Stratigraphy

Pierre Cotillon

Stratigraphy With 115 Figures

Springer-Verlag Berlin Heidelberg New York London Paris Tokyo Hong Kong Barcelona Budapest

Professor Dr. Pierre Cotillon Departement des Sciences de la Terre Universite Claude-Bernard Lyon I 27/43 Boulevard du 11 Novembre F-69622 Villeurbanne Cedex France Translated by Professor James P.A. Noble Department of Geology University of New Brunswick P.O. Box 4400 Fredericton, N.B. Canada E3B 5A3

Title of the original French edition: Pierre Cotillon, Stratigraphic

© Bordas, Paris, 1988

ISBN-13:978-3-540-54675-7 e-ISBN-13 :978-3-642-77025-8 DOl: 1O.l007/978-3-642-77025-8 Library of Congress Cataloging-in-Publication Data Cotillon, Pierre. [Stratigraphie. English] Stratigraphy/Pierre Cotillon; [translated by James P.A. Noble]. p. cm. Includes bibliographical references and index. ISBN-13:978-3-540-54675-7 1. Geology, Stratigraphic. 1. Title. QE651.C7313 1992 551.7-dc20 92-19762 CIP This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitations, broadcasting, reproduction on microfilm or in any other way, and storage in data banks. Duplication of this publication or parts thereof is permitted only under the provisions of the German Copyright Law of September 9, 1965, in its current version, and permission for use must always be obtained from Springer-Verlag. Violations are liable for prosecution under the German Copyright Law.

© Springer-Verlag Berlin Heidelberg 1992 The use of general descriptive names, registered names, trademarks, etc. In this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. Typesetting: Best-set Typesetter Ltd., Hong Kong 32/3145 - 5 4 3 2 1 0 - Printed on acid-free paper

Foreword

"The poor world is almost six thousand years old." Shakespeare, As you like it

Stratigraphy, the study of stratified rocks is with sedimentology, the science of sedimentary rocks, which recently has became independent from it. Its two principal objectives, to evaluate the course of time (geochronology) and to reconstruct past geographies (paleogeography), have, however, remained uniquely stratigraphic questions, unchanged by the progress associated with other sciences and techniques. Fossils may have attracted the attention of man since time immemorial, but the consequences of their study, such as the measure of time and the determination of ancient shorelines, were barely understood before the eighteenth century, when the Neptunists promulgated their extremist views that the entire crust of the Earth was precipitated from the oceans. It was only in the nineteenth century that stratigraphy in the proper sense established itself as an autonomous science. However, it could only solve problems of relative time, allowing the older to be distinguished from the younger, without being able to give a real age. The Earth was old, older than Shakespeare believed, but how old? Towards the middle of the twentieth century, radioactive isotopes began to provide answers to this question, giving stratigraphy its unit of time, millions of years. From that point on, the stratigraphic calendar was supplied with a reference system defined in relation to measurable units of time with names borrowed from geography. This first revolution was followed by another, resulting from the determination of former magnetic fields (paleomagnetism), which means that every point on the Earth could be tracked in its successive positions during time, giving a scientific foundation to the old concept of the mobility of continents, proposed earlier with such foresight by A. Wegener. From then on it was possible to reconstruct the sequence of past geographies as they unfolded in time, i.e. paleogeography. Many other techniques have been developed in recent years to make stratigraphy a new science. Pierre Cotillon, by his work on the'sediments of yesterday's

VI

Foreword

seas in the Alps and on the oceans of today is ideally suited to outline in this short volume the new approach to the history of the Earth, which is like an opera, with stratigraphy being the score. Jean Aubouin

Preface

The major purpose of this work is to outline the successive achievements of one of the oldest geological disciplines, whose basis and major principles date from the nineteenth century. The methods of stratigraphy have been improved, to the same extent as the other Earth sciences, not only by contributions from biology, paleontology, sedimentology, geochemistry, geophysics, and global tectonics, but also by the requirements of petroleum exploration and the large international programs of ocean drilling with respect to age dating. Stratigraphy enables the construction of paleogeographic syntheses which are the basis of all historical reconstructions. The histories of three very unequal segments of time, the Precambrian, the Paleozoic, and the Mesozoic-Cenozoic, are analyzed in the last chapter. For each of these periods, plate tectonics, variations of sea level, climatic trends, and marine and continental sedimentation are discussed successively. Only a few brief lines are devoted to biological phenomena, in spite of their close connections with geological aspects, but they have been treated fully in two books of this same series. A major effort has been to show the interdependence of all the events which constitute the history of the Earth and which have a principal driving force in common residing in the deeper layers of the Earth. Only the most relevant works and specialized articles are mentioned in the bibliography. I am very grateful to Prof. Jean Aubouin, Member of the Institute, who entrusted to me the writing of this book and who willingly criticized and corrected the first manuscript. I thank also, for their advice, my colleagues Raymond Enay, Jean Chaline, and Herve Charnley. Finally, I have benefited from the efficient assistance of Helene Trunde with regard to the text, and of Andre Duivon for the illustrations; I thank them warmly. Villeurbanne, July 1992

Pierre Cotillon

Contents

Chapter 1 Fundamentals of Stratigraphy . . . . . . . . . . . . . . . . . . . . . . . . . . .

1

1 2 3

1 1 3

Definitions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chronology of Events . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Principles of Correlation. . . . . . . . . . . . . . . . . . . . . . . . . . .

Chapter 2 Elaboration of the Fundamentals of Stratigraphy . . . . . . . . . . .

1 2 2.1 2.2 3 3.1 3.2 3.3 4

Lithostratigraphy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biostratigraphy. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Evolution, the Reference System for Age Dating. . . . . . The Zone Concept of Oppel ....................... Chronostratigraphy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. The Concept of the Stage. . . . . . . ... . . . . . . . . . . . . . . . .. Event Stratigraphy. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. The General Chronostratigraphic Scale. . . . . . . . . . . . .. Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

7

7 8 9 10 12 12 15 17 17

Chapter 3 Modern Stratigraphy

19

1 1.1 1.2 1.3

19 20 24

2 2.1 2.2 2.3 2.4 2.5

Refinement of Concepts and Time Scales. . . . . . . . . . .. Evaluation of Geologic Time Intervals and Rates ..... New Biostratigraphic Approaches. . . . . . . . . . . . . . . . . .. Search for a Rigorous and Universal Chronostratigraphy .................. New Methods of Correlation . . . . . . . . . . . . . . . . . . . . . .. Correlation by Sedimentary Rhythms. . . . . . . . . . . . . . .. Correlation by Mineralogic and Geochemical Markers. Correlation by Paleomagnetism. . . . . . . . . . . . . . . . . . . .. Extraterrestrial Correlations ....................... Conclusions......................................

33 37 37 47 56 62 62

x

Contents

Chapter 4 From Stratigraphy to Paleogeography ., . . . . . . . . . . . . . . . . ..

65

1 1.1 1.2 1.3 2 2.1 2.2 2.3 2.4 2.5 2.6

65 65 67 68 75 75 77 77 78 81

Principles and Methods of Paleogeography . . . . . . . . . .. Facies........................................... Paleobiogeography............................... Cartographic Syntheses. . . . . . . . . . . . . . . . . . . . . . . . . . .. Factors of Paleogeographic Evolution ............... Deformation of the Lithosphere ............ . . . . . . .. Volcanic Eruptions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. Interplay of Erosion and Sedimentation. . . . . . . . . . . . .. Eustasy......................................... Polar Wandering ................................. Conclusions: the Earth in Relation to Other Planets of the Solar System .......... . . . . . . . . . . . . . . . . . . . ..

82

Chapter 5 The Major Stages of Earth History . . . . . . . . . . . . . . . . . . . . . ..

83

1 1.1 1.2 1.3 1.4

83 83 85 86

1.5 2 2.1 2.2 3 3.1 3.2 3.3

The Precambrian . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. Boundaries and Subdivisions . . . . . . . . . . . . . . . . . . . . . .. Methods of Study. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. The Geography of the Precambrian ................. Early Segregation and Establishment of Fundamental Processes ......................... Conclusions on the Precambrian . . . . . . . . . . . . . . . . . . .. The Paleozoic: the Formation of Pangea ............. Lower Paleozoic. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. Upper Paleozoic .................................. The Mesozoic and Cenozoic: Breakup of Pangea. . . . .. The Mesozoic .................................... The Cenozoic .................................... Conclusions on the Mesozoic and Cenozoic ..........

87 100 100 101 114 132 133 155 171

General Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 173

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 177 Subject Index. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 185

Chapter 1

The Fundamentals of Stratigraphy

1 Definitions The aim of stratigraphy, or the science of geologic strata, is to study the distribution in space and time of these strata and the events which formed them, i.e. to reconstruct the organization and history of the outer crust of the Earth on the basis of the lithologic documentation obtainable from these superficial layers. The rocks record in their facies the signature of all or part of the dynamic events constituting this history, biological, physical, and chemical. In normal usage, the term stratigraphy is reserved for sedimentary rocks which occur as bedded successions; however, some stratigraphic methods are also applicable to crystalline rocks.

2 Chronology of Events Any history presupposes a succession of events of variable duration within a certain time framework; it is this succession of events, arranged against an appropriate time scale, which represents history in the most natural way. Just as the falling sand of an hourglass gives a notion of time, so does a sedimentary layer formed during a particular time interval also represent that interval, albeit fossilized. Prior to all historical reconstruction, therefore, a stratigrapher must establish the order of deposition of all beds under study, assuming, for normally stratified beds, that the lower bed of any superposed pair is the older (principle of superposition). However, a few exceptions to this principle are illustrated by alluvial terraces, sedimentary veins, cave deposits, etc. (Fig. 1). The order of deposition of different sedimentary beds defines ipso facto the relative chronology of the events which they represent. A succession of sedimentary beds provides a local or regional history, though generally incomplete, by virtue of the record of events it contains. These include metamorphic and plutonic rocks, volcanic flows, veins which cut one another on a regional or thin-section scale, continental erosional and depositional structures, tectonic deformations, and inclusions (Fig. 2). All

Fundamentals of Stratigraphy

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Fig. 1. Exceptions to the stratigraphic principle of superposition. A Benched alluvial terraces; order of appearance 1,2,3. B Volcanic flows: 1 of the plateau (the older); 2 of the valley (the younger). C Veins, the topographically higher (1) older than (2)

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Fig. 2. Local history. A Photographed on Mars by Viking. 1 Old impact crater partially filled with lava; 2 volcanic cone later than 1; 3 impact crater later than 2. B Regional observations: the granitic batholith 2 is later than formation 1 and its deformation. The erosional surface 3 is later than 2 but earlier than the discordant rocks 4. C Observations under the microscope: the foraminifer 1 included in the fragment 2 is older than it. The fragments 2, forming part of the rock, were deposited at the same time. The vein 3, cutting the shell fragment 2 is later than the formation of the rock but earlier than the vein 4 which cuts and offsets it

geological disciplines, therefore, must use stratigraphic principles whenever they wish to refer to the geologic time scale. A local history cannot be used directly to help reconstruct the general history of the globe. The duration and extent of any gaps that the succession contains are unknown. Thus, in a stratified succession the total active periods (i.e. of sedimentation) may be only a fraction of the "dead time" (represented by planes of nondeposition and diastems) during which no new geological documentation is added and some part of the old may be destroyed.

Principles of Correlation

3

w

E -.-------_'!4Permla".

~

easement --

1 - Granite of the basement; 2 - Permian sandstone Infilling depressions of basement reliefs; 3 - The "Conglom6rat principal" forming the first cuestas of the Paris basin and overlying the Vosges sandstones; 4 - Voltzia sandstones and Wellenkalk; 5 - anhydritgruppe; 6 - Upper Muschelkalk (second cuesta); 7 - Keuper; 8 - Rhetian carbonate sandstones with Avicula contorta; 9 - Levallols marls (Upper Rhetlan); 10 - Hettangian sandstones (basal Jurassic and third cuesta) (After Pommerol1975)

Fig. 3. Trias section from the Vosges to Lorraine (NE France): 2-3 sandstones; 4-6 dominantly carbonates; 7 evaporites

3 Principles of Correlation In order to contribute to general Earth history, the local histories must be related to one another by correlation, i.e. compared with respect to their characteristics and chronology. For example, in eastern France and the Germanic Basin the oldest rocks, constituting the basement, are covered with red beds, which pass upwards into a dominantly carbonate assemblage and then into varicolored evaporitic beds. This sequence of three sedimentary events, grossly simplified, constitutes the Trias (Fig. 3)1. But it has been demonstrated that the three lithological groups are not synchronous across the area in question. Furthermore, the Permo-Triassic red beds, or just the Trias, are often discordant on a basement of older, deformed, metamorphic rocks. This discordance can be considered as an important break in the continuity of a geologic history (see below). The correlations of local histories can have two results: 1. An inventory of events and determination of their lateral extent (paleogeographic stage). 2. Documentation of major events of widespread importance, useful for the erection of a global framework subdivided into distinct periods (geochronologic stage). Correlations are effected in two ways: 1. By attempting to follow beds or bed boundaries (litho horizons) from one region to another, the principle o~ lateral continuity is applied. This 1

Of which the lower sandy part is a continuation of the underlying Permian red beds.

4

Fundamentals of Stratigraphy

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Fig. 4. Isochronous and diachronous beds. A,8 Situation in space (A deposition by vertical accretion; 8 deposition by lateral accretion; arrow shows sense of displacement of sedimentary particles; 0 water surface). A', 8 ' Situation in space-time of the two cases; dashed lines are isochrones

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Fig. S. Correlation of events between two points. These events are symbolized by the beds in which they are expressed (see text)

method can only be applied to limited areas because overburden or erosion generally interrupt the outcrop continuity. The beds so followed are only isochronous if they formed by strictly vertical sedimentary accretion. In contrast, they are diachronous where sedimentary accretion, partly or totally lateral, is controlled by currents (delta front for example; Fig. 4). 2. By seeking comparable sequences in different places (sequence stratigraphy). Stratigraphic correlations are effected taking the following principles into account (Fig. 5):

Principles of Correlation

5

a) The duration of an event, as well as its beginning and end, can vary from place to place. For example, a faunal migration will result in such a variation. Therefore, a stratigraphic correlation is not necessarily a time correlation. b) New events can appear between two areas (C, G), and others can disappear (E). c) One event can be laterally replaced by another (lateral facies variation for example; A, A'). d) Gaps in events (lacunae), due to nondeposition or erosion, can exist in any lithologic sequence without necessarily being recognizable (B, E). e) Evidence for events may also be altered by diagenesis or metamorphism. Events of limited lateral extent are of little use in correlation. On the other hand, they can be useful in characterizing the environment. In contrast, major widespread events are very much sought, for they permit longdistance correlations, many of which are regarded as time correlative. The ideal would be a series of events of worldwide extent that are easily recognizable. The search for such a series is one of the major tasks of stratigraphy, as the history of this science demonstrates. To this end, tectonic, biological, climatic, eustatic, chemical, and paleomagnetic events have all been sought; so far, a truly universal stratigraphy has not been possible. However, the search for worldwide correlations today has the advantage of plate-tectonic theory, which does consider geologic phenomena and their causes on a global scale. This theory, if used cautiously, can enrich stratigraphy by providing new means of correlation. The value of an event in geochronologic correlation depends also on its duration. The shorter it is, the less diachronous its beginning and termination are likely to be. The disappearance of many groups of organisms at the Cretaceous-Tertiary boundary is not as abrupt as one might imagine from its supposed link with some cosmic cataclysm. This extinction is, in fact, gradual over a period of several hundreds of thousands of years. And no proof exists of the perfect synchronism of this event throughout the globe. The history of those outer layers of the Earth, capable of being described today, can thus be deduced from a juxtaposition of local, more or less well-correlated histories, allowing the recognition of the most important events. The latter are fundamental for long distance correlation and for the construction of a stratigraphic framework necessary for the division of geologic time. The recognition of these events is a precondition to all paleogeographic reconstructions. In other words, the task of stratigraphy is to solve a gigantic three-dimensional jigsaw puzzle. The pieces of the same age must first be assembled before it is possible to reconstruct the successive pictures of the Earth's history.

