Stratigraphy

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Pierre Cotillon

Stratigraphy

With 115 Figures

Springer-Verlag

Berlin Heidelberg New York

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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 DOl: 1O.l007/978-3-642-77025-8

e-ISBN-13 :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.

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

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"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 geo-graphies (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 strati-graphy 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.

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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.

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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.

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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, there-fore, 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

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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.

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Principles of Correlation

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--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 sand-stones; 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)

3

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 sedimen-tary 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

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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 accre-tion, partly or totally lateral, is controlled by currents (delta front for example; Fig. 4).

2. By seeking comparable sequences in different places (sequence strati-graphy). Stratigraphic correlations are effected taking the following principles into account (Fig. 5):

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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 necess-arily 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 varia-tion 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 long-distance correlations, many of which are regarded as time correlative. The ideal would be a series of events of worldwide extent that are easily re-cognizable. The search for such a series is one of the major tasks of strati-graphy, 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 strati-graphy 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 ter-mination 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 de-scribed 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.

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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 sedimen-tary 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 asso-ciated 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 appearedl . Figure 6 shows, for example, the stratigraphic

1 Suggesting the use of lithological characteristics and stratotype locality. Example: Comblanchian limestone (Bathonian, C6te-d'Or).

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8 Spherll. layera (ENAY 1966) Corallian or lubcorallian faclel

Elaboration of the Fundamentals of Stratigraphy

PseudolHhographic limestone. (RICHE 1898)

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 lithostra-tigraphic methods, cannot always be correlated with one another. The

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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 sedi-mentation 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 emer-gence of species, a phenomenon continuous, nonrepetitive and irreversible, but having the disadvantages of being more or less dependent on the en-vironment 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,

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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 post-mortem dispersal of their adult shells by virtue of their buoyancy. For this reason, correlations using ammonites are considered practically syn-chronous. 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

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due to increasing climatic zonation and consequent increased faunal pro-vincialism. Other groups evolved slowly (bradytely), are geographically restricted, and appear to be confined to certain types of sediment. They are, therefore, dependent on sedimentary facies, a concept also introduced in the last century by the Swiss geologist Gressly (1838). Finally, the fossils can be classified according to their usefulness in biostratigraphy. The data of Fig. 8, however, are only generalizations. Among the groups considered there can be exceptions. For instance, the hippuritids of the Upper Cretaceous, although dependent on a reef environment, evolved rapidly and are very useful stratigraphically.

Since Oppel, and following his lead, many biostratigraphic studies have been produced. Ammonite zones have been refined and increased in number, especially in the Jurassic where that group's evolution was par-ticularly rapid. The practice of biostratigraphy in a growing number of countries, however, has revealed that the horizontal extensions of biozones are limited. With the exception of the Liassic zones mentioned above, some are restricted to Europe, while the majority do not extend beyond certain faunal provinces (Fig. 9). Interprovincial extensions of a zone are only possible when certain taxa overlap in their geographic ranges. And these extensions can be complicated by the phenomenon of migration. Certain genera or species, responding most often to variations of the environment, modify their distributions through time, expanding or contracting them, or even displacing entire distributions (Fig. 10). In an invaded region, the

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12 _{Elaboration of the Fundamentals of Stratigraphy}

Fig. 9. The Tethyan domain and its faunal provinces (ammonites) from the Upper Bajocian to the Middle Bajocian (after Cariou et al. 1985). The Phylloceratina dominate the Mediterranean province of deeper-water environments. Faunas very diversified in the European Submediterranean province, less diversified in the Ethiopian. J Probable land; 2 epicontinental seas; 3 zones of ocean floor; heavy lines denote boundaries of provinces

sudden appearance will seem to be a new taxon. These phenomena show also that a biostratigraphic correlation is not necessarily a time correlation.

3 Chronostratigraphy

3.1 The Concept of the Stage

The biostratigraphic zones are related to the historically older chronostra-tigraphic stages. The stage was originally defined as a group of beds de-posited during a specific interval of geologic time, this interval being a geochronologic unit. It can be divided into subunits and is, by definition, universal. The first subdivisions of European sedimentary series into stages by d'Orbigny, Brongniart, Marcou, Oppel, etc. were based on both their lithologic character (facies) and some of their faunal elements. The facies refers to the nature of the beds in the region where the stage was defined.

