Department of Geology. University of Montreal Montreal, Que. (Canada) R.J. HORODYSKI Department of Geology University of California Los Angeles, Calif. 90024 (U.S.A.) D.J.J. KINSMAN DepartHent of Geological and Geophysical Sciences Princeton University Princeton, N.J. 08540 (U.S.A.) I.N. KRYLOV Geoiogical Institute Academy of Sciences of the U.S.S.R. Moscow Zh. 1 7 (U.S.S.R.) L. MARGULIS Department of Biology Boston University Boston, Mass. 02215 (U.S.A.) D.M. McKIRDY Bureau of Mineral Resources, Geology and Geophysics Canberra City, A.C.T. 2601 (Australia)

F. MENDELSOHN Maritime House Johannesburg 2001 (South Africa) C.L.V. MONTY Laboratoire de Paldontologie animale Universitd de Lidge 4000 Lidge (Belgium)

P.E. PLAYFORD Geological Survey of Western Australia Perth, W.A. 6000 (Australia) W.V. PREISS Department of Mines Adelaide, S.A. 5001 (Australia) M.E. RAABEN Geological Institute Academy of Sciences of the U.S.S.R. Moscow Zh.17 (U.S.S.R.) J.F. READ Department of Geological Sciences Virginia Polytechnic Institute and State University Blacksburg, Va. (U.S.A.) M.A. SEMIKHATOV Geological Institute Academy of Sciences of the U.S.S.R. Moscow Zh.17 (U.S.S.R.) S.N. SEREBRYAKOV Geological Institute Academy of Sciences of the U.S.S.R. Moscow Zh.17 (U.S.S.R.) R.C. SURDAM Department of Geology University of Wyoming Laramie, Wyo. 82071 (U.S.A.) J. THRAILKILL Department of Geology University of Kentucky Lexington, Ky. 40506 (U.S.A.)

R. TROMPETTE htudes gdologique Ouest-africaines C.N.R.S. Centre Universitaire de St-Jdrbme 13013 Marseille (France)

VII

LIST O F CONTRIBUTORS P.A. TRUDINGER Baas-Becking Geobiological Laboratory Canberra, A.C.T. 2601 (Australia) J.F. TRUSWELL Bureau of Mineral Rssources, Geology and Geophysics Canberra City, A.C.T. 2601 (Australia) C.C. VON DER BORCH Flinders University Bedford Park, S.A. 5042 (Australia)

M.R. WALTER Bureau of Mineral Resources, Geology and Geophysics Canberra City, A.C.T. 2601 (Australia) J.L. WRAY Marathon Oil Company Littleton, Colo. 80122 (U.S.A.)

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CONTENTS

.. . . . ... . . .. . . . .. . . .... . . .. . ... ... . ... .... 1. INTRODUCTION (M.R. Walter). . . . . . . . . . , . . . . . . . . . . . . . . . . . . . . . .

List of Contributors.

V 1

2.METHODOLOGY AND SYSTEMATICS Chapter 2.1.Basic field and laboratory methods for the study of stromatolites (W.V.Preiss). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chapter 2.2.Graphic representation of fossil stromatoids; new method with improved precision (H.J. Hofmann) . . . . . . . . . . . . . . . . . . . . . , . . .

...

Chapter 2.3.Biological techniques for the study of microbial mats and living stromatolites (T.D. Brock) . . . . . . . . . . . . . . . . . , . . . . . . . . . . . . . . .

5 15

.

21

Chapter 2.4.Approaches to the classification of stromatolites (I.N. Krylov)

31

Chapter 2.5.Stromatoid morphometrics (H.J. Hofmann)

.... ....., ..........

45

Chapter 3.1.Calcretes and their distinction from stromatolites (J.F. Read).

55

Chapter 3.2.Speleothems (J. Thrailkill).

.... . . . . ... . ....... . ... . . ... . . .

73

3.ABIOGENIC STROMATOLITE-LIKE STRUCTURES

Chapter 3.3.Geyserites of Yellowstone National Park: an example of abiogenic “stromatolites” (M.R. Walter) . . . . . . . . . . . . . . . . . .

.......

87

.. ...........

113

Chapter 4.2.Taxonomy of extant stromatolite-building cyanophytes (S.Golubic). . . , . . . . . . . . . . . . . . . . . . . :. . . . . . . . . . . . . . . . . . . ,

127

..

141

4.BIOLOGY OF STROMATOLITES Chapter 4.1.Organisms that build stromatolites (S.Golubic).

Chapter 4.3. Environmental microbiology of living stromatolites (T.D. Brock)

Chapter 4.4.Evolutionary processes in the formation of stromatolites (S.M. Awramik, L. Margulis and E.S. Barghoofn). . . . . . . . . . . . . . . . . . . . 149 Chapter 4.5.Biochemical markers in stromatolites (D.M. McKirdy).

. . . . .. .. .

163

X

CONTENTS

5. FABRIC AND MICROSTRUCTURE

Chapter 5.1. The origin and development of cryptalgal fabrics (C.L.V. Monty)

.

193

Chapter 5.2.An attempt to classify Late Precambrian stromatolite microstructures (J. Bertrand-Sarfati) . . . . . . . . . . . . . . . . . . . . . . . . . . .

251

6.MORPHOGENESIS Chapter 6.1.Stromatolite morphogenesis in Shark Bay, Western Australia (P. Hoffman) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

261

Chapter 6.2.Microbiology and morphogenesis of columnar stromatolites (Conophyton, Vacerrillo) from hot springs in Yellowstone National Park (M.R. Walter, J. Bauld and T.D. Brock) . . . . . . . . . . . . . . . . . . . . . . . . . .

273

Chapter 6.3.Gunflint stromatolites : microfossil distribution in relation to Awramik) . . . . . . . . . . . . . . . . . . . . . . . . stromatolite morphology (S.M.

311

Chapter 6.4.Biotic and abiotic factors controlling the morphology of Riphean stromatolites (S.N. Serebryakov) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

321

7.STROMATOLITE BIOSTRATIGRAPHY Chapter 7.1.Experience in stromatolite studies in the U.S.S.R. (M.A. Semikhatov) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chapter 7.2.Intercontinental correlations (W.V. Preiss)

337

.................

Chapter 7.3.Aphebian stromatolites in Canada: implications for stromatolite zonation (J.A. Donaldson) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

359 371

8. RECENT MODELS FOR INTERPRETING STROMATOLITE ENVIRONMENTS Chapter 8.1.Open marine subtidal and intertidal stromatolites (Florida, the Bahamas and Bermuda) (C.D. Gebelein). . . . . . . . . . . . . . . . . . . . . . . . . . .

381

Chapter 8.2.Modern algal stromatolites at Hamelin Pool, a hypersaline barred basin in Shark Bay, Western Australia (P.E. Playford and A.E. Cockbain). . . . 389 Chapter 8.3.Stratigraphy of stromatolite occurrences in carbonate lakes of the Coorong Lagoon area, South Australia (C.C. von der Borch) . . . . . . . . . . . . 413 Chapter 8.4.Algal belt and coastal sabkha evolution, Trucial Coast, Persian Gulf (D.J.J. Kinsman and R.K. Park). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

421

Chapter 8.5. Textural variation within Great Salt Lake algal mounds (R.B. Halley) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

435

Chapter 8.6.The geological significance of the freshwater blue-green algal calcareous marsh (C.L.V. Monty and L.A. Hardie) . . . . . . . . . . . . . . . . . . .

447

Chapter 8.7.Freshwater stromatolitic bioherms in Green Lake, New York (J.R. Eggleston and W.E. Dean) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

479

Chapter 8.8. Hot-spring sediments in Yellowstone National Park (M.R. Walter).

.

489

9.APPLICATION OF RECENT MODELS TO THE GEOLOGICAL RECORD Chapter 9.1.The effects of the physical, chemical and biological evolution of the earth (C.D. Gebelein) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

499

XI

CONTENTS 10. STROMATOLITES IN BASIN ANALYSIS

Chapter 10.1.Use of stromatolites for intrabasinal correlation : example from the Late Proterozoic of the northwestern margin of the Taoudenni Basin (J. Bertrand-Sarfati and R. Trompette) . . . . . . . . . . . . . . . . . . . . . . . . . .

517

Chapter 10.2.Paleoecology of Conophyton and associated stromatolites in the Precambrian Dismal Lakes and Rae Groups, Canada (J.A. Donaldson) . . . . .

523

.

Chapter 10.3.Lacustrine stromatolites, Eocene Green River Formation, Wyoming (R.C. Surdam and J.L. Wray). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 535 Chapter 10.4.Devonian stromatolites from the Canning Basin, Western Australia (P.E. Playford, A.E. Cockbain, E.C. Druce and J.L. Wray). . . . . . . . . . . . . . 543 Chapter 10.5.Lower Cambrian stromatolites from open and sheltered intertidal environments, Wirrealpa, South Australia (P.G. Haslett) . . . . . . . . . . . . . . . 565 Chapter 10.6.Stromatolites from the Middle Proterozoic Altyn Limestone, Belt Supergroup, Glacier National Park, Montana (R.J. Horodyski). . . . . . . . 585 Chapter 10.7.Environmental diversity of Middle Precambrian stromatolites (P. Hoffman) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

599

Chapter 10.8.Distribution of stromatolites in Riphean deposits of the Uchur-Maya region of Siberia (S.N. Serebryakov) . . . . . . . . . . . . . . . . . . .

613

Chapter 10.9.Palaeoenvironmental and geochemical models from an Early Proterozoic carbonate succession in South Africa (K.A. Eriksson, J.F. Truswell andA.Button) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 635

11. MINERALIZATION ASSOCIATED WITH STROMATOLITES Chapter 11.1. Mineral deposits associated with stromatolites (F. Mendelsohn).

..

Chapter 11.2.Biological processes and mineral deposition (P.A. Trudinger and F. Mendelsohn). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

645 663

12.CONTRIBUTIONS TO THE HISTORY OF THE EARTH-MOON SYSTEM Chapter 12.1.Geophysical inferences from stromatolite lamination (G. Pannella) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

673

APPENDICES Appendix I. Glossary of selected terms (M.R. Walter).

..................

Appendix 11. Table of time-ranges of the principal groups of Precambrian stromatolites (I.N. Krylov and M.A. Semikhatov) . . . . . . . . . . . . . .

.....

Appendix 111. List of available translations of major works on stromatolites (M.R. Walter). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

687 693 695

Appendix IV. Selective subject index to the Bibliography (S.M. Awramik) . . . . . 697 BIBLIOGRAPHY INDEX

............................................

...................................................

705 773


INTRODUCTION M.R.Walter”

It is 68 years since E. Kalkowsky coined and defined the word “stromatolith”, yet there is increasing controversy and confusion as to its use. Usage is not uniform in this book, nor have I attempted to make it so. The original definition is now of historical interest only. The only unifying feature of stromatolites is their genesis. The definition I find most useful is one modified from that presented in 1974 by S.M.Awramik and L. Margulis in the second Stromatolite Newsletter (unpublished) : Stromatolites are organosedimentary structures produced by sediment trapping, binding and/or precipitation as a result of the growth and metabolic activity of micro-organisms, principally cyanophytes.

Non-genetic definitions are so broad as to include a wide range of nonbiogenic structures, so that the term would cease t o be useful. The definition used here embraces the whole range of forms, including stratiform and unlaminated structures that some workers would exclude. It has generated the structure of this book. For some time the belief that stromatolites are mainly a marine intertidal phenomenon dominated the literature. The distribution of stromatolites is not restricted in this way now, nor was it during the past, particularly during the Proterozoic. Stromatolites can form in almost any free-standing body of water, providing the following conditions are met: (1)the environment is suitable for the growth of appropriate microorganisms; (2) the rate of growth of the constructing micro-organisms exceeds their rate of consumption by other organisms; (3) the rate of sedimentation is sufficient to produce a preservable structure but is not so great as t o prevent colonization by micro-organisms;

* Contributions by M.R. Walter in this book, except sections 3.3, 6.2 and 8.8, are published with the permission of the Acting Director, Bureau of Mineral Resources, Geology and Geophysics, Canberra City, Australia.

2

M.R. WALTER

(4)the stromatolites accrete faster than they can be destroyed by boring and burrowing organisms and erosive and other mechanical and chemical forces. These propositions may seem to be truisms yet they serve to explain the present and past distribution of stromatolites, as is discussed by many authors within this book. It is now generally accepted that stromatolites may form in a wide range of environments, but another simplistic notion is still prevalent. That is that all stromatolites are built by cyanophytes. This is not so now, nor was it necessarily the case in the past; many ancient stromatolites may have been built by bacteria or eucaryotic algae. The study of stromatolites is essentially one of morphogenesis. There are several good studies of the morphogenesis of Holocene stromatolites and these provide the basis for the uniformitarian interpretation of ancient stro.matolites. Such an approach is limited by the small range of stromatolite forms known from the Holocene, in contrast with the great variety described from the rock record, particularly the Proterozoic. Morphogenetic analysis has also been applied to ancient stromatolites with no Holocene homeomorphs. Both approaches will be found in this book. Neither approach is simple and the effort involved is worthwhile only because of the wealth of subtle palaeoenvironmental and palaeobiological information encoded in the shapes and microstructures of stromatolites. We are only just beginning t o crack the code. It is this basic role of morphology in the study of these structures, which are not true fossils but are more than sedimentary structures, that has produced the need for a taxonomy and has thus resulted in much debate. A fruitless part of the debate has concerned nomenclature; the fruit is to be found in discussions of classification, for those who have the fortitude to wade through them. Suffice it t o say that it is retrograde to force complex structures into a simple classification. The study of stromatolites has burgeoned in recent years, and at the same time it has become more interdisciplinary. Microbiologists are making increasingly significant contributions t o a field previously dominated by palaeontologists and sedimentologists. Now there are even geochemists, geophysicists and computer technologists hovering on the fringes of the field. The resultant abundance of new information and ideas made available over the past 5-10 years has not been assimilated by the geological profession at large and frequently there are false impressions, even among specialists, as t o the attitudes of other specialists. These points apply particularly where research is published in languages other than that of the user. For all these reasons, I believe the publication of this book is timely. All contributions were invited by me in an attempt to provide a comprehensive coverage of studies of stromatolites. No attempt was made, however, to make the book geographically representative. The 41 authors have been remarkably cooperative and I have received all but 3 of the contributions originally invited. I have attempted t o balance reviews of tried and tested work with reports of

INTRODUCTION

3

new methods and research, aiming both to reflect accurately the present state of knowledge and to produce a book useful to specialists and non-specialists alike. All authors were instructed to guide readers to the pertinent literature, and the bibliography is, I believe, complete within its defined limits. This, I hope, will compensate for gaps in coverage that readers may detect within the text. Gaps not covered by the text or the bibliography indicate those areas not yet studied. Some that can be perceived because research to fill them has just begun include the microbiology of Holocene stromatolites, the microfossil content of ancient stromatolites, the effects of diagenesis on stromatolite microstructure, the interpretation of deep-sea manganese nodules as bacterial stromatolites, and the geochemistry of stromatolitic carbonates. Much research remains to be done on the most controversial of all subjects in this field, stromatolite biostratigraphy. This subject is thoroughly discussed in this book, from many points of view, but the reader will not learn here whether or not the method is valid. The available microbiological and micropalaeontological data suggest that stromatolite biostratigraphy should be possible, but more and more evidence is being uncovered which is inconsistent with the simple biostratigraphic scheme used 5-10 years ago. It may be that the main problem lies in the taxonomies presently in use. We should remember that the history of stromatolite biostratigraphy essentially spans only the last 1 7 years; compared t o other fields of biostratigraphy, this one is in its infancy so we should not be too quick t o criticize. Even if stromatolite biostratigraphy eventually proves to be impossible, we will have accumulated abundant data for making much more precise palaeoenvironmental and palaeobiological interpretations than are presently possible. Editing this publication has occupied a significant part of my time for two years and would not have been possible without the support and assistance of many people. I want to take this opportunity to thank four individuals whose contributions have been indirect yet vital: M.F. Glaessner as my teacher and mentor, Preston Cloud as a source of inspiration and confidence, Brian J. Skinner who by inviting me to Yale University gave me the opportunity to broaden my experience and eventually to meet most of my colleagues who were to contribute to this book, and Marilyn, my wife, who has shared this burden with me. The Bureau of Mineral Resources, Geology and Geophysics generously allowed me to take on this task while in their employ, and for that I am truly thankful. Almost every paper in this book was scrutinised by at least one referee and while the authors and I must bear ultimate responsibility, the referees’ contribution. was indispensable. I shall not mention them all by name but several because of their skills and fortunate geographic location during the editing process bore more than their fair share of the work: they are W.V. Preiss, A.L. Donaldson and J.F. Truswell.


2. METHODOLOGY AND SYSTEMATICS

Chapter 2.1 BASIC FIELD AND LABORATORY METHODS FOR THE STUDY OF STROMATOLITES W.V. Preiss

INTRODUCTION

The methods employed in the study of stromatolites depend on the aims of one’s investigations and on the size limitations of the stromatolites. Thus, the field geologist will be mainly concerned with observations on the geometry and relationships of stromatolites and the information this provides in determining stratigraphic facings, characterizing mappable rock-units, and interpreting environments of deposition. Although he will not normally require the detailed laboratory investigation necessary for the identification and classification of stromatolites, it should be noted that taxonomy also provides valuable environmental data. Conophy ton, for example, is a stromatolite which largely formed only in subtidal environments (e.g. Donaldson, Ch. 10.2).

FIELD TECHNIQUES

Some of the methods suggested below may not apply in less than ideal exposures. The following check-list is intended as a guide to observations to be made wherever possible. The features described are illustrated in Fig. 1. Large-scale structures-the nature of stromatolitic beds

(1)What is the thickness of the stromatolitic bed? (2) Is it composed of a single cycle of stromatolitic morphologies and associated sediments, or of several superimposed cyclical units? (3) What is the lateral extent of the bed and its constituent units? (4) What is the shape of the bed and its constituent units, and what are the relationships of these to the surrounding rock types? (5) What are the lithologies of the stromatolites and surrounding sediments?

W.V.PREISS

6 MODE OF OCCURRENCE

DOMED

OA L

SLIGHTLY DIVERGENT

PARALLEL

MARKEDLY DIVERGENT

C

G COLUMNS

COLUMN SHAPE AND MARGIN STRUCTURE

PEAKS

BUMPS

PROJECTION

WALL

LAMINA SHAPE

STEEPLY CONVEX

RHOMBIC

WRINKLED

UNCONFORM lTY

CRESTAL ZONES IN CONOPHYTONS

NON-COLUMNAR STROMATOLITES

CUMULATE

LATERALLY -LINKED

COLUMNAR-LAYERED

Fig. 1. Diagrammatic illustrations of terms used in the description of diagnostic characters of stromatolites. Reproduced from Trans. R. Soc. S. Aust., 96(2); 69, with the permission of the Royal Society of South Australia.

METHODOLOGY AND SYSTEMATICS

7

(6) What is the shape and relief of successive growth interfaces of the stromatolites? (The “synoptic profile” of Hofmann, 1969a). The synoptic surfaces of several successive stages of growth of certain stromatolitic bioherms from Central Australia were constructed by Walter (1972a). Such information is important for palaeoecology since it provides a minimum depth of water by which the stromatolites were covered, at least intermittently. The mode of occurrence can then be categorized according to the definitions listed in the glossary (see also Preiss, 1972b; Walter, 1972a): bioherm (tabular bioherm, domed bioherm, subspherical bioherm, tonguing bioherm); biostrome (tabular biostrome, domed biostrome).

In termedia te-scale strom atoli t ic structures (1)What is the nature of the constituents of the stromatolitic beds? Any of the following may be present, in any combination: Stratiform stromatolites (flat-laminated, laterally linked, undulatory and pseudocolumnar), columns and cumuli (cumulate stromatolites). (2) What are the vertical and lateral relationships of these constituents t o one another? (3) If columns are present, are they branched? If so, note the style and frequency of branching: parallel branching (a-parallel, 0-parallel, y-parallel), slightly divergent branching, markedly divergent branching. (4) What is the shape of columns in longitudinal sections?, e.g. regular, sub-cylindrical, tuberous, gnarled, straight, or curved. ( 5 ) What is their shape in transverse section?, e.g. round, oval, rounded polygonal, lobate, or elongated. (6) What are the relative orientations of columns t o one another? (7) What is the nature of the column margins?, e.g. smooth, bumpy, ragged, or with large overhangs. (8) If the margins are smooth or bumpy, are they formed by laminae bending over and enveloping the lateral surface t o form a wall? (9) What is the shape of the laminae in the columns?, e.g. conical (this can be seen only in axial section), gently t o steeply convex, rectangular, rhombic, smooth, wavy or wrinkled. (10) Are there any lateral or vertical changes in any of these features when traced throughout the bioherm or biostrome, especially for instance, at bioherm margins? (11)What is the nature of the sediment in the interspaces between columns? This will provide valuable information on the environment of deposition. (12) The upward convexity of stromatolite laminae is often used as an indicator of stratigraphic facing. Nevertheless, upward concave laminae do sometimes occur (Fig. 2), and care must also be taken t o distinguish stromatolitic laminae from laminations in the interspaces, which are often concave

8

W.V. PREISS

Fig. 2. Pseudocolumnar stromatolites, illustrating both upward convex and upward concave lamination, Guck Creek Dolomite, west of Mount Stuart Homestead, Hamersley Basin, W.A. (Photo: M.R. Walter.)

upwards (Hofmann, 1973). Wherever possible, determination of facings should be based on columnar-branching stromatolites where the direction of growth is obvious. The presence of erosional micro-unconformities in the lamination is frequently helpful. Photography and sampling Where possible, photographs should illustrate the observations outlined above. Photo-montage of juxtaposed photographs may be necessary, for example, to show elongated structures such as biostromes. Stromatolitic structures are generally readily apparent on clean weathered surfaces free of lichen cover. If, however, there is insufficient contrast for photography, it may be necessary to outline columns with a felt-tipped marking pen (e.g. Walter, 1972a, pls. 19(1),20(5), 21(3)). Sampling should be commenced only after photography, and then a sketch


9

of the photographed outcrop showing location of samples will be useful for future reference. The number and size of samples required will vary with the nature of the stromatolites, and the following is only an approximate guide: (1) Samples should be selected such that they fit readily into the vice of a slabbing saw, e.g. roughly 25 cm cubes in the case of a 61 cm diameter saw, but longer specimens (to about 50 cm) can be accommodated sideways. (2) They should show on at least one surface a clear cross-section of several columns. (3) Different samples should be selected to be representative of the whole outcrop, including samples from the centre and margins of bioherms, and from the base to the top of the bed t o illustrate vertical changes. (4) Single samples should be selected which illustrate vertical changes from one morphology to another (e.g. flat-laminated to columnar) or changes in style of branching. (5) In the case of stromatolite columns too large t o be handled physically, careful field observations, preferably in different orientations, will be needed to show branching style, column shape, lamina shape and margin structure. Specimens can then be collected t o show portions of columns, e.g. marginal parts for reconstruction t o show margin structure. Thus, the number of samples required depends on the thickness of the stromatolite bed and the diversity of morphologies represented in it. LABORATORY TECHNIQUES

For the identification of previously described stromatolites and for the definition of new taxa, the following procedure is required.

Reconstruction Three-dimensional reconstruction is necessary t o show features which do not appear or cannot be differentiated in single sections. The method has been described by Krylov (1963) and Walter (1972a) and will be summarized here. The rock is cut on a saw as described above, parallel t o the length of the columns, into slabs up to 6 mm wide, depending on the width of columns. Generally 10-15 cuts are required, each cut providing two sections separated by the blade width (about 2 mm for a 61 cm blade). Two preliminary cuts at right angles to the slabs provide reference surfaces for reconstruction. The slabs are numbered, and their surfaces wetted to allow the columns to be outlined in pencil. Then a column or group of columns is selected for reconstruction and followed from one section t o the next. The outlines are then traced on to a block diagram framework on tracing paper, usually drawn with an angle of 45" between the front face and the line representing the top of the side reference face. Successive longitudinal sections are placed

10

W.V. PREISS SLABS

I

W

I

Actual Slab

W!dth 5mm / -

74-152

A

A c t u a l Cut Widlt I 2mm

De1.C.R.S.

Fig. 3. Method of reconstructing columnar stromatolites from serial slabs (left) and tracing on to a block diagram framework on tracing paper (right).

against this framework, turning over the tracing paper from one section t o the next; each slab is displaced along the edge of the reference face by the distance from the previous slab, corrected for perspective by multiplying by the cosine of 45" (0.7). For example, a series of slabs 5 mm thick and 2 mm apart will appear along the reference face 3.5mm and 1.4mm apart, respectively (see Fig. 3). The outlines of columns are traced on to the framework in such a way that only the portions of columns not hidden by the outlines on preceding slabs appear. This basic sketch is then redrafted using shading or stippling to show in three dimensions the features of the columns. Thin-section studies Thin sections are cut generally thicker than standard petrological thickness (0.03 mm); a thickness of 0.04-0.06 mm preserves sufficient contrast to


11

allow the structures and textures of the carbonate rocks to be studied at low magnification. Large thin sections (up to 15cm square) are ideal for the study of columnar stromatolites. It is important to choose the least altered, representative samples for thin sections, so that the following features of taxonomic interest can be studied in detail. (a) Lamina shape may be characterized as gently convex, moderately convex, or steeply convex, depending on the ratio of the height of a convex lamina ( h )t o its diameter ( d ) . It is useful to trace from thin sections a number of representative laminae from different parts of columns, and to measure their ratios of h / d . Laminae may also be characterized as domed, hemispherical, conical, sub-conical, rectangular or rhombic (i.e. flat-topped with parallel edges deflexed at about 90°, k d deflexed obliquely, respectively). On a finer scale, laminae may be smooth, wavy or wrinkled. (b) Margin structure. The behaviour of laminae at column margins is considered to be taxonomically important. If they thin and turn downwards sharply so as t o envelop the edges of the column, a structure termed a wall is formed, in which the internal lamination is sub-parallel to the margin. The margins of unwalled stromatolites may be smooth, bumpy, ribbed, ragged, or with pronounced overhangs. Pointed projections and projections set into niches are significant features if present. (c) Microstructure. This term refers to the thickness, continuity and mutual relationships of laminae, the nature of their boundaries, and any microscopic internal structure. These features are best observed at low magnification under a binocular microscope. Observation under high magnification is usually necessary to determine the textures of the fine-grained carbonate comprising the laminae, but this is probably of less taxonomic significance, being a reflection of the diagenetic history of the rock. The petrographic study of stromatolites is also aided by staining thin sections with alizarin red to distinguish calcite from dolomite. Davies and Till (1968) described a variety of stains useful for distinguishing various carbonates. (d) If the rock is wholly or partially silicified, there is a good chance of microfossils being preserved. Silicified areas should be scanned at medium magnification and the presence noted of any unicells, colonial unicells, filaments (with or without cross-walls, with or without branching, with or without differentiated cells) or any other organic structures. The orientation of filaments relative t o the lamination may be important.

Peels Acetate peels are a useful adjunct and are easy to prepare rapidly. A smooth-cut rock face is used; it may be necessary t o grind it flat if it is rough with saw marks. The surface is etched lightly with 10%hydrochloric acid (for about 10 sec if limestone, 30 sec or more if dolomite). The surface may be stained with alizarin red and is then washed, dried, wetted with acetone, and

12

W.V. PREISS

an acetate sheet (preferably about 0.2 mm thick) is placed smoothly over it, making certain that all air bubbles are pressed out. After drying for about half an hour, the acetate sheet is peeled off bearing an imprint of the structures and textures of the rock. Davies and Till (1968) describe an alternative method of making peels from carbonate rocks, and list additional stains.

Statistical studies Certain quantitative parameters can be measured, which when treated statistically, give additional criteria for the comparison and discrimination of stromatolite taxa. The following have been successfully applied. (a) The degree of lamina convexity ( h l d ) ,described above, is measured for as many laminae as possible in large thin sections. The results are divided into intervals of 0.1 units, and the percentage frequencies of these intervals calculated. (b) The thicknesses of laminae may be measured in stromatolites which have sufficiently distinct lamination. A graduated eyepiece is used with moderate magnification, since stromatolite lamina thicknesses generally fall within the range 0.01-2mm. The results may be divided into thickness intervals chosen according t o the predominant range of thicknesses (e.g. 0.05-0.10 mm intervals). (c) In the case of Conophyton, which generally has very distinct laminae, two additional parameters have been used (Walter, 1972a; Preiss, 197313) but the first of these could be applied also t o non-conical stromatolites. (i) The ratios of thicknesses of adjacent light and dark laminae. These can also be represented as contoured frequency plots (thickness of dark lamina plotted against thickness of adjacent light lamina, and contoured for frequency of points per unit area). (ii) Conophyton is characterized by a thickening and contortion of the conical laminae at their apices. The vertical structure that results from the superposition of these apices is termed the crestal zone. The coefficient of thickening is the ratio of the thickness of a lamina in the crestal zone to its thickness outside the crestal zone. These parameters can all be plotted as histograms or frequency diagrams, enabling rapid comparisons with other forms. If more than a visual comparison is required, it may be necessary to calculate means and standard deviations for the data.

SUMMARY

Field observations are an essential part of all stromatolite studies, not only for geological mapping, environmental interpretation and determination of stratigraphic facings, but also for detailed taxonomic study. The thickness,


13

shape, lateral extent, synoptic profile and associated sediments should be determined for the stromatolitic bed. The nature of its constituent units and their vertical and lateral relationships must be described. Columnar stromatolites should be categorized according t o shape and orientation of columns, style of branching, and margin structure. Stromatolitic outcrops should be photographed prior t o sampling. The size and number of samples to be taken depends on the size and variability of the stromatolites. In the laboratory, serial sectioning and reconstruction are needed to show column morphology in three dimensions. Lamina shape, margin structure and microstructure are studied in thin section and acetate peels. Quantitative measurements of features of the lamination aid in the identification and comparison of stromatolites.

ACKNOWLEDGEMENTS

Helpful discussions with Drs. M.R.Walter and H.J. Hofmann are gratefully acknowledged. The paper is published with the permission of the Director of Mines, South Australia.



Chapter 2.2 GRAPHIC REPRESENTATION OF FOSSIL STROMATOIDS; NEW METHOD WITH IMPROVED PRECISION H.J. Hofmann

INTRODUCTION

An historical review of the ways of studying and illustrating fossil stromatolites (stromatoids) reveals a number of developments. The earliest methods included verbal description and drawings of structures in outcrop, reproduced by wood cuts and lithographs (e.g. Steel, 1825). Later, the petrographic microscope facilitated the study of microstructure, and the invention of photography and new printing methods opened up new ways for rapid improvement in disseminating information pictorially. The first Precambrian stromatolite to be described, Archaeozoon acadiense (Matthew, 1890a, p. 40), was already illustrated photographically. The next major innovation did not come until the 1950’s with the method of “graphic reconstruction” from systematically spaced, serial longitudinal sections (Kry!ov, 1959b), adding a new perspective t o previously existing techniques. A subsequent variant of the graphic reconstruction method is that of Raaben (1969b), using systematically spaced, serial transverse sections. In both Krylov’s and Raaben’s methods the columns are reconstructed by shading of the surface features, using properly positioned tracings of the serial sections as reference lines. These shaded-in surface features are used to differentiate some groups and forms from others. It has now become evident that the process of shading can be very subjective, varying from one person to the next (Hofmann, 1973), and that consequently the distinction between certain taxa based on this method may be artificial. For these reasons a more objective and precise method of reconstruction is proposed, one which also may eventually lead t o automatic representation by computers. This is described below. PRINCIPLES

Basically, the method involves the preparation of an isometric drawing from systematically spaced, serial longitudinal sections, and may be thought

16

H.J. HOFMANN

of simply as combining the elements of the two methods developed by Krylov and Raaben. However, instead of subjectively shading the reconstructed stromatoid surfaces, the columnar outlines are actually depicted by two sets of lines: systematically spaced vertical profile lines visible in a particular chosen orientation (usually a 45” dextral or sinistral oblique view), and equally systematically spaced horizontal contours. In this manner, the surface of each column becomes outlined by a grid, whose lines and points of intersection are not only accurately placed in space, but which also automatically provides a shading effect that is mechanically produced, and therefore more objective and precise than conventional shading (Hofmann, 1973, p. 364). Further superimposed stippling or shading is optional. DESCRIPTION OF METHOD

Selection of sample Samples selected should give at least two or three columns for reconstruction, and include columns as long as possible, showing at least two successive points of branching. However, there is often a limit to the size of the specimen that can be taken, handled, transported, and processed in the saw, and for many columnar stromatoids it is impossible to apply the method because the columns are too large. If several morphologic types are present in a bioherm, a range of variants is sampled. Relevant field observations on the positional, lithologic, sedimentologic, or other aspects relating the structures to enclosing lithosomes are essential for later interpretation of the reconstructed stromatoids. Preparation of sample It is advantageous to first make a cut transverse to the columns, to obtain an upper planar “horizontal” surface that will serve as a reference plane ( x - y plane) from which later the grid lines for the horizontal contours are measured. This plane also gives the plan outlines of the concentric laminae, which are desirable for completion of the top surface of the reconstruction. A set of two converging, intersecting, or diagonal straight lines are drawn on the surface, with indelible ink markers t o permit the exact relative positions of the slabs to be established later on, after sawing. Likewise, it is desirable, but not essential, to have a second, longitudinal section perpendicular to the first one before proceeding with the serial sectioning. This second cut can provide the “front” ( x - z ) or “side” ( y - z ) face of the reconstruction and gives additional control for positioning the serial sections, as well as providing a further view of laininar profiles. After these preliminary cuts have been made (and the surfaces traced


17

and/or photographed alongside a metric scale for a permanent record), the specimen is cut into slabs of uniform thickness along the third direction. The thickness of slabs chosen depends on the diameters of the columns, and should be selected so that the finished thicknesses of the slices are equal to, or multiples (2,3,4)of the widths sawed and ground away. This will allow later addition of intermediate profile lines by interpolation. For example, if the slab is twice the width of the cut, the two faces of the slab are traced on the final drawing, but later a supplemental line may be drawn intermediate in position between the two actually traced. In this way the uniform spacing between successive profiles is maintained. The caption of the final figure should specify whether the reconstruction is based on actual tracings only, or has supplementary profile lines.

Hints Carbonate slabs as large as 10 by 30 cm can be cut 4-5 mm thick without breaking if a piece of foam plastic or rubber is placed in the receptacle underneath the slices being sawed off. For large and heavy samples serial sectioning frequently requires remounting of the specimen. The block can be reinserted in the vice with nearly the same orientation if the last-cut face of the remaining portion of the specimen is aligned against the side of the saw blade.

Tracings

.

The outlines, and prominent laminar profiles of the columns to be reconstructed, as well as all reference lines or points, are transferred from the rock slice onto the drawing surface. The preparation of this tracing will often be facilitated if contrast is enhanced by introducing a film of water between the plastic drafting film and the surface to be traced. If the columns are still not readily visible they should be brought out by special treatment such as etching or staining. Eventually, it may be possible to obtain such outlines automatically with computers with capability for imagedensity evaluation and suitable output peripherals. The tracings are labelled in sequence.

Graphic representation (Fig. 1 )

(1)Fasten a sheet of millimeter-paper onto a drafting table. This is the reticle or control grid, which provides the coordinate system for the reconstruction; it should be large enough to fumish orientation for all points represented within the sample. (2) Select a sufficiently large sheet of transparent drafting film or paper to carry the final graphic representation. This is the working sheet. (3)Place the working sheet over the control grid and fasten it with a piece

18

H.J. HOFMANN

Fig. 1. Method of graphic representation, illustrating reconstruction during stage of transferring fourth profile from tracing 4 onto working sheet. Contour lines have not yet been drawn through lowermost three sets of hypsometric control points.

of drafting tape at each of the two top corners. These pieces of tape act as hinges and allow the working sheet to be lifted for insertion and removal of successive tracings between it and the control grid. (4) On the working sheet select the position of the origin for the projection (point of intersection of the x - and z-lines of the first tracing used) at the intersection of two of the heavier lines of the control grid, and,mark it in pencil. This point should be so located that all columns to be represented can be accommodated on the working sheet. (5) For a representation with an obliquity of 45", draw a line at 45" from the origin, to the right if a dextral view is desired, or to the left if a sinistral view is preferable. This line is the y-reference line that gives, in isometric projection, the y-reference points of each of the serial tracings, displaced towards the rear (up and to one side) by a factor of 0.7 (cos 45") of the real distance. Mark off and label these reference points. (6) Select the columns to be represented by inspecting the serial tracings, and insert the tracing bearing the first oiltline in between the working sheet and the control grid. Move it until the reference point of the tracing exactly coincides with the origin on the working sheet, and the straight horizontal reference line coincides with the underlying grid line passing through the origin. The first tracing is now properly oriented for copying. (7) Trace the outline(s) of the column(s) of interest onto the working sheet. (If the first face to be depicted is substantially within the column, then the laminae of this surface may also be traced so as to illustrate the laminar profile in longitudinal section.) Note that the horizontal grid lines below the


19

x-reference line intersect the profiles and are hypsometric lines (contours). Mark or note all points of intersection of the profile lines with uniformly spaced contours, the contour interval equalling the actual uniform spacing between successive vertical profiles. (8) Remove the sheet bearing the first tracing and insert the succeeding one between the working sheet and the control grid. Superpose the reference points for this section on the tracing and working sheets and orient the tracing until the horizontal reference line is parallel to the corresponding x-coordinate of the underlying grid. The procedure is considerably simplified if the serial sections are cut and ground so as to be an even number of miIlimetres apart ( 4 , 6 mm, etc.): This is because the distance between successive horizontal reference lines in the projection chosen is equal to % the true distance as measured on the original sample (cos 45" x cos 45") allowing the horizontal reticle lines to be used directly as guides. Thus, if the sections are 4mm apart in reality, the horizontal x-reference line of the second tracing lies along the grid lines 2mm above the first, but shifted 45' to one side. This is equivalent to moving the reference points 4 x cos 45" = 2.8 mm along the projection's oblique. y coordinate. Trace the profile(s) of the same column(s) as in the first tracing onto the working sheet as well as any newly appearing ones of interest, taking care to show only those parts of the profile lines that are visible in the oblique view chosen, and not "behind" the profile(s) drawn for the previous section. Again, mark or note all points where the equally spaced horizontal grid lines intersect the profile lines. (9) The process is continued for all succeeding profiles of the columns of interest. After the third profile has been drawn, the set of lines joining points of equal elevation can be added. These are contour lines equidistant from the horizontal (x-y) reference plane, following the surface outline of the columns, and approximating the lines that would have been obtained by Raaben's . method of transverse sectioning. (10) To complete the view of the upper surface of the reconstruction, a projection of the original tracing of that surface can be used for highest precision. Alternatively, this surface can be reconstructed from the serial vertical profiles by joining correlative points previously positioned on each tracing along the upper horizontal reference lines. A projection of a second, vertical (lateral) section can be drawn in similar fashion if desired. (11)Where needed, intermediate supplementary profile lines may be added later between those actually traced. These may become necessary where the thickness of the slabs is large. The thickness should preferably be a multiple of the width of the sawed and ground portion of the specimen. In the case where the spacing is double, one supplemental profile line is added by interpolation halfway between those representing the front and back of the slabs; where it is triple, two supplemental lines are inserted.

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H.J. HOFMANN

SUMMARY

The result obtained by the method is an isometric diagram of vertical profiles and obliquely viewed horizontal contour lines, whose points of intersection lie on a systematic, three-dimensional coordinate system, on two sets of unifornily spaced and mutually perpendicular planes. This allows relative distances and mutual relations between morphological features t o be perceived quantitatively on a figure that is largely free from reconstructor’s bias. It also allows for uniformity in presentation, eliminating the possibility that reconstructions made by different persons look different. Moreover, because of the high degree of spatial control, it is expected that the method will be amenable t o computerization and lead t o eventual automatic reconstruction of stromatoids. Once computerized, the data could conceivably be processed or manipulated in such a way that any particular view can be chosen by electronic reorientation, allowing for the selection of the one or two views which most clearly show the geometric attributes of the columns. On the other hand, the method may be superseded by statistical methods such as described in Chapter 2.5.


Chapter 2.3

BIOLOGICAL TECHNIQUES FOR THE STUDY OF MICROBIAL MATS AND LIVING STROMATOLITES Thomas D.Brock

INTRODUCTION

Techniques are available for studying the growth and function of microbial mats in nature, and the data obtained with these methods are essential for the environmental interpretation of mat morphology and distribution.. This article briefly reviews these methods. It is hoped that workers will be encouraged to carry out ecological investigations on living mats, since such investigations will provide much useful information for the interpretation of fossil material.

ESTABLISHING STATIONS

For most purposes, stations should not be established randomly, but should be placed intelligently so that the maximum amount of information possible can be obtained from the study. In marine environments, transects from supratidal to low intertidal or subtidal are preferable, and a series of such transects should be made within the general study area, so that the generality of observations can be ensured. In thermal environments, stations are usually best established at various locations along the outflow channel of the spring, so that a variety of temperatures are obtained. Care should be taken to ensure that the various stations in the thermal effluent have similar flow rates, if it is desired to compare the specific effect of temperature. It should be emphasized that there is a large amount of variability in mats, even those formed by the same species, so that many replicate stations are desirable.

.

ENVIRONMENTAL MEASUREMENTS

'

In understanding the factors involved in the development of mats, it is essential t o know the environmental conditions under which growth is

22

T.D. BROCK

actually taking place. This means that environmental measurements should be made periodically at the various stations. It should be emphasized that measurements of dead mats have no biological meaning. It is important to know that the mats under study are actually growing; the procedures by which the existence of growth can be determined will be outlined in a subsequent section. Measurements in growing mats should be made at regular intervals, so that any changes can be detected. Especially in marine mats, measurements should be made over a number of tidal cycles, and because tide also varies with time of the month, measurements should be made over 'several months. In habitats where seasonal changes in temperature or water level occur, it is desirable t o make measurements over the whole annual cycle. It might be noted that a series of detailed measurements is essential since only a few days of unusual weather may be sufficient t o drastically affect the environmental parameters. Although automatic recording equipment is theoretically the best, reliability is sometimes a problem, and data reduction can require a significant amount of time. In most cases, it is probably simpler and more reliable to make manual measurements at frequent intervals. Once the investigator has obtained some experience with a habitat, he can more intelligently select the required periodicity of measurement. It should be emphasized that measurements must be made precisely at the locations where the mats are growing. Conditions may be quite different even a few centimeters distant. The most useful parameters are temperature, pH, salinity (in marine habitats), oxidation-reduction potential, oxygen concentration, sulfide concentration, and light intensity. Temperature measurements are most easily made with thermistor probes, which permit measurement at different levels within the mat. The pH can also be measuredcdirectly within the mat, using a combination electrode and a battery-operated pH meter (pH paper is generally unreliable, especially in salty or mineralized waters). It might also be desirable t o measure the pH of water within the mat itself, as metabolic processes can result in pH values quite different from those of the surrounding water. When measuring pH in mats, care should be taken to ensure that changes in pH do not take place during expression of the' water. Losses of gases should be especially avoided, since loss of C 0 2 can lead t o marked increases in pH. All pH measurements should be made directly in the field, or within a few hours in the laboratory on samples taken t o ensure no loss of gases, or oxidation of sulfide. In mats which are completely submerged at all times, salinity of the water within the mat should be the same as the salinity of the overlying water. However, in intertidal mats, exposed to the air part of the day, salinity of the water within the mats may vary widely and may at times be much higher than the salinity of the overlying water. Techniques for measuring chemistry of water within mats have not been published. Water could be expressed by squeezing, or the mat could be removed from the water, drained, and cut into segments for analysis. Doemel and Brock (1976) have used the latter method successfully to analyze for sulfur species in mats.


23

A key factor in growth and preservation of mats is oxidation-reduction potential. Again, measurement must be made directly in the mat, and at the level in the mat which is of particular interest, since differences in oxidationreduction potential may be great between surface and subsurface portions of a mat. Although more difficult t o measure, oxygen and sulfide concentrations at different levels in a mat might also be measured, since these parameters will influence oxidation-reduction potential and are perhaps more meaningful biologically. Cell biologists use microelectrodes for this purpose which could be easily adapted for use in mats (Lavallee et al., 1969). Light intensity at the surface of mats can vary with depth and with angle of incidence of the sunlight. Especially in rocky habitats,’ there may be quite significant variations in light intensity with angle of incidence. Underwater photometers can be used t o measure light intensity at the surfaces of mats in deep water. Light intensity at various levels within the mat are more difficult to measure, but are important in determining the depth in the mat at which photosynthetic activity ceases. Such light intensities can be measured by taking cores and cutting from them discs of various thicknesses. Each disc is placed on top of an opaque mask which contains a small hole through which light can pass. A light sensor such as a radiometer can be placed under the hole to measure light penetration. Because of the marked attenuation of light by even rather thin sections of compact mats, intense light sources must be used, and either sunlight or a high-intensity tungsten light is preferable. Natural sunlight is most environmentally relevant, although not especially subject to the investigator’s control. We have found 96-97% light attenuation by a mat 3 mm thick. SAMPLING

Cylindrical or square cores can be cut for analysis, and for many purposes cork borers can be used. If the mat is rather loose and lacking in structure, it may fall apart when removed from the coring device. We have found that the structure of such mats can be retained if they are allowed t o fall from the coring device gently into a container with melted 2% water agar (at 45-50°C). The agar is allowed t o harden without disturbing the container, and then the core can be easily handled, dissected, or otherwise studied. Another way to preserve the structure of a delicate mat is by freezing, preferably on dry ice. Thin sections can be cut from frozen cores upon a block of ice, using a razor blade. Mats can be embedded in resin (we.. have found Durcupan quite satisfactory) and thin sections (around l p m ) made with glass knives using an ultramicrotome. For detailed study of vertical zonation in algal mats, microscopic dissection is useful. We have found it preferable to take a dissecting microscope into the field and perform the dissections right at the habitat. If a series of replicate samples is to be taken from a single mat (for use,

24

T.D. BROCK

perhaps, in growth rate or photosynthesis studies as described below), it is essential that all samples be from the same population. Micro-environmental influences may result in populations of two or more types developing within a very small area, so that samples presumed to be replicates may not be. The only way to ensure that samples are indeed replicates is by making quantitative measurements on them, using some of the techniques described below. Sampling of mats with vertical relief (cones, biscuits, nodes, etc.) as opposed to flat mats presents some additional problems. For small cones or nodes, we have taken large cores which include the base of the structures and then excised the individual nodes with micro-scissors. Large cones or biscuits can be easily collected, but each structure is heterogeneous and must be dissected carefully to sort out the various components. Dissection is best done directly in the field under a dissecting microscope. TAXONOMY AND CULTURE

The taxonomy of the blue-green algae is in a very unsatisfactory state. As emphasized by Stanier et al. (1971),taxonomy of the blue-green algae can only be done by detailed study on a wide variety of pure cultures. Yet, the taxonomic distinctions which have been made and which are the basis of keys and species descriptions are based on rather unsophisticated microscopic examination of natural material. About the only thing that can be really told about blue-green algae from examination of natural material is the major group to which the alga belongs, and even here there is some uncertainty. Unsatisfactory as it must be to geologists, the taxonomy of blue-green algae is so uncertain as to be virtually completely useless. Indeed, some of the described taxa are not even blue-green algae (Brock, 1968) but photosynthetic bacteria (Bauld and Brock, 1973). For environmental and geological purposes, rather than to merely give the names, it is more useful to present a detailed microscopic description of the forms seen. Ideally, photographs of typical forms should be given, preferably using modern microscopic techniques, such as phase contrast or Nomarski interference contrast. For detailed study of typical forms, cultures are essential, and for many purposes, these must be pure cultures. (There is an excellent recent book describing algal culture methods: Stein, 1973.) However, this is really the domein of the microbiologist, and any team studying a mat-forming system should include a microbiologist. PIGMENTS

We have described in some detail the methods for measuring chlorophyll pigments in mats (Brock and Brock, 1967;Brock, 1969;Bauld and Brock,


25

1973) and details of these methods will not be repeated here. Briefly, the technique involves homogenizing mat material in acetone or methanol, centrifuging, and reading the light absorbance at the wavelength appropriate to the chlorophyll of interest. All chlorophylls absorb in two regions of the spectrum, the blue and the red or infra-red. The absorption peaks which serve to distinguish the various chlorophylls are in the red or infra-red region (Table I). TABLE I Absorption peaks of bacterial and blue-green algal chlorophylls Chlorophyll

Absorption peaks

in vivo Algal chlorophyll a Bacteriochlorophyll a Bacteriochlorophyll b Bacteriochlorophyll c Bacteriochlorophyll d

680 nm 8 2 0 nm 1025 nrt,~ 747 nm 725 nm

solvent extract 665 nm 770 nm -

660 nm 650 nm

Values listed are for the absorption peaks in the red or infra-red. All chlorophylls also have strong absorption peaks in the blue. Data are from Pfennig (1967), and Clayton (1963).

Blue-green algae have only chlorophyll a , which absorbs in acetone extract at 665 nm. We have found that blue-green algal chlorophyll is stable for several weeks if samples are placed in aqueous formaldehyde (4% final concentration) in the dark, but chlorophyll is not very stable in acetone extract, so that after extraction the absorbance must be measured within a day. If samples cannot be analyzed directly in the field, then cores should be placed in 4% formaldehyde (made up in the kind of water in which the organisms are growing), placed in the dark in a cool place, and extracted and analyzed as soon as possible in the laboratory. Vertical zonation of chlorophyll pigments will tell much about the structure and manner of growth of the mat (Brock, 1969), and can be determined by cutting segments of cores and assaying these separately. Bacteriochlorophylls may be an important part of the photopigments of a mat, and must be handled somewhat differently from the method described above. In general, the photosynthetic bacteria are usually in layers underneath the algae, although in some Yellowstone hot spring mats this is not the case, the photosynthetic bacteria being right on top with the algae. These bacteria may even be present in the absence of algae (Doemel and Brock, 1974). The absorption spectra of the various bacteriochlorophylls are given in Table 1. Note that the absorption spectrum is different in solvent extract than it is within the cell (in uiuo).It is the in uiuo absorption spectrum that is ecologically relevant, but for quantitative studies the measurement of absorption in solvent extracts is usually carried out. Note, however, that the

26

T.D. BROCK

absorption peaks of two of the bacteriochlorophylls, c and d , are quite close to the absorption peak of algal chlorophyll a. Particularly, it is impossible to distinguish bacteriochlorophyll c from algal chlorophyll a by spectrophotometry of solvent extracts. This became important to us because some of the mats we were studying consisted of a blue-green alga and a filamentous photosynthetic bacterium (Bauld and Brock, 1973), the chlorophylls of which absorbed at exactly the same wavelength in solvent extract. Indeed, this fact initially prevented us from knowing that a photosynthetic bacterium was involved (Brock, 1968). Thus, to distinguish these two chlorophylls in mats, in uiuo absorption spectra must be obtained, since in uiuo the two chlorophylls absorb at quite different wavelengths. To obtain in uivo absorption spectra, it is necessary to eliminate in some way the light scattering of the material. We use the technique first developed by Pfennig (1967) of suspending the homogenate in a concentrated sucrose solution, which removes most of the light-scattering properties of the homogenate. It should also be noted that the absorption spectra of several of the bacteriochlorophylls are far in the infra-red (Table I), so that a spectrophotometer with a photocell with appropriate sensitivity is necessary. PHOTOSYNTHESIS

In order to determine the activity of the organisms in mats, photosynthesis measurements are useful. The method of choice is the I4CO2method, fully described by Brock-and Brock (1967), and widely used in our laboratory (Brock, 1967; Brock, 1969; Bauld and Brock, 1973; Doemel and Brock, 1974). Photosynthesis studies can be carried out on cores of mat, or on dissected segments of cores, if natural conditions should be maintained, or they can be carried out on homogenates. Homogenates have the advantage that a series of replicates can be prepared from the same material. However, for many purposes it is essential that the natural structure of the mat be maintained. As described by Brock and Brock (1967), incubation with 14C02can be carried out on undisturbed cores, and then after the incubation is complete, the core can be homogenized and subsamples taken for separate determination of radioactivity and chlorophyll content. In this way, the radioactivity incorporated per unit chlorophyll can be determined, permitting normalization of values for different mats. It may be found that rate of photosynthesis per unit chlorophyll (photosynthetic efficiency) may vary markedly from mat to mat. This is a good indication that some of the mats are more active than others, an important piece of geomicrobiological information. Descriptions of how these techniques can be used to study the activity of mats under different situations can be found in Brock (1967) and Brock and Brock (196913). It should be clear that 14C02studies such as those just described present


27

data on the activity of the mat as a whole, but do not distinguish between the activity of individual organisms within the mat. To accomplish this, microscopic autoradiography must be done. This is done by placing samples of homogenized mat after incubation with 14C02 onto microscope slides, covering with liquid photographic emulsion, and allowing the radioactivity in the cells to expose the emulsion (M.L.Brock and Brock, 1968). By microscopic examination of the slides, the radioactivity can be localized over particular organisms. An example of a use of this technique with organisms from a marine environment can be found in Munro and Brock (1968).This technique would be especially useful in distinguishing between bacterial and algal photosynthesis in mats. Another way to distinguish bacterial from algal photosynthesis is by the use of the inhibitor 3-(3,4-dichlorophenyl)-l,ldimethylurea (DCMU). This agent inhibits photosynthesis in algae but has no effect on bacterial photosynthesis. At concentrations of 1-5 x lO-’M,virtually complete inhibition of algal photosynthesis occurs (Bauld and Brock, 1973). Because of the possible importance of bacterial photosynthesis in the anaerobic portions of mats, it seems essential to carry out studies with this inhibitor, t o determine the relative importance of bacteria and algae. GROWTH RATE

Probably the most sensitive indicator of environmental influence on mats is growth rate, and for many purposes measurement of growth rate will be desired. The most direct means of measuring growth rate is by carborundum marking, the carborundum layer providing a fixed horizon, on top of which the accreting mat continues t o grow. By taking cores through the area marked by carborundum at various intervals, the rate of accretion can be obtained. For simple studies, merely measuring the thickness of the mat above the carborundum layer may be sufficient, but since compression effects may lead to different thicknesses for the same biomass, it is probably preferable to make some chemical measurement, such as chlorophyll content, organicmatter content, or protein content, t o determine biomass. Another way of assessing growth rate is by darkening an area of mat with an opaque shade, and measuring the rate of disappearance of the mat material (T.D. Brock and Brock, 1968). If the original mat was in steady state (probably not common in marine mats), the rate of growth would be balanced by the rate of disappearance (due t o decomposition, erosion, and grazing), so that once growth is inhibited by darkening, only the loss processes can occur. Thus, by measuring the rate of disappearance of material, the rate of growth in the steady state can be determined.

28

T.D. BROCK

DECOMPOSITION

It is clear that preservation of mats can only take place if they have not previously decomposed. For geological interpretation, the whole process of mat decomposition is thus of great importance, and needs considerable study. Crude evidence of mat decomposition can be obtained by observation of gasfilled holes in mats, or other signs of degradation of cell material, but for any detailed studies, the rate of decomposition should be measured. This can most readily be done by use of carborundum marking. The procedure involves successive marking of the same mat with layers of carborundum at least twice. When the first carborundum layer is applied, the organisms will move on top and continue to grow, so that the mat will gradually build up .over the carborundum layer. Subsequently, another layer of carborundum is added. Cores are taken at the time the second layer is added, t o determine the distance of separation of layers. Afterwards, more cores are taken. As decomposition proceeds, the two carborundum layers move closer together, and may eventually merge. One can assess the decomposition rate by measuring the distance between the two layers, and detailed quantification can be obtained by separating the material between the two layers and determining chlorophyll content, organic-matter content, or some other biologically meaningful parameter. EXPERIMENTAL MODIFICATIONS

One of the attractive features of studying the ecology of mats is that they can be readily modified experimentally. These modifications reveal something about how the mats can respond to environmental change, and help in understanding the factors involved in the mat-forming processes. Darkening of portions of mat has been already described in a previous section. In addition, we have used neutral density filters t o reduce the available light over portions of mat, with the aim of studying the adaptation of the algae and bacteria to reduced light intensities. This work has shown (Brock and Brock, 196913) that the algae respond to light reduction by increasing chlorophyll content. In the case of the photosynthetic bacteria, we have shown (Doemel and Brock, 1974) that if the light intensity is reduced to a sufficiently low level, algal growth is no longer possible, but growth of the filamentous photosynthetic bacteria will continue and a strictly bacterial mat will be formed. In some earlier work, we used experimental channels (Brock, 1969) within which themats could develop. Other possible modifications which may be of interest are: alteration in water-flow rate over the mat, changes in temperature by diverting flows of hot water (in thermal habitats), poisoning of the components of the mat with selective inhibitors, elimination of the mat completely to determine the rate of mat regeneration (see Brock and Brock,


29

1969a), transfer of portions of mat from one location to another, change in sedimentation rate, change in periodicity of flooding. ACKNOWLEDGEMENTS

Research of the author in this area has been supported by research grants from the National Science Foundation (U.S.A.) (6B-35046). NOTE ADDED IN PROOF To avoid the necessity of measuring chlorophyll absorption spectra in uiuo, Madigan and Brock (in press) extracted mixtures of chlorophyll and bacteriochlorophyll in methanol, separated the chlorophylls by thin-layer chromatography, and eluted the spots. This technique permitted measurement of chlorophyll a and bacteriochlorophyll c in algal-bacterial mats.



Chapter 2.4

APPROACHES TO THE CLASSIFICATION OF STROMATOLITES I.N. Krylov

IS A CLASSIFICATION NECESSARY?

The necessity t o classify and name stromatolites inevitably arises in any attempts to use them for stratigraphic or paleofacies reconstructions. If a researcher thinks that the morphology (and the microstructure) of stromatolites is accidental and has no regular relation to the composition of the stromatolite-forming microbiota, or t o their growth conditions, then he will neither classify stromatolites nor name them. That is why Vologdin (1962) and Korde (1950) rejected any classification of stromatolites. They studied and described only algae, describing stromatolites only incidentally as forms of the growth of these algae. Another group of researchers is of the opinion that the shape of stromatolite constructions can be more or less regularly associated with their formative conditions: under similar conditions similar (or different) algal complexes could build similar constructions. Such constructions can be used for determining environmental conditions, or serve as local stratigraphic markers to correlate deposits within small regions or basins. In both cases the stromatolites should be described in some way and named. In classifying, “useful”, “operating” features are used: the extent of layers or the isolation of nodules and columns (Logan et al., 1964), the shape of the constructions (Robertson, 1960; Donaldson, 1963; Szulczewski, 1968) or a whole complex of features (Maslov, 1960). These workers rejected the system of type specimens and of the nomenclature-priority principle. The constructions may have names, but the nomenclature should not be Linnean. For example: (1)Uniform constructions may have Latin names, even if they are binomial, but these names should differ from “true” paleontological names, at least in the method of transcription (not italicized or capitalized, but Gothic, cursive, boldface or extended - see Cloud, 1942; Hofmann, 1969a). (2) Binomial Latin nomenclature should be replaced by a polynomial (Maslov, 1960) or uninomial system (Donaldson, 1963). (3)The name may consist of several letters forming an abbreviated description of the construction, for instance LLH (laterally linked hemispheroids; Logan et al., 1964).

32

I.N. KRYLOV

(4)Uniform constructions can be represented by letters or figures. Classifications of this type are usually temporary, pending the compilation of a fuller classification. So in papers by Korolyuk during the years 1956- 1958, stromatolites were designated by Roman and Arabic numerals (e.g. 11-3,1-2). Finally, a third group of researchers assumes that stromatolites can be successfully used for biostratigraphy. Such an opinion is based on the fact that in very distant regions we find identical stromatolites in a closely comparable order of superposition. Such identical constructions can have the same names and would enjoy all the rights of Linnean nomenclature. These stromatolites could and should be classified within the framework of formal paleontological classifications including strict observance of the nomenclatural codes. I t is most convenient t o use the provisions for fossils in the fnternational Code of Botanical Nomenclature (I.C.B.N.), as is the current practice of the majority of Soviet and many foreign researchers. The complicated nature of stromatolites as constructions formed by an association of algae and not by single species is no obstacle for the application of binomial nomenclature. Latin names are a part of international scientific terminology. Their only purpose is to give a name that is uniformly understood by all specialists. Binomial nomenclature is used not only for designating biological species, but also for the names of separate parts and organs of organisms (e.g. Musculus biceps), for chemical compounds and medicines (e.g. Oleum ricini), and for physiological processes and illnesses (e.g. Febris undulans). Latin binomial names are also widely used in geographical literature for regular plant associations (e.g. Pinetum myrtillosum). A Latin binary name is unambiguous. The nomenclature should be stable and the name should have only one meaning, determined by a type specimen for a form and a type form for a group. The priority principle serves t o protect this situation; new names are not accepted for previously named taxa. The I.C.B.N. specifically states (recommendation PB6) the necessity of a single binary name for taxa in formal classifications. Unfortunately, this recommendation has been repeatedly violated and one can find six diagnoses for the Gymnosolen group (Steinmann, 1911; Pia, 1927; Raaben, 1960; Krylov, 1963; Raaben, 1964b, 1969b), four diagnoses for the Boxonia group (Korolyuk, 1960c; Komar, 1964, 1966; Semikhatov et al., 1970), and so on. Urgent measures should be taken to re-establish and strictly protect the stability of the nomenclature. Otherwise it will become useless. WHAT ARE.WE CLASSIFYING: SAMPLES, COLUMNS OR BIOHERMS?

The codes of nomenclature regulate only the rules on the publication of taxa and do not deal with the features on which these taxa have been established. In starting to classify stromatolites, different researchers working with very different material based their classifications on different features.


33

Fig. 1. Identification of a stromatolite depending on the size of the sample. The entire hand specimen: columnar branching stromatolite (e.g. Minjaria Krylov, Gymnosolen Steinmann);A, C, D = non-branching columnar stromatolite (CollurnnacoZZenia Korolyuk).

As a result several parallel and independent stromatolite classifications have been published. The fundamental difficulties in the systematics of stromatolites result from the inevitably fragmentary nature of the material classified. As a rule, stromatolite bioherms and biostromes are rather large and of a complicated structure. And yet the classifications do not regard the bioherms as a unit, but treat only their separate fragments. There are no difficulties in classifying individual samples. The difficulties begin when we see the natural relationships between stromatolites of different gross morphology which have been referred to different taxa. The determination of a stromatolite means that the researcher must give a name to a specimen that is placed on his desk. It is easy to see that the result will depend not only upon the sympathy he feels for a certain classification scheme, but also upon the specimen itself. Fragments A and B on Fig. 1are parts of one construction. The researcher will call the stromatolite from sample A non-branching and will compare it with Cryptozoon, Colonnella, Collurnnacollenia, etc. The stromatolite in sample B will be called branched (Gymnosolen, Minjaria, Acaciella). If, however, sample B were divided into parts (C and D ) , the stromatolite (according to the observed features) would again be called non-branching. In this way, the application of a certain classification and, correspondingly, the choice of the group name, is sometimes determined simply by the size of the sample studied. Fig. 2 represents a bioherm from the Lower Riphean Kotuikan suite in the north of Siberia. At its base we see a large dome-like nodule with the features of the group Collenia Walcott. Columnar stromatolites B, C,D,E , would be

34

Fig. 2. Stromatolites of different morphological groups in a single bioherm. The Kotuikan River, Lower Riphean. A = Collenia Walcott; B, C, D, E = Kussiella Krylov; F = Collumnacollenia Korolyuk; G = Omachtenia Nuzhnov; H , Z = Stratifera Korolyuk.

referred to Kussiellu Krylov, the columnar “non-branching” stromatolite in sample F to Collumnacolleniu Korolyuk (= Colonnellu Komar), the columnar stratifonn constructions in sample G to Omuchtenia Nuzhnov, and CzO)compounds comprise a much greater proportion of the total aliphatic hydrocarbons in non-photosynthetic bacteria (>50%)than in photosynthetic bacteria ( Czo compounds in their aliphatic hydrocarbons, preferential metabolism of algal straightchain hydrocarbons) on the residual organic matter. Mats from different environments should be compared t o ascertain whether characteristic differences in their “biochemical facies” exist. Several organic geochemical parameters are t o a large extent environmentally controlled: pristane to phytane ratio and even-predominance in n-alkanes, by redox potential; and kerogen 613C, by temperature, pH and COz concentration. In view of the ability of carbonate minerals to form complexes with a wide variety of organic compounds, stromatolitic carbonates appear likely to be a more fruitful source of syngenetic chemical fossils than cherts. Finally a concerted attempt, employing a range of analytical techniques (including carbon isotopic analysis), should be made t o characterize as many stromatolite kerogens as possible, with the aim of detecting original biochemically derived (as distinct from geochemically imposed) differences which may be of evolutionary significance. Now that biological control of stromatolite microstructure (Gebelein, 1974) and some aspects of gross morphology (Serebryakov and Semikhatov,

190

D.M. MCKIRDY

1974) has been verified, the potential rewards awaiting further study of stromatolites by organic geochemists would seem t o far outweigh the present analytical and interpretational difficulties.

ACKNOWLEDGEMENTS

This paper would not have been written without the patient encouragement of Malcolm Walter who freely shared his knowledge of stromatolites during many discussions. Drs K.A. Kvenvolden, K.S.W. Campbell and D.J. McHugh kindly read the draft manuscript and made helpful suggestions for its improvement. Samples of stromatolites were donated by Drs M.R. Walter and W.V. Preiss. Mr Z . Horvath, Petroleum Technology Laboratory, B.M.R., provided skilled technical assistance. The organic geochemical data on Australian stromatolites was collected as part of the author's Ph.D. project at the Geology Department, Australian National University, Canberra. The support of an Australian Public Service Post-Graduate Scholarship for this work is gratefully acknowledged. The author publishes with the permission of the Director, Bureau of Mineral Resources, Geology and Geophysics, Canberra, A.C.T. NOTE ADDED IN PROOF Two recent papers on the organic geochemistry of algal mats from Baja California provide important new clues to the likely composition and derivation of stromatolitic lipids and kerogen. Cardosa et al. (1975) reported that hopane-type triterpenes and triterpanes are prominent in the lipid fraction, reflecting the predominantly prokaryotic nature of the mat biota. The extractable fatty acids include a homologous series of normal saturated acids in the range C14 to Cm with maxima at c16 and c 2 6 . This finding corrects an impression conveyed by earlier studies (summarized in Table 111) that long-chain (>C,,) fatty acids do not exist in Recent algal mats. Such acids constitute a plausible source for the long-chain n-alkanes isolated from ancient stromatolites. Treatment of the kerogen with chromic acid (Philp and Calvin, 1975) yielded oxidation products suggestive of a highly aliphatic structure. The kerogen appears to consist of a network of cross-linked polymethylene chains which probably formed by the condensation of unsaturated oxygenated lipids, including the phytyl moiety of chlorophyll. Eichmann and Schidlowski (1975) found the mean difference between the 6 13Cvalues o f inorganic and organic carbon in a group of 22 Precambrian stromatolitic carbonates to be about 3 ' / ~ greater than the average for Precambrian carbonates, but they were reluctant to attribute any significance to the discrepancy.

APPENDIX

2

Australian stromatolites for which new analytical data are presented in the text Sample

Reference No.

Formation

1 2 3

CMC ao CMC 68 CMC 37

Flaggy Limestone Member, Cavan Limestone. Munumbidgee Group

Taemas. N.S.W.

375 (Early Dev.)

4

S 105

Wilkawillina Limestone, Hawker Group

Wilkawillina Gorge, S.A.

565 (Early Cambrian) cumulate

Wonoka Formation, Wilpena Group

Bunyeroo Gorge, S.A.

“Etina Formation”. Umberatana Group

Kulpara, S.A.

5 6

WP 22

S 418

Location

Estimated age (m.y.)

0 r 0 0 4

Form

Lithology

Source

crenulate linked cumulate stratiform

carbonate carbonate carbonate

author author author

carbonate

M.R. Walter

E

n

Tungussia inna

carbonate

W.V. Preiss

0

650

Kulparia kulparensis

carbonate

W.V. Preiss

4

700

Gymnosolen ramsayi

carbonate

W.V. Preiss

Acaciella a u s t r a l i p

carbonate

M.R. Walter

Baicalia burra

carbonate

W.V. Preiss

carbonate

W.V. Preiss

Tapley Hill Formation. Umberatana Group

Wilson. S.A.

8

S 147

Loves Creek Member. Bitter Springs Formation

Williams Bore. N.T.’

9 10

S 405 S 172

11

09

Depot Creek, S.A.

cryptalgal laminate

chert

author

12

08

Mundallio Creek, S.A.

cryptalgal laminate

chert

author

13

WP 64

carbonate

W.V. Preiss

800

Depot Creek. S.A.

? Skillogalee Dolomite,

Depot Creek, S.A.

800

? Tungussia wilkatanna

unknown

Burra Group

? 800

14

MRW 12 (2/9/73)

Tooganinie Formation. Mc Arthur Group

Top Crossing. N.T.

15

s 95

Pillingini Tuff, Fortescue Group

Mount Herbert, W.A.

%

600

S 388

Skillagalee Dolomite, Burra Group

2 v 0

7

>

w

? Baicalia f.

1,600

Conophyton f.

carbonate

M.R. Walter

2.200

Alcheringa narrina

carbonate

M.R. Walter

E:

E



Chapter 5.1 THE ORIGIN AND DEVELOPMENT OF CRYPTALGAL FABRICS C.L.V. Monty

INTRODUCTION

The multiplication of studies on carbonate-building non-skeletal algae has led to the discovery of a wide range of algal structures which cannot always be called “stromatolite” if we want t o keep the meaning of this word within reasonable limits; that is why, following Aitken’s definition (1967),the term cryptalgd will be used t o designate all these biosedimentary structures which originated through the sediment-binding and/or the mineral-precipitating activities of non-skeletal algae, mainly blue-green algae, and of bacteria. Such cryptalgal structures are quite diversified in the geological column, and the purpose of this chapter is not t o account for all of them, but just to illustrate, with examples from the Recent, the way some of their fabrics originate. The study of cryptalgal fabrics is but one of the multiple approaches to stromatolitic carbonates, an approach which ranks third in the hierarchy of characteristics of these sediments: the stromatolite bioherm or biostrome the stromatolite* the fabric the m icrostruc tu re the ultrastructure

refers to the overall deposit refers to the individual cryptalgal structures constituting the deposit refers t o internal spatial properties of these structures such as the development of a lamination refers to the microscopic characteristics of the internal properties refers to the habit and arrangement of cryptocrystalline units

In a first approach, cryptalgal fabrics may be divided into two main groups:

* Sensu Kalkowski (1908)

194

C.L.V. MONTY

(1)the layered fabrics and (2) the non-layered fabrics according t o the presence or the absence of a conspicuous biosedimentary layering. LAYERED CRYPTALGAL FABRICS

General features Layered fabrics characterize three main types of cryptalgal deposits : Stromatolites, i.e., domal, club-shaped, columnar, etc., structures characterized by a generally non-planar lamination ranging from a few microns to a few millimeters, possessing welldefined threedimensional boundaries and which grow attached t o the substrate. Oncolites, i.e., unattached subspherical, ovoid, lobate or flattened structures otherwise similar to stromatolites. Cryptalgal laminites, i.e., laterally continuous stratiform deposits characterized by a subcontinuous planar lamination ranging from less than a millimeter t o several centimeters. Two main types of layered fabrics can be recognized: (1)The laminated fabrics are characterized by a prominent microstratification resulting from the superposition of well-defined individual layers or laminae of similar or different composition, microstructure, origin and/or color, separated, in most places, by a clear physical discontinuity, or microdiastem. This subtle notion of “discontinuity” should however be properly appreciated; in many cases the laminated fabric does not result from the stacking of individual laminae, but, for instance, from rhythmic changes affecting such features as the mineralization of an otherwise continuous algal thallus, producing stripes imprinted on a continuous framework; when this can be shown, it may be useful to speak of a striped laminated fabric. ( 2 ) The laminoid fabrics show no laminae, but the presence of well or poorly aligned structural features which tend t o outline an overall layering. Among other things, laminoid fabrics may result from: (a) the repetitive development of generally elongate primary or penecontemporaneous cavities or fenestrae (Tebbutt et al. 1965), open or filled with sediment and/or cement - the laminoid fenestral fabric; (b) a non-gravity-driven common orientation of elongate particles parallel to an overall lineation which pervades the cryptalgal body, showing that they have been bound by, or glued to, successive algal mat surfaces - the laminoid boundstone fabric;and (c) an irregular juxtaposition and superposition of algal cushions or mats much smaller than the width of the stromatoid -the lenticular laminoid fabric. The layering, which is one of the basic features of stromatolites, records a rhythmicity which affects the development of the constitutive algal population and/or one or several environmental parameters. The characteristics of the rhythmicity determines the organisation of the laminations and

FABRIC AND MICROSTRUCTURE

195

- B t C

....................... I

D

E

F

Fig. 1. Illustration of some frequent modes of organization of laminae in stromatolites: A, simple repetitive lamination; B, C, composite repetitive lamination; D, simple alternating lamination; E, composite alternating lamination; F, cyclothemic lamination.

the following modes can be distinguished: (1)Repetitive lamination. Simple: superposition of laminae of similar nature and configuration, separated by physical discontinuities (Fig. 1A). Composite: superposition of groups of laminae separated by physical discontinuities which determine the prominent lamination; may also result from the superposition of microstromatolites (Fig. lB, C). ( 2 ) Alternating lamination. Simple: alternation of two types of laminae texturally and/or mineralogically different (Fig. 1D). Composite: alternation of two types of layers, one or the two of which contain a second-order lamination (Fig. 1E). ( 3 ) Cyclothemic lamination. Succession of at least three different laminae which always appear in the same order and which can be grouped into genetic sedimentary units (Fig. 1F; Monty and Hardie, Ch. 8.6, Fig. 8). Besides the mode of organization of the lamination, the microstructure, the configuration, the linkage, the spacing, the relief of individual laminae are other important properties that have been reviewed and classified by Hofmann (1969a, Figs. 8,13) and will not be considered here.