Chapter 2

Elaboration of the Fundamentals of Stratigraphy

1 Lithostratigraphy The first European stratigraphers set out initially to describe local histories illustrated by vertical lithologic sequences. Among them, William Smith (1769-1839) is generally considered the founder of stratigraphy, including biostratigraphy. He saw in the succession of sedimentary deposits a sort of representation of the passage of time. He recognized their continuity in space and was able to use fossils to distinguish lithologically similar beds. Inspired by this, Quenstedt and Leopold de Buch subdivided the rocks of the Swabian Jura into three parts: (1) a lower group or "Black Jura" (Lias), formed of marls and dark shaly limestones; (2) a middle group or "Brown Jura" (Dogger), consisting of ferruginous layers; and (3) an upper group or "White Jura" (MaIm), composed of light-colored limestones. In addition, three superposed sequences of sands were soon distinguished in the Paris area: lower, middle, and upper sands, separated by shaly or calcareous formations. This objective lithologic stratigraphy, or lithostratigraphy, is still the basis of descriptive sedimentary geology. It is the basis of the measured section in the field and its representation as a stratigraphic column. It is also the starting point for sequential analysis. Finally, the cartographer is above all a lithostratigrapher who attempts to follow previously defined sedimentary units around the land surface. The first European geologic maps, like those of Guettard (18th century) and those of Dumont (19th century) were strictly lithologic, without any chronologic significance. The basic lithostratigraphic unit is the Formation, whose genetic basis implies deposition under uniform conditions. Its limits are placed where the lithology changes or where there are significant breaks in the continuity of the sedimentation. Formations are subdivided into Members and associated into Groups. They were originally named in various ways, by figures, numbers, lithologic character, and names of the places where the units were particularly well exposed (stratotypes). The present nomenclature is in many cases inherited from those original names, in spite of the stratigraphic codes that have since appeared l . Figure 6 shows, for example, the stratigraphic 1 Suggesting the use of lithological characteristics and stratotype locality. Example: Comblanchian limestone (Bathonian, C6te-d'Or).

8

Elaboration of the Fundamentals of Stratigraphy

PseudolHhographic Corallian or

limestone.

(RICHE 1898)

lubcorallian faclel

Spherll. layera

(ENAY 1966)

Dogger

Fig. 6. Lithostratigraphic relations of the Oxfordian sequences in the southern Jura. The author and date when each lithologic unit was defined are shown in parentheses (After Enay 1966, with names of authors added)

nomenclature of the Oxfordian in the southern Jura, according to Enay (1966). The formations are seen to have only limited distribution and their limits are not necessarily isochronous. They are named after place-names, lithologic or paleontologic characteristics, and even a particular position in the succession (passage beds, boundary beds). In many countries outside Europe, especially in the United States, lithostratigraphy remains the fundamental tool of the sedimentary geologist, a tool evidently used with objectivity in the descriptions and correlations of natural successions, until the local lithostratigraphic scale and the general chronostratigraphic scale (see below) can be tied together. However, this methodology unfortunately does produce a multiplicity of unit names; in 1938 the stratigraphic lexicon of North America counted more than 13000!

2 Biostratigraphy The history of the Earth must be reconstructed in its continuity, but the successive sedimentary events, arranged in time sequences using lithostratigraphic methods, cannot always be correlated with one another. The

Lithostratigraphy

9 Time

4

3

2

® Fig. 7. Discontinuity of sedimentary events. A Sedimentary events recorded in a stratigraphic section: 1 and 2, continuous deposition; 2,3 and 4 are beds separated by diastems. B The same events in a time framework; 3 slow deposition; 4 rapid deposition; 1 and 2 continuous deposition; 2,3,4 discontinuous deposition; hachures denote lacunae corresponding to diastems in stratigraphic section

simple phenomenon of stratification implies, in effect, a discontinuity in deposition, with a resulting hiatus of unknown importance (Fig. 7). Also, many sedimentary events are diachronous and likely to be repeated during the course of geological history. It quickly became apparent, therefore, that there was a need for a reference to phenomena independent of sedimentation and of a continuous nature more clearly representative of the flow of time.

2.1 Evolution, the Reference System for Age Dating Most important in this regard is biological evolution, manifest in the emergence of species, a phenomenon continuous, nonrepetitive and irreversible, but having the disadvantages of being more or less dependent on the environment and proceeding at variable rates. As early as 1831, Deshayes established in the Paris Basin that the fauna changed from one formation to another, leading to the concept of successive disappearances and creations as time markers. Thus, Alcide d'Orbigny (1850-1852) said "the first thing to obtain from a paleontologic study is the age;" and Albert Gaudry in 1896 added "of two different outcrops, I affirm that in one the animals will indicate a state of evolution less advanced than in the other. I conclude from this that the first is from an older epoch." Biostratigraphy was thus born. By consideration of fossil remains, their positions in the strata, and their place in the evolution of animals and plants,

10

Elaboration of the Fundamentals of Stratigraphy

it attempts to characterize the different segments of geologic history by a particular fossil or by an assemblage of fossils. The correlations between fossiliferous beds therefore, represent time correlations, and two beds possessing the same fossiliferous content are said to have the same age, i.e. within the limits of resolution they were formed at the same time. This method is obviously only valid for that epoch of Earth's history called the Phanerozoic, characterized by determinable and useful fossils, and it cannot be applied to rocks too severely affected by metamorphism. The principles of biostratigraphy were applied early. As long ago as 1829, Morton and Vanuxem proposed a correlation between the chalk of the Upper Cretaceous of Europe and certain formations of the east coast of the United States on the basis of their similar ammonite faunas. The same procedure was adopted for the limestones of Savoie and the chalk of Rouen by Cuvier and Brongniart (1822), who advocated the use of fossils rather than lithology to correlate different areas.

2.2 The Zone Concept of Oppel With Oppel (1856), all reference to lithology disappears. Faunas alone are considered stratigraphically useful, being considered, justifiably, as more stable than lithologic facies over long distances. Adopting the subdivisions of Quenstedt and choosing the fossil group showing the most rapid vertical changes, he proposed 33 ammonite zones for the Jurassic of Wurtemberg and showed, by 1856, that this zonation is repeated in northern Germany, England and France. Oppel's biozones can be defined as the volumes of rock corresponding to the vertical and horizontal ranges of two or more taxa, each not necessarily occupying the same space. These units are named from the most typical, frequent or characteristic fossil (index fossil), which may, however, be locally missing. The best zones are those with the shortest vertical ranges (high rates of evolution) and the widest horizontal ranges. Certain Oppel zones have been recognized as far away as Madagascar and South America. It was already apparent by the middle of the last century that certain fossil groups differed markedly in their rates of evolution. Some evolved rapidly (tachytely), for example the ammonites, especially in the Late Triassic and Jurassic, and the graptolites, whose taxa tend to be spread widely and rapidly independently of the nature of the sediments. This wide distribution is due to a biological cycle which includes a planktonic larval stage (planktotrophic larvae), and for the ammonites, extensive postmortem dispersal of their adult shells by virtue of their buoyancy. For this reason, correlations using ammonites are considered practically synchronous. Moreover, these zones are almost worldwide in the Lias since they are recognizable in Europe, North America and the Andes. They subsequently become more restricted during the course of the Mesozoic

11

Lithostratigraphy

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Refinement of Concepts and Time Scales

29

Numerous indicators of evolution (appearance, disappearance of species) are actually diachronous on a global scale because of very different reactions by different groups to environmental changes (temperature, salinity etc.) in space and time. This results in mismatches, sometimes significant, between different biostratigraphic zonations, and makes it necessary to calibrate them with the radiochronologic or magnetostratigraphic scales (see below), independently of biological processes. Despite this problem, the use of multiple zonation schemes has certain advantages, such as the following:

1. Possibility of dating where index fossils are absent or rare (dilution of fossils due to high sedimentation rate or sampling methods etc.). The rocks supposedly "azoic" are in fact rarely without such readily disseminated organic remains as coccoliths or pollen4. 2. Possibility of dating with very small volumes of material, using microand nannofossils. These fossils were used increasingly during the 1970s as a consequence of the early oceanic drillings. 3. Improvement of precision in stratigraphic subdivisions. Fifteen graptolites zones have recently been created in the Ordovician of West Texas and 25 zones of calcareous nannofossils in the Paleogene of northwestern Europe (Aubry 1983). This is as much progress as had previously been accomplished using trilobites in the first case and nummulites, vertebrates, and planktonic foraminifera in the second. 4. Better correlations between biozonations of faunal provinces, basins, or different environments. The 25 zones previously mentioned allow better correlations between the principal basins of Western Europe. In the Cretaceous, good correlations are possible between the rudistid zones (specific to the platform limestones) and the zones of the planktonic foraminifera (for the pelagic deposits), using the orbitolinids. Biozonations of marine and continental sequences are difficult to correlate in the absence of an intermediate sequence where the two interdigitate, but the palyniferous zones are useful for this since pollen can be dispersed on land as well as in the oceans.

1.2.3 The Different Concepts ofthe Zone Zones have always been characterized in a way that was as simple and convenient as possible, but several methods are possible, depending on the faunal elements available and on their evolutionary character (Fig. 19). Assemblage zones or zones of association designate a collection of beds characterized by a natural association of fossils. In most cases only a few

4The latter are very resistant except to metamorphism and prolonged oxidation (red beds).

=n= :c x

Modern Stratigraphy

30

Inlerzone

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Upper boundary of taxons Lower boundary of taxons

Fig. 19. Different types of biozonation. 1 Assemblage zones; 2 range zones; 3 overlapping range zones; 4 oppel zones; 5 abundance zones; 6 interval zones (After Hedberg 1979)

forms, often only two, are considered out of the total present; for instance, the Rotalipora montsalvensis-Rotalipora cushmani zone, which is the third subdivision of the Cenomanian according to Porthault (1974), and is designated Cn3. Range zones correspond to the vertical and horizontal ranges of a given taxon (species, genus or family), for example, the Acanthodiscus radiatus (ammonite) zone of the basal Hauterivian, or more simply the Radiatus zone. This zonal concept goes back to that of d'Orbigny. If an homophyletic biostratigraphy is practiced, i.e. one based on the evolution of a single phylum, as advocated by certain authors, then species in a continuous evolutionary lineage or clade have to be separated. This is not easy and often leads to more or less arbitrary zonal boundaries (phylozones). Moreover, the use of range zones assumes that the chosen species had short time ranges and that their distributions within a formation are perfectly known. Overlapping range zones are defined by the overlapping parts of ranges of several taxa. Oppel's zones are of this type. This method uses taxa of restricted vertical range within the zone. It follows, as we have already emphasized, that all taxa used in this way are not necessarily present in all locations and their coexistence may correspond to only the middle part of a zone. Abundance zones or acme zones are based on the abundance or maximum development (acme or hemera) of certain forms independent of their time range. Clearly, this is somewhat subjective and dependent on the original environment as well as on the hazards of collecting. Also, the actual moment of this maximum is often difficult to fix because it does not necessarily correspond to the beds enclosing the most abundant fossils. Finally, some species are mono, poly, or ahemeric.

Refinement of Concepts and Time Scales

31

Interval zones represent the stratigraphic interval between two biohorizons, i.e. between two surfaces possessing distinctive biostratigraphic characteristics (see below). Each of these different types of zones is useful according to the circumstances: faunal or floral abundance (often linked to the rate of sedimentation), rate of evolution and diversification, environment of deposition. The object is to gradually improve the resolution, the universality and the facility of use of biostratigraphy. These attributes are necessary in ocean drilling for precise and rapid correlation and have been made possible by the use of widely applicable time scales. From this point of view, refinements of the calcareous nannofossils have proven to be particularly useful. Nevertheless, the limits of ranges of taxa are not precisely known and not perfectly isochronous due to the hazards of sampling, especially in those beds in which first appearances or disappearances are supposed to take place. Biozonation can also be augmented by the use of biohorizons, which are surfaces or very thin beds corresponding to particular biological phenomena, such as first appearance, disappearance, evolutionary change etc. These are also called horizons, reference levels or marker beds. Invaluable for correlation, they are equally valid at the boundaries of biozones as they are within them. Examples include the extinction of Pseudoemiliana lacunosa and the first appearance of Emiliana huxleyi (coccoliths), synchronous with certain chemostratigraphic reference levels (stages 12 and 18 of the 8180 curve, see below). Both define horizons of worldwide validity. In practice, extinctions (which are first appearances during drilling) are preferable to first appearances because the latter are often gradual (phyletic gradualism) and difficult to detect during drilling because of contamination. Moreover, the emergence of new species by geographic isolation is a diachronous phenomenon by definition.

1.2.4 A Biostratigraphy Based on Degree of Evolution Another biochronologic method, used currently by mammologists, is based on the evolutionary level within anagenetic lineages (Gourinard 1984), rather than on vertical ranges of taxa. Population studies enable numerical indices to be defined whose average values are supposedly identical at the same time over very wide areas. The evolution of this mean index is calibrated in millions of years, so that it is possible to assign a numerical age to this or that population (Fig. 20).

1.2.5 Quantitative Stratigraphy For about the last dozen years, researchers have tried to eliminate unmeasurable factors and all subjectivity from biostratigraphy. These are related to

Modern Stratigraphy

32 _______ Datlngs Locations -..............

K/A r

IIquitaine (France) Provence and Corsica Italy Sardinia Algeria Morocco

•

15

*

•

••

Datum P1anea

.. o

6 V



5 10

o

15

20

20

25

25

30~--7~0--~6~r~~50~~4~0~3~0--~2±0--~1±0~~030 -400 -300 -200 -100 0 .. 100

Fig. 20. Curves expressing rates of evolution from biometric indices. A represents the calibration in millions of years of the Scott index, ratio of height to width of the principal opening of Globigerinoides of the primordius-trilobus line; the index multiplied by 100 in the abscissa varies from 0-70. B represents the calibration of the gamma index of Drooger for the Miogypsinidae of the evolutionary assemblage made up of the Miogypsinoides complanata and Miogypsina gunteri-intermedia groups. The index varies from -400 to + 100. Points shown by solid symbols correspond to potassium-argon ages; points with open symbols correspond to datum planes. Different symbols indicate different geographic locations of samples. The geographic region defined is the minimum area of validity of the curves (After Gourinard 1984)

the chance elements of samplingS, to the differential solution of calcareous microfossils, to the choice of biological events for the establishing of a time scale, and to the environmental influence. The observed data are therefore computerized and analyzed using appropriate programs (Gradstein et al. 1985). Data consist of events of low or high frequency or simple presence or absence. According to Davaud (1982) the most reliable type of zonation is that similar to Oppel's. It is defined by the "unitary association", which include the largest group of compatible species, i.e. those which have lived together for a certain time. The computer procedure consists of: 1. Eliminating all species not compatible with this unitary association (reworked fossils, as with many nannofossils, those due to sampling error, etc.). 2. Erecting a composite biostratigraphic scale, with relative positions of all events superimposed, based on as many sections as possible because a biostratigraphic sequence based on only one section reflects only the 5 In

the same section, the base of the NP25 zone, defined by the appearance of nannofossils, has a position which can vary up to 45 m according to different authors.

Refinement of Concepts and Time Scales

33

order in which that locality was colonized, and not necessarily the real chronology of biological events. 3. Selecting the most significant subdivisions of the scale by multivariate analysis. For example, from 100 species of Jurassic radiolaria determined in 210 samples coming from 43 localities, Baumgartner (1984) has defined 14 unitary associations distributed among 7 biozones. 4. Establishing correlations with certain confidence limits.