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Time

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Fig. 10. Change of areas of faunal distribution with time. A Restriction; B spreading; C displacement

The faunas provide a time reference as well as a means of recognizing the stage anywhere, even if the lithology has changed. Recognition may be direct if the faunas are the same, and indirect, through intermediate steps, if they change. Theoretically, the stage can be identified throughout the world even in the absence of the faunal suite characteristic of its type region. The faunas are also used to subdivide the stage into finer units.

The stages of d'Orbigny (ten in the Jurassic, seven in the Cretaceous) have been defined by reference to sections or stratotypes which are ade-quately fossiliferous, well exposed and well defined at their upper and lower limits. Such sections are common in epicontinental successions which are more easily subdividable into distinct lithologic units. For example, in the Paris Basin, the Cretaceous consists of green sands recognizable in the Aube department (Albian stage) while the Jurassic of Semur occurs as beautiful exposures of limestones with Gryphea and Arietites (Sinemurian stage). Figure 11 shows a list of Jurassic stages together with their stratotypes. However, in an epicontinental series, like that of the Paris Basin, the clear lithologic distinctions used to define the stages are often rendered more distinct by the sedimentary discontinuities which separate them. These include the depositional gaps (nondeposition and/or erosion) which are evidence of the numerous oscillations of sea level marking the basin's history.

These hiati accentuate also the contrasts between the paleontologic content of successive stages, giving the impression that each corresponds to a renewal of the fauna. The higher-order subdivisions are based on the same principle. Thus, Deshayes in 1831 introduced a major break between the top of the Chalk and the base of the Tertiary in the Paris Basin on the basis of a comparitive study of their respective faunas. The upper limits of the Devonian, the Permian and the Trias also coincide with massive disap-pearances of taxa, giving rise to the terms still in use today: trilobite era (Paleozoic); ammonite and reptile era (Mesozoic); and mammal era (Cenozoic). For d'Orbigny, imbued with the creationist ideas inherited from Cuvier, the history of the Earth consisted of 27 stages, each possessing its own fauna and each separated from the next by catastrophic events of tectonic origin (global revolutions). At each revolution, faunas and floras

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Subsystem Stages and sub-stages Authors of terms Origin of terms boundaries Neocomian Pubeckian facies Neocomian pro-parte A. Brongniart, 1829 Lacustrine deposits from Purbeck (Dorset, England) -135 M.A. Portlandian p.p. Portlandian A. Brongniart, 1829 Limestones and sandstones from Portland (Dorset) " Kimmeridgian stricto sensu. J. Thurmann, 1832 Black marls from Kimmeridge (Dorset) G,)'(i; Kimmeridgian Sequanian Sequanian s.s J. Marcou, 1848 Limestone from Franche-Comte, France (= Sequania) e OJ ~ Rauracian OJ -l'" 1. P. Greppin, 1870 Corallian limestones from Rauraces country ::s

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-. (dealing with a tribe from Jura bemois) Oxfor-Angoumian J. Marcou, 1848 Grey marls from Argovie (Swiss Jura) dian Oxfordian s.s A. Brongniart, 1829 Black marls from Oxford (England) Callovian A.d'Orbigny, 1852 Sandy limestone from Kelloways (England) Bathonian 1. Omalius d'Halloy, Oolithic limestone from Bath (England) '"

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!:8 1852 0 ::S~ Blijocian A. d'Orbignyi, 1852 Oolithic limestone from Bayeux (Calvados, France) Cl Aalenian C.Mayer-Eymar, 1864 Black marls from Aalen (Wurtemberg, Germany) Toarcian A.d'Orbigny, 1849 Marly limestone from Thouars (Deux Sevres, France) " Pliensbachian Domerian G. Bonarelli, 1894 Marls and limestones from Monte Domaro(LombardY,Italy) €'~ (= Charmouthien) A. Oppel, 1858 Marls from Pliensbach (Wurtemberg, Germany)

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~:; Carixian J. Lang, 1913 Marls from Carixia (= Charmouth, England) -. E. Haug, 1910 Limestones and marls from Lorraine (France) Sinemurian Lotharingian A. d'Orbigny, 1849 Limestone from Semur-en-Auxois (Cote d'Or, France) Sinemurian S.s. E. Renevier, 1864 Sandstone from Hettange (Moselle, France) -180M.A. Hettangian Triassic Rhetian W. Gumbel, 1861 Shales and limestones from Rhetic Alps (Engadine, and E.Renevier, 1864 Switzerland) Fig. 11. Origin of stage and substage names of the Jurassic. (Note that the substage names Argovian, Sequanian, and Rauracian are no longer used) I