196

C.L.V. MONTY


197

Origin o f layered cryptalgal fabrics

A number of biological, geochemical, physical and sedimentological factors account for the development of lamination in cryptalgal structures. Some of them are explained here to illustrate the diversity of the processes involved. Phototactic response o f the component organism Many motile microorganisms are characterized by daily movements in accordance with diurnal light variation. Such phototactic movements can induce the development of a primary biological lamination. Interesting examples have been described in colonies of Schizothrix calcicola sensu Drouet (Monty, 1965b, 1967; Gebelein, 1969) now classified as Phormidium hendersonii Howe (Golubic and Focke, 1976). During the day, upward gliding trichomes produce an organic lamina, up to 900pm thick, made of erect bundles of filaments; at night, filaments grow prostrate and at a much slower rate forming a thin dark organic lamina (100 pm thick) (Fig. 2A, B). This biological lamination may be outlined by sedimentary particles in environments characterized by low, more or less continuous sedimentation rates: during the day, the rapidly growing mat incorporates detrital particles that will remain scattered so that a definitely hyaline layer is formed; at night, on the contrary, as the filaments grow horizontally and much more slowly, the detrital particles can be concentrated in a well-defined corpuscular lamina (Fig. 2C, D). During the next day, the filaments will permeate and bind these grains firmly to grow a new hyaline lamina. The development of such organosedimentary laminated fabric relies on a balance between algal growth and sedimentation rate: if the latter is too low, a purely organic biolaminated structure is formed; if on the contrary it is too high, a massive boundstone is formed (Fig. 2E). Similar phototactic responses have been observed by Walter et al. (Ch. 6.2) in the case of siliceous flat-topped stromatolites built by Phormidium tenue var. granuliferum Copeland in Yellowstone National Park; laminae are however much thinner here. A reverse situation has furthermore been found by Fig. 2. Laminated domes and mats built by Phormidium hendersonii (formerly Schizothrix), Windward Lagoon, Andros Island, Bahamas. A. General view of biolamination in a sediment-poor colony; thin dark films, built at night, alternate with wider light laminae formed by erect phototactic filaments during the day. Scattered detrital particles appear black. Plane polarized light. Scale bar = 1mm. B. Detail of biolamination. Plane polarized light. Scale bar = 100pm. C. Section of a dome in which the balance between algal growth and sedimentation rate originated a well-laminated fabric due to concentration of detrital particles in biolaminae built at night (see text). View of impregnated slab. Scale bar = 400pm. D. Close up of Phormidium filaments binding detrital particles. Plane polarized light. Scale bar = 100 pm. E. Section of a dome swamped by intensive sedimentation. Basic biolamination is no longer outlined but is obliterated by oversedimentation, and a massive fabric tends to develop. Plane polarized light. Scale bar = 500pm.

198

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Doemel and Brock (1974) in stromatolites built by the photosynthetic filamentous bacterium Chloroflexus: as this bacterium responds to low light and darkness rather than to high light intensities, it migrates upward during the night, building a lamina about 100 pm thick, and grows horizontally during the day.

Alternating growth pattern of the component organism Some algae have a thallus combining both horizontal and vertical growth. This is for instance the case for Scytonema (Monty, 1965a, 1967; Golubic, Ch. 4.1, fig. 14). A laminated pattern can thus result from the alternation of phases showing horizontally growing filaments, with phases characterized by vertical bundles of filaments (Fig. 3).

Fig. 3. Vertical section in mat built by calcified filaments of Scytonema, subterrestrial, eastern Andros Island, Bahamas. Biolamination results from alternation of horizontally growing filaments with layers of erect bundles. Plane polarized light. Scale bar = 500pm.


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Periodic differentiation of an algal assemblage leading to an alternation of dominant algae Not infrequently, algal mats contain a pool of organisms which do not bloom at the same time or in the same conditions. This enlarges the adaptive range of the community and permits its perpetuation; by the same token, the periodic differentiation of one or another dominant organism may produce a laminated fabric in the mat. Interesting illustrations of such complex behavior can be found in the laminated mats built by the Scy tonema-Schizothrix calcicola association on eastern Andros, Bahamas (Monty, 1965a, 1967); these algae bloom in different conditions, as Scy tonema is a freshwater terrestrial alga whereas Schizothrix calcicola is more successful in flooded conditions and withstands saline waters. In vertical section, these mats and domes invariably show an alternation of whitish calcareous layers and brownish organic ones. This constant fabric is however genetically very complex and may result from at least three different behavior patterns of the Scy tonema-Schizothrix community, hence of three different mat structures: (1) A first type of mat (Monty, 1967, type 3) (3-4cm thick, 20-50cm wide) is composed of calcified layers from 5 m m thick at the surface compressing to 1mm thick at depth, built mainly by calcareous tubes of Scytonema; this alternates with hyaline layers (30-300 pm) made by dense growths of interwoven filaments of Schizothrix (Fig. 4E). These mats were found growing on rocks and on the mud of sinkholes bordering the Fresh Creek (Andros Island). Their genesis can be traced as follows: during the dry season, Scytonema grows at the surface of the mat either erect and vigorously if substratal moisture is important or sluggishly and oblique during drier intervals. A t this stage, Schizothrix concentrates beneath the surface, finding in between the bundles of Scytonema the moisture which enables it to survive. A calcified layer is then built; it results from the calcification of the sheaths of Scytonema and from the abundant precipitation of calcite in the intervening mucilages and the overgrowing felts of Schizothrix. When the mats are temporarily soaked by saline waters (* 20-30°/,) during a storm or an equinoctial high tide, Schizothrh rises to the surface of the mat and blooms; it does not calcify in such waters, however, and a loose organic layer is formed. After such an accident the salinity is progressively reduced by ground water and rain, and the mat progressively returns t o subaerial drier growth; Schizothrh recedes while Scy tonema starts producing a new calcified layer. The organic layers compact rapidly at depth where they are reduced t o brownish subhorizontal wavy streaks. (2) A second type of mat (Monty, 1967, type 2), built by the same community, shows the reverse situation, in which calcified layers are built by Schizothrix whereas the hyaline layers are built by Scytonema. In vertical section, it demonstrates an alternation of composite, micritic laminae

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(40-400pm thick) crowded with Schizothrix, alternating with irregular hyaline layers (20-800pm thick) built by erect bundles of Scytonema (Fig. 4B). In August 1964, such structures were observed actively blooming in sinkholes along the south shore of Fresh Creek in 1or 2 cm of brackish water (10-13°/oo) : here, the tops of submerged older algal polygons were covered with greenish brown mats, 2-4 cm thick, composed of a pile of wrinkled films of Schizothrix (about 1 0 p m thick) between which many gas bubbles and eventually colonies of unicells were entrapped. The films tended to be grouped into sets of 4-8, separated by thicker hyaline laminae (Fig. 4A). Each film was made of horizontal,tangled felts of Schizothrix loaded with micron-sized calcite crystals (Fig. 4F). The uppermost films contained motile reproductive units (hormogones) of Scytonerna, as well as very short filaments and isolated cells of other algae (Fig. 4D,F). On the other hand, the underlying films and interspaces were inhabited by fully mature algae including, in the wider interspaces, long greenish t o colorless erect filaments of Scytonerna. This microstructure reflects the following genesis: when the dormant mat is soaked or flooded by slightly saline waters, Schizothrix finds there ideal conditions, and blooms. By active division and phototropism it starts building subdaily films which pile on top of each other while calcium carbonate is precipitated in the mucilaginous felts. Metabolic gas bubbles accumulate between the films and distort the microstructure a little. The associated algae (Scytonerna and some Lyngbya), characterized by a generally much slower growth rate, respond t o burial under several hundred microns of calcite and mucilage by sending hormogones upward. These, as well as young unicellar stages, settle in and between the films where they Fig. 4. Laminated mats built by Scytonema and Schizothrix, subterrestrial, eastern Andros Island, Bahamas. A , B, C, D, F, mats controlled by Schizothrix, E, mat controlled by Scytonema. A. Rapid succession of thin, calcite-rich films of blooming Schizothrix (see F) forming a set overlying a hyaline layer crowded with Scytonema. Plane polarized light. Scale bar = 50pm. B. Detail of resulting lamination: calcitic laminae formed by compacted sets of films of Schizothrix alternate with hyaline layers of internally growing non-calcified filaments of Scytonema. Plane polarized light. Scale bar = 100pm. C. General view of resulting lamination. Some pellets are formed in situ by calcification of colonies of unicells and diatoms, some are left by burrowing organisms. Plane polarized light. Scale bar = 200pm. D. Growing hormogones sent by Scytonema into the Schizothrix surficial bloom. Plane polarized light. Scale bar = 2 5 pm. E. Section in mat controlled by Scy tonema : calcareous layers, made of erect calcified filaments of Scytonema, alternate with hyaline discontinuous ones built by Schizothrix. Plane polarized light. Scale bar = 500 pm. F. Smear of a surficial film of blooming Schizothrix: (see A) eu- and anhedral low-Mg calcite crystals are precipitated in the felt of filaments. In the upper right corner, there is a young stage of the blue-green Johannesbaptistia pellucida (Dickie) Taylor et Drouet. Plane polarized light. Scale bar = 20 pm.

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soon divide; thus progressively more mature stages are found from the surface downward. Blooms of Schizothrix are, however, not continuous, as seen in subtidal counterparts (Monty, 1967, p. 91); they may stop for some time during which the surface becomes colonized by “competing” algae; after a while, Schizothrix growth resumes and a new set of calcified films is formed. In the meantime, the young Scytonema which have settled in between the films, or rather the sets of films, keep on growing and expand these spaces into hyaline layers; as observed by the author (see also Schonleber, 1936), when Scytonema grows under water or separated from the air interface it may not calcify (see also below). During growth, the superposed films of Schizothrix progressively compact into thicker micritic composite layers, as the metabolic gases escape, while the intervening mucilages are consumed. A laminated mat composed of calcified and hyaline organic layers is formed (Fig. 4C). (3) A third type of mat, very common in the freshwater lakes of Andros (Monty, 1972), can be built during another behavior of the community. These mats show an alternation of calcified and hyaline layers built by Scytonema and Schizothrix (Fig. 5 ) . A cross-section of the living mat, examined at the end of the seasonal flood when algal structures emerge, shows the living portion, up to 3 cm thick, resting on a subfossil “basement” made of alternating light tan calcareous laminae and brownish organic ones. The living portion can be split into two parts. There is an upper spongy calcareous lamina up to 2 cm thick, made of calcified long vertical bundles of brownish Scytonema which may project above the surface, and which is zoned by horizontal calcified film of Schizothrix; the overall original fabric is thus reticulate. The lower part shows a loose green layer up to 1.5 cm thick with erect non-calcified filaments of Scytonema intermingled with films and draperies of Schizothrix delineating large vacuoles. Field observations show that the calcified lamina forms when, as the freshwater level drops, the mat emerges or is about to. In such conditions, Scytonema actually starts its freshwater subaerial growth phase and builds long filaments encased in a calcified sheath; as the mat is still fully soaked with water Schizothrix can equally grow and concurrently build its calcified films, so that a reticulate pattern is formed; spaces between the successive films can get wider and wider as the Scytonema filaments separating them grow. Beneath this surficial layer, life conditions for both algae are still good as shown by the deeply pigmented green filaments and the abundance of gas vacuoles; but conditions are not suitable for calcite to develop. (In this somewhat closed environment choked by mucilage and the overlying layer, any carbonate precipitated during the day may dissolve at night when the cells are only respiring.) This *layer remains mainly organic, except for scattered flocs of calcite. At the end of the growth period, both laminae collapse and compact severely (at least ten times) and the habitual laminated fabric of thicker calcareous layers separated by thinner organic ones is formed. In this case, as


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Fig. 5 . Section in a dome built concurrently by Scytonema and Schizothrix, freshwater marshes, eastern Andros Island, Bahamas. Gray to white calcareous layers show a reticulate microfabric resulting from overlap of erect calcified bundles of Scytonema, perpendicular to growth surface, with calcified films of Schizothrix parallel to this surface. Dark hyaline layers are built by internally or subaqueously growing phases of Scytonema. Note severe compaction of laminae inward. Polished impregnated slab. Scale bar = 1cm.

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in the first one, fragments of calcareous sheaths can still be found, but the original reticulate microfabric is completely obscured. These three cases show how subtle variations in moisture and salinity can contra1 differentiation of an algal consortium to finally originate a similar laminated fabric, although through different pathways.

Internal growth Geologists are prone to interpret the succession of stromatolitic laminae in stratigraphical terms, i.e., a lamina is younger than the one it overlies. The last two examples of the previous paragraph have shown that this is not always true and that algal layers can grow inside the mat. Many other examples have been found on Andros Island (personal observations). Generally, in algal mats submitted to intense insolation, the surficial layer becomes strongly pigmented to shield the cells from radiation, and/or very

Fig. 6 . Rivularia haematites, Lunzer Untersee, Austria. Development of striped laminated fabrics by periodic calcification. A. General view of a small colony showing dark concentric calcified stripes. Plane polarized light. Scale bar = 500pm. B. Detail of striped fabric; elongate, tubular, calcite crystals, oriented on the filaments, form conspicuous bands appearing dark. Interference contrast. Scale bar = 50 pm. C, D. Detail of calcified zones; C, view parallel to the filaments of elongated crystals surrounding the filaments; D, oblique view. Cross polarized light. Scale bars = 100 pm.


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compact t o prevent excessive evaporation. In this case it is not rare that, in zones where the substratal humidity concentrates, hormogones or dormant cells start actively to grow, originating the development of interstratified organic layers and a thickening of the mat from the inside (Monty, 1965b). Periodic calcification of dominant alga Periodic calcification of blue-green algal mats relies on patterns and processes which are still poorly understood today. If in some structures, one type of crystal habit seems to be preferentially deposited, in others a variety of calcite crystals appear t o be simultaneously precipitated in the mat (see for instance Monty and Hardie, Ch. 8.6, Fig. 4). In calcareous freshwater streams and lakes, a collection of blue-green algae are building laminated crusts or cushions resulting from the periodical deposition of calcite crystals organized into distinct concentric layers; this is the case for several species of

Fig. 7. Development of striped o r laminated fabrics by periodic calcification. Section of a laminated fluviatile calcareous crust built by Phormidium incrustatum (Ruisseau du bois d’Haumont, Belgium). Lamination results from alternation between densely calcified zones and loosely calcified ones (see detail on Fig. 8D). Aligned fenestrae ( f ) are tubes of chironomids. Plane polarized light. Scale bar = 200 pm.

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Schizothrix, Phormidium, Calothrix, Rivularia, etc. (e.g., Fritsch, 1929,1949, 1950; Geitler, 1932; Kann, 1933, figs. 7, 8, 11, 1941b). Two examples will be briefly illustrated here. The first concerns R iuularia haematites De Candolle, which builds strongly calcified dome-shaped colonies up t o 3 cm across and to 3 cm thick (Fritsch, 1950) in the splash zone of calcareous lakes. A cross-section in the colony reveals its typical striped fabric (Fig. 6A, B): the thallus, made of radially growing filaments, shows concentric bands of interlocking elongate crystals encasing the algal sheaths and alternating with non-calcified zones (Fig. 6C, D; see also Wallner, 1935). There are normally 3-4 calcified zones in colonies (Kann, 1933) but Geitler (1932) has counted up t o 33 zones in old ones. The reasons of this periodic precipitation are not yet clearly known. The second example refers to crusts built by the Phormidium incrustatum (Nageli) Gomont community in calcareous rivers (Fritsch, 1929 et seq.). In vertical section (Fig. 7), these crusts, up to 1 0 c m thick, show a prominent lamination resulting from alternating densely calcified and loosely calcified filamentous layers (Fig. 8D). Palynological studies on Belgian crusts (in the Ruisseau du bois d'Haumont; Geurts, 1976) permitted dating of the respective layers and showed that the lamination was seasonal. Between February and May, when the water is cold (ca. 4"C), Phormidium blooms and grows closely adpressed, radial filaments to form coherent calcareous layers up to 5 mm thick; the component filaments are heavily incrusted with small equant or rhombic crystals of low-Mg calcite. Then, as the water gets warmer (up to 16"C), between May and January, growth decreases and the filaments aggregate into loose and scattered calcified bundles separated by wide cavities. ~-

Fig. 8. Microfabric variations in laminated crusts of Phormidium incrustatum. A. Detail of lamination showing densely calcified flabellate tufts of Phormidium overlain by a loosely calcified layer, rich in detrital particles; films of Schizothrix form scattered concentric dark zones in the Phormidium lamina (second-order lamination). Here, calcification involves the precipitation of interlocking, elongate crystals on the filaments of Phormidium (see B, C) as in Riuularia (Hoyoux river, Belgium). Plane polarized light. Scale bar = 500 pm. B. Oblique section through crust illustrated in A, showing the interlocking crystals encasing the Phormidium filaments in two calcified layers sandwiching a poorly to noncalcified one (which appears black). Moulds of the filaments appear as black dots in the lower part of the photograph whereas longitudinal sections are seen in the upper one. Cross polarized light. Scale bar = 100 pm. C. Calcified tuft of Phormidium, illustrated in A, showing the elongate oriented crystals. Cross polarized light. Scale bar = 500pm. D. Detail of lamination of the Phormidium head illustrated on Fig. 7. From base to top: end of heavily calcified spring layer, passing to fenestrate loosely calcified summer layer, which grades into another densely calcified growth. Although growth appears continuous here, many cases exist where clear-cut discontinuities separate the different seasonal layers. Calcification results here from precipitation of small anhedral calcite crystals clustering around and between the filaments. Plane polarized light. Scale bar = 200 pm.

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Accordingly, a doublet, formed by a strongly calcified layer followed by a loosely calcified one, is deposited each year and a radial filament microstructure is generated in the component laminae (Fig. 8D). Precipitation o f crystals with different habits Seasonal variation in the algal coenose may originate the alternate precipitation of very different types of calcitic crystals. This is for instance the case for particular crusts built by Phormidium incrustatum and Schizothrix sp. in the Hoyoux river, Belgium. Here, the calcification of the Phormidium layer during each spring appears t o be much stronger than in the case reported above: instead of being surrounded by a microcrystalline sheath the filaments are encased in tubular interlocking elongate crystals (Fig. 8B,C) as was the case for Riuularia. Because of the flabellate, radial, growth of the filaments (Fig. 8A) these crystals tend t o form juxtaposed or intertwined composite fans. Such crusts may be periodically invaded by blooms of Schizothrix (minor invasions are shown on Fig. 8A) associated with the precipitation of small (less than 10pm) anhedral calcite crystals. Consequently, a vertical section of the crust will show a prominent lamination resulting from the alternation of microcrystalline laminae with layers of radiating elongate crystals which originates a crystalline laminated fabric (Fig. 9). Primary crystalline fabrics may be even more complex as is shown by crusts up to 20 mm thick built, in the same river, by a community including small oscillatoriaceans, abundant coccoids, and bacteria. The basic fabric, that may be called crystalline laminoid fabric (Fig. lOA), results from the alternation of rather porous layers, made of coarse-bladed crystals (up to 80 pm long and 30 pm wide) growing in all directions, with compact microcrystalline laminae (Fig. 1OC). Continuous observations revealed that clusters of large platy crystals, interspersed with mucilaginous matter and aggregates of unicells, were initially growing in horizontal subcontinuous cavities roofed by the living algal film, rich in coccoids, and floored by the older parts of the crust over which the organic film draped loosely (Fig. 10B). Such crystal aggregates probably formed by bacterial action on algal mucilage (W. Krumbein, pers. comm., 1975). As precipitation proceeds, the clusters of crystals become larger and more numerous to finally interlock and form a crystalline lamina such as illustrated on Fig. 1OC. These coarse-grained layers alternate with microcrystalline laminae deposited periodically in association with Schizothrix as in the Phormidium crusts (p. 207). Furthermore, erratically or along given horizons, the coarse crystalline laminae are invaded by microcrystalline equant crystals which are deposited on the blades and tend t o fill all the interstices, obscuring the coarse crystalline fabric; these microcrystals are here also a'ssociated with small oscillatoriaceans which colonize and overgrow the surfaces of the large platy crystals. All these interactions account for the resulting poor lamination of the crust (Fig. 10A).


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Fig. 9. Development of crystalline laminated fabric in a crust built by alternation of Phormidium layers, showing fan-shaped tufts of elongate crystals (as in Fig. 8C) with microcrystalline laminae (M.L. ) associated with Schizothrix (Hoyoux river, Belgium). Cross polarized light. Scale bar = 100pm.

Periodic enrichment in various chemicals Independently of the situation found in manganese nodules, cryptalgallaminated or striped fabrics may result from periodic enrichments in various mineral compounds of, for instance, iron and manganese. This is the case for the crusts reported by Kann (1940) from the shores of the Kellersee. These crusts are built by an intermingled continuous growth of Phormidium

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incrustaturn and the red alga Chantransia incrustans (Hansgirg); the lamination results from alternating light and dark bands, the latter representing periods of important enrichment of the mat in iron hydroxide (which may be related to the iron-rich springs surrounding the lake). Similar bandings have also been reported by Naumann (1924,1925)from globular colonies of Nostoc colonizing the surficial waters of volcanic African lakes; in the larger colonies, these bandings are repeatedly found every two millimeters and result from a heavy impregnation of the mucilaginous algal sheaths by ferric hydroxides. Finally, periodic enrichment in iron and manganese have also been reported by Howe (1931)from crusts and biscuits built by blue-green algal unicells in a creek in West Virginia, U.S.A. Periodic influx o f detrital particles Many blue-green algae, mainly oscillatoriaceans like Schizothrix, Phormidiurn, Microcoleus and Lyngbya are motile and are positively phototactic. They usually build patchy or subcontinuous mats and, because of their motility, they can glide up through a layer of sediment recently deposited on them t o re-establish a new surface mat, leaving empty sheaths, mucilaginous matter and dead cells behind. This way laminated fabrics can result from concurrent algal growth and sediment deposition. This has been illustrated previously in the case of discrete colonies (p. 197; see also Gebelein and Hoffman, 1968;Gebelein, 1969). Algal mats growing in shallow subtidal waters (Dalrymple, 1965)and particularly on intertidal and supratidal flats are faced with various periodical influxes of detrital particles, ranging from daily t o seasonal and yearly (e.g., Ginsburg et al., 1954; Hommeril and Rioult, 1965;Kendall and Skipwith, 1968;Davies, 1970).The thickness, texture and composition of the various constitutive corpuscular layers will vary as a function of the hydrodynamical agents which deposited them (e.g., tides, storm waves and hurricanes) as these agents have different frequencies and capacities, act during shorter or longer periods of time, and may drain particles from different regions. Furthermore, the superposition of these events will eventually produce several orders of laminae grouped into cycles (Dalrymple, 1965, fig. 40; Davies, 1970,fig. 11). Fig. 10. Development of crystalline 1arninoid.fabric in crusts built by unicells and oscillatoriaceans (Hoyoux river, Belgium). A. General view of part of the crust showing coarsely crystalline layers interspersed with microcrystalline laminae. Plane polarized light. Scale bar = 500 ym. B. Incipient coarsely crystalline layer. Elongate calcitic blades are being precipitated in the mucilaginous interspaces crowded with unicells, delimited upward by an arched film of unicells and oscillatoriaceans (dark streak) and downward by the surface of the solid crust. Plane polarized light. Scale bar = 100 ym. C. Detail of resulting fabric: contact between a coarsely crystalline layer, made of clusters of calcitic blades such as those forming in B, and a microcrystalline layer associated with Schizothrix. Cross polarized light. Scale bar = 100ym.

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Two main types of cryptalgal laminites can be separated as a function ot their lithotope and of climatic conditions: (1) On intertidal and supratidal flats of arid regions algae generally grow uncalcified leathery films or mats; they may be interspersed with detrital sands, .silts and eventually muds which may mold some of the filaments; when buried under sediment, these mats shrink and collapse t o form organicrich layers or streaks alternating with sediment-rich layers (Fig. 11).

Fig. 11. Cryptalgal laminite; laminated fabric results from alternating thicker, sedimentrich, layers and organic-rich dark laminae. Intertidal zone, Abu Dhabi, Persian Gulf. Courtesy of B. Purser.

(2) On supratidal flats from maritime tropical regions (e.g., Florida, Bahamas), submitted t o seasonal flooding by freshwater during the rainy season, the algal mat filaments may calcify as they grow, and very distinctive cryptalgal laminites are formed (Monty and Hardie, Ch. 8.6); the resulting deposit shows among other things, a prominent lamination resulting from the


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alternation of micritic layers of calcareous filaments, that formed during the rainy season, with layers of pellet packstone of wackestone deposited during hurricanes or severe storms (cf. Monty and Hardie, Ch. 8.6, Fig. 9). However, lamination of cryptalgal laminites is not always so clearcut and regularly bound to a small number of well-defined cosmic events. In many cases, it appears to be somewhat erratic and results from irregular variations in the organic matter/sediment ratio of thk successive laminae. Indeed, intertidal algal mats do not grow at the same rate throughout the year; they may show periods of rapid growth and periods when it slows down depending on the growth rhythm of the component algae or the onset of particular local conditions (e.g., dryer periods, higher salinities). Similarly, the amount of detritus shifted on t o the mats per unit time varies cyclicly (in response to normal tides, neap tides, spring tides and so on) or erratically (e.g., in response to accidental storms or stronger winds). In these conditions, subcontinuously growing algal mats will not show such a distinct lamination as illustrated on Fig. 11, but a succession of zones with different organic matter/ sediment ratios or with different detrital grain sizes. If cementation is rapid, a laminated fabric will result from the differentiation of laminae with different packing or size of their constitutive grains (Fig. 12).

Fig. 12. Laminated fabric resulting from cyclic changes in granularity and packing of bound grains. Lower intertidal zone, Shark Bay, Australia. Plane polarized light. Scale bar = 500 pm.

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There are, finally, cases where the production of laminae is completely erratic and episodic. An example is the mats built by Schizothrix, Scytonema, and Microcoleus on the intertidal flats of tidal swamps, eastern Andros (Monty, 1965b, 1967). Beside normal regular corpuscular laminae, these mats show many discontinuous, highly variable layers of detrital particles which result from very local and accidental factors. Adjacent mats, or even parts of mats, may be building hyaline algal layers in one place and binding detrital grains in another. Continuous observations revealed some of the responsible accidental factors: during high tides, the flats are covered by 10-20 cm of water harbouring puffers, voracious fishes common in mangrove channels and ponds. When disturbed by predator or prey, they rapidly swim out of their resting place, stirring up local clouds of sediments that will be redeposited on the surrounding mats to form a local detrital lamina. Another example is provided by burrowing annelids which build conical mounds on the flats, amid the algal structures. Muddy and silty waters expelled from active mounds flow down the cones and deposit their load over the adjacent mats.

Periodic inorganic cementation Periodic cementation due to change in (micro)-environmental,geochemical or biochemical conditions may result in an alternation of lithified and nonor poorly lithified laminae and hence in the development, or the re-inforcement, of a laminated or laminoid fabric. This is true of well-lithified structures growing in the lower intertidal zone at Carbla Point (Shark Bay, Australia), where calcareous particles, mainly ooids and bioclasts, are bound by filaments of Schizothrh (Fig. 13A). Specimens studied show a laminoid fabric resulting from the alternation of cemented layers of packstone about 0.5mm thick, with uncemented grainstone layers up to 3 mm thick (Fig. 13B, C). Cemented layers show generally a much closer packing of grains than the non-cemented ones where microstructure is looser. Detrital grains are embedded into a micritic aragonitic matrix (Fig. 13D, E) rich in organic matter, at least in the Fig. 13. Laminated fabric resulting from periodic cementation. A. General view of a poorly laminated dome built by Schizothrix sp. (so-called “smooth mat”) lower intertidal zone, Carbla Point, Shark Bay, Australia. Impregnated sample. Scale bar = 1cm. B. Detail of lamination: contact between a non-calcified grainstone lamina (lower half) and a strongly cemented packstone lamina (upper half ). Plane polarized light. Scale bar = 200 pm. C. General view of lamination. Beside differential cementation, a laminated fabric also results from differential packing which is usually much closer in the dark cemented laminae. Plane polarized light. Scale bar = 2 mm. D. Detail of an initial stage of cementation of grains by fibrous aragonite. Note relationships between packing and the development of the fibrous aragonitic rim. Cross polarized light. Scale bar = 100 pm. E. General view of cemented lamina with high porosity. Cross polarized light. Scale bar = 200 pm.

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upper layers, as shown by histological stains. Locally the cemented layers may dichotomize and surround small fenestrae. Grains are abundantly bored and micritized in contrast with those in the non-cemented layers. Cementation of grains comprises the deposition of fine rims of acicular aragonite around the ooids (Fig. 13D) and the filling of the interstices by microcrystalline aragonite (Fig. 13E). Meniscus cements are also found. Another occurrence of periodical cementation is found in the organosedimentary laminated crusts built by Scytonema along western Andros, Bahamas (Monty and Hardie, Ch. 8.6); here, the cement is a high-Mg calcite.

D iagenesis Variations in the chemical composition of (parts of) laminae, such as the presence of higher-Mg calcites or higher concentrations in magnesium in given algal sheaths, may not be conspicuous in fresh specimens unless these zones coincide with lithified laminae, as is often the case along western Andros (Shinn et al., 1969; Gebelein, 1975). During diagenesis deeper reactions can cause either the differentiation of dolomitic laminae alternating with calcitic ones, as indicated by Gebelein and Hoffman (1969), or the systematic dolomitization of parts of laminae as illustrated on Fig. 14. Alignment o f particles When the surface of an algal mat is smooth, i.e. when the constitutive filaments are small and have a regular tangled mode, elongate free-falling particles being deposited on the sloping flanks of the structure will be submitted to gravity forces as well as t o the “retention” forces of the sticky surface of the mat; as a result of the combination of these two forces, larger particles will orient themselves with their long axis parallel to the slope, i.e. to the growth surface of the mat. These particles will soon be bound in that position by overgrowing filaments, and incorporated into the structure. The continuation of this process will lead t o the development of an overall lineation and hence of a laminoid boundstone fabric (Fig. 15). A lignmen t o f fenestrae A fenestra has been defined by Tebbutt e t al. (1965, p.4) as a “primary or penecontemporaneous gap in rock framework, larger than grain-supported interstices (. . .) The distinguishing characteristic of fenestrae is that the spaces have no apparent support in the framework of primary grains composing the sediment”. Fenestrae are quite frequent in cryptalgal structures Fig. 14. Diagenetic development of laminated fabrics. Detail of lamination showing a microcrystalline algal layer, with filament molds, confined between two dolomitic laminae. Systematic gradient in the density of dolomitic rhombs (greater just above the algal layer, then diminishing progressively) probably reflects clines in the initial concentration in magnesium of the structure. Ordovician stromatolite. Arbuckle Mountains, U.S.A. Cross polarized light. Scale bar = 1 mm.

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Fig. 15. Outlining of laminoid fabric by commonly oriented elongate particles. Steep flank of a lower intertidal dome built by Schizothrix sp., Carbla Point, Shark Bay, Australia. Plane polarized light. Scale bar = 500pm. Ink line on left emphasizes lamination.

and, among other things, may result from: (1)Detachment of the surficial algal mat layers from underlying ones as a result of: (a) Generation of gas bubbles during photosynthesis or bacterial decomposition. The rising bubbles either distort the grain packing and/or accumulate under the surface algal film producing pustules. As predicted by Tebbutt et al. (1965) these bubbles generally extend laterally along physical discontinuities between laminae and originate an alignment of elongate cavities. (b) Shrinkage and drying of the mat. When the mats (described on p. 199) are submitted to intense seasonal desiccation, not only do the calcified layers shrink considerably, but widespread detachment from underlying layers may occur along the organic partings (compacted former hyaline layers) ;aligned elongated cavities appear which generally have smooth roofs and floors. Somewhat more complex situations combining separation of laminae aft9r desiccation and internal algal growth have been observed in laminated algal domes (10-15 cm across and up t o 2 cm thick) from the intertidal flats of tidal marshes, eastern Andros (Monty, 1965a). When prolonged periods of low water expose the mats to partial desiccation, the surficial layers, rich in water and mucilage, shrivel above the underlying more compact ones (Fig. 16). Lenticular cavities with flat floors and convex roofs progressively develop along a depositional surface and range from 400 t o 600 pm in height


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Fig. 16. Formation of fenestrae by detachment of surficial algal layers and internal growth in laminated mats built by Scy tonema, tidal marshes, eastern Andros Island, Bahamas.

and 0.5mm t o several millimeters in length. When the mat is soaked again, these new habitats are rapidly colonized by slender filamentous oscillatoriaceans growing erect filaments from floor t o roof. Vertical pressure resulting from the growth of these algae distorts the enclosing corpuscular layers and enlarges the organic cavities. As incipient micritic cementation occurs in the overlying corpuscular lamina, these internal algal phases will be preserved as aligned fenestrae separated by layers of particulate carbonate (the organic matter in the fenestrae will be oxidized or consumed). It should be noted that all intermediates exist between aligned discrete cavities and situations where internal growth extends subcontinuously throughout the whole mat originating a new lamina as seen on p. 204-205. (c) Active growth resulting in rapid lateral expansion. If the available space is limited, the blooming mat will dome over the older parts of the structure and become wrinkled, circumscribing hollow cavities underneath. Well-developed laminoid fenestral fabrics are found in subtidal heads and columns off Carbla Point (Shark Bay, Australia) (Fig. 17A). They will be described here in some detail as they provide an endpoint of the processes discussed above. The major fenestrae, which are about 5 mm wide and up t o 30 mm long, are arranged in zones parallel t o the surface of the mat, although many have irregular shapes. They are open, or partially or completely filled with sediment. They are surrounded by dense microcrystalline and cemented layers of wackstone (white on Fig. 17A) which emphasize the general laminoid fabric. The cement consists of fibrous aragonite which grows radially

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around the grains (Fig. 17D); the length of the fibres varies from 5 to 10 pm, although larger and smaller sizes are found. The development of the microcrystalline cement around the grains, which are deeply bored and micritized, obscures completely the original texture of the sediment (Fig. 17C). The cemented zones seem to develop from an algal mat (Fig. 17E), part of which domed when actively growing and formed internal cavities which remained as fenestrae; these mats actively bound detrital particles which are closely packed. Cementation must have been rapid as many flocs of organic matter as well as residual filaments have not been completely oxidised and can be extracted from the microcrystalline cement by gentle etching (Fig. 17F). It seems evident that cementation is bound to given algal mat phases, as no significant cement can be found elsewhere in the cryptalgal structure. In between the units composed of the fenestrae and their cemented surrounding laminae, are layers or lenses of unconsolidated sand (Fig. 17B); these are microstratified and show definite boundstone fabrics. The microstratification results from an alternation of grainstone and packstone laminae, 500-1500pm thick, with algal films, 100-250 pm thick; these are not always well preserved and may be recorded as thin micritic films, as clotted or as loose and unsupported laminae. (2) When algal layers, isolated bundles of filaments, or colonies of unicells are not calcified, their organic matter may be rapidly oxidized, mainly in coarse sediments, leaving elongate, irregular, horizontal or vertical cavities (Fig. 19; Monty and Hardie, Ch. 8.6, Fig. 14). A particular type of fenestra with a flat floor and an irregular, saw-toothed roof, quite frequent in the geological record, may be predicted from laminated mats observed in tidal marshes of eastern Andros Island and elsewhere. In cross-section, some of these mats show an alternation between erect growths of Scytonerna and layers of detrital carbonate (Fig. 18A). In many Fig. 17. Development of laminoid fenestral fabrics in a dome built by oscillatoriaceans (Schizothrix, Oscillatoria) in the subtidal zone, Carbla Point, Shark Bay, Australia. A. Laminoid fabric results from alignment of elongate fenestrae and is emphasized by calcified layers (white) which enclose them. Impregnated slab, scale bar = 1 cm. B. Contact between a non-cemented grainstone lamina (appearing medium gray on A) and the top of a cemented fenestral packstone lamina. Fenestrae here are open, which is not always the case. Plane polarized light. Scale bar = 2 mm. c. Detail of a cemented zone roofing a fenestra; note intense micritization and boring of grains (ooids) which fade into microcrystalline aragonitic cement. Cross polarized light. Scale bar = 200 pm. D. Production of microcrystalline fibrous aragonite around micritized grains to yield undifferentiated micritic patches such as visible o n middle left of C. Cross polarized light. Scale bar = 100 pm. E. Vestige of part of a domed algal mat (arrow), in cemented lamina, under which a fenestra formed. Plane polarized light. Scale bar = 100pm. F. Remains of coarse algal filaments extracted from cemented laminae by gentle etching. Plane polarized light. Scale bar = 100 pm.

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9

A

6

1 mm

Fig. 18. Formation of fenestrae with smooth floor and saw-toothed roof after oxidation of non-calcified growths of Scytonema growing o n a depositional surface. Prediction from observation of laminated mats built by Scytonema ; tidal marshes, eastern Andros Island, Bahamas.

cases, Scytonerna forms discontinuous layers with more actively growing tufts here and poorly growing filaments there, although belonging to the same horizon. The uncalcified algal mat grows on the subhorizontal depositional surface of an earlier detrital layer. The top of the algal growth is quite irregular due to the variously projecting bundles of filaments. This surface is accordingly replicated by the base of the overlying corpuscular layer which has a characteristic scalloped and saw-toothed outline. Oxidation of the algal mat and cementation of the corpuscular layers would leave typical fenestrae aslshown on Fig. 18B. (3) Algal or bacterial life within the mat may originate significant changes in COz concentration and accordingly in pH; this may result in solution of carbonate within the cryptalgal structure and the opening of cavities in given horizons (Fig. 20). (4) Animal organisms living within or on the mat may, at their death, leave aligned round or elongate cavities (Fig. 7). This is illustrated by the tubes of chironomids in freshwater algal structures: “they form long cavities up to 5 mm long and 0.5 mm wide (. . .) They are always parallel to the annual layers” (Irion and Muller, 1968, pp. 165-166, their Figs. 4,5). For the fenestrae to be preserved (mainly in processes 1 and 2 described in this section) $he roofing layer needs some degree of coherence - as may be provided by desiccation - and rapid cementation (or cementation of the surrounding sediment).


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Fig. 19. Formation of an open vaulted fenestra after oxidation of a flabellate non-calcified bundle of Scytonema over which a smooth Schizothrix mat draped. Lower intertidal zone. Hamelin Pool, Shark Bay, Australia. Plane polarized light. Scale bar = 500pm.

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Fig. 20. Opening of an elongate fenestra by internal solution of a microcrystalline layer of Scytonema beneath a smooth Schizothrix mat. Note characteristic solution surface corroding the Scytonema flabellate growth. Lower intertidal zone. Hamelin Pool, Shark Bay, Australia. Plane polarized light. Scale bar = 500pm.

Complex or composite lamination Combinations of the previously described processes are frequently met. When they act “in phase”, the resulting fabric will not be more complex than the one yielded by each of them separately; the lamination will eventually be re-inforced. This is, for instance, the case for the Phormidium (Schizothrix) laminated domes described on p. 207; here, the sedimentary event outlines the biolamination resulting from the phototactic behavior of the alga, as detrital particles concentrate in the lamina built by resting filaments. When the combined processes act “out of phase”, a composite lamination is formed. Such a situation can be found in the crusts built by Phormidium incrustatum; the basic fabric made of alternating laminae of strongly and poorly lithified filaments (Fig. 8) can be complicated by the superposition of various processes originating the development of sublaminae (Fig. 21). Among these processes the following can be found (together): (1) periodic blooms of calcified films of Schizothrix across the radial framework of Phormidium (Fig. 8A, 21); (2) the seasonal influx of detrital particles which contaminates the loosely cemented layer and concentrates on top of it (Fig. 21); and (3) the development of aligned elongated fenestrae due t o chironomids or t o the oxidation of seasonal clumps of diatoms (Figs. 7, 21). Another type of complex’lamination can be found in the crusts and


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cryptalgal laminites of western Andros (Monty and Hardie, p. 459-471), where biological, sedimentological and geochemical processes act at different moments, each contributing a particular (sub)lamina; the resulting cyclothemic lamination results from the repetition of the following series of events: (1) growth of a calcified Scytonerna mat; (2) influx of a detrital pelletal layer; (3) cementation of the upper half of this layer which becomes a pellet packstone overlying a pellet grainstone; and (4) precipitation of a micritic film. Composite lamination may also be built by ecotonal communities; an ecotone (Odum, 1959) can be briefly defined as a transition between two or more communities. The ecotonal community contains not only many of the organisms of each of the overlapping communities but also organisms proper to the ecotone. This concept, which has already been theoretically applied to the development of stromatolitic microstructures (Monty, 1973c, fig. 4) can be clearly seen when studying the evolution of algal-mat fabrics along a cline (e.g., tidal flat or a coast). In the ecotone where two blue-green algal communities overlap, algal mats are built in which each community imprints its own microstructure, the combination of which originates new fabrics. Such a situation can be found in stromatolites of the lower intertidal zone of Shark Bay where Schizothrix communities, originating the so-called “smooth mat” (Logan et al., 1974) illustrated on Fig. 13, overlap with the Scytonema community. Columnar structures are formed (Figs. 22A, 23), in which a laminoid fabric is imprinted by the recurrence of mats of Schizothrix, which bind detrital particles, whereas the doming of the lamination results from the radial flabellate growths of Scytonema over which Schizothrix mats are draped (Fig. 22B). As a result, a composite complex fabric is formed: a gross lamination may evolve from the succession of layers built mainly by Scytonema and layers built by sets of films of Schizothrix (Fig. 22B, C). This does not necessarily correspond t o seasonal successions but reflects the competition for space between the two dominant algae, one outcompeting the other for some time. Each layer will exhibit the fabric imposed by the component algae: the zones built by Schizothrix will show a laminoid boundstone fabric where elongate grains tend t o orient themselves parallel to the mat surface. In zones built by Scytonerna, grains are not primarily bound but trapped: they fall in between the erect filaments and lie without any preferential 6rientation. Furthermore, although most of the Scy tonema are not calcified here, this alga will tend t o imprint its typical radial filament fabric on its layers; this will be done in two ways: (1)development of tubular vertical microfenestrae separated by packstone, after oxidation of the filaments (Fig. 25); (2) development of moulds of filaments where microcrystalline aragonite has been precipitated in the pervading mucilages (Fig. 24). Finally, at a smaller scale, the superposition of the two fabrics may originate a reticulate microstructure where the radially growing filaments of Scy tonerna are regularly cross-cut by horizontal films of Schizothrix, as already seen on p. 202 (Fig. 25). It should be added that in such ecotonal situations, the

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overlapping communities may not only overgrow each other as time goes by, but also juxtapose themselves t o build a complex stromatolite with two different interfingering fabrics.

NON-LAYERED FABRICS

Non-laminated cryptalgal fabrics are quite varied and numerous; only three major types will be considered here: the thrombolitic, massive and radial fabrics. Throm bolitic fabrics Aitken (1967)proposed the term throm bolite for “cryptalgal structures related t o stromatolites, but lacking lamination and characterized by a macroscopic clotted fabric” (p. 1164).“The microfabric of thrombolite consists of centimeter-sized patches or clots of microcrystalline limestone (grain size 8-20 microns) with rare clastic particles, in part indistinctly ‘laminated, separated by spaces filled either with sparry calcite or silt- and sand-sized . . . sediments” (p. 1171). He adds “in some instances, microscopic filaments including rare examples assignable to Giruanella are present in the clots and in some specimens clearly make up the bulk of the clot forming material” (p. 1172). Present-day occurrences show that thrombolitic fabrics may originate in several ways and show a wide range of complexity. Oxidation of mamillate and pustular algal mats Irregular fenestral fabrics have been found in association with pustular colonies of unicellular blue-green algae, in Shark Bay, Australia (Logan et al., 1974). Colonies of Entophysalis up t o 3 cm wide and 2 cm thick (Fig. 26D) form a subcontinuous mat showing a strongly mamillated surface. Sediment particles are trapped and entombed in the depressions between the adjacent colonies, as well as on their surfaces, where they are soon cemented by microcrystalline aragonite. The oxidation of dead algal colonies leaves abundant irregular voids separated by patches of pellet or intraclast packstone, originating a thrombolitic fabric (Fig. 26A, B, C). This simple scheme is, however, complicated by a number of processes. ~~

~~

Fig. 21. Development of composite laminatton in a Phormidiurn incrustaturn head such as illustrated on Fig. 7. Here, the normal alternation of densely calcified with poorly calcified seasonal Phorrnidiurn layers (Fig. 8D) is complicated by the intercalation of (1) a lamina (B)of aligned fenestrae due to chironomids (see F on Fig. 7) overgrowing the heavily calcified spring layer (A); (2) a lamina (C) made of detrital terrigenous grains, and the superposition of Schizothrix films (E) over the next Phorrnidium growth (from D up). Plane polarized light. Scale bar = 500pm.

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(a) Microcrystalline carbonate may be precipitated within the pustular colonies or around them (Fig. 26D). In the first case, the oxidation of the algae will not leave an empty cavity but flocs of micrite. In the second, the peripheral micrite may form a coherent lining that would support the cavity when the algal material disappears. (b) Whereas some corpuscular patches appear to be constituted almost exclusively of detrital particles (ooids, bioclasts, etc.) others seem to form in situ, and several stages can be found: many ovoid to subspherical peloids (50-150 pm across) appear to derive from hollow shells of organic matter on the inner surface of which aragonitic microspherulites are first precipitated (Fig. 27A); they are followed by successive populations that fill up the shell from inside. Some of the peloids may show several stages of growth (Fig. 27C): they are composed of an inner compact core, made of imbricated microspherulites, surrounded by an accretionary zone delimited outward by an organic film “floating” around the core while new microspherulites are precipitated in the intervening spaces, As a result of the imbrication of the constitutive microspherulites, the in situ particles often appear as micritic peloids quite similar to the incorporated but otherwise completely micritized, detrital grains (in many cases, however, gentle etching can liberate the spherulites and help distinguish the grains). As these peloids grow amid members of the algal community, corpuscular patches or clots (sensu Aitkens) are progressively formed in which autochthonous micritic grains are enclosed in a pervading, sometimes filamentous, organic matter, where they seem to have been trapped and bound. In older parts of the structure, cryptocrystalline aragonite is progressively deposited in the interstitial organic matter, and clots of pellet packstone are constructed in situ (Fig. 27D). In the other cases, the microspherulitic precipitation is not limited to individual peloids but involves the greater part of a thrombolitic clot; in this case, a big patch of homogeneous cryptocrystalline (microspherulitic) aragonite is formed and accretes centrifugally by precipitation of new microspherulites between the clot and the surrounding organic film (Fig. 27E, F). Such in situ formation of micrite particles and clots has already been described by Monty (1965a, 1967), Dalrymple (1965), Friedman et al. (1973), ~~

Fig. 22. Stromatolites built by ecotonal communities. A. Vertical section in columnar stromatolite resulting from the overlap of the lower intertidal Schizothrix mat and Scytonema mat. Hamelin Pool, Shark Bay, Australia. Impregnated slab. Scale bar = 1 cm. B. Detail of Fig. 23 showing the radial growth of two Scytonema colonies (one in the lowermost part of the photo, the other in the upper half) which originated the doming of lamination (see the dark thin Schizothrix film ( s f . ) doming over the upper Scytonema growth) and the formation of column-like structures. Plane polarized light. Scale bar = 2 mm. C. General view of lamination: thin dense laminae built by Schizothrix binding detrital grains alternate with looser layers resulting from the entrapment of grains by tufts of Scytonema, most of which have been oxidized. Plane polarized light. Scale bar = 2 mm.

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Krumbein (1974), and Krumbein and Cohen (1974); aragonitic spherulites have been experimentally formed by bacterial action in marine waters by Oppenheimer (1965) and Krumbein (1974). It is evident that the formation of such thrombolites is not a simple matter, but results from a complex of sedimentary, organic and biochemical processes. Microspherulite precipitation as a factor of thrombolite genesis will be further substantiated below (p. 232-235).

Coalescence of calcified botryoidal colonies (a) Simple situations. Aitken (1967), in his study of Early Paleozoic cryptalgal carbonates, reported situations where filaments of Giruanella could be found in thrombolitic clots or even could form the bulk of them. Analogous situations are found in Recent thrombolitic carbonates where the constitutive clots represent calcified algal colonies endowed with a botryoidal surface growth form; as the various algal knobs merge numerous cavities are created, originating an irregular fenestral fabric. Two examples will be given here: freshwater cryptalgal tufa deposited in Green Lake, New York, U.S.A. is “exceedingly porous or spongy and consists of more or less closely intergrown arborescent masses that are richly nodose” (Bradley, 1929a, p. 205). The botryoidal upper surface of the deposit is obvious on Walcott’s (1916) pl. 4, fig. 3 and Bradley’s (1929a) pl. 29B, whereas the resultant thrombolitic fabric is illustrated in Walcott’s pl. 4, fig. 4. The microfabric of the clots is radial or laminated: radial flabellate growths of blue-green algal filaments (oscillatoriaceans), forming the calcareous knobs, are illustrated by Bradley (1929, pl. 30, figs. A, B); fine-grained calcite is first deposited in the interstices between the filaments and cementation follows.* When the algal felts decay and disappear from the clot, irregular radially elongated cavities are left that will also be cemented.

* The carbonate is algally precipitated according to Bradley and Walcott but detrital and trapped according to Dean and Eggleston (1975). although their paper is inconclusive as to how much of the fine-grained carbonate material is really detrital and how much is precipitated in situ .

Fig. 23. General view of section cut in sample shown on Fig. 22A. Frame delimits part of Fig. 22B. T w o columns are clearly visible: half of one covers the right-hand side of the photograph, part of the other appears along the left margin. They are separated by a gap infilled with detrital particles sometimes lying edgewise; in the upper half, Schizothrix films extend from columns over the depositional surface of the infilling material (arrows). Fabric results from alternation and/or interfingering of radial growth of Scytonema (often leaving vertical tubular microfenestrae (see Fig. 25) with smooth films of Schizot h r k doming over them). The right-hand column indicates the role of Scytonema in the shaping and framework of the structure. Plane polarized light. Scale bar = 1cm. Column margins have been partly emphasized with ink lines.

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Fig. 24. Detail of Fig. 23. Radial filament fabric of Scytonema preserved as moulds by microcrystalline aragonite. Plane polarized light. Scale bar = 500 pm.

A laminated pattern may also appear in the clots of Green Lake thrombolites, mainly near the surface (Dean and Eggleston, 1975, fig. 14, general view; Eggleston and Dean, Ch. 8.7, Fig. 7, close up). Such laminated knobs would result from the alternation of phases of algal-mat growth with phases of physicochemical precipitation, as proposed by Dean and Eggleston (Ch. 8.7). In other thrombolites, the clots are definitely concentrically laminated as a result of the episodic growth of the constitutive bluegreen algal colonies (Fig. 28). Such cases occur among the variety of cryptalgal carbonates that are deposited in the Hoyoux river, Belgium, where algae like Schizothrk and Phormidium are growing laminated botryoidal knobby encrustations. Here, the irregular fenestral fabrics result from two main factors: first, the merging of the various algal knobs which circumscribe internal cavities; second, the presence of globose masses of diatoms encrusted by carbonate-depositing blue-green algae. When the diatoms disappear, open irregular fenestrae are left in the tufa. (b) Complex situations. Another type of thrombolite fabric occurs in littoral deposits on the northern side of Mono Lake, California. The origin of these various deposits has been much disputed (Dunn, 1953;.Scholl and Taft, 1964). One case is here reported as an illustration of a variety of cryptalgal fabric


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Fig. 25. Detail of Fig. 23. Radial filament fabric of Scytonema preserved as vertical tubular microfenestrae after oxidation of the algal bundles. Overlap of ecotonal communities results in the superposition o f horizontal films of Schizothrix o n the radial fabric, leading to a reticulate microfabric. Plane polarized light. Scale bar = 500pm.

which also forms by coalescence of small calcareous knobs. It might also help understanding spherulitic microfabrics such as those described by Buchbinder et al. (1974). The samples, collected in 1964, are rather porous and show in section, a series of rounded (from less than a mm to 1cm across) to irregular or flattened crenulated carbonate flocs separated by open cavities (Fig. 29A). In thin section, these flocs have a “cellular” microstructure (Fig. 29B, C); each “cell” is composed of one or several imbricated spherulites (Fig. 29D), about 40 pm across, and is generally surrounded by darkish discontinuous organic matter. Observation near the surface of a mamillated knob shows that spherulites are actually forming within or beneath a brownish microfilamentous, apparently bacterial, film doming over the structure (Fig. 29F). The genesis can hence be viewed as follows: mono- or polyspherulitic microclots grow under the influence of bacterial colonies until they meet similar growing microclots in which case crowding prevents any further growth and a compromise “cellular” microstructure progressively appears. Macroscopic clots are progressively formed by juxta- and superposition of “cells” or spherules. These clots have a very knobby surface as the juxtaposition of spherulitic units tends to produce rounded protuberances. The projection of these knobby clots (such as illustrated on Fig. 29B) merge in turn with other similar bodies and the resultant internal cavities originate a conspicuous thrombolitic

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fabric. At this stage, the cavities are soon invaded by mucilage rich in bacteria and microfilaments which either fill the whole cavity or line it with a compact film. In between this continuous fi!m and the cavity wall, carbonate precipitation resumes, no longer as discrete spherulites, but as a continuous layer of closely packed needles up to 50pm long and very reminiscent of inorganic cements (Fig. 29C, E). In this way, the “cellular” microspherulitic clots become enclosed in a subcontinuous fibrous girdle. Further complications are found in these thrombolites due to overgrowth by various algal and bacterial colonies, but they will not be considered here as this is a problem in itself. Corrosion of deposited cryptalgal carbonate The development of irregular fenestrae leading to thrombolite-like fabrics may also result from the chemical processes of internal solution. This is illustrated by the large calcareous nodules (5-25 cm across) that are built by bluegreen algae on the bottom of the Rhine River, at Stein am Rhein, Germany (Baumann, 1913; personal observations), and which in section show a very porous fabric. These pores and holes that cut through the calcified algal mass are interpreted by Golubic (1973a) as developing “by secondary carbonate dissolution of the interior of the crust caused by local C 0 2 accumulation from bacterial action” (p. 442, see his fig. 21.4). It should be added here that the cavities and fenestrae of all these porous cryptalgal deposits generally harbour multitudes of organisms such as insect larvae, small crustaceans and nematodes; these organisms contribute to maintaining these cavities open and may eventually enlarge them. The massive fabrics Trapping mechanisms Massive cryptalgal fabrics show no internal lineation or spatial organization of their constitutive elements. Thus, they will not be easily identifiable as cryptalgal. Among the processes liable to originate massive fabrics, five will be briefly described. Fig. 26. Thrombolitic fabrics. A. General view of a thrombolitic cryptalgal structure associated with pustular colonies of Entophysalis major Ercegovih. Lower intertidal zone, Hamelin Pool, Shark Bay, Australia. Scale bar = 1cm. B. Vertical section showing the development of irregular fenestrae delimited by cemented corpuscular clots. Plane polarized light. Scale bar = 2 mm. C. Detail of a clot made of detrital and in situ particles, commonly held together by organic matter, and larger patches of microcrystalline aragonite. Plane polarized light. Scale bar = 500 pm. D. Pustular colony of Entophysalis major at the surface of the living mat; dark clouds and linings inside and around the colony are made of microcrystalline aragonite. Plane polarized light. Scale bar = 200 pm.

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Entrapment of detrital grains is common in communities of strong bluegreen algae (e.g., Dichothrix bornetiana (Howe), species of Rivularia, Scytonema, and Lyngbya) which grow erect tufts of filaments between which siltand sand-sized particles infiltrate and remain trapped (Fig. 30). Besides blue-green algae, filamentous green and red algae may also form mats actively trapping detrital grains; good instances are illustrated by the Enteromorpha (green) mat described from the Bahamas by Scoffin (1970, p. 265, fig. 22) and the Rhodothamniella (red) mat described by Hommeril and Rioult (1965, pp. 137-140, fig. 3A) from the coast of Normandy, France. Such algal filaments project freely from 0.5 t o 2 cm above the sediment surface. In areas where shifting of sand is continuous, grains constantly infiltrate in between the mesh of filaments, where they accumulate. In the subtidal zone of the Bahamas and the intertidal zone of the windward lagoon of Andros Island (Monty, 196513, 1967) the deposit progressively collapses as the filaments disappear and would yield an undifferentiated massive grainstone. It should be noted, however, that in cases of rapid cementation of the sand, as happens in Shark Bay, Australia, the fabric would evolve differently with the preservation of irregular piles of packstone separated by subvertical elongate cavities left by the oxidized bundles of filaments; here, a “tubular fenestral fabric” rather than a massive fabric would be formed. As is often the case in boundstones, the grains trapped by the algae are better sorted than the surrounding sediments and the average grain size is lower, as only the finer fractions are resuspended and transported t o the mats. However, as opposed to what happens when sedimentary grains are directly bound, the elongated particles will not show any common orientation parallel to the growing surface but may often stay edgewise (which may be a typical feature of “trapstones”). Fig. 27. Development of in situ grains and clots in Shark Bay thrombolites. A. In situ grain in process of formation: fibrous and microspherulitic aragonite is being precipitated on the inner surface of an organic shell and accretes inward to finally yield a micritic grain. Plane polarized light. Scale bar = 50pm. B. Collection of in situ grains in corpuscular patches such as appears on Fig. 26C. Note their microspherulitic microstructure. Plane polarized light. Scale bar = 75 pm. C. In situ microspherulitic grain with accretionary zone made of a lobate organic film (black lining at the surface) underneath which new microspherulites are being precipitated. Plane polarized light. Scale bar = 50pm. D. Detail of a clot made of in situ microspherulitic grains bound by pervading organic matter in which microcrystalline carbonate is being precipitated. Plane polarized light. Scale bar = 200pm. E. Clot made of microspherulitic aragonite and surrounded by an organic dark film. The clot is accreting mainly on its higher upper left margin (see detail in F). Black spots are air bubbles. Plane polarized light. Scale bar = 500pm. F. Detail of the upper left margin of clot shown on E. Accretion results from precipitation of new microspherulites in the interspaces between the organic film (arrow) and the older parts of the clot over which it domes. Plane polarized light. Scale bar = 50 pm.

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Fig. 28.Development of thrombolitic fabrics (continued). A. Fragment of knobby cryptalgal deposit built by clustered lobate colonies of oscillatoriaceans (Hoyoux river, Belgium). The merging of all these knobs and projections produces irregular internal cavities delimited by algal clots and hence a particular thrombolitic fabric. Scale bar = 5 mm. B. Section of structure illustrated in A showing algal clots with radially growing filaments (not visible on photograph); layering of the clots results from seasonal growth. Plane polarized light. Scale bar = 500 pm.


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Along with simple trapping mechanisms, other processes generally occur simultaneously. For instance, the mud fraction of the sediment, when present, does not tumble down in between the filaments, but sticks to their mucilaginous sheaths. In this case, the algal filaments may become encased in a micritic sheath and the original algal fabric will be variously replicated according to the abundance of mud and the fidelity of the mould. Also, the leading alga, which makes the sedimentary trap, forms a general framework in which other algae thrive (e.g., Schizothrix, Phormidium, Microcoleus). These will in turn overgrow and bind the detrital particles that have been trapped, and consolidate the deposit. They will not alter the “trapstone” fabric though, except that eventual precipitation of cryptocrystalline carbonate in their mucilage may alter the grainstone to a packstone. It should be clear that trapping mechanisms do not always yield nonlaminated cryptalgal deposits; if the influx of detrital particles is episodic or if the textural features of the sediment carried onto the mat vary seasonally or accidentally, a lamination will develop in the mat.

Oversedimentation Non-laminated, massive, fabrics may also form in rhythmically growing algal mats, like the Phormidium colonies described on page 197, when the mats are swamped by a continuous heavy influx of detrital grains, or when vertical algal movement is reduced. For instance, when traced onto exposed tidal flats, where sediment influx is very important during tidal movement and algal growth considerably slows down during low tide, or when traced toward those lagoonal floors where abundant silt is deposited, these welllaminated domes may pass to unlaminated massive formations (Fig. 31). In thin section, such bound sediments appear as an homogeneous sandy or silty deposit, composed of well-sorted particles significantly finer than the surrounding sediments, whereas tiny filaments of Phormidium run erratically from one particle to another. Beside what has been said before (p. 237), the normally good sorting of the component particles results (1)from the absence of protruding coarse filaments that could simultaneously agglutinate the fine and trap the coarser material, and (2) from the micron size of the algal filaments which is such that only the fine fraction of the bottom load can be stabilized. Furthermore, the coarse particles, which have less chance of being resuspended by normal agitation, are too voluminous or too heavy to be firmly bound during a single tidal event. If elongated particles are present in the sediment their orientation will not be gravity determined (see p. 217), which may give us clues as to the original boundstone nature of the deposit. In brackish or hypersaline environments micritic carbonate may be progressively precipitated in the interstitial organic matter by bacterial action and a massive packstone is formed.

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Binding of su bstratal sediment Unlaminated massive corpuscular deposits can also form in another way: sediment is not carried onto a growing algal mat, but algae invade and bind a previously deposited sediment. Putting aside the submarine algal films which periodically stretch over the sea floor and stabilize the sand (Bathurst, 1968; Neumann et al., 1970; Scoffin, 1970), I shall mention here only the case of sandy “crusts” found in the proces’sof formation on some beaches bordering the lower Fresh Creek (Andros Island, Bahamas; Monty, 1965b). These friable, white to grayish, sandy agglomerates, about 1cm thick, are composed of coarse organoclastic material and rest free on the loose skeletal beach sand. Their surface is rough, granular and devoid of any significant algal vegetation (Fig. 32). A thin section shows a poorly sorted biocalcarenite loosely bound by filaments of Schizothrix and glued by mucilaginous masses of Entophysalis, sometimes loaded with clouds of fine-grained carbonate. These structures originate by the local invasion of the loose beach sand by colonies of Schizothrix which bind the grains into friable flakes; once initiated, the sandy flakes thicken rapidly. Detrital particles, carried by the wind or the tides, are trapped and entombed in the irregularities of their rough surface where they will be bound. They produce a new and advantageous habitat for algal and bacterial colonies by providing a stable microenvironment (with no shifting of particles) in which moisture can be retained and protection against solar radiations is ensured. Therefore, most of the living material is inside the structure, amid mucilage. The organic material harbours bacteria that contribute to the precipitation of lime. Although friable, these structures harden considerably and curl up when dried, and can be incorporated into the beach deposit as chips of massive packstones.

Fig. 29. Development of thrombolitic fabrics (continued) A. General view of a littoral tufa from Mono Lake, California. Note variations in size and shape of component clots (grayish to white) which go from subspherical (upper left) to elongate. Impregnated slab. Scale bar = 1cm. B. View of a small clot made of a lobate, spherulitic inner core coated with a fibrous irregular lining. Cross polarized light. Scale bar = 500 pm. C. Detail of a clot. The groundmass is made of juxtaposed microspherulitic units (see D) originating its “cellular” microstructure. The rim of the clot is coated by a fibrous deposit (light band) growing under a probably bacterial organic film ( 0 . f . )Cross polarized light. Scale bar = 200pm. D. Detail of the microspherulitic units composing the clots. Cross polarized light. Scale bar = 50 pm. E. View of the bacterial organic film (arrow) under which is growing the fibrous deposit visible in B and C. The film split longitudinally during dehydration and staining of the preparation. Plane polarized light. Scale bar = 50 pm. F. Initiation of spherulitic units: close up of scattered spherulites growing inside a probably bacterial envelope appearing blurred on top of photograph. Plane polarized light. Scale bar = 50pm.

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Fig. 30. Entrapment of detrital particles between bundles of filaments of Rivularia. Intertidal zone, eastern Andros Island, Bahamas. Plane polarized light. Scale bar = 500 /Jm.

Massive cryptalgal diatom mats Massive cryptalgal structures can also result from the continuous precipitation of carbonate in growing mucilaginous algal masses. The intertidal mud flats bordering Fresh Creek, Andros Island, are locally covered with subpolygonal, grayish-black mats, about l c m thick and 3-5cm across (Monty, 1965a).The uppermost part of the mats is composed of a hyaline uncalcified film of diatoms, up to 100 pm thick (Fig. 33A); a faint layering may appear in this film as a result of the horizontal elongation of big clusters of diatoms. Beneath this is a layer up to 5 m m thick composed of a mucilaginous mass rich in diatoms and bacteria, with scattered colonies of Entophysalis; within this is abundant cryptocrystalline carbonate characterized by crystals mostly smaller than 1pm wide (Fig. 33B). This highly organic layer rests on an undifferentiated stratum of micrite where frustules of diatoms are still present. The superposition of these three layers illustrates the development of the mat: when conditions are favorable, diatoms bloom at the surface of the polygons and secrete a very mucilaginous stratum which is soon invaded by bacteria; calcium carbonate is progressively deposited at the expense of the mucilage and the living mat is progressively converted in a massive biomicrite.


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Fig. 31. Development of massive fabric due to oversedimentation on Phormidium films. Intertidal zone, cays off Fresh Creek, Andros Island, Bahamas. Plane polarized light. Scale bar = 5 mm.

,

Fig. 32. Development of massive fabric due to invasion and binding of beach sand by Schizothrix while mucilaginous colonies of unicells, developing internally, contribute to gluing the particles together. View of resulting friable crust. Sandy beaches of the Fresh Creek, eastern Andros Island, Bahamas. Scale bar = 1mm.

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Fig. 33. Development of massive fabrics in diatom mats. Intertidal beaches of the Fresh Creek, eastern Andros Island, Bahamas. A. Hyaline upper part of the mat made of piles of mucilaginous films of diatoms. Plane polarized light. Scale bar = 200pm. B. Lower part of the seasonal mat where microcrystalline calcite is being precipitated in the mucilage. Plane polarized light. Scale bar = 200 pm.

Similar phenomena have been reported from freshwater deposits by various authors (e.g., Wallner, 1935; Van Oye, 1938; Kann, 1940). Diagenesis Compaction, oxidation, precipitation of micrite, micritization of grains, dolomitization and recrystallization are among the many diagenetic processes that can blur or obliterate originally laminated or thrombolitic fabrics and produce massive crystalline fabrics. However, it is my experience that “ghost lamination”, visible only on weathered outcrop or in thin section, may persist even in crystalline fabrics. Radial fabrics Some algae, like Scytonema in the Recent or Orthonella and Garwoodia in the Paleozoic, frequently show a radial growth of skeletal (calcified) filaments. Such radial growths have already been described (e.g., p. 198) as part of laminated fabrics in which distinct laminae are composed of radially growing filaments. However, continuous or subcontinuous growth of such frame-building algae may lead to the formation of unlaminated to poorly


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Fig. 34. Radial filament fabric in lithified calcitic dome built by Scytonema. Higher parts of freshwater marshes, western Andros Island, Bahamas. Plane polarized light. Scale bar = 1 cm.

laminated structures characterized by a typical radial filament fabric (Figs. 34, 35;Monty and Hardie, Ch. 8.6, fig. 11).This fabric may be interrupted here and there by concentric layers due to the development of horizontal growth stages of Scytonema (p. 198). Furthermore, as already described, factors such as concurrent development of horizontal films of oscillatoriaceans in ecotones, episodic influx of detrital particles or physicochemical precipitation of cryptocrystalline laminae, can alter the radial filament fabric into a reticulate fabric. The fossilization of such algal structures can lead to the development of sparry calcite in between the skeletal filaments or to the deposition of cryptocrystalline carbonate (Fig. 35). THE IMPACT OF THE BIOLOGICAL STRATIFICATION OF THE ALGAL MAT ON THE RESULTING STRUCTURES

The layered fabric of stromatolites as it appears to us is in fact the total sum of two types of superposed stratifications, as has been developed in Monty (1965a): (1) a stratification in space or “instantaneous biological stratification” (Monty, 1973a) results from the taxonomical and/or metabolical differentiation of blue-green algal and bacterial communities which at any given time are superposed in a living algal mat; (2) a stratification in

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Fig. 35. Radial filament fabric in living Scytonerna crust (above) and subfossil one (below) where micrite has been precipitated between the calcified sheaths as well as in the lumen. Higher parts of the freshwater marsh, western Andros Island, Bahamas. Scale bar = 1 mm.

time or “historical biomineralogical stratification” results from the periodical superposition of layers. The instantaneous biological stratification reflects the complex structure of algal mats. A good example of such stratification studied by Sorensen and


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Conover (1962) is the Lyngbya mats of the Laguna Madre (Texas). These mats are 1-20mm thick and show a laminated fabric resulting from the superposition of layers characterized by different colours and different taxonomic composition. A t the surface are deeply stained blue-green algae which screen the incident light, in the inner layers the protected algae are actively growing, and in the basal parts there are purple bacteria and colorless algae. Heterotrophic processes predominate in the basal zone (see also Golubic, Ch. 4.1). Even when they are built by only one dominant alga, mats may show a vertical stratification of populations of conspecific individuals characterized by different metabolisms and even different morphological and histological properties. Fritsch (1953) has for instance described the various morphologies and habits of conspecific filaments of Phormidium at different levels in a film of Phorrnidium. This stratification in space determines the individualization of superposed and clearly circumscribed metabolical and hence biochemical microenvironments. In the case of polyspecific mats it also reflects the individualization of efficient cenotic pools from which the mat will be able to differentiate seasonally and keep on growing in spite of changing climatic or atmospheric conditions (see p. 199ff). The relationships between this instantaneous biological stratification and the historical stratification may vary considerably according to the growth pattern of the stromatolite and in fine to the particularities of its basic module: the algal film or mat. Two extremes can be illustrated (Monty, 1973c, fig. 2): in the case of stromatolites resulting from the yearly superposition of rather thick mats the instantaneous stratification will generally be limited to the growing mat, the underlying ones being already in the process of diagenesis. At this level, it may originate the differentiation of the yearly layer into secondary laminae. When the stromatolite results from the rapid superposition (daily, monthly) of thin algal films, the biological stratification will extend over a considerable number of historical laminae in which it will probably induce several types of auto bioturbation. Indeed, as the stratification in space is an energetic stratification, it may initiate fundamental reworking of the original fabrics and microstructures, for example: (1)Encysted populations within the mat may suddenly start growing, their optimal ecological threshold being suddenly reached, and thicken the mat from the inside, disturbing the original fabrics and structures (Fig. 36). (2) Filamentous populations moving upwards, as the stromatolite grows, to remain in the optimal energetic level, may originate the collapse of the laminae they were previously colonizing. (3) Formation of internal cavities, disorganizing the microstructure, as a result of the consumption of mucilages and organic accumulations by heterotrophic organisms. (4) The presence, at particular levels, of given metabolical organic acids may control the preferential precipitation of calcite, aragonite, or various

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Fig. 36. Autobioturbation of original lamination by localized and active internal growth of Microcoleus (stained with methylene blue) in laminated mats built by Scytonema. Tidal marshes, eastern Andros Island, Bahamas. Note how this phase of internal growth deformed both over- and underlying laminae. Plane polarized light. Scale bar = 500pm.

other permanent or fugitive carbonate phases; indeed, the degree of complexing of the calcium by the organic acids as well as the rate at which the calcium can be released, influences the rate of precipitation of the carbonate, hence the mineral phase. Furthermore, the release of hydroxyl groups by organic acids during the complexing of calcium ions, may originate at places significant variations of pH (see Trichet, 1967, 1972b).


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(5) The size, the crystallographic habit as well as the morphology of crystals (angles, sharp, rounded, etc.) are directly ruled by the physical and chemical properties of the algal mucilages in which precipitation takes place; in their turn, these properties vary with the horizon considered in the mat, with the seasonal metabolical modifications of the given cenose, with the taxonomic composition of the stratified cenoses, and so on. Finally, local variations in pH, concentration of C 0 2 , nitrates, sulfates can originate various degrees of corrosion in given parts of the mats, and reprecipitation of carbonate elsewhere. It is clear that all these processes of autobioturbation (see also Golubic, Ch. 4.1)can also proceed in other cryptalgal situations, for instance in thrombolites. Accordingly, without considering the action of external physical factors, a cryptalgal structure is always the total sum of two conjugated and interrelated processes : surface outward accretion (determining in stromatolites the superposition of successive historical laminae), followed by constant readjustment of the inner mat-dwellers which distribute themselves, from top to base, into distinct biological and energetical zones. Because of the multiple and sometimes antagonistic properties of these stratified microcommunities, the net result of the interference of this stratification in space on the accretionary stratification in time, will vary. Vertical accretion will be enhanced if, for example, one of the inner communities develops an internal growth phase, or induce a microenvironment in which precipitation and cementation will take place and consolidate the structure. The net result will be negative when one of the microcommunities takes the lead to determine solution processes. REMARK ON MUD MOUND

Studies in progress on Devonian micritic mud mounds strongly suggest that these would be unlaminated blue-green algal bioherms as evidenced by the abundance of filaments in the micrite, and the organic-like aspect of associated peloids. In first approach, they could then illustrate a megadevelopment of massive fabrics. ACKNOWLEDGEMENTS

The present study has been achieved thanks to Grant No. 10215 from the National Foundation for Scientific Research (Belgium) for which I am very grateful. I am particularly indebted t o Conrad Gebelein, Steve Golubic and Phil Playford for exchanging experience on Recent marine stromatolites as well as for their extensive collaboration in sending me samples from Shark Bay without which this paper would have been more incomplete yet.



Chapter 5.2 AN A’ITEMPT TO CLASSIFY LATE PRECAMBRIAN STROMATOLITE MICROSTRUCTURES J. Bertrand-Sarfati

INTRODUCTION

The stromatolite primary lamination reflects the growth pattern of the algal coenose and the habit of the carbonate precipitated or trapped within the filament framework, among other things. The fossil stromatolite microstructure includes furthermore the imprint of subsequent diagenesis. In fact, the significance and the functioning of the algal community building up the stromatolite laminae have been often neglected by geologists. In the ancient and especially in the Precambrian, because of the bad preservation of the “microstructure”, two ways have been followed. A carefully detailed description of the actual fabric of the microstructure mixing up original and diagenetic features (Russian school of Moscow) leads to an excessive number of microstructural patterns and a very artificial classification. A detailed description of the recognizable organic remnants (Vologdin, 1962), which are often suspect, neglects the building framework of the laminae. Recently an attempt has been made to introduce a genetic element to interpret more precisely the laminations (Bertrand-Sarfati, 1972c; Preiss, 1972b, 1973b; Walter, 1972a; etc.). The need of modern models was great because of the excessive attention paid by the geologists t o the sedimentary and physical processes. Only few of them (Hommeril and Rioult, 1965; Monty, 1965a, 1967; Gebelein, 1969; Davies, 1970b; Neumann et al., 1970; Walter et al., 1972, 1973; Awramik, 197313) described precisely the modem algal laminae and the role of the algae or the algal community. And even then, the fossilization of the laminae is often not observed. Having had the chance of working on Late Precambrian stromatolites with well-preserved microstructures, I proposed a classification that was well adapted for some of them: (1)simple microstructures where the laminations or the dominant fabrics follow the rhythmical growth pattern of the coenose; and (2) complex microstructures where the historical succession of laminations presents microstructural changes due to seasonal differentiation of the algal coenose.

h Fig. 1. Microstructure occurring as dark films. A. Conophyton ressoti var. jacqueti Sougy. B. Baicalia mauritanica Bertrand-Sarfati, 9 early Late Riphean, northern edge of Taoudenni Basin (scale-bar: 0.5 mm). 9

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SIMPLE MICROSTRUCTURE

Film microstructure The essential feature of this type of microstructure is represented by regularly banded dark, thin (mode 0.003 mm), micritic films. These micritic films alternating with clear sparitic (or microsparitic) laminae (0.03-0.05 mm) constitute the first-order lamination (Conophyton ressoti Menchikov, Fig. 1A). A second-order lamination occurs with the intercalation of much thicker layers of clear sparite (up to 2 mm wide). A variant of this last one shows groups of films, disrupted by diagenetic processes (Baicalia mauritanica Bertrand-Sarfati, Fig. 1B) in large sparry carbonate layers. Similar microstructures are found in some forms from the late Middle Riphean and the early Late Riphean: (a) Baicalia lacera Semikhatov, films composed of fibers (“razdel’no voloknistaya”: Raaben, 1972, in Raaben anc Zabrodin, 1972), with sporadic clear layers containing peloids; (b) Conophyton garganicum australe Walter (banded microstructure), where the films are regular with a thickness of 0.03-0.05mm. In some other forms of the Late Riphean and the Vendian, thin films of dark micrite occur within laminations of a more complex constitution : Inzeria nyfrieslandica, Tungussia indica, and Poludia polymorpha Raaben. We can find two present-day examples which, when fossilized, would display a film microstructure: (a) In mats dominated by Schizothrix calcicola in the sink holes of the Bahamas (Monty, 1965a, p. 204, fig. 20). The algal laminations were formed by the “daily” superimposition of thin calcified films, composed of very thin filaments arranged parallel t o the lamination. (b) In the effluent of a geyser in Yellowstone National Park, stromatolites resembling Conophyton (Walter et al., 1972; Ch. 6.2 herein) are built of very thin films (5-10pm). They are made of filaments of bacteria and cyanophytes. In the two cases we find thin filaments and the repetition of the same laminae as for our ancient stromatolites. Tussock microstructure In this type an irregular lamination is defined by the juxtaposition of separate hemispheric tussocks of different size (0.01-1 mm). Although composed of interlocking crystals, they retain traces of the primary radiating filament fabric. The tussocks are commonly overgrown either by a cement of pure sparite (Tungussia globulosa Bertrand-Sarfati, Fig. 2A) or a dark film (Tarioufetia hemispherica BertrandSarfati) or embedded in carbonate cement and detrital quartz (Serizia radians Bertrand-Sarfati). On these substrates, new tussocks begin to grow and so they define roughly parallel laminations. Some of the tussocks have a concentric growth pattern.

