Cyclic Sedimentation (Developments in Sedimentology 10)

DEVELOPMENTS IN SEDIMENTOLOGY 10

CYCLIC SEDIMENTATION

FURTHER TITLES IN THIS SERIES

L . M . J. U . VAN STRAATEN, Editor DELTAIC AND SHALLOW MARINE DEPOSITS 1.

2. G. C . AMSTUTZ, Editor SEDIMENTOLOGY AND ORE GENESIS

3. A . H. BOUMA and A . BROUWER, Editors TURBIDITES

4. F. G. TICKELL THE TECHNIQUES OF SEDIMENTARY MINERALOGY

5. J. C. INGLE Jr. THE MOVEMENT OF BEACH SAND

6. L . VAN DER PLAS Jr. THE IDENTIFICATION OF DETRITAL FELDSPARS

S. DzUEYI?SKY and E. K. WALTON 7. SEDIMENTARY FEATURES OF FLYSCH AND GREYWACKES

8. G. LARSEN and G. V. CHILINGAR, Editors DIAGENESIS IN SEDIMENTS

G. V . CHILINGAR, H . J. BISSELL and R . W . FAIRBRIDGE, Editors 9. CARBONATE ROCKS

DEVELOPMENTS IN SEDIMENTOLOGY 10

CYCLIC SEDIMENTATION BY

P. McL. D. DUFF A. HALLAM AND

E. K. WALTON Grant Institute of Geology University of Edinburgh, Edinburgh, Great Britain

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PREFACE

Despite the existence of a huge literature this is the first time that a textbook has been written on the subject of cyclic sedimentation. We cannot claim that our review of this literature is completely exhaustive, Russian work in particular being under-represented. We have, however, tried to cover as much of the relevant data as is necessary to allow for adequate consideration of all significant hypotheses. It has been found desirable to vary the style of treatment of the subject from chapter to chapter and both the metric and foot-inch scale of stratigraphical measurement have been used. In this we have been guided by the nature of the data and the existing literature. We should like to express our thanks to a number of people whose co-operation has been invaluable. Dr. D. F. Merriam kindly allowed us to study the unpublished manuscripts of a number of contributions to an important symposium on cyclic sedimentation, which appeared as Bulletin 169 of the Kansas Geological Survey just when our manuscript was going to press. Dr. Merriam also showed two of us (P. D. and A. H.) some of the classic sections of Late Palaeozoic Kansan cyclothems and Dr. H. R. Wanless showed one of us (P. D.) important Pennsylvanian sections in Illinois and Indiana. Grants in aid of travel were provided by the Commonwealth Fund (A.H.) and the Carnegie Trust for the Universities of Scotland (P.D.). Dr. J. H, Rattigan obligingly supplied an unpublished manuscript on some Australian Carboniferous cycles and Prof. F. H. Stewart made helpful comments on Chapter 8. Permission to reproduce text figures has been obtained from the authors or journals concerned. We should also like to acknowledge the considerable secretarial work of Miss A. Lord and the technical help given to us by members of the staff of the Grant Institute of Geology. Edinburgh

P. McL. D. DUFF A. HALLAM E. K. WALTON

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CONTENTS

PREFACE

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

VII

CHAPTER 1. INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . Cycles. rhythms and cyclothems . . . . . . . . . . . . . . . . . . . . . . . . . . . Nomenclature of cycles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Time series and harmonic analysis. 13 - Scale; phase and facies. 18 Classification and description . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1 1 3 8

20

.

CHAPTER 2 CYCLES I N FLUVIAL REGIMES . . . . . . . . . . . . . . . . Cycles in the Old Red Sandstone of Britain . . . . . . . . . . . . . . . . . . . . . . Molasse of Switzerland . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Flysch facies in molasse. 35 Fluvio-lacustrine coal-bearing sequences of Gondwanaland . . . . . . . . . . . . . Witwatersrand System of South Africa . . . . . . . . . . . . . . . . . . . . . . . .

.

CHAPTER 3 CYCLES I N LACUSTRINE REGIMES . . . . . . . . . . . . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Glacial varved clays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Periodicity. 53 - Transportation and sedimentation. 55 . Long-term variations. 60 Non-glacial lakes . . . ..................... Varves. 62 . Periodicity. 64 . Sunspot cycles. 66 . Larger cycles. 67

..

21 21 33

..

38 43

. . . . 49 .. 49 .. 49 .........

.

CHAPTER 4 TRANSITIONAL REGIMES. I-NORTH AMERICA . . . . . . . . . 81 United States of America . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81 Eastern Interior Basin. 83 - Mid-Continent Basin. 88 - Appalachian Basin. 97 - Rocky Mountain region. 102 Canada . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104 Nova Scotia. 104 Theories of origin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104 CHAPTER 5. TRANSITIONAL REGIMES. 11-EUROPE ............. Great Britain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Visean. Tournaisian and Namurian. 117 - Namurian and Westphalian. 132 Continental Europe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Environment of deposition. 148 - Cycle mechanisms. 149

.

CHAPTER 6 . EPICONTINENTAL MARINE ENVIRONMENTS. I . . . . . . . . . Calcareous and argillaceous rocks . . . . . . . . . . . . . . . . . . . . . . . . . . Cycles composed of differing types of limestone . . . . . . . . . . . . . . . . . . . . Limestonedolomite cycles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cycles composed of limestones and argillaceous beds . . . . . . . . . . . . . . . . Minor cycles. 163 - Major cycles. 170

.

117 117 141 148

.

157 157 158 161 163

61

.

CHAPTER 7 EPICONTINENTAL MARINE ENVIRONMENTS. I1 . . . . . . . . . Cycles with significant quantities of sandstone. ironstone and phosphorite: minor cycles with bituminous laminae . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Clay-sandstone-limestone cycles . . . . . . . . . . . . . . . . . . . . . . . . . . Clay-sandstone cycles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ironstone-bearing cycles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Phosphorite-bearing cycles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Minor cycles with bituminous laminae . . . . . . . . . . . . . . . . . . . . . . . .

.

183 183 183 185 187 191 192

CHAPTER 8 EPICONTINENTAL MARINE ENVIRONMENTS. I11 . . . . . . . . . Evaporite cycles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Major cycles with subsidiary evaporites . . . . . . . . . . . . . . . . . . . . . . . Cycles with dominant evaporites . . . . . . . . . . . . . . . . . . . . . . . . . . Major and intermediate cycles. 204 - Minor cycles. 209 Evaporite vames and solar cycles . . . . . . . . . . . . . . . . . . . . . . . . . .

199 199 200 204

CHAPTER 9. FLYSCH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Modal cycles and composite sequences . . . . . . . . . . . . . . . . . . . . . . . . Ideal (model) cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Turbidity currents. 223 - Combined action of turbidity currents and bottom currents. 226 Bottom currents. 227 Megarhythms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

215 216 221

212

228

.

CHAPTER 10 SEDIMENTARY CYCLES AND FAUNAL CHANGE . . . . . . . . . 233 Faunal succession within major sedimentary cycles . . . . . . . . . . . . . . . . . . 233 Faunal change between major sedimentary cycles . . . . . . . . . . . . . . . . . . . 236 CHAPTER 11 . GENERAL CONCLUSIONS . . . . . . . . . . . . . . . . . . . . Sedimentary control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Tectonic control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Eustatic control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Climatic control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cycles and time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

241 242 244 245 248 250

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

253

REFERENCES INDEX

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

271

“Those who accept rhythm in nature will find it even where it is rather indistinct, and they will arrive at proper conclusions. Those who do not want to, will not find it even where it is obvious.” (Yu. A. ZFJEMCHUZHNKOV, 1958.) “Science, to an extent matched by no other human endeavor, places a premium upon the ability of the individual to make order out of what appears disordered. Therefore, the scientist more than anyone else needs to maintain his objectivity about his work, and perhaps even more vigorously, 1964.) about himself.” (E. J. ZELLER,

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Chapter 1

INTRODUCTION

Certain topics within geology, like continental drift, geosynclines and granitisation, from their inception have been, and continue to be, centres of debate and controversy. Rhythmic or cyclic sedimentation is one of these. In general terms, which will be refined later, cyclic sedimentation refers to the repetition through a succession, of rock units which are organised in a particular order. But even now, after more than a century of discussion, there is still disagreement on the validity and usefulness of the concept. To some protagonists the subject is so general as to include the whole of sedimentation and so has no special meaningl; to others it provides, from apparent disorder, elegant generalisations which are satisfying in themselves as well as forming a basis for genetic interpretation. The simplest case of repetition involves only two components and at this lower end of the scale it is possible to regard tiny interbedded laminae of, say, silt and clay as examples of cyclic or rhythmic sedimentation. At the other end of the scale broad changes in sediment character can span whole systems or even longer intervals. Some authorities, for example VONBUBNOFF (1948) and SLOSS(1964), refer to cycles of the order of geological systems or more and WELLER (1964) has shown how stratigraphic thought in America in the early part of this century was dominated by theories of large-scale, world-wide cycles of sedimentation. We wish to exclude consideration of these larger sequences otherwise virtually every succession would have to be discussed. At the lowest level we would also exclude thin-bedded laminae as being of trivial, local significance. Thus we do not consider tidal laminae in this discussion but we do include annual layers or varves. Within these prescribed limits cyclic sedimentation ranges from clastic, organic or evaporitic varves often less than 1 mm thick, through sequences of intermediate size (say around a metre or more) up to thicknesses of tens and hundreds of metres.

CYCLES, RHYTHMS AND CYCLOTHEMS

The pattern of sedimentation which has come to be called cyclic or rhythmic involves a series of lithological elements (say A, B, etc.) repeated through a succession. The elements may be combined together (ABC) and referred to as a rhythm or a cycle, terms which go back at least to the latter part of the last century (see for example 1

“Essentially all deposition is cyclic or rhythmic” (Tvamom~,1939, p.502).

2

INTRODUCTION

PEACH,1888). In the simplest case we may have ABABAB and some authors (for example FEARNSIDES, 1950; FIEGE, 1952) would restrict the term rhythm to this type of succession. SANDER (1936), on the other hand, suggested that rhythm be restricted to those sequences where the rhythmic unit was fairly constant in thickness, whereas VON BUBNOFF (1948) added a genetic connotation in that he regarded a rhythm as being of climatic origin. Varves would conform to most of these requirements in that they are for the most part simple bipartite structures, fairly uniform in thickness per unit, of strict periodicity and climatic in origin. But the conditions could be fulfilled in so few cases that the term rhythm would be of very restricted application. There does not seem to be any strong case for restricting the term “rhythm” to simple successions of the “ABAB” type. There is always the possibility of lenticular lithologies coming in to change a rhythm into what would be called a cycle; in any case it is always convenient in descriptions to have a number of synonyms. There is little guidance from mathematicians on the subject. KENDALL (1947) suggested that the term cycle be used only when the repetition is regular in time and SCHWARZACHER (1964), in advocating this usage, recognised that most successions could only be claimed to show “sedimentary oscillations”. His scheme has some attractive features but the term “sedimentary oscillations” is very cumbersome and it would probably be difficult to obtain agreement on its use. Some authors lay importance on the difference between the sequences ABCDABCD and ABCDCBA and would restrict the word cycle to the latter. But, as ROBERTSON (1948) pointed out, one of the most common expressions in English is the “cycle of the seasons” which is of the ABCD ABCD type. Perhaps the simplest solution is to refer to the ABCD ABCD type as asymmetrical cycles and to the ABCDCBA type as symmetrical. “Cyclothem” is different from the other two terms in that it was introduced specifically to refer to sedimentary rocks. “The word cyclothem is therefore proposed to designate a series of beds deposited during a single sedimentary cycle of the type that prevailed during the Pennsylvanian period” (WANLESS and WELLER, 1932, p. 1003). Even so this original definition is not unambiguous. On one interpretation the term might be restricted to Carboniferous rocks, or even only to Pennsylvanian; on another, it might be applied more widely to sedimentary cycles “of the type that prevailed during the Pennsylvanian period”. In practice, and to the disapproval of WELLER (1961), later workers have tended to apply the term to rocks of different age and to rocks of very different lithologies from the Pennsylvanian cyclothems of Illinois (e.g., P. ALLEN,1959; HALLAM, 1961; J. R. L. ALLEN,1964a). There now seems no more reason to restrict the term to Carboniferous rocks than there is to regard cyclic sedimentation as uniquely confined to that system. In our opinion the three terms, rhythm, cycle and cyclothem should be regarded as synonymous except that the latter always refers to sedimentary deposits. Cycle or rhythm, though referring on most occasions to the deposits, might also denote the period of time during which certain sediments formed. The use of the terms will be clear from their context. BEERBOWER (1964) has objected to suggestions of this sort

NOMENCLATURE OF CYCLES

3

on the ground that the original definition of cyclothem implies asymmetry. But many workers have used the term to cover both types and the well known Kansan cyclothems, for example, include both symmetrical and asymmetrical cycles (R. C. MOORE, 1965). 1936; MERRIAM,1963; R. C. MOOREand MERRIAM, We see no prospect of general agreement on, nor any particular advantage in, the usage of these terms according to arbitrary, restrictive definitions. On the contrary it should be possible to retain the words as general terms and qualify them as necessary. If the repetition can be shown to be regular in time then it should be sufficient to apply a description such as “periodic” cycle; if the sequence is of the ABAB type then it could be referred to as a simple cycle or rhythm; and as indicated above, cycles may be symmetrical or asymmetrical according to the arrangement of elements within the rhythmic sequence. In this way the terminology can be unambiguous and yet retain a flexibility denied it by too rigid definitions. It may not be unreasonable to refer to thin cycles as microcycles, to those of intermediate size simply as cyclothems or cycles and to thicker sequences as megacyclothems or megacycles. But to attempt to link these sizes to specific causes (for example microcycles as climatic or epeirogenic and cyclothems as epeirogenic) as FEGE(1952) has done is dangerous in the extreme. It should also be recognised that in America megacyclothem tends to have the connotation attached to it by R. C. MOORE(1936; but see YOUNG,1955, 1957), that is to describe a “cycle of cycles” (see pp.88-94). Sequences larger than the megacyclothems of R. C. MOORE(1936) have been called hypercyclothems and magnacycles. WELLER (1961) introduced the term hyper(1963, following a cyclothem to cover a “cycle of megacyclothems” while MERRIAM suggestion of R. C. Moore), coined the term magnacycle to refer to rock units which represent major events in earth history. An example of a magnacycle given by Merriam is the Pennsylvanian-Permian sequence in Kansas. A hierarchy of cycles was also given by JABLOKOV et al. (1961). First order cycles were grouped into second order or mesocycles, the mesocycles combined into third order or macrocycles and the macrocycles joined in fourth order or megacycles. The recognition of these larger order cycles appears to be so arbitrary that their practical value at this stage seems doubtful. Nomenclature is confusing, as can be seen from the two systems cited above. In additionSAKAMo~o(1957) used lst, 2nd and 3rd order cycles in reverse order to Jablokov et al., i.e., Sakamoto regarded the “later” Palaeozoic system as constituting a 1st order cycle, divisions of the Middle Carboniferous as 2nd order cycles while 3rd order cycles are individual cyclothems.

NOMENCLATURE OF CYCLES

Historically, interest in successions showing continual and repeated changes of lithology (they were not at h s t called cyclic successions) seems to have been first aroused by coal-bearing sequences. Once it became accepted that coal represented the remains

4

INTRODUCTION

of plants and that the beds between the seams sometimes contained marine shells, speculation began as to the meaning of such combinations. Theories on the genesis of repetitive sedimentation in coal-bearing strata are dealt with in Chapters 4 and 5. At this stage it is instructive to trace the way in which generalisations have been made regarding cyclic sequences. In the earliest descriptions (for example DE LA BECHE, 1834; MACLAREN, 1839; MILNE,1839)it was considered sufficientto recognise an alternation ofcoal seams with marine strata. Some workers went a little further. DAWSON (1854) for instance emphasised the combination of lithologies, underclay-coal-bituminous limestone while PHILLIPS (1836) introduced a wholly admirable system which, had it been adopted, might have clarified thought on the whole topic and led to a much more rapid advance than has in fact occurred. He recognised that the Lower Carboniferous rocks in the north of England (which he called the Yoredale Series) were made up of repetitions of “limestone, gritstone and shale” and suggested that a particular combination of lithologies (say ABC or CAB) should be called a “term”. A number of such terms should constitute a “series”. Where a succession consisted of only two terms then it could be described as a “dimeric series”; the general case would be the “polymeric series” comprising many terms. If the successive terms are the same (ABC, ABC, ABC, . . .) then the series would be “homo-polymeric” but if the terms are dissimilar (ACB, ABC, BAC, . . .) then the series would be described as “heteropolymeric”. It is rather ironic that most British authors have quoted Phillips as being the first to recognise the Yoredales as being built up of a series of similar “Yoredale cycles”, when he in fact regarded the Yoredales as forming a hetero-polymeric series. Phillips’ suggestions were not taken up and for more than a century generalisation regarding rhythmic successions has been based on subjective assessment. Many authors, sometimes because they were dealing with a restricted succession or because they were dealing generally and superficiallywith thick successions over relatively large areas, have been content to note a simple repetitive unit. HIND (1902) for example described the Yoredale succession as repetitions of the unit, limestone-shale-sandstone. HUDSON (1924) incorporated the same lithologies (shale-sandstondimestone) but suggested a different starting point, because each limestone was thought to have an eroded top surface (surfaces since shown to consist of algal nodules of original shape, not eroded). In America, UDDEN(1912), dealing with a small sequence in the Pennsylvanian rocks of the Peoria Quadrangle, Illinois, had pointed out a similar rhythmic unit. “Each cycle” he wrote (UDDEN,1912, p.470) “may be said to present four successive stages, namely: ( I ) accumulation of vegetation; (2) deposition of calcareous material; (3) sand importation; and (4) aggradation to sea level and soil making”. Since all successions tend to vary both laterally and vertically such simple generalisation must be qualified to some extent when the area and the thickness of succession under consideration are extended. Thus we have PEACH (1888, p.17) Writing on the Lower Carboniferous of Scotland about the repeated cycles of varying lithologies: “When the succession is complete the following is the arrangement of strata in

NOMENCLATUREOFCYCLES

5

ascending order: (I) limestone charged with ordinary marine fossils; (2) shales, yielding stunted marine forms; (3) sandstone; (4) fireclay with the roots of plants which is overlain by a coal seam. In some cases one or more of these members may be absent, but the others preserve the same relative order.” This method, involving the subjective selection of lithologies in a rhythmic unit and then noting possible variations, remained the standard procedure in studies of cyclic sedimentation for seven or eight decades following Peach’s writing. Consider, for example, TRUEMAN’S (1954) description of the Coal Measures succession in Britain: “For many years it has been apparent to those who have examined coal-bearing sediments that there is a characteristic pattern in the sequence of rock types, varied in detail but consistent in essentials in rocks of all ages and of all countries. This repetition of a common motifthroughout the coal-bearing rocks may be described as of a cyclical or rhythmic nature. In the Coal Measures of Britain and in northwest Europe generally the unit (1-5) of the rhythmic pattern is conveniently stated as: (5) Coal. (4) Rootlet bed. (3) Sandstone. (2) Non-marine shale or mudstone. (I) Marine band. While the unit or cycle (cyclothem of American writers) is repeated in this simple form in some parts of the sequence, there are many minor variations. The thickness of the different members may vary greatly or some of them may be absent. The coal seam may be thick or thin, a mere streak in some places or absent in others, even if the rootlet bed is well developed. The marine band is generally thin when present, but in the majority of the units it is absent altogether. When a marine band occurs it is usually at no great distance above the coal seam although a thin non-marine layer may intervene and indicate more gradual submergence of the swamp. The sandstone may vary greatly in thickness as has been said, in different localities in the same unit. Occasionally a sandstone may immediately succeed a coal seam. The unit may also be extended by minor repetitions of sandy and muddy layers. But with every conceivable modifcation, the most significant feature in the sequence of rocks making up the productive (i.e., coal-bearing) part of the Coal Measures is the regularity of the simple pattern.” (TRUEMAN, 1954, p.10; our italics.) The variations are so carefully enumerated that the validity of the rhythmic sequence becomes extremely doubtful. Nevertheless the description in principle is the same as that of Peach. The pattern is the same elsewhere. In Germany, JESSEN(1961), having erected an elaborate “ideal” 14-unit ‘Toll-cyclothem” (p.3 12) for European coal-bearing rocks, qualified his remarks as follows (pp.316-3 17): “Wer alle diese Cyclothem-Glieder und ihre Position im Ablauf des zyklischen Sedimentationsvorgangeskennt, wird die naturgemassen Variationen verstehen, diegegeniiber dem “idealen Voll-Cyc1othem”-an Gliedern mehr oder weniger stark oder sogar extrem “an Gliedern verarmt” sind. Eine Variationsreihe beruht auf “Verarmungen” an (= Wegfall von) progressiv-hemizyklischenGliedern (1 + 2, 1-3,

6

INTRODUCTION

1-4, 1-5, 1-6, 1-7). Aus einem zyklischen Glieder-Aufbau wird dann schliesslich ein “rhythmischer” (Glieder 8-14), der aber gleichfalls ein echtes Cyclothem darstellt. Hieran schliesst sich eine zweite Variationsreihe an, bei der auch das rezessive Hemicyclothem an Gliedern verarmte (Glieder 15,14 13, 1 4 1 2 , 1 4 11,1410). Das durch Kombination beider Verarmungsreihen an Gliedern extrem verarmte Cyclothem besteht dann allein aus Schieferton. Es entstand durch besonders starke MeeresProgression, die jegliche Sand-Zufuhr in die Saumsenke verhinderte. Dem entgegengesetzt fuhrt eine Variationsreihe immer starkeren Ausfalls der feinkornigeren Glieder ( 6 8 , 5-9, 4-10, 3-1 1) zum Extremfall allein aus Sandstein bestehender Cyclotheme. Diese entstanden in Fallen, in denen sich die MeeresProgressionen iiberhaupt nicht oder nur eben andeutungsweise auswirken konnten.” And in Belgium VAN LECKWIJCK (1964, p.42), having described a five-fold “complete” cycle in the Namurian, had to add the comment “Les cinq phases ne sont pas prksentes dans tous les cycles. . .” In America the study ot cyclic sedimentation was greatly stimulated during the 1930’s by the work of WELLER (1930, 1931) and interest has continued undiminished. But nomenclatorial confusion has grown almost as rapidly as the accumulation of stratigraphic data. In 1930, Weller described the “typical” cyclothem in Illinois in terms of nine successive lithological units. This succession was modified a year later and yet again in 1932 when the term cyclothem was introduced to refer specifically to the sediments (see above). Then WANLESS and SHEPARD (1936) described the typical cyclothem of Weller in a number of ways (normal, complete, standard, common) yet referred to a slightly different set of lithologies. Later WELLER (1956) distinguished between an “idealised standard” succession and “real” cyclothems, real, presumably in the sense of naturally occurring units. It seems likely that Weller’s “idealised standard” cyclothem compares in its connotation with the “complete” cycle of PEACH(1888). There are therefore two categories of cycle which can be picked out, those referred to as “typical”, “normal”, etc., which might be expectedto beofcommonoccurrence and those, like the “idealised standard” which may be rarely developed but which express some characteristic order of the lithological units. Just as various terms have been used to describe the same succession and the same term has been used to describe different sequences in America, a similar confusion has arisen in British literature. The situation is summarised in Table I. It will be noticed that not only is there a profusion of terms but, what is more serious, there is also anumber, like “complete”, “normal”, “typical” which appear on both sides of the table and which have been used to describe the two categories of cycles distinguished above. There are two reasons why this confusion has arisen. The first is the subjective methods of assessing cyclic sedimentation and improved methods will be examined below. The second is the failure to isolate certain elements in the subject. We would separate these elements in this way. In any groups of rocks displaying cyclic sedimentation, it should be possible to identify that particular grouping of lithologies which occurs most frequently through the succession. This ordered sequence might naturally be correlated with what many

+

7

NOMENCLATURE OF CYCLES

TABLE I AMBIGUITY OF CYCLOTHEM NOMENCLATURE

(After DUFFand WALTON,1962) ~

~~

~

Cycles which reader might expect to occur frequently

Theoretical or partly idealised cycles not necessarily of frequent occurrence, if present at all

Typical (WELLER, 1930)l Typical (WELLER, 1931)l Normal (WANLESS and SHEPARD,1936)2 Complete (WANLESS and SHEPARD,1936)2 Standard (WANLESS and SHEPARD, 1936)2 Typical (WANLESS and SHEPARD, 1936) Usual (WANLESS and SHEPARD,1936) Common (WANLESSand SHEPARD,1936) Typical (ROBERTSON, 1948) Fully developed (EDWARDSand STUBBLEFIELD, 1 947)3 Normally developed (EDWARDS,1951)3

Full rhythmic sequence (ROBERTSON, 1948)

Standard (DUNHAM,1950)4 Commonly developed rhythmic unit (DUNHAM, 1953) “Normal” (R. A. EDENet al., 1957) Characteristic (GOODLET,1959)

Normal (ROBERTSON, 1948) Idealised standard (WELLER, 1956) Theoretical (WELLER, 1957) Full (R. A. EDENet al., 1957) Complete (GOODLET, 1959) Theoretically expectable composite succession (WELLS,1960)5 Typical (BEERBOWER, 1961)6

1 The unit of the two “typical” cyclothems actually differ though they purport to describe the same succession. (1930) cyclothem although not used byWeller. Used by Wanless and Shepard to describe WELLER’S Used to describe same cycle. Dunham cited “standard cyclothem or rhythmic unit” of WANLESS and WELLER (1932) although this qualifying term was not used by those two authors. 5 Used to describe WELLER’S (1930) cyclothem. 6 Referring to the “theoretical” of WELLER (1957).

workers have designated the “typical”, “normal” or “characteristic” cycle (Table I). In order to emphasise that this cycle has been picked out because of its frequent occurrence DUFFand WALTON (1962) proposed that it should be called the modal cycle. There are, however, also terms (Table I) which have a somewhat different meaning, for example Weller’s “idealised standard”, “composite” and “fully developed” cycles. These carry no implication of frequent development. Authors using such terms generally make it clear that seldom, if ever, do the units described as making up one of these cycles occur together in an actual cycle. Neverthelesscertain lithologies, although of infrequent occurrence, may have a preferred position with regard to the other beds of the modal cycle. For example a succession dominated by the rhythm ABCD may occasionally include an additional lithology, X, which, when present, lies between C and D. This is probably meaningful though there is no question that the sequence ABCXD is a common rhythmic unit. ABCXD is constructed from statistical data;

8

INTRODUCTION

it combines all the lithologies in a succession in the order in which they tend to occur and has been referred to as the “composite sequence” (DUFFand WALTON,1962). Both the “modal cycle” and the “composite sequence” are based on actual rock successions. But another concept runs through writings on rhythmic sedimentation. This is implicit in the terms “theoretical”, “idealised” and even, one supposes, in “the theoretical expectable composite succession” (WELLS,1960). There is suggested in these terms a theoretical cycle to which the observed sequences can be referred and through which the observed sedimentary successions can be understood. This “ideal” or “model” cycle is one which can be constructed from theoretical considerations and from accumulated data from modern environments and experimental evidence. It arises only in consideration of the observed groups of the modal cycle and the composite sequence. BEERBOWER (1964) has rightly pointed out that if any basin model is to be even approximately realistic it will generate a variety of cycles from which the most common can be picked out. This most common sequence he has called the “ideal modal cyclothem”. In our opinion the more correct title would be “modal ideal-cyclothem” but we deprecate the use of modal in this context because it was introduced specifically to refer to the results from statistical examination of actual successions.

METHODOMGY

In a provocative paper ZELLER (1964) has pointed out that “science, to an extent matched by no other human endeavour, places a premium upon the ability of the individual to make order out of what appears disordered”1. While this statement in its entirety may be debatable it is sufficiently valid to suggest that each subjective assessment on the presence or absence of cyclic sedimentation is suspect and, as a corollary, that every effort should be made to systematise descriptions of rhythmic successions on an objective and, where possible, quantitative basis. So far attempts at the latter have developed along two main lines: (a) statistical analysis to pick out the modal cycles, composite sequences, etc., and (b) refined mathematical techniques to test for periodicity in the data or to provide mathematical models for the geological data. Techniques of the second type are virtually restricted to simple successions such as varved clays though, as shown below, some attempt can be made to reduce more complicated sequences to a single variable. The picking out of the modal cycle represents an attempt to formalise the definition of the rhythmic unit in a system with several components. Cycle is defined as that “group of rock units which tend to occur in a certain order and which contains one unit which is repeated frequently through the succession” (DUFF and WALTON, 1 Zeller showed in the same paper that geology students were able to see correlations between actual successions in which each lithology was denoted by a number and sequences of numbers taken from the Kansas Telephone Directory!

9

METHODOLOGY

Cycle

Types

Fig.1. Histogram showing frequency distribution of cycle types from artificial succession given in text.

1962, p.239). The unit referred to in the latter half of the definition would normally be one to which a certain genetic significance could be attached, such as the coal seam or underclay of the Coal Measures. It is also obviously necessary to choose a unit which is relatively common in the succession. The procedure can be seen by reference to the succession indicated by the following letters: A, B, C and D which represent different lithologies: C, BC, ABC, ABC, BAC, DABC, ABC, DABC, BC, ABC, BABC, BABAC, ABC, BC, ABAC, ABDC, BABABC, DABC, BC, ABC, ABAC, ABC, ABABAC, ABC, ABC, ABC, BAC, DABC, BC, ABC, . . . Taking lithology C as the marker horizon because of its geological significance breaks up the sequence (as marked), into a number of units or cycles of which the sequence ABC is the most common (Fig. 1). It is therefore the modal cycle. In order to determine the composite sequence the position of additional lithologies, such as D, is examined with respect to the beds of the modal cycle. In this example it is clear that although D does not occur very often, when it does it tends to lie above C and below A. The composite sequence is therefore DABC. DUFFand WALTON (1962) analysed over 1,200 cycles from the Coal Measures of the East Pennine Coalfield, England and found the distribution of cycle types (Fig.2). The critical lithology for marking the cycles was chosen as the coal seam or, when this was not present, the seat earth. The seat earths were classified according to their grain-size so that where a mudstone seat earth lay on shale the two were classified together as A. The other lithologies considered were B, siltstone, C, sandstone and M, mixtures of sandstone with siltstone or shale. The modal cycles then appear as A (an alternation of shale with seat earth and sometimes coal), ABA and AMA. The dominant cycle is one made up of fine, then coarse, then fine sediment. Another way of approaching the problem is to pick out the dominant cycle in terms of the number of lithologies present (Fig.3). This turned out to be three and when those cycles with three units are analysed the same pattern, fine-coarse-he sediment, is found as before (Fig.3). The Coal Measures succession also contains marine shales as an important, but numerically insignificant, lithology. When the position of these bands and the coal is included in the sequence the composite sequence becomes:

11

METHODOLOGY

3 2 1

loo

200

300

No. of units in cycle

A

B

Fig.3. A. Numbers of lithological units in non-marine cycles. B. Relative position of rock types in three-unit non-marine cycles. Ornament: lines-A (shale), fine dots-B (siltstone), coarse dots-C (sandstone), lines and dots44 (mixture).

Coal. Seat earth. Shale, non-marine. Siltstone and/or sandstone. Shale, non-marine. Shale, marine. Having obtained the two sequences an ideal cycle can then be constructed from, for example, what is known of successions in deltaic regions of the present day. The comparison of the “ideal” and the “modal” can then be carried out and it is obviously desirable to be able to test the “goodness of fit” of the one with the other. A method proposed by PEARN(1964) may be applicable in this situation. His account does not distinguish between “modal” and “ideal” and in fact the term “ideal” is used in both senses. Transposed into the terms we have used the type of question which is posed is: does the ideal cycle proposed by R. C. MOORE(1936) for the Kansan rocks of Pennsylvanian age best describe the rhythmic sedimentation found in that succession?How near does the ideal cycle coincide with the modal cycle and how far does the observed sequence differ from a random distribution of the strata? For the purpose of answering these and related questions WARN(1964) introduced the Discordance Index G, a parameter calculated in the following way:

Fig.2. Histogram showing frequency distribution of cycle types in the Coal Measures, East Pennine Coalfield, England. (After DUFFand WALTON,1962.) A. Cycles with no marine fossils, divided into ( i ) those cycles containing no sandstone and (ii) those cycles with sandstone. B. Cycles containing marine fossils, divided into (i) and (ii) as in A. Lithological units of main cycle types shown in upward sequence: A = shale or mudstone; B = siltstone; C = sandstone; M = mixture of siltstone or shale and sandstone.

12

INTRODUCTION

Given a sequence such as: 24324532, and a proposed ideal sequence: 12345432123454321, the figures in italics are the lithologies common to both sequences and the units missing between the start and finish of the actual sequence total seven. This is one possible answer for G. But, as in the case of R. C. MOORE’S (1936) ideal cyclothem (see p.88) which is symmetrical, the observed sequence may begin in a regressive hemicycle (54321) rather than the transgressive one (12345) as taken above. The comparable ideal sequence would then be: 543212345432123454321 and the missing units in this case would total nine. G is taken as the smaller number so in this example it would be seven. The next step was to assess the probability of any value of G arising from a random distribution of lithologies in a sequence. The possible combinations of the five lithologies depend on how many units are present in the postulated cycles. As the cycles become larger then the amount of calculation involved becomes more and more unmanagable. A limitation is therefore imposed on this method which restricts the significance of any results obtained. PEARN (1964) chose to use sequences of seven units. All possible arrangements of five lithologies (such as 1234543, 2134543, etc.) in these seven units were considered (so long as no lithology was repeated consecutively). The values of G associated with each of these arrangements then gave the probability of any value arising by chance. Using chi-squared tests Pearn was able to show that observed values of G differed significantly from randomness. In order to determine the “goodness of fit” of the proposed ideal cycle of Moore the values of discordance were calculated for a number of other possible arrangements. It was found that the Moore hemicycle (12345) was the best fit for the Kansan rock column (a somewhat artificial sequence summarising the succession in that state and erected by R. C. MOOREet al., 1951) but that when a sample of actual sequences from different parts of the state was considered 78 different hemicycles gave discordance values lower than the Moore hemicycle. Although Pearn referred to these possible hemicycles as “ideal” the procedure corresponds to the search for a type of “modal” cycle; the cycle which best summarises the observed sequence. Moore’s ideal cycle based on a transgressive-regressive model of sedimentation apparently falls a long way short of coincidence with a “best-fit” cycle; it would seem desirable therefore to consider other models in order to see whether they would generate an “ideal cycle” closer to that observed. A possible alternative approach to the problem of finding the most suitable model or ideal cycle arises from recent work on cross-association (SACKIN et al., 1965; MERRIAMand SNEATH, in press). The method involves comparison of sequences and noting whether elements (lithologies) at similar levels within the sequences are the same or not. A measure of the agreement of one sequence with the other is thus obtained. Development of the method is in a preliminary stage but its application to sequences where only qualitative data are available should yield interesting results. Another possible treatment of complicated successions where a number of lithologies are present is to transform the data into a quantitative form. This has been done,

METHODOLOGY

13

for example, by VISTELIUS (1961) who allocated numerical values to the different lithologies (shale, sandstone, conglomerate, etc.) in proportion to their grain-size. In this form the data are amenable to the methods of analysis to be described in the next section.

Time series and harmonic analysis

In simple systems where one or two lithologies are involved, one variable measured through the succession, such as the thickness of each varve or the CaCO3 content of the sediments, gives directly a series referred to as a time series. This is a general term referring to the variation of one parameter through time and it will be realised at the outset that stratigraphical measurements refer to time at second hand as it were. The measurements are correlatable with variation through time only so far as the record is complete and the rate of sedimentation was relatively constant during the formation of the succession. Following JSENDALL (1947) we can first of all separate out the long-termvariation as a trend. This is convenient because we said at the outset that long-range variations would not be considered. Shorter-term oscillations may then be discerned in the time series which if strictly periodic according to Kendall could be termed “cyclical”. In addition, in natural sequences there is also an element causing random fluctuations. Given a time series which apparently shows oscillations the first problem is to show that these fluctuations are not random. This can be done by the “up and down” test (KENDALL, 1947; NEDERLOF, 1959) in which the number of turning points in the series is compared with the number which is likely to have arisen by chance. The number of runs ( R ) between turning points is compared with the number of observations (n) in the statistic K which is defined as: K=-

3R-2n +- 2.5 2/( (16n-29)/ 10}

The probability of a particular value for K arising by chance in a random series can be found from appropriate tables. If the value of Kindicates a non-random distribution the nature of the variations can then be investigated further. In a series where the value of the variable, x, is oscillatingaround a mean value, in so far as there is some regularity in the oscillation, successive values of x are not independent of one another. That is to say the value of x at different points in the series will show some correlation one with another. It is possible therefore to investigate the structure of the series by considering the correlation between successive values of x . The correlation coefficient can be calculated as in the case of two variables x and y , the procedure simply consisting of regarding values of x at successive points as values of a second variable, y . A number of correlation coefficients can be calculated according to whether x p (the value of x at position p

14

INTRODUCTION

-0.8-

-0.81 -1.0 -

1 0

2

4

6

a

0

Fig.4. A. Correlograms of Dartry Limestone thickness indexes (solid line) and fitted theoretical correlograms of a Yule-Kendall process (dotted line) and a harmonic process (fine dots). For Dartry Limestone the horizontal scaIe is in metres and the points calculated at intervals of 20 cm.(After SCHWARZACHER, 1964.) B. Correlopam of Benbulbin Shale (solid line) with fitted Yule-Kendall 1964.) process (dotted line). Scale as in A. (After SCHWARZACHER,

in the series) is compared with xp+l, or x p + 2 , or x p + k . The regression of x p and xp+l where the correlationcoefficientis found between ~ 1 x 2 ;X 2 X 3 ; . . . X n - l ; X n is referred to as the serial coefficientof the first order. The second order coefficient would relate x p and x p + 2 . In the general case the coefficient of the order k is given by:

and the plot of r k against k is the correlogram (Fig.4). When the correlogram has been

15

METHODOLOGY

determined for a given geological succession it can then be compared with suitable mathematical models. SCHWARZACHER (1964) in his study of a Carboniferous limestone succession, considered four different cases: Case I is a stochastic process of moving averages where x at any point is determined by the sum of a number of factors, u, some of which are common to successive values of x . The correlogram in this case appears as a straight line between l(r0) and zero ( r k ) . The case is described in the expression:

where cP is a random variable. Case 2 is that of the autoregressive series, which is defined by:

The first order of this series is given by:

+ Ep

x p = -UXp-1

while the second order is: x p = -axp-1-bxp-2

+ ep

The autoregressive series is comparable to a pendulum being struck at random by a stream of peas. Each one of the random impulses affects the oscillation and is integrated into the system. For the second order (the Yule-Kendall process) the correlogram has the form of a harmonic beginning as r,, equal to 1 and damping down towards zero. Case 3 is a special case of a harmonic process such as a pure sine wave. The correlogram of a sine wave is a cosine wave. Case 4 adds a stochastic, random variable to a harmonic process of type 3. The result can be expressed in the form: 27z x = Asin-p V

+ ep

The correlogram, after decreasing from 1 (To) takes the form of a cosine wave whose constant amplitude is determined by the variance of the random variable E. The application of these techniques can be seen in SCHWARZACHER’S (1964) study of a Carboniferous section in Ireland. The numerical data consists of percentage limestone or average thickness of limestone bands in 20 cm intervals. It was found useful to have these two measurements because in different sections one or the other was the more accurate or the more meaningful. Plotting the variables brought out a short-term oscillation and a long-term trend. The latter was estimated and subtracted

16

INTRODUCTION

from the data to isolate the oscillation. Correlograms were then computed for the oscillations seen in different parts of the succession-the Benbulbin Shale, and the Dartry and Glencar Limestones. With respect to the correlogram of the Dartry Limestone, it is apparent (Fig.4) that the Yule-Kendall autoregressive model gives a curve which is excessively damped but that a harmonic process with a superimposed disturbance gives a reasonable fit. A similar model could be applied to the correlogram of the Glencar Limestone but the Benbulbin Shale gives a curve which, though somewhat damped, corresponds to an autoregressive model. The geological significance of these results is discussed in Chapter 6. The structure of a cyclic sequence might best be analysed in terms of a periodic function-that function given in terms of the Fourier Series which can be expressed as (PRESTONand HENDERSON, 1964; PRESTON and HARBAUGH, 1965): n=m

U n COS

2nnz L

+ bn sin

-

n-1

1

-

2nnz L

where: L = half of the basic or fundamental period: in practice this is usually not known and can be taken as half the length over which the variable is sampled; z = independent variable of length through the succession; a,, = constant; an = the maximum value (or amplitude) of the cosine term at the “n”th harmonic; bn = the maximum value (or amplitude) of the sine term at the “n”th harmonic. The meaning of the parameters is illustrated from an artificial varve series in Fig.5 and the summation of sine and cosine terms up to n-5 is given in an example (Fig.6). Clearly as terms are added (with increase of n) the resultant curve becomes more and more complex. The procedure is that of finding a “best-fit” curve for the time series. The Fourier expression is perhaps the most powerful although any polynomial could be fitted (for example Fox, 1964).

year year

X

I

thickness A

0

I ‘Y 1..

C

Fig.5. Illustration of parameters used in harmonic analysis of time series C from an artificial vane succession A and varve diagram B. Value of y indicated at arbitrary position of z.

17

METHODOLOGY n-1

7oc

n-2

a

b

-

C

d

Figd. Synthetic single Fourier Series illustrating wave forms of individual terms and wave forms of series generated by summation of individual terms: a = cosine, b = sine, c = sine cosine, d = cumulative sine + cosine; for appropriate values of n, the harmonic number, from n = 1 to n = 5. (Adapted from PRESTON and HARBAUGH, 1965.)

+

For time series of any length the coefficients a,. . .a n and bn can be calculated (see for example PRESTON and HARBAUGH, 1965, or for more extended treatment, WYLIE, 1960). These coefficientscan then be used to derive a set of figures, ct, ci, cg . from:

..

This set constitutes the power spectrum of the series. The value of c; is an indication of the contribution of the “n”th harmonic to the series and the plot of c: against the harmonic number n gives an indication of the relative strengths of the different periodicities. Various refinements of analysis and presentation are available but for our present purposes it is sufficient to note that the power or amplitude in one form or another is plotted as ordinate against frequency or its reciprocal (time, if available as in vane series, or thickness, as in most successions). The more important periodicities can be easily read from the peaks which appear in these spectrum or amplitude diagrams. This type of analysis has so far been applied to varve successionsin an attempt to pick out any strong periodicities. In practice pronounced peaks are noticeably absent although there is often a general rise in amplitude pointing to a possible

18

INTRODUCTION

periodicity between 50 and 100 years in length (Fig.27, p.61). The average of glacial varve analyses (Fig.27, p.61) shows a rise in power at about 5 years while the Lake Superior varves show an increase involving frequencies between 6 and 14 years (Fig.7).

Scale; phase andfacies

Two further aspects remain to be mentioned at this stage. The first concerns the scale on which investigations are carried out. KRUMBEIN (1964) has set up a hierarchical scale involving a number of levels of investigationfrom the detailed study of a member of a cyclothem up to a group of cycles. He also analysed formally the relationship between the observed and inferred elements. Implicit in his analysiswas the relationship between the level of investigation and the inferences which can be drawn from the study and we would like to emphasise this rather obvious but sometimes overlooked point. Studies of tiny areas over trivial thicknesses of succession have often led to conclusions regarding mechanisms of formation involving world-wide and even cosmicevents. Our position is not to assert that these far-flung speculationsare completely unwarranted but to reiterate that there should be some approximate correlation between the scale of inference and the scale of observation. Secondly we might follow LAPORTE and IMBRIE (1964) in recognising that cyclic sedimentation can be studied both in phase and facies. Cyclic sedimentation refers to the development of lithologies in a pattern through a succession. It is appropriate therefore that successions at individual localities should be tested for cyclicity. This is to be primarily concerned with phase in sedimentation at different points in time at individual localities. But a system of sedimentation is one which has extent in space as well as time and cyclic sedimentation can therefore be regarded as the superimposition of lithologies due to the lateral migration of facies belts. In order to be complete therefore any analysis of sedimentation should take account of both phase and facies-the one implementing and illuminatingthe other (LAPORTE and IMBRIE, 1964).The essential combination of cycle-facies studies has also been stressed by ZHEMCHUZHNIKOV (1958; see Chapter 5). The erection of a modal cyclothem or a composite sequence is essentially a phase study. In a large basin of sedimentation separate modal cycles might be picked out for different sub-areas and the distribution of modal cycles would then reflect facies variation over the basin. KRUMBEIN’S (1964) analysisinvolveda similar approach. Other studies have laid emphasis on facies variation. WANLFSSet al. (1963) traced individual members of three Pennsylvanian cyclothems over a very large area in the mid-west of the U.S.A. (see Chapter 4). Interpretation of the environment of accumulation of the different lithologies led to the construction of successivepalaeogeographic maps. The changes in palaeogeography show very strikingly the gradual build-up of the cyclic succession and the influence of localised tectonic elements. The aim of most investigations like this is to build up a model of the basin and its sedimentary

19

METHODOLOGY

N = 106

?

I

1

px, M

3

1 10

1 ZQ

I

I '

1

I

8

6

5

4

1 (YEARS)

I-

I

4

I

1

1

1

3.S

3

2.75

2s

1

225

I

e

TIYE4RSl

Fig.7. Amplitude spectrum of Pleistocene Lake Superior vanes (N= 105). Large peak noticeable at low frequency (50-100 years) and broad rise between 6 and 14 years: f(CPY) = frequency, cycles per year. (After R. Y.ANDERSON and KOOPMANS, 1963.)

20

INTRODUCTION

fill. Certain individual lithologies can be studied separately towards the same end. POTTER(1962) for example, used data from sandstones in the Illinois area to reconstruct the physiography and filling of the Pennsylvanian basin of that region. Facies investigation is obviously facilitatedby rigorous time control. The marine Lias of western Europe is an especially suitable field for the study of widespread facies changes (HALLAM, 1961,1964b;see Chapter 6) because palaeontological zoning allows the recognition of synchronous surfaces of erosion or distinctive lithological horizons which mark the boundaries of so-called major cycles. Such research has also revealed the masking of cyclic episodes in areas where lithologies are not suitable. The importance of this approach, involving as it does inter- as well as intra-continental correlation, is that it provides one of the few known criteria for distinguishing eustatic changes of sea-level from local epeirogenic or sedimentary controls of cyclic sedimentation.

CLASSIFICATION AND DESCRIPTION

We have adopted a scheme of classification which is based on rather broad environments of sedimentation. Within the major categories separated by environment, the various types of cycle are treated either on a more detailed environmental basis or in terms of the lithologies present in the succession. While this scheme has the advantage of providing a ready link between description and genetic interpretations it has the chronic drawback, common to most systems, of generating a number of border-line cases, and what is perhaps more important, it divides seemingly coherent groups, such as varves, into different categories. We use the scheme because it seems to have the fewest disadvantages. The major divisions are: Continental

fluvial lacustrine

Transitional Marine

epicontinental geosynclinal

An attempt has been made to introduce some uniformity into the nomenclature but in many instances it has been necessary to be guided by what authors have found fit to call cycles. In view of our introductory remarks in this chapter the reader will realise that most so-called cycles, typical, complete or otherwise,should not be accepted uncritically.

Chapter 2

CYCLES IN FLUVIAL REGIMES

Deposits in fluvial regimes accumulate under different conditions which can be regarded as variations between two extremes. On the one hand, sedimentation in piedmont areas takes place in a number of overlapping alluvial fans. The fans are formed by mudflows, by deposition from sheet floods and heavily overloaded streams. Channels are continually choked with debris and the drainage deflected first one way and then another over the fan. At the other extreme there is sedimentationin the flood plains of large rivers which have well established channels. Deposition is not over-rapid and the main changes are due to meandering of the channel which carries different sedimenttypes across the plain. Under these conditions there tends to be a strong differentiation in the nature of the sediment from the coarse-grained material largely confined to within and near the channels to the much finer-grained sediment of the flood-basin outside. Between the two end members, the meandering river and the alluvial fan, braided rivers form an intermediate type in which there is rather less differentiation of facies than in the case of streams with near-permanent channels. The overloaded river forming the braided stream tends to break into many threads which cover much of the flood plain and so distribute coarse material more uniformly over the area. In so far as fluviatile deposits tend to be almost entirely clastic, and variability rather than patterned organisation is predominant, the recognition of cyclic sedimentation has been somewhat delayed. Nevertheless of recent years cycles in fluviatile deposits have been delineated from a number of successions. Their characteristics and associated problems have been described and discussed by J. R. L. ALLEN(1964a, 1965a,b) who also gave comprehensive bibliographies (see also HARMSand FAHNESTOCK, 1965).

CYCLES IN THE OLD RED SANDSTONE OF BRITAIN

Considering first Old Red Sandstone sediments, J. R. L. ALLEN(1964a) described a number of examples from England and Wales which he referred to as cyclothems. Each of these (Fig.8-13) shows the presence of three features which J. R. L. Allen regarded as essential in the development of this type of cyclothem. In ascending order these are: ( I ) at the base a sharp and scoured surfacesurmounted by(2) a conglomeratic sandstone often with large clasts of the immediately underlying sediments and culminating in (3) a fine-grained bed of siltstone with clays and interbedded fine sandstone. Only two of the cycles (Fig.8, 9) are relatively simple although even these show some

22

CYCLES IN FLWIAL REGIMES

MAIN

FACTS

INTERPRETATION

... Red. coarse siltstone devoid of bedding Sparse calcium carbonate concretions. Invertebrate burrows in lower part. Suncracks absent.

Vertical accretlon deposit from overbank floods. Probably deposlted in backswamp area, perhaps a nmre OT less permanent lake.

Variable thicknesr of red. ripple-drift bedded, very fine sandstone. Grades up into siltstane. Invertebrate burrows.

Vertical accretion deposit from overbonk floods. Possibly a levee deposit or a paint- bar swale filling.

White to purple, fine t o medium, well sorted sandstones. Siltstone clasts concen. troted at base and scattered throughout. Trough cross-strotlfied, units 10-90 cm thick. Contorted cross-strata near base and middle.

Channel deposit probably formed by lateral accretion on a point-bar. Sand trans ported as bed-load over river bed formed into lunate %dunes'. Strong, variable currents. Siltstone closts fwm lag concentrate where channel was deepest.

/

/

Cut on siltstone. Maximum relief 15cn Few directionol scour structures.

Erosion at deepest part of wandering river channel.

GENERAL LEGEND Scoured surface

Ripple-bedded fine to medium sandstone

introfarmational conglomerate

Ripple-bedded very fine sandstone

Cross-stratified coane to very coarse sandstone

Flat-bedded fine to medium sandstone

Cross-stratified fine to medium sandstme

Flat-bedded very fine sandstone

Trough crass-stratifled fine to medium sandstone Ripple-bedded coarse sandstone

n .... .

Massive

medium sand& ne,

Rippled bedding plme

m

Siltstone

El R

Carbonate concretions

Convolute lamination

Contorted cross-strata

~~

Massive very fine sandstone

Invertebrate burrows

Fig.8. Generalised succession and interpretation of Downtonian cyclothem (cycle A) at Ludlow, Shropshire. (After J. R. L. ALLEN, 1964a.)

23

CYCLES IN THE OLD RED SANDSTONE OF BRITAIN

I

MAIN

FACTS

INTERPRETATION

5

11 4

3

Red coarse siltstone dewid of bedding, grading up from very fine sandstone at base. Sandstone lenticle on erosional surface. No proofs of exposure.

Vertical accretion deposit on floodplain topstratum from overbank floods. Bockswanp deposit probably formed in more or less permanent lakes.

Medium to fine green sandstme on

Channel-fill deposit. Plug from longitudiml currents after channel wos cut.

parallel to channel side conccidont on scoured surface. Scattered siltstone

ul 1L

Channel cut

w I

occross

sand bar.

2

I

Medium to very fine green sondstonas with siltstone clask. planar cross-stratified or fiat-bedded and with primary current lineation.

Channel deposit probably formed by loteral accretion on point-bar. Strong variable currents.,, Sand transported in straight-crested dunes". Wave action on beaches exposed at low river Stage.

L a g deposit farmed in deepest parts

of channel. 0

Erosion In deepest parts of wandering river ctmnnel.

Fig.9. Generalised succession and interpretation of Breconian cyclothem (cycle B) at Brown Clee Hill, Shropshire. For legend see Fig.8. (After J. R. L. ALLEN,1964a.)

differences The first (Fig.8) has three members: member I begins with a coarse-grained deposit on top of a scoured surface and passes into a festoon-beddedsandstonewith scattered large siltstone clasts; member 2 is a thin band of ripple-drift-bedded sand; member 3 is a red, coarse, massive blocky siltstone with carbonate concretions and some burrows. Suncracks and plant remains arelacking. Similarly in the second example (Fig.9) there is no evidence of exposure and drying out, while the lower sandstone member is complicated by the presence of a large channel scour. Of the other examples taken as cyclothems each shows more or less deviation from the simple cases. Cycle C (Fig.10) has a number of scoured surfaces in the lower part of the succession and the lowermost sequence is made up of interbedded lenticular sandstones and very fine-grained greenish siltstones which differ from all other lithologies. Above this sequence there are at least two coarse-grained sandstones overlying a scoured surface and the upper part of the cyclothem is occupied by siltstone with thin sandstones. The siltstones have suncracks, burrows and abundant calcite concretions. Cycle D (Fig. 11) is remarkable for the episodes of channelling recorded in the lower sandstone beds; cycle E (Fig.12) has a number of intraformational conglomerates

24

CYCLES IN FLUVIAL REGIMES

MAIN

FACTS

/

INTERPRETATION

Thick red, coarse siltstones with lentides m d persistent beds af very fine. ripdebedded sondstone. Invertebrate burrows d severol horizons. No suncracks. Abundant calcium corbonote concretions.

Vertical accretion deposit from overbank flocds. Mostly backdeposits with c w s e intercoiations representing toes of ievees or crevasse-splays. Conaetirms sugqest fiuctudigroundwater table and exposure.

Thick red. coarse siiistones with lenticles ond persistent beds of very fine to medium sandstones. Suncrocks at three horizons. Sandstones ripple-bedded with sharp, rippled tops. Convolute lamination and dump bails. Invertebrate burrows.

Vertical accretion depusit from overbon floods. AltWMte submergence and exposure. Complex of levee, backswomp and perhaps crevosse-splay deposits. Active river channel ot o distance.

Planar craas-stratified, fine to medium, purple sondstone with contorted foresets locally. Thin siltstone and very fine ripple-bedded sandstone, both lenticular. Scattered siltstone clasts. in:raformalional conglomerate ot base.

Channel deposit proboMy fwmed by iaterol accretion on point-bor. Strong, voriobie currents. Sand carried 09 bed-load in straight-crested "dunes" moving mpidly at times. Conglomerate represents lag deposits formed in deepest ports of chonnei.

Cut on siltstone. Relief low. Smoilscale channels.

Erosion in deepest parts of wanderinq freshwater channel encroaching on tidal river.

Rapid alternation of lenticular sondstones ond siltstones. Sandstones mostly white, came, cross-stratified; sharp bases, Wen erosional. and sharp rippled or smooth tops. Siitstones pale green and unbrdded.

Tidal channel deposit. Variable currents with segregation of dock and moving water. Chonnel floor o complex of mud banks and sand banks covered with 'tiunes".

Cut on sinstone. Relief low.

Erosion at floor

of tidal channel.

Fig.10. Generalised succession and interpretation of Dittonian cyclothem (cycle C) at Lydney, 1964a.) Gloucestershire. For legend see Fig.8. (After J. R. L. ALLEN,

25

CYCLES IN THE OLD RED SANDSTONE OF BRITAIN ~~

MAIN

FACTS

FUlr and coven channd. Red. flat-

runwckod

INTERPRETATION

Channrl-fill and latmral accretion

<StOM.

In form of chonnal. ReIU about 4.0h.

5

Some siltstone.

Fig.11. Generalised succession and interpretation of Dittonian cyclothem (cycle D) at Tugford, 1964a.) Shropshire. For legend see Fig.8. (After J. R. L. LEN,

26

CYCLES IN FLUVIAL REGIMES r

MAIN

2

FACTS

INTERPRETATION

Red coarse siltstones with invertebrate burrows, ripple-bedded sandstone lentlcles, and convolute laminations. No evidence of exposure.

Vertical arcretion deposit from overbank floods. Backswamp area. probably a permanent lake.

Red coarse siltstones alternating with beds or “biscuitd’ of ripple-bedded, very fine sandstone. Invertebrate No proofs of exposure

Vertical accretion deposit from overbank floods. Levee and backswamp deposits with area possibly a lake for long periods.

Red, flat- or ripple-bedded very fine to fine sandstone with a channeled scoured surfoce in lower Dart. Scattered siltstone ciasts.

Probably mixed channel-fill ond lateral accretion deposits. Deposition of suspended and bed loads on channel bars and sand flats. Deepening or wandering of channel at times.

l i 1

.

. . . . . . . . , .. ,._., . ,. ,. . ,. ..,.

’’ I

Intraformational conglomerotes on scoured surfaces alternating with green siltstones and very fine to fins sandstones, showing ripple-bedding, flatbedding or convolute lamination. Concentrations of plant debris and ostracoderms. some af latter articulated.

Scoured surface

of low relief cut

Mixed channel-fill and channel lag deposits. Repeated migration and partial oggradation of channel. Flotsam of floodpialn plants and riverine ostracoderms deposited in or near active channel.

Erosion a t floor

of wandering river.

Fig.12. GeneraIised succession and interpretation of Dittonian cyclothem (cycle E) at Abergavenny, 1964a.) Monmouthshire. For legend see Fig.8. (After J. R. L. ALLEN,

in its lower portion and cycle F (Fig.13) has a notable series of potholes in one surface within the lower half while the lowest sandstone member is exceptionalin having flatbedded horizontal laminae rather than cross-stratification. Some objections might be raised to the way in which these cycles have been delimited. It is clear that no simple set of criteria has been used. For example in cycle D (Fig. 11) the presence of siltstone (9)and the channelling above might have been

27

CYCLES IN THE OLD RED SANDSTONE OF BRITAIN

I

MAIN

FACTS

INTERPRETATION

Verticol accretion dDposlt from overbonk floods. Backswamp deposit with intercalated levee tongue. Fluctuating groundwater table and periodic exposure.

Alternation of thin sandstones and siltstones. Red sandy coarse siltstones with invertebrate burrows and rare carbonate concretions. Very fine to fine poorly sorted sandstones. flot- or rlppls bedded or mossive. Commonly rest on suncracked or eroded surfaces. Tops gradational or sharp with ripples. Invertebrote burrows.

Verticol accretion deposit from overbonk floods. Deposition of suspended load vio bed-load on levees, crevasse splays, and in bockswamps. Repeated scour, aggradation. and expasure of floodplain top-strotum. Flow at times in direction awoy. from eariler channel.

Probably mixed channel-fill and lateral accretion deposit. Deposition of bed-load i n channels, shallow and probably shifting and braided, with some wave action on exposed bonks and bars. Local channel lag deposits.

Erosion at floor of river channel.

wandering

Fig.13. Generalised succession and interpretation of Breconian cyclothem (cycle F) at Mitcheldean, 1964a.) Gloucestershire. For legend see Fig.8. (After J. R. L. ALLEN,

taken to indicate a separate cycle. In this connection J. R. L. ALLEN (1965a) said: “although two siltstones are present, the sequence is considered to represent a single cycle of deposition, because of the essentially uniform palaeocurrents observed and the manner in which an existing facies controlled the deposition of a later one”. In speaking of a single phase of deposition J. R. L. Allen seems to indicate that the cycles have been demarcated in terms of a model based on studies of Recent sediments.

28

CYCLES IN n U V I A L REGIMES

POINT-BAR

CREVASSE- SPLAY

CHANNEL-FILL

- - - - -- -- Fig.14. Block diagram illustrating the development of flood-plain deposits in relation to a meandering 1964a.) channel. (After J. R. L. ALLEN,

Within these deposits a “depositional unit” can result from channel migration which has the essential features mentioned above, the scour surface, the coarse followed by the fine fill. This “fining-upwards unit” J. R. L. ALLEN (1965a) took to be typical of alluvial deposits. But each episode of channel migration is likely to be more or less complicated and variable. Recent-sediment studies suggest that in any one phase a number of different types of deposits may form (see J. R. L. ALLEN,1965b, for comprehensive survey). These are-together with their location of development (see Fig.14): (a) vertical accretion deposits (levee and back swamp); (b) splay deposits (crevasse from channels); (c) lateral accretion deposits (point bar and channel bar); (d) lag deposits (channel); (e) fill deposits (channel). Vertical accretion deposits form away from the channel and consist mostly of rather he-grained material which is spread over the flood plain. The sediments decrease in grain-size away from the levees and predominantly silty or sandy interbeds record increased water flow. Deposition occurs mainly from the suspended load to give horizontal lamination, though the fine-grained sands may be moulded into ripples with cross-lamination. Some of the sands have distinct lower surfaces but their upper margins are gradational. Drying-out periods are common with the development of suncracks, soil profiles and, under suitable conditions, calcite concretions. The sediments are penetrated by plant rootlets and invertebrate burrows. Towards the levees sandy layers become more frequent and in the levees themselves there is a rapid alternation of sand, silt and clay with small-scale ripple cross-lamination very common.

29

CYCLES IN THE OLD RED SANDSTONE OF BRITAIN

Crevasse-splay deposits are not unlike those of the levees but may be coarser in grain and almost entirely sandy and rippled. The sand layers have sharp bases. The deposits form fan-like wedges which spread into the back-swamp areas from cuts in the levees. Lateralaccretion depositsaccumulatewithin the channelas point bars in meanders, or channel bars within the stream course. The deposits are formed from the bed-load in variously sized and shaped cross-laminae. Some are very large laminae related to large dune formation; other form the typical festoon-beddingproduced by accumulating lunate dunes or by repeated scour and fill units. Ripple cross-lamination may be ubiquitous. Active bank erosion produces a scattering of penecontemporaneousclasts throughout the deposits. The size of the cross-laminated units frequently becomes smaller as the channel shoals. Horizontal bedding with primary current lineation on parting planes also occurs. The development of large and small cross-lamination or flat-bedding is determined by the flow conditions (Table 11). TABLE 11 SEDIMENTATfON STRUCTURES OF WELL WASHED SANDS AND SANDSTONES IN RELATION TO PLOW CONDITIONS

(After J. R. L. ALLEN,1964a) Internal structure

Bed surface roughness form

Flow conditions

Small-scale cross-stratification

Small-scale ripples

Low intensity lower-flow regime

Large-scale cross-stratification (sets assembled in cosets)

Large-scale ripples or dunes

High intensity lower-flow regime

Flat-beddingwith primary current lineation

Plane beds with sediment movement

Upper-flow regime

Channel-lag deposits form the coarsest-grained material of the alluvium. They comprisedetrital material from the sourcerocks and clasts derived from penecontemporaneous erosion of the river's own deposits. The large fragments are confined to the bottom of the channel and move relatively slowly, lagging behind at normal or low-water and being moved only at high-water stages. Channel-$11 deposits are found in abandoned channels, the nature of the deposits depending on the nature of the abandonment. If the cutting-off is complete and abrupt then the deposits are mostly fine-grained, arriving from overbank floods and produced by vertical accretion which results in predominantly flat lamination. If abandonment is gradual then the fill may be rapid, coarse material predominates, scour and fill episodes are frequent and cross bedding is very common. The base of any channel is characterised by scour structures which are usually somewhat indefinite in outline. Sometimes no preferred orientation of scours is pre-

30

CYCLES IN FLUVIAL REGIMES

sent, sometimes they have crudely fluted outlines, elongated parallel to the current. The depth of the scours tends to be limited to only a few cm (DOEGLAS, 1962). The model of sedimentation which J. R. L. ALLEN(1964a) proposed and appeared to refer to as one episode of sedimentation is that cycle which begins by active channelling followed by gradual deposition until filling is completed. Both the cutting and the filling are associated with the gradual migration of channels across a flood plain. As the migration proceeds at any one point coarse-grained channel lag deposits will be left on a scoured surface. The lag deposits will be followed by the sandy, festoonbedded deposits of the channel and the point bars. Thesecoarsedepositswill begradually superseded, perhaps with interdigitations of levee and crevasse-splay deposits, by the finer silts of the back swamps. With this model in mind the cycles described above can now be interpreted. Cycle A shows a scoured surface followed by a channel-lag and channel-fill deposits. The ripple-drifted sand above (member 2) may have formed a levee and the siltstones represent back-swamp conditions. But the absence of suncracks and plant remains may mean that the area was almost permanently flooded. The same might be true of the third member of cycle B whose sandstone member is complicated by the presence of a large channel cut-and-fill. The lowermost sequence of cycle C is so distinctive in its lenticular sands and very fine siltstones that J. R. L. ALLEN(1964a) suggested the influence of tides on the lower reaches of a river. Tidal effects are such as to cause ‘rapidchanges in current strength and hence of grain-size. The section above forms a fining-upwards succession with the sandstone member showing well developed planar cross-stratificationsimilar to that formed under straight-crestedas distinct from h a t e dunes. The back-swamp clays above with levee and crevasse-splay sands show numerous phases of drying out, burrowing and calcite formation. Cycle D is remarkable for the channelling phases recorded in the sandstones: cycle E has a number of intraformational conglomerates in the lower portion recording repeated bank erosion and small episodes of channel cutting. In cycle F the parallel lamination of the lower sandstone is reminiscent of beach sands but J. R. L. ALLEN(1964a) contended that conditions were probably very shallow and there may have been some “beach-like” deposition caused by wave action on exposed bars. Each of the individual cycles considered above forms part of a succession of repeated “fining-upwards units” (Fig.15, 16). The examples given from Britain show slight differences though the basic pattern seems to be identical. Several “standard” cycles can be picked out from different parts of Britain (J. R. L. ALLEN,1965a). Some of the cycles have a distinctive vertebrate assemblage whereas others contain lingulids. Others appear to differ in the number and nature of their sandstone bands but in the absence of any clear indication as to how these “standard” cycles are erected it is difficult to estimate the significance of any differences. Similar cycles to those of the British Old Red Sandstone have been recorded from other parts of the world and there seem to be no essential differences between the cycles recorded in Fig.15 and 16 from the Old Red Sandstone of Spitsbergen, the Catskill facies of the Devonian from the Appalachians, the Trias of the Deep River

[

CYCLES IN THE OLD RED SANDSTONE OF BRITAIN

31

...... ...... ........... :.-........