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;p

::: 0-po 8 ('b g

a

en So ~

a

~. ::r _'<

(23)

Eras Systems Orogenic stages Time in Ma Holocene Quaternary _Pleistocene

---

Pasadesian

---

- 1,6

---

- -- ---

-Pliocene Neogene Rhodanian

- -

-s Attican Miocene Cenozoic _Savian

_{- -}

- 23

Paleogene Oligocene Helvetan _{- 34}

or Pyrenean

-

-Nummulitic Paleocene 1======-= ==

=====

===== ::

~ Laramidian

---

- 65 Cretaceous Austrian Neocimmerian

-

_I-- - 130 Maim Andinianor _Nevadian

Mesozoic Jurassic DOJlSer

Lias Cimmerian

-

~ - 204 Triassic

---

--- _======:- Palatinian - 245

--- ---

_-

--Permian Saalian _-f-- - 21)0 Asturian Carboniferous Sudetian Bretonian

-

f-- - 360 Devonian Paleozoic Caledonian

-

f-- -400 Silurian Taconian

-

_~- 425 Ordovician _Salai"r

-

~ - 495 Cambrian

1::-====== ======: ======:

t= Assyntican

=

--

- 530 Precambrian

Fig. 12. Principal subdivisions of the general chronostratigraphic scale. Orogenic phases from Stille. Radiometric ages after Odin et al. (1982b)

were destroyed to be renewed in the succeeding strata. The principle of a stratigraphic paleontology capable of dating rocks far apart, without having to consider their lithologic similarities or differences was thus established.

3.2 Event Stratigraphy

D'Orbigny thus divided time according to a stratigraphy of events both biological and tectonic, the former being secondary to the latter. Others placed the major breaks exclusively at times of major lithospheric move-ments: Suess (1880) tied the rhythmic history of the Earth to marine

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oscil-16 Elaboration of the Fundamentals of Stratigraphy

lations related to positive and negative epeirogenic movements. This was also the theme of the German school under the influence of Stille during the years 1920-1930. Thirty orogenic phases were distinguished during the entire geologic time scale, each corresponding to a peak of activity. Their effects are identifiable not only in the orogen but also at some distance from it as emergence, synsedimentary deformation, instability of areas of deposition (slumps, turbidites), discordance, deposition of coarse detrital sediments (fault breccias, olistoliths, flysch, molasse), or by certain minerals (predominance of chlorite and illite

f.

It is generally supposed that the lateral extent of a tectonic event depends on its magnitude, and in principle there is no reason why a really major orogenic phase should not be felt throughout the world, even if only as a sea-level variation or as a slight epeirogenic movement. This is assumed implicitly in many geologic time scales, for example when the Late Jurassic and Late Cretaceous regressions are related to the Neocimerian and Laramide orogenic phases, which are hardly of worldwide extent (Fig. 12). In practice, such correlations are difficult to establish for several reasons, including the following:

1. Of variable intensity and often barely discernable, the manifestations of a tectonic event may be difficult to follow over large distances.

2. Tectonic events are often not synchronous from place to place, in so far as they may have variable durations in different locations and may migrate laterally. For example, the discordances which bound many chronostratigraphic units are often diachronous, and have only a regional significance. This diachronism becomes less clear in oldest successions, while the stratigraphic resolution is lessened.

The concept of tectonic phases is, therefore, out of date today, especially since the fundamental mechanisms for orogeny became known. The move-ment of lithospheric plates gives rise, in fact, to slow, gradual, continuous, and shifting deformations.

It follows that an orogen is active throughout the course of its history, but especially in its beginning, i.e. in the basin where synsedimentary tectonics are now commonly recognized. In this continuum some periods can, nevertheless, be discerned in the intensification or generalization of movements, sometimes accompanied by important regressions, detrital pulses, etc. Only such major events can be used in long-distance correlation, but they do not permit a fine division of time. At the best they can be used to indicate certain major cycles like orogenic cycles or sedimentary cycles. For example, the PaleozoiclMesozoic boundary has been related to a global geodynamic event, the beginning of fragmentation of Pangea, also

rep-2 However, characteristics of the clay sediments have as much climatic as tectonic significance.