1.3 Search for a Rigorous and Universal Chronostratigraphy 1.3.1 Weakness of the Initial Concept of the Stage. Revision of Stratotypes It has been seen (Chap. 2) that chronostratigraphic units were initially created from sedimentary sequences with evident discontinuities in lithology and faunal assemblages. The pioneers of stratigraphy considered these breaks to be simply episodes of short or no time duration. It is now known that these intervals may be very long, even longer than the periods of sedimentation separating them. This is shown by the presence of nonclassifiable faunas in the d'Orbigny stages. Occurring in the transition beds between units, these faunas prove that the stratotypic successions are generally incomplete. For instance, at Biarritz, the Lutetian-Stampian interval is represented by 1500 m of limestones, marls and marly limestones deposited in the relatively deep environment of the Aturian Gulf. It is a continuous sequence, apparently complete and difficult to subdivide stratigraphically. In the Paris Basin, the same interval is about one-tenth the thickness and consists of 18 distinctive lithologic formations evidently resulting from alternating marine and nonmarine conditions, and yielding, therefore, a sequence riddled with gaps and a very imperfect picture of the passage of time. However, the Paris Basin has been selected for the Eocene and Oligocene stratotypes. A revision of the European stratotypes was therefore undertaken during the 1960s, with a view to improving the definitions and zonal subdivisions. Two views have arisen: 1. Inappropriate stratotypes should be abandoned in favor of sections with continuous pelagic deposits. 2. Stratotypes should be preserved with these defects but should be restudied so as to evaluate the magnitude of their hiati.

The choice between these two solutions has depended on the actual case. At Valangin and Hauterive (Switzerland), urban development has resulted in the disappearance of most of the sections on which the Valanginian and Hauterivian stratotypes were based. Additional type sections (hypostrato-

34

Modern Stratigraphy

types) have therefore been selected in the Subalpine Basin of southeastern France, where sediments are thick and sedimentation was continuous. Ammonite biozonation has been refined and parallel time scales based on varied fossil groups have also been erected. However, for stratotypes reasonably well preserved, a thorough revision was undertaken as follows: 1. A precise relative dating of any sedimentary discontinuities, especially at the base or top of a type sequence. 2. A systematic inventory, both qualitative and quantitative, of the biologic content. 3. A description and relative dating of the bio- and lithohorizons. This type of study made it possible to verify whether the original definition of the stage in terms of physical and biological characteristics and subdivisions (substages, zones) corresponds to the objectively redescribed reality. If it does not, it had to be corrected. One of the consequences of all this revision work is that other countries have been able to propose new stratotypes which are better than the originals for any given time interval. Another consequence is the emphasis put on the heterogeneous character of stratotypes in general, distributed among two groups of faunal associations: those of the epicontinental regions with hiati now defined with respect to localization and significance; and those of the deep pelagic regions. From all this it is seen that stages, stratigraphically superposed, often represent a succession of ages separated by gaps but also with overlaps; thus the necessity for a redefinition of their boundaries.

1.3.2 Redefinition of Boundaries To be certain of the perfectly sequential nature of stages, it was decided that only their lower boundaries would be defined and this boundary would be taken as the upper limit of the underlying stage. Because of this practice, all subsequently discovered beds which fall between two stages are included in the upper part of the older stage. Therefore, interfaces between successive units, of theoretically zero duration, are substituted for the previously assumed breaks with their implied time discontinuities. The simplest scheme, following Oppel, would be to make the lower boundaries of stages coincide with biozone boundaries, which would make the biozone a part of the stage. It is known, however, that zone boundaries are not necessarily isochronous and therefore would be difficult to apply to chronostratigraphic boundaries. Today the method is more arbitrary: whether in a stratotype or in another sequence or representative part of a sequence called the boundary stratotype, a decision is made to define the base of a certain stage at a particular bed, coinciding perhaps with a biozone boundary at this point, but perhaps elsewhere with other events such as the appearance or disappearance of taxa, the beginning of a zone of mineralization, volcanic

35

Refinement of Concepts and Time Scales 55

60

Fig. 21. Position of some stratotypes of the Paleeocene and Lower Eocene in the chronostratigraphic scale of the Anglo-French-Belgium basin. Demonstration of the different time ranges for different stratotypes (After Berggren et al. 1985)

Late Paleocene

Early Eocene

(Selandlan)

(Ypresian)

Million years

_.

Selandlan Thanetlan

t--

--

Lutetian

Ypreslan

Sp~~_

-- -

Stratotype positions

ash beds, magnetic reversals, etc. In short, either a biomarker or a lithomarker can be used as a reference. Also, it is necessary that this boundary be related as precisely as possible to several biostratigraphic scales so that it is as widely applicable as possible. The stages redefined in this way between precisely defined boundaries related to a clear reference system then correspond to the totality of time, and in so doing clearly demonstrate the incomplete character of the majority of stratotypes (Fig. 21). Is it therefore necessary to determine precisely the lithologic and faunal content of a stage at one locality, knowing that this can change rapidly in a lateral sense? In other words, do the stratotypes, or even the stratotypic regions advocated by some, have any use? The higher-order boundaries have also been redetermined more precisely, with international working groups seeking the best boundary stratotypes. For example, for the Cretaceous-Tertiary boundary, two sections are in competition: that of Gubbio in Italy and that of The Kef in Tunisia. In both cases the boundary has been suggested where the greatest number of important events occur, such as climatic variation, faunal renewals, magnetic reversals, geochemical anomalies etc. Thus modern research on global phenomena, while helping to define major chronostratigraphic boundaries, rejects Lyell's principle of uniformitarianism (uniform distribution through time of all geodynamic processes). Instead, it reintroduces in modernized form the "revolutions" of d'Orbigny.

1.3.3 Refinement of the Chronostratigraphic Scale A time correlation of two geographically distant events implies that they could not be separated in time because of the inadequate resolving power of the stratigraphic tool employed. Since precision in correlation is necessary for true paleogeographic reconstructions, it is reasonable to try to improve this precision by using smaller units than the stages, which have average durations of 10, 5.7 and 4m.y. for the Paleozoic, Mesozoic and Cenozoic,

36

Modem Stratigraphy

respectively. One such smaller unit would be the zone, even if it is a biostratigraphic unit. However, for the system to be consistant, the zone used in a chronostratigraphic sense must become a chronozone. The duration of a chronozone is clearly inversely related to the rate of evolution of the paleontologic group used. The average duration of Ordovician graptolite chronozones is about 5 m. y., while the Devonian, Permian and Mesozoic ammonoid chronozones average about 1 m. y. The 185000 years average duration for the Toarcian (Sect. 1.1.2, this Chap.) chronozones in its type area almost certainly represents a record for precision. All requirements for greater precision, and all refinements of stratigraphic tools clearly result in new constraints, new revisions of boundaries and an increased need for rigor in definitions. As in all scientific disciplines, stratigraphy will, therefore, require continual adjustments.

1.3.4 Modern Trends: Biostratigraphy Slowly Replaces Chronostratigraphy Much more work is necessary before the chronostratigraphic scale becomes precise, reliable and universal. Because of this, its subdivisions remain somewhat abstract. In comparison, biostratigraphic units are more concrete and easier to use, especially when they are based on microorganisms generally well represented in the sediments. Founded on the irreversible evolution of living organisms, these units inevitably acquire a chronostratigraphic significance, even if they do not always lead to time correlations. The natural heirs of catastrophism, d'Orbigny's stages owed their existence to the unconformities bounding them. In the thick continuous sequences where stratigraphers later labored, such stages are difficult to recognize without recourse to purely paleontologic arguments such as the appearance or disappearance of a fauna, genus or species. This is more valid the smaller the stratigraphic unit sought. The zone, therefore, tends more and more to replace the stage as a material representation of a segment of Earth's history. This is certainly a deviation from the original concept of the zone, but it is justified for practical reasons. As Pomerol (in Pomerol et al. 1980) wrote: "One can observe a replacement of chronostratigraphy by biostratigraphy, and stratigraphy gains in effectiveness what it loses in rigor." This loss of rigor derives mainly, as we have seen, from the diachronism of certain biozones, especially at times of strong climatic gradients. Example: the Ericsonia subdisticha (coccolith) zone which cuts diachronously across the Eocene-Oligocene boundary from low to high latitudes in the northern hemisphere (Fig. 22). For this reason and for others which similarly prevent the synchronism of the appearance and disappearance of species (facies variations, fossil preservation, fossil abundance, faunal migrations), the establishing of biozonations of universal validity appears at the moment to be beyond realization.

37

New Methods of Correlation

w

Z

w

Zonation by planktonic Foraminifers (W.H.BLOW)

Germany 52° lat. N

Italy 45° lat. N

Florida 30° lat. N

g P19 (!)

~

P18

P 17 w ~ P16

u

ow

P 15 P14

2"

1

Fig. 22. Ericsonia subdisticha zone in the North Hemisphere, defined by two diachronous markers. 1 Disappearance of Discoaster barbadiensis and/or of D. saiponensis. 2 Disappearance of Cyclococcolithus formosus (After Cavelier 1979)

2 New Methods of Correlation After basing itself essentially on lithology and paleontology, stratigraphy turned during the 1950s towards physical, chemical and mineralogical methods for new forms of correlation or even new time scales. To a large extent this tendency stemmed from a desire to relate the stratigraphic scales to physical and chemical phenomena possessing certain properties such as: 1. A regular periodicity, giving them the character of clocks rhythmically ticking the march of time; 2. An instantaneous character on a geological scale, i.e. a duration not exceeding a few thousand years; 3. A very wide occurrence, in some cases worldwide. These new methods have been developed largely within the context of the remarkable technical progress in areas like geophysical exploration and improvement of analytical methods over the last few decades.

2.1 Correlation by Sedimentary Rhythms A rhythm is, by definition, a regular repetition of a certain feature or interval. The arrangement of sediments into beds is itself a rhythm. In order that correlations can be made by this method, the sedimentary rhythms should be related to periodic phenomena with a synchronism recognized over very wide areas. This type of correlation is now in general use due to

38

Modern Stratigraphy

1---1

Clay

E:] ....

Sands

E:i9

Platform carbonate

~

deep limestone

~

Conlinental red beds

~

Platform limestone and marl

c:

~

~

Fig. 23. Representation of lithologic units of a basin on a space-time diagram on the basis of seismic and drilling data, offshore West Africa (After Vail 1977)

the large amount of subsurface data available from petroleum exploration. The principal geophysical methods directly applicable to stratigraphy will, therefore, first be reviewed.

2.1.1 Subsurface Methods of Investigation 2.1.1.1 Seismic Methods Seismic waves generated at the ground surface are reflected by lithologic surfaces of discontinuity separating lithologies of different elastic and/or density properties. The recording of these reflections, therefore, permits the localization of the principal breaks in a sedimentary sequence (bed surface, surface of erosion or nondeposition, fractures, etc.) with a resolution of a few tens of metres. Sedimentary units and their relationships as well as the geological structure can then be reconstructed. According to Vail et al. (1977), bed surfaces and discontinuities revealed by seismic are useful in correlation but also have a chronostratigraphic significance (see below). Their characteristics enable the relative importance of stratigraphic gaps to be gauged and the lithology or environment of deposition to be known in some detail (Fig. 23). The seismic method, using data from ocean drilling as a reference, is also the basis of the geologic map of sediments overlying the ocean floor (Fig. 24) and thus provides a test of the reality of ocean-floor spreading at the mid-ocean ridges.

39

New Methods of Correlation

D

Pleistocene

i. . . ; "I

Jurassic

1=-: Ilower Cretaceous ~ Upper Cretaceous [](D

Basalt + Pliocene

Fig. 24. Geological map of sediments in contact with the Atlantic Ocean floor (from the Geological Atlas of the World of Freeman, Lynde and Tharp, in Daly 1984) . Isochronous bands symmetrical about the mid-oceanic ridge offset by transform faults

2.1.1.2 Well Logging Methods (Diagraphic)

These are measurements of different physical and chemical properties of rocks (lithology, mineralogy, nature and importance of fluids , texture , bedding, grain size distributions and dips) encountered during the drilling of a well. These measurements (natural radioactivity, spontaneous potential,

40

Modern Stratigraphy

.,:::>

o

'"o

~ U

Sandstone Dolomitic limestone Dolomite and anhydrite

:ii '

'is c

Limestones and marly intercalations

«I

t: o

0..

800

o

.,

Marls and marly limestones

"r;;

f!

Limestones

3

Marls and marly limestones

...,:::>

700

c «I "a' "C .~

E E

c «I

"c

:.::

~~.....I----H~+---4l-L,:+--+I-L~----.i+!::=t--+

800

Limestone and marly limestone

Sublithographic limestone Valiant-St.Georg.. 1 Grandvilia 101 (/) Gelannes1 Nozay 1 Mailly 102 ~

g

Mentioned heights (in meters) are calculated from soil surface and refer conventionally to the top of the Sequanian

Romi Ily-sur-Sei

Fig. 25. Example of electric-log correlations between holes in the Paris basin (After Perrodon 1968) abc

abc

a

a

b

I

T7 c

1

c

b

b

a

a

2

Fig. 26. 1 Cyclic and 2 rhythmic sequences a,b,c Lithologic units

41

New Methods of Correlation WEST

CONDAT

EAST

Sequences

---

Rhythm C

100 RhythmB2

80

--

--

60

Rhythm B1

"'!I-f--......~~-+--

__

---

Rhythm A

20

o Fig. 27. Sequence correlation (here the rhythms A, B1, B2, C) in the Middle Jurassic of Quercy (after Delfaud 1972). Ideal sequence: 1 lignitic marl; 2 micrite with gypsum pseudomorphs; 4 azoic micrite; 5 micrite with algal balls; 6 sparite with ooliths

density, porosity, resistivity and sonic) define for each bed what Serra (1972a,b 1986) has called an "electrofacies", expressed by the character of the recorded curves, and may be used in correlation. Especially the electrofacies makes it possible to determine the lithology and the internal structure of the rocks (beds, rhythms, discontinuities). For example, shales, porous and full of retained water rich in ions, are much less resistant to electric currents than a compact limestone. The two are, therefore, easily separable by a resistivity log. Reference markers such as unconformable surfaces (enriched in phosphates), cinerites and derived claystones (tonsteins) are recognizable by radioactive logs. The degree of resolution in the analysis of strata obviously depends on the resolving power of the various tools used: lOcm for the more classic methods, 1 cm for dipmeter logs. A "composite log" combining all types of measurement is a convenient method of defining a lithologic sequence. These can then be used by comparing the shape of the curves and their cyclic character to establish correlations between wells within a basin (Fig. 25). The degree and reliability of the correlations may be quantified using appropriate coefficients. In sum-

42

Modem Stratigraphy

mary, the logging technique is essentially lithostratigraphic. It can, however, provide results of chronostratigraphic significance in two situations: 1. In reference to pyroclastic horizons, which are more or less radioactive

and distributed independently of facies; 2. Where wells have a number of similar log characteristics, as shown by maximum coefficients of correlation over an interval limited in space and time. According to Serra (1972a), "the probability that over a certain time duration a non-synchronous cycle of sedimentation can be perfectly duplicated laterally in all its characteristics is so remote that one can say on the one hand that the reliability of the correlation is maximal and on the other that it must be synchronous".

2.1.2 Sequence Stratigraphy A stratigraphic sequence of sediments is a consequence of an evolution in sedimentation controlled by changing external factors. If these factors change cyclically and with more or less equal periods, they give rise to a rhythmic sequence of repeated lithologies. These rhythms can consist of continuous variations (= cycles in the strict sense of abcba type) or they may be syncopated, i.e. punctuated by sharp reversals (saw-tooth cycles of abcabc type) (Fig. 26). These reversals are generally marked by sedimentary discontinuities through erosion or nondeposition. In any sequence, rhythms of first, second, third or even higher orders may be defined (Delfaud 1972). Rhythm stratigraphy entails the definition, in a given location, of a particular vertical rhythm and its correlation with the same rhythm found elsewhere. To facilitate comparisons it is usual to express the rhythms as lithologic curves. Delfaud (1972) distinguishes three degrees of correlation (Fig. 27): 1. Between sequences of analogous facies. 2. Between sequences of different facies but within the same sedimentary domain. 3. Between sequences of different facies and different sedimentary domains (e.g. between a platform and a basin).