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

...... ........... ....r:.

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

....... ....... .......... ...-...... ........

G

I3 ........ ......

........ ,..... .. ...... ".. ........ ...... ......... ..:.:-, ::*.

F

A

C

E

H

["

D

B

Argilluceow rock

Fig.15. Sequencesshowing "fining-upwards" cycles:A = Lower Old Red Sandstone, Pembrokeshire, Wales; B = Lower Old Red Sandstone, Shropshire,England; C = Lower Devonian, Vestspitsbergen; D = Triassic, North Carolina; E = Triassic, North Carolina; F = Mesozoic, Sweden; G = Jurassic, 1965a.) Arizona; H = Molasse (Aquitanian), Switzerland. (After J. R. L. ALLEN,

Basin, North Carolina, the Kagerod Formation (Early Mesozoic) of southern Sweden, the Salt Wash member of the Jurassic Morrison Formation, Colorado Plateau and the Molasse formations in the vicinity of the Alps (J. R. L. ALLEN,1965a; see also VISHER, 1965; CONOLLY, 1965). The Wood Bay Series (Devonian) of Spitsbergen has cycles essentially the same

32

CYCLES IN FLUVIAL REGIMES

m m

Small scale croci-slralHIcalian Larqe scale aocrclrol(HiCotbn

Sillrlonr

Llmaclonr

Flat-brddlng

..r.wwg.

..-..I

?*% ,;

’.

conplomuate

Fig. 16. “Standard” and “representative” “fining-upwards’’ cycles: A = Standard cycle (thickness 2-15 m), Red Downton-Temeside Shale Groups (Lower Old Red Sandstone), Welsh Borders; B = Standard cycle (thickness 5-10 m), Holdgate Sandstones Group [Lower Old Red Sandstone), Welsh Borders; C = Cycle from Ditton Series (Lower Old Red Sandstone), near Tugford, Clee Hills (thickness of cycle 9.3 m); D = Cycle from Brownstones (Lower Old Red Sandstone), near Mitcheldean, Gloucestershire (thickness of cycle 8.1 rn); E = Standard cycle (thickness 1-11 m) (Upper Old Red Sandstone), Gloucestershire; F = Standard cycle (thickness range uncertain but average probably several metres) (Upper Old Red Sandstone), Clee Hills, Shropshire; G = Standard cycle (average thickness 15 m) (Lower Devonian), Vestspitsbergen; H = Standard cycle (average thickness order of 5-20 m), Catskill facia, Appalachian Mountains region; J = Standard cycle (characteristic thickness 10-20 m), Salt Wash member, Morrison Formation, Colorado Plateau; K = Standard cycle (thickness 2-15 m), Molasse, Swiss Plain and Aquitaine Basin. (After J. R. L.ALLEN,1965a.)

MOLASSE OF SWITZERLAND

33

as those of the British area, with erosion surfaces followed by coarse-grained sandstones passing up into siltstones. So regular is the development of this sequence that FRIEND (1965) ,curiously, suggested that it was not necessary to erect a modal cycle, without apparently realising that the modal cycle is just the obvious, invariate sequence which he described. His descriptions, however, add considerably to the knowledge of scour structures developed at the base of the coarse member. As remarked above, scour structures in stream channels are often irregular or crudely flute-like in shape. Friend confirmed the frequent development of flute markings in the Wood Bay Series but, in addition, recorded the occurrence of grooves, current crescents and a number of “welts”. The welts are apparently scour markings sometimes groove-like and regular, sometimes dendritic and in both cases elongated parallel to the current directions. Friend also included in this group, structures of polygonal type probably resulting from dessication cracks. The Falla Formation in the Beardmore-Shackleton area of Antarctica lies between fossiliferous coal measures dated as Permian below and probable Triassic rocks above. The succession is cyclic and according to MCGREGOR (1965) has a sequence corresponding to a modal cyclothem consisting of (5) Dark grey-green slightly calcareous siltstone and shale. (4) Pale, green-grey fine-grained sandstone, passing down into: (3) Current-bedded medium-grained, mostly buff to pink sandstone. (2) Thin mud-flake breccia. (I) Erosion surface with trace fossils, scour marks and shallow, dendritic channels. Individual cycles show considerable lateral variation and thicknesses which range between 2 and 12 m. MCGREGOR (1965) remarked on the similarity of these beds to those of the Enghsh Old Red Sandstone described above but interpreted the deposits as overlapping and superimposed alluvial fans developed in a slowly subsiding inland basin.

MOLASSE OF SWITZERLAND

BERSIER(1958) conceived a fluvial environment of formation for the Molasse in the Swiss Plain. He envisaged sedimentation in an on-delta region and drew comparison with the Coal Measures cycles. It appears that he may have been wrong in this interpretation and that the fining-upwards sequences are characteristic of a flood-plain environment. DE RAAFet al. (1965) have shown that deltaic sequences have a much more complicated cyclic unit even though coals and seat earths may be absent (Chapter 5). Nevertheless Bersier’s reconstruction is important because it extends J. R. L. Allen’s analyses of the Old Red Sandstone cycles in Britain and underlines the difficulties in delimiting cycles in fluviatile deposits. The Molasse of Switzerland is a very varied succession including marine, freshwater and fluviatile deposits which range in composition from conglomerates to

34

CYCLES IN FLUVIAL REGIMES

coals and limestones. This discussion is restricted to the deposits around Lausanne of Aquitanian age (BERSIER, 1958) although CROUZEL (1957) described similar sediments from the Aquitaine Basin. Bersier interpreted the succession in terms of a model which in the complete development consists o f Lacustrine limestone with coal above. Muddy limestone. Clays or shales. Siltstones. Sandstone with conglomerate, grading upwards. Scoured surface. If, however, sedimentation is taking place under fluviatile conditions then strong lateral variation would be expected (Fig.17). The immediate vicinity of the channel would show the complete cycle beginning with an erosive surface at the base and the succession would record the filling of this channel and the development of the backswamp deposits. But further away from the channel the contemporaneous cycle would be reduced in thickness and would often be lacking any erosive surface with coarsegrained beds. In BERSIER’S (1958) terminology the cycle would be stunted. On the other hand reduced cycles would be formed during the channelling phase and the top of the previous cycle removed to leave a truncated unit. It is also possible that two stunted cycles occurring towards the top of a thick cycle with sandstone and conglomerate would appear simply to be the top of the major one. Such a complex is referred to by Bersier as a composite cycle. The possibilities for complication and confusion in such an environment are very clear and suggest that while individual small sequences such as described above from the Old Red Sandstone are important in indicating the type of sedimentation they are in all probability local occurrences of no great lateral extent. This last consideration is important in discussing the origin of the cycles. The foregoing has emphasised one probable origin for the cycles, that of sediment or channel wandering. With this mechanism a thick varied succession could

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

...........

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

Fig.17. Lateral variation in molasse cycles: c = “composite”cycle formed from a number of individ1958.) ual cycles; n = “normal” cycle; t = “truncated” cycle; r = “stunted” cycle. (After BERSIER,

MOLASSE OF SWITZERLAND

35

have been built up under conditions of almost uniform subsidence. On the other hand DINELEY (1960; see also RICHEY, 1938) envisaged mainly tectonism acting independently or together with climatic changes to cause rejuvenation and changes in sedimentation. J. R. L. ALLEN(1965a) in addition to recognising these possibilities, pointed out that sea-level changes, superimposed on a continuous general subsidence of the depositional area, may have caused alternate periods of erosion and deposition. As J. R. L. Allen indicated it is perhaps premature to decide which one of these explanations is correct. It is our opinion, however, that far-acting tectonic and climatic changes may have had only a regional effect on sedimentation and perhaps determined the character of the deposits. With regard to the cycles, sedimentarycontrol in the form of channel wandering has the attraction of simplicity and economy and we regard this as the most likely cause of individual cycles. Sedimentation changes must take place in any flood plain and, even if an alternative mechanism is preferred, these changes must be allowed for. Once channel wandering is taken into account, we suggest it will be frequently found that the necessity for appeal to further mechanisms disappears.

Flysch facies in molasse

Although many molasse sediments were evidently deposited under fluviatile conditions and exhibit a cyclicity as described above it is important to note that a variety of conditions are represented in sequences which have been grouped under the heading molasse. Thus shallow marine conditions are evident in some successions and another intriguing facies is often described as “flysch in molasse”. In this facies there is a typical alternation of sandstones with shales or marls. The arenites have many of the features typical of flysch; the sandstones are graded, have no strong cross-lamination, have well developed sole markings and so on (see Chapter 9). KUENEN (1959) has described beds such as this from Bavaria-the Deutenhausener Schichten. Theseapparently accumulated in brackish waters and it was tentatively suggested that small turbidity currents formed periodically from a delta front which had built out and cut off the area of sedimentation from true marine conditions. More recently PANIN(1965) reported flysch facies from the molasse of the eastern Carpathians in which the sediments as well as showing turbidite features included a series of trace fossils-footprints of both birds and mammals. In order to explain this paradoxical association of turbidite features (generally taken to be deep-water in origin) and emergent features Panin invoked the operation of “courants turbides de surface” (a name suggested by Kuenen) that is to say flood currents charged with clastic debris which spread over an alluvial area sometimes partly under water, sometimes dry exposed. Panin suggested that the operation of these “turbides courants de surface” (similar currents had been envisaged by CUMMINS, 1958, from field evidence and by DZULYNSKI and WALTON,1963, from experimental studies) could explain the objections that MANGIN(1 962a,b) had raised to the formation of flysch in deep waters (see Chapter 9).

36

CYCLES IN FLUVIAL REGIMES

,1310 GRAHAMSTOWN LAKE FORMATION Varved slltrtonrs. mudstones sondslones and cohglomaratas

6110

4950

ITALIA ROAD FORMATION Chiefly graded ma88ive lllhic orenites. ahales and carbonacrour strata BALICKERA CONGLOMERATE Boulder conglomrrnle wllh tome ocid ignlmbrltr

3600

. Chlefly ondrsitic. dacltlc.to8conllic ond rhyollllc laws and pyrocIas1Ic8

I200

WALLARINGA FORMATION Chlefly red conglomerallc lllhla areniles wlth lhln red and green and tuffaceour h t r r b r d t

0

Chlefly dark mudrlone8.gray iilhic orenllrs, conglomarales and thln

llmrslones I marlnr

Fig.18. Carboniferous succession near Balickera. New South Wales. Thickness in feet. (After RATTI1967b.)

GAN,

Currents such as those envisaged by Panin may have operated in the formation of sediments of continental origin in New South Wales, Australia, which show a number of different cycle types (RATTIGAN,1967a,b). The succession consists of a greywacke (Burindi) facies followed by a molasse-type, fluviatile facies represented by the Wallaringa Formation (Fig.18). The Gilmore Volcanic Group is then followed by the King’s Hill Group which is succeeded in turn by another volcanic sequence of Permian age. The King’s Hill Group shows an overall grading from the Balickera Conglomerate through the Italia Road Formation comprising sandstones, shales and some coals and at the top, the Grahamstown Lake Formation, a set of varved, lacustrinesedimentswith some conglomerates. The last formation was clearly affected by nearby glaciation and tillites, dropstones and exotic boulders are fairly common among the varves. The Italia Road Formation presents an interesting series of cycles (Fig.19). The formation can be divided up lithologically into ten members and there is some variation in cycle type from member to member. The modal cycle of member M Zhas three

31

MOLASSE OF SWITZERLAND

New cycle initiated

B Sandstone. fine. laminated and

L

1150

riDDIe d r i f t bedded

1055

A Sandstone. massive. medium grainad

Old cycle compieted

New cycle initiated 2

' 7

CYCLOTHEMS

11

L

4 I-

z

Succession similar to that in (b) but upper uniis are thin-

U

n

CYCLOTHEMS

0 4

r! J 4

Old cycle completed

t

New cycle initiated C Kaotinllic shales with block car-

--I80

bonaceous. piant bearing pyritic shales and coal lenses. Plants preserved in situ.

CYCLOTHEMS

B Sandstone, fine. laminated and ripple drift bedded. A Liihic sandstone, medium to coarse. graded, massive. Angular intra formational phenoclasts ond rounded exotic phinociosts at base.

CYCLOTHEMS

1

20

0 Old Cyclb c o m p l e i ~ d (0)

(b)

Fig.19. Cyclothem developmentin the Italia Road Formation, New South Wales. Thickness in feet. (After RATTICAN, 1967a.)

units, graded massive sandstone (A), fine-grained, laminated and ripple-drifted sandstone (B), and (C) a sequence of thin beds of shale, seat earths, and carbonaceous beds which sometimes form an inferior coal. The cyclothem figured in Fig. 19b is an actual cyclothem and variations of course from this are found in M2. M3 has thinner B units and generally thicker A and C units: M4 has very thin C units and B units tend to be absent. M5 cyclothems resemble those of M3, M7-8 are like those of M2-4, and M Scycles have no C units. Occasional ash bands occur sporadically in C units throughout the succession.

38

CYCLES IN FLUVIAL REGIMES

At first sight the cycles resemble the fining-upwards units of the Old Red Sandstone described above but the sandstone units (A) have a number of distinctive features. They are graded, homogeneous poorly sorted sandstones in which there is no sign of lamination or cross-lamination. In this they contrast with the fluviatile “fining-upwards cycles” which can be seen for example in the underlying Wallaringa Formation. It is possible according to Rattigan that the A-type arenites might have formed as fluvial sandsheets produced by flash floods sweeping over flood-plain swamps and the B and C units formed during and after the waning of the flood. On the other hand if the similarity with turbidite structures is meaningful the outwash may have formed a “courant turbide de surface” as envisaged by PANIN(1965). RATTIGAN(1967a) also proposed that the outwash may have formed a “diving flash flood” which entered a shallow lake as a turbidity current. The B units would then have formed from slight bottom traction currents in the shallow waters on top of the turbidite and the C units similarly in very shallow-water conditions which occasionally formed swamps with soil and peat formation.

FLUVIO-LACUSTRINE COAL-BEARING SEQUENCES OF GONDWANALAND

Evidence of continental drift and the existence of Gondwanaland, a large land-mass in the Southern Hemisphere, has been claimed from the similarities in Upper Palaeozoic-Mesozoic rocks in India, Africa, South America, Antarctica and Australia. In the general area sedimentation during this period of the earth‘s history points to a change from a glacial to a warm-temperate climate and finally to desert or tropical conditions. Glacial deposits (Carboniferous-Permian) include tillites and varvites while continental sequences, in particular of Permian age, often have a coal-bearing facies. Cycles containing coal are treated in detail in Chapters 4 and 5 but those described below are different; they seem to have been formed under fluvio-lacustrine conditions and, for the most part, the coals are considered to be of “drift” rather than “in situ” origin. Such cycles can be found in other systems, for example, in the Cretaceous rocks of Nigeria (DE SWARDT and CASEY,1963). In India the coal-bearing facies is best developed in the Damuda System of Lower Gondwana (Lower Permian) age, where in 6,000 ft. of beds occurring in the eastern coalfields of the Damuda Valley, some 50-60 cycles have been recorded (KRISHNAN, 1956; RAO, 1964). Cyclicity is best developed in the Barakar Series where the sequence sandstone-shale-coal is repeated many times. In the Jharia Coalfield, for example, 25 seams (30 according to MEHTA,1964) were said to occur in 2,500 ft. of beds. The environment of deposition was considered to be of “piedmont type” (JACOB,1952) or fluviatile (KRISHNAN, 1956; RAO,1964) with peats having been formed from drifted vegetation accumulated in marshy depressions. NIYOGI(1964) considered Barakar sedimentation in terms of intermontane depressions, with piedmont alluvial sheets grading and interfingering into flood-plain and marsh deposits. Most

39

COAL-BEARING SEQUENCES OF GONDWANALAND

previous authors emphasised the presence of sandstone roofs to the coals but Niyogi gave the representative cycle as coal, carbonaceous shale, kaolinitic siltstone, and laminated micaceous shales with channel sandstones. There seems to be general agreement that the coal facies indicates a warm humid climate and that the cyclicity of the sedimentation is due to intermittent subsidence of the area of deposition (see Chapter 5 for discussion of this mechanism). TABLE I11 DMSIONS OF PART OF KARROO SYSTEM IN SOUTH AFRICA AND SOUTHERN RHODESIA

(After BOND,1952a) South Africa

Stormberg Series

Southern Rhodesia

Ape

Basalt Series

Jurassic

Forest Sandstone Pebbly Arkose Mudstone Group Escarpment Grit

Upper Trias/Rhaetic Upper Trias

Beaufort Series

Madumabisa Shales

Upper Permian

Ecca Series

Upper Wankie Sandstone Black Shale and Coal Group Lower Wankie Sandstone

Lower Permian

Glacial Beds

Upper Carboniferous

Dwyka Series

In South Africa, more or less similar environmental conditions have been envisaged for the Ecca Series of the Karroo System (Table 111) although the coals are regarded as having originated “in situ”. Literature on any cyclic aspects of the sedimentation, appears, however, to be lacking. The Gwembe Coal Formation (of Ecca age) in Zambia accumulated as a lacustrine deposit following glacio-fluvial conditions (TAVENER-SMITH, 1962). Coal seams (of “drift” origin) are supposed to have developed either on top of sandstones (formed at the margin of the lake basin) or on emergent mudbanks within the lake. The resultant muddy facies of the latter case gave rise to seventeen cycles in 40 ft. of strata at Nsanje, coal, coaly mudstone and carbonaceous mudstone constituting the rhythmic sequence. Despite a gradational change upwards from the coals the cessation of peat-forming conditions was considered to be due to sudden, spasmodic increases in the rate of subsidence. The extreme lateral variations were ascribed to the fact that “recurrent warping that caused the inception and termination of swamp conditions was local in effect.” (TAVENER-SMITH, 1962, p.56). Our doubts about this type of conclusion in the face of extreme lateral variations are expressed in Chapter 5 (pp.149-152). In the eastern Sebungwe area, Southern Rhodesia, about 100 ft. of tillites and varved sedimentsoccur below the Lower Wankie Sandstone (Table III). BOND(1952a,b) claimed that a well developed sunspot cycle of 12years could be detected in the varvites

40

CYCLES IN FLUVIAL REGIMES

(cf. p.61) but did not comment on any repetitive sedimentation in the Black Shale and Coal Group above. He emphasised that throughout the accumulation of the Karroo rocks, a major climatic cycle, from glacial to arid, occurred. The type of sedimentation was regarded as being partly due to this climatic change but intermittent, relative movements of the source areas and the basin of deposition were considered responsible for major recurrent changes in the type of debris washed into the basin. Four cycles were recognised (Table 111); the first comprised the Dwyka glacial deposits, the Lower Wankie Sandstone and the Black Shale and Coal Group; the second, the coarse Upper Wankie Sandstone (its base is marked by a non-sequence) and the Lower Madumabisa Shales (which in places have a coaly facies, BOND,1955). The third cycle-the pebbly Escarpment Grit and the finer, sometimes marly and sandy beds of the Mudstone Group-is followed by a fourth cycle-Pebbly Arkose and Forest Sandstone. Breaks in sedimentation, separating each of these cycles, were detected at the edges of the depositional basin. Each cycle is of the order of hundreds, sometimes even thousands, of feet thick and is regarded as being due to periodic, relative sinking of the basins with rejuvenation of the drainage systems which brought sediment into them. Gondwana sequences in Antarctica are generally referred to in terms of the so-called Beacon System (HARRINGTON, 1965), which can be divided as in Table IV. In certain areas, and in particular parts of the succession, the cyclic nature of the sedimentation has been recorded. As has been described already (p.33) MCGREGOR (1965) wrote of it in the Falla Group. In the underlying Buckley Coal Measures, however, he evidently did not consider it important and A. D. ALLEN (1962) stated specifically of the Bastion Coal Measures, that the coals did not occur in rhythmic sequences. GRINDLEY (1963), however, hinted at the presence of cycles and BARRETT (1965) described in the Queen Maud Range a composite sequence from his “unit C”-some 800-900 ft. of coal-bearing sediments presumed to be of the same age as the Bastion and Buckley Coal Measures. It begins “with the deposition, often on an eroded surface, of well-rounded quartz pebbles and light grey, trough cross-bedded, coarse TABLE IV SUBDMSIONS OF UPPER BEACON SYSTEM

Central Victoria Land

Jurassic

9

Jurassic Sandstones Triassic Plant Beds

Triassic

8

Upper Permian

0s Bastion Coal

Penno-Carboniferous” ? Upper Carboniferous

“

b

z

Measures ? Not deposited

Southern Victoria Land

Queen Maud Range

Falla Formation Buckley Coal Measures Mackellar Formation Pagoda Tillite

Unit D? Unit C Unit B Unit A

COAL-BEARING SEQUENCES OF GONDWANALAND

41

sandstone. The grain size decreases upwards and cross-bedding is replaced by microcross-lamination. Fine sandstone grades up into light and dark laminated shale, then to coaly shale and finally to coal. None of the cycles examined has all the features and lithologies of the cycle described above: in some cycles deposition began with microcross-laminated, fine sandstones,and in many the upper strata have been eroded away.” (BARRETT, 1965, p.352). While there is therefore some doubt as to the importance of cyclicity in the sedimentation of the coal-bearing facies of the Beacon rocks most authors seem to be in broad agreement as far as the environment of deposition is concerned. In general, from the Upper Carboniferous onwards, it appears that the temperature became less severe, as glacial conditions gave way to a more temperate climate, though the presence of coals may not imply warm-temperate or subtropical temperatures. Glacial varves within the coal measures in the Queen Alexandra range indicated a relatively cool, probably cold-temperate, periglacial climate during coal formation in that area (GRINDLEY, 1963). Most Upper Beacon sediments are taken as indicating deposition in fluviatile and lacustrine environments. Coals in general are considered to have been formed, mainly from transported vegetable material, in lake, lagoonal and marginal swamps. BARRETT(1965) described the fine-grained sediments of units A and B (Table IV) as having been deposited under lacustrine conditions. The coal measures (unit C)marked a change to a fluviatile environment-probably a broad, low-lying aggradation plain. Most of the carbonaceous material had accumulated in small ephemeral lakes and backwaters, though marginal swamps occasionally existed for some time, resulting ultimately in more coaly horizons. Barrett’s interpretation of the coal-bearing cycles is of note as it may well be applicable to the origin of other Gondwanaland coal-bearing sequences. “The vertical change in grain size and bedding features in cyclic units of the Cape Surprise Coal Measures shows that current velocities progressively decreased during the deposition of each cycle. The change from quiet waters to swift currents at the beginning of each cycle seems to have been rapid, for even where no erosion is evident the change from dark shale to sandstone occupies no more than a few inches of strata. Features of the cycles are consistent with the initiation of each cycle by “crevassing” when the nearby river overflowed its banks, inundating much of the surrounding country. Scouring occurred where the current was strongest, but elsewhere deposition was continuous. As deposition proceeded, current velocities were reduced until lakes and marshes once more covered most of the area” (BARRETT, 1965, pp.361-362). On the other hand, the change in lithology is consistent with the “fining-upwardsyysequence of J. R. L. ALLEN(1964a) as described in detail earlier in this chapter (p.23). It is interesting to speculate on the reason for the plant-bearing flood plains of Permian times in Gondwanaland and the barren nature of similar plains in Old Red Sandstone times in the Northern Hemisphere. Was it mainly due to climatic differences or had plants not evolved sufficiently to adapt to the environment envisaged, or to a combination of these factors?

42

CYCLES IN PLUVIAL REGIMES

In Australia, rocks of Gondwana age are conventionally referred to in terms of Carboniferous, Permian, etc., ages. Upper Carboniferous sequences display tillites and vanrites and some interesting new features of the succession in New South Wales have recently been revealed (see p.36). It is possible to divide the Permian rocks of New South Wales as in Table V (BOOKER, 1960),though it should be emphasised there is considerablelateral variation in thickness and lithology. TABLE V DIVISION OF THE PERMIAN ROCKS OF NEW SOUTH WALES

Group

Maximum thickness

(ft.) Newcastle Coal Measures Tomago Coal Measures Maitland Group Greta Coal Measures Dalwood Group

1 4,000

6,000 1 6,000

Sediments of the Dalwood and Maitland Groups (mainly sandstones and shales) contain marine fossils in places and these groups are generally considered marine in contrast to the non-marine Coal Measures. The Dalwood-Greta and the MaitlandTomago-Newcastle Groups have, in fact, been considered to represent two major rhythms, tectonic in origin, of marine and fresh-water sedimentation in an embayment of the Tasman geosyncline (BOOKER, 1960). Cyclic sedimentationin the coal facies was first recognised in 1953in the Tomago Group of the Singleton-Muswellbrook Coalfield. Since then it has been commented on in other Coal Measures Groups and in various coalfields (e.g., BOOKER et al., 1953; BOOKER and MACKENZIE, 1953; VEEVERS, 1960; BOOKER, 1960). In all cases the cycles appear to consist mainly of a fining-upwards sequence-conglomerate or “greywacke”, sandstone, siltstone, mudstone and coal, though because of the fluviatile nature of the coarser sedimentationlateral variations are very common. While BOOKER (1960) drew no definite conclusions as to the origin of the cycles, the evidence of glaciation during Permian times led him to suggest that meltwaters, during periods of ice recession, might have governed sedimentation in lakes where vegetational debris also accumulated. In the marine groups BOOKER(1960, p.51) recognised cycles of conglomerate, sandstone and shale, which passed into sandstone-shale-limestone cycles, with increased depth of water and distance from source areas. The evidence for these is, however, confusing and contradictory (see, for example, BOOKER,1960, p.22 and pp.32-36). Two regressions of the sea were identified in the Maitland Group comparable with

WITWATERSRAND SYSTEM OF SOUTH AFRICA

43

those which took place before the deposition of the Greta and Tomago-Newcastle Coal Measures. Epeirogenic movements were invoked to account for the cyclicity in the Tomago Coal Measures of the Howick area (VEEVERS, 1960). Three major cycles were recognised but the basis for their erection is not clear. The minor rhythms of major cycles 1 and 3, when complete, comprise “greywacke”-siltstone-shale-coal-shale. Major cycle 2 has thinner and less regular minor cycles. The Newcastle Coal Measures and their approximate equivalents, the Illawarra Coal Measures of the southern Coalfield, have been analysed using the methods of Duff and Walton (see pp.9-11 and 133-134). Cycles in the top part of the Illawarra Measuresincludetypes comparablewith the fining-upwards sequencesmentioned earlier (p.25). In addition, there are cycles, composed of fine, then coarse, then fine-grained beds, which can perhaps be compared to those of the European Coal Measures (Chapter5). Dum (in preparation) therefore consideredthat both flood-plain anddeltaic environments are represented in the successions and suggested that the cycles were probably the result of normal sedimentary mechanisms complicated by the periodic inflow of glacial meltwaters. In the Newcastle Coal Measures, however, conditions are different, with thick conglomerates and tuffs appearing as important features of the succession. Volcanic and tectonic activity is known to be contemporaneous with sedimentation in the area and the hinterland was undergoing glaciation. Such factors rendered it difficult to assign any one mechanism as the most likely in the formation of the cycles.

WITWATERSRAND SYSTEM OF SOUTH AFRICA

The Precambrian Witwatersrand System of South Africa is well known primarily for its gold-bearing conglomerates. Agreement has been reached neither on the origin of the ores nor on the environment of deposition of the sediments forming the System. As will be seen presently some authors regard the sediments as marine but others maintain a continental origin and it is because of the latter that the subject is dealt with at this stage. Some 25,000 ft. thick, the System can be split into Upper and Lower divisions. Lower Witwatersrand Beds (1 5,000 ft.) consist essentially of alternating quartzites and shales. The Upper Witwatersrand Beds, on the other hand, apart from one important shale horizon-the Kimberley Shale-comprise alternating quartzites and conglomerates. Within the matrix of some of the conglomerates gold, uraninite and other ore minerals occur. When the succession is considered as a whole (Fig.20) there is a general coarsening upwards and within this general change of grain-size major oscillations can be picked out by the alternations of groups of beds. In the Lower division, shale-dominated groups alternate with quartzitic groups and in the Upper Beds the finer groups (apart from the Kimberley Shale) are quartzites and these alternate with conglomerate-

44

CYCLES IN FL.UVIAL REGIMES

dominant groups. SHARPE (1949) referred to these large-scale oscillations as cycles and used the term cyclothem for the smaller-scalealternations of quartzite-shale (Lower Beds) and quartzite-conglomerate couplets (Upper Beds). Alternate rising and sinking of a marine area was invoked as apossible mechanism for the development of the cycles (SHARPE, 1949). Beginning at the shale horizons (e.g., Jeppestown and Kimberley, Fig.20) a period of uplift was succeeded by sinking and submergenceunder a transgressing sea. Sharpe saw the gold-bearing conglomerate (banket) as the result of working along the strand-line of this transgressing sea leaving resistant pebbles and paystreaks arranged parallel to successive shorelines (Table VI). The minor oscillations ((‘cyclothems”) recorded in alternating conglomeratic groups and quartzites according to Sharpe are the result of sedimentation under oscillating tectonism but without the long breaks in sedimentation represented below the main, economic conglomerates. As well as shoaling of the basin, elevation and erosion of the source area has been held responsible for the coarsening of the sediments arriving in the basin. Each cycle can then be related to a cycle of erosion during which the uplifted source is gradually reduced with concomitant reduction in grain size of the sediments (R. Borchers, quoted in: VISSER,1957). WIEBOLS (1955) denied the necessity for repeated tectonic movements either of oscillatory (up and down) character or of spasmodicsubsidence. He saw the generation of the cycles as the result of successive episodes of glaciation on a continuously subsiding basin. The basin was covered with a large inland sea which was periodically covered by spreading ice. This ice was responsible for the development of conglomerates as ground moraine. Some of this debris can be seen little modified in the coarse, poorly sorted conglomerates below the main bankets known as “Puddingstone”, Snowstorm Rock, Bastard Reef, etc. The ore-carrying beds differ in the concentration of the ore minerals, their better sorting and greater wearing of the included pebbles. These features Wiebols attributed to reworking of the till by a transgressing sea. The transgression would be expected during the interglacial period when the ice melted and retreated. The elongation of the paystreaks could have arisen by concentration of the ores in subglacial streams during the development in eskers. Associated with the conglomerates are some finely laminated rocks which Wiebols interpreted as varved sediments, although A. A. Truter (quoted in: WIEBOLS,1955) denied that these were varves pointing out that the laminae were lenticular with sharp margins. The sediments can also be interpreted as continental in origin, and the orientation of the paystreaks and of the long axes of pebbles has been regarded as the result of accumulation on a braided alluvial plain (REINECKE, 1927). Channellingwould of course be an essential feature of this environment. Pebble and paystreak orientation suggested sedimentation from the northwest, spreading out into the south and east (REINECKE, 1927). The orientation of current bedding substantiated these directions but the crests of transverse ripples were also found to lie in a northwest-southeast direction (HARGRAVES, 1962). Hargraves therefore postulated tidal in addition to river action, the tidal movement producing the ripples and moving in a direction southwest towards northeast. G.W. Bains (quoted in: LIEBENBERG, 1955) thought that movement

TABLE VI LITH0UX;IES AND INTERPRETATION OF MAJOR CYCLJ3.9 IN WITWATERSRAND SYSTEM ACCORDING TO SHARPE

(Adapted from SHARPE, 1949) -

(G) Hangingwall quartzites and Leader Reefs Leader Reefs having more than localsignificance andhigh payability, These may have been formed in areas where temporary reemergence of the d e position area has occurred.

P

1 2

Coarse grained quartzites and grits having local

developments of narrow pebble beds, containing generally small pebble sizes. Reconcentrated material from the shore line, carried over sands already deposited by currents and tides, may give rise to local payability in these Hangingwall Reefs or “Leaders”.

Conglomerate deposits containing a predominance of well rounded quartz pebbles. Concentration of heavy minerals and gold. Carbon usually in isolated granular form, but sometimes in narrow seams along bedding planes in which the carbon may have a columnar structure normal to the bedding. Unconformable plane of deposition (E) Channel and lagoon deposits (2) Lagoon deposits (c) Argillaceous beds often h e l y laminated of Meandering stream channel d e posits intersecting Lagoon type colours varying from blue-black through grey to beds, and occasionally Lagoon khaki. The mineral chloritoid is highly characteristic, beds overlying earlier stream sometimes forming an interlacing network of crystals, beds. this mineral may, however, be absent. (b) Hybrid type rock consisting of scattered pebbles and quartz grains in a dark grey to black or khaki coloured argillaceous matrix. This rock type has been variously termed “Snowstom Reef”, “Bastard Reef‘’, “Hybrid Reef”, “Puddingstone Reef”, etc. (a) Occasionally a basal pebble accumulation with overlying quartzites. (I) Channel deposits Coarse textured sediments generally of quartzite, grits, and extremely irregular lenticular conglomerate deposits. Angular and rounded pebble types occur. Local payable horizons in both quartzites: “pyritic quarzites” and conglomerate groups. Carbon: generally in granular form. (F)

Economic deposits

(0) Conglomerate be&

Irregular beds of conglomerates containing both angular chert and rounded quartz pebbles, local payability sometimes OCCUTS.

(C) Siliceous quartzites with

These quartzites frequently have a greenish tinge and are characterised by irregular angular chert pebble accumulations.

scattered angular cherty

Fine grained quartzites, argillaceous near the base, forming a transition phase to the shale beds, but becoming more siliceous with increasing distance from the shales. Transition phases contain characteristic micaceous laminae.

(A) Shale be&

Laminated grey to dark grey shales with intercalated beds of fine-grained argillaceous quartzites.

WITWATERSRAND SYSTEM OF SOUTH AFRICA

47

directions were probably rather complicated. He suggested that the paystreaks should converge in a downstream direction (rather than diverging as REINECKE, 1927, had supposed) in which case sediment movement during Main Reef cycle was from east to west. Supporting evidence for this movement comes in the observed decrease in pebble size and current bedding. During Kimberley times, however, the direction was reversed and sediment was moved from west to east. It was further pointed out that the tendency for the ore minerals to be concentrated towards the base of the conglomerates was typical of fluvial rather than beach sedimentation and according to Bains only one (Dominion) reef has concentrations of ore in the position expected of beaches. Furthermore wave action tends to concentrate heavy minerals in thin narrow streaks parallel to the beach but with very little branching. The parallelism is there in the ore deposits but branching is a general feature. REINECKE (1927) invoked earth movements for the increase in grain-size from the Lower to the Upper Witwatersrand Beds and for the presence of unconformities (as well as channelling) below the main productive horizons. The widespread distribution of the conglomeratic bands at different horizons he attributed to the sudden draining of lakes dammed by glaciers or landslides. Thinning of, and overlap within, the succession suggested to ANTROBUS (1956) that the original basin of sedimentation was not much larger than the present distribution of the rocks. Taking the sediments as continental he therefore made comparison between the Witwatersrand Basin and sedimentation in the Basin and Range province of the U.S.A. Episodes of sedimentation were probably controlled by tectonic movements of both source and basin. In particular, the auriferous deposits represent periods of stillstand when coarse material was left as a thin, widespread layer over a pediment surface developed on the outer margins of the Witwatersrand Basin. Presumably renewed movement of both “basin” and “range” would cause renewed sedimentation which might have taken the form of a series of coalescing alluvial fans. Evidently a considerable amount of sedimentological data is required to decide the probable environment of accumulation of the Witwatersrand Beds but there seems to be general agreement (with the exception of those advocates of glacial controls) regarding the presence of at least a disconformity at the base of the main cycles. It may be therefore that these are examples of tectonically controlled sedimentary oscillations. The smaller secondary cycles (“cyclothems” of SHARPE,1949) may also be tectonically controlled but glacial control cannot be entirely ruled out. They may, on the other hand, have been produced by periodic flooding as REINECKE (1927) suggested but it seems unnecessary to invoke special causes of flooding like dambreaking when changes in grain-size in alluvial fans are at the present time obviously the result of spasmodic storms.

This Page Intentionally Left Blank

Chapter 3

CYCLES IN LACUSTRINE REGIME§

INTRODUCTION

Probably the most obvious of all rhythmic accumulations are the glacial varved clays of the Pleistocene, described notably from northern Europe and North America. The similarity of some ancient sediments to glacial varves has been noted by a number of authors (e.g., COLLINS,1913; CALDENIUS, 1938; PETTIJOHN, 1957) but, because there has been little detailed work on these older rocks and because they illustrate no additional principles, attention here will be confined to the Pleistocene sediments. A number of rhythmic accumulations of non-glacial lacustrine conditions will also be considered. Both types of layering, glacial and non-glacial, appear to give information regarding annual pulses of sedimentation. This is in contrast to all the other cycles, discussed in this book, whose period in terms of years is not known or at best only hinted at. Moreover the individual varves differ (especially in thickness) from one another and there are suggestions of longer-term variations, whose period can be measured in years. Other long-term variations are also suggested by larger cycles made up of different clastic units. Since climate appears to be one of the major factors controlling sedimentation the variations appear to record periodic meteorological changes. A number of such periods have been proposed (especially the tantalising 1 1-year sun-spot cycle) on the simple basis of inspection and subjective matching up of the successions. Rigorous methods of testing (Chapter 1) may in time allow more precise conclusions regarding the postulated periodicities.

GLACIAL VARVED CLAYS

In their simplest rhythmic development successions of clays formed in glacial lakes consist of numerous couplets of sediment. One part of the couplet is coarse sediment (usually fine sand or silt) and the other portion fine-grained, of clay grade. Each couplet makes up the rhythmic unit or varve. lndividual varves are usually several centimetres in thickness but some reach a metre whereas others may be only a few millimetres. The last, the micro-varves, are of doubtful significance since they may be formed in the lake simply as laminae rather than as the deposit of one year. It may be recalled that the term “varve” was introduced by DE GEER(1912) to refer to a layer of sediment formed during one year.

50

CYCLES I N LACUSTRINE REGIMES

The couplets are repeated through tens, exceptionally hundreds, of metres of succession; in horizontal extent they are variable. Some, for example in Sweden and Finland, cover hundreds of square kilometres; others, particularly many of the Danish examples, are quite restricted (HANSEN, 1940). Clearly the main factor in this spread is the size of the host lake which varied from water bodies comparable in size to the present-day Baltic Sea to the tiniest lakes (for example the Plateau-Hill Ice Lakes in Denmark, HANSEN,1940), only a few hundred metres long. Thinning of the varves away from the source is general and can be seen when tracing individual varves or sequences laterally (Fig.2 1). Similarly thinning of varves occurs in vertical succession since successive varves are formed as the ice sheet retreats. Usually the change is slow but the rate of decrease in thickness may be accelerated by different current-strengths, distribution patterns and salinities. Some of the Finnish varves have an extent of 100 km or more but others may be restricted to 25-50 km, the difference in the latter being ascribed to flocculation and rapid sedimentation in relatively saline waters (SAURAMO, 1923). Flocculation in marine conditions may inhibit the development of varves entirely so that it appears that waters with well developed varves were fresh. In this connection, the deposits are essentially unfossiliferous although there are a restricted number of reports of invertebrates, especially bivalves and also, rarely, fish (HORNER,1948).

Thickness

of Vorves

(4

Distance from ice-margin

(miles)

Fig.21. Curve showing decrease in thickness of varve-sequence away from the ice margin in Lake Barlow, east Canada. (After ANTEVS, 1925.)

The varve usually considered in geochronological studies and made famous through de Geer’s classic studies (DE GEER,1940, has a full bibliography) is usually of the type just described, that is with a coarse layer below a fine-grained clay layer (Fig.22a). It will be convenient to refer to the lower layer as S (usually sand or silt) and the upper layer as C (clay). Within this simple type there are a number of possible variations--and all of these occur naturally (Fig.22). One variety shows a sharp base against the underlying clay layer, a near-perfect gradation in size from the

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

rrlm . ... .. .. .. .. .

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

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

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

A

0

C

D

GLACIAL VARVED CLAYS

51

........:< ...:.......:<;.

......

E

....... -. -.

F

... ... ... ..

G

Fig.22. The structure of different types of varves: A-E = diatectic varves; A = graded; B = graded, sharp junction with clay layer; C = very little gradation, sharp junction; D = thin coarse lamina in middle of summer layer; E = thin transition zone at base of graded layer; F = symmictic varves; C = composite varves.

light-coloured S-layer to the C-layer above. A second variety has an upward gradation but quite a sharp junction between the S- and C-layers of the varve. A third type shows little or no gradation within the S-layer and a sharp contact with the C-layer. Or, the S-layer may have a sharp lower contact but a thin, coarser lamina may appear in the middle, or even the upper part, of S (HORNER, 1948). There may be a thin transition zone at the bottom of the S-layer and yet another variety has little or no separation of the constituents according to grain-size apart from a very thin coarser layer right at the base. The last varves are generally thicker than the previous types. All of these varieties can be included in the simple type of ANTEVS (1951). The type-I3 varves of HANSEN (1940) are generally to be referred to this simple type. Where there is some separation of the grain-sizes and an overall gradation then the varves are diatectic; where virtually no separation can be seen then the varves are symmictic (SAURAMO, 1923). (1951) simple varves may also contain distinct laminae of According to ANTEVS silt or sand in the S-layer and of clay within the C-layer. The laminations can be of differing grain-size or of differing colour, the essential point being that although laminations occur, clay laminae do not appear in the S-layer and vice versa. But not all varves are of the simple type. S-layers may contain subsidiary laminae of clay and what appear to be distinct C-layers may have occasional thin laminae of silt or even, rarely, sand. Where these features are found then ANTEVS(1 951) would refer to the varve as composite. It might be noted here that the laminations referred to are almost invariably parallel, crossbedding on the scale of ripple marking is very rare but occasional contorted zones are

52

CYCLES IN LACUSTRINE REGIMES

Fig.23. A. Simple diatectic varves. B. Microlaminations in Danish clays. (After HANSEN, 1940.)

GLACIAL VARVED CLAYS

53

found. Composite varves are usually thicker than the simple type and the clay laminae within the S-layer produce thin couplets (micro-varves) which have a structure essentially the same as the simple varves. HANSEN (1940) recognised that an extreme development of these micro-varves in the Danish clays could lead to laminated but unvarved sediments (Fig.23). The varves with some lamination in the C-layer Hansen separated off as a “zonary” type but almost all of these could be included in the “composite” varves. A further class, distinguished by exceptional thicknesses has been called “drainage varves” by ANTEVS (1925). These units owe their thickness to the heavy supply of material resulting from a change in drainage following, for example, the failure of an earth or an ice dam. None of these classification systems is entirely satisfactory but it will be sufficient to use the terms simple and composite in the sense of Antevs. Grain-size analyses confirm the gross characters just described. Graded S-layers have been described by HORNER(1948) and W. J. EDEN(1955) although the latter author was impressed by the general lack of such gradation in the Steep Rock Lake Clays in Canada. The sorting is generally good and so far as the restricted data allow any conclusion the skewness is negative. Chemically a distinctive feature of most varves (some Danish varves are exceptional, HANSEN, 1940) is that the S-layers contain higher proportions of CaC03 and MgC03 than the C-layers.

Periodicity The well established view that the rhythmic couplets in glacial lake successions record annual pulses of deposition is based on a number of lines of evidence: ( I ) Apart from the change in thickness from proximal to distal parts the varves are extremely regular in thickness; there is no evidence of strongly variable currentaction. This suggests periodic supply of detritus to the basin rather than fluctuating current action within the basin. (2) The two layers, coarse and fine, suggest two different epochs; the one, shorter in length when the sand or silt arrived, the second a longer time when the clay particles slowly settled. ( 3 ) Bearing in mind the probable source, the supply and the distance of travel of the detritus, diurnal changes appear to be too short to produce the varves, whereas the year seems to be of the right order of time. This contention is supported by evidence from Recent sediments. The deposits in Lake Louise in the Canadian Rockies, for example, possess laminations with thicknesses corresponding to annual amounts of debris delivered to the lake from the feeding Victoria Glacier (JOHNSTON, 1922). ( 4 ) Perhaps the ultimate vindication of the view that the couplets are annual lies in the agreement of De Geer’s varve chronology and the chronology of the Pleistocene derived from I4C dating (HULTDE GEER,1951) (Table VII). That there should be no exact correspondence between the results from the two methods is not really surprising. When varve counting is considered it will be appreciated at once that a number of

54

CYCLES IN LACUSTRINE REGIMES

TABLE VII

RADIO-CARBON AND VARVE DATING (After HOLMES, 1965) ~~

Varve counts

Radio-carbon dates (All 150-200)

_ _ ~ _ Finland

~

Britain

~

w 1Ic

f 10,093

10,260

‘‘I Latest cirque Younger Dryas (cold) glaciers of Valders advance in upland North America regions ~~ ~

~~

10,760

1 10,213

10,075 10,175

Norway 10,500 10,880

~-

{

~

10,880

l0,SOO ~~ ~

~~

~~

Allernd (warm) Two Creeks interval in North America 11,960 _ _ _ W IIb Older Dryas Highland (cold) readvance > 12,800 Port Huron and Mankato in North America

~

12,070

12,000

12,140 Scanian moraine 13,300

inaccuracies are inherent in the procedure. First the method of correlation, involving simply the comparison of thicknesses in “runs” of varves, is always suspect and secondly the presence of composite varves a n d non- o r poorly varved zones renders interpretation very difficult. HANSEN(1940) recognised this difficulty when he wrote: “To judge this latter group of cases requires very great personal intimacy with the stratification of the particular clay sediments, and even when the observer is equipped with that experience,. . . the possibility of distinguishing varves in these sediments will always be governed by the variations of subjective judgement”. ANTEVS (1951) was more pointed: “it is in several cases not possible to determine to which category a clay lamina belongs”. The composite varves in Denmark led to a controversy between local geologists who insisted that only a short time span was represented and D e Geer, who tended to regard every clay lamina as a winter layer, so magnifying the total of varves (HANSEN,1940). This controversy with regard to the chronology is of only marginal interest to our present topic but it underlines the complicated nature of some successions and it is important to remember in the discussions below regarding longerterm variations which are based on thickness variation from varve to varve. Any uncertainty in the thickness of the annual layers is critical in such studies.

GLACIAL VARVED CLAYS

55

Transportation and sedimentation

In so far as the coarse layers represent summer melting and the fine layers are formed during winter freezing the main cause of varvity is climatic and seasonal. But it has been shown above that there are number of varieties of varves and in attempting to explain these it is necessary to consider other factors affecting the accumulation of the sediments. The importance of salinity has been long recognised (see HORNER,1948, for discussion and references). There is no doubt that clays are relatively quickly coagulated on entering the sea and there are abundant experimental data to confirm this. The character of the symmict clays was recognised by SAURAMO (1923) as being due to the effect of increased salinity. Marine incursions would certainly result in this type of varve as distinct from the diatectic varves in which clay grains must have been deposited more nearly in accordance with their size. The rapid decrease in thickness of the varves of Lake Algonquin may also be due to rapid depostion near the ice front. But even in diatectic clays it is likely that there was some flocculation. HORNER(1948) found a very large proportion of grains so small that to be deposited a t all over the winter requires that they were to some extent coagulated. The fact that the varves are diatectic and that some separation has taken place suggcsts that the coagulated grains had an upper size limit (Horner suggested sizes up to 1 p in diameter). W. J. EDEN (1955) measured the thixotropy of some of the Steep Rock Lake Clays and found that the results suggested some degree of aggregation in both the S- and the C-layers. It appears likely that flocculation occurred to some extent a t most times even in diatectic varves. This is also indicated by experimental results which show that flocculation can occur when the salinity is 1/50th that of normal sea waters (FRASER,1929). Whileit may be true that continuous successions dominated by symmict varves are the result of deposition in marine waters, ANTEVS(1951) was inclined t o think that SAURAMO (1923) had over-estimated this effect and that the water in which the symmict Fennoscandian varves accumulated was probably never truly marine. Other factors (type of electrolyte, grain-size, type and concentration of clay) apart from the marine nature of the waters may have governed the development of isolated symmict varves in predominantly diatectic successions. Teinperature is another obvious factor affecting sedimentation in that it controls the density and viscosity of the water. In addition it should be noted that the increased content of C a and Mg carbonates in the coarse layers in all probability is due to higher summer temperatures promoting precipitation. The density of the lake waters is critical in attempting to decide on the dominant mode of transportation of the clastic debris. Was it mainly moved in surface waters? as advocated by ANTEVS(1925, 1951) or by underflows? as maintained by DEGEER(1912) and re-iterated by KUENEN (1951). Judging from a number of present day lakes Antevs inclined to the view that the temperature of the lake waters may have been uniform during the summer though obviously in the winter a n inverse stratification occurred with the water at 0°C at the surface and around 4°C near the bottom. Some of the Jutenheim lakes in Norway

56

CYCLES IN LACUSTRINE REGIMES

TABLE viIr TEMPERATURE CONDITIONS IN SOME NORWEGIAN LAKES

(After ANTEVS,1951) ~~~

Locality

Altitude

Date

Surface ternperat lire

Temperature at

>rated cleptli

~ _ _ _ _ _ _ _

Flakevatn, Jotunheimen,

1,448 m (4,749 ft.)

Aug. 24, 1933

7"C (44.6"F)

Finsevatn, Jotunheimen

1,214 m (3,983 ft.)

Aug. 22, 1933

9 "C (48 O F )

Aug. 28, 1933

10°C (50°F) in topmost 5 m ( I 6.4 ft.) 7.5 'C (45.5"F) 4.4"C (40°F) at 90 m (295 ft.) 10°C (50°F) 5 " 4 T (41 O39.2"F) at 25-90 m (82295 ft.) 7.5"C (455°F) 4.5"C (40°F) in topmost 20 at 90 m (295 m (66 ft.) ft.)

Gjende, Jotunheimen Bessvatn, Jotunheimen

984 m (3,228 ft.)

Aug. 20, 1934

1,374 m (4,508 ft.)

Aug. 8, 1934

Sept. I , 1934

5.7"C (42.3"F) at 20-70 m (66-230 ft.) 8 "C (46.4 "F) at 20 m (66 ft.)

(Table VIII) seem to provide very close parallels to older lakes. Beginning with the proposition that a large amount of debris was transported and that the meltwaters arrived at the lake with temperatures about 0°C Antevs argued that if there had been a pronounced direct stratification in the lake waters during the summer the heavier meltwaters should have sunk directly to the floor and stagnated. Most of the mud would have sunk rapidly during the summer, leaving none for the winter layers. Since there are winter layers, however, much of the transportation must have been along the surface or at least in the upper "compartment" of the lake waters. Now consider the meltwaters (at about 0 "C) entering nearly isothermal waters with a temperature about 4 "C (Fig.24). Being lighter the meltwaters will flow up to the surface of the lake and spread out helped by anticyclonic winds blowing outwards from the middle of the ice-sheet. The spread of many of the varves suggests that these surface currents must

*-

J

--->

__

t

-

c--

Fig.24. Main circulation in a glacial lake according to ANTEVS (1925). Assumed direction and strength of currents indicated by arrows.

GLACIAL VARVED CLAYS

57

have been very effective although inevitably there is a size sorting. The distal equivalents of coarse sandy varves near the ice front are thin varves made up almost entirely of clay. Under conditions of uniform temperature ANTEVS (1925) envisaged any surface (wave) stirring action causing a circulation which could affect almost the whole body of water. If the circulation reached the floor then some erosion would take place. No erosion is evident in the varves but the effect of deep circulation may be seen, according to ANTEVS(1951), in the summer laminae of clay which result from downsinking currents carrying the fine particles to low levels from where they can sink to the floor before winter time. Further variations must be allowed for. First, in larger bodies of water the thermal conditions would be expected to vary away from the ice front. At the margin the effect of the ice, calving, etc., is to keep the temperatures low and an inverse stratification may hold throughout the summer. Then at a varying distance, gradual heating up may produce isothermal conditions which gradually give way distally to a direct stratification. These changes with distance may cause a complicated pattern of flow in any meltwaters which d o not mix rapidly with the lake waters. It is also possible (as in the case of Lake Louise) that a direct stratification of the lake water occurs because the lake is separated from the glacier by several miles. Although the meltwaters are heated to some extent in the feeding stream before they reach the lake they are still more dense than the surface waters and sink, probably to the level of the thermocline1, or at least to a level appropriate to their density and spread along that level (ANTEVS, 1951). The suggested model is then of a lake in which much of the transportation occurs near the surface. Although there are many complications with regard to thermal conditions, the meltwaters generally rise to the surface because of their smaller density. Isothermal conditions are not uncommon and the large-scale circulation inherent in these lakes may lead to the accumulation of summer clay layers in composite varves. Variationinthicknessof varvesis relatedto the position of the ice as shown above (Fig.21) but there are changes which appear to be related to depth. Thus the thickness of many varves increases with depth. ANTEVS(1925) appears to regard this as being due to the increased fall-out from a larger column of overlying water but it could also be due to the effect of underflow. DE GEER(1912) proposed that underflows had played a large part in the development of varves but received little support until KUENEN (195 I), extending the ideas of turbidity currents as applied to graded bedding in flysch and greywackes, re-iterated the likelihood of underflow and showed how many of the characteristic features of varves could be explained in terms of turbidity currents. Tn addition to increased thickness with depth, some varves are thicker on the proximal sides of rises of the floor, and in some instances deposition has clearly been controlled by flow around hillocks on the floor (KUENEN, 1951, fig.1).

The level, or narrow zone, at which there is a rapid change in temperature from dense stagnant water below to the upper, circulating water.

58

CYCLES I N LACUSTRINE REGIMES

,--Wind

I

FRESH WATER

I T -

L L I

SALT WATER

BRACKISH WATER

Fig.25. Types of spreading of meltwater under different conditions of salinity. (After KUENEN,1951.)

The crucial point which ANTEVS (1951) made regarding the movement of meltwaters once they reach the lake is that they are at about freezing point and being lighter than the lake waters they will therefore rise. Sufficient attention does not seem to have been paid to the effect of the suspended matter on the current as a whole. Information regarding the density of the meltwater plus suspended sediment is almost wholly lacking but KUENEN (1951) attempted t o arrive at a probable estimate. Kuenen emphasised that the figure for the increase in density (0.001 1) arrived at in his calculations is a conservative one; even so it suggests an effect ten times larger than the difference in density due to the difference in temperature and that so long as the waters in the lake are fresh then the meltwaters will form an underflow. If the flows enter sea water with a density of 1.028 then they would be expected to rise to the surface. With brackish waters a balance in densities might result in restricted movement of the meltwaters until eventually the lighter fractions spread over the surface and the coarser sink to the bottom perhaps in the form of a n underflow (Fig.25). Even though in fresh-water lakes the majority of flows would be expected t o follow the floor this does not necessarily mean that no clay would be available for the winter layers. Kuenen supposed that there would be a minimum of mixing between

GLACIAL VARVED CLAYS

59

the underflows and the overlying waters but the possibility cannot be entirely ruled out and a small amount of clay may be taken up into the lake waters. There is also another possibility. Delivery of detritus to the lake during summer must fluctuate and some of the thinner flows may be sufficiently light toform inter-, or even over-(surface) flows. The coarse material of these flows might contribute to the summer layers and the fine clay would be left for the winter crop. The formation of the summer layers is necessarily different from the graded sandstones of flysch and greywackes because supply extends over several months. But extensive flushing out of the drainage lines may occur in the early part of the summer to give the coarse and sharp bottoms to the varves. ANTEVS (1 95 1) pointed out that there is evidence that the heaviest supply of debris occurs later, in mid-summer. Where this does happen then the result may be varves with a gradational base or with a distinctly coarser band in the middle or towards the top of the summer layer as in the Uppsala varves described by HORNER (1948; Fig.22d). I n many cases the floor of the lake may slope towards rather than away from the ice but KUENEN (1951) supposed that heavy sedimentation of the coarser material in the region of the points of supply (forming an esker or kame) would be sufficient to reverse the slope and impel the underflows across the lake floor. Eventwlly the turbidity current would “pond up” on the floor of the lake to give a graded layer. If supply is slow and spasmodic, layers deposited from individual flows, each graded and with a thin clay lamina at the top, would give a series of “micro-varves” in a composite varve. But if the intervals between supplies were small then the flows might arrange themselves in the basin according to the density of the suspensions leaving a layered cloud of material from which grains would settle out according to size. A. J. SMITK (1959) considered this arrangement probable in the stratified glacial clays of Lake Windermere, England. The lack of bottom erosion in varved sequences does not raise any serious difficulty, nor does the extent of many varves. Experiments have shown that turbidity currents have a facility for spreading large distances on horizontal floors and even the ability to flow for certain distances up gentle slopes. Away from the immediate proximal regions turbulence damps down and only small erosional features are developed; in distal parts the current can only smooth over the floor (KUENEN and MIGLIORINI, 1950; DZULYNSKI and WALTON,1965). Moreover the fineness of many of the varves and the low slopes involved imply that movement of the currents would always be slow. It is neither desirable nor necessary to insist on one type of flow in glacial lakes. The picture which emerges is of material delivered by meltwaters at varying rates and forming at different times under-, inter- and over-flows according to the relative density of the lake waters. Any of these conditions could explain the recorded increase of clay in distal portions of varves. Graded bedding is possible from such types of flow, and composite varves may record individual turbidity currents rather than wholesale overturns of the water. Winter layers record the fine material brought into the lake by inter- or over-flows and probably spread over the surface by wind action.

60

CYCLES IN LACUSTRINE REGIMES

Long-term variations Varve diagrams form a time series which might show regular periodic variations measurable in years. A large number of authors have proposed regular periods varying from the biennial repetitions, which impressed de Geer, to those involving cycles of thousands of years. A summary of these suggestions is given in Fig.26. Apart from the 2-year cycle the most prominent periods appearing in these successions are those at about 5 and I I years in length. But the lack of agreement from area to area raises a suspicion about the validity of any other long-term periods and many reports are based on averages of maxima-a method which may conceal a considerable amount of variability. The lack of agreement also underlines Sauramo’s cautionary remarks on the tendency to ascribe variations in thickness to meteorological causes. An equally important and immediate factor controlling the thickness of the varves is the proximity to the ice. Periodic variations in varve thickness longer than a year are due to ice recession or advance. This may be due to climatic effects; but rates of advance and retreat are also affected by the sub-ice topography. Assuming a relatively flat surface to the glacier and a variable topography underneath, rapid retreat would be expected over high ground where the ice is thin and vice versa.

20-

2

4

6

8

10

12

14

16

18

20

22 7 2 3

Fig.26. Histogram summarising reported periodicities in varve series. (Based on data in R. Y. AN1961, from glacial, Pleistocene and Recent, non-glacial and marine, Recent and pre-Pleistocene varves.)

DERSON,

The special methods of harmonic analysis as described in Chapter 1 now provide a much more powerful and objective means of detecting and assessing periodic Auctuations in time series. Few successions have been subjected to this type of test but those which have show no marked periodicity (Fig.27,7). The varves from Steep Rock Lake and the Carboniferous of New South Wales show no more than a weak short term (about 5 years) variation and a slightly stronger, long-term variation of the order of a hundred years (R. Y. ANDERSON and KOOPMANS, 1963). An investigation which

61

NON-GLACIAL LAKES

STEEP ROCK LAKE U LN:5201

I t 1

I

I

1 1 1 1 ,

I

1 8 7 6’ 5

~db,bzszos 1°9

I

I

4 3.5 T(YEARS)

I

3

I

2.5

2

Id.;25i is Ibi 5 0 20

9

7

4

b

I

35

3

2.5

h

T(YEARS1

Fig.27. Power-spectra of Pleistocene (Steep Rock clays, Canada, ANTEVS, 1951) and Carboniferous (varve S, Australia, CALDENIUS, 1938) varve series. (After R. Y . ANDERSON and KOOPMANS, 1963.)

included a varve sequence from Sweden led BRYSONand DUTTON (1961, p.599) to remark: “The spectrum of the one varve series examined looks more like the spectrum of a series of random numbers than like the tree-ring spectra” while JACKSON (1965) also found no significant periodicity in spectra of Precambrian varves from Canada. These results are not disturbing because analysis of climatic data shows rather weak periods (at 2, 5 and 11 years, LANDSBERG et al., 1959). If these direct meteorological data afford only equivocal evidence of strong periodicities it would be surprising indeed if the climatic changes as recorded (at second or third hand as it were) in the glacial varves appeared distinctly cyclic.

NON-GLACIAL LAKES

Varved lake sediments which appear to have accumulated in areas unaffected by glaciation have been described from a number of regions. The key work in this subject is that of BRADLEY (1929, 1931) on the Green River sediments of Eocene age in the western United States. Other relevant successions are found on the Old Red Sandstone of Scotland (CRAMPTON et al., 1914; RAYNER, 1963), the Triassic Lockatong Formation of New Jersey and Pennsylvania (VAN HOUTEN,1962) and the Todilto Formation (Jurassic) of New Mexico (R. Y. ANDERSON and KIRKLAND, 1960).

62

CYCLES IN LACUSTRINE REGIMES

Fig.28. Varves of Green River Formation; dark laminae largely of organic material. Specimen 3.75 cm thick. (After BRADLEY, 1929.)

These formations along with similar recent sediments adequately illustrate the features of cyclic sediments in lacustrine conditions. Many of the problems are similar to those of the glacial lakes, but there are some which are confined to this environment. In addition to the small rhythmic units which we shall interpret as varves there are larger, more complicated sequences which for convenience can be referred to as “larger cycles”.

Vurves The smallest rhythmic unit is bipartite like the glacial varves but the couplets are made up of an organic (carbonaceous)-rich layer and an organic-poor layer (Fig.28). The latter often consists of carbonates though sometimes mineral material comprising quartz, felspar and clay minerals assumes greater importance. The carbonates are commonly calcite or dolomite, occasionally ferroan dolomite. In the Achanarras Limestone of the Middle Old Red Sandstone in Caithness (Scotland) the carbonate (calcite or dolomite) occurs predominantly as small anhedral grains forming a mosaic. Set in this mosaic are larger euhedral dolomite grains and on occasions even larger grains of calcite may be set in patches of euhedral dolomite crystals (RAYNER, 1963). Other minerals are often scattered irregularly through these carbonate-rich layers but

63

NON-GLACIAL L A W

and KIRKLAND (1960) recognised distinct in the Todilto Formation, R. Y. ANDERSON mineral layers in addition to the carbonate and carbonaceous laminae. An individual mineral-layer normally appears as an extremely thin, sparse scattering of sand grains over a carbonaceous layer, though sometimes it can be very indistinct because the sand grains have been pressed (?or have sunk) into the organic material. The black, opaque, sapropelic laminae often appear to be structureless though small fragments of vascular plants can sometimes be detected (R. Y. ANDERSON and KIRKLAND, 1960, fig.2). In some of the thicker dark layers there is developed a very thin micro-lamination. Pyrite is fairly ubiquitous and lenses of carbonate also occur. The carbonate may be oolitic; a rare example of spherulitic structure has been described from a varved Carboniferous sediment in Scotland (MUIRand WALTON, 1957). The thickness of the individual varve is almost invariably a fraction of a millimetre. The lateral extent of each couplet is difficult to judge but the thickness of each lamina seems to be fairly constant. Current bedding even on a small scale is noticeably absent, as are erosional features. Some trace fossils occur in the form of annelid trails along bedding planes, but these are not numerous and there is no disturbance of bedding due to other burrowing organisms (benthonic life must have been at a minimum). The contacts of the individual layers are usually sharp both top and bottom except in those couplets where clastic debris instead of carbonate is predominant. In these cases the couplets are very like glacial varves and have a gradational contact between lower and upper layer. Gradational contacts are also found in the rich oil-shales of the Green River region. The proportion of organic content varies in the Green River Formation along with other aspects of the composition and Bradley divided the sediments into four types, rich ojl-shales, moderate-, low-grade oil-shales and limy fine-grained sandstones. Only the last, like the clastic bands of the Achanarras Limestone, show grading similar to glacial varves. In all but the rich oil-shales mineral layers are thicker than organic layers. In the rich oil-shales the thick organic layers may have micro-laminations (BRADLEY, 1929, pl.14a). The thickness of the varves varies from one type to another being greatest in the limy sandstones and smallest in the rich oil-shales (Table IX). TABLE IX VARIATION IN THICKNESS OF VARVES

(After BRADLEY, 1929) Mean thickness

Range

Number

(mm)

(mm)

Limy sandstones Low-grade oil-shales Moderate-grade oil-shales Rich oil-shales

1.160 0.167 0.065 0.037

0.600-9.800 0.014-0.370 0.030-0.114 0.014-0.153

32 268 18 143

64

CYCLES IN LACUSTRINE REGIMES

The weighted mean for the thickness of the varves in the succession is 0.18 mm. Apart from the fauna and flora of the Green River Beds which is varied and abundant, in most successions fossils are often restricted to certain horizons and the faunas are poor in diversity. For example the Middle Old Red Sandstone rocks of Caithness (Scotland) and the Lockatong Formation (New Jersey and Pennsylvania) yield fish and certain invertebrates such as ostracods and eurypterids which are virtually restricted to dark platy mudstone horizons.

