10 - Cyclic Sedimentation

8/12/2019 10 - Cyclic Sedimentation

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DEVELOPMENTS IN SEDIMENTOLOGY 10

CYCLIC SEDIMENTAT ION


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FURTHER

TITLES

IN THIS

SERIES

1 .

DELTAIC A N D SHALLOW MARINE DEPOSITS

L . M . J .

U .

VA N ST RAATE N, Editor

2 . G. C . AMSTU TZ, Editor

SEDIMENTOLOGY A N D ORE GENESIS

3.

TURBIDITES

A . H. B OU MA and A. BRO UW ER, Editors

4. F. G. TICKELL

THE TECHNIQUES OF SEDIMENTAR Y MINERALO GY

5 . J.

C.

INGL E Jr.

THE MOVEMENT OF BEACH SA ND

6.

THE IDENTIFICATION OF DETRITAL FELDSPARS

L . V A N D E R P L A S J r.

7.

SEDIMENTARY FEATURES

OF

FLYSCH AN D GREYWACKES

S . DzUEYI?SKY and E. K. W A L T ON

8.

DIAGENESIS IN SEDIMENTS

G . LARSE N and G . V . CHILINGAR, Editors

9.

CARBONATE ROCKS

G. V . CHILINGAR, H .

J .

BISSELL and R . W . FAIRB RIDG E, Editors


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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

ELSEVIER PUBLISHING COMPANY Amsterdam London New York

1967


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WITH 91

ILLUSTRATIONS AND 35

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PRINTED IN THE NETHERLANDS


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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-rep-

resented. We have, however, tried to cover as much of the relevant data as is neces-

sary 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 meas-

urement 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Merriam kindly allowed us to study the unpublished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 NTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . .

1

1

ycles. rhythms and cyclothems

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

Nomenclature

of

cycles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3

Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Classification and description . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Time series and harmonic analysis.

13

-

Scale; phase and facies.

18

CHAPTER 2. CYCLES IN FLUVIAL REGIMES

. . . . . . . . . . . . . . . . . . 21

Cycles in the Old Red Sandstone

of

Britain

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

21

Molasse of Switzerland . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

33

Fluvio-lacustrine coal-bearing sequences of Gondwanaland

. . . . . . . . . . . . . . . 38

Witwatersrand System

of

South Africa

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

43

Flysch facies in molasse. 35

CHAPTER

3

.CYCLES IN LACUSTRINE REGIMES . . . . . . . . . . . . . . . .

49

Introduction

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

49

Glacial varved clays

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

49

Non-glacial lakes

. . .

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

61

Periodicity. 53

-

Transportation and sedimentation. 55 ong-term variations.

60

Varves. 62.eriodicity. 64.unspot cycles. 66 arger cycles. 67

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

Nova Scotia. 104

Canada . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104

Theories

of

origin

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

104

CHAPTER

5

.TRANSITIONAL REGIMES. 11-EUROPE . . . . . . . . . . . . . .

117

Great Britain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117

Continental Europe

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

141

Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

148

Visean. Tournaisian and Namurian.

117

-

Namurian and Westphalian.

132

Environment

of

deposition. 148

-

Cycle mechanisms. 149

CHAPTER

6.

EPICONTINENTAL MARINE ENVIRONMENTS. I . . . . . . . . . .

157

Calcareous and argillaceous rocks

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

157

Cycles composed of differing types

of

limestone

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

158

Cycles composed of limestones and argillaceous beds . . . . . . . . . . . . . . . . .

163

Limestonedolomite cycles

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

161

Minor cycles. 163 - Major cycles.

170


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CHAPTER

7.

EPICONTINENTAL MARINE ENVIRONMENTS.

I1 . . . . . . . . . 183

Cycles with significant quantities of sandstone. ironstone and phosphorite: minor cycles with

bituminous laminae . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

183

Clay-sandstone-limestone cycles . . . . . . . . . . . . . . . . . . . . . . . . . .

183

Clay-sandstone cycles

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185

Ironstone-bearing cycles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

187

Phosphorite-bearing cycles

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

191

Minor cycles with bituminous laminae . . . . . . . . . . . . . . . . . . . . . . . .