In all cases, the correlation can be lithologic and nonchronologic. In fact, the persistance of the lithologic characteristics of a formation over a wide area6 demands a contemporaneous uniformity of environment rarely observed, especially in the platform domain. Diachronism is, therefore, probably the rule. Among numerous examples known is that of the "lithoclinal sequences" making up the Dogger of the Paris Basin (Purser 1972). In this example, facies boundaries cut across time lines (Fig. 28). Diachronism 6 Sequence

1981).

correlation is possible from England to Germany for the Jurassic (Hallam

43

New Methods of Correlation Lorraine

Callovian IIthoclin. Upper

Bathonian

--v-v-

Discontinuities

• • • • • • • Isochronous lines

~ Etrochey limestone ~ "Daile nacr6e-

1_ _ I

Marls

Fig. 28. Relations between time units and lithologic units in the Callovian lithocline along the eastern border of the Paris basin (after Purser 1972). These relations imply a displacement of the zone of deposition with time

Isochronous line

Fig. 29. Israelski Principle. Sedimentary cycle determined by a transgression followed by a regression. The dashed line joining the points of furthest advance of each facies is an isochronous line representing a section of an isochronous surface (see also Kauffman and Hazel 1977, p. 228)

can result from progradation, as commonly seen in the deposition of sedimentary prisms at the edges of basins. Variation in sea level appears to be one of the major causes of this progradation, with diachronism being more pronounced, the slower the changes in sea level. When the eustatic movement is reversed, the direction of sedimentary progradation must also be reversed, and the positions of the points of reversal theoretically define an isochronous surface, in the absence of tectonic movements and given uniform subsidence (Israelski principle; Fig. 29). A generalization of the relation between sequences and sea-level fluctuations has been proposed by Vail et al. (1977). According to Vail, the sequences of strata defined on the continental margin by seismic reflection are objective stratigraphic units bounded by discontinuities. The majority of the major sequences are deposited under the double control of rate of sedimentation and the cyclic variations in relative sea level. The latter generally involve a rapid rise, a period of stability and then a rapid drop, with a total average estimated duration, according to Vail, of about 1 million years. The discontinuities which bound the sequences may be due to nondeposition during periods of stability, or both erosion and nondeposition

44

Modem Stratigraphy

during the fall of sea level. They are considered as practically isochronous, regionally, to within a few hundred thousand years. It has been possible to use such markers for the correlation of biostratigraphic scales established in different faunal provinces, for example in the Cenomanian between the Tethys and the temperate zones. Struck by the similarity in the character of continental margin submergence when compared by sequential analysis, Vail has proposed a curve of relative sea-level fluctuations of global scale (Fig. 30) 7 . Superimposed on this curve is a true stratigraphic scale of global eustatic cycles related to the chronostratigraphic scale. Seen from the eustatic point of view, therefore, the sedimentary sequences may be regarded as tools of correlation, as well as being useful in dating, but their resolving power remains rather low. Also, the chronostratigraphic significance of the Vail curves is refuted by some scientists. Although traditionally used to solve lithostratigraphic problems, the sequences are also very useful in the search for economically useful resources such as petroleum and water, and in paleogeographic studies (see below).

2.1.3 Use of Cycles of Punctuated Aggradation (PAC) According to Goodwin and Anderson (1985), basin sedimentation is controlled by periodic elevation of the depositional base level. These elevations, resulting generally from rapid transgressions, are marked by surfaces of nondeposition synchronous within a basin. These surfaces bound minor cycles of deposition (PAC), totally independent of formations, and these cycles are considered fundamental chronostratigraphic units deposited in a few thousand years, and therefore capable of very precise correlation.

2.1.4 Use of Binary Rhythms This method is applicable to low-energy environments where sedimentation is essentially of suspended fine-grained terrigenous matter and planktonic skeletons, deposited alternately in cyclic successions of the two components. 2.1.4.1 Pelagic Limestone-Marl Alternations Deep-water sediments often form monotonous sequences consIstmg of limestone beds and marl interbeds in intergrading units of decimetre thickness (Fig. 31). Many authors believe that these alternations represent cyclic global climate changes affecting both the oceanic environment, particularly temperature, dynamics, and productivity, and the continental environment, as source of clays. This relationship is clear for recent sediments where the 7 This

curve has been criticized because it is based mainly on data derived from the Atlantic margins, which have been appreciably affected by vertical movements.

45

New Methods of Correlation 2ND ORDER CYCLES PERIODS

RELATIVE VARIATIONS OF SEA LEVEL

EPOCHS

Plio-Pleistocene TERTIARY

...

P_ Rising_

1

...r:==~M~lo~ce~n~eL:==~_

......-..,p.?~a;r.IO;~~'!lcne~ene.....;..~_- -

s

CRETACEOUS

-

_

-~

M

TRIASSIC

......---:-I-..:..:..:c-S ---;

-fb-= _-_-_-_-_ :-~.:-

_~_~ ______

......- - - - - - I - - - I - - - - - - f S

PERMIAN

Lowering ~ _ = _ ~;Td.-O

Tc

_

I-------+....-_-_-_-_-_-_-_-_-~·:s'_·-_-_:r- - - - JURASSIC

~

NOTATIONS

- -

,- - - - -

p:s:(J-1 ~_~~~

Ka

UJ

J

~

sea l e v e l .

-----r ..----

1!;

~

FP

1-:300

\.

M

t---------ir--r--:I-===:;::~ CAMBRIAN

_200 ~

P

o-S ORDOVICIAN

~

TR -

"

PENNSYLVANIAN

-100

Kb

M

~

-

--------

PRECAMBRIAN

Fig. 30. Major cycles of global sea-level variations (After Vail et al. 1977)

Fig. 31. Example of limestone-marl alternations in the Lower Cretaceous of the Vocontian basin, southeastern France. The transition between the two lithologies is gradual

8 5 i=

. 1-500

COo

46

Modern Stratigraphy

Fig. 32. Correlation of a bundle of beds and interbeds in the Valanginian of the Vocontian basin, southeastern France. The sections occur in the deep zone indicated on the map (After Cotillon et al. 1980)

Site 534

t.. Core numbers

Vergons

Fig. 33. Example of correlation by cyclograms in the Campylotoxum zone of the Lower Valanginian. Vergons is a locality of the Alpes de Haute-Provence (France). 534 is the site of a drill hole in the Atlantic off Florida. The horizontal bars represent the relative thicknesses of the cycles defined by alternations of limestones and marls (After Cotillon and Rio 1984)

alternation of calcareous and terrigenous muds can be directly correlated with glacial and interglacial stages during the last 700000 years. Already by the end of the last century, Gilbert (1894), struck by the regularity of the Upper Cretaceous cycles of Colorado, interpreted them as due to climatic oscillations induced by the 21000-years cycles of the precession of the equinoxes. By counting the couplets in a formation, he was thus able to measure its duration. Today it is believed that cyclic variations of three orbital parameters, precession of the equinoxes with a period of 21000 years, obliquity of the Earth's rotational axis on the plane of the ecliptic with a period of 41000 years, and the eccentricity of the Earth's orbit with periods of 106000 and 410000 years, interfere to produce a complex fluctuation of solar heat at the Earth's surface, which in turn controls the

New Methods of Correlation

47

sediment rhythms. On this basis, it is believed that limestone-marl couplets can be correlated over very large distances with sometimes a resolving power at the level of the individual bed. These correlations are made directly bed by bed (e.g. Lower Cretaceous of the Vocontian Basin, Fig. 32), or by graphical representations of thicknesses of cycles plotted against time (cyclogram of Fig. 33). 2.1.4.2 Varved Microalternations Varves have been already mentioned as a means of calculating time durations. These indicators of anoxic lacustrine and marine environments can also be used for very precise correlation within basins if certain cycles or successions of cycles can be readily recognized.

2.2 Correlation by Mineralogic and Geochemical Markers Mineral or organic constituents, elements, even some isotopic ratios characterizing deposits of a certain age or region, can play an important role in correlation. The precision and geographic significance of this will depend on the spatiotemporal distribution of the markers. Many of them are related to cyclic phenomena or gradational processes, while others represent geologically brief events not necessarily repeated in the same place.

2.2.1 Clay Minerals Widespread because of their small particle size, clays are useful for precise correlation to the extent that they are inherited from the continents. According to Chamley et al. (1978), the clays of the North Atlantic are distributed through time as a function of major geodynamic events such as climate, plate mobility and orogeny. Smectite appears in appreciable amounts from the Upper Jurassic, but especially in the Cretaceous, and is indicative of the erosion of tropical soils. From the Eocene, the increase in content of the primary clays illite, chlorite, mixed-layer illite-smectite and chlorite-smectite, reflects the intensification of the Alpine Orogeny, and the global cooling evident from the Eocene-Oligocene boundary but especially in the Upper Miocene (first arctic ice) and in the Plio-Quaternary (growth of the Greenland and Alpine glaciers). There are also other indicators more time-specific; attapulgite in the Albian, attapulgite and sepiolite in the Paleocene and Lower Eocene. These also are minerals characteristic of a certain cratonic margin provenance. It appears, too, that newly formed minerals are also useful in correlation. Frohlich (1982) has observed that the vertical change in mineralogic composition of the azoic red muds of the Indian Ocean (composed of clays, clinoptilolite, phillipsite and amorphous silicates), represents a veritable stratigraphic sequence of extremely wide significance.

48

Modem Stratigraphy

2.2.2 Heavy Minerals These have the advantage of being very resistant, easily identifiable, and provenance-specific in their petrographic characteristics. Their associations are sometimes useful for characterizing lithologic formations. The Lower Trias Buntsandstein sands of the Vosges and the Rhenan region, for example, can be stratigraphically subdivided on the basis of the type of tourmaline (Henrich 1961) as follows: 1. Lower Vosgian sands: tourmaline rounded and angular. 2. Upper Vosgian sands: tourmaline rounded only. 3. Principal Conglomerate and Purple Boundary Beds: dominantly angular.

tourmaline

Useful for correlation, the heavy minerals can also help to trace the tectonic evolution of a region by their distribution in time. In the perialpine molasse, for example, glaucophane is a valuable marker indicating the first erosion of the Schistes Lustres of the Piemontaise zone. Figure 34 summarizes the distribution of the principal minerals of the alpine association during the Tertiary in the molasse of the region from Switzerland to Isere (Latreille 1969).

2.2.3 Volcanic Ash This is the basis of tephrostratigraphy. Explosive volcanic eruptions can eject pyroclastic material into the upper atmosphere where prevailing winds carry it over very great distances. In principle, the finer the ash, the more widely distributed will it be. For example, 300000 km 2 were covered by 8 billion tons of ash from the Quizapu volcano in the Chilian Andes in 1932; and 3500 years ago, 20 times that amount was ejected from the Santorin volcano. The ash falling into quiet environments forms beds of cine rite which tend to be altered later by diagenesis. Deposited with detrital sediments, they are little altered, although when included in beds of plant material (future coal for example) their feldspars are hydrolized in the presence of humic acids to form kaolinite. Every cinerite bed has its own characteristics identifiable in thin section, even when derived from the same volcano. Bouroz (1972), for instance, distinguishes five types of cinerites (called gores or tonsteins) in coal successions, according to their mineralogy and microstructure. This petrographic signature is often significant when tonsteins of different basins are compared. For example, in northern France the coal sequences of SaareLorraine and North Pas de Calais are correlatable by means of about ten beds of tonstein. Eight tonstein beds have allowed the sequences of the Cevennes Basin and the Jura Basin to be correlated much more precisely than by plant biostratigraphy (Bouroz 1972). At the boundary between the Lower and Middle Burdigalian, tuffites and cherts have been described from

49

New Methods of Correlation N Switzerland

Haute-Savoie (France) North Usses

Savoie (France)

South Usses

HELVETIAN

BURDIGALIAN

AQUITANIAN

OLIGOCENE

EOCENE

Fig. 34. Principal mineralogic groups in the detrital subalpine and peri alpine Tertiary (after Latreille 1969). Gr Gamet; Ep epidote; Gl glaucophane >5%

around the entire periphery of the western Mediterranean (Lorenz 1984). These are the products of acid volcanism from a source not yet located. Because they are products of very brief events, cine rites are perfect isochronous markers, chronohorizons in the sense of Hedberg (1979), allowing correlation between continental and marine sequences. Their only limitation is their moderate geographic distribution, rarely more than 1000km for the distinct beds, slightly more for those beds somewhat diluted but still detectable by their radioactivity.

2.2.4 Chemical Elements and Isotopes One of the implications of the principle of uniformitarianism is the constant chemistry of ocean water and, therefore, of the carbonates, sulphates and halides precipitated from it. However, this assumption has never been proven, even for deep oceanic sediments which have never been affected by meteoric diagenesis or other continental influences. Also, the names of periods such as Carboniferous and Cretaceous are an implicit recognition of global geochemical variations during geological history. In 1952, Arrhenius observed that two recent sediment cores taken from the East Pacific Rise showed synchronous fluctuations in their CaC0 3 content. Many other observations have since shown that in the Quaternary and the Neogene, such variations dated by microfauna and carbon 14 are correlatable throughout the Pacific, Indian and Atlantic oceans. The car-

Modern Stratigraphy

50

.."

-" "

0

"'u :2.,

90 100

~a;

0

110

~~ ~u

120

Fig. 35. Change in strontium content of pelagic carbonates since the Upper Jurassic (After Renard 1985; see also Renard in Pomerol et al. 1987)

bonate content of pelagic deposits must, therefore, depend on general factors such as primary production or rate of dissolution of carbonate sediment during or after deposition. In both cases, global changes in water chemistry are implied, perhaps related to the marked eustatic and climatic fluctuations of the recent periods. Whatever the causes, these variations are synchronous8 and are, therefore, very useful for correlation. They are the basis for chemostratigraphy. 8Given the short mixing-time constant of the oceans (less than 1000 years) all chemical changes of seawater are essentially synchronous in the world's ocean.