Periodicity

In the case of the glacial varves the annual freezing and thawing obviously provide a strong periodicity of supply which would impose itself on the sedimentation but in non-glacial areas there is the possibility of a lack of any strong seasonal changes and hence the varving may not be annual. BRADLEY (1929) was concerned to meet this problem and he considered a number of possible alternatives. BiochemicaZ reactions are known from work on Recent sediments to result in the precipitation of carbonates within organic ooze. While these may be sufficient to account for some indistinct lenses of carbonate within some oil-shales such reactions are probably inadequate to cause the thicker, continuous carbonate layers of the couplets. Furthermore the graded sandy layers of other varves are completely unaccounted for. Spasmodic storms with no definite period may have controlled the lamination. Their effect would have been primarily a stirring one, causing erosion of accumulated sediment, though they may have had a secondary effect in bringing renewed supplies of fresh water to the lakes. The main difficulty in envisaging storms as a control of the units is the lack of any signs of erosion and it seems unlikely that each storm would be capable of removing all traces of individual units instead of parts of couplets. Changes in the rate of supply of water and debris from storms might be recorded by a change in the nature of the sediment. But it turns out (see below) that the amount of sediment forming each varve compares with the annual supply of sediment rather than that derived during shorter periods. Differential settling might also be invoked. It is clear that the grains of carbonate even though they may not be so large (they are thought to be primary precipitate, see RAYNER, 1963) would settle more quickly because of their higher specific gravity. Furthermore the organic material would often be flat in shape and perforated by decay. The calcite grains (and any sand grains which were available in the upper waters) would reach the floor and be separated from the more slowly falling organic material. But once this process had been operating for some time, and assuming no periodic supply, earlier delivered organic material would reach the floor at the same time as relatively recently arrived mineral matter; the result would be no segregation into couplets. Periodic supply is therefore necessary, and a seasonal control seems the most plausible.

NON-GLACIAL LAKES

65

Compelling evidence is derived from recent sediments, in particular from Lake Zurich and Lake Baldegg in Switzerland (NIPKOW,1928). In an extended study covering the sedimentation from 1893 to 1919 it was found that in depths beyond 90 m the annual increment was a layer of calcareous sediment followed by a dark, carbonaceous layer. The annual succession could be checked from thicker layers formed after “bank collapses” whose dates of occurrence were known. The calcareous layer was found to accumulate during the summer and the organic-rich layer during the winter. Further evidence from recent sediments is cited by BRADLEY (1929) and RAYNER (1963). Rayner compared the deposition of carbonates in the Old Red Sandstone with that found in shallow Australian lakes by ALDERMAN and SKINNER (1957) and ALDERMAN (1959) where both calcite and dolomite form a fine-grained precipitate towards the end of summer following increased plant growth and a rise in pH. Even if the varves of Lake Zurich were compacted to a tenth of their present thickness (average 3 mm) they would still be much thicker than those of the Green River Beds but the Sakski Lake varves in the Crimea (B. V. Perfiliev, cited by: BRADLEY, 1929), at present 1.3 mm thick, after compaction would compare very closely with Eocene Green River Beds. A less direct but supporting consideration lies in the estimate which Bradley made of the likely thickness of annual layers in the Green River Basin. Knowing the approximate area and depth of the lake and making certain assumptions concerning climate it was possible to show that the observed thicknesses are of the correct order of size. More precise estimates of the probable climate of the Eocene during the formation of the Green River Beds can be made from the flora and fauna of the sediments and from hydrographic considerations involving size of lake in reference to total available basin, probable rates of evaporation and inflow. The flora of the Green River Beds compares somewhat closely with the present day flora of the South Atlantic and the Gulf Coast States and this together with hydrographic estimates suggested to Bradley that the region experienced a climate characterised by cool, moist winters and relatively long, warm summers with a mean temperature around 65 O F (18 “C)and a rainfall of about 85 cm/year. Under these conditions BRADLEY (1929) envisaged supply of detritus to the lake forming a localised marginal facies. Some dispersion of fine-grained material did, however, take place in the surface waters. This would settle slowly through the lake waters to form a mineral layer in late winter and early spring, or if very fine it might join the carbonates which began to precipitate in the early summer. Rise in temperature controlled the solubility of COZ and this together with increased plant activity abstracting COZcaused precipitation of the carbonate. Perhaps a little later planktonic “bloom” resulted in a rain of small organic particles which accumulated as an organicrich layer in late summer and autumn. Successivespecies reaching their peak at different times during the summer could have caused successive “crops” of organic material and would thereby be responsible for micro-laminations in some of the organic layers. Any thermal stratification (presumably a direct stratification in both winter and

66

CYCLES IN LACUSTRINE REGIMES

summer) would further retard the fall of the planktonic material (because of increased density in the lower waters). Overturns of water, due to changes in temperature stratification as in temperate lakes of the present time, seem unlikely due to the general lack of evidence of any erosional features. In present-day lakes where overturn occurs at the change of seasons very often large particles of organic muds are carried up from the floor to the surface. Below the directly stratified waters at least in the deeper parts (the critical depth which Nipkow found was 90 m) the accumulation of the sapropelic material was possible, because of stagnant conditions. In these foetid zones pyrite was produced in abundance. Bacterial activity produces calcium and magnesium carbonates (see BRADLEY, 1929, for references) and these processes could explain the lenses of carbonate associated with the dark layers. The foetid conditions-a direct result of the absence of currents and hence ripple structures-would also account for the scarcity of active benthonic forms and for the preservation of the fine laminae.

U

2.

E.

i

Fig.29. Varve diagram from Green River Formation showing recurrent peaks in thickness of varves. 1929.) Numbers indicate the separation of peaks in years. (After BRADLEY,

Sunspot cycles Like earlier workers on glacial varves Bradley was impressed with the apparently significant variation in varve thicknesses. It might be expected that periods of sunspot minima giving abnormally warm years would produce thicker annual layers of both organic and carbonate material and the Green River Beds show maxima at periods varying from 7 to 18 years but averaging just less than 12 (Fig.29). Averaging maxima in this way, however, can conceal not only a good deal of random variability but also significant changes of a shorter period. Analysis of a number of varved series suggest in fact that the 11-year period may not be so important as one about 12-14

0

2

4

6

8

10

12

14

16

18

20

22

24 >24

Fig.30. Histogram summarising prominent peaks in spectra from 10 varve series. (Adapted from R. Y. ANDERSON, 1961.)

NON-GLACIAL LAKES

67

years; in addition a 22-year period appears to be significant (Fig.30). A longer-term trend occasionally develops around 80 years. Solar sun-spot cycles may exercise ultimate control of all of these periods. R. Y. ANDERSON (1961) pointed out that sunspot activity was at maxima in alternate periods of 11 years (this would account for the 22-year peaks) but that superimposed on this was a longer cycle whose period lay between about 70-100 years (R. Y. ANDERSON, 1961;R. Y. ANDERSON and KOOPMANS, 1963).

Larger cycles

Sedimentary cycles of greater thickness than the varves were also considered to be present in the Green River Formation. The rhythm appears as an alternation of lowgrade oil-shale (or organic marlstone) and higher-grade oil-shale and the thickness of the beds varies around 1 m (Table X). Analysis of the varves gives an average figure for the rate of accumulation of each type of lithology and it is a simple matter to estimate the length of time taken for each of these beds to accumulate. The average time turns out to be 21,630 years (range 16,100-27,000). This average is remarkably close to the figure of 21,000 which is the period of the well-known precession of the equinoxes. The climatic changes involved during the cycle would consist of short hot summers with long cool winters alternating with long warm summers and short mild winters. The first combination of short summers and long winters would mean a thin layer of carbonate but the hot spell could mean a flourishing of the plankton and a relatively thick organic accumulation over the late summer and winter. This period would therefore produce the oil-shales. Thicker carbonate layers would tend to form during the long warm summers of the following part of the cycle. At this period, therefore, marlstones would tend to predominate. The changes in climate would be gradual; it is somewhat surprising therefore to find that the boundaries to the sediment layers are quite sharp. The range in cycle period estimated from the Green River Beds is not surprising considering the method of calculation and the fact that the astronomical cycle itself shows some variation. It is a remarkable confirmation of the method that radiometric measurements indicate the duration of the Eocene to be 20 m years; BRADLEY’S (1929) estimate was 23 ni years. Repetitive sedimentation in the Lockatong Formation (Triassic of New Jersey and Pennsylvania) has also been partly ascribed to precession cycles (VAN HOUTEN, 1962,1964,1965).Varved marlstone corresponding to the Green River lithology occurs in the Triassic rocks but to a subordinate extent. The cycles are of two varieties. The first, detrital, variety shows an upward passage from pyritic black shale at the base to interbedded mudstone and marlstone and is terminated by massive calcareous silty mudstone. The last unit shows indistinct often contorted laminae, the contortions apparently controlled by mud-cracking, and contains some fine cross-bedded sandstone bands and lenses. The thickness of the cycles ranges between 6 and 7 m.

68

CYCLES IN LACUSTRINE REGIMES

TABLE X GROUPS OF BEDS IN FOUR SUCCESSIONS REPRESENTING INTERVALS OF TIME SUGGEsnVE OF THE PRECESSION

CYCLE^

(After BRADLEY, 1929) Kind of rock

Thickness

(ft.)

Mean rate of accumulationlft. (years)=

Interval indicated by each bed (years)

Oil-shale Marlstone Oil-shale Marlstone Oil-shale Marlstone Oil-shale Marlstone

3.0 6.2 2.3 7.2 2.3 6.0 2.0 8.8

4,700 2,000 4,700 2,ooo 4,700 2,000 4,700 2,000

14,000 12,400 10,800 14,400 10,800 12,000 9,400 17,600,’

Marlstone Oil-shale Marlstone Oil-shale Marlstone Oil-shale Marlstone Oil-shale Marlstone Oil-shale Marlstone Oil-shale

6.8 0.6 6.9 2.0 3.8 1.9 6.1 2.8 4.8 1.6 5.4 2.2

2,000 8,200 2,000 4,700 2,000 4,700 2,000 4,700 2,000 8,200 2,000 4,700

13,600’ 4.900 13,800‘ 9,400 7,600‘ 8,930 12,200 ’ 13,350 9,600‘, 13,100 10,800‘ 10,300,

Oil-shale Marlstone Oil-shale Marlstone Oil-shale Marlstone

1.6 6.0 1.4 6.0 1.2 8.0

4,700 2,000 4,700 2,000 4,700

2,000

Oil-shale Marlstone Oil-shale Marlstone Oil-shale Marlstone

1.7 6.5 0.8 7.0 0.5 6.0

4,700 2,000 8,200 2,000 8,200 2,000

of beak

Interval indicated for each cycle (years) 26,500 25,200 22,800 27,000

18,500 23,200 16,530 25,350 22,700 21,Ooo

19,500 18,600 21,600

21,000 20,500 16,100

Average length of cycle, 21,630 years. For marlstone and related rocks yielding less than 15 gallons of oil to the ton the mean rate of accumulation is estimated at 2,000 years to the foot: for moderately good oil-shale yielding 15-35 gallons, 4,700 years; and for rich oil-shale yielding more than 35 gallons, 8,200 years. 1

NON-GLACIAL LAKES

69

The second, “chemical” variety is usually not so thick (range 3-5 m). The black shale unit is not present and the lower part of the cycle consists of interbedded mudstone and marlstone. The upper part is a dark-grey very hard mudstone (argillite) rich in analcime and dolomite. Sedimentary structures vary through the cycle, couplets of varve-type form the lowest unit, alternations of thin bands of mudstone and dolomite follow. Fossil fish, reptiles, estheriids, phyllocarids, ostracods and plants are found in dark mudstone layers in the lowermost part of the cycle. Analcime mudstone with disrupted fragments of marlstone follow in upward sequence to be succeeded by more uniform-appearing mudstone, microscopicallybrecciated, and with white patches of carbonate and analcime. Dark-grey mudstone at the top is extensively mudcracked. This feature is also found, however, in the mudstone-dolomite interbedded units lower in the cycle. Brecciation appears to be due to two causes; firstly the shrinking and hardening of the carbonated layers shortly after deposition and secondly, in the microbrecciated mudstone, the effect is that of de-watering of a colloidal mud. From the nature of the sediment it is to be expected that the rate of deposition of the chemical cycles should be lower than that of the detrital cycles and that both should be rather more rapid than the cycles in the Green River Formation ascribed to the 21,000 year period. Chemical and detrital cycles are interbedded with one another through the succession (VANHOUTEN, 1962, fig.2,3). If both result from the precession cycle some other factor must be responsible for the two different types. Van Houten saw the difference as being due to drainage conditions. The chemical cycle according to Van Houten arose when no through drainage occurred and the basin was subject to periodic filling and drying out. At most times (as witnessed by mud-cracks low down in the cycles) the lakes must have been very shallow, occasionally disappearing during very dry periods. Increased aridity is evidenced by the restriction of fossils to the lower part of the cycle and the increased precipitation of salts upwards. The detrital cycle shows some mud-cracking but no strong concentration of salts and the detritus becomes somewhat coarser up the succession. Supply of detritus was continuous and currents presumably increased in competency. This is in direct contrast to the chemical cycles and Van Houten concluded that during these periods there was a through-going drainage established; no excessive accumulation of salts was possible. In preferring a climatic control for the development of the Lockatong cycles Van Houten was influenced by the belief that climatic variations are more likely to be regular in period than diastrophic changes. This position cannot be maintained with any confidence when the precession cycle itself can show considerable variations, when the sediments themselves are of different thicknesses and when diastrophic movements might show an approximate regularity. Intermittent subsidence has been invoked by Crampton et al. to explain the detrital cycles (very similar to those of the Lockatong Formation) which are found in the Caithness Flagstones of the northeast of Scotland. The Middle Old Red Sandstone sediments show a number of different types at different heights in the succession (Table XI) and CRAMPTON et al. (1914) gave sections to illustrate the cycles.

70

CYCLES IN LACUSTRINE REGIMES

Comparison has already been made between the varves of the Achanarras Limestone and the Green River Beds. The Achanarras Band is only a small part of a succession which in general is made up of clastic sediments and carbonate horizons. The fauna is an abundant but rather restricted one of fish, eurypterids, ostracods, estheriids and occasional plant fragments and the fossils are associated with carbonate beds like the Achanarras Limestone. The ‘‘limestone’’ bands are subordinate in the succession. When they appear they usually form the base of the cycle and pass upwards into flagstone, dark or grey then greenish flagstones with sandstone and end with a sun-cracked mudstone. What Crampton et al. referred to as the “characteristic cycle” varies through the succession, for example a limestone unit can be expected in the Helman Head Beds cycle but not in the cycle, (Fig.31), characteristic of the Field Beds. The “characteristic cycle” seems to be comparable with a modal cycle though not so rigorously defined. If, for the present discussion this equivalence is accepted we can take the different modal cycles as indicating the relative position of each lithology and the composite sequence can be defined as: Pale-coloured suncracked mudstone. Sandstone grading upwards. Slaty or calcareous flags. Limestone (varved, dolomitic).

TABLE XI STRATIGRAPHIC SUCCESSION OF THE MIDDLE OLD RED SANDSTONE IN CAITHNESS

(After WATERSTON, 1965) John o’Groats Sandstone Thurso Flagstone Group

Mey Beds Thurso Flagstones Ackergill Beds Achanarras and Niandt Beds

Passage Bed Group

Noss Beds Castle Sinclair Beds Field Beds Papigoe Beds

Wick Flagstone Group

Wick Beds Red Beds Helman Head Beds Ellens Goe Conglomerate

Barren Group

Ulbster Sandstone Mudstones Sarclet Sandstone Sarclet Conglomerate

Sandstone, 10 ft.

. . . . . . . .

Black slaty flap with sandy layers, 12 ft.

. . .

Blark rdiTc:ireonsflap and limestoner;, 12 ft.

.

Mudatones with sun-cracks, 15 ft.

S d s t o n e , 20 ft.

. .

.

. .

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

Black daty flags and sandy layers, 6 ft. Black calcareous flags and limestones, 6 ft. Mudstones with sun-cracks, 10 ft. Sandstone, 12 ft.

.

.

. . . . .

. . . . . .

Black slaty flags and sandy layers, 15 ft.

.

Black slaty flags and limestones, 4 ft. Mudstones, 6 ft. . . . .

.

Sandstonee, 14 ft;

.

.

-

.

.

.

. . . . . . .

. . . . . . . .

Black slaty flags with sandy layers, 12 ft.

. . .

Black slaty flap and limestones, 3 ft.

.

Black flags in Helman Head Quarry.

.

.

. . . .

A Greenish-whitemudatoncs with sun-cmcks, 10 ft.

.

.

Sandstone beds, thinner at top, alteruatiag with greenish-white mudstones, 40 ft.

. . . . . . . .'

Black, slaty, bituminous shales with fish remains, 16 ft.

Greenish-whitemudstones with sun-cracks

. .,

. . . .

B Fig.31. Vertical sections of parts o f (A) Helman Head Beds and (B) Field Beds to illustr:ite the cyclic succession. (After CRAMPTON et al., 1914.) (Crown copyright, reproduced by permission of the; Controller of H.M. Stationery Office.)

72

CYCLES IN LACUSTRINE REGIMES

This composite sequence is comparable to certain Carboniferous cycles whose origin has been referred to delta outgrowth (Chapter 4, 5). A similar explanation is possible for the Caithness sediments and the “modal cycles” would then appear as variants of the composite sequence depending on the position of accumulation with respect to the point(s) of supply. The Noss Beds, in which the cyclic sequence is not well developed and thin bands of limestone, rippled sandstone and green mudstone occur in rapid alternation, represent an exceptional set of conditions, however, and deserve separate investigation. The recognition of the M.O.R.S. cycles as possibly “deltaic” in a broad sense raises the further possibility that “channel wandering” might be responsible for the rhythm. We have stressed the importance of sedimentary controls in other examplesbut it may be that in these lacustrine sediments other factors are more important, for example: (I) Lake basins are generally smaller than marine areas of sedimentation and in the case of supply from one end only of an elongate basin, channel wandering would be restricted simply because of the space available. (2) Lake levels (and therefore sedimentation)are much more sensitiveto changes in climate than the sea. (3) Changes in lake level are much more likely, e.g., due to the effect of drainage changes, as in the blocking of an old overflow or uncovering of a new one. Such considerations could be taken to support Van Houten’s contention that climatic controls are pre-eminent in these cases. P. ALLEN(1959) raised another possibility when he pointed out the synchroneity of marine transgressions (in France) and cyclic sedimentation in the Weald Lake (in the southeast of England) during the Lower Cretaceous. It should be realised, however, at the outset that these Cretaceous cyclothems are much thicker (up to 100 m in places) than those discussed so far and it may be preferred to follow P. Allen and use the term “megacyclothem”. Apart from thickness the Wealden cycles have a uniqueness (only one other occurrence of this type is reported, from the Dakota Sandstone, Colorado; P. ALLEN, 1959) which distinguishes them from the superficially similar Carboniferous, deltaic cyclothems. Without defining the term, P. Allen summarises the “standard” cycle from the Weald as (Fig.32): (gradual passage or sharp break with erosion) ( H ) Thick dark ostracod clays Pyritous; Viviparus; bands of Neomiodon with scattered fish scales. Thin sandstones and siltstones. Seams of clay-ironstone nodules towards base.

(a

Thin Neomiodon shell beds Gastropods rare. Clay-ironstone locally.

13

NON-GLACIAL LAKES FACfS c

North Weald

South Weald -c

PRO-DEL1A.UKE-CLAYS Mingling of reed and lake waters: iron precipitation

WICK DARK CLAYS Ostracods. Vivlparus, Neomiodon. fish, clay-ironstone, etc.

Shell and fish debris washed up on outer side of reeds

N. mrdius shell

beds with fish scales, local clay-Ironstone, rare gastropods

LAKE-CLAYS BEYOND REED BED

DARK CLAYS Partings with aerial debrls of E. lyelll

4 years’ life of reedswamp recorded

E. LYELLl SOIL BED

OFFSHORE REED BED Virtually continuous

WIN BASAL PASSAGE BEDS

DELTA-FRONTUKE-SILTS AND CUYS with other debris

Physa

Thin cross-laminated lenticular siltstones and clays. Local bone beds

foot of beach about here

REJREATINGSTRAND-LINE Headed by horsetailflotsam from reed beds

rHlN GRADED PEBBLE BED

Suncracked in lower part. Panlng of plant debris at b u e

.

sandy.

Levee, crevasse, back-

With occasional distributary channtls. mouth-bar sands. and eroded onshore soils DELTA-FRONTUKE-SILTS AND C&AVS With scattered small patchesof reed

pebbles, crossbedding and wuhCoarsening upwards. Lou1 roots (not horsetails) OU.I

!

1HlCK PASSAGE SILTSTONES AND SILTY CLAYS Local soil beds.(includlnp E.lye1li). Unio. Sphaeroriderite

THICK SILTY CLAYS Sphmroslderite

-

4.

North Weald

South Weald

--+

Fig.32. The “standard” Hastings cyclothem of the Weald, England. The lithologies described on the 1959.) left are interpreted on the right in terms of the model given in Fig.34. (After P. ALLEN,

(F) Thin dark clay Partings of aerial horsetail debris near base.

(a

11. Equisetites ZyelZi soil bed (with Physa) At top of alternating series of I. Thin cross-laminated lenticular sandstones, siltstones and clays forming perfect passage from ( D ) to Q. Local bone beds.

74

CYCLES IN LACUSTRINE REGIMES

(D)Thin graded pebble bed Top ripples; interior rippled and/or current-bedded, with local suncracks. Oversteps all sedimentary structures and changes of facies below. Exotic pebbles dominant. Scattered debris of horsetails and other plants frequently forms parting at base. (sharp break with erosion)

(C) Thick sandstone

In southern outcrops: Coarsens upwards; scattered pebbles, suncracks and roots (not horsetails) locally near top. Local flat-bedded well-sorted silversands, large-scale cross-bedding, washout structures, etc. (B) Thick lenticular siltstones and silty clays In south forming, by gradual coarsening, a perfect passage upwards into (C). Unio. Local soil beds (including E. lyelli).

( A ) Thick silty clays Thickest in south. Locally red or red-mottled. Grade upwards into (B).

Argillaceous sandy siltstones In northern outcrops: Replace top sandstones. Soil beds (including E. lyelli).

In north contain more sandstone, fining upwards into (C).

Northwards partially replaced upwards by siltier and sandier beds.

The development of this succession can be seen in the first “megacyclothem” which comprises the Fairlight Clays, Ashdown Sandstone and Pebble Bed, Passage Beds and Soil Bed and Wadhurst Clay. Particular attention should be paid to the nature of the Pebble Bed, and the Passage Beds below the Soil Bed. The Ashdown Pebble Bed is usually only a few inches thick; pebbles are usually small, siliceous in composition and can be related (as can the matrix) to the underlying beds: there is an upward gradation in clast size. In places strong concentrations of plant fragments have produced peaty rafts and lenticles. The upper surface of the Pebble Bed is moulded into symmetrical ripples which overlie asymmetrical forms, occasionally reaching up to about 60 cm in wavelength. In a northerly direction the pebbles spread over a pebblefree substratum and appear to come from the south. The Passage Beds are essentially clays with silts and sands. The occurrence of the coarser sediment is distinctive. It is found in thin discontinuous layers and lenses usually as sets of isolated ripple crests. The absence of silt and sand in the troughs suggests that there was just enough coarse sediment to form the crests. Some of the sandy ripples have minute terraces resembling many ripples found on intertidal areas at the present day. Occasional bone beds are found and the coarser material gradually dies out above. In turn the beds pass into the structureless clay of the Soil Bed.

75

NON-GLACIAL LAKES normal alluvial association colonired by horsefails

backswamp lake

shore-foce

gravelly sands

delta-front sands silts and clays

pro-delta silty clays

S.L.

S.L.

Fig.33. Interpretation of Hastings cyclothems supposing that most of the sequence formed during regression: S.L. = sea level static. (Adapted from P. ALLEN,1959.)

normal o l l w i a l aswciation

shore-face sands

delta-front rands silts and clays

pro-delta silty clays

S.L.

valley -plug alluvial association

shore-face uravels

horsetail reed

-K.L..

R.L.

Fig.34. Interpretation of Hastings cyclothems supposing upper portion (pebble bed and above) formed during transgression: S.L. = sea level static; R.L. = sea level rising. (Adapted from P. ALLEN, 1959.)

The sequence can be interpreted in the same way as Carboniferous cyclothems, i.e., in terms of one episode of delta outgrowth (Fig.33). But this is to ignore the peculiar features of the sequence and the nature of the Pebble Bed, which have much more in common with delta-front deposits than those formed upstream. As a more preferable alternative P. Allen proposed that the Pebble Bed and succeeding strata represent sedimentation during the transgressive rather than the regressive phase (Fig.34). The Pebble Bed with its accumulations of horsetail fragments would then have formed at the strand line of the encroaching lake. Upstream the effect of the transgression would be to cause sedimentation of the coarse detritus previously swept down over the delta

i

I

I

Fig.35. Possible interpretations of Wealden megacyclothems taking account of detailed succession including the presence of minor cyclothems. Interpretation 1 : cyclothem formed during regression. Interpretation 2: cyclothem formed mainly during regression but transgressive phase beginning after formation of soil bed. Interpretation 3: regressive phase representedup to top sandstone, transgressive

INTERPRETATION 2 PRO-DELTA AND DELTAFRONT CLAYS point-bar sequence \

IMERPRETATION 3 i

PRO-DELTA AND DELTAFRONT CLAYS

oint

0

I-

OFFSHORECLAYS.

5

J

'\bar finger, \

INTERPRETATION 4

n n

'\bar

finger,

reeds barrier', iflat reeds

1 !

bam'pr

',

1rl.d

channe

bnashoi barrie )ELTA-SHOREFACE

,n 1 I

n 1

hlgh-bar silts and clays

?

low-Lor rands and silts -

fringing reedswamp shoreface silts and clays beach rond

DELTA-SHOREFACE SANDS

I

,

ahoreface rilh and clays beach sand

C

DELTA-CHANNEL SAND-WAVES

NNER SHOREFACE SEQUENCE

/

DELTA-SHOREFACE SANDS

BARRIER-SHOREFACE SANDS

DELTA-FRONT SILTS AND CLAYS

OFFSHORE SILTS AND CLAYS

offshod channel /.'

v'

, I 1

I

! I I

DELTA-FRONT SILTS AND CLAYS offshore channel /

! I

I

oMan reedswamp

PRO-DELTA SILTY CLAYS

outer bar

P' o&re

reedswamp

PRO-DELTA SILTY CLAYS

off&& d w a m p

OFFSHORE SILTY CLAYS

phase from pebble bed upwards. Interpretation 4: regressive and transgressive phases as in 3 but major sandstone of arenaceous formation and sandstone of minor cyclothem in argillaceousformation interpreted as barrier sands. (After P. Allen, personal communication, 1965.)

78

CYCLES IN LACUSTRINE REGIMES

front; the valleys would become plugged and sandy silts would be spread widely over the delta surface. The cutting off of the debris would allow the reworking of the previous deposits as the strand line moved forward. Behind the winnowed pebble and piles of organic debris only finer material would be available for deposition as the Passage Beds. The silt and sand, in short supply, would be able to form only discontinuous rippled patches. As the water shoaled, colonising horsetails, perhaps growing in a foot or two of water, established themselves to form a soil bed below and a fringing screen lakewards of the advancing pebble bed. Beyond the horsetails in deeper water the finest lake clays formed the remaining beds of the cycle. If the Wealden succession is considered in a little more detail, in particular if the occurrence of minor cycles is noted then complicating factors must be allowed for in any interpretation. For example minor cyclothems a few metres thick may come into the succession in the lower, arenaceous formation or in the upper, argillaceous formation. These minor cyclothems are distinctly lenticular with an erosive base and channel form. They reproduce on a smaller scale the lithological features and sequence characteristic of the “megacyclothems”. In order to account for the development of the minor cycles it seems necessary to postulate the local growth of bar finger, off-shore barrier islands or longshore bars. This increases the number of possible interpretations of the succession and these are summarised in Fig.35. Although allowing the possibility that channel wandering could have been the cause of the Wealden “megacyclothems”, P. Allen was impressed with the probable correlation of the phases of transgression and regression with the events in the Paris Basin. On any reconstruction it is clear that the Wealden Basin (or the “Sussex TABLE XI1 POSSIBLE CORRELATION OF WEALDEN MEGACYCLOTHEMS WITH TRANSGRESSIONS IN THE PARIS BASIN

(After P. ALLEN,1959) Megacyclofhem ( Weald)

Formation (Weald)

Middle and Upper Weald Clay1 Horsham Stone Lower Weald Clay Upper Tunbridge Wells Sand1 Grinstead Clay

IV ? IT1

I1

I

c

Lower Tunbridge Wells Sand Wadhurst Clay‘ Ashdown Beds

1 Including minor cyclothems

Trend of lake level (Weald)

rising falling rising falling

Neocomian sea (Paris Basin)

Upper Barremian to Lower Aptian Transgression Barremian Regression Hauterivian Transgression

1I

minor rise falling rising falling

Later Valanginian to Early Haukrivian movements Valanginian Transgression Bemasian Regression

79

NON-GLACIAL LAKES

4,000

3,000

1H fl

a

A

Fig.36. A. Generalised sections through Lockatong and Brunswick Formations. B. Interpretation of environmental conditions. (Adapted from VANHOUTEN,1962.)

Morass” in P. Allen’s more evocative phraseology) must have been a marginal area and closely affected by any rise in sea level. The Paris Basin during this time provides clear evidence of major marine transgressions which might be linked (Table X I ) with the “megacyclothems” of the Weald. If they are correlated in time then a eustatic or a diastrophic control would seem to have operated in this region to form the rhythmic sequences. Another large-scalecyclewas reported by VANHOUTEN (1962) from the Lockatong Formation (Fig.36). The “detrital” cycles tend to occur in groups separated by “chemical” cycles, and the thickness of such groups is about 350 ft.1 The uppermost 1

More recently VANHOUTEN (1964) recognised groups of intermediate thickness, 70-90 ft.

80

CYCLES IN LACUSTRINE REGIMES

two of these "detrital" bundles coincide with brownish beds interbedded with the predominant grey measures of the Lockatong argillites. In the sequences above, the short cycles die out but interbedded brown beds occur in the grey strata at intervals between 325 and 375 ft. The Brunswick Shale above the Lockatong Formation is predominantly brown in colour but it has interbedded grey strata which occur at intervals from 350 to 425 ft. There appears therefore to be a long term variation around 345 ft. which, taking the average rate of sedimentation derived from the short Lockatong cycles and assuming these to be recession cycles, works out at an interval of half a million years. Continuing to prefer a climatic control, Van Houten interpreted the cycles as being related to long periods of alternating wet and dry conditions. During the accumulation of the Lockatong Formation when conditions were predominantly lacustrine and reducing, the drier periods of the long cycle would encourage evaporation in the basins and the exposure of large areas as flood plains and mud flats. Curiously, these periods coincided with some of the periods of through drainage as represented in the detrital cycles. Conversely during Brunswick sedimentation conditions were mainly flood plain and mud flat and only in wetter periods of the long cycle did conditions change to indundation and lacustrine conditions. During such periods, the grey measures were formed.

Chapter 4

TRANSITIONAL REGIMES, I-NORTH

AMERICA

Of all the sequences displaying cyclic sedimentation those bearing coal are probably best known. In 1854, DAWSON,in his account of the Coal Measures of Joggins Bank, Nova Scotia, described them as bearing witness to a “long succession of oscillations between terrestrial and aquatic circumstances.” His use of the term aquatic was wise because we are still not sure, over 100 years later and despite the advances in paleoecology and geochemistry, if, in many instances, we are dealing with predominantly marine or non-marine conditions during the non-continental periods. So far the most reliable indicators of salinity are considered to be fossils, which are used to distinguish beds of comparable lithology, as marine or non-marine. Beds without fossils are generally considered non-marine (TRUEMAN, 1946, 1954; WELLER,1957). Differences of opinion of course exist, as will be seen later, but in general it appears we can distinguish between coal-bearing sequences where there are frequent marine incursions and those where marine incursions are rare or absent. The evidence for a marine incursion is often seen in a limestone but in many cases the only proof is on a single bedding-plane of a shale. While we therefore have various types of cycle which could be classified and described separately, in many areas more than one type occurs, sometimes stratigraphically apart, sometimes at the same horizon but geographically separate. To avoid the confusion inherent in continually changing locale and to emphasise the importance of palaeogeography in the understanding of cyclic sedimentation the following accounts of coal-bearing cycles are given on an areal basis.

UNITED STATES OF AMERICA

Pennsylvanian rocks in the United States of America have received most attention and we shall concentrate on describing sedimentationin three areas, the Eastern Interior (or Illinois) Basin, the Western Interior (or Mid-Continent) Basin and the Appalachian Basin (Fig.37). The rocks under review consist in the main of shales, sandstones, limestones and coals. All the evidence available points to the shallow-water origin of the first three. Coal is generally assumed to indicate a period of emergence. Detailed petrography is not included here but the interested reader is referred to WELLER (1957), POTTERand GLASS(1958), FERM (1962), POTTER(1963), where he will find both detailed observations and extensive bibliographies. While our survey is confined mainly to Pennsylvanian rocks it will be realised that cyclic sedimentation of a comparable nature is also present in rocks of Mississippian and Permian age (e.g., SWANN,1964,

82

TRANSITIONAL REGIMES IN NORTH AMERICA

Fig.37. Distribution of Mississippian and Pennsylvanian rocks in areas mentioned in text. (After POTTER, 1963.)

and R. C. MOORE,1959, respectively). Rocks of these other two systems will be referred to where relevant. WANLESS (1950) aptly summarised the main features of Upper Palaeozoic sedimentation in the continental interior of America. He pointed out that the epicontinental seas, repeatedly transgressing across central America during Late Mississippian, Pennsylvanian and Early Permian times, advanced from the southwest. Consequently, in Arizona and southwest New Mexico there is a predominantly marine succession, in Kansas 7040 % of the succession is marine, in Illinois 25-10% while in West Virginia only 5 to less than 1% represents marine strata. Stratigraphic divisions of the Pennsylvanian are shown in Fig.38 along with a comparison of PennsylvanianCarboniferous divisions in western Europe and Russia.

Eastern Interior Basin

UDDEN (1912) was impressed by the cyclic nature of the Pennsylvanian rocks in part of Illinois (Fig.39). “Each cycle may be said to present four successive stages, namely:

83

UNITED STATES OF AMERICA

NORTH-\ GREAT BRlTAlN

I

RUSSIA EST GERIUIWY COUNTRiEl FRANCE

,w

NORTH 1lDCONTlNENT REGION

sA/\R KUSELER SCHICHTEN

3

-0

5

2j 4

b

i APPALACHIAN REGION

--- ----

DUNKARD

PSEUOOFUSULIW HORIZON

UPPER

*

VIRGIL

I

I

ZHELIAN

MONONGAHELA

MISSOURI

LOWER

0.

D

-

2

UPPER

cc

DES

MOINES 3

I

m C

%

;4

LOWER

i

-

3

C

-

MORROW

?-

POCAHONTAS SPRINGER

0

------------MAUCH CHUNK

-I

m VISZAN

I -

GREEN BRIER

330

MERAMEC

OSAQE

II:

-

320

IAMURIAN

A

-

KANAWHA

i-

ASHKlRlAF

-

-

ALLEGHENY ATOKA (LAMPASAS)

B

A

m.y

K ERICA

I

TOUIHAISIAN

KINDERHOOK

POCONO

Y O

345

Fig.38. Correlation chart for the Carboniferous of northwest Europe, Russia and North America. and WOODLAND, 1964.) (After E. H. FRANCIS

( I ) accumulation of vegetation; (2) deposition of calcareous material; (3) sand importation; and (4) aggradation to sea level and soil-making.” (UDDEN,1912, p.47.) WELLER(1930) published his classic paper Cyclical sedimentation of the Pennsylvanianperiod and its signi@ance, expanding on Udden’s work and laying the foundation for detailed studies of Pennsylvanian rocks in America (and elsewhere) which have produced such a wealth of literature and theories. His re-interpretation of Udden’s section is shown in Fig.39. The term cyclothem, “to designate a series of beds deposited during a single sedimentary cycle of the type that prevailed during the Pennsylvanian period,” was introduced in 1932 (WANLESS and WELLER,1932, p.1003). It will be noticed (Fig.39) that Udden commenced his cycles at the base of the coal seams, i.e., at the beginning of his “accumulation of vegetation stage.” WELLER (1930), however, decided that each cyclothem should be considered a formation and,

84

TRANSITIONAL REGIMES IN NORTH AMERICA

Fig.39. Cyclic sedimentation in the Pennsylvanian of Peoria, Illinois, as recognised by A. UDDEN (1912) and B. WELLER(1930). (After SHROCK, 1948.)

following stratigraphic practice, should therefore commence with some evidence of diastrophism. This latter was provided by the unconformity claimed to be present at the base of each sandstone unit. An eight-unit “typical Pennsylvanian formation” was erected in 1930 and later modified (WELLER, 1931; WANLESS and WELLER, 1932; WELLER and WANLESS, 1939). What is now regarded as the idealised Illinois cyclothem is shown in Fig.40. WELLER (1961, p.141), however, emphasised that it is “only a model, because it is neither ideal

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Upper Limestone Middle Limestone

Lower Limestone

Fig.40. Idealised Illinois cyclothem: a = lower, dominantly non-marine, hemicyclothem; b = upper, dominantly marine, hemicyclothem. 1 = sandstone, fine-grained, micaceous; and siltstone, argillaceous; variable from massive to thin-bedded; usually with an uneven lower surface; 2 = shale, gray, sandy; 3 = limestone, argillaceous; occurs in nodules or discontinuous beds; usually non-fossiliferous; 4 = underclay, mostly medium to light gray at top; upper part non-calcareous, lower part calcareous; 5 = coal; locally contains clay or shale partings; 6 = shale, gray; pyritic nodules and ironstone concretions common at base; plant fossils locally common at base; marine fossils rare; 7 = limestone; contains marine fossils; 8 = shale, black, hard, fissile, “slaty”; contains large black spheroidal concretions and marine fossils; 9 = limestone; contains marine fossils; 10 = shale, gray, sandy at top; contains marine fossils and ironstone concretions, especially in lower part. (After WELLER, 1956; KOSANKE et al., 1960.)

nor typical for all parts of the Illinois stratigraphic section nor for the Pennsylvanian sections of other regions.” He had earlier (1956, p.28) qualified the Illinois cyclothem by noting (our italics) “the most common clear development includes members 1 (and, or2),4,5,8,9 and lo”, thoughin 1957(p.330) he said “In its simplest obvious development a Pennsylvanian cyclothem consists of the following five members: (10)Shale. (9) Limestone. (5) Coal. ( 4 ) Underclay. ( I ) Sandstone.

86

TRANSITIONAL REGIMES IN NORTH AMERICA

A

C MACOUPIN (i)

TYPE

LA SALLE TYPE (11)

BOGOTA TYPE (iii)

Fig.41. Variants of Illinois idealised cyclothem-ornament as for Fig.40. A. Simplest obvious devel1961). C. Named variations (WELLER, opment (WELLER, 1957). B. Characteristic variations (WELLER, 1961).

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Other common variants of the idealised cyclothem have been described (WELLER, 1961) and are shown in Fig.41. That some of these cyclothem types may occur in the Illinois succession in a particular order is discussed on p.95. Both Weller and other workers (WELLER and WANLESS, 1939; WANLESS, 1950, 1962, 1964; GRAY,1962)have emphasised that there are considerable variations in the Pennsylvanian succession in the Eastern Interior Basin. The types of cyclothems, their number and their thickness, change with, among other factors, the overall thickness of the succession. WANLESS (1964) pointed out that the cyclic pattern varied from place to place and from time to time in the same place. In some areas variation was greater than in others and equivalent stratigraphic sequences could have different types and numbers of cyclothems. He thought that the Liverpool cyclothem, with certain regionally extensive units, perhaps composed a “representative cyclothem”. It consists basically of units 1,3,4,5,8,9,10 of the “ideal” of Fig.40. While differences in cyclothem type are reasonably well documented there is a lack of consistency in the recording of the number and thickness of cyclothems in a succession in any particular area. What information we can find from the literature on these points is given in Table XIII. Cyclic sedimentation has recently been described from rocks of Chesterian (Upper Mississippian) age in Illinois (SWANN, 1964). These differ from Pennsylvanian rocks in that they are made up of approximately 25 % limestone, 25 % sandstone, and 50 % shale compared with 4 % limestone, 33 % sandstone, and 63 % shale for the Illinois Pennsylvanian in general (POTTER,1963). SWANN(1964) made no attempt to define a cyclothem but merely emphasised that the succession consisted of alternations of limestone-dominated and clastic-dominated units. The latter include sandstones, shales, underclays and thin coals. It was thought that the succession, up to 1,500 ft.

TABLE XlII THICKNESS AND NUMBER OF CYCLOTHEMS IN PENNSYLVANIAN OF ILLINOIS

Area

Average thickness

(ft.)

__

Central U.S.A. Western Illinois Central States Northwestern Illinois lllinois East Central U.S.A. Eastern Interior Basin

Number

_____ 38

25 <50 221 55-80’

2

- ~~-_ _

~~

__

WANLESS and SHEPARD, 1936 WANLESS, 1950 WELLER, 1956 WANLESS, 1957 BRANSON, 1962a WANLESS, 1963 WANLESS, 1962

23 37 30 25 ___

1

-

Author

-

There are 500 ft. of Pennsylvanian rocks in this area (WANLESS, 1957). There are 2,000-3,OOO ft. of Pennsylvanian rocks in lllinois (KOSANKE et al., 1960).

88

TRANSITIONAL REGIMES IN NORTH AMERICA

thick at outcrop, showed fifteen major cycles of advance and retreat of the sea although there was evidence for at least 70 minor reversals in the direction of shore movement superimposed on these.

Mid- Continent Basin

R. C. MOORE(1936, 1949, 1950, 1959) described Pennsylvanian and Lower Permian cyclothems from Kansas and adjacent areas. The following members were proposed to describe the “ideal cyclothem” (R. C. MOORE,1936, p. 24-25): “ ( 9 ) Shale (and coal). (8) Shale, typically with molluscan fauna. (7) Limestone, algal, molluscan, or with mixed molluscan and molluscoid fauna. (6) Shale, molluscoids dominant. ( 5 ) Limestone, contains fusulinids, associated commonly with molluscoids. (4) Shale, molluscoids dominant. (3) Limestone, molluscan, or with mixed molluscan and molluscoid fauna. (2) Shale, typically with molluscan fauna. ( I c ) Coal. (Ib) Underclay. ( l a ) Shale, may contain land-plant fossils. (0) Sandstone. Members (0) and ( 1 ) in the initial part of the cyclothem and (9) at the end are non-marine. The remaining members are marine.” Cycles of this type occurred in the Virgilian Wabaunsee Group. (See Table XIV for stratigraphic divisions of Pennsylvanian in Kansas.) R. C. MOORE(1936) followed Weller in commencing the cyclothem with the sandstone. It was emphasised, however, that the fusulinid limestone (5) represented the culmination of a marine transgression while the beds above (6-8) recorded a regressive marine phase (Fig.42). This made the Kansas cyclothem nearly symmetrical and therefore different from the Illinois type. In the Shawnee Group below the Wabaunsee, however, cyclothems were different again. The group had been divided (R. C. MOORE,1936) into four limestone formations separated by relatively thick shale formations. In each limestone formation distinctive types of limestone occurred separated by usually thin shales, but always in the same order. These limestone members were known as the “lower”, “middle”, “upper” and “super” limestones respectively and, except in some cases, the “super” were typically fusulinid-bearing. The “lower” limestone is characteristically massive, brown-weathering, ferruginous, the “middle” thin, dense, blue and the “upper” comparatively thick and wavy bedded (R. C. MOORE, 1950). The thin shale members too were distinctive. An easily recognisable black fissile shale, for instance, occurred only between the “middle” and “upper” limestone members.

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Between the limestone formations the thick shale formations were frequently sandy and non-marine. In their middle parts, however, one or two thin marine, nonfusulinid limestones occurred while coals sometimes appeared in the upper part. The succession was therefore made up of two quite distinctive types of cyclic deposits-the complex multi-limestone portions and the simple cycle of the shale formations. The problem thus arose as to how these might be compared with the “ideal” Wabaunsee type cyclothem. Moore first considered whether each multilimestone portion of the succession might simply represent a rather complex type of “ideal” cyclothem, the three (plus) fusulinid limestones being the equivalent of the fusulinid member (5) of the Wabaunsee cycle (Fig.43). The shale-formation cyclothem compared quite well with the Wabaunsee one. The succession then could be considered as consisting of alternating complex and simple (or major and minor) cyclothems. This combination of different types of cyclothems, or “cycle of cyclothems”, Moore thought should be recognised and he proposed the term megacyclothem to describe it. There was, however, another possibility. Perhaps the different limestone members of the limestone formations were each part of incomplete individual cyclothems. In other words the succession Moore had previously postulated as being made up of two cyclothems might actually represent j i v e (Fig.43). Accepting the cyclic concept (i.e., units appearing in a particular order) then the position of certain distinctive members in the succession could now be more readily explained. Moore decided on this second hypothesis as being the more likely. A megacyclothem thus consisted of a “bundle” of five incomplete cyclothems. Fig.43 has been constructed from R. C . MOORE’S (1936) text to illustrate the two possibilities. We have used the same sequence of Shawnee rocks to demonstrate the conflicting hypotheses, though for some, not obvious, reason this was not done in the original paper. TABLE XIV DIVISIONS OF PENNSnVANIAN IN KANSAS

Series

Grows

Virgdian

Wabaunsee Shawnee Douglas

Missourian

Pedee Lansing Kansas City Pleasanton

Desmoinesian

Marmaton Cherokee

Atokan Morrowan

90

TRANSITIONAL REGIMES IN NORTH AMERICA I

I

Nonmarine Conti , nental rdimeni

nth

Mne Submerged by Shallow Sea Inter ml Mar mediate Far amp ginal Off- Offshare shore

44, Algal limestone (contains near-sbre and bmckish Urnstone (contains far off-shore invertebrates, especially furulinids) LlmesMne, impure to shdy (contains intermodfate off-shore invertebrates) Shale, marine (contains

mrar-shore invertebrates)

/ . . I -

Regressing

&

invertebrates) Urnstone (contains fm off-sbre invertebrates, especially fusulinids) WII-S~~IU

Llmestone, shaly (contains intermediate off-shore

...._ ....bnmarine shale, =3 DisconformiW Llmertone (contains furulinids)

-

/ -I-(

Fig.42. Diagrammatic section of Pennsylvanian rocks in Kansas showing cyclic sedimentation. (After R. C. MOORE, 1959.)

Megacyclothems from different stratigraphic levels are shown in Fig.44, but it should be realised that while in general the Pennsylvanian succession in Kansas consists of alternating carbonates and clastics, neither cyclothemsnor megacyclothems, as such, have been reported from all subdivisions. R. C. MOORE (1949), for instance, made no mention of cyclothems in rocks of Morrowan or Atokan age. As has already been noted the type of cyclothemscan vary stratigraphicallyand of course when traced laterally a cyclothem changes. Table XV shows further stratigraphical variations. (It is of interest to note how the Wabaunsee cyclothem given in Table XV differs from

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B

A f

A

v

h

\

_ _ _ _ _ ______----_ _ ----- --___----

*

Most prominent persistent and widely r e c o g n i s e d stratigraphic units

Limestone a l g a l marine L i m e s t o n e f u s u l i n i d marine Limestone morine Shale block f i s s i l e m a r i n e

Fig.43. Diagrammatic illustration (not to scale) of alternative interpretations (R. C.MOORE, 1936) of Shawnee Group succession as being: A, equivalent to fiveWabaunsee-typecycles; or B, two Wabaunseetype cycles. Central graphic column has been expanded for diagrammatic purposes, leaving gaps where units might be considered not to have developed, partial gaps where units are thin and unirnportant. (Based on succession given in R. C. MOORE,1936, pp.30-31.)

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TRANSITIONAL REGIMES IN NORTH AMERICA

TABLE XV VARIATIONS IN KANSAS CYCLOTHEMS~

Desmoinesian

____

Cherokee Group

(HOWE,1956)z (southeastern Kansas)

(Limestone and sandstone) (Shale, P Y )

Missourian

Virgilian

Marmaton Group

Kansas City Group Wabaunsee Group

(R. C. MOORE,

(R. C. MOORE,

(R. C. MOORE,

(R. C. MOORE,

1949)

1949)

1949)

1950)

Shale, calcareous Limestone Shale, gray

(Shale, dark and limestone)

Shale, black

Coal Underclay Limestone and sandstone Shale, gray

Coal Underclay Shale, sandy Sandstone

Shale, gray to Limestone brownish or limestone Shale, black, platy Shale or limestone Shale, marine containing marine fossils Coal Coal Coal Underclay Underclay Underclay Shale, non-marine Shale, sandy, silty Shale, sandy to and clayey clayey Sandstone, Sandstone, Sandstone, non-marine fine-micaceous non-marine

Shale, dark and limestone 1 2

Lithologies in brackets added to facilitate comparison. Described with coal as top unit.

the “ideal” described on p.88.) MERRIAM (1963) emphasised that cyclothems can be symmetrical or asymmetrical according to the arrangement of the marine and nonmarine components. He also pointed out that the “ideal” cycle is seldom seen. What is particularly noteworthy, however, is the great lateral persistence of some units. Limestones, for instance, are said to be traceable in some cases from Oklahoma to Pennsylvania (R. C. MOORE, 1959) and cover areas as great as 80,000 sq. miles. On the other hand it is not easy to understand in many instances how individual cyclothems are delimited (e.g., Fig.44). The fact that units are not infrequently assigned to different cyclothems in different studies presumably indicates how incomplete many cyclothems are. This is emphasised by BRANSON(1962a, p.449) who, in discussing the MidContinent area said: “A general misconception of the nature of cyclical sediments has been widely held. An ideal cyclothem has not been found, and in the Mid-Continent no cyclothem approaches the ideal. The normal well-developed cyclothem of the area, and there are few such, is shown below: Shale, dark unfossiliferous, with clay-ironstone concretions. Limestone, locally rich in fusulinids. Shale, gray, containing myalinid fossils.

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Index 0

too ton

AI

200

300

400

500

600

700

X- L i g h t gray, wavy thin beds Y- Blue, dense,vertical-jointed even bed Z-Yellow-brown. ferruginous, massive beds Cyclothems o f each rnegacyclothem are numbered i n upward order

Fig.44. Kansas megacyclothems: X = light gray, wavy thin beds; Y = blue, dense, vertical-jointed even bed; 2 = yellow-brown, fermginous, massive beds. Cyclothems of each megacyclothem are numbered in upward order. (After R. C. MOORE,1950.)

Limestone or clay-ironstone, marginiferids abundant. Shale, black, fissile, phosphatic. coal.

Underclay. Shale, silty, fossil plants. Sandstone, non-marine, locally conglomeratic”.

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TRANSITIONAL REGIMES IN NORTH AMERICA

.------

Illinois

Fig.45. Croweburg-Verdigris cyclothem. (After BRANSON, 1962a.)

He pointed out that a cyclothem which is fairly uniform over a large area is rare and gave as an example the Croweburg-Verdigris cyclothem (Fig.45). This figure is shown not only because it illustrates that even a “fairly uniform” cyclothem is quite variable but because it illustrates the difficulties in deciding just how the top and bottom of the cyclothem are identified. We shall return to this subject later. As with Illinois it is not easy to give a definite answer on the question of the number and thickness of cyclothems present in the area. BRANSON (1962a, p.450) noted that “On the Oklahoma-Kansas platform there seem to be more than 60 such cycles, 41 marked by coal beds, at least locally.” WANLESS (1950) recognised 25-30 cycles in Kansas while MERRIAM (1963, p.177) thought there might be as many as 85. No author to our knowledge has given an average thickness for the Kansas cyclothem though R. C. MOORE(1949) described 15 cyclothems of Cherokee age ranging in thickness from 10 to 70 it. (average of about 32 ft.). HOWE(1956) described rocks of similar age from southeast Kansas and while he redefined the limits of the cyclic units (he used the top of the coals instead of the conventional American choice of the base of the sandstones) it is interesting to note that the average thickness of these cycles also works out at about 30 ft. In the Wabaunsee Group, 320-520 ft. thick (MERRIAM, 1963), there are 15 cyclothems (R. C . MOORE,1950) which would

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again give an average thickness of the same order. Two “typical” Wabaunsee Group cyclothems illustrated by R. C . MOORE(1936, p.23) are about 35 and 45 ft. thick. Returning now to the question of megacyclothems, WELLER(1958) indicated that there are four present in each of the Marmaton, Kansas City, Lansing and Shawnee Groups and one in the Wabaunsee (Table XIV). R. C. MOORE(1959) considered that they are also present in the Lower Permian rocks in Kansas. Attempts to compare the Kansas and Illinois types of cyclic sedimentation (e.g., R. C. MOORE,1950, 1959; WELLER,1958, 1961; WANLESS, 1964) have led to the same type of problem encountered by R. C. MOORE(1936) when considering the Wabaunsee and Shawnee successions. Is the Kansas megacyclothem equivalent to a group of Illinois cyclothems or is it simply a variant of a single Illinois cyclothem? WELLER (1942, 1958, 1961) favoured the first possibility. On re-examination of the Illinois succession he recognised a repetition in order of three types of cyclothem (Fig.41,46). A cycle of cyclothems started with one of Macoupin type (more or less the “idealised” Illinois cyclothem but with no fresh-water “lower” limestone). This was succeeded by a La Salle type (lacking “lower” and “middle” limestones, and black shale, but with a very well developed “upper” limestone). One, but more often two or three, Bogota type cyclothems followed (marine “middle” and “upper” limestones absent but fresh-water “lower” limestone and black shale prominent). This cycle of cyclothems was then repeated. Weller used the black shale between the middle and upper limestones as the main key to his correlation (Fig.46). WANLESS (1964) also used the same black shale to show that the Kansas megacyclothem is the equivalent of one Illinois “fully developed” cyclothem. The coal of the Illinois sequence was equated with the Kansas “middle” limestone and the “lower” fresh-water limestone of the Illinois cyclothem with the “lower” marine limestone of Kansas. It is difficult to assess the two views adequately on the information presented. Some of the evidence is contradictory. WELLER (1958) for example, stated categorically that “no evidence is known certainly substantiating the possibility that the marine “lower” limestones of Kansas megacyclothems are equivalent to either the freshwater lower^' limestones or the rare marine zones in the basal sandstones of the Illinois cyclothems”, yet WANLESS (1964, p.603) maintained “that while underclay limestones in the Illinois Basin commonly formed in fresh-water lakes or brackish lagoons, they were contemporary with transgressive marine limestones to the southwest.” Weller gave no details of the Illinois succession when discussing it in terms of his three cyclothem types (other variants such as those shown in Fig.41 were not mentioned). As far as we know no actual section of Illinois rocks illustrating the cycle of cyclothems has been published. WELLER(1958, 1961) did, however, analyse the Kansas sequence in some detail (showing, incidentally, just how difficult it is to divide the sequence into cyclothems and megacyclothems in a generally acceptable manner). WANLESS(1964) used as the main comparison with the Kansas megacyclothem an Illinois cyclothem lacking in units 6 and 7 of the “fully developed” one. His careful

96

TRANSITIONAL REGIMES IN NORTH AMERICA

Fig.46. comparison of Kansas and Illinois cyclothems-ornarnent as for Fig.40. (After WELLER, 1958.)

areal study of individual cyclothems would seem to indicate that this type of variant is common; it unfortunately resembles none of Weller’s three main types! Areal facies studies such as those undertaken by LAPORTE (1962), WANLESS et al. (1963), IEWRIE et al. (1964), and WANLESS (1964), are, we consider, the most important need in attempting to solve this problem. On present evidence we consider it unlikely that a Kansas megacyclothem represents a series of incomplete cyclothems, perhaps

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equivalent to a series of Illinois cyclothems (after all the intercalation of cyclothems from place to place in Illinois makes correlation within the state difficult enoughsee, e.g., KOSANKE et al., 1960; WANLESS, 1964). As we have already pointed out it is not easy to separate, on a completely rational basis, the suspected cyclothems within a megacyclothem. What we can see is a vertical sequence of different limestone types. But until it is known what these changes represent in terms of depth of water (see p. 11l)and,just as important, how individual megacyclothems and their component units change facies laterally, it seems unprofitable to speculate further. BRANSON (1962a, p.451), discussing the Mid-Continent area, was very outspoken: “The imperfections of the megacyclothems.. . are such that they can now be regarded only as hypothetical units possibly indicating some sort of recurrent conditions.” Cyclic sedimentation in Kansas continued into Lower Permian times. Wolfcampian beds mainly consist of alternating thin shales and limestones. JEWETT (1933) is credited with first recognising their similarity to the underlying Pennsylvanian rocks. ELIAS(1937) paid particular attention to the evidence for the depth of sea water provided by the abundant fossils. Basically he considered the sequence to be one of continental red shales alternating with marine shales and limestones. Starting with the continental shales a symmetrical idealised cyclothem could be erected (Table XVI). Elias emphasised that no single cycle showed all phases of the ideal one though one or two are practically complete. The average thickness of a cycle is 50 ft. HATTIN (1957, p.6) examined in detail the Wreford Limestone and associated beds, part of the Chase Group, and recognised two “nearly complete” cyclothems making up the “Wreford megacyclothem”. “The typical transgressive hemicycle begins with red shale (locally with a sandstone) followed, in upward order, by green shale, mudstone, molluscan limestone, calcareous shale and cherty limestone (including chalkylimestone reef development). The typical regressive hemicycle includes the same rock types in reverse stratigraphic order except that the molluscan limestone equivalent is commonly algal.” R. C . MOORE (1959) considered that many Lower Permian sequences in Kansas displayed repetitions of a megacyclic nature and MERRIAM (1963) also emphasised their cyclic aspects.

AppaIachian Basin In contrast with Kansas where the succession is predominantly marine the Pennsylvanian rocks of the Appalachian region are predominantly non-marine. The axial region of the Appalachian trough contains the thickest succession, made up of sandstones, conglomerates, clays, shales and coals. Here and eastwards cyclicity is marked by the recurrence of coal beds. Westwards, however, the intercalation of recognisable marine beds makes the cyclic pattern more like that of Illinois. In some areas a cyclothem is more likely to resemble those above and below it than its lateral equivalents. In others there seems to be a fairly pronounced change in cycle type stratigraphically. STOUT(193 1) drew attention to the Ohio Pennsylvanian

98

TRANSITIONAL. REGIMES IN NORTH AMERICA

succession and the change from alternating marine and continental conditions near the base to predominantly continental conditions at the top. As a consequence there was a considerable change in cyclothem type up the succession (Fig. 47). He considered that the Ohio rocks were made up of cycles in which coal was the most constant unit and that there were seven main types of cycle present (Table XVII). There were of course local modifications of these types so that incomplete and irregular cycles occurred quite frequently. As far as Ohio was concerned he considered that no regional disconformities existed and thus chose the coal as the starting point of the cycles. Some 45 coal beds occurred in the succession and the cycles averaged 26 ft. in thickness (the range was from 2 ft. 0 inch to 110 ft. 2 inches).

Fig.47. Stratigraphic range of cycle types listed in Table XVII.

TABLE XVI IDEALISED BIG BLUE CYCLE OF DEPOSITION IN NORTH-CENTRAL KANSAS

(Em,1937, p.411) Number

Phases established chiefly on paleontologic evidence

Corresponding typical lithology

Red shale Green shale Lingula phase

to fine sandy shale, } Clayey rarely consolidated Sandy, often varved (?), rarely clayey shale Clayey shale, mudstone to bedded limestone Massive mudstone, shaly limestone

Molluscan phase Mixed phase Brachiopod phase Fusulinid phase Brachiopod phase Mixed phase Molluscan phase Lingula phase

Green shale

Red shale

}

Limestone, flint, calcareous shale Massive mudstone, shaly limestone Clayey shale, mudstone to bedded limestone Sandy, often varved (?), rarely clayey shale Clayey to fine sandy shale, rarely consolidated

99

UNITED STAT= OF AMERICA TABLE XVII SEVEN MAIN TYPES OF CYCLE IN OHIO ROCKS

(After STOUT, 1931)

A.

Clay, fresh-water Shale and sandstone, largely marine Iron ore, marine Limestone, marine Coal, fresh-water

B. Clay, fresh-water Shale and sandstone, largely marine Limestone, marine Coal, fresh-water

C. Clay, fresh-water Shale and sandstone, probably brackishwater or marine Shale, brackish-water Coal, fresh-water

D. Clay, fresh-water Limestone, fresh-water Shale and sandstone, partially marine Limestone, marine Coal, fresh-water

E. Clay, fresh-water Limestone, fresh-water Shale and sandstone, fresh-water Coal, fresh-water

F. Clay, fresh-water Limestone and calcareous shale, fresh-water Coal, fresh-water

G. Clay, fresh-water Shale and sandstone, fresh-water Coal, fresh-water

REGER(1931) described the West Virginian succession in terms of cycles with a “maximum succession” of beds in each cycle as follows: (9) Sandstone. (8) “A” coal or “rider” coal (ridercoal). (7) Ferruginous shale or sandy shale (overshale B). (6) Ferruginous or sandy limestone, with or without marine fossils (overlimestone). (5) Black fissile shale with plant fossils or a brackish-water fauna (overshale A). ( 4 ) Principal coal. (3) Fireclay or fireclay shale (undershale B). (2) Fresh-water limestone (underlimestone). (I) Sandy or red and variegated shale (undershale A). Mainly for practical reasons (e.g., mapping) the tops of the sandstones were chosen to divide the succession into cycles. REGER(1931) was so impressed with the cyclical arrangement of the beds he proposed, for incomplete successions, that missing members should be named and referred to as “phantoms~’.This scheme was considered applicable to the Mississippian and Permian successions of the Appalachians. It

100

TRANSITIONAL REGIMES IN NORTH AMERICA

TABLE XVlII VARIATIONS IN APPALACHIAN BASIN PENNSYLVANIAN CYCLOTHEMS ~

~

~~

_

Lower Pennsylvanian Pottsville Group Allegheny Group Conemaugh Group Monongahela Group Monongahela Group Ohio Dundee, Ohio Appalachians Ohio Central Appalachians Ohio (STURGEON et al., (GRAY,1961) (BRANSON, (BRANSON, 1962b) (BRANSON, 1962b) (BRANSON, 1962b) 1958) 1962b)

Shale, marine Limestone, marine Shale, brackish or marine Coal Underclay

Coal Underclay or mudstone Shale

Limestone (may contain fresh water fossils) Shale Sandstone

Shale, marine or Limestone, marine Roof shale limestone or shale Coal Coal Coal Underclay Underclay Underclay

Coal Underclay

Limestone, fresh Claystone, red water calcareous

Limestone, fresh water

Limestone, fresh water

Sandstone and shale

Claystone, red

Claystone, red

Mudstone Sandstone

Sandstone

Sandstone and shale

Shale, sandy Sandstone

Shale Iron ore and/ or chert Local disconformity Limestone Shale -~

~

_

_

.

~

~

~~

~

was calculated that there were 58 cycles in the Pottsville rocks, five in the Allegheny, eighteen in the Conemaugh and ten in the Monongahela. ASHLEY (1931), however, was not so impressed with the cyclical nature of the beds in Pennsylvania. While accepting the great lateral extent of some beds and the possibility of an unconformity at the base of most sandstones he did not feel there was any persistent order of units in the succession, apart from the common association of argillaceous deposits below, within and above coals. More recent work has underlined the areal and stratigraphical differences in the

_

_

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UNITED STATES OF AMERICA

-

MonongahelaGroup Pennsylvania, W. Virginia (BRANSON,1962b)

__

Lowerand Middle Monongahela Group Ohio, Pennsylvania, W. Virginia (STURGEON et al., 1958)

Monongahela Group Upper Monongahela and Dunkard Group, and Permian Athens Co., Ohio Ohio, Pennsylvania, W. Virginia (STURGEON et al., (STURGEON et al., 1958) 1958)

Shale, sandy; or disconformity Shale, silty, calcareous Shale, with plants Shale, with roof coals and plants Limestone, brackish Limestone, brackish Roof shale water water Coal Coal coal Underclay Clay shale Underclay Limestone, fresh water

Limestone, fresh water(?)

Sandstone

Siltstone, calcareous Redbeds

Sandstone

Limestone, fresh water

Upper Monongahela and Lower Dunkard Groups Ohio, Pennsylvania, W. Virginia (BEERBOWER, 1961)

Clay, grey plastic (This compares with succession given by CROSSand ARKLE Claystone, gray, limy/ (1952) save that their limestone description commenced Claystone, red, limy/ with clay shale) limestone Siltstone Sandstone Shale, silty calcareous; Siltstone or disconformity Shale, silty calcareous Shale, silty with roof coals Shale, with plants Shale, clay Coal, bony, shaly

Shale

Limestone, brackish water coal Clay shale

Limestone Coal

Limestone, microfossils

Limestone, argillaceous nonfossiliferous Shale and sandstone Limestone, silty Sandstone

Appalachian Basin. Table XVIII summarises many of the cyclothem types described. It is obviously impossible to speak of an “ideal” Appalachian cyclothem. The variety, both in type and in number, makes generalisation difficult. Some of the earlier estimates of thickness and numbers have been given. In Ohio recent information indicates a distribution like that given in Table XIX. Pottsvillian rocks perhaps show the most striking variations when considered over the whole basin. In southern West Virginia a coal seam some 4,000 ft. and 60 cyclothems from the base of the Pottsvillian is correlatable with a seam 300 ft. and 10 cyclothems from the base of the Pottsvillian in

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eastern Ohio and western Pennsylvania. “Part of the diminution is through loss of the lower cyclothems by overlap on an unconformity. Some additional changes are through interruptions by erosion surfaces within the cyclothems sequence, and by convergence within single cyclothems” (KAY and COLBERT, 1965, p.250). The same authors state that while the average thickness of a cyclothem is about 30 ft. in Pennsylvania 60 ft. is approached further south. A single cyclothem a few feet thick in northeastern Ohio swells to 500 ft. in Tennessee. The difficulties in trying to pick one representative cycle are apparent from Tables XVII and XVIII. This by no means exhausts the list. BRANSON(196213) also referred to 9-, 10- and 11- unit cyclothemsfrom the Conemaughgroup of the Pittsburgh area. CROSS and ARKLE (1952) described Conemaugh, Monongahela and Dunkard Group rocks in Ohio and West Virginia and said it was impossible (p. 103) “to outline a single type of cyclical sequence of component sediments for the entire area of any of the major series. . .” because of facies changes. They did, however, suggest an 1 1-unit cyclothem for the Monongahela series in the central and western part of the Dunkard area, pointing out that some of the members are quite different lithologically when traced east and north. CROSS and SCHEMEL (1956) described a variant as representative of the upper Monongahela and lower Dunkard in West Virginia while postulating an 8-unit cycle for the upper Dunkard. BEERBOWER (1961) considered that the Dunkard Group in Pennsylvania, West Virginia and Ohio includes about 40 cyclothems (cf. Table XIX). TABLE XIX THICKNESS A N D NUMBER OF CYCUlTHEMS IN OHIO

Group

Dunkard (permian)l Monongahela’ Conemaugh1 Allegheny1 Pottsville2 (highest part) 1

Number of cycles

15 12 15 12 10

Total thickness

Average thickness

(ft.)