192

CHAPTER

8

.

EPICONTINENTAL MARINE ENVIRONMENTS. I11

. . . . . . . . .

199

Evaporite cycles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

199

Major cycles with subsidiary evaporites . . . . . . . . . . . . . . . . . . . . . . .

200

Cycles with dominant evaporites

. . . . . . . . . . . . . . . . . . . . . . . . . . 204

Evaporite vames and solar cycles . . . . . . . . . . . . . . . . . . . . . . . . . .

212

CHAPTER

9.

FLYSCH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

215

Modal cycles and composite sequences . . . . . . . . . . . . . . . . . . . . . . . .

216

Ideal (model) cycle

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

221

Turbidity currents.

223

- Combined action of turbidity currents and bottom currents.

226

-

Bottom currents.

227

Major and intermediate cycles.

204

- Minor cycles.

209

Megarhythms

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

. . . . . . . . . . . . . . . . . . . . 241

Sedimentary control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

242

Tectonic control

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 244

Eustatic control

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

245

Climatic control. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

248

Cycles and time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

250

REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

253

INDEX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

271


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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,958.)

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,

about himself. (E.

J. ZELLER,964.)


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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 a t 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

VON BUBNOFF

1948)

and

SLOSS

1964),

refer to cycles of the

order of geological systems or more and WELLER1964) 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~,

939, p.502).


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2 INTRODUCTION

PEACH,1888). In the simplest case we may have ABABAB and some authors (for

example FEARNSIDES,950; FIEGE,952) would restrict the term rhythm to this type

of succession. SANDER1936),

on

the other hand, suggested that rhythm be restricted

to those sequences where the rhythmic unit was fairly constant in thickness, whereas

VON BUBNOFF1948) 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. KENDALL1947)

suggested that the term cycle be used only when the repetition is regular in time and

SCHWARZACHER1964), 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 ABCD-

ABCD and ABCDCBA and would restrict the word cycle to the latter. But, as

ROBERTSON1948) 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 (WANLESSnd WELLER,932, 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, 959; HALLAM,961; J.

R.

L. ALLEN, 964a). 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. BEERBOWER1964) has objected to suggestions of this sort


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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 cyclo-

thems, for example, include both symmetrical and asymmetrical cycles

(R.

C. MOORE,

1936; MERRIAM, 963; R.

C.

MOORE nd MERRIAM,965).

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 mega-

cyclothems 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.

MOORE1936; but see YOUNG, 955, 1957), that is to describe a cycle of cycles

(see pp.88-94).

Sequences larger than the megacyclothems of R.

C.

MOORE1936) have been

called

hypercyclothems

and

magnacycles.

WELLER1961) introduced the term hyper-

cyclothem to cover a cycle of megacyclothems while MERRIAM1963, following a

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 JABLOKOVt al. (1961). First order cycles were grouped into second order or

mesocycles, the mesocycles combined into third order or macrocycles and the macro-

cycles 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 addi-

tionSAKAMo~o1957) 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 lithol-

ogy (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


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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, 839) it was considered sufficient o recognise an alternation ofcoal seams

with marine strata. Some workers went a little further. DAWSON1854) for instance

emphasised the combination of lithologies, underclay-coal-bituminous limestone

while PHILLIPS1836) introduced a wholly admirable system which, had it been adopt-

ed, 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 combi-

nation 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 poly-

meric 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 hetero-

polymeric.

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 generalisa-

tion 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 superficially with 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-sand-

stone. HUDSON1924) 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, UDDEN1912), 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, 912, 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

PEACH1888, p.17) Writing

on the Lower Carboniferous of Scotland about the repeated cycles of varying litho-

logies: When the succession is complete the following is the arrangement of strata in


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NOMENCLATUREOFCYCLES

5

ascending order: (I) limestone charged with ordinary marine fossils;

(2)

shales, yield-

ing 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, remain-

ed the standard procedure in studies of cyclic sedimentation for seven or eight decades

following Peachs writing. Consider, for example, TRUEMANS1954) 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 repe-

tition 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 root-

let 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,

954, 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-Gliederund ihre Positionim Ablauf des zyklischen

Sedimentationsvorgangeskennt, wird die naturgemassen Variationen verstehen, die-

gegeniiber 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-hemizyklischen

Gliedern (1 + 2, 1-3,


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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 Hemi-

cyclothem an Gliedern verarmte (Glieder 15,14

+

13, 14 12 ,1 411,1410). Das durch

Kombination beider Verarmungsreihen an Gliedern extrem verarmte Cyclothem

besteht dann allein aus Schieferton.