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Outside the Taoudenni Basin (W. Africa), the only other comparable known microstructure comes from the Late Riphean: Alternella hyperboreica Raaben (intertwining films, “uzorchato-plenochnoy ”: Raaben, 1972, in Raaben and Zabrodin, 1972). However, in this case, the tussocks seem to be without filaments and occur as flat pillows superimposed randomly. The dark films are moulded onto the surface of the pillows and seem to anastomose. Recent colonies of Riuularia which form tiny tussocks encrustating various hard substrates in streams (Kann, 1941a; Bertrand-Sarfati, 1972c, plate XXIX, 4) have a microstructure of juxtaposed hemispheric tussocks. The Riuularia colonies are formed by radial growth of numerous trichomes, periodically embedded in calcite crystals (Fritsch, 1945; Monty, Ch. 5.1). The colonies, when indurated, often show a concentric pattern, due to different stages of growth. When the growth is interrupted, the colony may become covered by a coating of sediments. Vermiform microstructure In this kind of microstructure, thick, regular, pale and dark layers have the same composition, both consisting of “narrow, sinuous, pale-coloured areas (usually of sparry carbonate) are surrounded by darker, usually fine-grained areas (usually carbonate)” (Walter, 1972a, p. 14). Such a microstructure occurs in Madiganites mawsoni Walter (Fig. 2B). Diagenetic alteration breaks up the sinuous patches giving the layers a gnunelous aspect. Vermiform microstructures are found in Vendian and Cambrian forms: (a) Acaciella angepena Preiss, more regularly laminated, (b) Boxonia gracilis Korolyuk ( “globulyarnaya”: Raaben, 1972, in Raaben and Zabrodin, 1972), with granules more rounded than vermiform; (c) Uricatellu urica Korolyuk, with a polygonal network; (d) Boxonia diuertata Korolyuk, with a more variable network. Older forms such as Minjaria procera Semikhatov can show this structure sporadically. According to Walter (1972b, p. 159), the vermiform microstructure can be compared with the filaments of Giruanella, in spite of the fact that the clear patches of the vermiform microstructure are moulds of large filaments. Giruanella is not well defined with reference t o an actualistic model. The structure has been compared (Monty, 1965a, p. 231, plate 35) with that produced by compaction and collapse (3cm below the surface) in laminae composed of intertwined filaments of Scytonema and Schizothrix. This microstructure is most probably a polyspecific one and remains obscure. Fig. 2. A . Microstructure consisting of filamentous tussocks: Tungussia globulosa Bertrand-Sarfati (scale-bar: 1.25 mm). B. Vermiform microstructure: Madigunites mawsoni Walter, Middle to Late Cambrian, Jay Creek Limestone?, Jay Creek, Amadeus Basin. Width of field of view is 2.7 mm.

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COMPLEX MICROSTRUCTURE

The stromatolite is built of variable laminae and is probably controlled by one or several species or several coenoses.

Microstructures consisting of micritic mats Basically the laminae are thick, darkish in colour and composed of micrite; their upper boundary may be straight, crinkled or scalloped. The laminae are formed (Tungussia nodosa Semikhatov Fig. 3A) of two different layers, the upper one (micritic) is darker than the lower one (microsparitic). Many variants of this type are found in the same stromatolites: dark films at the top of the micritic laminae; rhythmical thick micritic layers separating lessdefined micritic mats; trapping (?) of peloids in the basal laminae. Frequently, sheet cracks separate adjacent laminae; these are filled with clear calcite. Diagenetic processes also disrupted the mats, which now appear patchy. Complex microstructures of this type are recorded in numerous forms: Jurusania burrensis, Baicalia burra and Acaciella augusta Preiss (banded microstructure);Jurusania nisuensis and J. cylindrica Krylov (disc-like microstructure, “plastinchataya”, or amorphous films, “amorfno-plenochnoy”: Raaben, 1972, in Raaben and Zabrodin, 1972; J. judomica Semikhatov), Baicalia capricornia and Inzeria intia Walter, and Gymnosolen ramsayi Steinmann (this one often has crinkled mats). Numerous forms of this type are known throughout the Riphean and Vendian; their usefulness in stratigraphy may be increased when more information on their possible range of variation is available. This kind of microstructure is quite difficult to interpret. Is the micrite an original pattern of growth of the calcite crystals or diagenetic micritization? Is the “doublet” the differential growth of one species of algae, or the succession of two dominant species? One possible comparison is proposed with Scytonemacrustaceum mats in the Bahamian upper tidal flats (Monty, 1965a, fig. 33, Pi. 55). The erect filaments form thick mats followed by calcitic cryptocrystalline layers accumulated around unicells and scarce Scytonema filaments. The erect filaments in the mat are completely reduced afterwards and enclosed within crystals as impurities. The variations can be due to trapping of particles within the calcitic layer or intercalation of a felt of Schizothrix.

Ca tagrap h-bearing mats These thick mats appear periodically in a succession of micritic mats. They contain catagraphs in a microsparitic matrix (Tifounheia ramificata Bertrand-Sarfati, Fig. 3B). Catagraphs are quite regular spheroids with a dark

Fig. 3. A. Microstructure in micritic mats: Tungussia nodosa Semikhatov (scale-bar: 0.5 mm), Late Riphean, northern edge of Taoudenni Basin. B. Catagraph-bearing microstructure: Tifounkeia ramificata BertrandSarfati, late Late Riphean, northern edge of Taoudenni Basin (scale-bar: 0.5 mm).

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envelope and clear calcite infilling. They are present also in the carbonate between the columns, where they are bigger. Sometimes they cannot be distinguished from micritic peloids. This microstructure is a dominant feature of a number of Vendian forms: Boxonia grumulosa, B. lissa Komar, Linella simica Krylov and above all Boxonia ingilica Semikhatov, where the laminations include numerous real catagraphs. Some other forms occasionally contain peloid-bearing layers (Linella ukka Krylov and Baicalia rara Semikhatov); these peloids seem to be trapped particles. This model is not very different from the preceding one, as far as the micritic laminae are concerned, but it differs by the presence of the catagraph layer. Are they ooids or peloids, or really calcified colonies of algal unicells with organic matter migrating on the surface? At present unicells proliferate at the bottom of Scytonema mats (in fresh water) and during recrystallization show this kind of feature (Monty, 1965a, plate 60). In this case, the catagraphs developed in situ, inside the micritic mats. But it is also possible that catagraphs are trapped on the surface of the mat, in the way that trapping occurs in tidal-flat algal mats (Hommeril and Rioult, 1965; Monty, 1965a; Gebelein, 1969; Neumann et al., 1970; etc.). DISCUSSION

(1)It is now clear that consideration of the microstructure must enter into the definition and classification of stromatolites. However, beyond the simple description of the mineralogy and petrofabrics of the microstructure, an attempt must be made to elucidate its genesis, and also the diagenetic alterations. (2) The nature of the microstructure has become more and more important in the definition of stromatolite forms (Conophyton, Komar et al., 1965) and even groups, especially in the case of simple microstructures. The delimitation of groups is more difficult in the case of complex forms and usually the stromatolite has to be determined on the basis of several morphological attributes (Hofmann, 1969a; Walter, 1972a). (3) Attempts at comparing fossil stromatolite microstructures with those of present-day algal communities are faced with several difficulties: (a) a paucity of description of present-day algal microstructures; (b) overemphasis of the importance of intertidal polyspecific algal mats whose structure is largely controlled by sedimentation rates; (c) difficulties for ancient stromatolites in defining basic types of microstructure due to frequent recrystallization, and lack of models. (4) At a first glance, some kinds of microstructure are easy to interpret: those presenting regular simple microstructures such.as films or tussocks or more complex vermiform microstructures, which are evenly repeated all


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along the stromatolite column. In the other cases, microstructures are composed of a historical succession of various kinds of laminae and the basic features of microstructure and the relation with other (variant) laminae are difficult t o understand, particularly in the Precambrian. The seasonal and climatic variations played an unknown role in this kind of microstructure. (5) Various approaches have been used t o study the relationship between morphology and microstructure. Krylov (1967a) was the pioneer. For Serebryakov and Semikhatov (1974, p. 567) “they are a combination of various structures originating under various conditions as the result of the life activity of one community (or species) of algae (Krylov, 1967a, 1972; Serebryakov, 1971; Serebryakov et al., 1972)”. Monty ( 1 9 7 3 ~ used ) the terminology of eurybiontic stromatolites for which different morphologies have originated due to environmental factors, and stenobiontic stromatolites which show very few morphological variations and are restricted t o a constant environment. (6) It is already clear that stromatolites with simple microstructures are widespread and seem t o have a restricted stratigraphical range: (a) Stromatolites with dark film microstructure appear all over the world, with a large range of morphological variation, but in a narrow interval of time, from the late Middle Riphean, to the early Late Riphean. They show great adaptability of the coenose to a wide variety of environments. (b) Filamentous tussock microstructure building columns with little morphological differences seem to inhabit more restricted environments during the Late Riphean. (c) During the Vendian and later, all the Cambrian vermiform microstructures show very little morphological variation also suggesting growth in a restricted range of environments. In conclusion, the definition and classification of stromatolite microstructures of the Late Precambrian confirm that the lamination and therefore the construction of stromatolites is often controlled by the algae (Raaben, 1964a; Monty, 1965a, 1972, 1973c; Krylov, 1967; Bertrand-Sarfati, 1972c; Walter, 1972a; Walter et al., 1973; Serebryakov and Semikhatov, 1974). This is in conflict with the assertion that lamination (Logan et al., 1964), columnar structure and ramification (Roehl, 1967) merely reflect variations in the activity of currents and sedimentation rates.


6. MORPHOGENESIS

Chapter 6.1 STROMATOLITE MORPHOGENESIS IN SHARK BAY, WESTERN AUSTRALIA Paul Hoffman

INTRODUCTION

Hamelin Pool, a hypersaline lagoon in Shark Bay, is apparently the only occurrence of modern stromatolites of comparable size and diversity with Proterozoic stromatolites. The Hamelin Pool stromatolites grow in environmental settings ranging from wave-raked shorelines of high physical stress, to protected tidal ponds of low stress. The most impressive stromatolites are in the high-stress environments and their morphogenesis is controlled by physical processes. Smaller stromatolites and sheet-like cryptalgal deposits occur in settings of low stress, where morphogenesis is dominated by the biotic processes of the algae. The Hamelin Pool stromatolites were first described by Logan (1961); enlarged upon by Logan et al. (1964);and treated comprehensively by Logan et al. (1974).What follows is a condensation, eliminating much important detail, of the most recent publication. BATH YMETRY AND ENVIRONMENTAL PROCESSES

Shark Bay (Fig. 1)is a bilobate embayment of the Indian Ocean on the west coast of Australia. Hamelin Pool, the southeasternmost lobe, is silled by a sea-grass barrier at its mouth. The climate is semi-arid and freshwater nm-off from inland is negligible. For these reasons, the salinity of Hamelin Pool is nearly twice that of normal seawater. In the less saline parts of Shark Bay, invertebrates graze the sediment surface and prevent the growth of algal mats in the lower intertidal and sublittoral zones. But in Hamelin Pool, the grazing invertebrates are inhibited by the high salinity and, as a result, algal mats grow in a broader range of environments than elsewhere.

P. HOFFMAN

262 supratidal flats beach ridges intertidal algal mats sublittoral shelf 10 krn

O

A

p......._... ........;.y ! F ’

;I..

HAMELIN POOL

Fig. 1. Map of Shark Bay showing the location of Hamelin Pool and the sea-grass banks and sills (stippled). Fig. 2. Bathymetric subdivisions of Hamelin Pool.

Bathymetric subdivisions Hamelin Pool (Fig. 2) consists of: (a) a central basin floor, 8-10m deep; (b) a sublittoral shelf, less than 5 m deep; and (c) a coastal strip, in the intertidal and supratidal zones. The coastal strip is made up of skeletal and ooid sand, produced mainly on the sublittoral shelf but driven onshore by waves during cyclonic storms and the prevailing southerly gales. The coastal strip is differentiated along its length according to different intensities of wave attack: (1)The most intense wave attack is at r o c b headlands, where the sublittoral shelf is relatively narrow and deep. Wave attack is multidirectional. The coastal strip is less than 100 m wide, and sediment is coarse and transient.

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263

(2) Intermediate are the beaches and sandflats located in arcuate bights, where the sublittoral shelf is relatively broad and shallow. Waves are refracted onto an onshore track during their passage across the shelf so that wave scour, concentrated along the seaward margin of the coastal strip, is perpendicular to the shoreline. Sedimentation rates are high and the intertidal sandflats prograde, producing a moderately broad supratidal zone, typically lined with beach ridges of bivalve coquina. Continued progradation will ultimately engulf the headlands with sediments and straighten the coastal strip, reducing its lungitudinal differentiation. (3) Most protected from wave attack are shorelines that face north (e.g. Nilemah Embayment) and those around tidal ponds behind barrier spits (e.g. Hutchison Embayment). Tidal currents may be strong in the outlets of the ponds but wave action is negligible. The supratidal zone is very broad, up to 2000 m, and generally lacks beach ridges. Factors controlling algal-mat distribution Algal mats are most prominent in the intertidal zone but, in places, extend into the lower supratidal and shallow sublittoral as well. There are seven basic types of mat (Fig. 3), each a complex community of several species of blue-green algae. Each mat type is dominated by a unique combination of species. Each is distinctive in its appearance, in the way it traps and/or binds sand to produce stromatolites, and in the laminated or unlaminated internal fabric it imparts to the stromatolites. The distribution of mats and their zonation by type are controlled by the following physical processes: (1)Desiccation. The zonation of mats is controlled most of all by the desiccation gradient (Fig. 4). As desiccation increases, the number of algal species decreases, thus allowing different combinations of species to dominate each successive mat type. The gradient is related mainly to elevation, the higher tidal flats being less frequently flooded than the lower ones. However, even the higher flats have depressions where water is ponded and desiccation is low. (2) Sedimentation. A secondary control of the mat zonation is the sedimentation rate (Fig. 4). In certain areas, particularly where the sublittoral shelf is very broad, sediment influx to the coastal strip is so rapid that algal growth cannot keep pace, and the intertidal zone is barren of mats. (3) Cementation. Precipitation of intergranular aragonite results in extensive lithified crusts in the intertidal and, to a lesser extent, sublittoral zones. Typically, the mats colonize exposed crusts because they offer a more cohesive substrate than loose shifting sand. Once established, however, the mats resist erosion in even the most turbulent environments. Furthermore, the stromatolites are themselves rapidly lithified during growth, accounting for their remarkable wave resistance. Thus, cementation is doubly important,

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264

’ILM cryptocrystalline rinds on lithified crusts

Hormatonema vrolacco -rugrum

3LISTER Microcoleus chtonop/sstes etc

-

- --_... - -- inorganic laminations -- with algal partings

TUFTED scallop fabric

Lyngbya mstuerii etc

GELATINOUS Symploce iwte-viriciis ? etc

SMOOTH Schizothrix helve etc

-

-,

--

ribbon fabric with polygonal prism cracks

laminated fabric with fine fenestrae

COLLOFORM Mictvcokws tmnwrimus etc

digitate fabric with coarse fenestrae

Fig. 3. Name, dominant bluegreen algal species, and characteristic internal fabric of the seven mat types in Hamelin Pool.

both in providing sites for the initiation of mats and in subsequent lithification of mat-bound sediment. (4) Erosion. Wave scour and, to a lesser extent, tidal run-off serve to expose cemented crusts to mat colonization. Wave scour also tends to prevent accretion in the depressions between stromatolites, thus allowing the tops of the stromatolites to grow well above the surrounding substrate. Where waves are strongly refracted, unidirectional wave scour results in elongate stromatolites.

MORPHOGENESIS

265

supratidal

intertidal

SMOOTH MAT sublittoral

COLLOFORM MAT I

(open coastline)

(tidal pond)

Fig. 4. Distribution of mat types on an environmental process grid. Mat types in capital letters produce columnar structures; those in lower case only stratiform cryptalgal sheets. DISTRIBUTION OF STROMATOLITE MORPHOTYF'ES

The stromatolites are most readily classified according to the mat types that build them: (a) colloform mat structures; (b) smooth mat structures; (c) pustular mat structures. The other 4 types produce only stratiform sheets. The distribution of stromatolite morphotypes is summarized in Fig. 5. Colloform mat structures Stromatolites of diverse morphology occur on the sublittoral shelf to depths of 2 m below mean sea level. All are formed by the colloform mat and inherit its coarse fenestrate internal fabric of multiconvex laminations. The mats support a dense growth of.the green alga AcetabuZaria and are populated by serpulid worms and small bivalves. These disrupt the mat growth and contribute to the generally cavernous margins and irregular morphology of the stromatolites. At rocky headlands, irregular columns, up to 1.0m in relief, having discrete bladed branches with flattened tops, grow on cemented crusts of bivalve coquina. In places, the columns coalesce to form bioherms, many meters in diameter, with a continuous outer shell. In the bights, the stromatolites become more prolate and generally have only 0.5 m relief. Individual structures are' elongate perpendicular to the shoreline (i.e. parallel to the direction of wave scour), but they may be arranged in rows parallel to the shoreline. The structures commonly coalesce laterally along the length of the rows, producing compound masses with elongation normal to that of the primary structures. On the sublittoral floor of the tidal pond in Hutchison Embayment, stromatolites with a digitate columnar internal structure occur beneath ovoid patches, less than 0.5m in diameter, of colloform mat. Unlike the

P. HOFFMAN

266 HEADLAND

.. . .

BIGHT

EMBAYMENT

.. ...

Fig. 5. Generalized distribution of stromatolite morphotypes.

free-standing stromatolites on the open shelf, these masses are almost buried in pellet mud.

Smooth mat structures Stromatolites built by the smooth mat have a fine fenestrate internal fabric of simple convex laminations. Except in protected embayments, the lower intertidal substrate is so unstable that the mats initially colonize only lithified crusts and crust fragments. Once established, the mat-bound sediment is itself rapidly cemented. Discrete columnar structures prevail because the mats are unable to breach the loose sand in depressions between the columns. The stromatolite columns are largest at rocky headlands, attaining a relief of 0.8 m. The columns are circular in plan, and tend to broaden and coalesce at the top (Fig. 6a), particularly where they project into the upper intertidal zone of pustular mat. Toward the bights, the relief of the columns is greatly reduced and the stromatolites become strongly elongate perpendicular to the shoreline (Fig. 6b). Some bights have bands, oriented parallel t o the shoreline, of

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267

“ridge-and-rill” structures (Fig. 6c). The “ridges” are extremely elongated stromatolites of low relief, oriented perpendicular to the shoreline, separated by “rills” of rippled sand. The seaward ends of the ridges tend to be swollen and oversteepened, and some of the ridges project seaward into rows of small discrete columns tilted seaward (Fig. 6d). Many of these columns widen upward and branch (Fig. 6e). In places, calyx-like structures occur where stromatolite columns have been decapitated, their less well cemented centers hollowed out, and their r i m s recolonized by mats.

Pustular mat structures Upper intertidal structures built by pustular mat have an irregular fenestrate fabric without laminations. Most of the structures are inherited from lower intertidal ones as a result of their upward growth. They range from high-relief circular columns at headlands, through medium-relief elongate columns, to low-relief ridge-and-rill structures in bights. The upper intertidal ridges tend to be broader and more sinuous than those in the lower intertidal. In places, the rills are colonized by gelatinous or tufted mats, whereby the unlaminated ridges become flanked by laminated cryptalgal deposits.

Stratiform cryptalgal sheets Wherever wave or current action is too weak or impersistent to prevent mats from colonizing loose sand, mats form continuous undifferentiated sheets that grow without forming structures with significant relief. This occurs throughout the intertidal zone of protected embayments and in protected parts of the upper intertidal zone in bights. The internal fabric of the sheets is controlled by the mat type’ (Fig. 3), distributed as follows: (a) smooth mat in the lower intertidal zone; (b) pustular mat in the upper intertidal zone; (c) blister mat in the lower supratidal zone; (d) gelatinous mat in poorly drained depressions and ponds; and (e) tufted mat in better drained depressions.

STROMATOLITE MORPHOGENESIS : SUMMARY AND CONCLUSIONS

From Fig. 5 , it is clear that the gross morphology of the stromatolites is not controlled by the type of algal mats. Smooth mat alone, for example, produces structures ranging from stratiform sheets to high-relief columns. Conversely, similar morphologies, the distinctive ridge-and-rill structure for example, are built by as biologically distinct mat types as the filamentous smooth mat and the coccoid pustular mat. The mat type does control the internal fabric of the stromatolites, filamentous mats producing laminated structures and coccoid mats unlaminated ones. But even the distribution of

268

P. HOFFMAN

MORPHOGENESIS

269

Fig. 6. Stromatolites near Carbla Point, 10 km south of the Hutchison Embayment. a. Circular columns, 0.8 m in height, at a rocky headland. b. Columns elongate perpendicular to the shoreline, in a bight near a headland. c. Low-relief ridge-and-rill structures, seaward to the left, in bight away from a headland. d. Small tilted columns, seaward t o the left, in a bight. e. Detail of branching columns in a bight.

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P. HOFFMAN

mat types is ultimately related, as is the stromatolite morphology, to physical processes of the environment. The following generalizations may be useful in interpreting the environment of ancient stromatolites: (1)Stratiform cryptalgal sheets occur where wave and tidal scour are weak. (2) Discrete columnar structures occur where wave and tidal scour are strong. (3) Relief of columnar structures is proportional to the intensity of wave action. (4) Elongation of simple stromatolites is parallel to the direction of wave and tidal scour, generally perpendicular to the shoreline. (5) Elongate stromatolites tend to occur in rows or bands parallel to the shoreline. (6) Stromatolites tend to be oversteepened or tilted seaward into the oncoming waves. Thus, shorelines can be subdivided into three bathymetric types according to their stromatolite assemblages (Fig. 7): (a) shorelines fully exposed to waves have large columns (diameter 30-100 cm) of high relief (greater than

Fig. 7 . Change in stromatolite morphology from circular columns at a headland to bands of ridge-and-rillstructures in a bight.

MORPHOGENESIS

27 1

50 cm); (b) shorelines with a shallow sublittoral shelf offering partial protection from wave attack have small tilted columns (diameter 5-15 cm) and ridge-and-rill structures of moderate relief (10-30 cm); ( c ) shorelines completely protected from waves by barrier islands or spits have stratiform sheets of low relief (less than 5 cm). CAUTIONARY POSTSCRIPT

Although Hamelin Pool contains the most diverse assemblage of modern stromatolites, many ancient stromatolites doubtless grew in environments not represented there. This is particularly true for sublittoral settings of low environmental stress, where the conclusions regarding stromatolite morphogenesis in shoreline environments, listed above, may not be applicable.


6.MORPHOGENESIS

Chapter 6.2

MICROBIOLOGY AND MORPHOGENESIS OF COLUMNAR STROMATOLITES (CONOPHYTON, VACERRILLA) FROM HOT SPRINGS IN YELLOWSTONE NATIONAL PARK M.R.Walter, J. Bauld and T.D. Brock

INTRODUCTION

The hot springs of Yellowstone National Park in Wyoming constitute an environment that in some ways is analogous to Precambrian marine environments. There is widespread inorganic deposition of silica, as during the Precambrian (Holland, 1971; Walter, 1972b). At higher temperatures there are no animals and the biota consists of cyanophytes and bacteria, as during much of the Precambrian (e.g. Schopf, 1970). It is in this “primitive” environment that we have found two kinds of columnar stromatolites; one of these is Conophyton, a group previously thought to have stopped forming near the end of the Precambrian; the other is Vacerrilla, a previously undescribed group containing only one form. The great amount of microbiological data on the Yellowstone springs (e.g., Castenholz, 1969; Brock, 1970) provides an excellent opportunity to study in depth the microbiology of columnar stromatolites. Furthermore, the significant environmental features of the outflow channels and pools can be determined readily. It is thus possible t o attempt to relate stromatolite morphology to both the environment and the constructing microbiota, and to attempt to resolve the conundrum of stromatolite morphogenesis, at least for two groups of columnar stromatolites. Earlier results of our work showed that. stromatolites can be primarily siliceous (Walter, 197213) and that bacteria can build stromatolites (Walter et al., 1972; Doemel and Brock, 1974), thus fulfilling the primary objectives of the study. The discovery of Conophyton was unexpected but is of great interest. This is one of the most distinctive groups of Precambrian stromatolites. We now have the opportunity t o analyse its morphogenesis t o discover what biological and environmental information is stored in the Precambrian forms. Walcott (in Anonymous, 1916) and Barghoorn and Tyler (1965) have also looked to Yellowstone for Recent analogues of the Precambrian stromatolites

274

M.R. WALTER E T AL.

they were studying. The Yellowstone stromatolites were first described in detail by Weed (1889a), in a comprehensive monograph on Yellowstone sinters. Korde (1961) has commented that Weed’s illustrations show some Conophyton-like stromatolites. The Yellowstone conophytons are similar to those described from the Precambrian by Hofmann (196913) and the Precambrian conophytons illustrated by Vlasov (1970) are astoundingly like those in Yellowstone. The interdisciplinary nature of this study presented certain problems based largely on the different experience of the three investigators involved. All of these problems were resolved except one: we were unable to agree that stromatolites should be given Linnean binary names. For this reason the stromatolites are named in an appendix t o this chapter, under the sole authorship of M.R. Walter. In his defence this author refers to the arguments of Cloud and Semikhatov (1969b), which apply as much to Recent as t o ancient stromatolites. METHODS

(1) Study areas Investigations were carried out at the following localities in Yellowstone Park: Main Spring, Jupiter Terrace (Mammoth Geyser Basin); Queens Laundry Spring (Sentinel Meadows); “Weeds Pool” (Fountain-Paint Pots area, 600 m W of Celestine Pool); Grand Prismatic Spring and an unnamed spring, 900 m SE of there on the east bank of the Firehole River (Midway Geyeer Basin); “Conophyton Pool”, 1OOOm W of the south end of Goose Lake, “Column Spouter”, 800 m NNW of the north end of Goose Lake, two unnamed springs 900m NW of the north end of Goose Lake (Fairy Creek meadows); White Crater Spring (Shoshone Geyser Basin). The names in quotation marks are unofficial. For localities see U.S.Geological Survey topographic map “Yellowstone National Park” and the 15’ quadrangle maps. (2) Environmental parame ters

Temperature, pH and light intensity were measured as described in Bauld and Brock (1973). Water-flow patterns and velocities were determined by injecting into the water a concentrated solution of methylene blue dye, and by photographing the distribution of the dye at intervals of 1-3 seconds. In some cases it was sufficient t o time the passage of the dye along a measured course, without photographing the distribution pattern. Vertical flow gradients in relatively deep pools were determined by injecting the dye at several depths.

MORPHOGENESIS

27 5

(3) Growth rate studies in the field To study the rate of formation of the stromatolites and the temporal significance of their laminae, fine carborundum powder was sprinkled over them to provide reference laminae. About fifteen sites in various pools were marked and sampled on several occasions. Soft, flat microbial mats were sampled with a cork borer and heavily mineralized mats were cut with a knife. The mats were marked during August 1971 and April, June and July 1972. Samples were taken during April, August and November 1972. One- to threeday experiments to check for diurnal lamination were run at three localities. (4) Identification of the microorganisms

Morphology Morphological observations were made using a Zeiss RA or a Zeiss Universal microscope equipped with phase-contrast and differential interference contrast optics. The cyanophytes were classified using Copeland's (1936) descriptions of Yellowstone species. Enrichment and isolation procedures Stromatolite samples from a variety of hot-spring effluents and pools were treated in the following manner. Algal enrichments were obtained by inoculating 10 ml of medium D (Castenholz, 1969) with natural material. The glass, screw-capped culture tubes containing the inoculated medium were incubated at temperatures of 30-40°C,' or 50°C, under fluorescent illumination. Cultures were examined by phase-contrast microscopy after 3-4 weeks. Unicellular blue-green algae are preferentially selected in liquid enrichment cultures (Stanier et al., 1971); to obtain pure cultures of the filamentous alga Phormidium , macerated stromatolite material was spread along a single line Difco Bacto-agar, medium D). The inocuon the surface of an agar block (1% lated agar surface was then placed in a light gradient. Phormidium filaments moved towards the light by gliding motility, away from immotile algae and bacteria. The most active filaments were transferred to a fresh agar surface and the process was repeated several times. Purity was assessed microscopically and by the absence of bacterial growth upon inoculation into suitable test media. Chloroflexus was enriched from stromatolite samples by inoculation into screw-capped culture tubes completely filled with medium D supplemented with 0.05% (w/v) yeast extract, 0.05% (w/v) NazS.9Hz0 and buffered with 0.5g/l of glycyl-glycine. The tubes were incubated at 5OoC under tungsten illumination. Cultures were examined microscopically after 3-4 weeks. Pure cultures of Chloroflexus were obtained under aerobic conditions as described by Bauld (1973).

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Pigment analyses Pigments were extracted from stromatolites and mat samples with methanol or acetone as described in Bauld and Brock (1973). Absorption spectra for the pigments were obtained with a Beckman DB-Ggrating.spectrophotometer equipped with an R136 photomultiplier tube having an extended response in th‘e red region. The in uiuo spectra of chromatophore preparations (Pierson and Castenholz, 1971; Bauld and Brock, 1973) were determined by R.W. Castenholz, University of Oregon, using a Cary model 14R recording spectrophotometer. Fluorescence microscopy Microorganisms from stromatolite samples were examined using a Zeiss Universal microscope equipped with a mercury arc lamp. When Phormidium and other algae are illuminated with blue exciting light, they can be seen because of the red fluorescence of their chlorophyll a . Chloroflexus, on the other hand, contains bacteriochlorophylls a and c which fluoresce in the near infra-red region of the spectrum and not in the visible region (Pierson and Howard, 1972). ( 5 ) Relative abundances of the microorganisms in the stromatolites Lengths of algal and bacterial filaments were estimated by a method similar to that of Bott and Brock (1970) using a Zeiss Universal or a Zeiss RA phasecontrast microscope. Samples fixed in 4% formalin at the time of collection were homogenized and placed in a Petroff-Hausser counting chamber; the number of intersections made by filaments with the vertical and horizontal lines of the grid was recorded. Intersections with non-filamentous microorganisms were also recorded to provide an approximate measure of their abundance. Between 700 and 2700 intersections were counted for each sample.

(6) Measurement of 14C02 fixation (photosynthesis) in the stromatolites Sampling and incubation Stromatolitic tufts 1-2 mm high from “Conophyton Pool” were excised from the adjoining flat mat and placed in spring water taken from the sampling site. Tuft preparations were placed in 5-ml glass vials in “Conophyton Pool” and allowed to equilibrate for 10 min before adding 0.1 ml of 2.3mM NaH 14C03 (specific activity 8.7 pCi/pmole). The samples were incubated in the environment from which they were obtained. Preliminary experiments had shown that isotope incorporation was linear for up to 2 hours under such conditions.

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277

Differentiation between algal and bacterial photosynthesis In order to estimate bacterial photosynthesis in the presence of Phormidium, 3-(3,4dichlorophenyl)-1,ldimethylurea (DCMU) was added to the M ,sufficient to inhibit incubation vials to give a final concentration of algal photosynthesis completely (Bishop, 1958; Bauld and Brock, 1973). Some incubations were carried out under a Wratten 88A filter in the form of a 10 x 10 cm gelatin layer sandwiched between glass plates. The transmission of light at wavelengths below 720nm was about 0.1% of the total incident light. Use of this filter effectively precludes algal photosynthesis, which does not occur at wavelengths above 700nm. However, transmission of light at 740 nm (in uiuo absorption peak of b;acteriochlorophyll a) was virtually 100% (manufacturer’s specifications). Processing of radioactive samples and determinations of pigment content in these samples were as described by Bauld and Brock (1973). (7) Testing for phototaxis Macerated stromatolite samples, or laboratory cultures, were spread along a line on an agar surface. The agar, contained in a petri plate wrapped in aluminium foil, was placed in a light gradient and oriented so that the “spread line” was normal to the direction of the light. A small hole in the foil allowed light to enter the container. Phototaxis of Phormidium filaments was quantified by measuring the rate of advancement of the front of algal filaments moving towards the light source. The rate of gliding motility of Phormidium filaments from macerated samples of conical stromatolites was also determined by placing a portion of the material on an agar surface, and measuring the movement of individual filaments using a Zeiss RA phase-contrast microscope equipped with a long-working-distancecondenser and an ocular micrometer. ( 8 )Laboratory growth of conical stromatolites

A bacteria-free culture of Phormidium isolated from a conical stromatolite was used. Cultures were grown under fluorescent lights in a 37°C incubator room where the heat of the lights raised the temperature of the culture vessels to about 40°C. Erlenmeyer flasks containing a thick layer of solid medium D were inoculated with a suspension of filaments. Sufficient inoculum was used so that a confluent growth of the alga occurred after about four days of incubation. At this time, liquid medium D. was carefully added to fill the flasks, and the organisms were allowed to grow up into the liquid. Within 24-48 hours after addition of the liquid medium, small nodes of algal growth appeared on the surface of the solid medium at the bottoms of the flasks. To simulate the periodicity of sunlight a light-dark cycle of 12 hours was used, the light being controlled by an automatic timer. This resulted in the formation of two laminae per 24 hours.

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(9)Identification of opaline silica The siliceous composition of the stromatolites was determined by energy dispersal X-ray microanalysis. X-ray diffractometry showed that the silica is opal A (Jones and Segnit, 1971).Scanning electron microscopy showed that the silica in’ these samples has the spherical habit characteristic of many “opaline” silicas (Jones et al., 1964).

(10)Embedding and thin-sectioning the stromatolites The stromatolites were dehydrated through an ethanol series, infiltrated with propylene oxide, and impregnated with Spurr Low-Viscosity Embedding Medium B (a blend of epoxy resins). This medium was chosen because of its low viscosity and high solubility in ethanol and propylene oxide (Spurr, 1969);these characteristics facilitated thorough impregnation of the samples. “Curing” of the resins was achieved by heating the samples at 70°C for 8 hours in a vacuum oven. In samples thicker than about 2 cm the resins did not polymerize fully and remained soft. After embedding, thin sections of the stromatolites (80-100Vm thick) were prepared by conventional methods (Kummel and Raup, 1965). RESULTS

Description of columnar stromatolites The stromatolites described here are formally named in the Appendix t o this chapter. Here the two types are referred t o as flat-topped columns and conical columns. Terms used t o describe stromatolite morphology are defined in the Glossary (pp. 687-692). Flat-topped columns (Vacerrilla walcotti) Mode o f occurrence. This stromatolite occurs in sheets 2-3cm thick. The shape of the sheets is determined by the environment in which they occur. In outflow channels they are long, narrow and frequently lobate. On extensive terraces they are irregularly lobate. In the examples known the sheets have maximum horizontal dimensions of about 10 m (see Ch. 8.8). Column shape. The columns are erect and subcylindrical with flat t o gently convex tops (Fig. 1).They are 0.5-1.0 cm in width and have about 1 cm of relief above the substrate. Adjacent columns may coalesce so that in transverse section they are lobate. No branching of columns has been seen.

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279

Fig. 1. Flat-topped columnar stromatolites in an outflow channel of “Column Spouter”, Fairy Creek meadows. The columns are about 1 cm wide.

Lamina shape. Laminae are gently convex with abruptly deflexed margins. They are rectangular, rhombic or, infrequently, steeply convex (Figs. 2, 3). They frequently coat the column margins, forming a wall (Figs. 2-4). Bridging between columns is moderately frequent and may produce a pseudocolumnar structure. Microstructure. The lamination is predominantly banded with pale laminae 30-150pm thick and dark laminae 20-60pm thick (Figs. 2-4). A striated microstructure is present locally within some columns: dark lenses 30-150 pm thick are set in a pale matrix. The walls are up to 600pm thick (Figs. 2-4). In them lamination is absent or is diffusely banded to streaky. Filament orientation. Where the microstructure is banded the algal and bacterial filaments alternate from predominantly subvertical, in the pale laminae, t o predominantly parallel to laminae, in the dark laminae (Fig. 5). In the walls the filaments are subvertical ji.e. parallel to the walls). In the dark lenses of the striated microstructure the filaments appear to be randomly oriented. In the light matrix the filaments are subvertical. Distribution. This stromatolite was studied in the outflow channels of two hot springs: “Column Spouter” (Fairy Creek meadows) and near “Weeds

z.

Figs. 2-4. Longitudinal thin sections of flat-topped columnar stromatolites from "Column Spouter". Transmitted light..Scale bar 2 mm. Figs. 2 , 3 , CPC 15460; Fig. 4, CPC 15461.

8

K 3s

MORPHOGENESIS

281

Fig. 5. Thin section normal to the lamination of a flat-topped columnar stromatolite from “Column Spouter”, showing Phormidium and Chloroflexus filaments (indistinguishable in this figure) forming two laminae, a lower one with horizontal filaments (which formed at night) and an upper one with erect filaments (which formed during the day). The filaments are about 1pm wide. Photographed with differential interference contrast optics. CPC 15460

Pool” (Fountain-Paint Pots area); it was subsequently recognised in a number of other springs which are mostly unnamed. Columns with conical tops (Conophyton weedii) Mode of occurrence. This stromatolite occurs in sheets a few millimetres to 30 cm or so thick. The shape of the sheets is determined by the environment of growth, ranging from long and narrow (in outflow channels) through very irregularly lobate (within large springs) to annular (around the margins of hot springs with little flow). The sheet-like bodies are biostromes in the sense of Walter (1972a). A few are so narrow and thick that they can be called bioherms. The occurrence is further discussed and illustrated elsewhere in this volume (Walter, Ch. 8.8). Column Shape. The columns are subcylindrical and erect and have conical tops (Figs. 6-16). Transverse sections are subcircular. The columns are about

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M.R. WALTER ET AL.

6

7 Figs. 6, 7. Tufts, small cones, and ridges at early growth stages of conical stromatolites, “Conophyton Pool”. Scale divisions are 1 cm.

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283

Fig. 8 . Small cones from an outflow channel of “Column Spouter”. Pine needles indicate scale.

Fig. 9. Subaqueous cones and ridges in “Conophyton Pool”. The nail marks the site of an accretion rate experiment and the dark patch is carborundum powder. Scale divisions are 1 cm.

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Fig. 10. Subaqueous cones in Jupiter Spring, Mammoth area. Scale divisions are 1 cm.

0.1-3.5 cm wide and from less than 0.1 to 10 cm high. Most are 1-2 cm wide by 2-3cm high. The smaller examples are usually at least l c m apart; the larger columns are contiguous, or nearly so. Columns less than about 0.1 cm wide are referred to here as tufts (Figs. 6 , 7). This basic shape is frequently modified by the presence of tiny spinose projections and radially arranged, narrow vertical ridges (Figs. 12-16). In some springs (e.g. “Column Spouter”) no ridges or spines are developed; in others there are only very weakly developed ridges (e.g. “Weeds Pool”, White Crater Spring); ridges may be strongly developed (“Conophyton Pool”, Main Spring), in some places t o the extent that no columns are recognizable (parts of “Conophyton Pool”) ; spines are restricted to the ridged stromatolites (“Conophyton Pool”).

Lamina shape. Laminae are irregularly conical (Figs. 17-25), with apices ranging from sharp (where the laminae have a radius of curvature of less than 0.2 mm) tp blunt (where the laminae are horizontal for 2-3 mm). The apical line, linking successive apices, is straight to gently wavy (with amplitudes of the flexures ranging up to 1mm); abrupt displacements of the apical line are moderately frequent. The laminae are mostly ‘smooth, but some are slightly wavy or wrinkled.

MORPHOGENESIS

285

Fig. 11. Large cones from “Weeds Pool”, Fountain-Paint Pots area. Scale divisions are 1 cm.

Laminae in the spines (Figs. 25,26) are widely variable in shape; they range from almost planar (oriented parallel to the associated cone flanks) through irregularly convex to obtusely conical.

Crestal zone. All three types of crestal zone recognized in Precambrian Conophyton stromatolites (Komar et al., 1965a) are present in the Yellowstone cones (Figs. 23,24,27-29). The zone of thickening and contortion of the laminae is 0.5-2.0 mm wide in the five specimens in which it was measured. The degree of thickening of the laminae in the crestal zone is difficult to measure because the laminae are frequently diffuse, but it appears that most laminae are about twice as thick in the crests as on the flanks of the cones: thickenings of up to five times were noted. Microstructure. The microstructure ranges from banded to streaky and from distinct to diffuse; differences in appearance depend partly on the magnification at which the microstructure is viewed (Figs. 17-31). At low magnifications, it normally appears distinctly banded, with laminae 30pm or more thick. Laminae range down to only 1pm in thickness but most are 5-15 pm

28 6

M.R. WALTER ET AL.

Fig. 12. Ridged cones from “Conophyton Pool”. Scale divisions are 1 cm.

thick. The thinnest laminae are composed of single algal or bacterial filaments encrusted with silica. These laminae were excluded from the measurements presented in Fig. 32 (p. 298). Subspherical t o lenticular thickenings of laminae are relatively irlfrequent and irregularly distributed (Figs. 30, 31). They are mostly 0.5-1.5mm thick by 0.5-9.0mm long.

Filament orientation. The algal and bacterial filaments are usually arranged parallel t o the lamination. There are three exceptions: (1)subspherical to subcylindrical masses of tangled unoriented filaments occur at column bases (Figs. 17-19);(2)locally on column flanks there are subspherical t o lenticular thickenings in which the filaments are rolled up into concentric layers (Figs. 30, 31);(3)in the axial zones filaments tend toward a vertical orientation. This last case has several expressions: there may be a tuft of more or less vertical filaments at the apex of a cone - these are perpendicular to the lamination and may penetrate through overlying laminae as though injected into them (Fig. 29);there may be a mass of unoriented filaments; filaments may be slightly oblique t o the laminae, and sigmoidally purved (Fig. 28). Distribution. Conical stromatolites are known from the following hot springs and geysers in Y ellowstone National Park: “Conophyton Pool”, “Column Spouter” and two unnamed springs (Fairy Creek meadows); Queens Laundry

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13

14 Figs. 13, 14. Cones with ridges and spines, “Conophyton Pool”. Scale divisions are 1 cm.

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M.R. WALTER ET AL.

Fig. 15. Top view of ridged and spinose cones from “Conophyton Pool”. Width of field of view about 10 cm.

Spring (Sentinel Meadows);“Weeds Pool” (on the flank of the Fountain-Paint Pots group); Grand Prismatic, Tromp Spring and an unnamed spring (Midway Geyser Basin); White Crater Spring (Shoshone Geyser Basin); and Main Spring (Jupiter Terrace, Mammoth area). In addition, they have been seen in a number of other small, unnamed springs in Yellowstone. Similar structures have been collected from a spring at Hveravellir in central Iceland by T.D. Brock. Conditions o f stromatolite formation The stromatolites described here occur in thermal waters with temperatures between 32°C and 59°C and with pH values of 7-9. It was demonstrated by carborundum marking that the stromatolites are actively growing in the environments in which they were observed. Filamentous cyanophytes of the genus Phormidium, which are an essential component of the biotas of both the flat-topped and conical columnar stromatolites, are apparently absent from waters hotter than 59”C (the filamentous photosynthetic bacterium Chloroflexus aurantiacus Pierson and Castenholz forms stratiform and nodular stromatolites at higher temperatures: Doemel and Brock, 1974). Below about 32”C, coarsely filamentous cyanophytes such as Calothrix coriacea Copeland dominate the stromatolitic mats, which are stratiform or nodular.

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289

Fig. 16. Cones with well-developed radial ridges, “Conophyton Pool”. Scale divisions are 1 cm.

Animals are present at temperatures up to 48-51OC (Brock, 1967c; Castenholz, 1969; Wickstrom and Castenholz, 1973). These include rotifers, ostracods, spiders and flies. Although some of these organisms graze the microbial mats (Brock et al., 1969), they apparently are not effective in shaping the stromatolites we have studied, as morphologically comparable stromatolites also form at higher temperatures than those at which the animals can survive. Encrustations of silica may protect some of the stromatolites from grazing animals. Air temperatures in Yellowstone Park average 15°C during mid-summer and - 10°C during mid-winter. The hot spring waters remain free .of ice and snow during winter but, where measured, the water in pools and outflow channels was 2-3OC cooler in winter than in summer. The maximum midsummer light intensity of 6.1*1051ux drops t o a mid-winter maximum of around 1.9*1051ux.This has a marked effect on the pigmentation of many of the cyanophytes in the springs: during summer they are yellow-orange (carotenoid pigments dominating) whereas in winter they are dark green due t o an increased chlorophyll content (Brock and Brock, 1969b). The flat-topped columns were studied during 1971-1972 in three outflow channels: two at “Column Spouter” (Fairy Creek meadows) and one immedi-

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M.R. WALTER E T AL.

Figs. 17-19. Thin sections of tufts and flat mats, which are the initial accretion stages of cones, "Conophyton Pool". Transmitted light. The black layer near the bottom of each photograph and the black, angular grains above the laminated mats are carborundum powder introduced during accretion-rate experiments. The filaments near the tips of the tufts are the cyanophyte Calothrix. CPC 15462.

z

s

d

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20 Figs. 20-22. Longitudinal thin sections of cones from "Conophyton Pool". Lateral surfaces of the cones were broken during collection and thin-sectioning. There were many bridging'laminae between adjacent cones. Transmitted light. Scale bars 1 cm. Fig. 20, CPC 15463, Fig. 21, CPC 15464; Fig. 22, CPC 15465

3 .

23

24 hl

Figs. 23, 24. Longitudinal thin sections of cones from “Conophyton Pool”, showing the crestal regions of the laminae. Note the deformed cone tip immediately above the scale in Fig. 23. Transmitted light. CPC 15466.

(D

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z

%

2r

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Figs. 25, 26. Longitudinal thin sections of cones from "Conophyton Pool" showing surficial spines. The spine in Fig. 26 projects from the side of a'cone. Transmitted light. Fig. 25, CPC 15467; Fig. 26, CPC 15468.

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295

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27 Figs. 27-29. Longitudinal thin sections of cones from “Conophyton Pool” showing the crestal regions of the laminae. Fig. 28 is an enlargement of part of Fig. 27. Note the vertical tuft of filaments in Fig. 29. Transmitted light. Scale bar in Fig. 27, 1 mm. CPC 15468.

29

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8 Figs. 30, 31. Longitudinal thin sections showing knoh of coiled filaments within the flanks of cones from "Conophyton Pool" Transmitted light. CPC 15464.

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MORPHOGENESIS

297

ately above “Weeds Pool” (Fountain-Paint Pots area). Stromatolites from all these localities are siliceous. “Column Spouter” erupts approximately hourly. During eruptions the columns are submerged by several centimetres of water with a temperature of up to 47’C. Eruptions last about 45 min. For about 15 min between eruptions there is very little water flow and during this period the stromatolites, which occur near the margins of outflow channels, are exposed to the atmosphere, and the temperature of any water flowing around the columns drops markedly, commonly to 20-25°C. Similar conditions prevail in the channel near “Weeds Pool”. In 1971-1972 the conical stromatolites were abundant in ten springs and geysers. In nine, silica is precipitating ftom the thermal waters; in the tenth (Main Spring, Jupiter Terrace, Mammoth area), calcium carbonate is precipitating in the form of aragonite (Friedman, 1970). The various waters range in temperature from 32’ t o 50°C and in pH from 7 t o 9. As most previous studies of stromatolites have attempted to relate their morphology to environmental influences, especially to directional water currents, particular attention was paid to these factors in Yellowstone. The waters in which conical stromatolites are growing range from intermittently flowing in. outflow channels, through gently and irregularly flowing sheets of water in shallow pools, to nearly still in pools up to 30 cm deep. Measured flow rates varied considerably. In the outflow channels of “Column Spouter” the cones are submerged in several centimetres of water flowing at 5-20 cm/sec at peak flow, but are exposed to the atmosphere between eruptions. In “Conophyton Pool” the cones are continually submerged in 2-10cm of water in which current directions and velocities are variable, depending on wind direction and intensity as well as on the rate of outflow from the spring (which discharges continually). In this pool there are large differences in flow rates between the water surface and the bottom: on one occasion flow at the surface was 3-6cm/sec, but one centimetre beneath the surface no flow was detected. Flow rates of 1-10 cm/sec were measured at the tips of 2-3 mm high cones on a windy day. Flow around the cones in White Crater Spring was measured only once and was about 1cm/sec. On a windy day, flow in “Weeds Pool” was 5-6 cm/sec at the surface and 0.5-1 cm/sec around the cones 5-10 cm below the surface; this pool is 30 cm deep, and is the deepest pool known to contain conical stromatolites.

Microbiology The microbiology of the flat-topped columnar stromatolites is very poorly known because before they could be studied in detail slight changes in outflow patterns destroyed them all. They were constructed by a cyanophyte which in one sample was identified as Phormidium truncutum var. thermule Copeland, accompanied by the photosynthetic filamentous bacterium Chloroflexus uuruntiucus Pierson and Castenholz. The unicellular cyano-

M.R. WALTER ET AL.

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MORPHOGENESIS

299

phytes Synechococcus liuidus Copeland and S. mineruae Copeland were also present. Other filamentous cyanophytes were sporadic. The conical stromatolites were studied in ten hot springs and geysers. In each case the microbiota was essentially the same: the dominant organism was the filamentous cyanophyte Phormidium tenue var. granuliferum Copeland. Next most abundant was the bacterium Chloroflexus aurantiacus. The unicellular cyanophyte Synechococcus Naegeli was usually present, with S. liuidus much more abundant than S. mineruae. Other filamentous cyanophytes were sporadic; these included forms resembling Pseudanabaena and Isocystis. Organisms resembling myxobacteria were present in some samples and Spirillum-like bacteria and spirochetes occurred rarely. The microbiota of the calcareous cones from the Mammoth area was somewhat different from that of the siliceous forms. The Phormidium species present was slightly coarser (trichomes 1.0-1.7 pm wide) than P. tenue var. granuliferum (trichomes 0.8-1.2 pm wide) though in other ways it was indistinguishable. As is shown in Table I, P. tenue var. granuliferum was usually much more abundant in the cones than in the mats between the cones. Conversely Chloroflexus was less abundant in the cones than in the mats. P. tenue var. granuliferum was most abundant in the tiny tufts and in the smallest, most recently developed cones. The ridges and spines frequently present on the cones had a microbiota that was indistinguishable from that of the cones generally. To further our analysis of the morphogenesis of the conical stromatolites we determined the relative photosynthetic activities of natural populations of Phormidium and Chloroflexus in the stromatolites from Conophyton Pool; these are the two most abundant organisms in the cones. To do this, lightstimulated 14C02 fixation (photosynthesis) was measured under the prevailing environmental conditions (see Methods section). The results are presented in Table 11. These data show that Phormidium was responsible for about 90% of the total 14C02 fixation in the tufts; the remainder (DCMU-insensitive photosynthesis) was attributable to Chloroflexus. Incubation under a Wratten filter which excluded light of wavelengths suitable for algal photosynthesis resulted in an inhibition of algal photosynthesis in the tufts similar t o that observed in the presence of DCMU. A statistical analysis of forty samples that included both tufts and the surrounding mat showed that the mean chlorophyll a content of the tufts, when standardized per unit protein, was about 1.5 times that of the mat from which they arise. This is consistent with the microscopic observation of more dead cells in the mat than in the tufts. It is apparent that Phormidium is primarily responsible for the construcFig. 32. Graphic plots of lamina thicknesses in Conophyton weedii. For the contouring of diagram D, the number of points within squares with 5 m m sides were counted (in this reproduction the squares have 4.0-mm sides). This is the standard manner of data presentation for Conophyton stromatolites.

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TABLE I The relative proportions of various microorganisms in the conical stromatolites from two springs, expressed as percentages of the total microbiota (see Methods, section 5 )

P.

C.

S.1.

S.m.

“P.”

“I.”

m.

“S.”

s.

0.4 0.3

-

0.5 0.2

-

0.6

-

-

0.3

“Conophytoa Pool” Tufts Mat between tufts Cone tips Mat between cones

82.9 64.0

8.2 25.2

2.3 6.5

0.3 0.2

5.4 3.1

0.6 0.5

-

72.1 59.7

16.3 28.7

4 .O 8.2

0.4 1.7

6.4

0.4 0.5

-

83.7 85.6

13.0 12.7

3.3 1.8

54.1 23.2

30.9 29.8

8.3 24.6

0.4 1.4

-

0.5

5.8 20.2

1.1

-

-

-

“Column Spo u ter ” Small cones Mat between small cones Large cones Mat between large cones

-

P. = Phormidium tenue var. ganuliferum ;C. = Chloroflexus aurantiacus; S.1. = Synechococcus liuidus; S.m. = Synechococcus mineruae; “P.” = Pseudanabaena-like cyanophyte; “I.” = Isocystis-like cyanophyte; m. = myxobacteria (?); “S.” = Spirillum-like bacteria; s. = spirochetes.

tion of the cones. Chloroflexus may play some role by utilizing the excretory products (e.g. mucus) of Phormidium as a carbon source (Bauld and Brock, 1974), thereby affecting the physical properties of the mats and cones (e.g. by reducing the cohesiveness of the aggregates of filaments). However, we have demonstrated in the laboratory that Chloroflexus-free cultures of P. tenue var. granuliferum are capable of independently forming conical structures (Figs. 33’34);these grow rapidly under ideal laboratory conditions. Conversely, natural Chloroflexus mats are frequently nodular but are never conical (Doemel and Brock, 1974).

Controls of stromatolite morphogenesis and lamination production Biological influence Many species of Phormidium are present in Yellowstone waters (Copeland, 1936) but apparently only P. tenue var. granuliferum constructs conical stromatolites of the kind described here. The flat-topped columnar stromatolites seem to be built by a different species, P. truncatum var. thermale (this correlation of cyanophyte morphotypes with stromatolite morphotypes has a validity independent of that of the cyanophyte taxonomy; Copeland’s (1936) taxonomy and nomenclature are used for convenience, with the

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TABLE I1 Photosynthetic 14C02 fixation in conical tufts from “Conophyton Pool” (see Methods, section 6 ) Component

cPm/Pg Chl a

1. Phormidium -I-Chloroflexus 2. Chloroflexus (4- DCMU) 2a. Chloroflexus (88A filter)* 3. Phormidium**

9419 681 (1295) 8738

fixation

Relative abundance (%)

100 7 (13) 93

100 9 9 91

% total

It is assumed that Phormidium and Chloroflexus account for all of the fixation of I4CO2. The amount of fixation is expressed as counts per minute (cpm) standarized to the chlorophyll a (Chl a ) content of the samples. The relative abundances of Phormidium and Chloroflexus in the samples are expressed as percentages, assuming that these two groups of microorganisms are the only components of the tufts (see Methods, section 5). Prevention of algal photosynthesis was by addition of DCMU (column 2) or incubation under a filter (column 2a). Each value for I4CO2 fixation is the average of four incubations. *No dark controls. The percentage of 14C02 fixation given in parentheses was corrected only for absor tion. **Values for’ CO2 fixation by Phormidium were obtained by subtracting component 2 from component 1.

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understanding that the named cyanophyte morphotypes may not represent true genetic taxa). These facts clearly suggest biological control of stromatolite morphogenesis, an interpretation that is supported by the growth of the conical stromatolites in a variety of hydrological regimes (including a laboratory flask).

Effects of light Light may be expected t o play a key role in stromatolite accretion since these structures are constructed by photosynthetic microorganisms. Previous studies of marine stromatolites (Monty, 1967;Gebelein, 1969)have shown that the orientation of cyanophyte filaments within stromatolites can be controlled by light and that lamination can be produced by the diurnal variation in intensity of light. In these examples, laminae with vertical filaments form during the day whereas those with horizontal filaments form at night. Diurnal light variation is responsible for similar lamination in the high-temperature bacterial stromatolites in Yellowstone, but in this case the filaments migrate upward at night (Doemel and Brock, 1974).Two interrelated aspects of our study involved investigating the role of light in morphogenesis and the temporal significance, if any, of the lamination. It was difficult to predict the effects of light because both bacteria and cyanophytes (which may respond differently) are abundant in the stromatolites studied and furthermore the filamentous bacterium present (Chloroflexus) is only facultatively phototrophic (Pierson and Castenholz, 1974).

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MORPHOGENESIS

303

To investigate stromatolite growth in the dark, a long-term experiment was conducted from August 1971 t o April 1972 in which a large light-proof black plastic box was used to shade about a square metre of cones in “Conophyton Pool”. The box did not prevent water movement; however, deprivation of light did prevent algal and bacterial photosynthesis and photoheterotrophism. The darkened stromatolites and those outside the box were marked with carborundum powder. After seven months the uncovered columns had accreted 0.5-2.0 mm and the new deposit was finely laminated. The columns under the box had a very uneven and almost completely unlaminated coating up t o 0.5 mm thick above the carborundum. These results demonstrate that the conical stromatolites will not accrete in the dark. In another experiment, run for three months, an infra-red filter was used to prevent photosynthesis by the cyanophytes but not by the bacteria. This also stopped the accretion of the stromatolites. It is apparent from these two studies that light of the wavelengths used for algal photosynthesis is necessary for accretion. The intensity of incident sunlight is subject t o two major variations, diurnal and seasonal, which may be reflected in patterns of stromatolite accretion. Like the diurnal laminae of some marine stromatolites (Monty, 1967; Gebelein, 1969) the laminae of the flat-topped columns in Yellowstone consist of alternations of vertical and prostrate filaments. The laminae composed of vertical filaments are pale and 30-150 pm thick, whereas those composed of prostrate filaments are dark and 20-60 pm thick. A fourday experiment at “Column Spouter” demonstrated that these laminae also form diurnally, with the filaments standing vertically during the day and lying prostrate at night. Other experiments in the same hot spring showed no accretion over the same period. A 12-month experiment in “Column Spouter” during which carborundum layers were emplaced on three occasions also indicated that the lamination is diurnal. However, laminae do not form every day. During summer about 70% of the days are represented by pairs of laminae (pale and dark), but very few laminae formed during winter. The following results were derived from a single specimen in which the three marked layers were visible. Between 2 August 1971 and 20 April 1972 only 27 laminae formed, mostly near the beginning and end of this period (i.e. not during winter). Winter is represented by a 1mm thick, dark, coarsely and diffusely laminated layer. Between 20 April and 20 June 1972 (61 days), 45-49 laminae formed. Between 20 June and 21 July (31 days), 20 laminae formed. In other examples there was either no growth during winter or else Calothrh coriacea overgrew the columns (because of a temperature drop resulting from changing water-flow patterns). Monty (1967) has observed that lamination is not strictly diurnal in Bahamian stromatolites either. Figs. 33,34. Conical structuresformed by a pure culture o f Phormidium tenue var.gronuliferum. Figures show cones as they appeared twelve days after addition of liquid growth medium to a preformed algal mat (see Methods section).

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M.R. WALTER ET AL.

Fig. 35. Submerged, inclined cones in White Crater Spring, Shoshone Geyser Basin. Scale divisions are 1 cm.

N o clearly diurnal lamination was found in the cones. One example from “Column Spouter” shows possibly diurnal lamination, but was overgrown by Calothrix coriacea before the accretion experiment was completed. Three scales of lamination are usually discernible in the cones. Laminae about 1 pm thick are ubiquitous; these consist of single layers of silica-encrusted filaments and their thickness is determined by that of the filaments (0.7-1.5pm). Up to six such laminae can form in one day. The other two scales of lamination may be distinct or diffuse and are not always present. Laminae 5-15pm thick occur frequently. They are difficult to .count but those from marked stromatolites in “Conophyton Pool” may be crudely diurnal. Frequently there are more laminae than days elapsed. The thickest laminae can be termed macrolaminae. In one experiment 7-8 macrolaminae 35-60 pm thick formed in 8%months. Thus, formation of laminae in the cones is not clearly controlled by diurnal variations in light intensity. It has been suggested that the inclination of some fossil stromatolite columns fesults from the phototropic (heliotropic) growth of the constructing organisms (Norderg, 1963;Vologdin, 1963). In Shark Bay, Australia, the inclination of stromatolite columns is due to tidal current action (Hoffman, Ch. 6.1)or other, unknown, factors (Playford and Cockbain, Ch. 8.2). In the environment of hot pools in which water movement is relatively gentle, heliotropism might be expected to be a significant process. In the relatively high

MORPHOGENESIS

305

latitude of Yellowstone Park (45’) heliotropism, if present, should be apparent. However, with only one exception, all columnar stromatolites observed there accrete more or less vertically. The exception occurs in White Crater Spring where the conical columns are inclined about 20” from the vertical in a direction of S 45’W (Fig. 35). Water flow in the pool was measured at 1cm/sec and the columns were inclined in the same direction, suggesting that deviation from the vertical in this example may have resulted from directional water flow. Possibly the lightdiffusing effects of clouds, refraction at the air-water interface, and scattering within the water, combine t o reduce any inclination due t o heliotropic growth. Inclination of stromatolite columns due to heliotropic accretion has not yet been demonstrated in either ancient or Recent stromatolites. On a smaller scale heliotropism may be significant. The vertical orientation of filaments in the flat-topped columns during daylight hours is best interpreted as a heliotropic response. In the cones the filaments predominantly are parallel t o the column lamination, which is very steeply inclined (mostly 50-80’ below the horizontal), or even vertical. At cone apices the filaments frequently form vertical tufts. In the laboratory it was shown that the most abundant filamentous cyanophyte in the cones, Phormidium tenue var. granuliferum, is phototactic and will glide towards a light source at’up to 16 pm/min. Such phototaxis will explain the observed filament orientation as well as the gross form of the stromatolites (as is discussed below).

Effects o f water flow The maximum height (synoptic, or growth, relief) of both types of stromatolites is determined by the depth of water in which they form. None extend above maximum water levels. They are both taller and broader in deeper water. The maximum water depth in which the cones were found was 30 cm; in this pool the cones were about 3.5 cm wide and had a synoptic relief of about 10cm. Greater size would probably have been attained had not the temperature of this pool fluctuated markedly over long periods, periodically allowing invasion by the coarse, filamentous cyanophyte Calothrix, which stopped cone accretion. Presumably, the maximum size of the structures is also limited by their mechanical strength, which would vary according to the amount of silica precipitated within the pools. The flat-topped columns are subcircular in transverse section (Fig. 1 ) despite growing in water with a strong unidirectional current flow. Similarly, the cones are almost invariably subcirculh t o subquadrangular in transverse section irrespective of the environment in which they were growing (Figs. 8-16). The development of ridges and spines on the cones varies from pool to pool; the origin of this variation is unknown. Ridges are rarely developed to the extent that columns become obscured. In one part of “Conophyton Pool”, ridges were both parallel and perpendicular to water flow directions. In the one case where the cones are inclined (Fig. 35), the inclination is in the direction of water flow (i.e., downstream).

306

a a u

M.R. WALTER ET AL.

-e-Q

mAbaa!m

1

2

3

6

5

4

Fig. 36. Proposed morphogenetic sequence of the Yellowstone conical stromatolites: 1 , A mat of horizontal filaments; 2, Development of a knot of filaments due to tangling during gliding; 3, A tuft of erect filaments forms over the knot (due to phototaxis), 4-5, Tuft enlarges, 6, Occasional lateral filament movement produces bridging laminae (some of which are suspended above the substrate). Silicification occurs continually during the growth process.

Suggested morphogenesis of the conical stromatolites The following morphogenetic sequence (Fig. 36) is a provisional interpretation. Everywhere in this section the word “filaments” refers t o P. tenue var. granuliferum. (1)A flat mat forms by the random gliding of filaments over their substrate. An example of this is the horizontal filaments forming the flat laminae visible in Fig. 17. (2) Here and there the randomly gliding filaments become entangled, forming knots 100 pm or so wide and high, which project above the surfaces of the mats (Figs. 6, 7). Tangling due to gliding is a well-known phenomenon (Castenholz, 1969). The tangled knots of filaments are visible in Figs. 1 7 and 18. (3)Filaments gliding over the mats may encounter the knots and be deflected upwards over them, towards the light source (the sun). This re-

MORPHOGENESIS

307

orientation may initiate positive phototopotaxis and additional upward gliding may result. It is known that in at least some species of Phormidium there is no phototopotactic response when filaments are perpendicular to the light rays (as in a flat mat under water), but phototopotaxis does occur when filaments are in other orientations (Haupt, 1965). Self-shading within the knots of filaments may also initiate positive photophobotaxis. Thus tufts of erect filaments form above the knots (Figs. 17,18). (4) As a result of steps 1-3, the substrate is now dotted with vertical protuberances. Thus, increasing numbers of randomly gliding filaments are diverted up the sides of the tufts, which therefore become enlarged. At this stage the tufts are about 1-2 mm high. Tuft formation as described in stages 1-4 occurred within a 3-month period (from August to November 1972) in a small area of “Conophyton Pool”. ( 5 ) There will be a strong tendency for the most actively gliding filaments to be concentrated in the tufts, because they are the most likely t o encounter them. The most phototopotactic filaments would probably glide the farthest, to the tips of the tufts. Those that exhibit weaker phototopotaxis would be left at the base or on the flanks of the tufts. Thus the tufts become pointed, and maximum accretion is concentrated at the tips. In cone formation,. the process of phototopotaxis is augmented by cohesion among the mucus-coated filaments (the filaments cohere like the hairs of a wet brush). (6) The cones accrete upwards at rates determined by the supply of nutrients and solar energy. Variations in these factors, particularly in light intensity, produce intermittent accretion, and therefore lamination. (7) If the directions of gliding of the filaments in a flat mat with protruding knots is not entirely random, then more filaments may impinge on each knot from some directions than from others. Thus lateral ridges will form on the knots. Once a slight ridge has formed it will enlarge because it will deflect more and more filaments upwards. This explanation is consistent with the occurrence of ridges converging on small tufts (Figs. 6, 7) as well as on larger cones (Figs. 11-16). (8) Local entanglement of filaments can occur on cone flanks (Figs. 30, 31), producing knots which may develop into spines. (9) At cone tips, filaments tend to stand vertically, forming tufts or thickenings within the laminae (Figs. 28,29). Filaments accumulate fastest at cone tips. The tips, therefore, are less silicified than the rest of the cone and so mechanically weaker, rendering them more susceptible t o structural deformation (Fig. 23). The three essential processes in the morphogenetic sequence suggested here are gliding, phototaxis and cohesion, all of which demonstrably occur in filaments of P. tenue var. granuliferum.

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M.R. WALTER ET AL.

DISCUSSION

The banded-streaky microstructure of the conical stromatolites is a result of parallel alignment of the Phormidium and Chloroflexus filaments, which in turn results from their phototaxis and cohesion. Lenticular and subspherical thickenings in the laminae on the cone flanks form by tangling of filaments during upward gliding. The apical complexities result from the rapid accretion and tendency for tuft formation in this region, again due t o phototaxis. Thus the microstructure is essentially a biologically determined characteristic. The morphogenetic analysis of both of the columnar stromatolites described here stresses the definitive role of responses to light by the constructing microorganisms. In environments with only gentle water movement, light responses can determine stromatolite morphology. This has two applications in the study of ancient stromatolites. Morphogenetic analysis of ancient stromatolites can provide physiological data on the constructing microorganisms; these data may be correlated with microfossil types in those Precambrian stromatolites where microfossils are preserved (e.g. Awramik, Ch. 6.3). Secondly, stromatolites which formed in quiet-water environments (e.g. lagoonal, deep subtidal) will in many cases have morphologies determined by the physiological characteristics of the constructing microorganisms. The morphology of such stromatolites can be expected to change in response t o evolutionary changes in microbiotas, and the stromatolites therefore should be biostratigraphically useful. Conical stromatolites (Conophy ton) are abundant in Precambrian rocks but are not certainly known from Phanerozoic rocks. They are presently forming in hot springs probably because these are sufficiently free of animal grazers t o permit development. It is likely that similar stromatolites will be found in other hot-spring areas. They can also be expected t o occur in other environments relatively free of animal grazers. It is probable that photosynthetic, filamentous, phototactic organisms other than P. tenue var. grunuliferum will be found constructing similar stromatolites. The abundance and diversity of Conophy ton stromatolites through geologic time was probably directly related to the availability of quiet-water environments largely free of grazing animals. The advent of active grazers near the end of the Precambrian (Awramik, 1971a) may well explain the apparently complete elimination of marine subtidal Conophy ton stromatolites at that time. CONCLUSIONS

In the hot springs of Yellowstone National Park two types of columnar stromatolites, conical and flat-topped, are currently forming. Here these are described (and in the Appendix are named Vucerrilla wulcotti gr. et f. nov.

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309

and Conophyton weedii f. nov.). Prior to this discovery Conophyton stromatolites were thought not t o have formed since the Precambrian. Field observations and laboratory experiments strongly suggest that the morphology and microstructure of the Conophy ton stromatolites result from the phototaxis and cohesion of the principal constructing microorganism, Phormidium tenue var. granuliferum. Conophyton-like structures have been induced to form in the laboratory, using pure cultures of this cyanophyte. The results of our work indicate that morphogenetic analysis of at least some fossil stromatolites can provide physiological data on the constructing microorganisms. APPENDIX (by M.R. Walter)

Stromatolite systematics Vacerrilla gr. nov. Type form : Vacerrilla walcotti f. nov. Name: From the Latin for small posts, referring to the appearance of the stromatolite columns. Characteristics : As for the type form. Content: V . walcotti only. Age : Recent Vacerrilla walcotti f . nov. (Figs. 1-5) Columnar stromatolites (partim) Walter et al. (1972, figs. 1,2a, b ) Holotype: CPC 15460. Name : In honour of Dr C.D. Walcott, who recognised the analogy between Precambrian stromatolites and those in Yellowstone hot springs. Diagnosis: Stromatolite with subcylindrical, unbranched, patchily walled columns, convex laminae and predominantly a banded microstructure except in the walls, where lamination is indistinct or absent. Description: See p. 278. Comparisons: In gross shape V. walcotti resembles forms of Colonnella Komar. It is distinguished from this group by its distinctive differentiated microstructure which changes from predominantly distinctly banded within columns to diffuse and frequently unlaminated on the margins. This same microstructural feature and its small size distinguishes it from Linocollenia angarica Korolyuk. Age : Recent. Conophyton Maslov T y p e form : Conophyton lituum Maslov. Diagnosis: Extremely rarely branching columnar stromatolites with conical laminae, many of which are thickened in their crestal parts. Content: See Walter (1972a, p. 102). A g e : Precambrian, Recent. Conophyton weedii f. nov.

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Conophyton f. nov., Walter, Bauld and Brock, 1972 Holotype: CPC 15468. Name: In honour of W.H. Weed, a pioneer of American geology, who described in great detail the sinters of Yellowstone National Park. Diagnosis : Conophyton with a diffusely banded and streaky microstructure in which laminae predominantly are less than lOpm thick. Description: See p. 281. Comparisons : This stromatolite belongs to the group Conophyton Maslov (emend. Komar, Raaben and Semikhatov) because of its erect, parallel, non-branching columns built of conical laminae with upwardly directed apices and thickenings in the crestal zone. Microstructure is used to divide the group into three subgroups (coarsely banded, striated and clotted). The microstructure of C. weedii cannot be classified into any of these types, although it is similar to the striated microstructure of C. garganicum Korolyuk (emend. Komar, Raaben and Semikhatov). In addition, the modal thickness of the laminae is in the 5-10pm range, less than half the thickness of the modes in previously described forms. It is these microstructural features that are used to define forms within the group Conophyton, and therefore a new form is here distinguished. To allow comparisons with fossil conophytons the data on lamina thickness are presented in the standard form (Fig. 32). A g e : Recent. Depository : Type specimens of the stromatolites are deposited in the Commonwealth Palaeontological Collection (CPC), Bureau of Mineral Resources, Geology and Geophysics, Australia.

ACKNOWLEDGEMENTS

The U.S. National Park Service permitted field work in Yellowstone National Park. Financial support for Walter was provided by the Department of Geology and Geophysics, Yale University, and for Brock and Bauld by the U.S. National Science Foundation; Bauld also received an AustralianAmerican Educational Foundation Travel Grant (Fulbright). J.H. Oehler critically read the manuscript and made many helpful suggestions.