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resented by a general transgression. This boundary coincides also with the beginning of the Alpine Orogenic Cycle. In general, the sharper and more important the boundaries as judged by the number and extent of coincident phenomena, the larger are the associated stratigraphic gaps. Between a folded and metamorphosed basement and the beds which lie discordantly above it, a very large part of the global history may be missing.

3.3 The General Chronostratigraphic Scale

This has been developed progressively since the eighteenth century, when geology began. It bears the stamp of the diverse schools which have inspired it and the regions where it was developed.

The principal units of this scale, recognized worldwide, are shown in Fig. 12 with their duration in millions of years. Excluded are the lower order units (stages), still used only locally or regionally, and which are mentioned in several figures in Chapter 5 (Figs. 74, 82, 90, 107). Systems are groupings of stages; their boundaries coincide with major discontinuities resulting from emergence and deformation3 • Systems have various origins noted in Chapter 5 (see the boundaries and subdivisions of the Lower and Upper Paleozoic, the Mesozoic, the Cenozoic). The boundaries of eras correspond not only to important geodynamic events but also to significant faunal renewals: the beginning of the Paleozoic, which is also that of the Phanerozoic

(= appearance of life), is distinguished, among other characteristics, by the appearance of the first abundant faunas, well preserved and widespread. At the end of the Paleozoic, trilobites and fusulinids disappear; at the end of the Mesozoic, the ammonites, rudistids and large reptiles disappear, while the nummulitids appear in the Cenozoic.

4 Conclusions

Such were the first steps of a stratigraphy aimed at a useful subdivision of time (stratigraphic scale) based on the lithologic, fossil, and deformation information observable in sedimentary sequences. The litho- and biostrati-graphic scales are the most objective and their scope is regional or pro-vincial. Lithostratigraphy can be applied to all rocks, sedimentary, volcanic or metamorphic, but each lithostratigraphic unit, a result of specific physical phenomena, can be repeated several times in the course of time.

Biostra-3This was not always the case. In 1830, Lyell subdivided the Cenozoic into three systems (Eocene, Miocene, Pliocene) characterized by an increasing percentage of modem species.

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18 Elaboration of the Fundamentals of Stratigraphy

tigraphy can only be applied to fossiliferous rocks, but biostratigraphic units, based essentially on the unique events of biological evolution, cannot be repeated during the course of time; thus their great utility. Theoretically, some of these units have a chronostratigraphic value, to the extent that appearances and disappearances of fossil species can be assumed to be synchronous everywhere. However, we will see in the following chapter how the influence of the environment often renders this assumption invalid. Biostratigraphy, therefore, has two components: one, irreversible, based on evolution, the other, reversible and related to factors of the environment. Chronostratigraphy, the normal end product of a regional study, follows from the other two aspects; it depends on a division of strata according to geologic time and therefore has a universal value.

Future studies will perfect the methods of basic stratigraphy, fill the gaps in our historical documentation, forge new tools of correlation and syn-thesize absolute data on the duration and speed of events.

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Modern Stratigraphy

Stratigraphy has contributed, like other subdisciplines, to the spectacular progress in Earth science made during the second half of the twentieth century, mainly due to three important factors:

1. A deeper knowledge of evolutionary phenomena;

2. The increase of petroleum exploration with its constant demand for greater precision in the recognition of specific stratigraphic units;

3. The development of plate tectonic theory and consequent programs of deep ocean drilling.

In the latter two cases, the drilling techniques have spawned new

strati-graphic tools, seismic and downhole logging methods in lithostratigraphy, micro- and nannofossil time scales in biostratigraphy. The stratigraphy of ocean sediments is constructed on oceanographic ships by international teams, an important factor in the establishment of a chronostratigraphic framework with worldwide validity, in the rigorous redefinition of old established units and in the sharpening of concepts used in stratigraphy. This need for consensus in the stratigraphic codes used has generated numerous discussions and yielded numerous resolutions at international meetings. A fortunate consequence of plate tectonics has been to elevate scientific debate to a planetary level, and stratigraphy, drawn into this movement, has felt compelled to find new methods of correlation dependent on events and phenomena of global significance: eustasy, oceanic geochemistry, climate and paleomagnetism. Similarly, striving to escape slowly from the relative nature of its chronology, stratigraphy is more and more supported by precise measures of absolute age. Finally, one can observe a will to adapt strati-graphy to the progress realized in paleontology (evolution, species concept), in paleoecology (influence of the environment) and in sedimentology (interpretation of sedimentary discontinuities, reworked beds and variable rates of sedimentation).