Es

entstand durch besonders starke Meeres-

Progression, 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 Meeres-

Progressionen iiberhaupt nicht oder nur eben andeutungsweise auswirken konnten.

And in Belgium VAN LECKWIJCK1964, 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

1930s by the work of WELLER1930, 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 WANLESSnd SHEPARD1936) 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 WELLER1956) distinguished

between an idealised standard succession and real cyclothems, real, presumably in

the sense of naturally occurring units. I t seems likely that Wellers 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


18/291

NOMENCLATURE OF CYCLES

7

TABLE I

AMBIGUITY OF

CYCLOTHEM NOMENCLATURE

(After DUFF nd

WALTON, 962)

Cycles which reader might expect to occur

frequently

~

~~ ~

Theoretical or partly idealised cycles not

necessarily of frequent occurrence, if

present at all

Typical (WELLER,930)l

Typical (WELLER,931)l

Normal (WANLESSnd SHEPARD, 936)2

Complete (WANLESSnd SHEPARD, 936)2

Standard (WANLESSnd SHEPARD,936)2

Typical

(WANLESS

nd

SHEPARD, 1936)

Usual (WANLESSnd SHEPARD, 936)

Common (WANLESS nd SHEPARD,1936)

Typical (ROBERTSON,948)

Fully developed (EDWARDS nd STUBBLEFIELD,947)3

Normally developed

(EDWARDS,1951)3

Standard (DUNHAM,

950)4

Commonly developed rhythmic unit (DUNHAM,953)

Normal (R. A.

EDEN

et al.,

1957)

Characteristic (GOODLET, 959)

Full rhythmic sequence (ROBERTSON,948)

Normal (ROBERTSON,948)

Idealised standard WELLER,956)

Theoretical (WELLER,957)

Full (R.

A.

EDENet al., 1957)

Complete (GOODLET,959)

Theoretically expectable composite

succession (WELLS,1960)5

Typical (BEERBOWER,961)6

1

The unit of the two typical cyclothems actually differ though they purport to describe the same

succession.

Used by Wanless and Shepard to describeWELLERS1930) cyclothem although not used byWeller.

Used to describe same cycle.

Dunham cited standard cyclothem or rhythmic unit ofWANLESSnd WELLER (1932)although this

qualifying term was not used by those two authors.

5 Used to describe WELLERS1930) cyclothem.

6

Referring to the theoretical of

WELLER1957).

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

DUFF

and WALTON1962) proposed that it should be called the modal

cycle.

There are, however, also terms (Table I) which have a somewhat different mean-

ing, for example Wellers 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. Nevertheless certain 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;


19/291

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, 962).

Both the modal cycle and the composite sequence are based on actual

rock successions. But another concept runs through writings

on

rhythmic sedimenta-

tion. This is implicit in the terms theoretical, idealised and even, one supposes,

in the theoretical expectable composite succession (WELLS, 960). 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 compo-

site sequence. BEERBOWER1964) 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 ZELLER1964) has pointed out that science, to an extent match-

ed by

no

other human endeavour, places a premium upon the ability of the individual

to make order out of what appears disordered1. 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 successionson 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 defini-

tion 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


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METHODOLOGY

9

C y c l e T y p e s

Fig.1. Histogram showing frequency distribution ofcycle 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. I t 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.

DUFF

nd WALTON1962) 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:


21/291


22/291

METHODOLOGY

11

No.

of

uni ts

in

cycle

A

3

2

1

loo 200 300

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 Penn-

sylvanian 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)

intro-

duced 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

DUFF

nd

WALTON, 962.)

A. Cycles with no marine fossils, divided into

i ) those cycles containing no sandstone and ( i i ) 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 siltstoneor shale

and sandstone.