6. MORPHOGENESIS

Chapter 6.3 GUNFLINT STROMATOLITES: MICROFOSSIL DISTRIBUTION IN RELATION TO STROMATOLITE MORPHOLOGY S . M . Awramik

INTRODUCTION

Aside from the search for metazoans, Precambrian paleontology has suffered from a great dichotomy of research concentrations. One area of research has been the detailed study of microorganisms preserved in cherts while the other has been the study of stromatolites and their use in paleoecology and biostratigraphy. Very few investigators have concerned themselves with the possible relation between the microfossils contained within stromatolitic laminae and the stromatolites themselves. A major problem is that most stromatolites are preserved as carbonates containing little if any direct evidence of the microorganisms responsible for their accretion (see Gebelein, 1974, for an interesting discussion of microstructure in relation to major groups of blue-green algae). Vologdin (1955c, 1962) and Korde (1953b) have identified what they interpret as microorganisms preserved within the laminae of carbonate stromatolites from the Precambrian and Cambrian of the U.S.S.R. Serious doubt exists whether all the microstructures illustrated in their publications are the actual cellular remains of the algae or degraded organic matter diagenetically and mineralogically altered (Korolyuk, 1960c, 1963; Krylov, 1963; Semikhatov et al., 1963a; Kirichenko, 1964; Raaben and Komar, 1964; Walter, 1972a). By far the most compelling evidence for cellular remains preserved within stromatolitic laminae is in the microfossiliferous stromatolitic limestones and cherts of the Transvaal Dolomite (MacGregor et al., 1974; Nagy, 1974), Gunflint Iron Formation (Barghoorn and Tyler, 1965; Cloud, 1965; Licari and Cloud, 1068), Belcher Islands (Hofmann, 1974), Paradise Creek (Licari and Cloud, 1972), and Bitter Springs Formation (Schopf, 1968; Schopf and Black, 1971) to name a few (see Schopf, 1975, for a more detailed list). Not all microfossiliferous cherts are stromatolitic, those of the Fig Tree Group (Barghoom and Schopf, 1966) and Pokegama Quartzite (Licari and Cloud, 1972) being notable exceptions. Aside from Vologdin (1962), little attention has been paid to the possible relationship between preserved microorganisms

WALTER first proof, Chapter 6.3, page 1

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S.M. AWRAMIK

and stromatolite morphology (Awramik, 1971b, 1973a; Licari and Cloud, 1972; see discussion in Walter, 1972a). The stromatolites of the Gunflint Iron Formation with their well-preserved microflora are the source of valuable information for determining the possible relationships between microorganisms, the physical environment, and stromqtolite morphology. I have observed and concluded the following about the Gunflint stromatolites: (1)they grew subtidally in shallow, frequently agitated, silica-rich water; (2) coccoid cyanophytes predominate over filamentous forms in stratiform stromatolites, whereas filamentous cyanophytes predominate over coccoids in columnar stromatolites; (3) a gradational change in the proportion of coccoids to filaments occurs in the change from stratiform to columnar stromatolites; (4)turbulence does not correlate with stromatolite morphology or contained organisms; and ( 5 ) each distinctive stromatolite morphology examined contains a distinctive assemblage of microorganisms peculiar to that morphology, reflecting, at least in part, environmental conditions and biological response to those conditions. THE STROMATOLITES

The Middle Precambrian Gunflint Iron Formation trends ENE to WSW along the northern shore of Lake Superior, from Ontario into Minnesota (Fig. 1).The age of the Gunflint has been variously assessed at somewhere between 1,600 and 2,000 millions years although mounting evidence suggests an age around 2,000 million years (see Awramik, 197313, for a discussion on the age of the Gunflint). Stromatolites are found along the entire strike of the formation, but are restricted to two stratigraphic levels: the Lower Cherty and Upper Cherty (Broderick, 1920; see also Hofman, 196913). The stromatolites in the eastern facies are generally black, including reduced iron compounds along with organic matter, while those from the western facies are red, with iron in the sesquioxide state (Barghoorn and Tyler, 1965). The microorganisms in the stromatolites of the eastern black cherts are preserved with greater fidelity than those from the western jasperoid facies. I have concentrated on stromatolites from the eastern facies. Four morphologically distinct stromatolites were studied with a view to specifying their systematics and the types of microorganisms present. The stromatolite systematics have been reported elsewhere (Awramik, 197313) and here need only be reviewed briefly. Three types of columnar and one type of stratiform stromatolites were identified : Gruneria biwabikia schreiberi, Kussiella superiora, an unnamed form, and Stratifera biwabikensis (Figs. 2, 3, 4 and 5). Vertical transitions between the unnamed stromatolite, Stratifera, and Kussiella were found (Fig. 6). Horizontal transitions between different stromatolite forms were not observed.

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313

ONTARIO

Fig. 1. Map showing general areal distribution of .the Gunflint Iron Formation (dotted region). “Frustration Bay”, “Discovery Point” and “Flint Point” are informal locality names. THE MICROORGANISMS

To date, 29 morphotypes have been described from the microfossiliferous cherts of the Gunflint Iron Formation (Korde, 1958; Barghoom and Tyler, 1965; Deflandre, 1968; Hofmann, 1971b; and Edhom, 1973). Of these 29 different morphotypes, 17 are of doubtful biological origin, doubtful taxonomic assignment and/or are morphologically indistinguishable from previously described morphotypes from the same rock unit (Awramik and Barghoom, 1975). Continuing research on the microorganisms by Barghoom and myself has shed new light on the biological specificity and taxonomic position of many of the organisms previously described and presently under study. Briefly, we have found that: (1)The microflora is wholly prokaryotic. (2) There are two different microfossiliferous chert facies. One, a thinbedded chert, contains what is interpreted as a planktonic assemblage of microorganisms dominated by Kakabekia urn bellata Barghoom, Eoastrion simplex Barghoom, and E. bifurcatum Barghoorn. The other is stromatolitic (Fig. 7). The microfloras found in these two facies are different, with very few common elements. (3)Huroniospora Barghoorn may not be represented by three species based on surface sculpture as previously reported (Barghoom and Tyler, 1965). Surface sculpture may be a product of mineral precipitation on the surface of the cell while growing or after death, cellular degradation, or diagenesis. Size, although dependent on growth and reproductive stage, may be

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Fig. 2. Polished slab of Gruneria biwabikia schreiberi. Bar equals 1cm. Fig. 3. Polished slab of Kussiella superiora. Bar equals 1 cm. Fig. 4. Polished slab of unnamed stromatolite. Bar equals 1cm. Fig. 5. Polished slab of Stratifera biwabikensis. Bar equals 1 cm. Fig. 6.Vertical transition; from the bottom: unnamed form + Stratifera + Kussiella. Bar equals 1cm. Fig. 7. Thin-section photomicrograph of a microfossiliferous stromatolitic chert of the Gunflint Iron Formation. Bar equals 10pm.

MORPHOGENESIS

315

a more reliable criterion. Based on size, I found two populations of Huroniospora in the four stromatolites studied systematically; one smaller than 3 pm and one larger than 3 pm. Schopf (1975) reports the presence of three populations of Huroniospora in Gunflint cherts with the following diameters: 1-2 pm, 3-8pm, and 9-16pm. There was no clear-cut population of Huroniospora in the 9- 16-pm size range in the four stromatolites I studied. (4) Six new morphotypes have been discovered bringing the diversity of the Gunflint to eighteen forms (Awramik and Barghoom, 1975). Only eight of the eighteen morphotypes recognized have probable taxonomic affinities with extant prokaryotes. (5) Enterosphaeroides amplus Barghoom may not be a distinct taxon but a degradation product or empty cyanophycean sheath with spherical microorganisms occupying the microenvironment within the sheath (Awramik, Golubic and Barghoorn, unpublished). The dominant microorganisms found in the stromatolitic cherts are the coccoid and filamentous blue-green algae Huroniospora spp. and Gunflintia minuta Barghoom, respectively (Plate I, 2, 4, 6, 7 and 9). The possibility that G. minuta is a photosynthetic flexibacterium has been suggested by Walter et al. (1972) but well-defined structures morphologically comparable to heterocysts and akinetes have been found (Licari and Cloud, 1968) making this suggestion, at present, untenable. All other microorganisms illustrated in Plate I, 1,3 , 4 , 8 , l O and 11are found within stromatolitic laminae but are very rare. The presence or absence of extremely rare forms is not considered critical to the interpretation of the possible relationship between stromatolite morphology and microbiota. Conceivably, the rare forms were a minor component of the microbiota contributing and depleting various resources, but present and preserved in such minor amounts as to not affect the stromatolite biocenose as a whole. In living algal mats it is quite common to find statistically rare microorganisms present with no apparent role in mat accretion living in an unoccupied niche or microenvironment as a “guest” (a term used by S. Golubic to describe this phenomenon). BIOGENICITY OF THE GUNFLINT STROMATOLITES

Hofmann (1969b, and personal communication, 1974) raised an interesting and critical question: Are the microorganisms found within the stromatolitic laminae responsible for the biological growth of the stromatolite, or do they represent a detrital accumulation of microorganisms? Hofmann notes that the filaments appear as a “hash” with random attitudes and that Huroniospora is found solitary and not in clumps as would be expected with mats built by coccoid algae. Although we may never be certain that the microfossils preserved within the laminae were responsible for the growth of the Gunflint stromatolites, or

316

S.M. AWRAMIK

MORPHOG ENESIS

317

any other microfossiliferous stromatolite for that matter, circumstantial evidence is consistent with a biogenic interpretation. By far the most compelling argument is that the filaments and Huroniosporu are restricted to the stromatolitic laminae while the microfossils characteristic of the non-stromatolitic facies are rarely found both within laminae and in the interspaces of stromatolites. Gunflintiu rninuta, the dominant morphotype, is occasionally found both prostrate and erect within laminae. I consider the Gunflint stromatolites biogenic. METHODOLOGY

Thin sections for optical microscopy were prepared from each of the four stromatolites studied. The microscope was fitted with a Zeiss mechanical pointcounting stage set for 0.5 mm intervals for both the 3c- and y-axes. Thin sections were examined under oil immersion (ca. 1OOOx) and all the microorganisms within the field of vision (150pm in diameter) were counted for each of the 0.5-mm intervals. The entire thin section was studied in this manner. The numerical density of the microorganisms varied at each stop a;S did the preservation. Upwards of 50,000 microorganisms were counted in this manner. The proportion of the different microorganisms encountered for each of the stromatolites examined (Table I) is presented in terms of the Braun-Blanquet (1932) scale of abundance as presented in Mueller-Dombois and Ellenberg (1974): 5 = > 75%; 4 = 50-75%; 3 = 25-50%; 2 = 5-25%; 1= < 5%; is rare.

+

PLATE I Thin-section photomicrographs of microfossils from the stromatolitic cherts. All photomicrographs taken at the same magnification. Bar equals 10p m. 1. Eosphaera tyleri Barghoorn. 2. Gunflintia grandis Barghoorn. 3 . Animikiea septata Barghoorn. 4. New form (Awramik and Barghoorn, in prep.). 5. Gunflintia minuta Barghoom. 6 . Huroniospora sp. 7 . ?Huroniospora. 8 . New form (Awramik and Barghoorn, in prep.). 9. Huroniospora sp. 10. Archaeorestis n.sp. (Awramik and Barghoorn, in prep.). 11. New form (Awramik and Barghoorn, in prep.).

S.M. AWRAMIK

318 TABLE I

+

refer to BraunMicroorganism composition o f Gunflint stromatolites (numbers and Blanquet scale of abundance as presented in Mueller-Dombois and Ellenberg, 1974) Gruneria biwabikia schreiberi Kussiella superiora Unnamed stromatolite Stmtifera biwabikensis

3 __

4 __

3 4

.~

3 -

+

7 4 - T4 3

1

~

+

30- 34% 42-46% 44- 48% 65- 68%

66-70% 54-58% 52-56% 32- 35%

u)

Y

Eha 0

E

d

6

s0 0

B u DISCUSSION

The three columnar stromatolites studied contained a microbiota dominated by Gunflintia minuta while the stratiform stromatolites contained an assemblage dominated by Huroniospora spp. Turbulence has been considered the factor responsible for mat and stromatolite growth into columns and domes, without attendant microbiological changes, in the Recent algal mats and stromatolites in Hamelin Pool, Western Australia (Logan et al., 1974). In order to determine if turbulence was a factor in shaping the Gunflint stromatolites, I measured the apparent maximum diameter of oolites and clasts trapped within laminae and in interspaces (see Friedman, 1958) (Table 11). Stromatolites growing in turbulent waters would have the largest size faction of particles associated with them. There was no relation between stratiform stromatolites and degree of turbulence; Stratifera grew in waters varying from quiet (as evidenced by the lack of oolites or clasts) t o agitated and turbulent (as evidenced by the presence of abundant oolites within the laminae, some approaching 1.5 mm in apparent maximum diameter). There were no corresponding changes in the microbiota with differences in degrees of turbulence. In contrast, columnar stromatolites are invariably associated with oolites and clasts of varying diameter. Turbulence may be associated with columnar growth, but other environmental factors were probably responsible

MORPHOGENESIS

319

TABLE I1 Summary of Gunflint stromatolite data Stromatolite

Algae

Range of oolites and clasts max. apparent diameter

Average max. diameter oolites and clasts

Gruneria biwabikia schreiberi

coccoid 30- 34% filamentous 66-70%

0.1-0.9mm

0 . 5 mm

K ussiella superiora

coccoid 42-46% filamentous 5 4 - 5 8 1

0.3-11 mm

4.0 mm

Stmtifera biwabikensis

mccoid 65-68% filamentous 32-35%

0.0- 1.5 mm

0.8 mm

Unnamed stromatolite

coccoid 44-481 filamentous 5 2 -56%

0.2-2.0mm

1.2mm

for the differences in the assemblages of the three columnar stromatolites studied. Unfortunately, these environmental factors appear to have left no detectable records. The presence of a distinct assemblage within a given stromatolite morphology suggests (but does not prove) a biotic influence on stromatolite morphology. Similarly, in Proterozoic stromatolites from the Paradise Creek Formation of Queensland, Australia, Licari and Cloud (1972)reported the presence of one or two distinctive fossil morphotypes in each of the different stromatolite morphologies examined ;diversity was extremely low and preservation relatively poor. In the four Gunflint stromatolites examined, a given stromatolite morphology does not contain microorganisms unique to that morphology (in contrast to the Paradise Creek stromatolites). The biotic changes in the Gunflint stromatolites are subtle and consist of changes in the relative abundance of microorganisms. Rare microorganisms are found too infrequently to give useful data at present. However, as the microorganism composition of a stromatolite changes, the morphology of that stromatolite also changes. This is clear in the vertical transition from the unnamed stromatolite to Stratifera and then to Kussiella through a distance of 4-5 cm (Fig. 5 ) . The biocoenose changes from one dominated by Gunflintia minuta in the unnamed columnar form to a biocoenose dominated by Huroniospora spp. in Stratifera. As Gunflintia minuta becomes more abundant relative to Huroniospora spp., doming of the laminae becomes apparent. Finally, where filaments constitute approximately 55% of the microbiota, well-defined columns of Kussiella are present. SUMMARY AND CONCLUSIONS

Four morphologically distinct stromatolites examined from the approximately 2,000 m.y. old Gunflint Iron Formation, north shore of Lake Superior, Canada, were found to contain distinctive microbiotic assemblages peculiar to the stromatolite morphologies. Assemblages are dominated by

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S.M. AWRAMIK

coccoid and filamentous morphotypes of presumed blue-green algae. Columnar stromatolites contain an assemblage of preserved microorganisms dominated by Gunflintia rninuta while stratiform stromatolites contain assemblages dominated by Huroniospora spp. Changes in the dominance of one morphotype over another are accompanied by a corresponding change in the morphology of the stromatolite. N o detectable paleoecological changes are associated with these macrostructural and microbiotic transformations. However, biologically, one cannot separate ecological changes from biological changes affecting stromatolite morphology at a given instant in time. In Recent algal and bacterial mats, changes in temperature, availability of water, water chemistry and exposure to solar radiation effect changes in the microbiology (Golubic, 1973a; Logan et al., 1974). Such ecological changes would be undetectable in the fossil record. The relative degree of turbulence (which is paleoecologically detectable), had little or no observable effect on the morphology assumed by the Gunflint stromatolites or on the microorganisms responsible for accretion. Thus, subtle environmental changes are thought to have occurred while the stromatolites were growing in the Gunflint sea. These environmental changes effected changes in the microbiota and, in turn, the stromatolite morphology changed. The evidence presented here and by Licari and Cloud (1972), coupled with the presence of timerestricted stromatolite forms in the Proterozoic (Serebryakov and Semikhatov, 1974), suggests that microbiota does indeed influence stromatolite morphology. The Gunflint stromatolites and living pertinent examples provide data for a biologicalenvironmental approach to interpretation of stromatolite morphology. In the Gunflint, it appears that minor changes in the physical environment caused changes in the microbial assemblage and stromatolite morphology. Viewing the distribution of stromatolites in time and space during the Proterozoic, I consider that the evolution of new members or the extinction of existing members in cyanophytic and bacterial mat-building communities among taxa possibly as low as the specific and generic level, in interaction with the environment, may be responsible for the increase in diversity and the time-restriction of these stromatolite morphologies. ACKNOWLEDGEMENTS

This paper is from my doctoral dissertation completed at Harvard University, the Department of Geological Sciences. I thank E.S.Barghoom, P.E. Cloud, and H.J. Hofmann for critically reading the manuscript. D. Doerner (UCSB) prepared the plates. Financial assistance was generously supplied by the Department of Geological Sciences (Harvard), Committee on Evolutionary Biology (Harvard), a Grant-in-Aid of Research from Sigma Xi, and NSF Grant GA-37140 to E.S.Barghoorn.

6. MORPHOGENESIS

Chapter 6.4

BIOTIC AND ABIOTIC FACTORS CONTROLLING THE MORPHOLOGY OF RIPHEAN STROMATOLITES S . N . Serebryakov

INTRODUCTION

There are two main concepts of morphogenesis of stromatolites. The first, which can be called ecological, is based principally on the evidence of Recent stromatolites; it was formulated most clearly by Logan et al. (1964). According to this concept, the morphology of stromatolites depends entirely on the conditions of their formation. The second, the biotic concept, outlined mainly in the works concerned with ancient stromatolites by Fenton and Fenton (1933, etc.) and Maslov (193713, 1939a, etc.) postulates the existence of some relationship between the morphology of stromatolites and the taxa of algae to which they owe their formation. The ecological concept of morphogenesis cannot provide a satisfactory explanation for the empirical observation of a temporal succession of Riphean stromatolites throughout the world or the presence of similar stromatolites in heterofacial coeval deposits (for details see Ch. 7.1, 7.2, 10.8). It is this evidence that indicates indirectly the existence of some relationship between the succession of Riphean stromatolites and the evolution of stromatolite-forming algae and, consequently, the existence of biotic control over the morphology of stromatolites. Observation of a correlation between the gross morphology of fossil stromatolites and the constructing algae, in the rare cases when it can be accomplished (see ch. 6.3), does not contradict this conclusion. The mechanism of biotic control over the form of stromatolites is still uncertain. The taxonomic composition of the algal community is generally considered to be the determining factor on which the form of the stromatolite lamina depends (Korolyuk, 1963; Krylov 1963; Raaben, 1969a, b; Walter, 1972a; etc.). The dependence of the microrelief of the mats on the kinds of algae present was found to hold for Recent stromatolites (Logan et al., 1974). Raaben (1969b), Hofmann (1969a) and Walter (1972a) showed a correlation of some gross-morphological features of ancient stromatolites with the morphology of their laminae.

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The part played by algae in the morphogenesis of the Riphean stromatolites, however, cannot be restricted to determining the morphology of the laminae, especially since this itself may vary even within one column (e.g. Krylov, 1963) and can be identical in morphologically different stromatolites. In all probability, the morphology of the structures as a whole was biotically controlled. This is supported by the following. The structure of the ancient columnar stromatolites resulted from a long-continued successive accretion of laminae formed contemporaneously with the accumulation of the enclosing sediments. Presumably, there was some balance between the rate of stromatolite formation and the rate of sedimentation which could be maintained by interruptions to the growth of the structures and/or partial destruction of the laminae. The frequent occurrence of the latter process can be judged from the occurrence of fragments of stromatolite layers which frequently fill the intercolumnar spaces in bioherms. Intermittent growth reported by Monty (1967) for Recent algal mats seems to be typical for ancient stromatolites also. Of particular importance for us is that in spite of the highly probable irregularity of their growth and the periodic destruction of laminae in fossil stromatolites the structures formed were of a definite shape with the morphological features typical for the given taxon. One possible mechanism of biotic control was suggested by Vlasov (1965, 1970). In his opinion the features typical for stromatolites are the “manifestation of the colonial life of mono-specific organisms”, the term “colony” implying not a simple multitude of individuals, but a “unification of individuals into a higher single whole” (Vlasov, 1970, p. 152). Various parts of such a colony were specialized to create definite intercorrelated elements of the structure. The colony itself underwent three stages of development (youth, maturity, old age) which resulted in the vertical morphological zoning of stromatolites. Unfortunately, this interesting hypothesis is based on numerous assumptions and practically ignores the role of environment in morphogenesis. The biotic concept of morphogenesis of the Riphean stromatolites does not, in principle, exclude the direct effect of environmental factors on the morphology of the stromatolites (Fenton and Fenton, 1933; Fenton, 1943; Keller et al., 1960; etc.). But the successful stratigraphic application of stromatolites in the early 1960’s resulted in the underestimation of the role of these factors by some researchers, who held the view that the environments affected the form of the structures only indirectly, by determining the distribution of coenoses of algae (Krylov, 1963; Nuzhnov, 1967). This view, in its extreme, inevitably requires the correlation of any changes in the morphologiqal characteristics of stromatolites with changes in the taxonomic composition of the algae which formed them (Raaben, 196913). Abundant evidence has been accumulated in recent years of the intrabiohermal morphological variability of stromatolites (Krylov, 1965, 1967a, 1972; Shapovalova, 1965, 1968; Shenfil, 196513; Vlasov, 1965, 1970;

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323

Bertrand, 1969; Bertrand-Sarfati, 1970, 1972b, c; Serebryakov, 1971, 1975; Serebryakov et al., 1972; Walter, 1972a; Preiss, 1973a; etc.). This variability has two principal features which suggest that it depended largely on environmental factors. Firstly, intrabiohermal variability is manifested in a similar way in taxonomically different stromatolites; secondly, it is not normally accompanied by changes in microstructures.

FACTORS INFLUENCING THE MORPHOLOGY OF RIPHEAN STROMATOLITES

The following is an attempt to consider, by giving some examples, the influence of biotic and environmental factors on the morphology of Riphean stromatolites and to determine the relative importance of these factors. The analysis is based mainly on the stromatolites of the Riphean deposits of various areas of Siberia. The effect of environmental factors on morphology is quite obvious for many Recent stromatolites (see Ch. 6.1), and in some cases can be regarded as proved and in others as highly probable also for Riphean stromatolites. The simplest example of such an influence is the elongation of bioherms or some individual structures of various stromatolites. Hoffmann (1967) and Trompette (1969) showed that the long axes of these structures are oriented along the direction of paleocurrents reconstructed from crossbedding in associated sediments. The Siberian observations support this; there we have measured the predominant orientation of ripple marks and channels with carbonate flat-pebble conglomerates. The bioherms concerned have, in plan view, the form of biconvex lenses and they are arranged in a chess-board fashion in such a way that the distance between them is practically constant. The bioherms normally have a similar orientation over a considerable area; the orientation is not related to ephemeral changes in the direction of currents as shown by ripple-mark patterns. For example, the orientation of ripple-marks in dolomites of the Platonov suite (Turukhan region, the Sukhaya Tunguska river) varies even in adjacent thin (1-3cm) layers. Meanwhile the same orientation of the bioherms, elongated in plan view and enclosed within these dolomites (over a thickness of 10-15m), has been traced over almost 20 km. The elongation of individual columns within bioh e m s is less constant and can successively follow several directions (see Serebryakov and Semikhatov, 1974, fig. 4). The lamination form of stratiform stromatolites seems to be associated, in some cases, with water movement. The bedding relief of the upper surface of Stratifera sometimes cannot be distinguished from ripple crossbedding. In the opinion of Hofmann (1969a) this is also true of Gongylina. Currents might be effective in causing asymmetry of structures as a result of a gradual displacement of their crests along the long bioherm axis (Hoffman, 1967; etc.). It is not improbable that this can be regarded as one

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cm -100

- 50

-0

Fig. 1. A block-diagram of a bed including elongated Platella stromatolites from the Debengda suite (Middle Riphean) of the Olenek Uplift. These Platella form the upper part of a large “Jacutophyton cycle”; they grade into columns which are isometric in plan view. The front of the blockdiagram is a drawing made from a large-scale photograph.

of the causes of the formation of uniformly inclined columns within some bioherms. This mechanism, however, is by no means universal. Specifically, it cannot explain the inclination of the elongated Platella columns shown in Fig. 1. One of the best examples of more complex effects of environmental factors on the morphology of structures is provided by the small bioherms of columnar stromatolites where both vertical and horizontal morphological zoning can be observed (Krylov, 1959a, 1965, 1967a; etc.). Horizontal zoning reflects lateral variations from the central to the peripheral parts of bioherms. In the marginal zones, inclined, horizontal and declined columns are frequently formed, while in a-parallel-branched unwalled stromatolites, the columnar structures may become stratiform here. The marginal zone is also characterised by morphologically indistinct columns which frequently have numerous bridges. Such changes seem to have been caused by the specific conditions in the marginal part of the growing bioherm (such as increased mobility of the water and the ability of the mat to extend laterally). The height of the bioherm during its lifetime was also of great importance. This depended, during early stages of bioherm formation, on the relief of the substrate, and subsequently on the relative rates of growth of the stromatolite and accumulation of the sediments. It could be comparable with, or greatly exceed, the height of individual columns during their lifetime. In the first case the columns were situated subvertically (Fig. 2A); in the second they were arranged like a fan. The height of the bioherm sometimes changed repeatedly during its formation as a result of change in the type of sediments or as a result of irregularity in the rates of sedimentation or the growth of

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B

Fig. 2. Drawings from photographs of two neighbouring bioherms with Kussiella kussiensis in the lower subsuite of the Kotuykan suite (Lower Riphean) of the Anabar massif. A. A bioherm in a lens of carbonate endoclastic and phytoclastic rocks. Stromatolites began growing on the concave section of the solid substrate. The height of the bioherm during its lifetime did not significantly exceed the height of individual columns during their lifetime. The columns are approximately vertical. B. A bioherm which formed on an erosional mound. In the marginal zone (on the right) the columnar structures become stratiform.

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Fig. 3. A section of the marginal zone of a large bioherm with Baicaliu. Various relations between the enclosing limestones and the bioherm reflect changes in its height during its lifetime. The orientation of the columns changes accordingly. Schematic drawing, Neruen suite (Middle Riphean) of the Uchur-Maya region.

the stromatolites. These changes, which are manifested in the relation of the enclosing stratified rocks with the bioherms, coincide with changes in the orientation of the columns in the peripheral zones of the bioherms (Fig. 3). The change from columnar to stratiform stromatolites in the marginal parts of bioherms is also related to the height of the bioherm. The division of a wide structure (or the stratiform base of a bioherm) into narrower columns, typical of the a-parallel-branched unwalled stromatolites, seems to have been facilitated by the accumulation of mechanical sediments in hollows in the mats. In the marginal parts of relatively high bioherms, where the inclination of the mats reached some critical value (in the studied examples, about 50’) this sediment is likely to roll down the slope, so allowing the formation of extensive layers (Fig. 2B). Finally, the height of bioherms during their lifetime determined, to some extent, their general form. Bioherms that rose markedly above the substrate normally have distinct outlines and can be easily distinguished from the enclosing rocks. Bioherms whose height during their lifetime was small normally have no clear lateral boundary and their outlines are irregular. The vertical zoning of bioherms can, in its simplest form, result from the fact that the form of structures in their lower and upper parts is less distinct and less “morphologically defined” than the form of the stromatolite in the central part. The study of the composition, texture and structure of the enclosing and intrabiohermal rocks showed that this phenomenon is linked, to some extent, with changes in the environmental conditions occurring

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during bioherm formation. These changes were, in the majority of cases, of transgressive or transgressive-regressive character. The bioherms normally overlie erosion surfaces (as a rule subaqueous) or may be separated from them by clastic and microphytolitic rocks. The lower and middle parts of bioherms are normally enriched in coarse carbonate fragments, their size gradually decreasing up the section. There are also variations in the composition of fragments; in the lower parts fragments of the substrate predominate, while higher, fragments of stromatolites are most abundant. The existence of directed changes of the environment are most pronounced where the stromatolites are members of small-scale sedimentary rhythms. Such stromatolite-bearing rhythms found in rocks of various ages have a rather uniform structure (Sarin, 1962; Korolyuk, 1966; Szulczewski, 1968; Serebryakov, 1971, 1975; Serebryakov and Semikhatov, 1974; and other authors). The rhythms are composed of various clastic (terrigenous and carbonate) rocks, phytogenic and chemogenic carbonates, and occasionally of argillites. Whatever the type (transgressive, transgressive-regressive or regressive) or the age of rhythms, stromatolites and microphytolites take a definite place in them, lying between the clastic rocks on the one side and the chemically precipitated ones on the other (Serebryakov, 1971, 1975; Serebryakov and Semikhatov, 1974). Columnar stromatolites often occur in a lower position in the transgressive rhythms than stratiform stromatolites (Fig. 4, see also Ch. 10.8, fig. 3). This suggests that the latter have been formed under calmer hydrodynamic conditions. Yet stratiform structures are sometimes present both at the top and the base of a stromatolite member of the rhythm. One has t o admit that in this case there were two different ranges of conditions favouring the formation of stratiform stromatolites. A similar phenomenon has been described by Gebelein (1969) for the Recent sublittoral Castle Roads stromatolites (Bermuda). When studying the vertical structure of bioherms one can often observe some insignificant changes in the morphology manifested in a similar way and simultaneously in a group of neighbouring columns or even throughout the whole bioherm. The Siberian observations provide the following examples of this type of change: (1)a sharp increase in the convexity of laminae without the appearance of any axial zone in Baicalia, Inzeria, etc.; (2) appearance of axial zones without any significant increase in the convexity of laminae in Kussiella, Baicalia and Inzeria; ( 3 ) change of column orientation from subvertical to inclined and then subvertical again at particular levels within bioherms of Kussiella, Baicalia, Platella and Colonnella; (4) Sharp changes (by 2-10 times) in the size of the columns of Colonnella, Anabaria, etc.; (5) temporary narrowing of columns of various stromatolites (Fig. 5); (6) simultaneous branching of structures, e.g. Kussiella - some examples of this type have been described in the literature (Krylov, 1967a; Raaben, 196913; Walter, 1972a; etc.). These changes in morphology are in some cases accompanied by changes in the enclosing rocks (e.g. enrichment in

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-

-

5

Fig. 4. Sedimentary rhythms of the upper subsuite of the Yusmastakh suite (Upper Riphean) of the Anabar massif. Legend: I = chemically precipitated dolomites; 2 = stratiform stromatolites; 3 = columnar stromatolites (Kotuikania);4 = oncolitic dolomites; 5 = elastic dolomites; 6 = clayey dolomites; 7 = silty dolomites; 8 = sandy dolomites; 9 = sandstones; 10 = submarine erosional surface.

argillaceous or coarser elastic material), but more often they are apparently not reflected in the character of the abiogenic sediments. Changes in the form of structures similar to those described above frequently occur many times in the vertical section of the bioherm, producing a cyclic structure. Obviously, they are the consequence not of a unidirectional “development of the bioherm” (Krylov, 1959a, 1965,1967a; Vlasov, 1965, 1970), but of some periodic changes in the environment which are often

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Fig. 5. Section of a bioherm with Kussiella kussiensis, lower subsuite of the Kotuykan suite (Lower Riphean) of the Anabar massif. One can see a sharp decrease in the diameter of the four left columns which coincides with the appearance of very thin clayey layers in the enclosing dolomites. The column on the right had a greater height during its lifetime and did not experience any changes. Evidently, this is also the reason why the decrease in the diameter in the two left columns occurs a little later than in the two central ones.

reflected only in the character of the stromatolites. This phenomenon may be called stromatolitic cyclicity. A more complex variety of stromatolitic cyclicity is the repeated alternation within a bioherm of extensive horizons composed of different morphological groups of stromatolites. For instance, in one of the bioherms of the Burovaya suite (Turukhan region) there is an alternation of horizons of Baicalia, Ornachtenia and stromatolites resembling Kussiella (see fig. 5 of Serebryakov and Semikhatov, 1974). In the bioherms of the lower members of the Kotuykan suite (Anabar massif), levels with Kussiella, Colonnella and Stratifera frequently alternate with specific columnar stromatolites which somewhat resemble Baicalia. The different strbctures are in both cases linked by gradual transitions and share a similar microstructure. A good example of stromatolitic cyclicity is provided by the so-called “Jacutophyton cycles” (Serebryakov et al., 1972),which are the rhythmic alternation of Conophy ton with various branching structures, observed by many researchers (Rezak, 1957; Votakh and Chayka, 1962; Shapovalova, 1965, 1968; Bertrand, 1969; Hoffman, 1972; Serebryakov e t al., 1972). “Jacutophyton cycles” are most typical of the Middle Riphean. Each

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Fig. 6. A section of a transitional zone between the horizons of Baicalia (at the bottom) and Jacutophyton (at the top). The zone of lamina-shape change in two neighbouring columns differs in height by 7-8 cm. The tablet with the figure is 6 x 3 cm. The Derevnya suite (Middle Riphean) of the Turukhan region.

bioherm representing such a cycle comprises several horizons (tens of centimetres to several metres each) of Conophyton, Jacutophyton, Baicalia, and, rarely, Colonnella and Strutifera. At the boundaries between these horizons there are thin (5-20 cm) transitional zones within which the morphology of the stromatolites changes rapidly (Fig. 6). The transition from Conophyton to Baicalia always occurs via a Jacutophyton stage. The Buicalia horizon is normally confined to the top of the complete cycles. It is covered by a conformable bed of non-stromatolitic rocks, separating the bioherm from the overlying one and producing a general rhythmicity in the section (alternation of stromatolitic and non-stromatolitic carbonates, Fig. 7). There are no regularities in the alternation of horizons other than those mentioned, but the order of this alternation is specific for each of the successive “Jucutophyton cycles”. Within tectonically and facially homogeneous zones, the individual features of the cycles persist over some area in spite of local changes in the structure of individual bioherms (Bertrand-Sarfati, 1972c; Serebryakov et al., 1972). Thus, application of only morphological features of stromatolites and their order of succession in cycles allowed precise correlation of four lithologically homogeneous sections of the Neruen suite (Uchur-Maya region);

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Fig. 7. Correlation of sections of the middle subsuite of the Neruen suite (Middle Riphean) of Uchur-Maya region according to “Jacutophy ton cycles”. (Amended from Serebryakov et al., 1972.) Legend: 1-3 = Stromatolite horizons with: I = Conophyton; 2 = Jacutophyton; 3 = Baicalia; 4 = non-stromatolitic limestones; 5 = argillities of the lower subsuite; 6-7 = boundaries of: 6 = cycles; 7 = cycle elements; 8 = boundaries of horizons with stromatolites differing in morphology. The two right columns are each composed of two parallel close sections; each of them gives an idea of local changes in the bioherm structure.

the sections were 2-50 km apart (Fig. 7). This shows that periodically operating factors causing cyclic changes may be expressed unambiguously over an extensive area, controlling the appearance in the section of structures of one shape or another. In all the four sections shown in Fig. 7 the horizons containing Buiculiu account for 58-60% of the total thickness of the stromatolitic horizons. The analysed features of the “Jucutophyton cycles”, and especially the rhythmic structure and persistence over some area, strongly suggest abiogenic causes for stromatolitic cyclicity. The question which is more difficult to answer is which ecological factors are responsible for these phenomena. The changes in morphology of stromatolites making up the “Jucutophyton cycles” normally are not accompanied by macro- or microscopic changes in the enclosing rocks; frequently stromatolites of the same morphology do cross the boundary between two lithologically different horizons (Fig. 8;

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Fig. 8. Contrasting levels of changes in the morphology of stromatolites and lithology of the rocks in the Derevnya suite (Middle Riphean) of the Turukhan region. Legend: 1 = level of changes in the morphology of stromatolites within the “Jacutophyton cycles”; 2-3 = strornatolite horizons with: 2 =Baicolia; 3 = Jacutophyton; 4 = surface of erosion; 5 = non-stromatolitic limestones; 6-9 = character of rocks: 6 = light chocolate porcelanous crypto-to microgranular limestones and dolomitic limestones; 7 = reddish-brown irregularly grained crypto- to fine-grained limestones and dolomitic limestones; 8 = brownish-grey (on weathered surfaces they are brick red) fine-grained calcareous dolomites and dolomitic limestones; 9 = grey irregularly grained, crypto- to finegrained limestones and dolomitic limestones.

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Serebryakov et al., 1972). The dependence of the morphology of stromatolites on factors of the environment which are not expressed by features of the sediment has been noted by Maslov (1959) and Preiss (1973a). We are often unable t o determine conclusively which environmental factors affected particular stromatolites, neither can we record all their possible combinations. For instance, according to the African evidence (Bertrand, 1969), and, to a lesser extent, the Siberian evidence (Serebryakov et al., 1972), Baicalia was found t o be associated in the “Jacutophyton cycles” with the most rudaceous varieties of the carbonate rocks. This as well as the cases of replacement of Jucutophyton by Baicalia in the marginal zones of bioherms suggests that hydrodynamics affect the morphology of stromatolites. Yet observations in the Derevnya suite (Turukhan region) showed that the change of groups cannot be related exclusively to changes in water movement. Passing up-section in this suite, there is increasing evidence from one cycle to another of the turbulence of the basin water. At the same time, the character of cycles remains essentially the same (provided the absence of horizons with Conophyton in the uppermost cycles is disregarded). The quest for some single cause responsible for the changes in the morphology of stromatolites often results in mutually exclusive conclusions. Both Rezak (1957), who considered alternations of Collenia and Conophyton, and Shapovalova (1965, 1968), who analysed the alternations of horizons of Baicalia, Jacutophyton and Conophyton, attributed the changes in morphology of the stromatolites t o changes in water depth. However, Rezak believes that Collenia was intertidal and Conophyton subtidal, whereas Shapovalova is of the opinion that Conophyton formed under shallower conditions than Buicalia. At the same time, the data now available suggest that the effect of bathymetry on the morphology of the stromatolites in “Jacutophyton cycles’’ was insignificant. The occurrence of stromatolitic cyclicity cannot be attributed solely t o depth fluctuations, particularly in cases of the lateral replacement of Conophyton by Baicalia in the marginal zones of bioherms (Krylov and Shapovalova, 1970a; Serebryakov et al., 1972). In some other cases, however, it is the fluctuation of depth that seems to be the main cause responsible for the changes in the morphology of the stromatolites. For example, the replacement of columnar-layered stromatolites by stratiform stromatolites in the sedimentary rhythms of the Omakhta suite (see Ch. 10.8, fig. 3) is likely to have been associated with the gradual deepening of the basin. The hydrodynamics of the basin do not seem to have undergone significant changes: the size of detrital quartz grains is practically the same in all rock-types comprising the rhythm, including the stromatolite dolomites (Serebryakov, 1971). From what has been said it follows that stromatolites have a considerable intrabiohermic morphological plasticity which can be attributed to the effect of environmental factors. The effects of these factors are frequently not reflected directly in the character of the enclosing sediment which makes

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their simple interpretation difficult. The normally uniform microstructure of all morphological variety of stromatolites of each bioherm suggests that the effect of the environment upon the morphology of the structures was direct and did not depend on changes in the community of the stromatolite-forming algae. This conclusion rests on the assumption that the features of microstructure (except for those having an obviously superimposed character) reflect, at least to some extent, the primary textures and structures of the original stromatolite and are correlated in some definite way with the taxonomic composition of the algae which gave rise t o it (see Ch. 7.1). Consequently, that morphologic variability of stromatolites which is not associated with changes in the microstructure, does not seem to be dependent on changes in the taxonomic composition of the algal communities either. Thus, we have good reasons to suppose that stromatolites of various morphologies but identical microstructure formed as a result of the life activity of a single community or species of algae (Komar and Semikhatov, 1965; Komar, 1966; Krylov, 1967a, 1972; Serebryakov, 1971, 1975; Bertrand-Sarfati, 1972c; etc.). However, some instances have been observed where the changes in morphology of stromatolites are accompanied by changes in the microstructure (Raaben, 196913; Serebryakov et al., 1972). In these cases, the microstructure normally does not change completely, but the relative abundances and shapes of its various elements alter. In such cases, those ecological factors which caused the gross morphologies t o change can also be supposed to have caused changes in the algal communities (for example, a change in the dominant species); i.e., we are dealing here with “ecads” in the sense of Fenton and Fenton (1933) and Fenton (1943). Though the form of stromatolites has been found t o change‘under the influence of environments, it should be emphasised that the range of these changes is not infinite. It seems to be controlled by the taxonomic composition of the stromatolite-forming algae. As we have previously mentioned, the existence of such a control can be seen not only from the existence of the definite succession of similar assemblages of Riphean stromatolites in the deposits of various regions. It can be also established by the direct study of the intrabioherm variability of the stromatolites. Krylov (1967a) was the first t o notice the regular combination of structures of different morphology in the bioherms of certain stromatolites. He coined the term “bioherm series” (Krylov, 1972, p. 56) to designate the combination of “the principal morphological varieties of structures belonging to the same bioherm or to uniform biohenns within a single bed”. Similar bioherm series occur in coeval deposits over vast areas. Thus, intrabioherm variability does not prevent the use of stromatolites in stratigraphical zonation; on the contrary, it opens up new possibilities for their stratigraphic application (comparison of bioherm series). Similarly, the taxonomic composition of the sets of stromatolites which occur in the otherwise identical

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stromatolite rhythms developed at different stratigraphic levels proved to be different. These sets differ greatly from one another, particularly in the morphology of their columnar structures (Serebryakov, 1971, 1975). A good example of the biotic control over the morphology of stromatolites is provided by the “Jacutophyton cycles”. The morphology of the branching members of such cycles is specific for the Lower, Middle and Upper Riphean (Serebryakov et al., 1972). In the Lower Riphean the cycles have KussieJa; in the Middle Riphean, Baicalia and Tungussia; in the Upper Riphean, Inzeria, and, according to Bertrand-Sarfati (1972c), Gymnosolen, Tilemsina and distinctive Baicalia. Thus, in spite of the variability of the morphology under the influence of environments, we can observe that due to biotic control over the morphology of stromatolites each Riphean subdivision has specific, unrepeated assemblages of structures. CONCLUSIONS

The morphology of Riphean stromatolites depends both on the taxonomic composition of the stromatolite-forming algae and on the environment. Macroscopic features of stromatolites can be subdivided into two groups. The features of one group (orientation of structures, elongated or isometric form and, possibly, the diameter of columns, etc.) were, evidently, fully controlled by abiotic factors. Others (style of branching, margin structure, shape of columns and laminae) are evidently of a dual nature, biotic and abiotic. In other words, they depended on the taxonomic composition of algae and could, at the same time, change within some definite range under the influence of the environment. Thus, in spite of the presence of inclined columns in the marginal zones of bioherms of Kussiella, Baicalia, Gymnosolen, etc., they can be easily distinguished from one another by characters of the second group. Though the transition from non-branching stromatolites with conical laminae t o branching structures with convex laminae can be observed at various stratigraphic levels, the morphological features of the branching stromatolites (as well as the microstructures) change in time. The consequence of the dual nature of the morphological features of stromatolites is, in all probability, the specific “convergence” of features; i.e. similar features might be the result of the operation of different factors. For instance, the appearance, in the Lower Riphean, of rare stromatolites with a smooth lateral surface and more or less distinct wall could be attributed to specific conditions of their formation, in particular to the fact that, in some localities, sedimentation might lag behind the rate of growth of the structure. On the other hand, the extensive development of such stromatolites in the Upper Riphean can be more reasonably attributed to the specific features of

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the stromatolite-forming algae, because it is unlikely that the total rate of sedimentation was significantly reduced in the Upper Riphean. However, the principal manifestation of the dual nature of the morphology of stromatolites is the existence of regular, definite sets of morphologically various stromatolites, fitting the diagnoses of various groups, supergroups and types in the current classification of stromatolites. The morphologically different members of each such set have a uniform microstructure and, evidently, were formed by the same community or the same species of algae. Included among the sets under consideration are Krylov’s (1972) bioherm series, which correspond to the sets of single bioherm or to several bioherms of a single layer. The analysis of the taxonomic composition of the sets indicates that each community (or species) of algae has a most typical and most frequently encountered morphological manifestation of the stromatolites formed by it. Therefore, the relative abundances of different stromatolites within a bioherm are almost always very different. One of the morphological modifications is normally dominant and comprises the bulk of the stromatolite body. This modification apparently results from optimal conditions for the development of the particular algae. In the sets under consideration there are, apart from taxa of extensive vertical distribution (e.g. Strutifera), stromatolites typical of limited stratigraphic intervals. I t is these that are responsible for the specific appearance of each of the sets and allow their stratigraphic application.

7 . STROMATOLITE BIOSTRATIGRAPHY

Chapter 7.1 EXPERIENCE IN STROMATOLITE STUDIES IN THE U.S.S.R. M . A . Semikhatov

INTRODUCTION

Most of the modern concepts in stromatolitology originated in the U.S.S.R. in Maslov’s works. Considering stromatolites as organo-sedimentary structures, he came to the conclusion that there was a relationship between the species composition of the stromatolite-building algal mats and the gross morphology of the stromatolites; he made the first attempt to clarify the main trends in stromatolite evolution and successfully used them for the first time for inter-regional correlation (Maslov, 1937b, 1938, 1939a, b, 1950; etc.). During the post-war years Maslov (1945, 195313, 1959, 1960) began carefully appraising the possibility of stromatolites acting as time markers and demonstrated the variability in their morphology and microstructures under the effects of the environment (in the case of the Ordovician forms). The concept that stromatolite morphology is completely dependent upon the environment was predominant in the U.S.S.R. during the 50’s. The most diligent in developing this concept were Korde (1950,1953b) and Vologdin (1955b, 1962) who concentrated their attention on the search for cellular algal remains in the carbonate stromatolite structures. EMPIRICAL EVIDENCE OF THE STRATIGRAPHIC SIGNIFICANCE OF STROMATOLITES

A new phase in these studies, which has undoubtedly provided evidence of the stratigraphic significance of stromatolites, began in the U.S.S.R. in the late 50’s. This phase was started by the work of Korolyuk (1958, 195913, 1960). She distinguished three types of stromatolites (layered, nodular and columnar), described several form-genera and -species for each type, and demonstrated for the first time the diagnostic significance and temporal variation of the margin structure of columns. The method suggested by Krylov (1959b, 1963) for reconstructing the form of columnar structures by serial sectioning (“graphical reconstruction”) made it possible to identify two new

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diagnostic features of columnar stromatolites: their branching style and the gross morphology of the columns. The application of this method in studying the Uralian stromatolites has shown that there is a distinct change in the morphological varieties of stromatolites through the geological column. The branching type, the margin structure and character of the microlamination change in a regular manner with time (Krylov, 1960b). Similar results were obtained with the Siberian material (Nuzhnov, 1960; Semikhatov, 1960). Somewhat earlier, it had been noticed that massive developments of conophytons were confined to the lower part of the Upper Precambrian (Riphean) in the U.S.S.R. (Dragunov, 1958; Keller and Khomentovskii, 1958, 1960) and specific branching structures were described for the Upper Riphean within the northeastern and eastern margins of the Russian Platform (Krylov, 1960c; Raaben, 1960). The generalization of the information gathered by 1960 had shown that there are three stromatolite complexes in the key sections of the Riphean in Siberia and the Ural (Keller et al., 1960) and that the composition and succession of these complexes coincide over vast areas. This coincidence was considered t o be evidence that the vertical change in the stromatolite complexes probably reflects the evolution of stromatolite-building algae and not changes in the local sedimentary environment. The first isotopic datings obtained by the K-Ar method confirmed this idea, and proved t o be similar in the rocks containing similar stromatolites both in Siberia and the Ural. The conclusions regarding the existence of time-dependent, consistent associations of Riphean stromatolites have been further developed in the works of a large group of geologists who investigated all the main platform and miogeosyncline sections in northern Eurasia (Korolyuk et al., 1962; Semikhatov, 1962; Korolyuk, 1963; Krylov, 1963, 1966c, 1967a, 196813, 1969, 1972; Semikhatov et al., 1963a, b, 1967a, 1970; Komar, 1964,1966; Komar et al., 1964, 1965a, 1970, 1973; Raaben, 1964a, b, 1967, 196913, 1972a, b; Furduy, 1965,1969; Keller et al., 1965; Kirichenko, 1965; Nuzhnov and Shapovalova, 1965, 1968; Semikhatov and Komar, 1965; Shenfil, 196513; Golovanov, 1966, 1970; Golovanov and Raaben, 1967; Nuzhnov, 1967; Krylov et al., 1968, 1971; Komar and Semikhatov, 196813; Korolyuk and Sidorov, 1969, 1971; Raaben and Zabrodin, 1969, 1972; Dolnik and Vorontsova, 1971, 1972; Rabotnov et al., 1971; Khomentovskii et al., 1972a; and others). New data obtained have confirmed the conclusion that the changes in the composition of the stromatolite complexes are irreversible in the Riphean section and make it possible to distinguish a fourth complex in addition to the above three. The fourth complex defines the uppermost strata of the Precambrian, designated the Vendian or Yudomian, on the basis of historical geological data (Sokolov, 1952,1964; Keller and Sokolov, 1962) and the assemblages of microphytolites (Zhuravleva and Komar, 1962; Zhuravleva, 1964a). It is important to emphasize that the levels at which the complexes change are largely independent of historical-geological boundaries.

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This is seen best of all in the Siberian sections (Semikhatov, 1962; Komar, 1966; Khomentovskii et al., 1972a). The stromatolite classification that served as a basis for these biostratigraphic schemes was suggested by Krylov (1961, 1963). It is a formal one and it provides for the specification of three categories of taxa - types, groups (form-genera) and forms (form-species) - and for Latin binary names for the groups and forms. Maslov’s suggestion (195313,1960)that three-fold or five-fold nomenclature should be applied to stromatolites has not been accepted, since such nomenclature is complicated, cannot replace descriptions and is no more informative than binomial nomenclature (Korolyuk, 1960c; Krylov, 1963). Columnar stromatolites are basic to the Subdivision of the Riphean. In Krylov’s classification they are grouped into form-genera according to a combination of three features: the gross morphology, branching style and margin structure. It has been suggested that forms should be distinguished by details of column morphology. However, microstructure has become a basic factor in their practical identification (e.g., Semikhatov, 1962; Komar et al., 1965a; Komar, 1966). Raaben (1964b) and Komar (1966) suggested slightly different schemes based on Krylov’s classification (see Ch. 2.4 and Hofmann, 1969a). Analyses of the diagnostic stromatolite features were published by Krylov (1963), Komar (1966), Raaben (1965, 1969a, b) and Raaben and Zabrodin (1972). The stromatolite distribution in the Riphean reference section of northern Eurasia is shown in Fig. 1constructed on the basis of data available from the Geological Institute of the U.S.S.R. Academy of Sciences and critical analysis of published information. With a minor exception, endemic forms are not plotted in Fig. 1. It should be emphasized that there are different concepts of a number of groups (form-genera) in the current literature (see Ch. 2.4). In compiling Fig. 1 the author kept as close as possible to the original concepts of the taxa. The published data on the vertical distribution of the most important groups of Proterozoic stromatolites are generalized in Fig. 2. Substantiation of the correlations implied in the construction of Figs. 1and 2 is given in Geochronology of the U.S.S.R. (Anonymous, 1973b) and in Semikhatov (1974). Four successive stromatolite complexes and adequate microphytolite associations (Zhuravleva, 1962, 1964a, 1968) served as a basis for constructing the general four-fold scale for the Riphean on paleontological grounds (Keller, 1964, 1966a; Komar et al., 1964; Semikhatov, 1966). This scale was derived from an earlier stratigraphic scheme which had been based on historical-geological criteria and used the unconformity-bounded lithostratigraphic rock groups of the Ural and Russian Platform as the types for chronostratigraphic units (Keller, 1952; Sokolov, 1952). The persistence of this thinking has caused a duality in concepts of the Riphean subdivision. On the one hand, the paleontological criterion is now regarded as fundamental in their distinction, and these subdivisions are accordingly understood to be

M.A. SEMIKHATOV

340

Fig. 1. Distribution of stromatolites in the Riphean reference sections in northern Eurasia. Legend: 1 = carbonate and terrigenous-carbonate formations; 2 = terrigenous formations; 3 = the pre-Riphean formations; 4 = discontinuities; 5 = acid intrusions; 6 = basic intrusions; 7-12 = stratigraphic boundaries: 7 = t h e Cambrian lower boundary; 8 = t h e Terminal Riphean lower boundary; 9 = t h e Upper Riphean lower boundary; 10 = the Middle Riphean lower boundary; 1 1 = the Lower Riphean lower boundary; 12 = boundariesof formations; 13 = unconformity; 14-24 = isotopic age in m.y.; 14 = K-Ar method (glauconite); 15 = K-Ar method (hydromica); 16 = K-Ar method (whole-rock, effusive rocks); 17 = K-Ar method (minerals of metamorphic and intrusive rocks); 18 = Rb-Sr method (whole-rock, granitoids); 19 = Rb-Sr isochron method (whole-rock, granitoids); 20 = U-Pb method (zircon); 21 = isochron U-Pb method (uranium from cement of clastic rocks); 22 = U-Pb isochron method zircons 23 = &Pb method (clastic zircons); h~ = 0.557 lO-''year-'; h ~ =b 1.39 * 10-I1 year . An.r. = Anabaria radialis Komar; B. = Baicalia sp., B.b. = B. baicalica (Maslov), B.in. = B. ingilensis Nuzhnov, B.1. = B. lacera Semikhatov, B.m. = B. minuta Komar, B.mc. = B. maica Nuzhnov, B.p. = B. prima Semikhatov, B.r. = B. rara Semikhatov, B.un. = B. unca Semikhatov; Bx.ab. = Boxonia allahjunica Komar and Semikhatov, Bx.g. = B. grumulosa Komar, Bx.1. = B. lissa Komar; C1. = Colleniella sp., C1.s. = C. singularis Komar; Col. = Colonnella sp., C01.c. = C. cormosa Komar, Co1.d. = C. discreta Komar, Co1.f. = C. frequens (Fenton), Co1.k. = C. kylachii Shapovalova, Col.1. = C. lineata Komar, Col.ul. = C. ulakia Komar; Con. = Conophyton sp., C0n.c. = C. cylindricum Maslov, C0n.g. = C. garganicurn Korolyuk, C0n.m. = C. miloradovici Raaben, Con.ml. = C . metula Kirichenko, Con.1. = C. lituum Maslov; G. = Gymnosolen sp., G.ab. = G. altus Semikhatov, G.as. = G. asymmetricus Raaben, G.f. = G. furcatus Komar, G.r. = G. ramsayi Steinmann, G.t. = G. tungusicus, G.l. = G . levis Krylov, Gn.d. = Gongylina differenciata Komar, Gn.m. = G . mixto Komar, Gn.n. = G . nodulosa Komar and Semikhatov, Gn.z. = G. zonata Komar; In. = Inzeria sp., 1n.c. = I. confragosa (Semikhatov), 1n.d. = I. djejimi Raaben, 1n.n. = I. nimbifera (Semikhatov), 1n.t. = I . tjomusi Krylov, 1n.vr. = I . variusata Golovanov; Jac. = Jacutophyton sp., Jac.m. = J. multiforme Shapovalova, Jac.r. = J. ramosum Shapovalova; Jur.c. = Jurusania cylindrica Krylov, Jur.j. = J. judomica Komar and Semikhatov, Jur.s. = J. sibirica (Yakovlev); Jur.t. = J. tumuldurica Krylov; K. = Kussiella sp., K.k. = K . kussiensis (Maslov), K.vt. = K . uittata Komar; Kt.t. = Kotuikania torulosa Komar;L.uk. = Linella ukka Krylov, L.s. = L. simica Krylov; Min.s. = Minjaria sakharica Komar, Min.ur. = M. uralica Krylov. M1.m. = Malginella malgica Komar and Semikhatov; M1.z. = M. zipandica Komar; N.in. = Nucleella inconformis Komar, N.f. = N. figurata Komar, Om. = Omachtenia sp., Om.om. = 0. ornachtensis Nuzhnov; Pan. = Paniscollenia sp., Pan.em. = P. emergens Komar; P1.p. = Platella protensa Komar; Prm.aim. = Parmites aimicus (Nuzhnov), Prm.c. = P. concrescens Raaben; S v s v . = Svetiella svetlica Shapovalova; St. = Stratifera sp., St.f. = S. flexurata Komar, St.ir. = S. irregularis Komar, St.un. = S. undata Komar; T. = Tungussia sp., T.b. = 2'. bassa Krylov, T.cl. = T. colcimi Raaben, T.cn. = T. confusa Semikhatov, T.n. = T. nodosa Semikhatov; Tn.p. = Tinnia patomica Dolnik. I-IV = formations; I = Kartochka; I1 = Krasnya Gora; III = Ostrov; I V = Migoedikha; V = Nemakit-Daldyn.

-2;

LP6

0,

%

a

s

Fig. 2. Vertical distribution of the most significant stromatolite groups (according to M.A. Semikhatov, 1974). Legend: I = Limits of vertical distribution of groups according t o data of 1968-1973; 2 = the same, according to data of 1960-1964; 3 = the same, according to data of 1965-1968. U h

I I 11

_-_

-

-~

-+

-

--

-

~

-

.

,

--

Aphebian

__-

-

.~

\

- --

Lower

i

t

I

I

\

A-.

1

\

Upper

-

-----

Middle

Ri p h e a n

Anabaria, 1964

icalia ,1962

ussiella ,1962

’ H I l i c t a ,1960 Vetella, 1967

4 Linocolhna ,1960 ~ U r i c a t e l l a1969 . - -#Columnoefacta ,1960

’

Schancharia. 196C Tunicata, 1969

Sa cculia.1960

Colleniella , 1960

F P a n i s c o l l e n i a , 1960

9

G e ol ogi c a l Age

1a n!

348

M.A. SEMIKHATOV

beds with a definite stromatolite assemblage. On the other hand, there is still an endeavor to determine the time-range of these units by the time-ranges of those lithostratigraphic rock groups which had been suggested as the types of the Upper Precambrian subdivisions. The situation is complicated by the fact that (a) not all the members of the stromatolite succession described in Siberia have been identified in the Uralian section historically chosen as the type for the Riphean and its subdivisions; and (b) there is some uncertainty in the precise correlation of this section with the Siberian ones. The above facts explain the difference in viewpoints on the subdivision of the Riphean and on the position of the boundaries of its subdivisions (discussions can be found in the works of Keller and Semikhatov, 1968, pp. 26-28, 90-92; Keller, 1971; and Semikhatov, 1974, pp. 260-264). , In spite of these differences, the four-fold stratigraphic scale of the Upper Precambrian based on stromatolite and microphytolite assemblages is more widely used in the U.S.S.R. than the other scales. Since 1961 it has served as a basis for all the correlation schemes approved at conferences held on Riphean stratigraphy in the main regions of its development in the U.S.S.R. and is used in a number of important generalizations (e.g., Anonymous, 1963, 1973b; Garris et al., 1964; Vinogradov, 1968; Keller et al., 1968). The four main subdivisions of the Upper Precambrian in this scale are considered to be chronostratigraphic. Following Keller (1966a), a number of geologists consider them to be units of a special rank: protosystems or phytems. The lower three phytems are usually described as the Lower, Middle and Upper Riphean, while the fourth one (Vendian or Yudomian) is either included in the Riphean as its terminal member (e.g. Keller and Semikhatov, 1968), or distinguished as an independent unit, sometimes referred to the Phanerozoic (e.g. Sokolov, 1968, 1 5 7 2 ~ )The . Yudomian is considered in this chapter as the Terminal Riphean. The data on the isotopic geochronology of the Late Precambrian of the U.S.S.R. obtained mainly by the K-Ar method, in general confirmed the stratigraphic conclusions based on stromatolites and made it possible t o date the boundaries of the complexes at 1350 f 50,950 f 50 (1000) and 675 k 25 (680 k 20) m.y., respectively (Garris et al., 1964; Keller, 1964; Semikhatov and Chumakov, 1968; Anonymous, 197313). Pre-Riphean stromatolites are of a very limited distribution in the U.S.S.R. and are poorly studied in comparison to those of the Riphean (Metzger, 1924; Butin, 1960, 1965, 1966; Krylov, 1966a; Vologdin, 1966; Kirichenko et al., 1967; Lyubtsov, 1973; Dolnik and Nikolskii, 1974). That is why the information now available about them is mainly based on the data obtained in other countries. However, the application of comparative methods is only beginning. Nevertheless, we can state even now that in the Aphebian as well as specific taxa there exist widespread groups with long time ranges extending through the Riphean and even into overlying formations.


349

DETAILED FOUR-FOLD SCALE OF RIPHEAN SUBDIVISION ACCORDING TO STROMATOLITES

The very considerable time-range of the four main subdivisions of the Riphean based on the assemblages of groups of stromatolites, explains the striving towards a detailed general scale. Three approaches to the use of stromatolites for this purpose may be outlined. In the first approach subdivisions are distinguished in one (Kabankov et al., 1967; Golovanov, 1970) or in several neighbouring sections (Nuzhnov and Shapovalova, 1965,1968) and are characterized by one or two forms. Thus, a relatively detailed subdivision is reached, but the problem of the lateral persistence of the subdivisions and of the time-ranges of the forms has still to be solved. It is not clear therefore whether the peculiarities of the distribution of forms are purely accidental and dependent on local conditions. The second approach is more promising. It has led to the present scheme of subdividing the Middle and Upper Riphean and made it possible to outline the two-fold subdivision of the Yudomian in Siberia. It provides for selecting certain assemblages of the stromatolite groups and forms on the basis of empirical data on their distribution in a series of sections in two or more regions. For stratigraphic purposes, only an integral picture of vertical distribution of the taxa is used, which summarizes the data available from the largest possible area. This makes it possible to balance, to some extent, the particular features of stromatolite distribution associated with environmental influences and to recognize the time-ranges of the taxa. The fossiliferous strata are grouped then with overlying and/or underlying formations into stratigraphic subdivisions so that the upper boundary of each subdivision coincides with the lower boundary of the next (Semikhatov et al., 1967a, 1970; Krylov et al., 1968; Raaben, 1969b, 1972; Raaben and Zabrodin, 1969; Komar et al., 1970; Krylov and Shapovalova, 1970b). As the stromatolitebearing strata usually form only a certain part of the section, peculiarities of the strato-type (such as the position of unconformities, sharp changes in lithology, etc.) are considered to be of decisive importance in determining the location of these boundaries. It is clear that tracing of such boundaries between regions is a very complicated matter and sometimes leads to contradictions. The third approach to Riphean stratigraphy lies in a detailed interregional correlation on the basis of similar stromatolite forms discovered (actually, of similar microstructures) (Komar and Semikhatov, l965,1968a, b; Semikhatov and Komar, 1965; Komar, 1966, 1973). Such an approach has two weak points. Firstly, microstructure becomes the basic diagnostic stromatolite feature according to this viewpoint; this leads to a violation of the principles of distinguishing taxa in the present classification (see below). Secondly, the tedious task of determining the distribution of stromatolites with certain microstructures in each of the regions must precede their use in the

3 50

M.A. SEMIKHATOV

inter-regional correlation. The fact is that the persistence of the succession of microstructures observed in a single section over a whole region is only one of the possible ways they can be distributed (see Ch. 10.8), as was revealed by the detailed investigations carried out in the Uchur-Maya Region (Komar et al., 1973). Moreover, some of the forms with a peculiar microstructure in one area may be considered as age datum markers in one succession but those in another area may be encountered in the reverse succession (e.g., Inzeriu tjornusi and I. confrugosu in the Uchur-Maya and Turukhan areas, Fig. 1and Ch. 10.8, fig. 5 ) . The most reliable results are obtained through the second of the approaches under consideration. This approach was used in establishing the phytems and now is applied in distinguishing their subordinate subdivisions, usually called “horizons”. They are established on stable assemblages of forms of different groups and sometimes even by specific groups. Three horizons are usually distinguished in the Middle Riphean, viz. the Svetlinian, Tsipandian and Lakhandian horizons (Krylov et al., 1968, 1971; Komar et al., 1970; Krylov and Shapovalova, 1970b), and two in the Upper Riphean, viz. the Katavian and Minyarian (Krylov and Shapovalova, 1970a; Krylov, 1972), or, to some extent of different content, the Biryanian and Minyarian (Raaben, 1969a, 1972). The other two approaches in detailing Riphean stratigraphy using stromatolites are aimed at detailed subdivision of the horizons. Their subordinate units are distinguished with confidence only on the regional scale at the present stage. All the Middle and Upper Riphean horizons, except the Svetlinian, extend over a very large area in northern Eurasia, and some of them can be traced beyond there (Krylov, 1972; Semikhatov, 1974). Nevertheless, they cannot be recognized in all the stromatolite-bearing sections in the U.S.S.R. since the specific stromatolite composition makes it possible, in a number of cases, to distinguish only a phytem as a whole. Moreover, Gymnosolen and Minjuriu urulicu, typical of the Minyarian horizon, occur in some of the sections above the beds with Inzeriu tjornusi and Jurusuniu cylindricu (the Katavian horizon), while in the other continuous sections the Katavian stromatolites are missing and the Upper Riphean begins with the beds containing Minjuriu urulicu and/ or Gymnosolen (see Fig. 1).This fact is probably explained by environmental causes as the first association of forms tends to occur in grey-coloured massive dolomites and the second one in yellow and red argillaceous dolomites and limestones. Some other stromatolites are also confined to certain facies (Semikhatov et al., 1970). However, there is also a contrary trend, i.e. spread of particular forms and certain associations of taxa into different facies. This trend, particularly in the Katavian association, is stressed by its discovery in clean, dark limestones on the Korean Peninsula and in the Uchur-Maya area (see Ch. 10.8). Preiss (1973a) arrived at similar conclusions for a number of taxa from the Adelaidean of South Australia.


351

CRITERIA FOR ESTABLISHING STRATIGRAPHIC BOUNDARIES ACCORDING TO STROMATOLITES

The investigations carried out in recent years have considerably widened knowledge of the systematic composition of the successive complexes of Riphean stromatolites, and have shown that the complexes have more groups in common than considered previously (see Fig. 2). As a result, establishing criteria for determining the stratigraphic boundaries according to stromatolites has become urgent. The majority of the Soviet stromatolite workers consider that the boundaries should be determined by the change of stromatolite complexes, i.e. by the change of the empirically established stable assemblages of certain groups and forms. Thus, the boundary of the Middle and Upper Riphean is defined, on the one hand, by the disappearance of the complexes of Lakhandian baicalias, conophytons, jacutophytons, tungussias and associated groups and, on the other hand, by the spread of another complex with the predominance of certain forms of gymnosolens, minjarias, inzerias, and jurusanias. The older groups are greatly reduced in the new complex and, as a rule, represented by new forms. This boundary is clearly traced in Siberia and, beyond its territory, established in the northern part of the Korean Peninsula and in some other areas (Semikhatov, 1974). However, the exact position of the boundary is not quite clear in the Ural. The Katavian suite contains the first Upper Riphean stromatolites and the underlying stromatolite-bearing (the Tsipandian according to Krylov, or Svetlinian, according t o Komar) part of the Middle Riphean deposits are separated here by the thick clastic Zilmerdak suite and its antecedent unconformity to which are confined gabbro-diabases with K-Ar age of 1,015-1,195 m.y. (Fig. 1). On the basis of historicalgeological and geochronologic data this boundary in the Ural is placed either at the base of the Zilmerdak suite (Raaben, 1964a, 1967, 1969a, 1972b; Krylov and Shapovalova, 1970a, b; Krylov, 1972) or at its top (Anonymous, 1973b; Semikhatov, 1973, 1974). If we accept the second alternative, which seems to be more consistent with the data now available (discussions are given in Geochronology of the U.S.S.R., Anonymous, 1973b, pp. 317-319), we are inevitably led to contradictions regarding the position of the Middle and Upper Riphean boundary. In the Ural this boundary is traditionally placed at the unconformity at the base of the Zilmerdak suite (age about 1,100 m.y.), but in Siberia the most probable time-equivalent of this suite is the lower part of the Lakhanda Group (Khomentovskii et al., 1972a; Semikhatov, 1974), which contains a stromatolite complex of the same name usually included in the Middle Ripheak Another criterion for placing boundaries - the first appearance of new groups - is adopted by Khomentovskii et al. (1972a, b). According to this, the Middle/Upper Riphean boundary is lowered by them down to the base of the Lakhandian horizon or to the underlaying Tsipandian horizon, as it is

352

M.A. SEMIKHATOV

in these horizons that the first rare minjarias, gymnosolens and inzerias are encountered among the baicalias, jacutophytons and conophytons typical for this part of the Riphean (Fig. 1).Such a lowering of the Middle and Upper Riphean boundary in Siberia has been supported by Keller (1973a). The following argument can be used against this viewpoint. Firstly, the Lakhandian And Tsipandian minjarias, gymnosolens and inzerias are known only in two areas of Siberia, are encountered within the Middle Riphean complex of stromatolites, and are represented as a rule by endemic forms having a microstructure common with baicalias dominant in the same beds ‘and bioherm. Secondly, adherence t o this viewpoint may prevent subdivision of the Precambrian using stromatolites since there are stromatolites in the Aphebian of Canada, Karelia and Australia (together with stromatolites unknown in the Riphean) which are close if not identical to Gymnosolen, Minjaria, Katavia and Tungussia by a number of criteria (Krylov, 1966a; Hofmann, 1969a, b; Walter, 1972a). The idea of lowering the Middle and Upper Riphean boundary in Siberia was supported by Komar (1973) from a different point of view. Defining the boundary not on the change of stromatolite complexes, but as the boundary of the Yurmatian and Karatavian Series of the Uralian section, he suggested a new correlation of the latter section with the Siberian sections. In this connection, two aspects must be emphasized. First, this correlation is based on comparison of stromatolite microstructures essentially without taking into consideration the morphology of the structures and sometimes in contradiction of it. Considering the microstructures as the most important diagnostic feature, Komar in a number of cases uses for his correlation microstructures belonging to different morphological groups of stromatolites (e.g., Baicalia and Svetliella, Inzeria and Appia) or those supposed to be at the transition between stromatolitic and abiogenic structures (Malginella malgica). Secondly, the correlation is contradicted by: (1)results of tracing the Katavian stromatolitic association (Krylov and Shapovalova, 1970b; Krylov, 1972); (2) some of the geochronologic data (in particular, the dating of the pre-Zilmerdak events in the Ural and sediments of the Turukhan section: Ivanovskaya et al., 1973; see Fig. l),and (3) that the certain correlatives of the upper three suites in the Upper Riphean of Turukhan, referred by Komar to the upper part of this phytem, are intruded by granites on the Yenisey Ridge which have U-Pb and Rb-Sr ages of 850 k 50m.y. (Semikhatov et al., 1973). The “level of development of characters” for the columnar structures, such as the occurrence of active branching in the Middle Riphean and of walls in the Upper or Middle Riphean, was considered several years ago to be a valid criterion for establishing stratigraphic boundaries according t o stromatolites (e.g., Komar et al., 1964; Krylov, 1966a; Raaben, 1969b). This scheme constructed on the example of Riphean stromatolites in the U.S.S.R. has not been confirmed by the studies of pre-Riphean forms; some of these


353

proved to possess active branching and/or walls (Cloud and Semikhatov, 1969b; Hofmann, 1969a, b; Walter, 1972a). However, it should be stated that the data now available have not disproved the conclusions on the qualitative differences in Riphean stromatolitic complexes of different ages, though they have made us revise the simplified concepts about these differences. So, for example, the Middle Riphean complex differs from its precursor not only in the unique combination of groups and forms of stromatolites but also in the domination of actively branching unwalled structures. The latter, though they are encountered also in older and younger sequences and do not completely characterize this complex, nowhere else reach such an abundance and variety. In the same way, the presence of actively branching and/or walled forms in the Aphebian cannot disprove the fact that such stromatolites attain abundance and, to a considerable extent, replace other groups in the Late Riphean. ENVIRONMENTAL VARIABILITY IN STROMATOLITE MORPHOLOGY

During the period of confirming the stratigraphic significance of stromatolites (1959-1964) attention was concentrated on the problem of determining the parallelism in the changes of complexes in remote sections. The main objects of investigations were, therefore, the columnar varieties which are encountered more often than others and which possess the clearest diagnostic features. The variability of stromatolites under the effect of the environment, which had been recorded by Fenton and Fenton (1933), Cloud (1942), Fenton (1943), Maslov (1959,1960) and other investigators, was beyond the attention of the new generation of stromatolite workers during these years. The successful use of stromatolites in stratigraphy corroborated the presence of biotic control of their morphology. All that resulted in the concept that ecological factors could affect the morphology of the structures only in an indirect way, defining the location of algal coenoses (Krylov, 1963; Nuzhnov, 1967; Raaben, 1969a), and that the stromatolites with stable diagnostic features were widely developed in the Riphean and Cambrian (Korolyuk, 1963; Krylov, 1963; Korolyuk and Sidorov, 1965; Keller and Semikhatov, 1968). The morphologic variability of stromatolites attracted attention again after Vlasov (1965, 1970) and Krylov (1965,1967a) began detailed studies of the structure of bioherms. According to Vlasov and Krylov, the morphologic features of stromatolites change in a regular manner from the centres of bioherms t o their margins, and there is a definite vertical and less apparent horizontal zonation in completely developed bioherms reflecting certain stages of their development. Conclusions on the universality of such regularities were criticized (Raaben, 1969b), but the fact of the variability of stromatolite morphology within a bioherm has received new corroboration (see Ch. 6.4).