(ft.)

330 40 1 547 263

22 33 36 22 18

180

STURGFON et al. (1958). BRANSON (1962b).

Rocky Mountain Region Mixed continental-marine cycles have of course been described from systems other than the Mississippian, Pennsylvanian and Permian in the U.S.A. YOUNG(1955, 1957) recognised cyclic sedimentation in Upper Cretaceous beds

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in central and eastern Utah and western Colorado. It proved possible to divide up the succession into marine shales and continental sandstones, shales and coals. The alternation was due to the intertonguing of continental deposits from the west with the marine Mancos shale from the east. Young interpreted the sequence as showing cycles of sedimentation which, when fully represented, would show a complete cyclothem as follows: (4) Coal. (3) Lagoonal deposits. (2) Littoral marine sandstone. ( I ) Marine shale. The marine shales, disconformable on the beds below, were regarded as being deposited in deeper water off-shore at the same time as part of the littoral marine sandstones. As the beaches grew seawards, however, sands covered the earlier deposited marine mud, hence the latter was designated unit 1. The sandstones (unit 2) can be up to 100 ft. thick, though in places they are represented by sandstone and shale, while they can be absent altogether. The lagoonal deposits (unit 3) are carbonaceous shales, silty shales, sandstone and coals which can reach 150 ft. in thickness. Unit 4 is a thick coal. Young interpreted the beds of unit 3 as being made up of incomplete cyclothems lacking in the marine sandstones and shales of units 1 and 2. He therefore, rather ambiguously, designated a complete sequence with widespread units I and 2 at the base a megacyclothem, which contained numerous cyclothems (e.g., in unit 3). SABINS(1964) described beds of similar age from the San Juan Basin in New Mexico. He interpreted the succession in terms of three lithological members: (a) Marine shale, containing two widespread thin limestones; (b) Marine sandstone, transgressive or regressive; and (c) Continental strata-interbedded coal, claystone, carbonaceous shale and fluvial sandstone. He considered a symmetrical cycle occurred as follows: Continental strata. Regressive marine sandstone. Marine shale. Transgressive marine sandstone. Continental strata. The cycles were defined (p.294) by “arbitrary divisions approximately through the midde of the continental units.” The marine shale represented both transgressive and regressive phases. Somewhere in the middle marked the greatest depth of water. Transgressive sandstones are rare so most cycles are in fact asymmetrical. The thicknesses of the constituent units of the cycles are measurable in hundreds of feet and the “continental strata” contain many coal seams. As with the sequences described by YOUNG(1955) it is apparent that these cycles from New Mexico are of a different order of magnitude from those described from the Palaeozoic. Periodic, relatively fast, subsidence was postulated as the main reason for the marine transgressions (SEARSet al., 1941; YOUNG,1955), the regressions being due to sediment

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accumulation during periods of slower subsidence. The theory of “intermittent subsidence’’ is discussed in Chapter 5 and objections raised to its validity for the formation of Carboniferous cyclothems might also be considered with regard to these Cretaceous examples.

CANADA

Nova Scotia

It is perhaps appropriate to end this descriptive section of North American coal-bearing sequences by returning to the Carboniferous succession of Joggins Bank, Nova Scotia, mentioned at the beginning of the chapter. Strictly speaking, as no marine beds have been identified, such cycles as are present should be dealt with elsewhere. Even so it is doubtful if the Joggins section should be dealt with under “continental cycles”. From published descriptions the rocks sound very similar in facies and fossil content (ROGERS, 1965) to the European Coal Measures and it is with them they should perhaps be grouped. On geographical and historical grounds, however, we prefer to describe them along with other North American coal-bearing cycles. COPELAND (1959, p.16) gave a “general sequence of deposition in each cycle” as: (7) Sandstone and shale, grey, interbedded. (6) Shale, grey, sometimes containing ironstone nodules. (5) Shale, black, carbonaceous with ostracods and pelecypods. (4) Shale, grey to black, highly calcareous with ostracods and pelecypods. (3) Coal. (2) Underclay. (1) Sandstone. “Some cycles may lack bed 2 or 3, or both, and in some places bed 4 may occur in an incomplete cycle below bed 2 or intercalated with bed 5”. Copeland also included graphic sections showing “types of cyclical deposition” as follows: 1,2,3,4,5,6,7,6; 1,2,4,2,3,4,5,6,7; 1,6,2,3,5,4,5,6,7; and 1,6,3,5,6,7. He pointed out that most cycles cannot be traced along the strike due to lateral variation, though the cycles containing the major coal seams were recognisable over a wide area. In some 5,000 ft. of beds, comprising part of the Upper Carboniferous (Westphalian) Cumberland Group, more than 60 cycles have been noted, varying in thickness from a few inches to several tens of feet.

THEORIES OF ORIGIN

As pointed out in Chapter 1 there is a crying need for more objective and quantitative data on cyclothems. In the preceding pages it is obvious that the separation of the

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modal cycle, the composite sequence and the ideal cycle is not easy. What seems to have been given in most cases is the composite sequence, while variants mentioned correspond presumably to modal cycles. The confusion in nomenclature is only partly due to the failure of most authors to separate properly observation from interpretation; it is also a reflection of the very great variation in cycle types present in the Pennsylvanian of the U.S.A. We think it is essential to correct the view, implicit in many publications, that the Illinois cyclothem typifies the American Pennsylvanian. It does not, as WANLESS (1950) and WELLER(1956, 1957, 1961), for example, have beenatpains to point out. Furthermore, while a few individual cyclothems are traceable over considerable distances, many are not. Even in those that are traceable through, say, Oklahoma, Kansas, Missouri, Illinois and Kentucky, areal variations are considerable (Fig.48), yet this part of the succession “shows the finest and most uniform display of cyclic sedirnentation” (WANLESS et al., 1963, p.441; our italics). Hardly a lithology persists throughout. There are considerable correlation problems between the various Pennsylvanian areas described in the text. Very few units in the stratigraphic column are correlatable over the whole area. Those correlations that are generally accepted involve, in the main, limestones. Between these marker bands, however, great changes in thickness, lithology and numbers of cycles takes place. Even within one state variations are such that cyclothems are not considered useful as units of stratigraphical correlation (KOSANKE et al., 1960; WANLESS,1962). Theories of origin therefore must take these factors into account. It seems quite unrealistic to try to explain the occurrence of numerous cyclothems, stretching over vast areas, each with a certain number of units, constant and always in order, as if such idealised cycles existed. They demonstrably do not. Theories to explain such an imaginary situation usually result in the authors trying to explain difficulties of their own making! Cyclic sedimentation in the coalbearing rocks of the Pennsylvanian does exist but, from the evidence we have reviewed, in a very much more modified form than that generally believed. DOTY and HUBERT (1962, p. 10) put the situation most clearly. “The principle of the “cyclic concept” of sedimentation in the Pennsylvanian coal measures may be generally applicable as stated by many authors. It may be more realistic, however, since ideal cyclothems are seldom encountered, to think in terms of specific environments of varying lateral extent that migrate across any given locality repeatedly but in poorly ordered sequence.” Before considering theories of origin, however, various environmental and sedimentological aspects must be discussed. Outside of the Appalachian Basin it is generally assumed, with Pennsylvanian cycles, that the beds above the coal are marine while those below are non-marine, as a glance at many of the foregoing figures illustrates. WELLER (1957) gave a very comprehensive summary of much of the evidence for these views. But KOSANKE et al. (1960, p.16), expressed a note of caution: “It is widely accepted that the sandstone-underclay coal sequence in each cyclothem is non-marine whereas the shale-limestone sequence above the coal is marine. However, most of the exceptions noted above indicate a trend toward accepting as marine some of the beds supposed to be non-marine and it is not impossible that more of the sequence is marine or brackish-water than is now

106 > 0 X

a IL W X

ti

z

0

z W ( I

-I

.. a

m a

ma

w

TRANSITIONAL REGIMES IN NORTH AMERICA

THeORIES OF ORIGIN

107

accepted. The fresh-water origin of the coals limits the trend in that direction.” WILSONand STEARNS (1960) held similar views regarding the Tennessee cycles. SIEVER(1957) drew attention to the difficulty of assigning a marine or nonmarine origin to the sandstones and POTTER (1963) postulated that the sheet sandstones (see later) were laid down during a marine regressive stage. SWANN(1963, p.26), writing on Mississippian rocks, also considered that the sandstones (similar to those in the Pennsylvanian) might be marine. WELLER(1957, p.329) supposed Pennsylvanian rocks to be “aqueous deposits laid down under a variety of piedmont, valley-flat, lake, marsh, delta, lagoon and shallow marine environments.” He emphasised that conditions changed from place to place and therefore gradations were possible both vertically and laterally. POTTERand GLASS(1959, after detailed petrological studies in southern Illinois, visualised the environment to be a coupled low-lying coastal plain and marginal shallow shelf. With oscillations of the strand line near-shore marine, littoral, tidal-flat and non-marine sediments would all occur. They adduced a wealth of data on sedimentary structures and petrography to support their views. WANLESS et al. (1963), in a detailed regional study of three cyclothems throughout the Mid-Continent, concluded that the clastic sediments were mainly pro-delta shales and deltaic sandstones and shales. The upper limestones and some of the shales were taken to be marine. R. C. Moore’s interpretation of the environment in Kansas (Fig.42) was, however, somewhat different, as like Weller he considered the basal sandstones to be continental deposits. BEERBOWER (1961), in discussing the non-marine Dunkard Group of the Appalachians, made detailed comparisons with the Mississippi Delta, as indeed did POTTER (1963), when considering Mississippian and Pennsylvanian sedimentation in Illinois in general. Sandstones are theoretically perhaps the most important rocks in the sequence as conflicting theories on the origin of cyclothems depend on the interpretation of their origin. As has been mentioned, WELLER (1930) decided that the base of the sandstone should be the starting point of the cyclothem (Fig.40). He noted that there was frequent erosion of the beds below and that the sandstones often occurred in channels cutting down a fair way into these beds. The sandstone therefore marked the base of an unconformity (or disconformity as it was later called) anduplift and erosion supposedly occurred before the deposition of each sandstone. Late Palaeozoic cyclothems were developed by “repeated oscillations, each consisting of a long gradual subsidence followed by a short sharp uplift, both centering in the area from which the sediments were derived.. .” (WELLER,1956, p.17). This is the essence of the diastrophic-control theory. The crucial part of the theory is the identification of the disconformity and its selection as the most important recurring event in the sequence of deposition. In choosing the base of the sandstone Weller differed from his predecessor, UDDEN (1912), and from many workers dealing with similar deposits in western Europe. The latter have generally chosen the cessation of coal-forming conditions as the most significant point to end the cycle of sedimentation. WELLER (1956) vigorously defended his theory but admitted that (p.30) “The selection of this boundary is a matter of

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TRANSITIONAL REGIMES IN NORTH AMERICA

opinion and choice is dependent on evaluation of various practical and theoretical considerationswhich are differently emphasised by different persons.” He believed in the disconformity and (p.29) “with this beginning, the cycle of deposition is brought into close connection with the cycle of erosion with which it is believed to be related.” What then is the evidence for a disconformity at the base of a sandstone and if there is one how significant is it in the development of a cyclic succession? POTTER (1963) classified Mississippian and Pennsylvanian sandstones in Illinois into two types; thin, relatively widespread, sheet sand bodies and thick lenticular, sometimes discontinuous, elongate sand bodies. Sheet sandstones can be lateral equivalents or lie above elongate sand bodies. The elongate bodies have four distinct distribution patterns: pods, ribbons, dendroids or belts. While the lower contact of the sheet sandstones is generally transitional (i.e., there is no evidence of a disconformity) the elongate bodies commonly have a non-transitional, unconformable basal contact. Many of the elongate bodies occur in channels, cut down into the underlying sediments and have been called “channel” sandstones to distinguish them from the “sheet” sandstones. They have been the subject of much study (e.g., MUDGE, 1956; RUSNAK,1957; SIEVER,1957; WANLESS, 1957; HOPKINS,1958; POTTERand GLASS,1958; FRIEDMAN, 1960; ANDRESEN,1961; DOTYand HUBERT,1962; POTTER, 1963), and differing views have been expressed concerning their mode of origin. Most writers have favoured a subaerial origin of the erosional channels with later infilling by stream aggradation. This latter was brought about by a change in base level due to advance of the sea into the area. Opinions have varied as to whether the infilling of the channel was alluvial, part alluvial-part marine, or almost exclusively marine. SIEVER (1957) pointed out the difficulties in assigning a marine or non-marine origin to Pennsylvanian sandstones in general and inferred that deposition took place in a combination of a variety of shallow-water marine, deltaic and coastal plain environments. He also said that while most of the channel sandstones of western Illinois showed clearly, from the sharp abrupt bases and borders, that they had been deposited in erosional channels, many in southern Illinois showed sedirnentational rather than erosional facies changes. RUSNAK (1957), in his study of the Pleasantview Sandstone in western Illinois, considered the channel to have been cut by tidal current action and later to have been filled with sediments through the action of both coastal and tidal currents. He compared the environment with that of the Dutch Wadden Sea where broad tidal flats are scoured by channels of tidal origin which are being filled with sand swept in from tidal flats and the North Sea. That channels can be cut and filled under water is also known from the work of FISK(1955) who showed that as a delta builds outwards distributaries scour channels into underlying sands and subsequently these channels are filled with sand. In the Pennsylvanian, scouring by delta distributaries has been considered sufficient to account for most sandstone channels even when these cut down into underlying shales, etc., to depths of 100 ft. (WANLESSet al., 1963, “Such a hypothesis diminishes the necessity for a series of tectonic uplifts .. p.447). KOSANKE et al. (1960) commented on the uncertain origin of the channels and emphasised that the frequency of channel-

.”

THEORIES OF ORIGIN

109

ing is relatively low. The physical evidence for an unconformity is present in less than 20 % of the area for any one cyclothem in Tllinois and no unconformities are known for some. BEERBOWER (1961) also considered channel sandstones of Dunkard age in Pennsylvanian and adjoining states to be of distributary origin. After reviewing much of the literature he thought that the case for a subaerial origin for the channel sandstones of the Illinois Basin had not been proved. He emphasised the contemporaneity of channel cutting and inter-channel deposition in many cyclothems. SWANN(1964) was adamant that the importance of discordant surfaces beneath sandstones in both the Mississippian and Pennsylvanian had been exaggerated. He pointed out, in his study of Mississippian rocks, that continuous deposition of con-

formable shallow-waterlimestone at the same time as channeling took place elsewhere made any theory of regional uplift untenable (see, however, p.115). Postulating an unconformity at the base of the sandstone meant that “the products of a single pulse of clastic deposition are unnaturally split between two adjacent units. Pro-delta shale is assigned to one cyclic unit, whereas the deltaic sand from which it was winnowed is placed in the overlying “younger” one.” (SWANN, 1964, p.652). Since the evidence for the disconformity at the base of each sandstone is therefore equivocal so must be the claim that the disconformity marks the beginning of a non-marine phase following on a marine period. On theoretical grounds its choice as the beginning of a cyclothem can therefore be criticised. There are also serious practical objections. Sandstones are the least laterally persistent of the common rock types present in a cyclothem (Fig.49). Attempted recognition of cyclothems in successions where sandstones are missing can introduce an undesirable element of subjectivity into the study, not to mention considerable difficulty. SIEVER (1957) and various Geological .Surveys have commented on this latter aspect. In Illinois, cyclothems, as recognised using the sandstone base, proved impossibleto use as rock-stratigraphic units (KOSANKE et al., 1960). Similar difficulty was encountered in Ohio where STURGEON et al. (1958) showed how the coal seam was the most logical place to complete a cyclothem but because of “nomenclature difficulties” retained the sandstone as the marker (surely a case of the tail wagging the dog!). In Kansas H o w (1956) used the coal seam to delimit the cyclothems. R. C. MOORE(1959, p.51) wrote: “ ( I ) Practical utility, accompanied by precision in ability to place boundaries in the field (coal horizons being identifiable where coal beds are absent) favours the top of coal boundary; (2) the ending of coal swamp sedimentation (or lacking coal, termination of non-marine conditions) and change to marine environments furnish the most nearly universal “punctuation point” in cyclic successions in all regions.” On “evaluation of various practical and theoretical considerations” (WELLER, 1956, p.30) it is clearly possible to disagree with the choice of sandstone as the first unit of the cyclothem. Theories of origin of Pennsylvanian cyclothems in America have fallen into two main groups-those that favour a tectonic mechanism involving intermittent move-

110 TRANSITIONAL REGIMES IN NORTH AMERICA

THEORIES OF ORIGIN

111

ment of the earth’s crust and those that produce the necessary sea-level changes mainly by climatic means. In the first group we find intermittent subsidence of the depositional area being invoked to account for the cycles (UDDEN,1912; STOUT,1931). Stout, writing on Ohio, thought that coal formation represented a period of virtual standstill. Rapid submergence brought the period of accumulation of vegetable material to a close. Filling up of the area with sediments continued until plant life could re-establish itself, then, after a period of extremely slow subsidence, the cycle commenced anew. It was considered that the weight of accumulating sediments played a part in producing the subsidence. Many European geologists have put forward a similar theory to account for Carboniferous cycles and a full discussion is given in Chapter 5. The diastrophic-control theory (WELLER,1930, 1931, 1956) also falls into this group. This theory, to recapitulate, seeks to explain an idealised cyclothem (Fig.40) which, in considerably modified forms (Fig.41), recurs throughout the Pennsylvanian succession in Illinois. The base of the cyclothem is taken as the disconformity thought to be present at the base of each sandstone. The implication is that the beds above the disconformity are non-marine while the beds above the coal, the top of which separates the lower from the upper “hemicyclothem”, are marine. Marine conditions were brought to a close by uplift of both depositional and source areas. The change to nonmarine (? continental) conditions is marked by the presence of the disconformity, which heralds in another cycle of deposition thought to be closely related to the cycle of erosion. The periodic episodes of uplift are, of course, superimposed on the overall subsidence of the depositional area. Doubt has already been expressed about the evidence for uplift provided by the sandstones. The occurrence of coal seams only a few feet above marine limestones, in some cyclothems, has also been taken to indicate that uplift must have occurred. Assuming that neither withdrawal (and consequent shallowing) of the sea nor uplift of the sea bed took place, then the depth of limestone deposition must be the uncompacted thickness of sediments between the limestone and the overlying coal. Figures calculated on this assumption are much smaller than the depths of deposition of the limestones estimated by some paleontologists, e.g., during the accumulation of certain Kansan Permian Limestones depths of 160-1 80 ft. were envisaged (ELIAS, 1937, 1964). Other work has shown, however, that many such limestones may have formed in waters considerably shallower, as discussed in Chapter 10 (LAPORTE, 1962; MCCRONE,1964). IMBRIEet al. (1964), after studying various facies of the Lower Permian Beattie Limestone in Kansas, went so far as to state that little more than a fathom of water was required for the fusulinid-bearing beds (still considered the deepest facies by ELIAS, 1964 - see also discussion, p.235). Thereis muchevidence that areal differences in the number of cyclothems making up a specific part of the succession are frequently due to the splitting of cyclothems (GRAY1961; KAYand COLBERT, 1965; WANLESS, 1964). Once splittingis conceded then it must be accepted that either adjacent portions of the earth’s crust are moving upwards and downwards at differing rates or that one portion is subsiding separately

112

TRANSITIONAL REGIMES IN NORTH AMERICA

and cyclothems in that area are being produced without uplift being involved. The first alternative implies most complicated movements of the earth’s crust. The second is perhaps not quite so difficult to accept as feasible. But it poses the question that if some cyclothems can be produced without uplift why not all? Objections were raised to the diastrophic-control theory from its inception and alternative hypotheses involving climatic control, both direct and indirect, have been put forward. WANLESS and SHEPARD (1936) introduced a theory of indirect climatic control involving repeated glaciations in Gondwanaland. They emphasised areal variations in Pennsylvanian sedimentation. Some areas were predominantly marine, some mixed continental-marine and others predominantly continental. This depended on the proximity of the depositional area to the land-mass supplying the sediments. Despite facies variation, however, a general picture emerged. More or less continuous subsidence, with sea level changing somewhat rhythmically in response to the waxing and waning of continental ice sheets, was postulated in the glacial-control theory. Evidence of glaciation during the Carboniferous period was cited from South America, Africa, India and Australia. With the onset of a glacial epoch sea level was lowered, upland vegetation became scarcer and the rate of erosion increased. Fans and deItas grew outwards from the upland areas towards the sea with channel cutting quite common. The conglomeratic beds of the piedmont areas passed into sandstones and sandy shales on the seaward margin of the deltaic areas. A slight increase of humidity associated with the beginning of glacial melting increased vegetation growth so that the rates of erosion and deposition decreased. As a consequence the sediments became finer grained and these were spread over the delta along with, probably, wind-blown loess. Weathering and soil-forming processes altered the fine material (which later became underclay). The continued rise of temperature and the beginning of melting of the ice sheets increased the humidity thus encouraging the growth of vast forests which spread across the mud-flats. Peat accumulated and swamps developed. Continued melting of the ice caused sea level to rise, drowning the swamps and covering them with mud, the only material then being washed down from the plant-covered uplands. As the sea deepened over the area more and more mud was deposited in the embayed valleys of the drowning hinterland. Further out to sea conditions were suitable for limestone formation. Lowering of the sea level at the onset of the next glacial epoch rejuvenated the rivers in the landward areas which first spread mud and then coarser material over the limestone and so the cycle commenced again. With this mechanism it was possible to explain the different facies found in, say, Kansas, Illinois and Pennsylvania. WELLER (1956) criticised the theory on, among other, grounds that Late Paleozoic glaciations did not correspond in time to the periods of cyclical deposition (Mississippian-Permian), and that there was no evidence for the large number of glacial and interglacial periods required to account for the number of cyclothems known. Further, he doubted if there was evidence for extensive upland Carboniferous floras existing during interglacial stages and thus inhibiting run-off, erosion and sedimentation. While it was thought at one time that glaciations in Gondwanaland were con-

THEORIES OF ORIGIN

113

fined to Upper Carboniferous-Permian times, recent work (see, for example, GRINDLEY,1963) shows that tillites are also found as far down as the Lower Carboniferous. The number of glaciations required in the glacial-control theory is a more serious problem but WANLESS (1960) considered that in Australia there is evidence for many more glaciations than was previously thought. Just how many glaciations would be required is a difficult question to answer, ofcourse, as the number of cyclothems present in the Pennsylvanian differs considerably, according to various authors, as we have seen. Splitting of cyclothems also poses a problem in this respect and WANLESS (1964) has emphasised the importance of separating the regional from the local in areal cyclothem studies, while pointing out the difficulties in attempting this. WHEELER and MURRAY (1957) carefully analysed both the diastrophic-control and the glacial-control theories. They considered that in each Pennsylvanian cyclothem there is evidence for two transgressions and two regressions of the sea. These were accounted for by correlation with the four phases of SIMPSON’S (1940) solar radiation glacial cycle. The required extra oscillations of sea level made the diastrophic-control theory completely untenable and rendered the glacial-control theory in need of considerable modification. The modified theory, however, is not one which commends itself at first sight. Firstly, Simpson’s theory is itself controversial and by no means accepted by all meteorologists. Secondly, some of the geological arguments can be criticised. That each cyclothem commences with a sandstone, beneath which is a disconformity, is accepted by Wheeler and Murray as fact. No correlations are given, yet we find the statement (p.1992) in the comparison of Kansas with Illinois, “it is generally agreed and only logical that the disconformity represents the same episode of sharp base-level lowering in both regions . . .”. Which disconformity is being considered is not mentioned, nor the fact that disconformities are usually absent. The difficulties of equating the type of cyclic sequences in Kansas and Illinois (see p.95) similarly receive no consideration. It is difficult to accept the theory, on the evidence produced, as other than a provocative exercise in geological speculation rather than as an hypothesis to explain observed facts. Theories involving direct climatic control have been advocated by SWANN (1964) and BEERBOWER (1961). Swann’s work concerned mainly Chesterian (Upper Mississippian) rocks in I!linois, a sequence differing from the Pennsylvanian in that it contains a much higher proportion of limestone (25 %) as compared to 4 % for the Pennsylvanian as a whole. No attempt was made to define a Chesterian cyclothem in terms of individual members but Swam emphasised that the succession consists of alternations of limestones and clastic units. The subsiding Illinois Basin was thought to have been repeatedly filled by either locally precipitated limestone or clastics carried from eastern Canada by the “Michigan River” System. A marine period, during which limestone accumulated, was brought to a close with the change to a drier climate. In the source area the consequent thinning of the cover of vegetation resulted in increased erosion and sediment yield. The Michigan River spread clastics for hundreds of miles over the limestone in the form of a huge delta which frequently developed an upper surface near

114

TRANSITIONAL REGIMES IN NORTH AMERICA

water level capable of supporting plants (thin coals occur in the upper parts of most clastic units). lncreased humidity followed and plant growth flourished in both source and depositional areas. As a result, in the former erosion and in the latter clastic deposition slowed down. Basin-sinking then proved too fast both for the plant growth in the sea level swamps and clastic deposition in other parts of the delta. The sea therefore encroached and limestone was deposited. Return to a drier climate started the cycle all over again. Swann recognised twelve to fifteen major cycles of advance and retreat of the shoreline. He considered that sea level remained relatively static and that variations in rainfall controlled the sequence of events. The diastrophic-control theory was rejected on the grounds that the importance of discordant surfaces beneath sandstones had been exaggerated, that channels when present were sub-aquatic, and that in any case continuous deposition of limestones on the nearby Cincinnati arch, while cycles wers being formed in Illinois, ruled out regional uplift. The non-rhythmic nature of those limestones also prevented acceptance of regional tectonic or worldwide eustatic changes in sea level as a possible cause of most of the cycles. Sea-level changes were conceded in the highest Mississippian Beds and in the Pennsylvanian. The nonmarine nature of the Permian Dunkard Group cycles (see later), supposedly of similar origin to Pennsylvanian cycles, together with the evidence from most of the higher Mississippian Beds, indicated that the primary cause of cyclic sedimentation was not to be sought in changes of sea level. Interruptions in the supply of sediments to an area, so that periodically carbonate deposition may proceed unhindered, have been explained by postulating changes in the drainage patterns of deltas (e.g., D. MOORE, 1958; GOODLET, 1959). In the present case, however, enough is known of Mississippian palaeogeography to make it extremely difficult to find a site to which clastics were diverted (SWANN, 1964, p.655). It has been suggested that the “Ontario” River, source of the lower Mississippian Berea Sandstone of the Appalachian basin, for a time joined the Michigan River System, during which period the “Michigan Berea” was deposited (PEPPERet al., 1954). Swann accepted that such switching might happen once or twice but doubted if such a process could regularly operate throughout Mississippian-Pennsylvanian times. It would imply, for instance, that clastic and carbonate deposition in the Illinois and Appalachian Basins were out of phase. Most correlations suggested that this was not so. It seemed therefore that only climatic fluctuations were left to explain the intermittent supply of sediments to the Illinois Basin. Swann’s case is well-argued and the theory an attractive one. But there are several points which require further discussion. It is stated that in the succession under discussion some seventy reversals took place in the direction of shoreline movement. Not all can be claimed to be due to climatic changes. The case for climatic changes hinges on the proof that periodically no clastics entered the Illinois Basin. Widespread limestone beds, correlatable throughout the area, provide the evidence. All limestones, however, are not basin-wide. Further, shifting of the Michigan River System, throughout the depositional period under review, seems well established (SWANN,1964, fig.2).

THEORIES OF ORIGIN

115

It appears therefore that only certain cycles can be claimed to provide evidence of climatic changes. The majority might be explained in terms of “delta-switching”. Twelve to fifteen major cycles are mentioned by Swann though it is not brought out in the text which these are and “It is difficult to define “major cycle” precisely, because cycles that appear important in one region seem insignificant or can not be recognised elsewhere” (SWANN,1964, p.639). The case against changes in sea level rests mainly on the assumption that there is no evidence of this in the successions of contemporaneous limestones formed in different environments nearby. But have these limestones been examined with, for example, cyclicity or the presence of non-sequences in mind, and by modern statistical methods? (p. 15). The argument that cyclic sedimentation occurs in non-marine successions, such as the Dunkard Group, and therefore sea-level changes are ruled out, is relevant only if the similarity of Dunkard to Mississippian cycles is accepted and a common origin for both assumed. Finally, while accepting the relationships between precipitation, run-off and sediment yield established by LANGBEIN and SCHUMM (1958), it must be remembered that during Mississippian times many unknowns, such as temperature and precipitation, and differences in these in both source and depositional areas complicate the simple analogy used. Dunkard Group cycles in Pennsylvania, West Virginia and Ohio (see p.101) were discussed at some length by BEERBOWER (1961). Me put forward a unit-by-unit correlation of two dissimilar (to us) idealised cyclothems (Illinois type and Dunkard type) which stretched credulity to the limits. A common origin was postulated for both and indeed for Carboniferous cycles the world over. Synchroneity of cycles was assumed. The necessary world-wide cause was considered to be periodic climatic changel. In summary, cyclic sedimentation took place during Pennsylvanian (and to a lesser extent Upper Mississippian and Lower Permian) times in America. Sedimentation involved in the main alternations of clastic and con-clastic material, the latter coals and limestones. The limestones were in the main marine, the coals non-marine. Some of the shales were marine but the bulk of the sandstones and shales were laid down in conditions which we are still not sure about. In the Mid-Continent conditions were predominantly marine, in the Appalachians apparently predominantly nonmarine. The order in which the beds occur is not as regular as one might be led to expect from the literature. Correlation from basin to basin is not refined enough for most individual cycles to be traced within a basin and certainly not from basin to basin. Certain limestone horizons, certain marine shale horizons and probably certain coal horizons are exceptional. Their deposition might represent periods when events took place on the continent-wide scale-the limestones and shales might even represent periods when there was a world-wide rise in sea level. The bulk of the cycles seems,

These (1961) views have been drastically altered (BEERBOWER, 1964).

116

TRANSITIONAL REGIMES IN NORTH AMERICA

however, to be explicable by more localised events such as the continually changing pattern of sedimentation at the junction of the sea with a broad deltaic or alluvial coastal plain, though still on a large enough scale to make modern analogies difficult to find. Further discussion takes place after the review of European cyclic deposits in Chapter 5.

Chapter 5 TRANSITIONAL REGIMES, II-EUROPE

GREAT BRITAIN

The Carboniferous rocks of Great Britain display a great diversity of cycles consisting of alternations of marine and non-marine strata. This diversity is a reflection of the complicated palaeogeography. In very general terms the Lower Carboniferous rocks of Britain indicate marine conditions in the south and what are taken to be deltaic conditions in the north. Periodically the northern areas were inundated by the sea. Complications due to the presence of various land-masses, occasional isolation of portions of the depositional area and areal tectonic differences considerably altered this broad picture from time to time (e.g., see WILLS,1951, and GEORGE,1958). Throughout the Upper Carboniferous deltaic conditions gradually spread and by Coal Measures times Britain was mainly an area of non-marine deposition, though still subject to occasional incursions of the sea.

Visean, Tournaisian and Namurian Because of the complexities of deposition in I ffering environments stratigraphic classification of the Carboniferous rocks in Britain has raised many problems. Fig.50 shows the currently accepted correlation and is useful in indicating the facies changes from south to north in Visean and Tournaisian times. As stated in Chapter 1 it was in the north of England and in southern Scotland that cyclic deposition was first recognised in Britain. PHILLIPS (1836), MILLER (1887) and HIND(1902) were all impressed with successions showing a cycle which is basically limestone-shale-sandstone-coal. HUDSON(1924, 1933), BROUGH(1928), DUNHAM (1950), D. MOORE(1958, 1959, 1960), JOHNSON(1959, 1960, 1961), WESTOLL (1962), JOHNSON and DUNHAM (1963) have all discussed at length the problems of this “Yoredale” rhythm. Recognised first in the type area of Wensleydale, Yorkshire, it was later realised that cycles of this type occurred throughout the Upper Visean and Lower Namurian in the north of England and in Scotland. A map showing the distribution of rocks of this age in northern England is given in Fig.51 while Table XX illustrates what at various times have been described as “ideal”, “fully developed”, “standard”, etc., cycles of Yoredale type. HUDSON(1924) interpreted the type section of the Yoredale Series of Wensleydale, Yorkshire, as indicating periodic elevation of the source area. Limestone was de-

118

TRANSITIONAL REGIMES IN EUROPE

ENGLAND

XOTLAHD I

RIDLAND VALLEY

d CInrifiotio

LIQDESDALE

Mew ClarrificatioP

c" ZI 0

1

2

N.E. CURBERUND NORTHUffBERLAND

1

PASSAGE GROUP I I I I

UPPER LIMESTONE GROUP

UPPER LIMESTONE GROUP

Catsbit Lrt.

u f

irlct Lst.

u

Penton

I

I

Z

-6

LIMESTONE

MID'DLE LIMESTONE GRBUP

' mkhousrs/ niddy or

I

Oxford Lrt.

mestone 1

I

' mestonc I

'Dun Limestone

GROUP

aworth BIRD-

l

i

I

~ LEMER ~ STON ~

l GROUP

~

~

E

'COAL GROUP

RAlGHlLL

\NDSTONEI GROUP

I

r--------

ELL S A N D ~ O N CROUP E CEMENTSTONE GROUP

I

----_7I - - - - - - - -

:EMENTSTONE GROUP I I 1

I 1

I

I

I

Fig.50. Classificationof Carboniferous rocks in England and Scotland. (After E. H. FRANCIS, 1965.)

GREAT BRITAIN

119

Fig.51. Carboniferousrocks in northern England. Rocks of Yoredale facies occur in areas designated Millstone Grit and Lower Carboniferous. (After JOHNSON,1960.)

posited during a period of minimum source erosion; uplift of the land resulted in first mud and then sand being brought into the area of deposition. Coal swamps flourished when the water was shallow enough, perhaps due to slight uplift in the depositional area. With planation of the land surface little sediment reached the area of deposition and limestone formed. He envisaged therefore fairly constant subsidence, though with perhaps occasional uplifts, the main reason for the cyclicity of the sediments being the continued rejuvenation of the land surface. BROUGH(1928), after a study of rocks of similar facies on the Northumberland coast, discussed various possible ways of producing a cycle of Yoredale type. Obviously the area of deposition was subsiding. If the downward movement was jerky so that from time to time relatively rapid depression took place then one would expect

120

TRANSITIONAL REGIMES IN EUROPE

TABLE XX YOREDALE-TYPE CYCLES -

West Northumberland “ideal” (JOHNSON, 1959) (10) A thin-bedded rhythmic sequence sometimes incomplete, or repeated several times as minor rhythms: coal Ganister or seatearth Sandstone and flags Shale Limestone or marine shale (9) Sandstone (8) Sandstone, thin-bedded flags, shale; and grey beds

North Northumberland “rhythmic unit” (BROUGH,1928)

Coal Fireclay Sandstone

.Sandstone, shaly

(interbedded shales, siltstones and sandstones) (7) Shale, relatively unfossiliferous, ferruginous

Shale, micaceous

(6) Shale, dark coloured, pyritic

Shale, fine dark

(5) Shale, fossiliferous, calcareous

Limestone

(4) Limestone, dark bluegrey, fragmental, with interbedded marl

bands and partings (3) Limestone, light grey, bioclastic, with many calcareous algae

(2) Limestone, dark blue-grey, muddy (1) Shale, marine and shale, sandy

1

Probably only local significance (DUNHAM, 1950).

a In 1924 HUDSON placed limestone at the end of cycle, in 1933 at the beginning.

successive deposits of sediment to be different. If on the other hand subsidence was slow and steady then he considered that some outside factor must be sought to account for the varying nature of the successive layers of rock. He postulated two more possibilities. Either there could be recurrent sharp uplifts of the land mass supplying the sediments or there could be continuing periodic changes in climate from humid conditions to arid. During the arid periods limestone would be deposited while with humid conditions detrital material would be brought into the depositional area to form sandstones and shales and coal swamps could flourish. On balance, in the light of the evidence then available, he thought the Yoredale rhythm was best considered

121

GREAT BRITAIN

Yorkshire “rhythmic succession” (HUDSON, 1924, 1933)

Yorkshire L‘fulydeveloped” (D. MOORE, 1959, 1960)

Northern Pennines “standard” (DUNHAM, 1950)

Sandstone or coaly shale Coal Fireclay or ganister

(8) Shale, upper (7) Coal (6) Seatearth

(7) Coal (6) Ganister or underclay

Sandstone

( 5 ) Sandstone, massive

Shale and sandstone, interbedded

Shale, non-fossiliferous, ferruginous Shale, fossiliferous, calcareous (limestone conglomeratel, surface below eroded and pot-holed) Limestone, algal phase or chert beds Shale, subsidiary, with “modified limestone fauna” Limestone, coral-brachiopod or brachiopod fauna Limestone, coral phase

( 5 ) Sandstone sometimes cross-bedded (4) Sandstone, flaggy (4) Shale, sandy, shaly sandstone or “grey beds” (interbedded shales, siltstones and sandstones) (3) Siltstone (3) Shale, unfossiliferous, ferruginous (?non-marine) (2) Shale (2) Shale, marine

(1) Limestone

(1) Limestone, marine

(Major cyclothems such as above may contain in upper part up to 6 minor cyclothems of limestone-shale-sandstone type)

as being due to uneven earth movements affecting both the areas of supply and deposition. D. MOORE(1958, 1959, 1960) re-studied the type area of the Yoredale rocks. While recognising eight major (sic) cyclothems, each typified by the occurrence of a prominent limestone at the base (Table XX), he pointed out that the upper part of some of the major cyclothems may contain minor cyclothems. Some confusion is apparent in the precise delimiting of these minor cyclothems. In 1958 (p.94) we read: “. . . the major rhythms from thick limestone to thick limestone often include minor rhythms with all the characters of the typical rhythmic unit except the persistence of

122

TRANSITIONAL REGIMES IN EUROPE

the limestone.” In 1959 (p.523): “. . . minor cyclotherns, which may be developed only in restricted parts of the outcrop area are invariably simple; they comprise a limestone at the base overlain by shale, which is in turn followed by sandstone” while in 1960 (p.218): “Each cyclothern, major or minor, comprises the following eight members. . None of the members is wholly persistent.” JOHNSON (1959) also pointed out (TableXX) the presence of “minor rhythms”in the top part of his “ideal” cyclothem. In both cases it is apparent that the significant feature of a major cyclothem is taken to be the presence of a relatively thick, laterally persistent, limestone. This view was challenged by SHIELLS (1963), who maintained that the study of cyclic sedimentation is primarily the investigation of a sequence of environmental changes. That being so the length of time a new environment persists and whether it is widespread or not is irrelevant. This point (perhaps representing the extreme of the “phase” approach, Chapter 1) is an extremely important one in discussing mechanisms of formation and will be returned to later. DUNHAM (1950) proposed a “standard cyclothem” for the Yoredales (Table XX) though he emphasised that various members may be missing. Interesting data were given on the thickness of various cyclothems, averages varying from 26 to 95 ft. The most variable members of the cyclothem are the sandstones while limestones tend to keep a much more constant thickness. JOHNSON (1960, 1961) amplified the remarks of Dunham on lateral variation within the beds of Yoredale age in northern England. He accepted the standard cyclothem of Dunham but pointed out that differences occurred as the succession was traced north from the Askrigg Block to the Northumberland Trough (Fig.52, 53). Johnson visualised marine conditions in the south and deltaic conditions in the north, the change in environment being marked by: ( I ) marine limestones in the south, which thinned northwards until ultimately the only evidence of marine conditions was the presence of thin marine shales; and (2) an increase in the thickness of shale and sandstone members and the number of cyclothems northwards. Most authors on the subject of Yoredale facies rocks seem to be in agreement as to the environment of deposition. Palaeontological evidence indicates the marine nature of the limestones and some shales, while their shallow-water origin was convincingly demonstrated by D. MOORE(1958) and JOHNSON (1960) in particular. Shales lacking marine fossils and the sandstones have many of the characteristics of deltaic sediments (D. MOORE, 1958,1959, 1960) and there is a close comparison between Yoredale facies and sediments of the Mississippi delta (Table XXI). All authors agree on the variability of Yoredale cycles from their idealised cycles though the persistence of the major limestone horizons is usually emphasised. Opinions on mechanism of formation are still divided. DUNHAM (1950) favoured a theory of intermittent subsidence. Isostatic adjustment of the crust due to the ever increasing weight of sediments perhaps played a part. JOHNSON(1960) favoured a similar type of origin. He envisaged the cycle commencing with a sudden marine transgression and shallow shelf-sea conditions, suitable for limestone deposition, continuing for a comparatively long period of time. There

.

123

GREAT BRITAIN

ASKRIGG BLOCK

ALSTON BLOCK

SOUTH SlOE NORTH NORTHUMBERLANO TROUGH NORTHUMBERLAND

SANDBANKS LST: ACRE LS7:

WATCHLAW LS%

WOODEND LS%

BANKHOUSES LST Fat

-100 -200

-300 -400

Fig.52. Succession of Yoredale-facies limestones and their correlation at points labelled in Fig.51. (After JOHNSON,1960.)

124

TRANSITIONAL REGIMES IN EUROPE

Stage 3 (Late p2 - E, times]

SIage 2

CD2- D j times]

Stage 1 [ D, t imesl

Horizontal scale

01

.in miles. 10

50

Vertical scale

0,

in feet 400

890

-

Fig.53. Cross-sections of Yoredale-facies rocks showing development of areal differences during DIEl times. Localities are as in Fig.51. Marine limestones are shaded and horizons numbered as in Fig.52. (After JOHNSON, 1960.)

followed a short period when terrigenous sediments were laid down and finally, during another long quiescent period, coal swamps flourished. D. MOORE (1958,1959,1960) considered that the major cyclothems resulted from the interaction of a sedimentary couple-a shallow epicontinental sea favourable to limestone deposition and a large delta comparable to that of the Mississippi. Subsidence, however, was continuous, though irregular. Periodically the delta-forming river was diverted (by crevassing) and invasion by the sea of the still subsiding area led to limestone formation. Re-establishment of the delta was marked by the appearance of mud on top of the limestone. As the delta-front approached, the fauna disappeared and an influx of non-calcareous mud, silt and sand took place. The delta built out distributaries, the troughs between being filled by occasional crevassing or over-bank flooding. Local depressions on the delta plain became the sites of coal swamps and these were eventually invaded by the sea due to the continued subsidence. Major crevassing of the main river of the delta complex and diversion of the clastics elsewhere would result in a particular delta lobe being covered once more by the sea. Limestone deposition would then take place over the area until the next period of diversion into the area of delta distributaries.

126

TRANSITIONAL REGIMES IN EUROPE

WESTOLL(1962) considered a 100 ft. thick “standard model cyclothem” and ingeniously attempted to assess the relative importance in the formation of the Yoredale-type cyclothem of compaction, isostasy and “tectonic forces”. He came to the conclusion that the maximum isostatic depression due to the weight of his model cyclothem would be 80 % of its final compacted thickness. Compaction played a part during the deposition of the sediments, of course, but it appeared that a “tectonic” contribution in the shape of additional subsidence to the isostatic subsidence was required. This Westoll considered to take place rapidly while subsidence due to compaction and isostatic adjustment proceeded at a constant rate. During Visean and Tournaisian times in Scotland the situation was rather more complicated than in the north of England. Reference to Fig.50 indicates how facies differences necessitate yet another classification of Carboniferous rocks distinct from those of central and northern England. The varying environmental conditions in Scotland during this part of the Carboniferous are reflected in the number of so-called “complete, full, etc.” cycles described from the various rock groups (Table XXII). Fig.54 shows the attempt made by E. H. FRANCIS (1965, following ROBERTSON, 1948), to summarise the differences in diagrammatic form. There is, however, disagreement between authors over both the order of the rocks in the cycles and where to start the cycles. Confronted with this rather bewildering array we can only assume that the differences of opinion reflect the fact that the actual order of the beds in the various cycles is not at all definite. In the Cementstone Group (Fig.SO), the Ballagan Beds in Stirlingshire consist of about 600 ft. of alternating green shales and cementstones with subordinate micaceous sandstones (MACGREGOR et al., 1925). The cementstones are argillaceous dolomites and are usually restricted to less than a foot in thickness as are the shale beds between. Similar rhythmic alternations occur elsewhere in Scotland of about the same age (e.g., EYLESet al., 1949). The occasional occurrence of gypsum with cementstones has been taken to favour the suggestion that the lagoon in which the sediments accumulated dried out periodically. Between intervals of dessication mud was carried into the lagoons to form the shales (MACGREGOR, 1929). It is possible, however, that the separation of argillaceous and dolomitic material took place during diagenesis like some of the alternations discussed in Chapter 6. Another facies of the Cementstone Group, in Ayrshire, consists of cycles starting with a conglomerate passing up into sandstone, followed by red marl with “cornstone” (calcareous) nodules and ending with shales and fine-grained sandstones. This second type of cycle is best considered as a variant of the fining-upwards cycles described by J. R. L. ALLEN (1965a) from the Old Red Sandstone and other successions (see Chapter 2). The Oil-Shale Group cycles also appear to be a special case and are dealt with later in this chapter. The Lower and Upper Limestone Groups display cycles more or less of Yoredale-facies type. The Limestone Coal Group is predominantly nonmarine. Limestones are absent, apart from the rare thin fresh-water variety, and coals

TABLE XXII

CYCLESIN

Carboniferous Limestone Group, Midland Valley (RICHEY,1937) “complete”

(11) Coal

(10) Fireclay (9) Shale (8) Shale, sandy

s

m

s CARBONIFEROUS ~

Carboniferous. Oil-Shale Scotland Group, Lothians (GOODLET,1959) (RICHEY,1937) “complete”

(10) Shale, sandy (9) Sandstone (8) Shale, sandy

(6) Shale, sandy

(7) Shale, with (6) Fireclay plants (6) Shalewith (5) Sandstone non-marine fossils

(2) Shale, calcareous with (b) Marine fossils (a) Lamellibranchs (1) Shale, often bituminous with (a) Lingula and (b) estuarine fish (11) Coal

Oil-Shale Grouj Midlothian (TULLOCH and WALTON, 1958)

(14) Seat-clay (1 3) Oil-shale (12) Limestone, fresh water (1 1) Mudstone

(7) Sandstone

(5) Shale, often bituminous with (b) estuarine fish (a) Lingula (4) Shale, calcareous with (b) Lamellibranchs (a) Marine fossils (3) Limestone, marine

Oil-Shale Oil-Shale Group, Lothians Group, Lothians (MACGREGOR, (GREENSMITH, 1938) 1962) “generalized “jiull,but composite succession” rhythmic unit”

(7) Coal

(5) Shalewith (4) Oil-shale Lingula

(4) Shale, (3) Shale marine (3) Limestone, (2) Shale, marine marine

(2) Shale, marinb

(1) Shale

(1) Coal

(7) Coal

(4) Mudstone with sandstone Partings (3) Sandstone

(10) Shale

.

(9) Oil-shale

(8) Sandstone

(7) Shale, sandy

(2) Mudstone, (8) Shale bituminous (1) Oil-shale

(7) Lime-

(5) Shale, marine

IB

(5) Oil-shale

(3) Limestone (usually non-mariot

(2) Ganister or fireclay (1) Sandstone lla

Notes see p.130.

~

Lower and Upper Limestone Groups, Scotland (MACGREGOR,1929) ‘yull sequence”

Lower Limestone Group, Limestone Coal Group, Stirling-Clackmannan Stirling-Claclcmannan (south) (north)l (READ,1959) (E. H. FRANCIS, 1956) “fully developed” “complete”

Upper Litnestone Group, ScotlandB (ROBERTSON, 1948) “normal rhythmic unit”

Coal Measures, Scotland (MACGREGOR, 1929) “typical lithological sequence”

(18) Fireclay (17) Fireclay, sandy (16) Mudstone, sandy

or silty (15) Sandstone, silty,

argillaceous (14) Sandstone, flaggy

(1 2) Seatearth

(1 1) Shale and sand- stone, interbedded (10) Sandstone (9) Shale and sand-

(9) Shale

stone, interbedded (8) Shale, non-marine (8) Seatearth

(13) Sandstone (12) Grit or sandstone, coarse (1 1) Sandstone, flaggy (10) Sandstone, silty,

argillaceous (9) Mudstone, sandy or silty (8) Mudstone, non-

marine

(8) Shale, with (7) Shale Lingula (7) Shale, calcareous, (6) Shale, marine calcareous (6) Limestone

(7) Shale and sand-

(7) Mudstone stone, interbedded (6) Sandstone (6) Mudstone, limy, usually shelly

’

(5) Shale and sand-

(5) Limestone

E (5) Shale, marine

:4) Shale with Lingula

(4) Shale,

calcareous (3) Shale

~

(5) Limestone

stone, interbedded

(4) Shale, non-marine

(4) Mudstone, limy,

(6) Shale

(3) Shale with Lingula

usually shelly (3) Mudstone with

(5) Shale, non-

(4) Shale, black,

13) Shale with plants

(2) Shale, carbonaceous

(2) Cannel and ironstone

(2) Mudstone, coaly,

‘2) Coal

(1) Coal

(1) Coal

(1) Coal

1) Sandstone

often canneloid

plants

(3) Coal

(2) Fireclay (1) Sandstone

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TRANSITIONAL REGIMES IN EUROPE

become prominent. Marine bands (sometimes only containing Lingula) occur and in fact we have cycles not dissimilar to those of the Coal Measures. Both ROBERTSON (1948) and GOODLET (1959) favoured deltaic conditions of sedimentation but with important qualifications. (1948, 1952) considered that the vegetation of the coal-swamp played ROBERTSON a far more important part in the formation of cycles than is generally recognised. He emphasised that the one item all Carboniferous cycles show in common is the periodic presence of coal-swamp conditions. Plants played an important part in slowing down incoming currents, thus promoting deposition. In turn this deposition increased the area available for colonisation by plants. Eventually, of course, areas of longestablished plant growth would be built up above general water level and thus prevent deposition of elastic material altogether. Robertson also postulated that sand-bars probably kept the sea from the coal-swamp areas. During subsidence the bar might be breached and marine invasion of the swamps would take place. GOODLET (1959) suggested that the Midland Valley of Scotland was the site of a delta-flank depression during “Mid-Carboniferous” times. This depression, he considered, lay to the north and northwest of the main southwest-flowing channel of a large delta. The main river channel was situated in the region of the North Sea and the main delta building took place in the Pennine area of England. Periodically one could expect some of the channel load to be diverted into the flank depression because of the continual subsidence of the area. Once sediment built up to a certain level, distributaries would cease to function once more in the depression area. Plants might then colonise it. Should conditions be not quite suitable for plant growth, fresh-water lakes could form, where the raw material of oil-shales or fresh-water limestones might accumulate. Occasionally during the non-clastic period, subsidence might be great enough to allow for eventual invasion of the area by the sea. Marine limestones could then form. The large number of cycles present (e.g., 240 in 8,400 ft. of TournaisianWestphalian B sediments in Fife) also indicated to Goodlet that any ofthemore spectacular theories of origin (e.g., WHEELER and MURRAY, 1957) was unlikely for the Scottish cycles. READ(1961), FORSYTH and READ(1962), and READ(1965) made detailed studies of the Limestone Coal Group. FORSYTH and READ(1962) demonstrated a satisfactory correlation for some 30 coal-bearing horizons in a succession 500-1,200 ft. thick between Glasgow and Stirling (30 miles). They were impressed by the lateral persistence of many individual horizons but noted that in some areas this regularity broke down. One particular horizon of interest is that of the Bannockburn Main Coal (READ,1961). NOTES TABLE XXII

READ(1961) gave same sequence for Stirling but commenced cycle at base of cannel(2). FORSYTH and READ (1962) gave same sequencefor Glasgow-Stirlingbut commenced cycle at base of sandstone (6). a E. H. FRANCIS (1956) gave sequence for this group in Stirling-Clackmannan(north) which apart from having cannel and ironstone above coal was the same as used by READ(1959) for Lower Limestone Group.

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This complex is represented by three cycles in the Glasgow area, one coal in the west of the Stirling Coalfield, fifteen cycles further east and one coal in the extreme east of the area under consideration. The uniform sedimentation of the Limestone Coal Group between Glasgow and the western part of the Stirling Coalfield was taken to indicate that sedimentation was controlled by a single dominant process-“an oscillation of land and sea levels relative to each other” (READ,1961, p.287) resulting in regular cyclic sedimentation. The “aberrant” cyclic sedimentation was thought to be due to differential subsidence of various unstable areas causing variations in thickness and number of cycles which obscured the regular pattern. Further work (READ,1965) indicated that fewer cycles may owe their origin to major oscillations of land and sea levels than was at first thought. A distinctive band of shales, the Black Metals, up to 180 ft. thick, occurs in the middle of the Limestone Coal Group east of Stirling. This band is considered part of an unusually thick and persistent cycle which was deposited following a eustatic rise of sea level. Traced eastwards, sandstones, seatearths and coals become intercalated, however, and in some cases apparently are the lateral equivalents of shale. Cycles, in other words, are developed which are very similar to those typical of the Limestone Coal Group elsewhere. Read postulated that these cycles were mainly due to local processes of subsidence and deltaic sedimentation on the fringe of the area affected by the eustatic rise of sea level. While certain other Limestone Coal Group cycles are thick and laterally persistent, like the normal Black Metals cycle, most are 10-30 ft. thick and some are subject to considerable lateral variation. Further, they tend to increase in number in any one area more or less in proportion to the total subsidence that has taken place there (see also READand DEAN, in preparation). These too might owe their origin to local processes of subsidence and sedimentation. While the Black Metals cycle in the west consists of shale it is of considerable interest to note that alternations of marine and non-marine fauna occur. Read considered that these faunal cycles may reflect the formation of coal-bearing cycles elsewhere. As already stated, the Oil-Shale Group cycles seem to be a special case. They differ from the ones we have been discussing in that they often contain fresh-water limestones (Table XXII, Fig.54). They do, however, occasionally contain marine shales. The surprising dissimilarities between the described cycles from what is, after all, a TABLE XXIII “COMMON PARTIAL UNITS” IN OIL SHALE GROW, SCOTLAND

(After GREENSMITH, 1962)

Shale Limestone, with ostracods Shale Sandstone, with seat earths

Shale Oil-shale Shale Sandstone, with seat earths

Shale Oil-shale Limestone, with ostracods Shale Sandstone, with seat earths

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very small area (ca.200 sq. miles) suggest to us that cyclicity is not well developed. GREENSMITH (1962) erected a “composite rhythmic unit” (Table XXII) and cited three ‘‘common partial units” (Table XXIII); “even these may be simplified further by omission of limestone, oil-shale or ganister beds” (GREENSMITH, 1962, p.358). The paucity of coals and marine shales and the frequent evidence of basal erosion led Greensmith to propose that the base of the sandstones be taken as the most significant point at which to begin the cycle. This point he considered marked an influx of fluvial debris. A later phase of lagoonal sedimentation followed. Intermittent subsidencecontinuedthroughout the deposition of the whole sequence of cycles whose incompleteness made it both difficult and unwise to estimate the number present. His interpretation of the environment of deposition (GREENSMITH, 1962, 1965)-lagoonal conditions in the flank of a delta, distributaries of which periodically spread clastics over the dominantly chemical deposits of the lagoons-seems to fit the facts as brought out by his careful petrological studies. We cannot, however, agree with the assertion (1962, p.358) that his composite unit bears a “superficial resemblance” to the Illinois ideal cyclothem.

Namurian and Westphalian The Namurian of England is characterised by a cycle differing in important aspects from those of Yoredale type. The “Millstone Grit” cycle contains no limestone but has in its place marine shale. A simple four unit cycle o f Coal, Sandstone, Shale, Marine shale, was early recognised by H. M. Geological Survey (e.g., WRIGHTet al., 1927; TONKS et al., 1931) and many such cycles have been subsequently described. As more detailed studies were made so elaboration of the description of cycles took place. In the Sheffield area of Yorkshire for instance R. A. EDENet al. (1957) described a “full cyclic unit” as follows: Thin coal. Seatearth. Sandstone, from fine to gravelly grade, and commonly transgressive into the underlying beds. Mudstone, sandy or silty. Mudstone, grey with nodules of ironstone. Marine Band-dark mudstone, commonly with large carbonate nodules (“bullions”): and with a goniatite-lamellibranch fauna. Thin coal. Thickness of the cycles is very variable and there is evidence that some might attain values of the order 10C200 ft.

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Though Coal Measures (Westphalian) sequences were at one time considered in terms of repetitions of sandstone (or conglomerate) followed by shale and coal (STRAHAN, 1901; GIBSON, 1905) it is obvious there is an essential similarity in the order of beds in both Millstone Grit and Coal Measures cycles (e.g., TONKSet al., 1931). The latter also pointed out that distinctive changes took place when the succession from the Millstone Grit through the Coal Measures was considered in detail. The cycles became thinner and more frequent. Marine bands appeared in fewer cycles, eventually becoming rare, while coals became common. ROBERTSON (1933) erected an idealised cycle for the Coal Measures in South Wales: Fireclay. Mudstone. Mudstone, sandy, plants. Sandstone. Grit or pebbly sandstone. Mudstone, sandy, plants. Mudstone with Carbonicola. Shale with Anthracomya, Naiadites and fish. Shale with Estheria and fish. Shale with Lingula, Orbiculoidea, etc. Shale, calcareous, pyritic, with goniatites, etc. Cannel. Coal. “It is seldom or never that all of these stages can be traced in any one cycle, but usually their potential occurrence is obvious” (ROBERTSON, 1933, p.88). The importance of the concept of faunal changes illustrated by this cycle was recognised by EDWARDS and STUBBLEFIELD (1947) who examined the succession in the East Pennine Coalfield from this and other aspects (see Chapter 10). TRUEMAN (1946, 1948, 1954), who wrote at length and with much insight on Coal Measures cycles, considered that the following was, in general, characteristic of the Coal Measures in both Britain and northwestern Europe: Coal. Rootlet bed. Sandstone. Shale or mudstone, non-marine. Marine band. He pointed out, as indeed have all the writers on Carboniferous cycles, that there are many exceptions to the “characteristic, typical, etc.” cycle (see Chapter 1). Conditions of sedimentation of both Millstone Grit and Coal Measures have been thought to be deltaic by most workers. Deposition took place very near sea level, the area of deposition undergoing intermittent subsidence. Periodic incursions of the sea led to the formation of thin marine shales, often widespread,and therefore extremely useful in correlation.

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DUFF and WALTON (1962) examined in detail the Coal Measures succession in the East Pennine Coalfield and devised a method of examining the sequence so that an objective appraisal of the order of occurrence of the beds separating the coals could be made (see Chapter 1). A good coverage of the coalfield, both areally and stratigraphically,was obtained from 41 logs of cored boreholes. Cycle types were identified depending upon the order of occurrence of the beds separating the coals. The cycle types were then plotted on a histogram (Fig.2), which shows that the commonest type of cycle (the modal cycle) is a simple one and consists of a non-marine shale followed by a seatearth and a coal. Secondary modal cycles, also of non-marine sediments, are as follows: Coal. Coal. Seat earth. Seat earth. Shale. Shale. Siltstone. Siltstone and sandstone. Shale. Shale. No other cycle types approach these three numerically and in fact a feature of the histogram is the overwhelmingly large number of combinations of sandstone, siltstone and shale that occur between coal seams. (In South Durham, CLARKE, 1963, also showed that variability of the sequence between coal seams is the rule, not the exception.) Most cycles do not contain marine fossils. Those that do appear to be very similar, as far as order and type of rock units is concerned, to the non-marine cycles. There is a tendency for the marine shales to be immediately above the coal, particularly in the lenisulcata zone (Lower Westphalian A) and the marine shales can be incorporated into the modal cycle to form the composite sequence for the Coal Measures of the East Pennine Coalfield: Coal. Seat earth. Shale, non-marine. Siltstone and sandstone. Shale, non-marine. Shale, marine. Areal study of particular, easily identifiable, cycles showed that each varies greatly when traced laterally (Fig.55). Stratigraphically, between horizons, there is also great variation. In the Upper similis-pulchru zone (Lower Westphalian C), for instance, the number of cycles ranges from 4-21; in the Lower similis-pulchru zone (Upper Westphalian B) from about 12 to at least 25; in the modiolaris zone (Westphalian A-B) 13-30. Furthermore every coal in the area exhibited splitting. Duff and Walton came to the conclusion that the stratigraphical and areal variation and the great variety of cycle types were best explained by a theory involving channel wandering in a deltaic area as the main factor in the development of the cycles. Three marine cycles are exceptional, however, in that they are much thicker than any other cycles and are traceable over most of Britain and northwestern Europe. Eustatic rise of sea level was thought to be a possible key to their development.

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SHAFTON M.B.

MANSFIELD M.B.

TWO FOOT MB.

TOP HARD COAL

CLAY CROSS M.B

TUPTON COAL

Fig.55. Histograms showing lateral variation in type of specific cycles in the Westphalian Coal Measures of the East Pennine Coalfield, England (see p.134). I. Occurrence of commoner types of cycles containing no sandstone. 11. Commoner types containing sandstone. 111. Less common types. Each rectangle represents a different type. IV. Number of cycles examined. V. Occurrences of different cycle types. The letter i after the horizon name denotes that the cycle includes that marker; a indicates and WALTON, that the cycle is above and b that the cycle is below the horizon specified. (After DUFF 1962.)

Recent work in the South Wales Coalfield (WOODLAND and EVANS,1964) has also indicated great variation in cycle types in the Coal Measures. Attempts to define a “standard” or “ideal” cycle were regarded as inappropriate because of this. Four main stratigraphical variations are however recognisable (Fig.56). The lack of coals

I36

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Lower part of LOWER COAL MEASURES typically 30 -lOOft.

GELLIDEO TO

TWO-FEET-NINE typically 20 -100ft.

Upper part of MIDDLE COAL MEASURES typically 40-

12QA.

PENNANT MEASURES tYPlcallY 100 4004t.

-

KEY

PENNANT SANDSMVE

wim coNaionmrE E

A N ~

WMrOW. AUINLY FlNE-

W N E O AND Q ~ T Z U l C

Y l W nuDlIw(E

KANT OEIWS E L M E L LldRAhWS W U ~ O E S

VMIED M I N E F U W

Fig.56. Westphalian cycles in South Wales. (After WOODLAND and EVANS, 1964.) (Lower Coal Measures are Westphalian A; Gellideg to Two-feet-nine, Westphalian A-B; Upper part of Middle Coal Measures, Westphalian C; and Pennant Measures, Westphalian C-D.) (Crown copyright, reproduced by permission of the Controller of H.M. Stationery office.)

in the lowest part of the succession was taken to indicate successive episodes of subsidence following on one another rather rapidly. Longer periods of stability were postulated to account for the thick coals in the cycles higher in the succession. Sandstones are uncommon in both these cycle types and lateral variations occur with thin partings in coal seams swelling to 100 ft. intervals of normal sediments within a few miles. Higher in the succession occur cycles similar to the “characteristic” of Trueman ( ~ 1 3 3 . but ) lateral variations are still great (Fig.57).

137

GREAT BRITAIN Cefn Coed Marine Band

Britannic Marine Band Upper Cockshot Rock

m

Hafod Heolog Marine Band Lower Cockshot Rock THREE COALS

TWO-FEET-NINE

UPPER

FOUR- FEET FOUR-FEET LOWER FOUR-FEET

SIX-FEET

UPPER SIX-FEET

I

COWER SIX-FEET

CAERAU VEIN

RED VEIN GROUP

UPPER NINE-FEET

,

NI NE-FEET

1

LOWER NINE-FEET BUTE

Amman Marlne Band

bzsj m--M

.

Fig.57. Generalised section of the lower part of the Middle Coal Measures (Westphalian B) in South Wales illustrating lateral variation. (After WOODLAND and EVANS,1964.) (Crown copyright, reproduced by permission of the Controller of H.M. Stationery Office.)