1 Refinement of Concepts and Time Scales

Since the 1950s, stratigraphy has had to respond to an increasing demand for more precise dating as science and subsurface exploration progressed. Since

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20 Modem Stratigraphy 100m more or less of drilling can have a considerable effect on the budget of an exploration company, there is a necessity for a finer subdivision of the stratigraphic scale in order to increase the resolving power of stratigraphy. This need has also led to the establishment of other scales (micro- and nannofossil) utilizable in drilling and related where possible to a radio-chronologic scale. Stratigraphy has also had to redefine its concepts more rigorously in light of new methods of correlation and dating and contri-butions from other disciplines.

1.1 Evaluation of Geologic Time Intervals and Rates

The estimation or calculation of geologic time intervals is necessary:

1. For the comprehension of past phenomena and their comparison with present-day or recent phenomena whose durations are known by direct reference to human history.

2. For the construction of a consistent geochronologic scale with correct relative placement of different divisions of geologic time. Two courses are possible:

1. Reference to sedimentary rhythms of known and constant time durations corresponding to seasonal or annual cycles, such as varves which are formed as alternations of light and dark millimetre-thick laminations, each couplet representing deposition during 1 year. Varves can be perfectly preserved over great thicknesses, indicating a lack of deforming compaction and an absence of bioturbation typical of anoxic lacustrine or marine environments. The calculation of duration to within a few years is, therefore, possible.

2. Reference to a physical transformation, which is unidirectional and irreversible and is a known function of time, for example, the trans-formation of a radioactive element into a stable one.

1.1.1 Radiochronology

A radioactive element A, contained in a mineral at its crystallization, will disintegrate progressively and be transformed into a daughter element B, said to be radiogenic. The ratio of concentration AlB will depend on the time duration of the disintegration and on the half-life T of the element (time required for the disintegration of half of the element) or on the decay constant (coefficient of decrease of the element as a function of time).

t = 11l0g(1

+

N'IN),

where N' is the number of atoms of the radiogenic element B (daughter element) and N is the number of atoms of the element A (parent element) after time t.

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Nand N' are measured in a mass spectrometer to yield a value for t which is the isotopic age (radiometric age) of the mineral containing the element. The measured age is from when the system closed, which generally causes the cessation of exchange of fluids between mineral and pore waters. Given certain conditions, it may then be possible to determine the age of the crystalline rocks or sedimentary formation in which the mineral occurs. The method is only valid if the decay constant is well defined and the mineral containing the radioactive element was a closed system throughout the decay time. Other methods also used include:

1. Measurement of the concentration of a radioactive element. The carbon atoms of CO2 derived from the atmosphere to form living matter or

biochemical carbonates have a 14C content

e

4_C/12_C

=

_1.2

x

₁₀₁₂₎_which

remains constant over time. After the system closes (death of organisms), the 14C content decreases with time

e

4C ~ 14N) so that its measurement yields the amount of time since the system closed (i.e. since death). 2. Measurement of the ratio between two stable elements A' and B' derived

from the decay of two radioactive elements A and B (e.g. 238U and 235U) of different half-lives. Since A'/B' varies as a function of time, its cal-culation yields the amount of time since the system closed.

Principal transformations used:

87 Rb~ 87 Sr 232 Th ~ 208 Pb 40K ~ 40Ar 238 U ~ 206Pb 235 U ~ 207 Pb 234 U ~ 230Th 230 Th ~ 226 Ra 14C ~ 14N 3T ~ 2H Half-life in years 5 or 4.7 x 1010 13.9 X 109 11.9 X 109 4.6 X 109 7.0 X 108 250000 75220 5568 12.26

The results of these age dates must be used with great caution, since the resolving power of the method decreases with increasing age so that the error can be 50-l00m.y. for the Precambrian. Also, the radiometric age may correspond to a first event (e.g. formation of a rock) when the chemical system closed and the radiometric clock was set, or to the latest of its transformations (metamorphic or deformational) which resets the clock at zero by reopening the chemical system.