354

M.A. SEMIKHATOV

The growth stages of bioherms were attributed by Vlasov (1970) t o biological regularities in the development of algal colonies. However, the ecological interpretation of the stromatolite variability in bioherms has become more popular. It has been consolidated by the correlation of changes in stromatolite morphology with changes in the composition of the enclosing rocks which reflect fluctuations of the environment (Miroshnikov, 1965; Shapovalova, 1965, 1968; Shenfil, 196513; Serebryakov, 1971; Serebryakov et al., 1972). However, it still remains t o be determined what the environmental factors are which have created different changes in the morphology. In addition, there is a definite contradiction with the examples of the morphological variability of stromatolites which are not accompanied by changes in the composition, texture and structure of enclosing rocks, and numerous examples of morphologically identical stromatolites encountered in deposits of different facies (see Ch. 6.4). The results of studies of stromatolite microstructure were of great importance in the interpretation of their morphological variability. The microstructures were sometimes identified as algae (Krasnopeeva, 1946; Korde, 1950, 195313; Vologdin, 195513,1962). However, the majority of Russian stromatolite workers consider them to be the products of the interaction of various carbonate-depositing processes (including biogenic processes) and subsequent modification of sediments; they regard them as one of the diagnostic features of stromatolites (usually as the main distinguishing feature of forms). Normally it is considered that the peculiarities of the microstructures are, in a certain way, connected with the systematic composition of stromatolitebuilding algae. This viewpoint, formulated by Walcott, the Fentons, Maslov and Korolyuk, was strengthened especially when the usual consistency of microstructures in bioherms, the stratigraphic limitation of a number of microstructures, and the increase in their variety from the Lower Riphean to the Cambrian were demonstrated (Komar, 1964, 1966; Komar and Semikhatov, 1965, 1968b; Semikhatov and Komar, 1965; Krylov, 1969; Raaben, 196913; Golovanov, 1970; Korolyuk and Sidorov, 1971). The dependence of stromatolite microstructures on the composition of the algae which formed them was strongly supported by studies of Recent stromatolites (Monty, 1967; Gebelein, 1969; Hoffman et al., 1969; Logan et al., 1974). The weak point in using microstructure as a diagnostic feature is that we can distinguish only some of their secondary transformations, namely, those which arise at relatively late stages (Kopeliovich and Krylov, 1960; Krylov, 1963; Komar, 1966; Raaben, 1969a; Semikhatov et al., 1970). In each particular case we cannot determine the significance of diagenesis in the alteration of the primary stromatolite microstructures. That is why all the stromatolite microstructures except those of definitely secondary origin are considered to be of diagnostic significance. Such an assumption is corroborated by the frequent recurrence of identical microstructures in rocks of different composition (including terrigenous) in various areas, and also by their survival


355

in the rocks found at different stages of alteration (up to regional metamorphism at lower greenschist facies). All the evidence taken together strongly suggests that the consistency of microstructure within bioherms reflects the unity of the algal communities which formed them. If so, examples of the persistence of microstructures through changes in gross morphology show (in full compliance with data on Recent phytogenic structures) the effect of environmental factors on the morphology of stromatolites (Shapovalova, 1965, 1968; Shenfil, 1965b; Komar, 1966; Krylov, 1967a, 1972; Komar and Semikhatov, 1968b; Serebryakov, 1971; Serebryakov et al., 1972; and others). This conclusion, on the face of it, cannot be reconciled with conclusions on the stratigraphic significance of stromatolite morphological groups. However, it has been found that the morphological variability resulting from changes in environmental conditions is not unlimited, and does have definite ranges probably determined by biotic factors. This was proved in three ways that are closely interconnected. Firstly, in studying Riphean and Cambrian stromatolites, Krylov (1967a) came t o the conclusion that the morphological varieties of certain stromatolites occur in regular combinations in bioherms; these same combinations are encountered in contemporaneous beds in numerous sections. Krylov (1972) later advanced the hypothesis of the bioherm series. These series are the combinations of morphological modifications present in a single bioherm or in several bioherms of the same kind in a single bed; each series may have been formed by one community (or species) of algae under various conditions. Each of the bioherm series as well as structures of a single characteristic group forming the central zone of the bioherm contains less-developed structures of some other groups forming the margins of the bioherm. Those groups can be common t o two or more series (see Ch. 2.4). Secondly, analysis of stromatolite cycles in stratigraphic sections (see Ch. 6.4 and 10.8) has shown that the cyclic alternation of groups depends upon the fluctuation of abiotic factors; but the systematic composition of the assemblages of groups present in such cycles, and their microstructures, are peculiar to certain subdivisions of the Riphean, and are therefore timedependent (Serebryakov, 1971; Serebryakov et d . , 1972). Finally, the existence of biotic control of the gross morphology of stromatolites was strongly supported by the intimate knowledge of the presence of successive complexes of stromatolites, generally of constant composition, on an inter-regional and even inter-continental scale. The relationships between microstructure and gross morphology led several authors t o the conclusion that microstructure must be taken into account not only in defining forms but also higher categories. Microstructural unity has been regarded as a decisive argument for referring all the morphologies found in a single bioherm to one and the same form (Komar, 1966; Semikhatov et al., 1967a, 1970; Komar and Semikhatov, 1968b). The morphology that is

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observed in the central part of a bioherm where the stromatolites were less subjected to the effects of the environment was considered to be typical for the form, and the appearance of peripheral morphologies was regarded as ecological variability of the one form. Thus, the position of Fenton and Fenton ( 1 9 3 9 ~and ) Fenton (1943) was actually repeated regarding the classification of ecologic varieties (ecads). In practice it leads t o an understanding that the groups are not morphological categories but the sum of the variable morphological forms of a certain microstructure. Hence, the principle adopted in modem taxonomy for the determination of stromatolite groups, i.e. the use of a set of morphological features, is violated, and the taxa selected acquire new meaning, though they formally remain members of the old classification (see Ch. 2.4). That is why it should be admitted after Krylov (1967a, 1972) that the various morphologies within bioherms and biostromes may and must have different group names in accordance with their morphological features and irrespective of the microstructure. Paleobotanical classification provides a precedent for such a procedure. This viewpoint is now approved by the majority of the Soviet stromatolite workers, including the author of this paper who previously took a different position. Unfortunately, there are now only a few examples of descriptions of different stromatolite groups derived from single bioherms and possessing a single microstructure (Krylov, 1967a, 1975). It was suggested that all stromatolites with particular microstructures be considered and all the morphological modifications (groups) of these stromatolites represented in available material enumerated (Serebryakov et al., 1972; Komar et al., 1973; Semikhatov, 1974). The absence of a widely adopted microstructural classification has not made it possible to use definite terms in this case and the investigators had t o apply complicated definitions as “the microstructure described in distinguishing such and such species”. I t is clear that this solution cannot replace paleontological descriptions of stromatolites within the classification now in use. On the other hand, the relations considered urgently require that an independent stromatolite classification, based entirely on microstructures, be elaborated. C0N CLU SI 0N S

Thus, the understanding of Precambriarl stromatolites in the U.S.S.R.has developed from the assertion of their stratigraphic significance (Maslov) t o its denial unde‘r the influence of ideas on the complete dependence of morphological features upon the abiotic factors, and then in a new phase to the corroboration of the stratigraphic significance of certain morphological groups on the basis of their empirically determined vertical distribution. It is the empiric approach to the solution of the problems regarding


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stromatolites which is a distinguishing feature of the Russian school of investigators. The analogies with Recent stromatolites, which dictated for a long period of time the genetic and ecological interpretation of stromatolites and determined a priori the negative estimation of their stratigraphic potentialities in the western literature, were always treated with great care in the Russian literature. Moreover, beginning from Maslov’s works there have developed in the Russian literature ideas on substantial differences in the ecology, mechanisms of formation and factors of morphogenesis of ancient and the most widely distributed Recent stromatolites. A t present, these ideas are acquiring greater and greater popularity. The influence of ecologic factors on the morphology of ancient stromatolites was either not accepted, or was denied, at the beginning of the new phase of studies in the U.S.S.R. The conclusion of the reality of such an influence - although its development was within certain limits probably determined by biological causes - was gradually accepted later. At present, a four-fold scale for subdivision of the Riphean in northern Eurasia has been established on the basis of changes in the complexes of certain groups and forms of stromatolites (mainly columnar). Recently it has been indicated that these complexes may be applied to the distinction of similar subdivisions on the other continents, and therefore the Russian stratigraphic scale of the Riphean based on stromatolites can be duplicated elsewhere. Subordinate subdivisions of more or less regional significance have been recognized in the three upper phytems of the Riphean in a number of sections in Siberia and the Ural. Some of them can be traced far beyond northern Eurasia.


7 . STROMATOLITE BIOSTRATIGRAPHY

Chapter 7.2 INTERCONTINENTAL CORRELATIONS W.V.Preiss

INTRODUCTION

Precambrian geologists have for many years entertained the possibility of using palaeontology for interregional and intercontinental correlations. Stromatolites are by far the most abundant Precambrian fossils, but their potential as index fossils was questioned as soon as their mode of origin became understood (e.g. Cloud, 1942). Logan et al. (1964)discovered a unique modern analogue of Precambrian columnar stromatolites at Shark Bay, Western Australia, but even here, there is nothing comparable to the enormous diversity of complexly branching forms known to be very abundant in the Precambrian. Realization of the potential value of stromatolites as index fossils can only follow from detailed taxonomic investigations. As many features of stromatolites as possible must be described and ranges of variation within single occurrences noted, so that form taxa can be erected and discriminated by serial sectioning, reconstruction and thin-section techniques. In the classification of stromatolites, it is inadmissible to refer t o “genera” and “species” since stromatolites are not fossil organisms. Nor are “form genus” and “form species” as used by palaeobotanists for parts of plants appropriate. The terms c‘grou”’ and “form” used by the Russians who revived interest in stromatolite taxonomy over fifteen years ago, have no strict biologic connotations and are therefore most suitable for the classification of these biogenic sedimentary structures. Much of the argument that has ensued arises from the semantics of classification. The taxonomic approach has a different aim from the descriptive classifications such as that of Logan et al., (1964). We are not concerned here with the geometry of an individual lamina or column, but with an entire structure passing through a number of growth stages and having a definite range of variability, whose mode and total extent must be determined in each case. For example, a progression from laterally linked t o columnar t o laterally linked forms (LLH + SH + LLH in descriptive formulae) is almost universal in columnar stromatolites and hence is of little taxonomic interest. A complex of characters is thus required for the discrimination of the various taxa.

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Both approaches are in themselves equally valid, but the advantage of the taxonomic approach is that it leads to the definition of a number of discrete taxa whose time-ranges can be investigated empirically. If these individual time-ranges turn out to be short, then the value of stromatolites for Precambrian biostratigraphy will have been demonstrated. If, on the other hand, the defined taxa span all or most of geological time, we will know with certainty the limitations of the method. Until this is achieved, there can be no agreement, for no amount of theoretical argument about modem analogues can prove or disprove that ancient stromatolites are useful index fossils. But even within the group of workers who accept the taxonomic approach, there are basic differences in philosophy which pose a problem t o any attempt to synthesize world-wide data on stromatolite time-ranges. THE EXTENT OF THE DATA

Despite these difficulties, there are sufficient data from the various continents to justify a preliminary appraisal of stromatolite time-ranges on a world-wide basis. The following discussion is far from complete but suggests the extent of our present knowledge. The subject is dealt with comprehensively by Semikhatov (1974). U.S.S.R. The first preliminary results of the Russian authors were summarized by Keller et al. (1960). Since then, the detailed descriptions of stromatolites from the southern Urals (Krylov, 1963), the Yenisey Mountains and Turukhan area (Semikhatov, 1962), northem Siberia (Komar, 1966), TienShan and Karatau (Krylov, 1967a), the Uchur-Maya region (Nuzhnov, 1967) and the Polyudov Mountains (Raaben, 1964b) have laid the foundations for modern stromatolite taxonomy and biostratigraphy . Within the U.S.S.R., the Late Precambrian has been subdivided into four chronostratigraphic intervals, each characterized by a specific stromatolite assemblage: the Early, Middle and Late Riphean, and the Vendian. But the considerable overlap in the ranges of some of the groups often necessitates identification to form level in order to characterize each assemblage. No group is restricted to the Early Riphean, for example, but the assemblage Kussiella kussiensis Krylov, with forms of Omachtenia Nuzhnov, Gongylina differenciata Komar and Nucleella figurata Komar, characterizes the Uchur Series, generally correlated with the type Lower Riphean in the Anabar region (Keller, 1973). Komar (1966) described the columnar forms Kussiella kussiensis Krylov, K . uittata Komar, and Microstylus perplexus, the stratiform stromatolites Stratifera flexurata, S. undata and the cumulate Gongylina differenciata and Nucleella figumta from the Early Riphean Kotuikan suite of the type section


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and Kyutingdin suite of the northern Siberian platform. The Burzyan Series (southern Urals), with, e.g., Kussiella kussiensis, Conophyton cylindricum has also been considered as Early Riphean, but Keller (1973) has expressed the view that it may be partly older. Conophyton is widespread in the Early Riphean, but none of its forms is restricted to it (Komar et al., 1965a). The Early Riphean can be characterized only by form-level identifications and by the absence of stromatolites first recorded from younger units. The Middle Riphean is characterized by the presence of new groups (Anabaria Komar, Baicalia Krylov, Suetliella Shapovalova) with abundant Conop h y t o n (C. cylindricum, C. lituum, C. metulum, C. garganicum Korolyuk) and Jacutophyton Shapovalova. Of these Anabaria has been found only in the Anabar vicinity (Komar, 1966) and Svetliella in the Uchur-Maya region (Krylov et al., 1968) and the Urals (Keller, 1973), but Baicalia is very widespread throughout the U.S.S.R. (e.g. southern and central Urals, Tien Shan, Turukhan region, River Maya, Anabar Massif, Olenek Uplift and Kharaulakh Highlands). Jacutophyton is known from the Aldan Anteclise, Turukhan area, Yenisey Mountains, southern Ural and other areas (Krylov and Shapovalova, 1970b). Tungussia Semikhatov is common in the Yenisey Mountains and Turukhan area (Semikhatov, 1962) and also in younger units elsewhere. The Late Riphean is marked in the U.S.S.R. by a significant increase in taxonomic diversity of stromatolites, a number of groups making their first appearance and/or expansion here. The most widespread groups are Gymnosolen Steinmann, Minjaria Krylov, Inzeria Krylov, and Jurusania Krylov, known from the Urals, Tien Shan and Siberian Platform. Kotuikania Komar, Boxonia Korolyuk, Katauia Krylov, Turuchania Semikhatov and Conophyton miloradouici Raaben are more restricted geographically. Rare Baicalia persists into the lower part of the Upper Riphean in the Turukhan and Uchur-Maya regions, together with typical Late Riphean stromatolites. The position of the base of the Upper Riphean in Siberia is currently under discussion (Keller, 1973; Komar, 1973). The stromatolites of Vendian or Yudomian sequences have been described by Krylov (1967a), Semikhatov et al. (1967a, 1970), Korolyuk and Sidorov (1969,1971) and Krylov et al. (in Rozanov et al., 1969). The Uk suite of the southern Urals, correlated with the Vendian, contains Linella ukka Krylov and L. simica Krylov. L. auis Krylov occurs in uppermost Precambrian beds of Tien Shan and Lesser Karatau together with Patomia ossica Krylov. Conophyton gaubitza Krylov also occurs in Vendian equivalents in these regions. Semikhatov et al. (1970) recognized L. simica also from the Yudoma suite of the Aldan Shield, together with Aldania sibirica Krylov. Boxonia, Patomia, Jurusania and certain cumulate stromatolites are widespread in the Yudoma suite and its Siberian equivalents. The chronostratigraphic ranges of stromatolites described from the U.S.S.R. have been summarized by Walter (1972a, table 6), Hofmann (1973,

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fig. 9 ) and Semikhatov (1974, figs. 13, 14). Kussiella and Omachtenia are characteristic of, but not restricted to, the Early Riphean. The association of Baicalia, Anabaria and Svetliella with Conophyton and Jacutophyton is typical of the Middle Riphean; Baicalia, Conophyton and Jacutophyton are the most abundant but also extend into the Late Riphean, which is characterized by a divexse assemblage of Gymnosolen, Minjaria, Boxonia, Znzeria, Katavia and others. The Vendian is characterized by Linella, Patomia, Aldania with Boxonia and Jurusania. Different forms of Conophyton and Jacutophyton characterize each subdivision. The data for the U.S.S.R. are promising and indicate the potential for worldwide correlation if the time-ranges of the defined taxa can be duplicated outside the U.S.S.R. Australia Taxonomic stromatolite studies were aimed at testing the Russian biostratigraphy. Preliminary results were published by Glaessner et al. (1969), amplified by Preiss (197213, 1973b) and Walter (1972a) and summarized by Walter and Preiss (1972). In summary: (a) The Lower Proterozoic Mt. Bruce Supergroup of Western Australia contains Alcheringa narrina Walter, Pilbaria perplexa Walter, Patomia f. indet. and Gruneria f. nov. (b)The unconformably overlying Bangemall Group contains Baicalia capricornia Walter and Conophy ton garganicum australe Walter, an assemblage closely resembling that of the Middle Riphean of the U.S.S.R. (c) The Burra Group of the Adelaide Geosyncline contains Baicalia burra Preiss closely resembling Russian forms from the upper Middle Riphean, and Tungussia, and Conophyton garganicum occurs out of stratigraphic context in a diapir. (d) The Bitter Springs Formation of the Amadeus Basin contains an assemblage including groups typical of the Late Riphean in the U.S.S.R. (Boxonia, Minjaria, Znzeria, Jurusania, Kotuihania), Linella avis Krylov found in the Vendian of Tien Shan and new groups Kulparia Preiss and Walter and Basisphaera Walter. Very widespread is Acaciella australica Walter, type form of a group as yet undescribed from outside Australia. (e) The Umberatana Group of South Australia, with basal glacial sediments unconformably overlying the Burra Group is also rich in stromatolites with Late Riphean and Vendian affinities - Inzeria, Boxonia, Jurusania, Katavia, Tungussia, Linella, Acaciella and Kulparia. (f) The Ringwood Member of the Pertatataka Formation, between two glacials in the Amadeus Basin, contains Tungussia inna Walter, now also identified from South Australia, just below the Ediacara fauna-bearing Pound Quartzite (Preiss, 1971a, unpublished). (g) Cloud and Semikhatov (1969b) described Kussiella in the Elgee Siltstone, Western Australia, Conophy ton garganicum from the Amelia Dolomite, Northern Territory (described in more detail by Walter, 1972a), Eucapsiphora paradisa from the Paradise Creek Formation, Queensland, and Znzeria cf. tjomusi from the Hinde Dolomite and,


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doubtfully, Dook Creek Dolomite, Northern Territory. The latter was disputed by Walter and Preiss (1972). Glaessner et al. (1969), Walter (1972a) and Preiss (1971a) proposed certain correlations with the U.S.S.R. assuming that the stromatolites have similar time-ranges in the two continents. The Bangemall Group and the Burra Group were correlated with the Middle Riphean; the Bitter Springs Formation (below an older tillite) was considered as Late Riphean; the Umberatana Group and the Ringwood Member, between two tillites, were considered to be close to the Late Riphean-Vendian boundary in age. The correlation of the early Adelaidean Burra Group with the Middle Riphean requires further comment: it was based on the presence of Baicalia burra, similar to 3.maica Nuzhnov from the Lakhandin suite, thought to be uppermost Middle Riphean (Nuzhnov, 1967, Krylov et al., 1968), and on the absence of typical Late Riphean forms. Russian data show that Baicalia extends into the Late Riphean in the U.S.S.R., and that the Lakhandin suite also contains some typical Late Riphean groups (Komar, 1973). (This is why the Middle/Upper Riphean boundary in Siberia is contentious.) New finds by Preiss (unpubl.) of Acaciella cf. austmlica and Gymnosolen cf. rammyi, from basal Adelaidean beds in South Australia (several thousand metres stratigraphically below Baicalia burra), indicate an overlap in the time-range of Baicalia with those of Acaciella and Gymnosolen. Correlation of the Skillogalee Dolomite with the Middle Riphean is therefore not justified, and a Late Riphean age is preferred for the basal beds and for the whole early Adelaidean. But the absence of Late Riphean forms from the Burra Group is a point in favor of some environmental control determining which stromatolites grew at these two stratigraphic levels. North Africa Most notable among the few other available taxonomic treatments is the work on the stromatolites of northern Africa (Bertrand, 1968a; BertrandSarfati, 1972c, d). In the Taoudenni Basin the Late Precambrian sequence contains five distinct stromatolitic horizons. In summary, the oldest assemblage has Conophyton, Baicalia, Tungussia, Tilemsina Bertrand-Sarfati, Parmites Raaben and Jacutophyton. The second is characterized by Gymnosolen. The third has Serizia Bertrand-Sarfati and Tarioufetia BertrandSarfati with Tungussia. The fourth with Jurusania, Nouatila Bertrand-Sarfati, Gymnosolen, Tungussia and Inzeria occurs only locally. The fifth, with Tifounkeia Bertrand-Sarfati and microproblematica, may have equivalents in the Schisto-Calcaire with Linella, in the West Congo. North African stromatolite assemblages were compared with those of the U.S.S.R. by BertrandSarfati and Raaben (1970).

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W.V. PREISS

India The great potential for detailed studies in India is shown by references t o stromatolites by numerous authors. In particular, Valdiya (1969a) has described and illustrated assemblages of forms from the Lesser Himalayas and the Vindhyachal Hills. His unfortunate decision to reject the modern nomenclature (he retains the Group name Collenia for many columnar stromatolites) renders comparison difficult and limits the precision of his identifications. The following taxonomically treated Indian stromatolites occur: (a) The Fawn Limestone (Lower Vindhyan) contains Conophyton cylindricum, Collenia columnaris. (b) The Bhander Limestone (Upper Vindhyan) contains Collenia baicalica (i.e. Baicalia in the modem nomenclature). (c) This form also dominates in the Gangolihat Dolomites of the Calc Zone of Pithoragarh together with Collenia columnaris, C. kussiensis (i.e. Kussiella), C. symmetrica, C. buriatica (i.e. Minjaria). (d) The Thalkedar Limestone, two formations above, contains Jurusania and Collenia symrnetrica. (e) The Deoban Limestone of Chakrata and the Shali Series of Himachal Pradesh have been correlated with the Calc Zone and contain C. baicalica, C. symmetrica, C. buriatica and C. columnaris (Lower Shali) and ?Jurusania (Upper Shali). Valdiya concluded an Early Riphean age for the Fawn Limestone, and Middle Riphean for the Calc Zone, Lower Deoban, Lower Shali and Bhander Limestone. The Gangolihat and Lower Shali could be transitional between the Middle and Late Riphean. The younger assemblage of the Thalkedar Limestone, Upper Deoban and Upper Shali compares with the Late Riphean. Other areas Here the information is as yet scanty. Cloud and Semikhatov (1969b) have given the only modem taxonomic account of stromatolites from southern Africa: (a) Archaean stromatolites from the Bulawayo Dolomite, Rhodesia; (b) Katernia africana Cloud and Semikhatov from the Dolomite Series, South Africa (a similar form appears in the Nash Formation, Wyoming, U S A ) ; (c) Baicalia aff. rara Semikhatov from the lower Abenab Formation, Tsumeb, S.W. Africa; (d) Conophyton ressoti Menchikov from the upper Abenab Formation. These authors also described the following forms from North America: (e) the Nash Formation forms mentioned above; (f) Gruneria biwabikia Cloud and Semikhatov from the Biwabik and Gunflint Iron Formations; Gruneria also occurs in the Ventersdorp System, South Africa and in the Fortescue Group, Western Australia (Walter, 1972a); (g) Conophyton cylindricum from the Siyeh Limestone, Belt Supergroup and Mescal Limestone, Apache Group; (h) Tungussia in the Mescal Limestone; (i) Linella aff. ukka and Boxonia cf. gracilis in the Johnnie Formation, California. The Crystal Springs Formation, unconformable below the


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possibly glacigenic Kingston Peak Formation, contains Baicalia and Jacu tophyton (Howell, 1971). White (1970) recorded Baicalia from the Belt Supergroup, whose possible correlatives in Alaska apparently also contain Middle Riphean stromatolites (M.A. Semikhatov, pers. comm. in Churkin, 1973). The equivalent Purcell Supergroup of Canada contains Baicalia and Conophyton and the Helikian Little Dal Formation of the Mackenzie Mountains contains Gymnosolen (M.A. Semikhatov, pers. comm.). Jacutophyton and Conophy ton are reported from the Athapuscow Aulacogen (Early Proterozoic) of Canada (M.R. Walter, pers. comm. in Hoffman, 1973). Hofmann (1969) discussed Early Proterozoic stromatolites from the Animikie Group, Canada, making kntative comparisons with previously named forms rather than firm identifications. These included Gymnosolen, restricted in the U.S.S.R. to the Late Riphean, which is a major anomaly requiring further investigation. However, Walter (1972b) has shown that at least some of these structures are of inorganic origin. Moeri (1972) and Cloud and Dardenne (1973) described Conophyton from the Bambui Group of central Brazil and Cowie (1961) noted Conophyton in the Fyn S6 Dolomite (Late Precambrian) of Greenland. M.R. Walter (pers. comm., 1973) has identified the latter as Conophyton cf. cylindricum. These finds all give encouragement for further searching in other continents. THE DATING OF STROMATOLITIC SEQUENCES

Unfortunately, reliable, precise radiometric ages for Precambrian sedimentary rocks are extremely scarce. What data are available are frequently controversial, yet they provide the only objective method of determining stromatolite time-ranges, and the .only ultimate test of intercontinental correlations. What is needed but cannot be attempted here is an independent evaluation by geochronologists of the varied and scattered data available from the various countries and rocks, which were based on different analytical methods and decay constants. Until this is achieved, the stratigrapher can only cursorily examine the published data and accept them at face value. Piper’s (1973) analysis of palaeomagnetic and geochronological data shows that many of the Late Precambrian tillites formed in low latitudes, and therefore represent global climatic events. In addition, the ages for an “upper” glaciation cluster in the 650- 700-m.y. interval, e.g., in Australia (Compston and Arriens, 1968; Dunn et al., 1971), the U.S.S.R. (Chumakov, 1971), Norway (Pringle, 1973), and Africa (Cahen, 1970). These data give encouragement that tillites may provide useful time markers in Late Precambrian stratigraphy, and therefore help define the time-ranges of stromatolites. Fig. 1 summarizes the relevant published radiometric ages of the sequences discussed above. It should be pointed out that the majority of Russian dates are single K-Ar determinations on glauconite. .

366 I

W.V. PREISS I

.R.

UCHURO-MAYA

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STROMATOLITE BIOSTRATIGRAPHY AUSl CENTRAL

SOUTH iAWKER GROUP

367

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Nash Formation

k--_ _ _ _ _

WYLOO

HAMERSLEY GROUP

---__

1 1 -FORTESCUE - - GROUP

B i-Il2a

De1.C

.s

l~DOLOMITESERIES

-

--I

*Iron? Gunflint 16 Formation

3 68

W.V. PREISS

TIME-RANGES OF THE BEST KNOWN TAXA

A brief summary of the ranges of some important taxa discussed is as follows: (1)Pre-Riphean stromatolites: Of these only Gruneria and Katernia africana have so far been identified in more than one continent, and their ranges may be of the order of 1,900 to 2,300 and 1,700 to 2,000 m.y., respectively. The occurrence of Gymnosolen-like stromatolites and Patomia of this age is suggested by Hofmann (1969) and Walter (1972a), and requires further investigation. (2) Late Precambrian stromatolites: The following important taxa appear at the present time t o have restricted time-ranges: Conophyton cylindricum, C. garganicum: Early and Middle Riphean (1,600 to about 1,000m.y.) Conophyton metula, Svetliella, Anabaria: Middle Riphean (1,350 t o about 1,000 m.y.) Tungussia: Middle Riphean t o Vendian (1,350 to 570 m.y.). Baicalia: Middle and early Late Riphean (1,350 t o about ?800 m.y.). Inzeria, Minjaria, Katauia, perhaps Gymnosolen : Late Riphean (about 1,000 m.y. to 680 m.y.). Boxonia, Jurusania, Linella: Late Riphean to Vendian (about 1,000 m.y. to 570 m.y.). 3

Fig. 1 (pp. 366-367). Geochronology and sequences of stromatolite assemblages. A. Type sections of the U.S.S.R. B. Sequences on the other continents. Notes: The age column in A applies to both figures, and indicates absolute ages in m.y. The black areas at the boundaries of the Russian chronostratigraphic intervals indicate the estimated uncertainty of their ages. Two alternative positions for the Middle Riphean/Late Riphean boundary are presently in use. The lower case names refer to suites in A and t o formations in B. Numbers 1 - 2 1 refer t o sources of age data. indicates a single age determination quoted; $ indicates a range of ages quoted; and $ indicates a single age determination with known limits of error. The rock units are placed in the figure wherever possible according to their absolute ages.indicates unconformity. Where a sequence is possibly older than indicated by its position, this extension is shown by an arrow. Roman numerals refer t o stromatolite assemblages: I = assemblages with Kussiella, Omachtenia, Conophyton cylindricum and others; I1 = assemblages characterized by Baicalia, Anabaria and Suetliella; 111 = assemblages with the first appearance of Gymnosolen, Minjaria, Boxonia, Inzeria, Jurusania and others; IV = assemblages with Linella, Boxonia, Patomia and Aldania. References: 1 = Krylov (1963); 2 = Komar (1973); 3 = Keller (1973); 4 = Semikhatov et al. (1970); 5 = Nuzhnov (1967); 6 = Komar (1966); 7 = Krylov, in Rozanov et al. (1969); 8 = Semikhatov (1962); 9 = Khomentovskii e t al. (1972a); 10 = Compston and Arriens (1968); 11 = Cooper and Compston (1971); 1 2 = Compston et al. (1966); 1 3 = Clauer (1973); 1 4 = Cahen (1970); 15 = Bertrand-Sarfati (1972d); 16 = Cloud and Semikhatov (1969b); 1 7 = Tugarinov et al. (1965); 18 = Obradovich and Peterman (1968); 1 9 = Shride (1967); 20 = Compston and Taylor (1969); 21 = Majoribanks and Black (1974);22 = Krylov and Shapovalova (1970b).


369

Linella ukka, L. simica, Aldania: Vendian (680 to 570 m.y.). In Australia, the geochronological ranges of most forms are known with even less precision owing to the poor radiometric control on the ages of the major Late Precambrian sedimentary basins. The following groups are so far known only from Australia: Acaciella is most widespread, and probably ranges from less than 1,000 m.y. (Late Riphean) t o Early Cambrian. Basisphaera, known only from the Bitter Springs Formation, is thus probably Late Riphean. Georginia, closely allied to Jacutophyton, is either Early Cambrian or latest Precambrian. Kulpariu is known only from Late Riphean or Vendian equivalents. Madiganites is Middle and Late Cambrian. Eucapsiphora occurs in rocks considered to be about 1,600 m.y. old. Certain sedimentary rocks from North Africa are now dated and suggest the best known stromatolitic sequence in the. Taoudenni Basin is entirely less than 1,000 m.y. old. The oldest assemblage (Conophyton, Baicalia, Tungussia, Tilemsina, Parmites, Jacutophyton) is therefore already of Late Riphean age. The youngest assemblage with Linella may well be Vendian. These correlations, based on geochronology, would fit the general timeranges known for the Riphean stromatolites, and confirm that Baicalia occurs in the Late Riphean together with other forms appearing here for the first time.

CONCLUSIONS

The biostratigraphic study of stromatolites is still in its infancy. In the last fifteen years of active publishing, a biostratigraphic scheme has been erected which appears to be valid throughout the Soviet Union. Preliminary indications from the other continents, notably Australia and North Africa, indicate that many of the Russian taxa (many of the groups and several forms) have widespread, perhaps global distribution. The determination of their time-ranges, however, depends on geochronology and direct datings of sedimentary rocks are scarce. However, what data we d o have suggest that at least certain groups and forms have restricted time-ranges, and that where assemblages of these are found in Precambrian rocks, tentative correlations can be proposed with known sequences, especially in the U.S.S.R. However, these correlations must be regarded with caution, since they are liable to change as more geochronologic data become available, thus changing the known time-ranges of the stromatolites. A major problem to be resolved is that of pre-Riphean stromatolites with a young aspect. These forms need to be carefully identified, and rocks intermediate in age need to be searched. This will indicate whether these stromatolites are the same as the Late

370

W.V. PREISS

Riphean forms, and hopefully, whether they represent convergent evolution or simply much longer time-ranges than previously suspected. ACKNOWLEDGEMENTS

Many of the ideas expressed arose from a study of South Australian stromatolites, commenced under the guidance of Prof. M.F. Glaessner, who first introduced me to the subject of stromatolites. I am indebted to Drs M.R. Walter, M.A. Semikhatov and I.N. Krylov for valuable discussions and correspondence. Drafting by the Drafting Branch, Department of Mines, Adelaide, S.A., is gratefully acknowledged. The paper is published with the permission of the Director of Mines.

7. STROMATOLITE BIOSTRATIGRAPHY

Chapter 7.3 APHEBIAN STROMATOLITES IN CANADA: IMPLICATIONS FOR STROMATOLITE ZONATION J.A. Donaldson

INTRODUCTION

Meticulous studies of stromatolites by Soviet workers have contributed significantly to subdivision of the Proterozoic rock record in the U.S.S.R. (Raaben, 1969a, and earlier references). Apparent restriction in the timeranges of distinctive “groups” and “forms” of stromatolites from widely separated regions within the Soviet Union has not only aided correlation within the U.S.S.R., but has also provided support for the possibility of using stromatolites as a basis for world-wide zonation of Proterozoic strata. This possibility has been enhanced by discoveries of comparable distinctive stromatolites in Proterozoic strata of other continents (Cloud and Semikhatov, 1969b; Glaessner et al., 1969; Valdiya, 1969a; Bertrand-Sarfati, 1 9 7 2 ~ )Particularly . impressive has been the detailed and systematic classification of numerous Proterozoic stromatolites in Australia by Preiss (1972) and Walter (1972a), following the Soviet methods of reconstruction and identification. In all these studies, stromatolites of Riphean age (time-range approximately 1,650- 570 m.y.) have received most attention, although Cloud and Semikhatov (196913) and Walter (1972a) also described some pre-Riphean forms. Most of the distinctive groups of stromatolites are reported to have ranges that fall entirely within the Riphean, and therefore an obvious method of testing the validity of presently designated ranges of Riphean stromatolites is to compare them with stromatolites from significantly older and younger strata. Because of .the greatly reduced abundance of stromatolites in the Phanerozoic, attributed by Garrett (1970b) to the evolution of grazing and burrowing organisms, strata of pre-Riphean age appear t o offer the most promise for comparisons.

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Fig. 1. Stromatolite-bearingsequences of Aphebian strata in Canada: 1 = Epworth Group; 2 = Snare Group; 3 = Goulburn Group; 4 = Great Slave Supergroup; 5 = Hurwitz Group; 6 = Gunflint Group; 7 = Sutton Lake Group; 8 = Belcher Group; 9 = Manitounuk Group; 10 = Mistassini Group; 11 = Kaniapiskau Supergroup. AF'HEBIAN (LOWER PROTEROZOIC) SEDIMENTARY ROCKS OF CANA,DA

Stromatolite-bearing Proterozoic rocks of Aphebian age (time-range approximately 2,500-1,750 m.y.) are well exposed in several regions of Canada (Fig. 1).The minimum age for Aphebian rocks has been established by numerous radiometric age determinations of events that clearly postdate Aphebian sedimentation, and thus, even allowing wide margins for the precision and accuracy of radiometric data (many Riphean ages in the U.S.S.R. derive from K - A r measurements on glauconite; most Aphebian ages in Canada are based on K- Ar and Rb-Sr determinations of igneous and metamorphic rocks), there is little doubt that rocks classified in Canada as Aphebian are also pre-Riphean. Several studies to provide data for comparisons of Aphebian and Riphean stromatolites have been initiated recently in Canada. Although considerable work remains, sufficient information has been accumulated to indicate that features characteristic of Riphean stromatolites are also displayed by some Aphebian stromatolites. Photographs of stromatolites from the Aphebian Manitounuk Group (Hofmann, 1969a) and a subsequently published graphic reconstruction of the same stromatolites (Hofmann, 1973) show

STROMATOLITE BIOSTRATIGRAF'HY

313

I

Fig. 2. Longitudinal sections of Aphebian stromatolites, Belcher Group: a. Furcate branching characteristic of kussiellid stromatolites such as Kussiella, Omachtenia and Boxonia *. b. Digitate branching characteristic of gymnosolenid stromatolites; most have smooth margins cf. Gymnosolen; the multibranched cluster resembles Anabaria. c. Ragged subcylindrical columns with numerous linkages, cf. Jurusania. d, e. Stubby tuberous stromatolites cf. Baicalia. f . Stromatolite in which part of one branch shows conical laminations and an axial zone, cf. the branched, conically 'laminated stromatolite, Jacutophyton. g, h. Bifid branching and development of wall structure, cf. Linella. i. Branching from niches in parent column, cf. Znzeria. j, k, 1. Markedly divergent branching characteristic of the tungussid stromatolite Tungussia.

* Although Raaben (1969a) classified Boxonia as a member of the gymnosolenid supergroup, it is logically a member of the kussiellid supergroup, according to its furcate style of branching.

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J.A. DONALDSON

characteristics of the Upper Riphean group Katauia. Stromatolites that resemble Gymnosolen, Tungussia, Baicalia and Jacutophyton occur in Aphebian strata of the Epworth Group and Great Slave Supergroup (see Hoffman, Ch. 10.7). Additional examples of stromatolites showing sets of morphological features characteristic of Riphean stromatolites occur in Aphebian strata of the Belcher Group; examples of these are illustrated herein.

STROMATOLITES OF THE BELCHER GROUP

The Belcher Group (Fig. 1) contains numerous units of limestone and dolostone that are richly stromatolitic (Jackson, 1960). The stromatolites display considerable diversity, ranging from low domal forms t o complexly branched columns. The branched columnar stromatolites are of principal interest here, because most of the presently designated stromatolite ranges apply to branched columnar forms that are characterized by distinctive sets of morphological features. Fig. 2 illustrates several stromatolite varieties common in the Belcher Group that resemble Riphean and Vendian stromatolites. Diameters of the Belcher columnar stromatolites commonly exceed 20 cm, and therefore satisfactory reconstruction by the method of serial sectioning is generally impractical. However, numerous sections parallel and perpendicular t o bedding are provided by excellent exposures in the field, and many details of branching, column ornamentation and configuration of laminae can be established by a study of such exposures, without recourse t o systematic sectioning. All of the varieties shown in Fig. 2 occur in laterally extensive units, some of which can be matched in measured sections as much as 50 km apart. It is important t o add, however, that the lithologies and primary sedimentary structures of some of these units are equally persistent, a relationship that has been established for Aphebian strata of other regions, such as the Knob Lake Group of the Kaniapiskau Supergroup, Labrador Geosyncline (Donaldson, 1963), and the Great Slave Supergroup of the Coronation Geosyncline (Hoffman, 1968b). Representative longitudinal sections of columnar stromatolites shown in Fig. 2 are grouped to show various styles of branching: furcate and digitate branching of subparallel columns (Figs. 2a, 2b, 2c), slightly divergent branching of stubby columns (Figs. 2d, 2e, 2f, 2g, 2h, 2i) and markedly divergent branching of dendroid stromatolites (Figs. 2j, 2k, 21). As a basis for comparison with Riphean stromatolites, the summary descriptions in Raaben (1969a), Cloud and Semikhatov (1969b) and Hofmann (1969a) have provided the primary references. Furcate branching of subparallel columns, in which division into smaller columns occurs without increase in total width of the stromatolite, can be widely recognised in the Belcher Group (Fig. 2a).


375

Fig. 3. Longitudinal section of Aphebian columnar stromatolites, Belcher Group. Furcate or kussiellid branching is apparent and the smooth-margined columns show some envelopment of successive laminations (wall structure).

Some have ragged margins and overhanging laminae suggestive of the named stromatolites Kussiella and Omachtenia, but more commonly they have smooth margins, locally enveloped by either selvages or wall structures (Fig. 3). Some of these display columns of very uniform diameter (left illustration of Fig. 2a), and thus are close in general appearance to Boxonia. Digitate branching of subparallel columns, in which there is a distinct increase in total width of the stromatolite at points of division, gives rise to abundant smooth-walled forms in the Belcher Group that closely resemble Gymnosolen; a less common style of ramification into numerous subparallel branches is displayed by bouquet-like stromatolites similar in appearance to Anabaria, but generally lacking the slightly divergent branching of this group (Fig. 2b). Some parallel columns display ragged margins, coalescence, and sporadic linkage (Fig. 2c), features that are common in published illustrations of Jurusania and Omachtenia. Stubby columnar stromatolites of the Belcher Group show a wide diversity of shapes and surface ornamentation, but most are characterized by dendroid branching, and resemble Baicalia (and t o a lesser extent, Tungussia) in longitudinal section (Figs. 2d, 2e, 4). Some Baicalia-like stromatolites display sharply convex laminations, with lamination thickening in the axial zone (Figs. 2f, 5). Such features in unbranched columnar stromatolites are

376

J.A. DONALDSON

Fig. 4. Longitudinal section of Aphebian stromatolites, Belcher Group, showing irregular Boicolia-like branching. Note distinct envelopment (wall structure) along margins of several stromatolites. Markings on white scale are at 5-cm intervals.

diagnostic of Conophy ton; their occurrence in branched dendroid stromatolites invites comparison with Jucutophy ton. Another common feature of the stubby columnar stromatolites is development of a “wall structure”, in which the marginal parts of laminae successively envelop the lateral surfaces of the columns. These stromatolites generally possess sharply defined smooth margins, and thus are somewhat similar to some forms of Linella (Figs. 2g, 2h, 6). Some of the stubby columnar stromatolites display niches in parent columns where branching occurs, a feature characteristic of Inzeria (Figs. 2i, 7 ) . Most distinctive of all are stromatolites showing markedly divergent branching, which is characteristic of Tungussiu (Figs. 2j, 2k, 21, 8). Margins of the columns are typically smooth and sharply defined, and individual branches range from uniform in diameter t o strongly flared. Niches occur at the base of some columns, giving profiles that can also be compared to, Znzeria (Fig. 2k). Segments of some columns are apparently walled, but these may merely represent oblique sections through columns that are steeply inclined to surfaces of exposure in outcrop.


377

Fig. 5. Longitudinal section of Aphebian stromatolite, Belcher Group, showing Conophyton-like segment in one branch, cf. Jacutophyton. SUMMARY

Although extensive serial sectioning and study of microstructures will be required to fully document and identify Aphebian stromatolites for accurate comparisons with Riphean stromatolites, some generalizations are warranted on the basis of present information. (1)Columnar stromatolites of pre-Riphean age display considerable diversity; the number of distinctive pre-Riphean stromatolite morphologies is clearly greater than indicated by Awramik (1971a) in his graph based on named columnar stromatolites. (2) Many Aphebian stromatolites display styles of branching characteristic of Riphean stromatolites. All four supergroups of Raaben (1969a) are represented (conophytonids, kussiellids, gymnosolenids and tungussids). (3) Specific characteristics used for classification of Riphean stromatolites, notably wall structures and niches, are common in Aphebian stromatolites. (4) Although comparisons presented here are preliminary, sufficient basic similarities are apparent, without detailed reconstructions, to suggest that some named Riphean stromatolites also occur in pre-Riphean strata.

378

J.A. DONALDSON

Fig. 6. Longitudinal section o f smooth-margined Aphebian stromatolite, Belcher Group, showing well-developed wall structure, cf. Linella.

Fig. 7. Longitudinal section of Aphebian stromatolites, Belcher Group, showing niches, a characteristic of Inzeria


379

Fig. 8. Divergent branching in Aphebian stromatolite, Belcher Group, cf. Tungussia. Width of view: 7 0 cm. Linear surface markings are glacial striae. CONCLUSIONS

The utility of stromatolites for intrabasinal correlation is well established (lithostratigraphy), and certain stromatolites have been used with considerable success for interbasinal correlation of Precambrian strata in the Soviet Union. Recently, hopes for extension of stromatolite correlation to the intercontinental level (biostratigraphy) have been raised by discoveries of distinctive stromatolites of similar Precambrian ages in Australia, India, Africa and North America. Serious problems for such world-wide correlation are posed, however, by discoveries of specific stromatolites outside their currently accepted time-ranges. An important example of this is the occurrence of Aphebian stromatolites that closely resemble some of the distinctive “index” stromatolites currently considered to have first appeared in Riphean (and hence post-Aphebian) time. Similarities of this type have been previously reported by Walter (1972a, p. 84), who suggested that certain stromatolites might have “disjunct distributions”, appearing briefly in pre-Riphean strata, and then again in Riphean strata. Considerable work is needed to firmly establish whether some preRiphean stromatolites, such as those briefly described and illustrated here, match in all respects (including microstructure) the Riphean stromatolites they resemble in longitudinal sections. Nevertherless, field and laboratory

380

J.A. DONALDSON

studies have progressed sufficiently to indicate the need for great caution in any attempt to assign geologic age on the basis of stromatolites alone. ACKNOWLEDGEMENTS

Field work on the Belcher Islands was supported by the National Research Council of Canada. Subsequent laboratory study at Carleton University has been advanced with the assistance of Peter Hews. The manuscript was prepared during sabbatical leave spent in the Department of Geology and Mineralogy, University of Adelaide, South Australia. I am indebted to Wolfgang Preiss for his helpful comments on the manuscript.

8. RECENT MODELS FOR INTERPRETING STROMATOLITE ENVIRONMENTS

Chapter 8.1 OPEN MARINE SUBTIDAL AND INTERTIDAL STROMATOLITES (FLORIDA, THE BAHAMAS AND BERMUDA)* Conrad D. Gebelein

INTRODUCTION

The most important factors controlling the distribution and morphology of Recent subtidal and intertidal stromatolites in open marine waters are: (1) the widespread distribution and high abundance of grazing and burrowing invertebrates; (2) the lack of rapid cementation of organic and organosedimentary structures.

OPEN MARINE, SHALLOW SUBTIDAL ALGAL MATS AND STROMATOLITES

Several types of algal sediments occur in the shallow subtidal'zone. Their form, distribution and vertical or lateral sequences provide detailed information on depositional environments. Algal mats Surficial mats composed of organic materials (blue-greens, greens, diatoms, red algae, invertebrate mucilage) and bound sediment grains (Bathurst, 1967b; Neumann et al., 1970) are absent only in shallow subtidal environments where: (1)rates of surface sediment turnover by grazers and especially by burrowers arevery high; and (2)rates of sediment movement over the surface preclude any binding of grains by organic mucilage (Gebelein, 1969). In very clear waters, as around Bermuda, algal mats have been observed t o depths of 5 0 m , on the forereef slope. In general, however, growth is most rapid in depths of less than 10 m. These surficial mats represent a dynamic interface at which sediments below and above the mat are constantly being reworked, deposited on the surface, reincorporated into the mat, etc. - i.e. the mat layer is always the surface layer. Hence, laminites are not produced. These

* Contribution number 617, Bermuda Biological Station for Research.

382

C.D. GEBELEIN

mats greatly retard sediment erosion (Bathurst, 1967b; Scoffin, 1970; Neumann et al., 1970). Erosion of the mats characteristically produces flat pebbles or aggregates of bound sediment. On portions of the Great Bahama Bank, these pebbles are slightly lithified (Purdy, 1963; Bathurst, 1971; Winland and Matthews, 1974).

Algal stromatolites (1) Distribution. Stromatolites occur in open marine subtidal environments in which, for various reasons, invertebrate grazers and burrowers are greatly reduced in numbers (Gebelein, 1969). Within these narrow environmental limits, stromatolite growth may begin on any surface irregularity (stabilized cipple, flat pebble of algally bound sediment, Thulussia rhizome, elevated reef or rock substrate) which elevates the surface above the surrounding matbound bottom (Fig. 1A). Maximum observed depths for subtidal stromatolite formation are 4-5m (Gebelein, 1969, and pers. obs.). (2) Morphology and internal structure. Subtidal stromatolites range in dimension from tiny bumps, only a few millimeters in diameter and height and containing 1 or 2 convex upward laminae, t o large domes, up to lOcm high and 30 cm wide (Fig. 1B). Structures are symmetrical in areas with a symmetrical (or no) sediment supply, and are markedly asymmetrical in areas with asymmetrical sediment supply. Stromatolites are elongate parallel t o the direction of predominant sediment supply. Where sediment supply is from one direction only, laminae are thickened in that direction. The lamination fabric consists of alternate thick layers (in which algal filaments are largely vertical) that form by growth and phototactic movement during the day, and thin layers (in which filaments are largely horizontal) that form during the night (Monty, 1965a, 1967; Gebelein, 1969). In areas of abundant sediment, sediment-grain density is highest in the thick layer, due to enhanced permanent binding by vertically oriented filaments. These stromatolites accrete at the rate of up to 1mm (one lamination couplet) per day. These forms are composed of thin-sheathed filamentous blue-green algae; Schizothrix and Oscillatoria are the most common genera.

Algal oncolites (1)Distribution. The environment of formation of oncolites at Joulters Cays, Bahamas is similar t o that for attached stromatolites, except that periodic turbulence, causing initial detachment of the forms, is higher. In the Joulters, this increase in turbulence corresponds t o a decrease in depth. Maximum formation occurs in depths less than 1 m . Concentrations of 10-100 oncolites per m2 are common in the shoal areas. Lags of oncolites carpet the floor of some tidal channels which cut through the shoals.

ENVIRONMENTAL MODELS

383

Fig. 1.A. Subtidal stromatolites in Tholassia bed; depth 2 m, Castle Roads, Bermuda. Black lens cap is 16 cm diameter. B. Subtidal stromatolites, Bermuda: 1 and 2, top view; 3, side view, showing bound sediment beneath,stromatolite; 4, cross section showing laminae. Scale for 1 , 2 and 3: bar = 1.5 cm; scale for 4:bar = 1.5 cm. C. Section through drape-over stromatolite which formed on the margin of a channel on intertidal mud flats, Cape Sable, Florida. Scale in cm. D. Thin section of high intertidal, domal stromatolite, Cape Sable. Arrows point to thin algal laminae between coarse sediment laminae. Bar = 0.2 mm. E. Close-up of upper right-hand corner of D, showing detail of algal lamina and sediment laminae. Bar = 0.1 mm.

384

C.D. GEBELEIN

( 2 )Morphology and internal structure. Oncolites range in size from 1mm t o a maximum of about 10cm. Lamination structure is very similar t o that in subtidal stromatolites. In the Joulters, tiny stromatolites become detached by wave or current turbulence, and form the cores of the oncolites. Once detached, the forms are kept in motion, which prevents reincorporation into the mat. Id very shallow (less than 15 cm) water, the stromatolitic core may have only one or two convex upward laminae: in some cases, no core at all is detectable. In deeper water, attached stromatolite growth may produce 10-20 laminae before detachment. Thus, as suggested by Logan et al. (1964), the degree of symmetry of oncolitic laminae provides a relative estimate of the turbulence of the environment. Oncolites from the Frazer’s Hog Cay area, Bahamas, contain an algalforaminifera1 consortium, with a characteristic anastornosing cellular pattern imposed on the algal laminae by the growth within the oncolite of an irregular opthalmid foraminifer (Buchanan et al., 1972). These structures, some of which are lithified, are very similar to the “Osagia” oncolites of the Paleozoic. MODEL FOR SUBTIDAL STROMATOLITES AND ONCOLITES

The distribution of subtidal forms described above fits a simple model (cf. Gebelein, 1969), in which the controls on distribution and morphology are rate of sediment movement (as it effects stability of the mat and the distribution of invertebrates), rate of sediment accumulation, and symmetry of sediment supply. Oncolites add the dimension t o this previous model of increased periodic turbulence, usually corresponding to decreased depth. Exceptions to the rule. Completely organic stromatolites have been found on hard substrates in reef, backreef and lagoon areas (Monty, 1967; Gebelein, 1969). These structures have approximately the same size and shape as the subtidal stromatolites described above. The completely organic forms have a very tough, leathery surface texture. These forms are quite rare (although a seasonal “bloom” was observed in Bermuda in the early fall of 1974). Their distribution appears to be random within the following limits: (1)they always form on hard substrates; (2) they occur in areas where the abundance of surface grazers is low. The occurrence of these Recent forms, combined with the discovery of abundant subtidal stromatolites lacking detrital textures, in the forereef sediments of the Devonian Canning Basin (Playford and Cockbain, 1969), leads to the speculation that such deposits could occur in any reef or forereef environment which: (1)lacks abundant grazers (as most reefs do); or (2) has the potential for rapid cementation (a condition found most commonly in reefs).


385

INTERTIDAL LAMINITES AND STROMATOLITES

In high-energy sandy shore zones with continuous scour and sediment movement, stromatolites and laminites are completely lacking, due to the absence of a stable substrate (contrast this situation t o that in Shark Bay). As scour decreases, stromatolites and laminites develop in the least energetic, upper portions of the tidal zone. However, development is limited to the upper half of the tidal zone, regardless of energy for scour, by reworking and ingestion of the surface sediment layer by grazing and burrowing invertebrates. The nature of this restriction has been documented by Gebelein and Hoffman (1968), Garrett (1970b) and Gebelein (1971): Grazing and burrowing invertebrates are restricted t o the lower portions of the intertidal zone by factors related to desiccation (drying of the animals, temperature, salinity and gas composition of the interstitial fluids) which adversely affect invertebrate viability and activity. In low intertidal areas, surficial algal mats cannot develop, as removal and reworking is more rapid than mat development. Above a certain flooding-frequency isograd, invertebrate activity and viability decrease sufficiently for surficial mats t o develop. At these latter elevations, laminated sediments rapidly form by the trapping and binding of sediments onto the mat. The position of the boundary between unlaminated and laminated sediments varies from place to place, and depends on the types of algae and invertebrates present. At Cape Sable, Florida, the boundary occurs at the 25% flooding-frequency isograd - i.e. the surface flooded 25% of the time (Gebelein, 1971). On Andros Island, Bahamas, the boundary occurs at the 50% flooding-frequency isograd (Ginsburg et al., 1970). Within the laminated portion of the tidal zone, a further zonati0.n of algal mat types may occur. This zonation is related to the distribution of various associations of blue-green and green algae according to flooding frequency. Each association generates a specific fabric, or microstructure (Gebelein, 1974). Hence, the upper intertidal area may contain laterally and vertically zoned laminites and stromatolites with different microstructures. Open tidal flats

(1)Morphology. In areas which are alternately flooded and uncovered on each tide, algal laminites are the characteristic form. Relief is limited t o that generated by the growth mode of the algal association, and is usually less than 2-3cm (see below for one exception). Various types of domes may occur with this small amount of surface relief. “Drape-over” stromatolites, in which the laminae follow the sediment surface curvature, may occur on the margin of tidal channels, and thus generate an asymmetrical form with several centimeters of total relief on the channel-facing side (Gebelein, 1971) (Fig. 1C).

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C.D. GEBELEIN

( 2 )Internal structure. Most intertidal algal associations contain a predominance of filamentous forms (Monty, 1965a;Gebelein, 1971; Golubic and Park, 1973) and hence generate laminated fabrics (Gebelein, 1974). The exact fabric varies from association to association. In general, the laminae are less than 5 mm thick and consist of an algal- and sediment-rich couplet (Fig. l D , E). Laminations are smooth, lack desiccation crinkling and crenulation, and drape over minor relief features. Lamination thickness is determined by the rate at which the dominant algal species can grow and move through the sediment layer, re-establish a surface mat, and hence permanently bind the sediment deposit. At Cape Sable, about a millimeter of sediment is deposited onto the mat surface during each tide. The blue-green algae are able to grow and move upward through this layer and permanently bind it before it is washed away by succeeding tides. On Andros levees, sedimentation occurs only during storms, when over a centimeter of sediment may be deposited (Hardie and Ginsburg, 1971).However, the rate of upward movement of the algae is such that all but about a millimeter or two of the sediment is washed away on the succeeding tides. Algae with vertical or radial growth modes form layered or domal structures either by the “sieve” effect causing the capture of sediment (cf. Hommeril and Rioult, 1965;Gebelein, 1971),or by cementation which generates lithified carbonate molds of the filaments and of the structure (as in the lithified Scytonema domes on the Andros tidal flats, where crusts are formed at the level of maximum “capillarity” on the flats). (3)Associated sedimentary structures. On the open tidal-flats a particular suite of sedimentary structures occur associated with stromatolites and laminites: prism cracks, ranging in depth from a few millimeters to several centimeters; flat pebbles of laminated sediment; current crescents, developed in the lee (flood tide direction) of surface irregularities; rare birdseye fenestrae and rare open burrows (Gebelein, 1971).

Ponded tidal flats (Fig. 2 ) On flats with slight surface irregularities, ponds may form. On portions of these flats, the flooding frequency is less than 5096,taken on a yearly average, but the duration of individual flooding and exposure events is quite long days or weeks. Increase in flooding duration is accompanied by the following phenomena: (1)prism cracks become better defined (often by drape-over laminae) and deeper; (2) individual sediment and algal laminae increase in maximum thickness; (3) lamination curling and crenulation increase. Increased time for algal growth leads to thicker (often several millimeter) organic laminae, which characteristically contain little sediment. Thick, storm-derived sediment layers are deposited on the flats. Lacking subsequent tidal floodings, these layers are not easily dispersed or washed away, and account for the thicker sediment layers. Intense desiccation of the surface


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Fig. 2. A. Domal stromatolites growing over curled algal-sediment laminae, ponded area, Cape Sable. Note well-developed prism cracks between stromatolites. Rule = 1 2 in. (30cm). B. Cross-section, high portion of ponded area, Cape Sable. Note thick algal laminae, storm deposits, crenulation and curling of laminated algal muds. Bar = 10 cm. C. Thin section of algal-laminated sediments, ponded area, Cape Sable. Note thick, multiple, crenulated algal laminae, and thick, discontinuous sediment laminae. Bar = 0.1 mm.

organic algal mats during the long periods of dryness leads to lamination crenulation, and often to the curling-up of a thick section of laminated sediment. Such curling can occur only where the organic content, and hence the cohesiveness of the deposit, is very high. Supratidal deposits on tidal flats Large expanses on tidal flats lie at elevations flooded only by major storms (less than 5% flooding frequency at Cape Sable; Gebelein, 1971). Sediments deposited in this environment are thin-bedded (1-3 cm thick). Each bed is the product of a single storm. Lack of tidal reworking leaves the beds intact. A single algal mat layer may develop immediately after storm flooding, but

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usually is desiccated to the extent that it dries up and blows away. Thus, truly supratidal environments are not characterized by laminated deposits. The most common internal structure of the thin beds is the birdseye fenestra (Gebelein, 1971). INTERTIDAL SEDIMENTATION MODEL (BAHAMAS, SOUTH FLORIDA)

Desiccation-related factors provide the basic division of intertidal flats into distinct invertebrate and algal associations. However, the position of boundaries between associations is largely the result of interaction between the associations. Such interactions produce sharp lateral changes in associ@ion composition, and hence in sediment fabric. These changes, seen in vertical section following progradation, are usually abrupt and very likely will generate bedding planes in analogous ancient carbonates. The general sequence found in intertidal sediments is: (1)a lower intertidal burrowmottled unit; (2) an upper intertidal algal unit (mostly thinly laminated), with the possibility for several distinct sub-units with different fabrics; (3) a supratidal thin-bedded unit, lacking or with rare algal lamination. Ponded tidal flats show the same sequence. However, in ponded tidal flats the algal section is characterized by thicker lamination, increased prismcrack formation, and crenulation and curling of the laminated deposits.


Chapter 8.2

MODERN ALGAL STROMATOLITES AT HAMELIN POOL, A HYPERSALINE BARRED BASIN IN SHARK BAY, WESTERN AUSTRALIA P.E. Playford and A.E. Cockbain

INTRODUCTION

Algal stromatolites are forming today over wide areas of the sublittoral platform and adjacent intertidal zone fringing Hamelin Pool, a hyperialine barred basin at the southeastern extremity of Shark Bay in Western Australia (Fig. 1).They are the most diverse and abundant shallow-water stromatolites known from modem seas. The Hamelin Pool stromatolites were found in 1954 by D. Johnstone, P.E. Playford and R.L. Chase during geological investigations of the area for West Australian Petroleum Pty. Ltd. (Wapet). In the following year Chase, with B.W. Logan, studied the Proterozoic Moora Group, which contains welldeveloped stromatolites (Logan and Chase, 1961). Chase recognized that the Hamelin Pool stromatolites were modern analogues of these Proterozoic forms, and at his request Playford collected some stromatolite heads from Hamelin Pool for further study. Algae in these samples were examined by D.M. Churchill. B.W. Logan commenced a Ph.D. project at Shark Bay in 1956, and included the Hamelin Pool stromatolites in this study. They were described in his thesis (Logan, 1959) and in a subsequent publication (Logan, 1961). More comprehensive studies of the stromatolites have since been carried out by P. Hoffman, B.W. Logan, and C.D. Gebelein, and their results are summarized by Hoffman (Ch. 6.1).Details have.been published by Logan et al. (1974). The Quaternary and modem environments of Shark Bay are described in the memoir by Logan et al. (1970a). This includes details of some aspects of the Hamelin Pool environment and sedimentation, and has comprehensive papers by Davies (1970% b) on seagrass banks and algal-laminated sediments. Study of the Hamelin Pool stromatolites by the Geological Survey of Western Australia began in 1968, and is still in progress. The occurrence of subtidal stromatolites was briefly reported by Playford (1973). Future

P.E. PLAYFORD AND A.E. COCKBAIN

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Bernier Island

250

Dorre Island

S H A R K BAY

Fig. 1. Locality map, Shark Bay and Hamelin Pool.


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detailed work will utilize low-level vertical colour air photos of important areas, which were taken for the Geological Survey in 1974. Regional geological mapping of the whole Shark Bay area will also be carried out. S.M.Awramik and S. Golubic are studying the microbiology of Hamelin Pool algal mats and stromatolites, with the cooperation of the Geological Survey. Their field work was carried out in 1973, and systematic work on the algae by Golubic is continuing. A meeting of stromatolite specialists was convened by the Environmental Protection Authority of Western Australia in 1973 in order to make recommendations on the preservation of Hamelin Pool and its unique stromatolites (Western Australia, Department of Environmental Protection, 1975). THE HAMELIN POOL ENVIRONMENT

Hamelin Pool consists of a broad central basin 5-10m deep surrounded by a sublittoral platform up to 5km wide and intertidal-supratidal platforms from a few metres to 3.5 km wide. Hutchison and Nilemah Embayments (Fig. 2) are subdivisions of Hamelin Pool characterized by very broad intertidal - supratidal platforms. The Hamelin Pool basin is barred to the north by the Faure Sill (Figs. 2 and 4), a seagrass bank (Davies, 1970a) extending from Kopke Point to Petit Point. Most of the sill is less than a metre below sea level, and it is cut by three main tidal-exchange channels, the Herald Loop, Faure, and Petit Channels (Figs. 2 and 4). Because of the restriction to inflow of oceanic waters caused by the Faure Sill, combined with the low annual precipitation (average 210 mm) and high evaporation (average about 2,200 mm), hypersaline conditions prevail in Hamelin Pool. Salinities range from 55'& to 70°/m throughout the year, the highest values being in the southern part. However, the restriction to circulation in the basin is insufficient to cause oxygen depletion in the bottom waters. Minor amounts of gypsum are precipitated on the intertidal- supratidal platforms. The prevailing wind in Hamelin Pool is overwhelmingly from the south. Winds from this direction are especially strong and persistent during the summer. There are no detailed tidal records for Hamelin Pool, but data from Logan et al. (1970a), combined with our own observations, indicate that the normal spring tidal amplitude ranges from about 1m in the central-northem part to 0.6m in the south, and the mean *diurnal amplitude ranges from about 0.5 to 0.3 m. However, the tides are very much influenced by the wind. When a northerly wind is blowing, water banks up in Hamelin Pool and high tides are commonly as much as 0.3m above normal levels. Conversely, persistent southerly winds can lower the water level by similar amounts.


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Fig. 3. Diagrammatic section illustrating distribution of living algal stromatolites in Hamelin Pool. Abbreviations are: M.S.L. = mean sea level; H.W.S.= high-water-spring tide level; L.W.S. = low-water-spring tide level.

The fauna of Hamelin Pool has not been studied in detail, but it is clearly very restricted in species numbers compared with the oceanic and metahdine parts of Shark Bay. However, individual species can be very abundant, especially the small bivalve Fragum hamelini Iredale. “Population explosions” of this species apparently occur in certain years. Other prominent elements of the fauna are peneroplid and miliolid foraminifers, a few species of fish, and sea snakes. The most remarkable elements of the Hamelin Pool biota are the stromatolites and associated flat algal mats, to be discussed later in this study. GEOLOGY

The oldest rock unit exposed in the Hamelin Pool area is the Toolonga Calcilutite, of Late Cretaceous age. It consists mainly of chalk (in part with flints) and is dolomitized at some localities. Eocene and Miocene limestones overlie the Toolonga Calcilutite in the Shark Bay area, but are not exposed around the margins of Hamelin Pool. An orthoquartzite which crops out prominently on headlands on the east side of Hamelin Pool is probably Tertiary in age. The main exposures in the area are Quate’mary,and these are discussed by Logan et al. (1970b). The oldest unit is the Peron Sandstone (possibly Early Pleistocene to Pliocene), which occurs on Peron and Nanga Peninsulas west of Hamelin Pool. It may be an eolian deposit. The Peron Sandstone is succeeded in coastal sections by thin marine Pleistocene units (Dampier Formation and Carbla Oolite) laid down during the Dampier Marine Phase (Logan et al., 1970b). The youngest marine Pleistocene deposit recognized in the area is the Bibra Formation, which contains a rich coral and molluscan fauna.

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Fig. 4. Earth Resources Technology Satellite image of Hamelin Pool and adjoining areas. The shallow Faure Sill and its associated tidalexchange channels are clearly shown extending from Petit Point t o Kopke Point along the northern margin o f Hamelin Pool (see Fig. 2 for localities).


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Hamelin Pool is believed to have had a configuration comparable with that of today during the Pleistocene marine transgressions, but circulation was less restricted than at present because the Faure Sill seagrass bank had not then developed. Stromatolites are not known from the Pleistocene deposits. Red sand dunes (the Nilemah Sands) of possible Late Pleistocene to Holocene age overlie older Pleistocene deposits on Peron and Nanga Peninsulas. The dunes are now fixed by vegetation. The Hamelin Pool area was completely emergent during the period of low sea level associated with the Wurm Glaciation. The Pleistocene sequences were eroded during this period, and calcrete crusts developed in some places. Playford and Chase (1955) suggestedethat coastal landforms in the Shark Bay area may have an underlying structural control. They pointed out that there is a strong north-northwest parallelism between Peron and Edel Land Peninsulas, Dirk Hartog Island, and Kopke Point and Yaringa Point headlands, and suggested that each of these features is probably controlled by folding or faulting below the Quaternary deposits. Wapet tested this hypothesis by drilling seventeen holes on Dirk Hartog Island in 1955-56. This showed that the Pleistocene dune limestone (Tamala Eolianite) forming the island has accumulated on an anticline in Tertiary rocks, having its axis parallel t o the length of the island and its culmination in the centre. Similar drilling has not been carried out on the peninsulas and headlands of Shark Bay, but as they are similar in configuration and trend to Dirk Hartog Island it Seems likely that they too overlie and are localized by Tertiary anticlines. It is possible that folding has continued into the Quaternary, as Pleistocene rocks are involved in folding of other anticlines in the coastal part of the Carnarvon Basin (Playford et al., 1975). If this hypothesis is correct, Hamelin Pool marks a synclinal downwarp, while the Hutchison and Gladstone Embayments overlie smaller synclines.

HOLOCENE SEDIMENTARY FACIES

The transgression following the Wurm Glaciation occurred rapidly, and the sea reached a height of some 1.5-2.5m above present sea level in Hamelin Pool about 4,000-5,000 years B.P. (Logan et al., 1970a). It is not certain whether the subsequent emergence that has occurred is eustatic or tectonic in origin, but we believe that it is probably tectonic. The Faure Sill seagrass bank formed across the northern margin of Hamelin Pool during the Holocene, restricting circulation in the basin, and leading to hypersaline conditions and the appearance of stromatolites. There are few published details of the sedimentary regime in the subtidal areas of Hamelin Pool (the basin floor and sublittoral platform). Nearly all the detailed work to date has been concentrated on the intertidal and supratidal areas. However, Logan et al. (1970a) state that sediments of the

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hypersaline basins of Shark Bay are dominated by coquinas of the bivalve Fragum hamelini, with species of peneroplid and miliolid foraminifers, associated with the green alga Acetabularia. Fragum coquinas and skeletalfragment sands are widespread on the sublittoral platform, and there are also ooid shoals. Logan and Cebulski (1970) record that in the hypersaline sublittoral sandflat environment “at certain times an incoherent layer of blue-green algal material coats all surfaces, but at other times it is absent”. However, our observations show that well-developed algal mats and stromatolites persistently cover large areas of the sublittoral platform in Hamelin Pool. The seagrass banks of Shark Bay are comprehensively described by Davies (1970a). The seagrasses Posidonia and Cymodocea trap and stabilize sedimentary material in these banks and provide the habitat for rich sedimentforming communities, dominated by foraminifers, algae, and molluscs. The intertidal and supratidal platforms of Hamelin Pool are generally wide in bays and narrow on headlands. The largest expanses are in the Hutchison and Nilemah Embayments, where broad supratidal areas have resulted from the Late Holocene regression. The intertidal zone is characterized everywhere by flat algal mats and stromatolites. Algal mats also extend over wide areas of the lower supratidal platforms. White sand and coquina beaches occur almost continuously around the shores of Hamelin Pool. The principal constituents of the sand are ooids, pellets, foraminifers, and shell particles, while the coquinas are composed almost entirely of Fragum hamelini shells. The beach deposits are crossbedded, and strongly cemented beach rock occurs at some localities. The beaches are actively prograding over wide areas, leaving successive beach ridges extending inland for tens or hundreds of metres. ALGAL STROMATOLITES

Introduction The stromatolites in Hamelin Pool are the best examples known in modem seas of algal stromatolites having morphological diversity and abundance comparable to that of Proterozoic and Early Palaeozoic stromatolites. The stromatolites were first described by Logan (1961), who reached the important conclusions that they are restricted to the intertidal zone and lithify as a result of subaerial exposure. As a corollary he deduced that their maximum height approximates the tidal range. Logan suggested that his conclusions on the origin of Hamelin Pool stromatolites may also be applied to ancient stromatolites, and this view was further expanded by Logan et al. (1964). For some years most stromatolite workers accepted that marine


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stromatolites are strictly intertidal phenomena. However, in recent years there has been increasing evidence presented that many ancient and modem stromatolites are subtidal, and Monty (1973a) has shown that bacterial stromatolites are forming today in abyssal water depths (see also discussion in Playford et al., Ch. 10.4). Even so, as pointed out recently by Serebryakov and Semikhatov (1974), many authorities have continued to believe that stromatolites are indicative of intertidal conditions. Our work has now shown that the original basis for the Hamelin Pool intertidal model of stromatolite genesis is inaccurate, as living stromatolites at Hamelin Pool are widely developed in subtidal as well as intertidal environments. The main areas we have studied at Hamelin Pool are in the vicinity of Flagpole Landing, Carbla Point, and Booldah Well (Fig. 2). Vertical colour air photos have been taken of each of these areas at a scale of 1:5,000 and will be used in our future studies. Different types of stromatolites and algal mats can be readily distinguished and mapped on these photos, and the wide distribution of the subtidal forms is clearly. shown. Some striking lineations and patterns can also be seen in the subtidal stromatolites.

Distribution Algal stromatolites and associated flat algal-mat sheets are widespread in the intertidal and inner sublittoral platform areas of Hamelin Pool. They are best developed in the central and southern (more saline) areas, south of Yaringa Point. In some areas (such as around Carbla Point) the principal intertidal stromatolite occurrences are on rocky headlands formed of Pleistocene or older rocks, while extensive flat algal mats occur mainly in the intervening bays. However, in other areas there are well-developed intertidal stromatolites along straight stretches of coastline lacking prominent headlands (such as the area south of Flagpole Landing). Subtidal stromatolites and flat algal mats occur over wide areas in front of both bays and headlands. Living intertidal stromatolites extend to about high-water-spring tide level (Fig. 3 on p. 393). However, in most areas the living stromatolites are backed by dead forms in the supratidal zone, and these commonly reach 0.5-1 m above mean sea level. The dead stromatolites are in various stages of disintegration. At some localities (such as 2.5 km south of Booldah Well) the dead forms occur in distinct levels, suggesting periodic emergence, but in others they simply slope gradually downwards to join living intertidal stromatolites (Fig. 5A). In some stromatolites that are only partly emergent, the exposed crests may be hard, cracked, and exfoliating, while the sides (which are still regularly submerged) have actively accreting living mats (Fig. 5B). The colour of the dead stromatolite surfaces changes progressively from grey

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where barely emergent, to orange, t o pale greyish-yellow in the highest examples. The dead forms are coated with a thin crystalline aragonite crust which is apparently an inorganic precipitate from sea spray. The grey crusts in the lower emergent forms are covered with films of coccoid epilithic and endolithic blue-green algae (film mat of Hoffman, Ch. 6.1) which is not participating in further accretion and is apparently aiding destruction of the stromatolites (S. Golubic, written communication, 1974). We believe that the emergence of these older stromatolites is most likely to be a result of periodic uplift in Holocene times. In some areas there are also inactive intertidal stromatolites, even though they are regularly submerged and appear to have grown when the sea was about at its present level. These forms are generally well lithified and are covered with films of epilithic and endolithic algae. It is not always clear why growth of these stromatolites has ceased, but in many cases it can be shown that they have been “smothered” by beach deposits, mud, or sand shoals, which have later been swept away. Some are subsequently reactivated as they become progressively covered with accretional algal mat. Most subtidal stromatolites are active today, being covered with soft sediment bound by algal mat. However, some are hard and are no longer growing. These have commonly been covered and later exhumed by moving sand or coquina. Underwater photos of subtidal stromatolites taken in different years show that the amount of individual columns protruding above the bottom sediment can vary considerably (Fig. 8). In such cases active accretion is continuing on the tops of the columns, while the lower parts (which are periodically buried) are inactive. The idealized form zonation of stromatolites shown by Logan (1961) does not represent the typical situation at Hamelin Pool. In most areas there is a general seaward slope of the upper surfaces of living stromatolites, and they pass below sea level to join the subtidal forms (Fig. 3, 5A). In many places there is an area of mobile sand with only scattered subtidal columnar stromatolites between the main intertidal and subtidal occurrences. The living subtidal forms reach depths of at least 3.5 m below sea level, and i t is possible that more detailed examination of the sublittoral platform will show that there are some even deeper examples, conceivably reaching as deep as 5m. Fig. 5. A. Columnar stromatolites at Carbla Point photographed a t low tide. Those in the foreground are now dead and emergent and are in an advanced stage of disintegration. The elevation of the tops of the stromatolites falls progressively seawards from the dead stromatolites through forms with dead crests and actively accreting sides to fully active intertidal forms (where the man is standing) and subtidal forms (which extend several hundred metres out to sea). B. Confluent club-shaped stromatolites (at low tide) 2.8 km southwest of Flagpole Landing. The crests of these stromatolites are now slightly above normal high-water-spring tide level (owing to recent emergence) and are hard, cracked, and exfoliating whereas the sides are still covered with actively accreting mat. The beach on the left is a Fragum hamelini coquina, which is advancing progressively over the stromatolites.

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Subtidal stromatolites commonly extend in patches for several hundred metres offshore, and they can be clearly seen from the beach as darker parts of the sea floor (as for example at Carbla Point). Stromatolites in Hamelin Pool commonly coalesce to form linear rigid bodies which are strongly wave-resistant (Fig. 7B). They were referred to by Wapet geologists and by Logan (1961) as stromatolite reefs. Davies (1970a) discussed reef and bank terminology in relation to his studies at Shark Bay. He followed the usage of Nelson and others (1962) and many other modem authors in restricting the term reef to structures having rigid skeletal frameworks. He did not discuss the problem of the Hamelin Pool stromatolite masses, but referred to them simply as “stromatolite ‘reefs”’, even though they lack skeletal frameworks. We believe that much of the nomenclatural argument about “when is a ‘reef’ really a reef?” has resulted from a lack of appreciation of the fact that stromatolites and other cryptalgal bodies, when subject to early cementation, can form raised, rigid wave-resistant bodies, as is the case at Hamelin Pool in both intertidal and subtidal environments. There seems to be no good reason why these bodies should not be termed reefs. Many Proterozoic and Palaeozoic reefs were apparently built of stromatolites in this way.

Forms represented

A wide variety of stromatolite morphologies is represented at Hamelin Pool (see Hoffman, Ch. 6.1 herein). The most characteristic intertidal forms are the club-shaped to cylindrical columns described by Logan (1961). They are well developed in the Flagpole Landing and Carbla Point areas (Fig. 5). Some columns are as tall as 0.75 m, but most are less than 0.5 m tall. In the Booldah Well area, on the west side of Hamelin Pool, stromatolites are developed as elongate flat-topped ridges 1-3 m wide, about 0.3 m high, and up to 400m long, separated by bare sandy areas (Fig. 6A). They are elongated nearly north-south at an angle of about 30” to the shoreline in this area but parallel to the strong prevailing wind direction. It seems clear that their orientation must be controlled by the wind, but the actual mechanism involved is unknown. Some of the ridges in the Booldah Well area have subsidiary longitudinal stromatolite structures that extend in the direction of wave translation, i.e. Fig. 6. A. Stromatolite ridges built by pustular mat near Booldah Well (at low tide). The ridges extend north-south, parallel to the prevailing wind, and at about 30° to the shoreline. The ridges slope seaward, passing below low-water-spring tide level. B. Longitudinal stromatolites near Booldah Well (at low tide). These forms are associated with the system of stromatolite ridges in this area (see A above) which are crudely developed in this view extending away from the observer (from right to left). The longitudinal forms are at an angle of about 50’ to the ridges and they are elongate in the direction of wave translation, perpendicular to the shoreline.