23/291

12

INTRODUCTION

Given a sequence such as: 24324532, and a proposed ideal sequence: 123454321-

23454321, 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. MOORES1936) ideal cyclothem

(see p.88) which is symmetrical, the observed sequence may begin in a regressive hemi-

cycle (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. PEARN1964) 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 consecutive-

ly). The values of

G

associated with each of these arrangements then gave the probabil-

ity 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. MOOREt 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. Moores 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(SACKINt al., 1965;

MERRIAM nd SNEATH,n 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 ob-

tained. 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 lithol-

ogies are present is to transform the data into a quantitative

form.

This has been done,


24/291

METHODOLOGY 13

for example, by VISTELIUS1961) 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,959) 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:

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 oscil-

latingaround 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 correla-

tion 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


25/291

14

-0.8-

INTRODUCTION

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

correlogramsof 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,

964.)

B.

Correlopam

of

Benbulbin Shale (solid line) with fitted Yule-Kendall

process (dotted line). Scale as in A. (After

SCHWARZACHER,964.)

in the series) is compared with x p + l ,or x p + 2 , or

x p + k .

The regression

of x p

and

x p + l

where the correlationcoefficient s found between

~ 1 x 2 ; 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 rk against k is the correlogram (Fig.4). When the correlogram has been


26/291

METHODOLOGY 15

determined for a given geological succession it can then be compared with suitable

mathematical models. SCHWARZACHER1964) in

his

study of a Carboniferous lime-

stone succession, considered four different cases:

Case I is a stochastic process of moving averages where x at any point

is

deter-

mined 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:

x p

= - U X p - 1

+

E p

while the second order is:

x p

=

-axp-1-bxp-2

+

e p

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 integrat-

ed 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 + e p

V

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 SCHWARZACHERS1964)

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


27/291

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 Lime-

stone, 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 dis-

turbance 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 signifi-

cance 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 (PRESTON nd HENDERSON,964; PRESTONnd HARBAUGH,965):

n = m

+ bn sin

nnz 1

nnz

L

Un COS

- 1

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 maxi-

mum value (or amplitude) of the cosine term at the nth harmonic; bn = the maxi-

mum value (or amplitude) of the sine term at the nth 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 poly-

nomial could be fitted (for example Fox, 1964).

year

A

y e a r

X

I

I

1..

thickness Y

0 C

Fig.5. Illustration of parameters used in harmonic analysis of time series

C

from

an

artificial v a ne

succession

A and varve diagram B. Value ofy indicated at arbitrary position of z.


28/291

METHODOLOGY

17

n - 1 oc

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, he harmonic number, from

n

= 1 to n

= 5.

(Adapted from PRESTONnd

HARBAUGH,

965.)

For time series of any length the coefficientsa,.

. .

n and

bn

can be calculated

(see for example PRESTONnd

HARBAUGH,

965, or for more extended treatment,

WYLIE,960).

These coefficients can then be used to derive

a

set of figures, c t , ci, cg

.

from:

This set constitutes the

pow er spectrum

of the series. The value of c; is an indication of

the contribution of the nth 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 pre-

sent purposes it is sufficient to note that the power or amplitude in one form or an-

other 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 dia-

grams.

This type of analysis has so far been applied to varve successions in 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


29/291

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

and

facies

Two further aspects remain to be mentioned at this stage. The first concerns the scale

on which investigations are carried out. KRUMBEIN1964) has set up a hierarchical

scale involvinga number of levels of investigation from 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 analysis was 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 cos-

micevents. Our position is not to assert that these far-flung speculations are completely

unwarranted but to reiterate that there should be some approximate correlation be-

tween the scale of inference and the scale of observation.

Secondly we might follow LAPORTEnd IMBRIE1964) 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 there-

fore any analysis of sedimentation should take account of both phase and facies-the

one implementing and illuminating he other (LAPORTEnd IMBRIE,964). 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. KRUMBEINS1964) analysis involved a similar approach.