Age dates of plutonic and volcanic rocks are the least problematic. Many silicates are suitable for radiochronology because their crystallization usually corresponds to the rock formation. In metamorphic rocks, the date

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22 Modern Stratigraphy

+

'-0:::---_--_---

_{Fig. 13. Dating of an}

azoic sedimentary formation by plutonic rocks. The age of the formation (2) is later than that of the granite

(1) but earlier than that of the laccolith (3)

+

-f-=--+

+

(j)+

+

HARLAND LAMBERT et aI. (1971) Ma (1964) Devonian .40( '. Devonian., ._ Silurian '"

...

Silurian ?

-

.-Ordovician Ordovician -500 ~?

...

I~. --Cambrian Cambrian

..

\ _~_/ :-600

+

o

ARMSTRONG (1978) Devonian I GALE GALE et al. (1982) (1980) ~Silurian ~ Silurian

...

Silurian Ordovician Ordovician Ordovician ~

.-

_~

-..

-_I I : Cambrian I CambriaJ!. _ii I I .. S Cambrian

...

---.-'

...

-

_.

.

-Fig. 14. Different geochronologic scales for the Lower Paleozoic, after Gale (1982). Note the indication of limits of error related to geochemical and analytical techniques for the latest scale

generally corresponds to the later recrystallization phase and not to the original formation of the rock; in sedimentary rocks, the minerals used are usually detrital and, therefore, older than the age of the rock. Glauconite is exceptional since this authigenic mineral is practically synsedimentary. It can be dated by K/Ar and Rb/Sr but it is sensitive to later diagenetic changes which can reset the radiogenic clock. This mineral is also often reworked and present in condensed sequences. Ages obtained by this method range from Cambrian to Pliocene. Sedimentary formations which are azoic, nonglauconitic and older than 40000 years are only datable relative to crystalline rocks which cut them or predate them (Fig. 13). For beds younger than 40000 years, 14C dates are the best, though subject to errors

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60 M a Va OdIn Hinle eill. 1976 1982 HulaDd Kenl 81 eill. Gladstein 1982 1985 "'" M • o - ~ M M.crichlilo M 5 a 7 Ca i - - - 74. ₅ :o-~ Ca 8 Co Sa 9 0 T ~ ~ Ce 100 ~ _AI AI 11 0- _Ap _~ n.. Ba Ha U f - - - _V Ha 12 o V ~ ~ f---=-13 14 0

Fig. 15. Different radiometric scales for the

Cretaceous 150 Ca _{c-p ...} Sa Sutoniu ~~.~ Ce c -... AI AJbiIII Ap Aptla Ba Barnmiul Ha liauterlrilll

"

VIIqiniu Be Benlullll

Fi

7. 91 97. 5 11 3 119 12 4 131 13 8 44 1

related to variations in the magnetic field!, to climatic variations or to isotopic fractionations due to "vital effects" in biological systems. In con-trast, the complex stratigraphy of Precambrian rocks can only be ascertained with the aid of isotopic dates which extend back to 3800m.y., the age of the Earth's formation being 4600m.y.

1.1.2 Radiometric Calibration of Stratigraphic Scales.

Durations of Stratigraphic Units

The first geologic time scale combining the principal chronostratigraphic boundaries and radiometric age dates was published by Holmes (1932). Since then it has been continually improved (Fig. 14), so that it is now possible to assign durations for all stages. At first, it was possible only to estimate average stage durations by dividing each system duration by the number of its stages. Subsequently, ages have been determined for stage boundaries, allowing separate and different durations to be calculated. Results vary according to different authors (van Hinte 1976; Kennedy and Odin 1982; Kent and Gradstein 1985; Fig. 15). Westerman (1984) and Kent 1 A reduction of the latter during the last 7000 years has resulted in an increase in the production of atmospheric 14C from the action of cosmic rays on nitrogen.

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24 Modern Stratigraphy

and Gradstein (1985) adopt the following principle for the Jurassic. Based on a system duration of 64-74 m.y. (according to different authors), they calculate a duration for each stage proportional to the number of ammonite subzones it contains. The reliability of this method assumes that all bio-stratigraphic units are defined homogeneously and that biological evolution proceeds at a constant rate. Knowledge of stage time durations then allows recognition of even shorter time spans. In the Toarcian, for example, a stage duration of 5 m.y. and the recognition of 27 ammonite zones suggests that an average ammonite zone lasted 185000 years. Many pelagic deposits are formed of a series of alternating limestone and shale couplets defining global cycles (see Chap. 3). From the number of cycles in different stages ranging from Jurassic to the Quaternary it has been possible to calculate the average time duration, varying from a few thousand years to a few tens of thousands for each cycle.