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at right angles to the shore (Fig. 6B). Such elongation also occurs elsewhere in the Hamelin Pool area among many intertidal stromatolites. Small columnar stromatolites 1 km south of Carbla Point form lines perpendicular to the shore, in the direction of wave movement, but the stromatolites themselves are inclined south into the prevailing wind (Fig. 7A). Again the means whereby the wind seems t o control stromatolite growth is unknown. Many different stromatolite forms occur in the subtidal environment at Hamelin Pool, from tiny bulbous bodies developed on otherwise flat algal mats, to large complex mounds. Columnar forms are most common, and these range from conical t o club-shaped (Fig. 9A). Narrow parallel-sided columns occur in some areas (Fig. 8). They have grown vertically and are apparently geotropic. The sun is to the north throughout the year (Hamelin Pool is situated at about latitude 26OS), but none of the columns have been seen t o have phototropic inclinations to the north. Some subtidal stromatolites in water up to 2 m deep are elongate perpendicular to the shoreline in the direction of wave movement (Fig. 9B), while other large masses are elongate parallel t o the shore (perhaps controlled by sand megaripples). The colloform mat which forms most subtidal stromatolites is soft.but coherent at the surface, and lithification of the structures begins a few millimetkes or centimetres below the surface, increasing progressively downwards. The lower parts of the columns are generally well lithified, and commonly have a “beard” of Acetabularia, which varies in abundance at different times (Fig. 8). Masses of tiny serpulids encrust cavities in the sides of the subtidal stromatolites, and they play a significant role in the construction of the margins. The small bivalve Irus irus (Linnd) also lives in the sides of many subtidal stromatolites. Hamelin Pool stromatolites range from unlaminated (thrombolites) to finely laminated; most show only crudely developed lamination (Fig. 10). Fenestral fabrics are characteristic in lithified forms (Figs. 10, 11). The fenestrae have apparently developed largely through the decomposition of enclosed algal material, and as a result of algal mat bridging indentations on the stromatolite surfaces. As pointed out by Hoffman (Ch. 6.1) there are some significant differences between the internal fabrics of stromatolites produced by the various mat types. The most distinct lamination occurs in stromatolites built by smooth and colloform mats, while those built by pustular mat are generally only weakly laminated or unlaminated. The largest fenestrae form from pustular mat, the smallest from smooth mat. Fig. 7. A. Stromatolites exposed at low tide 1 km southeast of Carbla Point showing pronounced elongation in the direction of wave translation (bearing 40-50 , perpendicular t o the shoreline). Individual columns are inclined in the direction 180’ (towards the prevailing wind). B. Stromatolite reef 1.7 km southwest of Flagpole Landing. The reef is now largely emergent and the upper stromatolite surfaces are disintegrating.

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The Hamelin Pool stromatolites are constructed of sedimentary material that has been trapped and bound by algae and has been cemented by aragonite t o form solid structures. The nature of the cementing process has not been studied; it may be wholly inorganic, but it is possible that the algae, or the products of algal decomposition, could also have played a significant role in lithification. The sedimentary material making up the stromatolites typically consists of pellets of problematical origin (Fig. l l B ) , ooids, foraminifers, Fragum hamelini, quartz sand, and bioclastic sand. Small dolomite rhombs occur in some stromatolites; these may be entirely detrital (derived from soft Cretaceous dolomite).

Algae The algae forming the Hamelin Pool stromatolites have been studied recently by C.D. Gebelein, and further detailed work by S. Golubic is in progress. The stromatolite-forming algal mats are broadly of three types: pustular and smooth mat (mainly intertidal), and colloform mat (subtidal). The dominant algal species in these mats (identified by Gebelein, quoted by Hoffman, Ch. 6.1) are Entophysalis major in pustular mat, Schizothrix helua in smooth mat, and Microcoleus tennerrimus in colloform mat. A number of other algal assemblages form flat algal-mat sheets; these are termed blister, tufted, and gelatinous mats, but they do not form stromatolites. They are discussed further by Hoffman (Ch. 6.1). The subtidal mats are dominated by eucaryotic algae, and the microflora has a much higher diversity than that in the intertidal mats (S. Golubic, written communication, 1975). The Hamelin Pool stromatolites can form an excellent basis for studying principles of stromatolite classification, as the morphology and biology of diverse living types can be studied through a range of environments. From work carried out to date it is clear that morphological characteristics of these stromatolites are controlled partly by the constituent algae and partly by environmental factors. It would be useful to apply defined names to the various stromatolite types represented at Hamelin Pool, just as it is useful to give names to stromatolites in the fossil record. However, further detailed study Fig. 8 . A. Subtidal columnar stromatolites 180 m offshore, 2.8 km southwest of Flagpole Landing, photographed at low tide in February 1972 in water about 1 m deep. The maximum height of columns in the foreground is about 0 . 4 m . The stromatolite crests are covered with living colloform mat, while the sides are coated with a dense “beard” of Ace tabu laria. B. The same group of subtidal stromatolites photographed in November 1974. Note that many o f the stromatolites visible in the background in the 1972 photograph were partly or wholly covered in 1974, owing to movement of sand megaripples. In addition, the Acetabularia “beard” was considerably less dense than in 1972.

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and description of the stromatolites and stromatolite-building algae represented at Hamelin Pool (especially those in the subtidal environment) will be required before a satisfactory nomenclatural system can be developed, whether it be Linnean or purely descriptive. The descriptive classification proposed by Logan et al. (1964) is inadequate to describe the wide range of morphological characteristics present in both modem and ancient stromatolites; moreover, it does not take account of internal fabric or of the algae in modem forms.

Growth rates The growth rates of various intertidal and subtidal stromatolites have been studied in our investigation, by placing non-corrosive nails in them and examining them periodically over a five-year period. Photographs taken of particular stromatolites over an interval of seventeen years were also compared, and tracks cut many years ago through stromatolites (by camel- and horsedrawn wagons that were once used to .load wool and sandalwood onto lighters) were examined. The conclusion reached is that growth of the Hamelin Pool stromatolites is very slow, and many of the living stromatolites are likely to be hundreds of years old. The maximum observed rate of growth amounted to about 1mm per year (over a period of five years) on one of the intertidal stromatolites, but in most cases there has been almost no observable growth, and net erosion has occurred in some. Many stromatolites seem to have essentially reached a state of equilibrium, with growth approximately balanced by erosion. Little or no regrowth has occurred over the paths cleared by wagons, even though they have not been used for decades. One of the most conspicuous of these paths cuts through the living stromatolite ridges opposite Booldah Well. It was last used during the early 1930’s t o load sandalwood, but the path is still sharply defined and is clear of stromatolites.

Stroma tolites and Uni f 0rmi tarian ism The modem stromatolites at Hamelin Pool are important as guides to some aspects of the origin and environmental significance of ancient stromatolites. However, it is clearly false t o assume that all stromatolites in the Fig. 9 . A. Club-shaped, cylindrical, and conical subtidal stromatolites 180 m offshore, 2.8km southwest of Flagpole Landing, photographed at low tide in November 1974, in water about l m deep. Note the living colloform mat on the crowns of the stromatolites and the sparse “beard” Acetabularia on the sides. B. Low mound-shaped subtidal stromatolites, 200 m offshore in water 2 m deep off Carbla Point, showing elongation o f mounds in the direction of wave movement. The mounds are built by colloform mat and are surrounded by sparse Acetabularia.

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Fig. 10. A. Section through an intertidal stromatolite built by pustular mat, from Flagpole Landing, showing crudely developed lamination and coarse fenestrae. B. Section through subtidal stromatolite built by colloform mat collected from water about 1m deep, 190 m offshore, 2.8 km southwest of Flagpole Landing. The stromatolite has grown on a cobble of indurated Cretaceous chalk. Note that lamination is better developed and fenestae are generally smaller than in the stromatolite shown in photo A.

past have formed under similar environmental conditions to those in Hamelin Pool. Stromatolites were widely distributed in Proterozoic seas, and were still abundant in the Early Palaeozoic, but since then they have steadily declined, until today Hamelin Pool is apparently the only place where diverse and abundant marine algal stromatolites are still forming. The reason for this is


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t o be found primarily in the unusual conditions prevailing in Hamelin Pool which have led to development of hypersaline conditions with a restricted metazoan fauna. Although the fauna has yet t o be studied in detail, it seems that organisms that could extensively graze and thereby destroy stromatolites and algal mats are rare. In particular, living gastropods are uncommon in Hamelin Pool, but are abundant in the oceanic and metahaline parts of Shark Bay. As suggested by Garrett (1970b), the decline of stromatolites through the Phanerozoic appears to be linked, at least partly, to the progressive rise of algae-consuming metazoans. Hamelin Pool, because of its hypersalinity, is now one of the few remaining places in world seas where metazoan grazers are sufficiently rare t o allow stromatolites to flourish. Hypersalinity thus appears t o be the factor of prime importance to development of stromatolites at Hamelin Pool, and although such abnormally high salinities can be expected t o have been associated with some stromatolites in the past, there is no reason t o believe that such an association was necessary for all stromatolites, especially during the Palaeozoic and Proterozoic. Sedimentary environments closely comparable with that of Hamelin Pool are probably only rare features of the ancient record, just as they are today. There are lessons to be'learned from the Hamelin Pool stromatolite story regarding the use of modem models to interpret ancient rocks. It shows the dangers of accepting incompletely known modern environments as definitive guides to the past. The nature of ancient environments should be deduced from evidence in the ancient rocks themselves, without the mandatory need for modem analogues, especially as ancient environments need have no modem counterparts. Moreover, it demonstrates that the past can be as much a key to the present as the present is to the past; recognition of subtidal stromatolites in ancient rocks led to reevaluation of the picture at Hamelin Pool and t o the realization that subtidal stromatolites are widely distributed there also. SUMMARY AND CONCLUSIONS

Hamelin Pool is a hypersaline marine barred basin, which is probably localized by a Cainozoic synclinal downwarp. It is barred to the north by the Faure Sill, a seagrass bank which developed in Holocene times. The fauna of Hamelin Pool is very restricted in species diversity because of the hypersaline conditions, and algae-consuming organisms are largely absent. As a result, algal stromatolites and flat algal-mat sheets flourish on the sublittoral and intertidal platforms. Living stromatolites in Hamelin Pool extend from depths of at least 3.5 m below sea level to about high-water-spring tide level. Older dead stromatolites in varying states of disintegration occur above this level, and these have

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probably emerged as a result of Holocene uplift, which may still be continuing. Lithification of stromatolites is taking place in both intertidal and subtidal environments, and coalescent stromatolites form raised wave-resistant reefs in many areas. Lamination is only crudely developed or absent in many Hamelin Pool stromatolites, and fenestral fabrics are characteristic. Growth of stromatolites at Hamelin Pool is very slow, and many living forms are probably hundreds of years old. The maximum observed growth rate of an individual stromatolite is about 1mmlyear, but the average seems to be much less. Many forms appear t o have almost reached an equilibrium, with upward growth balanced by erosion of the living mat. A considerable amount of additional work is necessary before the Hamelin Pool stromatolites can be fully described and classified. It is desirable that a comprehensive system of nomenclature be developed for them which will also have application in the classification of ancient stromatolites. ACKNOWLEDGEMENTS

We wish t o thank the following geologists for their assistance with our field work at Hamelin Pool: R.L Chase, R.W.A. Crowe, W.J.E. van de Graaff, D. Johnstone, and D.C. Lowry. S.M. Awramik and S . Golubic gave valuable advice and provided preliminary results of their investigations of the stromatolites, and useful discussions were held with P. Hoffman on aspects of his work. G.W. Kendrick kindly identified molluscs from Hamelin Pool.

Fig. 11. A. Thin section of a well-lithified intertidal stromatolite from Flagpole Landing showing characteristic fenestral fabric in weakly laminated pellet wackestone. Note scattered shelly fragments. B. Thin section o f the same stromatolite shown in photomicrograph A. This is a fenestral pellet wackestone, with scattered ooids, shell fragments, foraminifers, quartz sand, and dolomite rhombs (?detrital). Fenestrae are lined with acicular aragonite.



Chapter 8.3 STRATIGRAPHY OF STROMATOLITE OCCURRENCES IN CARBONATE LAKES OF THE COORONG LAGOON AREA, SOUTH AUSTRALIA Christopher C. uon der Borch

INTRODUCTION

Geologically Recent stromatolites have been described from several areas of modern carbonate sedimentation, the most notable of which is Shark Bay in Western Australia (Logan, 1961; Davies, 1970b). Other important areas include the Persian Gulf (Kendall and Skipwith, 1968; Kinsman et al., 1971), the Bahamas (Monty, 1967,1972), Bermuda (Gebelein, 1969) and the Great Salt Lake, Utah (Carozzi, 1962). In addition t o the above, a somewhat restricted but nevertheless interesting occurrence of stromatolites has been described from two ephemeral carbonate lakes associated with the Coorong Lagoon in South Australia (Walter et al., 1973). These stromatolites occur in carbonate muds composed of the minerals hydromagnesite, aragonite, dolomite and calcite (Alderman and Von der Borch, 1960, 1961; Skinner, 1963; Peterson and Von der Borch, 1965; Von der Borch, 1965). Small amounts of amorphous silica are associated with these carbonates. This assemblage has its analogues in the geologic record, particularly in the Precambrian (e.g. the Skillogalee Dolomite Formation of South Australia, Preiss, 1973a) where silicified dolomitic stromatolite-bearing rocks are relatively common. Because of this, as an aid in recognizing equivalent depositional environments in the ancient rock record, the sedimentological history and stratigraphy of the Coorong stromatolite association will be described in some detail. GEOLOGICAL SETTING

The Coorong Lagoon (Fig. 1)is the dominant physiographic feature in the area of the stromatolite lakes in question. It is located landward of a modern calcareous barrier island known as Younghusband Peninsula and is approximately 100 km in length and 3 km in width. There is a single marine pass at the northwestern extremity, where water depth in the lagoon reaches 10 m,

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Fig. 1. Locality map, showing Coorong Lagoon and ephemeral carbonate lakes. The localities of the detailed maps in Fig. 2 are indicated.

however water depth throughout most of the area averages only 2-3 m. Water salinity is generally higher than that of normal seawater, reaching values of 60 parts per thousand in southeastern areas. The Coorong Lagoon is the most recent of several that formed across a broad coastal plain during the Pleistocene. Several stranded barrier islands, now represented by calcreted calcareous eolianite ridges, occur subparallel to the present coastline and extend inland up to 65 km from the coast. These and their associated lagoonal deposits have been progressively stranded by a combination of glacially induced sea-level oscillation and gentle regional upwarping (Hossfeld, 1950; Sprigg, 1952). The landward shoreline of the Coorong Lagoon is formed in most areas by one of these Pleistocene calcreted barrier-beach-ridge complexes. Occasional inliers of a Pleistocene eolianite ridge occur within the present barrier of Ypunghusband Peninsula, particularly in northern areas (Brown, 1965). The Coorong Lagoon itself occupies one and sometimes two interdunal depressions between these ridges. Towards the southeastern extremity of the Coorong Lagoon, in the vicinity of the two stromatolite-bearing lakes (Fig. 2), a rather complex calcreted

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E C CALCAREOUS SMDS OF mDEWI BARRIER MD BEACH RIOGES. SILICEOUS SAND RlOGE NEAR SALT CREEK,

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MICROCRYSTALLINE WLORITE. CALCITE. ARAGONITE AND HYDROMGNESITE SEDIIILNTS OF LAKES AND ELEVATED FLATS. (LAYTS SHOW IN WHITE)

CALCAREOUS SANDS OF TIDAL DELTA FACIES. 0

CALCRETED PLEISTOCENE EOLlANlTE

S T R W l O L I T E OCCURKNCES ROADS

Fig. 2. Details of the two ephemeral carbonate lakes which contain stromatolites. A. Northern stromatolite lake area. B. Southern stromatolite lake area. C. Generalized cross section X-Y, showing typical stratigraphic relationships. The 120,000 and 80,000 year old barriers are indicated.

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eolianite topography exists, due possibly to a combination of Pleistocene dune blow-outs superimposed on the normal beach-ridge-barrier lineation. Low-lying portions of these areas were inundated between 3,000 and 5,000 years ago when the Holocene transgression reached its maximum.

GEOLOGICAL HISTORY

The sedimentological evolution of the Coorong Lagoon and marginal ephemeral lakes is most clearly understood by reference to the sea-level curve of the last 200,000 years of Veeh and Chappell (1970) constructed from a study of Late Pleistocene coral reef terraces in New Guinea (Fig. 3). Assuming that the degree of upwarping in the Coorong region has been essentially negligible for that time, it is evident that the last time the sea was near its present level was 120,000 years ago during the Sangamon interglacial. During this stage the presently calcreted beach-ridge-barrier system that forms the landward shoreline of the Coorong Lagoon was probably built (D. Schwebel, pers. comm., 1975). This system forms the basement of the stromatolite lakes shown in Fig. 2. A subsequent sea level approached t o within several metres of the present sea level about 80,000 years ago and this may have been responsible for the barrier island which now forms the eolianite inlier within the Younghusband Peninsula barrier. Next in sequence followed the major sea-level retreat of the Late Wisconsin Glaciation after which the Holocene transgression returned the sea to its present level, partly inundating the older barriers to form the initial stages of the modern Coorong Lagoon.

mi

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80 YEARS

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Fig. 3. Sea-level curve of the last 200,000 years, based on New Guinea data of Veeh and Chappell(l970). Relevant dates are as follows: A = high sea level of 120,000 years ago corresponding t o Sangamon interglacial: formation of older barrier that now forms most of the Pleistocene basement in the area: B = relatively high sea level of 80,000 years ago: formation of eolianite inlier within modern barrier. C = Holocene transgression: formation of modern barrier.

A t this early stage the lagoon water was about l m above its present level as evidenced by widespread stranded lagoonal sediments (Brown, 1965). It is uncertain whether this higher level was due to a sea level slightly higher than at present or was related t o unrestricted access of the ocean through numerous passes. However, this was the time during which low-lying areas of the


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Pleistocene topography marginal t o the lagoon, including the stromatolite lakes, were flooded by waters with oceanic affinities. As sea level stabilized in the Late Holocene, the Younghusband Peninsula barrier built over the older ridge causing the lagoon to become increasingly restricted until ultimately its southern waters reached their present hypersaline state. Lagoon waters at that time, possibly 3,000 years ago, receded l m to their present level which averages that of mean sea level, causing the stranding of the marginal stromatolite lakes. These lakes now have floors about 1m above present high lagoon level and receive their annual charge of water during winter months by local and regional groundwater seepage from surrounding calcareous eolianite ridges and deeper Tertiary aquifers. This water reaches a maximum depth of about 0.5 m in late winter and evaporates to dryness in early summer. The salts in the lake waters are derived in part by leaching from marginal flats, possibly augmented by magnesium ions from breakdown of high-magnesian calcite in the calcareous aquifers. It is in this environment that various carbonate minerals and the stromatolites are forming at the present day. STROMATOLITE DESCRIPTIONS AND STRATIGRAPHY

The Coorong stromatolites have been described in detail by Walter et al. (1973). Three morphological forms were described: globular, stratiform and crenulate (Fig. 4). They occur in marginal areas of the two shallow ephemeral lakes shown in Fig. 2 and are associated with a carbonate mineralogy of hydromagnesite, aragonite, dolomite and calcite. During winter and early spring months pH of the associated lake waters varies from 8.2 to 9.9 (Von der Borch, 1965) with salinities reaching a minimum of about loo/, . At this stage a sparse growth of the aquatic grass Ruppia maritima occurs on lake floors. Complete desiccation generally occurs during ensuing arid summer months. The actual stromatolite structures persist throughout the dry period with varying degrees of preservation, however during the wet season small myriapod arthropods burrow into them and destroy some of their features. Sediment cores taken through the stromatolite lakes to depths of several metres (Fig. 5) show a stratigraphy that reflects the evolutionary history of the Coorong Lagoon and its associated marginal lakes. In both cases basement is Pleistocene calcrete, which is developed on calcareous eolianite. Overlying this is a shallow-marine sedimentary unit (Protected Marine Phase, Fig. 5), consisting of skeletal grainstones and packstones containing a variety of bivalves such as Katelysh spp. and Venerupis sp. This was formed during and immediately following the Holocene transgression, before significant build-up occurred of the Y ounghusband Peninsula barrier. This grades upwards into pelletized organic-rich aragonite and magnesian calcite muds rich in a lagoonal fauna comprising the small gastropod Coxiella confusa, the

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Fig. 4. Coorong stromatolite morphologies. A. Globular stromatolites, southern stromatolite lake (coin 23 mm wide). B. Desiccation polygons in stratiform stromatolites, southern stromatolite lake (scale 30 cm long). C. Crenulate stromatolites on the margins of desiccation polygons of stratiform stromatolites, southern hydromagnesite lake (pencil 18 cm long).

foraminifera Ammonia beccarii and a variety of ostracods (Lagoonal Phase, Fig. 5). This change in sediment-type reflects a period of increasing restriction due to barrier development. Upper portions of cores, finally, are composed of white, microcrystalline carbonate muds and pellet packstones composed of the minerals hydromagnesite, calcite, aragonite and dolomite (Ephemeral Lake Phase, Fig. 5). This unit, which developed in response to the l m drop in lagoon level, contains stromatolite structures both at the surface and occasionally at depth. GEOLOGICAL SIGNIFICANCE

Stromatolites are rare in modem carbonate sediments, largely because of the browsing on algal mats by organisms such as crustacea. The fact that stromatolites occur in two lakes in the Coorong area is therefore somewhat sur-

419

ENVIRONMENTAL MODELS NORTHERN STROM. LAKE

0

-

SOUTHERN STROM. LAKE WITH NANTLY HYDROMAGNESITE, ARAGONITE,

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2 SKELETAL GRAINSTONE AND PACKSTONE W I T H B I V A L V E S K A T E L Y S I A AND V E N E R u P I S .

vl Y t

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.......J PLEISTOCENE CALCRETE

3PLEISTOCENE CALCAREOUS E O L l A N l T E

Fig. 5. Generalized stratigraphic columns from northern and southern stromatolite lakes. Terminology partly adapted from Brown (1965).

prising, particularly in view of their general absence in apparently comparable lakes nearby. One contributing factor may be that the two stromatolite lakes, due to greater localized ground-water influx, do not become as thoroughly desiccated as other lakes. This higher ground-water discharge in turn may lead t o the formation of the carbonate mineral hydromagnesite which is associated with the stromatolites. It would be quite possible for sediments formed under conditions similar t o the above to be preserved in the geologic record, particularly in structurally negative areas. Such occurrences have been described in the literature (Peterson, 1962). The deposits would be typified by an association of carbonate cycles similar to the one described from the Coorong sediment cores. A single upward-fining cycle would ideally measure a few metres in thickness, beginning at the base with a shallow marine carbonate unit. This would grade upwards into an organic-rich pelletized lagoonal carbonate unit and would culminate in a siliceous microcrystalline carbonate with associated stromatolite structures. Mineralogy of the upper unit would be variable in both the lateral and vertical sense and could comprise combinations of the minerals magnesite, dolomite or calcite. The carbonate units themselves would be lenticular and laterally discontinuous and would be closely associated with shoestrings of calcareous barrier sands.

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ACKNOWLEDGEMENTS

Sediment coring in the Coorong area has been supported by grants from the Australian Research Grants Committee and Flinders University. The manuscript was read by D. Schwebel. Photographs of the Coorong stromatolites were provided by M.R.Walter.


Chapter 8.4 ALGAL BELT AND COASTAL SABKHA EVOLUTION, TRUCIAL COAST, PERSIAN GULF David J.J. Kinsman and Robert K . Park

INTRODUCTION

The distribution of sabhkas in the Persian Gulf is shown in Fig. 1 and the area of detailed study in Fig. 2. The first studies of the Trucial Coast stromatolitic sediments were made by Kinsman (1964) and these were later expanded by Kendall and Skipwith (1968) who recognized a zonation of forms throughout the intertidal zone. Our very detailed work on the algal belt near Abu Dhabi (Park, 1973) has added considerably to these earlier studies. ALGAL-BELT ENVIRONMENT

A review of sedimentation along the Trucial Coast has been presented by Purser and Evans (1973). A general view of the lagoon immediately to the northeast of Abu Dhabi Island is shown as Fig. 3. The lagoon barriers comprise oolite and reef facies sediments; within the lagoons sediments are mainly pelleted aragonite muds but shoals downwind of extensive open lagoon areas may comprise winnowed accumulations of skeletal sands and gravels (predominantly molluscan debris). Algal mats colonise relatively stable sediment substrates in the intertidal zone and the algal belt may be as much as 1-2 km in width. High-energy intertidal zones where the substrate is mobile are not colonised to the extent that a permanent mat is established (Fig. 4). Low-energy intertidal zones along protected lagoon margins are typically colonised by algal mats; surface slopes are very low, and spring tidal range usually close to 120cm (profiles Area I and IV in Fig. 4). The physical, environmental variables can be summarised as follows: (a) predominant wind and wave direction from the NW; (b) tidal range diminishes from 210cm at the lagoon barrier t o 120cm in the innermost lagoon (tidal range is one of the important controls on the thickness of stromatolitic

D.J.J. KINSMAN AND R.K. PARK

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ARABIA

OCEAN

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Fig. 3. Diagram illustrating development of stromatolite bioherms over drowned stromatoporoid algal reefs.

SHALLOW-WATERSTROMATOLITES

Shallow-water cryptalgal limestones make up much of the limestone platforms in the Canning Basin. They are believed to have been deposited in supratidal to very shallow subtidal environments. The reef framework in many areas consists largely of cryptalgal limestone, generally having a fenestral fabric, associated with skeletal framebuilders (especially stromatoporoids, algae, and corals). Spaced columnar stromatolites make up part of this limestone, but much of it is massive and lacks recognizable growth forms. Stromatolite columns are indistinct where the inter-areas have been progressively filled with fenestral limestone similar to that forming the columns. However, columnar shapes are well preserved in some places, especially where the columns are covered by terrigenous detritus (Fig. 4A). Fenestral columnar stromatolites similar to those in the reef facies also occur in the back-reef deposits. They are associated with well-bedded fenestral (birdseye) and oncolitic limestones. In some areas, especially in the Famennian sequence, fenestral limestone constitutes the major part of the back-reef facies.

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Fig. 4. A. Shallow-water columnar stromatolites from the Pillara Limestone (reef-flat environment) at Windjana Gorge. The stromatolite heads are covered by calcareous sandstone. B. Thin section of modem intertidal columnar stromatolite from Hamelin Pool, Western Australia, showing well-developed fenestral fabric.

STROMATOLITES IN BASIN ANALYSIS

547

DEEP-WATER STROMATOLITES

General description Field evidence indicates that stromatolites in the fore-reef and inter-reef facies, and those capping drowned reefs, grew to considerable depths, probably exceeding 100 m below sea level. We refer to these forms as deep-water stromatolites. A diverse stromatolite assemblage occurs in the fore-reef deposits, but well-developed occurrences are uncommon and are confined to condensed sequences in the Virgin Hills and'Napier Formations. In some areas stromatolites extend below the foot of the fore-reef slope into the flat-lying inter-reef deposits (Fig. 2). Stromatolites similar to those in the fore-reef deposits cap ridges and pinnacles of drowned stromatoporoid-algal reefs (Fig. 3). Some of these form giant stromatolite bioherms, which are best developed on the north side of the Oscar Range (Figs. 6 and 7). Comparable, but smaller, stromatolite developments also occur on top of allochthonous blocks in the fore-reef deposits. The fore-reef stromatolites grew on depositional slopes with original inclinations ranging from a few degrees to near vertical. Algae (and possibly bacteria) are believed to have been responsible for maintaining slopes at angles steeper than the angle of rest for loose debris. The algae include skeletal forms (especially Renalcis and Sphaerocodiurn), but in many cases nonskeletal forms seem to have been more important. In areas where deposition was very slow, the non-skeletal algae (and/or bacteria) formed recognizable stromatolite heads, but where deposition was more rapid their action in stabilizing steep layers of sediment has largely been inferred rather than proved. Some of the deep-water stromatolites lack recognizable algal (or bacterial) filaments. These types are believed to have formed through the trapping and binding of biogenic and terrigenous sedimentary particles by non-skeletal algae and/or bacteria, with or without concomitant precipitation of calcium carbonate. Other stromatolites contain recognizable algae, belonging to the genera Sphaerocodium, Girvanella, Frutexites, and Renalcis. It is not known whether the deep-water algae were photosynthetic. Some of the stromatolites could well have been formed by heterotrophic algae or bacteria. Many, but not all, of the columnar stromatolites grew vertically on depositional slopes, but it is not clear whether this was controlled by light (phototropism) or gravity (geotropism). C. Thin section of Devonian shallow-water columnar stromatolites from the Windjana Limestone (reef facies) at Geikie Gorge. The stromatolite is growing on a stromatoporoid. The resemblance between the fenestral fabric o f this Devonian stromatolite and that of the modern form from Hamelin Pool is striking.

P.E.PLAYFORD ET AL.

548 TYPE

PLAN

SECTION

TYPE

PLAN

SECTION

..

A R

I

LONGITUDINAL

B

~

I

RDOMAL

1

Fig. 5. Diagram illustrating principal types of deep-water stromatolites.

Fig. 6. Map of part of the northern Oscar Range near Elimberrie Spring showing Elimberrie stromatolite bioherms nos. 1, 2, and 3.


A

-

..-,

-__---

CROSS-SECTION /

549

(natural scale)

A-B

B

,

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_ _ - /

W STROMATOLITE

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STROMATOLITE BIOHERM FORE-REEF

I ~~

_ _ _ _ FORE-REEF

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STRATIFICATION

JOINT (OF DEVONIAN AGE)

LIMESTONE

+

AXIS OF STROMATOLITE BIOHERM DIP & STRIKE OF STRATIFICATION

~

~~

~

~~

Fig. 7. Map and cross-section of Elimberrie stromatolite bioherm no. 2.

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P.E.PLAYFORD ET AL.


551

Holdfasts of corals and crinoids encrust the surface of some stromatolites (Fig. l l A ) , and ammonoids, nautiloids, and conodonts are conspicuous elements of the associated open-marine pelagic fauna. The main types of deep-water stromatolites are illustrated on Figs. 5, 8, 9, and 10. Most of these forms grade from one to another, the named types being essentially end-members of series. The following descriptions briefly summarize some of the characteristic features of these stromatolites. Spaced-columnar stromatolites are the most abundant forms. They commonly grew vertically on the fore-reef slopes. The maximum known height of such columns is l m , but their relief during growth was normally not more than about 5 cm, and they often show lateral linkages (Fig. 8A). Such linked forms can also be referred to as “pseudocolumnar”. Contiguous columnar stromatolites (Fig. 9A) are less common. Most of these also grew nearly vertically on the depositional slopes. Branching columnar stromatolites consist largely of the alga Sphaerocodium, and this genus also occurs in other columnar forms. A continuous range is found between columns composed entirely of Sphaerocodium filaments and those in which the filaments are rare. Longitudinal stromatolites occur in some areas. In cross-section they resemble columnar types, but they actually form elongate ridges, directed down the fore-reef depositional slopes (Fig. 9B). This elongation was probably caused by down-slope currents. Scalloped stromatolites are similar in vertical section to contiguous columns, but in plan they have a scalloped form, extending parallel to the strike direction of the original depositional slopes. The points of the scallops are directed up-slope. The down-slope face of each stromatolite is thickened and is composed largely of white micrite, whereas the rest is laminated and is commonly red. The micritic margin often contains abundant encrusting foraminifers. Reticulate stromatolites are unusual forms, which developed low on the fore-reef slopes and extended into the flat-lying inter-reef deposits. They are characterized by reticulate patterns of meandering grooves between very low flat-topped mounds (Fig. 8B). The best lamination in these stromatolites is that marking successive grooves. Undulous stromatolites show simple to intricate undulating to bulbous growth forms. Many have the appearance of being secondarily contorted, but as successive layers are commonly encrusted by crinoid and coral holdfasts Fig. 8 . A. Laterally linked spaced-columnar deep-water stromatolites in the Virgin Hills Formation (fore-reef facies) at Ngumban Cliff, Bugle Gap. These stromatolites grew vertically (apparently phototropic or geotropic growth) on a depositional slope of about 12 . They grew extremely slowly and their relief during growth did not exceed 5cm. The inter-areas contain fragmental crinoid material. Note the horizontal geopetal filling in an ammonoid near the bottom right-hand corner of the photo. B. Reticulate deep-water stromatolites from the Virgin Hills Formation (inter-reef facies) at Bugle Gap.

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(which require a hard surface for attachment) it is clear that they must have grown in their undulous form as hard, rigid bodies (Fig. 9C). Domal stromatolites occur in the fore-reef deposits of some areas. They show some resemblance to the undulous forms, but have a more regular and well-defined mound shape. They tend to be elongated in the strike direction of the depositional slopes on which they grew. Bioclastic debris commonly accumulated on the up-slope side of each mound, and crinoid holdfasts are also concentrated there. Many of the domal stromatolites have grown over, or in association with, coral thickets composed mainly of the genus Aulopora. Most domal forms are about 0.5-2 m long, and these had a relief during growth of 0.2-1.5 m. Depth relationships

The fore-reef deposits were commonly laid down with steep initial dips on the flanks of the limestone platforms. The depositional nature of these dips was first recognized by Guppy et al. (1958) on the basis of observed stratigraphic and structural relationships. This interpretation was confirmed by other workers, including Rattigan and Veevers (1961) and Playford and Lowry (1966), but they did not describe any method of separating the initial component of an observed dip from later components due to compaction or tectonism. More recent studies have shown that the amount of post-depositional tilt can be deduced, within limits of a few degrees, using geopetal fabrics that show the orientation of the rock in space at the time of deposition (Playford and Cockbain, 1972). Many types of geopetal fabrics have been recognized in these rocks, and they can be broadly grouped into two types-cavity fillings and organic growth forms. The cavity fillings are “fossil spirit levels”, generally consisting of laminated sediment in the lower part and sparry calcite above, with the lamination and the spar-sediment contact approximating the original horizontal. They are best developed in fossils, especially closed brachiopods. The geopetal organic growth forms consist mainly of calcareous algae and stromatolites that grew nearly vertically, presumably under the influence of light or gravity. Fig. 9. A. Polished slab (parallel to the depositional dip) of contiguous columnar deepwater stromatolites from the Virgin Hills Formation at McWhae Ridge. They grew vertically on a depositional slope of about 35’. Note two crinoid holdfasts attached t o the dark (iron-rich) lamina on the left-hand side of the slab. B. Polished section (parallel to the depositional strike) through a slab of longitudinal deep-water stromatolites from the Virgin Hills Formatioz at Bugle Gap. They grew on a depositional slope (away from the viewer) of about 20 and are elongated in the slope direction. C. Polished section through a slab of undulous deep-water stromatolites from the Virgin Hills Formation a t Ngumban Cliff. Note the cluster of crinoid holdfasts overgrown by the stromatolite on the left. Other fossil material consists of crinoids, nautiloids, and ammonoids.

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555

The sediment which has filled or partly filled cavities in fossils consists mainly of calcarenite (including bioclastic material) and calcilutite which is identical to sediment in the surrounding matrix. Lithification of the fore-reef limestones occurred very soon after deposition, as is shown by such features as penecontemporaneous neptunian dykes and slide breccias. Sediment must have entered the shells while the fore-reef sediments were being deposited, and lithification of both the geopetal sediment and the surrounding fore-reef strata occurred soon afterwards. The spar-sediment contact and the layering in the sediment are normally parallel from one cavity to another, and it is clear that the sediment must usually have settled close to horizontal in each cavity. However, in some cases where sediment poured in on one side it maintained a significant depositional dip inside a cavity. It is therefore necessary to average numerous geopetal measurements when using cavity fillings to determine dip components. The evidence of steep depositional dips in the fore-reef facies has important palaeoecological implications, as it points to a means of estimating relative water depths at the time of deposition. Where an observed dip is entirely depositional, the difference in original water depth between any two points on a bedding plane is the same as the present elevation difference between those points. Appropriate corrections need to be applied in order to estimate depths where there has been post-depositional tilting. There are a number of localities where beds containing stromatolites in their original growth positions are exposed over considerable elevation ranges, allowing relative water-depth estimates to be made. Notable examples are at McWhae Ridge, Geikie Gorge, and Windjana Gorge. McWhae Ridge is a faulted reef spine with associated off-reef deposits at the southern end of the Lawford Range platform. At this locality the Sadler Limestone and overlying Virgin Hills Formation were laid down with steep depositional dips against and over the reef spine. Geopetal fabrics are well developed in the Sadler Limestone, and detailed measurements of these have been made. The average of measurements on the geopetal fillings of 106 brachiopods, 39 Receptaculites, and 8 other fossils indicates that the observed dip of 41’ is made up of two components, a depositional dip of about 34’, and a post-depositional dip (due to tectonism and/or compaction) of about 7’. At the same locality the Sadler Limestone contains beds of Giruanella oncolites which developed on top of the reef spine after reef growth had ceased (apparently due to drowning) and were intermittently swept off, coming to rest on the steep depositional slopes on each side of the spine. Fig. 10. A. Thin section o f a deep-water stromatolite showing dark “microcolumns” of iron-rich Giruanella filaments. Quartz silt and fossil fragments are incorporated in the stromatolite. B. Thin section of a deep-water stromatolite showing dark iron-rich “shrubs” of Frutexites associated with quartz silt, fossil debris, and some Girvanella “microcolumns” in the lower part.

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P.E.PLAYFORD ET AL.

Fig. 11. A. Thin sectipn of a deep-water stromatolite showing abundant crinoid and coral holdfasts which encrusted successive stromatolite surfaces. The stromatolite contains abundant Frutexites. B. Girvanella oncolites and capped oncolites at McWhae Ridge. .The oncolites developed on a drowned reef spine and were periodically swept from the top (probably by wave and current action), coming to rest on a depositional slope of about 34O, in water at least


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Conical Sphaerocodium caps grew vertically on some of the oncolites, at angles of about 35' t o the depositional slope (Fig. 11B). The average direction of elongation of 95 caps is almost at right angles (within about 1%') t o the original horizontal as indicated by the geopetal cavity fillings. The oncolite-bearing bed is exposed up the sides of the ridge t o the top, and elevation data indicate that the lowest exposed Sphaerocodium caps must have grown in water at least 35 m deep. The Virgin Hills Formation at McWhae Ridge was laid down over the Sadler Limestone with similarly high depositional dips. It is bright red in colour and contains a rich pelagic fauna of goniatites, nautiloids, and conodonts. Conspicuous stromatolite beds occur in the formation, interfingering with other limestones, and pinching out down the flanks of the ridge. This pinching out may have been the result of decreasing light penetration with increasing water depths, or it could have been a response to increasing rates of sedimentation on the flanks of the ridge. The difference in elevation between the lowest stromatolites and the eroded crest of the reef spine is nearly 55 m, and making allowance for post-depositional structure it is conservatively estimated that the minimum depth of water in which those stromatolites grew was about 45 m. However, palaeotopographic crosssections through the ridge suggest that the actual water depth was considerably more, probably exceeding 100 m. The evidence at Windjana Gorge and elsewhere similarly indicates that stromatolites grew on fore-reef slopes and over drowned pinnacle reefs and allochthonous blocks in water up to 100 m or more in depth. Age relationships Druce (1976), using conodonts, has shown that well-developed deep-water stromatolites in the Virgin Hills Formation of the Bugle Gap area are of three different ages. The oldest horizon is late Frasnian, the middle horizon marks the base of the Famennian, and the youngest horizon is early Famennian. The youngest is of the same age as the stromatolite-bearing horizon in the Napier Formation at Narlarla (Napier Range). The well-developed deep-water stromatolites are associated with strongly condensed sequences (i.e. very slow deposition). This is shown by the fact that in stromatolite-bearing beds the conodont zones are very closely spaced and conodont concentrations are high. A Devonian conodont zone represents an average of about 0.5 m.y., and on this basis the growth rate of some deepwater stromatolites may have been as low as 2 p m per year. Growth could have taken several hundred thousand years for the taller individual columns, and a few million years for the large stromatolite bioherms. 35 m deep. Vertical (phototropic or geotropic) conical caps of Sphaerocodium grew on some of them, especially those in the final layer. Most oncolites have cores of crinoid columnals or brachiopods.

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At McWhae Ridge stromatolite beds in the Virgin Hills Formation contain abundant palmatolepid conodonts (probably deep-water forms, see Druce, 1973) associated with relatively large numbers of icriodid (probably intermediate depth) conodonts having high degrees of morphological variability. Icriodids are lacking and palmatolepids are abundant in sediments above and below the stromatolite horizons. Druce (1976) suggests that this could mean that these stromatolites represent periods of relative shallowing in the prevailing deep-water environment. It is important to note that in addition to the Canning Basin occurrences other Phanerozoic stromatolites are known t o be associated with strongly condensed sequences. Examples have been reported from the Jurassic and Cretaceous of Europe by Szulczewski (1968), Radwansky and Szulczewski (1966), Marcinowski and Szulczewski (1972) and Jenkyns (1971). Modern oceanic ferromanganese stromatolites (Monty, 1973a) are also features of areas of very slow sedimentation. It therefore seems likely that there is a characteristic association of Phanerozoic deep-water stromatolites with condensed sequences. Iron and manganese deposition Much of the Virgin Hills and Napier Formations is coloured bright red by finely divided hematite. Minor amounts of black manganese oxide also occur. The reddest sediments are those containing the smallest amounts of platformderived debris, laid down in relatively deep water, and exhibiting the slowest rates of sedimentation. Red is also the dominant colour of deep-water stromatolites in the Canning Basin. In many cases the iron-oxide colouring is finely and uniformly dispersed, but in others there is a marked concentration of iron oxide, and smaller amounts of manganese oxide, in and around algal filaments. This is especially so with the genus Frutexites, which is preserved as minute “shrubs” of branching filaments containing high iron concentrations (Fig. 10B). Electron microprobe analysis (carried out by courtesy of C.S.I.R.O., Perth) indicates that the metallic iron content of these filaments is as high as 30%. Concentrations of iron oxide also occur in and around Giruanella filaments (Fig. 10A) and (less commonly) in Sphaerocodium filaments. It is possible that some algae precipitated iron, but it seems more likely that iron bacteria living in close association with the algae were responsible. Such associations are common today (Choldony, 1922), and they no doubt also occurred in the Rast. The finely dispersed hematite in fore-reef and inter-reef sediments associated with the stromatolites is also likely to have a bacterial origin. It was probably precipitated originally in the form of iron hydroxide. The genus Frutexites also occurs in Ordoviciari stromatolites of the U.S.S.R. (Maslov, 1960), and in Jurassic stromatolites of Poland

STROMATOLITESIN BASIN ANALYSIS

559

(Szulczewski, 1963). In each case it is characterized by the concentration of iron oxide in its filaments. Red basinal deposits associated with Phanerozoic carbonate complexes also occur in other parts of the world, for example, in the Jurassic of Sicily (Jenkyns, 1971), the Triassic of the Alps (Fischer, 1964), the Carboniferous of Ireland (Schultz, 1966), the Devonian of the Camic Alps (Bandel, 1972), and the Silurian of the Michigan Basin (Mesolella et al., 1974). Among these occurrences, the red sediments of Sicily, Ireland, and the Michigan Basin are also associated with stromatolites. Maslov (1960) states that many stromatolites (of different ages) in the Soviet Union are red, and Walter (1972a) notes that some Precamljrian stromatolites in Australia occur in red sediments. Iron and manganese are the only base metals known to have been concentrated in the Canning Basin stromatolites. However, the sole economic lead-zinc orebody found to date in the area, at Narlarla in the Napier Range, occurs above beds containing deep-water stromatolites. A genetic relationship seems possible, although there are no known lead-zinc orebodies near other deep-water stromatolite occurrences in the Devonian outcrop area. Elim berrie stromatolite bioherms On the north side of the Oscar Range in the vicinity of Elimberrie Spring there is a group of three giant stromatolite bioherms, referred to as Elimberrie bioherms 1 , 2 , and 3 (Figs. 6 and 7). The largest bioherm, no. 1, is approximately 1km long. It has developed over two stromatoporoid-algal pinnacle reefs. These reefs are thought to have been drowned as a result of abwpt subsidence t o depths (possibly in excess of 100m) too great for continuing growth of the reef-building organisms. Successive stromatolite layers then developed over the drowned pinnacles (Fig. 3), the separate caps merging with continuing growth to form a single bioherm. Elimberrie bioherms 2 and 3 are also thought to have formed in this way, but the present level of erosion is not low enough to expose pinnacle reefs below these bioherms. The best exposed bioherm is no. 2 (Figs. 7, 12). It is ca. 500 m long and has a similar shape, but on a greatly increased scale, to the domal stromatolites previously described. Its relief during growth is thought to have been at least 25 m, and possibly more than 100 m. Large convex outgrowths on the flanks of the bioherm meet at typical v-junctions of the same type as those found on the small domal mounds. Dips on the flanks are up to near vertical, and welldeveloped geopetal fabrics show that the dips are depositional. Some stromatolite layers in the bioherms contain spaced-columnar or scalloped stromatolites, but others are massive. Skeletal algae, especially Sphaerocodium, occur in some beds. Receptaculites (possibly an alga) is abundant in parts of bioherms 1and 3.

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Fig. 12. A. Aerial view of the southern crestal part of Elimberrie stromatolite bioherm no. 2, looking north. The width of the area covered by the photo in the centre is about 150 m, and the total length of the bioherm is about 500 m. Note the characteristic v-junctions of major convex outgrowths.


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Conodonts indicate that the Elimberrie bioherms are of Famennian age, and that they represent condensed sequences. The core of bioherm no. 2 yields abundant icriodid (probably intermediate-depth) conodonts, whereas the flanks yield palmatolepids (probably deep-water forms) only. Jenkyns (1971) described red stromatolites associated with ferromanganese nodules and crusts in red pelagic Jurassic limestones capping drowned reefal platforms in western Sicily. This occurrence appears to be analogous to that of the reef-capping stromatolites in the Canning Basin. Machielse (1972) has reported a thin discontinuous layer of stromatolites capping Upper Devonian pinnacle reefs in the subsurface at Rainbow Lake, Alberta. The stromatolites extend nearly 1 7 0 m down the flanks of the reefs, and Machielse favoured the hypothesis that they developed during intermittent cessations of reef growth associated with lowered sea levels. However, in view of the Occurrence of deep-water stromatolites capping drowned pinnacle reefs in the Canning Basin it is suggested that the Rainbow reefs may have been drowned by rapid subsidence and then capped by deep-water stromatolites. This idea is also important in assessing deep-water versus sabkha hypotheses for the origin of the immediately overlykg Muskeg evaporites. COMPARISONS WITH MODERN STROMATOLITES

The best examples of columnar stromatolites known from modem seas are at Hamelin Pool, a barred hypersaline embayment forming part of Shark

Bay in Western Australia. They were discovered by geologists of West Australian Petroleum Pty Ltd in 1954 and were first described by Logan (1961), who stated that the stromatolites are confined to the intertidal zone. By analogy he suggested that ancient stromatolites are also intertidal phenomena. The Hamelin Pool intertidal model was accepted by many authorities. However, there were some early dissenters, notably Fischer (1965) and Monty (196513). Playford and Cockbain (1969) stated that Devonian stromatolites in the Canning Basin must have grown t o depths of at least 45 m, and accordingly concluded that the Hamelin Pool model could not be applied to all ancient stromatolites. Achauer ixid Johnson (1969), Walter (1970a, 1972a), and Hoffman (1974) reached similar conclusions. Monty (1971) suggested on theoretical grounds that, far from being necessarily restricted to the intertidd zone, there was no reason why stromatolites should not grow to great depths of the ocean, in complete darkness. Monty (1973a) has now shown that ferromanganese nodules and crusts B. Ground view of the northern crestal part of Elimberrie stromatolite bioherm no. 2, looking north. The man standing in the centre left gives the scale. The structure is depositional, formed by successive accretion of stromatolite layers.

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forming today in abyssal water depths and capping oceanic seamounts are bacterial stromatolites. Hofmann (1969a) had been the first t o suggest that such ferromanganese crusts could be regarded as modem deep-water stromatolites, but he had not shown that they were of organic origin. Recent work by the Geological Survey of Westem Australia at Hamelin Pool has shown that stromatolites there are not confined to the intertidal zone as previously supposed. Living subtidal stromatolites are widespread in Hamelin Pool, and they extend t o water depths of at least 3.5m (Playford and Cockbain, Ch. 8.2). The Hamelin Pool stromatolites (both intertidal and subtidal) show close similarities to shallow-water stromatolites in the Devonian of the Canning Basin (Fig. 7). A characteristic feature of most Hamelin Pool forms is that they have well-marked fenestral fabrics. Lamination is generally, but not always, crudely developed or absent, and the columns commonly have irregular margins. These features are similarly characteristic of the shallowwater Devonian stromatolites. The Devonian deep-water stromatolites do not closely resemble the Hamelin Pool forms, but show some similarities to the ferromanganese oceanic stromatolites described by Monty. In common with oceanic stromatolites, the Devonian deep-water forms are generally finely laminated, lack fenestral fabrics, show evidence of iron and manganese precipitation, mark condensed sequences, and are associated with pelagic faunas. The reefcapping stromatolites in the Canning Basin are also analogous to stromatolites capping modem seamounts. However, the Canning Basin deep-water forms generally contain only small amounts of iron and minor manganese, whereas modem oceanic stromatolites are composed largely of iron and manganese oxides. Moreover, there is no evidence t o show that any of the Devonian forms grew in abyssal water depths. SUMMARY AND CONCLUSIONS

Stromatolites are important constituents of Devonian reef complexes in the Canning Basin. They grew through a wide range of environments, from shallow reef-fringed limestone platforms to deep inter-reef basins. Cryptalgal limestone, ranging from massive in the reef facies to well bedded in the back-reef facies, makes up much of the platforms. Discrete columnar stromatolites occur in these deposits, which are thought to have formed in shallow subtidal to intertidal environments. Deep-water stromatolites occur in the fore-reef and inter-reef facies and as cappings on drowned reefs. They grew to depths of at least 45 m, and probably more than l o o m , below sea level. They are best developed in strongly condensed sequences, and grew very slowly; their average growth rates probably amounted t o no more than a few microns per year. The deep-water


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TABLE I Differences between deep-water and shallow-water stromatolites from Canning Basin Feature

Deep-water stromatolites

Morphology fenestral fabrics absent usually finely laminated diverse assemblage of forms

Shallow-water stromatolites fenestral fabrics common usually weakly laminated or unlamin ated columnar forms only, associated with oncolitic and fenestral limestones

Occurrence characteristically in condensed sequences grew on depositional slopes, drowned reefs, and allochthonous blocks

not in condensed sequences

Associated fauna

some forms encrusted with crinoids and corals

associated with reefal and b i o stromal organisms, especially stromatoporoids and Renalcis not encrusted by corals or crinoids

Chemistry

iron and some manganese precipitation important in certain forms

n o significant iron or manganese precipitation

Colour

commonly red or reddish brown

commonly white or pale yellow

pelagic faunas common

grew on near-horizontal limestone platforms

stromatolites characteristically grew on hard surfaces which stood above the surrounding sea floor, received little or no sediment, and were at depths too great for extensive colonization by skeletal reef builders. The largest examples are the Elimberrie stromatolite bioherms, which grew on drowned stromatoporoid-algal pinnacle reefs. Distinguishing characteristics of deep-water and shallow-water stromatolites in the Canning Basin are summarized in Table I. Shallow-water stromatolites from the Devonian limestone platforms show close similarities to modem intertidal and shallow subtidal stromatolites known from Hamelin Pool. The Devonian deep-water forms have certain characteristics in common with modem oceanic stromatolites; they do not closely resemble the Hamelin Pool forms.


10. STROMATOLITES IN BASIN ANALYSIS

Chapter 10.5

LOWER CAMBRIAN STROMATOLITES FROM OPEN AND SHELTERED INTERTIDAL ENVIRONMENTS, WIRREALPA, SOUTH AUSTRALIA P.G.Hasle t t

INTRODUCTION

During his early investigations of the Lower Cambrian sequences in the northern Adelaide geosyncline, Mawson (1925) interpreted many of the structures found within the limestones as being of algal origin. Recent work of Preiss (197213, 1973b) and Haslett (1976) has confirmed the presence of algal stromatolites in most of the Lower Cambrian sequences. These sequences are largely composed of carbonate rocks which are considered to have formed in broad shallow seas (Daily, 1956; Dalgarno, 1964). Local zones of faulting and diapiric intrusion have, however, resulted in areas of considerable complexity within this broad and uniform depositional environment (Dalgamo, 1964; Dalgarno and Johnson, 1968; Daily and Forbes, 1969). Detailed study of the Lower Cambrian stratigraphy and carbonate sedimentology at Wirrealpa (Fig. 1) (Haslett, 1976) has revealed numerous stromatolite beds associated with marked lithofacies changes in a sedimentary sequence adjacent to the northern edge of the Wirrealpa Diapir (Fig. 5). This sequence is laterally equivalent to the Wilkawillina Limestone, which forms the basal carbonate unit of the Cambrian over large areas of the central Flinders Ranges (Daily, 1956). The environmental model proposed herein for the stromatolite-bearing sequence at Old Wirrealpa is consistent with information collected over a far wider area of the central Flinders Ranges (Haslett, 1976). At Old Wirrealpa Spring, 11km NW of Wirrealpa, excellent outcrop and a lack of structural complexity allow variations in stromatolite morphology and distribution to be readily mapped. Selected samples were cut into three mutually perpendicular slabs, acidetched and stained with alizarin red S for laboratory study. Large thin sections of selected specimens were also examined. Preservation of features is generally good, but certain microscopic features have been obscured by diagenesis. No attempt has been made to apply a rigorous classification to the stromatolites studied. Instead, the structures have been described in general

P.G. HASLE’IT

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I

1

1 k ’

0

I

I

SOUTH AUSTRALIA

Scale

200 Precambrian

1

I I

.I

Fig. 1. Wirrealpa locality map.

terms, chiefly following the orderly scheme of Hofmann (1969a). Associated carbonate lithologies are referred to the classification of Dunham (1962). The term “gravelly” (see legend, Fig. 5) is used in the sense of Folk (1968) to indicate that lithoclasts within the grainstones are in the pebble to boulder size range. LOCAL GEOLOGY

WirrealpaDiapir

The Wirrealpa Diapir forms an area of complex, poorly exposed rocks, to the south and west of the stromatolite-bearing sequence at Old Wirrealpa. The mode of emplacement of this diapir is not well understood. It is mainly composed of sedimentary rocks, which may correlate lithologically with the lowermost part of the late Precambrian sequences elsewhere in northern


567

South Australia (B. Murrell, pers. comm., 1972). Evidence from detailed mapping in the vicinity of the diapir suggests that periodic intrusion of the Wirrealpa Diapir took place over a considerable span of Lower Cambrian time. These episodic movements had marked effects on sedimentation. Uplifted areas acted as sources of carbonate and non-carbonate detritus which was incorporated into nearby Cambrian sediments. Large boulders up to 10 m in diameter, composed of characteristic diapiric lithologies, occur within the Cambrian sequence adjacent to the diapir, particularly on the western extremity of the stromatolite-bearing sequence. Although the southern contact of the diapir with the stromatolite-bearing sequence is poorly exposed, better exposed areas at Wirrealpa suggest that this contact represents an onlap of Cambrian sediments over an older eroded surface of diapiric material. The large size of exotic boulders within the Cambrian, and the development of a Cambrian karst topography associated with areas of diapiric intrusion, attest to there having been considerable, and at times, subaerial, relief of these areas. Furthermore, the palaeogeographic distribution of local, Lower Cambrian lithofacies indicates that the areas of rising diapir acted as barriers to wave and tidal action during sedimentation (Haslett, 1976).

Local Cambrian sedimen tology The geology of the Lower Cambrian stromatolite bearing sequence at Old Wirrealpa is summarized on the lithofacies map and diagrammatic crosssection (Fig. 5 ) . The sequence basically consists of massive cross-bedded ooid and lithoclast grainstones to the west, passing laterally into finely laminated calcareous mudstones to the east. A marked cyclicity in sedimentation, which is particularly evident in the western part, has probably occurred in response to local diapiric movements. Stromatolite beds occur over the whole lateral extent of the sequence. Evidence of other organic activity is limited to rare worm trails and bioturbation in the mudstone facies, and rare phosphatic skeletal remains as nuclei to ooids in the grainstone facies. The eastern part of the sequence consists of dark-coloured calcareous mudstones, most showing fine lamination. Much of the lamination is believed to be of algal origin, because most beds display sporadic lateral development of low domes and nodular stromatolites. Desiccation mudcracks, although rare, are present throughout the sequence. Thin interbedded flat-pebble conglomerates are common, and are composed of locally derived cryptalgalaminite pebbles. These flat-pebble conglomerate beds are broadly lenticular, with sharp basal contacts and pebble-rich lower units. Uppermost boundaries are usually gradational into calcareous mudstones or stromatolites. Columnar stromatolites associated with the conglomerate beds are best developed where the conglomerate lenses are thickest. However,

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columnar stromatolites are also found on the margins of narrow, steep-sided channels which cut through both older stromatoltes and underlying flatpebble conglomerates (see cross-section, Fig. 5). The desiccation features and flat-pebble conglomerates indicate shallow intertidal to lower supratidal environments of deposition (Lucia, 1972). Deposition of the flat-pebble conglomerates is believed to have taken place preferentially on broad depressions in a shallow tidal flat, the pebbles apparently having been derived from adjacent shallower areas, some of which were periodically exposed. With the exception of the conglomerate beds, the whole of the eastern sequence is believed to represent carbonate sedimentation under lowenergy conditions, on a sheltered, shallow intertidal mudflat. Massive grainstone wedges, which thicken markedly toward the diapir edge, are the dominant lithological units in the western part of the sequence. Nearest the diapir the grainstones consist almost entirely of diapirderived debris, a large proportion of which is non-carbonate material. These grainstones contain poorly sorted, angular lithoclasts as well as blocks up to l O m in diameter. The upper parts of the large blocks are commonly encrusted by columnar stromatolites. Eastward there is a gradual lateral change in the nature of each grainstone unit, the size and amount of noncarbonate clasts decreasing, and the grainstones showing much better sorting. Light-coloured ooids, well-rounded quartz sand and well-rounded pebbles are the major clastic components of the grainstones. Pronounced medium-scale cross-stratification, commonly of the herringbone or chevron type occurs, indicating a strong tidal influence on sedimentation (Conybeare and Crook,

Fig. 2. Channel cutting pikt of a thick stromatolite bed within the grainstone sequence. Doming of the stromatolites may be seen at the right centre of 'the photograph. Note the rounded pebbles near the hammer in the channel bottom. Fig. 3. Domed biostromal stromatolite bed within the grainstone sequence.


569

1968). Most upper bedding surfaces on the ooid grainstones show welldeveloped symmetrical ripple marks. Farther east, as the grainstone beds thin into the dominantly mudstone sequence, non-carbonate lithoclasts disappear and ooids become very dark in colour. Here the grainstones are poorly sorted and commonly merge at their easternmost extremities into packstones and wackestones. Grainstone beds are usually overlain by thick units of columnar stromatolites. Clasts within these stromatolite beds show a similar decrease in grain size and non-carbonate component to the east, as described above for the underlying grainstones. Channels partly filled with well-rounded lithoclasts intersect the stromatolite beds t o the west (see Fig. 2). This sequence of grainstones and stromatolites is considered to represent shallow, highenergy marine deposition, probably adjacent to an exposed headland of diapiric material. Stromatolite beds in the west are typically overlain by a sequence of paleyellow dolomitic mudstones, minor flat-pebble conglomerates, calcareous mudstones and thin irregularly bedded but well-sorted quartz sandstones, which locally fill welldeveloped desiccation cracks in t h e dolomitic mudstones. The shallow and restricted conditions suggested by this sequence may have been the result of shoaling due to local stromatolite growth under stable environmental conditions. Continued stability and lowenergy conditions are indicated by calcareous mudstone sedimentation for brief periods, even in the westernmost parts of the sequence, prior to renewed grainstone deposition. STROMATOLITES

Thickness and macrostruct ure The thickest units of columnar stromatolites occur within the western grainstone sequence, where stromatolitic beds are up to 10 m thick. Most, however, have a maximum thickness in the order of 1-1.5m and show gradual thinning t o the east, where the underlying grainstone lenses thin into calcareous mudstones. Stromatolite beds to the west generally have planar basal contacts with underlying grainstones. Stromatolite biostromes with relief of up to 25 cm (Fig. 3) occur adjacent to the diapir, and these pass gradually into tabular stromatolite beds to the east. There is a corresponding change within the bed from radiating t o erect columns. In those cases where stromatolites develop on rippled ooid grainstone surfaces, stromatolite elongation is perpendicular to the ripple crest, indicating common current directions for both structures (see Fig. 4). Beds of columnar stromatolites within the mudstone sequence are more lenticular and thinner than their counterparts to the west. Maximum

570

e

P.G. HASLETT

CURRENT DIRECTIONS Measured from a. Stromatolite bioherrn elongation in .open tidal flat.

n=9

b. Current ripples

open tidal flat.

C.

Stromatolite bioherm elongation in restricted tidal flat. n = 17

Fig. 4. Current directions from open and sheltered intertidal areas.

thickness of the beds does not exceed 2m, and the relief of constituent domed stromatolites is less than 10cm. Columnar stromatolites are invariably associated with flat-pebble conglomerate beds, and the maximum thickness of stromatolites appears to occur where conglomerate lenses thicken. In many places both stromatolites and flat-pebble conglomerates are cut by channels. The columnar stromatolites may be seen to pass laterally into domeshaped stromatolites and algal laminites.

Current directions A scarcity of bedding-plane exposures limits the number of measurements of stromatolite elongation that can be taken. A plot of the readings that have been obtained, however, shows good agreement within particular beds (Fig. 4). Current directions measured from stromatolites in the western part of the sequence, also show good agreement with those measured from ripple marks in associated grainstones. Stromatolites in the eastern part of the Fig. 5. Lower Cambrian lithofacies map and diagrammatic cross-section north of the Wirrealpa Diapir.


575

sequence do not show such marked elongation, and give a wider spread of current directions. Some biohermal stromatolite beds in the eastern sequence show weak asymmetric growth, similar t o that reported by Hoffman (1967). The asymmetry indicates a supply of carbonate mud from the northwest.

Lateral changes in stromatolites In mapping the nature and distribution of stromatolites in the sequence, all beds were traced laterally and their characteristics noted. More detailed work was concentrated on two beds in the lower, more markedly cyclic part of the sequence. The two beds were selected for their good exposure and the apparent wide range of sedimentary environments which are laterally represented. Details of differences in stromatolite morphology from west to east can be most easily described with reference to representative samples A-K as marked on the diagrammatic cross-section (Fig. 5). Some very large allochthonous boulders derived from the diapir form sites for growth of columnar stromatolites (A). These boulders, which presumably remained immobile after initial deposition, have their topmost parts encrusted by stromatolites in layers up to 20cm thick. The stromatolites are slender, erect, cylindrical, close-spaced and about 5 mm in diameter. Rare branching is dendroid to anastomosed (Hofmann, 1969a). Laminae tend to be diffuse, slightly wavy and convex, with low relief. Wall structure is poorly developed. Intercolumnar sediment is up to 2mm in grain size, but most sediment within the columns is less than 0.3 mm. Stromatolites are not generally well developed within the coarse lithoclast grainstones between points A and B; however, minor crusts cap some boulders, and detached thumb-sized columns also occur. At point B, stromatolite beds become well developed. The stromatolite columns tend to be radiate and constringed, with ragged margins. Anastomosing of columns is common, although furcate branching also occurs. The average diameter of individual columns is about 5 mm and most columns tend t o be short. Thin-section examination reveals diffuse to lumpy laminations, with a moderate to low degree of inheritance. Some laminae are wavy and convex with a very slight tendency t o lap over column margins. Microstylolitization has removed many column edges. Intercolumnar lithoclasts are up to 2cm in size, whereas material within the columns is usually less than 2 mm in grain size. Stromatolites of the type which occur at B have a very notable feature about their microstructure. Within columns, generally at the same level from column to column, irregular patches of micrite and pseudospar occur. Rare cellular structures similar to Renalcis (Johnson, 1966, p. 25) may be found (Fig. 6). Other globular, branched areas of pseudospar are also apparent within the columns (Fig. 7). Branching of these areas is invariably divergent

576

P.G. HASLETT

Fig. 6. Preserved Renalcis-like algal remains from stromatolite in the grainstone sequence. (Thin section JW 10.) Fig. 7 . Globular patches of pseudospar within a stromatolite column in the grainstone sequence. Note the tendency of the patches to branch upward. (Thin section JW 10.)

upwards. Although preservation is f a r from perfect, it appears that algal species capable of carbonate precipitation have at times played a part in the development of these stromatolites. Some distance farther east (C),lithoclasts seldom exceed 1cm in diameter. Stromatolites are similar to those described above, being short, highly constringed columns with common anastomosing and dendroid branching. Laminations are extremely irregular and bulbous, with low inheritance. Stromatolites at point D are associated with well-sorted grainstones which only rarely contain large lithoclasts. The erect cylindrical columns, some strongly constringed, may be up to 1.5cm in diameter. The columns are close-spaced, with anastornosed to umbellate branching. Wall structures are strongly developed and commonly show intense black pigmentation (Fig. 8). The diffuse to lumpy laminations show high relief. A basic difference appears to exist between the irregular but relatively continuous laminae in the central parts of the columns, and the clotted irregular fabric of the pigmented areas. The pigmented areas sometimes form laminae but are mainly concentrated along column walls (see Fig. 8). As the stromatolite beds thin to the east, and associated grainstones lens out into mudstones and flat-pebble conglomerates, the stromatolite columns generally become broader in cross-section ( E ) . Branching of the erect to decumbent columns is usually umbellate. Laminations show diffuse to clotted textures with low to moderate inheritance. As seen in profile, the


577

Fig. 8 . Stromatolite with highly pigmented column margins, grainstone sequence. (Rock slab NW 30.) Fig. 9. Broad columnar stromatolite with associated flat-pebble conglomerates from the mudstone sequence. Arrows indicate irregular laminations which may be traced from one column to the next. (Thin section JW4.)

laminae are convex t o penecinct with moderate t o high relief. Large flat mudstone pebbles which appear within the stromatolite beds often cap columns and appear to have prevented continued growth. They subsequently became colonized and acted as a base for new columns. The columnar stromatolite beds are in general overlain by irregular stratiform stromatolites. At point F, squat columnar stromatolites are closely associated with flatpebble conglomerates within the mudstone sequence (Fig. 9). Columns are 2-5 cm in breadth, and are cylindrical to turbinate, with furcate branching. The erect columns show distinct, even laminations which are convex and have low relief. Partial linking of the close-spaced columns occurs locally. Several thick irregular layers (Fig. 9) contrast with the more normal thin, regular and strongly inherited laminations. The irregular layers are readily traceable from one column to the next. Stromatolites at G are dominantly stratiform, but show a strong similarity to the broad columnar forms with respect to their laminations. The porous, irregular layers are thick, and loaf-shaped. The high degree of inheritan& in overlying regular laminations means that many irregularities are reproduced with subsequent stromatolite growth. Some irregularities soon disappear, but some are enhanced by further growth, and furcate branching may result. To the east, cryptalgalaminites and nodular stromatolites occur. Columnar stromatolites are associated with flat-pebble conglomerate beds, particularly

578

P.G. HASLETT

Fig. 10. Walled columnar stromatolite from the mudstone sequence, showing spar-filled cavities within columns. (Rock slab JW 30.)

in areas where the conglomerate beds are thickened. Some columnar stromatolites which have grown on flat-pebble conglomerates have fine carbonate mudstone between and above the columns (H). These stromatolites are upright and slender, with diffuse laminations but welldeveloped, highly pigmented wall structures (Fig. 10). Large irregular sparand sediment-filled cavities are generally found within the columns. The cavities show radial shrinkage cracks and probably formed during periods of subaerial exposure. Another type of columnar stromatolite forms low, discrete bioherms (K ). Digitate branching of the close-spaced cylindrical columns is common. Laminae are convex, regular to wavy, and have low relief. Walls are not well developed. Intercolumnar sediment consists of carbonate mud, and of carbonate intraclasts up to 5 mm in length. Columns usually coalesce at the tops of the bioherms and pass into algal laminites. Many columnar stromatolites associated with the thickest parts of flatpebble conglomerate beds (I)have grown from oncolites which have finally come to rest (Fig. 11).Broad columnar stromatolites, upon umbellate or furcate branching, form numerous smaller constringed columns. The branching typically occurs at particular levels throughout the stromatolite. Laminae may be planar or low-relief convex, and are markedly down-curved on column margins. Most laminations are regular and somewhat diffuse, but thick irregular clotted layers with considerably higher relief and lower inheritance than the more normal laminations occur at certain intervals. These irregular layers are similar to those described above (Fand G) and are traceable from one column to the next. Textures of the irregular layers are fairly well preserved, and are somewhat similar to those found in Madigunites mawsoni, and described by Walter (1972a) as a vermiform microstructure. Stromatolites which grow on the edges of steep-sided channels which


579

Fig. 11. Branched columnar stromatolite from the mudstone sequence, showing irregular porous laminations within parts of the columns. (Thin section JW 19.) Fig. 12. Stromatolite from the margin of a tidal channel within the mudstone sequence. The effect of rare irregular laminae on the shape of later laminations with high inheritance may be seen. (Thin section JW 25.)

intersect flat-pebble conglomerate beds (J),have formed adjacent to, but at a topographically lower level than, those just described (I).They consist of cylindrical t o turbinate close-spaced columns which show digitate branching and distinct convex to geniculate laminations (Fig. 12). The regular laminae have a moderate t o high relief and generally show a moderate degree of inheritance. Lenticular bands with an irregular clotted structure also occur within these stromatolites. They tend t o have higher relief than normal regular laminae and cause irregularities in the column structure. The irregular layers correspond in adjacent columns, and commonly occur at levels of column branching. DISCUSSION

Workers on Recent stromatolites (Logan, 1961; Monty, 1965b, 1967; Gebelein, 1969) have recognised that the characteristics of a particular stromatolite are moulded by complex interrelationships between physical, chemical and biological aspects of the environment in which that stromatolite forms. Recent stromatolites have been shown to form by the binding, trapping or precipitation of sediment, mainly by communities of blue-green algae. It is only recently that the biological complexity and

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environmental sensitivity of such algal communities has been really appreciated (Carr and Whitton, 1973). There still exists, however, a polazation af views on whether the major factors that determine stromatolite morphology are biological (Cloud and Semikhatov, 1969b; Walter, 1972a) or physico-chemical (Logan et al., 1964; Hofmann, 1969a). The contentious points, which bear on the validity of stromatolite biostratigraphy , have been summarized by several different workers (Hofmann, 1969a; Cloud and Semikhatov, 196913; Walter, 1972a), and will not be repeated here. At Old Wirrealpa, stromatolite morphology appears to be related to lithofacies distribution. Most variation in form occurs laterally, with little or no vertical change within equivalent lithofacies. Precise correlation between stromatolite morphology and lateral changes in physical, chemical or biological factors is difficult to make in ancient sequences. At Old Wirrealpa no preserved organic matter has been found, chemical factors such as pH and salinity are indeterminable, and even physical factors are difficult to assess, due t o outcrop limitations and diagenesis. Despite these difficulties, it is possible to make some tentative environmental analogies with modern stromatolites.

Mechanisms of growth Algal stromatolites at Old Wirrealpa appear to have grown predominantly by the trapping and binding of sediment. This is most obvious in the western sequence where large amounts of non-carbonate sand and silt occur as layers within stromatolite columns. The ability of algae to trap and bind sediment in modem environments has been found to depend on factors such as current velocity, sediment size and supply rate, and the species of algae involved (Gebelein, 1969). Stromatolites do not occur in the bottom of channels at Old Wirrealpa, where energy levels and abrasive action must have been high. Columnar stromatolites which encrust large boulders in the western sequence appear to have formed under high-energy conditions but would, because of their elevation, have been less subject to abrasion. Carbonate precipitation appears to have contributed minor amounts t o stromatolite growth, and although evidence is scarce, much of the carbonate precipitation appears t o have taken place under algal influence. Clotted, highly pigmented and cellular structures of possible organic origin have been observed within columnar stromatolites associated with clastic grainstones (see p. 575 and Figs. 6 and 7). Such bands of apparent precipitation may be traced from column to column at approximately equivalent levels. Although best devebped on column margins, some bands may be traced from within columns, down margins and as bridges across t o adjacent columns. The thickening of these pigmented bands on the vertical column margins in contrast to the more normal laminations, which are thickest on column tops,


581

suggests that dominant growth was by precipitation. An almost total absence of noncarbonate detritus from within these bands, even in the western sequence, indicates that their development was associated with low current velocities and low sediment-supply rates. It may be that such layers are ancient analogues of calcified layers found in Recent algal mats, correlated by Monty (1967) with periods of emergence. Similarly, the irregular sparry layers described from well-laminated columnar stromatolites within the mudstone sequence may also be the result of carbonate precipitation. Most of these once very porous bands occur on column tops (see Fig. ll), but some form continuous dome-shaped layers in stratiform and nodular algal limestones. The greatest thickness and frequency of these layers seems t o occur in columnar stromatolites from slightly more elevated parts of the tidal flat (compare samples I and J, Figs. 11 and 12). These layers may represent growth of different algal species, or a reaction of certain species to a period of subaerial exposure.

Effectso f sediment grain size Despite the ill-sorted nature of sediment supplied t o stromatolites in the western part of the sequence a t Old Wirrealpa, trapped and bound material within the columns is restricted to sand- and mud-sized grains. Detrital material caught between columns commonly contains larger well-rounded pebbles. Persistent agitation in this environment probably caused such pebbles to be dumped on growing algal mats at various times, terminating growth in these positions. The size of such pebbles, however, would have reduced their chances of being bound, and further agitation could have removed them and allowed renewed growth upon columns. Such a mechanism probably played an important role in the development of irregular laminations and to some extent may have influenced column branching in this part of the sequence (see p. 583). Columnar stromatolites to the east are composed of very fine calcareous muds, with minor fine quartz sand and silt. Sediment between columns is usually of equivalent grain size, but may consist of intraclast chips, coarser quartz sand and occasional larger flat conglomerate pebbles which were trapped edgewise between columns. The periodic incursion of high-energy conditions into these sheltered environments, probably during storms, has resulted in local accumulation of flat-pebble conglomerates upon the stromatolite beds. Unlike their grain-size equivalents to the west, these pebbles appear to have been seldom shifted after initial deposition. Their flattened shape and the relatively rare incursion of high-energy conditions into this environment probably account for this. Depending on the thickness of the flat-pebble conglomerate beds, columnar stromatolite growth may have been effectively stopped. The selective trapping and binding of finer sediment fractions by algae, as

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illustrated by these Cambrian stromatolites, has been reported in studies of Recent environments (e.g., Black, 1933; Gebelein, 1969). The greater range of grain sizes within columns in the western part of the sequence at Old Wirrealpa, compared with that to the east, probably reflects t o some degree the greater range available in the west, due to a poorly sorted supply. The possibility that this wider range of grain sizes might be contributed to by differences in the nature of algal communities living in the two environments cannot be discounted.