New Methods of Correlation

51

2.2.4.1 Trace Elements A number of elements occurring in carbonates are now being used for correlation. Strontium in pelagic sediments generally decreases from the Cambrian to the Jurassic and then increases to the Present while showing marked variations in certain periods (Fig. 35). Its maximum concentration occurs in the Miocene. According to Renard (1985), the strontium curve may be related to changes in submarine hydrothermal activity and its calcium supply at the mid-oceanic ridges. The variations in concentration of strontium may also be related to variations in the CCD (Carbonate Compensation Depth), which controls the dissolution of calcite and, therefore, the release of strontium for incorporation into the residual sediment. However, the CCD curve is related to the sea level curve, therefore the observed coincidence between the strontium curve and transgressiveregressive cycles, as pointed out by Renard, supports this hypothesis. Iron and manganese vary through time very much like strontium. Their concentrations in calcareous pelagic deposits probably depend on the amounts produced at the mid-oceanic ridges and, therefore, on the latter's activity. Other attempts at correlation have been based on Na, K, Ca/Mg. Finally, some elements appear in exceptional concentrations at certain brief moments of geological time, during what may be called geochemical events. This is the case for Iridium, which is concentrated at the CretaceousTertiary boundary at about 50 oceanic and continental sites distributed worldwide. The same concentration is found at the Eocene-Oligocene boundary. The significance of these concentrations is still controversial. Two causes have been suggested: one extraterrestrial (fall of giant meteorites), the other volcanic (periods of intense magmatic activity). 2.2.4.2 Stable Isotopes In 1955, Emiliani demonstrated variations in the () 18 09 of the calcareous shells of planktonic foraminifers from the Quaternary of the North Atlantic and the Caribbean. These variations may be used to define isotopic stages (Fig. 36) which correspond rather closely to the glacial and interglacial stages. This correspondance derives from the fact that, at thermodynamic equilibrium, the 180/160 ratios of a carbonate and the water from which it precipitated (chemically or biochemically) are different, and that this difference increases as the temperature of precipitation decreases. Many subsequent studies have shown the synchronism of these variations in the marine environment (Fig. 37) and, therefore, their utility as a correlation tool. The generalization of this method to pre-Quaternary formations poses numerous problems related to diagenesis, such as recrystallization and temperature effects which change the original isotopic composition of oxygen. Nevertheless, data as far back as the Paleocene show the existence 9

52

Modern Stratigraphy

IX!

o

a.. o

~

o CD -co

NORMAL BRUHNES

o

2

3

4

REVERSE MATUYAMA

5

6

7

8

9

10

11

12

13

14

Depth (m)

Fig. 36. Curve of /) IH O of planktonic foraminifera correlated with the magnetostratigraphic scale in a core from the Pacific (after Shackleton and Opdyke 1976). The numbers up to 17 refer to Emiliani's (1955) isotopic stages

-2

Fig. 37. Variation in composition of oxygen isotopes from tests of foraminifera in four cores from 1 the Caribbean sea; 2 the Indian Ocean; 3 the Mediterranean; and 4 the Pacific. Time scale in millions of years. On the ordinate, variations of /)180 in parts per mil relative to PDB (Peedee Belemnite Standard) (after studies by Emiliani 1966; Shackleton and Opdyke 1973; Be and Duplessy 1976; Cita et al. 1977)

of variations related to fluctuations of arctic and antarctic ice and, therefore, to oceanic temperatures (Fig. 38). In contrast, the isotopic ratio of carbon (13 CP2C), expressed as 013C, is practically unaffected by burial diagenesis. Moreover, it appears to vary, synchronously, in the world's ocean, showing sharp changes principally at the Cretaceous-Tertiary boundary (65m.y.), at the Paleocene-Eocene boundary (53 m. y.), and in the Upper Miocene (about 6.1 m.y.; Fig. 39). This synchronism derives from the fact that the 013C of pelagic carbonates seems to be an indicator of the paleodepth,

53

New Methods of Correlation

Ma

PPliol

Miocene

lp

2,0

10 I igocene I

-2

l

j

1

0

0

o 2

p 10

o

0

JPalaeocene

of

+-0

~

I0"),+

00

6.0

~o

o/~'w

: +

0

~O+•• O

1

Eocene

40

30

C\)

t

0

0

o

0

0

00 0

0

t

o h;\/o 1}ty"'~+"++.o .. ....++ .. +'!.o >,\-o 0

~40 ,\.t • I +

101801

Fig. 38. Temporal variation of the oxygen isotope composition of total carbonate (0) at two sites in the northeastern Atlantic and planktonic foraminifera ( +) at three sites in the South Pacific (after Vergnaud-Grazzini 1979). Arrows indicate events

Ages ~ ot

g

Paleo cene

Miocene

Late

Late Cretaceous

Jurassic

+6:

u

M

+

('t)

'Ma

b

,

10

,

20

jo

, , 50

40

6'0

,

70

I

80

I

90

i i i

100

110

120

1!m

,

140

Fig. 39. Change in carbon isotope ratios of pelagic carbonates (total carbonate) since the Upper Jurassic (After Renard 1985)

and the depth of the euphotic zone. It increases during transgressions and decreases during regressions. Modifications of the continental and marine biomass (fixing 12C preferentially) lead to modifications of the 013C of sediment. For example, sediments enriched in organic matter on the shelves during transgressions, or significant deposits of coal, increase 013C. In summary, the isotopic variations of oxygen and carbon in marine carbonates reflect changes in the temperature, geochemistry, and other parameters of the environment of formation related more or less directly to global changes in climate and sea level. These variations are sharp at certain times and may then be used as stratigraphic markers. Other isotopic ratios under study may also become tools of correlation. The 034S curve, expressing changes in the 34SP2S ratio, defines a megacycle

54

Modern Stratigraphy Total volume of evaporite. (t O· km')

0,25

0,75

Neogene Paleogen

Cretaceous

Jurassic

,-, , I

Triassic Permian

Carbonlfero~'s .. ,

,

\

\ ,- .-

Devonian I

Silurian

---- --

,,-- --

Fig. 40. Temporal variation of sUlf.hur isotope ratios 4 S/ 3 S) in sulphates (solid line) and of the volume of evaporites (dashed line). Inspired by Odin et al. (1982a) and Tardy (1986)

... - -"

"

e

, f

Ordovician ,

10

20

--'

rn a:: c(

w

>z

0

:J ...J

~

~

w

~ ~

20 ",' 60 ./ ...... J 100 I 140 I 180 ....... 220 ~ 260 \ '\ 300 ........... ",' 340 380 I .~ 420 460 500 540 580 0

Tertiary Cretaceous

\

Jurassic

.

Triassic

---

Permian Carboniferous

'.

........

...o

...

o· 87 Sr/86 Sr ratio

Devonian Silurian Ordovician Cambrian Precambrian

o

01

o

~.

Fig. 41. Change of the strontium isotopic ratio (After Faure 1982)

New Methods of Correlation

55

(Fig. 40), with a minimum value in the Permo-Trias. In general, but not always, times when evaporites (rich in 34S) were important were also times when marine waters were enriched in 32S and, therefore, had lower 034S values. The 87Sr/86Sr ratio reflects the contributions of submarine volcanism, related to ocean spreading, and continental erosion. When plotted against time, this ratio varies inversely with the activities of the mid-oceanic ridges, the lows in the curve corresponding to periods of major oceanic spreading or, as in the Permian, to periods of major fragmentation (Fig. 41). 2.2.4.3 Organic Matter Geological history has been punctuated by periods characterized by sediments abnormally rich in organic matter. This imparts a dark color to the sediments ("black-shales") and results in a high content (greater than 1 or 2%) of organic carbon. The classic black shales include the graptolitic shales of the Paleozoic, the bituminous shales of the Upper Lias, the CallovoOxfordian black marls of the Tethys and those of the Cretaceous known from the North and South. Atlantic, from the United States, from Western Europe and from North Africa, deposited during phases of maximum extension between the Upper Aptian and the Coniacian. The most remarkable black shales, by virtue of wide geographic distribution and short stratigraphic duration, were formed at the Cenomaniah-Turonian boundary (Fig. 42). This was truly a black shale event. Similarly, the Upper Lias bituminous shales were deposited over a large part of western Europe. They are known in France as "carton shales". In the Quaternary of the eastern Mediterranean, beds of black sapropels have been encountered in drilling. The apparent causes of such events are varied, but all appear to be related to global phenomena such as climate, eustasy, and distribution of continental masses, guaranteeing their synchronism over very large distances. The composition of organic molecules can also be a basis for stratigraphic correlation. Recent studies (Brassell et al. 1986) have shown that Quaternary marine sediments of the last million years contain unsaturated organic components (alkenones) derived from certain coccolithophorids. The index of unsaturation of these products is a function of the surface water temperature at the time of their synthesis. The variation of this index through a sequence parallels the 0180 curve, which is also (partly) related to temperature. The index of unsaturation curves may, therefore, be used in correlation in the same way as the 0180 curves. They are especially useful for low-carbonate sediments deposited below the CCD. Alkenones have been used as far back as the Cretaceous, but the importance of further diagenetic change in older formations has yet to be evaluated. Of all the mineralogic and geochemical markers, it is the latter which promise to be of greatest use. The majority of them express the chemistry of former seas, with temporal variations in this chemistry being immediately valid for all the world's ocean; thus the great significance of chemostratigraphy for correlation. The use of this method, however, is limited by

Modem Stratigraphy

56

Fig. 42. Anoxic deep water (dotted areas) at the Cenomanian-Turonian boundary (After Graciansky et al. 1986)

diagenesis, the influence of which increases with the age of the sediments, decreasing the precision of measurements and, therefore, of correlation.

2.3 Correlation by Paleomagnetism 2.3.1 The Principle The ferromagnetic minerals (magnetite, iron sesquioxide, ilmenomagnetite) possess the property of taking on a magnetism when they are placed in a magnetic field. Included in substances undergoing cooling (lavas), these minerals preserve a thermoremanent magnetism acquired during their cooling at temperatures below their Curie point (578°C for magnetite, 600°C on average for most minerals). These minerals acquire the characteristics of the Earth's magnetic field at that time (intensity, declination lO , inclination), but they are wiped out if the ambient temperature subsequently exceeds the Curie point. On the basis of measurements taken from carefully oriented and dated material of the last 2000 years, it has been possible to show changes of inclination and declination with time. These parameters have varied even during historical time as secular variations of 20° to 30° lOVery close to the direction of the geographic meridian.

57

New Methods of Correlation Age (Mal

Fig. 43. Magnetic reversal scale based on recent volcanic rocks

about an average direction. Having established these temporal changes, it is possible to date archeological material or recent lavas, for example, by measuring their remanent magnetism.

2.3.2 Magnetic Polarity It has been observed that throughout the Quaternary and Tertiary, geological materials such as volcanic rocks have preserved magnetic fields whose directions and inclinations have about the same degree of scatter about the modern values as those values for the last 2000 years. In contrast, the actual direction of magnetization can be either similar to the present (normal magnetization) or opposite (reversed magnetization). These reversals, practically instantaneous on a geological scale, with a duration of only a few thousand years, have been recognized as worldwide synchronous events. Following advances in dating of relatively young rocks by the K-Ar method, it has been possible since 1963 to construct a detailed stratigraphy of magnetic reversals. Four major periods of about 1 million years average duration have been defined. They are, from the youngest, the Brunhes period (normal), the Matuyama (reversed), the Gauss (normal) and the Gilbert (reversed; Fig. 43). Within each major period, shorter periods of reversals less than 10000 years long have also been recognized and given names. These are called "excursions" by anglo-saxon authors and cor-

58

Modem Stratigraphy

respond to shifts of the magnetic pole of more than 45° of latitude in relation to their normal position. Paleomagnetism is also measureable in sediments and sedimentary rocks, where the preserved permanent magnetism can be attributed to three principal phenomena: 1. Orientation of magnetic particles at their deposition or shortly after (detrital remanent magnetism); 2. Magnetization of crystals at their formation during diagenesis or alteration in the Earth's magnetic field (iron oxides and sulphides for example) creating a secondary magnetization (chemical or crystalline remanent magnetism) ; 3. Parasitic magnetization (viscous, anhysteretic, etc.). Only remanent magnetism of the first type (primary signals) can be used as a stratigraphic tool, and using this a succession of magnetic reversals has been constructed similar to that derived from volcanic rocks. The other types of magnetization can be removed in the laboratory by various processes, such as by heating or by applying alternating magnetic fields.

2.3.3 Magnetostratigraphy The utilization of paleomagnetism in stratigraphy is difficult. The Earth's history shows that only two types of magnetic polarity are possible, as against an infinity of other types of events, especially those nonrepetitive events related to biological evolution. A magnetic reversal, therefore, has in itself little chronologic significance, other than allowing conclusions on the different ages of rocks in different locations by virtue of their different polarities or similar polarities but different magnetic declinations. However, stratigraphic information can be greatly improved by reference to a standard paleomagnetic time scale. Figure 44 illustrates such a time scale with the superimposed oceanic magnetic anomalies numbered 1 to M29 and several polarity periods or magnetozones (5 to 22) following the four previously designated (Bruhnes to Gilbert). Since anomalies and magnetic zones are correlated with the radiometric time scale, their relative durations are known and they consequently are valid geochronologic units. In addition, some higher order groupings are apparent, specifically "disturbed" periods with numerous reversals of polarity and "quiet" periods, with mainly normal or mainly reversed polarities. How has this time scale been constructed? The first succession of magnetic reversals was constructed by Cox et al. (1963a,b) for the last 7 million years and based on exposed lavas. This time scale was continued back into the Cenozoic and Mesozoic using positive and negative magnetic anomalies measured on the seafloor. It is known that the latter is formed from a continuous supply of basaltic rocks with ferromagnetic minerals which fossilize the Earth's field as they cool. This takes the paleomagnetic time scale back to the Upper Cretaceous, on the assumption

New Methods of Correlation

59

." ;;)

0 III U ~

~

'"'

III

~

'"

u

>-

• ...l ~

III

•

NORMAL POLARITY

D

REVERSE POLARITY DISTURBED ZONE FROM THE SINEMURIAN AT ~EAST

Fig. 44. Magnetic polarity scale for the Mesozoic and Cenozoic (After Channell 1982; and, for the Jurassic pre-Kimmeridgian, after Kent and Gradstein 1985)

60

Modern Stratigraphy 200m

250

300

-

II

.....0 - - - - -

2

-

-

3 34

...

_

II

K'T

6 2 5 k m - - -...... ~

II

.... 0-----

350 I

I

I

I

1_

I

I

9 5 5 k m - - -.........

III.,. 33

5 1 5 k m - - -.........

4

Fig. 45. Comparison of different polarity sequences established for the Upper Cretaceous. 1 Gubbio limestones (Italy); 2 North Pacific (40 N); 3 North Indian Ocean (88 E); 4 South Atlantic (38 S); (After Channell 1982) 0

34

,-3_3",-_

0

0

of a constant rate (1.9 cm/year) of ocean floor spreading for the South Atlantic l l . This method has two disadvantages: 1. It is difficult to date the ocean basalt anomalies, the potassium-argon method being inaccurate beyond 5m.y. This often makes it impossible to separate radiometrically two adjacent magnetozones. 2. The paleomagnetic signals weaken considerably with the age of the oceanic crust because of the alteration of its upper part. The sequence of paleomagnetic polarities has been subsequently compared with events used in biostratigraphy, such as first and last appearance of taxa, using sections on land and wells drilled in the ocean. These have made it possible to extend backwards the magnetostratigraphic time scale. The Cretaceous has been most studied on land, in central Italy, where it has been possible to correlate the biostratigraphy with the succession of polarities. This vertical sequence is then comparable directly with the horizontal sequence of oceanic magnetic anomalies, to provide them with the same calibration (Fig. 45). The anomalies should also be datable by microfossils in the sediments immediately overlying the basalts, but this is somewhat inaccurate because of the discontinuities which can exist between the basalt and this sediment. The data for the Jurassic come from boreholes in the ocean floor and from sections in Italy and northern Spain. The oldest oceanic data are from anomaly 29 of Oxfordian age. Before that was a long period of stability or weak magnetic field, as in the Middle and Upper Cretaceous, which lasted until the Callovian. This followed a disturbed

11 It has since been shown that this approximation is correct for many sectors of the world's ocean.

New Methods of Correlation

61

Fig. 46. Example of zonation by calcareous nannofossils in the Paleogene, and relation to the chronostratigraphic and magnetostratigraphic scales (After Aubry 1983)

period recognized in recent studies from the Bathonian to the Sinemurian (in Hallam 1984). In summary, magnetostratigraphy appears to be the best method of calibrating biostratigraphic zonation against absolute age derived from the radiometric time scale (Fig. 46). Widely used to date ocean spreading, it can aid considerably in the correlation of offshore wells and sequences on land. Magnetochronology is now being combined with biochronology and radiochronology to establish a unified geologic time scale.

62

Ma

Modem Stratigraphy

Moon stratigraphy

0

1000

COPERNICIAN

1800

ERATOSTHENIAN

3000' IMBRIAN

~OOO

4?1V1

NECTARIAN PRENECTARIAN

Fig. 47. Geologic time scale for the Moon (Van Eysinga 1985)

2.4 Extraterrestrial Correlations The interpretation of images of celestial bodies provided by space probes is based partly on relative dating and correlation of rocks according to their surface morphology. The planetary reliefs all show meteoritic impacts, particularly numerous about 4.5 billion years ago, i.e. very shortly after the creation of the solar system. Since that time, both the frequency of impacts and the size of meteorites have decreased. A stratigraphy based on surface formations is therefore possible for those planets, such as Mars, the Moon, and Mercury, which have experienced little crustal evolution. Two outcrops, for example of volcanic flows, having essentially the same density of large craters per unit surface area are correlatable in the sense that they began being bombarded by meteorites at the same time. Their formation must, therefore, have been also about the same time. For the Moon these observations, together with dating of samples brought back to Earth, have led to the construction of a rudimentary chronostratigraphic scale (Fig. 47).