Other studies have laid emphasis on facies variation. WANLFSS t 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 accumu-

lation of the different lithologies led to the construction of successive palaeogeograph-

ic 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


30/291

METHODOLOGY

19

N =

106

? I

1

1

1 I I 1 I

px

M

ZQ 10 8

6 5

4

1 YEARS)

3

I

I

I

1

1 1

1

I

3.S

3

2.75 2s

2 2 5

e

TIYE4RSl

Fig.7. Amplitude

spectrum

of Pleistocene Lake Superior van es (N= 105). Large peak noticeable at

low frequency

(50-100 years) and broad rise between 6 and 14years: f ( C P Y ) = frequency, cycles

per

year. (After R .

Y. NDERSON

nd KOOPMANS,963.)


31/291

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 recon-

struct the physiography and filling of the Pennsylvanian basin of that region.

Facies investigation is obviously facilitated by rigorous time control. The marine

Lias of western Europe is an especially suitable field for the study of widespread facies

changes (HALLAM,961,1964b; 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 im-

portance 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 sedimen-

tation.

CLASSIFICATION

AND

DESCRIPTION

We have adopted a scheme of classification which is based on rather broad environ-

ments 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:

fluvial

lacustrine

ontinental

Transitional

epicontinental

geosynclinal

arine

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 accept-

ed uncritically.


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Chapter 2

CYCLES IN FLUVIAL REGIMES

Deposits in fluvial regimes accumulate under different conditions which can be regard-

ed 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 sedimentation n 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 sediment-

types 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 sedimen-

tation 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.

ALLEN1964a,

1965a,b)

who

also gave comprehensive bibliographies (see also

HARMS

nd

FAHNE-

STOCK, 1965).

CYCLES IN

T H E

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 surface surmounted by(2) a conglomeratic

sandstone often with large clasts of the immediately underlying sediments and cul-

minating 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


33/291

22

CYCLES

IN

F L W I A L REGIMES

MAIN FACTS

...

Scoured surface

Red. coarse siltstone devoid of bedding

Sparse calcium carbonate concretions.

Invertebrate burrows in lower part.

Suncracks absent.

Variable thicknesr of red. ripple -drift

/

bedded, very fi ne sandstone. Grades up

into silts tane . Invertebrate burrows.

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-stra ta near base

and middle.

/

Cut on siltstone. Maximum re lief 15 cn

Few directionol scour structures.

GENERAL LEGEND

Ripple-bedded fine to

medium sandstone

Ripple-bedded very

fine sandstone

introfarmational conglomerate

Flat-bedded fine to

medium sandstone

Flat-bedded very

fine sandstone

Cross-stratified coane to

very coarse sandstone

Cross-stratified fine to

medium sandstme

Trough crass-stratifled fine

n assive medium s&

to medium sandstone

....

.

INTERPRETATION

Vertical accretlon deposit from overbank

floods. Probably deposlted in backswamp

area, perhaps

a nmre OT

less permanent

lake.

Vertical accretion deposit from overbonk

floods. Possibly a levee dep osit or a

paint- bar swale filling.

Channel deposit probably formed by

latera l 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.

Erosion at deepest part of wandering

river channel.

Rippled bedding plme

Siltstone

Carbonate concretions

m

El

R

onvolute lamination

Contorted cross-strata

~~

Invertebrate burrows

ipple-bedded coarse sandstone Massive very fine sandstone

Fig.8. Generalised succession and interpretation

of

Downtonian cyclothem (cycle A) at Ludlow,

Shropshire. (After

J. R.

L. ALLEN, 1964a.)


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CYCLES IN THE OLD RED SANDSTONE OF BRITAIN

23

MAIN FACTS

I

5

4

3

ul

1L

w

I

2

0

Red coarse siltstone dewid of

bedding, grading up from very fine

on erosional surface.

No

proofs of

exposure.

11 sandstone at base. Sandstone lenticle

Medium to fine green sandstme

on

parallel to channel side concc idont on

scoured surface. S cattered siltstone

Medium to very fine green sond-

stonas with siltstone clask. planar

cross-stra tified or fiat-bedded and with

primary current lineation.

INTERPRETATION

Vertical accretion deposit

on

flood-

plain topstratum from overbank floods.

Bockswanp deposit probably formed in

more or less permanent lakes.

Channel-fill deposit. Plug from

longitudiml currents after channel wos

cut.