Stromatolite laminae As a general rule, stromatolite laminae in the carbonate mudstone lithofacies show far greater regularity than those associated with grainstones in the western sequence. Columnar stromatolites within the mudstones have finer laminae of more uniform thickness and with a greater degree of inheritance than those t o the west. Cryptalgalaminites and domed stromatolites have even finer and more uniform laminations than columnar varieties. As has been reported by Gebelein (1969) for some stromatolites of Bermuda, high degrees of lamina regularity may be related to areas of lower rates of sediment supply. As discussed above, other factors such as the degree of sorting of supplied sediment, and the nature of the actual algae, may also influence the regularity of the laminations. Stromatolites within the mudstone sequence commonly show down-folding of laminae at column margins to form walls. Such walls are usually best developed where intercolumnar sediment is fine grained. Broad, squat columnar types have betterdeveloped walls than the slender columnar varieties. The irregular nature of laminations within stromatolites in the western part of the sequence may be partly due to large pebbles temporarily preventing growth (see above, p. 581) but erosion may also play a part in this irregularity. The thickness of individual laminae varies markedly from one lamina to another, and within a single lamina there may be pronounced thickness variation. Wall structures such as described above are rare, the column margins usually being ragged. An exception occurs, however, in that some column peripheries have clotted and highly pigmented margins of what is interpreted to have been organically precipitated carbonate. These margins differ from the dominantly laminated central portion of the column, and are probably best described as mantles (Raaben, 1964: see Hofmann, 1969a, p. 18).

Distribution of columnar stromatolites All stromatolites within the grainstone sequence at Old Wirrealpa are columnar. Columnar stromatolites in the mudstone sequence only occur in close association with flat-pebble conglomerate beds, which appear to have


583

accumulated in areas which were slight topographic depressions on the sheltered tidal flats. The growth of columns and domes in present-day stromatolites is due, fundamentally, to enhanced stromatolite growth in certain parts of a mat and/or limited growth in adjacent parts. Substrate irregularities are believed to initiate column growth in certain instances (Logan, 1961; Walter, 1972a). Continued column development may take place due to the restriction of active mat growth to column tops and sides, and the retardation of growth between columns by such factors as excessive wetting, mechanical erosion, and very heavy sedimentation (Logan et al., 1964). Monty (1967) suggests that initiation and growth of columns may be related t o the growth of certain distinct algal species. In areas with high rates of sediment supply Gebelein (1969) has reported nodular, rather than flat laminar, stromatolite growth. Cambrian columnar stromatolites at Old Wirrealpa do not necessarily form on very irregular substrates, and some conglomerate beds are overlain by only slightly domed forms. The columnar stromatolites appear to have grown most prolifically in areas where sediment of suitable size for dgal trapping and binding was supplied at a high rate, mainly as a consequence of topographic position on the very shallow tidal flats. Judging by the greater frequency of desiccation mud cracks in those parts of the sequence in which flat-laminated and domed stromatolites are found, these stromatolites appear to have formed in even shallower areas, where sediment-supply rates might be expected to have been very low. Thus, sedimentation rate may well have been an important contributing factor in the distribution of columnar stromatolites within the Cambrian sequence. Branching Much of the above discussion of columnar-stromatolite distribution is also relevant to stromatolite branching. All of the columnar stromatolites show branching of one type or another. Stromatolites from within the western or grainstone sequence usually exhibit markedly divergent types of branching. This branching tends to occur a t irregular intervals during column development. Growth constraint (Hofmann, 1969a) caused by pebbles resting on active algal mats was probably the most common cause of initial branching of stromatolites in the western grainstone sequence. However, the low inheritance of laminae in these stromatolites means that there is a fairly small chance that any single growth constraint will give rise to branching, without the largely fortuitous intervention of other environmental factors. In most instances constraints in lamina growth are “evened out” by succeeding laminae. In contrast, the branching of columnar stromatolites in the mudstone sequence is mostly parallel to slightly divergent. Columns within a particular

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bed commonly branch a t about the same level, and their structure, to use the term of Walter (1972a), appears to “anticipate” branching. The regularity and high inheritance shown by column laminae means that any irregularities in the structure are reproduced for a considerable time by successive laminar accretions. These variations are frequently enhanced by other environmental factors, such as erosion or preferential deposition in hollows, and distinct branches result. Irregular bands of probable precipitative origin may provide the necessary irregularity within stromatolite columns to enable this sort of branching to begin (see Fig. 11). SUMMARY AND CONCLUSIONS

Lower Cambrian algal stromatolites at Old Wirrealpa formed in open to protected intertidal environments. The nature, sorting and rate of supply of sediment to the growing stromatolites varied markedly from one environment to the next. Stromatolite growth took place predominantly by trapping and binding of the finer sediment fractions, although minor distinct intervals of carbonate precipitation also occurred. These intervals commonly show an irregular black pigmentation and contain poorly preserved cellular structures. Columnar stromatolites formed where rates of supply of finer sediment were relatively high. Cryptalgalaminites and domed stromatolites developed in upper intertidal areas where sediment supply was lower. Columnar stromatolites which grew on open tidal flats show irregular laminae and poorly developed wall structures. The slender, ,commonly constringed, columns branch at irregular intervals, and the branches tend to be divergent. Columnar stromatolites from sheltered tidal flat environments are usually regularly laminated, with individual laminae showing high inheritance. Wall structures are commonly developed by lamina downfolding on column margins. The columns, most of which are closely spaced, have parallel branching which often occurs at distinct levels within the stromatolite. ACKNOWLEDGEMENTS

The author wishes to acknowledge the expert assistance of the School of Surveying, South Australian Institute of Technology, in providing map control over the study area. Financial support for this contribution by the Centre for Precambrian Research, University of Adelaide, is also appreciated. Finally, the author thanks B. Daily, J.A. Donaldson and W.V. Preiss for their helpful criticisms of this paper.


Chapter 10.6

STROMATOLITES FROM THE MIDDLE PROTEROZOIC ALTYN LIMESTONE, BELT SUPERGROUP, GLACIER NATIONAL PARK, MONTANA Robert J. Horodyski

INTRODUCTION

The Belt Supergroup, a thick Middle Proterozoic sedimentary sequence consisting of argillaceous, arenaceous, and calcareous rocks, extends from western Montana and northern Idaho into adjacent parts of Alberta, British Columbia, and Washington state (Ross, 1970;Harrison, 1972). The section exposed at Glacier National Park is near the eastern margin of the Beltian depositional basin; it is very well exposed, structurally simple, essentially unmetamorphosed, and contains an abundant and diverse assemblage of stromatolites. The Altyn Limestone (actually a dolostone) is of restricted distribution, occurring only in a narrow outcrop band which immediately overlies the Lewis Overthrust on the east side of Glacier National Park and an adjacent part of Canada (Ross, 1959). The interval exposed in Glacier National Park attains a thickness of several hundred meters and is composed predominantly of silty and sandy dolostone. Although rocks of the Altyn Limestone have not been dated radiometrically, dates are available for other units of the Belt Supergroup and for the underlying metamorphic basement. These data indicate that the Altyn Limestone is approximately 1.3 ? 0.2 billion years in age (see Harrison, 1972). Stromatolites have long been known from the Altyn Limestone in Glacier National Park, having been described previously by Fenton and Fenton (1931,1937), Rezak (1957),and White (1970,197313). The stromatolites are particularly well exposed on the southeast side of Appekunny Mountain immediately east of Appekunny Falls. The stromatolite beds range from several decimeters to about 2 m in thickness; locally, several of these beds are superimposed to form composite stromatolitic units several meters thick. Individual stromatolites are columnar, are commonly branched, and are composed of silty dolomite.

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DEPOSITIONAL ENVIRONMENT

A 26 m thick interval, representative of the stromatolite-bearing portion of the Altyn Limestone at Appekunny Falls, was selected for detailed study. Two lithologic types predominate in this interval: (1)medium-bedded sandy dolarenites, forming meter-thick units which are overlain by (2) thinly interbedded silty dololutite and rippled sandy dolarenite, occurring as 1-2m thick units. In the second lithologic type, interbedding of rippled coarsergrained sediments with non-rippled finer-grained sediments resembles the “wavy bedding” of Reineck and Wunderlich (1968), suggesting a tidally influenced depositional environment, probably a tidal flat. The underlying medium-bedded dolarenite units have curved to slightly undulatory lower erosional surfaces, contain sparce centimeter-sized dolostone intraclasts near their base, and commonly display rather poorly developed, low-angle, planar cross-stratification. Although the precise depositional setting for these lower sands is not known, they evidently were deposited under somewhat higherenergy conditions and in deeper water than the overlying thinly interbedded dololutites and dolarenites. Deposition as intertidal sand bars seems unlikely because they lack the reactivation surfaces and type of cross-stratification characteristic of such deposits (see Klein, 1970). The curved erosional surfaces and rapid lateral thickness changes exhibited by some of these dolarenites suggest deposition within laterally migrating tidal channels cut to depths of about 0.5-2m. However, the geometry of other dolarenite beds in the unit suggests that deposition also might have occurred on an undulatory, rather than channeled, surface in the shallow subtidal zone. Centimeter-sized dolostone intraclasts and medium sand- to gknule-sized quartz grains between stromatolite columns indicate that relatively highenergy conditions prevailed during stromatolite growth. As is shown in Fig. 1, the stromatolite beds occupy the same general position as the mediumbedded dolarenites: they typically overlie a sandy dolarenite bed and underlie thinly interbedded dololutite and dolarenite. This relationship indicates that they formed at water depths greater than that of the thinly interbedded dololutite and dolarenite; and, because shrinkage features in the overlying dolarenites are rare (and are not necessarily the result of subaerial desiccation), it seems likely that the stromatolites were generally, or perhaps continuously, submerged during growth. Irregular erosional surfaces typically are present at the base of the stromatolite beds. These surfaces have somewhat greater relief than those associated with the dolarenite beds (e.g., one erosional surface beneath a stromatolite bed exhibits a relief of 80cm throughout a lateral distance of several meters); some of the stromatolites may therefore have formed in tidal channels. The markedly streamlined shape of some Altyn stromatolites, evidently the result of current rather than wave activity, further suggests a tidal channel environment for at least some of the stromatolites.

STROMATOLITES IN BASIN ANALYSIS SANDY

587

DOL AR E NITE

-erosional

surface

INTERBEDDED RIPPLED SANDY DOLARENITE AND NONRIPPLED S ILT Y DOLOLUTIT E

p?( #@, a

-gradational

contact

STROMATOLITE BED WITH DOLOSTONE INTRACLASTS AT BASE

erosional surface SANDY DOLARENITE

-erosional

surface

INTERBEDDED RIPPLED SANDY DOLARENITE AND 'NONRIPPLED SILTY DOLOLUTITE

Fig. 1. Diagrammatic representation of a stromatolite bed and adjacent strata in the Altyn Limestone. LAMINAR STRUCTURE

Stromatolites in the Altyn Limestone are composed almost entirely of two types of laminae (Fig. 2A); compositionally, they are thus much simpler than stromatolites which occur in other formations of the Belt Supergroup (e.g., eight types of laminae have been recognized in stromatolites from the lower Missoula Group in Glacier National Park: Horodyski, 1975). The two main types of laminae in the Altyn stromatolites are compositionally similar, both being composed primarily of dolomite pseudospar. They differ principally in texture, one being finely (15-60 pm) crystalline, and the other very finely (5 - 15pm) crystalline. Laminae of the coarser variety are typically smooth and continuous, range from 0.04 to 5mm in thickness. and, as shown in Fig. 2B, they generally exhibit fining-upward size grading. Medium silt- to very fine sand-sized detrital quartz and feldspar compose 4-15% of these laminae, indicating that the accumulation of detrital particles played a role in their formation. In fact, the importance of detrital sedimentation was probably greater than suggested by the abundance of terrigenous detritus, because submillimetersized dolostone intraclasts are abundant constituents of the Altyn sediments. Due to their small size, such intraclasts are difficult to detect in the stromatolitic laminae; however, an estimate of their abundance may be made from the somewhat coarser-grained dolarenites which form a major part of

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Fig. 2. Lamination of Altyn stromatolites. A. Transmitted light photograph of a thin section of a stromatolite showing very finely crystalline precipitated laminae (p = darker laminae) and finely crystalline, organically stabilized detrital laminae (s = lighter laminae). B. Enlargement of a portion of A showing precipitated laminae (p),organically stabilized detrital laminae (s), and mixed precipitated and organically stabilized detrital laminae ( m ). C. Photomicrograph of a thin section of a sandy dolarenile illustrating the predominance of sand-sized dolostone intraclasts (i) over terrigenous detritus (t). D. Transmitted-light photograph of a stromatolite thin section showing interlayering of organic films (darker layers) and calcareous laminae. The rosette-like crystals are pseudomorphs after barite.


589

the Altyn Limestone. In the very fine and fine sand-sized dolarenites the ratio of detrital quartz and feldspar to dolostone intraclasts of approximate hydraulic size equivalence is about 1 : l O (see Fig. 2C). Thus, it appears that the coarser-grained laminae in the Altyn stromatolites formed chiefly by the accumulation of detrital particles (both autochthonous and allochthonous). In situ carbonate precipitation in laminae of this type was probably of secondary volumetric importance ; however, carbonate cementation might have been important in strengthening the growing stromatolite. That this lamina variety accumulated through organic stabilization (viz., by either trapping or agglutination) of sedimentary particles, rather than solely through the physical accumulation of sedimentary particles, is demonstrated by the laminae maintaining a fairly uniform thickness as they pass from the horizontal portions to the steeply inclined sides of stromatolitic growth surfaces. Laminae of the finer-grainedvariety (Fig. 2B) range from 0.01 to 1mm in thickness, are somewhat wavy and discontinuous, and commonly occur in couplets with the coarser lamina type. These finer laminae generally have sharp rather than gradational contacts with underlying comer laminae (see Fig. 2B), suggesting that the two lamina types might have been deposited in somewhat differing manners. This possibility is supported by the observation that in such couplets the thickness of the upper finer-grained laminae is not directly related to the thickness of the lower coarser-grained laminae (see Fig. 2A). The finer-grained laminae could have been produced either by trapping or agglutination of a very fine-grained carbonate detritus in an organic mat, or by in situ carbonate precipitation within such a mat. It should be possible to distinguish between these alternatives by comparing the abundance of terrigenous material in these laminae with that of texturally equivalent, physically deposited sediment occurring in associated strata. Specifically, if these stromatolitic laminae resulted from the accumulation of very fine-grained particles, they should contain about the same proportion of terrigenous material as texturally equivalent, physically deposited sediment; on the other hand, if these laminae originated largely as a result of in situ precipitation, they should be relatively deficient in terrigenous material. Scanning electron microscopy of etched slabs and optical microscopy of petrographic thin sections revealed the finer stromatolitic laminae to be deficient in terrigenous material relative to texturally equivalent, physically deposited sediment. Thus, it seems evident that in situ precipitation was the major process resulting in formation of the finer-grained lamina variety. Stromatolitic laminae texturally intermediate between these two distinct lamina types are also of common occurrence. Such intermediate types typically contain less terrigenous material than the coarser-grained laminae, and more than that occurring in the finer-grained laminae. This suggests that E. Quartz and hyalophane pseudomorphs after barite, obtained by dissolving part of a calcareous stromatolite in hydrochloric acid.

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they probably formed through a combination of both sediment stabilization and in situ carbonate precipitation. Wavy 1-5pm thick organic films were detected in the upper portion of one stromatolite bed (Fig. 2D). These films probably represent sedimentpoor organic mats, subsequently compressed during diagenesis. Associated with these organic films are pseudomorphs after barite (Figs. 2D, E), presently composed largely of quartz and a lesser amount of the barium-rich feldspar hyalophane. (Mineralogic composition was determined by X-ray diffraction and an approximate chemical composition, Ko.sB%.z(Al,Si),OS, was determined by electron microprobe analysis.) The association of barite with stromatolites of both the Altyn Limestone and lower Missoula Group (Horodyski, 1975) of the Belt Supergroup, and its apparent absence in associated non-stromatolitic sediments, suggests that barium might have been .concentrated by organisms of the stromatolitic microbiotas and precipitated as barium sulfate during diagenesis. ORIGIN OF THE CARBONATE

The origin of the carbonate occurring in both stromatolites and sediments of the Altyn Limestone is a subject of major importance in understanding the mode of formation of the Altyn stromatolites. As previously discussed, the finer-grained stromatolitic laminae appear to have been produced by in situ carbonate precipitation within an organic mat; however, these laminae account for only about 20% of the total volume of the stromatolites. The carbonate occurring in these stromatolites seems to have been derived mainly from the accumulation of submillimeter-sized carbonate intraclasts. Such intraclasts are also a major component of the associated dolarenites. The composition and texture of these intraclasts closely resembles that of the finer-grained (precipitated) stromatolitic laminae and, being relatively deficient in terrigenous material, differs from that of the finer-grained, physically deposited associated sediments; thus, it seems likely that the intraclasts in both the dolarenites and stromatolites of the Altyn Limestone were derived mainly by mechanical fragmentation of carbonate laminae, originally precipitated within algal mats. STROMATOLITIC MACROSTRUCTURE

The Altyn stromatolites occur in discrete beds ranging from several decimeters to about 2 m in thickness. Some beds are distinctly pod-shaped and pinch out laterally; others extend continuously for several tens of meters (and possibly much further). Three varieties .of macrostructure occur; all stromatolites within a single bed exhibit similar macrostructural attributes.


591

Fig. 3. Two sets of stereophotographs of a single model of a somewhat divergently branched columnar stromatolite from the Altyn Limestone. This model was made by cutting serial sections through a stromatolite specimen, cutting cardboard templates of the column shape for each section, assembling these templates in their true positions, and filling open spaces with modelling compound. Representative laminae are outlined and part of a column was removed from the front of the model in the lower photographs. Bar for scale is 5 cm.

592

R.J. HORODYSKI


593

T w o of these macrostructural varieties exhibit columnar branching; both types are composed of 4-20cm diameter, 0.1-2m high columns that are subcircular, oblong, or lobate in transverse section (Fig. 3). The two forms differ in branching pattern, one exhibiting markedly divergent branches (Figs. 4A, 5A), and the other being much less divergently branched (Fig. 5B). The columns of the former type developed under conditions of openspace packing on broad surface irregularities standing several decimeters above the surrounding depositional surface. The columns of the less divergently branched variety developed under more closely spaced conditions giving rise to stromatolite beds that extend laterally for tens of meters or more (Fig. 4B). A third macrostructural variety also occurs. In contrast to the forms described above, the columns of this variety typically are unbranched, are elongate and smooth in transverse section, and commonly are. inclined. Column dimensions are similar to those of the two branched forms except that they are elongate in transverse sections, attaining lengths of several decimeters (or a few meters in some extremely elongate examples; Fig. 4C). Inclinations from the vertical of 30-45" are common (Fig. 4D),and inclinations locally exceed 60' (Fig. 5C). Stromatolites of this third macrostructural type are very closely spaced (Fig. 5C), occurring in beds that extend laterally over distances of at least a few tens of meters (Fig. 4E). Although the columns within a 10 m long segment of a single bed typically are inclined in the same direction, stromatolitic columns in overlying beds commonly are inclined in a different direction (Fig. 5C). In situ precipitated laminae and organically stabilized detrital laminae are present in these inclined elongate stromatolites; both types of laminae are asymmetric in thickness and shape. These laminae are similar in composition, grain-size of detrital constituents, and lamina thicknesses to those of the branched forms; they differ, however, in degree of uniformity and lateral continuity. The laminae of the inclined elongate stromatolites are smooth and continuous (Figs. 6A, B), whereas those of the branched columnar forms tend to be irregular and somewhat discontinuous (Fig. 6C).

Fig. 4. Altyn stromatolites in outcrop, near Appekunny Falls. A. Branched columnar stromatolites showing marked divergence of branches. These stromatolites occur as an isolated moundshaped structure in an intraformational conglomerate bed separated from underlying strata by an undulatory erosional surface (e). Columns and representative laminae are outlined. Scale in 5-cm divisions. B. A bed (5') composed of slightly divergently branched columnar stromatolites. C. Plan view of extremely elongate stromatolites. Scale in 5-cm divisions. D. Erect, branched, columnar stromatolites overlain by inclined, unbranched, highly elongated stromatolites (shown in plan view in C). Columns and representative laminae are outlined in parts of the outcrop. Scale in 5-cm divisions. E. A massive unit composed of eight superimposed stromatolite beds. Sketch of the stromatolites in part of this unit is shown in Fig. 5C.

594

R.J. HORODYSKI

I

c

L.

A

~

~

I

l m

I

J B r

l m

1

C

Fig. 5. Shape of representative Altyn stromatolites. Markedly divergently branched, columnar stromatolites (A); less divergently branched, columnar stromatolites (B); and inclined, generally unbranched, elongate stromatolites (C). Sketches were made from photographs of outcrops in which columns and representative laminae were outlined with a felt-tipped pen. Stippled areas indicate portions of the outcrop where detailed structure is not clear. FACTORS INFLUENCING STROMATOLITIC MACROSTRUCTURE

One of the most significant features of the Altyn stromatolites is that they clearly demonstrate the influence of physical conditions on the development of stromatolitic macrostructure. That the two branched columnar varieties of stromatolite were produced by the same (or very similar) microbiologic components is indicated by the close similarity of their microstructure and is consistent with their stratigraphic proximity, being separated by only 2.5 m of strata. Differences in the branching style in these two stromatolite types seems to reflect differing degrees of spacing during growth. The divergently branched forms grew as isolated mounds (Figs. 4A, 5A) elevated somewhat above the syrrounding substrate; a lack of lateral constraints on the developing stromatolitic columns permitted lateral expansion which resulted in their highly divergent branching pattern. Stromatolites with less divergently branched columns occur in beds that are laterally more extensive; this form developed under relatively closely spaced conditions (Fig. 4B, 5B). Lateral


595

Fig. 6.Laminar structure of Altyn stromatolites. A. Slabbed section of an unbranched elongate stromatolite. B. Thin section (transmitted-light photograph) of an unbranched elongate stromatolite. C. Thin section (transmitted-light photograph) oE a branched columnar stromatolite. D. Thin section (transmitted-light photograph) of part of an unbranched elongate stromatolite showing two erosional surfaces (e). Note the smooth, uniform lamination in the unbranched elongate forms (A, B, D) and the irregular, discontinuous lamination in the branched columnar form (C).

expansion of these stromatolites was inhibited by adjacent columns; the resultant stromatolites therefore developed only slightly divergent branching patterns. In addition, the shape of successive biohermal growth surfaces influenced the branching pattern of the constituent stromatolites. Develop-

596

R.J. HORODYSKI

ment of inclined branches seems to have been favored on the inclined sides of mound-shaped bioherms (Figs. 4A, 5A), whereas more erect branches developed on the generally horizontal growth surfaces of laterally extensive beds densely packed with stromatolites (Figs. 4B,5B). The.marked degree of elongation and streamlining exhibited by the third variety of Altyn stromatolite (Fig. 4C) was probably the result of unidirectional (or bipolar) currents flowing parallel to the direction of elongation (see Hoffman, 1967). However, other evidence of paleocurrent direction could not be obtained from any of these stromatolite beds; the stromatolites are so closely spaced that the beds are essentially devoid of physically deposited sediments and thus do not contain primary sedimentary structures of the type needed for paleocurrent analysis. Continuity of the stromatolitic columns and asymmetry of shape and thickness of the laminae indicate that the inclinations exhibited by these stromatolites are primary, rather than being the result of tilting or sliding of vertically growing stromatolites. Either increased biologic activity on one side of the stromatolites or an asymmetry in direction of sediment supply could account for these inclinations; however, the precise nature of the casual factors have not been identified. Effects of currents are also evident in individual stromatolitic laminae. The smooth and uniform laminae of the elongate forms (Figs. 6A, B) are probably the result of rather vigorous currents; the irregular and discontinuous laminae exhibited by the branched columnar stromatolites (Figs. 6C) apparently reflect relatively less intense current conditions. Stronger currents during growth of the elongate stromatolites are further suggested by the common occurrence of erosional features in these forms (Fig. 6D). SUMMARY

Stromatolites occurring in the Middle Proterozoic Altyn Limestone of Glacier National Park, Montana, are dolomitic, frequently branched, columnar structures that attain heights of several decimeters to about 2 m . Strata of the stromatolite-bearing portion of the Altyn Limestone appear to have been deposited in a tidally influenced setting, probably the lower portions of a tidal flat and adjacent subtidal areas, with the stromatolites having formed in a shallow subtidal environment. The Altyn stromatolites are composed largely of two types of laminae. One of these formed through the organic stabilization of submillimeter-sized dolostone intraclasts and detrital particles; the other appears to be the result of in situ carbonate precipitation, probably occurring within an algal mat. Millimeter- and submillimeter-sizeddolostone intraclasts are the major carbonate component of the Altyn sediments and stromatolites. The texture and composition of these intraclasts is similar to that occurring in the finergrained, apparently precipitated stromatolitic laminae, and differs from that


597

of the finer-grained, physically deposited Altyn sediments. It seems likely, therefore, that most of the carbonate in both the Altyn stromatolites and sediments was derived ultimately from carbonate precipitated within algal mats. Three macrostructural varieties of stromatolites have been identified; two of these are branched columnar forms and the third is an inclined form which is elongate in transverse section. Similarities in microstructure of these stromatolites, in conjunction with their stratigraphic proximity, suggest that the two branched forms, and probably also the elongate inclined forms, were produced by identical or very similar microbiotas. The macrostructure of the Altyn stromatolites was strongly influenced by the physical environment. The closeness of spacing of columns and the shape of successive growth surfaces of the entire stromatolitic bioherm were of major importance in determining the branching pattern of the columnar forms. Stromatolites growing on isolated mounds elevated somewhat above the surrounding substrate developed in to divergently branched forms. The lack of lateral constraint on the developing columns permitted lateral expansion and the development of a highly divergent branching pattern; in addition, the inclined sides of growth surfaces of the entire stromatoliti'c bioherm favored the development of inclined branches. Stromatolites growing under more closely spaced conditions and forming relatively more horizontal biohermal growth surfaces developed into less divergently branched forms. Lateral expansion of these stromatolites was inhibited by adjacent columns and the growth of relatively more erect branches may have been favored by the more nearly horizontal nature of the bioherm growth surface. The inclined elongate forms apparently developed under relatively strong current conditions, their elongate nature probably being a streamlining effect caused by unidirectional (or bipolar) currents; ACKNOWLEDGEMENTS

This study is part of a graduate research project undertaken at the University of California, Los Angeles. It was supervised by Prof. J.W. Schopf, to whom I am deeply indebted. I thank the National Park Service and the many rangers who provided valuable assistance. Thanks are also due to Prof. J.W. Schopf, Prof. J.A. Donaldson, and Dr. M.R. Walter for critically reading this manuscript. Financial support for this study was received from NASA Grant NGR 05-007-407 and NSF Grant GB-37257 (Systematics Biology Program), both awarded to Prof. Schopf,-and from Geological Society of America Penrose Grants.



Chapter 10.7 ENVIRONMENTAL DIVERSITY OF MIDDLE PRECAMBRIAN STROMATOLITES Paul Hoffman

INTRODUCTION

Stromatolites occur in a wide variety of facies -carbonate and noncarbonate, shallow and deep-water, marine and non-marine. Four formations are selected to document this variability. All are from the Middle Precambrian of the northwesternmost part of the Canadian Shield. (1)Stromatolite biostromes in the upper part of regressive cycles in the interior of a broad carbonate shelf (Rocknest Formation). (2) Stromatolite bioherms between tidal channels along the outer margin of a carbonate shelf (Taltheilei Formation). (3) Deep-water columnar stromatolites in terrigenous sediments of a foreshelf basin (McLean Formation). (4) Non-marine stromatolites at the distal ends of alluvial fanglomerates (Murky Formation). It is concluded that the mere presence of stromatolites is not in itself sufficient t o interpret environment of deposition, but that specific types of stromatolites may be environmentally diagnostic. Above all, stromatolites must be studied within their local lithologic and regional stratigraphic context, not in isolation. The following publications may be consulted for more detail: regional geologic setting (Hoffman, 1973b); Taltheilei and McLean Formations (Hoffman, 1974a); Rocknest Formation (Hoffman, 1976).

REGIONAL GEOLOGIC SETTING AND GEOCHRONOLOGY

The major Middle Precambrian tectonic elements of the northwestern part of the Canadian Shield (Fig. 1) are: (1) the Slave craton; (2) the intracratonic Athapuscow and Bathurst aulacogens; (3) the pericratonic Wopmay orogen. The facies transitions from the craton into the foreland of the orogen are shown in Fig. 2, and from the craton into the Athapuscow aulacogen in Fig. 3.

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P. HOFFMAN

Fig. 1. Major Early Proterozoic tectonic elements of the northwesternmost part of the Canadian Shield.

Ages of the four stromatolitic formations, based mainly on Rb-Sr isochrons of relevant intrusive rocks, are: (1)Rocknest Formation - between 1,865 and 2,200m.y.; (2) Taltheilei and McLean Formations -between 1,795 and 1,865m.y.; (3) Murky Formation .-between 1,300 and 1,865 m.y. Thus, all but the Murky Formation are of pre-Riphean age. The geochronology of the pre-Riphean formations is important because they contain certain columnar stromatolites with modes of branching typical of Middle and Upper Riphean stromatolites elsewhere (Raaben, 1969a). These pre-Riphean stromatolites have not been subjected to the rigorous serial sectioning required to elucidate their intricate detail, but the generalijies of their modes of branching are evident in the field (Fig. 4). STROMATOLITES OF A CARBONATESHELF INTERIOR

The Rocknest formation constitutes the upper part of the miogeoclinal shelf in the foreland of the Wopmay orogen (Fig. 2). Sedimentary cycles are

601

STROMATOLITES IN BASIN ANALYSIS TECTONliE FRONT

East

Wesi

I

I

2

3 km

4

5

1

1 ;

1

-

1-

slromatolilic shaly and cherty dolomite silty mudstone with turbidites

crossbedded orthoquartzite and quartz-pebblestone pillow basalt and basalt breccia

(BASEMENT UNKNOWN)

arkose and granite-pebbleslone

Fig. 2. Facies transitions from the Slave craton into the foreland fold belt of the Wopmay orogen (see Fig. 1 for location).

well developed, except along the outer margin of the shelf and in the foreshelf basin to the west. Stromatolite biostromes within individual cycles can be traced for tens of kilometers across, and hundreds of kilometers along the shelf. A typical cycle is shown in Fig. 6. There are nearly 200 such cycles, each

ATHAPUSCOW AULACOGEN

SLAVE PLATFORM NOR13

Q,

0

N

GROUP

CHRISTIE BA'

RTHEl

K*HO'btLLL*

lTHS SUBAERIAL BASALT

MSAN

ASH- AND LAPILLI.FA*LL TUFF

u

FlNE CROSSBEDDED OUARTZITE

0

RED CROSSBEDDED LITHWENITE GREYWACKE TURBlDlTES

~

B

R

E

D P L A T Y SILTSTONE

RED FISSIIE MUDSTONE

UNlON 1SL

m

ARKOSIC DOLOMITE

STROMATOCITIC DOLOMITE

-

STROMATOLITK. LIMESTONE

DIGITATE FENESTRAL LIMESTWE

@PLATY RHYTHMIC MIRLSTONE

REDCONCRETONARI MUDSTONE

DIGITATE MNUSTONE

GREEN TO BLACK MUDSTONE

RED FISSILE MARLSTONE

RED MUDSTONE WLTH SlROYATOLlTlC LIMESTONE OLISTOLITES

Fig. 3. Facies transitions from the Slave craton into the Athapuscow aulacogen (see Fig. 1 for location).

cd


603

Tungussiform

Kussieiiiform

Baicaiiform

Fig. 4. Modes of branching of columnar stromatolites (after Raaben, 1969),all of which occur in the Early Proterozoic.

2-20 m thick, in the formation as a whole. At the base of each cycle is a thin intraclast packstone bed, interpreted t o be the condensed record of transgression. The remainder of the cycle is regressive, and results from progradation of intertidal and supratidal dolomite over sublittoral marlstone. The upper part of each cycle is stromatolitic. In simple cycles, there is a vertical succession of stromatolites from laterally linked domes and columns of Omachtenia (Fig. 5a), 0.5 m in diameter, to flat-laminated and fenestrate sheets of Stratifera. The domes and columns tend to be elongate in plan and preferentially oriented, and are associated with edgewise flat pebbles and oncolites, and lenses of intraclast or ooid grainstone. The overlying stromatolite sheets have prone tabular oncolites and are more silicified. The vertical change in stromatolite morphology is interpreted to reflect decreasing wave action from lower to higher tidal flats. The upper parts of more complex cycles have, in addition to the stromatolites described above,.units of black cherty dolomite up to several meters thick. These units contain a different stromatolite assemblage, in which the following types are prominent: (1)Discrete smooth-walled columns of 2-5 cm diameter with a Gymnosoleniform (Raaben, 1969a) mode of branching (Fig. 5b). (2) Tiny digitate forms resembling Alenia (Fig. 5c), in which vertical filament moulds are preserved where silicified. (3)Closely spaced vertical columns, less than 1cm across, of Conophyton (Fig. 5d), forming low domal bioherms 1-3 m across. (4) Flat-laminated sheets, some with incipient Alenia-like growths, others with fenestrate or clotted (thrombolitic) fabrics. The black dolomite units have little evidence of turbulence and are interpreted as forming in stagnant ponds within or behind the main tidal flats. Although lacking cycles, the uppermost part of the formation in the shelf interior also has extensive stromatolite biostromes, including a thick columnar unit with a Tungussiform (Raaben, 1969a) mode of branching (Fig. 5e).

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P. HOFFMAN

Fig. 5. Stromatolites in the Rocknest Formation. a. Laterally linked domes. b . Columns with Gymnosoleniform mode of branching. c. Alenia-like form. d. Conophyton. e. Columns with Tungussiform mode of branching. The scale is divided in 3-cm intervals.

605


dark cherty stromatolitic dolomite (Alenia-I ike , Gymnosolen)

supratidal ponds

light cherty stromatolitic dolomite

intertidal sandflats

I

IColonnella. Omechtanie, Stratifera)

reg1

jive ~~

light intraclastic oolitic dolomite

sublittoral nearshore

I

beach lag

I

dark argillaceous dolomite

t .

transgressive I

m a s tic dolomite

Fig. 6. Typical cycle in the Rocknest Formation.

In conclusion, the shelf interior facies is characterized by extensive stromatolite biostromes occurring mainly in the upper parts of regressive cycles . STROMATOLITES OF A CARBONATE SHELF MARGIN

The Pethei Group (Fig. 7) undergoes a facies change from carbonate shelf over the Slave craton to foreshelf basin in the Athapuscow aulacogen. The outer margin of the shelf is sharply defined and well exposed in the Taltheilei Formation. In the shelf interior, the Taltheilei Formation consists of extensive biostromes of alternating, columnar and flat-laminated stromatolites. In the thickest columnar biostromes, there is a basal zone of Conophyton in which the columns are tilted toward the shelf interior. Above, there is a thin zone of Baicaliform (Raaben, 1969a) branched columns, which passes into a thick zone of Ornachtenia (Fig. 8a), in which the columns are strongly elongate parallel to paleocurrents and normal to the shelf edge (Fig. 8b). Toward the shelf margin, the columnar biostromes become thicker at the expense of the flat-laminated ones. Within 1-5 km of the outer edge of the shelf, the columnar biostromes reach 10-20 m in thickness and develop

P. HOFFMAN

606 S

ATHAPUSCOW AULACOGEN

SLAVE CRATON

N

Fig. 7. Facies transitions in the Pethei Group from the Slave craton into the Athapuscow aulacogen.

depressions within them filled with cross-bedded intraclast grainstone. Reconstruction of these depressions in plan suggests that they were anastornosing channels oriented normal to the shelf edge. Cross-bedding indicates bimodal transport, suggesting tidal current action. As the channels become deeper, the intervening stromatolites appear in cross-section as large bioherms (Fig. 8c), the tops of which stand several meters above the channel floors. Both the bioherms and their constituent columns tend to be elongate normal to the shelf edge. At the shelf edge itself, the channels widen at the expense of the bioherms. Less than a kilometer into the basin, the stromatolites disappear and the cross-bedded grainstones change to thinly flat-bedded, finer-grained, poorly sorted packstone. The lower slopes are made up of monotonous very thinbedded lime mudstone with shale partings, and sheets of slump breccia composed of blocks of slope and shelf edge facies. In conclusion, the outer margin of the shelf is marked by a belt of transverse tidal channels, between which are large columnar stromatolite bioherms. This contrasts with the extensive biostromes of the shelf interior.

STROMATOLITESOF A FORESHELF BASIN

The McLean Formation is in part the basinal stratigraphic equivalent of the Taltheilei Formation described above (Fig. 7). The lower foreshelf slope


607

Fig. 8. Stromatolites in the Pethei Group. a. Laterally linked columns of Omachtenia. The scaleis1.5 m long. b. Bedding surface showing elongate columns of Omachtenia. c. Margin of bioherm flanked by intraclastic channel fill in foreground; 3 m of section shown. d. Poorly laminated Conophyton-like columns in dark marlstone of the foreshelf basin floor. e. Calcareous columns in dark marlstone with thin inorganically deposited carbonate beds. Scale is divided into 3-cm intervals.

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P. HOFFMAN

facies interfingers with stromatolitic marlstones of the basin floor. The marlstones also interfinger with thick tongues of greywacke turbidites, transported along the axis of the aulacogen parallel to the carbonate shelf edge. Absolute water depths in the basin cannot be determined, but the basin must have been a sufficient bathymetric trap to accommodate hundreds of meters of turbidites that nowhere lap out onto the adjacent shelf. The basinal stromatolites are important in view of the general assumption that stromatolites are strictly shallow-water features. The stromatolites consist of poorly laminated, vertical to inclined columns (Fig. 8d), 3 cm in diameter. The columns must result from direct precipitation of carbonate, for the surrounding sediment is terrigenous and growth of the columns is snuffed out by thin beds of mechanically deposited carbonate (Fig. 8e). The columns are circular in plan and their inclination is parallel t o the axis of the aulacogen. The laminations, although indistinct, are conical and there is, in places, an indistinct apical zone, suggesting that the columns are closely related to Conophyton. The basinal stromatolites lap out of the aulacogen onto the adjacent shelf at the top of the Taltheilei Formation (Fig. 7). The passage from basin (McLean Formation) to shelf (Utsingi Formation) is gradational, the columns becoming more closely spaced and of slightly greater diameter, and the intercolumn sediment changing to dolomite relatively free of terrigenous admixture. That the shelf was submerged below wave base is suggested by the absence of a sharply defined shelf-edge facies change, and by the extreme monotony of the Utsingi Formation, which consists of hundreds of meters of poorly laminated Conophyton-like columns, unbroken by any other lithology. This reinforces the impression that Conophyton is a sublittoral form. The vertical transition from the shallow Taltheilei shelf to the submerged Utsingi shelf is interesting. Here, there are cycles involving a reversible but ordered succession of the forms Conophyton, Jacutophyton, Baicalia and Omachtenia (in some cycles, the first or last named are absent). This succession is interpreted to reflect decreasing water depth (Fig. 9). In conclusion, Conophyton is believed t o be a sublittoral form in the Pethei Group. Poorly laminated Conophyton-like columns grew by direct carbonate precipitation in foreshelf basins of water, many tens, perhaps hundreds of meters deep.

STROMATOLITES IN A CONTINENTAL FAULT BASIN

The Murky Formation consists of alluvial fan deposits shed into localized continental fault basins during the final phase of tectonic activity in the Athapuscow aulacogen. It is made up of little deformed sheets of friable red


,,sea level

.. . Omachtenia

lm

10 m

100m-

water depth

2 :onophyton? (poorly laminated)

Fig. 9. Postulated relation of stromatolite type to water depth.

Fig. 10. Morphology of stromatolites in the Murky Formation.

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P. HOFFMAN

Fig. 11. Murky Formation. a. Small isolated colonies (scale in cm). b. Geologist with foot on the basal stromatolite sheet and hand on the bulbous crown. Crown in the foreground has fallen out of place.

conglomerate and breccia. Near the distal ends of the fanglomerate sheets axe units up to 10 m thick of red terrigenous silstone, interpreted as ephemeral lake deposits. It is in one of these silstone units that the calcareous stromatolites occur. The unusual morphology of the stromatolites is shown Fig. 10. In


611

development, small isolated colonies (Fig. l l a ) coalesced to form an extensive basal sheet, from which grew discrete narrow pedestals with broad bulbous crowns (Fig. l l b ) standing nearly 2 m above the basal sheet. Internally, the stromatolite masses consist of closely spaced columns with a Kussielliform (Raaben, 1969a) mode of branching. The carbonate of the stromatolites must have been precipitated in place as the surrounding sediment is entirely terrigenous. In conclusion, the occurrence of stromatolites in a non-marine noncalcareous basin adds to the diversity of environmental settings in which even the most ancient stromatolites have been found. SUMMARY AND CONCLUSIONS

The mere presence of stromatolites is not sufficient to induce environment of deposition. They may be marine or non-marine, shallow or deep-water. However, certain types of stromatolites may be diagnostic of specific facies. (1)Extensive stromatolite biostromes occurring in the upper parts of stacked regressive cycles are typical of the interior of a carbonate shelf. (2) Large stromatolite bioherms separated by tidal channels are typical of the outer margin of a carbonate shelf. (3) The distinctive columnar stromatolite Conophyton may be an exclusively subaqueous form, and poorly laminated Conophy ton-like columns may grow in foreshelf basins of water tens or even hundreds of meters deep. (4) The elongation of stromatolites in plan is generally parallel to paleocurrent direction and commonly normal t o the depositional strike. (5) Stromatolite growth by direct precipitation, rather than sediment trapping, occurs in non-marine and deep-water marine basins where the particulate sediment is terrigenous.



Chapter 10.8

DISTRIBUTION OF STROMATOLITES IN RIPHEAN DEPOSITS OF THE UCHUR-MAYA REGION OF SIBERIA S.N. Serebryakov

INTRODUCTION

The use of Riphean stromatolites in basin analysis is rather limited due to inadequate knowledge of their paleoecology. The widely employed extrapolation of the ecological features of the best known (intertidal) Recent stromatolites to Precambrian stromatolites meets well-grounded objections from many scientists (e.g., Maslov, 1959, 1960; Korolyuk and Sidorov, 1965; Trompette, 1969; Walter, 1970a, 1972a; Serebryakov, 1971, 1975; Bertrand-Sarfati, 1972b, c; Hofmann, 1973; Serebryakov and Semikhatov, 1973, 1974; Monty, 197313, c; and others). The most popular method of studying the paleoecology of the Riphean stromatolites, by analyzing microfacies of individual bioherms (e.g. Bertrand-Sarfati, 1970, 1972b), is not faultless either. In particular, the effect of biotic (associated with the systematic composition of algae) factors on the morphogenesis of stromatolites can hardly be detected in local material. This makes, in turn, the determination of the relative role of the environment more difficult. Furthermore, the interpretation of microfacies itself is based mainly on evidence from Recent stromatolites. Some of these faults can be avoided by considering broadly the location of stromatolites in the Riphean deposits of an extensive region. This article is an attempt to determine a relative effect of biotic and environmental factors on the lateral and vertical distribution of stromatolites in the Riphean of the Uchur-Maya region. REGIONAL GEOLOGY

The Uchur-Maya region of southeastern Siberia covers an extensive area (650 x 750 km) in the basins of the right tributaries of the Aldan river and includes three large structural elements (Figs. 1,2): the eastern slope of the Aldan shield, the Uchur-Maya platform of the Siberian craton and the

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Fig. 1. Schematic lithologic-facies profiles of the pre-Yudomian deposits of the UchurMaya region. Legend: 1 = sandstones (predominantly quartz) and conglomerates, grey-coloured; 2 = the same, red-coloured; 3 = siltstones; 4 = argillites; 5 = sandyklayey red-coloured rocks; 6 = variegated argillites of the Neruen suite; 7 = limestones;8 = dolomitic limestone and limey dolomites; 9 = dolomites; 10 = variegated clayey limestones, mark and calcareous argillites of the Totta suite; 11 = rhythmic interstratification of sandstones, sandy oncolitic and stromatolitic dolomites; 12 = the same and chemogenic dolomites; 13 = rhythmic interstratification of sandstones, siltstones, argillites and dolomites; 1 4 = variegated and dark-grey pelitomorphic platy limestones of the Malgina suite; 15 = light-grey and grey, massive and platy dolomites; 16 = grey dolomites with reddish-brown crust of weathering; 17 = dark-grey bituminous limestones of the Neruen suite; 18 = grey-coloured clastic and microphytolitic limestones and dolomites; 19 = red-coloured chemogenic, clastic and stromatolitic limestones and dolomites of the Ignikan suite; 20 = grey and dark-grey chemogenic, clayey-bituminous, clastic and microphytolitic limestones and dolomites of the Ignikan suite; 21 = pre-Riphean formations; 22 = surfaces of regional unconformity; 23 = surfaces of regional weathering; 24 = suite boundary lines; 25 = surface of preYudomian erosion; 26 = K-Ar age of glauconite in m.y. (according to Kazakov and Knorre, 1970); 27-37 = forms of stromatolites (in brackets - morphologies of stromatolites found in the studied material analogous to the given form in microstructure but with a different morphology) :27 = Omachtenia omachtensis Nuzhnov (Stratifera,nodular, rare Kussiella); 28 = Nucleella figurata Komar, Gongylina differenciata Komar, Kussiella kussiensis Krylov (Stratifera, Omachtenia); 29 = Baicalia baicalica (Maslov) (Stratifera), Colonnella kylachii Shapovalova, Suetliella tottuica Komar and Semikhatov (nodular, Stratifera), S. suetlica Shapovalova (Stratifera), Conophyton garganicum Korolyuk; 30 = Malginella malgica Komar and Semikhatov (Stratifera);31 = Malginella zipandica Komar (Stratifera, Irregularia); 32 = Parmites aimicus (Nuzhnov) (Irregularia); 33 = Colonnella ulakia Komar (Baicalia, Tungussia);34 = Minjaria sakharica Komar (Baicalia, Jacutophyt o n ) , Conophyton metula Kirichenko (Jacutophyton), Conophyton lituum Maslov (Jacutophyton,Baicalia, Colonnella, nodular), Baicalia lacera Semikhatov (Jacutophyton, Conophyton), Baicalia ingilensis Nuzhnov (Jacutophyton);35 = Baicalia maica Nuzhnov (Jacutophyton); 36 = Inzeria tjomusi Krylov (Jacutophyton); 3 7 = Inzeria confmgosa (Semikhatov) (Stratifera, Omachtenia?); 38 = area of deep burial of Riphean deposits; 39 = boundary of largest positive structures (Al. Sh. = Aldan shield, Om. Up. = Omnia uplift); 40 = zone of Nelkan fault; 41 = depressions in the Uchur-Maya platform (Uch. D. = Uchur, M.D. = Maya); 42 = Yudoma-Maya trough (Yud. M.T.); 43 = geographic position of lithologic-facies profiles (a-b and c-d, Fig. 1 ; e-f-g-h-i, Fig. 2 ; j - k , Fig. 6); 44 = position of sections shown in Fig. 5.

h

i

h

I

N. SLOPE OF

1

Fig. 2. Schematic lithologic-facies profiles of lower and upper subsuites of the Yudoma suite (Terminal Riphean). For position of profile line see Fig. 1 ;e , f , g , h , i - points of inflection of profile line. Legend: 1 = quartz and feldspar-quartz sandstones and gritstone with subordinate limestones and oncolitic dolomites; 2 = quartz and feldspar-quartz sandstones with subordinate gritstones, siltstones and argillites; 3 = complex, occasionally rhythmic alternation of predominating microphytolitic dolomites with stromatolitic, granular and sandy dolomites and sandstones; 4 = variegated cherty and clayey dolomites, argillites and cherty argillites with subordinate phytogenic and sandy dolomites; 5 = argillites, clayey and indistinctly granular dolomites with rare lenses ofsandstones, stromatolitic and microphytolitic dolomites; 6 = indistinctly granular dolomites with intercalation of argillites, sandy and microphytolitic dolomites; 7 = bituminous and clayey dolomites with rare stromatolites, intercalations of argillites, siltstones and sandstones; 8 = bituminous limestones; 9 = distinctly granular dolomites with particular stromatolites and microphytolites; 10 = indistinctly granular limestones containing glauconite; I 1-1 8 = stromatolites with microstructures described from the following stromatolite forms: 11 = Boxonia grumulosa Komar; 12 = B. ingilica Komar and Semikhatov; 13 = Pankcollenia emergens Komar; 14 = Colleniella singularis Komar; 15 = Jurusania judomica Komar and Semikhatov; 16 = Gongylina nodulosa Komar and Semikhatov; I7 = J. sibirica (Jakovlev); 18 = Linella simica Krylov; 19 = undefined stromatolites.

cL

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Yudoma-Maya trough. The latter is distinguished from the typical platform area by greater and more variable thicknesses, and, in some cases, by the facies of the Riphean deposits. It has been treated lately as an aulacogen. A number of lesser structures can be distinguished within the Uchur-Maya platform; the largest among them are the Uchur and Maya depressions separated by the Omnia uplift. The composition of the deposits of the four subdivisions (phythems) of the Riphean is shown in Figs. 1 and 2. There are terrigenous, terrigenouscarbonate and carbonate units forming extensive sedimentary cycles (e.g., Nuzhnov and Yarmolyuk, 1959, 1963; Komar et al., 1970). Five cycles of this kind (which are regarded as groups) have been distinguished in the pre-Yudomian deposits (Fig. 1).At the bases of all the groups (except the Uii) there are unconformities; these are most distinct on the Uchur-Maya platform. The Yudoma suite (Fig. 2) corresponds to the Terminal Riphean (Yudomian) and overlies the older rocks with regional unconformity. It is intimately associated with the deposits of the Early Cambrian epoch, so that they form a single sedimentary cycle. In the Uchur-Maya region the completeness of the sections and the thickness of the Riphean deposits gradually increase from west to east; the terrigenous suites are subject to the most significant lateral changes (Fig. 1). The variations of the carbonate suites are marked by changes in the proportions of limestones and sedimentary-diagenetic dolomites, and the amount and character of allochthonous material. Unfortunately, the marginal (nearshore) facies of the deposits are preserved only in the Omakhta suite and in the lower subsuites of the Yudoma suite. The variations in the composition of the carbonate rocks can be partially accounted for by fluctuations in the salinity of the basinal waters (e.g. Akulshina et al., 1969). Supply of continental waters containing sodium and magnesium carbonate was evidently also of great importance; this facilitated accumulation of dolomite and limestone-dolomite facies (Serebryakov, 1968, Grigorev et al., 1969). Further from the coast (or as the transgressions developed), these facies were replaced by limestones, much like the phenomenon observed in the Neruen, Ignikan and Yudoma suites. In the carbonate suites of all the zones of the Uchur-Maya region, one can commonly observe traces of local submarine erosion, cross-bedded and wavy-laminated structures, and ripple marks; clastic carbonate rocks (mainly endoclastites) are also widespread in occurrence. Desiccation cracks, typical of terrigencus suites, are found occasionally. This suggests deposition under generally shallow water and relatively high hydrodynamic energy for all of the Riphean sediments of the Uchur-Maya region. This conclusion is supported by the practically ubiquitous occurrence of stromatolites and microphytolites. Stromatolites have been found in all the suites of the region, except the Ustkirba suite. The only stromatolites considered below are those of the carbonate and terrigenous-carbonate

623


--

IDESICCATIO)(I

DEEPER WATER

/

E5ii m 2 1703

7n4 ... ... .... ..

Fig. 3. Sedimentary rhythms of the lower part of the Omakhta suite, Uchur River. Legend: 1 = stromatolites; 2 = oncolitic dolomites; 3 = sandy dolomites and dolomitic sandstones; 4 = finegrained silty sandstones; 5 = silty argillites; 6 = desiccation cracks; 7 = pseudomorphs after halite; 8 = ripple marks.

suites. (The stromatolites were studied jointly by the author, Komar and Semikhatov. Komar and Semikhatov made all the taxonomic determinations.) The study of the Uchur-Maya stromatolites indicated that they formed subtidally. This conclusion is based on the uniformity of the stromatolitebearing members of the suites over vast areas, persistence of microstructures within the same bioherm irrespective of its growth relief, and absence of any evidence that the bioherms were subjected to intense erosion. Neither is there any evidence of subaerial exposure of the stromatolites; in particular,

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no desiccation cracks have been found. According to Gebelein (1969), these are the only reliable indication of intertidal or supratidal formation of stro matolites . Among the stromatolite-containing suites of the Uchur-Maya region, the evidence of shallowness is most pronounced in the Omakhta suite of the Uchur depression. There the suite is represented by rhythmically alternating terrigenous and carbonate rocks (Fig. 1). In the direction to the Aldan shield, the rhythms become gradually thinner, the chemogenic dolomites disappear, the rocks get more and more red-coloured, and numerous desiccation cracks and halite pseudomorphs are observed in the clastic members of the rhythms. Rhythmicity in the Omakhta suite in the western, near-shore part of the Uchur depression is manifested by the repeated occurrence of the following rock types (Fig. 3): fine-grained silty sandstones with quartz and, then, dolomitic cement, then sandy dolomites, then oncolitic and stromatolitic dolomites with variable amounts of sand. Some of these rocks recur in the reverse order forming closed asymmetric rhythms (Serebryakov, 1971). In the sandstone beds which begin and conclude each rhythm, thin clay layers are common. Desiccation cracks, ripple marks and salt crystal moulds are confined to these layers, providing evidence of short periods of exposure. At the same levels there are some erosional surfaces. The carbonate members of the rhythms show no evidence of subaerial exposure. It is apparent that the rhythms are of a transgressive-regressive character, stromatolites being confined to the periods of greatest submergence. It is significant that in the eastern zone of the Uchur depression, where there is less evidence of shallow deposition, the transgressive part of the rhythms is terminated by chemogenic dolomites overlying stromatolites. Thus, there is every reason to suppose that even the Omakhta stromatolites, which formed in the shallowest water, formed below the ebb tide level. Thus, the observed distribution of stromatolites in the Uchur-Maya region should reflect their original distribution in the various subtidal environments. These environments differed at least in their position relative to the shore and in some cases in the conditions of sedimentation. Therefore, the first question to be answered is whether these differences affected the lateral distribution of the stromatolites. The second question is whether there exists any correlation between the vertical changes in the stromatolites and the variations in the composition and character of the enclosing rocks. These questions are considered below independently for the various morphological groups of stromatolites and for the stromatolites of a definite microstructure irrespective of their morphology. The stratigraphic basis for analysing the distribution of the stromatolites is provided by the suites (formations) and subsuites which are litho-stratigraphic subdivisions; we are obliged to regard their boundaries as isochronous.


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DISTRIBUTION OF ASSEMBLAGES OF MORPHOLOGICAL GROUPS OF STROMATOLITES

Study of stromatolites of the Uchur-Maya region (Nuzhnov, 1960,1967; Nuzhnov and Shapovalova, 1965, 1968; Semikhatov and Komar, 1965; Voronov et al., 1966; Komar et al., 1970, 1973;Krylov and Shapovalova, 1970b;Komar, 1973)indicated that their groups, defined on a morphological basis, form laterally persistent stable assemblages confined to particular stratigraphic intervals. The spatial distribution of the assemblages was found to have three general features: (1) Stromatolite assemblages occur over practically the whole area of outcrop of the carbonate and terrigenous-carbonate suites; they are not subject to any significant lateral changes. The variations in thickness and facies of the sediments are accompanied by changes in the relative abundance of some groups, the total abundance of the stromatolite and the thickness of their bioherms. The Neruen suite provides a good example. Its thickness (Fig. 1)in the Yudoma-Maya trough is twice that in Uchur-Maya platform. A t the same time there appear in the trough, among the carbonate rocks, beds of chemogenic, bituminous limestones and dolomites which result in a lesser abundance of the phytogenic and phytoclastic rocks. Extensive carbonate bodies appear in the trough in the lower and upper part of the suite represented on the platform mainly by argillites. In spite of these changes in the suite, Baicalia, Conophyton and Jacutophyton, normally forming “Jacutophyton cycles”, dominate everywhere (Shapovalova, 1965, 1968; Serebryakov et al., 1972).In the Yudoma-Maya trough, in comparison with the Maya depression, the individual stromatolite horizons get thicker and bioherms composed of any one group of stromatolites (predominantly Baicalia) are of greater size and more numerous. As a result, the relative abundance of the above groups is somewhat different. The horizons containing Baicalia account, in four sections of the Maya depression, for 58-60% of the total thickness of the stromatolite horizons and, in two sections of the Yudoma-Maya trough, for 67-7076. (2)The composition of the assemblages may remain the same, irrespective of vertical changes in the composition and character of the enclosing deposits. Most spectacular in this respect are stromatolites of the Neruen suite. Conophyton, Jacutophyton, Baicalia and Colonnella are enclosed in its lower part in clastic and chemogenic dolomite varying in their fabrics and colour, and in its upper part, in less variable limestones and dolomitic limestones. These stromatolite constructions frequently cross the boundary of the lithologically different carbonate rocks without changing their morphology. Moreover, all these groups (except Jacutophyton) have been found enclosed in argillites (Fig. 4). Absence of any direct interdependence between the morphology of

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8 not exposed 740m

I

Fig. 4. Stromatolite horizon of the upper subsuite of the Neruen suite, Lyaki River. Legend: 1 = limestones; 2 = argillites;3-5 = stromatolite morphology;3 = Jacutophyton; 4 = Buicaliu; 5 = Colonnella; 6-9 = stromatolite microstructures typical for the following forms: 6 = Baicalia lacera Semikhatov; 7 = B. ingilensis Nuzhnov; 8 = Conophyton lituum Maslov; 9 = C . cylindricum Maslov (for jacutophytons the denominator indicates the microstructure of central column, the numerator that of the branches); 10-14 = colour of rocks: 10 = red; 11 = mottled (red and green); 1 2 = variegated; 13 = grey of various shades; 14 = dark grey to black.

stromatolites and the character of the enclosing rocks in the Neruen suite has been already considered in the description of the “Jacutophyton cycles” (Serebryakov et al., 1972). Yet the superposition in bioherms of different stromatolite morphologies can evidently be attributed to ecological factors which may have operated over extensive areas (see Ch. 6.4,fig. 7). This contradiction can obviously be accounted for by the fact that cyclic variations in the morphology of stromatolites and local variations in the character of rocks were controlled by different causes. For instance, the rate of sedimentation and intensity of subsidence are poorly manifested in the character of carbonate sediments; the role of these factors in the morphogenesis of stromatolites seems to be significant (Bertrand-Sarfati, 1972c; Serebryakov et al., 1972). The possibility that the environment may affect the shape of stromatolites but not the character of the sediment has been suggested by Maslov (1959) and Preiss (1973a).


627

(3) Different assemblages of stromatolites are found in lithologically similar beds of different ages; these assemblages may vary through a homogeneous section of carbonate rocks. Such a change in assemblages occurs in the uniform dolomites of the Tsipanda suite (see Fig. 1).In its lower part there are stratiform stromatolites (Mulginellu, Strutiferu, Irregulariu); in the middle part, stratiform and stratiform-columnar types (Irreguluria, Purrnites) are encountered, and in the upper part, these are replaced by columnar structures (Colonnellu, Buiculiu, Tungussiu, occasional Minjuriu, and forms resembling Tilernsinu). There is no lithological evidence that these changes in stromatolite assemblages are dependent on any changes in ecological conditions. Though such a dependence cannot be absolutely denied, the fact that the changes in morphological assemblages coincide with variations in the microstructures of the stromatolites indicates that these changes are probably due to changes in the communities of stromatolite-forming algae. Dolomites similar in composition, colour and fabric to the Tsipanda ones occur in the Yudoma-Maya trough a t the base of the overlying Neruen suite. They are separated from the rocks of the Tsipanda suite by a regional weathering surface and a thin (5-20m) bed of argillites. Minjuriu and Tilemsinu (?) have not been found in these dolomites but Jucutophyton and Conophyton are abundant. This change in the morphological assemblage is accompanied by a partial change in the microstructures; but some of the stromatolites of the boundary horizons of the Tsipanda and Neruen suites have similar microstructures. The Neruen stromatolites assemblage differs from the Tsipanda one not only in composition but also in the appearance of the cyclic structure of the bioherms (Jucutophyton cycles). Consequently, the change in the associations at the boundary of the suites under consideration was caused not only by the change in algal communities but also on the changes in the environment. It is significant that these changes are not manifested in the macroscopic and petrographic appearance of the rocks. This is not an exceptional example of the repetition of uniform carbonate beds a t various stratigraphic levels. The dolomites of the Svetly and the Tsipanda suites are similar in their composition and position in the sedimentary cycles. Dolomites of the Svetly type have been encountered in the Neruen and Ignikan suites. Yet the stromatolite assemblages in these suites are different. The lithologically similar deposits are unlikely to have formed, in all cases, under different conditions. It is much more probable that we have to deal here with the age variability of stromatolite morphology; this view is reinforced by the fact that there are corresponding variations in their microstructures (see below). The specific features of the distribution of the stromatolite assemblages allow two conclusions. Firstly, the vertical change in assemblages often has no direct relationship to changes in the enclosing deposits. Though the effect of environment on the stromatolites in the sections can be established in

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some cases, the general sequence of assemblages can hardly be explained without admitting that it was dependent on the evolution of the stromatoliteforming algae. Secondly, there is very little relationship between the distribution of assemblages, the conditions of sedimentation and the distance from the coast. This does not exclude the effect of environmental factors on stromatolite morphology within the bioherms or even in certain areas of basins (see Ch. 6.4). The effect of these factors is not manifested in the characteristics of the rock, which makes their interpretation more difficult. Those ecological factors which affected the morphology of the stromatolites hardly influenced their microstructure. In fact, the intrabioherm variability of stromatolites is normally unaccompanied by microstructural changes (Komar and Semikhatov, 1965, 1968b; Komar, 1966; Krylov, 1967a, 1972; Serebryakov, 1971; Walter, 1972a). The abundant evidence of the Recent and ancient stromatolites (see Ch. 7.1) suggests that particular microstructures owe their formation to particular communities or species of algae. If this is correct, the distribution of stromatolites with particular microstructures must to some degree reflect the original distribution of the stromatolite-forming communities. DISTRIBUTION OF STROMATOLITES WITH PARTICULAR MICROSTRUCTURES

The specific features of microstructure usually provide the basis for distinguishing forms (“species”) of stromatolites; the concept of “group” is sometimes identified with the concept of “stromatolites of a certain microstructure”. Krylov (1967a, 1972) has repeatedly discredited such identifications, stating that whether a form does or does not belong to some group, according to the current classification, must be determined by the morphology of the structures. Meanwhile, the same microstructure frequently occurs in stromatolites with diverse morphologies characteristic of different groups. Because the classification of microstructures remains an unsolved problem, I have referred in the following analysis to the microstructures described at the time of the establishment of each particular form. The distribution of stromatolites with particular microstructures in Riphean deposits of the Uchur-Maya region has recently been considered by Komar et al. (1973). Four types of distribution have been recognized: (1) The same microstructure is characteristic of all the stromatolites of a suite or some portion of a suite over the whole area of its distribution. For example, a considerable part of the Malgina suite is composed of Malginella malgica Komar et Semikhatov and Stratifera of a similar microstructure. In the lower part of the overlying Tsipanda suite, Malginella, Stratifera and Irregularia have a common but different microstructure (that described from Malginella zipandica Komar). The boundary between the two microstructures coincides (not always exactly) with the sharp change in the composition of


629

the rocks at the boundary of the suites. The next level of microstructural change is within the uniform Tsipanda dolomites: in their middle part, there occurs Pamites aimicus (Nuzhnov) and Irreguluriu with a similar microstructure. Y A 2

. CHELASIN 3

PLLYW-VU

4

L +@

1) Fig. 5. Vertical range of stromatolites with particular microstructures, in some sections of the Neruen and Ignikan suites (ranges in proportion to thicknesses). For position of sections see Fig. 1. Letters given in circles are stromatolites with microstructures described from the following forms: c = Conophyton cylindricum; It = C. lituum; mt = C. metula; s = Minjaria sakharica; u = Colonnella ulakia; i = Baicalia ingilensis; 1 = B. lacem; m = B. maica; t = Inzeria tjomusi; cn = I . confragosa.

The distribution of microstructures in the Ignikan suite is particularly interesting (Fig. 1).This suite is composed of clastic, microphytolitic, and chemogenic carbonate rocks of diverse composition in which dolomites typical for the Uchur-Maya platform are gradually replaced by limestones in the Yudoma-Maya trough. Yet the vertical sequence of stromatolites persists over the whole area of occurrence of the suite (Fig. 5). This allows three regional biostratigraphic subdivisions to be distinguished in the suite. The facies variations in the deposits consist only of some irregularities in the distribution of stromatolite bioherms and thickness variations in the beds containing stromatolites of a particular microstructure. (2) Mass concentrations of stromatolites of some definite microstructure are confined to definite associations of deposits. The Yudoma suite (Fig. 2) can be used to illustrate this. The distribution of its stromatolites exhibits two opposite tendencies (Semikhatov et al., 1970). On the one hand,

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Fig. 6 . Schematic lithologic-and-facies profile of the lower Yudoma subsuite. For position of profile line see Fig. 1. Legend: I = sandstones; 2 = argillites; 3 = siltstones; 4 = dolomites; 5 = limestones; 6 = darkcoloured bituminous and clayey-bituminous limestones; 7 = sandy dolomites; 8 = variegated cherty-dolomite and clayey-herty-dolomite rocks; 9 = catagraphs; 10-1 1 = stromatolites with microstructures typical for the following forms: 10 = Boxonia grumulosa; I 1 = Jurusania judomica; 12 = K-Ar age of glauconites in m.y. (according to Kazakov and Knorre, 1970).

stromatolites with the same microstructure are present in rocks of diverse composition and genesis, for example in cross-bedded quartz sandstones, horizontally laminated clayey dolomites and massive pelitomorphic limestones. On the other hand, mass concentrations of stromatolites with a definite microstructure are confined to definite associations of rock types, which causes the stromatolites to be irregularly distributed. Thus, most


631

common among sandstones and microphytolitic dolomites are stromatolites which have the microstructures described for Boxonia grumulosa Komar, B. ingilica Komar and Semikhatov, Paniscolleniu emergens Komar and Colleniella singularis Komar; stromatolites with the microstructures described for Jurusaniu judomica Komar and Semikhatov occur in cherty and clayey dolomites which probably formed in the deeper sea; those with the characteristic Gongylina nodulosa Komar and Semikhatov microstructure are found in various types of pure carbonates. The boundaries of the rock associations with their contained stromatolite assemblages may be diachronous (Fig. 6). Some of the rock associations occur repeatedly in the section due to the cyclic structure of the Yudoma suite. However, among its stromatolites there are forms which are confined to only one of the subsuites. Thus, Boxonia grumu Zosa and some other morphologically different stromatolites of a similar microstructure do not pass into the deposits of the upper subsuites despite the passage upwards of its characteristic rock association. (3) Stromatolites of various microstructures are distributed in a disorderly fashion within a suite. Though some sequence in the change or alternation of microstructure can be observed in some sections, it is not maintained throughout the depositional area. The Svetly and Neruen suites can provide an example of this. Among the stromatolites of the Neruen suite, seven varieties of microstructures were distinguished (Fig. 5 ) . Five of them are encountered practically in the whole time-range of the suite, overlapping and replacing one another in vertical and lateral directions without any visible regularity. Moreover, in some cases one stromatolite structure may have different microstructures in its different parts (e.g. some Jacutophyton, see Serebryakov et al., 1972). (4) Stromatolites occurring in different sections of the suites have different microstructures. This type of distribution has been found in the Omakhta suite in two isolated sections: in the Uchur depression and in the northern part of the Yudoma-Maya trough (see Fig. 1). These considerations do not contradict the previously made suggestion that the spatial distribution of stromatolites of a certain microstructure reflects the distribution of communities of the stromatolite-forming algae. If this is so, it is unlikely that there was any direct dependence of the change of communities in time on environmental variations. This conclusion is supported also by vertical distribution of assemblages of morphological groups of stromatolites (see above). The condition in the Riphean basins of the Uchur-Maya region allowed, in some cases, the development of one or several coenoses over the whole of its ar6a; in other cases they were predominantly confined to limited areas. In some comparatively rare cases, changes in the algal communities might be caused by the same environmental factors which affected the morphology of the stromatolites. This view is supported by the occasional coincidence in

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a change of stromatolite morphology and change in their microstructure (some of the “Jucutophyton cycles”). In general, the following hypothesis may be proposed t o explain the distribution of stromatolites in the Neruen suite (type 3): in the Neruen basin there existed at the same time five similar and closely related communities (species?) of algae which gave rise to stromatolites with five different microstructures. Each of the communities could develop within the entire range of the conditions present. For this reason the spatial distribution of the communities was irregular and their appearance in the given point of the basin was fortuitous. The difference in the microstructures of the Omakhta stromatolites (type 4 distribution), can be, in all probability, accounted for by the existence of two isolated basins populated by different communities (species?) of algae. The same type of distribution of stromatolites with definite microstructures is observed in suites differing in composition (for instance, in the Svetly and Neruen) while different types are encountered in the lithologically similar suites: in the Svetly and Tsipanda, Neruen and Ignikan. This suggests that the realization of one type or another was determined not by the conditions of sedimentation but by the particular character of the algal communities. CONCLUSION

(1) In the Uchur-Maya region, there were, during the Riphean, extensive shallow-water basins where the conditions of sedimentation were rather stable in space and time. As already pointed out in the literature (e.g., Keller et al., 1968; Trompette, 1969; Bertrand-Sarfati, 1972c) such basins were, evidently, characteristic of the Riphean. (2) Stromatolites of the Uchur-Maya region formed subtidally as did, probably, the majority of other Riphean stromatolites (see Monty, 1973b, c; Serebryakov and Semikhatov, 1973,1974). They formed practically over the entire area of the basin. Therefore, u priori use of Riphean stromatolites as indicators of an intertidal or a nearshore environment is unfounded. (3) Variations in the distribution of different communities (species?) of algae seem to have been the result of their different residence against environmental changes. The range of conditions allowing formation of stromatolites by one kind of alga or another was rather wide, which reinforces the interpretation of the subtidal formation of the stromatolites. (4) The dependence of the distribution of the Uchur-Maya subtidal stromatolites on the conditions of sedimentation is poorly manifested. A moderate supply of terrigenous material did not limit the development of the stromatolites. It was not infrequent that short periods of terrigenous sedimentation failed to change the stromatolite assemblages (for example, in the Svetly and Neruen suites). Such changes, however, did occur within


633

homogeneous carbonates. Stromatolites were, normally, unaffected by changes in the carbonate composition of the rocks. Stromatolites of similar morphology and microstructure occur in limestone, sedimentary-diagenetic dolomites and mixed limestone-dolomite rocks. Thus, there is no reason to regard the Riphean stromatolites as indicators of definite salinity conditions in the basin. (5) The morphology of the stromatolites, and sometimes also the distribution of the algal communities was greatly affected by environmental factors which are poorly, or not at all, manifested in the lithological characteristics of the rocks. The effect of these factors is more apparent in the analysis of the intrabioherm variability of stromatolites (see Ch. 6.4). (6) Vertical variations in the algal communities and, as a result, in the morphological and microstructural assemblages of stromatolites, seem to have been associated mainly with the evolution of the stromatolite-forming algae. It is not mere chance that a similar sequence of stromatolites has been observed in other Riphean sections within the U.S.S.R. (see Ch. 7.1). (7) The conditions under which the Riphean stromatolites formed were, probably, essentially different from the conditions of growth of Recent algal structures.



Chapter 10.9 PALAEOENVIRONMENTAL AND GEOCHEMICAL MODELS FROM AN EARLY PROTEROZOIC CARBONATE SUCCESSION IN SOUTH AFRICA K.A. Eriksson,J.F. Truswell and A . Button

INTRODUCTION

The lower part of the Transvaal Supergroup is largely nondetrital, made up in the main of carbonates and chert and lesser amounts of banded iron formations. It is developed in two structural basins, firstly around the Bushveld Complex in the Transvaal and secondly in the northern Cape (Fig. 1).Within these rocks several palaeoenvironmental models have been developed. In this paper these are related in space, defined wherever possible in terms of their geochemistry, and used in an analysis of the basin. The rocks considered are about 2,250 m.y. old. Environmental aspects of the model may have relevance through the Proterozoic. But as will become apparent, aspects of the carbonate chemistry, and the presence of both widespread carbonates and banded iron formation in the basin, suggest that the geochemistry could only be fully developed in the time period 2,300-2,000 m.y.

STRATIGRAPHIC RELATIONSHPS

A full succession is developed in the eastern Transvaal. Here within the Olifants River Group carbonates and cherts of the Malman'i Dolomite form the bulk of the succession, and are overlain by the banded iron formations of the Penge Formation, above which are further carbonates and terrigenous clastics in the Duitschland Formation (Button, 1 9 7 3 ~ )The . Malmani Dolomite has been subdivided into five lithostratigraphic zones in the eastern Transvaal (Fig. 2). Zones A , C and E are composed largely of darkcoloured carbonates and are essentially chert-free. Zones A and C are dolomitic; in zone E the dolomite is more ferruginous and there are remnants of limestone and minor development of proto-iron-formation and carbonaceous shale. The thicker zones B and D contain light-coloured recrystallized dolomite with abundant chert (Button, 1 9 7 3 ~ ) .

K.A. ERIKSSON ET AL.

636 G H A A P ond OLIFANTS RIVER G R O U P S

.

..... ...-,

,

---

DEPOSITIONAL AXIS

z

[27 Iron Formation . .. . SUBOUTCROP UPPER C O N T A C T

--

Fig. 1. Outcrop and locality map of the Ghaap and Olifants River groups.

Legend Banded Iron Formation Limestone, Fe-rich Dolomite 8 Proto BIF

Zwartruggens Zwortkops

/

I

Palaeoerosional

surfTe

Fe 8 Mn-rich Dolomite Barrier-forming, clostic textured carbonates Oolitic Barrier Dark, Fe-poor Dolomite Recrystallized Dolomite 8 chert Shale Black Reef Quartzite Arenite Control Sections (thickness in metres) .Controlled 8 Projected Stratigraphic Boundaries Unconformity

Fig. 2. Stratigraphic relationships in the Malmani Dolomite.