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TABLE XXIV LITHOLOGICAL FACIES OF

THE ABBOTSHAM FORMATION, NORTH DEVON

(After DERAAFet al., 1965) ~-

-

Facies

Description

L: “Major-sandstone facies

Thicker sandstone beds up to 100 ft. Coarse- tofine-grained, erosive at base, cross- and parallel laminated; ripple-drift and wavy-bedded; sometimes structureless

K: “Fining-upwards”

Erosive sandstone at base, cross-bedded with mud flakes, grading up into silty sandstones and silty mudstone with rippledrift bedding. Units up to 30 ft.

units J: Cross-stratified sandstones and mudstones

Lenticular, cross stratified sandstones with erosive bases with horizontally laminated siltstones and mudstones

H: Oscillatory beds3

Alternations of dark, poorly laminated or structureless mudstone and wavy laminated silty sandstone

G: Oscillatory beds-2

Alternations of dark grey, faintly laminated mudstone and silty and sandy laminae sometimes rippled, often extensively burrowed

F: Oscillatory beds-1

Alternations of structureless or poorly laminated black mudstone and burrowed silty mudstone

E: Sandy streak

More or less continuous bands of sandstone with small scale cross-lamination, muddier bands between sandstones contain lenses and laminae of siltstone and fine sandstone

D: Silty streak

Thin (1-2 mm) silt laminae (up to 30 cm long) with wavy or rippled bedding

C: “Turbidite”

Graded sandstone-mudstone layers, parallel bedded, with sole markings

B: Silty mudstone

Thin (1-2 mm) lenticular bands of siltstone occurring in bands (up to 5-7 cm thick) separated by mudstone. Occasional colour banding (2-3 cm thick)

A: Black mudstone

Banded mudstone, black and very dark grey (1-2 cm thick); parallel banded; with ferruginous concretions

The cycles from the topmost Coal Measures (Westphalian C-D), the “Pennant Measures”, are quite different from those below. They are thicker and consist mainly (80 %) of sandstone, the conglomeratic base of which frequently lies directly on coal. Mudstone sometimes intervenes. The top of the sandstone however grades upwards into finer material and sometimes several horizons of seatearth occur between the mudstones before the coal appears. Changing environmental conditions were considered responsible for the different cycle types. WOODLAND and EVANS(1964) seemed to avoid deliberately the use of the word “deltaic” in their discussion of the environment

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of deposition. They postulated sedimentation taking place in shallow water ona shelf of considerable extent. The cycles resulted from intermittent subsidence of this shelf. Occasional eustatic changes of sea level emphasised this subsidence. The Lower Westphalian succession in north Devon, England, is lacking in coal seams but it has been interpreted as a marginal, deltaic accumulation and is appropriately considered here. Though the sequence is almost entirely clastic, variation in grain-size and sedimentary features (structures, inter-lamination, etc.) has allowed the recognition of six cycles in 1,200 ft. of the Abbotsham Formation (DEF ~ A A Fet al., 1965). The number of cycles examined did not warrant strict statistical treatment but the authors set up a “standard’y cycle, admittedly subjective but nevertheless largely based on the observed order in which the lithologies tend to occur. The “standard” cycle approximates to a composite sequence and is made up of three members. A lowermost mudstone member is followed by one which is “intermediate” in lithology which in turn is capped by a sandstone member (Fig.58). The significant horizon chosen to start the cycle is black mudstone with a sharp, usually erosive, base lying on a burrowed horizon often with rootlets. The black finely laminated mudstone is usually succeeded by a silty mudstone of lighter colour which may include some thin graded sandstones interpreted as turbidites. Also associated with this member is a lithology in which black and silty mudstone laminae are interbedded (oscillatory 1, facies F, Table XXIV). The intermediate member is very variable and consists of a number of lithologies referredto as silty streak, sandy streak and oscillatoryfacieswhich are made up of inter-laminated beds of sandstone, siltstone and mudstone in different proportions, sometimes with ripple cross-lamination, sometimes with burrows. The uppermost sandstone member may comprise channel sandstones often cross-laminated, sometimes arranged in a unit which grades upwards from coarse- to fine-grain (fining-upwards unit) or sandstones without an erosive base, some parallel laminated, others showing cross-bedded units interbedded with mudstones (facies H, J, K, L, Table XXIV). The top of the sandstone member is often finer in grain, burrowed and may contain rootlets. Fossils are rare in the succession. Non-marine lamellibranchs have been found in the upper parts of the cycles and minute goniatites in a mudstone member (PRENTICE, 1960). The six cycles described are very variable. The order of lithologies can be appreciated by taking the succession of facies as lettered by DE RAAFet al. (1965, fig.8.). Examined in this detail the cyclic sedimentation is not at all obvious. Generalising in terms of the three members Mudstone ( I ) , Intermediate (2), and Sandstone (3) and allocating the 11 facies types (A-L) as indicated in Fig.58 to the appropriate member the succession appears as: Cycle 1: Cycle 2: Cycle 3: Cycle 4: Cycle 5 : Cycle 6:

13. 12123. 1. 1212123. 123. 13.

Q Y

co

S W

I W

L 0 Iv)

a L

a

v)

Q W

m t W

I w t

i

a w

S Q W

k

I

I I

P W

m I W

E Y

z 0

c Y)

n 2

t

Fig.58. Facies of the Abbotsham Formation and their mutual relationships Within the “standard” cycle. (After DERAAFet al., 1965.)

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(Cycles 1-6 numbered as in DE RAAF et al., 1965, fig.8, with the direction of upward sequencefrom left to right). Even at this level there is considerable variation and it is evident that many more examples would be necessary to establish a modal cycle. Nevertheless there is a repetition which can be explained in terms of a sedimentary model. Mudstone, intermediate and sandstone members can be compared with the sequence of lithologies in a delta. Thus the black mudstone can be taken as basin muds accumulated off the margin of a delta pile whose bottomset beds are seen in the silty mudstones. It is in this zone that sediment supply could occasionally produce turbidity currents to form the occasional graded beds. The oscillatory facies (F), a mixture of black mudstone and silty mudstone, would also be expected in this position. Submarine topset beds, and foreset (deltaslope) environments are represented by the facies with interbedded sands, silts and muds (facies D, E, G, H and perhaps the finer-grained, lower portions of the facies J), the precise upper and lower limits of the sub-environmentsbeing difficult to d e h e . On-delta (topset, sub-aerial) beds evidently are formed of the sandstone member (facies J, K, L), a conclusion supported not only by the coarsening of grain size and the obvious similarity of the “fining-upwards’’ units to filled-in channels (FISK,1960), but also by the presence of rootlet beds and nonmarine lamellibranchs. Of the Abbotsham cycles only number 5 might be taken to represent one simple episode of delta buildings. It was therefore suggested that the formation as a whole does not represent the advance of one single delta into a basin but a complex of deltas interfingering one with another. In the context of Westphalian conditions in the southwest of Britain this complex of deltas seems more likely than drainage wandering connected with one immense delta. The land mass supplying debris from the northwest (a direction based on directional structures within the Abbotsham Beds) was probably of restricted extent. Immediately to the north of the Bristol Channel the Coal Measure of South Wales accumulated in a trough of similar size and dimensions to the present basin and some of the debris was derived from the south (BLUCKand KELLING,1963). The absence of appreciable coals in the north Devon Beds is one of their most interestingfeatures and perhaps reflects different conditions of subsidence and supply. Under conditions of slow subsidence and sedimentation there are times when swamp conditions may establish themselves and persist to form coal seams. On the other hand, and this may have been the case in the north Devon area during the Lower Westphalian, subsidence and sedimentation may be so rapid that at no time are swamps established for a long enough period to allow vegetation to establish itself and hence ultimately produce coals.

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We have treated at length the work done in the U.S.A. and Great Britain because of our familiarity with the areas concerned and with the literature. This bias must not be taken to mean that we do not recognise the contributions of workers in other countries.

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TRANSITIONAL REGIMES IN EUROPE

In Germany and France, for example, SCHMIDT (1924), BARROIS (1927) and PRWOST (1930) were among the first to appreciate the problems raised by coal-bearing cycles. Since then innumerable papers have been written by authors in continental Europe and they, in general, have used a similar approach to most workers in the U.S.A. and Great Britain and have produced as great a diversity of opinions (see for example DELMER,1952; LOMBARD, 1952, 1956; RUTTEN,1952; GRIBNITZ,1954; HAVLENA, 1957; BERSIER,1950, 1958; KOREJWO,1958; BOUROZ,1960; ZEMAN,1964). Apart from these papers, however, we should like to consider in more detail works where a more fundamental difference in the mode of attack on the problem is apparent. Discussion in Germany and Belgium for example has been greatly influenced by the work of Fiege and Van Leckwijck respectively. They recognised cycles of different orders of thickness within the Carboniferous succession (FIEGE,1937, 1952, 1960; FIEGE et al., 1957; FIEGE and VANLECKWIJCK, 1964; VANLECKWIJCK, 1949, 1960, 1964) and postulated different modes of origin for the different sizes of cycles. Describing the Lower Namurian Beds in Westphalia, FIEGE (1937) provided in great detail the evidence for different types of cycles. He separated small cycles, 1 cm1 m in size, mainly recognizable by changes in grain size, from larger 4-5 m cycles consisting of various rock types. The latter cycles he considered to be due to periodic changes in the rate of erosion, perhaps due to epeirogenic movements, but more probably to climatic changes. The Lower Namurian beds of the Ruhr were discussed (FIEGEet al., 1957) in terms of “Grosszyklen”, which were compared with the American cyclothems. An argillaceous phase at the base of the cycle gradually, by inter-lamination, became predominantly sandy upwards. Three phases-“Ton-Phase; Ton-Sand Phase; Grauwacken-Phase”-were recognised. A sharp boundary at the base of the Ton-Phase emphasised the asymmetry of the cycles which averaged about 4 m in thickness. Laterally, cyclic sequences passed into non-cyclic sequences due to change of facies. Larger cycles of about 10 m thickness, (mainly argillaceous beds) and smaller cycles (graded sandstones 50-1 50 cm thick) were also identified. In the Upper Namurian, coal seams occurred above the “Grauwacken-Phase” and an ideal cycle for the Ruhr Coal Measures was described as follows (following GRIBNITZ, 1954): (e) Coal. (a) Clay-sandstone, mixed, frequently with rootlets. (c) Sandstone. (b) Clay-sandstone, mixed. (a) Shale, often bituminous at base; marine band. The thickness of cycles was considered statistically and three maxima were observed: 4-5 m, 7 m and a weaker one at 10 m. In Belgium, Lower Westphalian B coal measures were described by VAN LECKWIJCK (1949). He noted that between coals the beds occurred in a particular three-fold order, though each unit was not invariably present. The roofs of the coals, generally shales containing fossils, were followed by fairly thick, predominantly

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sandy units which in turn were capped by argillaceous beds. These last were sometimes wholly in the form of seat earth. Should there be no argillaceous beds then the top of the sandy unit-usually interbedded mudstone and sandstone-had the characteristics of a seatearth. The roof shales could be 6-9 m thick, the sandy unit 20 m and the top argillaceous beds 1-2 m but generally the overall thickness of the beds between coal seams was of the order 9-10 m. Van Leckwijck emphasised that the number of cycles present in the succession examined did not vary much, increasing from about 32 to 36 as the succession increased from 226 to 330 m in thickness. In The Netherlands there were 35-40 cycles in the corresponding interval which reached 385 m in thickness there. Van Leckwijck considered that there was evidence for individual sandy units having a great lateral spread and therefore favoured a more general mechanism to account for the cycles than one which assumed shifting of drainage channels, distributaries, etc. He outlined a complicated series of positive and negative sea-level changes which might account for both the order of the beds and their periodic absence. In 1964, VAN LECKWIJCK proposed a “complete cycle” for the Namurian of Belgium: (e) Coal. (d) Shale, sandy with some inter-laminated sandstone. (c) Sandstone, with sandy shale. (6) Shale, sandy, with some inter-laminated sandstone. (a) Shale. It was pointed out that while in the Westphaliansuch a complete and symmetrical cycle was common this was not the case in the Namurian. Asymmetrical cycles with only units a, b and c (“Dachbankzyklus” of German authors) occurred in the latter as did asymmetrical cycles containing only units c, d and e (“Sohlbankzyklus”). In fact, exceptions to the complete cycle given above were the rule. In addition to the broad measure of agreement between Belgium and Germany, as far as the order and type of beds in the cycles were concerned, different orders of cycles were recognised (e.g., FIEGE,1937; FIEGE et al., 1957; VANLECKWIJCK, 1952, 1964). The average thickness of a complete cycle was regarded as about 10 m. Within such a cycle, however, it was often possible to determine thinner, usually incomplete cycles (e.g., Dachbankzyklus) of about 4 m thickness. Perhaps three 4 m cycles formed a 10m cycle, though it was evidentlyonly rarely that a 10m cycle could be brokendown entirely into 4 m cycles. Larger cycles, sometimes referred to as 50 m cycles (FIEGE,1960),50-75 m cycles (VANLECKWIJCK, 1952), or 60 m cycles (FIEGEand VANLECKWIJCK, 1964; VAN LECKWIJCK, 1964), were also recognised, consisting of about six to eight 10 m cycles. It is not at all clear how 4 m and 10 m cycles are differentiated and the authors themselves admit difficulty (FIEGEet al., 1957; VANLECKWIJCK, 1960, 1963). There seems also to be some confusion as to the thickness of the larger cycles and they too cannot always be easily picked out (FIEGEet al., 1957; FIEGE and VANLECKWIJCK, 1964). There does, however, appear to be a more objective basis for their recognition.

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The beginning of a 50 m cycle is identifiedby the presence in its lowermost 10 m cycle of a basal marine shale. Succeeding 10 m cycles change from comparatively thick, predominantly argillaceous, types to thinner ones containing sandstones and coals. The succeeding 50 m cycle is recognised by the appearance of the next 10 m cycle carrying a basal marine shale. Further division of the succession, on the 100 m to several 100 m scale, was also proposed by FIEGE (1960), based on the alternations of cycles carrying workable coals with cycles lacking coals. In this paper he considered the origin of the coal-bearing rocks of northwestern Europe in terms of 4,

10 anrl50 m cycles. The furmaiun of 4 m

cycles was considered to be due mainly to topographic and sedimentationalirregularities. On the other hand 10 m cycles were usually more widespread and contained fossils in the basal shales. They were thought to be due to epeirogenic movements, though local topography and sedimentation could still modify them. The 50 m cycles, of similar origin to the 10 m ones, were very extensive, were marked by marine bands and in fact formed the basis of the major stratigraphic subdivisions of the Carboniferous in northwestern Europe. The use of thickness as a criterion to distinguishcycles, particularlyin successions which show considerable vertical and lateral variations, seems to us quite inappropriate. At the very least the system is arithmeticallyunsatisfactory. There is always the danger too that cycles will be identified at specific intervals because the system carries the implication that they must exist. The doubts regarding the validity of this type of analysis are strengthened when the situation in The Netherlands is considered. The impression gained of the Carboniferous rocks is that cyclic sedimentation is very poorly developed (THIADENS and HAITES,1944) although “little” and “great” cycles have been picked out (VANDER HEIDE,1950). The little cycle was said to consist of a coarsening-upwards succession from coal through shale, sandy shale and sandstone, at the top of which are found rootlets under a capping of coal. Bracketed between marine horizons, a great cycle consisted of a succession of little cycles, the upper set of which was rich in coal horizons, in contrast to the barren lower ones. The great cycle might therefore be compared to the 50-60-75 m cycles of the Ruhr and Belgium (p.143), savethatvander Heide cited as an example the great cycle between the Wasserfall and Catenna Marine Bands (Upper WestphaIian A), some 500 m thick! In Britain we also doubt whether such an analysis could be applied. Taking the Westphalian succession in the East Pennine Coalfield of England as an example, the Clay Cross Marine Band, in the modiolaris zone, lies near the middle of a non-marine sequence comprising some 2,000 ft. (610 m) of beds! Difficulties would also be encountered in the lower part of the Ienisulcata zone where as many as nine out of the ten cycles making up the 400-1,000 ft. (120-305m) of beds can possess marine shales (R. A. EDEN,1954). Apart from the obvious difficulties of classifying the above parts of the succession in terms of 4 1 0 and 50 m cycles, lateral variations (see e.g., DUFFand WALTON, 1962)would surely frustrate any attempt at meaningful analysis in terms of size of cycles.

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All German authors do not, of course, accept themethods of F ~ ~ ~ ( 1 9 6and 0); JESSEN(1956a, by 1961) has written at length on cyclic sedimentation in both the Ruhr and Europe in general. The Ruhr he considered in terms of a normal cyclothem (“Das Regel-Cyclothem”, JESSEN, 1956a) as in Table XXV (cf. p.142). TABLE XXV THE RUHR CYCLE

(After JESSEN, 1956a) (4) Sandstone (3c) Sandstone, shaly (3b) Shale, sandy (3a) Shale (2) Coal (1) Rootlet bed

Regressive hemicyclothem

I

Progressive hemicyclothem

In Jessen’s opinion, the roof shale (3a) marked a most significant point in the genesis of the cycle viz., a change from transgressive to regressive conditions. In practice incomplete cycles were the rule and the succession could be considered in terms of “cyclothem-pairsYy, each pair averaging about 18 m in thickness. Individual cycles might lack either the transgressive or regressive phase or one or other phase might be developed more fully. Such irregularitieswere considered to be due to oscillations of sea-level of less magnitude than those required to produce a normal cyclothem. About fifteen cyclothem-pairs constituted a megacyclothem and some twelve megacyclothems, each of the order of 250 m thick, made up the 3,000 m succession in the Ruhr. Megacyclothems could evidently be picked out by the occurrence of welldeveloped and widespread sandstones periodically throughout the succession. Jessen was impressed by the constancy of numbers of cyclothems occurring within given stratigraphic limits and feIt that only some unspecified “extra-terrestrial” phenomenon could produce the numerous and regular oscillations of sea-level he thought necessary. In 1961 he erected a “Voll-Cyclothem” as representative of the northwest European Carboniferous coal-bearing rocks in general (see Table XXVI). This composite sequence is much idealised, most units are in practice missing and in the absence of detailed information on the successions on which it is based it is difficult to assess. As with the “Regel-Cyclothem” the pivotal lithology between progressive and regressive hemicyclothems is the shale (8) and the symmetry about this shale is a somewhat unusual feature. Again it was alleged that the number of cycles was constant over large areas, for example between marine bands recognisable in Germany and Britain. This is demonstrably not true for within an individual coalfield in Britain variation in the number of cycles making up a particular succession is considerable (see p.134).

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TRANSITIONAL REGIMBT IN EUROPE

TABLE XXVI CARBONIPEROUS COAL-BEARING CYCLE OF NORTHWESTERN EUROPE

(After JESSEN,1961)

(14) Conglomeratic sandstone (13) Coarse-grained sandstone (12) Fine-grained sandstone (11) Inter-laminated sand and shale (10) Sandy shale (9) Shale, slightly sandy (8) Shale (7) Coal (6) Shale (seatearth) (5) Shale, slightly sandy (4) Sandy shale (3) Inter-laminated sand and shale (2) Fine-grained sandstone (1) Coarse-grained sandstone

I

Regressive hemicyclothem 8a-14

'I'

}

y:mressive hemicyclothem

I

The extra-terrestrial influencesinvoked by Jessen may have had some effect but constancy of cyclothem development does not appear to have been one of them! In Russia, particular interest has been shown in the Carboniferous system of the Donetz Basin (Fig.38). The succession is some 10,0W12,000 m thick and the Tournaisian and Visean stages are predominantly marine sediments (limestone, dolomites, shales) though in the upper part of the Visean and in the Lower Namurian a coalbearing facies is developed. The Middle Carboniferous (Bashkirian-Moscovian fi Westphalian) bears the most coal. Unlike the Westphalian in western Europe, however, limestones, though thin, are of frequent occurrence. JABLOKOVet al. (1961), for instance, recorded in 1,800 m of Middle Carboniferous rocks in the Donetz Basin some 108 cycles, up to 115 seams of coal and 40 limestones. The erosive bases of sandstones in the Moscovian are usually taken as the beginning of the cycles with sandstones becoming finer-grained upwards and passing into siltstones and argillites (KOPERINA, 1958). The last contain numerous rootlets below the overlying coal seam. A marine or brackish-water fauna is carried by the limestones and argillaceous beds which occur above the coals. Inevitably, however, this is not the whole story and often some of the components are either lacking or very poorly developed. Koperina was more interested in changes of facies than in establishing cyclicity, pointing out that the environment of deposition was different in the Lower Carboniferous from that of the Middle Carboniferous. In the former, coal measures were considered to have formed along a flat littoral zone fringed by a band of shallow water, not more than 10 m deep and separated from the open sea by a belt of sand-bars and spits. Only fine sediment accumulated in the lagoonal conditions behind the sand-bars and as the lagoons silted up swamp conditions spread over the area. Perhaps for a

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while peat accumulation could offset subsidence but eventually, Koperina supposed, the sea would overwhelm the swamp and initiate the next cycle with the establishment of a new sand-bar. In the Middle Carboniferous coal measures deltaic accumulation was thought to be operative. Outbuilding of the delta-plain led to marsh and peat formation and the relative slowness of the peat accumulation would eventually lead to marine invasions and the start of a new cycle. Limestone formed before the next influx of clastic material brought by the growing delta. Koperina rejected arguments that channel cutting at the base of some sandstones indicates uplift and favoured a sedimentary control of cyclic sedimentation. In this she opposed the view of many Russian geologists who are strongly in favour of tectonic controls (e.g., ZHEMCHUZNIKOV, 1958; FEOFILOVA, 1959). ZHEMCHUZNIKOV (1958, p.5) is particularly outspoken, first on those sceptics who doubt the reality of cyclic sedimentation; “. . .those who accept rhythm in nature will find it even where it is rather indistinct and they will arrive at proper conclusions” (from A.G.I. translation; our italics) and on the origin of cyclothems; “If [geologists] . . do not connect cyclicity and tectonic factors, they. . . underestimate that [tectonic] factor”. Discussion is obviously very similar to that which we have reviewed in the previous pages, although there seems to be very general support for mechanisms involving tectonic oscillations superimposed on general subsidence. Techniques appear to be dominated by the “facies-cyclic” method of Zhemchuzhnikov wherein lithologies are assigned to environments and sub-environments with a confidence which we find somewhat surprising. Environments then reflect transgressive and regressive phases of cycles. FEOFILOVA (1959), in a detailed study of 600-1,100 m of the Lower Carboniferous rocks in part of the Donetz Basin, described various types of cycles and differed from KOPERINA (1958) in the interpretation of some facies. Asymmetry of the cycles was emphasised, coals tending to be succeeded by lithologies recognised as the most marine facies present. For the Visean succession the generalised sequence of deposition appeared as: Coal. Fossil soil. Siltstone with rootlets. Siltstone. Sandstone, fine-grained. Siltstone, banded, with plants. Siltstone with plants and shells. Mudstone with shells. Limestone, marine. Coal. This type of cycle exhibits regressive facies, transgressive facies being almost entirely absent. In this, these Donetz Basin sediments differ considerably from the Permian coal measures of the Pechora Basin in the Urals where approximately equal development of regressive and transgressive components give a symmetrical structure

.

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TRANSITIONAL REGIMES IN EUROPE

(the so-called “balanced” cycles of KRUMBEIN, 1964). Variation in cycle type could be picked out by FEOFILOVA (1959), according to the particular part of the basin in which the sediments accumulated. Five environments were recognised according to lithology -marine, shallow-marine, embayment, lagoon and delta. Cycles representative of various types were identified and statistical data used to show the percentage distribution of each facies in the different cycle types. Marine cycles averaged about 5 m in thickness and typically consist, in upward succession, of limestone, mudstone and siltstone; coal is rare and never of workable thickness. Shallow-marinecycles comprise three main variants made up of different combinations of siltstone and mudstone; no coal is present, average thickness is 5-7 m. Lagoonal types bear commercial coals. Deltaic types have sandstone: and so on with minor variations. The Lower and Middle Carboniferous succession in the Donetz, Karaganda and Moscow Basins have been described as showing a complicated intercalation of deposits of different facies; marine (in-and-off-shore), transition (lagoons, bars, bays, submerged parts of deltas, etc.) and continental (river, bog, etc.) (JABLOKOV et al., 1961).The authors were impressed with the occurrence of alluvial deposits beneath coal seams. These alluvial sediments rarely exceed 20-30 m though occasionally several episodes of alluvial deposition can be picked out. Scouring and channelling were taken to indicate periodic uplift and further evidence of this was cited in the occurrence of bog deposits on the foraminifera1limestones, the latter being considered to have formed at depths of 50-100 m. In contrast to the “facies-cyclic” method advocated by Zhemchuzhnikov, V.S. Jablokov (quoted by ZHEMCHUZHNIKOV, 1958) preferred to examine grain-size changes through a succession as an aid to picking out cycles before detailed facies studies. This “cycle-to-facies” method aimed at being more objective in application but some difficultiesarose when, for example, cycles indicated by the granulometric studies were at variance with the cyclic sedimentation suggested by facies studies. We cannot claim that the foregoing is an adequate r h m 6 of the Russian contribution nor would it be reasonable, because of language problems, for us to be dogmatic in our evaluation of the work and ideas. Nevertheless there does seem to be a certain arbitrariness in the way in which different facies are recognised and ascribed to different environments. Larger-scale groups of cycles have been suggested. Thus 1st order cycles are grouped into meso- or 2nd order cycles and mesocycles into macroor 3rd order cycles and macrocycles into mega- or 4th order cycles. But again the basis of such groupings is not clear, nor is it possible to correlate these larger cycles with those set up in different parts of the world.

DISCUSSION

Environment of deposition

As stated at the beginning of this chapter, on the basis of available evidence, the environment of deposition of the British Carboniferous rocks under discussion (and

DISCUSSION

149

their European counterparts) appears to have been shallow-water and in the main non-marine. GREENSMITH (1956, 1962, 1965), D. MOORE,(1958), JOHNSON (1960), BLUCKand KELLING (1963), CLARKE(1963), KELLrNG(1964a), E~L10T~(1965), to name but a few, have all provided evidence supporting this thesis. Many workers have considered the cyclic sequences in all divisions of the British Carboniferousto represent deltaic sedimentation interrupted periodically by marine invasions. There were, of course, differences in emphasis of the marine phases in Yoredale facies, Millstone Grit facies and Coal Measures facies, deposition. D. MOORE’S (1958) comparison (Table XXI) of the Mississippi delta and Yoredale rocks seems apposite while DUFFand WALTON(1962) also pointed out similarities between Coal Measures cycles and sequences met with in present-day deltas. The possible importance of coastal sand barriers was emphasised by ROBERTSON (1948) and KOPERINA (1958). CLARKE (1963) considered Coal Measures sedimentation in Europe in terms of a large coastal plain, emphasising the not infrequent river-like channels filled by sandstones, as also did ELLIOTT (1 965). HEMINGWAY (in preparation) likened Coal Measures sedimentation to that seen in intertidal flats on which lacustrine and deltaic environments frequently encroached. The uniqueness of this combination in Carboniferous times was the vast areal extent.

Cycle mechanisms

It has been pointed out that theories of origin of Pennsylvanian cyclothems can be regarded as falling into two main groups-those involving a tectonic mechanism and those that rely on climatic control. In Europe, it is also possible to consider theories of origin under these headings but additional groups must be added (Table XXVII). The table is a generalisation and it must be emphasised that not all the described sequences are necessarily comparable and that many theories contain elements from those in other groups (for a more detailed classification see, e.g., WESTOLL, 1962, p.767). Those theories under “others” are mainly of historical interest and are not discussed here. Table XXVII is useful, however, in containing a large enough sample to show important differences in thought between European and American geologists. Obviously, in Europe, climatic control has not been thought important. By contrast the assumed great lateral extent of individual cycles in America has presumably led to sedimentational control being barely considered. Overwhelmingly, European geologists have favoured tectonic control but, with a few exceptions, most authors have not supported the diastrophic-control theory of WELLER (1956). The objections raised to this theory in Chapter 4 are even more strongly supported by the evidence in Europe. Intermittent subsidence (tectonically controlled) of the depositional area, (with, of course, modifications in detail) has received most support as a possible mechanism for producing coal-bearing cycles. The concept implies that, periodically,

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TRANSITIONAL REGIMES IN EUROPE

TABLE XXVll THEORIES OF ORIGIN OF

COAL-BEARlNG CYCLES FAVOURED

BY EUROPEAN AUTHORS

._

Tectonic

Sedimentational

DELA BECHE,1834 MACLAREN, 1838 GEIKIE,1882 MILLER,1887 GOODCHILD,1891 Hmm, 1902 BISAT,1920 HUDSON, 1924 SCHMIDT,1924 BROUGH, 1928 PRUVOST,1930 TONKSet al., 1931 1932 TROTTERand HOLLINGWORTH, ~ G E 1937, , 1960 MAKOWSKI, 1937 TRUEMAN, 1946, 1948, 1954 VANLECKWIJCK, 1949 BERSIER, 1950 DUNHAM, 1950 DELEAU, 1952 RVrreN, 1952 HAVLENA, 1957 K o m o , 1958 ZHEMCHUZHNIKOV, 1958 FEOFILOVA, 1959 JOHNSON,1960 WELLS,1960 JABLOKOVet al., 1961 GREENSMITH, 1962 WESTOLL, 1962 BOIT, 1964 and EVANS,1964 WOODLAND ZEMAN, 1964

PHILLIPS, 1836 ROBERTSON, 1948, 1951 (Plants have controlling influence) DELMER, 1952 BERSIER, 1958 KOPERINA, 1958 D. MOORE,1958, 1959, 1960 GOODLET, 1959 DUFFand WALTON,1962 READ,1965 CIimatic

SCHWARZACHER, 1958 HOLLINGWORTH, 1962 ?JESSEN, 1956a, b, 1961 (Unspecified “extraterrestrial” control) Compactioml THIADENS and HAITES,1944 VANDER HEIDE,1950 Others

MILNE,1839 SIMOENS, 1918 W. FRANCIS, 1961

after silting up of the area of deposition, subsidence slowed down or ceased altogether, then after a prolonged period of plant growth, commenced again with renewed vigour, with consequent drowning of the coal swamp. How then does this theory fit the facts? In the Coal Measures, it must be emphasised once more that we are not dealing with successions where the same number of beds keeps regularly appearing, again and again, in the same order. Rather we see a sequence of events where spasmodically, a return to the same environment for varying periods of time is recorded. Between these periods, changes in sedimentation occurred, sometimes in a predictable manner. Most important, it appears that in any area of deposition, other than very occasionally, the environment is not identical everywhere. Splitting of coal seams indicates that while swamp conditions obtained in one place, simultaneously, sand, silt or mud was being deposited elsewhere. Tracing of a particular horizon laterally may also show marked

151

DISCUSSION

10 miles

W.. CAIIIZIUS WATER BORE

CARTAKRY

TOLL

BORE

BOCSIDE No.3 BORE

E.

COMRrE 30.24

COMRlE

COMRIE

No.25

No.8'

BORE

BORE

BOIUP

mhfudstonr

Fig.59. Relationship of tuffaceous horizon to beds of a cycle in Scottish Carboniferous over a distance of 10 miles. (After E. H. FRANCIS, 1965.) (Crown copyright, reproduced by permission of the Controller of H.M. Stationery Office.)

variations in the faunal content of that horizon, and in the beds below and above it, indicating facies changes. Few horizons seems to be true time-horizons. This was (1961), in the Limestone graphically illustrated, on a small scale, by E. H. FRANCIS Coal Group in Fife, Scotland, where the tuffaceous band, presumably as near a true time-horizon as is possible to find, is seen to cut right across the rock units of a cycle (Fig.59). While marine sediments accumulated in one part of the area a coal swamp existed some miles away. A second point of importance in considering the origin of cycles is that it has been demonstrated, for instance by DUFFand WALTON (1964), and READ and DEAN (in preparation), that the number of cycles in a succession is intimately related to the thickness of the succession. The thicker the succession the greater is the number of cycles present. Lateral variation, then, makes it difficult to appeal to change of environment brought about by some important event controlled by either a general tectonic mechanism (e.g., intermittent subsidence) or by climatic and/or eustatic changes, for the formation of each and every cycle. If at one place the drowning of a coal forest is taken to indicate the occurrence of such an important event then that event must appear quite unrepresented (or unrecognisable) in a succession some short distance away-perhaps in the middle of an apparently homogenous sandstone or shale!

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TRANSITIONAL REGIMES IN EUROPE

If this is so, how does one decide which sections provide proof of such events and which sections record the number of times such events occurred? Certainly, detailed palaeontological work has shown the presence of rhythmic alternations of fauna in apparently homogenous shale sequences (see e.g., RAMSBOTTOM et al., 1962; READ, 1965) and these may be correlatable with the formation of coal-bearing cycles elsewhere. But these instances are exceptional. On the whole, at present, there is simply not the evidence for the general synchronous development of individual cycles over large areas. There are exceptions-some of the European Upper Carboniferous marine horizons are very widespread-but they are the only evidence at present for large-scale sea-level changes. Most Upper Carboniferous cycles seem to be quite adequately explained in terms of sedimentationalvariations in an area undergoing general, though often areally differential, subsidence. Deltaic conditions in perhaps a delta coastal plain appear to supply a possible environment. BOTT(1964) put forward a theory claiming an isostatic origin for the intermittent subsidence thought necessary to explain Carboniferous cycles in Britain, which were stated to be about 100 ft. thick. This figure is quite unrealistic and it is clear from the text that Bott was primarily concerned with the limestone-bearing Yoredale-facies of the Carboniferous in northern England. Certain limestones in this facies are traceable over perhaps 2,000 sq. miles or more (Fig.51-53) and this persistence has been taken to indicate that periodicaly intermittent subsidence with resultant invasion by the sea has occurred. But again there is considerable lateral variation which Bott appears to have ignored, and the number of cycles varies from area to area. The socalled “minor” cycles in the Yoredales are identical in all but thickness and areal extent to the “major” cycles (SHIELLS, 1963). It then becomes a question of degree whether the origin of any particular cycle is assigned to sedimentary fluctuations or whether intermittent subsidence over a “large” area is required. The proposed “deltaswitching” theories of D. MOORE(1958) and GOODLET (1959) seem more acceptable. The picture which emerges from this discussion is firstly, that of an area undergoing general subsidence. Secondly, this subsidence was probably differential from area to area since it seems unlikely that a large part of the crust should behave as a monolithic block. Sedimentation inevitably took place in this basinal region but the development of cycles was controlled by sedimentary distributive mechanisms. If this is accepted for coal-bearing cycles what general problems still remain? The Kansas megacyclothems raise considerable difficulties, as noted earlier. There seems no doubt that in certain parts of the succession, limestones, of distinctive types, appear in a particular order. Their significance, in terms of environment of deposition, is a matter for debate (see particularly, MERRIAM, 1964). Another difficulty inassessingthecyclicnature of the Kansas succession, in terms, say, of Illinois, or even Europeancycles,is the evident lack of seat earth or coal horizons. An objectiveappraisal of cyclic sedimentation in Illinois and the Appalachians using seat earth/coal horizons as markers can obviously be made. In Kansas, however, the more marine nature of the beds makes this difficult. Regional facies studies may, however, yield answers to the problems more quickly then continual attempts to explain complicated, often hypo-

153

DISCUSSION

thetical, vertical sequences. It may well be that the Kansas cycles are not strictly comparable with the majority of coal-bearing cycles in America and Europe. The question of scale is an obvious general problem. Given, however, a number of deltas, often coalescing along coastal plains with the sea generally not far distant, comparison with recent and present-day deltaic areas makes this difficulty appear to be not too great. In America the scale is larger but the problem not insuperable if it is conceded that most individual cyclothems are not continuous from, say, Kansas to the Appalachians. Further work may confirm that a number of the American cycles are persistent and perhaps even correlatable with some in Europe. This would provide strong evidence for eustatic changes in sea level. In general, however, regular and numerous periods of world-wide change of sea level seem unnecessary. The asymmetry of cycles, in particular those in which coal is immediately overlain by marine beds, has been taken as indicative of sudden subsidence. It should be realised, however, that a marine transgression will normally lead to alluviation upstream and consequently slower sedimentation in the area encroached by the sea. TABLE XXVIII OF CORE HOLE 105, R H ~ N EDELTA (After LAGAAY and KOPSTEIN, 1964)

STRATIGRAPHY

Environment of deposition

Gross lithology

Topsets

(7) Coastal plain (undifferentiated) (6) Distributary channel

Sand

Foresets

(5) Fluviomarine barrier face (4) Proximal-fluviomarine (3) Distal-fluviomarine

Deltaic structural elements

Marine clay Bottomsets

(1) Slow (2) Moderate deposition deposition

)Middle-neritic

Alluvial valley (Pleistocene)

Sand and gravel

Interesting information on this point is available from a borehole sequence through and KOPSTEIN, 1964). The sediments of the Rh8ne Delta (Table XXVIII; LAGAAY late Pleistocene marine transgression resulted in alluvial sands and gravels being covered with a thin marine clay. At a much more recent date the delta encroached and there followed rapid deposition of deltaic silts and sands. “The spectacular changes in numbers and in composition of the microfauna are credited primarily to the increase in the rate of deposition as the delta advances. The scarcity of microfauna in the rapidly deposited upper half or two thirds of the sequence contrasts markedly with the great

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TRANSITIONAL REGIMES IN EUROPE

abundance of microfauna in the thin basal “Bryozoa Bed” which took several thousand years to accumulate. This kind of development, from an abundant microfauna at the bottom to a scarce microfauna at the top, seems to be characteristic of certain transgressive-regressive marine cycles.” (LAGAAY and KOPSTEIN, 1964, p.226). This situation is true of most Coal Measures cycles as far as fossils are concerned. Most “mussels” and marine fossils are found near the base of the cycle, above the coal. JESSEN (1956) and WELLS(1960), for example, emphasised that the juxtaposition of a marine roof and a coal need not imply the sudden or intermittent subsidence invoked by many authors (e.g., PRWOST,1930; TRUEMAN, 1946; DUNHAM, 1950; WOODLAND and EVANS,1964). Returning to the general question of coal-bearing cycles it will be obvious from the foregoing that sedimentational controls are regarded as the main reason for the succession of beds of varying type between seatearth/coal horizons. But can sedimentational controls alone explain why swamp conditions periodically ceased? If intermittent subsidence (of tectonic origin) and eustatic changes of sea level (of climatic or tectonic origin, see Chapter 11) are rejected, how can this flooding be brought about? Four possibilities might be considered: ( A ) The breaking down of an off-shore barrier has been put forward by, for example, ROBERTSON (1948) and KOPERINA (1958), with some conviction. While the evidence of such barriers in Carboniferous sequences is not conspicuous, sand-bars may well have played an important part in analogous environments in other parts of the geological column, e.g., the Wealden cycles of southeast England (discussed in Chapter 3). Furthermore, TEICHMULLER and TEICHMULLER (in preparation) have described how a 300 ft. seam of lignite of Miocene age in the Lower Rhine District of Germany formed in a coastal swamp protected from the North Sea by a large sand-bar. Obviously, the breaking down of such a barrier would result in the sea flooding the swamp area, but this mechanism might only apply to those cycles containing marine horizons. It might be extended by supposing that, after breakdown of the barrier, the salinity of the water decreased inland and the upper reaches of the flooded delta-swamp supported a flourishing “non-marine” bivalve fauna. Evidence for the lateral passage of marine into non-marine horizons, is, however, scarce (but see GOODLET, 1959, pp.232-233). (B) Compaction of the sediments between the coal horizons (and of course of the peat itself), may, as THIADENS and HAITES(1944) and VAN DER HEIDE(1950) suggested, play an important part in cyclic sedimentation. In Europe it can be demonstrated that there may exist a direct relationship between the thickness of a coal seam and the shale/sandstone ratio below the seam (TRUEMAN, 1954). The greater compaction of the mud is thought to have allowed more peat (and hence a thicker coal) to form. Obviously, below any swamp there could be variable thicknesses of sand, silt, mud and peat, all of which were undergoing compaction. Is the rate of compaction constant or does it vary through time? Might not the load of sediments in a particular area reach a critical level, causing the rate of compaction in underlying material to increase rapidly so that the swamp is flooded?

155

DISCUSSION

If there is any truth in this suggestion, flooding through the agency of compactionsubsidence could be liable to take place both on the delta-front, allowing a marine incursion, and on the delta-top, allowing non-marine flooding and providing another way of producing splits. A point in favour of this mechanism is that it could happen at any time during sedimentation, thus accounting for the irregularity of most cycles. ( C ) Sedimentational control alone, in the form of distributary changes, could conceivably affect the growth of forest swamps and hence the accumulation of peats. It is known for instance, that changes in salinity have a very marked effect on the type of vegetation that can thrive in a water-logged environment. A major drainage switch might result in large areas of forest being deprived of the fresh water (in addition to the rain water) necessary for continued survival of the peat-forming plants. If plantgrowth, and hence peat accumulation, does not keep up with subsidence then flooding will occur. As in ( A ) this mechanism would perhaps have its greatest effect in the production of cycles containing marine horizons. (D)It is implicit in all theories concerning coal-bearing cycles that peat accumulation would continue indefinitely unless changes of climate, rates of subsidence or some combination of externalfactors led to flooding of the peat swamp. The assumption is that flooding is the reason for the decline of the swamp vegetation but the reverse could be true, i.e., flooding occurred because the swamp vegetation ceased to flourish. It is worth considering what causes changes in the flora colonising a particular area. A change of climate will obviously have a profound effect on the vegetation. Alterations in the drainage, which may or may not be the result of climatic change, are also critical. Examination of the stratigraphical distribution of spore types within coal seams shows that striking changes in the flora contributing to the peat took place. Such changes in coals of the Yorkshire Coal Measures, England, were compared by SMITH (1962) with evidence from present-day peat bogs. The accumulation of peat can so alter the drainage and nutritional properties of the layers where the plants root that the type of flora growing on top of peat may change independently of climatic fluctuations. In Carboniferous peats it was difficult to assign changes in flora to either climatic or edaphic factors. The coastal and deltaic peat swamps of Sarawak (J. A. R. ANDERSON, 1963), which cover some 5,660 sq. miles, provide, perhaps, conditions most analogous to Carboniferous coal swamps. In Borneo there is abundant evidence of the dying out of a forest vegetation due to edaphic factors. J. A. R. ANDERSON (1963, p.131) describedhow “Alluviumcarrieddown by the river draining the interior has been deposited at the mouths of rivers or in bays along the coast and as the coastline progressed seawards so peat has developed and accumulated under the dense forest on the plain behind. The Rejang, the largest river on the northern coast of Borneo, and to a lesser extent other rivers, has divided to form a complex deltaic system. Each island on the delta forms a distinct and self-contained swamp unit bounded by a fringe of mangrove or riparian forest. . .The coastal and deltaic peat swamps. are entirely of the raised bog type with a stilted water table and surface drainge.” From the shores of each island inwards, marked changes in plant species occur. Trees become smaller and scarcer.

..

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TRANSITIONAL. REGIMES IN EUROPE

The central parts, higher above water level than the peripheries, cannot support a forest vegetation. “The vegetation types are found in a catenary sequence from the perimeter to the centre of a raised bog. . . pollen analysis indicated that the horizontal pattern of vegetation types found on the ground is also likely to be found in a vertical succession in the centre of the raised bogs” (J. A. R. ANDERSON, 1963, p.142). Application of such considerations to Carboniferous coal-swamps leads to the suggestion that perhaps plant growth and hence peat accumulation changed so that the latter could not indefinitely offset continuous subsidence. Whether the main reason for the cessation of forest growth was climatic or edaphic is difficult to assess but the evidence from Sarawak indicates that edaphic factors alone could be sufficient. It must be emphasised that in the context we are discussing neither climatic nor edaphic changes need take place with any regular periodicity. Rather it seems they would each act within the framework of the irregular cyclic sedimentation inherent in a delta coastal plain environment. Given a suitable climate, swamp forests would flourish wherever sedimentation made the water shallow enough. Once established, the length of time peat would accumulate would depend on many factors but at any time conditions could have become unsuitable for plant growth, and these conditions need not have been uniform throughout the area. Thus we have in a sedimentary-edaphic control mechanism adequate allowance for all the irregularities of cyclic sedimentation known from coal-bearing sequences. In summary, a theory of cyclothem development which includes only “deltaswitching” as a mechanism we conclude is inadequate. Some method of ending coalswamp conditions must be found and we have given four possibilities. If drainage changes are combined with edaphic controls we have a sedimentational mechanism which accounts not only for the order of the sediments, and their lateral complexity, but also for the cessation of one cycle and the beginning of another. Periodically, during Carboniferous times, as evidenced by the few very widespread and thicker marine bands in the European Coal Measures (see DUFFand WALTON,1962) it does appear that an overall rise of sea level took place. Eustatic changes in sea level in these few instances seem possible, and might coincide in time with important changes in Pennsylvanian sedimentation in America but these are considered incidental in the cyclic sedimentation of Carboniferous rocks. Other mechanisms may have played their part but the combination of sedimentary-edaphic processes seems built in to the envisaged palaeogeography, and, if we are correct, must have led to cyclicity.

Chapter 6 EPICONTINENTAL, MARINE ENVIRONMENTS, I

CALCAREOUS AND ARGILLACEOUS ROCKS

In this and the two succeeding chapters we shall consider those cyclic deposits laid down in “stable shelf” seas, corresponding essentially with the neritic zone, though with inevitable gradations into the bathyal and littoral zones. The usual abundance of benthonic fossils signifies shallow water conditions, with depth of sea normally less than 200 m. By restricting consideration to “stable shelf” deposits we intend to exclude those beds suggestive of rapid deposition in tectonically unstable areas, the so-called flysch-type facies. Although these deposits are normally considered to have been laid down in deep water this may not be easy to prove, and so it is better to separate them as “geosynclinal” on the basis of their distinctive lithological characteristics (see Chapter 9). Stable shelf regimes are characterised in contrast by comparatively slow deposition, and sandstones are generally subordinate to argillaceous, biogenic and chemical deposits. Volcanic detritus is rare or absent and the arenaceous beds are characteristically well sorted. The most typical sedimentary structures in these rocks are crossbedding and ripple marks. As the lithologies in stable shelf regimes show considerable variety, in contrast to, say, “flysch” deposits, it has been found convenient to subdivide sedimentary cycles primarily on a lithological basis. In this chapter we shall be concerned with carbonate rocks, mainly limestones, and argillaceous rocks, which may on occasion be collectively referred to as shales or clays. Mainly because of diagenetic alteration, limestones may pose difficult problems of interpretation. This is most acute in those fine-grained types known variously as calcite mudstone, calcite siltstone, calcilutite and micrite. It is normally impossible to tell whether such rocks, recrystallised to a fine mosaic of calcite crystals, had an inorganic or organic origin. However, limestones have two great advantages. Fossils are frequently abundant, and allow important deductions about the environment of deposition and, in many instances, correlation over large distances. Secondly, the absence of coarse terrigenous detritals simplifies interpretation of cyclic sequences by effectively eliminating the influence of migrating stream channels, which is so important in other environments. The classification to be adopted in this chapter is as follows: ( I ) Cycles composed of differing types of limestone. (2) Limestone-dolomite cycles. (3) Cycles composed of limestones and argillaceous beds: (a) minor cycles, (b) major cycles.

158

EPICONTINENTAL MARINE ENVIRONMENTS, I

CYCLES COMPOSED OF DIFFERING TYPES OF LIMESTONE

A classic account of major cyclic alternations within a limestone sequence, measurable in tens of metres, was published early this century by DIXON and VAUGHAN (191 I), on the Lower Carboniferous in the Gower Peninsula, south Wales. A contrast was made between the so-called standard and lagoon phase limestones. The standard limestones consist predominantly of crinoidal and oolitic limestones, some of them dolomitised, rich in a variety of marine fossils. These were considered to be shallow water but open sea deposits. The lagoon phase limestones are characteristically finer-grained and less fossiliferous, the fauna being restricted mainly to a few species of bivalves. They include calcite mudstones (calcilutites) and beds with algal structures. Certain sedimentary breccias suggest reworking of material broken up by desiccation. These deposits were supposed to be of extremely shallow water origin and effectively isolated from the open sea. It is likely that salinities were abnormal.

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Dixon and Vaughan related the large-scale repetitions of these two types of facies to changes in depth of sea (Fig.60) with the standard limestones signifying slightly deeper water. These depth changes were considered to be a result of local earth movements superimposed on a regional subsidence. From a comparison of slight facies changes in neighbouring regions it was concluded that these earth movements were differential, and that regions of deeper water were depressed to a greater extent. A comparable sequence, cyclic on a smaller scale, in the Upper Visean of Belgium, has recently been described by PIRLET(1963). The cycles, ranging in thickness from 60 cm to 10 my consist of two alternating phases. Phase A is composed of skeletal calcarenite with an interstitial drusy calcite cement ("calcaire organoclastique") while phase B consists of poorly fossiliferous calcilutite ("calcaire cryptogrenu") and stromatolitic limestone. The base of phase A is marked characteristically by an erosion (1963) interpretation of the or scour surface, the top by a gradual transition. PIRLET'S cycles (Fig.61) is similar to that of DIXON and VAUGHAN (1911). Phase B rocks were deposited on extremely shallow lagoonal platforms (as signified by the algal horizons) protected from strong marine currents, an environment that was evidently rather unfavourable for animal life.

159

CYCLES OF DIFFBRING TYPES OF LIMESTONE

The change to phase A deposits reflects a subsidence, which allowed the open sea to break in. Conditions were favourable for normal benthonic organisms and fine calcareous mud was winnowed away by current and wave action. As subsidence diminished and the still-shallow sea filled up with sediment, so extensive lagoons formed once more. Small-scale cycles in the Pennsylvanian limestones of northeastern Nevada (DOTT,1958) do not differ notably from those described by PIRLET(1963).Thereisthe same alternation between mechanically deposited rocks (skeletal calcarenites, oolites, subsidiary calcirudites) and fine-grained limestones, some of them cherty (Fig.62). The most important difference is that the fine-grained beds in Nevada contain shelly fossils and are not stromatolitic. Dott was cautious in his interpretation, observing that the cycles resulted from periodic variations in strength of waves and currents but hesitating to go further. It is worth noting that WANLESS and PAJTERSON (1952), in their article on limestone-shale cycles in the Pennsylvanian of neighbouring states, attributed calcarenites to formation above and calcilutites to formation below wave base. Hence calcilutites were held to signify deeper water in this case, an interpretation diametrically opposed to that of PIRLET(1963) and DIXONand VAUGHAN (1911). This is a possible interpretation when the calcilutites are not stromatolitic or mudcracked. Where such alternations are small-scale and frequent through a thick succession, it may be doubted whether each cycle marks a change in depth of sea, however slight. It is possible that periodic storms might have stirred up fine-grained sediment leaving thin horizons of winnowed shells as their record, a process that can be observed occasionally, for instance, in the Bahamas.

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Fig.61. Minor cycles in the Visean limestones of Belgium. (Adapted from PLRLET, 1963.)

160

EPICONTINENTAL MARINE ENVIRONMENTS, I

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SCHWARZACHER (1958) has divided up the Lower Carboniferous Great Scar Limestone (northwest Yorkshire) into cycles in an unusual way. Ten cycles averaging about 9 m thick are marked off by master bedding planes traceable laterally over a considerable distance. Most of the limestones are fossiliferous, with crinoid ossicles being the most abundant, and have a matrix varying from microcrystalline calcite mud to coarsely crystalline interstitial cement. Schwarzacher claimed that the proportion of fossils in the limestones increases towards the bedding planes, suggesting a correlation of these with phases of increased current strength and reduced precipitation of calcium carbonate.

LIMESTONE-DOLOMITE CYCLES

161

The regularity of the bedding planes was thought to favour climatic rather than tectonic control. Schwarzacher made the suggestion that several of the cycles in the Great Scar Limestone correspond with limestones in the classic Yoredale succession further north.

LIMESTONE-DOLOMITE CYCLES

The best-described cyclic alternations of limestone and dolomitic rock occur in the so-called Lofer facies of the Upper Triassic in the Austrian Alps. These were made famous by SANDER(1936, 1951) in his pioneer studies of fabrics in carbonate rocks. Sander distinguished between so-called metre-rhythms and millimetre-rhythmites,well exemplified in the Dachstein and Wetterstein Limestones. The former consist of cyclic alternations, generally a few metres thick, of massive limestones containing abundant bivalves (Megalodon) and finely laminated dolomitic beds without fossils. The latter comprise the millimetre rhythmites, with laminae ranging mostly between 1 and 5 mm in thickness. Traces of algal structures are present in the rhythmites. Sander regarded the rhythmites as varve deposits, without being able to produce convincing evidence. They were held to signify deposition in stagnant water below wave base, perhaps between 100 and 200 m depth (a greater depth was ruled out by the presence of algae). The massive Megalodon limestone signified shallower water conditions, partly above wave base. Sander thought that the periodic changes in water depth implied by this interpretation were more likely the result of climatic control operating through glaciation than local epeirogenic control, because counting of the millimetre rhythmites per cycle gave a figure approaching 21,000 years, which is supposed to mark an equinoxial cycle. SCHWARZACHER (1954) made a statistical study of this cyclic succession in the Dachstein Limestone of Lofer and concluded that every fifth dolomitic bed was thicker than its neighbours. He thought that such a regular repetition probably signified an ultimate astronomical control, but this was not specified in detail. A further detailed study of the Lofer cycles has recently been made by FISCHER (1964). They were attributed to a lagoonal, back-reef facies north of a major reef belt. Fischer described a typical Lofer cycle (corresponding to a composite sequence) as follows: ( d ) Subtidal massive limestone (calcarenite and calcilutite) with marine shells. (c) Intertidal dolomitic member with a variety of shrinkage features attributed to desiccation. (b) Argillaceous member, red or green in colour, restricted to solution or desiccation cavities in the underlying rock. (a) Disconformity. Some sediments probably lost through erosion. The laminae of unit c, the dolomitic member, which would have been interpreted by Sander as varves, were compared with algal mat laminated sediments of the type found, for instance, in the Bahamas and Florida. These, combined with desiccation

162

EPICONTINENTAL MARINE

ENVIRONMENTS, I

features such as prism cracks, sheet cracks and shrinkage pores, were held to signify intertidal conditions. They are thus regarded as shallower water deposits than the massive shell limestone, which is thought to have formed in several metres depth of water. Such an interpretation, the opposite of Sander’s, parallels in an interesting way the conflicting interpretations that have been proposed for alternations of different types of limestone. Of the two interpretations, Fischer’s appears the more probable in the light of modern knowledge of algal-flat and back-reef environments. Fischer related the changing depth of water to eustatic control. Since there must have been local tectonic subsidence to accommodate the sedimentary succession, this interpretation was thought to be simpler than that invoking a complex set of local epeirogenic movements, though the latter was not discounted. It was tentatively suggested that the oscillations of sea level might have had an amplitude of up to 15 m and a periodicity of between 20,000 and 100,000 years. Comparable cycles have been described from the Lower Muschelkalk of north(1938) .The cyclic sequence in its full development (composite west Germany by FIEGE sequence) was described as follows: Dolomitic limestone, devoid of fossils. Marl and marly limestone. Fine-grained limestone with marine fossils. Oolitic or skeletal calcarenite with marine fossils. The cycles begin usually with a sharp contact with the underlying rock; they are best developed in the central part of the Muschelkalk Basin. Most range in thickness between 2 and 6 m. In developing his interpretation Fiege discussed whether the calcarenites signify deeper or shallower water than the fine-grained rocks. He favoured the first alternative because the sequence up the cycle is the same as that laterally towards the old shoreline. The dolomitic limestone hence signifies extreme shallowing and probably lagoonal conditions unfavourable to animal life. This interpretation is clearly close to Fischer’s. Unlike Fischer, however, Fiege attributed the changes in water depth to epeirogenic movements of the sea floor. It was pointed out that the asymmetry of the cycles does not necessarily imply an equivalent asymmetry in the tectonic control, since the sharp break at the base might signify a phase of slow deposition or erosion. One can make a similar observation about the Lofer cycles. Limestone-dolomite cycles in the Lower Ordovician of Maryland, with features similar to those in the central European Triassic, have been described by SARIN(1962). Sarin recorded thirty five cycles in a 43 m section; his ideal (composite?)sequence is as follows: Pisolitic limestone. Algal limestone (with stromatolitic structures). Massive structureless calcilutite. Mottled limestone. Massive structureless dololutite. Laminated dololutite.

CYCLES OF LIMESTONES AND ARGILLACEOUS BEDS

163

Sandy dolomite. Intraformational dolomite conglomerate. The “ideal” sequence diverges from Sarin’s “typical” sequence insofar as the pisolitic limestone unit is developed in only two of the thirty five cycles. The dolomite conglomerate normally overlies limestone of the previous cycle with a sharp contact, whereas the dololutite passes up to the overlying limestone by means of a gradual transition. Only the limestones are fossiliferous and only the dolomite beds contain appreciable quantities of detrital quartz. Sarin believed that the rocks were deposited in a shallow marine basin similar to the Bahama Banks. The limestones were deposited in conditions of normal water circulation. Uplift of the sea bed led to limited erosion and influx of detrital quartz. The increased shallowness together with the rise of sills restricted circulation and resulted in the formation of unfossiliferous laminated dolomite, which was considered a primary precipitate. Gradual subsidence led eventually to a renewal of normal circulation and the further deposition of limestone. It seems clear in all the examples discussed that the rocks were laid down in very shallow water and highly probable that the dolomitic beds formed in a shallower regime than the limestones. The dolomite is characteristicallyfine-grained and confined to particular horizons and thus of the sort often termed primary. Bearing in mind modern research on dolomite formation in the Persian Gulf, the Caribbean and the Bahamas (DEFFEYES et al., 1965; ILLINGet al., 1965; SHINNet al., 1965) it seems likely that it formed by the early diagenetic replacement of aragonite in a hypersaline supratidal regime. Abnormally high salinity rather than loss through diagenetic replacement is the likely cause of the rarity or absence of fossils.

CYCLES COMPOSED OF LIMESTONES AND ARGILLACEOUS BEDS

Minor cycles

Regular cyclic alternations of shale (and/or marl) and fine-grained argillaceous limestone, of the order of a few decimetres per cycle, are a common feature of many shallow water marine successions but their detailed study has been largely neglected. An exception to this is the Blue Lias Formation at the base of the Lower Jurassic in southern England and Wales, which has received considerable attention over many years and has been the subject of a long-standing controversy (HALLAM, 1964a). It will accordingly be considered here in some detail. The limestone bands are composed of argillaceous calcilutite mostly 5-30 cm thick and are separated by mark and shales of a somewhat wider range in thickness. Though shelly fossils are generally abundant, there is no doubt that the variations in calcium carbonate content are not due to variations of shell content. The dispute has been whether the cycles are original sedimentary features or products of diagenesis. There now appears to be evidence favouring both interpretations to some degree.

164

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EPICONTINENTAL MARINE ENVIRONMENTS, I

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The strongest single piece of evidence favouring a sedimentary origin is the mottling of many rocks, well seen in the classic section on the Dorset coast, as a result of the burrowing activities of various organisms which have produced a number of distinct trace fossils. Lighter, more calcareous rock is seen to penetrate downward into darker, less calcareous rock and vice versa (Fig.63). The fine preservation of trace fossils can only be due to the original deposition of alternating layers of lighter, more calcareous, and darker, more argillaceous mud. Supporting evidence comes from the presence at certain horizons in Dorset of thin bands of finely laminated bituminous shale. These bands differ sharply from the other sediments in their fine lamination and comparatively poor and small-sized benthonic fauna (ammonites, bivalves, ostracods) which tend to occur in paperthin layers in the midst of shale which is virtually barren of all but fish fragments. These rocks must have been laid down in anaerobic or near-anaerobic bottom waters, in contrast to the vast majority of limestones and marls embracing them. Now these shales occur characteristically between successive limestones and marls and tend to be

CYCLES OF LIMESTONES AND ARGILLACEOUS BEDS

165

less calcareous (Fig.63). The location of such distinctive bands in the midst of typical cyclic units is held to constitute good evidence for primary origin. A further point, which is less conclusive but has an important bearing on the mode of origin, is the extraordinary lateral constancy of many limestone bands. There are several lines of evidence suggesting that at least part of the cyclic sequence owes its origin to segregation of calcium carbonate during early diagenesis. A number of limestones have a concretionary aspect and sometimes pass into bands of ellipsoidal nodules symmetrical about planes of bedding. There are also indications of a secondary accentuation of the contrast produced by a primary variation of sedimentation. As illustrated diagrammatically in Fig.63 the dark rock (1) penetrating down into the limestone bands is generally harder and more calcareous than the overlying marl (2), and the light-coloured rock (3) immediately below the limestones, softer and less calcareous than the latter. This suggests some migration of calcium carbonate both upward and downward toward the limestones, so increasing the sedimentary contrast. The Blue Lias limestones in Dorset average about 85 % CaC03 and the marls about 35 %. There is a little evidence suggesting that segregation may account for a quarter of the calcite present in the limestone today (HALLAM, 1964a). Perhaps a more decisive line of evidence favouring large-scale segregation is as follows. The thickness of the alternating sedimentary units tends to remain constant regardless of the thickness of the rock succession. The thicker the succession the more limestone beds it contains in proportion. This may be clearly seen by reference to Fig.64 and 65. The loss of limestone units in thinner successions cannot be attributed to the intervention of non-sequences since there is good palaeontological control. It seems that the thickness of many limestone units is controlled by some factor independent of the original conditions of deposition and that a regular cyclic alternation may in some cases be produced entirely by segregation. The occurrence of what SUJKOWSKI (1958) has termed “rhythmic unmixing” during diagenesis may be far more widespread than has been assumed hitherto and is equally relevant to cyclic alternations of bands of limestone and chert, where there may be clear evidence of diagenetic mutual replacement. The process poses a number of problems but these will not be pursued here since they are not strictly relevant to cyclic sedimentation. “Rhythmic unmixing” at least relieves the embarrassment of having to account for numerous small-scale but significant alterations in lithology in a sedimentary regime suggestive of slow sedimentation and tectonic stability. This is probably an important reason why climatic variations have been suggested as a controlling factor for this type of cycle (e.g., GILBERT, 1895; ZIEGLER, 1958). A smaller number of sedimentary alternations have nevertheless to be accounted for. It is assumed that the calcium carbonate was precipitated from sea water either 1964a). Because of the lack of inorganically or through some organic agency (HALLAM, well-studied modern analogies it is very difficult to decide between several alternative

EPICONTINENTAL MARINE ENVIRONMENTS, I

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CYCLES OF LIMESTONES AND ARGILLACEOUS BEDS

167

explanations and the following suggestions are only tentative. To consider only the more plausible explanations, fluctuations in the rate of calcium carbonate precipitation could result from slight changes in water depth or temperature, while the amount of terrigenous clay could vary with humidity or tectonic activity on the land. If the cyclic variations were the result of pulses of terrigenous sedimentation superimposed upon a “background” of steady calcium carbonate precipitation one would expect to find the proportion of clay increasing towards the old shoreline. This is the reverse of the situation in southern England and Wales. Moreover, tectonic or climatic pulses on the land might be expected to produce coarser sediments than in the Blue Lias. Of the remaining alternatives,periodic changes in water depth are preferred provisionally, because some of the lateral variations in calcium carbonate content seem too pronounced to be accounted for purely by climatic change. For instance, the Glamorgan Blue Lias close to the old shoreline is both thicker and more calcareous than elsewhere, suggesting a higher rate of carbonate precipitation in shallow coastal waters. A similar type of cyclic sequence that has been described in detail occurs in the Upper Jurassic of southwest Germany. SEIBOLD (1952) undertook a chemical study of a 120 m profile containing some 460 beds of alternating fine-grained limestone and marl. The calcium carbonate content, which is evidently the only inorganic component that varies appreciably, ranges from 46 to 96 % but in the bulkofthemarlsrangesfrom 65 to 80 % and the limestones have a mode at 85 %. The mean limestone-marl fluctuation is in fact only 13 %, and the terms “limestone” and “marl” acquire a relative significance in some instances, being determined essentially by depth of weathering compared with adjacent beds (Fig.66, 67). These cycles are evidently more calcareous than those in the Blue Lias but otherwise similar in most respects. Nevertheless only a primary origin was considered. Seibold thought that the cycles were produced by periodic variations of calcium carbonate precipitation against a constant “background” of clay deposition. In support of this it was pointed out that this facies suggests very quiet conditions of deposition, not of the type to be expected in an environment where numerous pulses of sediments were being brought in from the land. The fine preservation of ornament on Foraminifera in the marly “Lettenlagen” was held to rule out appreciable loss of calcium carbonate by solution in the less calcareous beds. In a further study, on the relationship of foraminiferal and carbonate content of these beds (SEIBOLD and SEIBOLD, 1953), it was noted that the density of Foraminifera per unit volume of matrix declines with increasing carbonate content (a similar relationship, with macrofossils, has been discerned in the Blue Lias; HALLAM, 1960). This was attributed to a slower rate of sedimentation of the clays. Assuming the cycles are primary we can suggest the alternative explanation that the mark may have suffered more compaction than the limestones. It was further claimed that a few foraminiferal species were sensitive to calcium carbonate content. It appears, though, that, while there may be a rough correlation, the samples were taken not from successive beds but were chosen at larger intervals, so that it is hard to judge the relevance of these data to the mode of origin of the cycles.

168

EPICONTINENTAL MARINE ENVIRONMENTS, 1

ZIEGLER (1958) has made a detailed stratigraphical study of Upper Jurassic rocks of this facies in southwest Germany and his work complements Seibold and Seibold’sin a highly satisfactory way. Ziegler has been able to correlate in great detail over wide areas and has demonstrated a remarkable constancy of horizon of individual beds. In discussing the mode of origin of the cycles Ziegler tentatively preferred a climatic to a tectonic control, with the marls reflecting more humid or cooler phases. Unlike Seibold he did not rule out periodic impulses of terrigenous sedimentation. Nothing in the descriptions given by Seibold or Ziegler excludes the possibility that the cyclic sequence under discussion owes its origin wholly or in part to diagenetic segregation. Suggestive in this respect is an observation by Ziegler that thinning of a group of beds corresponds usually with a reduction in the number of component units, just as in the Blue Lias. Further European examples of this type of cycle are given by LOMBARD (1956). SPRENG (1953) has described an example from the Mississippian of Alberta. SCHWARZACHER (1964),for his time-series analysis of a limestone-shale sequence in the Lower Carboniferous of northwest Ireland, assumed without discussion that the sequence was primary in origin. He came to the conclusion on the basis of his statistical analysis that the small-scale fluctuations, i.e., between thin bands of limestone and shale, originated from truly cyclic changes in the environment which might have had an astronomical control. As Schwarzacher failed to give petrographic details it is difficult to judge whether the possibility of diagenetic segregation can be confidently excluded. If not, there is a danger that the statistical results may be meaningless in the context of sedimentation control.