1.1.3 Rates

Once time durations are known, it is possible to calculate rates, especially rates of sedimentation. These are generally average rates, but in the case of continuously deposited pelagic sediments they are also close to instanta-neous rates2 • Rates of erosion, uplift, subsidence, and lithification are also determinable.

1.2 New Biostratigraphic Approaches

1.2.1 Factors in Biostratigraphic Development

1.2.1.1 The Modern Species

Taxonomic concepts of modern biology have had an important influence on paleontology and stratigraphy. Typological concepts based on the rep-resentation of a species by an arbitrarily chosen individual have rightly given way to the dynamic concept whereby the population and as many factors as possible are taken into account in the definition of the species. The paleontologist-stratigrapher then has the problem of distinguishing between species and the problem of their transitions.

Distinction. Only biometrics and statistics allow a proper study of variation

in a population and enable conclusions to be drawn on its monospecific or polyspecific character.

2 These are calculable in near-surface sediments by the decrease in the 230 Th/232 Th ratio compared to this ratio at the surface.

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A

-dm A

/

Sub-~-+--f--:i~H"f-~ speCieS} e SpeclesE .~+-;/,~H'-T--'¥--t _{_____ 'L _____ _}Subspecies SpeclesB a b c d e f dm Morphotypes B

Fig. 16. A Cladogenetic evolution and B anagenetic (after Tintant 1972). dm Morphologic variation; T time;

f

frequency; A,B,C,D,E distinct species; B,B' ,b,c,d,e subspecies

Transitions. The change from one species to another in a lineage can occur

by continuous transformation (anagenesis; Fig. 16), but the problem is how this can be used to establish discrete biostratigraphic units, or in the words of H. Tintant (1972) "how does the continuous flux of the evolution of life flow within the discontinuous framework of stratigraphic division?" If the morphologic stages of a lineage represent a succession of barely perceptible changes, it is only by using numerical indices that biostratigraphic sub-divisions can be constructed (autochronology). However, a species may also originate abruptly by the branching of a lineage (cladogenesis; Fig. 16), and this sudden event is frequently used to define higher-order stratigraphic units. It should be noted, however, that biostratigraphic units are not all based on single lineages but may also be based on the succession of species belonging to different lineages (allochronology).

1.2.1.2 Relationships Between Paleontology and Other DiscipHnes

The multidisciplinary approach using paleontology, paleoecology, sedi-mentology, and paleogeography to solve problems is now a common practice, to the benefit of biostratigraphy. For example, knowledge of a paleoenvironment (bathymetry, degree of energy, temperature, salinity, etc.) from a study of the sediments is indispensible for understanding the true significance of faunal changes over time, because it can allow the environmental component of the change to be separated. Faunal changes due to the environment are reversible because they are generally not yet fixed in the genotype; they are repetitive and can be diachronous within a single basin. Changes due to evolution have the opposite characteristics. Thus, in the Late Pleistocene, the direction of coiling of the two species of

Globigerina, Gl. pachyderma and Gl. hirsuta is temperature controlled; in

the cool waters of glacial periods, the coiling is dominantly sinistral, while in the warmer waters of the interglacial periods it tends to be dextral. It is only because we know a great deal about the biology of these globigerines that we can avoid confusing these intraspecific variations with mutations in the

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26 Modem Stratigraphy J J J I J I I J I I _I J _{Haplophragmoides} I _I _I I _"ocontionus I I _MOUllADE I I I _I ~ ;,- ----j---i I J I _J I Lenticulina I ouachensis bartens-I I _I _{tein; MOULLADE} I I _J I _I _I _Lenticulina eichen-iii I I I bergi BARTENS· I I I TUN el BRAND I _I Dorothia zedlerae I I I _MOUllADE I I I I _I I J

_--

Gaudryinella eichenbergi I I _I I I _I MOULLADE I _J _I I I I _Dorothia

haule-:

I J I _{riviana (MOUllA·} I I I de) I I I I I I Lenliculina bus· I J I _{nardo; MOUllA·} I I I _HE I I I