Penge

rl


637

Zones B and D are truncated by chert breccias, interpreted as a chert rubble following short-lived subaerial exposure. Similar material, marking a more significant unconformity, occurs between the Olifants River Group and overlying sediments. This latter chert breccia truncates different horizons around the basin in the Transvaal. The Duitschland Formation is only locally present in the northeastern Transvaal near Penge. The Penge Formation can be traced westwards to west of Thabazimbi (Fig. l), and zone E of the Malmani Dolomite extends little further. The Penge Formation reappears near Zeerust, but traced eastwards it and zone E are again truncated (Grootenboer et al., 1974). In the Potchefstroom synclinorium and at Zwartkops the upper part of zone D has also been removed (Eriksson, 1972; Eriksson and Truswell, 1974a). Proceeding southwards in the eastern Transvaal the Duitschland and Penge Formations and zones E , D and C are all successively truncated by the unconformity. Throughout most of the Transvaal the Black Reef can be regarded as a basal clastic member of the Malmani Dolomite (Eriksson and Truswell, 1974a); but in the northeastern Transvaal it locally thickens within an early proto-basin and is there afforded formational status (Button, 1974). In the northern Cape the Black Reef is overlain by the Ghaap Group (Beukes, 1973). Within this the Campbell Rand Dolomite and Kuruman Iron Formation are regarded as equivalents of the Malmani Dolomite and Penge Formations, respectively. The Griquatown Jasper lies conformably above the Kuruman Iron Formation, and above that is an unconformity. The Campbell Rand Dolomite is poorly exposed beneath the Ghaap Plateau but in general contains less chert and more limestone than the Malmani Dolomite. The Formation commences with a clastic unit, which grades up to limestones and ferruginous dolomites, followed by a thick iron-poor dolomite unit (Visser and Grobler, 1972; Truswell and Eriksson, 1973). In the vicinity of Prieska the Campbell Rand Dolomite is similar in nature to zone E of the Malmani Dolomite, and comprises ferruginous dolomite, proto-ironformation and shale. This chapter is concerned with aspects of the Malmani Dolomite, Campbell Rand Dolomite, and Kuruman and Penge Iron Formations, but offers no comment on either the structurally disturbed Duitschland Formation or on the Griquatown Jasper Formation. PALAEOENVIRONMENTAL MODELS

A number of depositional environments have been recognized in these rocks. Their distinction as facies leans heavily on Walther’s (1894) Law which recognises that unbroken vertical sequences are a reflection of lateral facies. The environments established involve uniformitarian analogy where possible, but of necessity conceptual models are more commonly applied.

638


The basal marine transgression The base of the Malmani Dolomite represents a classical marine transgressive unit which frequently commences with a conglomerate veneer passing up to orthoquartzitic beach and shallow marine sands (Button, 1974). The sands grade up to and interfinger with carbonaceous shale and structureless dolomite (rich in iron and manganese), which grades in turn into dolomites containing what are believed to be subtidal domes (Truswell and Eriksson, 1975). The thickness of the basal arenaceous unit (the Black Reef Quartzite) is largely dependent on the configuration of the pre-Transvaal topography. The formation grades laterally into cobble conglomerate adjacent to what were islands during the transgression, and may wedge out altogether over the crest of an island. The tidal-flat model Around Zwartkops (Fig. l), an intertidal assemblage is developed within light-coloured chert-rich dolomite of Malmani zone D . In this assemblage, finely laminated sediments are ‘associated with small-scale symmetrical algal domes, flat-pebble breccia and bird’s eye and palisade structures (Eriksson and Truswell, 197413). These features are analogous t o those in contemporary tidal-flat sediments in protected intertidal to marginal supratidal environments, as for example at Shark Bay, Western Australia (Davies, 1970b; Logan e t al., 1974). Larger domes with relief ranging to more than 1m are the most common large-scale structures of zones B and D. Increases in dome size are thought to reflect successively greater intertidal depths (Logan, 1961; Monty, 1972). The elongation of domes probably indicates the influence of tidal currents, an interpretation supported by the presence of breccia between domes, lenses of oolite and by the sporadic development of linear and ladder ripples. Intertidal conditions are also suggested by mud-cracked clay drapes over ripple marks and by small stromatolite domes swamped by rippledominated carbonates (Truswell and Eriksson, 1975). The more steeply shelving slope At Boetsap in the northern Cape (Fig. l), a complex of subenvironments has been related t o a steeply shelving slope. The model is a modification of Irwin’s (1965) limestone shelf model. In addition, the concept that stromatolite domes increase in size outwards into the intertidal has been extended into the subtidal regime (Truswell and Eriksson, 1973). In this model, sedimentation occurred in a telescoped range of environments, extending from the intertidal, through a highenergy agitated zone, into the subtidal. Columnar stromatolites characterize the intertidal, but


639

more delicate columns are also developed within domes in the shallow subtidal environment. In the agitated zone, the carbonates contain breccias, ripple marks, oolites and oncolites. Deeper subtidal carbonates are dominated by elongate stromatolitic mounds, up to 40m long and 1Om wide, with a vertical inheritance of up to 1 3 m , and a relief of up to 2.5m. The microstructure of these mounds is a delicate crinkled lamination. Elongate mounds of a similar order of size have been described from zone C of the Malmani Dolomite at Zwartkops and elsewhere in the Transvaal. At Zwartkops they are considered to have formed offshore from aripple-marked tidal-flat assemblage. Talus breccia marks the assumed steeper slope between the prograding tidal flat and the subtidal domes (Truswell and Eriksson, 1975).

The lagoonal environment In the eastern Transvaal, iron formation has been shown to be an end member in a succession of carbonate sedimentary cycles (Button, 1976). It has been shown that iron formation was probably precipitated from solution in a barred basin or lagoon. The barrier behind which the precipitation took place was formed by carbonate detritus consisting of oolitic and intraclastic breccia. Transgressive and regressive shifts of the barrier resulted in sedimentary cycles in which iron formation, iron formation precursor and carbonate detritus are interstratified with limestones and iron-rich dolomites. Near the base of the Malmani Dolomite at Zwartkops a lagoon which developed behind an oolite barrier has been recognized (Truswell and Eriksson, 1975). In contrast to the lagoon described above, rock types here are confined to iron- and manganese-rich dolomites (Fig. 2). PALAEOENVIRONMENTAL AND GEOCHEMICAL SYNTHESIS

As an introduction t o a synthesis of the palaeoenvironmental models and their geochemistry it is necessary to set the general scene during the formation of these rock types in Early Proterozoic times. It is envisaged that sedimentation took place in an epeiric sea during a marine transgression across the craton. The atmosphere at. that time is considered to have been oxygendeficient (Cloud, 197313; Garrels et al., 1973) and probably richer in COz than today. The atmosphere appears to have become more oxidizing from 2.0 b.y. onwards (Cloud, 1973b); thus, the synthesis which will be presented here has application only to sedimentary basins greater than 2.0 b.y. in age. From the palaeoenvironmental models discussed it is possible to recognize four major lithologic associations; namely: banded iron formation, limestones with ferruginous dolomites, limestones with iron-poor dolomites, and recrystallized dolomites and chert. The limestones and ferruginous dolomites


640

are those which constitute the barrier to the iron formation lagoon, the carbonates which formed immediately seawards of the barrier, and also the intertidal facies at Boetsap. The deepest subtidal carbonates are iron-poor pure dolomites; those which have been interpreted as shallower subtidal sediments at Boestsap are also iron-poor and contain limestone remnants; these are grouped together as limestones and iron-poor dolomites. It is now necessary to relate these associations in space. Isopach data for the Olifants River and Ghaap groups are sparse away from the eastern and central Transvaal. Indications are, however, that the axis of the depositional basin had a roughly northeast to southwest orientation (Fig. 1). Thus, much of the original northwestern extent of the basin in the Transvaal and all of the southeastern margin in the northern Cape have been removed by erosion. , The preserved stratigraphic thickness in the Transvaal is composed almost wholly of recrystallized dolomites with chert and iron-poor dolomites with large domes, the other two associations only appearing at the top of the succession along the northwest margin of the basin (Fig. 2). The very shallowwater structures often developed in the recrystallized dolomites and chert suggest that deposition took place close to the basin-edge while towards the depositional axis subtidal conditions prevailed in which the iron-poor dolomites with domes developed. The intertidal and subtidal domes are all elongated in a northwesterly direction (Truswell and Eriksson, 1975),providing further evidence for a northeast to southwest orientation of the depositional axis (Hoffman, 1967).Furthermore, the intertidal domes in the southeast are oversteepened to the northwest, again indicating a basin-edge to the southeast. In contrast to the Transvaal, lithologies in the northern Cape are almost devoid of recrystallized dolomites and cherts. Banded iron formations with which substantial thicknesses of shales are associated, pass through clastictextured carbonates into limestones and ferruginous dolomites and eventually into iron-poor dolomites. This sequence is considered to represent increasing distance from a basinedge to the northwest. A lagoon in which iron formations were formed was separated by a elastic-textured carbonate barrier bar (Button, 1976) from a more open-marine regime where limestones and iron-rich dolomites graded seawards eventually into iron-poor

NW

Barrler-formlng CIastlc Textured Carbonates

SE L.W L.-

Recrystalltzed Dolomltes

Dolomites

Fig. 3. A palaeoenvironmental reconstruction.


641

dolomites with domes (Fig. 3), the latter being the only unifying lithology between the Transvaal and northern Cape. The four recognized lithologic associations thus represent distinct facies. Before discussing the geochemistry of the four facies it is necessary to set some probable constraints on the composition of surficial waters draining into the Proterozoic sea. The reducing nature of the atmosphere at that time has already been mentioned and the fact that both Fe and Mn in the rocks under consideration are principally divalent strongly supports this contention (Eriksson et d., 1975). In the absence of oxidation of Fe and Mn it is considered likely that these metals would have been dissolved in river waters in their crustal abundances relative to Ca. If it is assumed that the concentration of Ca in surface waters has not changed significantly through geologic time, then the waters draining into this Early Proterozoic sea probably had Ca, Fe and Mn concentrations of around 15,20, 35 ppm. An atmosphere significantly richer in COz would have affected the absolute amounts of metals in solution but their relative concentrations would have remained essentially the same. With these concentrations, solubility graphs predict that on increasing pH, Fe carbonates would precipitate first, followed at higher.pH’s by Fe-Ca and finally Fe-Ca-Mn carbonates (Eriksson et al., 1975), the amount of Fe in solution decreasing with time. It is contended that these three predicted geochemical facies are represented in the northern Cape by iron formations, limestones and iron-rich dolomites, and subtidal iron-poor carbonates, respectively (Fig. 3). These are considered to have developed away from an active northwestern basin-edge along which not only substantial amounts of argillaceous sediments but also high concentrations of cations, most notably iron, were being introduced into the depository. The source of Fe, in iron formations has always been a controversial subject. Eugster and Ming Chow (1973)consider that in the absence of free atmospheric oxygen sufficient ferrous iron would be carried in solution to account for iron formations. This becomes even more plausible when the tectonic environment in which these Early Proterozoic iron formations formed is considered. The extremely stable and low-lying hinterland would have been an ideal site for deep chemical weathering with the release of Fe and other cations. A tectonic pulse would have flushed out the groundwater causing it to flow basinwards, carrying with it a suspended clay fraction and dissolved or colloidally dispersed .iron and silica (Button, 1976). Whereas the iron formations and limestones with associated ferruginous dolomites developed in and close to a barred lagoon, the limestones with iron-poor dolomites formed in a more disw subtidal environment which was least influenced by the high concentrations of Fe being washed in from the land. The iron-rich dolomites are invariably associated with limestones and on the basis of field evidence a two-stage process for their formation is envisaged, involving: (1)the precipitation of primary limestone, and (2) the dolo-


Bonded Iron twmatlons ( overage FeOIMnO rotlo)

No.Of analyses 112 Tldal flat dolomites Limestones and Fe-rlch dolomites 31 Subtidal limestones and Fe-poor dolomites 32

4-

0

0 0

0

0

3. 0

0

0 0

2 . 0 8 O

.. ... a0 .

0

0

a 0

1

1

2

3 4 MnO %

5

6

7

8

Fig. 4. FeO vs MnO plot illustrating the grouping of points into four geochemical facies.

mitization of this limestone by solutions rich in both Fe and Mg and also containing significant amounts of Mn (Button, 1976).In contrast, the deeper subtidal carbonates which are composed of pure dolomites formed either primarily or, as seems more likely, through alteration by normal epeiric sea waters. The subtidal carbonates furthest up the paleoslope contain most Mn (Fig. 4) and probably formed as that metal became saturated in solution, substantial amounts of manganese having been taken into the calcite lattice (Eriksson et al., 1975). A major portion of the Malmani Dolomite is composed of an intertidal facies, in which pure recrystallized dolomites alternate with chert. The recrystallized dolomites have low and variable Fe and Mn concentrations and erratic Fe/Mn ratios (Fig. 4). The recrystallization and diagenetic formation of chert are considered to reflect the influence of low-pH meteoric waters (Truswell and Eriksson, 1975). Both processes are promoted by dissolution by meteoric waters, a phenomenon which also enhances dolomitization (Badiozamani, 1973). In contrast to the northwestern margin, that in the southeast appears to have acted in a passive manner with only minimal


643

amounts of clastics and cations introduced from this direction. A similar phenomenon has been observed for clastic sedimentary basins on the craton in which maximum supply of sediments was always from the northwest (Pretorius, 1974). The similar lithologic association to that of the northern Cape which occurs at the top of the succession in the Transvaal is considered to have developed during a southward overlap of the northern iron formation/iron-rich dolomite facies, the majority of which has been removed by erosion. The Fe-Mn-rich lagoonal assemblage which is present towards the base of the Malmani Dolomite is somewhat problematical. It is known that clastic sedimentation was a factor in early Malmani times and it is conceivable that cations could have been intrdduced in some abundance prior to the development of the broad tidal-flat environment. In summary, there are considered to be three distinct environments of dolomitization, which resulted in chemically distinct dolomites. Immediately seawards of the lagoon iron-rich dolomites developed. The preservation of limestones in these rocks is considered to relate to a freshwater dilution with respect to Mg and Fe, resulting in incomplete dolomitization. In the tidal-flat regions, the presence of lower-pH meteoric waters both enhanced dolamitization, and catalyzed the diagenetic formation of chert and recrystallization of dolomites. These dolomites are characteristically poor in Fe and Mn. In the deeper subtidal environment, dolomitization was effected by the alkaline waters of the epeiric sea which had attained a critical Mg/Ca ratio. The vertical stratigraphic relationships observed are related to transgressions and regressions in the epeiric sea. The most dramatic evidence of such a phenomenon occurs towards the top of the succession where proximal lagoonal banded iron formations have come to rest on more distal marine carbonates as a result of a major regression or overlap towards the southeast. In Western Australia rocks of a similar age make up the Hamersley Group and are dominated by iron formations with only a minor development of carbonates. Carbon-isotope determinations led Becker and Clayton (1972)to conclude that those iron formations were precipitated in a basin isolated from the ocean but probably in close proximity to it, and that the carbonates developed in marine environments. The interlayering of the iron formations with carbonates was attributed to transgressions and regressions. Many shallow-water cratonic carbonate sediments give way to a deeperwater clastic facies (e.g. Hoffman, 1974).No evidence of such a relationship was observed in the present study, with an epeiric sea being the only recognizable depositional setting. It is feasible that deep-water conditions may have existed to the south of Prieska. The-recognition of such a facies, however, is most unlikely as a 1.0 b.y. old metamorphic belt occurs in this area. It is also presently unclear what such metamorphic belts represent in terms of Precambrian plate-tectonic movements.


11. MINERALIZATION ASSOCIATED WITH STROMATOLITES

Chapter 11.1

MINERAL DEPOSITS ASSOCIATED WITH STROMATOLITES Felix Mendekohn

INTRODUCTION

Many mineral deposits are closely associated with stromatolites which occur either as bioherms (reefs), or biostromes that formed as widespread stromatolitic beds or algal mats. It is likely that there are many more than the ones dealt with here, but these are sufficiently numerous and widespread to suggest that they give a fair sampling of the class. The information available on different deposits shows a wide range in quality and quantity, probably reflecting the knowledge of and interest in the stromatolites themselves, the degree of preservation, and the effects of metamorphism. The term reef has been rather loosely used in many discussions of ore deposits to indicate any mound-, ridge-, or reef-like organic structure; since it is not always possible to be certain of their precise meaning, “reef” is used as the authors use it, and should be understood to include bioherms as well as reefs in the restricted sense of protruding wave-resistant structures. In the descriptions that follow, deposits are grouped according to the valuable element(s) they contain; a summary is presented in Table I. COPPER

The association between stromatolites and ore deposits is well demonstrated and documented in the copper province of Zambia/Zaire. On the Zambian Copperbelt the two main groups of ore deposits are those in argillite (ore shale) and those in quartzite. Bioherms present in many of the ore-shale deposits were not recognized as stromatolitic rocks for a long time because the moderate grade of metamorphism, local shearing along the stromatolite zone, and supergene alteration t o talcose cherty dolomite resulted in the partial destruction of the internal structure. However, the intersection of a number of well-preserved bodies at depth leaves little doubt that all or most are stromatolitic. Stromatolitic bioherms, forming a nearshore dolomitic limestone facies of the argillaceous Ore Formation (Garlick,

646

F. MENDELSOHN

Fig. 1. Portion of the Roan Antelope orebody, in the vicinity of the Irwin shaft barren gap, shown unrolled and restored as nearly as possible to its attitude and relations before folding, but with the vertical scale much exaggerated. The barren dolomite, considered to be part of a stromatolite bioherm, occupies the whole of the Ore Formation and rests on a hill of basement rocks; beyond, the Ore Formation rests on footwall beds. The barren dolomite thins to a feather edge and is then replaced by argillite; beyond, the widespread carbonate at the base of the Ore Formation, containing copper/iron sulphides and forming a second orebody where marked “mineralized”, is an impure dolomitic limestone representing an original algal mat. The copper-rich zone in the argillite spreads outward from the bioherm and away to the right like a plume, but decreases to normal grade in this direction.

1964), occur in many instances over ridges or hills of basement that formed promontories or islands, projecting to or slightly above the base of the Ore Formation during deposition. The bioherms themselves are barren of copper and related minerals except in shaly inclusions, but the adjacent ore shale is richly mineralized, the metal content decreasing outwards and being reflected by a mineral zoning of bomite-chalcopyrite-pyrite (Fig. 1;Garlick, 1964). A number of stromatolitic bioherms are now known for 50 km along the eastem margin of the ore shale deposits, from Roan Antelope through Nkana and on to Chambishi (Annels, 1974; Clemmey, 1974);associated layers of impure dolomite, notably at the base of the Ore Formation (Fig. l), are considered to represent original algal mats with included detritus. The

MINERAL DEPOSITS

647

Fig. 2. Stromatolite bioherms at Mufulira, in argillite immediately beneath the quartzite of the ‘B’ orebody. It can be seen how stromatolite growth started in the lower shale, but was inhibited and almost stopped by a thin layer of sand (white); in the succeeding dark shale growth again became vigorous, t o form the bioherm, which is about double the width shown. The ‘B’ orebody quartzite can be seen at the top, starting at the handle of the hammer; the quartzite immediately above the bioherm is mottled for a few feet by poorly developed algal growths.

zone of bioherms and sandy sediments marks the eastern shoreline, offshore is a zone of shale about 2 km wide that contains the copper orebodies, and beyond is a 3-8 km zone of pyritic carbonaceous shale; beyond this again are (deeper-water) carbonates (Garlick, 1961, p. 157), or terrestrial sediments (Annels, 1974). These dimensions and rock types indicate that deposition took place in an interdeltaic coastal marine lagoonal environment (Hamblin, 1973). Van Eden (1974)concludes that broadly similar conditions of deposition, with offshore shoals, existed during the deposition of the sands that now form the Mufulira C orebody, one of the major quartzite deposits. The quartzite orebodies are in general devoid of stromatolites, but the first stromatolites recognized on the Copperbelt were in a bioherm in argillaceous beds immediately below the Mufulira B orebody fringe (Malan, 1964;Figs. 2, 3);Paltridge (1968)found that an extensive algal biostrome, containing bodies of less well-developed stromatolites, spread away from the original occurrence. Malan (1964) reports a low copper content in the

648

F. MENDELSOHN

Fig. 3. Close-up of stromatolites of the Mufulira bioherm, in shale, showing shape and structure. The dark lines in the stromatolites are micaceous material similar t o the adjacent argillite and represent sediment that collected on the flat tops of the stromatolites during growth; little or none is present on the steep flanks.

bioherm, proportional to the amount of argdlaceous material included in or between the stromatolites; Paltridge reports a similar low copper content in the carbonate part of the biostrome, but as much as 7% in fine-grained algalbedded sandy sediment, which in places forms the foundation for stromatolites. In the continuation of the copper-bearing Katanga formations in the Shaba (Katanga) province of Zdire, the Sene des Mines is considered to be the stratigraphic equivalent of the ore-bearing formations of Zambia (Franqois, 1974). The ore deposits within the Sene des Mines consist of two mineralized formations, the upper a dolomitic shale like that of the Zambian argillite deposits, and the lower a finely laminated siliceous dolomite (Oosterbosch, 1962). The intervening regular bed of dolomitic limestone in places consists wholly or partly of stromatolites, identified as Collenia undosa (Oosterbosch, 1962,p. go),forming an algal biostrome; in places bioherms protrude into the overlying dolomitic shale (Fig. 4).As in Zambia the stromatolite formation is barren of copper, except for occasional inclusions of cupriferous shale. The stromatolites here are cylindrical, and on weathering are partly replaced by silica to form hollow cylinders and the rock is known as “roches siliceuses cellulaires” (Fig. 5 ) .

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Fig. 4. A bioherm in the Musonoi open pit, Shaba, Zaire, projecting about 4 m into the dolomitic shale o f the upper orebody, which is in part draped over the bioherm. The underlying stromatolitic carbonate formation (roches siliceuses cellulaires, RSC) from which the bioherm grew is hidden under the broken rock on the right.

At Matsitama in Botswana, in an isolated basin of metamorphosed formations that might be correlated with the Katanga, a deposit of potential economic significance occurs in quartz-carbonate rock, dolomite, and shale. A quartz-carbonate body that is probably a bioherm is more richly mineralized with copper than the laterally adjacent shale and biostromal beds. A t Mount Isa, Queensland, the Urquhart shales of the Middle Proterozoic Mount Isa Group (1,600 m.y.) contain important lead-zinc and copper deposits; the Urquhart shales consist of dolomitic shales and siltstones, dolomite, and tuffaceous shale. Galena, sphalerite, and pyrite occur in distinct concordant layers throughout the Urquhart shales, and where sufficiently concentrated, form the fourteen known orebodies, which are arranged en echelon, plunge south, and have in places suffered folding and faulting (Bennet, 1965). There is a broad but distinct mineral zoning, from pyrite in the north into sphalerite-pyrite-pyrrhotite, then rich galena-sphalerite orebodies that terminate against silica-dolomite bodies. The silica-dolomite contains pyrite, pyrrhotite, and chalcopyrite, and in places close to the lead-zinc bodies but separate from them, are the copper orebodies. The coarse ‘disordered-looking’ silica-d olomite bodies have been interpreted as

650

F. MENDELSOHN

Fig. 5 . Siliceous tubes of the RSC stromatolitic carbonate that led to the naming of the formation the “roches siliceuses cellulaires”. The outer, presumably purer carbonate portions of the columnar stromatolites (see Fig. 3) were preferentially replaced by silica, and the interior portion was weathered away; remnants of the stromatolite layering can be seen inside the tubes.

being algal reefs and reef-breccias, the finely bedded shale being off-reef organic tuffaceous sediment (Garlick, 1964;Stanton, 1972). In the Middle Proterozoic Belt Supergroup in Montana, a stratiform copper deposit has been found in feldspathic quartzite and siltstone of the Revett Formation, and subeconomic or unproven copper- silver concentrations are known in various places through the higher Belt formations (Harrison, 1974). In the Helena Formation of the middle Belt carbonate sequence, disseminated copper minerals are associated with algal stromatolite mounds and other structures of probably algal affinity (Fig. 6). The Infracambrian formations of the western Anti-Atlas of Morocco, consisting of sandstones, schists, and limestones, contain a number of small copper deposits at various stratigraphic horizons, many on the flanks of basement highs (Chazan and Fauvelet, 1962). The Tamjout series, consisting of recrystallized reef dolomites containing Collenia, overlain by a layer containing lenses of white quartz, is widely but weakly mineralized with lead, zinc, vanadium, and pyrite, and has several copper deposits.

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Fig. 6. Stromatolitic mounds in the Helena dolomite of the Belt series, Montana, U.S.A. In the cliff face behind the mounds are other features that are probably algal, and disseminated copper minerals occur in similar rocks nearby.

In the Bijawar formations of Madya Pradesh, in an old copper-mining district, Collenia-bearing dolomite is overlain by a more siliceous layer containing cyclindrical Conophyton (Balasundaram and Mahadevan, 1972). Chalcopyrite and galena occur disseminated and as veinlets in the formations containing the stromatolites, in places within the stromatolite zones.

LEAD-ZINC

Lead-zinc (barite- fluorite-copper-cobalt) deposits are widespread in the Mississipi Valley of the United States, the great majority in carbonate rock ranging from limestone to dolomite (Beales and Onasich, 1970), and the class of deposit represented here is known as the Mississipi Valley type. The deposits are spread through almost the complete range of Phanerozoic rocks, but over 80% of the ore lies in formations of Cambrian-Ordovician and Mississipian -Pennsylvanian age. Stromatolite reefs are associated with a number of the deposits that are present in Late Cambrian and Early Ordovician formations in southeast Missouri (Wagner, 1961;Myers, 1966;Snyder

652

F. MENDELSOHN

PLAN

0

S ECT 10N

-

30 60 r rn

VERTICAL EXAGGERATION l o x

/

RIDGE

Fig. 7. Stromatolite reef over topographic high in the southeast Missouri lead belt; lead minerals occur mainly on the upper and lateral margins of the reef, and also in the overlying formations (after Wagner, 1961).

and Gerdemann, 1968) and to a lesser extent in Tennessee (Kendall, 1960), but in the upper Mississippi district stromatolites are absent from the ore formations (Heyl, 1968). The southeast Missouri lead belt is situated on the flank of a basement high and here Late Cambrian reefs, now interpreted as algal banks (Larsen, 1973), of digitate or columnar Collenia or Gymnosolen, lie over ridges of either basement rocks or Early Cambrian sediments (Fig. 7), accompanied by widespread discontinuous biostromes formed from stromatolite mounds. The sulphide minerals are closely associated with the stromatolitic formations, the major concentrations commonly being along the tops or edges of the elongate reefal structures. Associated black pyritic shales interrupted in places by patch reefs represent the muddy deposits accumulated in lagoons within fringing reefs (Myers, 1966). In east Tennessee, zincsulphide concentrations associated with reefs in the Late Cambrian Longview

MINERAL DEPOSITS

653

Formation form a significant proportion of the ore in some mines in this area. These reefs are 3- 5 m across and up to 2 m high and are built up of fairly even laminae, without columnar structures, but are considered to be stromatolitic (Kendall, 1960);sphalerite occurs mainly as distinct caps, up to 30 cm thick, on the reefs, with some along other margins but little internally. On Baffin Island, Northwest Territories, Canada, the Middle Proterozoic Society Cliffs Formation is predominantly an algal carbonate that is brecciated and contains specks and massive bands of sphalerite, galena, and pyrite (Geldsetzer, 1973). The carbonate consists mainly of finely to irregularly laminated stromatolitic dolomite with occasional biohermal and nodular structures, interpreted by Geldsetzer as being of subtidal to intertidal origin. In Bahia, Brazil, lead-zinc orebodies in the Bambui are associated with Collenia (Cassedanne, 1966,1969). PHOSPHATE

In the Aravalli metasediments of Udaipur, India, phosphorite is.closely associated with stromatolitic dolomite overlying orthoquartzite (Banejee, 1971b), the stromatolites concerned being Collenia symmetrica, Minjaria, and Baicalia. In detail, the phosphate occurs as: concentrations in stromatolite columns, laminar algal (stromatolitic) phosphorite, reworked fragments of stromatolites forming silicified conglomeratic or brecciated phosphorite, massive-bedded phosphorite with sandy and clayey laminae, and disseminated pellets and nodules in dolomite. Phosphorites are widely distributed, the P,O, content ranges from 10%to 37%, and the major ore mineral is apatite. Bannerjee also refers to the occurrence of Collenia and oncolites in other Late Precambrian phosplioritic formations in India (Gangolihat Dolomite), Russia, and China. IRON

Several of the major deposits of iron formation of Early Proterozoic age, around 2,000-2,200 m.y. old, are associated with stromatolites and carbonate rocks, and there is evidence that there was algal and bacterial activity in a substantial number of them (Cloud, 1973;La Berge, 1973). The Biwabik Iron Formation in the Mesabi district of the Lake Superior iron province contains two zones of cherty columnar stromatolites (Bayley and James, 1973), and underlying dolomite also contains stromatolites. On the Ontario side of Lake Superior, the equivalent Gunflint Iron Formation consists of interbedded stromatolitic chert, bedded chert, taconite, jasper, limestone, and black shale (Hohann, 1969b; Douglas, 1970, p. 118); Moorehouse and Beales (1962)describe jaspery and carbonate stromatolites

654

F. MENDELSOHN

(Collenia and Gymnosolen ?), and also “reefs” (bioherms), one with a core of stromatolitic chert and jasper but a surface consisting mainly of calcarenite and taconite. In black shale of the overlying Rove Formation at Kakabeka Falls powerhouse a cherty reef-like structure has a capping of pyrite about 5 cm thick, and oncolites occur nearby in the shale. At Steep Rock, Ontario, iron ore in the Archaean to Early Proterozoic formations (Jolliffe, 1966; Cloud, 197313) overlies dolomite containing columnar stromatolites. In South Africa, iron formation of the Transvaal sequence overlies a thick dolomite that contains abundant stromatolites (Truswell and Eriksson, 1972), and stromatolitic limestone occurs immediately beneath the iron formation in both the Cape and Transvaal Provinces. In the Cape region, pyritic stromatolitic limestone and pyritic carbonaceous shale underlies the ferruginous chert and pyritic carbonaceous shale of the transition zone (Beukes, 1973).In the eastern Transvaal the transition zone between the two consists of interbedded stromatolitic limestone, pyritic dolomite and mudstone, and iron formation; the limestone contains laminar, columnar, and domal stromatolitic structures (Button, 1973c), and Button (1976) also emphasizes the relation between carbonate and iron formation. MANGANESE

A t Kisenge in the western part of Shaba province, Zaire, a stratiform manganese deposit in sericite schists of the Lukoshi Complex is cut by a pegmatite dated at 1,850 m.y. (Doyen, 1973). Below the oxide zone, the deposit consists of alternating beds of manganese carbonate, “gondite”, and graphite schist; the carbonate consists of almost pure rhodochrosite, and the gondite of spessartite, a manganese-bearing garnet. The carbonate ore, with about 40% manganese, contains spessartite and small amounts of nickel and cobalt sulphides. Within the carbonate ore are several layers of stromatolites that are stated by Doyen to resemble Collenia. Doyen concludes that the manganese was of sedimentary origin and that subsequent metamorphism led to the development of garnet in the more impure carbonate layers. In the Transvaal Black Reef Formation in Botswana small stromatolites consisting of and embedded in manganese oxide, are associated with a layer of manganese nodules having a general similarity to those being formed today on the ocean floors; it is not known whether the manganese oxide replaces carbonate or is an original constituent (Litherland and Malan,

1973). Monty (1973)found that matiganese nodules from the Blake Plateau and the south Atlantic have a very fine wavy and regular lamination, characteristic of stromatolitic structures, and consisting of fine dark brown and lightcoloured laminae; in the dark laminae, 6-8pm thick, are manganese-bearing bacterial rods and tiny filaments. The nodules result from the rhythmic

MINERAL DEPOSITS

655

growth of filamentous bacteria concentrating manganese and iron minerals within their sheaths, and it is possible that regional variations in composition might be dependent on the biochemical activity of the bacteria. Monty considers that they can be regarded as deep-water oceanic stromatolites. GOLD AND URANIUM

The Early Proterozoic (2,300-2,600m.y.) Witwatersrand formations of South Africa consist of an upper arenaceous division and a lower more argillaceous division. Most of the gold-uranium deposits occur in the lower part of the upper division, in or adjacent to conglomerate layers at the bases of cycles of sedimentation (Pretorius, 1974). In the western and southwestern parts of the goldfield, the bulk of the gold and uranium is present in carbon seams at the top of certain of the cycles of sedimentation; Pretorius (quoted in Payne, 1974) estimates that in recent years, 40% of the gold produced came from carbon seams, and the proportion is probably increasing. These carbon seams are considered to represent algal mats (Pretorius, 1974) or, according to Hallbauer and Van Warmelo (1974),mats of a material resembling modem lichen and probably consisting of algal and fungal organisms. Hallbauer and Van Warmelo present evidence to show that gold and uranium were assimilated by these plants during growth, and also physically trapped as detrital grains between carbon columns. In the Rum Jungle area of Northern Territory, Australia, uranium and associated copper, lead, and gold deposits occur in Early Proterozoic (1,800-2,200m.y.) formations. The deposits are closely associated with silicified limestone breccias, partly reefal in origin, or occur in sediments of off-reef facies, siltstone and carbonaceous shale with intercalated limestone lenses (Condon and Walpole, quoted in Garlick, 1964). Crohn (in Walpole et al., 1968) reports both agreement and disagreement with the interpretation of a reefal environment by subsequent workers. Based on his work at Nabarlek, Cooper (1973)suggests that regional uranium mineralization took place in Late Proterozoic times (710-815m.y.). FLUORITE

In the western Transvaal, South Africa, fluorite is present in significant amounts at the top of the Dolomite of the Transvaal Supergroup, in what was a minor lead-zinc district in the past. The presence of abundant stromatolites in the Dolomite has already been referred to, and a good deal of the fluorite in this area is associated with stromatolites, economic concentrations being largely confined to stromatolite bioherms.

TABLE I Summary of data on mineral deposits and associated stromatolites Formation

Age

Lo cat ion

Form of stromatolites

Elements

Nodules Bonneterre

Recent L. Cambrian

Ocean SE Missouri, U.S.A.

Bacterial Collenia (Gymnosolen)

Mn, Fe (Cu, Co, Ni) Pb, Zn

Longview Tamjout Unknown Unknown Aravalli

L. Cambrian Infracambrian Middle Riphean Sinian Precambrian (?)

Tennessee, U.S.A. Morocco Russia China Rajasthan, India

?

Zn cu P P P

Gangolihat Bambui Katanga Katanga

Middle Riphean Middle Riphean L. Proterozoic : 900 m.y. L.Proterozoic: 900 m.y.

Pithoragarh, India Bahia, Brazil Copperbelt, Zambia Shaba, Zaire

Society Cliffs Belt Mount Isa Matsitama Gunflint

L. Proterozoic: 1,000 m.y. M. Proterozoic: 1,200 m.y. M. Proterozoic: 1,700 m.y. L.Proterozoic (?) E. Proterozoic: 2,000 m.y.

N.W.T., Canada Montana, U.S.A. Queensland, Australia Botswana Ontario, Canada

Biwabik Steep Rock ?

E. Proterozoic: 2,000 m.y. E. Proterozoic/Archaean E. Proterozoic: 1,800-2,200m.y.

Transvaal Transvaal

E. Proterozoic: 2,250m.y. E. Proterozoic: 2,250m.y.

Lukoshi Bijawar

E. Proterozoic: Precambrian

Minnesota, U.S.A. Ontario, Canada Northern Territory, Australia Cape Province, S. Africa Transvaal, South Africa; Botswana Shaba, Zaire Madya Pradesh, India

Witwatersrand

E. Proterozoic: 2,500m.y.

> 1,850m.y.

South Africa

Collenia Collenia oncolites Baicala Minjaria Collenia sy m me trica ?

Collenia Collenia Collenia undosa

P Pb (Zn) c u (CO, U) c u , co (U)

? ? ? ?

Collenia Gymnosolen (?) ? ? ?

Fe Fe U (Cu, Pb, P)

?

Fe Fe, Mn

?

Collenia Collenia Conophy ton stratiform

Mn (Co, Ni) Cu, Pb Au, U

9

MINERAL DEPOSITS

657

ANALOGOUS DEPOSITS ASSOCIATED WITH OTHER BIOHERMS (Table I)

A substantial number of mineral deposits in Phanerozoic host rocks are associated with bioherms built by corals, rudistids, orbitilinids, stromatoporoids, and other animals, or with bodies loosely referred to as reefs, biohems, etc., and with unknown (or partial algal) affiliation. Because of their general similarity to many of the deposits in stromatolitic formations already described, some of these are listed or briefly described below. Lead-zinc deposits in or closely associated with reefs of Devonian or Carboniferous age are those of Pine Point, Canada (Campbell, 1967);Meggen, West Germany; and Tynagh and Silvermines, Ireland (Pereira, 1967).In reefs associated with lead- zinc deposits in the Canning Basin, Australia, Monseur and Pel (1973)list the reef-building organisms as stromatoporoids (see also Ch. 10.4 herein). Maucher and Schneider (1967)show the Triassic Alpine type lead-zinc deposits to be restricted to a few distinct beds of the thick limestone-dolomite sequence; the Ladinian deposits are bound to reef complexes, occurring mainly in a few layers in the back-reef facies, with a few iron (zinc) bodies in the coral reef itself. In northwest Africa, Monseur and Pel (1973)list the following Jurassic stratiform lead-zinc deposits that occur in reefs, many also extending into the back-reef facies: Bou Dahar (Pb), Bou Arhous (Pb), Tagount (Pb), Touissit (Zn, Pb) in Morocco; El Abed (Zn, Pb), Dominique Luciani (Zn, Pb), and Deglen (Zn, Pb) in Algeria. In addition there are a number of nonstratified deposits in reef complexes, and several deposits containing manganese and/or iron, such as Tiharatine, Youdi, and Bou Arfa in Morocco. Lower Cretaceous deposits include Quenza and Bou Khadra (Fe), and Mesloula (Pb) in Algeria, and Reocin (Zn, Fe, Pb) in Spain, as well as a number of non-stratified deposits. The Ruby Creek deposit in Alaska is a stratiform copper deposit in interbedded dolomite, limestone, and phyllite that form part of a Middle Devonian reef complex, containing stromatoporoids, corals, brachiopods, and gastropods (Runnels, 1969). In the Jurassic Todilto Limestone, Grants, New Mexico (Perry, 1963),uranium is present within and especially on the edges of elongate to arcuate reefs. Perry considers that the reefs are of algal origin, though they are coarsely crystalline and no internal structure is preserved. Fluorite pipes occur in reef complex limestone of the Carboniferous limestones of Derbyshire (Ford, 1969). DISCUSSION

The mineral deposits associated with stromatolites fall into the broad class of stratiform deposits in sedimentary rocks, since they are with few exceptions either stratiform in habit, or are portions of stratiform deposits. They

658

F. MENDELSOHN

occur in biohermal or biostromal stromatolitic rocks, or in closely associated rocks of both the fore-reef and back-reef environments, so that the hosts are dominantly carbonates, dolomite and limestone, or argillites , but also range to arenaceous and even coarser sediments in places; interbedded volcanic rocks are common, but intrusive magmatic rocks are conspicuously absent. These deposits occur in rocks of Proterozoic or Early Phanerozoic age, encompassing a 2-b.y. period (2.6-0.5 b.y) during which stromatolites reached their greatest development (e.g. Glaessner, 1968). In rocks of postEarly Ordovician age, broadly similar deposits are associated with bioherms built by corals, stromatoporoids, rudistids, etc., rather than the blue-green algae that were the main builders of the stromatolites. It is not clear to what extent algae and bacteria were concerned with the building of the Phaneroozoic bioherms; perhaps they were important but were not preserved, or their role was originally unimportant because they were destroyed by the action of grazing and burrowing animals, as described by Garret (1970b). There have been several references to the temporal partition of elements in the broad class of stratiform mineral deposits in sedimentary (volcanic) rock sequences, such as those by King (1965), Mendelsohn (1967), and Pereira (1967). For example, King describes a broad depositional pattern in which iron concentrations appear first, then copper, followed by lead-zinc and phosphate. Turning to the biohermal environment, Monseur and Pel (1973) find that copper is frequent in Precambrian and Infracambrian reefs, whereas zinc and to a lesser degree lead prevail in the Paleozoic and Mesozoic reefs, and that the associated rock also changes gradually from dolomite to limestone over this period. A wide range of valuable elements is concentrated in the deposits associated with stromatolites, and though the data are not sufficient to provide a complete picture, there is a suggestion that the different elements tend to be confined to host rocks laid down in particular periods. The major Superior-type iron deposits of the world were laid down during the Early Proterozoic (Goldich, 1973), and it is therefore not surprising that the iron deposits associated with stromatolites are confined to host rocks of this age. The Witwatersrand gold-uranium hosts are also Early Proterozoic, slightly older than the iron deposits, as are other gold-uranium-pyrite conglomerates. The first major copper and lead-zinc deposits associated with stromatolites appear at 1,700 m.y., though there seems to be a concentration of cupriferous deposits between about 1,200 and 700 m.y.; lead and zinc hosts are confined mainly to the period from 1,000 to 500 m.y., though they again become important in Late Paleozoic and Mesozoic biohermal deposits. Phosphorus is concentrated in midRiphean rocks, laid down between 1,350 and 950 m.y. Manganese occurs in Early Proterozoic hosts, then again in the modem oceanic nodules. The genesis of stratiform deposits in sedimentary rocks has been the subject of considerable debate for many years, and there has been a substantial swing away from earlier concepts of introduction of elements from late

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659

intrusive bodies, to those concerned with syngenetic and diagenetic processes. The genesis of mineral deposits associated with stromatolites must be considered as a part of this broad picture, though they have special aspects and problems, some of which are considered below. Banerjee (1971b) concludes that the Udaipur phosphorite was directly related to the action of stromatolitic algae, which led to the precipitation of phosphate and the trapping of phosphatic sediment in a shallow basin. Trudinger and Mendelsohn (Ch. 11.2) discuss the precipitation of phosphorus in shelf areas by chemical activity, or its accumulation by phytoplankton, though there is no direct evidence of phosphorus concentration related to stromatolite formation. There is little doubt that the gold and uranium in the Witwatersrand formations were deposited and concentrated during the deposition of the host rocks (Pretorius, 1974). The organisms that form the “algal” mats, whether they were algae or a lichen-like form, seem to have played an active part in collecting a substantial part of the gold and uranium, both physically and organically. In the case of the uranium (gold-lead-copper) deposits of northern Australia, the evidence is conflicting (Walpole et al., 1968), and it does not seem likely that stromatolite growth contributed directly to ore deposition, though the role of the bioherms as a host during later processes could have been important. It is generally accepted that the Early Proterozoic iron-formations are chemical sediments, though there are many differences regarding the mechanisms and conditions of derivation, transport, deposition, and diagenesis of the iron, silica, and associated elements (James and Sims, 1973). In many basins the iron formation is closely associated with stromatolitic carbonate rocks, although they may not be exactly contemporaneous, and with bacteria and blue-green algae. The ability of microorganisms to precipitate iron is discussed by Trudinger and Mendelsohn (Ch. 11.2). Cloud (1973b) suggests that, in Early Proterozoic time, the interaction of oxygen-producing photosynthetic blue-green algae and ferrous iron led to the removal of excess free oxygen, the conversion of ferrous iron to ferric iron, and the precipitation of iron as ferric oxides or hydroxides. Cloud suggests that at this time the pH was generally too low for the formation of carbonates, though Button (1975) suggests that in the Transvaal, offshore bars of clastic limestone led to the formation of barred basins within which evaporation led to iron deposition. It seems that, while microorganisms were probably associated with iron deposition, and algal mats might occur, stromatolitic carbonate rocks are unlikely to be intimately associated with iron formations, though they are an integral part of the ore-forming cycle and some forms could tolerate a substantial amount of iron in the environment. Manganese is known to accumulate in stratiform deposits during sedimentation (Park and McDiarmid, 1964), and the work of Doyen (1973) and Litherland and Malan (1973) suggests the possibility that stromatolites were

660

F.MENDELSOHN

at least able to grow under conditions where manganese was precipitating. Evidence suggesting the direct precipitation of manganese in modern oceanic nodules by stromatolite-forming microorganisms is given by Monty (1973), and Trudinger and Mendelsohn (Ch. 11.2) show the close relation that can exist between manganese deposition and the action of microorganisms. It seems likely that some manganese deposits formed during, and perhaps related to, the growth of stromatolites. Copper, lead, zinc, and iron sulphides, and associated minerals such as other sulphides, barite, and fluorite, provide some of the more spectacular and important examples of mineral deposits associated with stromatolites, as well as with other biogenic rocks. For the Mississipi Valley lead-zinc and related deposits, and many of the Phanerozoic deposits related to biogenic rocks that have been listed, a widely accepted idea is that the valuable elements were introduced by circulating or connate brines that were heated and moved up-dip from the depths of the depositional basins during diagenesis. The porous reef rocks provided channel ways for the solutions, and contained organic material and sulphur that created a reducing environment and led to precipitation of these elements as sulphides (Campbell, 1966;Beales and Onasich, 1970,etc.). The evidence is persuasive, and there can be little doubt that this process did operate in a number of these deposits. For the Baffin Island lead-zinc deposit, Geldsetzer (1973) attributes syngenetic dolomitization and sulphide mineralization to seawater-derived brine that circulated through karst-brecciated algal carbonates. In the Zambia-Zaire copper-cobalt province, Garlick (1965,1972) has summarized many of the arguments for syngenesis, while Bartholomd (1974) makes a convincing case for the diagenetic introduction of copper and cobalt at Kamoto; Annels (1974) proposes a similar process for the Zambian Copperbelt, and Van Eden (1974) suggests a combination of the two at Mufulira. Clemmy (1974)suggests a combination of tidal-flat, marine, and lagoonal condtions of sedimentation for the Ore Formation at Nkana; the copper content is related to shoreline, facies, and tidal directions, and mineralization took place during sedimentation. The bioherms in this province, which could be expected to have had the greatest permeability during diagenesis, are barren of sulphide minerals, whereas formations that represent algal mats and other stromatolitic biostromes are mineralized, the copper content commonly being related to the argillaceous content. These features suggest that mineralization was contemporaneous with sedimentation for the ore-shale deposits, and that the metal was introduced into the host rock together with the clay minerals; the stromatolitic formations and the organisms that formed them played an important role in the sedimentation of the host rock. Diagenetic processes were active in creating or enriching some deposits in this province. The partition of metals is evident in many of 'these deposits, and this is well illustrated at Mount Isa, where copper in biohermal rocks is closely

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661

associated with the stratigraphically equivalent bedded lead- zinc deposits. Bennett (1965) considers that all sulphides there were formed by the precipitation of metals during sedimentation, the sulphur being biogenic, and that the copper bodies formed by local re-mobilization during folding. Trudinger and Mendelsohn (Ch. 11.2) show that the deposition of metal sulphide minerals could take place during sedimentation as a result of biogenic sulphate reduction, or of evaporitic concentration and the subsequent formation of organic complexes. Rickard (1974)considers that biogenic synsedimentary copper sulphide ores are possible, provided an adequate metal source exists; potential environments for stratiform copper-sulphide ore formation are sediments of active oceanic ridges, partly connate subsurface water systems, and near-shore organic-rich sediments near a mineralized hinterland. CONCLUSIONS

The process of mineralization of stratiform deposits associated with stromatolites is an integral part of the whole development of sedimentary basins during sedimentation and diagenesis, and it is perhaps significant that many of the deposits discussed are in intracratonic basins. This general statement can be extended to include deposits associated with other biogenic formations, and even to a large part of the class of stratiform mineral deposits in sedimentary (volcanic) rocks. The favoured sedimentary environment is the barred basin-lagoon- tidal flat-evaporite-sabkha group of ocean-edge environments, of which stromatolitic bioherms and biostromes and algal mats often form an integral part. The valuable elements could be supplied from the hinterland by erosion, from the ocean itself, or from volcanic emanations in the vicinity; depending on the rate and nature of the supply and the conditions of sedimentation, these elements would be widely dispersed in the sediments or concentrated locally. Where mineral deposits are formed, the contribution of the microorganisms could be indirect, helping to create a favourable physical and chemical environment, or more direct in trapping enriched material, helping to precipitate the valuable elements organically, or supplying other necessary elements such as sulphur. Altematively the organisms could merely survive in an environment where deposition of these elements is taking place, without playing any part in the process. Soon after sedimentation and through the diagenetic period, mineralization would take place through the agency of connate brines that leach the valuable elements from their wide dispersion or from already-formed deposits, or perhaps through brines bringing them from a source external to the sediments; the brines would move, generally up-dip, till precipitation takes place in a favourable environment. In this process, the biogenic

662

F.MENDELSOHN

formations could serve by providing channel ways for the solutions, organic material to create a reducing environment, and sulphur in the form of sulphate or biogenically reduced sulphide to precipitate metal sulphides. Later processes could be superimposed on earlier ones, leading to further enrichment or leaching of existing deposits, and also providing conflicting evidence of ore genesis. Late diagenetic and metamorphic processes could be expected to modify deposits by recrystallization or even remobilization and redeposition, so that original textures, characteristics, and processes become obscured; this could be expected to be significant in many stromatoliterelated deposits, which are among the earliest known stratiform deposits. The reason for the temporal partition of elements in these deposits is not entirely clear, but it is in part related to the changing supply of particular elements at or near the surface of the Earth. An example of this is iron, which became abundantly available when the evolving conditions at the surface reached a particular stage in Early Proterozoic times, to form a group of major deposits that has not been duplicated since. Lead does not appear in any stratiform deposit before about 1,700 m.y., and indeed in few known major deposits formed before this time. Copper and zinc are common in Archaean volcanogenic deposits, and copper occurs in Early Proterozoic stratiform deposits, but both appear, with lead, for the first time in stromatolite-related deposits in the Middle Proterozoic at around 1,700 m.y. In addition to the changing supply of the elements to the environment in which stromatolites formed, perhaps there was an evolution of the microorganisms or changes in their ability to survive in or react with concentrations of these elements.

11. MINERALIZATION ASSOCIATED WITH STROMATOLITES

Chapter 11.2

BIOLOGICAL PROCESSES AND MINERAL DEPOSITION Philip A . nudinger and Felix Mendelsohn

INTRODUCTION

Mendelsohn (Ch. 11.1) has discussed the frequent association between mineral deposits and stromatolites and one may enquire whether this association is fortuitous and whether stromatolite-building organisms play some role in mineralization. There are two ways in which reefs and biostromes can affect sedimentation: the building of a reef or the creation of an algal mat affects the physical and chemical environment of sedimentation, and the organisms can participate directly in chemical processes, either actively or passively. Reef building has the major effect of creating lagoons in back-reef areas or within patch reefs, which would result in quiescent, anaerobic conditions favourable for the accumulation and preservation of organic material and metals. It is also possible that a fringing reef could protect the fore-reef area from wind and wave action and thus create similar conditions there. A reef could also form a front between waters of different composition, where reaction could take place and metal become precipitated (Hofmann, 1973). The trapping of sediment by an algal mat is also a significant factor in sedimentation, and it has been shown at Mufulira, Zambia, that the amount of copper present in a stromatolite body is proportional to the amount of sediment present (Malan, 1964). Along the Zambian Ore shale deposits, offshore from the zone of reefs and sandy sediments, a zone of shale about 2 km wide contains the copper orebodies, beyond which is a zone 5-10km wide that contains pyritic carbonaceous shale; beyond this again are (deeper-water) carbonates (Garlick, 1961, p. 157). These are the dimensions and types of formations (apart from the copper?) that could be expected in an interdeltaic coastal marine lagoonal environment (Hamblin, 1973). Van Eden (1974) concludes that broadly similar conditions of deposition, with off-shore shoals, existed during the deposition of the sands that now form the Mufulira ‘C’ orebody. It is perhaps significant that these major orebodies are associated with such an environment, in which conditions similar to those described for back-reef lagoons existed over wide areas.

P.A. TRUDINGER AND F. MENDELSOHN

664 TABLE I

Examples o f element concentration by marine plants (from Bowen, 1966) Element

Concentration in sea water (PPd

Concentration in marine plants (PPm dry wt)

Enrichment factor

Ca cu Fe Mn Ni

400 0.003 0.01

10,00CL300,000 11 700 53 3 3,500 8 150

25-7 50 3,700 70,000 26,500 600 50,000 267,000 15,000

P Pb Zn

0.002 0.005 0.07 0.00003 0.01

Direct contributions to mineralization by organisms may take place by concentration of mineral-forming elements within living cells or by active metabolic transformation of elements. The ability of marine plants (principally algae) to accumulate elements is illustrated in Table I. It is apparent that, following death and sedimentation of cellular material, these organisms are potentially capable of inducing considerable enrichments of a number of elements in sediments. The proportion of cellular material actually reaching the sediment surface, however, will depend on the rates of sedimentation versus those of cell degradation. In some instances, the proportion may be quite low: Deuser (1971), for example, calculated that only about 4% of primary organic carbon is incorporated into the sediments of the Black Sea. On the other hand, in shallow-water environments where growth of microflora may take place primarily at the sediment- water interface, it might be reasonable t o conclude that a high proportion of intracellular metals and other elements would be available for mineralization at the sediment surface. As well as “active” accumulation of elements, “passive” formation of metallo-organic complexes has a profound influence on the mobilization, transport and fixation of metals (Berge, 1950; Saxby, 1969). The frequently observed positive relationship between organic content and metal enrichment in sediments (Krauskopf, 1955) is probably the result of active and passive biological processes. Metabolic reactions leading t o the fixation of mineral-forming elements have been discussed by Silverman and Ehrlich (1964). The reactions are primarily oxidations or reductions and fall into two groups: (1)those which cause direct precipitation, e.g. the oxidation of soluble ferrous and manganous ions to ferric and manganic oxides or hydroxides; and (2) those in which the products react with other elements to form precipitates, e.g. metabolic production of the metal precipitant, hydrogen sulphide. In this chapter examples of both direct and indirect contributions of

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665

biological activity are discussed with reference to the formation of minerals of the types associated with stromatolites. PHOSPHORITES

Large-scale deposition of phosphate minerals frequently occurs in regions where deep, cold ocean waters containing up t o 0.3 ppm PO$- upwell along a shelf and merge with warmer surface waters which have an average PO:concentration in the order of 0.01 ppm (McKelvey, 1967; see discussion by Cook, 1975). Phosphate precipitation has been attributed to purely chemical factors such as loss of C 0 2 and increases in pH resulting from warming of the phosphate-rich waters (Kazakov, 1973). On the other hand, due to the increased availability of nutrients, upwelling regions are also zones of intense planktonic productivity (Ryther, 1963) which may be as much as 4 g C rn-' day-' (Steemann-Nielsen and Jensen, 1957). This has given rise to the hypothesis that organisms, particularly phytoplankton, may be instrumental in accumulating phosphorus which, after deposition and degradation of the organisms, is then released and fixed in surface sediments (see discussions by Bushinskii, 1966; Gulbrandsen, 1969). Based on a phytoplankton productivity of about 1-1.5gC m-2 day-', Gulbrandsen (1969) and Cressman and Swanson (1964) concluded that large phosphorite deposits, such as those of the Permian Phosphoria Formation, could have been derived from planktonic sources in the relatively short time of 2 l o 5 years. A diagrammatic representation of the upwelling-biochemical hypothesis for phosphorite formation is shown in Fig. 1. S H O R E

Increasing temperature, pH salmlty High organic Organic degradation and deposition Apatite precipitation

O C E A N

W a r m bottomwater high pH high saltnity

' /

Cold water low pH High phosphorus

Fig. 1 . Diagrammatic representation of the biochemical-upwelling hypothesis for phosphorite deposition (after Gulbrandsen, 1969).

666


As far as the authors are aware there are no known instances of modern phosphoritic stromatolites. Nevertheless, with some reservations, the arguments advanced for biochemical deposition of phosphate could be equally applicable t o stromatolite-building organisms. The reservations are introduced by lack of agreement of some aspects of the mechanism of phosphorite formation and the conditions under which it takes place. Proponents of “biochemical accumulation’’ hold that shallow-water conditions are necessary to allow for rapid deposition of dead organic matter and a minimum of degradation, which could lead to premature release of phosphorus, during sedimentation (Bushinskii, 1966). Gulbrandsen (1969) proposed that the environment must also be highly aerobic (see also Dietz et al., 1942; Berge, 1972) to facilitate rapid decomposition of sedimented organic matter. These conditions are similar to those under which calcareous stromatolites are formed (Bathurst, 1971). On the other hand uranium in sea-floor phosphorites is frequently in the reduced tetravalent state (Altschuler et al., 1958), which, together with the frequent association of bedded phosphates with organic matter, pyrite and black shales, points to an essentially reducing environment for phosphate deposition (Youssef, 1965). A requirement for lo’w oxygen tension in phosphorite formation had been suggested earlier by Blackwelder (1916) and by Mansfield (1927). A second point of contention is whether phosphorites are composed of primary or secondary minerals. Ames (1959), Youssef (1965) and Pevear (1966, 1967), for example, suggest that at least some phosphorites are formed by secondary phosphatization of carbonate minerals rather than by direct precipitation. In such situations a relationship between stromatolitebuilding organisms and phosphate mineralization could be coincidental, or it might reflect the permeability of many stromatolites.

MANGANESE NODULES

Monty (1973) concluded that manganese nodules from the Blake Plateau are oceanic stromatolites and several others have alluded to the association of organisms with manganese nodules (Crerar et al., 1972; Crerar and Barnes, 1974). Graham (1959) detected organic matter in nodules from the Blake Plateau and suggested that organisms were important in nodule formation (Graham and Cooper, 1959). Greenslate (1974) described structures resembling tubes of Saccorhiza (foraminifer) in nodules from the Pacific Ocean and also reported that Saccorhiza and other, unspecified, organisms on nodule surfaces build structures containing large amounts of manganese and iron compounds. Oborn (1964) reported the presence of abundant Micrococcus (bacterium) in a manganese nodule from deep Pacific Ocean water but, as there was a lapse of nine years’between collection and microbiological examination of the nodule, this finding must be viewed with

MINERAL DEPOSITS

667

reserve. More reliable are the results of Ehrlich (1972 and references quoted therein) who isolated both manganese-oxidizing and -reducing organisms from Atlantic Ocean nodules. The ability t o promote manganese oxidation appears to be quite widespread amongst bacteria, fungi and algae (see, for example, Silverman and Ehrlich, 1964; Alexander, 1965; Schweisfurth, 1970). Deposition of manganic oxides from fresh waters has been particularly associated with chlamydobacteria (Zapffe, 1931; Wolfe, 1960), Hyphomicrobium (Tyler, 1970) and Metallogenium (Perfilev and Gabe, 1965; Sapozhnikov, 1970). These bacteria are characterised by the presence of sheaths, stalks or filaments around which metal oxides are deposited, and this property could possibly contribute t o the development of structure within the oxide precipitates. Filamentous and stalked bacteria are frequently found in ferromanganese nodules in lake sediments (Sapozhnikov, 1970) and in metalliferous deposits in filters (Wolfe, 1960) and pipelines (Tyler and Marshall, 1967a). In the latter instances thick deposits containing 1-50%Mn and 3-25%Fe may be derived from waters containing trace amounts of metals, and the Mn:Fe ratio of the deposit may be the reverse of that in the waters (see Trudinger, 1975). Tyler and Marshall (196713) showed experimentally that biological factors are necessary at least for the initiation of manganese deposition in pipelines. The means by which microorganisms promote oxidation of manganous ions is, however, not known for certain. Three main mechanisms have been proposed each of which might operate in particular circumstances. Sohngen (1914) concluded that oxidation is catalysed by hydroxy-acids such as malate and citrate and suggested that microorganisms contribute by creating an alkaline environment (and, presumably, by producing hydroxy-acids) which favours the chemical reaction. Mose and Brantner (1966) and Brantner (1970) proposed that manganese precipitation is due to microbial metabolism of the organic moieties of soluble organo-manganese complexes. A similar proposal was advanced by Sorokin (1972) for the formation of marine manganese nodules. There is evidence, however, that some organisms possess specific manganese-oxidizing enzymes. Johnson and Stokes (1966), for example, reported that Sphaerotilus discophora developed the ability to oxidize manganous ions only when grown in the presence of manganese and Ehrlich (1968) prepared a protein fraction, which catalysed manganese oxidation, from an Arthrobacter isolated from a marine manganese nodule. Ehrlich (1963, 1966) noted that preformed manganese oxide, or certain other solid materials, were necessary for manganese oxidation by Arthrobacter. He concluded (Ehrlich, 1968, 1972) that manganous ions were initially absorbed on preformed manganese oxide and subsequently oxidized by bacteria with the creation of new absorption sites:


668 chemical

H2Mn03 + Mn2+ -MnMn03 MnMn03

+ 2 H 2 0 + 402biological

+ 2H+

(H2Mn03)2

Whatever the precise oxidation mechanism may be, the expected slow and periodic growth of microorganisms and localized variations in microfloral communities might account for the observed characteristics of nodule growth and for differences in shape, structure and composition of manganese nodules from different locations (Greenslate, 1974).

IRON MINERALS

Species of chlamydobacteria, Hyphomicrobium and Metallogenium (loc. cit.) are associated with the precipitation of ferric iron at near neutrality. This property is shared with Gallionella spp. which are kidneyshaped bacteria characterized by the presence of long, slender, twisted ribbons or stalks (Choldony, 1924). The latter structures appear to be composed largely of oxidized iron compounds bound to an organic matrix. The general properties of the neutral iron bacteria have been discussed by Pringsheim (1949a, b). Acidophilic bacteria of the genera Thiobacillus and Ferrobacillus oxidize ferrous iron enzymically for the production of energy but because of their highly acidic habitats they are probably not relevant to the present discussion. The mechanism of bacterial iron precipitation by the neutral iron bacteria is unknown: there is at present no evidence for enzymic catalysis of ferrous oxidation. Ellis (1907) and Pringsheim (1949b) have suggested that the bacteria may possess specific binding sites for preformed ferric hydroxides while others (e.g., Aristovskaya, 1961; Clark et al., 1967) propased that ferric-organic complexes are absorbed into the bacterial cell and that iron is fixed in capsular material after metabolism of the organic moieties. The iron bacteria are widely distributed in ferruginous waters and they precipitate, ferric hydroxides from solutions containing less than 1ppm of iron (Starkey, 1945). They are frequent inhabitants of environments favouring the formation of bog-iron ore (Harder, 1919). Kuznetsov et al. (1963) suggest that microorganisms play a dual role in bog-iron ore formation: ferric iron is reduced by the microflora in subsoils to the mobile ferrous form which migrates towards the oxidizing zone and becomes reoxidized by iron bacteria. Puchelt et al. (1973) observed Gallionella ferruginea in ferric hydroxide sediments presently forming in bays of the Kameni Islands of the Aegean Sea. They concluded (Puchelt et al., 1973, p. 767) that the bacteria “occur in such masses, that there is no doubt concerning the presence, activity and share of these bacteria in the process of iron deposition”. Periodic mass development of iron bacteria in Lake

MINERAL DEPOSITS

669

Krasnoe of the Isthmus of Karelia is accompanied by a decrease in both ferrous and total soluble iron again indicating a role for these organisms in iron oxidation and deposition (Drabkova and Stavinskaya, 1969). Algae have also been implicated in iron deposition. Pringsheim (1946) noted incrustations of iron oxides on the surface of algal cells and Choldony (1926) suggested that algae participate indirectly by creating an oxidizing environment which facilitates oxidation of ferrous iron. A similar suggestion was advanced by Cloud, 1968a, b, 1973b) who equated the deposition of banded iron formations prior to 2 * l o 9 years B.P. with the development of blue-green algae (see also L.Y. Kodyush, 1969, cited by Alexandrov, 1973). Lepp and Goldich (1964) believe that changes in pH, Eh and C02 concentration brought about by algae and bacteria may have been important factors in the genesis of Precambrian iron formations. The possible contributions of iron bacteria to the formation of massive iron formations has been discussed theoretically by Harder (1919) and by Zapffe (1933) and, more recently, structures resembling modern iron bacteria have been detected in stromatolitic cherts from the Gunflint Iron Formation (Barghoorn and Tyler, 1965). Cherts from a number of iron formations also contain alga-like structures (Gruner, 1922; Tyler and Barghoorn, 1954; Cloud and Hagen, 1965; Cloud and Licari, 1968; LaBerge, 1967, 1973), and on the basis of such fossil evidence it has been suggested that microorganisms were possibly instrumental in precipitating the original constituents of iron formations (Moorhouse and Beales, 1962; LaBerge, 1973). In this respect the similarities noted by Walter (197213) between siliceous stromatolites of the Lake Superior iron formations and those formed by algal and bacterial action in the hot-spring and geyser effluents of Yellowstone National Park, Wyoming (Walter et al., 1972), take on added significance.

SULPHIDE MINERALS

The formation of metal sulphides as the result of bacterial reduction of sulphate is now welldocumented (Miller, 1950; Baas Becking and Moore, 1961; Rickard, 1969; Hallberg, 1972). The responsible organisms are the socalled dissimilatory sulphate-reducing bacteria belonging to the genera Desulfovibrio and Desulfotomaculum. These organisms are strict anaerobes which oxidize organic matter at the expense of sulphate reduction (Postgate, 1959, 1965; Trudinger, 1969), and the overall synthesis of metal sulphides (MeS) may be depicted as follows: reduced organic matter

+ SO:-

-

oxidized organic matter

+ H2S

Me C -+

MeS

670


Rates of biogenic sulphide production in modem euxinic environments appear to be adequate to account for the formation of a number of stratiform mineralized deposits (Temple, 1964;Trudinger et al., 1972;Rickard, 1973; Goldhaber and Kaplan, 1974). Some support for the concept of biogenic sulphide deposits is the fact that patterns of sulphur isotopic composition in such deposits often resemble those predicted from the known isotopic fractionation effects during bacterial sulphate reduction (e.g., Wedepohl, 1971;Sangster, 1968,1971b;Schwarcz and Burnie, 1973). Recently, microfossils of bacterial dimensions have been detected in mineralized zones of the McArthur River lead-zinc-silver deposit, Northern Territory, Australia (Hamilton and Muir, 1974). The major factor controlling the extent of bacterial sulphate reduction 5n natural environments is the availability of organic matter (Sorokin, 1962; Ivanov, 1968; Berner, 1970; Chukrov, 1970) derived from decaying algae and other flora and fauna. It is perhaps not surprising therefore, that mineralized zones of copper, lead, zinc and iron sulphides are frequently associated with stromatolites (Mendelsohn, Ch. 11.1). Renfro (1974) considered that the depositional environments of Kupferschiefer, Roan and certain other evaporite-associated stratiform metalliferous deposits may be likened to evaporite flats known as coastal sabkhas. He proposed that, during periods of regression, oxidized terrestrialformation waters a t low pH mobilize metals from underlying rocks and pass upward through H,Scharged layers of decomposing algal mats where metals are immobilized as sulphides. Jackson and Beales (1967)in a discussion on the genesis of Mississippi Valley-type ores, proposed that in the presence of organic matter such as oil or bituminous sediment, gypsum and anhydrite in, or adjacent to, carbonate horizons, are reduced by sulphate-reducing bacteria to hydrogen sulphide which reacts with lead and zinc transported as metalchloride complexes from adjacent basinal sediments. An alternative role for algae in sulphide mineralization was proposed by Roberts (1973) in relation to the genesis of the Woodcutters lead-zinc prospects in the Northern Territory, Australia. The host rocks of this prospect are highly dolomitic and contain abundant stromatolitic structures (W.M.B. Roberts, 1974, personal communication). Roberts concluded that the initial stages of the formation of the Woodcutters deposit took place in a shallow restricted basin where dense algal growth and evaporitic conditions caused co-precipitation of lead and zinc with precursors of the host-rock dolomite. During subsequent dolomitization the metals, in the form of complexes with products of algal decomposition, were released into pore solutions i h i c h then migrated to the sites of metal-sulphide deposition. Roberts (1973)also concluded that the concentration of lead and zinc in the basin waters from which the Woodcutters deposit was derived, would need t o have been in the order of 0.5-lppm. Heavy metals are often considered t o be extremely inhibitory to microorganisms but there are

MINERAL DEPOSITS

671

several reports of reasonably high tolerance amongst algae (Table 11). In many instances the precise levels of tolerance listed in Table I1 are probably overestimates because of complications introduced by partial precipitation of metals by anions, such as phosphate, in the growth media. Nevertheless, it does not seem unreasonable that vigorous algal growth could take place in the presence of the metal concentrations required by Roberts’ (1973) model, particularly since tolerance to metals may be increased by such factors as the presence of organic matter, ion interactions and adaptation (Sadler and Trudinger, 1967). TABLE I1 Toxicity of metals to algae Alga

Level of tolerance (PPm) Pb cu

Zn

Chlorella vulgarus (Chlorophyta)

110

1

400

Scenedesmus quadricauda (Chlorophyta)

-

0.5-6*

-

Khobotev et al. (1969)

Variety of fresh-water Chlorophyta

0.1-5**

0.1-1**

3-60**

Whitton (1970)

Variety of fresh-water Cyanophyta

-

>25***

-

Mendelsohn (1973)

Reference

Den Dooren de Jong (1965)

*Cu-organic complexes used. **Geometric mean o f “just non-inhibitory ” and “just-lethal” concentrations ***Growth at pH 1 1 : major part of Cu in ionic form as determined polarographically

CONCLUDING REMARKS

The association of stromatolites and other fossil structures with mineral deposits has often been taken as evidence for a possible role of biological activity in mineral formation. In the presentday environment there are several examples of biological processes which contribute, directly or indirectly, t o the fixation of mineral elements and there is evidence that analogous processes may have operated during the genesis of some mineral deposits in the past. Should there have been great “blooms” of algae and bacteria, as seems likely in the presence of sufficient nutrients and the probably higher carbondioxide content in the atmosphere in Precambrian times (Rutten, 1966), the effect on the accumulation of metals could have been substantial. It is considered that, given an adequate supply of metals or other valuable elements and the appropriate conditions, stromatolite formation during

672


sedimentation could be accompanied by the accumulation of these elements to form ore deposits.

ACKNOWLEDGEMENTS

The Baas Becking Geobiological Laboratory is supported by the Bureau of Mineral Resources, Commonwealth Scientific and Industrial Research Organization and the Australian Mineral Industries Research Association Ltd.

12. CONTRIBUTIONS TO THE HISTORY OF THE EARTH-MOON SYSTEM

Chapter 12.1

GEOPHYSICAL INFERENCES FROM STROMATOLITE LAMINATION Giorgio Pannella

INTRODUCTION

In the preceding pages different aspects and uses of stromatolites have been emphasized. Potentially, if not practically at the moment, the use of stromatolites as recorders of geophysical and astronomical conditions of the Earth during its evolution could be even more important. Since growth of stromato!ites is a direct outcome of environmental dynamics, it has been suggested that growth structures could be used to check geological changes in the speed of the Earth’s rotation (Runcom, 1966; McGugan, 1967; Pannella et al., 1968; Pannella, 1972a) and to determine the amplitude of paleotides and polar shifts (Cloud, 1968a; Nordeng, 1963; Vologdin, 1963, 1964; Cloud, 1968a). Fossil organic structures which may provide the first type of information have been called paleontological clocks (Runcorn, 1966). In order to separate the effects of changes in the Earth’s moment of inertia from those due to tidal dissipation, a paleontological clock must provide, from continuous sequences, two sets of data: one on the length of the sidereal day, the other on the length of the synodic month (Runcom, 1964). The crucial and controversial question with regard to the use of stromatolites as clocks is with what precision and continuity stromatolites record, during growth, the astronomically driven environmental fluctuations and how much of this record is left for us to decipher. For several reasons, which will be mentioned in the following pages, stromatolites, in general, are not as reliable paleontological clocks as other fossils. Hofmann (1973) attributes to them either zero or a very low degree of reliability. In this paper it is not intended to take an opposite and over-optimistic view of these applications because stromatolitic growth records in most instances are so incomplete as to justify Hofmann’s conclusion. However, in exceptional instances and with the help of statistical techniques, stromatdites are going to provide precious insights into the largely unknown Precambrian. The strongest reservation to these applications of stromatolites depends on the danger of extrapolating from one localized situation to world-wide generalizations. Another reservation is the uncertainty in respect to determining the absolute age of Precambrian material.

674

G. PANNELLA

Notwithstanding the many difficulties and the rather tenuous results of the first naive attempts, there is ample room for improvement and, perhaps, the ultimate goal of tracing in broad lines the Precambrian evolution of the Earth-Moon system, is not too far in the future. The geologist and the paleontologist with the help of stromatolites will undoubtedly contribute.

STROMATOLITES AS RECORDERS OF PERIODICAL ENVIRONMENTAL CHANGES

The modem analogs of stromatolites provide insight and models of their growth. Light stimulation, by controlling metabolism, plays an essential role in the growth of stromatolites. Short-term experiments, extending from a few days t o two weeks, have shown that laminae of many modem stromatolites form with a daily frequency because of light stimulation of algae or bacteria. Monty’s experiments, in which Bahamian algal oncolites formed laminae consisting of organic-rich (deposited during the day) and inorganicrich lamellae (deposited at night), suggest daily growth is a basic feature of some stromatolites, but add little to the question of the accuracy and continuity of stromatolitic sequences (Monty, 1965b, 1967). Monty noted that there is size limitation of growth for the oncolites which are carried away after about two months growth. Gebelein (1969) described daily layers of about 1mm thick from subtidal algal oncolites from Bermuda and attributed them to the day-night algal growth cycle. He also emphasized the role of currents, and of velocity and rate of sediment movement in the building of stromatolitic structures. Currents over 20 cm/sec d o not permit the formation of algal mats, and only bottom sediment movement of less than 60-80 g/h per foot allows mats and domes t o accrete. The thickness of lamination is inversely proportional t o current velocity. Tidal fluctuations, which control current velocity and direction, can thus leave a record in sequences of laminae. In Bermuda, for example, more sediments are transported at flood tides than at ebb tides (Gebelein, 1969). In intertidal algal mats high-tide flooding creates sedimentrich lamellae, the thickness of which is proportional to the period of flooding, whereas organic-rich lamellae are related to low tides (Gebelein and Hoffman, 1968). The relationships between tidal fluctuations and stromatolitic deposition in the intertidal zone could produce, in areas of semidaily tides, two laminae per day. The possibility that this could reduce the accuracy of stromatolites by producing more laminae than days is not relevant here since intertidal stromatolites, limited and discontinuous in their growth, are unlikely candidates for geochronometry. But the tidal mechanism of stromatolitic growth must be kept in mind when interpreting sequences of laminae. Whereas flooding and air exposure are the controlling parameters of

HISTORY OF THE EARTH-MOON SYSTEM

675

TABLE I Hypothetical frequencies of periodical growth sequences in modem stromatolites Growth-pattern frequencies Tidal bands/year

2 variable Y 0 U

I

Remarks

Daily increments/tidal band 1- 3 (only exceptionally higher)

7 8 - Extreme - - - -high - -water - - of spring - - - tides --- ---

m--

< 5-

< 12

3-7

the frequencies are highly variable depending on tidal types, diurnal inequality

< 20

4-10

columnar stromatolites and algal mats are common forms

a d

a - - - Extreme - - - -low - -water - - of- spring - - -tides - -- -

;

2

z1 rn

laminae are rarely daily; discontinuous sequences ; algal mats are common forms

Stromatolites

Stromatolites: Interaction of Microbes with Sediments

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