Fig.66. Chemical characteristics of limestones and mark in Upper Jurassic cyclicsequence at NeufIen, Germany. Above: relative amounts of marl and limestone. Carbonate content of individual beds given as percentage. Limestones: hollow squares. Below: absolute amounts of clay and carbonate. Clay fraction in cm: white and black ornament. Carbonate: lined ornament. Total thickness of bed given by clay and carbonate fractions together. (Adapted from SEIBOLD, 1952.)

CYCLES OF LIMESTONES AND ARGILLACEOUS BEDS

CARBONATE CONTENT OF INDIVIDUAL BEDS

169

A

Fig.67. Carbonate content of individual beds in Neuffen profile (see Fig.66), showing overlapping of compositional curves for “limestones” and “mark”. (Adapted from SEmom, 1952.)

A very different type of cycle from those discussed above is that in which variations in the quantity of calcium carbonate in a clay sequence are produced largely or entirely by calcareous fossils. Good examples in the Oxford Clay (Callovian) of Peterborough in southeastern England have been well described by BRINKMANN (1 929). The cycles, which range up to about 1 m in thickness, may be expressed thus: Sharp boundary (denoting a considerable pause in sedimentation). Pavement of lamellibranch and ammonite shells, often overgrown with encrusting oysters. Comminuted shells, coarsening upwards. Greenish shale. Brownish shale, with green mottling in the upper part. Greenish shale. The brownish shale is highly bituminous (organic carbon content about 9 %), the greenish shale somewhat less so (3.3 % C). The brownish shales are poorly fossiliferous, containing only benthonic bivalves known to tolerate poor aeration (Nuculu) and others which might have been planktonic

170

EPICONTINENTAL MARINE ENVIRONMENTS, I

(Posidonia, Pseudomanotis). These obviously anaerobic or near-anaerobic beds alternate with shell beds containing a variety of benthonic bivalves and ammonites. This small-scale alternation of aerobic and anaerobic deposits recalls that in the Blue Lias. BRINKMANN (1929) related the cycles to local epeirogenic movements of the sea bottom. The shell beds marked periods of increased current action in shallower and better aerated water, the bituminous shales periods of deeper, stagnant water. FUCHTBAUER and GOLDSCHMIDT (1964) has recently described rather similar cycles of bituminous shale and shell beds in brackish-water WeaIden beds (straddling the Jurassic-Cretaceous boundary) in northwest Germany but did not commit himself on an interpretation. Though not strictly relevant to epicontinental sedimentation it is convenient here to consider an important case of cyclic sedimentation on the deep sea floor. Investigation of sediment cores from the east Pacific (ARRHENIUS, 1952) has revealed that the calcium carbonate content varies cyclically from less than 30 to more than 60 %. The thickness of the cycles ranges mostly between 0.25 and 1 m and correlation between cores shows that it varies laterally. The cycles extend back at least into the Pliocene, the average carbonate content increasing and the cycle amplitudes decreasing with age. The cycles are apparently the result of fluctuations in calcium carbonate rather than clay. The carbonate is entirely organic in origin, being composed of coccoliths and planktonic Foraminifera. According to Arrhenius the cycles have ultimately a climatic origin. During the glacial periods of the Pleistocene the rate of circulation of deep water was probably higher and hence more calcium carbonate was dissolved. But this was more than counteracted by a considerable increase in the rate of production of plankton including lime-secreting organisms because of an increased rate of upwelling in areas of ocean current divergence. Hence the glacial phases in the sediments, stratigraphically determined, are the more calcareous. There has been a general tendency to equate glacial periods with reduced production of calcium carbonate (KUENEN, 1950). This belief was influenced by investigations in the Atlantic Ocean. Conditions evidently vary according to regional circumstances. Where the rate of dissolution exceeds the rate of production or where the latter decreases during unfavourable life conditions for lime-secreting plankton the concentration of carbonate on the sea bed will be diminished. In the Atlantic it seems that the inflow of polar bottom waters has played a bigger role than in the Pacific. Major cycles

Somewhat different problems are posed by certain limestone-shale cycles of the order of a few metres to a few tens of metres in thickness. These may themselves include minor cycles of the type just described. An example of such cycles will be taken from the Pennsylvanian of the southwestern United States. From the entirely marine succession of this region WANLESS

CYCLES OF LIMESTONES AND ARGILLACEOUS BEDS

171

Fig.68. Major limestone-shale cycles in Lower and Middle Jurassic of Lorraine. The Greek letters refer to Liassic stage boundaries (see text), the irregular lines to erosion surfaces (with borings) and the crosses to coral masses. (After KLUPFEL,1917.)

and PATTERSON (1952) briefly described cycles ranging in thickness from about 10 to 30 m. The limestones are of varied lithology, ranging from oolites and skeletal limestones (biosparites) to algal and dense, fine-grained limestones. The sparry limestones were thought to have formed above wave base, which allowed fine mud to be winnowed away, and the dense limestones, marls and shales below wave base, in quieter water. The changes in depth of sea implied by this were referred to some sort of eustatic control. It will be recalled from an earlier section in this chapter that the alternation of fine-grained and coarse-grained sparry limestones can be interpreted in several ways but Wanless and Patterson gave no data to support their own contention. As there is no discussion on the point, it is not clear that they were even aware of the alternatives. Moreover, no convincing evidenceof eustatic control was put forward, since no detailed correlation between widely separated sections was attempted. A paper of outstanding importance in this connection was published many years ago by KLUPFEL (1917) on cycles in the Lower and Middle Jurassic of northeastern France (Lorraine). The cycles contain three major units which may be termed clay (at the base), marl and limestone. They are asymmetrical in the sense that the clay rests with sharp contact on the underlying limestone but passes up gradually into more calcareous rocks (Fig.68). The basal clay is comparatively poor in fossils, which consist characteristically of thin-shelled bivalves such as Nuculu and Nuculunu and ammonites. The more

I72

EPICONTINENTAL MARINE ENVIRONMENTS, I

fossiliferous marl contains subordinate layers of argillaceous limestone and limestone nodules, whose proportion tends to increase upwards, producing a facies resembling the Blue Lias. The limestone unit is often fine-grained and compact, but may be coarser-grained, exhibiting cross bedding (e.g., oolite in the Dogger). The most characteristic fossils are thick-shelled bivalves such as Ostrea, Trigonia, Cardinia and Astarte. Corals and brachiopods may occur, and echinoderm debris is common, but ammonites may be rare if the rock is calcarenitic. The upper boundary of the cycle is frequently marked by an erosion surface (certain Dogger cycles may show more than one such surface, see Fig.68). This is characteristically encrusted by oysters and other invertebrates and bored by bivalves; there can be no doubt that such surfaces were hardened soon after deposition. They correspond to the “hardgrounds” of European authors. The clay overlying the erosion surface often has a basal layer of derived limestone fragments, sometimes with a ferruginous surface, encrusted by oysters, serpulids and bryozoans and bored by bivalves. There may also be derived fossils. Another common feature of this basal layer is the abundance of phosphorite nodules, some of which appear to have been derived from the underlying limestone. Two other important points should be mentioned. If detrital quartz is present, it tends to increase in size and quantity up the cyclic succession. There is an intimate correlation between the ammonite sequence and the sedimentary cycles, with the cyclic boundaries normally marking sharp breaks in the sequence. As examples of cyclic boundaries KLUPFEL (1917) cited those between units in the old German stratigraphic classification such as Lias a, j3, y, 6 and E, which correspond to ammonite stages or substages. Kliipfel interpreted the facies change up the cyclic succession as due to shallowing of the sea, with the erosional features of the limestone ccroofbed”(“Dachbank”) signifying emergence. He thought that this shallowing could not be explained merely by sedimentary infilling, since this would be inadequate to account for the amount of shallowing implied by the facies changes (some cycles are only a few metres thick). Sedimentary infilling need only be seriously considered where thick sandstone wedges enter the succession, suggesting a possible deltaic influx. Hence the shallowing was, like the subsequent deepening, attributed to local epeirogenic movements of the sea floor. These tectonic movements were supposed to be asymmetrical in time, gradual uprise being followed by comparatively sudden subsidence. This was proposed to account for the evident asymmetry of the cycles. This epeirogenic interpretation, and the intimate correlation with ammonite zones, has been supported by workers on the German Lias (FREBOLD, 1925; HEIDORN, 1928; SOLL,1957). FREBOLD (1925) was the fist to point out the considerable lateral extent of certain distinctive roof bed^" (in the Sinemurian). Cycles of the type KL-FEL (1917) described have also been recognised in the British Lias (HALLAM, 1961). As Kliipfel himself noted, a givenunitin the sequencemay be missing locally, but there is no doubt that his scheme provides a valuable frame-

CYCLES OF LIMESTONES AND ARGILLACEOUS BEDS

173

work for the study of changes in the vertical succession. Departures from the “ideal” are discussed in detail in HALLAM (1961). A few points may be added to Xliipfel’s generalised description. When finely laminated bituminous shales, signifying anaerobic conditions, occur, their characteristic position is close to the base of a given sedimentary cycle. This point is well exemplified by the famous group of such beds in the Lower Toarcian known variously, according to region, as Jet Rock, “schistes cartons”, “Posidonienschiefer”, etc. Such shales appear to be relatively shallow-water deposits, contrary to what has been widely assumed in the past (HALLAM, in preparation). The phosphatic horizons at the base of a complete cycle are also normally rich in glauconite, a further pointer to slow deposition. While these Liassic cycles are not so impressive in their development as some of those in the Carboniferous, they possess the incomparable advantage of ease of long-range correlation by means of the abundant ammonites. Zones and sometimes even subzones can be correlated generally without difficulty over the whole of northwest Europe. The study of lateral facies vanations over vast areas that is rendered possible by this means provides a very powerful control on the interpretation of Klupfelian cycles. An important point in Kliipfel’s interpretation is the alternation of deposition in a shallow sea and erosion just above sea level. Though a careful study of limestone roofbeds leaves little doubt that consolidatedrock was subjected to erosion and organic incrustation, there is some debate as to whether such erosion necessarily signifies emergence above the sea. JAANUSSON (1961), for example, has argued cogently in support of this notion, with special reference to similar widespread “discontinuity surfaces” in the Lower Palaeozoic of Sweden. He pointed out that such surfaces, which may have phosphatic or ferruginous staining, are commonest close to old shorelines as deduced from general palaeogeographic considerations, and that exposure above sea level leads to rapid cementation and corrosion of recent carbonate sediments whereas this does not occur in those that remain submerged. There does not in fact seem to be any known mechanism whereby superficial layers of carbonate sediment may be rapidly consolidated on the sea floor. Against this line of argument it can be pointed out that there are now known to be many areas swept by strong currents even on the deep sea floor where erosive forces appear to predominate over depositional, and HOLLMANN (1964) has recently described evidence of penecontemporaneous corrosion of limestones, including obviously hard ammonite shells, in the Ammonitico Rosso Superiore (Upper Jurassic) of Italy; this facies is usually considered on a variety of grounds to have been deposited in fairly deep water and it appeared to Hollmann unnecessarily extravagant to invoke a major lowering of sea level for each corrosion surface. Presumably compaction some distance below the sediment surface played an important role in consolidation, with the hardened surface subsequently being exhumed and corroded. As regards the Liassic “discontinuity surfaces”, slight emergence above sea

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EPICONTINENTAL MARINE ENVIRONMENTS, I

level poses no serious difficulty, since the varied facies are indicative of deposition in shallow water, perhaps never more than a few hundred feet in depth and usually much less. Although emergence seems the most likely explanation for the more striking erosional horizons at least, it is nevertheless not necessary to assume this in claiming that the different deposits composing the cycles were laid down in different depths of water. Evidence based on lateral facies variations does support, however, the belief that the roofbeds were formed in shallower water than the other deposits, and this is after all the significant point at issue. In the first place, such beds are often extremely widespread, and rock sections hundreds of miles apart may show almost identical features. A particularly striking example is the top of the Obtusum Zone, which is marked in southwest England by a bored and encrusted calcilutite, in eastern France by a bored limestone, in southwest Germany by a calcilutite containing bored calcilutite pebbles and in Lower Saxony by a similar bed. Elsewhere there is a stratigraphic gap between this and the overlying zones. In the face of this sort of evidence, which is not atypical, it seems unconvincing to argue that erosion of consolidated limestone is local and random with respect to depth, being merely related to varying current strength (cf. ALDINGER, 1957). In the second place, the interpretation that erosion of roofbed followed by shale deposition signifies deepening of the sea, however slight, is supported when the facies varies laterally to an appreciable extent. To persist with the same example, the Obtusum Zone in the English north Midlands is represented by a limonite oolite giving clear indications of deposition in shallow, agitated water (HALLAM, 1963b). The oolite terminates exactly at the top of the zone, and is sharply overlain by smooth shales of the next zone, which must in this case signify deeper water (Fig.69). s. w.

N.

ENGLAND

MIDLANDS

\

I --I

\ .. A- -- :.

__--

Oxynotum

I

- - _- I-

-R W. SCOT LAND

Ob t usum

0

0

0

0 0

0

0

0 0

0

0 0 0

0

P

0

0

0 0

-

0

.

.. Turncri

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

Fig.69. Lateral facies variations at the Lower and Upper Sinemurian boundary in different parts of

Great Britain.

CYCLES OF LIMESTONES AND ARGILLACEOUS BEDS

175

Barry- Lavcrnoek

c----l Southerndown

CAR RHAETIC x x Skeletal Calcarenite

Calcilutitar

#Shales

Fig.70. Lateral facies variations in the Hettangian and Sinemurian of South Wales; near-shore facies at Southerndown, off-shore facies at Barry and Lavernock.

We must next turn to a consideration of the depth relationships of the limestones and shales. Limestone beds of various ages have often formed by default if terrigenous sedimentation was diminished and it is not unusual for palaeogeographical analyses to show that strata become progressively less calcareous and more argillaceous towards old shorelines. Hence near-shore shales can signify shallower or at least equally shallow water conditions. On the other hand the precipitation of calcium carbonate is facilitated in shallower, warmer water and one may expect some calcareous facies to mark shallower water conditions than shales. Although the former situation is valid for the northwest European Lias when viewed regionally, e.g., comparing Yorkshire with Dorset, it is believed that most limestone-shale cyclic sequences in a given region fall into the latter category. This is supported by several lines of evidence. Firstly, in passing towards land it is a common situation that the beds become more, not less calcareous. This is not simply due to diminished terrigenous sedimentation near the shore because the area has been by-passed by rivers, since the nearshore deposits of Glamorgan and the Hebrides are both more calcareous and thicker than average, and there is a greater proportion of oolites, skeletal calcarenites and hermatypic coral limestones. These facts suggest a higher rate of calcium carbonate deposition in shallower water. Fig.70 illustrates the point convincingly. In the nearshore facies of Glamorgan, at Southerndown, skeletal calcarenite, with hermatypic corals, is sharply overlain at the top of the Planorbis Zone by alternating thin bands of calcilutite and marl. In the off-shore facies west of Cardiff, only a few miles away, alternating calcilutite and marl are overlain at exactly the same horizon by argillaceous beds. It is implausible to invoke strongly differential tectonic movements within such

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EPICONTINENTAL MARINE ENVIRONMENTS, I

a small area, and far simpler to accept that the sea deepened at both localities. Secondly, where the faunal evidence is relevant, it is consistent with the limestones being deposited in shallower water (cf. KLUPFEL,1917). Finally, where a limestone-shale sequence can be traced into a sandstone-shale (or oolitic ironstonelshale) sequence the sandstones or ironstones usually correspond in age with the more calcareous strata. A good example is the Lias a-p transition of the German classification,whichin Swabia and elsewhereis marked by a sudden change to less calcareous fine-grained deposits, in Yorkshire and Skye from sandstones to silty shales. An even better example is the change from Lias 6 to Lias E (Domerian to Toarcian). This last point, coupled with the evidence suggesting alternating phases of erosion or deposition, effectively rules out any suggestion that the alternation of more or less calcareous beds is the result of purely climatic variations, as for instance changes in temperature. We are left with the alternative of changes in depth of water. Whether or not this was controlled by local epeirogenic movements of the sea bottom, as Klupfel and other German geologists have maintained, must now be discussed. Regional facies variations in the Lias of northwest Europe may be referred to a series of basins and swells, basins being areas of comparative downwarping, swells consisting either of land margins or marine shoals, relatively resistant to subsidence. These areas appear to have retained their tectonic distinctness throughout the Lias. A survey of the successions in different parts of northwest Europe has allowed the recognition of eleven stratigraphic units corresponding locally with Klupfelian cycles (HALLAM, 1961). The most striking result of the survey was the recognition that facies changes up the succession suggested that Klupfelian cycles are the local expression of variations in depth of sea in the same sense over an extremely wide area, apparently independent of the pattern of basins and swells. The changes appear to have been essentially synchronous in most cases, as far as the ammonite evidence allows. These facts suggest that the underlying control might have been the eustatic rise and fall of sea level, superimposed upon local epeirogenic movements effecting differential subsidence of the basins of deposition. To establish beyond reasonable doubt the operation of eustatic control it is necessary not merely to correlate likely changes of sea level at particular horizons. One must also attempt to relate them to major transgressionsand regressions which affected different continents, thus firmly excluding the possibility of local tectonic control. This is not an easy undertaking at present since we lack fine stratigraphic detail in most areas outside Europe. Furthermore, local epeirogenic movements can complicate the picture, and the evidence for transgressions over swells may often have been destroyed by penecontemporaneous erosion. Another point to be borne in mind is that a change in sea level will not always have an appreciable effect on marine sediments, particularly, if, as envisaged in this case, such changes involved only a few metres. Therefore one cannot expect marine successions at every locality to provide supporting evidence. One useful working rule can be proposed here to ameliorate this handicap. If,

177

CYCLES OF LIMESTONES AND ARGILLACEOUS BEDS

considering a particular horizon, evidence of transgression or sea deepening at one locality can be matched by evidence suggesting the opposite in another locality, then one item of evidence cancels out the other and nothing can be deduced about possible change in depth of sea. If, however, the second locality reveals no evidence favouring either deepening or shallowing of the sea, then nothing can be deduced one way or another. The case is progressively strengthened as evidence of transgression or deepening accumulates over a wide area, if no contrary evidence is forthcoming. Despite the type of practical difficulty outlined above there is already enough evidence in several instances to make a convincing case for eustatic control of major sedimentary cycles in the Jurassic (HALLAM, 1963c, 1964b, 1965). The best example in the Lias concerns the boundary between the Domerian and Toarcian. The Upper Domerian Beds form the natural top of a major sedimentary cycle (ARKELL,1933; HEMINGWAY, 1951) and consist of obviously shallow-water ironstones, sandstones and limestones over most of northwest Europe; there may or may not be an erosion surface at the top. These beds are generally sharply overlain by Lower Toarcian Shales, with an extremely widespread band of bituminous shales in the Falcifer Zone. This change is the clearest indication in the whole of the Lias of a deepening of the sea, and appears to coincide more or less precisely with a major transgression in several continents (see also Fig.71). Accepting the likelihood of eustatic control in some cases, we must now turn back to the influence of regional epeirogenic movements on the sedimentation. It is generally accepted that while the basins have been subject to more or less continued downwarping the intervening land masses have undergone periodic uplift. In the first place this may help to explain the comparative constancy of palaeogeography throughout the Lias in Europe, with only limited transgressions for the most part because uplift of the land would act in opposition to rise of sea level. Secondly, it

5. Mendips

N. Bath

Sodbuq

SW. Stloud

N.E. North Cotrwoldt

UPPER lNFERlOR OOLITE

noRizoNTAL?

“MILES

Fig.71. Diagrammatic section across southwest England to illustrate (a) Lower Toarcian (Junction Bed) transgression and (b) diachronous sandstoneshale transition in Toarcian (Upper Lias). (Adapted and WELCH, 1961.) from KELLAWAY

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EPICONTINENTAL MARINE ENVIRONMENTS, I

may help to account for the characteristic asymmetry of Klupfelian cycles. At first glance the abrupt transition at the top of a cycle from limestones or sandstones to clay suggests a comparatively sudden deepening of the sea following gradual shallowing (this is in fact what Klupfel himself proposed). However, subsidence of the basins and their margins, where most of the deposition took place, would accentuate the change due to eustatic rise. Moreover, the concomitant retreat of shorelines would probably serve to reduce the supply of sediment from the land so that the early, transgressive phase of a cycle should normally be signified by condensed beds. This seems in general to be the case since, as already noted, the basal bed of a cycle is commonly marked by abundant phosphatic nodules, glauconite grains and/or shell accumulations. The ammonites often provide supporting evidence for slow deposition at this horizon. Intracontinental uplifts should have strictly local effects on the sedimentation in contrast to the more extensive though possibly milder results of eustatic movements. Thus regional uplift became important in Britain only towards the end of the Lias, as indicated by the diachronous sandstone formations in the Upper Lias of southwest England (Fig.71) and the irregular and local pre-Bajocian warping and erosion in the Midlands and Yorkshire where several ammonite zones disappear below the Dogger within the distance of a few miles. Sedimentary cycles showing many similarities with those described by Klupfel also occur in the Lower and Middle Cretaceous (and less conspicuously in the topmost Jurassic) of the Helvetic Zone in Switzerland (FICHTER, 1934; BRUCKNER, 1951, 1953; CAROZZI,1951). Fichter’s account is admirably detailed and his interpretation carefully argued. Some cycles pass up gradually from shales into limestones, others are more calcareous and pass up from marly, fine-grained, thin-bedded limestones containing ammonites, radiolaria and sponge spicules into skeletal echinoderm sparry limestones near the top. The abundance of shell fragments and the grain size of detrital quartz also tend to increase up the succession (Fig.72). The cycle boundaries (“Zyklengrenzen”) are taken directly above the echinoderm limestones and are marked by thin bands rich in glauconite and shelly fossils, which are sometimes phosphatised. Ammonites are concentrated in and largely confined to these boundary beds. The cycles are asymmetric, as in the Lias, with the basal beds beginning abruptly. Fichter favoured a bathymetric interpretation. While he was not prepared to commit himself firmly on the ultimate cause of the changes in depth of sea, he was inclined to relate them to transgressions and regressions. This is evident in the following passage quoted from FICHTER’S paper (1934, p.99): “Die eigentliche Ursache, welche diesen tektonischen Bewegungen zugrunde liegt, bleibt vorlaufig im Dunkel. Es diirfte jedoch dieselbe Ursache gewesen sein wie die, welche die grossen Transgressionen und Regressionen hervorgerufen hat; die Zyklengrenzen sind ja in vielen Fallen nichts anderes als kleine Transgressionen, und in andern Fallen liegt wenigstens ihr Zusammenhang mit Transgressionen klar zutage”. Fichter noted that the Cenomanian marks the base of a new cycle and cited the wellknown Cenomanian transgression in support of his contention.

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CYCLES OF LIMESTONES AND ARGILLACEOUS BEDS

AGE OFAM~KJNITE

STRATIGRAPHIC

FAUNA AT .CYCLE BOUNDARIES

AXENDECKEALPENRAND

UNITS

ROCK

MAXIMUM CRAlN S I Z E

DRUSBERGT E l LDECKE

I N THE DRUSBERGTE ILDECKE

E

E

2

Fig.72. Major limestone-shale cycles in Upper Jurassic and Cretaceous of the Helvetic zone, Switzer1934.) land. The grain-size data refer to detrital quartz. (Adapted from FICHTER,

180

EPICONTINENTAL MARINE ENVIRON"TS,

I

A new technique was brought to bear on the problem by CAROZZI (1951). On the basis of microscopic examination of samples taken at short intervals up the successions at several localities, he produced curves of quartz frequency and other parameters. He was able to show that the amounts of quartz and the supposed pelagic organism CaIpioneIIa vary inversely, the one increasing, the other decreasing up the cyclic succession. The ratio of benthonic to pelagic organisms was also found to increase up the cycles. The sequence, compact limestone-minor limestone-marl alternations-marl-marly limestone-pseudo-oolitic and sparry limestone, was interpreted as one of decreasing depth of sea water. The minor limestone-marl alternations did not show any corresponding variations in detrital quartz and were interpreted not in terms of sea depth but variations in chemical equilibrium. This point was not pursued at length. BRUCKNER (1951, 1953) has argued in favour of climatic control of cyclic deposition in the Helvetic Cretaceous, and was the first to take lateral variation seriously into account. He divided the Helvetic Zone into northern, middle and southern sectors (BRUCKNER, 1951). The major cycles range from a few metres thick in the north to about 300 m in the south. The northern zone has a more condensed succession and a higher proportion of limestone to marl and shale than the southern. In addition, algae, thick-shelled bivalves, echinoderms, corals and benthonic Foraminifera are dominant in the north, CaIpionelIa and pelagic Foraminifera are commoner in the south, and the grain size of detrital quartz is larger in the north. These data were interpreted as signifying that the northern zone was closer to the old shoreline. It may be noted that the facies relationships described by Bruckner compare closely with those of the Lias (HALLAM, 1961). Some of the argumentation is similar too. For instance BRUCKNER (1953) pointed out that if the cyclic changes under consideration were caused by dilution of terrigenous material more argillaceous beds would be expected towards the old shoreline, which is not the case. Bruckner was also concerned with the origin of the small-scale limestone-marl cycles, 5-100 cm thick, which he thought had a considerable bearing on the main problem. They could hardly be due to depth changes, since the fauna was hardly affected, despite the remarkably high variation in calcium carbonate content. Hence they must signify temperature changes. One might here add parenthetically that this interpretation also lacks plausibility, since temperature is known to affect animal distribution profoundly. The difficulty may readily be overcome if it can be established that these minor cycles are diagenetic in origin, as seems to be the case in much of the Blue Lias, discussed earlier in this chapter. Returning to the major cycles, Bruckner accepted that the limestones with benthonic fossils were deposited in shallower water than the fine-grained marly limestones and shales with CaIpionelIa, but argued that this was due to more rapid deposition of calcium carbonate during periods of higher temperature, so that the basin of deposition filled more readily. At times of lower temperature and colder water, much less carbonate was precipitated and the sea deepened as the basin subsided. In support of his belief that moderate temperature variations are the main cause of

CYCLES OF LIMESTONES AND ARGILLACEOUS BEDS

181

variations in the quantity of calcium carbonate in the rocks, Briickner made a comparison with the shelf deposits off the eastern coast of the United States. Almost pure carbonate deposits are found off Florida but the proportion of calcium carbonate diminishes northwards until north of Cape Hatteras only negligible amounts are deposited. It was argued, in view of these facts, that the major cycles in Switzerland might have been an expression of the wandering of climatic zones involving a 5 "C change of temperature, and the minor cycles changes of 2 "C or less. There are a number of serious objections to Briickner's ideas, besides that already noted. The more condensed major cycles of the sort under discussion, only a few metres in thickness, embracing a wide range of facies but excluding sandstones, are difficult to explain purely by the sedimentary infilling of an epicontinental marine basin, as both Kliipfel and Fichter clearly appreciated. Climatic controlfails, moreover, to account for the feature of condensation in the glauconitic boundary beds. Briickner's modern analogy is weak because it totally fails to take into account a significant variable, namely the influx of terrigenous matter from rivers along the American coast. The deposits around Florida and the Bahamas are calcareous partly because of the warm climate but principally because no major drainage system empties its sediment into the area. The region around the Mississippi delta has, after all, a comparable latitude, and it would be absurd to argue that the increasingly terrigenous nature of the coastal sedimentation westward from Florida signifies decrease in temperature. Briickner's principle objection to the ideas propounded by Carozzi and others seems to be that in the sort of stable shelf regime exemplified by the Cretaceous of the Helvetic Zone it is implausible to invoke periodic strong oscillations of the sea bottom. There is some merit to this objection, which would, however, lose its weight if it could be shown by means of correlation over wide areas that these Cretaceous cycles are controlled, as Fichter hinted, by eustatic changes of sea level.

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Chapter 7

EPICONTINENTAL MARINE ENVIRONMENTS, I1

CYCLES WITH SIGNIFICANT QUANTITIES OF SANDSTONE, IRONSTONE AND PHOSPHORITE: MINOR CYCLES WITH BITUMINOUS LAMINAE

In this chapter we shall deal with a wider variety of lithological types than in the previous chapter. Generally speaking they are of subordinate importance to Iimestoneshale cycles.

CLAY-SANDSTONE-LIMESTONE CYCLES

The best-known cycles composed of sequences of argillaceous, arenaceous and calcareous deposits, such as those in the late Palaeozoic of northern England (Yoredale facies) and the mid-continent region of the United States, are dealt with fully in Chapters 4 and 5 , since they have generally been regarded as indicative of oscillating marine and continental environments. The few other examples discussed here may be dealt with most appropriately under the heading of epicontinental marine environments. In his classic work on the Jurassic System in Great Britain ARKELL (1933) observed that as long ago as 1822 Conybeare and Phillips had remarked on the regular manner in which clay is followed by sandstone and the latter by limestone in the socalled Oolitic sequence. Arkell went on to elaborate this tripartite scheme, dividing almost the whole of the Jurassic in southern England into major cycles. His scheme is given below, with the clay, sandstone and limestone units signified by capital letters: Portland Stone (L) Portland Sand (S) (9) Kimmeridge Clay (C) Westbury Ironshot Oolite (L) Sandsfoot Grit ( S ) (8) Sandsfoot Clay (C) Osmington Oolite, etc. (L) Bencliff Grit, etc. (S) (7) Nothe Clay, etc. (C)

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EPICONTINENTAL MARINE

ENVIRONMENTS, I1

Berkshire Oolite (L) Lower Calcareous Grit (S) (6) Oxford Clay (C) Kellaways Rock (L) Kellaways Sand (S) (5) Kellaways Clay ( C ) Cornbrash Main Forest Marble Limestone (L) Hinton Sand, etc. (S) (4) Bradford Clay, etc. (C) Great Oolite Series (L) Stonesfield Slate beds ( S ) (3) Fuller's Earth (C) Inferior Oolite Series (L) Upper Lias Sands ( S ) (2) Upper Lias Clay (C) Marlstone Rock-bed (L) Middle Lias Sands (S) ( I ) Middle Lias Clay (C) Lower Lias clays, etc. It is easy to criticise aspects of this scheme as an example of cyclic sedimentation. As Arkell himself freely admitted, there was no question of clay, sandstone and limestone being deposited over the whole area simultaneously. No lithological details are given, distinguishing, for instance, different types of limestone. Distortion is involved equating major formations like the Oxford and Kimmeridge Clay with minor and local units like the Sandsfoot Clay; the Marlstone consists of sandstone and ironstwe besides limestone, and so on. Nevertheless one cannot disagree with Arkell that there is a striking overall periodicity involved, phases of widespread deposition in very shallow water (oolites, coral limestones, sandstones) being interrupted by phases of deeper water (clays and subsidiary argillaceous limestone). Arkell followed the Germans in attributing these oscillations to periodic epeirogenic movements of the sea floor. We may note, however, that the beginnings of cycles 2 , d (best taken together with the much thinner and less conspicuous cycle 5) and 9, all suggesting widespread and pronounced deepening of the sea, correspond with important worldwide transgressions and hence signify eustatic control (HALLAM, 1963~).The same may be true of others of Arkell's cycles.

CLAY-SANDSTONE CYCLES

185

Another example where the cycles are major stratigraphic units, ranging up to several hundred metres in thickness, comes from the Upper Pliocene and Pleistocene deposits of Wairarapa, New Zealand (VELLA,1963). Vella gave the following as a typical marine cycle, together with his interpretation: (Disconformity) shallowing culmination. Sandstone with shell beds shallowing. Sandy mudstone with scattered fossils deepening culmination. Sandstone with shell beds deepening. Coquina limestone deepening. (Disconformity) shallowing culmination. Vella attempted an estimation of depth of deposition based on known distribution of modern relatives of the contained fossils, and concluded that the cycles were expressions of depth changes ranging from about 50 to 160 m. These were in phase at least over half of North Island and, allowing for local tectonic disturbance, glacioeustatic control was proposed. To conclude this section an instance of a minor cyclic sequence will be cited from the Rhaetian in southwest England (HAMILTON, 1962). In the Cotham Beds (for example) four cycles can be distinguished, about a metre in thickness, with the following characteristics: ( I ) a basal calcareous horizon, above which there is usually a gradual reduction in the amount of carbonate, (2) shell or sand beds, or ripple lenticles, most common near the base of a cycle but which decrease in frequency upwards, and (3) an increase in the proportion of clay upwards in each cycle. Hamilton considered that the basal bed of each cycle was deposited under conditions of greatest current and/or wave action, with a gradual reduction of energy input from the environment as the cycle progressed. Such conditions were thought to be related possiblyto relatively small oscillations of the strand line. It is interesting to note, as Hamilton pointed out, that these minor cycles occur within a Rhaetian sequence which itself forms a good example of a cycle of Kliipfelian type.

CLAY-SANDSTONE CYCLES

As in the previous case, it is difficult to find many cited instances in the literature of clay-sandstone cycles in stable shelf regimes which do not involve non-marine deposits. Infact, both of the examples which will be considered here pass laterally into nonmarine beds within short distances. They are nevertheless most appropriately dealt with here. Major marine and non-marine units of the Eocene of the Anglo-Franco-Belgian Basin (STAMP, 1921) interfinger diachronously as indicated diagrammatically in Fig.73. The marine part of the sequence tends, in its fullest development, to show the sequence: basal gravel-sand-clay-sand-gravel. The gravel may be only poorly developed or absent, but the plane of erosion between the marine cycle and the underlying beds is almost invariably well marked. To give a specific example, the London Clay

186

EPICONTINENTAL MARINE

-.

2

I,

%I

LOWER

HEADON

BEDS

ENVIRONMENTS, I1

OLIGOCENE

1BARTON I AN LEDlAN

LUTETIAN BAGSHOT

'SANDS

7-7 YPRESIAN

READING,

BEDS

1

Fig.73. Major cycles in t& Lower Tertiary of the Hampshire Basin. Stippled areas represent marine beds, unornamented areas hon-marine beds. (After STAMP,1921.)

in the Hampshire Basin begins with thin sandy pebble beds and passes up into silty clay; the top is sandy once more. This succession is thinner and sandier in the west, where it is interrupted by non-marine beds. The thickness of the London Clay cycle varies from nearly 100 m in Whitecliff Bay to 70 m in Alum Bay. Thin intercalations of gravel within the clay were not regarded by Stamp as marking minor cycles. Stamp was able to demonstrate the essential contemporaneity of major cycles within the whole area considered. They were related to periodic transgressions and regressions of the sea, with the clays signifying deeper water conditions than the sands. Cycles very similar to those just described also occur in the Tertiary deposits of the Gulf Coast Region of the United States (BORNHAUSER, 1947; LOWMAN, 1949; FISHER, 1964; and Table XXIX). Gulfwards the deposits pass from a fluvial and lagoonal facies into an argillaceous marine facies. The cycles occur where marine clays are intercalated between marginal marine and continental arenaceous beds. Fisher (1964) has given a detailed account of cyclic deposits in the Eocene. His transgressive phase of a given cycle consists of sands and marls overlain by so-called restricted or marginal marine clays. These are laminated and have an apparently dwarfed fauna of thin-shelled bivalves. This phase overlies a basal condensed unit, marking a transgressive marine disconformity, rich in glauconite, phosphorite pebbles, reworked fragments and shark teeth. The inundutive phase (Bornhauser's term) marks the maximum advance of the sea and is characterised by normal argillaceous beds. Fisher's regressive phase has a basal arenaceous unit (fluvial or marginal marine) and an upper argillaceouscarbonaceous unit (mostly lagoonal or flood plain) in the northern Gulf Coast Region. These strata pass into entirely marine beds southeastwards. Fisher recognised five cycles in the Claibornian of Alabama but only one in the

IRONSTONE-BEARING CYCLES

187

Jacksonian (cf. Table XXIX). The thickness of the marine phases in this region rarely exceeds 30 m while the non-marine phases may be much thicker. While there appears to be general agreement that these Tertiary cycles in both Europe and North America result from the advance and retreat of the sea, no-one seems to have considered that such oscillations might have been the local expressions of eustatic movements until HALLAM (1963a) pointed out that the major transgressions and regressions in the two regions appeared to coincide in time, at least to the degree of refinement indicated by such age designations as end-Palaeoceneand Middle Eocene. kn this connection it is noteworthy that the basal transgressive beds appear to be condensed, giving an overall asymmetry to the cycles which compares with the Mesozoic limestone cycles discussed in the previous chapter. Another point of comparison is that the laminated shales, most likely anaerobic deposits, occur in the lower part of the cycle. IRONSTONE-BEARING CYCLES

One of the major problems in sedimentary geology concerns the origin of the banded ironstones or itabirites found in the Precambrian of every continent (GEIJER,1957; JONES,1963). Though most abundant in the Late Precambrian, where they compose formations up to 450 m in thickness (SAKAMOTO, 1950) they also occur less commonly in the Archaean and Palaeozoic. We are not concerned here, however, with the general problem of iron enrichment but with the nature and origin of the cyclic alternations between chert on the one hand and one or more iron-bearing minerals (silicates, carbonates and oxides) on the other. Specific examples will be taken from the famous deposits of banded ironstone in the Huronian of the Lake Superior region in North America, considered to have been laid down in a restricted marine environment (JAMES,1954). One common type of banding consists of regular alternations of chert and siderite averaging 1 cm in thickness. Stylolites are common and slump structures not rare. A more spectacular rock is the jaspilite of the Marquette Range, composed of alternations of reddish jasper and grey haematite typically 0.25-1.25 cm thick. These layers may themselves be laminated, with laminae about 0.025 mm thick. The bedding is wavy rather than straight and individual layers show pinch-and-swell structure. Parts of the succession are oolitic, both in the chert and haematite, though oolitic structure is more obscured in the latter lithology. A notable feature of the various types of banding is the remarkable constancy of individual horizons. James was primarily concerned with the general environment of deposition and the conditions governing the formation of various iron minerals and did not address himself to accounting for the banding. Sakamoto, however, dwelt on this subject at length in a general review of Precambrian ores (SAKAMOTO, 1950). He developed an ingenious hypothesis treating the banding as annual. A monsoonal climate was proposed, with alternating wet and dry seasons. During the wet season, it was argued, acid conditions would have prevailed in the weathering regime, allowing iron to migrate in

188

EPICONTINENTAL MARINE ENVIRONMENTS, I1

TABLE XXIX MAJOR CYCLES IN THE TERTIARY OF THE GULF COAST OF THE U.S.A.

(After FISHBR, 1964) Lithology

Sand, shale, lignite -.

Group

Formation Mississippi

Louisiana

Catahoula

+ post-Catahoula

Cycle Phase

Pliocene

Regression

-.

?

?

Miocene

Inundation

Chickasawhay L. Byram Marl Marianna L. Forest Hill/ Red Bluff

Vicksburg

Vicksburg (Oligocene)

Transgression

Grey shale, marl

Yazoo

Jackson Shale

Jackson

Inundation

Marl, glauconitic sand

Moody’s Branch

Moody’s Branch (Eocene)

Transgression

Sand, grey shale, lignite

Cockfield

Cockfield

Regression

Dark shale

Limestone, marl, shale

V

VI

-

Dark shale

Cook Mountain

Limestone, shale, sand

Inundation

Cook Mt. Shale Clayborne _ _ _ ~ (Eocene) Cook Mt. L.

(Wantubee)

Transgression

Regression

Sand, shale, lignite Sparta (Kosciuska) Sparta Dark shale

111

_

-

Zilpha

Inundation

Cane River Shale

I1

~~

Glauconite sand, marl, shale, glauconite sand

Sand, shale, lignite Wilcox Dark shale Marl, calc. shale UPPER CRETACEOUS

Transgression

Winona Tallahatta Cane River Marl

Wilcox

Wilcox (Eocene) Regression

Midway

Midway

Inundation

Porter’s Creek Clayton

Transgression

I

IRONSTONE-BEARING CYCLES

189

solution, whereas silica would remain behind as insoluble material. Conditions would have been reversed in the dry season. SAKAMOTO (1950) considered that the banded ores were deposited in lakes. Iron hydroxide would have been precipitated in alkaline waters in the dry season, with the pH varying from 9 to 5. JONES(1963) has also proposed that the primary control involved was pH (in contrast to the Eh control of different iron minerals, as James argued). Sakamoto thought that banded ironstones were formed only during a period in the Precambrian under conditions which have never recurred since. A major difficulty in Sakamoto’s interesting hypothesis is the assumption of striking changes in pH at frequent and regular intervals. They would be remarkable even in lakes, but it is highly questionable whether the Precambrian banded ironstones formed in lacustrine environments. JAMES (1954, p.243) has argued cogently for marine conditions in the case of the Lake Superior ores, and the wide lateral extent of such deposits in different parts of the world is more readily explicable on this assumption. It is well-known that sea water is a well-buffered chemical system, with pH varying only between narrow limits (about pH 7.6-8.1). The problem therefore remains. Perhaps the possibility of a diagenetic control of the banding should be explored, as in the case of some small-scale cycles in limestone, shale and chert (see Chapter 6). KREJCI-GRAF (1964, p.485) has argued, for instance, that the cause of the banding might have been processes of solution and precipitation within the sediment due to periodic or episodic changes of Eh in connection with the rhythm of sedimentation of organic matter. There seem to be one or two pointers towards a hypothesis of diagenetic unmixing in James’ description of the Marquette jaspilites. The bands are themselves laminated, and where oolitic structure occurs it evidently ignores the banding, occurring in both lithologies. It is true that James argued for the primary origin of the chert, but the evidence he put forward (remarkable lateral constancy, stylolites and chert veinlets cutting bands, slump structures involving chert) is not decisive and the socalled primary chert could be accounted for by diagenetic migration fairly soon after deposition. A totally different type of cycle involving ironstone has been described from the Lias of Yorkshire, England, by HEMINGWAY (1951). Three major cycles were distinguished. The best developed corresponds with the Toarcian and is about 100 m thick. Following silty grey shales at the base come finely laminated dark brown bituminous shales of the Jet Rock. These pass gradually upwards into fine grey shales and thence via silty shales into fine-grained sandstone (Fig.74). At the top of the cycle in Rosedale are a few metres of sideritic chamosite oolite. Another cycle corresponds almost exactly with the Domerian, shales passing up via sandstone into shales with nodules of siderite mudstone and thence into chamosite oolite (Cleveland Ironstone). The third cycle is taken to embrace almost the whole of the Lower Lias, but in this case a regular sequence is not apparent and the relationships of the different lithological units are more complex than Hemingway assumed. Hemingway interpreted the cycles as resulting from tectonically-induced vertical

EPICONTINENTAL MARINE ENVIRONMENTS, I1

STRATA Rosedale Ironstone Blea Wyke Beds Striahitus Shales

SUB STAGES

Peak Shales

'TOARCIAN

Alum. Shales

The Hard Shales The Bituminous Shale; Jet Rock The Grey Shales

1

Cleveland Ironstone Series

DOMERIAN

The Sandy Series

Shales with ironstone concretions

CAR I X I A N

Shales with hard sandstones 5 I NEMUR I AN

Shales with occasional shell limestones

J R Iu1

5 0 0 feet omitted

Grey qrccn shale Black shale

-

HETTANGIAN

RHAETIC

Fig.74. Major cycles involving ironstones in the Liassic succession of Yorkshire, with interpretation of relative depth of deposition of the different facies. (After HEMINGWAY, 1951.)

movements altering the local depth of sea. Based on a comparison with recent deposits in the Black Sea, a gradual shallowing was deduced up the succession (Fig.74). It was presumed that the topographically subdued land masses which bordered the Liassic sea were subjected to deep chemical weathering in a warm humid climate. Ironstone was formed in small, shallow restricted arms of the sea only after the cessation of mechanical erosion at an advanced stage of peneplanation. Certain aspects of Hemingway's hypothesis may be criticised, for instance the dubious assumption that sand and clay were derived from different sources, and the claim that each of these marine cycles corresponds with a cycle of erosion on the land. Nevertheless the general interpretation in terms of changing depth of sea in a special chemical environment seems reasonable. It is interesting in this connection that a

PHOSPHORITE-BEARING CYCLES

191

cycle very similar to that in the Toarcian of Yorkshire is present in rocks of the same age in northeastern France (THEOBALD and MAUBEUGE, 1943), while the Domerian cycle and the major change in depth of sea between Domerian and Toarcian is rec1961). ognisable over a much wider area than Yorkshire (HALLAM, Further examples of cyclic sedimentation involving ironstones were described by REID (1965) from the Precambrian of Yampi Sound, Western Australia. Two types of cyclic variation are distinguishable. In individual beds, often less than 3 cm thick, there is a textural gradation from a predominance of haematite at the base to a predominance of quartz at the top. Reid compared this with graded bedding, with the difference that the variation here is expressed by the specific gravities of two different minerals rather than grain size. In addition to this there is a regularly repeated cycle, from about 10 to 40 m thick, of haematite-rich, schistose and quartzose beds. Within a succession of 17 such cycles a few lithological phases are missing locally but no phase occurs out of position. The Yampi iron ores were considered to be clastic sediments but the origin of the cyclic sequence was not discussed.

PHOSPHORITE-BEARING CYCLES

Although many sedimentary cycles deposited in shelf seas contain layers of phosphatic nodules we are concerned here with deposits containing thick beds of economically exploitable phosphorite. One of the best-known sequences of such deposits is the Permian Phosphoria Formation of western Wyoming and neighbouring states. SHELDON (1963) has recognised two major cycles in these deposits, ranging from about 20 to 60 m in thickness. The idealised (composite?) sequence, not completely present at any one locality, is as follows: The underlying sequence in reverse order, i.e., 1 1 passing up to 1. (11) Dark carbonaceous mudstone. (10) Dark pelletal phosphorite. (9) Dark dolomite. (8) Light-coloured bioclastic phosphorite. (7) Bedded chert. (6) Nodular or tubular chert. (5) Interbedded light-coloured bioclastic limestone and calcareous sandstone. ( 4 ) Light-coloured dolomite and dolomitic sandstone. (3) Light-coloured mudstone. (2) Red beds. ( I ) Conglomerate overlying erosion surface. These various units were considered to be approximate time-rock horizons and represent transgressive and regressive phases of areally zoned depositional environ-

192

EPICONTINENTAL MARINE

ENVIRONMENTS, I1

ments. The erosion surface and conglomerate mark the phase of maximum regression, the dark mudstone maximum transgression. Lateral facies variations (Fig.75) support this interpretation, with land lying to the east and deep water to the west. Sheldon stated that the cycles tend to be skewed towards the base, with the upper half of the sequence being more fully developed. This suggested to Sheldon that the transgressions took place more rapidly than the regressions, but we may note that transgressive-regressive cycles are frequently asymmetrical in this way, with transgressive phases being marked by comparative condensation of the sedimentary sequence, and do not necessarily signify asymmetry intheunderlyingcontrol (see 178p.). Sheldon thought that the cycles, which extend over a very wide area, were the result of tectonically controlled fluctuations in depth of sea.

MINOR CYCLES WITH BITUMINOUS LAMINAE

A common feature of many marine bituminous shales, siltstones and limestones is a fine lamination consisting of layers of organic matter alternating regularly with mineral matter. BRADLEY (1931) seems to have been the first to suggest that the organicmineralic couplets corresponded to a year’s deposition and hence were true varves. Bradley studied several examples, from the Genesee Shale (Devonian) of New York, the Hannibal Shale (Mississippian) of Illinois, the Hartshorn Sandstone (Pennsylvanian) of Oklahoma and the Modelo Formation (Miocene) of California. The mean varve thickness ranged from 0.025 mm in the Modelo Formation to 0.40 mm in the Genesee Shale. Unlike glacial lake varves the mineralic layers showed no signs of graded bedding, hence rendering unlikely an origin by selective settlement from suspension in the sea water. The dark organic layers were thought by Bradley to represent autumn or winter settlement of dead plankton, such as diatoms and dinoflagellates, following intensive summer growth in the surface waters, a phenomenon that has been observed in certain large lakes. It was thought unlikely that successional phases of organic productivity in spring and summer would give more than one sharply defined layer each year. In fact it is now known that there may be several blooms of diatoms each year, with corresponding deposition more than once in the year, but the supernumerary layers can usually be recognised as such (DEEVEY, 1953). Direct support for Bradley’s principal contention, that the couplets are annual, comes from work on recent deposits. The deposition of sediment in the Clyde Estuary in Scotland was studied over a period of several years by H. B. MOORE(1931), who was able to show that each year correlated with the formation of a thin, peaty layer not more than 2 mm and a light band from 3 to 7 mm thick. Apparently the silica of the diatoms, composing the bulk of the organic matter, was quickly dissolved, so that all trace of organised structures disappeared at an early stage. This observation may heIp to account for the evident lack of structure in similar fossil deposits. Further confirmation comes from the work of SEIBOLD (1958) on euxinic sediments in an isolat-

193

MINOR CYCLES WITH BITUMINOUS LAMINAE Phosphatic Shale

LightChert

Carbonate Rock

co o r e d Mudstone

Red

BJE . ..l ,::\\\\\

Precycle

*

W Southeastern Idaho

Central Wyorni n 9

I

A PERIOD OF INITIAL SEDIMENTATION OF CYCLE AND OF MAXIMUM REGRESSION

B

C PERIOD OF MAXIMUM TRANSGRESSION

D PERIOD OF MAXIMUM REGRESSION

Fig.75. Interpretation of the formation of major cycles involving phosphates in the Phosphoria Formation of Wyoming. (After SHELDON,1963.)

194

EPICONTINENTAL MARINE ENVIRONMENTS, I1

ed bay in the Adriatic. He recognised fine laminations in the deepest part of the bay, with laminae averaging about 0.25 mm. These could be proved by historical events and seasonal variations to be annual deposits. Seibold considered that the light layers were deposited in summer and the dark layers, rich in organic matter and iron sulphide, throughout the remainder of the year. Two further examples of laminated recent sediments attributed to an annual cycleare the black euxinic muds in the Black Sea (ARCHANGELSKY, 1927) and diatomrich clays in the Santa Barbara Basin off southern California, also deposited in conditions of oxygen deficiency (HULSEMANN and EMERY,1961). Taken together with the evidence from similar deposits in lacustrine environments (NIPKOW,1928) there seems to be good empirical support for Bradley’s ideas, even though there is no general agreement as yet on the exact time of deposition of the organic-rich layers. Evidently the possibility of more than one phytoplankton flowering in the year does not seriously distort this picture. Recently, in a study of diatom-rich varved sediments from the central Gulf of California, CALVERT(1966) has produced evidence to suggest a radically different mode of origin. While phytoplankton production is reasonably constant throughout the year, river discharge varies greatly in different seasons. The varves in this case are therefore attributable to an increased sedimentation rate as a result of summer floods. The term varve may be ascribed with confidence to the type of minor cycle under discussion. On the other hand, indiscriminate use of the term for laminated sediments of a variety of other types (e.g., KORN,1938) is to be discouraged. It seems highly probable that organic-mineralic varves can only form under anaerobic or near-anaerobic conditions. If oxygen is available in abundance in the bottom waters benth0ni.c organisms rapidly destroy the fine lamination and the organic matter is oxidised (cf. CALVERT, 1964). Though such varves may form at any depth they are most likely to be common where organic productivity is high, namely in shallow coastal waters and shelf seas or in areas of upwelling. A number of examples of lamination interpreted as fossil marine varve deposits has been described, in addition to those already mentioned. The oldest so far recorded occur in the Precambrian Nama Limestone in South West Africa (KORNand MARTIN,1951). Fine lamination in the limestone is composed of couplets of lightishcoloured calcite bands and organic-rich quartz bands. Calcite precipitation was evidently the principal control on variations in thickness. Bituminous shales occurring in the Blue Lias of southern England are seen in thin section to consist of alternations of layers of dark reddish-brown structureless organic matter ranging from 9 to 17p in thickness, and clay-calcite layers from 16 to 25p thick (HALLAM, 1960). Some of the intervening limestones are also laminated, with individual laminae being much thicker (average 0.23 mm) because of the greater abundance of calcite. North American examples include, besides those cited by Bradley, laminae from 0.1 to 0.2 mm thick in the Miocene Monterey Formation of California (BRAML E m , 1946) and laminae in the Upper Cretaceous Beds of the Black Hills region

195

MINOR CYCLES WITH BITUMINOUS LAMINAE

THE VARVED CLAStlC-ORGANIC-EVAPORITE ANNUAL CYCLES

LAMlNATlONS (diaqrommatic)

STRATIGRAPHIC SECTION

CYCLE

FORMATION APPROXIMATE CLIMATE TIME REQUIRED FOR DEPOSITION

I

COOL WARM OR

(erriron 2- FOLD

ormation

CYCLE clartic and wsum

WET

OR

DRY

\ I

ndrtona

Fig.76. Minor cycles (varves?) in the Jurassic Todilto Formation of New Mexico. (After R. Y. ANand KIRKLAND, 1960.)

DERSON

(RUBEY,1930). Rubey actually distinguished three types of lamination, grading into each other and averaging about 0.2 mm in thickness, only one of which showed a variation in content of organic matter. The others consisted of alternations of calcite and quartz silt and of silt and clay. All three types were attributed to annual climatic cycles of unspecified type. R. Y. ANDERSON and KIRKLAND (1960) have described limestone varves from the Jurassic Todilto Formation of New Mexico which have three components. The thickest consist of limestone bands variable in thickness but averaging 0.13 mm (Fig.76). They were considered to have been deposited in summer, in conditions of higher temperature, evaporation and/or photosynthetic activity. The organic layers, described as sapropel, are more constant and average 8,u in thickness. They contain subordinate

196

EPICONTINENTAL MARINE ENVIRONMENTS, I1

10-

5-

Fig.77. Amplitude spectrum of Upper Devonian Ireton Shale varves. (Adapted from R. Y. ANDERKOOPMANS, 1963.)

SON and

fragments of vascular plants and were regarded as autumn-winter layers resulting from plankton mortality. The third component is detrital quartz sand, intermittent but persistent areally. This is most probably a winter deposit, both wind- and streamborne. Varved deposits possess a special interest for those who hope to detect the past

MINOR CYCLES WITH BITUMINOUS LAMINAE

197

existence of solar cycles of varying magnitude. The eleven-year sunspot cycle is by far the most well-known (KORN,1938; R. Y. ANDERSON, 1961) but a number of others of greater period has been claimed from a variety of data. Sunspot cycles are supposed to affect the weather and hence ultimately the thickness of tree rings and varves. Thus increased precipitation of rain results in the thickening of the clastic component of organic-silt varves in certain Russian lakes (SHOSTAKOVICH, 1936). Temperature variations may affect the thickness of chemical precipitates. The subject of climatic cycles and their influence on sedimentation will be dealt with separately in the last chapter. We are concerned here only with the detection of thickness periodicity within varve sequences in non-evaporitic marine deposits. A number of attempts has been made to detect such periodicity in varves of all types by personal estimations of abnormal thickness, but these have been insufficiently objective to be convincing. Statistical techniques such as power spectrum or time series analysis are available for more rigorous analysis and should be used wherever possible in this type of study in an attempt to separate “noise” of a stochastic nature from genuine non-random “signals” (see Chapter 1). SEIBOLD and WIECERT(1960) undertook a type of sequential Fourier analysis of the Adriatic varves described by the first-named author in 1958 and recognised weak periods close to 6,8, 11 and 14 years throughout the sequence. R. Y. ANDERSON and KOOPMANS (1963) made power and amplitude spectrum analyses of several extensive varve sequences. They found, for instance, that varves in the Upper Devonian Ireton Shale of Alberta (R. Y. ANDERSON, 1961) registered a significant peak at 22 years with both techniques, while the amplitude spectrum analysis revealed lesser peaks at 1 1 + , 12+ , 8 + and 6 + years (Fig.77). R. Y . Anderson and Koopmans also analysed a varve sequence in the Nama Limestone, using data from KORNand MARTIN (1951). A strong long-term trend was not obvious, though a weak peak at about 100 years was present, overshadowed by stronger peaks near 25 and 12 years. A cluster of high peaks occurs at about 6-8 years, with other peaks at 3.7 and 2 years.

This Page Intentionally Left Blank

Chapter 8

EPICONTINENTAL MARINE ENVIRONMENTS, I11

EVAPORITE CYCLES

Given a climate in which evaporation exceeds precipitation, evaporite deposits may form in continental environments such as lakes but there can be no doubt that the major evaporite deposits of the past were precipitated from sea water. Though the environments of deposition can be classed as epicontinental, water depth need not invariably have been shallow, as we shall see when the origin of certain small-scale cycles is discussed. The origin and distribution of evaporite deposits is fully discussed elsewhere (BORCHERTandMUIR,~ ~ ~ ~ ; B R A I1962; T S CLOTZE, H , 1957; STEWART, 1963a). Attention will be confined here to certain features of the basins of deposition having an important bearing on the origin of the cyclic sequences to be considered later. STEWART (1963a) has distinguished two types of depositional area. The first type, exemplified by the northern European Zechstein Sea, has more soluble salts towards the centre of the basin, signifying the existence of denser brine at depth. The second type, exemplified by the back-reef Guadeloupian evaporites of the Delaware Basin of Texas and New Mexico, has more soluble salts away from the ocean connection, and signifies progressive brine concentration through evaporation as sea water was drawn in towards the land. It has been generally accepted that salt basins must have been partially isolated from the open sea by some sort of sill or bar, in order to account for the brine enrichment. Otherwise dense brine would tend to escape to the ocean by means of a counter current. While most workers have thought of a physical barrier such as an organic reef, sand bar or basefnent swell, SCRUTON (1953) has argued for a dynamic barrier produced by friction between water bodies of different density, such as occur in the Mississippi River; the effectiveness of this is increased by constriction of the channel. Hence the barrier should be considered to be in a state of dynamic equilibrium subjected to fluctuations in such factors as temperature, wind stress and sea level, all of which will affect water density at a given location. An iconoclastic note has recently been struck by ARKHANGEL’SKAYA and GRIGOR’YEV (1960) in their study of the Lower Cambrian evaporite basin of the Siberian Platform. They pointed out that while salt basins of the present day, such as the Karabogaz Gulf, conform to the bar hypothesis, this was not necessarily always true in the past, when, unlike today, extensive shallow epicontinental basins occurred in arid belts. The two Russians constructed a theoretical model which made no use

200

EPICONTINENTAL MARINE ENVIRONMENTS, III

of an offshore bar but assumed that sea water entering the salt basin was already hypersaline, and went on to show how their predictions from this model conformed with the actual distribution of various types of salt in the Siberian deposits. SUGDEN(1963) concluded from a hydrological study of the Persian Gulf that the bar theory was probably needed to account for the formation of potash salts in the past but was not necessary for calcium sulphate or halite deposits. Extensive shallowing would also reduce circulation in a large, partly isolated shallow sea, with resulting precipitation of these evaporites. Before we go on to consider evaporite cycles in the past it will be helpful to take note of what is happening, and what has been happening, in Postglacial Time, in the famous Karabogaz Gulf (DZENS-LITOVSKIY and VASIL’YEV, 1961). The Karabogaz Gulf is virtually an evaporating pan, 18,000 sq. km in area, connecting with the Caspian Sea (which feeds it with water) by the Karabogaz Strait, which is constricted by sand spits. The inflow of water through the strait is determined solely by the height of the Caspian Sea level relative to that of the Gulf. This has fallen almost 3 m in the last 30 years, due to a multiplicity of causes which need not concern us. This has led to the concentration of surface brines in the Gulf, the highest brine concentration being in the north and northeast, farthest removed from the strait. Precipitation of salts naturally varies with brine concentration. In the so-called intermediate zone of concentration halite and epsomite (MgS04.7 HzO) precipitate in the summer, mirabilite (NazS04.4HzO) in the winter. A slight solution of salts takes place in spring. A series of older beds occurs beneath the upper salt layer. They consist of three layers of m’lxed salts including halite separated by horizons of gypsiferous carbonate oozes with a Caspian fauna. These fluctuations are related to cycles of transgression and regression in Postglacial Time, the periods of transgression being marked by the carbonate and the regressions by the mixed salts. Before we deal with cycles dominated by evaporites, it will be convenient to consider a number of cases in which evaporites are a subsidiary feature, these forming a transition group between evaporite cycles proper and those cycles described in the previous two chapters.

MAJOR CYCLES WITH SUBSIDIARY EVAPORITES

The term “major” in this context is meant to exclude those cycles measured in centimetres or millimetres. A characteristic feature is the rarity or absence of halite and potash salts; evaporation has not usually extended beyond the stage of precipitation of calciumsulphate (or the more soluble salts have been redissolved). An important group of Late Palaeozoic cycles has come to light in recent years as a result of the exploration for oil in the northern Great Plains region of North America, including the Williston Basin of Saskatchewan, Manitoba, North Dakota and neighbouring states. These cycles are composed mainly of carbonates with subsidiary shales and evaporites.

MAJOR CYCLES WITH SUBSIDIARY EVAPORITES

201

TABLE XXX CYCLES IN OFF-SHORE AND NEAR-SHORE REGIMES

(After ANDRICHUK, 1951) “Off-shore” limestone-evaporite cycle

“Near-shore’’ ablomite-evaporite cycle

Dense or fossiliferous, fragmental Brown, dense and saccharoidal Anhydrite Salt (rare) Anhydrite Buff and brown, dense Green and brown, slightly argillaceous Brown saccharoidal Dense to sublithographic Dense to crystalline, slightly fossiliferous Fossiliferous, fragmental

Black, bituminous Green and maroon Grey and reddish grey Red, brown, green Argillaceous, silty Dolomitic, shaly anhydrite Brown and red Green Buff and brown, dense Dense, slightly fossiliferous or fragmental fossiliferous

A generalised account of Devonian cycles was given by ANDRICHUK (1951) who distinguished “idealised” (composite?) sequences in off-shore and near-shore regimes. These sequences are given in Table XXX, but no exact age equivalence of individual units is implied by placing them side by side. It will be seen from the above that the evaporites diminish, the carbonates become dolomitic and the proportion of clastics increases in passing towards the old shoreline. Andrichuk claimed that the carbonate-anhydrite cycles can be correlated over several hundred miles. Comparable cycles, with thickness of the order of a few tens of metres, were described from the Devonian Manitoba Group of the Williston Basin by BAILLIE (1955) although the lithological sequence was treated in a different way. The basal beds are thin and consist of red and green shale. These rest with sharp contact if the underlying beds are biohermal limestones, but are gradational if the topmost beds of the underlying cycle consist of anhydrite (this is much commoner in the subsurface). The shales grade up into argillaceous limestone, commonly finely laminated and associatedwith dolomite, and thence into reefoid beds with corals and stromatoporoids. Baillie confirmed Andrichuk‘s claims of wide lateral persistence of cycles over the basin. Both Andrichuk and Baillie agreed that the evaporites signified periods when the seas were restricted while the carbonates signified freer circulation of water. The local influx of clastics related either to proximity of shorelines or to slight tectonic movements on adjacent positive areas. The periods of restricted seas, when appropriate climatic conditions led to excess evaporation, were thought by Andrichuk to have been induced either by lowering of sea level or by upward movements in the area of reefs to the north.

202

EPICONTINENTAL MARINE ENVIRONMENTS, 111

Cycles not unlike the types described, and again of the order of a few tens of metres thick, continue up into the Mississippian of the same region (FULLER,1956; ANDRICHUK, 1955). Fuller’s idealised cycle is illustrated diagrammatically in Fig.78. Unit .4consists of a reworked weathered mudstone residue, sometimes containingmarkings resembling rootlet impressions. It was held to imply a subacrial surface. Unit B, the clear-water phase of oolitic and algal limestone, contains marine fossils and marks a marine transgression. Unit C, the argillaceous limestone with shaly intercalations, marks the beginning of a regression, continued by the anhydrite of unit D.Seven such units are recognisable in the Madison Limestone, with the evaporites dying out towards the basin centre and exhibiting diachronous relationship to the shales. Cycles in the Madison Group have also been considered by ANDRICHUK (1955). To him the oolites signified extreme shoaling conditions, and the anhydrite followed on naturally as the sea became progressively more restricted. As in the case of the Devonian cycles, alternative explanations are possible. Either sea level changed periodically or local areas rose through organic reef growth or tectonic movement, influencing circulation in the whole basin. Several other examples of shale-carbonate-calcium sulphate cycles have been described from different parts of the United States. HAM(1960) recognised four such cycles in the Middle Permian Blaine Formation of Oklahoma, each cycle being a few metres thick. Ham gave the following as the composite sequence: Green shale. Red shale. Gypsum (the thickest unit). Dolomite. The green shale contains hystricospherids indicating marine conditions and is associated with the transgressive phase signified by the dolomite and the lower part of the gypsum. Ham took the red shale to mark the phase of maxiniumregression(Fig. 79).

Fig.78. Idealised representation of major carbonate-evaporitecycles in the Madison Limestone of Saskatchewan. (Adapted from FULLER, 1956.)