Channel cut occross sand bar.

Channel deposit probably formed by

loteral accretion

on

point-bar. Strong

variable currents.,, Sand transported in

strai ght- cres ted dunes . Wave action on

beaches exposed

at

low river Stage.

La g deposit farmed i n deepest parts

of

channel.

Erosion

In

deepest parts of

wandering river ctmnnel.

Fig.9. Generalised succession and interpretationof 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-grain-

ed 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 lithol-

ogies. Above this sequence there are a t 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 con-

cretions. Cycle

D

(Fig. 11) is remarkable for the episodes of channelling recorded in the

lower sandstone beds; cycleE (Fig.12) has a number of intraformational conglomerates


35/291

24

CYCLES IN

FLUVIAL

REGIMES

MAIN FACTS

Thick red, coarse siltstones with lentides

m d persistent beds

af

very fine. ripde-

bedded sondstone. Invertebrate burrows

d

severol horizons.

No

suncracks.

Abundant calcium corbonote concretions.

/

Thick red. coarse siiistones with

lenticles ond persistent beds

of

very fine

to medium sandstones. Suncrocks at

three horizons. Sandstones ripple-bedded

with sharp, ripp led tops. C onvolute

lamination and dump bails. Invertebrate

burrows.

Planar craas-stratified, fine to medium,

purple sondstone with contorted foresets

locally. Thin siltstone and very fine

ripple-b edd ed sandstone, both lenticular.

Scattered siltstone clasts. in:raform-

aliona l cong lomerate ot base.

Cut on siltstone. Relie f low. Smoil-

scale channels.

Rapid alternation of lenticular sond-

stones ond siltstones. Sandstones mostly

white, ca m e, cross -stratified ; sharp bases,

Wen erosional. and sharp rippled

or

smooth tops. Siitstones pale green

and unbrdded.

Cut on sinstone. Re lief low.

INTERPRETATION

Vertical accretion deposit from overbank

flocds. Mo stly back- deposits with

c w s e intercoiations representing toes

of

ievees or crevasse-splays. Conaetirms

sugqest fiuctud i- groundwater table and

exposure.

Vertical accretion depusit from overbon

floods. AltW Mte submergence and

exposure. Complex of levee, backswomp

and perhaps crevosse-splay deposits.

Active river channel ot

o

distance.

Channel deposit proboMy fwm ed 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

chonne

i

Erosion in deepest parts of wanderinq

freshwater channel encroaching on tidal

r iver.

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 .

Erosion at f loor of t idal channel.

Fig.10. Generalised succession and interpretation of Dittonian cyclothem (cycle C) at Lydney,

Gloucestershire. For legend

see

Fig.8 . (After J.

R. L. ALLEN,

964a.)


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CYCLES

IN

THE

OLD RED SANDSTONE OF BRITAIN

25

~~

MAIN

FACTS

INTERPRETATION

FUlr

and coven channd. Red. flat - Channrl-fil l and latmral accretion

r u n wc k o d & l t S t O M .

In

form

of

chonnal. ReIU

about 4.0h.

5

Some siltstone.

Fig.11. Generalised succession and interpretation

of

Dittonian cyclothem (cycle D) at Tugford,

Shropshire. For legend see Fig.8. (After J. R.

L.

LEN964a.)


37/291

26

CYCLES IN FLUVIAL REGIMES

r

MAIN FACTS

Red coarse siltstones with invertebrate

burrows, ripple-be dded sandstone lentlcles,

and convolute laminations. No evidence

of exposure.

Red coarse siltstones alternating

with beds or biscuitd of ripple-bedded,

very fine sandstone. Invertebrate

No proofs of exposure

Red, flat - o r ripple-bed ded very

fine to fine sandstone with a channeled

scoured surfoce in lower Dart.

Scattered siltstone ciasts.

2

Intrafo rmation al conglomerotes on

scoured surfaces alternating with green

1 siltstones and very fine to fins sand-

stones, showing ripple-bedding, flat-

bedding or convolute lamination.

Concentrations of plant deb ris and

l

. .

. . .

.

.

. .

. , . ._. ,

,. ,.

.

,. ..,. I

ostracoderms. some af latter articulated.