I _I I _I _{chensisouachensis}Lenticulina

oua-I _I I J I (SIGAl) J _I _I _I I _Lenliculina nodo--l- _I I I I -I I _I sa nodosa (REUSS I _I I _I I I I I Frondh'ularia d. .~ I I I

:

_bit/(,IIIDIU I I I CUSHMAN I I I I 7

61

5

I

4

₁

3

₁

2

11

Ammonite Zones

Fig. 17. Comparison between the distribution of the principal foraminifers of the Ardeche border (thick lines) and of the Vocontian Basin (thin lines) during the Valanginian (Moullade 1979). 1 Otopeta; 2 Pertransiens; 3 Campylotoxum; 4 Verrucosum; 5 Trinodosum; 6 Callidiscus; 7 Radiatus (After Darmedru 1984)

direction of other species. This allowance for the effect of the environment on fossil species is necessary before any biostratigraphic subdivision is attempted; and those index forms too dependent on facies should be eliminated. The benthonic representatives are especially affected, for example the nummulites, which favor shallow calcareous facies, and num-erous small foraminifers of the platform and the basin. In the Valanginian of southeastern France, for example, several calcareous (Lenticulina) and agglutinated (Dorothia) species have vertical distributions which differ in the southern sub-alpine chains (Drome, Hautes-Alpes, Alpes de Haute-Provence) from those in Ardeche (Fig. 17). These areas correspond re-spectively to a basin and its border and therefore represent very different

(35)

environments, and especially depths. Therefore, when biozonations over large distances are attempted, the influence of the environment on the temporal distribution of species is apparent even for planktonic forms; the less obvious this environmental dependence is, the more rapid is the evolution of the taxon (e.g. ammonites). The rate of evolution is, in turn, also influenced by climatic and eustatic variations3•

A close scrutiny of any sedimentary structures or directional indicators should be included in any study of a fossiliferous unit if it is required to reconstruct its formation and to classify it as one of the two categories: life assemblage or death assemblage. This study may also confirm the existence or not of a reworked fauna, i.e. the presence of older fossils eroded else-where and transported into the indigenous community. However, low rates of sedimentation can lead to condensed faunas which may appear like reworked faunas, especially if the beds have been bioturbated and bedding destroyed. Understanding of some of the diagenetic history should allow the discrimination between the effects of mechanical (deformed or crushed fossils) or chemical (dissolution) processes on the fossils at the time of burial.

Finally, biostratigraphy should consider the role of certain paleogeo-graphic factors in the space-time distribution of taxa. Climate is undoubtedly the most important, as illustrated in the following two examples:

1. In the Paleogene, nummulites are abundant in the Paris Basin, but never extend further north than a line approximately between London and Brabant and, therefore, never invaded the North Sea Basin.

2. In the Callovo-Oxfordian, boreal ammonites never extended beyond the North Tethys margins.

Many other groups, especially among the nannoplankton, have a distribu-tion very dependent on climate, but natural barriers also play an important role. For example, in the Plio-Pleistocene, the Atlantic and Pacific were separated by the formation of the Isthmus of Panama with a consequent divergence of faunas in the two oceans, but in a unique climatic setting. The opposite effects occurred in the terrestrial faunas of the two Americas, because their faunas were able to mingle.

1.2.2 Diversification and Perfectioning of Biostratigraphic Scales Certain fossil groups were selected in the establishment of the first bio-stratigraphic zonation schemes: trilobites, graptolites and vascular crypto-gams for the Paleozoic; ammonites for the Mesozoic; and pelecypods, echinoids, large benthonic foraminifera and mammals for the Cenozoic. Subsequently, other groups have been sought with the characteristics of 3Transgressions, by favoring speciation, increase the rate of evolution.

Stratigraphy

Pierre Cotillon

Stratigraphy

Springer-Verlag

Berlin Heidelberg New York

London Paris Tokyo

Hong Kong Barcelona

Budapest

Contents

Chapter 1

The Fundamentals of Stratigraphy

1 Definitions

2 Chronology of Events

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Elaboration of the Fundamentals

of Stratigraphy

1 Lithostratigraphy

2 Biostratigraphy

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2.1 Evolution, the Reference System for Age Dating

2.2 The Zone Concept of Oppel

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3.1 The Concept of the Stage

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