2.5 Conclusions Geophysics, geochemistry, mineralogy, sedimentary cycles, and geomorphology all now contribute to stratigraphic methodology. This is not to say that they replace or even compete with classical lithostratigraphy and biostratigraphy, which remain the basic tools, but they are an indispensible complement, allowing, for example, the correlation of different paleon-

New Methods of Correlation

63

to logic zonation schemes and the testing of the synchronism of the appearance and disappearance of taxa at different latitudes. For instance, the comparison of various biological events with the paleomagnetic scale shows that many of them are synchronous to about O.1-0.4m.y. According to Johnson and Nigrini (1985), the disappearances of species are much more synchronous than their appearances, based on Cenozoic radiolaria of the Indo-Pacific region. In this way, a more useful biostratigraphy and lithostratigraphy not influenced by facies is slowly being established. Conversely, biostratigraphy can be used to test the value of a physical or chemical marker. This reciprocal control by different methods is one of the more significant factors contributing to the progress in stratigraphic methodology during the last two decades.

Chapter 4

From Stratigraphy to Paleogeography

The principal task of stratigraphy is dating. This allows the correlation of contemporary events, a necessary prerequisite to the reconstruction, chapter by chapter, of the Earth's history. This paleogeographic stage is dominantly spatial rather than temporal.

1 Principles and Methods of Paleogeography Paleogeography aims to paint the successive pictures of the Earth's surface from the beginning of its history. This synthetic discipline is therefore fundamental to geology. In a narrow and classical sense it attempts to trace the former boundaries between land and oceans and to reconstruct the vanished continental surfaces with regard to their topography, climates, life, and geodynamics. Paleogeography in the wider sense is also concerned with the oceans and their sedimentation, currents, depths, chemistry and living components. To achieve its synthetic purpose, paleogeography must extract information from the lithological and paleontological documents at its disposal in order to interpret particular environments of deposition.

1.1 Facies This term, introduced by Gressly in 1838, refers to all the physical, chemical and biological characteristics of a sedimentary rock reflecting its depositional environment. Thus, a facies (or isopic) map is implicitly a paleoenvironmental map. Lithofacies and biofacies represent often the two major components of a facies, the one physical and chemical, the other biological (fossils and/or traces). For example: 1. The Triassic red sandstones of the Vosges have characteristics which suggest a slightly inclined alluvial plain, with meandering rivers and sparse vegetation (Voitzia, Equisetum), and a climate like modern Sudan with contrasting wet (when silicate iron dissolves out) and dry (when ferric iron precipitates on sand grains) seasons.

From Stratigraphy to Paleogeography

66

EC9 Tithonian facies

Transltlonallacles

I;:;·.~.:J

'Calcaires blancs de Provence'

o

Gravelly limestone Pelagic Neocomlan

o

II7a1 High energy biohermalfacles ~

B

White limestones with green clay interbeds

Neritic Neocomian

Fig, 48. 1 Geometric relations of different facies near the Jurassic-Cretaceous boundary (dashed line) in Haute-Provence (Castellane region). 2 Restored section for the beginning of the Cretaceous, based on facies interpretation. Note the platform carbonate to basin transition (After Cot ilion 1975)

2. The Quaternary sapropels of the eastern Mediterranean (Chap. 3, Sect. 2.2.4.3) consist of dark shaley beds, decimetric, laminated, with organic material and planktonic remains, alternating with calcareous muds. Detailed analyses, including geochemical, of their lithofacies and biofacies allow precise conclusions to be drawn with respect to their environment of deposition: marine waters, stratified due to lack of vertical mixing caused by a hyposaline surface layerl. The deep water becomes therefore anoxic, so preserving the organic matter. The names given to facies show clearly their close relationships with the environment. They may refer to a place characterized by a particular type of deposition (Germanic facies of the Trias, Dauphinois facies of the Jurassic) or they may refer to a particular environment (reef facies, pelagic facies). Thus the first step in paleogeography is the analysis of rocks as indicators of the environment. The second step is the recording of all contemporary facies in a region and an examination of their mutual relations, the purpose being to reconstruct a terrestrial or submarine landscape; for example, the transition from the "Calcaires Blancs" of Provence to the Tithonian limestones of Haute-Provence in the Portlandian-Berriasian (Fig. 48). The geometric rela1 This freshening comes from continental waters brought by the Nile during climatically wet periods.

Principles and Methods of Paleogeography

67

tions between these two facies show that the transition from one to the other is both gradational and by interdigitations. The change of facies along a N-S cross-section suggests the transition from a platform to a basin with significant bathymetric variations. In addition, the subfacies within the "Calcaires Blancs" enable variations in the environment of the platform to be mapped.

1.2 Paleobiogeography Contributing to the knowledge of the environment (water depth, oxygenation, temperature, climate etc.), the biofacies is a fundamental element in paleogeographic syntheses. It provides information especially on the distribution of biological populations, the basis of paleobiogeography, whose aim is the study of the relations between the evolution of life and the evolution of the Earth. The first paleobiogeographic synthesis (Neumayr 1872) involved Jurassic rocks and introduced the idea of faunal provinces, generally very large regions often containing smaller areas defined by the occurrence of different taxa. Uhlig (1911), for example, distinguishes in the northern hemisphere in the Mesozoic: 1. A temperate zone in the north, or Boreal province. 2. A warm zone in the south, or Tethyan province, subdivided into several subzones (Mediterranean, Caucasian, sub-Mediterranean, Himalayan, Japanese, Pacific)2. None of these regions can be defined precisely because of faunal exchange across their boundaries. Western Europe was often the locus of such exchanges, espcially in the Toarcian, Callovian and Neocomian. These events complicate, as we have seen, the problems of time correlation but they do allow correlation between adjacent provinces with different biostratigraphic zonations. They also help to differentiate certain paleogeographic domains: for example in the Callovian-Oxfordian, the faunal expansions of boreal origin never crossed the north Tethyan boundary in Europe (Cariou et al. 1985) which is, therefore, identifiable by its immigrant faunas. For much older epochs, the faunal distributions may not always be related to climatic gradients; this could be the case, for example, for the two major trilobite provinces of the Lower Cambrian (Fig. 49), the Olenellus province of North America, Scotland, and Scandanavia and the Redlichia province of Asia and Australia.

The paleobiogeography helps to verify the paleogeographic reconstructions deduced from continental movements, which must be compatible with the known migration paths of this or that species, suggesting, for example, is now common to assume for the Jurassic a Boreal domain, a Tethyan domain (with Mediterranean, Indo-Southwest Pacific and East Pacific provinces), and from the Tithonian an Austral domain (Enay 1980). 2 It

68

From Stratigraphy to Paleogeography

Fig. 49. Faunal provinces in the Lower Cambrian, from trilobite distributions. The arrows indicate the possible exchange of faunas between provinces. Land indicated by cross-hatching (After Cowie 1971 in Pomerol and Babin 1977)

barriers or freeways. Several periods in the Albian - Turonian have been proposed for the opening of the communication between the North and South Atlantic oceans. Moullade and Guerin (1982) prove, using benthonic and planktonic foraminifera, that the South Atlantic was clearly open to the Central Atlantic from the Middle Albian (Fig. 50). The warm water Tethyan species are present at this time in the northern part of the South Atlantic but disappear to the south, due without doubt to the cooler waters. This shows that the only possible migration route was north-south from the Central Atlantic and not from the south via an eastern route around Africa. Moreover, the presence of certain planktonic forms shows that the arm of the sea separating Africa and South America was at least lS0-200m deep in the narrowest part. We should note, however, that the propagation of even planktonic species does not always require open ocean. According to Enay (1980), an opening can be preceded by faunal exchanges across shallow epicontinental waters. Thus, from the Lower Jurassic "pre-oceanic freeways" above a sialic basement may have linked the Tethyan and East Pacific oceans.

1.3 Cartographic Syntheses Just as geography cannot be conceived without cartographic illustrations, so paleogeography is usually illustrated by synthetic maps of variable scales, including, for different epochs, the maximum of information on continental

69

Principles and Methods of Paleogeography

\(

~

30

30

•• 60

Fig. 50. Migration of Central Atlantic foraminifera to the South Atlantic in the Albian. Points represent studied wells (After Moullade and Guerin 1982); arrow denotes direction of migration

and marine distributions and their boundaries. Paleogeographic maps have been improved over time, but whatever their degree of perfection, their image of the globe is never instantaneous but is always the average of a series of superimposed images representing a certain interval of time. The more recent the time and the larger the scale, the smaller need be the interval of time illustrated.

1.3.1 Facies Maps They are the basis of paleogeographic maps. They represent the lateral distributions of the different facies (therefore different environments) of chrono- or biostratigraphic units (stages, zones). Marine and continental deposits are clearly differentiated and their boundaries sometimes delineated (Fig. 51), thus representing a transition towards a true paleogeographic map. The facies boundaries do not necessarily correspond to depositional boundaries, but often to erosional boundaries as seen in the Upper Jurassic formations which surround the French Massif Central today, but probably covered it entirely at the time of deposition. Isopachous contours indicating the thickness of various formations may be superimposed on the facies maps. For regions which have been strongly deformed tectonically, it is often useful to restore the facies to their pre tectonic

70

From Stratigraphy to Paleogeography

Fig. 51. Facies map of the Upper Cretaceous showing the distribution of continental basins (vertical hachuring) and the maximum extension of marine sediments (dashed line and dotted area) in the Rhodanian basin (After Debrand-Passard et al. 1984)

positions. The maps resulting from this unfolding are palinspastic maps (Fig. 52).

1.3.2 Paleogeographic Maps These are interpretive maps showing basically the boundaries between oceans and continents, but may include continental relief, ocean depths, shoreline movements, faunal migrations, tectonic movements, provenance of sediment, or sediment transport directions. On the continents, the zones of deposition with their facies and the zones of erosion with their lithology and age may also be indicated. For example, in southeastern France, basement movements of Middle Cretaceous age have caused the erosion of Permian rhyolites and the deposition of bipyramidal quartz derived from the rhyolites. Formerly, because of insufficient data to determine the positions of the major cratons, the paleogeographic maps were based on the present distribution of land and sea (Fig. 53). The interpreted shorelines were shown, therefore, as landward encroachments more or less beyond the present coastline or conversely exposing parts of the continental platform. Such simple distinctions obviously could not represent all the observed sedimen-

71

Principles and Methods of Paleogeography

Fig. 52. Palinspastic map of the western Alps in the Senonian (After Ricou 1984)

ft~t~!&J

Continental areas

o

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Marine area

Fig. 53. Paleogeographic map of the Upper Jurassic showing the uncertainty regarding the distribution of oceans of that time (After Furon 1972)

tary and tectonic phenomena. For example, when some detrital sediments implied the existence of some offshore continental source, subsequently submerged beneath the ocean, this "source land" could be located and its extent determined only with great difficulty. Moreover, the great distances

72

From Stratigraphy to Paleogeography

between continents bearing identical biological communities posed the problem of migration across vast oceans by swimming, or on rafts of vegetation, or via intercontinental bridges, somewhat as a present-day isthmus allows the exchange of faunas between emerged lands. In a general way it was difficult to imagine, assuming the fixed nature of continents, the paleogeography of areas corresponding to the large oceans of today (Fig. 53). Since the 1960s, the petroleum exploration of the continental margins, the theory of plate tectonics and deep ocean drilling have revolutionized the way of paleogeographical expression which is now presented as global maps on which the ocean floors (continental platforms, slopes and abyssal plains) are given as much significance as the continents. The boundaries between land and sea are placed in their most probable positions for each epoch of geologic history (Fig. 54). From the present day back to the Middle Jurassic, the successive geographies have been deduced from the following: 1. Paleomagnetic data, collected on land as well as from submarine oceanic crust. The magnetic declinations measured on different continents allow the reconstruction of their relative positions at different epochs, and the magnetic inclinations their latitudinal positions. 2. Unfolding of tectonically deformed continental crust. For older periods only continental data can be used, because the corresponding oceanic crust has disappeared by subduction or has been transported as blocks on to the edge of cratons (obduction). According to whether an ocean is now bounded by a stable or an unstable margin, estimates of its original position at different epochs can vary between 100 to 1000 km (Dercourt 1984). These estimations also depend on the particular sequence of paleogeographic evolution chosen from all the possible sequences: once constructed, the global paleogeographic maps will show gaps which correspond generally to vanished oceanic segments, like the Tethys, the only evidence for which are the ophiolitic sutures (Fig. 111). Other information helpful in the localization of oceans and continents are the following: 1. The paleo depths of oceanic domains are calculated on the basis of the subsidence curve for the ocean floor basalts as a function of time and thickness of overlying sediments. 2. The rate of detrital deposition within or marginal to the craton yields information on the extent of continental relief. 3. The tracing of former shorelines proceeds from two approaches: a) From the character of the most landward deposited marine facies, it is possible to determine whether the shoreline was close to or well beyond the present outcrop limits of this facies (Fig. 55). b) By examination of the eustatic curves (Vail et al. 1977; Hallam 1978) some corrections of estimates based on the first approach are possible (Fig. 56).

73

Principles and Methods of Paleogeography

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Fig. 54. Example of a global paleogeographic map at the Jurassic-Cretaceous boundary (130m.y.; Dercourt et al. 1985). A relatively narrow Tethys separated Africa and Europe, partly covered by an epicontinental sea. To the south of Spain, the ocean was reduced to a narrow trough whose dynamics reflected the sinistral movement of Africa

4. The percentage of land covered by sea can lead theoretically to a calculation of the altitude of sea level in relation to the present day (Fig. 57). In fact, knowledge of the exact part played by eustasy in determining shoreline positions is necessary, since the latter is also controlled by subsidence, sediment compaction, and tectonic movements. 5. The oceanic and atmospheric paleocirculations depend on two factors: a) The general global climate: the distribution of facies on a global scale gives a good illustration of this because it indicates the significance of latitudinal temperature gradients, which control the major circulations of air and water. b) The distributions of land and sea on the Earth's surface. Taking into account these distributions, several models have attempted to recreate

From Stratigraphy to Paleogeography

74

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Sea level

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Bajocian Aalenian Toarcian Pliensbachian Sin em urian Hettangian

Fig. 56. Variation of average sea level on the continents in the Jurassic (After Hallam 1978)

75

Factors of Paleogeographic Evolution

E 8000 .~ 6000 "C

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Fig. 57. Hypsometric curve representing the percentage area of land above a given sea level. A Present area; B area in the Upper Cretaceous

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the circulations on the basis of locations of supposed high- and lowpressure atmospheric zones (Fig. 58). It appears that in the Cretaceous, the traditionally assumed relation between low thermal gradients and slowness of current flow is doubtful, the atmospheric circulation appearing not to be too low at this time (Parrish et al. 1982).

1.3.3 Mapping of Volcanics One particular case of cartographic synthesis concerns rapidly evolving volcanic zones. A computer-aided evolutive cartography at a scale of 1 :25000 has recently been produced by the BRGM (Bureau de recherches geologiques et minieres) for Piton de la Fournaise (Reunion island). These data will be used for a survey of modern volcanic activity.

2 Factors of Paleogeographic Evolution The face of the Earth, represented cartographically, is continually modified through time. This evolution results from the interaction between two dynamics: one, internal and manifested by a continual deformation of the lithosphere, the other external, shaping the surface of the Earth through its enveloping fluids and biosphere.

2.1 Deformation of the Lithosphere 2.1.1 Plate Tectonics This is the essential agent of paleogeographic evolution. It is the basis of orography, whose main characteristics, continental and oceanic rifts, transcurrent faults, accretionary prisms, subduction trenches and mountain chains are associated with plate boundaries. In addition, the global tectonics

From Stratigraphy to Paleogeography

76

Conttnents and epicontinental seas Upwelling areas

H: High pressures

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_

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Fig. 58. Model of winter atmospheric circulation in the North Hemisphere in the Tithonian (After Parrish and Curtis 1982)

control the distributions of continents around the Earth, which in turn control the oceanic circulation and the climate. Geologic history seems to indicate periods of dispersion of continental crust and periods of assembly, ending in the formation of supercontinents of the "Pangea" type (Wilson cycles). Two cycles have been recognized within the last 900 million years; their cause probably has to be found in the asthenosphere or deeper within the Earth (variations of heat flow). The consequences of such cycles are fundamental for the history of the Earth, because the creation of Pangea was generally associated with a lower sea level and a deteriorating climate, marked by the appearance of ice sheets and profound changes in the course of biological evolution .