204

EPICONTINENTAL MARINE ENVIRONMENTS, 111

The origin of these cycles was seen in eustatic rises and falls of sea level superimposed on regional tectonic movements leading to basinal subsidence. No independent support was adduced for this notion however. Shale-gypsum cycles in the Jurassic Carmel Formation of Utah, ranging considerably in thickness from 3 to 70 m, have been similarly related to transgressions and regressions of the strandline, with gypsum being deposited in restricted, hypersaline conditions on the sea margin and signifying the regressive phase (RICHARDS, 1958). Similar cycles occur in the Triassic Moenkopi Formation of Arizona (McKEE, 1954). Finally, certain cycles in the Upper Jurassic Purbeck Beds of southern England have been referred by SHEARMAN (1966) to so-called “sabhka cycles of sedimentation and early diagenesis”. Based on a comparison with the Trucial Coast of the Persian Gulf, he suggested an alternation of environmentsin which lagoonal sediments passed up through intertidal-zone algal mats and thence into supratidal-flat sediments with diagenetic calciumsulphate. Shearman went on to suggest that many other ancient evaporites might have a similar origin, even some of those including potash salts.

CYCLES WITH DOMINANT EVAPORITES

Major and intermediate cycles Symmetrical cycles including halite occur in the Pennsylvanian rocks of the Paradox Basin, which extends from central Utah to the Four Corners region of the southand BARKELL (1957) gave the following “simple ideal western United States. HERMAN cyclothem” from the Paradox member: Limestone. Dolomite. Anhydrite. Halite. Anhydrite. Dolomite. Limestone. The thickness of such cycles is of the order of tens of metres. An important feature is the occurrence of black shales interbedded with the evaporites. These contain conodonts, inarticulate brachiopods and plant remains. Laterally the cycles grade into open sea limestones, and the vertical facies changes have their corresponding lateral equivalents (Fig.80). The origin of this type of cycle will be best dealt with later in the chapter, when better-known examples have been described. The deposits of the Upper Permian Zechstein Sea in Germany and England can be grouped into several major cycles of flooding and desiccation marked by alternating units of evaporites with dolomitic carbonates and shales. Correlation between the two countries is of necessity lithological and is based on the presumed time

CYCLES WITH DOMINANT EVAPORITES

205

Fig.80. Halite-bearing cycles in the Pennsylvanian of the Paradox Basin, western U.S.A. (Adapted from HERMAN and BARKEL.L, 1957.)

equivalence of potash zones. Table XXXI is a simplified version of table XIX in STEWART (1963a). The correlation adopted by Stewart is that of LOTZE(1958). An alternative correlation (STEWART, 1954) equated the three principle English evaporite cycles with the upper three German cycles. These major cycles are themselves composed of alternations of more and less soluble salts, as can be seen from the table. These are sometimes sufficiently regular to be considered as cycles of intermediate grade (measurable in metres rather than tens of metres). STEWART (1963b), for instance, has subdivided the Lower Evaporites at Fordon in Yorkshire into three cycles, the boundaries being marked by reversals up the succession from more to less soluble salts. Thus the lower cycle passes from basal carbonate to an anhydrite subzone and thence to a halite-anhydrite subzone, with a subsequent reversal to another anhydrite marking the commencement of the next cycle. The reversals in the sequence are all relatively sharp, giving an overall asymmetry. This feature is not uncommon in the Zechstein evaporite cycles. BORCHERT and MUIR(1964) note, for example, that in the case of the Second Evaporite Bed in Lower Saxony there is a comparatively rapid

206

EPICONTINENTAL MARINE ENVIRONMENTS, I11

TABLE XXXI CORRELATION OF THE ZECHSTEIN EVAPORITE SEQUENCES OF ENGLAND AND GERMANY

-

Beds

Hanover and Thuringia

Whitby district, England Approximative thickness

(4 -

Oberer Zechsteinletten Grenzanhydrit und Fourth Tonmischsalz evaporite Jiingstes Steinsalz Pegmatitanhydrit

5

}

Top anhydrite

1

Salt clay

3

I25

____

Jiingerer (Roter) Salzton

16

Tonbrecciensalz Oberes jiingeres Kalilager Third Oberes jiingeres Steinsalz evaporite Unteres jiingeres Kalilager Unteres jiingeres Steinsalz Hauptanhydrit

up to 200

Upper Permian marl

I210

Upper halite Upper Potash zone eva- [Lower halite porites Anhydrite Carbonate

1

50

-

Alterer (Grauer) Salzton

Carnallitic marl

15

Upper halite

Decksteinsalz Second Alteres Kalilager evaporite Alteres Steinsaiz Basalanhydrit

Anhydrite ~~

Hauptdolomit und Stinkschiefer, mit Schlamm Grundkonglomerate J lo

1

Limestone (marl in part)

50

I

Upper halite1 anhydrite Lower Upper anhydrite evaup to 300+ Lower haliteanhydrite I Lower anhydrite 1

Oberes mittlerer First Zechsteinanhydrit evaporite Altestes Steinsalz Unter mittler Zechsteinanhydrit

Zechsteinkalk Kupferschiefer Zechsteinkonglomerat

Upper magnesian

f

1

4 0.3

2

Lower magnesian Limestone, with some anhydrite basal sands, breccias, mark

CYCLES WITH DOMINANT EVAPORITES

207

transition upwards in the main basin from highly soluble salts through anhydrite to clay. The Permian evaporitic deposits of Texas and New Mexico occur in the Ochoa Series which is divided into three formations, several hundred metres in thickness, the Castile, Salado and Rustler (KROENLEIN, 1939; KING,1948). The Castile Formation consists predominantly of anhydrite and is confined to the Delaware Basin. Halite is dominant in the overlying Salado, which also contains subordinate potash salts. Deposits of this formation spread over a much larger area than the Castile, possibly as a consequence of minor tectonic activity. The Rustler rests unconformably on the Salado and marks a return to sulphate and carbonate facies. These formations may be thought of as corresponding to major cycles in the Zechstein. Within them there are numerous alternations of calcium sulphate and halite of the order of a few metres thick, which may be compared with the intermediate cycles (ADAMS,1944; Fig.81). The cyclic sequences discussed above, including carbonates which are almost certainly chemical precipitates, clearly signify periodic variations in salinity, ranging from normal or slightly hypersaline water from which carbonates were precipitated to highly saline brines from which the potash salts and complex chlorides came down. Beyond this matter of general agreement, interpretation depends to some extent on the nature of the environmental model preferred, namely whether one envisages a basin restricted by a static or dynamic bar, or merely a shallow shelf sea with no bar at all. LOTZE(1957) and BORCHERTand MUIR(1964) favoured tectonic movements of the bar zone as the controlling factor, in fact they did not consider any alternative. Upwarping would progressively restrict marine circulation and hence induce conditions of high salinity in an arid climate, while downwarping would lead to a return of open sea or at least less saline conditions. The attraction of this hypothesis is that, given the appropriate environmental framework, minor tectonic movements in a

4

ANhYDRlTE GYPSUM

BANDED @ ANHYDRITE

DOLOMITE LIMESTONE

Fig.81. Major evaporite cycles in the Permian of Texas and New Mexico. (Adapted from ADAMS, 1944.)

208

EPICONTINENTAL MARINE

ENVIRONMENTS,

111

restricted area could lead to extremely widespread changes in salt concentration and precipitation. On the other hand there is no direct evidence of the existence of a physical bar in the Zechstein Basin. The changes accounted for by tectonic movements of the bar zone can be accounted for equally by slight changes of sea level, and as direct evidence of this we may cite the Postglacial sequence in the Karabogaz Gulf. Unfortunately it is difficult if not impossible at present to obtain direct support for this hypothesis by means of intercontinental correlations because of the paucity of good stratigraphical fossils. There is, however, one suggestive point in favour of the eustatic hypothesis. The asymmetry of certain cycles has already been referred to, a gradual upward change towards increasingly soluble salts being followed by a comparatively rapid return to less soluble ones. This has been held to signify the operation either of relatively rapid subsidence of the bar (and presumably the basin) or the relatively rapid rise of sea level followed by slower upward growth of the bar with or without fall in sea level (F. H. Stewart, quotedin: HALLAM and WALTON, 1963). In other words asymmetry of the cycles implies a corresponding asymmetry in the controlling mechanism. We have observed a comparable asymmetry in other types of marine cycle, in which it is clear upon close examination that the transgressive part of the succession is more condensed than the regressive part, and reasons why this should be so with changing sea level have been discussed (seep. 178). The asymmetry of the evaporite cycles could perhaps be explained in a similar way, the transgressive beds being the open sea carbonates in the case of the major cycles or less soluble salts in the case of the intermediate ones. (Entry of less saline waters into the depositional area could accentuate the asymmetry by causing re-solution of precipitated salts). Hence if sea level changes were the underlying control, movements might well have taken place at similar speeds. If the concept of a topographic bar is rejected in favour either of SCRUTON’S (1953) dynamic bar model or the hypothesis of ARKHANGEL’SKAYA and GRIGOR’YEV (1960), then sea level changes provide the most plausible controlling mechanism. Climatic control, involving variations in temperature or precipitation, seems less likely for the following reasons. If atmospheric temperature increased over the basin of deposition, increased evaporation would induce a correspondingly increased compensatory flow from the sea, which would tend to counter increase in salinity. The net result would probably be an increased thickness of a given salt precipitate rather than a transition to a more soluble salt. If the climate became more humid, the addition of fresh water to the basin from land streams would presumably increase, with the consequent lowering of salinity. To have more than a local effect, however, in basins as large as the Zechstein, the climatic change involved would have to be very great indeed. While it is conceivable that there were indeed major changes in humidity and/or temperature, related to the Late Palaeozoic glaciation, affecting the European and North American Permian evaporites, we still have to account for major cycles in other geological periods, and it seems more economical provisionally to prefer a mechanism of either tectonic or eustatic control, where a minor change could have a considerable effect on the sedimentary sequence.

209

CYCLES WITH DOMINANT EVAPORITES

Where salts alternate with clastic deposits the operation of climatic control and GALE(1958) described Late Pleistocene sediseems a distinct possibility. FLINT ments from Searles Lake, California, and were able to demonstrate by means of radiocarbon dating that muds were deposited during a pluvial phase corresponding with the Wisconsin Glaciation. These deposits are overlain by evaporites, which were precipitated during the succeeding arid phase.

Minor cycles Major and intermediate evaporite cycles are often seen to be themselves composed of minor cycles which are measurable in terms of millimetres or centimetres. UDDEN (1924) was one of the earliest to note their existence in his description of millimetrelaminated anhydrite in the Castile Formation of Texas, but they have since been recorded from many different deposits. Apart from the Permian evaporites of the Delaware and Zechstein Basins, for which there is a large literature, minor evaporite cycles have been observed in the Upper Silurian of Michigan (DELLWIG,1955), the Upper Devonian of the Pripyat Salt Basin in the U.S.S.R. (SHCHERBINA, 1960), the Jurassic of New Mexico (R. Y. ANDERSON and KIRKLAND, 1960) and the Oligocene of the Rhine Valley (BORCHERT and MUIR,1964), and this list is far from exhaustive. The cycles commonly have two components, but there may be as many as four, as STEWART (1963b) has found in the Zechstein Lower Evaporites of Fordon, Yorkshire. The wide range of possible lithologies has been well illustrated by LANG(1950) for the Delaware Basin evaporites on the assumption that the laminae are seasonal phenomena. The range of variation of mineral precipitates at different seasons is given in Table XXXII. It will be seen that no laminae would have formed when halite and anhydrite were deposited both in winter and summer. The organic layers were taken to mark the base of the cycles and considered to represent winter layers mainly.

TABLE XXXII RANGE OF VARIATION OF MINERAL PRECIPITATES AT DIFFERENT SEASONS (DELAWARE BASIN)

(After LANG,1950) Place

Layer

Mineral precipitates1

Castile

Summer layer Winter layer

C, Ca C, Ca

Salado

Summer layer Winter layer

A A

1

C

S

=

organic matter; Ca sylvite.

=

=

A C, Ca

Ca C, Ca A

A

M

M, G

calcite; M = magnesite; G

=

A Ca, A

H M, G

gypsum; A

=

A A

H S

H H

anhydrite; H

=

halite;

210

EPICONTINENTAL MARINE

ENVIRONMENTS,

I11

Besides the alternations of various types of chemical precipitate there may be alternations of evaporites and clastic layers, as in the Top Anhydrite of the Yorkshire Permian (STEWART, 1954). This could be a seasonal effect also, related to periodic floodings by fresh water or periodic wind action blowing in dust from the land. A noteworthy feature of the cycles is that they tend to be thicker the more soluble the salt (potash salts apart). Thus RICHTER-BERNBURG (1960) noted that cycles in the Linienanhydrit of the German Zechstein consisted of anhydrite units a few mm thick separated by skin-thin layers of bituminous carbonate. The overlying cycles consist ofanhydritebandsless than 1 mm alternating with halite bands 5-10 cm thick. A similar result was recorded by ADAMS (1944). If it is assumed that the cycle periodicity remained constant, as would of course be the case if they are annual, then it follows that the more saline salts were deposited more rapidly, a result which is of course consistent with our knowledge of sea water chemistry. STEWART (1963b) found that a general upward increase in the proportion of halite in the Fordon Lower Evaporites was proportional to an increase in the thickness of the halite layers. This was interpreted as signifyingthat the deposition rate increased towards the top of the succession (Fig.82). Another point of interest is the widespread lateral constancy of some minor cycles. RICHTER-BERNBURG (1958, 1960, 1964) has claimed to correlate sequences of minor cycles over distances of nearly 200 miles with such confidence that borehole cores with only about fifty cycles could be located stratigraphically without difficulty.

HALITE- POLYHALITEANHYDRITE u

6440

"'4

ANHY:TE;FO,HAl,E,

,'y.,:!

.

6760

-

6eOO 20

;

40 bo m 100 1 2 0 140 '160' 180 TOTAL NUMBER OF LAYERS PER FOOT

Fig.82. Relation of small-scale layering to position in sequence of the middle cycle of the Lower evaporites at Fordon, Yorkshire. The graph is believed to indicate relative rates of deposition. (After STEWART, 1963b.)

CYCLES WITH DOMINANT EVAPORITES

21 1

There has been widespread agreement that minor cycles of the type described result from climatic fluctuations, since tectonic or eustatic changes of the high frequency implied seem implausible. Accepting a climatic control, the regularity of thickness for a given lithological type in vertical succession suggests that many such cycles are indeed varves (or “Jahresringe” in German literature) as they have commonly been called. The reality of annual climatic cycles is obvious and unquestioned, un!ike any cycles of greater period, and recent deposits in Russian and American salt lakes show annual layering (LOTZE,1957). Nevertheless a sceptical note has been sounded by some authorities such as BRAITSCH (1962) and LOTZE (1957). One difficulty concerns the thickness of some anhydrite beds. The normal thickness of less than 1 mm is unobjectionable for annual deposits, but not the beds greater than 5 mm and even exceeding 20 mm. For the precipitation of 5 mm of anhydrite some 14 m of normal sea water or 4 m of calcium sulphate-saturated sea water are necessary (BRAITSCH, 1962). The amount of evaporation implied seems abnormally large if they are in fact varves. Again, assuming the fine banding to be annual, it can be calculated that the German Altere Salzfolge was deposited at a rate of 6 cm a year and as there are some 500 m of these deposits locally this would imply an excessive rate of subsidence for an area of undoubted tectonic stability if the sea were shallow all the time. The conclusion seems inescapable that, if the minor cycles are annual, then the basin of deposition must have been deep originally (RICHTERBERNBURG,1950). We can perhaps agree with Lotze that only in the case of especially good regularity and small thickness are annual cycles the most likely explanation. Abnormally thick anhydrite or halite bands could consist of several imperfectly separated varves in a sequence of homogeneous lithology. Explanations of the actual factors controlling the cyclic precipitation of various salts are extremely diverse. ADAMS(1944), discussing the origin of calcite-banded anhydrite in the Castile Formation, favoured a mechanism whereby a sand-dune bar protected by organic reefs was alternately breached and sealed, probably seasonally. The calcite was thought to be a summer deposit. In the cooler, wetter season the increased influx of river water would have diluted brines, while increased evaporation in the summer might have allowed the precipitation of more soluble salts. This process could account for cycles of thick halite (summer) and thin clay-dolomite-anhydrite (winter) (BORCHERT and MUIR, 1964).Work on the bromine content of salts in two German mines suggests that periodic dilution of brines by rain water might have been the significant factor in some cases (KUHN,1953). However, it can be argued that seasonal effects are dependent on the temperature coefficients of the solubilities of the various salts. As anhydrite is more soluble in cold than in hot water it could represent the summer layer in anhydrite-halite varves (STEWART, 1963a). STEWART (1953) also stressed this factor in discussing the origin of alternations of anhydrite-magnesite and gypsum-halite in the English Zechstein. Other presumed varves may involve alternating salt precipitation and solution.

212

EPICONTINENTAL MARINE ENVIRONMENTS, III

Summer convection currents may produce mixing of brine layers and partial resolution of the topmost bed. Hence many cycles have a clear-cut base (BORCHERT and MUIR, 1964). DELLWIG (1955) described interesting deposits in the Upper Silurian Salina Salt of Michigan, consisting of layers of halite, 3-8 cm thick, alternating with tissuethin laminae of anhydrite and dolomite. The halite consists of two alternating varieties, clear and cloudy. The latter contains minute gas bubbles and hopper crystals and was thought to have been formed by the accumulation of surface-formed hoppers on the bottom. Seasonal rise in temperature was presumed to lead to partial solution and a subsequent fall to the recrystallisation of clear salt. LOTZE(1957, p.203) has given another example involving possible periodic solution. If a halite bed originally contained anhydrite and clay impurities, periodic dilution of the solute, through either increased rainfall or river influx, might cause solution of part of the precipitate. This could result in the formation of a thin layer of insoluble residue. This interpretation could apply, for instance, to cases where the anhydrite-claylayer has a completely smooth undersurface (e.g., in the Oligocene of Alsace). Evidently the original irregular halite crystals have been rendered smooth by solution. Many of the mechanisms proposed for various sequences seem plausible enough. No doubt a multiplicity of causes has operated.

EVAPORITE VARVES AND SOLAR CYCLES

The recognition of minor evaporite cycles as varves has inevitably stimulated some geologists to seek evidence of greater periodicities marked out by slight thickness changes, which might relate to solar cycles of varying period. Indeed, BORCHERT and MUIR(1964), accepting published evidence for 11-year sunspot cycles in the Zechstein deposits, have gone so far as to argue that this evidence is the most cogent reason for believing that the laminated sediments are varves! To seek support for an annual cycle by invoking something as controversial as the 11-year sunspot cycle seems rather foolhardy. The most enthusiastic and indefatigable supporter of solar cycles is RichterBernburg who has, over the years, made some 40,000 varve measurements on haliteanhydrite rocks and especially anhydrite-carbonate rocks in the German Zechstein. He has also made briefer studies of Tertiary salt deposits from Sicily and Arabia. Using a method of visual matching for correlation, and graphical plotting of the composite sequence, he has claimed to find evidence of an 11-year cycle as the commonest periodicity larger than the annual (RICHTER-BERNBURG, 1960). Weaker periods were claimed of 22, 34 and about 45 years, which could be multiples of the 11-year cycle (Fig.83). There are apparently differences relating to rate of sedimentation. The rapidly deposited halite was said to reveal shorter periods more clearly while the more slowly deposited anhydrite showed longer ones, minor anomalies being suppressed. These longer periods were recognised by grouping varve means together

213

EVAPORITE VARVES AND SOLAR CYCLES N

Depth

“ANObMLY DISTANCE” IN VARVES

Number of:

“ A N O M L Y DISTANCE”

Fig.83. Frequency of appearance of evaporite varves of anomalous thickness in a sequence from the Permian Zechstein depositsof Germany. Note the maxima at approximately 1 1. and 22 years. (Adapted from RICHTER-BERNBURG, 1960.)

and tentative claims were made for 33.4, 95, 180-200, and 380400 year periods. It will be noted that the last two are approximate multiples of 95. Much as one must respect the industry which Richter-Bernburg has shown in collecting much valuable data, it is a pity that he did not seek the assistance of a mathematician to undertake some form of harmonic analysis, since his own technique necessarily involves subjectivejudgement of which varves have “anomalous” thickness. Until this is done one must share the scepticism of LOTZE (1957) and BRAITSCH (1962) about the evidence for solar cycles. R. Y. ANDERSON and KOOPMANS (1963) have, in contrast, undertaken both power and amplitude analyses from supposed

214

EPICONTINENTAL MARINE ENVIRONMENTS, I11

limestone and gypsum varves from the Upper Jurassic Todilto Formation of New Mexico, described by R. Y. ANDERSON and KIRKLAND, 1960 (see Fig.76). The analysis was based upon the measurement of 943 sedimentary units and failed in fact to find any evidence in support of an 11-year cycle, although there were a few weaker peaks, most prominently at 180 years.

Chapter 9

FLYSCH

One group of sediments, in addition to varves, owes its name essentiallyto the repetitive nature of the succession. This is theftysch of the Alps, so-called because of the tendency of the beds to slip very easily over one another and to form unstable, easily erodable ground. The movement is the direct result of coarse- and fine-grained beds alternating with one another often through great thicknesses of sediment. The coarse-grained beds are usually sandstones and the fine-grained shales or mudstones, though other lithologies do occur. The term flysch during its long history has acquired a number of different connotations; it is therefore important to make clear that it is being used here in the lithological sense. The alternation of coarse and fine sediments is perhaps the most important characteristic. But it is not completely diagnostic. To the alternation should be added (DZULYNSKI and SMITH,1964): ( A ) The sediments are marine in origin. (B) The coarse-grained beds (usually sandstones but occasionally clastic limestones) may have an original detrital clay-grade matrix. They are commonly graded from bottom to top. (C) The interface of sand and mud at the base of the sandstone is commonly sculptured into many markings produced by the movement of the sand over the mud both before and after deposition. These are the so-called sole-markings. (D)Sole-markingsproduced by the current often show a fairlyconstant direction of movement over large areas. (E) Fossils are rare though they occur in both shales and sandstones. ( F ) Large-scale cross-stratificationis usually absent. The successions to be dealt with in this chapter are marine in origin. They are also intimately connected with geosynclinal development, in particular eugeosynclinal conditions and generally, as in the Alps and the Carpathians, associatedwith the preparoxysmal stage immediately before the major mountain-building movements. In so far as many greywacke successions are also associated with this tectonic framework and because the greywacke successions compare lithologically with the flysch, in addition to the Alpine sediments we must include sediments from Caledonian, Hercynian and other mountain chains. The unstable tectonic conditions of these environments contrast with the stable regimes treated in other chapters. Examples of the successions to be considered outside the Alpine Belt are Devonian-Carboniferous rocks in the Hercynian Belt of western Europe, Lower and Upper Palaeozoic rocks of the Appalachians, Carboniferous rocks of the OuachitaBelt

216

FLYSCH

in south-central U.S.A., Lower Palaeozoic rocks of Britain, Permian and Cretaceous rocks in Japan, and Tertiary rocks in California, the East Indies, New Zealand and the Caucasus. Though differing in detail, for example in the composition of the sandstones, or the presence or absence of volcanics, all these successions show the characteristics listed above. The likelihood is that the mode of formation of the sediments was common to all types and the underlying cause of the repetitive nature of the sedimentation the same throughout.

MODAL CYCLES AND COMPOSITE SEQUENCES

Leaving aside for the moment the occurrence of other rock types such as boulder beds and slump deposits we shall consider a simple sequence in which only sand and shale units are present. In normal flysch these units would be in almost equal proportions, in shaly flysch lutites would predominate and in sandy flysch the arenaceous bands would be in greater proportion. The succession presents a repetition of simple cycles each consisting of a sandstone bed and an overlying shale and being bounded above and below by sharp changes in grain size, if not scoured surfaces. It may be objected that the sandstone may be a complex unit formed by a number of depositional episodes. This may be so but these complications can be considered at a later stage when the generation of the cycles is under discussion. Interbedded laminae (defined by PAYNE, 1942, as less than 1 cm thick) are usually not regarded as cycles with the same significance as the beds of sandstone and shale. Within the basic rhythm so defined there may be a variety of features but generally there is a characteristic gradation in size from bottom to top. This may take place within an apparently homogeneous bed or the homogeneous “massive” horizon may be followed by a laminated portion. The two together form a composite bed (KSIAZKIEWICZ, 1954). More detailed analysis has led to the development of a number of descriptive systems. VASSOEVIC (1948) proposed the recognition of a number of elements (I, 11,111). These major divisions within the rhythm are defined on grain-size, I, being the coarsest through to 111-the finest, muddy horizon. Sub-elements (a, b, c, etc.) are set up as more detailed analysis is necessary. These sub-elements may consist of any character within the element such as mineralogical composition or sedimentary structure. The “complete” sequence in the sense of Vassoevic then would be 1-11-111, an “incomplete” sequence would be designated 1-11 or 11-111 and any sequence may have a different combination of sub-elements so that the “formula” for a particular rhythm may appear as (say) la,b-IIb-IIIa,b. A system proposed by BOUMA (1962; SIGNORINI, 1963, and SCHAUB, 1951, made similar suggestions) has received widespread support and interest. Bouma concluded from a study of flysch sediments of the Alpes Maritimes, France, that the sequence, sandstone (a-+shale (e), was made up of five “intervals” which he designated: (e) Pelite.

MODAL CYCLES AND COMPOSITE SEQUENCES

217

( d ) Upper interval of parallel lamination. (c) Interval of ripple cross-lamination (and convolute lamination). (b) Lower interval of parallel lamination. (a) Graded interval. WALKER (1965) pointed out that the use of “interval” in this way was somewhat unusual and proposed the term “division”. We prefer the latter term as being more conventional in its connotation. While Bouma regarded the five divisions as “typical” it is clear from his analysis that this type of sequence (TaVe) occurs somewhat rarely. He recognised many incomplete cycles. Cycles with lower divisions missing were called base cut-out (for example TePe)and those with some of the top units absent were referred to as truncated sequences (for example Tab). The system is therefore amenable to the treatment suggested in Chapter 1. For the sandstones which Bouma analysed the composite sequence would appear to but the modal cycle from the figures be that represented by the “complete” cycle, Tape, shown in Fig.84 is clearly TcPe.The ideal (model) cycle we shall consider at a later stage.

.280 v)

240

U

Fig.84. Frequency distribution of bed thickness and type of sequence in sandstones of the PeiraCava area. Types classified according to the presence of the divisions u-e as described in text. (After DZULYNSIU and WALTON,1965, based on data from BOUMA,1962.)

Any 3ysch sequence can be analysed in the way in which Bouma suggested. Present data are rather meagre and lack the statistical basis which is desirable. Nevertheless enough information is available to allow the provisional erection of modal cycles and composite sequences from a number of different areas. The Lgota Beds (Aptian-Albian, Carpathians) compare closely with the Peira Cava Sandstone of Bouma in that 71 % of the beds are laminated and graded divisions are not numerous (Table XXXIII). The Waitemata Group (Miocene, New Zealand)

218

FLYSCH

TABLE XXXIII FREQUENCY OF TYPES OF BEDDING IN THE LGOTA BEDS, CARPATHIAN FLYSCH

(After UNRUG, 1959) Type of bedding

Frequency ( %)

Laminated Laminated and cross-laminated Cross-laminated Graded Composite (graded, laminated and cross-laminated)

71 .O 18.6 2.6 2.6 5.3

is also similar though BALLANCE (1964) adopted a different scheme of sub-units and his interpretation of the formation of the sediments is slightly different. At this stage it is sufficient to note that Ballance recognised as the composite sequence: (6) Very fine sandstone with ripple-drift lamination. (5) Muddy grey siltstone. (4) Sandstone with ripple-drift lamination. (3) Sandstone with ripple cross-lamination and convolute units. (2) Division with horizontal lamination. ( I ) Massive graded sandstone. The Aberystwyth Grits (Silurian, Wales) also have many units which are of the character (Te-Jbut they also have a large number of sandstone beds with a complicated contorted internal structure-the so-called “slurried” beds (WOODand SMITH, 1959). The composite sequence for this succession would therefore have to contain a slurried horizon. Within the basin the position of this slumped horizon varies. In the south it is normally found below the graded unit while in the north the position is reversed. The composite sequence as a whole could be referred to as in Fig.85. DZULYNSKI and SLACZKA (1958) also described units from the Krosno Beds (Oligocene, Carpathians) which include a slump horizon. In this case the slumped horizon is found consistently above the graded portion so that the composite sequence corresponds to that of the northern part of the Aberystwyth Grits.

northern part of basin

-7 A

laminated division

slurried bed

graded division

southern part of basin

Fig.85. Position of slurried horizons in composite sequence of Aberystwyth Grits, Silurian,Wales.

MODAL CYCLES AND COMPOSITE SEQUENCES

219

The Hell’s Mouth Grits (Cambrian, Wales) are rather coarser grained than many of the sequences referred to above and tend to have a much thicker graded division (BASSETT and WALTON,1960). In addition many of the sandstones show an additional phase of parallel lamination appearing at the base of the unit and the pelitic division (e) may be divided into a lower dark grey band and an upper white-weathering layer. Calcareous lenticles as well as lenses of a curious “pellet”-rock are also intercalated in the pelitic horizons. The composite sequence reads: White-weathering mudstone. Dark mudstone with calcareous lenticles and “pellet”-rock. Laminated division with some ripple marking. Graded division. Lowermost division of parallel lamination. Some of the units show multiple grading, i.e., repeated units of graded sandstone within the sandstone bed. This is a characteristic of successions predominantly of coarse sandstone or sandy flysch. The Istebna Beds (Campanian-Palaeocene, Carpathians) often appear to be similar judging from the bedding types described by UNRUG(1963). Cross-lamination appears in some of the sandstones and interbedded conglomerates and slumped horizons are not uncommon. The coarse-grained beds are taken by UNRUG to represent inshore conditions of sedimentation with fluxoturbiditel deposition (DZULYNSKI et al., 1959) predominating. Similar lithologies occur in the Southern Uplands of Scotland where the Ordovician contains a number of lithological types (WALTON,1963). The Corsewall and Portpatrick types compare with the Istebna Beds. The first (Corsewall) type is made up of coarse-grained sediments fairly well sorted, laminated and bedded without good grading; some largescale cross-stratification is found. Where grading does occur then it is usually in multiple units. Multiple grading is also typical of the Portpatrick type which tends to be slightly finer in grain than the Corsewall type and lacks the coarse conglomerates. Some conglomeratic layers are found as local concentrations of pebbles at the base of graded layers; elsewhere occasional pebbles are scattered sporadically through the sandstone beds. Shale layers between the sandstones are usually very thin. The Kirkcolm type of the Southern Uplands represents a finer-grained succession more nearly approaching normal flysch. But it differs from the sandstones described by Bouma from the Alpes Maritimes in that the graded portion seems to be of more frequent occurrence. The Hawick and Wenlock Rocks (Silurian, Scotland) correspond more closely with the Pefra-Cava Sandstones and the Lgota Beds in that laminated beds appear to be more numerous. The analysis of the rhythmic unit so far has been confined to the internal sedimentary features. But the external structures, in particular the sole-markings should also be considered. The sole-markings are a particular aspect of the asymmetry of the rhythmic unit in that they often represent the response of a mud floor to a sedimentladen current and the beginning of the phase of sedimentation. Moreover the response 1

Mass movement of material probably involving grain flow along the floor rather than suspension.

220

FLYSCH

of the floor is related to the energy of the current. Since the latter also determines the grain size and other internal features it may be expected that sole-markingsand internal structures are correlated. In a broad way this is true, as can be seen in the lithological types from the Southern Uplands described above. The conglomeratic, Corsewall type has only large, rather irregular scours on the base of the sandy beds. The Portpatrick sandstones tend to exhibit either channels or large flute moulds and occasional large tool moulds, whereas the finer-grained Kirkcolm rocks have small flute moulds, longitudinal ridge patterns and many tool markings. In addition the directions of the current structures also vary from type to type. Corsewall directions are very variable and often directed away from the margin of the basin, Portpatrick-type rocks also show variable directions with a tendency to longitudinal flow (i.e., parallel with the length of the trough), a direction which is predominant in the Kirkcolm-typerocks. Modal cycles and composite sequences vary from one area to another in the same trough. It should now be clear that some of this variation can be related to within-basin variance. Consideration of the Southern Uplands types makes it clear that the grain-size and other sedimentary features (the characteristics of the rhythmic unit) are determined by nearness to source. Before discussing the origin of the rhythmic units it is useful to note that whereas most flysch units are of sandstone and shale, occasionally limestone flysch is found, as in the Teschen (Cieszen) Limestones of Poland (Cretaceous, Carpathians). These beds have all the characteristic structures of flysch apart from their composition. HUBERT (1966) has recently described an interesting example of limestone flysch from the Southern Uplands of Scotland. The Whitehouse Beds (Ordovician) have a lower limestone member which is clearly flysch. An analysis of the internal structures shows the presence of the units u-e as described by BOUMA (1962). The modal unit is: (e) Pelite. (a) Upper division of parallel lamination. (c) Division of ripple cross-lamination (and convolute lamination). The composite sequence appears as: (e) Pelite. ( d ) Upper division of parallel lamination. (c) Division of ripple cross-lamination (and convolute lamination). (b) Lower division of parallel lamination or dune division. (a) Graded division An important feature of the units is the replacement of the graded “a” division by a cross-laminated division. The size and appearance of this division leave no doubt that it is formed of mega-ripples or dunes with wave-lengths reaching 6 m and amplitudes about 20-30 cm. The sequence is also important because it throws light on the nature of the pelite horizon. It is clear that in most cases this pelite must have two different origins, one part coming from the fine-grained portion of the influx which now forms the graded unit, the other part being independent of the influx. Where the composition of these two parts is the same then it is very difficult to estimate their relative amounts but in the Whitehouse Limestone member the influx contributes a fine-grained lime

IDEAL

(MODEL) CYCLE

22 1

mud while the second, “background”, material is a grey-green shale. In this particular case the contributions are in the proportions 75:25 which compares with RADOMSKI’S (1968) estimate from foraminifera1 assemblages in flysch that most of the pelite would normally be associated with the sandy influx. In another member of the Whitehouse Beds, however, HUBERT (1966) found a ratio of 50:50 “influx” to “background” shale. The relationship described from the Southern Uplands of the lateral movement of coarse-grained deposits and the axial directions associated with finer-grained sandstones has been found many times in the Carpathians of Poland (KSIAZKIEWICZ, 1962) and elsewhere. But in some areas there is very little indication of lateral movement as in the Martinsburg Formation (Ordovician) of the Appalachians (MCBRIDE,1962) and the Stanley-Jackfork Groups of the Ouachita Mountains (Oklahoma - Arkansas) (CLINE,1960; BRIGGS and CLINE,1963). Dispersal of sand in the latter basin presents a problem because KLEIN(1966) found that the composition of the sandstones suggested derivation from the sides rather than longitudinally from the east (the up-current direction). A somewhat similar situation is found in the Cretaceous Cerro Torro Formation in Chile (SCOTT,1966). Sole-markings indicate a constant axial (N-S) movement of currents but palaeogeographical considerations, occasional large-scale current bedding and large slump over-folds, point to a derivation of material and a down-slope movement from the east. DEWEY (1962) distinguished two types of sandstone unit in the Ordovician of Ireland. The coarser sandstone is generally well graded, with well developed solemarkings, particularly flute and groove moulds, whereas the second type is finer grained, has only poorly developed flowage structures on the base and internally contorted laminations. The sole-markings of the first type are oriented axially with respect to the basin and the second type shows directions of overfolding consistently across the basin, i.e., lateral movement. IDEAL

(MODEL) CYCLE

Erection of an ideal cycle and interpretation of the observed rhythmic units is dependent very largely on one major question, i.e., the depth of sedimentation (see DZULYNSKI and WALTON,1965, for bibliography). The assemblage of sedimentary features, especially the predominance of graded bedding rather than large-scale cross bedding, suggests that the deposits were not subject to strong reworking by wavestirring action. This indicates accumulation at least in the deeper neritic areas if not bathyal or abyssal depths. The fossils tend to c o d r m this though there is some difference of opinion. Foraminifera on the Tertiary flysch comprise both shallow-water, benthonic forms and species characteristic of deep water, the former being restricted to the coarser horizons (NATLAND, 1963;NATLAND and KUENEN, 1951).The assemblage of trace fossils (biohieroglyphs)is different from that of neritic sedimentsand SEILACHER (1962) has concluded that it is diagnostic of deep waters. Perhaps the most interesting and conclusive evidence lies in the fish fauna of the Jaslo Shale in the Carpathian

222

FLYSCH

Flysch. The fossils have specialised light organs characteristic of fish in present-day deep seas and some of the same species still exist in this habitat (JERZMANSKA, 1960). Accumulated evidence of the presence of sand and even gravel layers in modern deep seas also supports the contention that flysch and most graded greywackes accumulated at considerable depths. Some workers continue to resist this evidence and persist in regarding the flysch as shallow-water deposits. MANGIN(1962a, b, 1963), for example, cited the presence of bird’s footprints in Tertiary rocks of the Pyrenees, compared the facies with topset beds of the Mississippi delta and appealed to climatic variations as the main cause of cyclicity in the sedimentation (but see Chapter 3). DERAAF(1964) also pointed out the presence of salt pseudomorphs as evidence that certain flysch-like sequences may represent shallow-water conditions. No exceptional structures were described from New Zealand flysch sequences by KINGMA (1960) but that author regarded the sedimentary characteristics of lamination and small-scale cross bedding together with rare shell beds as suggestive of shallow-water deposition. The diagrams of internal structures (see for example KINGMA, 1958, fig.6) suggest that these New Zealand rocks are very similar to many in the Alps and the Carpathians and there seems no reason to reject the turbidity-current hypothesis for their formation; especially since the mechanism of tectonic control of current velocities and deposition in a shallow basin put forward by Kingma is singularly unconvincing. Admitting that rare features in certain successions present equivocal evidence it seems most probable on present evidence that most successions of flysch are deepwater deposits. Assuming such bathymetric conditions we can now proceed to consider mechanisms involved in the production of the rhythmic units. Certain suggestions deserve note but have received little support. OULIANOFF (1965) for example, in a summary of his work, proposed that vibrations of the crust were responsible for a slow down-slope creep of material. As well as causing the spread of sand he also envisaged the possibility of graded bedding originating from these vibrations. There is clear evidence of the effect of earthquake shocks in the liquefaction and flowage of sediment to form such features as sandstone dykes and sills, pseudo-nodules and some load structures (DZULYNSKI and WALTON,1965). But indications of current movement in the rhythmic units of the flysch suggest that the mechanism suggested by Ouilianoff can only be of marginal significance. The evidence of current action is so compelling that serious consideration can only be given to those hypotheses involving such movement. Pre-eminent amongst such hypotheses is that of turbidity current action as proposed by KUENENand MIGLIORINI (1950). The simplest models using this hypothesis will be examined first. It will emerge that the examples of rhythmic units described above demand more complicated schemes and consideration of oceanic currents capable of affecting the sea floor on abyssal and bathyal depths. These currents have come to be referred to as “bottom currents”. Furthermore the possibility has to be examined that the rhythmic units have resulted not from turbidity currents but from the operation of bottom currents only. The models to be considered therefore are those involving:

IDEAL

(MODEL) CYCLE

223

( 1 ) Turbidity currents, with: (a) supply from one end of the trough (longitudinal supply), (b) longitudinal combined with transverse supply, (c) other mass movements. (2) Turbidity currents and bottom currents: (a) graded beds and current-affected inter-beds, (b) bottom currents reworking the whole of the sandstone units. (3) Bottom currents.

Turbidity currents

On this hypothesis the development of the currents is sporadic, probably not periodic and most probably triggered off by earthquake shocks. The sand supplied by the turbidity currents provides the material of the graded units and interrupts background sedimentation of fine pelagic detritus. This detritus appears now as the pelitic horizon between the coarser bands. Model cycles are set up from experimental data, field evidence, hydraulic considerations and general conditions of supply to and dispersal of material in basins. Model l a : turbidity currents with longitudinal supply This simplest model compares with experimental conditions (KUENEN and MIGLIORINI, 1950; KUENEN and MENARD,1952; DZULYNSKI and WALTON,1963) and the results suggest that the deposits from a single turbidity current (i.e., one rhythm) would consist of a bed varying from proximal to distal portions. Grading may be absent from the near-source areas where lamination and sometimes cross-lamination may appear (DZULYNSKI, 1965). Most of the bed away from the immediate “in-shore” area would however be graded and the thickness would decrease distally. There may be more admixture of clay in the remote areas due to incorporation of this material in the highly eroded proximal regions. Sole-markings would vary from large scours near source through tool markings and longitudinal ridges (which become smaller distally) to a smooth lower surface (DZULYNSKI and WALTON, 1963). The experiments support the contention that graded beds result from the operation of turbidity currents. DZULYNSKI (1965) also produced laminated and crosslaminated proximal beds resembling flux0 turbidites and near-source greywackelithologies. But the lamination so often found in the upper parts of the cycle has not yet been successfully reproduced. KSIAZKIEWICZ (1954) for example envisaged the formation of laminations from weaker, less dense turbidity currents which either lagged behind the main currents and came along at intervals after the main flow or which formed inter-flows (as distinct from the bottom flow of the main current) and deposited material from some height above the floor. A rather more economical and elegant explanation links the laminated and graded portions together as deposits from a single turbidity current. WALKER(1965) envisaged the formation of the internal structures in the rhythm in terms of the succession of bed-forms found in hydraulic

224

FLYSCH

experiments under conditions of falling competency. The lowermost division (a, seep.217)Walkerinterpretedin terms of the nature of the current. If this were a current in which a thorough size sorting of grains had occurred (a mature current in Walker’s phraseology) so that the grain size decreased from head to tail and upwards in the current then deposition without reworking would produce a graded division. Walker envisaged a number of variations in the structure of the lowermost division according to whether the current showed a simple gradation of sizes or whether a more complex structure prevailed. Above the a division the lower laminations in b, with current parting lineation, can be interpreted as representing a transition flow regime (WALKER, 1965; J. R. L. ALLEN,1964b), which leads with falling velocity into the lower flow regime structures of the ripple lamination (c). The origin of the uppermost layer (d) with parallel lamination is problematical but Walker suggested that at this stage deposition takes place through a thin boundary layer developed just above the surface resulting in a slight sorting and gradation within the laminae. Another feature which is still unresolved is the lack of dune-structures which appear in hydraulic regimes above ripple-marking. This makes the Whitehouse occurrences (p.220) of exceptional interest. With regard to the mechanism of formation of parallel lamination, KUENEN (1965) has advocated one rather similar to that proposed by Moss (1962, 1963). Kuenen rejected the widely held idea that current pulsations are necessary for the formation of laminae and envisaged lamination as a result of “like seeking like”. That is to say, random patches of sediment of a particular size tend to grow because of the tendency to trap grains of the same size; mica flakes for example, are unlikely to be depositedwithacoarse sand because of the turbulence associated with such a floor, and sand grains may well roll right over a patch consisting merely of clay or mica flakes because of the smoothness of the surface. Laminae tend thereby to be of limited extent and can be formed from a current without pulsations. KUENEN (1965) demonstrated experimentally that sand laminae with grain diameters three times those of neighbouring layers could be formed in a uniform current. The interpretation of the sequence of structures is far from complete but the situation is clear enough to envisage the nature of the cycles as they are developed in different parts of the basin (Fig.86). At a point corresponding, for example, to the position d (Fig.86) the most frequent cycle (modal) will tend to consist of laminated beds with small tool markings and longitudinal ridges. This type may be interbedded with rather finer-grained beds from weaker flows or with coarser beds showing the graded division, produced from stronger flows. Where a number of flows have been generated in quick succession then multiple sandstone beds may occur. Current directions as shown by sole-markings, by internal grain orientations and by uppersurface ripple-markings should be nearly uni-directional, variations only occurring through local swings in the current and perhaps cross-currents affecting deposition in the ripple stage. KELLING (1964b) envisaged surge-waves produced by the passage of the turbidity current sweeping back and rippling the finer deposits in directions normal to the previous movements.

225

IDEAL (MODEL) CYCLE

a

b

C

d

e

Fig.86. Hypothetical lithological variation in elongate trough with source at one end. Above: sections; below: plan. a = conglomerates and sandy flysch of flux0 turbidite type; b = sandy flysch with multiple grading; c = normal flysch (cycle type Ta-e probably dominant); d = shaly flysch (cycle type Tb-sprobably dominant); e = pelagic (open-sea) deposits: shale-silt succession, remote K I WALTON,1965.) from source. (Based on a number of sources, see D ~ Y N S and

The sequence of structures inherent in this model can be seen in most local successions, say on the exposure level (level 2 of KRUMBEIN’S, 1964, hierarchy; p.18). But when modal cycles and sequences are compared over a large area, and when palaeo-current directions are taken into account a more sophisticated construction is necessary. Model Ib: longitudinal combined with transverse supply Consider the simple situation of Model 1 complicated by the presence of a number of active sources lying at points on the side along the trough. The main supply may be from one end as envisaged by KUEN~N (1957) or a longitudinal movement may predominate because of the tendency of transversecurrents to swinginto the axial direction along the length of the trough (DZULYNSKI et al., 1959). But the longitudinal pattern is confused by movements from a number of nearby transverse sources. The type of unit accumulating at any one point will then be controlled by the distance from the sources of supply and the fact that in this case the distribution cone of the turbidity

226

FLYSCH

current will gradually be rotated through 90 O from an initial transverse movement into a longitudinal one. This situation is characterised by a mixture of cycle types in the succession as well as strong variations in current directions-a mixture of transverse directions and longitudinal or even in the extreme-in the middle of the troughs-sometimes by opposing directions of movement. Multiple beds are fairly common.

Model Ic: turbidity currents with other mass movements Any of the models described above may have an additional complication: that of sliding of material in mass movements other than turbidity currents. Sand flow (in grain by grain flow), or slumps-characterised by a chaotic assortment of fragments at the one extreme or a simple arrangement of folds due to plastic gliding at the other-can be expected to originate on the lateral margins of the trough and to cut obliquely across the longitudinally derived debris in the middle of the basin. In terms of material and direction of movement they should be similar to material from lateral sources. More information is required from ripple marking before the precise conditions can be inferred but from present information the flysch troughs in the Carpathians provide excellent examples of both longitudinal movement patterns and many lateral sources. The Krosno Beds of the Carpathians (DZULYNSKI and SLACZKA, 1958) are fairly typical. In addition there are many cases of slumped beds and pebbly mudstone intercalated in the succession (UNRUG,1963; MARSCHALKO, 1963). These beds have moved downslope laterally from the intra-geosynclinal sources which contributed most of the debris. The rocks described by DEWEY (1962) from Ireland provide a rather exceptional example of longitudinal and laterally derived beds differing markedly in internal and external structures. Combined action of turbidity currents and bottom currents Model 2a: graded beds and current-affected inter-beds Consider an elongate trough occasionally suffering turbidity currents and also stirred by a water movement sufficient to move material along the floor. Transverse or axial movement of turbidity currents may be combined with any direction of bottom currents. Perhaps the most obvious manifestation of these bottom currents would appear in the non-alignment of sole- and upper-markings. Supposing the turbidity currents to be the more competent the major portion of any one bed could be laid down as a turbidite but the upper fine-sand or silt portion would then be reworked by bottom currents which would impose their orientation on any directional structure-ripples seem to be the most likely. This situation seems to be very common in nature. PRENTICE (1956) first drew attention to it in the flysch rocks of Devon and later CRAIGand WALTON(1962), KELLING(196413) and Hsu (1964) recognised similar occurrences elsewhere. In these examples the reworking current is at right angles to the turbidity currents and lies

IDEAL (MODEL) CYCLE

227

across the axial direction of the trough. (Kelling suggested surge waves associated with the movement of the turbidity current and this alternative cannot easily be ruled out.) Where there is a difference in movement direction then the deposits from the different currents may be picked out, but where current directions are found to be uniform throughout it becomes extremely difficult to pick out bottom current-formed rippled beds from distally formed, current rippled turbidites.

Model 26: wholly reworked units Another possibility arises: that of bottom currents strong enough to rework coarse sand grains through a considerable thickness. Given sufficient reworking the sand derived from a turbidity current may be reorganized from top to bottom and the sole markings destroyed so that all directional structures conform to the movement of the bottom currents. On any sandy floor it seems not improbable that reworking might take place down to the next smooth-surfaced lutite. Any combination of turbiditycurrent direction and bottom-current direction is possible and this type of reworking may have occurred in some of the sequences mentioned above in 2a. But a more common situation may be that of laterally derived mass movements (including turbidity currents) affected by bottom currents which have an axial long-shore movement. For the reasons outlined above both SCOTT(1966) and KLEIN(1966) were led to propose the (at first sight) improbable situation that each sandstone bed may have been reworked by bottom currents.

Bottom currents If reworking of sands is envisaged as a possibility is it necessary to postulate turbiditycurrent action at all? Suppose bottom currents were competent enough to disperse large masses of sand over the floor of a trough. HUBERT (1964) has recently urged that the whole question of deep-sea sands requires re-examination. Grain-size parameters and internal structures in most of the fine sands (from the North Atlantic) examined by Hubert were indistinguishable from those of shallow,current-formed sands. Hubert, therefore, in the light of recent evidence for bottom currents suggested that much of the sand dispersal in the Atlantic may be due to bottom currents rather than to turbidity currents. In considering setting up a model along these lines it is immediately apparent that there is a minimum of control in the form of evidence. Granted the operation of bottom currents we must then suppose that these are variable and that each wanes in competency over a period to produce the rhythmic unit, we must further suppose that the current operates over one area and dies in a distal (deeper?) direction. We may then end up with a current which has all the characteristics of a turbidity current! MURPHY and SCHLANGER (1962) have suggested the operation ofbottom currents but in their section (Rec8ncavo Basin, Brazil) the beds they were concerned with (like the sands described by HUBERT,1964, from the North Atlantic) were almost all

228

FLYSCH

laminated and cross-laminatedand there was no suggestion of gradations from coarse near-source beds to distal lithologies. In summary many of the natural sequences described so far appear to coincide in terms of modal cycles and composite sequenceswith a model involving longitudinal movement of turbidity currents derived ultimately from a number of transverse points of supply. There is growing evidence that bottom currents have played a part in forming parts of the rhythmic units. In some successions these bottom currents may have been dominant but at the moment it is very difficult to assess their precise importance. MEGARYTHMS

Larger-scale repetitive units were designated as megarhythms to distinguish them from the small-scale features discussed above (KSIAZKIEWICZ, 1960). Developing the idea of N o ~ ~ ~ ( 1 9and 2 7others, ) Ksiazkiewicz had in mind large units of formational or group significance (hundreds of metres in thickness) and he pointed out that the whole of the Carpathian flysch was made up of these megarhythms. Each unit began with coarse sediments and ended with fine-grained or sometimes organogenic sediments. Two types of megarhythms were picked out. One begins with coarse-grained sediments appearing suddenly above clays and shales, the second shows a more gradual transition from the fine-grained beds with the coarsest conglomerates some distance above the base. Examples of the first are the Lgota Beds (Albian) coming in abruptly over the Verovice Shales and of the second-the Inoceramian Beds (Senonian) where the sandy beds gradually dominate over the Cieszyn limestones and shales (Neocomian). Siluro-Devonian rocks of Victoria, Australia, show similar large-scalepulsations of the order of several thousand feet (SCHLEIGER, 1964). Six coarse-grained members occur in about 20,000 ft. of sediment and each member becomes finer upwards. Bedding thickness also decreases with decrease in grain size, although there may be some slight reversalsin this tendency through the succession. Like Ksiazkiewicz, Schleiger attributed these large-scale features to diastrophic movements. Within the members there are said to be smaller units, which Schleiger referred to as cycles and sub-cycles. These divisions consist of bundles of greywackes and siltstones and appear to be separated from one another by changes in the size of the greywacke beds; for example one cycle is picked out in the sequences shown in Fig.87 because of the incoming of a thicker greywacke after a number have shown a gradual reduction in thickness. The sub-cycles also are delimited by changes in the thickness of the greywacke. The procedure, however, appears to be quite arbitrary (even Schleiger admitted to a difficultyin resolving some of the “sub-cycles”) and of very little significance except perhaps to indicate that there may be groups of beds which show a decrease in the thickness of greywackes from bottom to top and that there may be oscillations in the thickness of greywacke beds through the succession. These should only be picked out by more rigorous methods of investigation than Schleiger apparently employed.

IDEAL

229

(MODEL) CYCLE

CYCLE2

TOP

Fig.87. Cycles and sub-cycles from the Siluro-Devonian sequence in Victoria, Australia, according to SCHLEIGER, 1964.

Other authors have commented on groups of beds of the order of several metres which appear to form larger rhythms. The Aberystwyth Grit succession (Silurian, Wales: WOODand SMITH,1959) shows in parts groups of thin beds separated by groups with thicker bands. On occasion the group of beds begins with a thick greywacke and through the cycle the greywacke bands gradually decrease in thickness. The situation is reversed in part of the Ordovician succession in the Rhinns of Galloway (Scotland; KELLING,1961) where the cycle (of the order of 10 m), consisting of a number of greywacke beds terminates upwards in a thick (up to 3 m) bed. NEDERLOF (1959) found a significant fluctuation in bed thicknesses through a Carboniferous succession in Spain, the size of which compares with those from the Aberystwyth Grits. He also detected a longer period oscillation which he referred to as a “trend” (see Chapter 1: NBDERLDF,1959). But not all successions show such fluctuations or trends. Similar methods to those used by Nederlof for example failed to reveal any fluctuations in the Lower Palaeozoic successions in areas in Scotland and Ireland (T. B. ANDERSON, 1962; WELSH,1964). SUJKOWSKI (1957) also achieved negative results from his analyses of flysch successions. For the larger rhythms in the Carpathian flysch KSIAZKEIWICZ (1960) favoured a tectonic control, envisaging sporadic uplift of the intra-geosynclinal areas which acted as sources for the sediment. The origin of the smaller groups of beds is little understood. For the most part they have been remarked on without further comment. NEBF(1964), however, produced an ingenious idea to account for certain “bundles” of beds in early Pliocene rocks of New Zealand. These bundles, or rhythms in Neef’s terminology, consist of mudstone surmounted by a series (6-9) of thin graded sandstones (Fig.88) although in some sections the place of mudstone may be taken by siltstone. Fault movements were invoked to account for the rhythms in this way. The area is cut by northeast-southwest trending faults which Neef supposed operated,

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FLYSCH

RHYTHM 5 15 ft 9 t Graded Beds

RHYTHM 4

13 ft.

7 Graded Beds RHYTHM 3 16ft.

6 Graded Beds

RHYTHM 2 20 ft.

9 Graded Beds

Mudstonc

63Sandstone and Siltstone

6] Lenticular Concretionary Band Fig.88. Rhythmic groups of graded beds from the Pliocene, hlfredton, New Zealand. (After NEEF, 1964.)

perhaps at intervals of about 20,000-30,000 years, to produce a small scarp against the downdropped portion which sloped gently upwards away from the scarp (Fig.89). Neef suggested that turbidity currents would be channelled along the deeper portion alongside the scarp and that the coarser sediment would be deposited in these lower regions. Some of the upper part of the turbidity current would spread over the top of the scarp but this would be only the finer material, and would appear in the geological record as a bed of mudstone. So long as no further movement along the fault intervened sedimentation would fill up the deeper portion and sand sedimentation would spread over the whole area. During this time the upper graded beds of the top of the rhythm probably formed. Another movement along the fault would initiate another rhythm.

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Chapter 10

SEDIMENTARY CYCLES AND FAUNAL CHANGE

Fossils have so far only been mentioned incidentally in this book. Because of their great value in environmental interpretation and stratigraphic correlation, however, something must be said about their relationship with sedimentary cycles. A comprehensive review would be beyond the scope of this work and all that is intended in this chapter is to point out a number of significantfindings based on some selected examples of research. It will be convenient first to consider faunal variation within major cycles, where the principle concern is with facies change, and pass on to faunal variation between cycles, where the emphasis shifts towards evolution and extinction.

FAUNAL SUCCESSION WITHIN MAJOR SEDIMENTARY CYCLES

The alternation in the European Upper Carboniferous Coal Measure cycles between “marine bands” and beds containing so-called “non-marine lamellibranchs” (Carbonicola, Naidites, Anthraconaia, etc.) is well known. What is perhaps less familiar is that the marine bands are themselves often cyclic from a faunal point of view, and have been interpreted as showing transitions from open-sea to brackish-water or fresh-waterconditions.The Clay Crossmarineband of the Pennine Coalfieldinnorthern England provides a good example. As described by EDWARDS and STUBBLEFIELD (1947) the lowest few cm of the marine band in its area of maximum development are characterised by Lingula alone, occasionally associated with the marine ostracod Hollinella; this unit is followed by one containing the goniatite Anthracoceras and thin-shelled bivalves including Dunbarella; inarticulate brachiopods and gastropods also occur here. The top part of the marine band is again in Lingula facies, with interdigitations of thin layers containing non-marine bivalves. JESSENet al. (1952) have given the schematic representation of faunal cycles within the Ruhr Coal Measures shown in Table XXXIV. This is an idealised scheme, and one or more elements are usually missing. It is interpreted in terms of varying salinity, with the goniatites marking the most fully marine conditions. Similar cycles in the Belgian Coal Measures have beendescribed by VANLECKWIJK (1948). Such faunal cycles, testifying to varying environmental conditions, may occur in a lithologically apparently uniform shale sequence, as in parts of the Namurian (Miustone Grit Series) of Derbyshire. RAMSBOTTOM et aL (1962j have recordedfhe

234

SEDIMENTARY CYCLES AND FAUNAL CHANGE

TABLE XXMV SCHEMATIC REPRESENTATION OF FAUNAL CYCLES WITHIN TFE RUHR COAL MEASURES

(After JESSENet al., 1952)

1 Non-marine lamellibranchs 2 Planolites (trace-fossil) 3 4 4 3 2 1

Lingula Goniatites Goniatites Lingula Planolites Non-marine lamellibranchs

i

Regressive hemicycle

Progressive hemicycle

following faunal phases, which oscillate in a regular manner: (6) Typical thicker-shelled goniatite phase: fully marine with numerous bivalves and occasional trilobites and brachiopods. (5) Anthrococeras and Dimorphoceras phase: thin-shelled goniatites, perhaps signifying slightly brackish conditions. (4) Molluscan spat phase: abundant spat of marine molluscs, probably not fully marine. (3) Lingula phase: regarded as more saline environment thari I and 2, but still not fully marine. (2) Planolites phase: no marine fossils. ( I ) Fish phase: palaeoniscids and acanthodians mainly; no marine fossils. Though the sedimentation might not have varied cyclically, it is clear that significant environmental changes would be missed by anyone who chose to ignore the fossils. The same is true of the Jurassic Oxford Clay of southeastern England. Before BRINKMANN (1929) pursued his detailed studies and revealed the minor faunal cycles described in Chapter 6, no-one had suspected the existence of numerous environmental oscillations during deposition of the formation. The notion that salinity has been the main controlling factor affecting faunal distributions in the Carboniferous Coal Measures has recently been disputed for certain instances by BOGER(1964). Boger made an intensive study of two marine bands in the Ruhr Coalfield and recognised the following six fossil associations, which may occur in sequence: (6) Brachiopods: rare; may be accompanied by members of the fifth association; only occurs in sandy sediment. (5) Gastrioceras: includes Pterinopecten. (4) Anthracoceras: includes Posidoniella. (3) Nuculanids: small bivalves. (2) Planolites and Lingula. ( I ) Jonesina: ostracods and fish remains.

235

FAUNAL SUCCESSION WITHIN MAJOR CYCLES

Resemblance of these associations, occurring in succession, to the previously described faunal cycles or phases is obvious. Boger argued, however, that such factors as direction and strength of water currents, the quantity and clasticity of sediment deposited in the area and the depth of sea might have played an important role in affecting the fauna. The influence of salinity might have been an indirect one. The first three associations are dominated by infauna (living within the sediment), the last three by epifauna (living above the sediment). This suggested to Boger the possibility that the former group inhabited relatively shallow, perhaps intertidal waters of fairly normal salinity, where they were well protected by their mode of life from periodic dessication and salinity fluctuations. The latter group would have been more exposed to such fluctuations and were in consequence probably confined to slightly deeper waters offshore. Faunal variations up the succession may point to changes in degree of aeration of the sea bottom (e.g., BRINKMANN, 1929;HALLAM, 1960) or changes in depth of sea. Depth determinations from fossils are fairly reliable for Middle Tertiary and younger deposits, though rarely for differences of only a few metres. As noted in Chapter 7, VELLA(1963) ascribed depths of deposition to his Plio-Pleistocene sedimentary cyclic units with confidence on the basis of the contained fossils. For much older deposits there is far greater uncertainty, as is well illustrated by the controversy over the depths of deposition of various lithological units in the Pennsylvanian and Permian cyclothems of Kansas and neighbouring states in the North American Mid-Continent region. ELIAS(1937) wrote an influential paper in which depth of sea was considered the principal factor controlling deposition of the sediments, using fauna as the main criteria. The beds were thought to have been deposited in depths of up to about 55 m (180 ft.) of water, with the fusulinid limestones marking the greatest depth (160-180 ft.). This interpretation has been disputed for one of Elias' best examples, the Wolfcampian Beattie Limestone Formation, by IMBRIE et al. (1959, 1964) and LAPORTE (1962).

TABLE XXXV LAPORTE'S INTERPRETATION OF THE RELATIONSHIP OF FACIES AND ENVIRONMENTIN THE PERMIAN COTTONWOOD LIMESTONE OF KANSAS AND NEIGHBOURING STATES

Facies

Bioclastic Fusu1ine Platy algal Shelly Lime-rich Clay-rich Silty Osagia

Water circulation

Turbulence

influx

Terrigenous

low low low

poor poor intermediate

intermediate intermediate intermediate

intermediate high high

good good good

low low high

236

SEDIMENTARY CYCLES AND FAUNAL CHANGE

Laporte undertook a detailed palaeoecological study of the Cottonwood Limestone, a member of the Beattie Limestone which can be followed from Oklahoma to Nebraska, and suggested three main environmental controls of faunal facies, none of them directly involvingdepth. The proposed relationshipsare given in Table XXXV. In contrast to Elias, I ~ R IetEal. (1959) believed the deposits to be of extremely shallow-water origin, with the Osagia beds being laid down in water of a fathom or less and the fusuline and bioclastic facies in waters only slightly deeper. MCCRONE (1963, 1964) has returned to the belief in the primary importance of depth control in the case of another of Elias’ examples, the Lower Permian Red Eagle cyclothem. The importance of other factors is not neglected, however, and McCrone supported Laporte’s contention that fusulinids lived in shallow depths, probably no greater than about 20 m. As evidence for this, it was pointed out that rich fusuline beds are associated with crustose calcareous algae whose living analogues certainly dwell in very shallow waters. McCrone recognised a number of environmental index fossils: Usagia (algae): clear, shallow (0-10 ft.), warm, gently agitated water. Lingula, Orbiculoidea: shallow (0-10 ft.), somewhat turbid conditions, restricted circulation. Triticites: clear, shallow (10-40 ft.), warm, normal marine water, gentle, free circulation. Trochiliscus (charophyte): shallow (0-10 ft.), mildly brackish water rich in CaCOs. The researches of these different workers indicate the great scope that exists for detailed palaeoecological investigations of fossiliferous sedimentary cycles.

FAUNAL CHANGE BETWEEN MAJOR SEDIMFNTARY CYCLES

It is also profitable to investigate broader relationships between fauna and sediments by consideringfaunal distribution in whole sequences of major sedimentary cycles over large areas. Continuing with examples from classic Late Palaeozoic sequences we may first note the work of CALVER(1956) on the distribution of Westphalian non-marine bivalves in the English Pennine coalfields. Three major marine bands occur in an otherwise largely non-marine succession, the Clay Cross, Mansfield and Top. As illustrated in Fig.90 the genus Carbonicoh continues up to the Clay Cross and then disappears suddenly. Anthracosia comes in just below the Clay Cross and continues right up to the Mansfield. Naidites, in contrast, continues right through the succession before disappearing almost exactly at the horizon of the Top Marine Band. A few other less common genera show no such clear relationship, although it should be noted that fossils of the Anthraconauta phillipsi group come in almost exactly at the level of the Top Marine Band. Evidently the disappearance up the succession of these three genera marks extinction over the Pennine region and maybe a more extensive area. It appears highly probable that this

237

FAUNAL CHANGE BETWEEN MAJOR CYCLES

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was intimately bound up with three widespread transgressions of the sea, destroying at least temporarily the old habitats. Another British Carboniferous example was given by DIXONand VAUGHAN (1911) who pointed out that some major cyclic boundaries in the Carboniferous Limestone of Gower, Glamorgan, coincide with the boundaries of faunal zones. Thus the new fauna of zone D enters the succession where the facies change suggests a deepening of the sea (see Chapter 6). VELLA (1963) gave a Plio-Pleistoceneexample of faunal zones correlating with major cycles. The best examples so far, come, however, from shelf sea successions in the

238

SEDIMENTARY CYCLES AND FAUNAL CHANGE

European Mesozoic. An intimate correlation between sedimentary cycles and ammonite zones in the Jurassic was noted a long time ago by KLUPFEL (1917), FREBOLD (1925) and others, and a similar relationship, less well documented, observed in the Swiss Cretaceous by FICHTER (1934). The Lias has received the most attention from this point of view, being particularly favourable for study because of its rich and easily correlatable ammonite faunas in northwest Europe. It has been possible to confirm and extend the findings of Klupfel and Frebold and demonstrate that ammonite zonal boundaries and sedimentary cycles usually coincide (HALLAM,1961). Furthermore, the more striking the sedimentary change the more striking tends to be the faunal turnover. Though condensation of the succession or the occurrence of minor stratigraphical gaps at cycle boundaries may accentuate the faunal changes, they do not “artificially” create them, since zonal boundaries normally remain distinct in a variety of facies, including those where the lithological evidence does not favour strong fluctuations in rates of sedimentation. The objection might also be raised that ammonites are first used to correlate sedimentary cycles and subsequently the rock succession is used to inform on changes in the ammonites. The implication is that the argument, like the sedimentation, is cyclic, or at least circular. This objection is not valid; the argument would be circular only if changes in the ammonite succession alone were used to establish the existence of sedimentary cycles. What in fact has been done is simply to correlate sedimentary and faunal change. There is no reason, for instance, why important changes in the ammonite succession should not frequently fall in the middle of major sedimentary cycles in given areas, if the relationship were merely random. Other invertebrate groups were evidently less sensitive to environmental changes than the ammonites and had much longer-ranging species. Nevertheless there were two major changes in sea level that had a profound effect on the fauna as a whole. The first episode was a widespread marine transgression at the base of the Pliensbachian. This is not marked in most areas by a striking sedimentary change and, ammonites apart, there is no suggestion of widespread disappearance of Sinemurian invertebrates. The transgression correlates, however, with the appearance and proliferation of new organisms, notably among the ammonites, belemnites, bivalves, brachiopods and Foraminifera. The second change is the most notable in the whole of the Lias from both sedimentary and faunal viewpoints. A phase of widespread shallowing of the sea in the Late Domerian was followed by a deepening in the early Toarcian, marking the beginning of a new sedimentary cycle. This is clearly indicated by the sedimentary record (see Chapter 6). The change correlates with an almost complete faunal turnover, virtually all major groups being affected. An episode of widespread disappearance of older species, marking at least local extinction, correlates not exactly with the Domerian-Toarcian boundary but with the presence of an extremely widespread unit of laminated bituminous shale in the Lower Toarcian, clearly an anaerobic deposit. It seems likely that bottom stagnation was largely responsible for extinguishing the benthonic fauna, together with some nektonic elements. Subsequently, in the later

239

FAUNAL CHANGE BETWEEN MAJOR CYCLES

Hettangian Astarte obsoletu

Cardinia listeri Chlamys? calvu Chlamys textaria

Entolium lunare Geruillella hugenowi Gryphuea aff. urcuuta Grypliaeu cymbinm

Sinemurian

Pliensbachian

.Toarcian

,

IIippopodiunz ponderosum Inoccramus ventricosus Lima gigantea

Lima succincta Mactronzya cardioides Meleugviliella substria f a

Modiolus scalp~iim Myoplzorella literata Niiczrlu hamnzeri

Nucula nauis Nuculana ovum Oxytomu inaequiualue

Pulueoneilo gahtea Pamllelodon buckniani Pholnd~myau m b i p a Piniia hartmaniii

Pmtocardia truncata Pseudolimea acuticosta Pseudotimea pectinoides Pseudopecten aequiualuis Terquemia arietis Variamussum gumilum

Velata veluta

Fig.91. Stratigraphical ranges of some common bivalve species in the British Lias. (Based on data in HALLAM, 1961.) Domerian is equivalent to Upper Pliensbachian.