Scoured surface of low relief cut

INTERPRETATION

Vertica l arcretion deposit from

overbank floods. Backswamp area. proba bly

a permanent lake.

Vertical accretion deposit from

overbank floods . Levee and backswamp

deposits with area possibly a lake

for long periods.

Probably mixed channel-fil l ond

lateral accretion deposits. Deposition of

suspended and bed loads on channel

bars and sand flats. Deepening or

wandering of channel at times.

Mixed channel-fil l and channel lag

deposits. Repeated m igration and

partial oggradation of channel. Flotsam

of floodpialn plants and riverine

ostracoderms deposited in or near

active channel.

Erosion a t floo r of wand ering river.

Fig.12. GeneraIised succession and interpretation of Dittonian cyclothem cycle E) at Abergavenny,

Monmouthshire.

For

legend

see

Fig.8. After

J. R. L. ALLEN,

964a.)

in its lower portion and cycleF (Fig.13) has a notable series

of

potholes in one surface

within the lower half while the lowest sandstone member is exceptional in having flat-

bedded 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

cycleD (Fig. 11) the presence of siltstone

(9)

nd the channelling above might have been


38/291

CYCLES

IN

THE OLD RED SANDSTONE OF BRITAIN

MAIN FACTS

Alterna tion of thin sandstones and

siltstones. Red sandy coarse siltstones

with invertebrate burrows and rare

carbon ate concretions. Very f ine to fine

poorly sorted sandstones. flo t- or rlpp ls

bedded

or

mossive. Commonly rest on

suncracked or eroded surfaces. Tops

gradat ional or sharp with ripples.

Invertebrote burrows.

27

I

INTERPRETATION

Verticol accre tion dDposlt from

overbonk floods. Backswamp deposit with

intercalated levee tongue. Fluctua ting

groundwater table and periodic

exposure.

Vertico l accret ion deposit from

overbonk floods. Dep osit ion of

suspended load vio be d-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-fi l l and

lateral accretion deposit. Deposition of

bed-loa d in channels, shallow and

probably shif ting an d braided, wit h some

wave act ion on exposed bonks and

bars. Local channel lag deposits.

Erosion

at

floor of wandering

river channel.

Fig.13. Generalised succession and interpretation of Breconian cyclothem (cycle F) at Mitcheldean,

Gloucestershire. For legend

see Fig.8.

(After J. R.

L. ALLEN,

964a.)

taken to indicate

a

separate cycle. In this connection J . R.

L.

ALLEN1965a) 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.


39/291

28 CYCLES IN n U V I A L REGIMES

POIN T-B AR CREVASSE- SPLAY CHANNEL-FILL

- - - - - - - - -

Fig.14.

Block

diagram illustrating the developmentof flood-plain deposits in relation to a meandering

channel. (After J. R. L. ALLEN,964a.)

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, 965b, for comprehensive survey).

These are-together with their location of development (see Fig.14): (a)vertical accre-

tion deposits (levee and back swamp); (b) splay deposits (crevasse from channels);

(c)

lateral accretion deposits (point bar and channel bar);

( d )

ag 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 de-

crease 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 sun-

cracks, 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.


40/291

CYCLES IN THE OLD

RED SANDSTONE

OF BRITAIN

29

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 accumulat-

ing lunate dunes or by repeated scour and fill units. Ripple cross-lamination may be

ubiquitous. Active bank erosion produces

a scattering

of

penecontemporaneous clasts

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 fo rm

Flow

conditions

Small-scale Small-scale

cross-stratification ripples

Low intensity

lower-flow regime

Large-scale Large-scale High intensity

cross-stratification ripples or dunes lower-flow regime

(sets assembled in cosets)

Flat-bedding with primary Plane beds with sedim ent Upper-flow regime

current lineation movement

Channel-lag

deposits form the coarsest-grained material of the alluvium. They

comprise detrital material from the source rocks and clasts derived from penecontempo-

raneous 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-


41/291

30 CYCLES I N 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,962).

The model of sedimentation which

J.

R. L. ALLEN1964a) proposed and ap-

peared 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 fill

Documents

10 - Cyclic Sedimentation