2.1.2 Epeirogenic Movements These are vertical movements of the lithosphere, with no apparent relation to orogeny but without doubt also linked to variations in heat flow. When the latter increases, the lithosphere dilates, becomes less dense and rises (thermal updoming). The opposite effects occur when the heat flow decreases (thermal subsidence). The structures seen as broad cratonic domes and basins could be of this origin. For example, in central Siberia, a dome brings up the basement to an elevation of 1700m, while the Amazonian Shield is depressed into as immense synclinal basin. Thermal subsidence also plays a role in epeirogenesis, as defined above. It effects not only all parts of the progressively cooling lithosphere, especially the oceanic crust just after

Factors of Paleogeographic Evolution

77

its formation, but also the continental margins and cratonic regions. It also influences the loci of sedimentary basins and their evolution. Finally, epeirogenesis appears also to effect continents as a group. According to Worsley et al. (1984), the thermal tumescence of continental masses, which are weak conductors of heat, is proportional to their surface areas and inversely proportional to their rate of displacement above the asthenosphere. Under these conditions, the supercontinents of the Pangea type would not be invaded by seas because of their relatively high mean elevations. Epeirogenic movements result also from isostatic adjustments. The lithosphere, overloaded by an ice sheet, sediments or lavas, sinks into the asthenosphere, indicating that a part of the subsidence of basins is due to the weight of sediments. Periodic volcanic eruptions (the Hawaiian islands, for example) similarly create a sagging of the lithosphere, thereby reducing a part of the former relief. Conversely, a lessening of the load on the lithosphere leads to its uplift. When the Wiirm ice sheet covering northern Europe began to melt, Scandinavia rose. This movement began 12000 years ago and continues today. The maximum uplift, centred on the Gulf of Bothnia, reached about 400m. Unloading of the lithosphere can also result from erosion of its upper part. Since the intensity of this process is generally a function of its elevation, it follows that an elevation of the lithosphere by thermal tumescence will tend to increase the effect of erosion.

2.2 Volcanic Eruptions Continental volcanism is the cause of some of the highest mountains of the Earth (Kilimandjaro in Africa of nearly 6OO0m; Aconcagua in the Andes, 7026m). Enormous flows have built great basaltic piles (200000km 2 wide and 2000 m thick) in the Upper Cretaceous Dekkan plateau of India and massive trachyandesites in Chile and Patagonia (700 000 km2 wide and several thousands of meters thick). Oceanic volcanism is active in the zones of plate divergence, building the great mid-oceanic ridges, a major morphologic feature of the oceans. Their length totals about 60000km, with a width of l000-3000km and an average height of 2000 m. These ridges are cut by transform faults. Apart from ridges and volcanic arcs associated with subduction, oceanic magmatism is rare. The alignment of volcanic belts parallel to the direction of movement of the plate carrying them suggests the presence of a hot spot beneath the plate. This is the explanation for the Hawaian islands.

2.3 Interplay of Erosion and Sedimentation All topographic relief is inexorably doomed to destruction by erosion. The products of erosion are then carried by several agents, the main one being gravity, towards the low areas of the Earth's crust, i.e. the oceans, where

78

From Stratigraphy to Paleogeography

they are deposited. This continental transfer of material from the continents to the oceans should eventually flatten the continents. It has been calculated that in only 50 million years the continents would disappear in spite of isostatic readjustments due to this unloading. Such a scenario appears to be reflected in the peneplains which are developed at the end of orogenic cycles, before the transgression which initiates the following cycle advances over the erosional surface. However, as long as the internal motor of the Earth remains active and therefore capable of deforming its crust, the destruction of all continental relief can only be transitory for any given area and cannot be a condition of the whole globe at anyone time. Submarine relief also seems to be largely due to mechanical and chemical erosion, as direct observations from submersibles have recently revealed. Sedimentation acts in a sense opposite to that of erosion. It fills basins and can in some situations lead to emergence. In the marine environment, this may occur only in the relatively shallow epicontinental regions. This is illustrated in nearshore areas by the progradation of deltas and coastal spits and the construction of barrier reefs. Beyond the continental platform, sedimentation plays an important morphologic role at the base of the continental slope, where submarine deltas are built in front of large rivers.

2.4 Eustasy 2.4.1 Paleogeographic Effects Whatever their causes, variations of sea level have profound repercussions, especially on the continental margins, although the shifts of paleoshorelines are generally due more to local or regional deformation of the cratonic margin than to eustatic fluctuations. Drowning terrestrial topography, and especially the low-lying valleys, a marine transgression creates an incised coastline with isolated islands. Regressions are characterized by smoother coastlines. The degree of immersion of the continent controls the base level of the fluvial system, and, therefore, the interplay of erosion and sedimentation, both continental and marine. A high sea level decreases erosion in the low fluvial valleys and increases the sedimentation on the continental shelf; a low sea level causes erosion of the shelf and the terrigenous products are transferred directly to the base of the slope, where they accumulate as submarine deltas or may reach as far as the abyssal plain (Fig. 59). The variations in sea level also affect oceanic circulation. A eustatic rise creates new communications by drowning sills and highs. It therefore facilitates interprovincial exchanges. A eustatic drop tends on the contrary to isolate basins from each other and may cause the development of restricted, brackish or hypersaline environments. Paleobiogeographic evolution is directly affected by these variations.

79

Factors of Paleogeographic Evolution

Low sedimentation rate

Fig. 59. Influence of eustacy on oceanic detrital sedimentation. 1 High sea level; 2 low sea level

Finally, eustasy controls to some extent the global climate. Transgressions increase the ocean's surface and decrease the average albedo of the Earth, thereby creating a warmer and more humid climate (if other factors remain the same). Regressions have the opposite effects. The Jurassic and Cretaceous, characterized by two major transgressions, were generally warm and humid periods. On the other hand, the end of the Cretaceous, marked by a major regression, was typically cold and dry. The end of the Jurassic, which also coincided with a regression, but of less significance, was also a time of relatively dry conditions, according to Hallam (1984).

2.4.2 Causes of Eustasy The curve according to Vail et aI. (1977), which attempts to represent eustatic variations (see above, Fig. 30), can be useful in distinguishing the principal factors involved. This curve consists of cycles of several orders:

1. First order cycles of very long periods (200-400m.y.), of which there are only two from the end of the Precambrian to the Present. They are parallel to the Wilson cycles and consequently follow the rhythm of the contraction and dispersion of continental crust. 2. Second order asymmetric cycles of 10-100m.y., taken generally from the major subdivisions of geologic time. Their limits coincide, therefore, with several of the discontinuities separating the systems and subsystems of the stratigraphic scale. 3. Third and fourth orders cycles of 1-10 and of 10000-lm.y. The larger

80

From Stratigraphy to Paleogeography

cycles are asymmetric and may correspond to stages, especially in the Jurassic and Tertiary. The smaller cycles can be seen from the end of the Miocene (Messinian) to the Present. This curve has been revised by Haq et al. (1987). Three major types of causes of eustasy are generally accepted: 1. Geotectonic causes. All are effective in modifying the volume of oceans. They can, according to Vail, be applied to the first-, second- or thirdorder cycles, at least in part. For example, the orogenies resulting from plate convergence induce a shortening of the continental crust with a corresponding increase of ocean widths. If the ocean volume remains constant, this must result in slightly lowered sea level. This effect is obviously greater when the continents are assembled into a supercontinent, when the thermal updoming is also at a maximum (see above). Consequently, the formation of a Pangea is always accompanied by a very low sea level. Conversely, cratonic tensional regimes, while creating sedimentary basins and rifts, reduce the areas of the oceans and therefore raise sea level. According to Bureau (1985), these tensional effects alone would have caused a eustatic rise of about 80 m in the Mesozoic. Variations in mid-oceanic ridge activity also modify the ocean floor morphology and, therefore, the space available for the ocean (Pitman 1978). The marked expansion of the basaltic crust results in a growth of ridge volume and a rise in sea level 3 . This was the situation in the Upper Jurassic and in the middle of the Upper Cretaceous. Some authors do not accept the global character of Vail's secondorder cycles, suggesting that they result from an interaction between firstorder cycles and local deformation, thermal subsidence of continental margins, flexure of the lithosphere etc. 2. Climatic causes vary the volume of ocean waters much more abruptly than magmatism and are at least partly responsible for the third- and fourth-order cycles. During glacial periods, water is bound as ice at the poles, lowering sea level. The alternation of glacial and interglacial periods, well known since the Miocene, thus induces eustatic cycles. Recent work has concluded that such cycles have existed since the Trias, when the North Pole was located to the northeast of Siberia. It should be added that the ocean volume depends also on temperature, a drop of 1°C resulting theoretically in a 1 m drop in sea level. 3. Causes related to deformation of the geoid. Of variable origins (coremantle relations, distribution of oceanic masses, variations of gravity and of global rotation, etc.), these deformations can cause local eustatic variations so that a transgression in one part of the globe could correspond to a regression in another part (Marner 1981). 3 According

to Pitman, a variation of 3 cm/year at a rate of expansion along a ridge 40 000 km long can raise sea level by 1 cm/lOOO years.

81

Factors of Paleogeographic Evolution 90W

Fig. 60. Apparent displacement of the North Pole relative to Europe (solid squares), and North America (solid circles) from the Cambrian to the Present (after McElhinny 1973). Cb Cambrian; S Silurian; D Devonian; Cl Lower Carboniferous; Cu Upper Carboniferous; P, Permian; Tr Triassic; Trl Lower Triassic; Tru Upper Triassic; J Jurassic; K Cretaceous

In summary, the paleogeographic evolution produced by transgressions and regressions represents a complex interplay between eustasy and vertical movements of the continental crust (subsidence, isostasy, epeirogenesis, tectonism), and even of the sediments (compaction).

2.5 Polar Wandering We have seen above that the directions and inclinations of paleomagnetic fields vary within certain constant limits about a mean value, the mean declination giving the direction of the geographic poles. It is therefore assumed that the magnetic and geographic poles have always coincided throughout geologic time. The numerous paleomagnetic measurements made all over the world since the 1950s have shown that, after making allowance for continental movements, the poles must have drifted slowly at an average rate of 4 cm per year since the Precambrian. The polar wandering curves are peculiar to each continent because of the latter's relative movement (Fig. 60). They record the major geodynamic phenomena. Thus, the northern polar wandering curve, seen from North America, changed its direction between 120 and 50m.y. ago and also around 200m.y. ago. The Laramide and Nevadan orogenic phases may be related respectively to these changes. The mobility of the poles, in addition to that of the continents, has played a very important role in regional climatic change because it has resulted in the displacement of the latitudinal climatic zones, especially the

82

From Stratigraphy to Paleogeography

evaporite zone indicative of aridity and high temperatures. It is known that this zone moved across much of Europe during the Mesozoic (see below). Pole migration is, in fact, only apparent in so far as the Earth's mean rotational axis is considered stable with respect to the Earth as a whole. It is therefore necessary to assume that the totality of the lithosphere is mobile and can move with respect to the deeper zones of the Earth.

2.6 Conclusions: the Earth in Relation to Other Planets of the Solar System The paleogeographic changes evident at the Earth's surface are characteristic of a planet often called "living", not by reference to its biosphere, but rather to the dynamic nature of each of its envelopes. This leads to continual modification of its surface and its climate. The energy needed for this is of thermal origin, coming mostly from internal sources and partly from the sun (atmospheric and hydrospheric movements). Terrestrial relief, therefore, does not depend on external agents such as meteorites, which cannot leave any durable imprint on the surface. Other planets of the solar system, especially the inner "telluric" planets, owe their surface morphology primarily to very intense crater-forming impacts at approximately 4600m.y., i.e. at the end of the period of planetary accretion. Mercury is most typical; being very close to the sun, it lost its fluid envelopes very soon and preserved its primitive surface, pockmarked by large craters. On the Moon, also lacking water and an atmosphere, volcanic outpourings of lava testifying to an internal activity have locally masked its primitive surface. Mars is much more like the Earth, but tectonism there has remained somewhat rudimentary, partly due to a very thick lithosphere (200km). However, erosion and deposition due to wind and runoff, as well as to volcanic flows, have destroyed the initial relief of its surface. The Earth long ago lost all trace of its primitive morphology. It is also the only planet (together with Venus?) manifesting the processes of plate tectonics.

Chapter 5

The Major Stages of Earth History

1 Precambrian Time We will define the Precambrian as the period of time from the formation of the Earth to the lower boundary of the Cambrian, although a narrower definition starts the Precambrian from the first dated rocks (3800m.y.). This traditional term is not the best because it does not fit with the names of later eras, Paleozoic, Mesozoic, Cenozoic. Thus, some people prefer the term Archeozoic. The Precambrian is five times as long as all the other eras combined, which are commonly grouped into a single chronostratigraphic higher-order unit (eon) named the Phanerozoic. The Precambrian should, therefore, be regarded as the first, or even first two eons of geologic history.

1.1 Boundaries and Subdivisions The Precambrian can be subdivided according to two concepts (Fig. 61): 1. Classic concept. According to this concept, the Precambrian covers two eons: the Archean, including in North America very metamorphosed rocks, bounded at the top by a major break at 2S00-2600m.y. marking the end of strong tectonic and magnetic activity recognizable on all the shields; then the Proterozoic (from the Greek Proteros, "first"), discordant on the Archean and much less metamorphosed. The Proterozoic itself includes several units corresponding to orogenic cycles. These units are often named according to local terms and therefore are equivalent to formations. At the top of the Proterozoic is a unit, the Eocambrian, which is equivalent to a part of the French Infracambrian. Its lower part (600 m. y.) coincides with glacial formations and its upper part is overlain by the first Cambrian beds with trilobites. The Eocambrian contains the first remains of metazoa with mineralized skeletons, shells, spicules, etc. 2. The concept of Salop (1979). In this concept, the term Archean disappears because it is too poorly defined at the base. It is integrated partially into a very large unit, the Protozoic, or rocks of primitive life,

84

The Major Stages of Earth History

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..

I

Early Archean

Late Middle Early Proterozoic :t.rchean~rcheanl

Heliklan Gothian

It)

Aphebian Presvecocarelian

Svecocarelian

Icartian

Pentevrlan Suggarlan

I

Karelian

Riphean

Epiprotozoic

Neoprotozoic

pr::::~i~

Byelomorian

Katarchean

Paleoprotozoic

Katarchean

---~~~------------~------~~----~------~I~------~ ~ Pha-

;;j

Eons

.. 'e." o

~~I~

EozolC

Protozoic c: c:

"c: >

.!! .. ";::

....

~~

00..

. c:

.~

c:

~

c:

.!!!

~

::I

o

Fig. 61. Some stratigraphic scales of the Precambrian now in use

which is subdivided into Paleo-, Meso-, Neo-, and Epiprotozoic, defined by orogenic megacycles. Units older than 3500m.y., which seems to be the time of a major break, are placed in a new eon, the Eozoic (dawn of life), the beginning of which is difficult to define precisely but whose termination is defined by the era called Katarchean (lit. "below the Archean"). Finally, the Eocambrian can be placed in the Phanerozoic (organized life) since it already contains the remains of complex organisms lacking a skeleton or shell. Moreover, this unit coincides with the beginning of a major transgression which culminates in the Cambrian. These two contrasting stratigraphic concepts are mentioned in order to show the complexity in the study of this initial period of Earth history, which is not only the longest but also the poorest in decipherable records.