Toarcian, northwest Europe was colonised by new forms which often range into the Middle Jurassic. These points are brought out by Fig.91, which gives a list of some of the commonest Liassic bivalve species. It will be seen that the species are divisible into four groups. One group ranged through most or all of the Lower and Middle Lias (Hettangian-Domerian) and even into the basal Upper Lias (Toarcian) before becoming extinct. The second entered the succession in the Lower Pliensbachian before suffering a similar fate and a third entered in the Toarcian, above the bituminous

240

SEDIMENTARY CYCLES AND FAUNAL CHANGE

shale horizon. Only a small minority of species, composing a fourth group, were able to survive the episode of widespread bottom stagnation. The question naturally arises, do changes of this type reflect world-wide events? Evidence is beginning to accumulate to suggest that they do at least in some instances (HALLAM, 1963b). The later stages of a major cycle in the Jurassic, at times of relatively shallow water and contracting seas, seem to be bound up with the extinction of old groups of organisms and the early stages with the evolutionary radiation and migration of new groups. This is very well shown in the Middle Jurassic. In Bathonian times the seas reached an extreme condition of contraction and the ammonites have unusual features, with the development of extreme oxycones, sphaerocones and cadicones, and exhibiting various eccentricities of growth. It seems to mark in fact a time of “evolutionary stagnation”, to use ARKELL’S (1956) term. The succeeding transgression in the Callovian, marking the start of a new cycle, was associated with the evolution and migration of new groups, Tethyan macrocephalitids being followed by boreal kosmoceratids. Very rarely indeed does an individual ammonite genus cross a major cyclic boundary and striking changes are often apparent even at family level. These phenomena seem to represent instances of a major sort of evolutionary process associated with the eustatic rise and fall of sea level (R. C. MOORE,1954; NEWELL, 1962, 1963). Periods of low sea level when shelf seas were reduced to a minimum often correlate with phases of widespread extinction Qfinvertebrates. As the sea level rose subsequently the transgressing seas created once more a large variety of ecological niches which led to rapid multiplication and radiation of the survivors. Outstanding examples include the Permian-Trias and Cretaceous-Paleocene transitions. It is no coincidence that the boundaries of these systems correspond with those of the major eras of Phanerozoic time. Here is a much-neglected field for research, since surprisingly little intensive and systematic work has been done seeking relationships between major changes in the sedimentary and faunal successions. The bearing of this type of research on problems of both cyclic sedimentation and faunal evolution is not inconsiderable.

Chapter I1

GENERAL CONCLUSIONS

It is still arguable that the attempt to discern a pattern of repetitive sedimentation in successions has led to more misrepresentationand difficultiesthan it has enlightenment. Nevertheless such a search is probably inevitable at some stage in sedimentary studies and efforts will no doubt continue. It is a chastening observation that methods of investigation and data assessment did not change essentiallyfrom the early recognition of possible rhythmic sedimentation in the earlier part of the last century to the middle of this. There is hope now that at least the more undesirable subjective elements will be eliminated from future analyses. This does not mean a Iess important role for the assessment of data on geological grounds. Indeed, as indicated in Chapter 1, statistical analysis of complex successions can usually only begin after decisions have been made regarding, for example, the most significant lithology in a mixture of lithologies making up the succession. But once geological decisions of this sort have been made it is incumbent on workers at this stage to carry out as objective and quantitative analyses as possible. A number of different methods is available. The methods of D m and WALTON(1962) and PEARN (1964) represent one type where there is an attempt to pick out what may be the most common of cycles and to compare this (modal) cycle with ideal (model) cycles. This type of analysis may be essential in complicated successions, although VISTELIUS(1961) has shown how it may be possible to allocate figures to different lithologies and apply rather more rigorous mathematicaltechniques. Of these techniques the method of power-spectrum analysis, which picks out the predominant periodicities, in a complex series of measurements through time, is of great potential. It is also possible, as SCHWARZACHER (1964) has done, to pick out the variation in particular parameters through a succession and compare this variation with mathematical models represented by, for example, simple sine curves or more complex expressions. Although it should be always borne in mind that statistical methods are not always the most appropriate and that geological significance should not be lost in the search for more and more elegant mathematical techniques, the use of these methods should provide a sounder basis for genetic considerations than is currently available. From the way in which much evidence has generally been presented it is often difficult to be sure of field sequences (see especially Chapters 4 and 5). There is no doubt too that there has been a strong tendency to overemphasise the regularity of rhythmic successions. This has certainly been the case in studies of Carboniferous rocks. Very many workers have assumed that cyclicity is present in a certain form, and that variants from this form are unimportant so far as the overall theory of origin is concerned. But it is difficult to know where the process of making

242

GENERAL CONCLUSIONS

allowances for variations from an expected development should stop in this subjective approach. Carried to absurdity it would be possible to “see” complex cycles in a thick perfectly homogeneous successionof one lithologicaltype-facies variations throughout the basin of deposition having led to the absence of expected units in the area under consideration. Furthermore, once it was realised that Carboniferous sediments displayed somewhat similar cycles in different parts of the world, many workers began a misguided search for a worldwide mechanism and, often, for one single cause of cyclothems. VELLA(1965), returning to ideas of H. Stille and E. Suess on worldwide synchronous eustatic or diastrophic events, has even recommended the adoption of cyclothems as time-stratigraphic units. When it is realised that it is often not possible in America to correlate Carboniferous cycles between say Kansas and Illinois or even within each state and that in the British Coal Measures there are often correlation problems between Sheffield and Nottingham, a distance of about 30 miles, we find it difficult to take Vella’s suggestion with any seriousness. We have considered cyclic successions from a variety of environments and it seems probable that only in a few regimes can worldwide controlling mechanisms be recognised. Thus, certain of the major cycles of the Jurassic can be ascribed to eustasy but this is possible only because of the remarkably fine zonal scheme available and after painstaking comparisons on an intercontinental scale. To begin by asserting a world-wide synchroneityis clearly to pre-judge the question before the trial has begun. Lithologies in cyclic successions are obviously controlled by the environment of deposition. Our survey has shown that cycle types, irregularities and lateral changes in types are themselves largely a function of the environment. It is therefore manifestly ludicrous to attempt to find a single control for cyclic sedimentation. “An argument over the origin of cyclothems is absurd; the argument must always deal with a particular cyclothem or a carefully defined class of cyclothems” (BEERBOWER, 1964, p.41). When considering possible controls we follow ROBERTSON’S view (1952, pp.515-516): “Only if these [local mechanisms] fail to convince us of their adequacy is it permissible to go further afield into the more speculative realms of climate, regional earth-pulsations, planetary movements or cosmic influences”. The obvious local effects to be considered at first are those which are inherent in the sedimentary processes responsible for the accumulation of the succession. It will of course be remembered that in all cases it is necessary to suppose an overall subsidence and in the first instance this can be taken as effectively uniform in rate.

SEDIMENTARY CONTROL

Awareness of the importance of this type of control has had to await detailed evidence from sedimentological studies, many of which have been forthcoming only in the last two decades. This is especially evident in facies which are recognised as marginal in development and probably deltaic in origin.

SEDIMENTARY CONTROL

243

The work of H.N. Fisk and many others has led to an understanding of delta growth which throws an entirely new light on the origin of many cycles. The sequence of lithologies encountered in many Carboniferous cycles bears precise comparison (1960) from the Mississippi. And the with that sequence summarised by SCRUTON periodic changes in sub-delta active growth must have played a large part in cyclothem development. It is evident that these changes are inevitable in any delta. As outward growth takes place the gradient of the main channel is reduced. Areas around the active sub-delta continue to subside to lower levels and crevassing upstream from the main channel may succeed in diverting much of the drainage to the new low spots. Such a diversion is likely to be maintained, until again, sub-delta growth is successful in extending the river, gradients are decreased and crevassing again becomes likely. The system is largely self-regulating and incorporates an in-built feed-back mechanism which like many processes in engineering inevitably sets up a rhythmic succession of events. A further feed-back mechanism is found in the growth of peat which in its accumulation contains the mechanism of its own decline. Combined in the sedimentaryedaphic control theory the two systems perhaps provide a comprehensive explanation of cyclicity for the majority of coal-bearing cyclothems. Some anomalies arise when comparison is made between ancient and modern sequences. One of these is the apparent development of a sand layer over the previous delta sequence (SCRUTON, 1960). The sand layer is formed by reworking of the underlying deltaic sediments as the sea transgresses across the subsiding area. This situation is very rare in older rocks. It is uncommon because many of the local cycles, say in the Coal Measures, are probably the result of local subsidence in on-delta areas and the region never became truly marine; the area was never exposed sufficiently to allow strong stirring and reworking of previous deposits. But it is also true that in those cases where true marine conditions penetrated at the beginning of the cycle, sandstones are still almost unheard of. Only in the exceptional cyclothems of the Wealden area (see pp.72-78) is there any sign of reworking of previous deposits as the transgression goes on. Does the absence of this sandstone mean that the delta model with sedimentation control must be abandoned? One would think not. 1s it possible that thelocal geography of the Mississippi area does not cover all conditions? It would be surprising if it did. An alternative geographical model and one which seems to be very likely in the context of northern Europe or America in Carboniferous times is of deltas being built out very rapidly on to a shallow, widespread platform rather than into a deep region like the Gulf of Mexico. Such conditions in Carboniferous times might cut down the generation of waves and inhibit reworking of previous deposits. Another factor might have been the very extensive deposits of peat which in themselves would resist reworking, and in any case could not provide material for a sand. Sedimentation may have exerted an indirect effect on cyclic development by way of compaction. It is possible that the variable accumulation of sand, shale and peat would lead to differential subsidence on compaction. If compaction were spasmodic, perhaps after a critical load of overburden were reached, this mechanism may have caused the development of some cycles.

244

GENERAL CONCLUSIONS

Flood-plain, fining-upwards sequences as recognised in the Alpine Molasse, the British Old Red Sandstone and deposits of other regions are a similar case to deltaic sequences in that experimental work and studies on recent rivers have led to an immense improvement in the interpretation of older sediments and the recognition of cycles (J. R. L. ALLEN, 1965a,b; VISHER, 1965). These cycles are also the result of the inherent instability of channel development in the flood plain, and a succession built up in this environment inevitably has a repetitive nature. In BEERBOWER’S (1964) nomenclature the cycles are due to an autocyclic mechanism inherent in the sedimentation regime. Deltaic and flood-plain cycles represent what ROBERTSON (1948) called distributive sequences, in which differentiation of materials took place during transportation. Supply of debris could be continuous or nearly so. Robertson also distinguished a group of sequences which involved successive deliveries of material. He supposed that differentiation of the debris took place during settling and graded beds resulted. Varved clays were given as one example of these “settlement-grading sequences”. If varved clays involve underflows as suggested in Chapter 3 then the name “settlement-grading” is not really appropriate. A more general term such as “distributive sequence, supply spasmodic” would cover deposits resulting from settlement as well as varved clays and turbidite sequences where material in discontinuous supply is transported by underflows. The sequence of structures within the sandstone units of flysch sediments appears to be entirely controlled by a sedimentary process. Apart from a number of workers who appeal to climatic and other factors, most would favour an origin involving turbidity currents with a varying influence from bottom currents (Chapter 9). A feed-back mechanism to give rise to repeated sandstones in this case is not so clear as in the delta model and there are a number of ways in which turbidity currents might be triggered OR.Supposing a fairly constant accumulation of material inshore then a certain periodicity is possible in the turbidity currents because they are most likely to be generated after the accumulation of a considerable amount of debris. Earthquakes may be the most likely cause of the currents but they will obviously be much more successful in originating turbidity currents from an unstable mass of sediment. Where submarine canyons are situated off river mouths there is also the possibility of currents originating from flood debris. In this instance the ultimate control is one of climate, just as in glacial varves when the coarse layers are derived from summer meltwaters, some of them probably also from underflowing turbidity currents.

TECTONIC CONTROL

Even when sedimentary factors have been considered, some workers still think they are inadequate. The complicated nature of certain cyclothems, the widespread nature of certain horizons (mainly limestone, sometimes coals, occasionally marine shales) and the apparent sudden incoming of a marine phase into, for example, the Yoredale rhythm of the north of England are appealed to in an attempt to show the inadequacy

EUSTATIC CONTROL

245

of sedimentarycontrols. We have shown in the case of many Carboniferous cycles that there is doubt concerning the nature and constancy of many cyclothems and that for most occurrences cutting off of the supply of debris allied with continuous subsidence would be enough to cause a transgression. Rising base-level means aggradation in upstream portions of rivers and inevitably the transgressive portion of the cycle reverts suddenly to marine facies. Suggestionsof variability of supply due to successive movements in the basin coupled very often with uplift in the source (WELLER, 1956; HUDSON,1924; etc., see Chapters 4 and 5) would appear to be due to inadequate appreciation of sedimentary distributive mechanisms. Sudden downdropping of the floor would provide an effective cause of cycle development and it is maintained with justificationby proponents of diastrophic controls that faulting tends to be intermittent. BOTT(1964) contended that erosion in source areas would lead to isostatic adjustments, made possible by flow of material in the mantle. Most of this flow, taking place locally, would cause subsidence in the basins around the source-land. Subsidence, according to Bott, would be controlled by pre-existing lines of weakness along which faultmovements would be spasmodic. Some of the arguments concerning diastrophic control of cycles arose before detailed sedimentological evidence was accumulated. But it would be foolish to deny that diastrophic movements were not important in sedimentation. They almost certainly control the overall environment of sedimentation and determine the general character of the sediment but it is difficult in the present state of knowledge to point to any cyclothem and say in this case there must have been diastrophic control. If it is a cycle of small extent then sedimentary controls may have been sufficient to cause its development. If it is very widespread, say, continent-wide, then questions of correlation arise and it may be suggested that further efforts at correlation could prove the event to be world wide and more likely to be due to a climatic or a eustatic mechanism.

EUSTATIC CONTROL

Eustatic changes can be induced either by alternating glaciations and deglaciations, during which sea water is locked up on land as ice and subsequently remelted, or by changes in the cubic capacity of the ocean basins as a result of vertical (epeirogenic) movements of sectors of the ocean floor. The former phenomenon has been termed glacio-eustasy, the latter tectono-eustasy (FAIRBRIDGE, 1961). Many years ago Suess proposed that positive movements of sea level could result from the displacement of sea water by sedimentation from the continents but it is now known that the rate of sedimentation over the ocean floor is inadequate to produce a significantdisplacement. The world-wide changes of sea level most familiar to geologists are glacioeustatic, and took place during the Pleistocene. It is generally believed, however, that the present world climate is abnormal, and that during most of well-recorded geological history polar ice caps did not exist or were negligible in size, since reliable evidence of extensive glaciation is confined to the Pleistocene, the Late Palaeozoic and, more

246

GENBRAL CONCLUSIONS

doubtfully, the late Precambrian. It follows that any important world-wide changes of sea level deducible from the stratigraphical record at other times must almost certainly owe their origin to oceanic epeirogeny (HALLAM,1963a; DOTT, 1964). Moreover, the role of glaciation and deglaciation in sea-level changes in, for instance, the Late Palaeozoic has not been evaluated and it cannot be assumed without question that such changes necessarily resulted from the operation of this factor alone. Though pioneered by such eminent geologists as E. Suess in Europe and T. C. Chamberlain in North America, GRABAU (1936) was perhaps the first to give a clear formulation of the relationship of epeirogeny and eustasy and its influence on the stratigraphical record in his so-called Pulsation Theory. He maintained that a series of major transgressions and regressions of the sea over extensive parts of the continents was the result of changes of sea level rather than orogenic or epeirogenic movements on the continents, as others had suggested. The thesis was illustrated by examples from the Palaeozoic, with one or two transgressions and regressions within each period. With regard to the nature of the epeirogenic movements responsible for changes of sea level, modern oceanographic research has indicated some possibilities. Much attention has been directed recently to the huge ridges or rises on the sea floor, such as the Mid-Atlantic Ridge, the East Pacific Rise and the Darwin Rise. The latter, in the West Pacific, is studded with flat-topped seamounts or guyots, and atolls, and it can be proved from shallow-waterfossils dredged from the tops of guyots or collected from borehole cores from the atolls that the Darwin Rise has been subsiding since about the mid Cretaceous (MENARD, 1964). Menard estimated that sinking of the rise must have resulted in a lowering of sea level of some 100 m in the last 100 million years, a rate of change of 0.1 cm/lO3 years. On the other hand Tertiary elevation of the East Pacific and other youthful oceanic rises must have raised sea level some 300 m, a rate of change of 0.3 cm/lO3 years. This rate of change is much slower than that due to the formation and melting of Pleistocene glaciers, which ranges between 102 and 103 cm/103 years. If there had been no other major epeirogenic movements affecting the ocean floor since the Cretaceous there would have been a net rise instead of a net lowering of sea level. There are a number of indications, however, that substantial areas along the continental margins have subsided during this time (HALLAM, 1963a). The wellknown deep sea trenches of the Pacific borders and Caribbean are Tertiary or Quaternary features, while extensive areas now occupied by the Mediterranean and Black Seas and the seas of Indonesia and Melanesia, to name just some, appear to have undergone some subsidence within the last 25 million years or so. Extrapolating backwards in time according to uniformitarian principles we can appreciate that at a given time a summation of the epeirogenic movements is far more likely to have resulted in a net positive or negative eustatic change than in a condition of exact balance. Hence a condition of stable sea level would be exceptional even if polar ice caps and substantial glaciers were missing from the earth’s surface. Turning to cyclic sedimentation, we cannot agree with the contention of WELLS(1960) that eustatic changes of sea level are the most important controlling

EUSTATIC CONTROL

247

mechanism, since we are satisfied that the great majority of so-called cycles are the result of processes intrinsic to sediment transport and deposition. Nevertheless eustatic control seems a plausible explanation for a minority of Late Palaeozoic cycles in transitional niarine-continental environments, characterised by laterally extensive marine limestones or shales. Whether the ultimate control in a given instance was climatic or tectonic appears impossible to determine. While glacio-eustatic transgressions and regressions are evidently far more rapid than tectono-eustatic ones, there is still more than enough time available for the deposition of major sedimentary cycles with significant clastic components. Reasons are given in Chapters 6 and 7 for believing that sea-level changes were responsible for the deposition of major epicontinental marine cycles in the Mesozoic and Tertiary of Europe and North America. Eustasy provides one of the most plausible mechanisms to account for major evaporite cycles and might even have had an influence on certain non-marine clastic cycles. In none of the instances quoted need the sea level have altered by more than a few metres, possibly, in extreme cases, a few tens of metres. One can only speculate about the actual amount of change in given instances. Let us take 15 m as a reasonable figure for the rise of sea level in the Early Toarcian (see Chapter 6). Taking 20 million years as a round figure for the duration of the Lias gives 1 million years as the average duration of an ammonite zone. A 15 m eustatic rise during the three Lower Toarcian zones is equivalent to a rate of change of 0.5 cm/lO3 years. Making due allowance for the assumptions involved in this calculation, itisgratifyingto find that this figure is of the same order of magnitude as those calculated by Menard. The types of sedimentary cycle that are being discussed are presumably the consequence of comparatively minor eustatic oscillations superimposed on the more familiar rises and falls of sea level which caused the major transgressions and regressions known from the stratigraphical record. As a result it will only rarely be possible to seek the best kind of independent supporting evidence for eustatic control, namely intercontinental correlation by means of fossils. Nevertheless there are a number of distinguishing characteristics which can be used to establish a fair degree of probability: ( I ) Widespread lithological horizons developed more or less constantly over an area measurable in thousands of square miles (correlation being based either on distinctive fossil content or on continuity of exposures). (2) Facies indicative of deposition in a shallow-water stable tectonic regime, e.g., limestones, shales, subordinate well-sorted and relatively pure sandstones. (3) Lithological changes up the stratal succession suggestive of deepening sea correlating with marine transgressions elsewhere and vice versa. ( 4 ) Independence of such changes from local epeirogenicmovements (producing sedimentary basins and intervening swells) as indicated by regional variations in rock thickness and facies. An attractive feature of the eustatic control hypothesis is that it provides a

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satisfying explanation of the so-called asymmetry of many cycles of different types without having to invoke intermittent subsidence. This “asymmetry” has the common characteristic that the transition to the presumed deeper water or more off-shore marine facies is relatively sharp, and such evidently transgressive deposits frequently bear evidence of relatively slow deposition. As discussed in Chapter 6, positive and negative movements of sea level of similar speeds must act in such a way that the transgressive parts of eustatically-controlled sedimentary cycles will normally be more condensed than the regressive parts. It should of course be borne in mind that regional epeirogenic movements on the continents may complicate matters. Thus an epeirogenic rise of a continental sector will act in opposition to a rise of sea level and may even induce a regression locally. The deposits resulting from a rise of sea level are indeed bound to be diachronous to some degree unless the land being transgressed is completely flat! There is the additional complication that geodetic changes involving migration of the poles may induce movements of sea level over fairly extensive sectors of the earth’s surface (FAIRBRIDGE, 1961). For such reasons as these some uncertainty is likely to remain even when data are abundant.

CLIMATIC CONTROL

In a recent presidential address to the Geological Society of London, HOLLINGWORTH (1962) suggested that climatic variations were the primary cause of many widely differering types of sedimentary cycle. He included among the causal factors the oscillations of sea level due to the advance and retreat of ice caps during alternating glacial and interglacial periods. This process, in which climatic fluctuations only indirectly affect sedimentation, is more appropriately dealt with in the section on eustatic control. We shall confine our attention here to the direct influence that variations in temperature and precipitation may have on sedimentation. Climatic variations are most evident in the different seasons of the year, and most obviously revealed in lacustrine and marine varve deposits, consisting of alternating layers of different grain size, of organic and mineral matter and of different types of evaporite. There can be little dispute about such annual cycles, even though complications of interpretation exist, as discussed in Chapters 3, 7 and 8. Longer-period astronomical cycles have also been invoked, however, and about these there is considerable uncertainty, both about the response of sedimentation to climate and the response of climate to astronomical cycles. The difficulties and disagreements that currently inhibit understanding are well brought out in a recent symposium on climatic change, published in the Annals of the New York Academy of Sciences in 1961. In the present context, the most familiar astronomical cycle after the annual is the so-called 11-year sunspot cycle. Meteorologists cannot agree even about this. Thus BRYSONand DUTTON(1961) undertook a power-spectrum analysis of yearly

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sunspot numbers from 1760 to the present day, which clearly revealed to them an ll-year cycle. On the other ‘nandWILLIAMS (1961) has pointed out that the so-called 11-year cycle may vary in length from 8 to 16 years, so that the 11-year period may be a mathematical abstraction. One analysis revealed a 22, not an ll-year cycle. It appears that the same data may give different results according to the technique used! Sunspot cycles of longer period have also been claimed. WILLETT(1961) has suggested that an 89-year cycle is probably related to significant world-wide climatic changes. Owing to the variable gravitational effects of themoon, sunandplanets, ( I ) the position of the equinoxes and solsticeswith respect to the perihelion, (2) the obliquity of the earth’s ecliptic, and (3) the eccentricity of the earth’s orbit, vary with average periodicities of approximately 21,000,41,000and 97,000 years respectively. The earth‘s climate has supposedly been influenced by these major cycles, which have frequently been invoked by students of cyclic sedimentation (see for instance GILBERT,1895; WZNKLER, 1926; BRADLEY, 1929; SANDER, 1936; VANHOUTEN,1962). It is still not clear just how fluctuations in solar energy affect the earth’s climate. Variations in ultraviolet radiation intensity appear to affect ozone concentration in the stratosphere and this in turn affects temperature at the earth’s surface (PLASS, 1961). Whereas increase in the ozone mixing ratio probably results in increased surface temperatures combined with reduced precipitation, increase in the solar constant or in the atmospheric carbon dioxide mixing ratio probably leads to increased temperatures combined with increased precipitation (KRAUS,1961). Climate need not vary, furthermore, in a uniform manner over the whole earth. Any factor which causes increased surface temperature probably also causes increased evaporation over the oceans and hence increased cloudiness. In low latitudes and warmer seasons this increased cloudiness would tend to offset the original temperature increase. In high latitudes and colder seasons increased cloudiness would, in contrast, tend to amplify the temperature increase. Precipitation might also be expected to vary regionally. It appears that it is not yet possible to relate sunspot cycles in a simple way to climatic changes expressible in terms of temperature and precipitation. The difficulties are compounded when the influence of climate on sedimentation is considered. A few examples will illustrate some of the complexities involved. LANGBEIN and SCHUMM (1958) made a study of sediment yield in a drainage basin in relation to mean annual precipitation. They found no simple relationship, with sediment yield increasing steadily in proportion to precipitation. The yield reached a maximum at 10-14 inches precipitation/year and decreased sharply both in more arid conditions (because of decreased runom and more humid conditions (because of the increased density of vegetation cover). GARNER (1959) studied the influence of climate on alluviation in a part of the Andes. Changes in precipitation evidently played an important but not readily predictable role. In arid conditions weathering is predominantly mechanical and results in the production of coarse residuals which are not readily transported effectively to base level. The sediments deposited during such climatic phases are characteristically aeolian silts and sands. In humid conditions

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GENERAL CONCLUSIONS

chemical weathering is of major importance. Residuals are gradually removed, together with soluble mineral matter, and clay and sand transported by rivers, to be deposited at base level. Garner proposed a “principle of sedimentary lag”, that is, sedimentary products of the arid phase are only removed to base level during the succeeding humid phase. The relationship of rainfall precipitation to sedimentation may sometimes, however, be relatively straightforward, as revealed for instance in Searles Lake, California, by FLINT and GALE(1958). These workers were able to demonstrate that evaporites were deposited during an arid phase, and clays during a pluvial phase of the Pleistocene. A major objection to invoking astronomically-controlled climatic cycles (other than the annual) to account for cyclic sedimentation in the past is that it involves extrapolating from a limited and not well understood body of data extending back no more than a few centuries. It should surprise no one that BRYSON and DUTTON (1961), using the same power spectrum technique which so clearly revealed an 11-year sunspot cycle, had no comparable success with a glacial varve sequence from Sweden. The results from this sequence resembled those obtainable from a series of random numbers! We are provoked to comment that even if they had recognised significant periodicities their interpretation would remain equivocal. For the reasons discussed above, attempts to relate sedimentary cycles to longterm climatic or astronomical cycles are bound to remain highly speculative. Without ruling out completely the operation of such long-term cycles we wish to stress our belief that there are adequate mechanisms to account for most types of cyclic sedimentation, which have nothing to do with the direct influence of climate. Those types of cycle, such as minor alternations of clay and fine-grained limestone, that seem less readily explicable by these mechanisms, discussed elsewhere in this chapter, may well have a diagenetic origin (see Chapter 6).

CYCLES AND TIME

Various figures have been given for the time involved in the formation of cycles (apart from varves). VANLECKWIJCK (1949), for example, took 50,000 years as a likely length for the formation of one cycle in the Belgian Carboniferous; JESSEN (1961) suggested 20,000-30,000 years for a cyclothem in the Carboniferous in Germany, WESTOLL (1962) 200,000 years for a model Yoredale cycle and MERRIAM (1963) 350,000 years for a Kansan cyclothem. In another facies, KUENEN (1953) suggested intervals varying between 1,000years and 100,000years between successiveturbidites in Lower Palaeozoic rocks in Britain. These estimates provoke the reflection that such exercises may be unprofitable. Besides the very real difficulties involved in absolute dating and correlation there remains the fundamental objection that regular periodicity of the controlling mechanism is assumed. Even when this is the case there is no guarantee that such regularity will be expressed in the sedimentation.

CONCLUDING REMARKS

25 1

CONCLUDING REMARKS

We have felt obliged in these concluding remarks to strike a cautionary note with regard to certain aspects of cyclic sedimentation. But it is our belief that the search for and the discussion of cyclic sedimentation has an important role to play in understanding geological successions. There has been recently an enormous interest and effort directed to the detailed analysis and interpretation of individual lithologies. Whatever one’s ultimate conclusions about the regularity or even the reality of sedimentary cycles, their study, perforce, directs attention to the relatively neglected subject of stratigraphic relationships, that is, the distribution of facies through space and time. This, in effect the study of changing environments, is the very essence of geology.

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REFERENCES INDEX

ADAMS, J. E., 207, 210, 211 ALDERMAN, A. R., 65 ALDINGER, H., 174 ALLEN,A. D., 40 ALLEN,J. R. L., 2, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 35, 41, 126, 224, 244 ALLEN,P., 2, 77, 73, 75, 77, 78, 79 ANDERSON, J. A. R., 155, 156 ANDERSON, R. Y.,19, 60, 61, 63, 66, 67, 195, 196, 197,209, 213,214 ANDERSON, T. B., 229 M. L., 108 ANDRESEN, ANDRICHUK, J. M., 201,202 ANTEVS, E., 50, 51, 53, 54, 55, 56, 57, 58, 59, 60 E. S. A., 47 ANTROBUS, ARCHANGELSKY, A. D., 194 W. J., 117, 183, 184,240 ARKELL, N. A., 199, 207 ARKHANGEL'SKAYA, ARKLE,T., 102 G., 170 ARRHENIUS, ASHLEY,G. H., 100 BAILLIE,A. D., 201 BAINS,G. W., 44 BALLANCE, P. F., 218 BARKELL, C. A., 204,205 P. J., 40, 41 BARRETT, BARROIS, C., 142 BASSETT, D. A., 219 BEERBOWER, .I. R., 2, 7, 8, 101, 102, 107, 109, 113, 115,242, 244 BERSIER, A., 33, 34, 142, 150 BISSAT,W. S., 150 BLUCK,B. J., 141, 149 BOGER,H., 234 BOND,G., 39, 40 BOOKER, F. W., 42 BORCHERS, R., 44 H., 199, 205, 207,209, 211,212 BORCHERT, M., 186 BORNHAUSER, BOTT,M. H. P., 150, 152,245 BRANSON, C. C., 87, 92,94, 97, 100, 101, 102 BRIGGS,G., 221 BOUMA. A. H.. 216.217. 219.220 Bomoz, A., 142 _

_

I

BRADLEY, W. H., 61, 62, 63, 64,65, 66, 67, 68, 192.249 B ~ S C HO., , 199,211,213 BRAMLETTE, N., 194 BRINKMANN, R., 169, 170,234,235 BROUGH, J., 117, 119, 120, 150 W. D., 178, 180, 181 BRUCKNER, R. A., 61,248,250 BRYSON, C. D., 49 CALDENIUS, CALVER, M. A., 236,237 S. E., 194 CALVERT, CAROZZI, A., 178, 180 0. P., 38 CASEY, CLARKE, A. M., 134, 149 CLINE,L. M., 221 COLBERT, E. H., 102, 111 W. H., 49 COLLINS, CONOLLY, J. R., 31 M. J., 104 COPELAND, CRAIG,G. Y.,226 CRAMPTON,C. B., 61, 69, 70, 71 CROSS, A. T., 102 CROUZEL, F., 34 CUMMINS, W. A., 35

DAWSON, J. W., 4 DEAN,J. M. A., 131, 151 DEEVEY, E. S., 192 K. S., 163 DEFFEYES, DE GEER,G., 49, 50, 54, 55, 57, 60 DE LA BECHE,H. T., 4, 150 DELEAU, P., 150 DELLWIG,L. F., 209,212 DELMER, A., 142, 150 DE RAAF,J. F. M., 33, 138, 139, 140, 141, 222 DE SWARDT, A. M. J., 38 DEWEY, J. F., 221, 226 DINELEY, D. L., 35 DIXON,E. E. L., 158, 159,237 DOEGLAS, D. J., 30 DOTTJR., R. H., 159, 160,246 DOTY,R. W., 105, 108 DUFF,P. McL. D., 7, 8, 9, 11,43, 134, 135, 139, 144, 149, 150, 151, 156,241 DUNHAM, K. C., 7, 117, 120, 121, 122, 150, 154

272 DLITTON, R. A., 61,248, 250 A. I., 200 DZENS-LITOVSKIY, S., 35, 59, 215, 217, 218, 219, 221, DZULYNSKI, 222, 223,225,226 EDEN,R. A., 7, 53, 132, 144 EDEN,W. J., 55 W., 7, 133,233 EDWARDS, ELIAS,M. K., 97, 98, 111, 235, 236 R. E., 149 ELLIOTT, K. O., 194 EMERY, W. B., 135, 136, 137, 138, 150, 154 EVANS, V. A., 126 EYLES, R. K., 21 FAHNESTOCK, R. W., 245, 248 FAIRBRIDGE, W. G., 2 FEARNSIDES, FEOFILOVA, h. P.,147, 148, 150 FERM,J. C., 81 FICHTER, H. J., 178, 179, 181, 238 FIEGE,K., 2, 3, 142, 143, 144, 145, 150, 162 A. G., 162, 188 FISCHER, W. L., 186 FISHER, FISH,H. N., 108, 141, 243 FLINT,R. F., 209, 250 FORSYTH, I., 129, 130 Fox, W. T., 16 E. H., 83, 118, 126, 127, 130, 151 FRANCIS, FRANCIS, W., 150 FRASER, H. J., 55 FREBOLD, H., 172,238 S. A., 108 FRIEDMAN, P. F., 33 FRIEND, FUCHTBAUER, H., 170 FULLER, J. G. C. M., 202 GALE,W. A., 209, 250 GARNER, H. J., 249 P., 187 GEIJER, GEIKIE,A., 150 T. N., 117 GEORGE, W., 133 GIBSON, GILBERT, G. K., 165, 249 GLASS, H. D., 81, 107, 108 GOLDSCHMIDT, H., 170 GOODCHILD, J. G., 150 GOODLET, G. A., 7, 114, 128, 130, 150, 152, 154 A. W., 246 GRABAU, GRAY,H. H., 87, 100, 111 GREENSMITH, J. T., 128, 131, 132, 149, 150 K. H., 142 GRIBNITZ, GRINDLEY, G. W., 40, 41, 113 V. N., 199, 207 GRIGOR’YEV, HAITES,T. B., 144, 154 A., 2,20, 163, 164, 165, 166, 167, 172, HALLAM, 173, 174, 176, 177, 180, 184, 187, 191, 194,

REFERENCE INDEX

207, 235, 238, 239, 240, 246 HAM,W. E., 202, 203 HAMILTON, D., 185 HANSEN, S., 50, 51, 52, 53, 54 HARBAUGH, J. W., 16, 17 R. B., 44 HARGRAVES, HARMS, J. C., 21 HARRINGTON, H. J., 40 D. E., 97 HATTIN, U., 142, 150 HAVLENA, F., 172 HEIDORN, HEMINGWAY, J. E., 149, 177, 189, 190 HENDERSON, J. H., 16 HERMAN, G., 204, 205 HIND,W., 4, 117, 150 S. E., 150, 248 HOLLINGWORTH, HOLLMAN, R., 173 A., 54 HOLMES, HOPKINS, M. E., 108 HORNER, N. G., 50, 51, 53 55, 59 HOW, W. B., 92 Hsu, K. J., 226 3. F., 105, 108,220, 221,227 HUBERT, R. G. S., 117, 120, 121, 150, 245 HUDSON, HULSEMAN, J., 194 HULTDE. GEER,E., 53 ILLINC,L. V., 163 IMBRIE, J., 18, 96, 111, 235, 236 JAANUSSON, V., 173 JABLOKOV, V. S., 3, 146, 148, 150 T. h.,61 JACKSON, K., 38 JACOB, JAMES, H. L., 187, 189 JERZMANSKA, A., 222 W., 5, 145, 146, 150, 154, 233, 234, 250 JESSEN, JEWETT,J. M., 97 JOHNSON, G. A. L., 117, 119, 120, 122, 123, 124, 149, 150 W. A., 53 JOHNSTON, JONES, 0. A., 187

KAY,M., 102, 11 1 KELLAWAY, G. A., 177 KELLING, G., 141, 149, 224, 226, 227, 229 M. G., 2, 13 KENDALL, KING,P. B., 207 J. T., 222 KINGMA, KIRKLAND, D. W., 61, 63, 195, 209, 214 KLEIN,G. DE V., 221, 227 KLUPFEL,W., 171, 172, 173, 176, 178, 181, 238 L. H., 19, 60, 61, 67, 196, 197, 213 KOOPMANS, V. V., 146, 147, 149, 150, 154 KOPERINA, F. P. H. W., 153, 154 KOPSTEIN, K., 142, 150 KOREJWO, KORN,H., 194, 196

REFERENCE INDEX

KOSANKE, R. M., 85, 87, 97, 105, 108, 109 KRAUS,E. B., 249 KREJCI-GRAF, K., 189 M. S., 38 KRISHNAN, G. A., 207 KROENLEIN, W. C., 18, 148,225 KRUMBEIN, KSIAZKIEWICZ, M., 216, 221, 223, 228, 229 KUENEN, P. H., 35, 55, 57, 58, 59, 171, 221, 222, 223,224, 225,250 KUHN,R., 211 LAGAAY, R., 153, 154 LANDSBERG, H. E., 61 LANG,W. B., 209 W. B., 115, 249 LANGBEIN, LAPORTE, L. F., 18, 96,235,236 LIEBENBERG, W. R., 44 LOMBARD, A., 142, 168 L~TZE, F., 199,205, 207,211,212,213 LOWMAN, S. W., 186 MACGREGOR, M., 126, 128, 129 C., 4, 150 MACLAREN, A., 150 MAKOWSKI, MANGIN,J. P., 35,222 MARSCHALKO, R., 226 MARTIN,H., 194, 197 MAUBEUGE, P. L., 191 MCBRIDE,E. F., 221 MCCRONE, A. W.. 111. 236 M C G ~ G O R , v. R., 33,’ 40 MCKEE,E. D., 204 MCKENZIE, P. J., 42 MEHTA,D. R. S., 38 H. W., 223, 246,247 MENARD, D. F., 3, 12, 92,94,97, 152, 250 MERRIAM, C. I., 59, 222, 223 MIGLIORINI, MILLER,H., 117, 150 MILNE,D., 4. 150 114, 117, 121, 122, 124, 125, 149, MOORE,d., 150, 152 MOORE,H. R., 192 MOORE,R. C., 3, 1 1 , 12, 82, 88, 89, 90, 91, 92, 93, 94, 95, 97, 107, 109 Moss, A. J., 224 MUDJE,M. R., 108 MUIR,R. O., 63, 199, 205, 207, 209, 211, 212 MURPHY,M. A,, 227 MURRAY, TI. H., 113, 130 NATLAND, N. L., 221 H. M., 13,229 NEDERLOF, NEEF,G., 229,230, 231 NEWELL, N. D., 240 NIPKOW,H. F., 65, 66, 194 NIYOGI,D., 38

273 NOWAK,J., 228 OULIANOFF, N., 222 PANIN, N., 35, 38 PAW, T. G., 216 J., 159, 171 PATTERSON, PEACH,B. N., 2, 4, 5, 6 PEARN,W. C., 11, 12,241 PERFILIEV, B. V., 65 PEPPER, J. F., 114 PETTIJOHN,F. J., 49 J., 4, 117, 150 PHILLIPS, PIRLET,H., 158, 159 PLASS,G. N., 249 P. E., 20, 81, 82, 87, 107, 108 POTTER, J. E., 139, 226 PRENTICE, F. W., 16, 17 PRESTON, PRWOST,P., 142, 150, 154 RADOMSKI, A,, 221 RAMSBOTTOM, W. H. C., 152,233 RAO, P. V., 38 RATTIGAN, J. H., 36, 37, 38 RAYNER, D. H., 61, 62, 65 READ,W. A., 129, 130, 131, 151, 152 REGER,D. B., 99 REID,I. W., 150, 191 REINECKE, L., 44,47 RICHARDS, H. G., 204 RICHEY, J. E., 35, 128 RICHTER-BERNBURG, G., 210, 211, 212, 213 T., 2, 7, 126, 129, 130, 133, 149, ROBERTSON, 150, 154,242,244 ROGERS,M. J., 104 RUBEY,W. W., 195 G. A., 108 RUSNAK, RUTTEN,M. G., 142, 150

SABINS, F. F. J., 103 SACKIN,M. J., 12 T., 3, 189 SAKAMOTO, SANDER, B., 2, 161, 249 SARIN, D. D., 162, 163 SAURAMO, M., 50, 51, 55, 60 H., 216 SCHAUB, SCHEMEL, M. P., 102 S. O., 227 SCHLANGER, SCHLEIGER, N. W., 228,229 SCHMIDT,c.,142, 150 SCHUMM, S. A., 115,249 SCHWARZACHER, W., 2, 14, 15, 150, 160, 161, 168,241 SCOTT,K. M., 221,227 SCRUTON,P. C., 199,208,243 J. D., 103 SEARS, SEIBOLD, E., 167, 168, 169, 192, 196

274 SEIBOLD, I., 167 SEILACHER, A., 221 SHARPE,J. W. N., 44, 45, 46, 47 SHCHERBMA, V. N., 209 SHEARMAN, 0. J., 204 SHELDON, R. P., 191, 192, 193 SHEPARD, F. P., 6, 7, 87, 112 SHIELLS,K. A. G., 122, 152 SHINN,E. A., 163 SHOSTAKOVICH, V. B., 196 SHROCK,R. R., 84 SIEVER,R., 107, 108, 109 SIGNORINI, R., 2, 16 SIMOENS,G., 150 SIMPSON,G. C., 113 SKINNER, H. C . W., 65 SLACZKA, A., 218, 226 SLOSS, L. L., 1 SMITH, A. H. V., 155 SMITH,A. J., 59, 215, 218, 229 SNEATH,P. H. A., 12 SOLL,H., 172 SPRENG,A. C., 168 STAMP,L. D., 185, 186 STEARNS, R. G., 107 STEWART,F. H., 199, 205, 208, 209, 210, 211 STILLE,H., 242 STOUT, w. E., 97, 99, 111 STRAHAN, A., 132 STUBBLEFIELD, C. J., 7, 133, 233 STURGEON, M. T., 100, 101, 102, 109 SUESS, E., 242, 246 SUGDEN,W., 200 SUJKOWSKI, 2. L.,165,229 SWANN,D. H., 81, 87, 107, 109, 113, 114, 115 TAVENER-SMITH, P., 39 TEICHM~LLER, M., 154 TEICHM~LLER, R., 154 THEOBALD, N., 190 THIADENS,A. A., 144, 150, 154 TONKS, L. H., 133, 150 TROTTER, F. M., 150 TRUEMAN, A. E., 5, 81, 133, 136, 150, 154 TRWR A. A.,44 TULLOCH, W., 128 TWENHOFEL, W. H., 1 UDDEN,J. A., 4, 82, 83, 84, 111

REFERENCE INDEX

UNRUG, R., 218,219,226 VANDER HEIDE,S., 144, 150, 154 VANHOUTEN, F. B., 61, 67, 69, 79, 80, 249 VAN LECKWIJCK,W., 6, 142, 143, 150, 233, 250 VASIL’YEV,G. V., 200 VASSOEVIC, N. B., 216 VAUGHAN, A., 158,237 VEEVERS, J. J., 42, 43 VELLA,P., 185, 235, 237, 242 VISHER,G. S., 31, 244 VISSER,D. J. L., 44 VISTELIUS, A. D., 13, 241 VON BUBNOFF, s., 1 , 2 WALKER,R. G., 217,223, 224 WALTON,E. K., 7, 8, 9, 11, 35, 43, 59, 63, 134, 135, 144, 149, 150, 151, 156, 208, 217, 219, 221,222,223,224,226,241 WALTON,H. S., 128 WANLESS,H. R., 2, 6, 7, 18, 82, 83, 84, 87, 94, 95, 96, 37, 105, 106, 107, 108, 110, 111, 112, 113, 159, 170 WATERSTON, C. D., 70 WELCH,F. B. A., 177 WELLER,J. M., 1, 2, 3, 6, 7, 81, 83, 84, 85, 86, 87, 88,95,96, 105, 107, 109, 111, 112, 145, 245 WELLS,A. J., 7, 8, 150, 154, 246 WELSH, W., 178,229 WESTOLL, T. S., 117, 126, 149, 150 WHEELER, H. E., 113, 130 WIEBOLS, J. H., 441 WIEGERT,R., 196 WILLETT,H. C., 249 WILLIAMS,D., 249 WILLS, L. J., 117 WILSON,C. W., 107 WINKLER,A., 249 WOOD,A., 218,229 WOODLAND, A. W., 83, 135, 136, 137, 138, 150, 154 WRIGHT,W. B., 1, 132 WYLIEJR., C. R., 17 YOUNG,R. G., 3, 102, 103 ZELLER,E. J., 8 ZEMAN,J., 142, 150 ZLEGLER, B., 165, 168 ZHEMCHUZHNIKOV, Yu, A., 18, 147, 148, 150

SUBJECT INDEX

Abergavenny, 26 Aberystwyth Grits, 218, 229 Achanarras Limestone, 62, 63, 70 Adriatic Sea, 194 Africa, 38 Alabama, 186 Alberta, 168, 196 Albian, 217, 228 Alfredton, 230, 231 Algae, 88, 90, 91, 93, 97, 120, 121, 161, 162, 235,236 Allegheny Group, 98,99, 100, 102 Allerrad, 54 Alpes Maritimes, 219 Alps, 215, 222 Alsace, 212 Alum Bay, 186 Ammonites, 164, 170, 171, 172, 173, 178, 238 Ammonitico Rosso Superiore, 173 Amplitude spectrum, 19 Andes, 249 Anglo-Franco-Belgian Basin, 185 Antarctica, 33, 38, 40 Appalachians, 32, 215,221 Appalachian Basin, 81, 82, 97, 100, 101, 102 Aptian, 217 Aquitanian, 31, 32, 34 Arabia, 212 Archaean, 187 Arizona, 31, 82, 204 Arkansas, 221 Ashdown Pebble Bed, 74 Ashdown Sandstone, 74 Atlantic Ocean, 170 Australia, 38, 42, 65, 191 Austria, 161 Bahamas, 159, 161, 163, 181 Bajocian, 178 Balickera, 36 Ballagan Beds, 126 Barakar Series, 38 Barry, 175 Bashkirian, 83, 146 Basin and Range province, 47 Bathonian, 280

Bavaria, 35 Beacon System, 40, 41 Beardmore, 33 Beattie Limestone, 235, 236 Beaufort Series, 39 Belgium, 6, 142, 143, 158, 233, 250 Benbulbin Shale, 14, 16 Berea Sandstone, 114 Bessvatn, 56 Big Blue cycle, 97, 98 Bivalves, 93, 97, 98, 128, 132, 133, 134, 161, 164, 165,169,170, 171,172,233,234,236,237 Black Hills, 194 Black Metals, 131 Black Sea, 190, 194,246 Blaine Formation, 202 Blue Lias, 163, 165, 166, 167, 168, 172, 180, 194 Bogota-type cycles, 86, 95 Borneo, 155, 156 Brachiopods, 98, 118, 121, 128, 129, 172, 233, 234,236 Brazil, 227 Breconian, 23, 27 Brereton cyclothem, 106 Brownstones, 32 Brunswick Formation, 79, 80 Burindi facies, 36 Caithness Flagstones, 69 California, 192, 194, 209, 216, 250 Callovian, 169, 240 Cambrian, 199, 219 Campanian, 219 Canada, 53, 57, 60, 61, 104 Cape Hatteras, 181 Cape Surprise Coal Measures, 41 Carboniferous, 2, 3,4, 15, 36, 38, 39,41,42,60, 63, 158, 160, 168, 215, 229, 234,237, 241, 242, 243,250 - cycles, 81-102, 104-156, 158-161 - Limestone, 237 - stratigraphic classification, Europe, 83, 118 _ _ - , U.S.A., 83, 89 Cardiff, 175 Caribbean Sea, 163,246 Carmel Formation, 204

276 Carpathians, 35, 215, 217, 218, 219, 220, 221, 222,226, 229 Caspian Sea, 200 Castile Formation, 207, 209, 21 1 Catskill, 30, 32 Caucasus, 216 Cementstone Group, 126 Cenomanian, 178 Cerro Torro Formation, 221 Chase Group, 97 Cherokee Group, 92 Chert banding, 187, 189 Chesterian Series, 87, 113 Chile, 221 Cieszen Limestones, 220, 228 Clay Cross Marine Band, 236, 237 Clee Hills, 32 Cleveland Ironstone, 189 Climatic control, 67, 68, 69, 79, 80, 112, 113, 150, 161, 168, 180, 187, 192, 194, 195, 196, 209,211, 212,248 Clyde, 192 Coal Measures, 5, 9, 11, 33, 127, 129, 133-138, 141, 142, 146, 147, 149, 150, 154, 155 Colorado, 72, 103 Compaction, 150, 154, 155 Composite sequence, 8, 134,220 Conemaugh Group, 98, 99, 100, 102 Corals, 118, 121, 172 Correlogram, 14, 15, 16 Cotham Beds, 185 Cottonwood Limestone, 235, 236 Cretaceous, 38, 72, 103, 170, 178, 179, 194, 216, 220,221,238,240, 246 Crimea, 65 Cross association, 12 Croweburg-Verdigris cyclothem, 94 Cumberland Group, 104 Cycles, duration, 250 -, ideal (model), 8, 217, 221, 241 --,modal, 7, 134, 216, 220, 224, 225 -, nomenclature, 3 Cyclothems, 2 -, splitting, 111, 113 Dachstein Limestone, 161 Dakota Sandstone, 72 Dalwood Group, 42 Damuda System, 38 Dartry Limestone, 14, 16 Darwin Rise, 246 Deep River Basin, 30 Delaware Basin, 199, 207, 209 Denmark, 50, 54 Derbyshire, 233 Devon, 166,226 Devonian, 30, 31, 32, 192, 196, 197, 201, 202,

SUBJECT INDEX

209, 21 5, 228, 229 Deutenhauser Schichten, 35 Diagenetic segregation, 165, 189 Diastrophic-control theory, 107, 111 Dittonian, 24, 25, 26, 32 Dogger, 172, 178 Domerian, 176, 177, 189, 191, 238, 239 Donetz Basin, 146 Dorset, 166, 175 Downtonian, 22, 32 Dryas, 54 Dunkard Group, 101, 102, 107, 109, 114, 115 Dwyka Series, 39, 40 Eastern Interior Basin, 81, 82 East Indies, 216 East Pacific Rise, 246 East Pennine Coalfield, 9, 11, 133, 134, 135 Ecca Series, 39 Echinoderms, 178 Edaphic factors, 155, 156 England,4,9, ll,21, 72, 132, 166, 174, 178, 185, 234, 244 Eocene, 65, 67, 185, 186, 187 Esteriids, 69 Europe, 5, 20, 49, 83, 176, 177, 187, 215, 243 Eurypterids, 64 Eustatic control, 20, 115, 131, 139, 156, 162, 167, 171, 176, 177, 178, 181, 185, 187, 202, 204,208,245-247 Extra terrestrial control, 145, 146 Fairlight Clay, 74 Falla Formation, 33 Field Beds, 70, 71 Fife, 151 Fining-upwards units, 30, 244 Finland, 50, 54 Finscvatn, 56 Fish, 69, 72, 128, 129, 133, 164, 234 Flakevatn, 56 Flood-plain sequences, 244 Florida, 161, 181 Flysch, 35, 216,222 Foraminifera, 88, 90, 91, 93, 98, 167, 170, 180, 221, 235 Fordon, 205, 209,210 Forest Sandstone, 40 Fourier Series, 16, 17 France, 171, 174 Gastropods, 72, 233 Genesee Shale. 192 Germany, 5, 142, 154, 162, 167, 168, 172, 174, 213. 250 Gilmore Volcanic Group, 36 Gjende, 56

277

SUBJECT INDEX

Glacial-control theory, 112, 113 Glamorgan, 166, 175, 237 Glencar Limestone, 16 Gloucestershire, 24, 27, 32 Gondwana beds, 38,40 Gondwanaland, 38, 41, 112 Goniatites, 118, 132, 133, 233, 234 Gower Peninsula, 158, 237 Grahamstown Lake Formation, 36 Great Britain, 5, 54, 174 Great Scar Limestone, 160, 161 Green River Formation, 61,63,64,65, 67,69,70 Greta Coal Measures, 42 Guadeloupian, 199 Gulf Coast, 65, 186, 188 Gulf of Mexico, 243 Gwembe Coal Formation, 39 Hampshire, 186 Hannibal Shale, 192 Harmonic analysis-see Time-series analysis Hartshorn Sandstone, 192 Hawick Rocks, 219 Hell’s Mouth Grits, 219 Helman Head Beds, 70, 71 Helvetic Zone, 178, 179, 180 Hemicyclothem, 111 , 145, 146 Hettangian, 239 Holdgate Sandstones, 32 Horsetails, 73, 74 Huronian, 187 Hypercyclothems, 3 Illinois, 4, 20, 82, 83, 87, 192, 242 -basin, 81, 82, 95, 96, 109 - cyclothem, 85 Illawarra Coal Measures, 43 India, 38 Indonesia, 246 Inferior Oolite, 177 Inoceramian Beds, 228 Ireland, 15, 168, 221, 226, 229 Lreton Shale, 196, 197 Isostatic control, 122, 126, 152 lstebna Beds, 219 Italia Road Formation, 36, 37 Jackfork Group, 221 Japan, 216 Jaslo Shale, 221 Jeppestown, 44 Jet Rock, 173, 189 Jharia Coalfield, 38 Joggings, Bank, 81, 104 John 0’Groats Sandstone, 70 Jurassic, 31, 39, 163, 167, 168, 170, 171, 173, 179, 183,204,209,214,234,240,242

Jutenheini, 56 Kagerod Formation, 31 Kansas, 3,8, 11,12,82,89,94,109,152,235,242 - City Group, 92 Karabogaz Gulf, 199,200, 208 Karaganda Basin, 148 Karroo System, 39 Kimberley, 44, 47 - Shale, 43 Kimmeridge Clay, 184 Kings Hill Group, 36 Krosno Beds, 218,226 Kuttung facies, 36 Lake Baldegg, 65 - Louise, 53, 57 - Superior, 18, 19, 187, 189 - Windermere, 59 - Zurich, 65 La Salle-type cycles, 86, 95 Lausanne, 34 Lavernock, 175 Lgota Beds, 217, 218, 219, 228 Lias, 20, 172, 175, 176, 177, 178, 189, 190, 239 Limestone Coal Group, 126,127,129,130,131, 151 Liverpool cyclothem, 87 Lockatong Formation, 61, 64, 67, 69, 79 Lofer facies, 161 London Clay, 185, 186 Lorraine, 171 Louisiana, 188 Lower Limestone Group, 126, 127, 129, 130 Ludlow, 22 Lydney, 24 Macoupin-type cycles, 86, 95 Macrocycles, 3 Madison Group, 202 Madumabisa Shales, 39, 40 Magnacycles, 3 Maitland Group, 42 Mancos Shale, 103 Manitoba, 200 Manitoba Group, 201 Mansfield Marine Band, 236, 237 Marmaton Group, 92 Marquette, 187, 189 Martinsburg Formation, 221 Mediterranean Sea, 246 Megacycles, 3 Megacyclothems, 72,89,95,96,97, 103, 152,153 Megarhythms, 228 Melanesia, 246 Mesozoic, 31, 38, 187, 238, 247 Michigan, 212

278 Michigan (continued)

- Basin, 82

-River, 113, 114 Mid-Atlantic Ridge, 246 Mid-Continent Basin, 81, 82, 88, 92 Midlands, English, 174, 178 Midlothian Coaliield, 128, 129, 130 Millstone Grit, 132, 233 Miocene, 154,217 Mississippi Delta, 125 Mississippi River, 181, 199, 222, 243 Mississippi State, 188 Mississippian,82,109,113,114,115,168,192,202 Mitcheldean, 27, 32 Modelo Formation, 192 Moenkopi Formation, 204 Molasse, 31, 32, 33, 35, 244 Monrnouthshire, 27 Monongahela Group, 98, 99, 100, 101, 102 Monterey Formation, 194 Morrison Formation, 32 Moscovian, 146 Moscow Basin, 148 Muschelkalk, 162 Nama Limestone, 194, 196 Namurian, 6, 117, 132, 142, 143, 144, 146, 233 Nebraska, 236 Neocomian, 78,228 Netherlands, The, 143, 144 Nevada, 159 Newcastle Coal Measures, 42, 43 New Jersey, 61, 64 New Mexico, 61,82,103, 195, 199,207,209,214 New South Wales, 36, 37, 42, 60 New York State, 192 New Zealand, 185, 216, 217, 222, 229,230,231 Nigeria, 38 North America, 49, 81, 82, 83 North Atlantic, 227 North Carolina, 31 North Dakota, 200 North Devon, 138-141 Northern Pennines, 119, 121 Northumberland, 119, 120 - Trough, 119, 122, 123, 124 Norway, 54, 56 Noss Beds, 72 Nottingham, 242 Nova Scotia, 81, 104 Ochoa Series, 207 Off-shore Barrier, 77, 78, 154 Ohio, 97-99, 100, 101, 102 Oil-shales, 63, 64, 67, 68 -- Group, 126, 127, 128, 131, 132 Oklahoma, 92, 94, 192, 202, 203, 221, 236

SUBJECT INDEX

Old Red Sandstone, 21, 30, 31, 32, 33, 34. 38, 41, 61, 62, 63, 64, 65, 69, 70, 244 Oligocene, 209,212,218 “Ontario River”, 114 Ordovician, 219,220,221,229 Ostracods, 64, 69, 131, 233, 234 Ouachita Mountains, 215, 221 Oxford Clay, 169, 184, 234 Pacific Ocean, 170 Palaeocene, 219 Palaeozoic, 3, 38, 173, 183, 187, 200, 208, 215, 229, 236, 245, 246, 247, 250 Paradox Basin, 204, 205 Paris Basin, 79 Pechora Basin, 147 Pefra Cava Sandstones, 219 Pennant Measures, 136-139 Pennines, 236 Pennsylvania, 61, 64 Pennsylvanian, 2, 3, 4, 11, 20, 82, 107, 108, 109, 159, 160, 170, 192, 204, 205, 235 Permian, 3, 36, 38, 39, 41, 42, 82, 97, 98, 101, 102, 107, 114, 115, 147, 202, 203, 204, 207, 208,209,210,213,216,235,240 Persian Gulf, 163, 200, 204 Peterborough, 169 Phosphoria Formation, 191, 193 Phyllocarids, 69 Peoria, 4, 84 Plant control, 130 Plateau Hill Ice Lakes, 50 Pleistocene, 19, 49, 53, 60, 185, 209, 235, 237, 245,246, 250 Pliensbachian, 238,239 Pliocene, 185, 229, 230, 231 Poland, 220, 221 Posidonienschiefer, 173 Post-glacial, 200 Pottsville Group, 98, 99, 100 Power spectrum, 17, 61, 196, 241, 248 Precambrian, 43, 61, 187, 191, 194, 246 Pripyat Salt Basin, 209 Purbeck Beds, 204 Pyrenees, 222

Queen Alexandra Range, 41 Radiocarbon dating, 53, 54 Radiolaria, 178 Recent, 60 RecBncavo Basin, 227 Rejang River, 155 Khaetian, 185 Rhine Valley, 209 Rhinns of Galloway, 229 RhBne Delta, 153, 154

SUBJECT INDEX

Rocky Mountains, 102 Rosedale, 189 Ruhr, 142, 145,233,234 Russia, 83, 146, 147, 197 Rustler Formation, 207 Sabhka, 204 Sakski Lake, 65 Salado Formation, 207 Sandstones Channel, 108 - Sheet, 108 San Juan Basin, 194 Santa Barbara Basin, 194 Sarawak, 155, 156 Saskatchewan, 200, 202 Saxony Lower, 174,205 Scanian moraine, 54 Scotland,4, 61,62,63, 64, 69, 118, 126-131,219, 229 Searles Lake, 209, 250 Sedimentary control, 20, 150, 155,242-243 - edaphic control, 155, 156, 243 Senonian, 228 Shackleton, 33 Shawnee Group, 88, 89,91 Sheffield, 132,242 Shropshire, 22,23,25, 32 Siberian Platform, 199 Sicily, 212 Silurian, 209, 212,218, 219, 228, 229 Sinemurian, 238, 239 Skye, 176 Somerset, 166 South Africa, 39, 43 South America, 38, 112 - Atlantic, 65 Southerndown, 175 - Rhodesia, 39 - Uplands, 219,220,221 Spain, 229 Spitsbergen, 30. 31, 32 Sponges, 178 Stanley Group, 221 St. David cyclothem, 106 Steep Rock Lake, 53, 60 Stirlingshire Coalfield, 129-131 Stormberg Series, 39 Summu cyclothem, 106 Sunspot cycles, 49,66,67, 196,212,213,248,249 Swabia, 176 Sweden, 31, 50, 54, 173,250 Switzerland, 31, 33, 65, 178, 179 Tasman geosyncline, 42 Tectonic control, 111, 122, 149, 150, 154, 158, 163, 167, 168, 170, 172, 176, 184, 189, 190, 192, 202, 207, 208, 229, 230, 244, 245

279 Temeside Shale, 32 Tennessee, 107 Tertiary, 186, 187, 212, 216, 222, 235, 240, 246, 247 Teschen Limestones, 220 Texas, 199, 207, 209 Thermocline, 57 Tidal laminae, 1 Time-series analysis, 13, 60 Toarcian, 176, 177, 189, 191, 238, 239, 247 Todilto Formation, 61, 63, 195, 214 Tomago Coal Measures, 42, 43 Top Marine Band, 236, 237 Tournaisian, 117-132, 146 Triassic, 30, 31, 33, 39, 161, 162,204, 240 Trucial Coast, 204 Tugford, 25, 32 United States, 1, 6, 47, 61, 81, 83, 170, 181, 186, 205,242,243 --,classification of Pennsylvanian, 83 Upper Limestone Group, 126-1 31 Uppsala, 59 Urals, 147 U.S.S.R., 83, 146, 147, 196, 209 Utah, 103, 204 Varves, 1, 8, 13, 16, 17, 18, 19, 36, 41, 51, 53, 55,248 -, evaporitic, 211, 212 -, glacial, 4 9 4 1 -, marine, 187, 192-196 -, non-glacial, lacustrine, 62-66 Verovice Shales, 228 Victoria, 228, 229 - Glacier, 53 Visean, 117-132, 146, 147, 158 Wabaunsee Group, 88, 89,91,92,94, 95 Wadden Sea, 108 Wadhurst Clay, 74 Wairarapa, 185 Waitemata Group, 217 Wales, 21, 136, 158, 166, 175, 218, 219, 229 Wallaringa Formation, 36, 38 Wankie Sandstone, 39,40 Wealden, 170,243 Weald Lake, 72 Welsh Borders, 32 Wenlock Rocks, 219 Wensleydale, 117 Western Interior Basin, 81, 82 Westphalian, 132, 136, 236 West Virginia, 82, 99, 102 Wetterstein Limestone, 161 Whitecliff Bay, 186 Whitehouse Formation 220,221,224

280 Williston Basin, 200, 201 Witwatersrand System, 43, 45, 46, 47 Wolfcampian, 97, 235 Wood Bay Series, 31, 33 Wreford Limestone, 97 Wyoming, 191, 193 Yampi Sound, 191

SUBJECT INDEX

Yoredale Beds, 4, 117-126, 152, 161, 183,244, 250 Yorkshire, 117, 118, 166, 175, 176, 189, 190,205, 209,210 Zechstein Basin, 208, 209 - evaporites, 205, 206, 209, 210, 211, 212, 213 - Sea, 199, 204