85

Precambrian Time

1.2 Methods of Study These illustrate the special situation of the Precambrian among geological formations. The problems inherent to the study of these rocks derive from their composition, their correlation, and the difficulty in applying the principle of uniformitarianism to them. The oldest deposits of the Earth's crust were intensely altered during several orogenies. They include crystalline schists, more or less metamorphosed, migmatites, and granites, all strongly deformed and recrystallized. In these conditions the criteria for determination of upper and lower parts of a layer do not exist; therefore, the fundamental stratigraphic principle of superposition is often not applicable. However, even though badly altered, many Precambrian rocks still preserve important primary structures such as grading, erosion marks, current ripples, oblique stratification, cross-bedding, and even certain sedimentary cycles. Correlation of such sequences is always difficult because they are generally azoic (originally, or due to metamorphism) or they lack organisms having any stratigraphic value. The only fossils usable are stromatolites and certain unicellular organisms (see below). Moreover, many Precambrian cratons formed continental blocks which were subsequently fragmented during the course of several orogenies. The historical reconstructions of the separate parts of these gigantic puzzles are not easily correlated with one another. Also, generally speaking, the only practical correlations are lithostratigraphic, although it has been necessary to adapt the method to metamorphic rocks by assuming that the alterations were isochemical, i.e. they preserve the geochemical identity of successive beds which can therefore be distinguished by their compositions. In the old shields, the most obvious stratigraphic breaks generally mark the end of orogenic cycles and separate units of different metamorphic grade. They have allowed the recognition so far of 16 orogenic cycles in the entire Precambrian. The basic data of relative age associated with these breaks must first be established in the field, especially discontinuity surfaces and plutonic intrusions. In thin section, the successive tectonic phases are interpreted from successive recrystallizations, which also help to establish the timing of plutonic emplacements. These different methods lead to a regional lithostratigraphy which can be applied in anyone region as far as tectonic structures can be continuously traced. If, however, it is necessary to correlate separate and structurally different domains, then radiochronology must be used. Three methods are commonly used for the Precambrian: Rubidium-Strontinum (87Rb/86Sr) on whole rock or micas; Potassium-Argon (40K/40Ar) on whole rock or on minerals rich in potassium; and Uranium-Lead or Thorium-Lead 38UI 206Pb, 235Upo7Pb, 232ThPo8Pb) on zircons (ZrS04) and apatites (Ca5(P04hOH, F, CI). The use of these three methods on the same sample makes it possible to date with an error of 2-3% the formation of the

e

The Major Stages of Earth History

86

o

Phanerozoic

_

Proterozoic Archean

fZ:iJ

Fig. 62. Distribution of Precambrian cratons of Pangea in the Permian (After Windley 1984)

igneous rock and sometimes the phases of deformation and metamorphism. In practice, only coarsely crystalline and unaltered igneous rocks will give reliable results . As information on the Precambrian accumulates, the difficulty of a strict application of the principle of uniformitarianism for a period of time so distant becomes increasingly apparent. The laws of physics have remained the same since the formation of the Earth, but the conditions of their application have changed considerably . Thus the atmosphere, the hydrosphere, erosion, weathering and the entire dynamics of the lithosphere were very different in the Precambrian from what they are today. Their evolution to the present situation occurred either gradually or sporadically, with certain stages marked by metal concentrations, first red beds (iron oxide), first ophiolites, first organized metazoa, weathering surfaces, igneous eruptions of particular compositions, glacial formations, etc.

1.3 The Geography of the Precambrian The Precambrian terranes constitute the backbones of the eXlstmg continents. Their outcrops define the shields which are covered peripherally by weakly deformed Phanerozoic rocks. These peripheral zones, where the Precambrian basement is accessible only by drilling, correspond to the platforms. In addition, some more or less important elements of the basement are reactivated in various orogenies postdating the Precambrian. Examples are known from Scotland, Bohemia, Spain, and France, where they occur in the

Precambrian Time

87

Fig. 63. Section in the Grand Canyon of Colorado (after Pomerol and Babin 1977) . I Basalts and diabases; 2 Hotanta conglomerate; 3 Bass limestone; 4 Hakatai variegated shale; 5 Shinumo quartzite; 6 Dox stromatolitic sandstone; 7 Chuar shale; 8 Tapeats sandstone; 9 Bright Angel trilobite shale; 10 Muav limestone; I I Temple Butte lenticular limestone; 12 Red Wall Productus limestone; 13 Supai sandstone; 14 Hermit shale; 15 Coconino sandstone; 16 Kaikab fusulinid limestone

majority of the old massifs. The map in Fig. 62 shows the distribution of shields and platforms in the Permian Pangea. A famous section is that of the Grand Canyon of Colorado (Fig. 63) where two major unconformities are visible. One between the Archean and the Proterozoic, the other between the Proterozoic and the Cambrian (Huronian unconformity).

1.4 Early Segregation and Establishment of Fundamental Processes 1.4.1 The First Crust Once the original material of the Earth had accreted and become heated internally by the energy derived from gravitation, radioactivity and meteorite impacts, the principal elements and minerals were segregated by density to form the different layers of the Earth. Water and gases were expelled to the surface, where they formed the primitive hydrosphere and atmosphere, the latter replacing the essentially hydrogen-rich initial atmosphere. In contrast, the heaviest elements sank to the centre of the Earth where they formed the dense nickel-iron core. Between the atmosphere and the core, a mantle of iron and magnesium alumino-silicates was formed, with an upper silica-rich part which became differentiated by partial fusion and cooling into a crust

88

The Major Stages of Earth History

composed largely of the lighter elements such as Si, K, Na and Ca. The subsequent history is still being debated. One possible scenario, according to Kroner (1984) is as follows. From the stage when a crust was established above a mantle, an embryonic plate tectonics began to operate. The mantle, undoubtedly hotter than at present, would be stirred by vigorous convection currents. At the surface where these currents emerged, the crust would be thinned and broken into rigid fragments, much like one sees today in lava lakes. These plates would be rapidly recycled in the underlying mantle, into which they would sink by virtue of their high density, or under the impact of meteorites whose maximum effect was between 4500 and 4000m.y. From 4000m.y. the heat flux and temperature of the mantle diminished, the primitive crust therefore persisted longer, and the volcanism resulting from partial melting of the upper mantle was able to thicken certain plates and thus make them more buoyant. The first silica crust, composed essentially of granitoid plutons, then developed progressively at the expense of subcrustal magmatic differentiations. The end result was a relatively light crust composed of two constituents: at the base, high-grade gneiss complexes and at the top, a mixture of volcanics and granitoid intrusions. When this crust emerged above sea level, erosion produced the first sediments, some of which are believed to be still extant: for example, the gneisses of the Limpopo belt in South Africa, dated at 3800m.y. (Fig. 64). The evolution of the continental nuclei into blocks too light to be assimilated into the mantle occurred slowly by vertical accretion, leading to thickening of their roots. This scenario therefore suggests vertical accretion as the mechanism for the formation of primitive continental crust, which was perhaps 25 - 30 km thick by 3500 m. y. It also rejects the uniformitarian explanation of marginal accretion during collision, although it seems to be valid for Phanerozoic times.

1.4.2 The Primitive Hydrosphere and Atmosphere Study of the gaseous envelopes of cosmic bodies less evolved that the Earth, or differently evolved, can shed much light on the Earth's primitive atmosphere; all planets of the solar system should have started out with similar atmospheres. Figure 65 summarizes the principal atmospheric components of these planets today. The outer planets, of greater mass and strongly shielded from solar radiation, have certainly retained most elements of their primitive atmosphere, which should, therefore, resemble that of the Earth very early in its history. To be noted are the abundance of the light elements and the reducing character of this atmosphere with CH 4 , NH 3 , N2 , H 2 , H 2 0, H 2 S. However, the atmospheres of the telluric planets, which include the Earth, were rapidly enriched in CO2 and H 2 0 coming from the degassing of the crust and mantle, and, therefore, became very dense (pressures of 70 bar at the Earth's surface, according to some estimates). The greenhouse effect resulting from this huge quantity of CO2 could have led to very high

Precambrian Time

89

Fig. 64. Simplified mantle plume model for the origin and growth of primitive continental crust. Arrows denote movements of convection (Kroner 1984, after Condie 1980)

Outer or gaseous planets (and satellites)

Inner or telluric planets Mercury and Moon

Venus

Earth

Mars

Jupiter

Saturn

Uranus

Neptune

N2 O2 CO 2

CO H 2O H2 He Ne

A NH3

----

CH4

Fig. 65. Principal atmospheric constituents of the planets in the solar system

temperatures at the surface (70-100°C in the oceans). The Earth is at the moment the only inner planet to still have small amounts of the lighter elements (hydrogen, helium). It has also been able to retain its nitrogen derived from the dissociation of ammonia under the influence of solar

90

The Major Stages of Earth History

radiation. The presence of free oxygen in large quantities, a condition specific to the Earth, is due principally to the photosynthetic activity of plants (see below), and slightly to the photo-dissociation of CO 2 and H 20 by UV radiation. In the high atmosphere, an ozone (03 ) layer makes a protective screen against UV radiation. Whatever the hypothesis for the origin of dissolved salts in seawater, and their proportions, no real idea of the composition of primitive seawater yet exists. It was probably very acid because of the very large quantities of CO2 dissolved in it and the presence of strong acids. It was in this perhaps ubiquitous oceanic environment, already containing most of the salts we find today (except sulphates) and in the presence of a reducing atmosphere, that the first organic compounds were synthesized. Later, by about 3500 m. y. or earlier, these would lead to the emergence of life. Soils may also have played a role in this emergence. This first period corresponds to the Katarchean of Salop (1979, 1983). It terminates, according to this author, with the Saamian orogeny (37503500m.y.) characterized by deformations, plutonic and hypabyssal intrusions and metamorphism.

1.4.3 From the Archean to the Eocambrian: the Establishment of a Dynamic System 1.4.3.1 Internal Dynamics In the Archean, the first evidence of lithospheric activity is seen in the greenstone belts i.e. the zones of sediments and volcanics, folded and metamorphosed (chlorite, epidote, serpentine) with gneisses and granitic plutons. The greenstones represent the fillings of old basins, and some authors interpret them as the product of intraoceanic or marginal subductions analogous to those of the Phanerozoic. Others, like Kroner (1983a, 1984), propose an ensialic origin linked to continental rifting (Fig. 66); in fact, the existence since 3600m.y. of fragments of continental crust up to several hundred kilometers across is generally acknowledged. At point or linear hot spots the still unstable crust broke up, thinned, and then sank along elongate grabens widened by the classic mechanisms of tilted blocks. Continental basins were so created and became filled with sediments and essentially basaltic basic igneous rocks! from partial melting of the upper mantle. Crustal melting can also lead to granitoid intrusions. If the crust breaks completely, small oceanic basins appear. Their closure, after disappearance of the point or linear hot spots, results in deformation of the basin filling as recumbent folds and thrust faults. In total, the formation of greenstone belts results in an enlargement of continental crust as well as its thickening by magmatic differentiation and 1 In the Archean, many lavas were peridotitic komatiites derived from the deep mantle. These types disappear in the Proterozoic.

91

Precambrian Time

c

B

o

\(

T .ripnou. ledimentl,

biotenic c.bonate.

1l1li.................

E

Malic: 10 u1tr.101ic RICO or oc_ floor llllnity + •

c........ or .......... of... ...., or_.de""",

..

Fig. 66. Dynamics of the Archean lithosphere. A Evolution after small-scale convection in the upper mantle. B-E Different stages of formation of a green stone belt (After Kroner 1983a)

cooling. This latter process is estimated to have formed 85% of the crustal section between 2800 and 2500m.y. In the Lower and Middle Proterozoic (2500-900m.y.), the strongly cratonized continental masses were relatively stable, with large basins which became filled with volcano-sedimentary sequences, especially in West Africa, Brazil and Canada. These basins sometimes developed along rifts or aUlacogens. These fold belts always arise, according to Kroner (1983b), from an intracrustal or ensialic tectonism, implying weak plate movements

92

The Major Stages of Earth History

(verified by paleomagnetism), delamination of the mantle lithosphere and crustal imbrication (Fig. 67). The delamination is itself followed by metamorphism and intense granitization. Relatively deep basins were created by these movements and received the first flysch sediments. Once formed, the belts remain zones of weakness for a long time, later becoming loci for large continental faults, for example, the Grenville belt (Fig. 68), which is often interpreted in modern plate tectonics terms as the opening of a Proto atlantic around 13oom.y., oceanic closure and collision around llOOm.y. Also, according to Kroner (1984), the subduction at that time did not affect a hydrated lithosphere as it did in the Phanerozoic, thus explaining the absence of island-arc calc-alkaline volcanism. Several phases of intensified plutonism resulted in intrusions of gabbros, ultrabasites and granitoids, as well as migmatizations, granitizations, and mineralizations of Cr, Ni, Pt and Cu. At the beginning of the Upper Proterozoic, around 900m.y., plate tectonics of the Wilson cycle type 2 was initiated and resulted in continental accretions by collision. This is demonstrated by evidence of active margins and island arcs, by crustal shortening suggesting collisions, by the appearance of ophiolites, and by high pressure-low temperature minerals. This new dynamics, evidence perhaps for an acceleration of plate movement, must have been established gradually between 900 and 650m.y., as shown, for example, by the occurrence of the Pan-African orogeny (see below). But Kroner's (1984) interpretations are rejected by many authors, who believe they see evidence for a modern type of plate tectonism from the Lower Proterozoic. For instance, they interpret the greenstone belts as back-arc basins. The absence of HP-LT minerals and ophiolites before the Upper Proterozoic is explained as being due to a hotter oceanic crust than today and narrower oceans. Whichever is true, it seems certain that by plate movements at least four amalgamations of continental crustal blocks into a supercontinent of the Pangea type have taken place during the course of the Precambrian (Worsley et al. 1984), with each time a renewal of volcanic activity and a major emergence of the continental platform. It was not until after 1oo0m.y., when the oceans increased significantly in size, that the first paleogeographic characteristics were outlined. The African and South American shields were formed, solidly attached to one another, while two immense continental assemblages, one Northern, the other Gondwanian, also gradually took shape. The movements of these two megablocks were to influence practically all tectonic history of the globe, outside the Pacific, from the Precambrian to the Present. For example, at the end of the Precambrian the destruction by subduction of the Celtic ocean, which was a precursor of the North Atlantic, was the principal motor of the Cadomian orogeny (Fig. 69). 2 See

Chapter 4, Section 2.1.1.

93

Precambrian Time

A

+ -

n

Pyrenean

uisian

Landenian Vitroliian Laramldlan

Fig. 107. Subdivisions of the Cenozoic, Two radiometric age scales: a according to Odin et al. (1982b); b according to Van Eysinga (1985)

3.2.2 Major Geodynamic Stages 3.2.2.1 Paleogene: Initial Frontal Collision Between Eurasia and Parts of Gondwana; Complete Opening of the North Atlantic (Fig, 108)

At many locations, the Laramide orogenic phase continued into the Paleocene or even the Eocene, the latter epoch being particularly active tectonically.

The Mesozoic and Cenozoic: Breakup of Pangea

157

Fig. 108. Paleogeography of 1 the Lower Eocene and 2 the Upper Oligocene about 50 and 25m.y. ago: Complete opening of the North Atlantic (Daly 1984, modified after Irving 1977). Dashed lines indicate boundaries between oceans and epicontinental seas

Tethyan Domain. Increased tectonic activity resulted from the contacts between Eurasia, the Arabian-African block, and India, initiating the final phase of the closure of the Tethys. Until the Eocene, the convergence of Africa and Europe continued at an accelerating pace (Fig. 97), provoking the formation of the Rhodope Volcanic Arc in Iran, and ended in the Upper Eocene with a general collision. Principal consequences were a tectonic phase in the Betic Cordillera and North Africa, and mountain building from the internal Alps (origin of the ophiolitic and "schistes lustres" nappes) to central Iran. Very schematically it could be said that in the west, Africa was overriding Europe, particularly the Apulian block which continued its northern overthrusting, while from the Balkans to Indonesia Gondwana was being overridden by Eurasia (Aubouin and Debelmas 1980). The movement of microplates between the two continents can also explain certain tectonic features of the Tethyan chain. For example, after sliding to the east and moving away from Europe during the Cretaceous, the Iberian block moved back again with a SE-NW movement beginning in the Maestrichtian. The result of this was the closure of a trench by subduction and the underthrusting of the Iberian plate beneath the European plate in the western

158

The Major Stages of Earth History

Pyrenees, as well as an activation of the North Iberian margin adjacent to the Gulf of Gascogne (Boillot et al. 1984). This important tectonic phase, called the Pyrenean-Proven